GreenCarbon ETN Book

See discussions, stats, and author profiles for this publication at: https://www.researchgate.
net/publication/334697497
Refined biocarbons for gas adsorption and separation
Chapter · May 2019
CITATIONS READS
0 1,222
4 authors, including:
Sabina Alexandra Nicolae Xia Wang

Queen Mary, University of London Stockholm University
7 PUBLICATIONS 25 CITATIONS 8 PUBLICATIONS 48 CITATIONS
SEE PROFILE SEE PROFILE
Magda Titirici
Imperial College London
308 PUBLICATIONS 20,352 CITATIONS
SEE PROFILE
Some of the authors of this publication are also working on these related projects:
Pyrolysis View project
Postdoc UCSB View project
All content following this page was uploaded by Sabina Alexandra Nicolae on 26 July 2019.
The user has requested enhancement of the downloaded file.

GreenCarbon European Training Network
Edited by Joan J. Manyà

May 2019
Advanced Carbon Materials from Biomass: an Overview
Edited by Joan J. Manyà
May 2019

This project has received funding from the European Union’s Horizon 2020 Research and Innovation
Programme under grant agreement No 721992
Copyright © GreenCarbon Project and Consortium

http://greencarbon-etn.eu
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License
(https://creativecommons.org/licenses/by-nc/4.0)
A free online version is available from zenodo.org
DOI: 10.5281/zenodo.3233733

2
Preface
This book aims at providing a state-‐of-‐the-‐art review about the production, refining and application of
biomass-‐derived carbon materials. The energy crisis, environmental pollution and global warming are
serious problems that are of great concern throughout the world. A round 40% of world´s current energy
consumption is dedicated to the production of materials and chemicals, which are mostly derived from
fossil fuels. These materials need to be simple to synthesise, as cost effective as possible and ideally
based on renewable resources. Carbon materials are ideal candidates for performing many of these
functions. In the past decade, the nanostructured forms of crystalline carbon (fullerenes, carbon
nanotubes and graphene) have received the most attention due to remarkable and unusual
physicochemical properties. However, the main disadvantage of using these crystalline nanocarbons for
energy and environmental related applications is their high production costs. Alternatively, carbon
materials derived from renewable resources (e.g., lignocellulosic biomass) could play a very powerful
role in this d irection in the n ear future.
The main objective of the GreenCarbon European Training Network (MSCA-‐ITN-‐ETN-‐721991) is to

develop new scientific knowledge, technology, and commercial products and processes for biomass-‐
derived carbons. This will be accomplished through outstanding research and training programmes for
fourteen early-‐stage researchers (ESRs). As part of the training programme, all the ESRs have co-‐
authored at least one chapter of the present book. The respective chapters served as pieces of literature
summing up the aims and science of the network. Furthermore, this collaborative training activity has
provided the ESRs with additional insights into many a spects of the process of writing/editing.
Regarding the organisation and contents of the book, its chapters can be divided into four parts: (1)
production of pristine pyrochars from dry biomass feedstocks; (2) production of pristine hydrochars
from wet biowastes (including lignin); (3) refining of pristine pyrochars and hydrochars for advanced
applications in the fields of adsorption, catalysis and electrochemical conversion; and (4) carbon stability
and sequential uses of biochars (i.e., biomass-‐derived carbon applied to soil). The detailed contents for
each part a re summarised b elow.
Chapters 1–4 focus on the production of biomass-‐derived chars through slow pyrolysis. In chapter 1, the
effects of pyrolysis operating conditions on the physicochemical properties of the resulting char are
discussed. In chapters 2 and 3, the technologies currently available for the continuous production of
char are reviewed. For its part, chapter 4 provides an overview of modelling approaches applied in the
thermochemical conversion of biomass.
Since the development of value-‐added lignin-‐based materials will be crucial to the economic success of
wood-‐to-‐chemicals biorefineries, chapter 5 deals with the production and characterisation of lignin-‐
containing streams produced within a lignocellulosic biorefinery framework. These lignin-‐containing
wastes can b e converted into value-‐added activated carbons via sequential hydrothermal carbonisation
and activation. Both p rocesses are widely d escribed in chapter 6.
Chapters 7–10 focus on the functionalisation and nanostructuring of pyrochars and hydrochars to be
used in adsorption, heterogeneous catalysis, electrocatalysis and direct-‐carbon fuel cells. In chapter 7,
the combination of hydrothermal carbonisation and salt templating is presented as an interesting
pathway to produce engineered porous carbons for adsorption of CO2 and SO2 in gas phase. The
potential of biochar-‐derived activated carbons as catalysts or catalyst supports is reviewed in chapter 8.
In chapter 9, an overview of the most recent advances in the use of hydrochars in catalysis and
electrocatalysis is provided. Finally, chapter 10 d eals with the use of b iomass-‐derived carbons as fuels in
direct-‐carbon fuel cells.
The most common biochar stability test methods, which are aimed at predicting the long term
behaviour of biochar after its addition to soil, are explained and compared in chapter 11. Finally, chapter
12 contains a d escription of potential synergies in sequential biochar systems. In this sense, recycling of
used biochars could maximise the value a chieved a cross the chain. Examples of applications relevant to
these systems a re also briefly d iscussed.
4
Acknowledgements
I would like to express my d eep appreciation to the many p eople who contributed to this book. First of
all, a big thank you to all the Early-‐Stage Researchers involved within the GreenCarbon European
Training Network for their enthusiastic participation: Gianluca Greco (ch. 1), Filipe Rego (ch. 2), Jorge
López-‐Ordovás (ch. 3), Przemyslaw Maziarka (ch. 4), Qusay Ibrahim (ch. 5), Pablo Arauzo (ch. 6), Sabina
Nicolae and Xia Wang (ch. 7), Christian Di Stasi (ch. 8), Pierpaolo Modugno and Anthony Szego (ch. 9),
Maciej Olszewski (ch. 10), Dilani Chathurika (ch. 11), and Christian Wurzer (ch. 12).
I’m also grateful for the ongoing support of supervisors from the different nodes within GreenCarbon:
Jiawei Wang, Yang Yang, Katie Chong and Tony Bridgwater from Aston University; Frederik Ronsse f rom
Ghent University; Moritz Leschinsky from the Fraunhofer Centre for Chemical-‐Biotechnological Process;
Andrea Kruse from University of Hohenheim; Magdalena Titirici from Queen Mary University of London;
Niklas Hedin from Stockholm University; and Saran Sohi and Ondrej Masek from the University of
Edinburgh.
Thanks also to Belén González for her help throughout the editing process and Pompeyo Planchart for
his volunteer contribution to the cover and formatting.

Joan J. Manyà, PhD
Associate Professor of Chemical Engineering, Universidad de Zaragoza
Coordinator of the GreenCarbon ETN

6
Table of contents
Chapter 1 ............................................................................................................................................. 9
Operating conditions affecting char yield and its potential stability during slow pyrolysis of
biomass: a review.
Chapter 2 ........................................................................................................................................... 21

Pyrolysis of biomass and wastes in a continuous screw reactor for char production.
Chapter 3 ........................................................................................................................................... 33

Industrial t echnologies for slow pyrolysis.
Chapter 4 ........................................................................................................................................... 45

Matter of modelling in thermochemical conversion p rocesses of biomass.
Chapter 5 ........................................................................................................................................... 57

Production and characterisation of lignin and lignin-‐containing streams obtained through a t wo-‐
stage fractionation process of lignocellulosic b iomass.
Chapter 6 ........................................................................................................................................... 71

Review of the conversion f rom hydrochar to a ctivated carbon.
Chapter 7 ........................................................................................................................................... 83

Refined biocarbons for gas adsorption and separation.
Chapter 8 ......................................................................................................................................... 101

Biochar as sustainable platform for pyrolysis vapours upgrading.
Chapter 9 ......................................................................................................................................... 111

Hydrothermal carbonisation and its role in catalysis.
Chapter 10 ....................................................................................................................................... 125

Bio-‐based carbon materials for a direct-‐carbon fuel cell.
Chapter 11 ....................................................................................................................................... 135

An overview of the m ethods used in the a ssessment of biochar carbon stability.
Chapter 12 ....................................................................................................................................... 147

Synergies in sequential b iochar systems.
8
GreenCarbon ETN Book Chapter 1
Operating conditions affecting char yield and its potential stability during slow
pyrolysis of biomass: a review
Gianluca Greco, Belén González, Joan J. Manyà
Aragón Institute of Engineering Research (I3A), University of Zaragoza, crta. Cuarte s/n, Huesca E-22071, Spain
Abstract
The energy crisis, environmental pollution, global warming and the food produtivity are serious problems, which have recently
generated a growing interest in developing new technologies focused on reducing the greenhouse gas emissions and increasing the
carbon sinks. A promising solution for such issues is char, a form of charred organic matter, which is possible to apply to soil in a
deliberate manner as a means of potentially improving soil productivity and carbon sequestration. For this purpose, a common route
to produce char with high yield from biomass is slow pyrolysis. Given the high number of variables affecting the process and the wide
range of available biomass sources, a large variability in the yield and properties of the produced char should be expected. Therefore,
one of the main challenges nowadays is to optimise the process conditions of the pyrolysis process for a given biomass feedstock with
the aim to obtain a char with the desired properties to be used for a given application. This chapter aims to provide a review on the
available alternatives to produce char and the main effects of the process parameters on the char yield and its potential stability.
The Intergovernmental Panel on Climate Change (IPCC)

1. Introduction published the Fourth Assessment Report (AR4) in 2007, which
concluded that the average global surface temperature
1.1. The global problem of CO2 emissions
increased by 0.74 ± 0.18 °C over the 20th century, and that
The fossil fuel resources of our planet, estimated between 4000 “most of the observed increase in global average temperatures
and 6000 Gt of carbon1 are the product of biological and since the mid-20th century was very likely (>90% probability)
geologic processes that occurred over hundreds of millions of due to the observed increase in anthropogenic GHG
years ago and still continue nowadays. The carbon sequestered concentrations”.1 Anthropogenic change also reduced the
in these resources was originally a constituent of the effectiveness of certain climate feedback mechanisms: changes
atmosphere of a younger Earth. Around 360 million years ago, in land use and land-management practices have reduced the
such atmosphere possessed about 1500 parts per million (ppm) ability of soils to build soil carbon inventory in response to
CO2.1 Since the dawn of the industrial age (year 1750 ca), about higher atmospheric CO2 concentration, while the ocean
5% of these resources was combusted and an estimated acidification has reduced their capacity to take up additional
amount of 280 Gt-C released back into the atmosphere in the CO2 from the atmosphere.1 In the absence of CO2 mitigation,
form of CO2. In the same period, a further 150 Gt-C was released the resulting emissions will lead to further increase in
to the atmosphere from soil carbon pools as a result of changes atmospheric CO2 concentration, causing an enhancing warming
in land use. The atmospheric, terrestrial and oceanic carbon and inducing many changes in the global climate. Even if the
cycles dispersed the most part of these anthropogenic CO2 concentration is stabilised before 2100, the warming and
emissions (i.e., by locking the CO2 away by dissolution in the other climate effects are expected to continue for centuries,
oceans and in long-lived carbon pools in soils1). Since the due to the long-time scales associated with climate processes.
industrial age, the CO2 concentration in the atmosphere has The climate predictions suggest warming over a multicentury
increased from 280 ppm to 368 ppm in 2000. This increase in time scale in the range from 2 to 9 °C.1 Hence, it is necessary to
CO2 concentration in air influences the balance of incoming and urgently find a solution to reduce the CO2 emissions and,
outgoing energy in the Earth-atmosphere system. This is why consequently, to mitigate the long-term climate change.
CO2 is considered the most significant anthropogenic
1.2. Ameliorating physical and chemical properties of
GreenHouse Gas (GHG).1,2 Two of the first researchers on the
soils
influence of CO2 on global warming were Tyndall (1859)3 and
Arrhenius (1896). Through their experiments, they determined An intensification of agricultural production on a global scale is
that the fossil fuels produce a substantial amount of carbon required in order to ensure the food supply for an increasing
dioxide and observed that their combustion would cause an world population. Glaser et al.8 conducted a study on the
increase in the average global surface temperatures.4 The most improvement of the physical and chemical properties of soils in
part of CO2 emissions is due to the energy production5, followed the tropics. They stated that such worldwide need caused a
by industry. Around 40% of global anthropogenic CO2 emissions reduction of fallow periods in shifting cultivation in the humid
comes from industrial processes. For example, 60% of the CO2 tropics, leading to irreversible soil degradation and to the
produced during cement manufacture comes from the required destruction of remaining natural forests due to cultivation of
calcination of limestone6, producing CaO and CO2 from CaCO3. new areas after slush-and-burn.9 In many tropical
Another example is that iron and steel manufacture requires CO environments, soils lack nutrient contents, meaning that the
to reduce Fe2O3 ore, forming CO2 and Fe. The release of CO2 is sustainable agriculture is constrained in such areas. The low
inevitable without a radical redesign of such processes and nutrient contents typically accelerate the mineralisation of soil
many others like them.7 organic matter (SOM).10,11 Consequently, the cation exchange
This book is published under a CC BY-NC 4.0 license

9
Chapter 1 Greco et al.
capacity (CEC) of the soils, which is often low due to their clay lignin24. Unlike cellulose, hemicelluloses are heterogeneous
mineralogy, decreases further. In these circumstances, the groups of branched polysaccharides. The structural elements
efficiency of applied mineral fertilisers becomes lower.12,13 In are monomers such as glucose, galactose, mannose, and xylose.
addition, many farmers cannot afford the costs of regular Their structures are amorphous, with low physical strength.
applications of inorganic fertilisers.8 Lignin (see Fig. 1c) is mainly present in the outer layer of the
The most common route to improve cultivations in the tropics fibres and is responsible for the structural rigidity and holding
is to employ slash-and-burn techniques. When the biomass the fibres of polysaccharides together. It plays a binding role
burns, the nutrients are rapidly released into the soil. However, between hemicelluloses and cellulose. Lignin is an aromatic,
they have positive effects on soil fertility only for a short three-dimensional and cross-linked phenol polymer consisting
period.14–16 Furthermore, the biomass burning releases certain of a random assortment “hydroxyl-“ and “methoxy-“
amounts of the greenhouse gases CH4 and N2O, enhancing the substituted phenylpropane units, whose monomers can be
level of global warming.17 categorised as syringyl, p-hydroxylphenyl and guaiacyl
Organic matters such as manures, mulches and composts have units.26,27 The lignin’s structure varies with the type of biomass;
frequently been applied to increase soil fertility.8 However, for instance, the hardwood lignin has a high methoxyl content,
organic matter is usually mineralised very rapidly under tropical due to the presence of both guaiacyl and syringyl units, whereas
conditions10 and only a small portion of the applied organic the softwood lignin is only composed of guaiacyl units.28
matter will be stabilised in the soil in the long term, and it will Extractives and inorganic components are also present in
then be released to the atmosphere as CO2.18 smaller quantities in the biomass. Extractives such as alkaloids,
An alternative pathway for the soil amendment is to employ essential oils, fats, gums, and proteins act as intermediates in
more stable compounds, such as carbonised materials. This metabolism, as energy reserves, and as plant defences against
class of materials comprises a wide range of materials, from microbial and insect attack.27 The inorganic fraction mainly
charred material (e.g., char) to graphite and soot particles.19 consists of compounds of potassium, calcium, silicon, sodium,
Several investigations20,21 have been carried out on this topic, chlorines and phosphorus.24 Their fraction in the biomass
showing that carbonised materials produced from the ranges from less than 2% to as much as 15%.24
incomplete combustion of organic material (i.e., black C,
pyrogenic C, char) are responsible for maintaining high levels of
a)
SOM and available nutrients in anthropogenic soils of the
Amazonia. These soils are called Terra Preta and they are
characterised by a large amount of black C, indicating a high and
prolonged input of carbonised organic matter, probably due to
the char production in hearts, whereas only low amounts of
char are added to soils as a consequence of forest fires and
slash-and-burn techniques.17,18
2. Biomass and char b)

2.1. The potential of biomass resources
Among renewable energy resources, biomass is one of the most
plentiful and well-utilised sources in the world.22 Biomass is
defined as the plant material derived from the reaction
between CO2 in the air, water and sunlight by photosynthesis
process which produces carbohydrates, the building blocks of
biomass.23 In particular, the term lignocellulosic biomass refers
to non-edible dry plant matter, which is an abundant and low-
cost source of renewable energy.24 Biomass resources include c)
woody, herbaceous and aquatic plants as well as animal
manures and processing residues.25 The structure of
lignocellulosic biomass is typically composed of three main
building blocks: cellulose, hemicelluloses and lignin. Cellulose
(see Fig. 1a) is the most abundant organic polymer24, present in
the cell wall of the plant cells. It is a natural polymer of
repeating D-glucose unit, a six-carbon ring. Cellulose shows
unique properties of mechanical strength and chemical
stability, due to its crystalline structure, originated from the Figure 1. Chemical structures of a) cellulose, b) a fraction of
hemicellulose, and c) a fraction of lignin.
three hydroxyl groups in the six-carbon ring which can react
among themselves, forming intra- and intermolecular hydrogen
bonds. Hemicelluloses (see Fig. 1b), which surrounds the
cellulose fibres, represent the link between cellulose and
10
2.2. The biochar concept • Energy industry to produce electricity.

Concerns about climate change and food productivity have • Chemical industry for production of carbon
recently generated interest in biochar29, a form of charred disulphide, sodium cyanide and carbide.
organic matter, which is applied to soil in a deliberate manner • Smelting and sintering of iron ores or purification of
as a means of potentially improving soil productivity and carbon nonferrous metals. Most valued reductant in
sequestration.30 The idea of adding char into soil to obtain a metallurgical industry.
fertility improvement was inspired by the ancient agricultural
practices which led to the creation of terra preta soils31. A • Water and gas purification, solvent recovery and
relevant number of studies are focused on the benefits of using wastewater treatment.
char in terms of mitigating global warming and as a solution to • Medical industry for poisons adsorbent
manage soil health and productivity.8,32–35 However, these • Skincare, as part of a scrubber or toothpaste
studies were often geographically limited and constrained by
limited experimental data, due to the complexity of the Depending on the final use of the char, different properties are
research tasks29. desirable. Therefore, different processes are needed to obtain
The term char refers to a carbon-rich, fine-grained, porous such wide variety of chars. For example, the desired property in
substance, produced by thermal decomposition of biomass energy production is a high heating value41, whereas a high
under oxygen-limited conditions and at relatively low surface area and a good compressive strength are desired
temperatures (i.e., below 700 °C).30,31 The definition stated by properties in the field of catalytic activation42 and metallurgical
the International Biochar Initiative (IBI) clarifies the need for industry39, respectively.
purposeful application of this material to soil for both
agricultural and environmental gains.31 Nevertheless, the
3. Char production
commercial exploitation of char as a soil amendment is still in
its infancy31, since the relationship between its properties and 3.1. Pyrolysis
its applicability into soil is still unclear29. For this reason, a In recent years, converting biomass into energy has been based
synergic collaboration between different fields of research is mainly on biochemical and thermochemical processes.
needed: production and characterization of char on one side, Among all the thermochemical processes, pyrolysis is
and investigation of the char behaviour in the agricultural soils considered one of the most promising pathways, since its
on the other side, measuring both environmental and conditions could be optimised to produce any of the three
agronomical benefits29. Differently from char, charcoal is used products, pyrolytic oils with a high energy density, the char with
as a fuel for heat, as an adsorbent material or as a reducing key properties for soil amendment or the gas to produce
agent in metallurgical processes.30 energy43. Pyrolysis is the term given to the thermal
One of the most interesting properties of char is its porous decomposition of organic matter within an environment with a
structure, which is believed to be responsible for improved limited oxygen content. The heating of biomass in an inert
water retention and increased soil surface area.31 Furthermore, atmosphere results in the production of permanent gases (e.g.,
an increase in the nutrient use efficiency has been observed CO2, CO, CH4 and H2 and light hydrocarbons), and an organic
when the char was added to the soil. This is due to the nutrients volatile fraction from the cellulose, hemicelluloses and lignin
contained in the char as well as the extent of physicochemical polymers. These vapours can be condensed, giving an organic
processes, which allow a better utilisation of soil-inherent or liquid known as bio-oil. The bio-oil composition mainly depends
fertiliser-derived nutrients. In addition, a key property of char is on the lignocellulosic composition of the biomass.44 The non-
its biological and chemical stability36,37, allowing it to act as a condensable gases leave the reaction system and can be
carbon sink.31 Therefore, the conversion of biomass to long- employed to provide heat for self-sustain the pyrolysis process.
term stable soil carbon species can result in a long-term carbon The remaining solid carbon-rich product is named char.
sink, as the biomass removes atmospheric carbon dioxide Yang et al.45 investigated the pyrolysis behaviour of cellulose,
through photosynthesis38, leading to a net removal of carbon hemicelluloses and lignin using thermogravimetric analysis
from the atmosphere. (TGA). They pointed out that the decomposition of
According to the above-explained considerations, three hemicelluloses occurs at 220–315 °C, whereas the cellulose
complementary goals can be achieved by using char decomposes at 315–400 °C. Differently from the cellulose and
applications for environmental management: hemicelluloses, the decomposition of lignin occurs in a wider
• Soil improvement, from both pollution and range of temperature: from 160 to 900 °C. Furthermore, the
productivity point of view. highest solid residue derives from the pyrolysis of lignin.
• Energy production, if energy is captured during the The operating conditions of the pyrolysis process, such as
char production process. highest (or peak) temperature, heating rate, residence time,
• Waste valorisation, if waste biomass is used for this pressure, etc. markedly affect the nature and yields of
purpose. products.46 Thus, the process conditions can be adjusted as a
Beyond its usefulness as soil amendment and as a mean for the function of the desired product. It is well known that high
carbon sequestration, char can be used in a wide range of temperatures and long residence times of the vapour phase
applications39,40: enhance the secondary reactions, leading to a higher
production of non-condensable gases, whereas high
11
temperature and shorter residence times favour the formation pressure of 1‒2 MPa, and a flash fire is ignited at the bottom of
of condensable products. On the other hand, solid products are the packed bed. Air is delivered to the top of the packed bed
usually favoured at low temperatures.47 Hence, the pyrolysis after a short time and the biomass is converted into char. The
processes can be categorised in two main types (slow and fast) reaction lasts less than 30 min and the temperature profile is
depending on the operating conditions employed. affected by several factors, such as the type of biomass, the
total amount of air delivered, the heating time and the moisture
3.1.1. Slow pyrolysis
content of the feedstock60. Regarding the process yields, Antal
Slow pyrolysis is one of the most promising ways to obtain an et al.60 carried out a study on the flash carbonisation behaviour
elevated char yield. It is carried out at temperatures of of some woods and agricultural byproducts. They realized a
250–400 °C39,40, despite there are wider ranges in the literature char yield ranging between 29.5% and 40%, fixed carbon yields
(300–700 °C48) and slow heating rates (usually lower than from 27.7% to 30.9%, and energy conversion efficiencies of
10 °C min–1), for long solid and vapour residence times48 (from biomass into char of 55.1%–66.3%.
minutes up to hours and even days). Long residence times of The flash carbonisation process seems to be a promising
vapours within the reactor allow intermediate volatiles to alternative route to produce char, since the reaction times are
continue to react among them, producing further char and very short compared to those of the slow pyrolysis process (in
gas27. Therefore, slow pyrolysis is aimed to produce large batch operating mode), and even because it would be possible
amounts of char, although significant quantities of bio-oil and to retain a larger amount of fixed carbon from the feedstock.60
gas are produced49. Slow pyrolysis has been used for thousands
3.3. Hydropyrolysis
of years for char production and sometimes it is called
conventional pyrolysis, although the technology used at the Hydropyrolysis is an alternative pyrolysis process, which is
beginning was based on kilns39,50 and there are current performed in a reductive hydrogen environment instead of an
technologies with a higher level of development50,51. inert one. In hydropyrolysis, the reducing H2 gas generates
Slow pyrolysis is sometimes confused with torrefaction, which hydrogen radicals, which react with the volatiles released from
is normally performed at lower temperatures in order to partly the biomass, leading to the production of H2O, CO and CO2, and
decompose the feedstock.50 Torrefaction leads to a partial hydrocarbons.65 Furthermore, many of the reactive volatiles
degradation of the hemicellulose for densification purposes.52 released by the process are capped by the hydrogen radicals,
This process is typically performed at a temperature comprised avoiding to undergo polymerisation.66,67 This leads to the
between 200 °C and 350 °C39,48,52,53, and it is characterised by a formation of hydrocarbons with higher selectivity, if compared
higher solid yield (around 80%) than that of slow pyrolysis to that of the fast catalytic pyrolysis.68,69 One of the advantages
(30%–35%).48,53 of hydropyrolysis is that the process is globally exothermic.
However, this process requires the supply of H2 and an
3.1.2. Fast pyrolysis
operating pressure of 3.0 MPa. Further studies are needed to
Fast pyrolysis uses high heating rates (above 200 °C min–1) and assess the viability of this technology from both technical and
short vapour residence time (around 2 s) at temperatures of economic points of view.
500‒550 °C.29 These operating conditions particularly promote
the formation of liquid products (bio-oil) at the expense of
char.54 Generally, fast pyrolysis processes produce 60%‒75% 4. Influence of process parameters on the
(wt.). of bio-oil, 15%‒25% of solid char, and 10%‒20% of non- char yield and its stability during slow
condensable gases, depending on the feedstock used.
pyrolysis
From a chemical point of view, the char obtained at high
heating rates shows a higher oxygen content55 and a lower Given the high number of variables affecting the process (e.g.,
calorific value56, compared to the char produced through slow pressure, peak temperature, heating rate, gas residence time)
pyrolysis. This could be due to the shorter residence time of the and the wide range of available biomass sources, a large
vapour fraction, which inhibits the extent of secondary charring variability in the char properties should be expected.29 In other
reactions. However, the char obtained through fast pyrolysis words, the carbon sequestration potential of produced char can
can possess a larger specific surface area for two reasons: first, largely be dependent on process operating conditions for a
the relatively small size of the feedstock usually required for given feedstock. Therefore, one of the main challenges
such process29, and second, the faster devolatilisation, which nowadays is to optimise the process conditions of pyrolysis for
can lead to more fragmented particles57 and a certain a given biomass feedstock70,71 with the aim of obtaining an
development of large micropores and mesopores at the appropriate yield of char with high stability, an essential
expense of narrow micropores (which are dominant in slow requirement for its use to capture CO2 and as soil amendment.
pyrolysis-derived chars). In addition, analysing the effects of pyrolysis process conditions
on additional physicochemical properties of char (e.g., porosity,
3.2. Flash carbonisation
functional groups in surface, conductivity, etc.) is also needed
Flash carbonization is a novel technology developed at the to explore further uses of char in, for instance, adsorption,
University of Hawaii.29 The process allows the biomass to catalysis and electrochemical applications.
convert into char in a more efficient way than conventional
slow pyrolysis.58–60 The experimental apparatus consists of a
pressurised vessel, which contains a canister with a fixed bed of
a given biomass. Air is used to pressurise the vessel to an initial
12
4.1. Effect of peak temperature 600 °C. Furthermore, the fraction of aromatic C markedly
Peak temperature is generally defined as the highest increased with the peak temperature, leading to an increase in
temperature reached during the pyrolysis process.40 In contrast pH. This could be due to a decrease in the acid functional
to the effect of pressure and the pyrolysis environment, a large groups, resulting in a decline in the cation exchange capacity of
number of studies are available in literature concerning the char in the soil. In other words, the operating conditions leading
influence of peak temperature on the char yield and its to a maximisation of the potential carbon stability can also
potential stability. As a general trend, the char yield decreases result in a char with lower capacity for soil improvement
when the peak temperature increases56,72–75, whereas the purposes.
fixed-carbon content in the final char gradually rises with an 4.2. Effect of absolute pressure
increasing temperature40,76–79. Furthermore, an increase in The pressure effect on the pyrolysis behaviour of any feedstock
peak temperature generally leads to an increment of the has not been properly demonstrated yet, as many studies
aromatic C fraction78,80,81 and a decrease in both H:C and O:C reported in literature are inconsistent with each other. Most of
atomic ratios77,82,83, suggesting that the chemical recalcitrance these studies have revealed an increase in the char and gas
of char (i.e. its ability to resist abiotic and biotic degradation) is yields, and a decrease of the liquid yield, when both the
improved. pressure and the residence time of the vapour phase were
McBeath et al.84 performed several hydropyrolysis runs increased.76,86–90 For instance, Antal et al.87 carried out
producing char from common feedstocks, at ten temperatures experiments using a lab-scale process unit development (PDU)
between 300 and 900 °C, in order to assess their influence on to identify the effects of operating pressure on char yields from
carbon stability. Higher temperatures resulted in lower char macadamia nut shells. They stated that only 0.4 MPa were
yields for each feedstock. The initial decline in char yield, which sufficient to get a yield of 40.5% wt. (see Table 1). A further
was visible in the range 300–450 °C depending on the feedstock increase in pressure (i.e., 3.30 MPa) raised the char yield to 51%
type was mainly due to the thermal decomposition of cellulose wt. However, part of this increase was attributed to the higher
and hemicelluloses. In addition, differences in ash content volatile matter content with higher pressures. Hence, the
among the feedstocks led to changes in the char yield at the pressure effect on the char yield was less strong than it
same temperature, probably due to ash-char interactions. appeared. Then, the pressure influence on the char yield was
When pyrolysis temperature increased from 300 to 600 °C, the slighter when the pressure value was above 1.0 MPa. In
char yield decreased to approximately half of the value addition, they highlighted a heat transfer improvement within
obtained at 300 °C. Then, the temperature effect above 600 °C the reactor at higher pressures, leading to the production of a
(and up to 900 °C) on the char yield was slighter than that more uniform char and to a reduction of the required heating
observed at lower temperatures. On the other hand, potential time. According to this, Qian et al.90 reported, for the pyrolysis
char stability was promoted by the increase in temperature. of rice husk, an increase in the yields of char, water, and gas
The fraction of stable polycyclic aromatic carbon to total from 0.1 MPa to 1.0 MPa at a constant linear velocity of the gas
organic carbon (SPAC/TOC) was used as an index for the char flow (i.e., constant gas residence time) at the expense of the
stability. More in detail, the SPAC/TOC ratio just increased up yield of organic condensable products (bio-oil). This was
to 20% at temperatures lower than 450 °C, whereas an increase explained by an enhancement of polycondensation,
greater than 80% was observed for temperatures above 700 °C. dehydration and cracking of the volatiles to form more char,
Furthermore, the most part of the feedstocks showed a water, and gas; respectively. However, the effect of pressure
sigmoidal-like progression for the SPAC/TOC, with a minimal became negligible when the pressure was raised from 1.0 MPa
formation of SPAC structures in the range 300–400 °C, followed to 5.0 MPa. The same study90 was then conducted with the
by a rapid increase in SPAC/TOC up to 700 °C, and then a plateau same values of pressure, but keeping constant the volume flow
above 700 °C. Hence, a trade-off, in terms of temperature, rate (i.e., the higher the pressure, the higher the gas residence
between the char yield and the char potential stability was time). The yields of char and gas obtained under atmospheric
found, whose optimum was generally observed between 500 pressure were similar to the previous ones, whereas the yield
and 700 °C. Manyà et al.85 confirmed such findings, examining of bio-oil raised to 36.2% wt. and the water yield decreased to
the combined effect of pressure and peak temperature on the 14% wt. This suggests that the vapour residence time plays an
potential stability of the char produced from two-phase olive important role under atmospheric pressure in promoting the
mill waste through slow pyrolysis in a laboratory-scale fixed-bed dehydration of volatiles. The yield’s trends for char, gas, bio-oil,
reactor. In this case, the fraction of aromatic C, the fixed-carbon and water as a function of pressure were similar to those
yield and the atomic H:C and O:C ratios were used as rough observed at constant linear velocity of gas flow.
indicators of the char stability. They observed a statistical On the other hand, Manyà et al.85 observed a significant
consistency of such parameters (i.e., no contradictory findings decrease in char yield when the pressure was increased,
were found), confirming their usefulness as indicators for the keeping constant the gas residence time within the reactor.
potential stability of char. They agreed with the previous results Such effect could be attributed to the enhancement of the
found in literature, stating that the temperature negatively kinetics of the steam gasification reaction with the pressure
affected the char yield, whereas had a favourable effect on the (and catalysed by the alkaline metals inherently present in the
fixed-carbon yield, improving the long-term C sequestration biomass source), Furthermore, the effect of pressure,
potential of char. The authors established the best operating combined with the temperature effect, resulted positive on the
conditions (in terms of maximising the char potential stability long-term C sequestration potential of char (i.e., higher fixed-
as well as obtaining an acceptable char yield) at 1.1 MPa and
13
carbon content and lower H:C and O:C atomic ratios). Similarly, the produced chars obtained in study by Biswas et al.92. By
Azuara et al.91 observed a very slight decrease in char yield increasing the temperature, the carbon content became higher
(from 32% to 30% wt.) when the pressure increased from 0.1 more markedly under the CO2 atmosphere (from 44.90% to
MPa to 1.1 MPa, during pyrolysis of vine shoots in a lab-scale 50.95%) than under the N2 atmosphere (from 42.24%w. to
fixed bed reactor. This was attributed to the above-mentioned 45.33%w.). The corresponding decreases in hydrogen and
promotion of the steam gasification reaction, as well as a oxygen contents were attributed to a higher degree of
dilution effect of pyrolysis volatiles (caused by an increase in the carbonisation, which is mainly explained by an increased
mass flow rate of carrier gas to keep constant its residence time temperature. In contrast to the results reported by Biswas et
within the reactor). This dilution effect results in a decrease in al.92, Azuara et al.91 did not observe any influence of the
the partial pressure of volatiles, which can lead to a lower pyrolysis environment on the yield of vine shoots-derived char.
extent of secondary charring reactions. In addition, the fixed- The effect of the atmosphere was only evident for the gas
carbon yield was practically unaffected by pressure, suggesting distributions obtained after the pyrolysis runs. In particular, the
that other process parameters, such as peak temperature, main effect was a marked decrease in the CO2 production,
could mainly explain the fixed-carbon content in the produced accompanied by a proportional increase in the CO production,
char. In conclusion, the pressure could affect the potential due to the high CO2 partial pressure, which can be related to a
stability of char and, to a lesser extent, the char yield. The promotion of the reverse Boudouard reaction. Furthermore,
magnitude of its influence will depend on the nature of the the extent of the reverse Boudouard reaction enhanced the
feedstock (since a higher content in alkaline metals will further conversion of carbon, leading to a certain porosity
promote gasification) as well as the selected operating development. Regarding the potential stability of the produced
conditions in terms of vapour residence time, reactor chars, the use of CO2 instead of N2 as pyrolysis atmosphere led
configuration, and partial pressure of volatiles. to similar values of fixed-carbon contents and slightly lower H:C
and O:C atomic ratios.
Table 1. Effect of pressure on air-dry macadamia nut shell char Lee et al.93 investigated the pyrolysis behaviour of red pepper
yield using a lab-scale reactor (source: Antal et al.87). stalk under a CO2 environment. The thermal degradation under
CO2 occurred faster than under N2, suggesting that CO2
Pressur Char Yield Volatile matter in
expedited the thermal degradation of amorphous substances
e (% wt. dry basis) char
such as lignin. Further effects related to the CO2 presence were
(MPa) (% wt. dry basis)
higher degrees of carbonisation, and a clearly enhancement of
0.40 40.5 20.6
the thermal cracking of condensable volatiles, resulting in a
0.70 40.2 17.7 favourable condition of the syngas generation. As a
1.00 44.4 25.3 consequence, the CO2 presence greatly reduced the liquid
1.10 50.8 28.8 formation, since condensable volatiles were used as reaction
3.30 51.0 29.3 substrates for the syngas production. The higher degree of
carbonisation was confirmed by the analysis of the char
4.3. Effect of pyrolysis environment composition. In particular, the char produced under CO2
contained higher aromaticity matter and less aliphatic matter
Another important parameter affecting the pyrolysis behaviour
than those of the char produced under N2. Since the aromatic
of biomass is the pyrolysis atmosphere.91 It could be interesting
carbon is more stable than aliphatic carbon under conditions of
to study in terms of energy efficiency, since the flue gas
biotic and abiotic oxidation, the char produced under CO2 could
generated by combustion of pyrolysis gas can be used as
result more recalcitrant than that produced under N2.
pyrolysis gas environment. This approach can lead to important
cost savings70, resulting in an improvement in the char Table 2. Product yields of pyrolysis runs conducted between
production process in terms of economic feasibility, 300 and 450 °C (source: Biswas et al.92).
environmental impact, and thermal efficiency. Nevertheless,
further investigations are needed to assess the effects of Pyrolysis peak
Char yield, Gas yield, Bio-oil yield,
temperature
modifying the inert environment (i.e., from pure N2 to a flue gas wt.% wt.% wt.%
(°C)
containing CO2) on the pyrolysis products distribution as well as
N2 CO2 N2 CO2 N2 CO2
on the char properties. So far, only few studies have been
300 39.2 47.0 31.6 21.6 29.2 31.4
focused on the effect of pyrolysis environment. Biswas at al.92
investigated the pyrolysis behaviour of rice straw under carbon 350 35.6 42.0 23.9 33.8 30.6 34.1
dioxide at temperatures ranging from 300 to 450 °C using a 400 34.2 38.0 27.5 34.8 31.0 34.5
fixed-bed reactor. They pointed out a marginally improvement 450 33.2 37.0 28.9 37.0 29.8 34.1
of char and bio-oil yields in the presence of CO2 instead of N2,
whereas the gas yield was slightly reduced, as shown in Table 2.
The authors stated that the high partial pressure of CO2 could
explain the observed higher yields in condensable organic
compounds. Moreover, CO2 could also promote some
repolymerisation reactions, resulting in a higher char yield.
Table 3 reports the carbon, hydrogen, and oxygen contents of
14
Table 3. Ultimate analysis of chars produced between 300 and formation by means of secondary reactions.72,106,107 The
450 °C (source: Biswas et al.92). possibility of using large biomass particles can also lead to
Pyrolysis peak important cost savings: (1) by avoiding the need of severe
temperature C, wt.% H, wt.% O, wt.% milling processes (with a high energy consumption), and (2) by
(°C) improving the self-sustaining nature of the pyrolysis process,
N2 CO2 N2 CO2 N2 CO2 since the secondary reactions are exothermic.87,108
300 42.2 44.4 3.00 4.38 54.0 50.3
350 43.2 43.4 2.46 3.92 53.6 51.9 5. Conclusions
400 44.0 45.3 2.22 3.53 53.0 50.38
From the present review, the following conclusions can be
450 45.3 50.9 1.46 3.45 52.5 44.8 drawn:
• Slow pyrolysis seems to be one of the most
4.4. Other effects suitable pathways to produce char with
appropriate yields and high potential carbon
Peak temperature, pressure, and pyrolysis atmosphere can be
sequestration potentials. However, further
considered as the process variables that mainly affect the char
research is needed to establish the best operating
yield and its potential stability. However, there are further
conditions to efficiently produce char for soil
factors to mention which could have remarkable effects on the
amendment and other advanced applications. For
final char properties. Firstly, it should be noted that char yield
this reason, nowadays the process optimisation
is influenced by the feedstock composition (i.e., lignin,
of slow pyrolysis is a deal of great concern.
cellulose, hemicelluloses, extractives and inorganic matter).
• An increase in peak temperature results in lower
More in detail, chars produced from feedstocks with high lignin
char yields. However, increasing the peak
contents generally show higher yields.40,94 Furthermore,
temperature leads to higher potential stabilities
pyrolysis of extractive-rich woods (e.g., chestnut) seemed to
for the char. Hence, in terms of temperature, a
lead to higher char production than that obtained from
reasonable trade-off between char yield and its
pyrolysis of wood species with lower content of extractives.95
stability needs to be investigated. According to
The moisture content could also affect the char yield, since
the current state of knowledge, temperatures
previous studies73,87,96,97 indicated that high moisture contents
between 500 and 600 °C are believed to be the
(i.e., ranging from 42% wt. to 62% wt.) can improve the char
most reliable choice to achieve a good
yield at elevated pressures.
compromise between char yield and potential
In addition, and as mentioned above, the inorganic fraction
stability.
present in the feedstock can play a non-negligible role in the
• Among the wide range of process parameters
pyrolysis process. The presence of alkali and alkaline earth
that could affect the pyrolysis process, pressure is
metals (AAEMs) is always associated with low temperatures
one of the most interesting one. It is believed that
required for pyrolysis, higher yields of char and gas, and lower
its effect may be positive on char’s potential
yields of condensable products98,99. In this sense, Wang et al.100
stability. However, the most part of the results
reported an increase in the yields of char and gas produced
reported in the literature are discordant,
from pine wood through slow pyrolysis by adding K2CO3. On the
especially in the mass yield of produced char.
other hand, in another work101, it was observed that CaO had a
Hence, further investigations are needed in order
catalytic effect on the cracking of volatiles, promoting the
to clarify the true effect of pressure.
decarboxylation of organic acids and leading to the formation
• Since the char yield and its stability are apparently
of light hydrocarbons. In addition, CaO is a good low-cost
slightly affected or even unaffected by the
sorbent for CO2. Manyà et al.102 conducted a study on the
pyrolysis environment, using CO2 (or a CO2-
effects of pressure combined with the addition of a rejected
containing flue gas) instead of an expensive inert
material from municipal waste composting on the pyrolysis
gas (e.g., N2) would be interesting in terms of
behaviour of two-phase olive mill waste. Unexpectedly, they
energy efficiency and economic viability.
observed that the addition of such AEEMs-rich material led to a
Nevertheless, further research is needed in order
decrease in the char yield at any pressure. This finding could be
to assess the effects of a higher CO2 partial
related to a higher catalytic role of AAEMs during the primary
pressure on the products’ distribution (especially
devolatilisation as well as heterogeneous secondary reactions
for the gas composition) as well as the surface
(e.g., steam gasification). In other words, the secondary
and textural properties of the resulting char.
charring reactions were not sufficiently promoted by AAEMs.
Using CO2 at different partial pressures and peak
Another factor to consider is the particle size. It is well known
temperatures could be an interesting route to
in literature103–105 that an increase in particle size leads to more
produce chars with an engineered porosity and
pronounced gradients of temperature within the particles, with
surface chemistry.
the core temperature lower than that of the particle surface,
resulting in higher char yield and lower bio-oil and gas yields. In
addition, as the particle size is greater, the diffusion rate of
volatiles within the char decreases, leading to a further char
15
• Other factors related to the biomass feedstock, (10) Tiessen, H.; Cuevas, E.; Chacon, P. The Role of Soil
such as its composition (in terms of Organic Matter in Sustaining Soil Fertility. Nature 1994,
hemicelullose, cellulose, lignin, and extractives), 371, 783–785.
moisture content, inorganic fraction’s (11) Zech, W.; Senesi, N.; Guggenberger, G.; Kaiser, K.;
constituents, and particle size could also affect Lehmann, J.; Miano, T. M.; Miltner, A.; Schroth, G.
both the yield and final properties of char. Factors Controlling Humification and Mineralization of
• In light of the current lack of knowledge about the Soil Organic Matter in the Tropics. Geoderma 1997, 79,
relationship between the char properties and its 117–161.
applicability into soil, great collaborative efforts (12) Melgar, R. J.; Smyth, T. J.; Sanchez, P. A.; Cravo, M. S.
between different fields of research are required. Fertilizer Nitrogen Movement in a Central Amazon
In this sense, the GreenCarbon’s Early-Stage Oxisol and Entisol Cropped to Corn. Fertil. Res. 1992,
Researcher #1 (G. Greco) is contributing 31, 241–252.
throughout the investigation of the most (13) Cahn, M. D.; Bouldin, D. R.; Cravo, M. S.; Bowen, W. T.
appropriate slow pyrolysis conditions (with a Cation and Nitrate Leaching in an Oxisol of the Brazilian
particular focus on the pressure effect and the Amazon. Agron. J. 1993, 85, 334–340.
pyrolysis environment) in order to improve the (14) Cochrane, T. T.; Sanchez, P. A. Land Resources, Soil
char properties for its employment as a carbon Properties and Their Management in the Amazon
sequestration agent. Region: A State of Knowledge Report. In International
Conference on Amazon Land Use and Agricultural
Research; 1980.
Acknowledgements (15) Kauffman, J. B.; Cummings, D. L.; Ward, D. E.; Babbitt,
“This project has received funding from the European R. Fire in the Brazilian Amazon: 1. Biomass, Nutrient
Union’s Horizon 2020 research and innovation programme Pools, and Losses in Slashed Primary Forests.
under the Marie Skłodowska-Curie grant agreement No Oecologia 1995, 104, 397–408.
721991”. (16) Kleinman, P. J. A.; Pimentel, D.; Bryant, R. B. The
Ecological Sustainability of Slash-and-Burn Agriculture.
Agric. Ecosyst. Environ. 1995, 52, 235–249.
References
(17) Fearnside, P. M.; Graca, P. M. L.; Filho, N. L.; Rodrigues,
(1) Rackley, S. A. Carbon Capture and Storage; Joe Hayton: F. J. A.; Robinson, J. M. Tropical Forest Burning in
Chennai, 2010. Brazilian Amazonia: Measurement of Biomass Loading,
(2) IPCC. Summary for Policymakers: Emissions Scenarios. Burning Efficiency and Charcoal Formation at Altamira,
A Special Report of Working Group III of the Pará. For. Ecol. Manage. 1999, 123, 65–79.
Intergovernmental Panel on Climate Change. Group (18) Fearnside, P. M. Global Warming and Tropical Land-
2000, 20. Use Change: Greenhouse Gas Emissions from Biomass
(3) Tyndall, J. Note on the Transmission of Radiant Heat Burning, Decomposition and Soils in Forest
through Gaseous Bodies. 1859, pp 37–39. Conversion, Shifting Cultivation and Secondary
(4) Arrhenius, S. In the Air upon the Temperature of the Vegetation. Clim. Change 2000, 46, 115–158.
Ground. Philos. Mag. J. Sci. 1896, 41, 237–279. (19) Schmidt, M. W. I.; Noak, A. G. Black Carbon in Soils and
(5) SMMT. Total CO2 emissions from cars in use Sediments: Analysis, Distribution, Implications, and
https://www.smmt.co.uk/reports/co2-report/ Current Challanges. Global Biogeochem. Cycles 2000,
(accessed May 19, 2015). 14, 777–793.
(6) Dean, C. C.; Dugwell, D.; Fennell, P. S. Investigation into (20) Glaser, B.; Balashov, E.; Haumaier, L.; Guggenberger,
Potential Synergy between Power Generation, Cement G.; Zech, W. Black Carbon in Density Fractions of
Manufacture and CO2 Abatement Using the Calcium Anthropogenic Soils of the Brazilian Amazon Region.
Looping Cycle. Energy Environ. Sci. 2011, 4 (6), 2050. Org. Geochem. 2000, 31, 669–678.
(7) Fennell, P. Calcium and Chemical Looping Technology (21) Glaser, B.; Haumaier, L.; Guggenberger, G.; Zech, W.
for Power Generation and Carbon Dioxide (CO2) The Terra Preta Phenomenon - a Model for Sustainable
Capture; Elsevier, 2015. Agriculture in the Humid Tropics. Sci. Nat. 2001, 88,
(8) Glaser, B.; Lehmann, J.; Zech, W. Ameliorating Physical 37–41.
and Chemical Properties of Highly Weathered Soils in (22) Demirbaş, A. H.; Demirbaş, A. S.; Demirbaş, A. Liquid
the Tropics with Charcoal - a Review. Biol. Fertil. Soils Fuels from Agricultural Residues via Conventional
2002, 35, 219–230. Pyrolysis. Energy Sources 2010, 26, 821–827.
(9) Vosti, S. A.; Carpentier, C. L.; Witcover, J.; Valentin, J. (23) Aysu, T.; Küçük, M. M. Biomass Pyrolysis in a Fixed-Bed
F. Intensificied Small-Scale Livestock Systems in the Reactor: Effects of Pyrolysis Parameters on Product
Western Brazilian Amazon. In Agricultural technologies Yields and Characterization of Products. Energy 2014,
and tropical deforestation; 2001; pp 113–133. 64, 1002–1025.
16
(24) Dhyani, V.; Bhaskar, T. A Comprehensive Review on the (41) Industrial Charcoal Production, TCP/CRO/3101 (A)
Pyrolysis of Lignocellulosic Biomass. Renew. Energy Development of a Sustainable Charcoal Industry. In
2017. Food and Agriculture Organization of the United
(25) McKendry, P. Energy Production from Biomass (Part 1): Nations (FAO); Zagreb, Croatia, 2008.
Overview of Biomass. Bioresour. Technol. 2002, 83, (42) Hao, W.; Björkman, E.; Lilliestråle, M.; Hedin, N.
37–46. Activated Carbons Prepared from Hydrothermally
(26) Carrier, M.; Loppinet-serani, A.; Aymonier, C. Carbonized Waste Biomass Used as Adsorbents for
Thermogravimetric Analysis as a New Method to CO2. Applied Energy. 2013, pp 526–532.
Determine the Lignocellulosic Composition of Biomass. (43) Demiral, I.; Şensöz, S. Fixed-Bed Pyrolysis of Hazelnut
Biomass and Bioenergy 2011, 35, 298–307. (Corylus Avellana L.) Bagasse: Influence of Pyrolysis
(27) Mohan, D.; Pittman, C. U.; Steele, P. H. Pyrolysis of Parameters on Product Yields. Energy Sources 2006,
Wood/Biomass for Bio-Oil: A Critical Review. Energy & 28, 1149–1158.
Fuels 2006, 20, 848–889. (44) Mythili, R.; Venkatachalam, P.; Subramanian, P.; Uma,
(28) Liu, Q.; Wang, S.; Zheng, Y.; Luo, Z.; Cen, K. Mechanism D. Characterization of Bioresidues for Bio-Oil
Study of Wood Lignin Pyrolysis by Using TG-FTIR Production through Pyrolysis. Bioresour. Technol.
Analysis. J. Anal. Appl. Pyrolysis 2008, 82, 170–177. 2013, 138, 71–78.
(29) Manyà, J. J. Pyrolysis for Biochar Purposes: A Review to (45) Yang, H.; Yan, R.; Chen, H.; Lee, D. H.; Zheng, C.
Establish Current Knowledge Gaps and Research Characteristics of Hemicellulose, Cellulose and Lignin
Needs. Environ. Sci. Technol. 2012, 46, 7939–7954. Pyrolysis. Fuel 2007, 86, 1781–1788.
(30) Lehmann, J.; Joseph, S. Biochar for Environmental (46) Varma, A. K.; Mondal, P. Pyrolysis of Sugarcane
Management: An Introduction. In Biochar for Bagasse in Semi Batch Reactor: Effects of Process
Environmental Management; Eartschan, 2009; pp 1– Parameters on Product Yields and Characterization of
10. Products. Ind. Crops Prod. 2017, 95, 704–717.
(31) Sohi, S.; Lopez-Capel, S.; Krull, E.; Bol, R. Biochar, (47) Bridgwater, A. V. Review of Fast Pyrolysis of Biomass
Climate Change and Soil: A Review to Guide Future and Product Upgrading. Biomass and Bioenergy 2012,
Research; 2009. 38, 68–94.
(32) Lehmann, J. A Handful of Carbon. Nature 2007, 447, (48) Sadaka, S.; Negi, S. Improvements of Biomass Physical
143–144. and Thermochemical Characteristics via Torrefaction
(33) Laird, A. D. The Charcoal Vision: A Win-Win-Win Process. Environ. Prog. Sustain. Energy 2009, 28, 427–
Scenario for Simultaneously Producing Bioenergy, 434.
Permanently Sequestering Carbon, While Improving (49) Yang, Y.; Brammer, J. G.; Ouadi, M.; Samanya, J.;
Soil and Water Quality. Agron. J. 2008, 100, 178–181. Hornung, A.; Xu, H. M.; Li, Y. Characterisation of Waste
(34) Fowles, M. Black Carbon Sequestration as an Derived Intermediate Pyrolysis Oils for Use as Diesel
Alternative to Bioenergy. Biomass and Bioenergy 2007, Engine Fuels. Fuel 2013, 103, 247–257.
31, 426–432. (50) López-Ordovás, J. Industrial-Scale Waste Pyrolysis in a
(35) Gaunt, J. L.; Lehmann, J. Energy Balance and Emissions Novel Pyrolysis Reactor; 2018.
Associated with Biochar Sequestration and Pyrolysis (51) Garcia-Nunez, J. A.; Pelaez-Samaniego, M. R.; Garcia-
Bioenergy Production. Environ. Sci. Technol. 2008, 42, Perez, M. E.; Fonts, I.; Abrego, J.; Westerhof, R. J. M.;
4152–4158. Garcia-Perez, M. Historical Developments of Pyrolysis
(36) Swift, R. S. Sequestration of Carbon by Soil. Soil Sci. Reactors: A Review. Energy & Fuels 2017, 31, 5751–
2001, 166, 858–871. 5775.
(37) Nguyen, B. T.; Lehmann, J.; Kinyangi, J.; Smernik, R. J.; (52) Kolokolova, O.; Levi, T.; Pang, S. Torrefaction and
Riha, S.; Engelhard, M. H. Long-Term Black Carbon Pyrolysis of Biomass Waste in Continuous Reactors. In
Dynamics in Cultivated Soil. Biogeochemistry 2009, 92, Proceedings of the 13th International Conference on
163–176. Environmental Science and Technology; 2013.
(38) McHenry, M. P. Agricultural Bio-Char Production, (53) Arias, B.; Pevida, C.; Fermoso, J.; Plaza, M. G.; Rubiera,
Renewable Energy Generation and Farm Carbon F.; Pis, J. J. Influence of Torrefaction on the Grindability
Sequestration in Western Australia: Certainty, and Reactivity of Woody Biomas. Fuel Process.
Uncertainty and Risk. Agric. Ecosyst. Environ. 2009, Technol. 2008, 89, 169–175.
129, 1–7. (54) Zhang, L.; Chunbao, X.; Champagne, P. Overview of
(39) Basu, P. Biomass Gasification, Pyrolysis and Recent Advances in Thermo-Chemical Conversion of
Torrefaction; Elsevier Applied Science: Burlington, Biomass. Energy Convers. Manag. 2010, 51, 969–982.
2013. (55) Yanik, J.; Kornmayer, C.; Saglam, M.; Yüksel, M. Fast
(40) Antal, M. J.; Gronli, M. The Art, Science, and Pyrolysis of Agricultural Wastes: Characterization of
Technology of Charcoal Production. Ind. Eng. Chem. Pyrolysis Products. Fuel Process. Technol. 2007, 88,
Res. 2003, 42, 1619–1640. 942–947.
17
(56) Duman, G.; Okutucu, C.; Ucar, S.; Stahl, R.; Yanik, J. The (71) Ronsse, F.; van Hecke, S.; Dickinson, D.; Prins, W.
Slow and Fast Pyrolysis of Cherry Seed. Bioresour. Production and Characterization of Slow Pyrolysis
Technol. 2011, 102, 1869–1878. Biochar: Influence of Feedstock Type and Pyrolysis
(57) Scala, F.; Chirone, R.; Salatino, P. Combustion and Conditions. GCB Bioenergy 2013, 5, 104–115.
Attrition of Biomass Chars in a Fluidized Bed. Energy & (72) Di Blasi, C.; Signorelli, G.; Di Russo, C.; Rea, G. Product
Fuels 2006, 20, 91–102. Distribution from Pyrolysis of Wood and Agricultural
(58) Nunoura, T.; Wade, S. R.; Bourke, J.; Antal, M. J. Studies Residues. Ind. Eng. Chem. Res. 1999, 38, 2216–2224.
of the Flash Carbonization Process. 1. Propagation of (73) Demirbaş, A. Effects of Temperature and Particle Size
the Flaming Pyrolysis Reaction and Performance of a on Bio-Char Yield from Pyrolysis of Agricultural
Catalytic Afterburner. Ind. Eng. Chem. Res. 2006, 45, Residues. J. Anal. Appl. Pyrolysis 2004, 72, 243–248.
585–599. (74) Abdullah, H.; Wu, H. Biochar as a Fuel: 1. Properties
(59) Wade, S. R.; Nunoura, T.; Antal, M. J. Studies of the and Grindability of Biochars Produced from the
Flash Carbonization Process. 2. Violent Ignition Pyrolysis of Mallee Wood under Slow-Heating
Behavior of Pressurized Packed Beds of Biomass: A Conditions. Energy Fuels 2009, 23, 4174–4181.
Factorial Study. Ind. Eng. Chem. Res. 2006, 45, 3512– (75) Méndez, A.; Terradillos, M.; Gascó, G. Physicochemical
3519. and Agronomic Properties of Biochar from Sewage
(60) Antal, M. J.; Mochidzuki, K.; Paredes, L. S. Flash Sludge Pyrolysed at Different Temperatures. J. Anal.
Carbonization of Biomass. Ind. Eng. Chem. Res. 2003, Appl. Pyrolysis 2013, 102, 124–130.
42, 3690–3699. (76) Antal, M. J., J.; Allen, S. G.; Dai, X.; Shimizu, B.; Tam, M.
(61) Pan, Y. G.; Velo, E.; Roca, F. X.; Manyà, J. J.; Puigjaner, S.; Gronli, M. Attainment of the Theoretical Yield of
L. Fluidized-Bed Co-Gasification of Residual Biomass. Carbon from Biomass. Ind. Eng. Chem. Res. 2000, 39,
Fuel 2000, 79, 1317–1326. 4024–4031.
(62) Rezaiyan, J.; Cheremisinoff, N. P. Gasification (77) Enders, A.; Hanley, K.; Whitman, A.; Joseph, S.;
Technologies - A Primer for Engineers and Scientists; Lehmann, J. Characterization of Biochars to Evaluate
CRC Press: Boca Raton, 2005. Recalcitrance and Agronomic Performance. Bioresour.
(63) Brewer, C. E.; Schmidt-Rohr, K.; Satrio, J. A.; Brown, R. Technol. 2012, 114, 644–653.
C. Characterization of Biochar from Fast Pyrolysis and (78) Zhao, L.; Cao, X.; Mašek, O.; Zimmerman, A.
Gasification Systems. Environ. Prog. Sustain. Energy Heterogeneity of Biochar Properties as a Function of
2009, 28, 386–396. Feedstock Sources and Production Temperatures. J.
(64) Fernandes, M. B.; Brooks, P. Characterization of Hazard. Mater. 2013, 256–257, 1–9.
Carbonaceous Combustion Residues: II. Nonpolar (79) Manyà, J. J.; Roca, F. X.; Perales, J. F. J. TGA Study
Organic Compounds. Chemosphere 2003, 53, 447–458. Examining the Effect of Pressure and Peak
(65) Resende, F. L. P. Recent Advances on Fast Temperature on Biochar Yield during Pyrolysis of Two-
Hydropyrolysis of Biomass. Catal. Today 2016, 269, Phase Olive Mill Waste. Anal. Appl. Pyrolysis 2013, 103,
148–155. 86–95.
(66) Thangalazhy-Gopakumar, S.; Adhikari, S.; Gupta, R. B. (80) McBeath, A. V.; Smernik, R. J.; Schneider, M. P. W.;
Catalytic Pyrolysis of Biomass over H+ZSM-5 under Schmidt, M. W. I.; Plant, E. L. Determination of the
Hydrogen Pressure. Energy Fuels 2012, 26, 5300–5306. Aromaticity and the Degree of Aromatic Condensation
(67) Melligan, F.; Hayes, M. H. B.; Kwapinski, W.; Leahy, J. J. of a Thermosequence of Wood Charcoal Using NMR.
A Study of Hydrogen Pressure during Hydropyrolysis of Org. Geochem. 2011, 42, 1194–1202.
Miscanthus x Giganteus and Online Catalytic Vapour (81) Wu, W.; Yang, M.; Feng, Q.; McGrouther, K.; Wang, H.;
Upgrading with Ni on ZSM-5. J. Anal. Appl. Pyrolysis Lu, H.; Chen, Y. Chemical Characterization of Rice
2013, 103, 369–377. Straw-Derived Biochar for Soil Amendment. Biomass
(68) Krishna, B. B.; Biswas, B.; Ohri, P.; Kumar, J.; Singh, R.; and Bioenergy 2012, 47, 268–276.
Bhaskar, T. Pyrolysis of Cedrus Deodara Saw Mill (82) Sun, H.; Hockaday, W. C.; Masiello, C. A.; Zygourakis, K.
Shavings in Hydrogen and Nitrogen Atmosphere for Multiple Controls on the Chemical and Physical
the Production of Bio-Oil. Renew. Energy 2016, 98, Structure of Biochars. Ind. Eng. Chem. Res. 2012, 51,
238–244. 3587–3597.
(69) Singh, R.; Krishna, B. B.; Mishra, G.; Kumar, J.; Bhaskar, (83) Ghani, W. A. W. A. K.; Mohd, A.; da Silva, G.;
T. Strategies for Selection of Thermo-Chemical Bachmann, R. T.; Taufiq-Yap, Y. H.; Rashid, U.; Al-
Processes for the Valorisation of Biomass. Renew. Muhtaseb, A. H. Biochar Production from Waste
Energy 2016, 98, 226–237. Rubber-Wood-Sawdust and Its Potential Use in C
(70) Mašek, O.; Brownsort, P.; Cross, A.; Sohi, S. Influence Sequestration: Chemical and Physical
of Production Conditions on the Yield and Characterization. Ind. Crops Prod. 2013, 44, 18–24.
Environmental Stability of Biochar. Fuel 2013, 103, (84) McBeath, A. V.; Wurster, C. M.; Bird, M. I. Influence of
151–155. Feedstock Properties and Pyrolysis Conditions on
Biochar Carbon Stability as Determined by Hydrogen
Pyrolysis. Biomass Bioenergy 2015, 73 (155–173).
18
(85) Manyà, J. J.; Laguarta, S.; Ortigosa, M. A.; Manso, J. A. (100) Wang, Z.; Wang, F.; Cao, J.; Wang, J. Pyrolysis of Pine
Biochar from Slow Pyrolysis of Two-Phase Olive Mill Wood in a Slowly Heating Fixed-Bed Reactor:
Waste: Effect of Pressure and Peak Temperature on Its Potassium Carbonate versus Calcium Hydroxide as a
Potential Stability. Energy Fuels 2014, 28, 3271–3280. Catalyst. Fuel Process. Technol. 2010, 91, 942–950.
(86) Rousset, P.; Figueiredo, C.; De Souza, M.; Quirino, W. (101) Wang, D.; Xiao, R.; Zhang, H.; He, G. Comparison of
Pressure Effect on the Quality of Eucalyptus Wood Catalytic Pyrolysis of Biomass with MCM-41 and CaO
Charcoal for the Steel Industry: A Statistical Analysis Catalysts by Using TGA–FTIR Analysis. J. Anal. Appl.
Approach. Fuel Process. Technol. 2011, 92, 1890–1897. Pyrolysis 2010, 89, 171–177.
(87) Antal, M. J., J.; Croiset, E.; Dai, X.; DeAlmeida, C.; Mok, (102) Manyà, J. J.; Alvira, D.; Azuara, M.; Bernin, D.; Hedin,
W. S.; Norberg, N.; Richard, J. R.; Al Majthoub, M. High- N. Effects of Pressure and the Addition of a Rejected
Yield Biomass Charcoal. Energy Fuels 1996, 10, 652– Material from Municipal Waste Composting on the
658. Pyrolysis of Two-Phase Olive Mill Waste. Energy &
(88) Noumi, E. S.; Blin, J.; Valette, J.; Rousset, P. Combined Fuels 2016, 30, 8055–8064.
Effect of Pyrolysis Pressure and Temperature on the (103) Şensöz, S.; Kaynar, I. Bio-Oil Production from Soybean
Yield and CO2 Gasification Reactivity of Acacia Wood (Glycine Max L.): Fuel Properties of Bio-Oil. Ind. Crops
in Macro-TG. Energy Fuels 2015, 29, 7301–7308. Prod. 2006, 23, 99–105.
(89) Recari, J.; Berrueco, C.; Abellò, S.; Montané, D.; Farriol, (104) Şensöz, S.; Angin, D.; Yorgun, S. Influence of Particle
X. Effect of Temperature and Pressure on Size on the Pyrolysis of Rapeseed (Brassica Napus L.):
Characteristics and Reactivity of Biomass-Derived Fuel Properties of Bio-Oil. Biomass Bioenergy 2000, 19,
Chars. Bioresour. Technol. 2014, 170, 204–210. 271–279.
(90) Qian, Y.; Zhang, J.; Wang, J. Pressurized Pyrolysis of (105) Encinar, J. M.; Gonzalez, J. F. Fixed-Bed Pyrolysis of
Rice Husk in an Inert Gas Sweeping Fixed-Bed Reactor Cynara Carduncules L., Product Yields and
with a Focus on Bio-Oil Deoxygenation. Bioresour. Compositions. Fuel Process. Technol. 2000, 68, 209–
Technol. 2014, 174, 95–102. 222.
(91) Azuara, M.; Sáiz, E.; Manso, J. A.; García-Ramos, F. J.; (106) Manyà, J. J.; Ruiz, J.; Arauzo, J. Some Peculiarities of
Manyà, J. J. Study on the Effects of Using a Carbon Conventional Pyrolysis of Several Agricultural Residues
Dioxide Atmosphere on the Properties of Vine Shoots- in a Packed Bed Reactor. Ind. Eng. Chem. Res. 2007, 46,
Derived Biochar. J. Anal. Appl. Pyrolysis 2017, 124, 9061–9070.
719–725. (107) Varhegyi, G.; Szabo, P.; Till, F.; Zelei, B.; Antal, M. J.;
(92) Biswas, B.; Singh, R.; Kumar, J.; Singh, R.; Gupta, P.; Dai, X. TG, TG-MS, and FTIR Characterization of High-
Krishna, B. B.; Bhaskar, T. Pyrolysis Behavior of Rice Yield Biomass Charcoals. Energy Fuels 1998, 12, 969–
Straw under Carbon Dioxide for Production of Bio-Oil. 974.
Renew. Energy 2018, 129, 678–685. (108) Stenseng, M.; Jensen, A.; Dam-Johansen, K.
(93) Lee, J.; Yang, X.; Cho, S. H.; Kim, J. K.; Lee, S. S.; Tsang, Investigation of Biomass Pyrolysis by
D. C. W.; Ok, Y. S.; Kwon, E. E. Pyrolysis Process of Thermogravimetric Analysis and Differential Scanning
Agricultural Waste Using CO2 for Waste Management, Calorimetry. J. Anal. Appl. Pyrolysis 2001, 58, 765–780.
Energy Recovery, and Biochar Fabrication. Appl.
Energy 2017, 185, 214–222.
(94) Mok, W. S.; Antal, M. J.; Szabo, P.; Varhegyi, G.; Zelei,
B. Formation of Charcoal from Biomass in a Sealed
Reactor. Ind. Eng. Chem. Res. 1992, 31, 1162–1166.
(95) Di Blasi, C.; Branca, C.; Santoro, A.; Hernandez, E. G.
Pyrolytic Behavior and Products of Some Wood
Varieties. Combust. Flame 2001, 124, 165–177.
(96) Dai, X.; Antal, M. J. Synthesis of a High-Yield Activated
Carbon by Air Gasification of Macadamia Nut Shell
Charcoal. Ind. Eng. Chem. Res. 1999, 38, 3386–3395.
(97) Varhegyi, G.; Szabo, P.; Mok, W. S.; Antal, M. J. Kinetics
of the Thermal Decomposition of Cellulose in Sealed
Vessels at Elevated Pressures. Effects of the Presence
of Water on the Reaction Mechanism. J. Anal. Appl.
Pyrolysis 1993, 26, 159–174.
(98) Raveendran, K.; Ganesh, A.; Khilar, K. C. Influence of
Mineral Matter on Biomass Pyrolysis Characteristics.
Fuel 1995, 74, 1812–1822.
(99) Di Blasi, C.; Galgano, A.; Branca, C. Effects of Potassium
Hydroxide Impregnation on Wood Pyrolysis. Energy &
Fuels 2009, 23, 1045–1054.
19
20
Pyrolysis of biomass and wastes in a continuous screw reactor for char

production
Filipe Rego, Jiawei Wang, Yang Yang, Anthony V. Bridgwater
European Bioenergy Research Institute (EBRI), School of Engineering and Applied Science, Aston University, Birmingham,
B4 7ET, UK
Abstract
This chapter addresses the pyrolysis of biomass and wastes, with a focus on processes with the objective of continuous production of
the solid product, char. The different types of pyrolysis processes are described, along with a characterisation of the products. Some
of the properties and applications of the char product are described, focusing on the potential uses in soil and for adsorption, along
with a brief description of the process of activation. The recent research on the use of screw reactors for the pyrolysis of biomass and
wastes is reviewed, and the potential, advantages and disadvantages of this type of reactor are also discussed. Some screw reactor
systems, such as the lab-scale and the pilot-scale Pyroformer systems at Aston University, are described.
sugarcane, and many others, compose the herbaceous

1. Introduction biomass. The biofuels produced from these crops are the
so-called first generation biofuels, but there is a concern
Nowadays it is clear that there are several problems
that producing these crops leads to competition with food
related to the way energy and resources are produced and
production, and other negative effects, such as over-
handled. The generation of energy and products from fossil
occupation of land.1,2 The wastes from the processing of
fuels is releasing greenhouse gases and other pollutants to
these herbaceous crops (straw, husk, stover, etc.) can also
the environment, leading to increasing ocean acidity,
be used, producing the second generation biofuels.1 The
climate change, and risks to fauna and flora worldwide. It
so-called third generation biofuels are produced from algal
is also known that resources are being consumed at an
biomass (micro and macroalgae).3 The wastes from
unsustainable pace, and, furthermore, there is an urgent
activities such as farming, food production and
need to reduce the growing amount of waste that is
consumption, the paper industry, water treatment plants
produced all over the world. Therefore, there is a necessity
and other activities (e.g., pruning and food residues,
for investment in “clean” technologies, besides solar, wind
sewage sludge), are also considered as biomass.
and others, which can produce the energy and products we
Biomass and wastes are resources available all over the
use daily, utilising renewable or sustainable resources such
world, and can be converted by biochemical and
as biomass or wastes.
thermochemical processes into valuable biofuels or
Pyrolysis is an example of the technologies that can be
chemicals, promoting the sustainability of resources. What
used for these purposes, and it has been known and used
is obtained from the processing of biomass and wastes
for centuries, for the production of charcoal, for example.
depends on the composition of the feedstock and the
Recently, there has been an increase in pyrolysis-related
process route and conditions.
research, with the aim at producing biofuels and other
As mentioned and described in Chapter 1, biomass is a
valuable products, and some commercial applications with
widely available resource, consisting of a collection of
this objective are in place. Solid, liquid and gas products are
lignocellulosic compounds and other minor components
obtained from the pyrolysis process, with different yields
(extractives, inorganics, and proteins).
and quality depending on factors such as feedstock and
Non-biological wastes can be made up of many different
process conditions. Although all of the pyrolysis products
materials, including plastics, textiles, paper, wood and
are useful, the solid product is particularly interesting due
other lignocellulosic materials. The distribution of these
to the broad spectrum of applications: energy production,
materials in waste has wide variation according to: the
soil applications, removal of contaminants from water and
origin and collection method, how the wastes were
gas streams, among many other uses.
processed, etc. These wastes are inherently
heterogeneous materials in comparison with biomass.
2. Pyrolysis of biomass and wastes Wastes from the consumption of plastics, textiles, and
other materials from household, commercial and industrial
2.1. Characteristics of biomass and wastes
origin are generated worldwide in large quantities. The
Biomass and wastes are two interconnected groups of wastes that are not separated, recycled and reused (the
resources, with various subtypes. Biomass can be main strategy that should be used), can be used for energy
categorised into woody, herbaceous, algal, and waste- production, avoiding the process of landfilling. In some
based biomass. Woody biomass covers woody products cases, they are processed in dedicated facilities to produce
and the residues derived from wood and forestry Refuse-Derived Fuel (RDF), using a series of steps, including
industries (branches, leaves, bark, sawdust, etc.). Crops shredding, screening, separation of inerts and metals.4
and plant species such as wheat, corn, switchgrass, RDF, usually pelletised, is used in some coal power plants

21
Chapter 2 Rego et al.
and also in the cement industry for energy production. slagging and fouling by inorganic species due to exposure
Pyrolysis is another option to obtain energy (and other to high temperatures can be reduced.
products) from RDF. Pyrolysis can be applied to several types of materials, such
A very important constituent of biomass and wastes is the as plastics and its derivatives, waste tires, coal, and
inorganic matter. The inorganic material exists in the form biomasses.
of carbonates, sulphates, oxalates, oxides, and others5, and In pyrolysis, the volatile matter of the material is broken
many different inorganic elements can be present, usually down and produces vapours, which can then be partially
consisting of Ca, K, Mg, Na, P, Cl, Si and S, and also Fe, Mn, condensed, forming the liquid and non-condensable gas
Al, and others.6 In biomass, many of the inorganic elements products. The solid left behind is the solid product from
are essential for the growth of plants, especially the pyrolysis, called char. An overall scheme of the pyrolysis
nutrients Ca, K and Mg7, and also N and P (main process is represented in Figure 1.
constituents of fertilisers, along with K).8 The compounds
with the elements Na, K, Ca, and Mg, are the so-called alkali
and alkaline earth minerals (AAEM), due to being formed
with elements from the first and second groups of the
periodic table, and they have an important effect in the
Figure 1. The process of pyrolysis.
pyrolysis process. It should also be noted that high
temperatures can cause chemical reactions between The gas product is mainly composed of CO, CO2, CH4, H2
inorganic compounds, forming other compounds (such as and light hydrocarbons, and it can be used for heat and
silicates, sulphates, phosphates, etc.), which can cause power generation in a gas turbine or engine.12 The liquid
slagging, fouling and corrosion problems in equipment product consists of, besides water, organic compounds
such as boilers, heat exchangers, and piping.9 such as sugars and their derivatives, acids, aldehydes, and
Wastes such as sewage sludge can have a significant others11, in an aqueous and an organic fraction13, which
amount of inorganic materials and even of heavy metals, can be in a single phase or in two phases. The organic
compared to lignocellulosic biomass, and this will affect fraction can be upgraded into a biofuel and used for heat
the products and yields during the thermal processing. and power production.12 In some cases, individual
Another important constituent of both biomass and compounds can be extracted from the liquid, with
wastes is moisture, which affects storage, transport, applications for example in food flavouring and
handling, and processes such as drying and conversion. A adhesives.14 The solid product from pyrolysis, referred to
relatively high moisture content can render a as char, is a black and brittle material, with a high
thermochemical process economically unfavourable due proportion of carbon and fixed carbon.15 The pyrolysis
to the energy needed for drying. products are sometimes used to provide the energy
When processing biomass or wastes via pyrolysis, each necessary for the process (e.g., heating the reactor and/or
group of components have different behaviours in the drying the feedstock11).
thermochemical process. The composition of biomass and In the pyrolysis process, the feedstock undergoes a
wastes relates to their physicochemical properties, and complex series of chemical reactions, with different
their knowledge is fundamental for the design and activation energies and kinetics16. An overall reaction
implementation of logistics, pre-treatment, and (over-simplified, not balanced) for the pyrolysis of biomass
conversion processes.10 (represented by CxHyOz) is presented in Eq. 117.
∆
2.2. The pyroysis process 𝐶𝐶𝑥𝑥 𝐻𝐻𝑦𝑦 𝑂𝑂𝑧𝑧 → (𝐻𝐻2 + 𝐶𝐶𝐶𝐶 + 𝐶𝐶𝐶𝐶2 + 𝐶𝐶𝐶𝐶4 + ⋯ ) + (𝐻𝐻2 𝑂𝑂 +
As already introduced in Chapter 1, pyrolysis is a 𝐶𝐶𝐶𝐶3 𝑂𝑂𝑂𝑂 + 𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 + ⋯ ) + 𝐶𝐶 (1)
thermochemical process in which a material is exposed to
heat in an oxygen-limited atmosphere, and produces In Eq. 1, the first part of the product side corresponds to
gaseous, liquid and solid products.11 Combustion and the gaseous products, the second part comprises the liquid
gasification are other thermochemical processes, in which product, and the third part (C, carbon) represents the solid
either an excess of oxygen or an amount below the product, char. The inorganic components of the feedstock
stoichiometrically necessary for complete combustion is (ash) mainly remain in the solid product.18 In terms of
applied, respectively.11 This higher proportion of oxygen in water, it is present in the liquid product, originating from
the process atmosphere leads to mainly obtaining gas the feedstock moisture, and some water is produced by
products and solids (ash in the case of combustion), while dehydration and other reactions (water of reaction).18, 19
pyrolysis obtains gas, solid, and liquid products, which is More details about the mechanism of degradation in
advantageous due to the value and possible applications of pyrolysis can be found in Chapter 4 of this book.
those products. Further differences are in terms of The pyrolysis process can be influenced by a great variety
temperature, which in combustion and gasification needs of variables: the feedstock properties, the temperature
to be higher than 700 °C, while in pyrolysis it can be as low and pressure, the residence times (of solids and vapours),
as 400 °C.2 The lower temperatures provide a lower heat the heat transfer method, and the atmosphere. Different
requirement and energy consumption. Also, the risk of combinations of these variables determine the product
distributions and qualities.
22
Heat transfer is a feature with great influence on the Table 1. A comparison between pyrolysis types according
pyrolysis process, governing the selection of the reactor to their typical process conditions.2, 12, 22
type, the heating methods, the use of heat carriers, forms
of the feedstock, etc. This has led to the classification of HR T SRT VRT
pyrolysis according to how fast the heat transfer occurs, Slow Low Approx. Long or Long
which is in turn related to other parameters. Table 1 pyrolysis (under 1 400 °C very long (over
summarises different pyrolysis types and a relative °C s–1) (minutes seconds)
comparison of typical process conditions, although there to days)
are other types (e.g., flash pyrolysis). In Table 1, HR stands Intermediate Medium Approx. Long Medium
for heating rate, T stands for temperature, SRT for solid pyrolysis 500 °C (minutes) (some
residence time, and VRT for vapour residence time. seconds)
Fast pyrolysis allows to obtain a higher liquid yield, of up to Fast Very Approx. Short (a Very
75 wt. % from wood (dry basis).12 In the fast pyrolysis pyrolysis high 500 °C few short
process, heat transfer must occur very rapidly and (over 10 seconds (under
efficiently inside the reactor and within the feedstock °C s-1) or less) seconds)
particles. For this purpose, the feedstocks are processed
with a small particle size, in the order of millimetres or Slow and intermediate pyrolysis, comparing with fast
lower.12 The feedstock is also required to be dried, typically pyrolysis, have a more balanced production of all three
to a maximum water content of 10 wt. %, so as to reduce pyrolysis products. Typical yields from these pyrolysis
the process heat required and minimise water content in types (with wood as feedstock) are presented in Table 2.
the liquid product.14 To provide a faster heat transfer, heat For char production, the employed temperatures and
carriers may also be employed in the reactor, for example, heating rates are relatively low, and solid residence times
in the form of fine heated sand particles, heated steel shot, are in the order of minutes or higher15, 23, which is the case
etc.12 of slow and intermediate pyrolysis. In these processes, a
The most used reactors for fast pyrolysis are of the higher contact time between the solid and the produced
fluidised bed type (bubbling and circulating), and other vapours contributes to a higher char yield through
reactor types are the rotating cone, the ablative system, secondary chemical reactions14, 15. The use of larger
etc.20 The solid residence time and of the vapours should particle sizes (e.g. chips, pellets) also contributes to char
be minimised, so as to prevent secondary reactions and to production15. Using larger feedstock particles is
extract the maximum possible amount of condensable advantageous: it avoids mechanical operations of pre-
volatiles from the system and thus a higher liquid yield.12 treatment such as grinding, sieving, etc., increasing the
For this purpose, relatively high flow rates of inert carrier overall process efficiency and reducing the carbon
gas are usually used. footprint, and adds flexibility to the pyrolysis process.
The liquid product from fast pyrolysis (also referred to as Further advantages of the lower temperatures in slow and
bio-oil) is typically dark brown and free flowing, with a intermediate pyrolysis is a lower energy requirement, and
distinctive smoky odour.14 Its high heating value (HHV) is a reduced risk of formation of inorganic species that can
usually between 16 and 19 MJ kg–1, with typically 25 wt. % lead to slagging, fouling and corrosion, as well as reducing
of water content.14 Water content depends on production tar formation.24
and collection methods, and its maximum value in the The catalytic effect of alkali and alkaline earth metals
liquid product is indicated to reach 50 wt. %.12 The heating (AAEM) on secondary reactions is undesired in fast
value is about 40 % of diesel by weight, while it increases pyrolysis, since it can lower liquid product yields. Therefore
to about 60 % by volume, due to a relatively high density19 there has been significant research on reducing or
of about 1.2 kg L–1. It has a substantial amount of organic minimising the AAEM content, usually by washing with
acids, which results in an acidic pH which can be as low as water or an acid or alkaline solution25, 26. This pre-
2, making it corrosive to construction materials such as treatment could however adversely affect the biomass
carbon steel and aluminium.19 The ASTM specification21 for structure (i.e., hydrolysing the hemicellulose and
pyrolysis liquid for use in industrial burners states that the cellulose), which will reduce liquid product yields.12 For
ash content should be below 0.25 wt. %, and the solid slow and intermediate pyrolysis, however, where the liquid
content below2.5 wt. %. It has a relatively poor stability, product is not the main focus, the catalytic effect of AAEM
due to continuous secondary reactions and to its high species is tolerated and therefore there is higher flexibility
oxygen content (about 35-40 wt. %, similar to the original in terms of ash content in the feedstocks. Furthermore, the
biomass)12. Due to this instability, its properties can change process of removing these species is time and resource
with time (i.e., aging); for example, viscosity is increased. consuming, leading to higher costs.
Phase separation usually occurs with high water contents
or due to long term storage, developing an upper aqueous
and acidic phase, and a lower organic phase that is very
viscous.19
23
Table 2. Typical product yields (dry wood basis) from (<15 wt. %)15. Metallurgical applications require a reducing
different pyrolysis types.14 agent (carbon) with high purity.
For pyrolysis focused on the production of char, the most
Liquid yield Gas yield Solid yield important measure of efficiency of the process is the fixed
(wt. %) (wt. %) (wt. %) carbon yield15, 18, 29, given by Eq. 2, by multiplying the fixed
Slow pyrolysis 30 35 35 carbon content in char (wt. %, dry ash-free basis) by the
Intermediate 50 25 25 char yield (wt. %, dry ash-free feedstock basis). This
pyrolysis measure is more advantageous than char yield or others,
Fast pyrolysis 75 13 12 since it takes into account both the yield and quality of the
solid product.30
An inert carrier gas is usually used in pyrolysis, to maintain 𝑦𝑦𝐹𝐹𝐹𝐹 = 𝐹𝐹𝐹𝐹𝐶𝐶ℎ𝑎𝑎𝑎𝑎 𝑦𝑦𝐶𝐶ℎ𝑎𝑎𝑎𝑎 (2)
an oxygen-free atmosphere, and, in the case of fast
pyrolysis, to minimise contact time between the solid and The fixed carbon yield is related to material stability, and
product vapours. On the other hand, if liquid production is so it is relevant for the solid product applications. For
not the main focus, the carrier gas may be absent during example, in combustion, it affects thermal stability, heat
operation. Using carrier gas also leads to dilution of the release, efficiency, etc., and in soil applications, it affects
produced gas27, which reduces its calorific value. characteristics such as stability, recalcitrance, and carbon
Employing different flow rates of carrier gas and different sequestration potential. It has been reported that the fixed
gases can also affect the product distribution and quality. carbon yield increases with pyrolysis temperature, up to a
certain point, limited by the ash content and nature of the
feedstock15. With an increase in pyrolysis temperature,
3. Char properties, applications, and their more stable chemical structures are formed, making the
relation with the process parameters solid more resistant to thermal and biological
degradation.22
3.1. General properties and applications of char
Char, as already mentioned, is the solid product from 3.2. Use of char for soil applications
pyrolysis, mainly composed of carbon, and also hydrogen Char can be a sustainable option for soils that have been
and oxygen, and inorganic elements. It is black in colour, adversely affected by factors such as intensive agricultural
dry, and brittle15. An example of a char produced from management practises and climate change.
biomass pyrolysis is shown in Figure 2. Char is more stable Applying char from biomass pyrolysis in soils has been used
than biomass, due to the loss of moisture and volatiles, and
to improve soil fertility and increase crop yields. Char can
it can have a calorific value of about 30 MJ kg–1.15
lead to a higher nutrient availability and retention,
improvements in soil structure (e.g. density, porosity,
stability), and water-holding capacity31. Char is usually
found favourable for soils when it has certain qualities,
including: high carbon content, high proportion of
aromatic carbon (for stability), high chemical and biological
stability, low density, high porosity and surface area, and
high cation exchange capacity (CEC, proportional to
availability of nutrients)32. Advantages of applying char in
agriculture include carbon sequestration, interception of
contaminants and improvement of plant growth.33
Char quality for soil applications can be improved by
Figure 2. Char produced from pyrolysis (600 °C, 3 min SRT) changing the pyrolysis parameters, but they can also have
of wheat straw pellets in the lab scale screw reactor at contradictory effects: higher temperatures (600-700 °C)
Aston University. lead to higher proportions of aromatic carbon and lower
contents of H and O functional groups, at the same time
Char has been historically used as a fuel for domestic
lowering the CEC; on the other hand, relatively lower
heating and cooking, as well as in metallurgy for smelting
pyrolysis temperatures lead to a higher proportion of C-O
(as the reducing agent)15. Besides these, char has been
and C-H functional groups, and thus a higher CEC32, but
employed in soil conditioning and amendment (i.e.,
stability in terms of fixed-carbon is lower.
biochar), as an additive in cattle feed, as an activated
Chars can be used to increase the pH of acidic soils, since
carbon precursor, and there has been research for its use
they are usually alkaline materials33. However, this is
as a catalyst support, for electrochemical applications (e.g.
dependent on the ability of the char to retain its pH value
for electrodes), and others43–45. Char is also a porous
(buffer capacity). Electrical conductivity, related to the
material, which can develop a microporous structure with
ability of plants to extract water from the soil can also be
potential for adsorption applications.28
relevant for soil applications33. The surface area, which is
Char for domestic heating (e.g. for cooking) typically has
reported to increase with pyrolysis temperature30, is also
20-30 wt. % (up to 40 wt. %) of volatile matter, whereas for
related to exchange and sorption capacity33.
metallurgical applications it should have a lower value
24
The addition of char to soils has also been found to avoid chemical activation, although both types involve chemical
greenhouse gas emissions (CH4, N2O), possibly through reactions.
better aeration and stability of the soils34. Furthermore, The activation conditions greatly affect the activated
due to its adsorption capability, chars can help prevent carbon properties (e.g., the temperature and time of
leaching of compounds such as nitrates, phosphates, and activation, activating agent and proportion used, etc.39, 40).
other ionic solutes.31, 34 The starting material is also a very important factor.
Recycling of nutrients (e.g., K, Mg, Ca, Na, and P) to the soil Activated carbon usually originates from materials
can be done if char is applied on them, with benefits for characterised by a high carbon content and hardness:
the soil and crops35. Regardless, the ash composition of the coconut shell, coal, peat, etc.44. Activation is a process with
chars must be assessed in order to check for harmful heavy a high production cost39, due to energy and resource
metals, which can pollute soil and water and be harmful to needs. Because of this, interest is growing on the possibility
life. of using other biomasses such as agricultural wastes in
Precautions should also be taken about the particle size of order to reduce costs and to benefit the environment39.
char for soil applications: it should not be very small (i.e., Chemical activation is done by mixing a concentrated
powder form), since it could be scattered in the air and solution of a chemical agent with the precursor (coal,
cause health hazards. It should also be noted that the biomass, or char if it is a 2-step activation), e.g.
choice of a char for soil applications depends not only on impregnating the precursor. The most commonly used
the quality of the char, but also on the properties and chemical agents are ZnCl2, H3PO4 and KOH45. The
needs of the soil. mechanism of chemical activation varies according to the
activation agent, but the following principles are
3.3. Activated carbon
common45: chemical bonds in the precursor structure start
Although there have been examples of research using char to break during mixing, and ions from the activation agent
for adsorption36, 37, the porosity and surface area of chars bind to the surface and occupy voids, defining the porosity
are relatively low (in the order of 100 to 102 m2 g–1)38, created in the subsequent activation. During activation,
compared with activated carbon (up to 3000 m2 g–1)39. the chemical agent avoids formation of tars in the pores,
Producing activated carbon from char is an increasingly and the pores become available post-activation, after
popular application, due to its usefulness for removal of washing the material to remove the activation agent. In
pollutants, in both gaseous and aqueous streams40. this way, the porosity and specific surface area increase.
Activated carbon had a market of about 1.6 Mton in 2015, However, at high activating agent doses or activation
and it grows regularly41. There has been an increasing times, small pores can collapse and cause an adverse effect
interest in this application due to stricter environmental of a decrease in surface area. The peak temperatures to
requirements in some countries such as the US, and which the material is exposed in chemical activation are in
promising results from research.40 general lower (as low as 400 °C) than in physical
Examples of specific applications of activated carbon in gas activation39.
streams are in gas masks, in inhabited spaces such as In physical activation, the carbonaceous material
offices, and in the purification of exhaust gases in industrial undergoes partial oxidation to increase porosity and
facilities. Activated carbon can also be used as an surface area, at temperatures usually above 700 °C44. The
adsorption medium in the biogas production industry to oxidant used is usually steam, CO2 or oxygen/air. The
separate unwanted gases such as CO2 and H2S from the process occurs in two stages: firstly, tar is oxidised and
methane42. In terms of liquid streams, activated carbon has clogged pores are opened; and secondly, part of the
been used to remove components such as heavy metals, carbon structure is also oxidised. The second stage, besides
organic molecules (for example tetracycline, phenol), dyes, creating new pores, or interconnections between pores,
and others, in wastewater, pools and aquariums. can also change the chemical functionalities of the surface.
Activated carbon works with the process of adsorption, in These functionalities include phenolic and carboxylic
which the target substance to be adsorbed gets groups, created on the aromatic surfaces of the material.
concentrated in the interface between the adsorbent Since the physical activation normally occurs in two steps
(activated carbon) and the bulk fluid with the substance43. (carbonisation and then activation), there is a higher
This process is different than the process of absorption, in energy consumption and more equipment may be needed,
which chemical bonds are formed. Instead, intermolecular compared with an activation process in one step
forces of attraction and repulsion are involved. (carbonisation and activation at the same time).
Adsorption is a capability of char that can be enhanced by Furthermore, if the char is cooled down between the two
activation. In the activation process, the physical structure steps, it may adsorb some volatiles and tars from the
of the char is altered: the porosity and surface area are surrounding atmosphere, which will reduce quality
greatly increased, with the disadvantage of consumption (plugging of pores for example). However, chemical
of some part of the material. With an increase in surface activation also has disadvantages (e.g., the requirement of
area, more molecules can be adsorbed on the surface. The a washing step to remove the chemical agent, thus
process can be performed on the char from pyrolysis (2- incurring in further costs and resource usage).
step), or directly on the material before pyrolysis (1-step).
Furthermore, activation is generally classified in physical or
25
David and Kopac46 studied the activation of chars from a a screw, in the case of Auger reactors. Further description
mixture of rapeseed oil cake and walnut shell in pellet of different reactors used for slow pyrolysis (in industrial
form. Pyrolysis was performed in a vertical fixed bed scale) is done in Chapter 3.
reactor, at temperatures between 400 and 750 °C, with A screw reactor is heated externally generally using a
5 °C min-1 heating rate and 60 min hold time at peak furnace, and in some cases heat carriers are employed to
temperature. Activation was then performed with CO2 promote heat transfer inside the reactor (e.g., hot sand,
atmosphere, with the same temperatures of 400 to 750 °C. steel particles or metal/ceramic spheres12). After being
transported to the other end of the reactor, the solid
It was found that the surface area and porosity of the
product (char) and the heat carriers (if existent) exit the
activated carbons depended mainly on activation
reactor by gravity into a container. The heat carriers are
temperature and activation time. The surface areas (N2, recycled into the reactor with an appropriate separation
BET method) obtained at 600 °C were between 300 and and transport system. The vapours that exit the reactor
500 m2 g–1 (0.5 to 3.5 h activation time), and 400 and pass through a condensing system, where the condensable
500 °C yielded small N2 adsorption. A surface area up to fraction forms the liquid product. An example
around 1000 m2 g–1 was obtained at 750 °C and 3 h of representation of an Auger reactor is shown in Figure 3.
activation time.
Using different atmospheres can have advantages such as
increasing the concentration of combustible gases and
thus its energy content (15 to 20 MJ m–3 with steam)14.
Furthermore, some studies have shown that using a CO2
atmosphere in pyrolysis decreases the tar content in the
solid, improving its quality47.
By injecting CO2 and transforming it in the pyrolysis
process, the release of this harmful greenhouse gas into
the atmosphere can be reduced. There is much interest
nowadays in methods for carbon capture and storage (CCS)
and for utilising this gas in other processes, for which Figure 3. Generic scheme representing a typical Auger
reactor.
activated carbon production is a possibility. Using different
carrier gases such as CO2 in the pyrolysis process can also Industrial use of Augers started in the beginning of the 20th
bring economic advantages, due to avoiding the use of century, in applications such as conveying, drying, and
nitrogen as carrier gas. pyrolysis, firstly of coal. The first known reference51 of
As a conclusion, to make the best use of char, the effect of biomass pyrolysis in a screw reactor was in 1969.
different feedstocks and pyrolysis conditions must be Comparing with other reactors, Auger reactors have some
better understood. Standardising char characteristics and advantages. Besides continuous processing, the handling
analyses is very important, and there have been some and operation are relatively simple52, and the design is
examples, by the International Biochar Initiative (IBI)48, and compact, allowing for it to be more easily transported53. A
by the European Biochar Foundation49, for char produced study demonstrated the feasibility of using modular and
for soil applications (commonly referred to as biochar, if transportable Auger reactors for pyrolysis of biomass,
produced from biomass with certain characteristics). which allows for the production of energy in decentralised
Besides the environmental advantages of all applications and/or remote areas54, and consequently reduces the
of char from biomass, it can be a source of income, costs if biomass is available on site.
increasing the feasibility of the pyrolysis process and of the Another study used an Auger reactor for continuous
bioeconomy. pyrolysis of forestry waste and demonstrated that the
design was robust and that scaling-up was possible,
supported by the fact that the feedstock did not require a
4. Pyrolysis with screw reactors significant particle size reduction (i.e., to powder), which
Slow pyrolysis can be performed in many different types of makes the process more economic53. Auger reactors are in
systems. Early on, wood logs (or other biomass materials general more flexible to different feedstock types, shapes
with a large particle size) were processed in batch systems: and sizes, and more adequate for heterogeneous
earth-made kilns, retorts, wagons, and other systems50. materials12, and also the use of carrier gas is optional52. The
Continuous operation was then developed, along with ability to control the solid residence time by changing
using a smaller particle size (e.g., chips, pellets and crushed screw speed is also an advantage.
residues). Continuous mode systems improve the Some limitations of screw reactors are mechanical issues
economics of the process, since less time is spent switching due to the moving parts, bearings and sealing exposed to
batches. Examples of continuous pyrolysis systems are high temperatures inside the reactor27, and the risk of
rotary drums, paddle reactors, moving bed reactors, and blockage in the system, due to the accumulation of
screw (also called Auger) reactors50. The way of biomass and/or products inside the inlet, reactor tube or
transporting the feed along the reactor changes between the outlet. Heat transfer also limits scale-up due to the
systems: by using rotation and inclination, in the case of design of the reactor27. Heating the screw and reactor tube
rotary drums; using paddles or other similar tools; or using
26
with larger dimensions becomes more difficult and energy- from the char, and recycled back to the system with a
demanding. screw conveyor60. Multiple feedstocks were tested in the
There is significant research on pyrolysis in Auger reactors reactor to produce pyrolysis liquid, for example, coconut
(although not as much as with for example fixed and and rape residues, wheat straw, olive stones, and rice husk,
fluidised beds), with important examples in universities in with particle sizes up to 5 cm.56
the US (e.g. Mississippi State, Iowa State), at the Karlsruhe The Pyroformer, patented in 2009, is a pilot-scale system
Institute of Technology (KIT, Germany), at Ghent University installed and operated at Aston University61. This reactor,
(Belgium), and at the European Bioenergy Research represented in Figure 562, has a very important feature: it
Institute (Aston University, UK). has two co-axial screws, instead of one screw as usual.
At Aston University there is a lab-scale (300 g h–1) single
screw Auger reactor. The reactor is a stainless steel
cylindrical pipe with 83.5 cm length and internal diameter
of about 2.5 cm, with a furnace covering about 40 cm of
the pipe. The screw is driven by an electrical motor with
controllable speed. The biomass is fed through a vertical
inlet pipe, with a valve that can minimise air entrance, and
with an input for nitrogen gas. In the system, temperature
is monitored with thermocouples in four locations: inside
the reactor tube (middle section), around the wall of the
pipe, in the vapour exit (similar to the freeboard in a
fluidised bed), and in the start of the condensing system.
The solid transported along the reactor is collected in a pot, Figure 5. Representation of the Pyroformer system
while the vapours go through the cooling system. The (adapted62).
cooling/condensing system is comprised by a water-cooler
(around 5 °C), two ice fingers with dry-ice, and a cotton These two screws can be moved independently and driven
filter for trapping aerosols. The produced gas can be forwards and backwards, and there are slots through
analysed online using a micro-GC after passing through a which material can pass from one screw to another. This
gas flowmeter. The liquid products are collected in flasks feature allows to recycle char internally and to mix fresh
for posterior analysis. The lab-scale screw reactor system feedstock with the recycled char, which acts both as a heat
at EBRI is represented in Figure 4.55 carrier and as a catalyst for char-producing secondary
reactions24. The pilot-scale system has a processing
capacity of up to 20 kg h–1, and there is also a 100 kg h–1
demonstration system built at Aston University.
In the Pyroformer, the vapours produced leave the reactor
and pass through two hot gas filter candles to remove solid
particles. After that, the clean vapour passes through a
shell and tube heat exchanger where it is condensed and
the liquid is collected63. The non-condensable gases pass
through an electrostatic precipitator (ESP) to remove
aerosols, while the char product is collected at the bottom
of the reactor in a char pot64. Generally, product yields
Figure 4. Lab-scale Auger reactor available at EBRI (Aston from biomass are: 40-60 wt% of liquid, 15-25 wt% of char,
University). and 20-30 wt% of gas62.
Examples of pilot or demonstration scale Auger reactors
that operate in slow or intermediate pyrolysis conditions
are the Haloclean system, the Pyroformer, and the
Thermo-Catalytic Reformer (TCR).
The Haloclean system was created in 2002 by the Italian
company Sea Marconi and the German
Forschungszentrum Karlsruhe56, 57. After being used for
processing waste electronic and electrical equipment into
fuels and noble metals58, it was found to be a suitable
technology for biomass pyrolysis59. The system was
claimed to produce less tars, a liquid and gas product with Figure 6. Representation of the TCR system.
lower solids content, and lower acidity in the products,
More recently, the Thermo-Catalytic Reformer (TCR)
compared with a fast pyrolysis system59. A lab-scale system
system was developed in Germany by the Fraunhofer
of 3 kg h–1 was developed, along with a pilot-scale
UMSICHT Institute. There are units installed in Germany (2
100 kg h–1 system56, 60. During operation, heat carriers
kg h-–1, 30 kg h–1, and a 300 kg h–1), and in Tyseley Energy
(metal spheres) were employed, which are then separated
Park in Birmingham (30 kg h–1).
27
The TCR system has a pyrolysis screw reactor combined the particle size and form (e.g. pellet or powder form), and
with a post-reforming reactor. The feed is transported on the atmosphere inside the reactor.
through the first reactor and the products from pyrolysis
are directed into the secondary reactor. The char acts as a
catalyst to upgrade the quality of the product vapours,
before they pass through a condensation system and are
separated into the liquid and gaseous products65. The
pyrolysis reactor works at temperatures usually between
400 and 500 °C, while the reforming reactor operates
between 500 and 750 °C. The system is said to be flexible
to various feedstocks and with relatively high ash and
moisture contents, producing products with improved
Figure 7. Proportion of articles that study different
quality66. A representation of the TCR is presented in Figure
parameters of the pyrolysis of biomass and wastes in
6.67
auger reactors, excluding the research that focuses on
There are some examples of commercial units, and, for
liquid production.
example, the Canadian company ABRI-Tech develops 1 ton
per day screw reactor units50. The German company Pyreg
has also developed commercial screw reactors for Conclusions
pyrolysis, with units in Germany and Switzerland.68
In the literature, a limited number of studies focuses on Slow pyrolysis has been performed for a long time, at first
char production from biomass in Auger reactors. It was also (and still in many cases) with traditional and simple
found that research mainly analyses the effect of a limited systems, which are not the most efficient or safe for the
number of parameters, focusing in most cases on the peak environment. More recent systems, such as the Auger
temperature and the feedstock type, and in some cases on (screw) reactor, have been developed for pyrolysis with
the solid residence time and the feedstock flow rate. advantages over other systems, e.g. continuous operation,
Since research with screw reactors mainly focused on simplicity of operation, flexibility for different particle
liquid production (considered fast pyrolysis), most use a sizes, among others. Char production can be enhanced by
very small particle size (millimetres or lower), heat carriers, using optimised parameters such as temperature and solid
and carrier gas, to prevent extended contact between the residence time, taking into account char quality as well.
volatiles and the solid that favours secondary reactions and The production of char and its derivatives has received
thus char and gas production. The solid residence time increasing attention in terms of research, and many
used, however, is higher than in typical fast pyrolysis, and promising and valuable applications have been found, for
vapour residence times range from 5 to 30 s depending on example in soil treatment, adsorption, catalysis and for
design and size12. Screw reactors could therefore be a electrochemical applications.
useful reactor to produce char in significant quantities. The various and useful applications of char make it a
Char yields from Auger reactors with woody biomass are worthwhile product and more research must be done in
reported50 to be between 17 and 30 wt. %. order to improve the understanding of the relation
In terms of feedstock, research mainly focused on woody between its properties and production methods, and the
biomass (mostly pine wood), and there are a few accounts feedstocks. As an example, activated carbon from char
of herbaceous biomasses being used: wheat straw made from biomass or wastes brings economic and
(pellets)56, barley straw (pellets)64, rice husk and corn environmental benefits that can, among other things,
stalk55, switchgrass69, rice husk and bran56, and rapeseed56. increase the sustainability of resources.
Examples of wastes studied in Auger reactors are MSW70, The aim of the research of the GreenCarbon’s Early-Stage
71, digestate from anaerobic digestion66, 72, 73, tyres74, and Researcher #2 (F. Rego) is to study the pyrolysis of biomass
wastes from the wastewater treatment industry (sewage and wastes in a continuous screw reactor, with a focus on
sludge)75, 76, the paper industry (de-inking sludge)62, 77, and the production of char. Objectives include assessing the
the brewing industry24. Some of these wastes were influence of process parameters such as temperature and
processed in pellet form: de-inking sludge and sewage solid residence time on the product yields and properties,
sludge62, 77, paper sludge72, and digestate from anaerobic as well as the possible applications of the produced char.
digestion.66, 72 The production of a solid product suitable for use as an
A literature survey based on 21 articles found that over half adsorbent of contaminants from aqueous solutions is also
(about 57%) were focusing on the liquid product an objective.
production, and that mainly temperature, feedstock type
and solid residence time were the process variables. A
Acknowledgements
graphic representation of the proportion of articles
(excluding the ones focusing on liquid production) that This project has received funding from the European
studied temperature, feedstock type, solid residence time, Union’s Horizon 2020 research and innovation programme
and feedstock flow rate is shown in Figure 7. Limited or no under the Marie Skłodowska-Curie grant agreement No
research was found on other parameters, for example, on 721991.
28
References (19) Oasmaa, A.; Czernik, S. Fuel Oil Quality of Biomass

(1) Hayes, D. J. M. Second-Generation Biofuels: Why They Pyrolysis Oils - State of the Art for the End Users.
Are Taking so Long. WIREs Energy Env. 2013, 2 (June), Energy Fuels 1999, 13 (4), 914–921.
304–334. (20) Bridgwater, A. V. The Production of Biofuels and
(2) Tripathi, M.; Sahu, J. N.; Ganesan, P. Effect of Process Renewable Chemicals by Fast Pyrolysis of Biomass. Int.
Parameters on Production of Biochar from Biomass J. Glob. Energy Issues 2007, 27 (2), 160–203.
Waste through Pyrolysis: A Review. Renew. Sustain. (21) ASTM International. ASTM D7544-09, Standard
Energy Rev. 2016, 55, 467–481. Specification for Pyrolysis Liquid Biofuel; West
(3) Alam, F.; Mobin, S.; Chowdhury, H. Third Generation Conshohocken, PA, 2009.
Biofuel from Algae. In 6th BSME International (22) Demirbaş, A.; Arin, G. An Overview of Biomass
Conference on Thermal Engineering (ICTE 2014); Pyrolysis. Energy Sources 2002, 24 (5), 471–482.
Elsevier B.V., 2015; Vol. 105, pp 763–768. (23) Demirbas, A. Biomass Resource Facilities and Biomass
(4) Ragland, K.; Bryden, K. Combustion Engineering, 2nd Conversion Processing for Fuels and Chemicals. Energy
Ed.; Ragland, K., Bryden, K., Eds.; CRC Press, 2011. Convers. Manag. 2001, 42, 1357–1378.
(5) Hon, D.; Shiraishi, N. Wood and Cellulosic Chemistry; (24) Mahmood, A. S. N.; Brammer, J. G.; Hornung, A.;
Marcel Dekker, Inc.: New York, 1991. Steele, A.; Poulston, S. The Intermediate Pyrolysis and
(6) Van Loo, S.; Koppejan, J. Chapter 8 - Biomass Ash Catalytic Steam Reforming of Brewers Spent Grain. J.
Characteristics and Behaviour in Combustion Systems. Anal. Appl. Pyrolysis 2013, 103, 328–342.
In The Handbook of Biomass Combustion and Co-firing; (25) Banks, S. W.; Nowakowski, D. J.; Bridgwater, A. V.
Van Loo, S., Koppejan, J., Eds.; Earthscan, 2008. Impact of Potassium and Phosphorus in Biomass on
(7) Rowell, R. M.; Pettersen, R.; Han, J. S.; Rowell, J. S.; the Properties of Fast Pyrolysis Bio-Oil. Energy Fuels
Tshabalala, M. A. Chapter 3 - Cell Wall Chemistry. In 2016, 30 (10), 8009–8018.
Handbook of wood chemistry and wood composites; (26) Eom, I. Y.; Kim, K. H.; Kim, J. Y.; Lee, S. M.; Yeo, H. M.;
Rowell, R. M., Ed.; CRC Press, 2005. Choi, I. G.; Choi, J. W. Characterization of Primary
(8) Maguire, R.; Mark, A.; Flowers, W. Fertilizer Types and Thermal Degradation Features of Lignocellulosic
Calculating Application Rates; 2009. Biomass after Removal of Inorganic Metals by Diverse
(9) Holubcik, M.; Jandacka, J.; Malcho, M. Ash Melting Solvents. Bioresour. Technol. 2011, 102 (3), 3437–
Temperature Prediction from Chemical Composition of 3444.
Biomass Ash. Holist. Approach to Environ. 2015, 5, (27) Bahng, M. K.; Mukarakate, C.; Robichaud, D. J.; Nimlos,
119–125. M. R. Current Technologies for Analysis of Biomass
(10) Cai, J.; He, Y.; Yu, X.; Banks, S. W.; Yang, Y.; Zhang, X.; Thermochemical Processing: A Review. Anal. Chim.
Yu, Y.; Liu, R.; Bridgwater, A. V. Review of Acta 2009, 651 (2), 117–138.
Physicochemical Properties and Analytical (28) Khalil, L. B. Porosity Characteristics of Chars Derived
Characterization of Lignocellulosic Biomass. Renew. from Different Lignocellulosic Materials. Adsorpt. Sci.
Sustain. Energy Rev. 2017, 76 (March), 309–322. Technol. 1999, 17 (9), 729–739.
(11) Basu, P. Biomass Gasification and Pyrolysis: Practical (29) Antal, M. J.; Allen, S. G.; Dai, X.; Shimizu, B.; Tam, M. S.;
Design and Theory; Elsevier Inc., 2010. Grønli, M. Attainment of the Theoretical Yield of
(12) Bridgwater, A. V. Review of Fast Pyrolysis of Biomass Carbon from Biomass. Ind. Eng. Chem. Res. 2000, 39
and Product Upgrading. Biomass and Bioenergy 2012, (11), 4024–4031.
38, 68–94. (30) Manyà, J. J. Pyrolysis for Biochar Purposes: A Review to
(13) Hossain, A. K.; Davies, P. A. Pyrolysis Liquids and Gases Establish Current Knowledge Gaps and Research
as Alternative Fuels in Internal Combustion Engines - A Needs. Environ. Sci. Technol. 2012, 46 (15), 7939–7954.
Review. Renew. Sustain. Energy Rev. 2013, 21, 165– (31) Tian, X.; Li, C.; Zhang, M.; Wan, Y.; Xie, Z.; Chen, B.; Li,
189. W. Biochar Derived from Corn Straw Affected
(14) Bridgwater, A. V. Renewable Fuels and Chemicals by Availability and Distribution of Soil Nutrients and
Thermal Processing of Biomass. Chem. Eng. J. 2003, 91 Cotton Yield. PLoS One 2018, 13 (1), 1–19.
(2–3), 87–102. (32) Li, Y.; Hu, S.; Chen, J.; Müller, K.; Li, Y.; Fu, W. Effects of
(15) Antal, M. J.; Grønli, M. The Art, Science, and Biochar Application in Forest Ecosystems on Soil
Technology of Charcoal Production †. Ind. Eng. Chem. Properties and Greenhouse Gas Emissions: A Review.
Res. 2003, 42 (8), 1619–1640. Springer 2018, 546–563.
(16) Radlein, D.; Quignard, A. A Short Historical Review of (33) Allaire, S.; Lange, S.; Auclair, I.; Quinche, M.; Greffard,
Fast Pyrolysis of Biomass. Oil Gas Sci. Technol. 2013, 68 L. Analyses of Biochar Properties; Quebec, 2015.
(4), 765–783. (34) Lehmann, J.; Gaunt, J.; Rondon, M. Bio-Char
(17) Overend, R.; Milne, T.; Mudge, L. Fundamentals of Sequestration in Terrestrial Ecosystems - A Review.
Thermochemical Conversion of Biomass; Overend, R., Mitig. Adapt. Strateg. Glob. Chang. 2006, 11 (2), 403–
Milne, T., Mudge, L., Eds.; Elsevier, 1985. 427.
(18) Di Blasi, C. Modeling Chemical and Physical Processes (35) Lehmann, J.; Joseph, S. Biochar for Environmental
of Wood and Biomass Pyrolysis. Prog. Energy Combust. Management : An Introduction. Biochar Environ.
Sci. 2008, 34 (1), 47–90. Manag. - Sci. Technol. 2009, 1, 1–12.
29
(36) Nartey, O. D.; Zhao, B. Biochar Preparation, (50) Garcia-Nunez, J. A.; Pelaez-Samaniego, M. R.; Garcia-
Characterization, and Adsorptive Capacity and Its Perez, M. E.; Fonts, I.; Abrego, J.; Westerhof, R. J. M.;
Effect on Bioavailability of Contaminants: An Garcia-Perez, M. Historical Developments of Pyrolysis
Overview. Adv. Mater. Sci. Eng. 2014, 2014. Reactors: A Review. Energy and Fuels 2017, 31 (6),
(37) Wang, Y.; Liu, R. Comparison of Characteristics of 5751–5775.
Twenty-One Types of Biochar and Their Ability to (51) Lakshmanan, C.; Gal-Or, B.; Hoelscher, H. Production
Remove Multi-Heavy Metals and Methylene Blue in Of Levoglucosan By Pyrolysis Of Carbohydrates. I&EC
Solution. Fuel Process. Technol. 2017, 160, 55–63. Prod. Res. Dev. 1969, 8 (3), 261–267.
(38) Budai, A.; Wang, L.; Gronli, M.; Strand, L. T.; Antal, M. (52) Thangalazhy-Gopakumar, S.; Adhikari, S.; Ravindran,
J.; Abiven, S.; Dieguez-Alonso, A.; Anca-Couce, A.; H.; Gupta, R. B.; Fasina, O.; Tu, M.; Fernando, S. D.
Rasse, D. P. Surface Properties and Chemical Physiochemical Properties of Bio-Oil Produced at
Composition of Corncob and Miscanthus Biochars: Various Temperatures from Pine Wood Using an Auger
Effects of Production Temperature and Method. J. Reactor. Bioresour. Technol. 2010, 101 (21), 8389–
Agric. Food Chem. 2014, 62 (17), 3791–3799. 8395.
(39) Dias, J. M.; Alvim-ferraz, M. C. M.; Almeida, M. F.; Sa, (53) Puy, N.; Murillo, R.; Navarro, M. V.; López, J. M.;
M. Waste Materials for Activated Carbon Preparation Rieradevall, J.; Fowler, G.; Aranguren, I.; García, T.;
and Its Use in Aqueous-Phase Treatment : A Review. Bartrolí, J.; Mastral, A. M. Valorisation of Forestry
2007, 85, 833–846. Waste by Pyrolysis in an Auger Reactor. Waste Manag.
(40) Schröder, E.; Thomauske, K.; Oechsler, B.; Herberger, 2011, 31 (6), 1339–1349.
S.; Baur, S.; Hornung, A. Chapter 18 - Activated Carbon (54) Badger, P. C.; Fransham, P. Use of Mobile Fast Pyrolysis
from Waste Biomass. In Progress in Biomass and Plants to Densify Biomass and Reduce Biomass
Bioenergy Production; Shaukat, S., Ed.; InTech, 2011. Handling Costs - A Preliminary Assessment. Biomass
(41) Grand View Research Inc. Activated Carbon Market Bioenergy 2006, 30 (4), 321–325.
Analysis and Segment Forecasts To 2024; San (55) Yu, Y.; Yang, Y.; Cheng, Z.; Blanco, P. H.; Liu, R.;
Francisco, 2016. Bridgwater, A. V.; Cai, J. Pyrolysis of Rice Husk and Corn
(42) Esteves, I. A. A. C.; Lopes, M. S. S.; Nunes, P. M. C.; Stalk in Auger Reactor. 1. Characterization of Char and
Mota, J. P. B. Adsorption of Natural Gas and Biogas Gas at Various Temperatures. Energy Fuels 2016, 30
Components on Activated Carbon. Sep. Purif. Technol. (12), 10568–10574.
2008, 62 (2), 281–296. (56) Roggero, C. M.; Tumiatti, V.; Scova, A.; De Leo, C.;
(43) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Binello, A.; Cravotto, G. Characterization of Oils from
Powders and Porous Solids - Principles, Methodology Haloclean Pyrolysis of Biomasses. Energy Sources, Part
and Apps; Academic Press, 1999. A Recover. Util. Environ. Eff. 2011, 33 (5), 467–476.
(44) Prauchner, M. J.; Rodríguez-Reinoso, F. Chemical (57) Hornung, A.; Bockhorn, H.; Appenzeller, K.; Roggero, C.
versus Physical Activation of Coconut Shell: A M.; Tumiatti, W. Patent EP 1 217 318 A1 - Plant for the
Comparative Study. Microporous Mesoporous Mater. Thermal Treatment of Material and Operation Process
2012, 152, 163–171. Thereof. EP 1 217 318 A1, 2002.
(45) Hagemann, N.; Spokas, K.; Schmidt, H. P.; Kägi, R.; (58) Hornung, A.; Koch, W.; Seifert, H. Haloclean and PYDRA
Böhler, M. A.; Bucheli, T. D. Activated Carbon, Biochar - a Dual Staged Pyrolysis Plant for the Recycling Waste
and Charcoal: Linkages and Synergies across Pyrogenic Electronic and Electrical Equipment (WEEE). In Metals
Carbon’s ABCs. Water (Switzerland) 2018, 10 (2), 1–19. and Energy Recovery : Internat.Symp.in Northern
(46) David, E.; Kopac, J. Activated Carbons Derived from Sweden; Skelleftea, 2003; p 7.
Residual Biomass Pyrolysis and Their CO2adsorption (59) Sea Marconi. Haloclean BioEnergy - Pyrolysis Liquid for
Capacity. J. Anal. Appl. Pyrolysis 2014, 110 (1), 322– Power Generation; PyNe IEA Bioenergy, 2007.
332. (60) Hornung, A.; Apfelbacher, A.; Koch, W.; Linek, K.; Sagi,
(47) Dutta, T.; Kwon, E.; Bhattacharya, S. S.; Jeon, B. H.; S.; Schoner, J.; Stohr, J.; Seifert, H. Thermo-Chemical
Deep, A.; Uchimiya, M.; Kim, K. H. Polycyclic Aromatic Conversion of Straw – Haloclean , an Optimised Low
Hydrocarbons and Volatile Organic Compounds in Temperature Pyrolysis. In 14th European Biomass
Biochar and Biochar-Amended Soil: A Review. GCB Conference; Paris, 2005; p 5.
Bioenergy 2017, 9 (6), 990–1004. (61) Hornung, A.; Apfelbacher, A. EP2300560B1 - Thermal
(48) IBI, I. B. I. Standardized Product Definition and Product Treatment of Biomass. EP2300560B1, 2009.
Testing Guidelines for Biochar That Is Used in Soil; (62) Yang, Y.; Brammer, J. G.; Ouadi, M.; Samanya, J.;
2015. Hornung, A.; Xu, H. M.; Li, Y. Characterisation of Waste
(49) European Biochar Foundation (EBC). European Biochar Derived Intermediate Pyrolysis Oils for Use as Diesel
Certificate - Guidelines for a Sustainable Production of Engine Fuels. Fuel 2013, 103, 247–257.
Biochar; Arbaz, Switzerland, 2012. (63) Hornung, A.; Apfelbacher, A.; Sagi, S. Intermediate
Pyrolysis: A Sustainable Biomass-to-Energy Concept-
Biothermal Valorisation of Biomass (BtVB) Process. J.
Sci. Ind. Res. (India). 2011, 70 (8), 664–667.
30
(64) Yang, Y.; Brammer, J. G.; Mahmood, A. S. N.; Hornung,

A. Intermediate Pyrolysis of Biomass Energy Pellets for
Producing Sustainable Liquid, Gaseous and Solid Fuels.
Bioresour. Technol. 2014, 169, 794–799.
(65) Jäger, N.; Conti, R.; Neumann, J.; Apfelbacher, A.;
Daschner, R.; Binder, S.; Hornung, A. Thermo-Catalytic
Reforming of Woody Biomass. Energy Fuels 2016, 30
(10), 7923–7929.
(66) Neumann, J.; Meyer, J.; Ouadi, M.; Apfelbacher, A.;
Binder, S.; Hornung, A. The Conversion of Anaerobic
Digestion Waste into Biofuels via a Novel Thermo-
Catalytic Reforming Process. Waste Manage. 2016, 47,
141–148.
(67) Fraunhofer UMSICHT. The Biobattery Thermo Catalytic
Reforming TCR ®; 2016.
(68) Hammond, J.; Rödger, J. Biochar - Climate Saving Soils
- Newsletter 1; 2012; Vol. 1.
(69) Dalluge, D. L.; Daugaard, T.; Johnston, P.; Kuzhiyil, N.;
Wright, M. M.; Brown, R. C. Continuous Production of
Sugars from Pyrolysis of Acid-Infused Lignocellulosic
Biomass. Green Chem. 2014, 16, 4144–4155.
(70) Ouadi, M.; Jaeger, N.; Greenhalf, C.; Santos, J.; Conti,
R.; Hornung, A. Thermo-Catalytic Reforming of
Municipal Solid Waste. Waste Manag. 2017, 68, 198–
206.
(71) Yang, Y.; Heaven, S.; Venetsaneas, N.; Banks, C. J.;
Bridgwater, A. V. Slow Pyrolysis of Organic Fraction of
Municipal Solid Waste (OFMSW): Characterisation of
Products and Screening of the Aqueous Liquid Product
for Anaerobic Digestion. Appl. Energy 2018, 213
(November 2017), 158–168.
(72) Conti, R.; Jäger, N.; Neumann, J.; Apfelbacher, A.;
Daschner, R.; Hornung, A. Thermocatalytic Reforming
of Biomass Waste Streams. Energy Technol. 2017, 5
(1), 104–110.
(73) Neumann, J.; Binder, S.; Apfelbacher, A.; Gasson, J. R.;
Ramírez García, P.; Hornung, A. Production and
Characterization of a New Quality Pyrolysis Oil, Char
and Syngas from Digestate - Introducing the Thermo-
Catalytic Reforming Process. J. Anal. Appl. Pyrolysis
2015, 113, 137–142.
(74) Martínez, J. D.; Murillo, R.; García, T.; Veses, A.
Demonstration of the Waste Tire Pyrolysis Process on
Pilot Scale in a Continuous Auger Reactor. J. Hazard.
Mater. 2013, 261, 637–645.
(75) Yang, Y. Energy Production from Biomass and Waste
Derived Intermediate Pyrolyis, Aston University, 2014.
(76) Tomasi Morgano, M.; Leibold, H.; Richter, F.; Stapf, D.;
Seifert, H. Screw Pyrolysis Technology for Sewage
Sludge Treatment. Waste Manage. 2017.
(77) Ouadi, M.; Brammer, J. G.; Yang, Y.; Hornung, A.; Kay,
M. The Intermediate Pyrolysis of de-Inking Sludge to
Produce a Sustainable Liquid Fuel. J. Anal. Appl.
Pyrolysis 2013, 102, 24–32.
31
32
Industrial technologies for slow pyrolysis

Jorge López-Ordovás, Katie Chong, Anthony V. Bridgwater, Yang Yang
European Bioenergy Research Institute (EBRI), School of Engineering and Applied Science, Aston University, Birmingham,
B4 7ET, UK
Abstract
This chapter classifies and analyses the technologies available in industry for slow pyrolysis. Depending on the mode of operation,
these technologies are classified into batch, semi-continuous and continuous processes; each of them with different characteristics,
advantages and disadvantages. Different criteria to help with the choice of a reactor are briefly introduced with all the gathered
information presented. Furthermore, the introduction of the reactor in the system is briefly explained, and different options for pre-
and post-treatment of the materials are described. In addition to the reactor design, Techno-Economic Assessment (TEA) is explained
with all associated costs and economic evaluation methods. The chapter concludes with an explanation of the challenges in the design
of this type of reactor, addressing different possibilities for the upcoming research.
1. Introduction 2. Batch Processes

Research carried out in academia is very important for
The batch processes are the most straightforward
both scientific and industrial development. However, most
technology to produce char and have been used for many
of the studies completed within academia never reach
years. They typically consist of concrete rectangular kilns
industrial stakeholders for a variety of reasons, and only
that are loaded with the feedstock and the temperature is
when the research is shared does it have an impact on
raised to the pyrolysis temperature through internal
society.
heating, adding some oxygen for the partial combustion of
One of the challenges to overcome in pyrolysis is the
the feedstock. They require significant time for operation
method or technology used to produce char. Despite being
(25-30 days residence time, including cooling down period)
made since ancient times and the continuous
with low char yields (5–20 wt. %) (See Figure 2). These
improvements on the existing technologies, there is no
processes operate without heat recovery, and a significant
preferred method to produce char. The existing reactors
fraction of the feedstock is burnt off, but they can cope
can be classified into different types. Some articles1-2 divide
with a range of feedstocks and are very cheap to
the industrial processes based on the heating method
construct2, 6. The feedstock does not move within the
whilst others make the classification based on the process
reactor, and, the product only can be collected once it is
operation mode3-6: batch, semi-continuous and continuous
cooled.4
processes. This classification is more intuitive and makes
the differences between the technologies much clearer.
Regardless of the technology used or the operation mode
in the reactor, the feedstock always undergoes the same
processes represented in Figure 1.
Feeding Drying Pyrolysis Cooling Collection
Figure 1. Main process steps.
One of the characteristics mentioned in pyrolysis is the

absence of oxygen. The oxygen-free environment begins at
the feeding step where all the air is removed from the raw
material. The feedstock has a moisture content, which
might need an independent pre-treatment step to reduce Figure 2. Missouri kiln.
it to the required value or this can be done in the reactor if
The use of kilns to produce char is still widespread as they
it is not too high. Once the drying phase is complete,
are relatively cheap and simple to operate, which makes
pyrolysis begins and starts to degrade the feedstock to the
them attractive despite the low char yield. It is a good
products mentioned. These products need cooling, to
option for developing countries like Ghana due to their low
avoid auto ignition, to stop any reactions occurring or to
investment cost6, and it is still used in some industrialised
condense the vapours to obtain the liquid product. After
countries like the United States5. Kilns are chiefly used in
the process, the products are collected to use them for a
the Brazilian state of Minas Gerais, for instance,
range of applications. There might be other pre-or post-
particularly by a company called Biocarbon Ind. Com. Ltd4.
treatment steps, but drying and cooling are always
The company Horner Charcoal Company in Taneyville,
required in pyrolysis processes.5

33
Chapter 3 López-Ordovás et al.
Missouri, uses a set of Missouri kilns to produce char5 (see such as gas or fuel oil. In the meantime, the second vessel
Figure 2). is charged with the raw material. The first vessel will
undergo carbonisation reactions, and it will release the
pyroligneous vapours that will be burnt in an external
3. Semi-continuous processes chamber to provide energy to heat up the second retort,
The semi-continuous processes have a continuous as shown in Figure 4. Once the carbonisation finishes in the
operation, but the feeding of the feedstock and the first vessel, the reactor will be replaced with a new one
discharge of the products occur in batches. This with fresh feedstock, leaving the first one to cool down
characteristic allows the systems to recover part of the covered with a lid. The time required per vessel for the
energy used in the product. For instance, the gas produced, whole process is 8–12 h and the char yield obtained is 30-
comprising of the condensable and non-condensable 32 wt. %2. Although it is called twin retort more than two
fractions, is usually burnt in a combustion chamber to units can be used, for example, 12 vessels would need
provide energy for the process, both in the drying and the three workers per shift working on a 24-h basis, and the
pyrolysis part.4, 6 whole system can produce between 6000 and 7000 tons of
char per year. There are some operating plants using this
3.1. Waggon retort technology in Carbon France (France), Green Coal Estonia
Waggon retort technology consists of a 45 meter-long (Estonia) and Carbo Group (The Netherlands) and USA.2, 5
tunnel divided into three chambers: drying, carbonisation
and cooling, separated by doors. The wall of the tunnel is
made of perforated steel, which enables the removal of
vapours from carbonisation to burn them in an external
furnace and recirculate them to the drying and the
carbonisation chamber. Two-thirds of the gases from the
combustion chamber enter the carbonisation chamber and
Figure 4. Carbon twin system.
the one-third left goes to the drying chamber. The cycle of
each wagon varies from 25 to 35 h. The process is
commercialised by Impianti Trattamento Biomassa with 4. Continuous processes
plants in Milazzo and Mortera (Italy), Belišće (Hungary) and
Alterna Biocarbon in Prince George (Canada). The average Continuous operation and feeding characterise continuous
production of this technology is 6000 tons of char each processes. The capital cost is higher, there is a need for
year. A typical waggon retort arrangement is shown in external sources of energy, and the systems are not usually
Figure 3. A significant disadvantage of this system is the portable. However, the yields of the char are higher, and
high operating cost compared to similar technologies such some product characteristics such as surface area and high
as the Carbo Twin Retort.2, 4-5 heating value are, overall, higher. Some by-products like
acetic acid can be recovered and the heat required can be
partially produced by one or more of the products.2, 5
There are different ways to classify the continuous
processes further: they can be ranked by heating method
or by the feeding method. Some of the technologies are
fed in batches of different sizes such as the Lurgi or
Reichert reactor, whereas other technologies like rotary
kilns or screw reactors use continuous feeding; however,
the classification used in this chapter is based on the
particle size. Reactors such as Lurgi, Reichert or Lambiotte
can deal with big particles like logs, other reactors like the
Figure 3. Waggon retort system.
Paddle Pyrolysis Kiln or Herreshoff Multiple-Hearth
3.2. Shelf reactor Furnace usually process smaller particles. There is a
comparison of all characteristics of the most common
This technology consists of a horizontal retort fed with
reactor types at the end of this chapter in Table 1.
shelves which enter through two sliding doors. In one of
With a focus on particle size, the reactors designed to work
the sections, biomass is dried and then carbonised.
with big particles cannot handle small particles because
Eventually, the shelves go to a cooling chamber. The main
they would pack tightly and would hinder circulation of
characteristic is the need for manual loading of feedstock
gaseous products. The particles would not act as heat
and discharge of the product and low costs in comparison
carriers, decreasing the heat transfer within the reactor. In
with more developed technologies.3-4
contrast, the reactors working with small particles will
3.3. Carbon twin retort produce char in a powdery form which is not so easy to
handle, and it will need some post-treatment to be sold in
The system contains two retorts with the same capacity
the form of char briquettes or pellets.4
(usually 4.5 m3). One of the vessels is loaded with the
feedstock and heated using an external source of energy
34
4.1. Lambiotte 4.2.Reichert

There used to be two types of pyrolysis processes called This retort consists of a conical top and brick-lined bottom
Lambiotte, SIFIC or CISR, whose main difference was the fed by a conveyor from the top (see Figure 6). After
treatment of pyrolysis vapours. The SIFIC variant took the feeding, a valve is closed, and another valve is opened to
vapours through some treatment to obtain components to let hot gases into the reactor, and the carbonisation starts
be burnt afterwards. A portion of the gases are re-injected at the top and moves slowly to the bottom before the char
into the retort, and the other part is cooled and used at the is moved to the cooling bunker. The vapours pass through
bottom of the reactor to cool the char. There is a scheme a cooler and scrubber, and the condensable and non-
for this reactor in Figure 5. The limitation of the system is condensable fraction are separated. The non-condensable
the moisture content of the feed which should be below 25 fraction is separated, heated to the pyrolysis temperature
wt. %. (450−550˚C) and recirculated to the reactor.2
Figure 6. Reichert system.

Figure 5. Lambiotte reactor.
The capacity of the system is 7 104 tons of beech wood per
The process consists of a vertical retort divided into year, and the annual production is 24000 tons of char, so
different hearths, fed from the top into a lock-hopper the yield is around 34 wt. %. It is a system used by Degussa
which prevents gases from escaping and air from entering. (also known as Chemviron Carbon) and ProFagus, located
The biomass moves slowly down the reactor with a in Bodenfelde (Germany).1-2
counter-current flow of inert hot gas which dries the wood 4.3.Lurgi
and increases the temperature. The cycle takes 11 hours
since the feedstock enters until it is removed at the bottom This reactor operates using a similar concept to Lambiotte.
as char2, 5. The feedstock is fed from the upper part, and it It is divided into an upper pyrolysis zone and a lower
goes to a chamber where it is dried with the rising gases cooling zone. The particular characteristic of the system is
coming from the carbonisation zone, which is the hearth the recirculating gas stream (See Figure 7). The gas
below. After the carbonisation hearth, the char goes to recirculation comes from the combustion of the
other chambers where colder gas is used to cool down the pyroligneous vapours with air in a combustion chamber.
product before it is stored. The fraction reinjected in the pyrolysis zone (around one-
The Lambiotte process has several advantages: the quality third of the gases of the combustion chamber) passes
and yield of char are high, the vapours produced can be through a conditioning step to reach the desired
used for cogeneration, and there is a high level of temperature (550–700 ˚C). Before being reinjected in the
automation. On the other hand, it requires the biomass to cooling zone, the gas is pre-cooled in a scrubber by
be pre-dried, so if the feedstock has a high moisture exchange with water and is injected in the bottom of the
content, some auxiliary fuel for heating would be needed. reactor.2
Also, some corrosion can take place due to the acetic acid There is a large plant, part of the Silicon Metal Complex
produced. The largest plant using this technology was (SIMCOA) in Bunbury, Western Australia, which is
located in Prémery (France)5 and produced 25000 tons of producing 27000 tons of char per year in two Lurgi retorts.2
char per year, but was closed in 2001 due to economic
problems. Some units are still operating in Europe and
Russia.3-4
35
Figure 8. Herreshoff multiple-hearth furnace.

Figure 7. Lurgi reactor.
4.6.Moving agitated bed
4.4.Badger-Stafford process The process consists of a horizontal surface that is heated
by molten salts such as potassium nitrate, sodium nitrate
The Badger-Stafford process is an old technology which has
or sodium nitrite. The biomass is transported to the surface
a char yield of 20 wt. %, 4.5 wt. % of acetic acid and 10 wt.
with a conveyor (as shown in Figure 9). Pyrovac used this
% tar. Due to the low char yield, it was shut down and,
technology as part of the vacuum pyrolysis unit, and some
among other disadvantages, it had to be dismantled every
yields for bio-oil were over 50 wt. %. Presently, NewEarth
two weeks to burn the tar inside the reactor5, 7-8. The
is commercialising a moving bed system. The advantage of
reactor consists of a wall composed of firebrick,
this process is the size of the equipment, which is
diatomaceous earth and insulating brick. A number of
comparable to the height of the fixed bed in a laboratory
barrel valves regulate the flow of wood particles entering
scale (few cms.). However, in a pilot plant with a capacity
and char removal at the top and the bottom of the reactor.
of 3 ton h–1 constructed in Saguanay (Quebec, Canada),
The reactor uses the heat of the exothermic carbonisation
some problems with the condensation towers were
to heat the descending column of wood up to 515 ˚C, which
found.3-4
was the set temperature for pyrolysis —although the heat
transfer was not very effective and near the wall, the
temperature could be as low as 225 ˚C.7
4.5.Herreshoff multiple-hearth furnace

This system includes a vertical kiln made of a refractory-
lined steel shell with several plates or circular hearths (4–
10 hearths). A shaft rotates at a slow speed
(1–2rpm) moving radial toothed-arms located in each
furnace. These arms have ploughs attached to move the
biomass across each hearth. The biomass falls to the next
hearth from the centre or the side, and the system is
designed to make the biomass move through the hearths
(see Figure 8). Before going to the next hearth, the biomass
is agitated and makes contact with hot gases supplied by Figure 9. Moving agitated bed.
combustion air blowers from burners or ports. The
4.7.Paddle pyrolysis kiln
temperature for the process is within a range of
500–600 ˚C.3-4, 9-10 The Paddle Pyrolysis Kiln consists of a horizontal vessel with an
The system was patented by R.D. Pike in 1921 and was internal mechanism that moves and mixes the biomass (see
proposed for large capacity units with one ton of char per Figure 10). Overall, the mechanism used to move the biomass
hour and four tons of biomass capacity per hour. It was is a group of paddles which increases the heat transfer and
widely used in the 1980s with the construction of 16 mixes the biomass. It is used by BEST Energies and is part of the
furnaces in the United States. Although it can efficiently design of Choren.3-4
and flexibly use fine-grained materials, the char made
needs to be briquetted to be commercialised, and the
capital cost is high. NESA commercialised a large pyrolysis
unit for waste treatment, and BIG Char from Black Is Green
Pty Ltd commercialised a mobile reactor that produces
char briquettes.3-4
36
4.9.Rotary kiln
The rotary kiln is a rotating cylindrical shell heated
externally. The angle of the drum and the rotation speed
control the residence time of the particle within the
system, which takes several minutes to go through the
reactor (see Figure 12). The residence time is short if it is
compared to the traditional batch carbonisation (which
takes a day to pyrolyse the biomass)3, 6, 12. To improve the
heat transfer, there is a sequence of radial steel fins
supported on an insulated mantle4. Rotary kilns can be
described as heat exchangers where energy from the gas
phase is transferred to the bed material and, along with
this transfer, several thermal processes such as drying,
Figure 10. Paddle pyrolysis kiln. heating and chemical reactions occur at a broad range of
temperatures12. The rotation speed, and the radius of the
4.8.Screw reactor drum will determine the mixing inside the reactor, which
This type of pyrolysis reactor is also called an auger reactor should be considered for the possible design of a process
and consists of a screw inside a horizontal tubular reactor where a rotary kiln is involved.13
(see Figure 11). As explained in Chapter 2, it can be a single
screw or a twin-screw design. In general, the rotation of
the screw moves the biomass along the axis while it is
heated and pyrolysed6. Due to this characteristic, some
screw reactors are designed to produce bio-oil and char,
and others are used to produce char and heat.3-4
Figure 12. Rotary kiln reactor.
Garcia-Nunez et al.3 highlight the good balance between

yields of oil (37–62 wt. %) and the char obtained (19–38 wt.
%). It is a technology widely used for tires, sewage sludge,
Figure 11. Screw (Auger) reactor. MSW and plastics. There is a plant in Burgau-
Unterknöringen that produces 2.2 MW and another one
Char yields are 17–30 wt. % and the bio-oil 48–62 wt. %, with a capacity of 105 tons per year at VEW Energie AG
although the amount of water is higher (30–55 wt. %) Westfalen in Hamm-Uentrop, both in Germany. Garcia-
compared to the oil obtained from fast pyrolysis. It is one Nunez et al. point out that the systems that already exist
of the most common systems used for pyrolysis, and some could be adapted to work with biomass. Although the
companies such as ABRI-Tech have commercialised several yields of char and bio-oil are typically lower than fluidised
1 one-per-day capacity units4. Renewable Oil International, bed reactors, Amaron Ltd. got close to that technology
Biogreen, Advanced Bio-refinery and Forschungszentrum through multiple high-intensity burners to optimise heat
Karlsruhe (FZK) focus their designs on producing bio-oil and transfer, an auger arrangement in the feed section and a
char, whereas International Tech Corporation, eGenesis char outlet below the end of rotating section.3-4
and Agri-tech design the screw reactors to produce char
and heat. It is very popular due to the simplicity of
construction and operation, and the capacity to handle a 5. Industrial application
wide variety of feedstock with high char yields. However, 5.1.Reactor choice
there are some issues regarding the heat transfer within
the system, which could limit the capacity to process From the section above, it can be seen that there is a wide
biomass4, and it is challenging to scale-up the experimental range of technologies available to use in waste pyrolysis
units to the industrial scale11. The description of the process. Each technology has advantages and
different heating methods and the systems where an auger disadvantages. The auger reactor has got very critical
reactor is used have been previously described in Chapter issues with the scale-up, especially with the heat transfer
2. to the middle of the particle, and it has got a risk of
plugging if it is overfeed; the rotary kiln is challenging to
seal; the agitated moving bed has some condensation
problems; Lambiotte plant was not profitable; Reichert
37
technology only works with biomass, and it has not been partial removal of the ash, it also removes the stones and
tested with waste. An optimised design is needed which some extractives from the feedstock. The extractives are
can be safe to operate and will meet the needs of industry mainly organic substances like alkaloids, essential oils and
in terms of capacities and feedstocks. fats16. There are other methods at laboratory scale to
Table 1 makes a comparison of the different characteristics remove the ash content such as a mixer, vibratory sieve
of the reactors. From this table, some criteria can be shaker and hammer mill treatment15, but they have not
established to make the best choice, depending on the been tested at industrial scale.
weight of each parameter on the final decision. Some of The reactor is the central part of the system which
the criteria could be whether it needs size reduction, transforms the feedstock into the products and
moisture limit, portability, heat transfer methods and its determines the product distribution from the conditions.
efficiency or the maximum capacity designed. The reactor needs energy to increase the temperature of
This data must be interpreted with caution because the the biomass and start the degradation of the biomass.
interests can be different depending on the stakeholder. There are several types of reactors to be used as described
We can only look at the char yield, but it should be previously.
considered the final use of each technology, which will Pyrolysis reactors are very sensitive to water content from
depend on the process. Moreover, the developing an energy point of view, because this increases the energy
countries will place more importance or weight on the demand and, consequently, the operating cost. For that
CAPEX and the OPEX than the developed countries where reason, a drying step is needed to control the moisture
the primary interest can be the production capacity or the content at the entrance of the reactor. Depending on the
yield.6 feedstock, it may also contain some metal contamination,
especially if the feedstock has its origin from municipal
5.2.Block diagram
waste. The metal screening step will remove the metals,
The reactor is the heart aspect of the pyrolysis process. which can then be recycled. With pyrolysis, the heat
However, it is essential to design the pre-treatment steps transfer is a fundamental issue, and it is clear that the heat
for the feedstock to ensure its properties at the entrance transfer will take less time if the particle is smaller.
of the reactor as well as the treatment post reactor to add Therefore, there should be a milling step followed by
the maximum value to the products. Different possibilities screening to specify the particle size which enters the
for these pre- and post-treatment steps are represented in reactor in order to ensure adequate heat transfer.
Figure 13 for the feedstocks studied within the Once the feedstock is processed, there are different
GreenCarbon project. The feedstock and targeted products treatments for the products, always focused on the final
profoundly influence the pre-treatment and cleaning use. Before storing the char, it needs post-treatment. First,
steps. The economics of each stage has also got a strong the char requires cooling and, it has to go through a
influence on its selection. For example, the washing is an briquetting process before being stored17. The vapours
excellent step to remove the stones and partially the ash obtained from the reactor contain the condensable and
content; however, it means that the feedstock will have to the non-condensable fraction and leave the reactor at a
be dried after it, and the energy demand of the system very high temperature (400-700˚C). Due to the high
including that step may be too high to be economically temperature, a cooling system is needed to condense the
feasible. bio-oil and separate it from the permanent gas. There are
Figure 2. Block diagram.
The feedstock usually needs treatment before being stored many different options for this heat exchanger such as a
and reaching the reactor. It can contain some impurities tube and shell, spray tower or a quench column. Once
that will affect the final product such as ash, glass, metals, cooled the vapour from the cooling step can still contain
stones and moisture content. The ash content will mainly some particles of solids and aerosols. The aerosols can be
remain in the solid and, consequently, the char, increasing removed from the gas with an electrostatic precipitator
the burnout temperature14 and the burners can be (ESP). The solid particles can be retained in a ceramic filter
severely damaged if the ash content is high enough. at the end of the process.
Furthermore, the resulting heating value is lower with high Biomass, char, bio-oil and gas have to be stored or
ash content within the feedstock 15. The only industrial disposed of, unless the plant is located next to an end user
method to reduce the ash content1 is washing, although it or any of the products is used as a fuel for the process itself.
should be studied more closely to examine the feasibility The treated biomass can be stored in bins protected from
of the step due to its high energy demand. Besides the moisture until it is required for its processing18. The bio-oil
38
should be stored in tanks19; the char is stored in sealed Fixed Capital Investment is Inside Battery Limits (ISBL),
vessels or containers to prevent moisture or oxygen whose direct costs cover the expenses associated with the
ingress20 and in a covered area, avoiding the wind and rain. material and installation of the plant and the indirect costs
Ash can be sprayed with a small amount of water or mixed have some possible expenses related to the construction,
with clay slurry to avoid spontaneous combustion18. Part of such as the insurance and fees of the equipment and field
the char could be used in the system for heat generation. expenses.
The storage of the gas produced is not practical at The Working Capital is the other contributor to the Capital
industrial scale; the most common practice is its disposal Cost and involves all costs after the construction until the
to the atmosphere or to flare it at an industrial scale. first income is generated. The costs included in this part are
5.3.Techno-Economic Assessment (TEA) the start-up of the plant and the materials, the salaries of
the workers, the money for possible emergencies and to
The reactor has a significant effect on the performance of
maintain the plant in operation. It also used to pay the bills
the whole plant. Therefore it is essential that the entire
and sustain operation before the product is sold and
plant is considered from a chemical engineering point of
income is generated.23-25
view. In industry, the techno-economics of the entire
system has to be considered to understand the feasibility Operating Costs. When the plant is in operation, the costs
of such a plant21. The TEA of the whole system calculates associated with it are called Operating Costs. Similar to the
and evaluates the costs associated with the complete Capital Costs, the Operating Costs are divided into
plant, allows the comparison between different scenarios Production Costs and General Costs. The production costs
and compares different configurations of the process22. are directly related to the production process, unlike the
The TEA brings together mass and energy balances and the General Costs which don’t alter with the production rate
flow of information to work out how much the but are related to the sales, research and administration.24
construction and operation of the plant can cost. The The Production Costs are divided into Variable Costs, Fixed
benefit of TEA is that it provides a relatively quick and low- Costs and Overheads. The Variable Costs are strongly
risk method to study the process without building a influenced by the plant design, the feedstock and the
physical plant. It identifies the viability of the system and location of the plant. Besides the raw material and labour,
the process design can be improved when the critical areas packaging and shipping are also included here. These
for optimisation are determined.23 values are not so closely related to production rate,
because they can be heavily influenced by market
5.3.1. Costs and Income conditions. To calculate the labour costs, the number of
The costs are estimated to calculate the profitability of the shift staff must be estimated. For this work ,this estimate
plant. A classification of the costs is shown in Figure 14.The is based on the hypothesis of 4.58 operators per shift
three central values, capital cost, operating cost and position in a four-shift-rotation if the shifts are 12-hours-
income, in Fig. 14 are estimated based on the mass long, and it takes into account holidays and 40 working
balance. The value of the plant at the end of its working hours per week26. The Fixed Costs within Production Costs
life, called scrap value should also be taken into account. In takes into account the maintenance needed, the
general, only 5-10% of the cost is recovered in this value depreciation, insurance, interest and rates and taxes. The
because just the building and some standard items keep depreciation is crucial because it indicates the value that
their value over the plant lifetime 23. the equipment loses during a period of time26. The
Overheads keep the plant operating but are not related to
the primary activity of the plant, services such as security
or the canteen for the workers23.
The General Costs are the Administration where
accounting, human resources and IT are included. The
Distribution and Marketing involve the sales and
advertising; Research and Development invest money in
finding improved methods for the product and the
corresponding tests24.
Figure 14. Costs and income.
Income. It is the earnings for the sale of the product or
Capital Cost. This term includes all of the capital products of the plant. It is calculated as a function of the
expenditure to buy the necessary equipment, its sales price and the market size. However, the forecast is
installation and the cost to start up the plant. There are sometimes difficult to calculate. The demand for a product
two types of Capital Cost: the Fixed Capital Investment and is not constant along the time. Moreover, there can be a
Working Capital. The Fixed Capital Investment is the cost time-lag between the plant construction, the operation
of the design, the construction and the installation of the and the income. The technology also changes, and better
plant, including the engineering costs and contingency equipment can come up to the market. Furthermore, there
are factors like changes in the policies which are difficult to
charges. The whole infrastructure of the plant with services
control 23. Depending on what is included in the income, it
such as security, offices and canteen are included in a term
can be called net income, from which all costs and taxes
called Outside Battery Limits (OSBL). The other word of the
39
are already deducted, or Gross Income, which is the money interest rate or investing it in the project. This method
earned from the sales.24 takes into account the time value of money. 23, 27-28
5.3.2. Economic evaluation (𝑁𝑁𝑁𝑁𝑁𝑁 𝐶𝐶𝐶𝐶𝐶𝐶ℎ 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹)𝑛𝑛

𝑁𝑁𝑁𝑁𝑁𝑁 = ∑𝑛𝑛0 (3)
The profitability is a relationship between the investment (1+𝑟𝑟)𝑛𝑛
needed for a project and the income obtained from it. The
profitability is related to the flow of money although it can Discounted Cash Flow Rate of Return (see Eq. 4). This method
also be measured with an interest ratio, a rate or time, establishes the Net Present Value as 0, and the interest
which can sometimes be a better way to compare different rate calculated is the actual rate of return of the project,
projects 23. Regarding the investment, it is important to and it can also be called Internal Rate of Return (IRR).
highlight that, the higher the risk taken, the higher the Based on these values, the profitability of the projects can
reward level that may be obtained but there is the be compared between them. Also, if the value i is
potential that it could all be lost. There are many methods compared with the coefficient r from NPV, the profitability
to compare profitability, among them are the minimum of the project is being compared to the one in the bank,
acceptable rate of return, payback time, return on which would be a low-risk option for the money from the
investment, net present value and discounted cash flow project and would be a general idea about the risk taken.23,
27-28
rate of return.23
The minimum acceptable rate of return (see Eq. 1). The value (𝐶𝐶𝐶𝐶𝐶𝐶ℎ 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹)𝑛𝑛
∑𝑛𝑛0 =0 (4)
obtained is the division of the expected net profit by the (1+𝑖𝑖)𝑛𝑛
total capital investment and, depending on the value
obtained, the level of risk is determined. A value of 4–8 is 5.4.Role of the industrial partner BPP
classified as a safe investment, but values of 32-48 are very Biomass Power Projects (BPP) is a firm created 20 years
risky, and the investment may not be recovered.23 ago, which engaged in several European and UK projects in
𝑛𝑛𝑛𝑛𝑛𝑛 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 emerging low carbon technologies. The company designs
𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 𝑜𝑜𝑜𝑜 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 = (1) manufacture and install waste-to-energy systems for a
𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 𝑐𝑐𝑐𝑐𝑐𝑐𝑖𝑖𝑡𝑡𝑡𝑡𝑡𝑡 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖
range of different waste streams. GreenCarbon’s Early-
Payback time. The method calculates the time necessary Stage Researcher #3 (J. López-Ordovás) is closely working
to recover the initial investment, indicating how long the with BPP to get the in-depth industrial insight for the
investment is at risk. It is a useful measure to know when project. Due to the lack of industrially based information in
the investment is recovered. It is the simplest method to the literature, the interaction and collaboration between
compare profitability. Also, it is straightforward to the researcher and the company are significant to get the
understand and gives a quick method to estimate the best design in each situation.
profitability. On the other hand, it does not include the Additionally, the company can use the results and learn
factors that impact the profitability such as the interest or from the project to aid future construction of an industrial
the plant lifetime. Overall, it is not recommended for high scale reactor with an average throughput of 3 ton h–1. This
investments.23 would be very beneficial to validate the results obtained
during the PhD project and generate a valuable impact on
Return on Investment (ROI) (see Eq. 2). It is a very common
the GreenCarbon network.
criterion used to compare different options. It calculates
the percentage of return on that money invested 5.5.Challenges in the design of a slow pyrolysis reactor
previously on the project. It is the relationship of the
The information available for the design of a slow pyrolysis
cumulative net cash flow, the plant life and its initial
reactor at industrial scale is very limited compared to
investment. It is essential to clarify what parameters are
laboratory tests. The model can be divided into three
used for its calculation. The advantage is that it gives a
different parts: the behaviour of the bed of solids, the heat
good idea about the profitability of the project and makes
transfer, and the kinetics of the process. Besides this, a
it easier to compare projects between them. It is also a very
challenge for the research is the validation of the model,
common criterion used in the projects. The disadvantage is
which represents an important step to demonstrate that
that it does not include the time value of money, so it
the assumptions did work correctly and the results are as
assumes money does not change its value over the time of
close as possible to the real behaviour of the system.29
the project, it does not consider the cash flow on each time
and capital investment interest23. It includes the original 5.5.1. Behaviour of the bed of solids
investment but does not include interest.
The real behaviour of the bed of solids involves many
𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑛𝑛𝑛𝑛𝑛𝑛 𝑐𝑐𝑐𝑐𝑐𝑐ℎ 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 different aspects in the rotary kiln. There are six different
𝑅𝑅𝑅𝑅𝑅𝑅 (%) = 100 (2)
𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑥𝑥 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 motions in the transverse plane, which depend on the
radius of the reactor and the rotational speed; they are
Net Present Value (NPV) (see Eq. 3). This method calculates ordered from high to low rotational rates12:
the cash flow for each year applying a constant interest
Centrifuging: it occurs at very high speed. The material
rate for the following years. It gives a measure of the
rotates with the drum wall; it would involve very high
difference of investing the money in the bank with the
energy consumption and a poor mixture of the solids.
40
Cataracting: it has lower velocities than centrifuging, but parameters for the equations for the heat transfer of the
they are still high. The solids rise over its level and drop, freeboard and the bed of solids are different.12
provoking a perturbation of the system causing instability.
It also mixes the biomass axially, which would make the 5.5.3. Kinetics
system more difficult to control. Kinetics are a challenging part to evaluate in pyrolysis. It is
Cascading: this motion is very similar to cataracting with known that there are many different reactions within
lower rotational speed and smaller drops on the surface of pyrolysis with their own parameters and orders. Trying to
solid. achieve a comprehensive set of reactions, Peters et al.29
Rolling: this rolling motion causes an excellent axial mixing defined 149 individual reactions previously set up by Ranzi
due to the rolling of the bed of solid on its surface and the et al.34 in a pyrolysis reactor without being too simple.
wall on the bed. It is a steady motion with reasonable However, there is a lack of the secondary cracking
uniformity in the properties within each axial profile. reactions in this model, and the whole set of reactions
Slumping: cyclical variation of the angle of repose and would be composed of thousands of still unknown
material at the shear edge becomes unstable. reactions with different parameters. The critical
Slipping: this motion occurs at the lowest rotational rates. information for the design of the reactor is the product
Due to the low speed, the material slides on the wall, and distribution based on the feedstock and the conditions, the
there is not proper movement within the bed of solids properties should be studied once the reaction has been
which could remain as a solid piece of material. carried out.29
Besides the behaviour within the bed of solids, the
macroscopic behaviour is also essential in the design of the
6. Conclusions
reactor. The height of the bed of solids is likely to be
increasing or decreasing along the reactor. This variation of Pyrolysis has been done for centuries for char making
height is another issue to take into account the design of purposes; however, none of the technologies used until
the reactor because it will influence some critical now is apparently the best choice. Each of them has
parameters, such as the heat transfer contact area. different advantages and disadvantages. Despite being the
According to Boateng12, the behaviour of solids has been char the main product, the significant amounts of bio-oil
studied through granular flow model in rock falls, snow and gas produced have to be taken to account in the
avalanches or mudflows but the application in rotary kiln process and the post-treatment.
has not been defined. However, there is a model made by With this wide range of technologies available, it is
Saeman30 which describes the movement of the bed of essential to define the criteria for the reactor choice and
solids inside the kiln and is able to estimate the height of establish a classification based on some characteristics
the bed.31 such as feeding capacity, yields or heating methods, among
others. Weighting should be associated with each feature
5.5.2. Heat transfer for a better evaluation of the alternatives. One example of
Heat transfer is a critical step in pyrolysis because all the this type of classification was done by Duku et al.6 In our
particles have to reach the process temperature to react. case, however, the criteria will be different because it is
In addition, the only source of heat for the reactor is more focused on the efficiency of the process rather than
external, although the solid might also be heated by the the investment needed for the technology.
gas inside. The particle size can vary the product yields by In addition to the reactor selection, the process
10 wt. % for each product when the size varies from parameters such as temperature, residence time of solids
0.3 to 1.5 mm, although this effect is negligible for higher and liquids, and heating rate will influence the quality of all
sizes32.This study shows how important the heat transfer is products, and the influence will vary from each technology.
because the yields differ significantly with the particle size. Moreover, the reactor is a vital part of the plant, but the
All the heat transfer mechanisms are present in the reactor pre-treatment and the post-treatment processes are
(conduction, convection and radiation). Conduction is required to get the maximum value from the feedstock.
intuitive; there are interactions between energy carriers Once the reactor is defined, the role of the Early Stage
within a material. The energy is transferred from the more Researcher #3 will be its design, which is still very
energetic carrier to the less energetic ones, transferring challenging due to the lack of technical information for it.
the energy from hot to cold. This conduction can occur The methodology is divided into four interconnected parts
within the same material or different ones. Convection is a after selecting the reactor: the behaviour of the solids in
particular type of conduction which is treated separately; the reactor, the heat transfer, the kinetics of pyrolysis and
the heat transfer occurs in a moving medium. This change the mechanical specifications. None of these parts is fully
makes the heat transfer equation a bit different because, defined in the literature for the design of a pyrolysis
besides the conduction, there is an enthalpy carried by the reactor, and this project gathers all the information
macro-scale flow. This enthalpy includes the effect of the available in the literature to design an industrial pyrolysis
pressure and the volume related to the medium moving.33. reactor.
The radiation is produced by the agitation of the atoms,
which release energy in the form of electromagnetic waves
and is absorbed by other atoms in a less agitated state. The
41
Table 1. Comparison of the reactor technologies available for slow pyrolysis.
Herreshoff
Lambiotte Moving Agitated Paddle Pyrolysis
Criteria Reichert Process Lurgi Waggon Retort Twin Retort Shelf Reactor Multiple-Hearth Rotary Drums Auger Reactor
Process Bed Kiln
Furnace
1. Technical Aspects
1.1. Reactor Size
1.1.1. Dimensions Large Large Large Small-Large Small Small Large Small Small Medium/large Small
Horizontal/ Horizontal/ Horizontal/
1.1.2. Position Vertical Vertical Vertical Horizontal Vertical Horizontal Horizontal Horizontal
Vertical Vertical vertical
1.1.3. Reactor Volume [m3] 100 600 12 5-16.5 na
Height: 8.5 Height: 27 Height: 16.3-18.4 Length:8-9
1.1.4. Size [m] 0.3x0.3x0.1 Height: few cm
Diameter: 5 Diameter: 3 Diameter: 3-4.3 Diameter: 2.5
1.1.5. Capacity [t/h] 8.7 3.4-6.2 0.4-2.5 2.1 1.1 1-12.5 1.2-12 <2.1 3.0-3.5
1.1.6. Production [t/h] 3 1.6-1.7 0.25-3 0.75 0.3-0.9 1.9-2.75 0.36-3.6 <0.36
1.2. Process conditions
Softwood/
1.2.1. Raw material Beech wood Oak wood
Hardwood
Chips/ Fine Chips/ Fine Chips/Fine Chips/ Fine
1.2.2. Material Shape Cordwood Coorwood Coordwood Cordwood Cordwood/chips Chips Chips
particles particles Particles particles
Length 30-35 cm. 1-1.2 m length
35 cm length
1.2.3. Particle Size Thickness 10-15 0.08-0.12 m
10 cm thickness
cm. Diameter
1.2.4. Temperature [˚C] 450-550 550-700 547-560 580 500
1.2.5. Pressure Atmospheric Atmospheric Atmospheric Atmospheric Atmospheric Atmospheric Atmospheric Atmospheric Atmospheric Atmospheric Atmospheric
None Drying
Pre-dried Ground in
1.2.6. Pretreatment needed Max. Moisture 30% maximum 25% moisture None Pre-dried Ground into chips None Pre-dried None
30% maximum cips/fine particles
25% maximum
1.2.7. Feeding method Batches Batches Batches 5-7 m3 batches 5-16.5 m3 batches Shelves Continuous Continuous Continuous Continuous Continuous
Direct Direct Direct Direct Direct Direct Direct Direct Direct

Direct Direct
1.2.8. Process Control measurement of measurement of measurement of measurement of Mmeasurement measurement of measurement of measurement of measurement of
measurement measurement
temperature temperature temperature temperature of temperature temperature temperature temperature temperature
1.2.9. Cycle Time [h] 16-20 11 25-35 22-36
Biochar and bio- Biochar and bio- Biochar and bio- Biochar, bio-oil Biochar and bio- Biochar/bio-
1.2.10. Targeted product Biochar Biochar Biochar/heat Biochar/ bio-oil Biochar
oil oil oil and gases oil oil/gases
1.2.11. Average Gas yield [wt.%] 20-30 18-35
1.2.12. Average Bio-oil yield [wt.%] 30-50 37-62 48-62 >50
1.2.13. Average Char yield [wt.%] 33-38 30-40% 30-35 30-38 22-33 25-30 19-38 17-30
Herreshoff
Lambiotte Moving Agitated Paddle Pyrolysis
Criteria Reichert Process Lurgi Waggon Retort Twin Retort Shelf Reactor Multiple-Hearth Rotary Drums Auger Reactor
Process Bed Kiln
Furnace
1.3. Heat transfer
Contact with hot
Contact with hot
gases Direct contact
gases
Combustion of with hot gases/ Indirect heating Indirect heating
Direct contact Contact with heat Contact with heat Indirect Heat Combustion of
1.3.1. Heating method Indirect heat Indirect heat auxiliary fuels Use of a hot heat (sometime (sometime
with hot gases gases gases through the walls pyrolysis gases
and direct or carrier/ Indirect molten salts) molten salts)
and/or auxiliary
indirect contact of heating
gases
combustion gases
Heating with an Volatile external Heating with an

Hot gases Hot gases External heat and External heating
external volatiles Direct combustion external
1.3.2. Charge ignition method N/A generatedin an generatedin an volatiles and volatile External oven External heater
combustion measurement chamber to combustion
external oven external oven combustion combustion
camber produce hot gases chamber
Typically slow
Slow or
Slow or Fast pyrolysis (could Slow or fast
1.3.3. Heat transfer rate achieved Slow Pyrolysis Slow pyrolysis Slow pyrolysis Slow Pyrolysis Slow Pyrolysis potentially fast Fast/slow Slow pyrolysis
Pyrolysis be used for fast pyrolysis
pyrolysis
pyrolysis)
2. Economics and Construction
2.1. Construction
Metal/
Steel
Steel Vessel and
2.1.1. Materials Metal Metal Metal High Temperature Steel/iron/brick Metal Steel Metal Metal Metal
piping
Steel
components
Stationary or
2.1.2. Portability Stationary Stationary Stationary Stationary Stationary Stationary Stationary Stationary Stationary Stationary
portable
Manual/
2.1.3. Loading and discharge methods Mechanical Mechanical Mechanical Waggons use Manual Mechanical Mechanical Mechanical Mechanical Mechanical
mechanical
2.2. Economics
2.2.1. CAPEX [pound/annual char ton] 1.15 60-62.5 120-175 68.5
2.2.2. OPEX [pound/ char ton] 280 315 332
3.Others
Siemens, Mitsui
Renewable Oil
Babcock, Ethos
International,
Lambiotte & Cie energy, Graveson
Biogreeen,
(France and Energy
Advanced Bio-
Bunbury Belgium) Impianti Carbo Group Management,
BIG Char (Black is refinery, Pyrovac
(Australia) Units in Europe Trattamento (Estonia, Ghana Metso,
Green Pty LtD) Forschungszentru International
3.1. Companies with the technology Biochar and Balt Carbon Biomasse (Italy) and China) Umwelttechnik Choren, BEST
(Australia) m Karlsruhe (Canada)
Engineering Ltd. (supplier for Alterna Biocarbon JCKB GmbH & Co, ML
NESA (FZK), NewEarth
(Colorado) Russia) (Canada) International Entosorgungs and
International
Biochar Solutions Energianlagen
Tech Corporation,
Inc. (USA) GmbH, Noell-KRC,
eGenesis, Agri-
Amaron Energy,
tech
Pyreg
42
Acknowledgements (16) Dhyani, V.; Bhaskar, T., A comprehensive review on the

pyrolysis of lignocellulosic biomass. Renew. Energy
This project has received funding from the European
2018, 129, 695–716.
Union’s Horizon 2020 research and innovation programme (17) OY, C., CARBOFEX OY- Sustainable Industrial Production
under the Marie Skłodowska-Curie grant agreement No of Biochar. In WASTE TO VALUABLES, OY, C., Ed.
721991. CARBOFEX OY: Hiedranranta, Tampere, Finland, 2017.
(18) Lynch, J.; Stephen, J. Guidelines for the Development
and Testing of Pyrolysis Plants to Produce Biochar;
References International Biochar Initiative, 2010; p 32.
(1) Antal, M. J.; Grønli, M., The Art, Science, and (19) Rogers, J. G. A techno-economic assessment of the use
Technology of Charcoal Production. Ind. Eng. Chem. of fast pyrolysis bio-oil from UK energy crops in the
Res. 2003, 42 (8), 1619–1640. production of electricity and combined heat and power.
(2) Grønli, M.; Antal, M. J.; Schenkel, Y.; Crehay, R. The Aston University, 2009.
Science and Technology of Charcoal Production; (20) Major, J. Guidelines on Practical Aspects of Biochar
9781872691923; PyNe Subject Group: 2005. Application to Field Soil in Various Soil Management
(3) Garcia-Nunez, J. A.; Pelaez-Samaniego, M. R.; Garcia- Systems; International Biochar Initiative: 9 November
Perez, M. E.; Fonts, I.; Abrego, J.; Westerhof, R. J. M.; 2010, 2010; p 23.
Garcia-Perez, M., Historical Developments of Pyrolysis (21) Chong, K. J. A methodology for the generation and
Reactors: A Review. Energy Fuels 2017, 31 (6), 5751– evaluation of biorefinery process chains, in order to
5775. identify the most promising biorefineries for the EU.
(4) Garcia-Perez, M.; Lewis, T.; Kruger, C. E. Methods for Doctoral Thesis, Aston University, Birmingham, 2011.
Producing Biochar and Advanced Biofuels in (22) Olszewski, M.; Kempegowda, R. S.; Skreiberg, Ø.; Wang,
Washington StatePart 1: Literature Review of Pyrolysis L.; Løvås, T., Techno-Economics of Biocarbon
Reactors; 11-07-017; Washington State University: Production Processes under Norwegian Conditions.
Pullman, WA, July 2011, 2011; p 137 pp. Energy Fuels 2017, 31 (12), 14338–14356.
(5) Industrial charcoal production, TCP/CRO/3101 (A) (23) Chong, K., CE3001: Process economics and loss
development of a sustainable charcoal industry; Food prevention course. Aston University, 2017.
and Agriculture Organization of the United Nations (24) Peters, M. S.; Timmerhaus, K., Plant design and
(FAO): Zagreb, Croatia, June 2008. economics for chemical engineers. 4th ed ed.; McGraw-
(6) Duku, M. H.; Gu, S.; Hagan, E. B., Biochar production Hill: New York, 1991.
potential in Ghana—A review. Renew. Sustain. Energy (25) P. Towler, G.; Sinnott, R. K., Chemical Engineering
Rev. 2011, 15 (8), 3539–3551. Design: Principles, Practice and Economics of Plant and
(7) Nelson, W. G., Waste-wood Utilization by the Badger- Process Design. 2013.
Stafford Process1 the ford Wood-Distillation Plant at (26) Garrett, D. E., Chemical engineering economics. Van
Iron Mountain. Ind. Eng. Chem. 1930, 22 (4), 312–315. Nostrand Reinhold: New York, 1989.
(8) Emrich, W., Handbook of Charcoal Making: the (27) Olivares-Olivares, P., Module 04: Finance and Economic
Traditional and Industrial Methods. 1985. Project Management. In Master in Project
(9) Industrial Charcoal Making. Food and Agriculture Management, Kühnel Business School: Zaragoza, Spain,
Organization of the United Nations (FAO): Rome Bernan 2017.
Press(PA), 1985. (28) Perez-Perez, M., Quality Management and Production
(10) Perry, R. H.; Green, D. W., Perry's chemical engineers' Organization. In Chemical Engineering Degree,
handbook. New York : McGraw-Hill, ©2008. 8th ed. / Zaragoza, Ed. University of Zaragoza: Zaragoza, 2013.
prepared by a staff of specialists under the editorial (29) Peters, J. F.; Banks, S. W.; Bridgwater, A. V.; Dufour, J.,
direction of editor-in-chief, Don W. Green, late editor, A kinetic reaction model for biomass pyrolysis
Robert H. Perry: 2008. processes in Aspen Plus. Appl. Energy 2017, 188, 595–
(11) Brassard, P.; Godbout, S.; Raghavan, V., Pyrolysis in 603.
auger reactors for biochar and bio-oil production: A (30) Saeman, W. C., Passage of Solid through Rotary Kiln.
review. Biosystems Eng. 2017, 161, 80–92. Chem. Eng. Progress 1951, 47 (10), 508–514.
(12) Boateng, A. A., Rotary kilns: transport phenomena and (31) Babler, M. U.; Phounglamcheik, A.; Amovic, M.;
transport processes. Amsterdam ; Boston : Ljunggren, R.; Engvall, K., Modelling and pilot plant runs
Elsevier/Butterworth-Heinemann: 2008. of slow biomass pyrolysis in a rotary kiln. Appl. Energy
(13) Fantozzi, F.; Colantoni, S.; Bartocci, P.; Desideri, U., 2017, 207, 123–133.
Rotary Kiln Slow Pyrolysis for Syngas and Char (32) Shen, J.; Wang, X.-S.; Garcia-Perez, M.; Mourant, D.;
Production From Biomass and Waste—Part I: Working Rhodes, M. J.; Li, C.-Z., Effects of particle size on the fast
Envelope of the Reactor. 2007; Vol. 129, p 901-907. pyrolysis of oil mallee woody biomass. Fuel 2009, 88
(14) Mayer, Z. A.; Apfelbacher, A.; Hornung, A., A (10), 1810–1817.
comparative study on the pyrolysis of metal- and ash- (33) Nellis, G.; Klein, S. A., Heat transfer. New York :
enriched wood and the combustion properties of the Cambridge University Press, 2012.
gained char. J. Anal. Appl. Pyrolysis 2012, 96, 196–202. (34) Ranzi, E.; Cuoci, A.; Faravelli, T.; Frassoldati, A.;
(15) Pradhan, U. Physical treatments for reducing biomass Migliavacca, G.; Pierucci, S.; Sommariva, S., Chemical
ash and effect of ash content on pyrolysis products. Kinetics of Biomass Pyrolysis. Energy Fuels 2008, 22 (6),
Auburn University, Auburn, Alabama, 2015. 4292–4300.
43
44
Matter of modelling in thermochemical conversion processes of biomass

Przemyslaw Maziarka, Frederik Ronsse
Department of Green Chemistry and Technology, Ghent University, Coupure links 653, 9000 Gent, Belgium
Abstract
Modeling is a complex task combining elements of knowledge in the field of computer science, mathematics and natural sciences (fluid
dynamics, mass and heat transfer, chemistry). In order to correctly model the process of biomass thermal degradation, in-depth
knowledge of multi-scale unit processes is necessary. The biomass processing model can be divided into three main subparts depending
on the scale of unit processes: molecular, single particle model and reactor model. Molecular models describe the chemical changes in
biomass structural compounds. The model of the single particle corresponds to the description of the biomass structure and its
influence on the thermophysical behavior and the subsequent reactions of the compounds released during biomass decomposition.
The largest subpart and at the same time the most difficult to describe is the reactor model, which describes the behavior of a vast
number of particles, the flow of the reactor gases as well as the interaction between them and the reactor. This chapter contains a
basic explanation about what the model is and how it works from a practical point of view. In addition, an extended description of each
subpart is presented.
improvements of technologies. In the last four decades,

1. Introduction due to social pressure and though research initiatives, the
knowledge gaps started to be filled and new, more efficient
Biomass was the primary source of energy and materials
solutions start to appear. Unfortunately, despite the
until the Industrial Revolution1. With the increase of
pressure on fossils replacement, the alternative materials
availability and usage of the cheaper and more abundant
produced with novel technologies are in many cases not
fossil resources, biomass, a crucial resource so far, was put
sufficiently engineered or their price is uncompetitive on
aside. Since the negative influence of greenhouse gases
the current market. For this purpose, new and more
(GHG) on the climate became to be a recognised
sophisticated methods of research as well as new
phenomenon (mainly related to CO2), strong actions
technological ideas are being developed to meet both
against its emissions started to take place. Additionally,
economic and engineering ends of the problem.
society self-awareness about the depletion of fossil
resources and the ecological capacity of the environment
regarding pollution gave strong and positive feedback on
2. Biomass conversion - modelers approach
the research and development of more sustainable
technologies. Reduction of GHG and pollution has taken 2.1. Simulation and its uncertainties
place by the partial replacement of energy from fossil fuels Substantial improvement in the last 30 years of the
by more sustainable and eco-friendly energy sources like, computer science brought ease and spread the access to a
e.g., sunlight or water movements. There is also a steady robust tool —the numerical modeling. Simulations
trend of replacing the fossil origin carbonaceous materials conducted on numerical models allow to significantly
with materials of biomass origin. In the case of the improve the pace of research and development in the
production of materials, there is no other natural resource, biomass processing field.
like in case of energy, so its production is only possible Some commonly used words need to be defined and
through biomass processing. The replacement has two clarified before the explanation of the computational
main advantages. Firstly, it lowers the environmental modeling. A model is the mathematically described (by
impact of materials during its production, usage, and algorithms and equations) representation of system
utilisation through its significantly lower or in some case existing in real life, and a simulation is an action of
even neutral GHG emissions2-3. The second main performing a test on a model. The term numerical means
advantage is the sustainability regarding the resource that the mathematical model is translated into a numerical
renewal and variety of feedstock type.4-5 language (through informatics) to conduct computation on
One of the most important processes of the primary a numerical tool, more straightforward, and on the
biomass conversion into carbonaceous materials is computer7. Models can be various, depending on the field
pyrolysis. It can be defined as the thermal conversion of where they are used, but in natural sciences and
biomass in an atmosphere with reduced oxygen content to engineering, the most commonly used ones are the
prevent its burnout. The “idea” of the process is not a new numerical models.
concept and has been known since ancient times6. As one It needs to be kept in mind that models are only a
can presume, older technologies are based on very basic representation of the real system, and in most cases, they
solutions, like kilns or burning pits, which are simple in assume simplifications and approximations. Moreover, the
operation, but its efficiency and process control are model background is often based on experimental data,
relatively poor. Not without significance is the fact that in which could be burdened with an error. Therefore,
the past the knowledge about the conversion process itself simulation results in most cases show discrepancies from
was not broad and did not allow for significant “true” results, caused by unknown deviations of the model

45
Chapter 4 Maziarka and Ronsse
elements, which are known as uncertainties. To be able to 2.2. Simulation and profit
diminish the deviation of the result from reality, Simulations on the properly constructed model result in
uncertainties need to be found, quantify and clarified. The valuable information about the system behavior, which
sources of uncertainty can be divided into8: often cannot be obtained through measurement. Such
• Parameter uncertainty. The parameters used in the knowledge can give a significant boost for innovative
model, which cannot be experimentally measured (too solutions in development and help to identify the critical
hard or too expensive) and had to be assumed in the points of the system (bottlenecks). In general, using the
model. modeling in studies of technology development brings four
• Model inadequacy. Lack of full knowledge about the main advantages:7
theory behind the modeled system or influence of the • To allow to conduct Proof of Concept analysis at the very
simplifying assumptions. beginning of the project (low sunk cost in case of failure).
• Residual variability. Simulation output differs from the • To allow to perform numerous tests with a low unit cost.
experimentally obtained output through random • To increase the knowledge about dependencies in a real
fluctuations of parameters in a real situation (low system.
repeatability of the real system). • To accumulate the obtained datasets and simplify their
• Parametric variability. The modeled system is not treatment and sharing (Big Data processing).
sufficiently described/measured, and input values have
to be assumed. All of the mentioned advantages can have crucial meaning
• Observation error (experimental uncertainty). The for the economic feasibility of new technological solutions
deviation in values due to the variability of experimental development. As it is shown in Figure 2, the application of
measurements. the simulation can reduce the overall cost of new solution
• Interpolation uncertainty. Assumption of the tendency of implementation and decrease the risk of the project's
the parameter in the range of experimental results unprofitability, which in the development of new
between measured data points. technologies is a strong benefit.
• Code uncertainty (numerical uncertainty). The strongest
related with numerical procedures, caused by the lack of
possibility of exactly solving the problem (technical
boundaries) and use of the approximation while solving
(e.g., a partial approximation of the partial differential
equation by a finite element solution method).
A clear indication of the individual share of each
uncertainty on the total is not simple, but a proper
clarification of each improves the model's accuracy. In the
modeling, researchers are advised to keep a critical and Figure 2. Changes of the new idea implementation costs,
very careful approach. through the project time.7
A simplified scheme of simulation study with the linkage
between the experiments, theory, and model is shown on Models are more flexible than real processes, so changes
Figure 1. As it can be seen, the simulation has to be in modeled systems and its influence can be quickly
validated to have certainty about its usefulness. Models verified. It allows for solving technical problems in the early
based on experimental data are reliable only in a specified stage, which is the most costly option. Modeling can also
range of experimental value and, only for this range, expand the knowledge about the investigated process. If
results are valid. Therefore, it is always better to set the the model is detailed and mimics the real system
foundation of the model on the fully established theories, adequately, there is a possibility to investigate and validate
which in general have a broader range. new correlation and theories through large and detailed
databases of the process history.
3. Idea behind the CFD-DEM method

3.1. Computational Fluid Dynamics
As defined, Computational Fluid Dynamics (CFD) is the
analysis of systems involving fluid flow, heat transfer and
associated phenomena (e.g., chemical reactions) by using
computer-based simulation10. In general, it can be stated
that CFD is the integration of the fields of natural sciences
(physics and chemistry), mathematics and computer
Figure 1. Simplified scheme of simulation study.9 science11, as it is shown on Figure 3.
46
equations). The accuracy and precision of the result from a

CFD model are based on a number of cells contained in the
grid (mesh coarsens). The increase in the number of cells
will result in an improvement of model accuracy, but only
until the point when the addition of new cells does
influence the result, so until the point when the result
becomes grid independent. The most coarse grid, in which
further mesh densification does not lead to improvement
of the solution, is called independent grid10. Conducting
simulations on an independent grid has a major advantage:
smallest numerical error with simultaneous less possible
Figure 3. Scientific fields included in the CFD.11 coarse mesh (lowest computational burden).
A detailed explanation of the procedure for a CFD solution
According to the fluid dynamics principles covered in CFD, is robust and very complex, and goes beyond the purpose
the flow of matter (fluid) is treated as a continuum of this chapter; therefore, it will be omitted. Nevertheless,
(Eulerian approach). In the Eulerian approach of fluid a brief introduction to the matter is provided. A simplified
dynamics descriptions, the points in the entire geometry scheme of CFD data treatment and computing procedure
do not change their position with the fluid motion12-13. is shown in Figure 5. The framework procedure for the CFD
Therefore, the Eulerian approach allows only for the code consists of three main elements solution:11
description of parameters changes at a certain point of the • Pre-processor. It is a part of the code responsible for
investigated geometry as the change of the parameter implementation of investigated geometry, and its
field. Such an approach also indicates no distinction for consecutive meshing. Mesh obtained in the pre-
specific molecules/particles, so their time-based processor is the foundation for the future
investigation is not possible. The model behavior is based implementation of governing equations and shapes the
on governing equations —the mathematically described process of simulation.
physical behavior in the form of differential equations (e.g., • Solver. It is responsible for the performance simulation.
Navier-Stokes equations). To solve mathematical It contains and processes all information regarding the
equations, computer scientists developed high-level applied physics and chemistry located in the nodes of the
computer programming languages, which convert them grid. Through implemented solution methods, it
into computer programs or software packages possible to simulates the changes of the parameters in nodes
understand by a computing machine. CFD, for its according to the applied governing equations and
computation, uses the dimensional geometrical domains, boundaries parameters.
which are the backbone for the mathematical construction • Post-processor. It is responsible for the visualization of
of the model. Prior to performing a simulation, the the results (data sets) obtained in simulations. Most
specified geometry (domain) needs to be subdivided into a post-processors allow for quick creation of 1D plots and
finite number of smaller, non-overlapping subdomains presentation of parameters of interest on the applied
called cells. The process of domain division into the geometry (2D and 3D plots). As well as for the solver,
subdomains is called meshing and results in a grid of cells foundations of the visualisation are the mesh. Therefore,
overlying the whole domain geometry, as shown in Figure the results can be as detailed as an applied mesh.
4.
Figure 4. Visualization of meshing based on finite volume

method (from COMSOL Multiphysics 5.3).
Figure 5. CFD code solution scheme.11
The cell can be defined as representative of an element or
a volume, depending on the division method (finite 3.2. Lagrangian method
element or finite volume). In most commercially available
The method that allows for the distinction of individual
CFD software, geometry division method is already
particle behavior in the system is called the Discrete
implemented in the code. Each cell consists of a node,
Element Method (DEM). The single particle movement in
which hold all the information about a certain point of the
the DEM method is based on the Lagrangian approach,
geometry. Information stored in the node changes
built on Newton’s second law of motion14-15. In the
according to the applied physics and chemistry (governing
Lagrangian approach, each particle is modeled with its own
47
body (subdomain), which moves in applied geometry subdomain along applied geometry causes change in the
according to forces affecting the particle. It allows for fluid grid. Therefore, the movement of solids has an
investigation of the particle's trajectory. The visualization influence on fluid behaviour. Combination of the Eulerian
of the difference between Eulerian and Lagrangian and Lagrangian approaches is called CFD-DEM method14 or
approaches is shown in Figure 6. The particles movement XDEM (eXtended DEM)16. With the increase in the amount
independence leads to their different thermal and of investigated particles, as well as the complexity of the
chemical behavior, due to different position and different model of single particle behaviour, the quantity of data
residence time in the processing environment. Moreover, that needs to be handled by the solver grows exorbitantly.
the Lagrangian description allows for the implementation Therefore, proper implementation of the Eulerian-
of particles mechanical interactions. It opens the possibility Lagrangian method into the investigated system, besides
for the investigation of the particle-particle and particle- the in-depth knowledge on its description and variety of
wall interactions. input data, needs a robust numerical software and
powerful hardware.
3.4. Structure of a comprehensive model

First, it needs to be mentioned that all the schemes
described in this section are treated as first-order
Arrhenius reactions, with the pre-exponential parameters
set as constant or temperature-dependent. Furthermore,
to narrow down the description possibilities and limit the
Figure 6. Simplified visualization of the difference data regarding biomass properties, the wood will be
between Eulerian and Lagrangian approach. treated as the exemplary lignocellulosic biomass type.
The building of a comprehensive/multi-scale model for
3.3. Two phase flow description biomass conversion can be divided into parts according to
For single-phase flow description, the Eulerian description the scale in which the crucial processes take place. It is
can cover all significant unit processes. Unfortunately, such illustrated in Figure 7. Besides combined implementation,
a method can be insufficient in the case where the flow is each model part can be studied separately, experimentally
two-phased (gas-solid systems). In peculiar cases, when or through simulation, leading to expanding the knowledge
of certain biomass conversion process phenomena.
the particles size is significantly small, there is a possibility
of simplification. Such approach leads to the assumption
that particles suspended in the fluid are “dissolved” in it.
Therefore they can be treated as a part of the fluid and
behave as such (quasi-continuous models). The
assumption leads to a change from the two-phase model
to the one-phase model. Models that describe the two-
phase model as one-phase are called Eulerian-Eulerian
models. It indicates that both gases and solids are
described by the Eulerian approach, and during the
simulation the distinction of the particles in the flow is not Figure 7. The idea of comprehensive biomass conversion
necessary nor possible. This simplification is very model construction, adapted from Anca-Couce.17
convenient in terms of mathematical description because
The smallest considered scale in the comprehensive model
it does not require the implementation of different terms
is named molecular model. It describes the chemical
for suspended particles movement, and simultaneously
reactions of organic compounds and catalytic effects of
can cope with their physical and chemical behaviour. As it
inorganic compounds, which take place during the biomass
can be expected, applying the simplification to relatively
conversion process. Chemical reactions are not dependent
large size particles introduces larger deviation between
on spatial dimensions. Therefore, implementation of the
reality and model. It results in significant model
geometry can be omitted. The amount of data that is in use
inadequacy and low result accuracy.
for this scale allows for simulations without robust
A comprehensive and complete description of the
numerical solvers.
behaviour of the two-phase gas-solid system is provided by
A model covering sizes larger than molecular is named
the Eulerian-Lagrangian approach. The first part of the
single-particle model. It describes the behavior of one
approaches name is related to the gaseous phase
particle of biomass for which temperature and pressure
description (fluid with continuum properties). In a model,
gradients during the process play a crucial role and cannot
its properties are stored in grid nodes of applied geometry.
be diminished. In respect to the real system, the single-
The fluid movement does not interfere with the grid
particle model needs to contain a description of the heat
arrangement. The second part is related to the solid
and mass transport and fluid dynamics within the particle.
particles. They are not constringed with the fluid grid, and
The model covers changes in particle size, shape and
particles subdomains can move freely through the applied
structure (porosity) as well as bio-polymers chemical
geometry. Nevertheless, the movement of the particles
reactions and the water evaporation process. The behavior
48
of gaseous product mixture has a strong influence on the kinetic parameters between biomass pyrolysis
final products yield and composition17. Thus, the fluid investigations. Fluctuations in the kinetic values available
movement within the particle as well as its chemistry has in the literature are also caused by application of feedstock
to be described in a very detailed manner. In the model, with different bio-composition, size, and morphology as
gases and liquids are treated as fluids, and biomass as well as the application of different research
stagnant solids, which is indicated in the model description methodologies.
as separate phase system. Therefore, the Eulerian
description is sufficient to cope with such physical
behaviour. As mentioned before, the model is strongly
dependent on the geometry, so the use of a numerical
solver is necessary to perform simulations.
The last subpart of biomass conversion model is the
reactor model. It covers the description of every relevant
unit process in rector for biomass thermo-chemical
conversion. The behaviour of each biomass particle in most Figure 8. Shafizadeh and Chin’s competitive biomass
cases should be described separately, according to the pyrolysis scheme including high-temperature tar
single particle model. Besides particle conversion, the cracking.17
model also consists of flow and thermal behaviour of
gases, particle movement (collisions with each other and In the literature, extensions/improvements of the
walls) and thermo-physical interactions between gas and Shafizadeh and Chin competitive scheme (e.g., with
solid phase. If a simplification is not applied, the total flow intermediate components) can be found, but all of them
through the reactor is two-phased due to the immersion of shows only moderate improvement regarding the model
the biomass particles in the reactor gas. Therefore, in the accuracy20-21. For the more accurate outcome of the
reactor model, the Eulerian description for fluids needs to model, they needed to undergo significant development in
be combined with the Lagrangian description for biomass terms of bio-components thermal degradation, together
particles. As it can be anticipated, the quantity of equations with a detailed description of the degradation of
and the amount of data needed to be processed in this consecutive (secondary) products.
model is the largest among other sub-parts of a
comprehensive model. Besides of the need of a numerical 4.2. Detailed reaction schemes: Ranzi scheme
solver to conduct the simulation, its performance in an A very detailed description was firstly introduced by Ranzi
efficient manner requires appropriately large et al.22, and it was further improved by him and his co-
computational power resources. workers23-27. The Ranzi’s model combines all findings
related to the thermal decomposition of each biomass
major component: cellulose, hemicellulose, and lignin28-30.
4. Molecular model It also distinguishes total lignin between 3 artificial types of
4.1. Single component and competitive schemes lignin: LIG-H, LIG-O, and LIG-C, according to their chemical
Historically, research started with the introduction of the structure. Another innovation of the Ranzi’s model is the
simplest biomass pyrolysis models, due to low technical char description, which distinguishes “pure” char and the
feasibilities of the detailed investigation. It had only limited volatiles “trapped” within a char metaplastic phase (e.g., G
ability for outcome prediction. Simple, single component [CO]). Thermally unstable “traps” degrade according to the
models aim at predicting the yield of the three main applied kinetic reaction, releasing captured volatiles. Such
products from biomass pyrolysis: char, tar, and gas, a description allows for the introduction of char
without distinguishing their detailed composition. They devolatilisation into the kinetic scheme. Reactions of
include one reaction (decomposition of biomass into three extractives were introduced in the Ranzi model by Debiagi
products) and therefore, they cover only primary pyrolysis et al.31. The extension assumes reactions of two
reactions18. Further improvements of the single models representative compounds (tannin and triglyceride) and it
were made after the discovery of the cracking reaction of significantly broadened the range of the biomasses for
condensable vapors (named tars) at temperatures higher which the model is valid. Ranzi’s model does not cover all
than 500 oC. The single component model with possible evolved species, but reduces its amount to 20
implemented consecutive tars reactions is named representatives, most abundant in non- and condensable
competitive kinetic scheme. The most often used is the one vapors. The Ranzi scheme allowed for the derivation of
proposed by Shafizadeh and Chin19, which is shown in complex reaction schemes, combining separate chemical
Figure 8. This model, however, does not consider low reactions into the consolidated model, which is
temperature (below 500 oC) tar-char interactions, called summarised in Figure 9. Most up to date kinetic
secondary charring reactions17. In most cases, secondary parameters and heat of reaction for Ranzi’s scheme,
charring reactions in single component models are composition of vapors, kinetic parameters, and reaction
hindered in the values of the primary kinetic reaction heat can be found in the work of Ranzi26-27 and Anca-
parameters. It leads to major discrepancies in values of Couce.32
49
Figure 9. Ranzi’s kinetic scheme, adapted from Anca-

Couce.17 Figure 10. RAC kinetic scheme adapted from Anca-
Couse.17
For an implementation of the Ranzi’s model, knowledge
about the biomass bio-composition is necessary. The main events that increase the value of the adjustable
Investigation of the analytical method in order to obtain parameters are17:
the most reliable bio-components composition was made • A decrease in temperature.
by the NREL research group33-34. It resulted in a detailed • A decrease in heating rate.
analytical procedure35. Despite the high accuracy and • An increase in volatiles retention time into the particle
repeatability of the NREL method, it is very time- (larger particle or slower gas flow rates).
consuming. Therefore, only a minority of authors • An increase in pressure.
conduct/commission the bio-components measurement. • An increase in the content of inorganics (especially
To by-pass experimental measurements, Debiagi et al.31 AAEM).
proposed a correlation between biomass elemental and In the theory behind the scheme, the amounts of
bio-components composition. Elemental analysis has a secondary reactions, indicated by the value of the “x”
much more simple procedure and the equipment needed parameter, differ for each bio-component. Unfortunately,
for its performance is much more available in academia the lack of a quantitative correlation between the
and in industry. For now, the correlation has low accuracy, conditions of pyrolysis, biomass parameters and amounts
due to the low amount of input data, but its further of secondary charring reactions causes the need for an
improvement will have a beneficial effect on the iterative fitting of those parameters to the experimental
simplification of obtaining initial parameters. result. A common approach is to set the adjustable
The Ranzi’s model is a milestone in the description of parameter as the same for all bio-components a priori,
pyrolysis kinetics, but there are a few areas in which based on the available experimental data. From examples
improvement or extensions can be made. The kinetic of data fitting to experimental results, it can be stated that,
scheme does not cover tar-char reactions at low for pyrolysis and torrefaction in a fixed bed, values of x can
temperatures, and it does not consider the catalytic be adjusted in the range of 0.3–0.4; for fluidized bed they
influence of the mineral mater (mainly alkali and alkaline- should be lower than for fixed bed, due to higher gas
Earth species, AAEM) contained in biomass. Moreover, the movements; and for experiments in micro-TGA, the proper
pyrolytic mechanics of the phenolic compounds evolution value should be around 0.2, due to significant
is not considered. Such limitations of the model result in fragmentation of investigated material17, 38. It is worth to
over-estimation of the sugar products and under- mention that the proportion of the secondary charring
estimation of the production of BTXs.17, 36 reactions have a noticeable influence on the heat of the
reactions, as it was observed by Rath et al.39 during the
4.3. Detailed reaction schemes: Ranzi and Anca-Couce investigation of spruce wood pyrolysis.
model The RAC scheme does not cover all areas, in which the
A partial extension of secondary charring reactions to the Ranzi scheme lacks (e.g., a detailed description of AAEM’s
Ranzi’s scheme was introduced by Anca-Couce et al.32 This influence or insights into polycyclic aromatic compounds
adaptation was named as RAC (Ranzi and Anca-Couce) formation). Constant work and recent findings on the
model, which is shown in Figure 10. A full description of the subject give promise of improvement and further
model and its kinetic and thermal parameters can be found extension of the pyrolysis scheme, which would allow for
in the works by Anca-Couce et al.36-37 better understanding of biomass pyrolysis process and
As can be observed in Figure 10, the RAC model introduces predict its outcome with higher accuracy.40-42
an adjustable parameter “x,” which defines the proportion
of the alternative route of degradation (secondary
reactions, e.g., charring reaction) in the overall 5. Single-particle model
degradation process. The adjustable parameter also The single-particle model focuses on the influence of the
partially takes into count the influence of inorganics, which boundary conditions and thermo-physical properties of
have a role in leading the way of the degradation reactions. the particle and other products on the behaviour of a single
50
biomass particle during the processing. The biomass < 𝜑𝜑 >= 𝜀𝜀𝛾𝛾 < 𝜑𝜑 >𝛾𝛾 (4)
particle, due to its structure, cannot be treated as an un- 𝛾𝛾
In other words, it can be stated that < 𝜑𝜑 > is the intrinsic
permeable solid block, since the description of the porous
or true value of the quantity, and < 𝜑𝜑 > is the average
structure needs to be implemented. In applications outside
value of the quantity in the representative volume. For
research, biomass is rarely fed to a pyrolysis process in a
example, < 𝜌𝜌𝑆𝑆 > 𝑆𝑆 will be defined as the true density of the
dry state. Therefore, for boarding application of the model,
solid from which the porous structure is made of, and
besides the description of the pyrolytic behaviour, it needs
< 𝜌𝜌𝑆𝑆 > will be defined as the density of the solid contained
to cover the drying process and a description of the water
in a representative volume of the porous structure, so
movements within the particle.
averaged volume density of the solid with porous space
5.1. Definitions of phases in particle structure (bulk density). The notation with the <,> brackets is based
on the fact that the authors belief that it is clearer in
A wet biomass feedstock used for the process consists at reception and of course, is not mandatory.
least of four different phases: liquid water, bounded water, As it was mentioned, the particle is made at most out of
gas mixture and solid. The bounded water is distinguished four phases, therefore the representative volume can be
from liquid water due to its significant difference in treated as the sum of the volume of each phase:
behaviour. Each of the mentioned phases needs to be
identified and described for further investigation. All of the 𝑉𝑉 = 𝑉𝑉𝑆𝑆 + 𝑉𝑉𝐿𝐿 + 𝑉𝑉𝐵𝐵 + 𝑉𝑉𝐺𝐺 (5)
phases are treated as a continuum, for which conservation where the subscripts S, L, B, and G represent a phase of
laws must be satisfied. A primary but detailed description solid, liquid water, bounded water and gas, respectively.
of each phase was made by Whitaker43, for which the The sum of the volume fraction occupied by each phase
boundary surface between each phase has to be sums into one, so:
differentiated in order to calculate phase fractions. Wood
𝜀𝜀𝐺𝐺 = 1 − (𝜀𝜀𝑆𝑆 + 𝜀𝜀𝐿𝐿 + 𝜀𝜀𝐵𝐵 ) (6)
structure has a very complex geometric structure.
Therefore, it would be hard to describe the boundary
< 𝜌𝜌𝑆𝑆 > < 𝜌𝜌𝐿𝐿 > < 𝜌𝜌𝐵𝐵 >
surfaces and keep track on them during the pyrolysis 𝜀𝜀𝐺𝐺 = 1 − � 𝑆𝑆
+ 𝐿𝐿
+ � (7)
< 𝜌𝜌𝑆𝑆 > < 𝜌𝜌𝐿𝐿 > < 𝜌𝜌𝐵𝐵 >𝐵𝐵
process, and the amount of computation for such
sophisticated model would be very burdensome for the This means that the volume fraction occupied by the gas is
solver. The problem of efficient description was further possible to obtain knowing the intrinsic and average
investigated by Perre and co-workers44-45, and an elegant density of the solid, liquid, and bounded water. A visual
description of this issue was presented by Grønli46. The representation of the real system in the Whitaker theory is
improved approach assumes the application of the given in Figure 11.
conservation equations volume averaging over a
representative volume, which contains all existing phases.
It results in a set of conservation equations for every phase,
valid within the applied particle geometry.
For a further description it will be helpful to define the
artificial variable 𝜑𝜑. Its spatial average defined over the
applied geometry is the averaged value of geometry total
volume and contains all the phases existing in the particle.
The spatial average is defined as:
Figure 11. Visual representation of the conversion of the
1 real system into the model system according to the
< 𝜑𝜑 > = � 𝜑𝜑𝜑𝜑𝜑𝜑 (1)
𝑉𝑉 Whitaker theory.
𝑉𝑉
The average for one of the phases is defined in general as 5.2. Governing equations
𝛾𝛾 and the phase average is defined as:
In this section, the explanation of the conservation laws is
1 introduced, but no explanation of the theory behind the
< 𝜑𝜑 >𝛾𝛾 = � 𝜑𝜑𝜑𝜑𝜑𝜑 (2)
𝑉𝑉𝛾𝛾 conservation laws is provided. For the sake of the
𝑉𝑉𝛾𝛾
description clarity and simplicity, the one-component
where < 𝜑𝜑 >𝛾𝛾 is the averaged value of the quantity in the kinetic scheme is used for explanation of principles. The
phase 𝛾𝛾 , and 𝑉𝑉𝛾𝛾 is the volume of the phase in the authors will keep to the most general description;
representative volume. The volume fraction occupied by therefore, the negative sign will be originated only from
the phase 𝛾𝛾 is defined as: the mathematical derivations, not from the values of
𝑉𝑉𝛾𝛾 actual parameters. The equations given in this subsection
𝜀𝜀𝛾𝛾 = (3)
𝑉𝑉 are valid only within the applied geometry, and they do not
where 𝜀𝜀𝛾𝛾 is the fraction of total volume occupied by the describe the interactions of the particle with the
phase 𝛾𝛾. The relation of the averaged value quantity environment outside of it. In general, it is worth to keep in
between the one located in phase 𝛾𝛾 and the spatial mind that all conservation equations are referred to the
average can be calculated as follows: single representative volume.
51
5.2.1. Mass conservation equations: Solids where 𝐾𝐾𝐺𝐺,𝑒𝑒𝑒𝑒𝑒𝑒 is the effective gas permeability, 𝜇𝜇𝐺𝐺 the gas
At any time of the reaction, the volume of the solid is dynamic viscosity, and < 𝑃𝑃𝐺𝐺 >𝐺𝐺 the pressure of the gas
represented by either biomass or biochar, and hence, it can mixture.
be stated that: 5.2.3. Mass conservation equations: Liquid water
< 𝜌𝜌𝑆𝑆 >=< 𝜌𝜌𝐵𝐵𝐵𝐵 > +< 𝜌𝜌𝐵𝐵𝐵𝐵 > (8) The mass conservation equation for liquid water can be
where < 𝜌𝜌𝑆𝑆 >, < 𝜌𝜌𝐵𝐵𝐵𝐵 > and < 𝜌𝜌𝐵𝐵𝐵𝐵 > are the volume- written as:
averaged density of solid, biomass and biochar; 𝜕𝜕
< 𝜌𝜌𝐿𝐿 > +∇< 𝑢𝑢𝐿𝐿 𝜌𝜌𝐿𝐿 >= 𝜔𝜔̇𝐿𝐿 (15)
respectively. The mass conservation equation for the 𝜕𝜕𝜕𝜕
biomass can be written as: where < 𝜌𝜌𝐿𝐿 > is the volume-averaged liquid water density,
𝜕𝜕 < 𝑢𝑢𝐿𝐿 𝜌𝜌𝐿𝐿 > the transport term, and 𝜔𝜔̇𝐿𝐿 the mass change rate
< 𝜌𝜌𝐵𝐵𝐵𝐵 >= 𝜔𝜔̇𝐵𝐵𝐵𝐵 (9)
𝜕𝜕𝜕𝜕 caused by evaporation and/or re-condensation. It is
where 𝜔𝜔̇𝐵𝐵𝐵𝐵 is the biomass mass change rate caused by assumed that liquid water migrates though the structure
degradation reactions. As it was mentioned, despite the entirely due to the pressure changes (convectively), and
fact that the degradation reaction leads to the reduction of therefore its transport term can be expressed as:
mass, in the equation the negative sign is not used, < 𝑢𝑢𝐿𝐿 𝜌𝜌𝐿𝐿 >= 𝑢𝑢𝐿𝐿 < 𝜌𝜌𝐿𝐿 > (16)
because the value of the time derivative will be negative.
Similarly, the mass conservation equation can be derived where 𝑢𝑢𝐿𝐿 is the superficial velocity of the liquid water. The
for the biochar: Darcy law is also applied to obtain the superficial liquid
velocity:
𝜕𝜕
< 𝜌𝜌𝐵𝐵𝐵𝐵 >= 𝜔𝜔̇𝐵𝐵𝐵𝐵 (10) 𝐾𝐾𝐿𝐿,𝑒𝑒𝑒𝑒𝑒𝑒
𝜕𝜕𝜕𝜕 < 𝑢𝑢𝐿𝐿 >= ∇(< 𝑃𝑃𝐿𝐿 >𝐿𝐿 ) (17)
𝜇𝜇𝐿𝐿
Therefore, in the most general form the mass conservation
is defined as: where 𝐾𝐾𝐿𝐿,𝑒𝑒𝑒𝑒𝑒𝑒 is the effective liquid water permeability, 𝜇𝜇𝐿𝐿
𝜕𝜕 the liquid water dynamic viscosity, and < 𝑃𝑃𝐿𝐿 >𝐿𝐿 the
< 𝜌𝜌𝑆𝑆 >= 𝜔𝜔̇𝑆𝑆 (11) pressure of liquid water. < 𝑃𝑃𝐿𝐿 >𝐿𝐿 is connected with gas
𝜕𝜕𝜕𝜕
pressure and capillary pressure through the following
where 𝜔𝜔̇𝑆𝑆 is the total mass change of the solid obtained equation (𝑃𝑃𝐶𝐶 is the capillary pressure):
from the sum of the reaction of biomass and char.
< 𝑃𝑃𝐿𝐿 >𝐿𝐿 =< 𝑃𝑃𝐺𝐺 >𝐺𝐺 + 𝑃𝑃𝐶𝐶 (18)
5.2.2. Mass conservation equations: Single component of
the gas mixture 5.2.4. Mass conservation equations: Bounded water
The equation for the mass conservation of the ith The mass conservation equation of bounded water is
component of the gas mixture: defined as:
𝜕𝜕 𝜕𝜕 𝜕𝜕
(𝜀𝜀 < 𝜌𝜌𝑖𝑖 >𝐺𝐺 ) + ∇< 𝑢𝑢𝑖𝑖 𝜌𝜌𝑖𝑖 >= 𝜔𝜔̇𝑖𝑖 < 𝜌𝜌𝐵𝐵 > +∇< 𝑢𝑢𝐵𝐵 𝜌𝜌𝐵𝐵 >= 𝜔𝜔̇𝐵𝐵 (19)
𝜕𝜕𝜕𝜕 𝐺𝐺 𝜕𝜕𝜕𝜕 (12) 𝜕𝜕𝜕𝜕
< 𝜌𝜌𝑆𝑆 >= 𝜔𝜔̇𝑆𝑆 where < 𝜌𝜌𝐵𝐵 > is the volume-averaged bounded water
where < 𝜌𝜌𝑖𝑖 >𝐺𝐺 is the density of the ith component in the density, < 𝑢𝑢𝐵𝐵 𝜌𝜌𝐵𝐵 > the bounded water transport term, and
gaseous phase, < 𝑢𝑢𝑖𝑖 𝜌𝜌𝑖𝑖 > the ith compoennt transport 𝜔𝜔̇𝐵𝐵 the mass change rate caused by unbounding. In
term, and 𝜔𝜔̇𝑖𝑖 the mass change rate caused by opposition to liquid water, it is assumed that bounded
formation/degradation reaction of the ith gas component water migrates entirely by diffusion, and therefore, its
(e.g., water vapour, gas or tar). Transport of the gas takes transport term is defined as follows:
place by two phenomena (convection and diffusion) and < 𝜌𝜌𝐵𝐵 >
< 𝑢𝑢𝐵𝐵 𝜌𝜌𝐵𝐵 > = −< 𝜌𝜌𝑆𝑆 > 𝐷𝐷𝐵𝐵 ∇ � � (20)
therefore, the transportation term is defined as: < 𝜌𝜌𝑆𝑆 >
< 𝑢𝑢𝑖𝑖 𝜌𝜌𝑖𝑖 > where 𝐷𝐷𝐵𝐵 is the bounded water diffusion coefficient.
= 𝑢𝑢𝐺𝐺 < 𝜌𝜌𝑖𝑖 >𝐺𝐺
(13) 5.2.5. Energy conservation equation
< 𝜌𝜌𝑖𝑖 >𝐺𝐺
−< 𝜌𝜌𝐺𝐺 >𝐺𝐺 𝐷𝐷𝑒𝑒𝑒𝑒𝑒𝑒 ∇ � �
< 𝜌𝜌𝐺𝐺 >𝐺𝐺 The energy conservation equation is based on the
assumption of local thermal equilibrium for gas, liquid, and
where 𝑢𝑢𝐺𝐺 is the superficial gas velocity, < 𝜌𝜌𝐺𝐺 >𝐺𝐺 the total
solid in the particle:
density of the gas mixture in the gas phase, and 𝐷𝐷𝑒𝑒𝑒𝑒𝑒𝑒 the
𝜕𝜕𝜕𝜕
effective gas diffusion coefficient. Due to the low 𝜕𝜕𝜕𝜕
�< 𝜌𝜌𝑆𝑆 > 𝐶𝐶𝑃𝑃,𝑆𝑆 +< 𝜌𝜌𝐿𝐿 > 𝐶𝐶𝑃𝑃,𝐿𝐿 +< 𝜌𝜌𝐵𝐵 > 𝐶𝐶𝑃𝑃,𝐵𝐵 + 𝜀𝜀𝐺𝐺 < 𝜌𝜌𝐺𝐺 >𝐺𝐺 𝐶𝐶𝑃𝑃,𝐺𝐺 �
permeability of the biomass structure, which results in 𝑁𝑁
(21)
+ ∇𝑇𝑇 �< 𝑢𝑢𝐿𝐿 𝜌𝜌𝐿𝐿 > 𝐶𝐶𝑃𝑃,𝐿𝐿 +< 𝑢𝑢𝐵𝐵 𝜌𝜌𝐵𝐵 > 𝐶𝐶𝑃𝑃,𝐵𝐵 + 𝜀𝜀𝐺𝐺 � < 𝑢𝑢𝑖𝑖 𝜌𝜌𝑖𝑖 > 𝐶𝐶𝑃𝑃,𝑖𝑖 � =
general low superficial velocities of fluids, the Darcy law is 𝑖𝑖=1
valid to obtain the value of superficial gas velocity: = ∇�𝜆𝜆𝑒𝑒𝑒𝑒𝑒𝑒 ∇𝑇𝑇� + 𝑄𝑄
𝐾𝐾𝐺𝐺,𝑒𝑒𝑒𝑒𝑒𝑒 where 𝐶𝐶𝑃𝑃 is the specific heat, whereas subscripts S, L, B and

< 𝑢𝑢𝐺𝐺 >= ∇(< 𝑃𝑃𝐺𝐺 >𝐺𝐺 ) (14)
𝜇𝜇𝐺𝐺 i indicate solid, liquid water, bounded water, and ith
component of the gas mixture; respectively. 𝜆𝜆𝑒𝑒𝑒𝑒𝑒𝑒 is the
52
effective thermal conductivity and 𝑄𝑄 is the total heat 6.2. Identification of the scale on single particle model
produced by occurring reactions, which is defined as: As it was mentioned in previous sections, there is a
𝑁𝑁
possibility for simplification of the comprehensive model
𝑄𝑄 = � 𝐻𝐻𝑖𝑖 𝜔𝜔̇𝑖𝑖 + 𝐻𝐻𝐿𝐿 𝜔𝜔̇𝐿𝐿 + 𝐻𝐻𝐵𝐵 𝜔𝜔̇𝐵𝐵 + 𝐻𝐻𝑆𝑆 𝜔𝜔̇𝑆𝑆 (22) (from two-phase fluid into one) when the single particles
𝑖𝑖
are sufficiently small. In order to assess if such approach is
Where 𝐻𝐻 is the heat of the reaction. In the most general valid, the investigation of two non-dimensional numbers is
case, the transport term is treated in conservative form, done: thermal Biot number (Bi) and pyrolysis number (Py),
and the energy conservation equation takes into account which has the Thiele modulus as its inverse15. Based on
the heat transfer through the convective transport and those numbers, the particle can be assigned to one of four
diffusion46-48. Some authors apply simplification in defining thermal regimes: pure kinetic, thermally thin, thermal
the transport, omitting the heat transported through wave, and thermally thick. Each of the regimes indicates
diffusion, assuming that the amount of heat exchanged that thermal phenomena (chemical reactions, intra-
through this phenomenon is negligible15, 20, 49-50. With the particle or extra-particle heat exchange) have the
mentioned assumption, the energy conservation equation strongest influence on the particle thermal behaviour18, 51-
takes the following form: 54. The most complex description of particle behaviour is
𝜕𝜕𝜕𝜕 valid for all regimes, but in case it is not necessary, there is
�< 𝜌𝜌𝑆𝑆 > 𝐶𝐶𝑃𝑃,𝑆𝑆 + < 𝜌𝜌𝐿𝐿 > 𝐶𝐶𝑃𝑃,𝐿𝐿 +< 𝜌𝜌𝐵𝐵 > 𝐶𝐶𝑃𝑃,𝐵𝐵 + 𝜀𝜀𝐺𝐺 < 𝜌𝜌𝐺𝐺 >𝐺𝐺 𝐶𝐶𝑃𝑃,𝐺𝐺 �
𝜕𝜕𝜕𝜕 no need to overcomplicate the model.
+ ∇𝑇𝑇�𝑢𝑢𝐿𝐿 < 𝜌𝜌𝐿𝐿 > 𝐶𝐶𝑃𝑃,𝐿𝐿 + 𝑢𝑢𝐵𝐵 < 𝜌𝜌𝐵𝐵 > 𝐶𝐶𝑃𝑃,𝐵𝐵 + 𝑢𝑢𝐺𝐺 𝜀𝜀𝐺𝐺 < 𝜌𝜌𝐺𝐺 >𝐺𝐺 𝐶𝐶𝑃𝑃,𝐺𝐺 � (23)
= ∇�𝜆𝜆𝑒𝑒𝑒𝑒𝑒𝑒 ∇𝑇𝑇� + 𝑄𝑄 The simplification is most valid for particles in pure kinetic
regime, for which the size, in general, is lower than 1 mm17.
The particles in the thin thermal regime, due to their
5.2.6. Reactions
structure, do not show thermal or pressure gradient, and
The mass change rate of every reaction in the kinetic thus, the application of the simplification should not
scheme is defined as: introduce a significant error to the model. For particles in
𝜔𝜔̇𝑗𝑗 = 𝑘𝑘𝑗𝑗 < 𝜌𝜌𝑗𝑗 >= 𝑘𝑘𝑗𝑗 𝜀𝜀𝛾𝛾 < 𝜌𝜌𝑗𝑗 >𝛾𝛾 (24) the thermal wave regime, application of the simplification
is not advised due to the existence of significant gradients
where 𝜔𝜔̇𝑗𝑗 is the mass change rate of jth species (e.g.,
during their processing. Nevertheless, there is the
biomass, tar, gas), 𝑘𝑘𝑗𝑗 is the reaction rate of jth species,
possibility of partial simplification of their complex
< 𝜌𝜌𝑗𝑗 > is the averaged volume density of the jth species,
thermo-physical description by the implementation of an
and < 𝜌𝜌𝑗𝑗 >𝛾𝛾 is the intrinsic density of the jth species in
unreacted shrinking core model, a volumetric model or a
phase 𝛾𝛾. Depending on the applied drying/evaporation
layer model17, 55. It is possible because, in the mentioned
model (e.g., equilibrium, heat sink, kinetic), the mass
regime, the conversion of the particle takes place in the
change rate for the liquid water and bounded water will
thin surface front, and therefore; the assumption that the
take a form suitable for the chosen model.
conversion front thickness converges to 0 is not a large
deviation from reality.
The most complex model is needed for particles in the
6. Reactor model and multiscale
thermally thick regime, which is controlled by both internal
The reactor model is the final and, at the same time, the heat transfer and chemical reactions, and shows the
most difficult and time-consuming subpart of the highest temperature gradients among regimes. There is no
comprehensive modeling of the process of biomass stiff boundary from where the thermally thick regime
thermal conversion. In this subpart, the division between starts, but in the literature, it can be found that a particle
the reactor gases and processed material phase is in the mentioned regime for Bi number higher than
description have to be considered due to significant 40−100 and thermal Thiele modulus (1/Py) number higher
physical, chemical and thermal differences. than 100−1400, depending on the source.53, 56
6.1. Gases in reactor 6.3. Particles size and motion

The reactor gas as the fluid is always modeled via the For fixed-bed reactors, the only limitation for the particle
Eulerian approach, which makes it a less challenging part size is the reactor dimensions. Therefore, the largest
of the reactor modeling, in comparison to particles’ particles can be processed. For fixed beds where the
description. In terms of the fluid in the reactor, the movement of the particles is negligible, and the mixing of
following has to be described: fluid motion within the solids is not significant, Wurzenberger et al.57 proposed the
reactor geometry, taking into account its moving parts, application of the Representative Particle Model (RPM). It
changes of reactor volume due to particles movement, and assumes that the processing parameters for the whole
heat transport from reactor walls to fluid and from fluid to reactor can be treated as homogenous, so all processed
particles. Moreover, the description of the reactor gas has particles will have the same behaviour. Hence, the single
to cope with the chemical behavior of the species particle model has to be solved only once for the applied
contained in it (e.g., secondary tar cracking). boundary and initial conditions (for one representative
particle). Application of the RPM model for fixed-bed
modeling reduces the computational time significantly and
shows good agreement with experimental results.16-17
53
In general, it can be stated that the cases when the The main disadvantage of incorporating a particle model
particles are in motion are more challenging. Particles can into a reactor model from the practical point of view is the
be forced to move due to changes in pressure need for a high amount of computational power resources
(pneumatically), or due to the physical force of a rotary and knowledge on its use in a sufficient manner17. Such
element (mechanically). In the last scenario, the moving computation power demand is caused mainly by the large
element also has an influence on the gas motion in the number of particles that have to be considered within the
reactor, which needs to be implemented. model. Besides the quantity of the particles, also the
The selection of the reactor technology (factor inducing complexity of the description of their conversion plays a
the movement of particles) puts the boundaries on the size part in the amount of power needed for computation.
of the particles applied into the model. For fluidised beds, It needs to be mentioned that the very complex single
the particle size has to be significantly small to be able to particle model and the limited computation power
be dragged by the fluidising gas. In general, it can be stated resources result in computation time, which does not allow
that particle size in fluidised-bed reactors does not exceed for relatively rapid adaptation of the investigated model.
2−3 mm. For small particles in a fluidised-bed reactor, a Currently, the computational power is not the only factor
simplification in the flow description can be applied, which hindering the reactor modeling development, but also the
significantly reduces the complexity and simultaneously lack of a detailed description of the solid surface
reduces the computation burden. interactions, and particle mechanical changes. From a
For rotary reactors (auger/screw or rotary kiln reactors) practical point-of-view, insufficiently developed software
particle size is usually larger than in fluidised-bed reactors, have problems with mesh adaptation58 (e.g., in the case of
but its maximum size is limited to the moving parts complex rotary reactors, which in many cases represents a
dimensions (screw size), mechanical durability of material, barrier in its application in models for biomass processing).
and mixing and homogenous distribution of solid material
within the reactor. Due to the size of the particles, the
application of the flow description simplification is not 7. Conclusion
possible, and thus, the full Eulerian-Lagrangian description Numerical modeling is a very robust tool, which allows for
of the two-phase flow has to be implemented. low cost research and development of technologies within
Additionally, the movement of the rotary element and its the thermal processing of biomass field. As indicated in this
influence on the flow behaviour have to be consistent in chapter, for its proper application, knowledge from
the reactor description. Based on all of this, it can be stated different areas of science have to be combined in order to
that models for rotary reactors are, among the others, the obtain reliable and valuable results. In theory, there is no
most demanding, both for the model builder and the limitation in modeling any processing technology, and
hardware requirements to conduct simulations. applying it to broad processing parameters or different
feedstock. From a practical point of view, it must be kept
6.4. Reactor model consolidation
in mind that the selected environment or material imposes
The applied equations and their shape in the reactor model strong boundaries on the validity of the applied molecular,
strongly depend on the type of reactor, and therefore, single-particle, and reactor models. The complexity and
their detailed description will be omitted. An extensive selection of proper models have a significant influence on
review of this matter can be found in the work by Jurtz et the accuracy and realism of the investigated case. In
al.58 and Subramaniam59. The model with coupled gas general, it can be stated that it is better to apply a
phase (continuous) and solid particles (discrete) can be description as detailed as available. Nevertheless, the
found in the work of Mahmoudi et al.16, who used balance between computing power, computation time,
eXtended Discrete Element Method (XDEM). It is worth to and model complexity also have to be taken as main
mention the work by Funke et al.60, where for the first time priorities. It is highly recommended to check if the model
the heat transfer between particles in an auger type cannot be simplified due to the fundamental phenomena
reactor was calculated, using a combined fundamental occurring in the process, without loss of the model
heat transfer model and DEM simulation. accuracy.
6.5. Reactor model and computation power

Acknowledgements
Complex kinetic models with constringed relations
between compounds lead to the improvement of the “This project has received funding from the European
model predictions accuracy and bring the model closer to Union’s Horizon 2020 research and innovation programme
the description of reality. The application of detailed under the Marie Skłodowska-Curie grant agreement No
models brings the issue of implementation of the vast 721991”.
amount of input data (biomass initial composition and
thermo-physical parameters). When the complex
molecular sub-model is implemented into the
comprehensive larger scale model, it leads to a significant
increase in the computation load, which in most cases
results in a significant increase in computation time.
54
References (20) Park, W. C.; Atreya, A.; Baum, H. R. Experimental

(1) EIA, Energy sources have changed throughout the and theoretical investigation of heat and mass
history of the United States transfer processes during wood pyrolysis. Combust.
[https://www.eia.gov/todayinenergy/detail.php?id Flame 2010, 157 (3), 481−494.
=11951]. (21) Koufopanos, C. A.; Lucchesi, A.; Maschio, G. Kinetic
(2) Norgate, T.; Haque, N.; Somerville, M.; Jahanshahi, modelling of the pyrolysis of biomass and biomass
S. Biomass as a Source of Renewable Carbon for Iron components. Can. J. Chem. Eng. 1989, 67 (1), 75−84.
and Steelmaking. ISIJ Int. 2012, 52 (8), 1472−1481. (22) Ranzi, E.; Cuoci, A.; Faravelli, T.; Frassoldati, A.;
(3) Lehmann, J.; Joseph, S. Biochar for environmental Migliavacca, G.; Pierucci, S.; Sommariva, S. Chemical
management: science and technology. Earthscan: Kinetics of Biomass Pyrolysis. Energy Fuels 2008, 22
London; Sterling, VA, 2009. (6), 4292−4300.
(4) Faaij, A. Large Scale International Bio-Energy Trade. (23) Cuoci, A. F. T.; Frassoldati, A.; Granata, S.;
In 12th European Conf. and Technology Exhibition Migliavacca, G.; Ranzi, E. In A general mathematical
on Biomass for Energy, Industry and Climate Change model of biomass devolatilization. Note 1. Lumped
Protection, Amsterdam, 2002. kinetic models of cellulose, hemicellulose and
(5) de Wit, M.; Faaij, A. European biomass resource lignin,, Proceedings of the 30th combustion meeting
potential and costs. Biomass Bioenergy 2010, 34 (2), of the Italian Section of the Combustion Institute VI,,
188−202. 2007; pp 2.1–2.6.
(6) Antal, M. J.; Grønli, M. The Art, Science, and (24) Cuoci, A. F. T.; Frassoldati, A.; Granata, S.;
Technology of Charcoal Production. Ind. Eng. Chem. Migliavacca, G.; Pierucci, S. A. general mathematical
Res. 2003, 42 (8), 1619−1640. model of biomass devolatilization. Note 2. Detailed
(7) Dubois, G. Modeling and simulation : challenges and kinetics of volatile species. Proceedings of the 30th
best practices for industry, 2018. combustion meeting of the Italian Section of the
(8) Kennedy, M. C.; O'Hagan, A. Bayesian calibration of Combustion Institute VI, 2007, 3.1–3.6.
computer models. J. Royal Statistical Soc. Series B (25) Corbetta, M.; Frassoldati, A.; Bennadji, H.; Smith, K.;
(Statistical Methodology) 2001, 63 (3), 425−464. Serapiglia, M. J.; Gauthier, G.; Melkior, T.; Ranzi, E.;
(9) Balci O. In Guidelines for successful simulation Fisher, E. M., Pyrolysis of Centimeter-Scale Woody
studies, Winter Simulation Conference Proceedings, Biomass Particles: Kinetic Modeling and
9-12 Dec. 1990; 1990; pp 25−32. Experimental Validation. Energy Fuels 2014, 28 (6),
(10) Versteeg, H. K.; Malalasekera, W. An introduction to 3884–3898.
computational fluid dynamics: the finite volume (26) Ranzi, E.; Debiagi, P. E. A.; Frassoldati, A.
method. Pearson Education: Harlow, 2011. Mathematical Modeling of Fast Biomass Pyrolysis
(11) Tu, J.; Yeoh, G. H.; Liu, C. Computational fluid and Bio-Oil Formation. Note I: Kinetic Mechanism of
dynamics: a practical approach, 2018. Biomass Pyrolysis. ACS Sustain. Chem. Eng. 2017, 5
(12) Childress, S. An introduction to theoretical fluid (4), 2867–2881.
mechanics, 2009. (27) Ranzi, E.; Debiagi, P. E. A.; Frassoldati, A.
(13) Batchelor, G. K. P. An introduction to fluid dynamics. Mathematical Modeling of Fast Biomass Pyrolysis
Cambridge University Press: Cambridge, 2013. and Bio-Oil Formation. Note II: Secondary Gas-Phase
(14) Sakai, M. How Should the Discrete Element Method Reactions and Bio-Oil Formation. ACS Sustain.
Be Applied in Industrial Systems? A Review. KONA Chem. Eng. 2017, 5 (4), 2882–2896.
Powder Particle J. 2016, 33, 169−178. (28) Mamleev, V.; Bourbigot, S.; Le Bras, M.; Yvon, J. The
(15) Anca-Couce, A.; Zobel, N. Numerical analysis of a facts and hypotheses relating to the
biomass pyrolysis particle model: Solution method phenomenological model of cellulose pyrolysis:
optimized for the coupling to reactor models. Fuel Interdependence of the steps. J. Anal. Appl.
2012, 97, 80−88. Pyrolysis 2009, 84 (1), 1–17.
(16) Mahmoudi, A. H.; Hoffmann, F.; Peters, B. Detailed (29) Piskorz, J.; Radlein, D. S. A. G.; Scott, D. S.; Czernik,
numerical modeling of pyrolysis in a heterogeneous S. Pretreatment of wood and cellulose for
packed bed using XDEM. J. Anal. Appl. Pyrolysis production of sugars by fast pyrolysis. J. Anal. Appl.
2014, 106, 9−20. Pyrolysis 1989, 16 (2), 127–142.
(17) Anca-Couce, A. Reaction mechanisms and multi- (30) Faravelli, T.; Frassoldati, A.; Migliavacca, G.; Ranzi, E.
scale modelling of lignocellulosic biomass pyrolysis. Detailed kinetic modeling of the thermal
Prog. Energy Combust. Sci. 2016, 53, 41−79. degradation of lignins. Biomass Bioenergy 2010, 34
(18) Di Blasi, C. Modeling chemical and physical (3), 290–301.
processes of wood and biomass pyrolysis. Prog. (31) Debiagi, P. E. A.; Pecchi, C.; Gentile, G.; Frassoldati,
Energy Combust. Sci. 2008, 34 (1), 47−90. A.; Cuoci, A.; Faravelli, T.; Ranzi, E. Extractives
(19) Shafizadeh, F.; Chin, P. P. S. Thermal Deterioration Extend the Applicability of Multistep Kinetic Scheme
of Wood. In Wood Technology: Chemical Aspects, of Biomass Pyrolysis. Energy Fuels 2015, 29 (10),
American Chemical Society: 1977; Vol. 43, pp 57−81. 6544–6555.
55
(32) Anca-Couce, A.; Sommersacher, P.; Scharler, R. (46) Grønli, M. G. A theoretical and experimental study
Online experiments and modelling with a detailed of thermal degradation of biomass, 1996.
reaction scheme of single particle biomass pyrolysis. (47) Jalili, M.; Anca-Couce, A.; Zobel, N. On the
J. Anal. Appl. Pyrolysis 2017, 127, 411–425. Uncertainty of a Mathematical Model for Drying of
(33) Sluiter, J. B.; Ruiz, R. O.; Scarlata, C. J.; Sluiter, A. D.; a Wood Particle. Energy Fuels 2013, 27 (11), 6705–
Templeton, D. W. Compositional Analysis of 6717.
Lignocellulosic Feedstocks. 1. Review and (48) Melaaen, M. C. Numerical Analysis of Heat and Mass
Description of Methods. J. Agric. Food Chem. 2010, Transfer in Drying and Pyrolysis of Porous Media.
58 (16), 9043–9053. Numer. Heat Transfer A 1996, 29 (4), 331–355.
(34) Templeton, D. W.; Scarlata, C. J.; Sluiter, J. B.; (49) Fatehi, H.; Bai, X. S. A Comprehensive Mathematical
Wolfrum, E. J. Compositional Analysis of Model for Biomass Combustion. Combust. Sci.
Lignocellulosic Feedstocks. 2. Method Technol. 2014, 186 (4–5), 574–593.
Uncertainties. J. Agric. Food Chem. 2010, 58 (16), (50) Shi, X. Computational fluid dynamics modelling of
9054–9062. biomass slow pyrolysis in screw reactors for the
(35) National Renewable Energy Laboratory (NREL). production of biochar and charcoal. Ghent
Determination of structural carbohydrates and University, 2017.
lignin in biomass: laboratory analytical procedure, (51) Mettler, M. S.; Mushrif, S. H.; Paulsen, A. D.;
2011. Javadekar, A. D.; Vlachos, D. G.; Dauenhauer, P. J.
(36) Anca-Couce, A.; Mehrabian, R.; Scharler, R.; Revealing pyrolysis chemistry for biofuels
Obernberger, I. Kinetic scheme of biomass pyrolysis production: Conversion of cellulose to furans and
considering secondary charring reactions. Energy small oxygenates. Energy Environ. Sci. 2012, 5 (1),
Conversion Manage. 2014, 87, 687–696. 5414–5424.
(37) Anca-Couce, A.; Scharler, R. Modelling heat of (52) Paulsen, A. D.; Mettler, M. S.; Dauenhauer, P. J. The
reaction in biomass pyrolysis with detailed reaction Role of Sample Dimension and Temperature in
schemes. Fuel 2017, 206, 572–579. Cellulose Pyrolysis. Energy Fuels 2013, 27 (4), 2126–
(38) Anca-Couce, A.; Obernberger, I. Application of a 2134.
detailed biomass pyrolysis kinetic scheme to (53) Pyle, D. L.; Zaror, C. A. Heat transfer and kinetics in
hardwood and softwood torrefaction. Fuel 2016, the low temperature pyrolysis of solids. Chem. Eng.
167, 158–167. Sci. 1984, 39 (1), 147–158.
(39) Rath, J.; Wolfinger, M. G.; Steiner, G.; Krammer, G.; (54) Di Blasi, C. Heat, momentum and mass transport
Barontini, F.; Cozzani, V. Heat of wood pyrolysis. through a shrinking biomass particle exposed to
Fuel 2003, 82 (1), 81–91. thermal radiation. Chem. Eng. Sci. 1996, 51 (7),
(40) Norinaga, K.; Shoji, T.; Kudo, S.; Hayashi, J.-i. 1121–1132.
Detailed chemical kinetic modelling of vapour- (55) Mehrabian, R.; Zahirovic, S.; Scharler, R.;
phase cracking of multi-component molecular Obernberger, I.; Kleditzsch, S.; Wirtz, S.; Scherer, V.;
mixtures derived from the fast pyrolysis of cellulose. Lu, H.; Baxter, L. L. A CFD model for thermal
Fuel 2013, 103, 141–150. conversion of thermally thick biomass particles. Fuel
(41) Nowakowska, M.; Herbinet, O.; Dufour, A.; Glaude, Process. Technol. 2012, 95, 96–108.
P.-A. Detailed kinetic study of anisole pyrolysis and (56) Villermaux, J.; Antoine, B.; Lede, J.; Soulignac, F. A
oxidation to understand tar formation during new model for thermal volatilization of solid
biomass combustion and gasification. Combust. particles undergoing fast pyrolysis. Chem. Eng. Sci
Flame 2014, 161 (6), 1474–1488. 1986, 41 (1), 151–157.
(42) Trendewicz, A.; Evans, R.; Dutta, A.; Sykes, R.; (57) Wurzenberger, J. C.; Wallner, S.; Raupenstrauch, H.;
Carpenter, D.; Braun, R. Evaluating the effect of Khinast, J. G. Thermal conversion of biomass:
potassium on cellulose pyrolysis reaction kinetics. Comprehensive reactor and particle modeling.
Biomass Bioenergy 2015, 74, 15–25. AIChE J. 2004, 48 (10), 2398–2411.
(43) Whitaker, S. Simultaneous Heat, Mass, and (58) Jurtz, N.; Kraume, M.; Wehinger Gregor, D.
Momentum Transfer in Porous Media: A Theory of Advances in fixed-bed reactor modeling using
Drying. In: Advances in Heat Transfer, Hartnett, J. P.; particle-resolved computational fluid dynamics
Irvine, T. F., Eds. Elsevier: 1977; Vol. 13, pp 119-203. (CFD). Rev. Chem. Eng. 2019, 35, 139–190.
(44) Perré, P.; Turner, I. W. A 3-D version of TransPore: a (59) Subramaniam, S. Lagrangian–Eulerian methods for
comprehensive heat and mass transfer multiphase flows. Prog. Energy Combust. Sci. 2013,
computational model for simulating the drying of 39 (2), 215–245.
porous media. Int. J. Heat Mass Transfer 1999, 42 (60) Funke, A.; Grandl, R.; Ernst, M.; Dahmen, N.
(24), 4501–4521. Modelling and improvement of heat transfer
(45) Nasrallah, S. B.; Perre, P. Detailed study of a model coefficient in auger type reactors for fast pyrolysis
of heat and mass transfer during convective drying application. Chem. Eng. Process.: Process
of porous media. Int. J. Heat Mass Transfer 1988, 31 Intensification 2018, 130, 67–75.
(5), 957–967.
56
Production and characterisation of lignin and lignin-containing streams obtained

through a two-stage fractionation process of lignocellulosic biomass
Qusay Ibrahima,b, Moritz Leschinskyb, Andrea Krusea
a Department of Conversion Technologies of Biobased Resources, Institute of Agricultural Engineering, University of Hohenheim,
Garbenstrasse 9, 70599 Stuttgart, Germany
b Pretreatment and Fractionation of Renewable Feedstocks. Fraunhofer Center for Chemical-Biotechnological Process CBP,
Amhaupttor (Tor 12, Bau 1251), 06237 Leuna, Germany
Abstract
One of the main challenges in lignocelulosic biorefinery framework is to develop and optimise an efficient fractionation process to
recover and produce high purity grades of lignin and lignin-rich side streams to be used in further processes and applications. This
chapter aims to give a review on the two-stage processes to fractionate lignocellulosic biomass and the effect of their operating
conditions on the yield and the specifications of the resulted fractions. The focus is on the main composition of lignocellulosic biomass
(lignin, cellulose, and hemicellulose). Furthermore, kraft, sulphite and the organosolv pulping process are discussed, including the
advantages of organosolv pulping over the other processes. In addition to that, the enzymatic hydrolysis, as a further step to produce
sugars from the cellulose stream as well as hydrolsis lignin as a side-stream, is introduced.
Sugar units are the building blocks in cellulose and

1. Introduction hemicellulose, and are considered as the main potential source
of fermentable sugars after biomass saccharification. The most
Lignocellulosic biomass is the most bountiful biomass on the
important monosaccharides are glucose, mannose and xylose.
earth. Compared to fossil fuels sources, lignocellulosic biomass
Additionally, lignin is the main potential source for phenolic
has the ability to diminish carbon dioxide emissions by
compounds (e.g., as platform chemicals for chemical industry).
75%–100%, because the carbon dioxide bounded by the
The process of sugar production from hemicellulose through
lignocellulosic biomass during photosynthesis is the same that
hydrolysis is partially easier than that from cellulose. The
that released during the combustion of biofuel produced from
crystalline structure of cellulose requires harsher treatment and
it1. In addition, lignocellulosic biomass does not compete with
more severe conditions like using acids, the presence of
food as a source for chemicals and materials, it has low costs
enzymes or high temperature for hydrolysis to
and comes in most cases from sustainable sources.
monosaccharides (glucose). Typical challenges in lignocellulose
Lignocellulosic biomass is mainly comprised of lignin, cellulose
fractionation processes are: (i) poor separation of both cellulose
and hemicellulose, as well as other minor components like
and lignin, (ii) the formation of compounds that inhibit ethanol
extractives, water, minerals and protein. The percentage of
fermentation (formation of furans from sugars, acetic acid from
these components differs with the source of lignocellulosic
hemicellulose and phenolic compounds from lignin fractions),
biomass. Table 1 shows the major composition of common
(iii) the requirement of high amounts of energy and/or
sources of biomass.
chemicals, and (iv) the production of wastes at the end of the
Table 1. Major constituents for different biomass sources.2 process.
Biomass Cellulose Hemicellulose Lignin 1.1. Composition of Lignocellulosic biomass
source (wt. %) (wt. %) (wt. %)
Hardwood 40–55 24–40 18–25 1.1.1. Cellulose
Softwood 45–50 25–35 25–35 As one of the major components of lignocellulosic biomass,
Nut shells 25–30 25–30 30–40 cellulose is composed of glucose monomers that are connected
Corn cobs 45 35 15 into cellobiose subunits by ß–1,4–glycosidic bonds. Cellulose
Grasses 25–40 35–50 10–30 has (C6H10O5)n as a molecular formula3. Figure 1 shows the
Paper 85–99 0 0–15 structure of a single cellulose molecule.
Wheat Straw 30 50 15
Sorted refuse 60 20 20
Leaves 15–20 80–85 0
Newspapers 40–55 25–40 18–30
Swine waste 6 28 NA
Switchgrass 45 31.4 12.0
Miscanthus 35–40 16–20 20–25
Figure 1. Molecular chain structure of cellulose (adapted
from 3).

57
Chapter 5 Ibrahim et al.
Cellulose characteristics depend on the degree of The solubility of hemicellulose in aqueous solutions depends on
polymerization (DP) which is defined as the number of glucose its chemical structure, the DP and the pH of the solution11.
units that form one polymer molecule. Depending on the Normally, hemicellulose has good solubility in alkaline
source of cellulose, the DP ranges can vary from 3000 to 15000 solutions. Moreover, acidity and high temperatures lead to the
(for instance, it is about 10000 in wood4). 1,4– glycosidic bonds hydrolysis of hemicellulose.
force cellulose to form long and robust straight chains that
interact with each other through hydrogen bonds to form
fibres. Hydroxide groups on both sides of the monomers lead to
the formation of hydrogen bonds between the linear glucan
chains, which is the reason for the crystalline structure as
shown in Figure 2.5-7
Figure 3. Schematic representative of Glucuronoxylan in

hardwood (Adapted from 12).
Figure 2. Demonstration of the inter- and intra-chain hydrogen

bonding pattern in cellulose type I. Dashed lines are inter-
chain hydrogen bonding and dotted lines are intra-chain
hydrogen bonding (adapted from7).
Cellulose is found in one of these two forms: crystalline or non–

crystalline structure. It has six different known structures
(Cellulose I, II, IIII, IIIII, IVI, and IVII)7. Under normal atmospheric Figure 4. Schematic representative of galactoglucomannans
conditions (20 °C, 60% relative humidity), cellulose absorbs and arabinoglucuronoxylan in softwood (adapted from 12).
water. At normal temperatures, cellulose is insoluble in water,
dilute acidic solutions, and alkaline solutions5,8. Concentrated 1.1.3. Lignin
acids mainly hydrolyse the cellulose and normally only the low-
Besides cellulose, lignin is considered as the second most
DP fractions are soluble. In addition, cellulose is only soluble in
abundant renewable source and the largest source for aromatic
very specific solvents like aqueous solutions of copper (II) tetra
compounds on earth. It constitutes up to 10%–25% (wt.) of
hydroxide (Schweitzer’s reagent/Cuoxam). Additionally,
lignocellulosic feedstock.9 Valorisation of lignin is considered as
cellulose does not melt but decomposes starting from 180 °C.9
a challenge because of its amorphous and heterogeneous
1.1.2. Hemicellulose structure. Lignin has been applied in surfactants, stabilisers,
epoxy resins, superabsorbent hydrogels and in different
The second component of lignocellulosic biomass,
polymeric applications. Furthermore, lignin is a water-insoluble
hemicellulose, is a branched polysaccharide that consists of
material, stable in nature and connected to both cellulosic and
pentoses, hexoses and sugar acids such as xylose, glucose and
hemicellulosic layers. Lignin is a three- dimensional, highly
4–O–methylglucuronic acid, respectively9. The extraction
cross-linked macromolecule, which consists of three major
method and the species of the plant affect the composition and
units of substituted phenols: sinapyl, coniferyl and p–coumaryl
the structure of these sugars10. The main hemicelluloses of
alcohols as shown in Figures 5 and 6. Hardwood lignin contains
softwood are galactoglucomannans and
more than 90% of sinapyl alcohol with some coniferyl units. On
arabinoglucuronoxylan, while in hardwood glucuronoxylan and
the other hand, softwood lignin contains mainly coniferyl
glucomannan are the dominant species. Hemicelluloses are
alcohol units.14
partly actetylated. Figure 3 shows the principal structure of
hemicellulose in hardwood, whereas Figure 4 shows
hemicelluloses in softwood. In hardwood, the main component
of hemicellulose is xylose. One of the most important features
of hardwood xylan is that it lacks crystallinity due to the
branched structure and acetyl groups that are connected to the
polymeric chain.4
58
Today, Kraft and sulphite pulping are the most common

chemical processes used in paper making industry. Kraft pulping
is an alkaline process that uses sodium hydroxide and sodium
sulphide to delignify lignocellulosic biomass. For its part,
sulphite pulping is an acidic process that uses magnesium
bisulphide and excess of sulphur dioxide. Furthermore, sodium
sulphide is used in Kraft process to break the bonds that
connect lignin, cellulose and hemicellulose (enhance ether
cleavage) as well as to prevent unwanted condensation
reactions. As a result, strong and weak fibres are produced by
Kraft and sulphite processes, respectively. Despite the high
Figure 5. Building blocks of the three-dimensional polymer amounts of lignin extracted from both processes, they have
lignin. many disadvantages, such as the release of sulphide-containing
compounds and other volatile sulphur compounds.20
2.1. Organosolv delignification
Compared with the sulphite and Kraft processes, organosolv
pulping can be understood as an environmentally friendly
chemical process for the fractionation of lignocellulosic
biomass; in which delignification is achieved using organic
solvents (generally ethanol) mixed with water21-23. The
treatment of lignocellulosic biomass typically occurs at
temperatures in the range of 130–200 ˚C with or without
catalyst. Alcohols like ethanol, methanol, butanol, ethylene
glycol and glycerine are regularly used as solvents for the
delignification of lignocellulosic biomass. Moreover, mineral
acids such as HCl, H2SO4 and H3PO4 are used as catalysts to
enhance the delignification process24. Through the process, the
structure of lignin is fragmented into smaller fractions and
dissolved from the lignocellulosic biomass. The extract is
separated in the form of liquid phase, which is rich in phenolic
Figure 6. Suggested structural model of a section of hardwood compounds like dissolved lignin, vanillin, syringaldehyde,
lignin.13 phenol and substituted phenols but also contains part of the
dissolved and degraded hemicellulose.22, 23
There are many reported advantages of the organosolv process,
2. Delignification of lignocellulosic biomass
for example, the production of sulphur-free lignin; therefore, it
Delignification is the chemical separation of lignin from has a significantly lower effect on the environment. It can be
lignocellulosic biomass. Lignin needs to be separated from conducted at a smaller scale compared with the Kraft process,
lignocellulosic biomass to allow the production of high quality and resulting by-products might have a significant commercial
lignin and lignin-rich side streams. Pre-treatment methods have interest like furfural and lignin. The recovery of the used solvent
many goals, such as improving the accessibility of lignocellulosic is relatively easy (it could be used again for another pulping
biomass and easing enzymes’ work during enzymatic hydrolysis process) and less reagents are used during the process.25,22,26-28
to increase the yield of reducing sugars.15 2.2. Pre-hydrolysis
According to Wyman et al., there are many factors to consider
for the selection of a pre-treatment method16: avoiding the size Pre-hydrolysis is defined as the process in which the
reduction of biomass, avoiding the formation of degradation lignocellulosic biomass is treated by saturated steam or hot
products and inhibitory toxic by–products, allowing lignin water with or without the addition of small amounts of acids or
recovery, decreasing energy demands and using low cost alkali. Typically, pre-hydrolysis is used as a means to remove the
catalyst and/or low cost regeneration process for catalyst hemicellulose prior to Kraft pulping when dissolving pulp is
recovery. produced (i.e., the so-called pre-hydrolysis Kraft process,
Many physical and/or chemical pre-treatment methods to “PHK”). The principle of autohydrolysis or hydrothermolysis is
change the properties or fractionate lignocellulosic biomass applied in other processes like steam explosion. In this process,
have been suggested, including steam explosion, pre- most of hemicellulose can be recovered in the liquid phase as
hydrolysis, auto hydrolysis, enzymatic hydrolysis, kraft pulping, xylooligosaccharides and xylose without causing significant
sulphite pulping, organosolv, ammonia, aqueous lime, sodium dissolution of cellulose and lignin29. Lignin and cellulose remain
hydroxide, and wet oxidation15,17-19. A sequential combination in the solid phase and show enhanced capability to further
between two or more processes is possible for fractionation of fractionation30,31. Pre-hydrolysis is an effective, simple and
lignocellulosic biomass like the combination between pre- selective pre-treatment, which enables the production of
hydrolysis and organosolv delignification. soluble hemicellulose oligoscaccharides from the liquid phase
and allows high recovery of lignin and cellulose in the solid
59
phase. This is why pre-hydrolysis might be investigated as a first temperature and time were combined. It was employed
step in biorefinery concepts prior to the delignification successfully for describing the harshness of treatment
process32,33. Furthermore, pre-hydrolysis will reduce conditions of hot water treatment, steam treatment and steam-
carbohydrate loss for the subsequent processes35-37. On the explosion, and Kraft pulping. Moreover, it was used successfully
other hand, applying the organosolv delignification process in organosolv and liquefaction of the polymers that resulted
directly to the lignocellulosic biomass results a liquid stream from the treatment process.39
containing both lignin and hemicellulose that will require Lora et al.40 studied the effect of a two-stage process consisting
further purification and separation for lignin recovery.34 of autohydrolysis and organosolv delignification on aspen,
2.3. Why a sequential pre-hydrolysis and organosolv eucalyptus and fast-growing poplar wood in separated
process is suggested? experiments. All feedstocks were autohydrolysed at
temperatures between 175 and 225 °C for 13–21 min.
In an effort to overcome the drawbacks of concentrated and
Afterwards, the treated wood was delignified separately by
diluted acids, a two-stage process is proposed. A two-stage
extraction with 9:1 dioxane-water or 1% sodium hydroxide
concept can enhance the hydrolysis of hemicellulose by
solutions at mild conditions (soxhlet extraction, refluxing for 3–
diminishing the degradation into furans, which play a role as
4 h). The results showed that the maximum delignification was
enzyme and fermentation inhibitor. A pre–hydrolysis step
achieved at higher autohydrolysis temperatures and short
recovers the cellulose in a less recalcitrant form, thereby
times (i.e., 215 °C and 4 min). The main finding was that the
facilitating its reactivity to enzymes. If conducted under mild
solubility of lignin in dioxane-water and NaOH in the second
conditions, the lignin becomes very reactive towards
step manifested a clear maximum at certain autohydrolysis
subsequent delignification. This eases the development of a
conditions. Too severe and long autohydrolysis conditions led
process with lower energy costs through decreasing the process
to an important decrease in delignification, indicating the
temperature and lowers equipment costs by decreasing the
occurrence of lignin recondensation.
generated pressure in the subsequent organosolv step.
April et al.41 also tested a two-stage process of pre-hydrolysis
2.4. Overview of two-stage process combination and organosolv delignification of southern yellow pine wood.
including organosolv delignification This study was done in order to know the effect of pre-
Given the high number of factors affecting the two-stage hydrolysis on organosolv delignification by comparing the
fractionation process (e.g., temperature, residence time, results obtained through direct organosolv with those obtained
amount and type of catalyst, and the used solvent and its ratio) from delignification of pre-treated wood. Pine wood was pre-
as well as the wide range of lignocellulosic biomass sources hydrolysed at 180–250 °C for 0–3 h. Afterwards, the treated
(e.g., softwood, hardwood and agricultural residues), it is pine wood was delignified with 1:1 n-butanol: water solution at
expected to obtain three different fractions (lignin, cellulose temperatures of 180–250 °C for 30–180 min. The results
and hemicellulose) with different yields and properties. obtained from this study showed that the yield of pulp of pre-
Previous variables and their effects on the two-stage treated wood after delignification was the same than that
fractionation process were studied extensively. These variables obtained for the non-pretreated wood. Moreover, higher
were used to fractionate variety of lignocellulosic biomass and hydrolysis rate was obtained at higher temperatures, where the
their effects were evaluated according to the yield of lignin, the optimum conditions for pre-hydrolysis without significant
yield of sugars and the formation of degradation compounds degradation of hemicellulose were 200 °C and 105 min. The
during the process. In addition, analysing the effects of the two– lignin yield from treated pine wood increased with time at a
stage fractionation process on the characteristics of the constant temperature (180, 200 or 250 °C).
extracted lignin (e.g., the average molecular weight, Mw; the In another study, Patel et al.42 studied the effect of pre-soaking
number average of molecular weight, Mn; the dispersity index, with 1% H2O2 and the acidic pre-hydrolysis on organosolv
Mw/Mn; phenolic-hydroxyl and carboxyl groups contents, delignification of sugarcane bagasse. As a first step prior the
thermal properties, and sugar content) is also needed to pre-hydrolysis, sugarcane bagasse was soaked with 1% H2O2 in
explore additional applications of lignin. These include its use as order to increase the susceptibility of sugarcane bagasse to
a starting material in hydrothermal processes as used in the enzymatic hydrolysis. After that, the soaked material was pre-
production of chemicals and polymer industry. Therefore, hydrolysed in the presence of 0.1% H2SO4 at temperatures of
researchers were conducted several studies in this field and an 125–175 °C for 4 h. The treated wood was delignified with
overview of the conducted studies (grouped into categories 50 wt. % aqueous ethanol at 200 °C for 1–2 h. As a result, about
according to the studied variable) is given below. 41 wt. % of the hemicellulose was recovered without any
significant degradation of lignin and cellulose, and without the
2.4.1. The effect of temperature and residence time formation of degradation side products like furfural. Moreover,
Before the description of the effects “harshness” of the about 90 wt. % of the lignin was recovered and most of the
treatment conditions on the fractionation process and the final remaining hemicellulose without significant degradation of
fractions (lignin, cellulose and hemicellulose), we have to define cellulose.
a term/expression that it used to describe them. In other words, A similar study was conducted by Thring et al.43, who used a
treatment conditions are combined together in one expression two-stage process of pre-hydrolysis and organosolv to delignify
called the “reaction ordinate” to determine the hydrolysis populous deltoids. In terms of severity factor, populous deltoids
extent. The severity factor (R0) was the first reaction ordinate were aqueous–steam pre-hydrolysed at a severity factor of 3.8
that was developed by Overend et al.38, where both and the pretreated wood was delignified with ethylene glycol at
60
a severity factor of 6.2 (temperature range of 110– 170 °C).

Ethylene glycol was used to prevent the degradation of
cellulose during the organosolv process even at high
temperatures. As a result, the recovery of hemicellulose was 90
wt. % in the pre-hydrolysis step and the recovery of lignin was
20 wt. % from the original wood. Moreover, about 73 wt. % of
the lignin was recovered as acid insoluble lignin.
In another study, Hongzhang et al.44 investigated the
fractionation of wheat straw through a two–stage process of
steam explosion and organosolv delignification. Steam
explosion was done using the optimum conditions determined
by Chen et al.84 (pressure: 1.4 MPa, moisture: 34%, and
treatment time: 4.5 min). The treated wheat straw was
delignified with 20–70 wt. % of aqueous ethanol at 80–180°C
for 5–100 min in the presence of 0.1 wt. % of sodium hydroxide
as catalyst (see Figure 7). About 75 wt. % of lignin with high
purity was recovered by acid precipitation.
Figure 8. Schematic representation of sulphuric acid pre-

soaking and ethanol organosolv delignification (Brosse et al.46).
In order to enhance the efficiency of delignification process, El

hage et al.47 studied a two-stage process of catalysed
autohydrolysis and organosolv delignification of miscanthus
giganteus (see Figure 9). In this process, miscanthus was
autohydolysed at temperatures of 130–150°C, with/without 2–
naphthol (used to prevent lignin recondensation) and severity
of 4.1 for 0–43 h. The treated miscanthus was then delignified
with 80 wt. % of aqueous ethanol at 170 °C in the presence of
0.5 wt. % H2SO4 as catalyst for 60 min. It was found that the
higher temperature of autohydrolysis stage had a large effect
on the delignification process and the structure of resulting
Figure 7. Schematic representation of the fractionation lignin. Further, the used catalyst had a large impact on
process followed by Hangzhang et al.44 autohydrolysis, which led to enhanced delignification and
better sugar recovery.
Brosse et al.45 studied the effect of two-stage process of pre-
soaking with dilute acid and organosolv delignification of
miscanthus giganteus. The feedstock was pre-soaked with
0.9 wt. % of H2SO4 at 180 °C for 2 min. The treated miscanthus
was delignified using aqueous ethanol at temperatures ranging
from 170 to 190 °C for a holding time of 2–80 min, using H2SO4
as a catalyst to enhance the delignification process. Better
recovery of xylan was achieved with 71 wt. % of lignin being
recovered, since the pre-soaking step prior to the
delignification step enhanced the digestibility of resulted
cellulose for enzymes. In a related study, Brosse et al.46 studied
the same process under mild conditions. Thus, miscanthus was
pre-soaked using 0.9 wt. % H2SO4 at 100 °C overnight. The
treated material was then delignified with aqueous ethanol at
170–190°C for 2–10 min. As compared to the previous study,
pre-soaking step at lower severity enhanced the fractionation
of miscanthus with good yields and low amounts of degradation
compounds (like furans) were produced. Figure 8 shows a Figure 9. Schematic process followed by El hage et al.47
schematic representation of the process.
Ruiz et al. 48 applied a two-stage process of autohydrolysis and
organosolv delignification to fractionate wheat straw. In the
first stage, wheat straw was autohydrolysed at 180 °C for 30
min. The treated wheat straw was delignified with 40 wt. %
61
aqueous ethanol at different temperatures range from 180 to uncataylsed ethanol organosolv was inconsistent on the basis
200 °C in the presence of 0.1 wt. % sodium hydroxide catalyst of previous studies.
for 20–40 min. The conclusion was that both temperature and
time played a key role in lignin recovery. The maximum yield of
lignin after delignification step was 3.25 g lignin per 100 mL of
liquor. Moreover, about 78 wt. % of sugars were recovered as
xylooligoscaccharides. In a similar way, but using a different
lignocellulosic biomass, Romani et al.49 carried out a study of a
two-stage process consisting of autohydrolysis and uncatalysed
organanosolv delignification to fractionate eucalyptus globulus
wood as shown in Figure 10. The authydrolysis stage was
performed according to the parameters adopted by Romani et
al.85. Furthermore, organosolv delignification was carried out
using 48–72 wt. % of aqueous ethanol at 172–203 °C for a
holding time of 1 h. They found that using 60 wt. % of ethanol
without using any catalyst at 180–200 °C were the best Figure 11. Flow diagram representing the process followed by
conditions for the delignification process in terms of yield of Liu et al.50
lignin. Moreover, sugars from hemicellulose were recovered as
Another two-stage process of acidified pre-hydrolysis and
mono- and oligosaccharides.
organosolv delignification was carried out by Huijgen et al.52 to
fractionate wheat straw. The study was based on two steps: a
first one consisting of an acidified pre-hydrolysis to recover
sugars from hemicellulose, and a second one consisting of
organosolv delignification. Pre-hydrolysis stage was used to
prevent degradation of hemicellulose sugar during organosolv
delignification process. As shown in Figure 12, wheat straw was
pre-hydrolysed by acidified water at temperatures of
160–190 °C for 30 min. Likewise, delignification of treated
wheat straw was carried out using 60% aqueous ethanol at
temperatures of 190–220°C for 60 min. The pre-hydrolysis
stage enhanced the sugar yield and, at the same time, reduced
the yield of lignin, probably due to the formation of pseudo
lignin and lignin recondensation. In addition, Huijgen et al.
found that increasing the temperature of organosolv process
Figure 10. Schematic representation of autohydrolysis and
resulted in improved enzymatic hydrolysis, thus the glucose
ethanol organosolv delignification (Romani et al.49).
yield was about 93 wt. %. Furthermore, temperature could
Not only single lignocellulosic biomass was studied but also a partially reduce yield problem by increasing the temperature of
mixture of them was investigated. In the study by Liu et al.50, organosolv delignification.
the fractionation of a mixture of lignocellulosic biomass (which
was composed of maple, poplar and birch) using two-stage
process of pre-hydrolysis and ethanol extraction was executed.
As shown in Figure 11, the pre-hydrolysis stage was carried out
using steam at 170 °C for 30 min to separate hemicellulose.
Lignin was precipitated by acidifying pre-hydrolysis liquor using
polyethylene oxide and poly-aluminium chloride. It was found
that the two-stage process of pre-hydrolysis and ethanol
extraction led to the recovery of lignin, hemicellulose and pure
cellulose from this wood chips mixture.
One of the most attractive features of organosolv process or
derived two-stage process is that it can use any lignocellulosic
biomass. For instance, Amendola et al.51 used red grape stalks
in their study of a two- stage process of autohydrolysis and
uncatalysed organosolv delignification. In this study, red grape
stalks were autohydrolysed at 180 °C for 30 min. An uncatalysed Figure 12. Scheme representing pre-hydrolysis and organosolv
process of ethanol organosolv was used to recover lignin from delignification process (Huigen et al.52).
autohydrolysis grape stalks at 180 °C for 90 min. Autohydrolysis
successfully recovered hemicellulose as free sugars and
oligomers. Lignin recovery from pre-hydrolysed grape stalk by
62
Tunc et al 53 observed the effect of using organic acids like acetic Amiri et al.57 carried out a two-stage process of autohydrolysis
acid and formic acid to catalyse the autohydrolysis process and organosolv followed by enzymatic hydrolysis to delignify
during the organosolv delignification of pre-treated wood chips. and fractionate pine wood and elm wood. Woods were
A mixture of southern hardwoods chips were pretreated with autohydrolysed separately at 180 °C for 60 min. The treated
10 g L–1 of formic acid at 160 °C for 90 min. Then, the pretreated woods were delignified at 180 °C with 40 wt. % aqueous ethanol
wood chips were delignificated (without catalyst) with 50 wt. % for 60 min in the presence of 1% H2SO4. The two-stage process
aqueous ethanol at 160 °C for 90 min. Consequently, the severe played various vital roles in the composition of final treated
acid conditions in autohydrolysis stage increased the selectivity woods. The first stage (autohydrolysis) removed about
of lignin removal in organosolv stage. Thus, about 9.7 wt. % of 53%–61% of hemicellulose, whereas in the second stage
lignin was recovered from the original lignin in the wood chips (organosolv) the removal was lower than 23 wt. %. About 95 wt.
and selective sugar removal was achieved in the autohydroylsis % of the delignification was observed in the second stage
stage without lignin recondensation. Furthermore, Tunc et al. (organosolv). About 100–140 g of lignin was produced from
found that formic acid was more effective than acetic acid, due each 1 kg of wood.
to its higher acidity. Gurgel et al.58 carried out a similar process with the assistance
Vallejos et al.54 studied the two-stage process of hot water of CO2 (as a catalyst) organosolv step to delignify pre-
autohydrolysis and organosolv delignification of sugarcane hydrolysed sugarcane bagasse, which was pre-hydrolysed with
bagasse. The innovative aspect of this study was the use of a liquid hot water at 180 °C for 20 min. The treated bagasse was
relatively low liquid/solid ratio (with minimum consumption of then delignified with the assistance of high pressure CO2 and
hot water in the autohydrolysis step) to enhance the efficiency using 50% of aqueous ethanol at different temperatures in the
of organosolv delignification step. Autohydrolysis was carried range of 112–168 °C for 24–66 min. As a result, a lower lignin
out at 170 °C for 60 min using a liquid/solid ratio of 1:6. recovery and higher delignification at low temperatures (112–
Organosolv delignification was accomplished using 50 wt. % of 140 °C) was obtained compared with regular organosolv
aqueous ethanol at different temperatures from 160 to 190°C delignification process.
during 30–150 min. As a result, sugarcane bagasse was A recent study based on using a two-stage process of
fractionated into three main components (lignin, cellulose and autohydrolysis and glycerol organosolv delignification was
hemicellulose) with lower yield than that achieved using higher conducted by Meighan et al.59. Sugarcane bagasse was auto-
liquid/solid ratio. Similarly, in the study by Zhu et al.55, lignin hydrolysed at 176 °C during 49 min. The pretreated material
with good characteristics and chemical reactivity was obtained was delignified at temperature of 210 °C for 40 min with 80% of
through a two- stage process of hydrothermal autohydrolysis pure glycerol. The study showed a decrease in the
and organosolv delignification of Eucommia ulmoides Oliver. delignification rates as well as important changes in the lignin
Firstly, the feedstock was hydrothermally auto hydrolysed at structure, probably due to the increase in the severity of the
180 °C for 30 min after soaking with water. The second stage autohydrolysis step.
(organosolv delignification) was conducted using 50 % of More recently, Matsakas et al.60 conducted a study on birch
aqueous ethanol for 30 min in the presence of 1% of HCl as wood chips using a novel hybrid organosolv process, in which
catalyst. As a result, 14.5 g of lignin, 9.5 g of the inclusion of a steam explosion pre-treatment led to better
xlyooligosaccharides and 41.0 g of cellulose-rich residues were results than those reported by Nitsos et al.61, who utilised an
obtained from 100 g of feedstock. The resulting lignin had organosolv delignification process at higher temperature.
distinctive characteristics such as high purity, narrow Matsakas et al. evaluated the effect of reaction time, amount
polydispersity, low molecular weight, and high chemical of sulphuric acid and the percentage of aqueous ethanol on the
reactivity. fractionation process and the resulting product fractions. Wood
In a different study, Moniz et al.56 conducted experiments of chips were pre-treated with steam inside a steam explosion
autohydrolysis and mild organosolv to fractionate rice straw reactor and subsequently delignificated at 200 °C with different
into lignin and hemicellulose. At the first stage, rice straw was amounts of H2SO4 (0–1% wt. basis) and different aqueous
autohydrolysed at 195–220 °C with severity factors ranging ethanol percentages for various reaction times (15–60 min). As
from 3.66 to 4.36. Moniz et al. observed that the best a result, they achieved a delignification yield of 86.2% with high
temperature for autohydrolysis was 210 °C. For the second purity lignin and 87 wt. % of cellulose. Moreover, the remaining
stage, treated rice straw was delignificated at 30°C (nearly room pulp contained lower amounts of residual lignin, which had a
temperature) using different ratios of aqueous ethanol ranging positive effect on the enzymatic hydrolysis (increased cellulose
from 33.05 to 71.90 wt. % and holding times of 0–24 h. The digestibility). Definitely, it seems clear that the organosolv
temperature has a significant effect on the delignification as
optimised autohydrolysis at 210°C enhanced the digestibility of
well as on the residual lignin in pulp. However, it should be kept
cellulose residues to enzymes, produced liquid phase rich in
in mind that additional variables, such as the conditions at
pentoses (particularly in oligomeric form), and produced a solid
which the pre-treatment step is conducted, can also play an
phase rich in lignin and glucan. The mild organosolv of treated
important role.
rice straw allowed the production of low molecular weight
lignin in yields lower than those obtained from the regular
organosolv delignification process.
63
3. Enzymatic hydrolysis attack less ordered regions of the cellulose chain, which cause
a decrease in DP. On the other hand, exo-glucanases attack the
Regardless of the delignification process used (single-step or
ends of cellulose chains, leading to a release of cellobiose as a
multi-step process), cellulose is obtained in the form of
product with slight effect on the DP. Nevertheless, regardless of
cellulosic fibres or pulp. The recalcitrance of cellulose to
the substrate to be hydrolysed, the DP is associated with an
enzymatic hydrolysis is related to the diversity, heterogeneity
increased recalcitrance of crystalline cellulose, as illustrated in
and complexity of cellulosic biomass62. Due to this complexity
Figure 14.
and heterogeneity of cellulose substrates, the mechanism of
cellulose hydrolysis is not fully understood.
The main objective for most of pre-treatment processes for
lignocellulosic biomass is to separate hemicellulose or lignin.
The main purpose of any pre-treatment method is to obtain
high yields of products through sequential enzymatic hydrolysis
at minimum costs. Otherwise, formation of by-products or
degradation products from lignin or sugars (like furfural and its
derivatives, which inhibit the enzymatic hydrolysis) should be
minimised. As mentioned in the literature, the lignin content
and its distribution have an impact on the performance of the
enzymatic hydrolysis process63, 64. High rates of enzymatic
hydrolysis conversion of cellulose have been obtained from
extensively delignified wood. On the other hand, partial lignin
removal has resulted in decreased hydrolysis yields.65, 66
Amorphous cellulose is rapidly hydrolysed with the increase in Figure 14. The decrease in DP of cellulose with time during
the degree of crystallinity and thus, a typical time of hydrolysis enzymatic hydrolysis of phosphoric acid inflated cellulose
was suggested by Mansfield et al.67. Furthermore, the (PASC) by Trichoderma ressei cellulose complex
conventional time for enzymatic hydrolysis of lignocellulosic (adapted from 71).
biomass is characterised by a fast initial rate of hydrolysis
followed by a slower and insufficient hydrolysis rate, as shown Another important characteristic that prompts enzymatic
in Figure 13. hydrolysis is the accessible surface area of the substrate.
Usually, accessible surface area is measured using the BET
(Bennet–Emmit–Teller) method from N2 adsorption isotherms
at 77 K72. As reported in the literature, this method has many
defects that include the drying of the substrate, which does not
allow measurements in inflated state of substrate.
Consequently, specific surface area (SSA) can be overestimated,
since the relatively small nitrogen molecules have the ability to
access to pores and cavities on the substrate surface that
enzymes cannot enter73. Therefore, enzyme molecules
attached to cellulose surface cover a number of glucose units,
as long as the hydrolysis reaction proceeds, only on one β-
glucosidic bond (see Figure 15).
Figure 13. Typical time course of the enzymatic hydrolysis of
lignocellulosic biomass (adapted from Mansfield et al.67).
According to Fan et al. 68, the rate of enzymatic hydrolysis of

lignocellulosic biomass is strongly affected by structural
characteristics of cellulose; for instance: degree of
polymerisation, available surface area, crystallinity of cellulose,
structural organisation, and the presence of other materials like
hemicellulose and lignin. Furthermore, Fan et al.68, 69 suggested
that the most important factor affecting the hydrolysis rate is
the crystallinity of substrate, while other researchers suggested Figure 15. Representative drawing of enzyme-catalysed
that the degree of crystallinity has no effect on the hydrolysis reaction on insoluble substrate.73
rate.70
Moreover, the degree of polymerisation (DP) is identified as the Both the shape and particle size of any substrate are closely
number of glycosyl residues per cellulose chain. The effect of DP connected to the external surface area. Accordingly, more
on the hydrolysis rate is linked to the other characteristics of adsorption positions per mass of substrate could be achieved in
substrate, like crystallinity. Zhang et al.71 argued that the case of high surface area to weight of substrate ratio. Therefore,
depolymerisation is mostly a function of the nature of the any pre-treatment method of lignocellulosic biomass usually
cellulosic substrate being attacked. Endo–glucanases prefer to involves reduction in size by cutting the lignocellulosic biomass
64
substrate, which results in an increased SSA. Additionally, pre- A general scheme of the proposed approach is shown in Figure
treatment methods include the removal of both lignin and 16. Several parameters have to be evaluated and compared on
hemicellulose that causes extended changes in the structure the basis of pulp yield, acid insoluble lignin content,
and accessibility of cellulose. Moreover, removing lignin and carbohydrate hydrolysis, and lignin yield. Moreover,
hemicellulose facilitates enzyme attack.70 hemicelluloses will be separated in the process prior to the
3.1. Hydrolysis Lignin organosolv delignification as xylan, while lignin will be
separated through the organosolv process using a mixture of
Lignocellulosic biomass is generally pre-treated in order to
ethanol and water. Afterwards, solid residues will be further
produce lignin, cellulose fibres and hemicellulose. Moreover,
processed through enzymatic hydrolysis to produce hydrolysis
these products are further processed using different methods.
lignin and enhance the production of glucose.
For instance, lignin is used for the production of phenolic
compounds, vanillin, phenol derivatives, etc.; whereas
hemicellulose is used to produce xylose. In addition, cellulose
fibres are enzymatically hydrolysed in order to enhance the
production of glucose and generate hydrolysis lignin as by-
product.
The enzymatic hydrolysis is a heterogeneous reaction; thus, the
first step in cellulose hydrolysis is the attachment of the enzyme
molecules to the substrate by adsorption. Cellulose is converted
to cellobiose by the bounding part of endo-glucanase and exo-
glucanase. Otherwise, cellobiose is converted into glucose by
the unbound part of β-glycosidase. As a result, the bound part
of both endo- and exo-glucanase is responsible for cellobiose
formation and the unbounding part of β-glycosidase is
responsible for glucose production74. After finishing the Figure 16. Scheme of the two-stage fractionation process
adsorption step, hydrolysis reaction occurs. considered in future work.
The negative effect of lignin on enzymatic hydrolysis of cellulose
substrates has been reported in different ways: 1) lignin inhibits
the accessibility of cellulose to the enzyme by acting as a 5. Conclusions
barrier; and 2) lignin absorbs cellulose via hydrophobic, ionic
A two-stage process is a promising pathway for the
and hydrogen interactions and thus, cellulose becomes less
delignification and fractionation of lignocellulosic biomass
available for enzymatic hydrolysis75–77. Moreover, recent
into lignin, cellulose and hemicellulose with feasible yields
studies stated that lignin content, composition of functional
and distinctive characteristics of final products. However,
groups of lignin and physical distribution could affect the
the following points have to be considered:
enzymatic hydrolysis.78, 79
On the other hand, several studies have been conducted using • Lignin can be degraded or hydrolysed during the
different types of enzymes. These studies assessed the auto-hydrolysis or pre-hydroylsis step and its
influence of degradation products from lignin (e.g., furfural and chemical structure can change, causing the
its derivatives) on the inhibition or deactivation of enzyme generation of phenolic fractions in the reaction
activity80, 81. Nevertheless, Ko et al argued that the biomass type medium.
and/or the pre-treatment method played a vital role in the • It has been reported in previous studies that the
characteristics of lignin, which affect enzyme adsorption82. For auto-hydrolysis or pre-hydrolysis of the
example, lignin extracted from hardwood at higher harshness lignocellulosic biomass is generally conduced at
conditions resulted in a severe inhibition on the hydrolysis of 170–200 °C (near the glass transition temperature
cellulose83. Furthermore, a negative effect was also observed of lignin)83,89. Thus, the possibility of lignin
on the hydrolysis of cellulose from softwood, which was recondensation could be relatively high, leading
previously pre-treated at harshness conditions.80 to a lignin in viscous state.
• Lignin droplets will be formed due to the
precipitation of high molecular weight lignin
4 Future work fragments on the surface of fibres during the
From analysing the previous studies available on the two-stage cooling of the reactor.89
fractionation process, the GreenCarbon’s Early-Stage • Residual lignin in remaining pulp after two-stage
Researcher #5 (Q. Ibrahim) plan to develop and optimise a fractionation process can reduce the efficiency of
process of two sequential steps including organosolv the enzymatic hydrolysis process. Lignin can act as
delignification of beech wood at both lab and pilot scales. The a physical barrier and inhibit the access of
effect of sequential processes for delignification of enzymes to cellulose cell wall.83
lignocellulosic biomass on the cellulose digestibility will be
evaluated by enzymatic hydrolysis of the remaining solids
(pulp).
65
• The difficulty of lignin removal after pre- (12) Sjostrom, E.; Wood Chemistry: Fundamental and
hydrolysis or auto-hydroylsis can be explained by Applications; Academic press: Sand Diego, 1993.
a certain lignin repolymerisation, which also leads (13) Qiu. W. H. ; Chen, H. Z. Structure, Function and
to the formation of carbonium ions intermediate, Higher Value Application of Lignin. Cellul. Sci.
thereby, promoting the formation of new Technol. 2006, 14, 52–59.
carbon–carbon bonds like ß–ß, ß–1 and ß–5.87, 88 (14) Brinchi, L.; Cotana, F.; Fortunati, E.; Kenny, J. M.
• The resulted fractions could be used for further Production of nanocrystalline cellulose from
applications or processes; for example, cellulose lignocellulosic biomass: technology and
fibres for enzymatic hydrolysis, lignin for applications. Carbohydr. Polym. 2013, 94,154–169.
hydrothermal processes and chemicals (15) Alvira, P.; Pejo, T.E.; Ballesteros, M.; Negro, M. J.
production, and sugars for biofuel production. Pretreatment technologies for an efficient
• Alternative pre-treatment methods (e.g., fungal bioethanol production process based on
and enzymatic pre-treatments) prior organosolv enzymatic hydrolysis: A review. Bioresour.
delignification could be used to avoid the Technol. 2010, 101, 4851–4861.
problems related to the chemical pre-treatment (16) Wyman, C. E. Biomass Ethanol : Technical Progress,
methods. Opportunities, and Commercial Challenges. Ann.
Rev. Energy. Environ. 1999, 24, 159–226.
(17) Carvalheiro, F.; Duarte, L. C.; Girio, F. M.
Acknowledgements Hemicellulose biorefineries: a review on biomass
This project has received funding from the European pretreatments. J. Sci. Ind. Res. 2008, 67, 849–864.
Union’s Horizon 2020 research and innovation programme (18) Hendriks, A. T. W. M.; Zeeman, G. Pretreatments
under the Marie Skłodowska–Curie grant agreement No to enhance the digestibility of lignocellulosic
721991. biomass. Bioresour. Technol. 2008, 100, 10–18.
(19) Taherzadeh, M. J.; Karimi, K. Advances in
lignocellulosic biotechnology: A brief review on
References
lignocellulosic biomass and cellulases. Bioresour.
(1) Fulton, L.; Jowes, T.; Hardy, J. Biofuels for Technol. 2008, 9, 1621–1651.
transport: An International Perspective; (20) Viikari, L.; Suurnäkki, A.; Grönquist, S.; Raaska, L.;
International Energy Agency: Paris, 2004. Ragauska, A. Forest Products: Biotechnology in
(2) Sun, Y.; Cheng, J. Hydrolysis of lignocellulosic Pulp and Paper Processing; Elsevier: Helsinki, 2009.
materials for ethanol production: a review. (21) Aziz, S.; Sarkanen, K. Organosolv pulping—a
Bioresour. Technol. 2002, 83, 1–11. review. Tappi J. 1989, 72, 169–175.
(3) Zugenmaier, P. Conformation and packing of (22) Young, R. A.; Akhtar, M. Environmentally Friendly
various crystalline cellulose fibers. Prog. Polym. Technology for the Pulp and Paper Industry; Wiley:
Sci. 2001, 26, 1341–1417. NY, 1998.
(4) Otmer, K. Encylopedia of chemical technology; (23) Shirkolaee, Y. Z.; Rovshandeh, J. M.; Charani, P. R.;
John wiley & Sons: New Jersey, 2001. Khajeheian, M. B. Study on cellulose degradation
(5) Faulon, J. L.; Carlson, G. A.; Hatcher, P. G. A three– during organosolv delignification of wheat straw
dimensional model for lignocellulose from and evalmation of pulp properties. Iran. Polym. J.
gymnospermous wood. Org. Geochem. 1994, 21, 2007, 16, 83–96.
1169–1179. (24) Sun, Y.; Cheng, J. Hydrolysis of lignocellulosic
(6) Harmsen, P. F. H.; Huijen, W. J. J.; Lopez, B. L, M.; materials for ethanol production: a review.
Bakker, R. R. C. Literature Review of Physical and Bioresour. Technol. 2002, 83, 1–11.
Chemical Processes for Lignocellulosic Biomass, (25) Garrote, G.; Eugenio, M.E.; Diaz, M. J. Ariza, J.;
Biosynergy project, 2010. Lopez, F. Hydrothermal and pulp processing of
(7) Festucci–Buselli, R. A.; Oroni, W. C.; Joshi, C. P. Eucalyptus. Bioresour. Technol. 2003, 88, 61–68.
Structure, organization, and functions of cellulose (26) Gilarrahz, M. A.; Oliet, M.; Rodriguez, F.; Tijero, J.
synthase complexes in higher plants. Braz. J. Plant Ethanol–Water Pulping: Cooking Variables
Physiol. 2007, 19, 1–13. Optimization. Can. J. Chem. Eng. 1998, 76, 253–
(8) Krassig, H.; Schrz, J.; Ullmann’s Encyclopaedia of 260.
Industrial Chemistry; Wiley–VCH: Weinheim, 2002. (27) Jimenez, L.; Garcia, J. C.; Perez, I., Ariza, J.; Lopez,
(9) Timell, T. E. Wood Hemicelluloses: Part I. Adv. F. Acetone Pulping of Wheat Straw. Influence of
Carbohydrate Chem. 1964, 19, 247–302. the Cooking and Beating Conditions on the
(10) ThermoWood Handbook; International Resulting Paper Sheets. Ind. Eng. Chem. Res. 2001,
ThermoWood Association: Helsinki, 2003. 40, 6201–6206.
(11) Ban, L.; Chai, X.; Guo, J.; Ban, W.; Lucia, L. A. (28) Gargulak, J.; Lebo, S. In Lignin: Historical, Biological
Chemical Response of Hardwood Oligosaccharides and materials prespectives; American Chemical
as a Statistical Function of Isolation Protocol. J. Society: Washington DC, 2000.
Agric. Food Chem. 2008, 56, 2953–2959.
66
(29) Garrote, G.; Dominoguez, H.; Parajo, J. C. (43) Thring, R. W.; Chornet, E. Fractionation of
Manufacture of xylose-based fermentation media woodmeal by prehydrolysis and thermal
from corncobs by posthydrolysis of autohydrolysis organosolv. Process strategy, recovery of
liquors. Appl. Biochem. Biotechnol. 2001, 95, 195– constituents, and solvent fractionation of lignins
207. co-produced. Can. J. Chem. Eng. 1993, 71, 116–
(30) Kim, Y.; Mosier, N.S.; Ladisch, M. R. Enzymatic 123.
digestion of liquid hot water pretreated hybrid (44) Hongzhang, C.; Liying, L. Unpolluted fractionation
poplar. Biotechnol. Prog. 2009, 25, 340–348. of wheat straw by steam explosion and ethanol
(31) Laser, M.; Schulamn, D. Allen, S. G.; Lichwa, J.; extraction. Bioresour. Technol. 2007, 98, 666–676.
Antal, J. M.; Lynd, I. R. A comparison of liquid hot (45) Bross, N.; Sannigrahi, P.; Ragauskas, A.
water and steam pretreatments of sugar cane Pretreatment of Miscanthus x giganteus using the
bagasse for bioconversion to ethanol. Bioresour. Ethanol Organosolv Process for Ethanol
Technol. 2002, 81, 33–44. Production. Ind. Eng. Chem. Res. 2009, 48, 8328–
(32) Carvalheiro, F.; Silva, F. T.; Duarte, L. C.; Girio, F. M. 8334.
Wheat straw autohydrolysis: process optimization (46) Brosse, N.; El Hage, R.; Sannigrahi, P.; Ragauskas,
and products characterization. Appl. Biochem. A. Dilute sulphuric acid and Ethanol Organosolv
Biotechnol. 2009, 153, 84–93. Pretreatmnet of Miscanthus x Giganteus. Cellulose
(33) Moniz, P.; Pereira, H.; Quilho, T.; Carvalheiro, F. Chem. Technol. 2010, 44, 71–78.
Characterisation and hydrothermal processing of (47) El Hage, R.; Chrusciel, L.; Desharnais, L.; Brosse, N.
corn straw towards the selective fractionation of Effect of autohydrolysis of Miscanthus x giganteus
hemicelluloses. Ind. Crop. Prod. 2013, 50, 145–153. on lignin structure and organosolv delignification.
(34) Harmsen, P.; Huijgen, W.; Bermudez, L.; Bakker, R. Bioresour. Technol. 2010, 101, 9321–9329.
Literature review of physical and chemical (48) Ruiz, H. A.; Ruzene, D. S.; Silva, D. P.; da Silva, F. F.
pretreatment processes for lignocellulosic M.; Vicente, A. A.; Teixeira, J. A. Development and
biomass. Bioenergy project, Report 1184; 2010. characterization of an environmentally friendly
(35) Huijgen, W. J. J.; Reith, J.H.; den Uil, H. process sequence (autohydrolysis and organosolv)
Pretreatment and Fractionation of Wheat Straw by for wheat straw delignification. Appl. Biochem.
an Acetone–Based Organosolv Process. Ind. Eng. Biotechnol. 2011, 164, 629–641.
Chem. Res. 2010, 49, 10132–10140. (49) Romani, A.; Garrote, G.; Lopez, F.; Parajo, J.C.
(36) Bozell, J. J.; Black, S.K.; Myers, M.; Cahill, D.; Miller, Eucalyptus globulus wood fractionation by
W. P.; Park, S. Solvent fractionation of renewable autohydrolysis and organosolv delignification.
woody feedstocks: organosolv generation of Bioresour. Technol. 2011, 102, 5896–5904.
biorefinery process streams for the production of (50) Liu, Z.; Fatehi, P.; Jahan, M. S.; Ni, Y. Separation of
biobased chemicals. Biomass Bioenergy 2011, 35, lignocellulosic materials by combined processes of
4197–4208. pre–hydrolysis and ethanol extraction. Bioresour.
(37) Toledano, A.; Serrano, L.; Balu, A. M.; Luque, R.; Technol. 2011, 102, 1264–1269.
Pineda, A.; Labidi, J. Fractionation of organosolv (51) Amendola, D.; De Faveri, D. M.; Egües, I.; Labidi, J.;
lignin from olive tree clippings and its valorization Spingo, G. Autohydrolysis and organosolv process
to simple phenolic compounds. ChemSusChem for recovery of hemicellulose, phenolic
2013, 6, 529–536. compounds and lignin from grape stalks.
(38) Overend, R. P.; Chornet, E.; Gascoigne, J.A. Bioresour. Technol. 2012, 107, 267–274.
Fractionation of lignocellulosics by steam– (52) Huijgen, W. J. J.; Smit, A. T.; de Wild, P. J.; den Uil,
aqueous pretreatments. Phil. Trans. R. Soc. 1987, H. Fractionation of Wheat straw by prehydrolysis,
321, 523–536. organosolv delignification and enzymatic
(39) Chornet, E.; Overend, R. P. In: Steam Explosion hydrolysis for production of sugars and lignin.
Techniques: Fundamentals and Industrial Bioresour. Technol. 2012, 114, 389–398.
Applications. Proceedings of the International (53) Tunc, M. S.; Chheda, J.; van der Heide, E.; Morris,
Workshop on Steam Explosion Techniques. CRC J.; Heiningen, A. Two. Stage fractionation of
Press, Milan: 1988, p. 21–58. hardwoods. Bioresour. 2013, 8, 4380–4395.
(40) Lora, J. H.; Wayman, H. Delignification of (54) Vallejos, M. E.; Zambon, M. D.; Area, M. C.; da
hardwoods by autohydrolysis and extraction. Silva-Curvelo, A. A. Low liquid–solid ratio
Tappi J. 1978, 61, 47–50. fractionation of sugarcane bagasse by hot water
(41) April, G. C.; Bharoocha, R.; Sheng, J.; Hansen, S. autohydrolysis and organosolv delignification Ind.
Prehydrolysis achieves higher organosolv Crops. Prod. 2015, 65, 349–353.
delignification. Tappi. J. 1982, 65, 41–44.
(42) Patel, D.P.; Varshney, A.K. The effect of presoaking
and prehydrolysis on organosolv delignification of
bagasse. Indian J. Technol. 1989, 27, 285–288.
67
(55) Zhu, M. Q.; Wen, J. L.; Su, Y. Q.; Wie, Q.; Sun, R. C. glucosidase activity on the inhibition pattern.
Effect of structural changes of lignin during Biotechnol. Bioeng. 1992, 40, 663–671.
autohydrolysis and organosolv pretreatment on (66) Ooshima, H.; Burns, D. S.; Converse, A. O.
Eucommia ulmoides Oliver for an effective Adsorption of cellulase from Trichoderma reesei
enzymatic hydrolysis. Bioresour. Technol. 2015, on cellulose and lignacious residue in wood
185, 378–385. pretreated by dilute sulfuric acid with explosive
(56) Moniz, P.; Lino, J.; Duarte, L. C.; Roserio, L. B.; decompression. Biotechnol. Bioeng. 1990, 36,
Boeriu, C. G.; Pereira, H.; Carvalheiro, F. 446–452.
Fractionation of hemicelluloses and lignin from (67) Mansfield, S. D.; Mooney, C.; Saddler, J. N.
rice straw by combining autohydrolysis and Substrate and Enzyme Characteristics that Limit
optimised mild organosolv delignification. Cellulose Hydrolysis. Biotechnol. Prog. 1999, 15,
Bioresour. 2015, 10, 2626–2641. 804–916.
(57) Amiri, H.; Karimi, K. Integration of Autohydrolysis (68) Fan, L. T.; Lee, Y. H.; Beardmore, D. R. The influence
and Organosolv Delignification for Efficient of major structural features of cellulose on rate of
Acetone, Butanol, and Ethanol Production and enzymatic hydrolysis. Biotechnol. Bioeng. 1981,
Lignin Recovery. Ind. Eng. Chem. Res. 2016, 55, 23, 419–424.
4836–4845. (69) Fan, L. T.; Lee, Y. H.; Beardmore, D.R. Mechanism
(58) Gurgel, L. V. A.; Pimenta, M. T. B.; da Silva-Curvelo, of the enzymatic hydrolysis of cellulose: Effects of
A. A. Ethanol-water organosolv delignification of major structural features of cellulose on enzymatic
liquid hot water (LHW) pretreated sugarcane hydrolysis. Biotechnol. Bioeng. 1980, 22, 177–199.
bagasse enhanced by high-pressure carbon (70) Grethlein, H. E.; Allen, D. C.; Converse, A. O. A
dioxide (HP–CO2). Ind. Crops. Prod. 2016, 94, 942– comparative study of the enzymatic hydrolysis of
950. acid pretreated white pine and mixed hardwood.
(59) Meighan, B. N.; Lima, D. R. S.; Cardoso, W. J.; Biotechnol. Bioeng. 1984, 26, 1498–1505.
Baeta, B. E. L.; Adarme, O. F. H.; Santucci, B. S.; (71) Zhang Y.-H. P.; Lynd, L. R. Toward an aggregated
Pimenta, M. T. B.; de Aquino, S. F.; Gurgel, L. V. A. understanding of enzymatic hydrolysis of
Two-stage fractionation of sugarcane bagasse by cellulose: noncomplexed cellulose systems.
autohydrolysis and glycerol organosolv Biotechnol. Bioeng. 2004, 88, 797–824.
delignification in a lignocellulosic biorefinery (72) Masamune, S.; Smith, J. M. Adsorption rate
concept. Ind. Crops. Prod. 2017, 108, 431–441. studies—significance of pore diffusion. AIChE J.
(60) Matsakas, L.; Nitsos, C.; Raghavendran, V.; 1964, 10, 246–252.
Yakimenko, O.; Persson, G.; Olson, E.; Rova, U.; (73) Brown, R. F.; Holtzapple, M. T. A comparison of the
Olsson, L.; Christakopoulos, P. A novel hybrid Michaelis–Menten and HCH-1 models. Biotechnol.
organosolv: steam explosion method for the Bioeng. 1990, 36, 1151–1154.
efficient fractionation and pretreatment of birch (74) Fenila, F.; Shastri, Y. Optimal control of enzymatic
biomass. Biotechnol. Biofuels 2018, 11, 1–14. hydrolysis of lignocellulosic biomass. Resour.-Effic.
(61) Matsakas, L.; Nitsos, C.; Raghavendran, V.; Technol. 2016, 2, S96–S104.
Yakimenko, O.; Persson, G.; Olson, E.; Rova, U.; (75) Zhao, X.; Zhang, J.; Liu, D. Biomass Recalcitrance
Olsson, L.; Christakopoulos, P. Isolation and Part I: The Chemical Compositions and Physical
Characterization of Organosolv and Alkaline Structures Affecting the Enzymatic Hydrolysis of
Lignins from Hardwood and Softwood Biomass. Lignocellulose. Biofuels, Bioprod. Biorefin. 2011,
ACS Sustain. Chem. Eng. 2016, 4, 5181–5193. 6, 465–482.
(62) Arantes, V.; Gourlay, K.; Saddler, J.N. The (76) Berlin, A.; Balakshin, M.; Gilkes, N.; Kadla, J.;
enzymatic hydrolysis of pretreated pulp fibers Maximenko, V.; Kubo, S.; Saddler, J. J. Inhibition of
predominantly involves “peeling/erosion” modes cellulase, xylanase and ß–glucosidase activities by
of action. Biotechnol. Biofuels 2014, 7, 1–10. softwood lignin preparations. J. Biotechnol. 2006,
(63) Vinzant, T. B.; Ehrman, C. I.; Himmel, M. E. 125, 198–209.
Simultaneous saccharification and fermentation of (77) Pan, X. Role of Functional Groups in Lignin
pretreated hardwoods: effect of native lignin Inhibition of Enzymatic Hydrolysis of Cellulose to
content. Appl. Biochem. Biotechnol. 1997, 62, 97– Glucose J. Biobased Mater. Bioenergy 2008, 2, 25–
102. 32.
(64) Mooney, C. A.; Mansfield, S. H.; Touhy, M. G.; (78) Nakagame, S.; Chandra, R. P.; Kadla, J. F.; Saddler,
Saddler, J. N. The effect of initial pore size and J. N. The isolation, characterization and effect of
lignin content on the enzymatic hydrolysis of lignin isolated from steam pretreated Douglas–fir
softwood. Bioresour. Technol. 1998, 64, 113–119. on the enzymatic hydrolysis of cellulose. Bioresour.
(65) Gusakov, A. V.; Sinitsyn, A. P. A theoretical analysis Technol. 2011, 102, 4507–4517.
of cellulose product inhibition: effect of cellulose (79) Yuan, T. Q.; Wang, W.; Zhang, L. M.; Xu, F.; Sun, R.
binding constant, enzyme/substrate ratio, and β- C. Reconstitution of cellulose and lignin after
[C2mim][OAc] pretreatment and its relation to
68
enzymatic hydrolysis. Biotechnol. Bioeng. 2013,

110, 729–736.
(80) Ximenes, E.; Kim,Y.; Mosier, N.; Dien, B.; Ladlisch,
M. Inhibition of cellulases by phenols. Enzyme
Microb. Technol. 2010, 46, 170–176.
(81) Ximenes, E.; Kim,Y.; Mosier, N.; Dien, B.; Ladlisch,
M. Deactivation of cellulases by phenols. Enzyme
Microb. Technol. 2011, 48, 54–60.
(82) Ko, J. K.; Kim, Y.; Ximenes, E.; Ladisch, M.
Adsorption of enzyme onto lignins of liquid hot
water pretreated hardwoods. Biotechnol. Bioeng.
2015, 112, 447–456.
(83) Ko, J. K.; Kim, Y.; Ximenes, E.; Ladisch, M. Effect of
liquid hot water pretreatment severity on
properties of hardwood lignin and enzymatic
hydrolysis of cellulose. Biotechnol. Bioeng. 2015,
112, 252–262.
(84) Chen, H. Z.; Chen, J. Z.; Liu, J.; Li, Z. H. Studies on
steam explosion of wheat straw I. Effects of the
operating conditions for steam explosion of wheat
straw and analysis of the process. J. Cellulose Sci.
Technol. 1999, 7, 60–67.
(85) Romani, A.; Garrote, G.; Alonso, J.L.; Parajo, J. C.
Experimental Assessment on the Enzymatic
Hydrolysis of Hydrothermally Pretreated
Eucalyptus globulus Wood. Ind. Eng. Chem. Res.
2010, 49, 4653–4663.
(86) Zhang, Y.; Qin, M.; Xu, W.; Fu, Y.; Wang, Z.; Li, Z.;
Willför, S.; Xu, C.; Hou Q. Structural changes of
bamboo–derived lignin in an integrated process of
autohydrolysis and formic acid inducing rapid
delignification. Ind. Crops. Prod. 2018, 115, 194–
201.
(87) Leschinsky, M.; Zuckerstatter, G.; Weber, H.K.;
Patt, R.; Sixta, H. Effect of autohydrolysis of
Eucalyptus globulus wood on lignin structure. Part
1: Comparison of different lignin fractions formed
during water prehydrolysis. Holzforschung. 2008,
62, 645–652.
(88) Leschinsky, M.; Zuckerstatter, G.; Weber, H.K.;
Patt, R.; Sixta, H. Effect of autohydrolysis of
Eucalyptus globulus wood on lignin structure. Part
2: influence of autohydrolysis intensity.
Holzforschung. 2008 62, 653–658.
(89) Zhuang, X.; Yu, Q.; Yuan, Z.; Kong, X.; Qi, W. Effect
of hydrothermal pretreatment of sugarcane
bagasse on enzymatic digestibility. J. Chem.
Technol. Biotechnol. 2015, 90, 1515–1520.
69
70
Review of the conversion from hydrochar to activated carbon

Pablo J. Arauzoa, Maciej P. Olszewskia, Michela Lucianb, Catalina Rodríguez-Correaa, Yvonne Ringelspachera, Andrea
Krusea, Luca Fiorib
a
Department of Conversion Technologies of Biobased Resources, Institute of Agricultural Engineering, University of
Hohenheim, Garbenstrasse 9, 70599 Stuttgart, Germany
b
Department of Civil, Environmental and Mechanical Engineering, University of Trento, Via Mesiano 77, 38123 Trento,
Italy
Abstract
Conversion of agricultural wastes using biogas plants is a possibility for the production of renewable energy. The residues produced
after biogas treatment (i.e., digestates) can be used for the production of activated carbon. On the other hand, hydrothermal
carbonisation (HTC) is considered a pretreatment for the conversion of initial wet biomass into carbonaceous material called hydrochar.
The aim of this chapter is to show the different parameters that influence the conversion from the initial biogas residues to hydrochars
as well as the subsequent activation of hydrochars.
investigated; however, there is not enough data about the

1. Introduction influence of catalysts and starting pH.11
It is known that the overall reaction rate of HTC is improved at
Biogas is an important resource for renewable heat, electricity
weakly acidic conditions. During HTC, organic acids, such as
and fuel generation. A large variety of biomass (for instance,
levulinic acid, formic acid and acetic acid, are formed, exhibiting
agricultural or forestry resources, sewage, wastes, such as
autocatalytic effects on the reaction. Acid addition is used to
municipal solid wastes, industrial and household wastes) can be
enhance the reaction, while the type of acid used plays an
used as raw materials, giving the possibility of organic waste
important role12. Nevertheless, there is few literature data
disposal in biogas plants1. Due to benefits of biogas production,
discussing the role of the type of acid. Reiche et al.13 proved an
the number of installed biogas plants in Europe is increasing.
oxidising effect of nitric acid during HTC of glucose at low pH
Biogas is produced by anaerobic digestion of biomass, at which
ranges (pH ≤ 1.5). Lynam et al.14 showed that acetic acid
an additional residual product, referred to as digestate, is
addition during HTC of loblolly pine led to a shift in reaction
formed.
equilibrium, resulting in a lower production of acetic acid. The
The large amount of generated digestate must be disposed, or
impact of acid addition depends on the type of feedstock and
better to find applications, to avoid constraints in the
acid addition does not always show catalytic effects.15
development of biogas production. Until now, digestate is
mainly used as fertiliser and soil conditioner. It provides a good
source of available nutrients (especially nitrogen and
2. Characterisation of activated carbon (AC)
phosphorus) and has positive effects on the biological
properties of soil; however, its use in agriculture is restricted, Due to their high adsorptive capacity, activated carbons are
since it needs to fulfil certain quality standards (for instance in widely used in chemical, food and pharmaceutical industries for
hygiene and biodegradability). Additionally, an improper use purification and separation processes and in catalysis. Typical
can lead to ecological problems, such as metal accumulation in applications are, for instance: water purification, gas storage or
soil (in particular Cu and Zn) or soil salinization.2,3 separation16. Recently, ACs are also used as electrode materials
Recent studies have reported the generation of highly in supercapacitors5,17 or Li-S battery cells.18
adsorptive activated carbons by hydrothermal carbonisation The applicability of an activated carbon depends on its
(HTC) of lignocellulosic biomass and subsequent activation. adsorptive characteristics, which are on the one hand
These activated carbons can, for instance, be applied in attributed to its textural properties and porous structure and
supercapacitors, for selective gas storage or as catalysts4-9. on the other hand, to functional groups bonded on the carbon
Since digestate also contains lignocellulosic compounds, the surface. Besides carbon, AC also contains small amounts of
production of activated carbon provides an alternative oxygen, hydrogen, nitrogen and sulphur, contributing to the
utilisation.10 adsorptive properties. Therefore, pore structure analysis (such
During HTC, a raw material (in principle any type of biomass) is as determination of pore size distribution, surface area and
heated in presence of liquid water at elevated temperatures pore volume) and analysis of functional surface groups are
(usually 180–250 °C) under autogenous pressure, whereupon a important to characterise ACs.
solid carbon-rich material, referred to as hydrochar, is
generated. The process conditions of HTC, such as temperature, 2.1. Adsorption capacity of model substances
residence time and starting pH influence the characteristics of To determine the adsorption capacity of AC, the adsorption of
both the hydrochar and subsequent activated carbon. The model substances, typically methylene blue, iodine and phenol
effects of temperature and residence time are well can be tested, giving also an indication of present pore sizes in
the AC. Methylene blue and iodine adsorption is usually used to

71
Chapter 6 Arauzo et al.
study mesopores or large micropores; the methylene blue when the pore width exceeds a certain critical width (about
molecule is accessible to pores with diameters larger than 1.5 4 nm for N2 adsorption at 77 K) and pore condensation takes
nm, while the iodine molecule can be adsorbed in pores larger place at a pressure less than the saturation value. Types of
than 1 nm. Phenol can be used for micropore analysis, since it hysteresis loops are classified by IUPAC as shown in Fig. 2.
can be adsorbed in pores smaller than 2 nm or even 0.7 nm19.
Adsorption capacities of activated carbons are additionally
influenced by the presence of surface groups20.
2.2. Pore structure analysis by gas adsorption

experiments
More detailed information about pore structure and pore size
distribution can be gained by gas adsorption experiments,
allowing an assessment of pore sizes from 0.35 to 100 nm21.
According to IUPAC recommendation22, pores are classified by
their size:
(i) Pores with widths exceeding about 50 nm are called
macropores.
(ii) Pores of widths between 2 nm and 50 nm are called
mesopores.
(iii) Pores with widths not exceeding about 2 nm are
called micropores.
Figure 2. Types of hysteresis loops in adsorption isotherms.22
During adsorption experiments, a known amount of gas is
admitted to a confined, calibrated volume containing the As adsorbate, several gases at different temperatures can be
sample. Due to adsorption, the pressure in the confined volume used. Nitrogen at its boiling temperature (77 K at atmospheric
falls. As the temperature is held constant during the pressure) and carbon dioxide at 273.15 K are one of the most
experiment, the adsorbed gas amount can be determined from commonly used. Since CO2 at 273.15 K diffuses faster in
the pressure decrease by applying the ideal gas law. The micropores and CO2 adsorption starts at higher pressures
adsorbed gas amount is the difference between the admitted compared to N2 at 77 K, the use of CO2 is advantageous for
gas amount and the required amount of gas to fill the void micropore analysis. CO2 adsorption experiments are faster and
space, which has to be determined previously. By varying the do not require high vacuum systems. However, from CO2
amount of admitted gas, an adsorption/desorption isotherm, adsorption isotherms only pore sizes up to 1.5 nm can be
showing the amount of adsorbed gas at various relative determined. Therefore, it is reasonable to combine CO2
pressures, can be obtained. Isotherm shapes are classified isotherms for micropore analysis with N2 measurements for
according to IUPAC as shown in Fig. 1.22 mesopore determination.23
For the evaluation of gas adsorption experiments, there are
several models, allowing estimations of surface areas, pore
volumes and pore size distribution. Table 1 lists the most
common used methods, as described in the operation manual
of a commercial gas adsorption device.24
Figure 1. Classification of adsorption isotherms according to

IUPAC.22
If the amount of adsorbed gas during desorption at a respective

pressure deviates from the amount during adsorption, the
isotherm shows a hysteresis loop. Hysteresis is associated with
capillary condensation resulting in delayed desorption. It occurs
72
Table 1. Evaluation methods for gas adsorption experiments24. 2.3. Analysis of functional surface groups
To determine functional surface groups, FTIR spectroscopy is
Method
Purpose Description often used25,26. A simpler method is the Boehm titration, which
name
determines the surface acidity or basicity of AC. Surface groups
Linear plot of BET equation, such as carboxylic groups, lactones, lactoles and hydroxyl
describing the relation
Brunauer- groups of phenolic character have acidic characters and can be
between weights of gas
Emmettt- Specific surface neutralised by alkaline titration. Since these groups have
adsorbed and adsorbate
Teller (BET) area
constituting a monolayer of different acidic strengths, they can be differentiated by
method
surface coverage at a relative neutralisation with 0.05 M solutions of bases with different
pressure p/p0. strengths. Sodium hydroxide (NaOH) neutralises all acidic
Assumes that at p/p0 = 1 all
pores are filled with liquid;
groups, sodium carbonate (Na2CO3) neutralises carboxylic and
as the pressure is lowered, lactonic groups and sodium bicarbonate (NaHCO3) neutralises
adsorbed liquid evaporates only carboxylic groups. Basic groups, which origin is still in
Barrett, and desorbs; according to discussion, can be neutralised with hydrochloric acid. There are
Pore size
Joyner, Kelvin equation, the pressure
distribution several hypotheses about the origin of the surface basicity, such
Halenda (BJH) at which evaporation or
(mesopores) as π basicity of graphene layers or pyrone-type structures;
method condensation occurs
depends on pore radius; however, basicity caused by these structures are weaker than
thus, the corresponding pore proven basicity. Stronger basicity can be obtained from pyrone-
volume is calculated for type structures, where the carbonyl group and the ring oxygen
different pore radii.
are distributed on polycyclic aromatic compounds.27,28
Surface area of
microporous Limiting case of BET equation
Langmuir
samples in the in the absence of meso- and
method
absence of meso- macropores. 3. The activation process
and macropores
Calculation of t, the The activation process leads to an increase in internal surface
statistical thickness of an areas of a carbonaceous precursor. As precursors, for instance
Micropores
surface area and
adsorbed film at a coal, pitch, agricultural by-products and polymeric materials are
V-t method corresponding relative used29. There are various patented methods for activation,
micropore volume
(t-method) pressure p/p0 using de Boer,
in the presence of
Carbon Black or Halsey
where it is generally differed between physical and chemical
mesopores activation. Physical activation includes two steps: carbonisation
equation; plot of adsorbed
volume V versus t. and activation. During carbonisation, the precursor is pyrolysed
Micropores in an inert (mostly nitrogen) atmosphere at 600–900 °C. The
surface area and carbonised precursor is then activated with oxidising gases,
Alpha-s Empirical analogue to t-
micropore volume
method method such as carbon dioxide or water steam at 600–1200 °C. During
in the presence of
mesopores chemical activation, the precursor is mixed with a chemical
Fraction of adsorption reagent and heated in an inert atmosphere (usually nitrogen) to
volume V occupied by liquid 450–900 °C25. Various reagents, such as ZnCl2, H3PO4, K2CO3,
Micropore
Dubinin- adsorbate is expressed as a
volume, pore size Na2CO3, AlCl3, KOH or NaOH can be used, leading to different
Radushkevich Gaussian function; it
distribution of chemical reactions with the precursor. Chemical activation has
(DR) method assumes homogeneous pore
micropores
size distribution; adoption of several advantages compared to physical activation: it requires
model parameters. lower temperatures, produces much higher yields and allows to
Micropore Generalised form of DR obtain very high surface areas. The microporosity can be well
Dubinin-
volume, pore size method, also for
Astakohov developed and the pore size distribution controlled by the
distribution of heterogeneous pore size
(DA) method process conditions. However, corrosive reagents have to be
micropores distributions
Calculation of pore size used and an additional washing step is required30. Chemical
distribution of micropores activation can be performed as a one-step method, but with an
Horvath- Pore size
from low relative pressure additional pre-carbonisation step higher adsorption capacities,
Kawazoe (HK) distribution of
region, independent of
method micropores
Kelvin equation; it assumes higher surface areas and pore volumes can be obtained.26
slit-like pores
Pore size Alternative to HK method; it 3.1. Process conditions of chemical activation
Saito-Foley
distribution of assumes cylindrical pore
(SF) method 3.1.1. Type of chemical reagent
micropores geometry.
Density The type of activation agent plays the major role in the
Functional
More realistic description of activation process. H3PO4, ZnCl2, NaOH and KOH are the most
Theory (DFT)
micropores; mumerical commonly used activation agents. A different type of chemical
and Monte Pore size
solution of Generalized
Carlo distribution
Adsorption Isotherm (GAI)
reagent leads to different chemical mechanisms during
Simulation activation and therefore to different AC characteristics.
equation
(MC)
Studies comparing different chemical reagents show that ZnCl2
methods
and H3PO4 do not work efficiently at temperatures above
600 °C, and that using alkali hydroxides (NaOH or KOH) or their
73
carbonates much higher pore volumes and surface areas can be temperature range from 50 to 85 °C, they only observed minor
obtained31,32. Compared to activation with NaOH, KOH effects of impregnation time or temperature on pore volume
activation can lead to a narrower pore size distribution.33 Since and BET surface area.
very high surface areas (up to 3000 m² g–1) and well defined
micropore volumes can be obtained, there are various studies 3.1.4. Temperature
about activation with alkali hydroxides (NaOH and KOH) for the The effect of temperature depends strongly on the activation
production of new carbon materials5,17,18, and mechanisms agent. In principle, high temperatures promote the activation
during KOH and NaOH activation are widely discussed. Otowa process and the higher the temperature, the more pores are
et al.34 proved the formation of larger amounts of H2, smaller formed to certain extents; however, if a certain temperature
amounts of CO, CO2, CH4 and K2CO3 and observed the formation level is exceeded, total surface areas in AC decrease. Therefore,
of metallic potassium during KOH activation. They considered to obtain a maximal surface area, for every activation agent
the following reactions to contribute to activation: there is an optimal activation temperature, depending also on
𝟐𝟐𝟐𝟐𝟐𝟐𝟐𝟐 → 𝑯𝑯𝟐𝟐 𝑶𝑶 + 𝑲𝑲𝟐𝟐 𝑶𝑶 (1) the type of precursor.
If the temperature is increased to a certain level, pores will
𝑲𝑲𝟐𝟐 𝑶𝑶 + 𝑪𝑪𝑶𝑶𝟐𝟐 ⇄ 𝑲𝑲𝟐𝟐 𝑪𝑪𝑶𝑶𝟑𝟑 (2)
grow and combinations of pores are formed, resulting to a
𝑪𝑪 + 𝑯𝑯𝟐𝟐 𝑶𝑶 ⇄ 𝑪𝑪𝑪𝑪 + 𝑯𝑯𝟐𝟐 (3) decrease in specific surface area and micropore volume, and an
𝑪𝑪𝑪𝑪 + 𝑯𝑯𝟐𝟐 𝑶𝑶 ⇄ 𝑪𝑪𝑶𝑶𝟐𝟐 + 𝑯𝑯𝟐𝟐 (4) increase in mesopore volume. This effect plays an important
role during activation with alkaline hydroxides and was
𝑲𝑲𝟐𝟐 𝑶𝑶 + 𝑯𝑯𝟐𝟐 ⇄ 𝟐𝟐𝟐𝟐 + 𝑯𝑯𝟐𝟐 𝑶𝑶 (5)
observed during KOH activation above 800 °C by Hu and
𝑲𝑲𝟐𝟐 𝑶𝑶 + 𝑪𝑪 ⇄ 𝟐𝟐𝟐𝟐 + 𝑪𝑪𝑪𝑪 (6) Srinivasan38 and Hayashi et al.32.
Lillo-Ródenas et al.35 showed that there was a reaction between During H3PO4 and ZnCl2 activation, at a certain temperature
the alkali metal hydroxide and carbon during KOH activation. level, carbon shrinkage occurs leading to decreases in surface
They proposed the following reaction during KOH activation: areas and pore volumes; this was observed above 500–600 °C
by Ahmadpour and Do31, Teng et al.37, Hayashi et al.32 and Fierro
𝟔𝟔𝟔𝟔𝟔𝟔𝟔𝟔 + 𝟐𝟐𝟐𝟐 ⇄ 𝟐𝟐𝟐𝟐 + 𝟑𝟑𝑯𝑯𝟐𝟐 + 𝟐𝟐𝑲𝑲𝟐𝟐 𝑪𝑪𝑶𝑶𝟑𝟑 (7)
et al.40 On the other hand, with increasing temperature, a
Three main effects during activation with alkali metal decrease of functional surface groups was observed.40
hydroxides contributing to pore formation can be summarised:
(i) etching of the carbon framework by redox reactions (3), (6) 3.1.5. Gas flow rate
and (7) with carbon; (ii) formation of H2O and CO2 via reactions Nitrogen is the gas most commonly used in chemical activation.
(4) and (5), favouring further carbon gasification; and (iii) However, there are few studies about the role of nitrogen
intercalation of metallic potassium leading to micropore during chemical activation. Lillo-Ródenas et al.30 compared
formation after washing.18,29,36 chemical activation with NaOH in a nitrogen, carbon dioxide
and steam atmosphere and varied also the flow rate. They
3.1.2. Reagent to precursor ratio
showed that there was no porosity developed when carbon
Several studies showed a positive effect on total pore volume dioxide was used and that the surface areas of AC produced in
and surface area of high reagent to precursor ratios during a steam atmosphere were lower compared to AC generated in
activation with H3PO437, KOH26,33,38,39 or NaOH33, regardless of a nitrogen atmosphere. AC produced at a 500 mL min–1 STP
the type of precursor. For KOH activation, a KOH to precursor nitrogen flow showed much higher BET surface areas compared
weight ratio of 4:1 was found as an optimum, resulting to to AC made at lower flow rates (100 and 40 mL min–1). This
highest pore volumes and surface areas26. An increasing indicated that nitrogen flow plays an important role during
reagent to precursor ratio leads to pore widening and chemical activation. While carbon dioxide is not suitable for
therefore, to a change in pore size distribution, indicating that chemical activation, steam could be alternatively used;
the reagent to precursor ratio can be used to control the however, using nitrogen results in higher pore volumes.
porosity of AC.26,33,38.
3.1.6. Carbonisation time
3.1.3. Mixing method
Usual activation times range from 1 to 3 hours. In this range,
There are two ways to mix the precursor with the chemical comparably low effects of activation time were observed. For
reagent agents: the pure, dry reagent can be mixed with the KOH and H3PO4 activation, there was a slight increase in BET
precursor and directly activated or, alternatively, the precursor surface area and pore volume with increasing carbonization
can be impregnated by mixing it with the reagent solution and time31,37,41. For ZnCl2 activation, however, a slightly negative
drying the mixture prior to activation. The advantage of the effect on BET surface area and micropore volume was observed
latter method is a better distribution of chemical agents into by Ahmadpour and Do.31
the solid mass; however, an additional drying step is required.
Ahmadpour and Do31 showed that impregnation prior to 3.1.7. Type of precursor and material pre-treatment
activation led to AC with higher BET surfaces and higher As precursors for activation, coal, peat, different types of
micropore volumes compared to AC prepared by physical biomass or biochars are used. Since the type of precursor
mixing with the reagent. Teng et al.37 have investigated the plays an important role on AC characteristics, mixture of
impregnation time and temperature of coal prior to chemical different materials42 or pre-treatments of the raw materials
activation with H3PO4. In a time range from 1 to 3 hours and can be used to tailor AC porosity. For instance, acid pre-
74
treatment is used to reduce the mineral content of the 4. Hydrothermal carbonisation

precursor31,40. However, Ahmadpour and Do31 found only a
In recent years, HTC has widely been investigated. In
negligible effect of acid pre-treatment during KOH or ZnCl2
contrast to other coalification processes, such as dry
activation of coal. Fierro et al.40 showed that minerals had a
pyrolysis, during HTC the raw material must be submerged
positive effect on the activation of lignin with H3PO4 and AC
in water and hence, no pre-drying step is required. This
from untreated lignin exhibited higher pore volumes and
offers the possibility of the use of a new range of feedstock,
better methylene blue adsorption compared to AC from
such as sewage sludge, wet agricultural residues or
sulphuric acid pre-treated lignin.
digestate48. Most of the research focuses on the application
Several authors pyrolised the raw material prior to activation
of hydrochars as an alternative energy carrier or as soil
by heating it in an inert gas atmosphere in a conventional
ameliorant. Table 3 shows a summary of the existing
device39,43 or using microwave ovens.42,44 Nevertheless, HTC
literature about HTC and the corresponding aim of study.
is considered to be a more suitable pre-treatment for
Even if research about the application of HTC is still in
activation compared to conventional pyrolysis. Since it uses
progress, it is not a new technology. It was already used in
lower temperatures, hydrochars exhibit a lower degree of
the beginning of the 20’s century by the chemist Friedrich
aromatisation, a higher oxygen content and are more
Bergius as a simulation process for natural coalification49. In
reactive. Hydrothermal treatment increases the content of
his review from 1928, he describes how he used elevated
oxygenated functional groups (OFG) contributing to
temperatures (170 to 340 °C) to accelerate coal formation,
adsorptive characteristics of AC9,11 and increasing the
lasting millions of years in nature. To avoid local overheating,
chemical reactivity. Therefore, activation works effectively
he used water as a heat transfer medium in a pressure
used hydrochars as precursors.4,8 Table 2 lists some
resistant container to exclude evaporation. As raw materials,
publications where hydrochars were used as precursors for
he used cellulose, wood, turf, sugar and grass.
AC production. As can be seen in this table, first publications
Independently of the feedstock, the chars showed an
were released in 2011 and the amount of studies focusing on
increasing carbon content with increasing applied
HTC activation routes have increased from 2013.
temperature or residence time. Chars with a similar
Functionalities and pore structure of AC can be controlled by
composition to brown coal could be obtained49. Berl and
HTC and activation parameters. The following paragraphs
Schmidt 50 showed that water was not only a heat transfer
abstract the literature available on HTC and its most
medium, but also an important reaction partner during the
important reaction parameters.
carbonisation process.
Table 2. Previous studies available on activation pathways During the 20th century, many publications about
based on hydrochar as precursor. pressurised heating of biomass in the presence of water
were released. The aim, rather than applications of artificial
HTC Activation coal, was to get a deeper understanding about the natural
Feedstock Reference
conditions conditions coalification. In 2007, two reviews on HTC were published
KOH 4:1, 600
suggesting the use of hydrothermal treated biomass as an
Glucose 190 °C, 4 h 45 alternative energy carrier to treat the CO2 problem51,52. From
°C, 2 h, N2
CO2, 850 °C, then on, the interest in the application of HTC have increased
Walnut shell,
30 min over the years.
sunflower stem, 220 °C, 20 h 8
Air, 250 °C,
olive stone
30 min
4.1. Definition of hydrothermal carbonisation
250 °C, 7.5 h,
KOH 4:1,
Hazelnut shell citric acid 9 Hydrothermal carbonization (HTC) is an exothermal process
600°C, 2 h, N2
addition increasing the degree of carbonisation of biomass; i.e., it
Glucose, cellulose, KOH 3:1, 750 lowers its hydrogen and oxygen content (leading to lower
180–240 °C 4
rye straw °C, 2 h, N2
KOH 4:1, 700
atomic O/C and H/C ratios). In principle, any organic material
Spruce, corncob 200 °C, 24 h 5 can be used as feedstock12.
°C, 1 h
Grass cuttings, HTC is performed in the presence of water at elevated
horse manure, CO2, 800 °C, 2 temperatures (typically in a range of 180 to 250 °C) under
Purchased 7
beer waste, bio- h subcritical conditions. Since the feed must be submerged
sludge
with liquid water during the whole process, pressure must
Glucoseamine in
180 °C, 12 h, KOH 2:1, 600 exceed the saturation pressure of water. According to Funke
the presence of 6
pH 11, NaOH °C, 4 h, N2
graphene oxide and Ziegler12, the pH should be neutral or weakly acidic,
200 °C, 20 since at alkaline conditions, different reactions occur.
CO2, 800 °C,
Coconut shells min, H2O2 46 However, some authors have also conducted HTC under
2h
addition
alkaline conditions.6
200–350 °C, ZnCl2, 1:1,
Coconut shell 20 min, ZnCl2 2:1, 3:1, 800 47
addition °C, N2, CO2
75
Table 3. Summary of available previous studies on HTC. Table 3 (continued)
HTC process 200 °C; 4–12 Conversion of

Feedstock Aim of study Reference Sewage sludge h, 85.7% w/w sewage sludge 64
conditions
moisture to solid fuel
Cellulose, Simulation of Empty Palm Oil 220 °C; 4 h;
170–340 °C; HTC of EFB as
wood, turf, natural 49 Fruit Bunches 7%–32% w/w 65
1–230 h energy carrier
sugar, grass coalification (EFB) water content
Understanding 280–365 °C; 2 Study on
150–350 °C; 6 Black liquor
Cellulose the degradation 50 h; 3% w/v degradation of 66
h lignin
of cellulose solid load lignin
Comparison of Investigation of
Wood, lignin, 150–350 °C; 200–230 °C;
artificial and 53 HTC parameters:
cellulose 8–72 h 1–5 min; 0.6–
natural coal Loblolly pine temperature, 67
2.38 mm
Understanding time, and
Biomass waste, 120–390 °C; 1 particle size
the effect of particle size
subbituminous min–6 54
conditions on 220 °C; 6 h;
coal months Influence of
HTC pH of 0–6,
oxidative
Conversion of Glucose nitric acid and 13
180–250 °C; strength and pH
Pine needles, biomass to hydrochloric
16 h; 12.5%– on HTC results
pine cones, oak mesoporous acid addition
50% w/v of 51
leaves, orange carbons as an 190–245 °C;
biomass, citric
peels alternative 150–570 min;
acid addition
energy carrier citric acid
Wheat straw 68
180–200 °C; addition
Application of
3–5 h; 20 % (pH of 3, 5, Prediction of
Cellulose HTC in a 56
dry mass, pH and 7) product mass
continuous plant
of 1.97–4.93 190–245 °C; yields using
Assessment of 140–560 min; severity models
215–290 °C;
Mixture of the effect of citric acid
5–60 min; Digestate 15
Jeffrey pine and temperature 57 addition
12.5% w/w
white fir and time on HTC (pH of 3, 5,
biomass
results and7)
Comparison of Investigation of
240 °C; 22 h;
General comparative review HTC and dry 48 structural
Kraft lignin H2SO4 and 69
pyrolysis changes of lignin
FeCl2 addition
230 °C; 5 min; during HTC
Investigation of
20 % w/w
the effect of
biomass,
acetic acid and 4.2. Chemical mechanisms during HTC of lignocellulosic
Loblolly pine acetic acid/ 14
lithium chloride
lithium biomass
addition during
chloride
HTC The complex reaction mechanisms occurring during HTC are
addition
190–270 °C; Investigation of not fully understood yet. Hydrolysis, dehydration,
2– 10 h; citric the effect of decarboxylation, polymerisation and aromatisation are
Digestate from
acid addition, temperature, 58 considered as the main mechanisms during HTC, forming a
maize silage
pH of 3, 5, time, and pH on
parallel network of different reaction paths.
and 7 HTC results
180 °C; 5–30 Investigation of Generally, hydrochar formation can be summarised as
min; 10%– the effect of follows: biomass components are hydrolysed to oligomers
Sugarcane 22% w/v time, acid
59 and monomers, partly soluble in water. Solved fragments
bagasse biomass, addition, and degrade further, while highly reactive intermediates, such as
H2SO4 solid-liquid ratio
addition on HTC outputs
5-hydroxymethylfurfural (HMF), are formed. Polymerisation
Feasibility study of solved fragments leads to the generation of larger, water
200–220 °C;
of HTC as a insoluble molecules forming the solid product. Parts of the
Sewage sludge 5–7 h, citric 60
drying raw material, which are not degraded to water solvable
acid addition
technology fragments, are also part of the hydrochar.12
185–245 °C;
Spent grains 1–6 h; 8.8%–
Description of Hydrolysis is the cleavage of chemical bonds, such as ether
HTC by kinetic 61 and ester bonds, by addition of water. Hydrolysis of cellulose
(draff) 16.8% w/v dry
models
mass was already described by Berl and Schmidt50 during pressure
Comparison of heating of cotton. By cleavage of β-1,4-glycosidic linkages,
Wheat straw HTC and dry oligosaccharides and monosaccharides are formed.
230 °C; 6 h 62
digestate vapothermal
carbonisation Dehydration and decarboxylation are considered as the
HTC of main reasons for carbonisation of the biomass by lowering
220 °C, then
agricultural the H/C and O/C ratios. Dehydration leads to the formation
Agricultural 180 °C; 4 h;
residues for 63 of water, generally explained by the elimination of hydroxyl
residues 15% w/w dry
solid fuel
matter groups. During decarboxylation, carboxyl and carbonyl
production
groups are degraded forming carbon dioxide and carbon
76
monoxide. The elimination of hydroxyl and carboxyl groups composition11. Since lignin exhibits a more stable structure
leads to unsaturated compounds, which polymerise easily to compared to cellulose and hemicelluloses, higher
lager molecules. Polymerisation occurs mainly by temperatures (or residence times) are required for biomass
condensation under the formation of water.12 degradation if the lignin content is high. A higher solid
Under hydrothermal conditions, aromatic structures are concentration also requires higher temperatures (or
formed. Aromatisation is significantly temperature residence times) to obtain same effects. For these reasons,
dependent and is enhanced by alkaline conditions. Aromatic the temperature optimum is shifted to higher temperatures,
structures are stable under hydrothermal conditions and are if the lignin content of the biomass or the solid to liquid ratio
therefore considered as basic building blocks of hydrochars is increased (see Fig.5a).
and as well as of natural coal.12 Falco et al.4 performed HTC trials with glucose, cellulose and
Due to the high thermal stability of aromatic structures, the rye straw prior to KOH activation, varying the HTC
effect of hydrothermal treatment decreases with the temperature from 180 to 280 °C. They observed that the
amount of aromatic structures in the biomass12. Therefore, highest BET surface area was obtained for the activated
HTC at 240–280 °C induces only small structural changes of hydrochar obtained at 240 °C. Jain et al.47 found an optimal
lignin, which contains a high amount of aromatic HTC temperature of 275 °C for the production of ZnCl2
structures66,69. Structural changes of lignin occur mainly by activated carbons from coconut shell.
the cleavage of β-O-4 ether bonds and demethylation under
the formation of radicals (see Fig. 3)69. Lignin degrades
further at higher temperatures in the range from 310 to
365 °C.66 Mechanisms of hydrochar formation from cellulose
are described by Sevilla and Fuertes55, as shown in Fig. 4.
Figure 3. Homolysis of the C-O and O-C bonds during heat

treatment of lignin.69
4.3. Effect of process conditions on HTC
4.3.1. Temperature
Temperature is the main influencing factor on HTC. Higher
Figure 4. Mechanisms of formation of hydrochar particles
temperatures promote degradation and carbonisation of
from cellulose by HTC.55
biomass and usually result in a decrease in solid yield, an
increase in liquid and gas yields, and a higher carbon content 4.3.2. Residence time
of the solid product12,15,58,68. At temperatures above 260 °C,
HTC tends to liquefaction.69,70 Residence time is the second most important influencing
HTC temperature also affects AC’s characteristics, if factor12,15,58,68. Typically, an increase in residence time leads,
hydrochars are used as precursors. Due to partial hydrolysis like a temperature increase, to a decrease in solid yield and
of polysaccharides and lignin, hydrothermal treatment to a higher carbon content of the resulting hydrochar. But in
increases the amount of oxygenated functional groups (OFG) some cases, an increase in the solid yield with increasing
to some extent. However, if a certain level is exceeded, OFG residence time was observed, probably due to ongoing
are decomposed under formation of gaseous products and polymerisation. Residence time should thus be high enough
the chemical reactivity of the hydrochars is lowered5,11,47. to complete hydrothermal reactions12. The effect of
Therefore, there is an optimal HTC temperature leading to residence time on the amount of OFG is similar to that
hydrochars with a maximal OFG content, which depends on observed for the temperature. At lower residence times, the
residence time, solid to liquid ratio, and biomass amount of OFG increases, due to ongoing hydrolysis.
However, a further increase in the residence time could
77
result in an excessive dehydration/carbonisation, leading to 4.3.3. Pressure

the formation of more stable oxygen groups (ether or Pressure is not controlled during HTC and its influence is
quinones) or decomposition of OFGs. The time where the hardly discussed in literature. The reaction pressure
amount of OFG is maximal depends on temperature, solid influences the reaction network according to the principles
content and lignin concentration (see Fig. 5b)11; for instance, of LeChatelier (i.e., with increasing pressure the reaction
a time of 6 h was the most appropriate during HTC of sewage equilibrium shifts to solid and liquid phases and to reactants
sludge with a moisture content of 86 % at 200 °C.64 with a lower number of moles). However, the effect of
pressure on HTC is low.12
4.3.4. Solid to liquid ratio

Funke and Ziegler12 suggested to choose the solid to liquid
ratio as high as possible to promote polymerisation. The
solid mass must, however, be completely submerged in
water during the whole process, since water is important as
heat transfer medium, solvent and reactant. Chen et al.59
showed that a higher solid to liquid ratio during HTC of sugar
cane led to higher mass and energy yields. Sevilla and
Fuertes55 observed less condensed products (high O/C and
H/C ratios) using higher solid loads, due to a lower extent of
hydrolysis of the biomass. According to Jain et al.11, similar
results can be obtained at higher solid to liquid ratio, if
higher temperature or residence times are used or if the
lignin content of the biomass is lower (See Fig. 5c).
4.3.5. Starting pH and use of catalysts

A weakly acidic environment is necessary for hydrothermal
degradation. Acid formation during HTC leads to a pH drop
during the reaction and to autocatalysis. Nevertheless, acid
addition can enhance the reaction process and increase the
degree of carbonisation, while the type of acid plays an
important role12. Literature data show that acid addition
prior to HTC has diverse effects. Mumme et al.58 investigated
the effect of citric acid addition on HTC of digestate from
maize silage. Citric acid addition did not show any significant
effect on carbon content, but lowered the mass yield of the
solid product. Lynam et al.14 observed a decrease in mass
yield and an increase in higher heating value (HHV) when
acetic acid was added before HTC of loblolly pine. Acetic acid
addition led to a shift of reaction equilibrium, while less
acetic acid was formed. Chen et al.59 found a lower O/C ratio,
a decreased solid mass yield and an increase in the energy
enhancement factor (EEF) by sulphuric acid addition prior to
Figure 5. Trends of oxygenated functional groups (OFG)
HTC of sugarcane bagasse. Reiche et al.13 investigated the
with temperature (a), residence time (b) and solid
effect of nitric acid and hydrochloric acid during HTC of
concentration (c).11
glucose at pH ranges from 0 to 6. They showed that type of
The effect of temperature and residence time is often acid and acid concentration (starting pH) play an important
described by reaction severity. The model by Ruyter role. A transition of colour from brown to black and an
describes the reaction severity f (as a function of the increase in powder density and particle size of the resulting
residence time, in s, and the absolute temperature) as hydrochars were observed when the pH value was
shown in Eq. (8). decreased below 3. Above pH 3, hydrochloric acid as well as
3500
nitric acid concentration did hardly affect the carbon yield.
f = 50 ⋅ (𝑡𝑡)0.2 ∙ exp �− � (8) Below pH 3, a decrease in carbon yield with increasing
T
The higher the reaction severity (i.e., higher temperature hydrochloric acid concentration was observed. The carbon
and/or residence time) the higher the carbon content and yield decreased between pH 3 and 1.5 and increased again
the lower the mass yield, within a certain temperature and at pH 1 when nitric acid was added. The authors proved an
time range. oxidising effect of nitric acid at low pH ranges. Wikberg et
al.69 proved a promotion of lignin degradation by sulphuric
acid addition. The formed hydrochars showed a higher
thermal stability when sulphuric acid was added prior to
78
HTC. Moreover, Suwelack et al. observed that citric acid hydrochars obtained through HTC, which is a promising
addition increased reaction severity of HTC of digestate, but conversion technology for the conversion of wet feedstock.
did not have any reasonable impact on the HTC of wheat High water content of the HTC is taken as an advantage.
straw15,68. They applied a severity model, which includes The porosity of ACs derived from HTC chars are generally
temperature, retention time, as well as citric acid addition dominated by micropores. Chemical activation seems more
(see Eq. 9): promising than physical one. NaOH and KOH are promising
X−Xref Tp −Tp,ref chemical agents for achieving high surface areas, up to 3000
f = exp � � ⋅ exp � � ⋅ dR (9)
λXref ω m2 g–1.
where X and Xref are the initial and reference concentrations, The understanding of the chemical mechanisms during the
respectively, of citric acid (in mass fraction). Tp and Tp,ref are production of AC and during the HTC is a complex study. The
the process and reference temperatures, respectively; dR addition of catalysts during the HTC process, which improve
corresponds to the retention time, whereas both λ and ω are the hydrolysis reaction mechanism, and/or the use of
model parameters, which depend on the biomass feedstock. chemicals during hydrochar activation result in important
The severity model is suitable to predict gas, liquid and solid changes in the textural properties of the resulting ACs.
yield of HTC and degree of carbonisation of the hydrochar.
For every type of feedstock, experiments are necessary to
Acknowledgements
estimate the value of the model parameters λ and ω. Besides
temperature, time and pH, other parameters are not “This project has received funding from the European
considered in severity models yet. Union’s Horizon 2020 research and innovation programme
Apart from acids, other catalysts can be used to enhance under the Marie Skłodowska-Curie grant agreement No
HTC. A decreased mass yield and increased HHV was found 721991”.
after HTC of loblolly pine when lithium chloride was added.
The effect was higher when it was added in combination
References
with acetic acid. LiCl reduces the reaction pressure
improving safety and lowering costs of the operation14. (1) Mao, C.; Feng, Y.; Wang, X.; Ren, G. Review on
Some authors used iron or zinc salts, such as Fe(NH4)2(SO4)2, research achievements of biogas from anaerobic
Fe2O3, FeCl2 or ZnCl2 to improve dehydration during HTC. digestion. Renew. Sustain. Energy Rev. 2015, 45,
Using Fe2O3, BET surface areas of 400 m² g–1 for hydrochars 540–555.
from starch could be obtained11. Wikberg et al.69 showed (2) Alburquerque, J. A.; La Fuente, C. de; Campoy, M.;
that FeCl2 had only slight effects on HTC catalysis of lignin. Carrasco, L.; Nájera, I.; Baixauli, C.; Caravaca, F.;
Higher contents of oxygenated functional groups (OFG) Roldán, A.; Cegarra, J.; Bernal, M. P. Agricultural use
could be obtained when ZnCl247 or H2O246 were used during of digestate for horticultural crop production and
HTC, which resulted in an efficient activation and higher improvement of soil properties. European J.
porosity of the hydrochar-derived ACs. Agronomy 2012, 43, 119–128.
(3) Alburquerque, J. A.; La Fuente, C. de; Ferrer-Costa,
4.3.6. Particle size of feedstock A.; Carrasco, L.; Cegarra, J.; Abad, M.; Bernal, M. P.
A feedstock with a lower particle size exhibits larger surface Assessment of the fertiliser potential of digestates
areas, theoretically enhancing chemical and physical from farm and agroindustrial residues. Biomass
processes. Yan et al67 showed that a smaller particle size Bioenergy 2012, 40, 181–189.
promoted HTC of loblolly pine, although the effect was (4) Falco, C.; Marco-Lozar, J. P.; Salinas-Torres, D.;
relatively low over the particle size range investigated. Morallón, E.; Cazorla-Amorós, D.; Titirici, M. M.;
Lozano-Castelló, D. Tailoring the porosity of
4.3.7. Technical aspects chemically activated hydrothermal carbons:
HTC is performed in pressure resistant containers usually Influence of the precursor and hydrothermal
equipped with a temperature and pressure sensor, carbonization temperature. Carbon 2013, 62, 346–
sometimes additionally with a stirring unit14,57,58,60. In some 355.
cases, oxygen is replaced by an inert gas, such as nitrogen or (5) Falco, C.; Sieben, J. M.; Brun, N.; Sevilla, M.; van der
helium prior to reaction14,57. For industrial applications, Mauelen, T.; Morallón, E.; Cazorla-Amorós, D.;
continuously working HTC plants have already been Titirici, M.-M. Hydrothermal carbons from
developed56. Technical realisation of HTC can influence hemicellulose-derived aqueous hydrolysis products
reaction mechanisms. For instance, condensation reactions as electrode materials for supercapacitors.
are enhanced by stirring the suspension during HTC.13 ChemSusChem 2013, 6, 374–382.
(6) Fan, X.; Yu, C.; Yang, J.; Ling, Z.; Qiu, J. Hydrothermal
synthesis and activation of graphene-incorporated
5. Conclusion nitrogen-rich carbon composite for high-
performance supercapacitors. Carbon 2014, 70,
Using a digestate for the production of carbonaceous
130–141.
materials appears to be a very interesting option, leading to
a higher product (AC) value than current applications such as
fertiliser or soil conditioner. ACs can be produced from
79
(7) Hao, W.; Björkman, E.; Lilliestråle, M.; Hedin, N. (20) Pereira, M. F. R.; Soares, S. F.; Órfão, J. J.M.;
Activated carbons prepared from hydrothermally Figueiredo, J. L. Adsorption of dyes on activated
carbonized waste biomass used as adsorbents for carbons: Influence of surface chemical groups.
CO2. Appl. Energy 2013, 112, 526–532. Carbon 2003, 41, 811–821.
(8) Román, S.; Valente Nabais, J. M.; Ledesma, B.; (21) Lowell, S. Characterization of porous solids and
González, J. F.; Laginhas, C.; Titirici, M. M. powders: Surface area, pore size and density. In:
Production of low-cost adsorbents with tunable Particle technology series v16; London; Kluwer
surface chemistry by conjunction of hydrothermal Academic: Dordrecht, 2004.
carbonization and activation processes. (22) Thommes, M.; Kaneko, K.; Neimark, A. V.; Olivier, J.
Microporous Mesoporous Mat. 2013, 165, 127–133. P.; Rodriguez-Reinoso, F.; Rouquerol, J.; Sing, K. S.W.
(9) Unur, E. Functional nanoporous carbons from Physisorption of gases, with special reference to the
hydrothermally treated biomass for environmental evaluation of surface area and pore size distribution
purification. Microporous Mesoporous Mat. 2013, (IUPAC Technical Report). Pure Appl. Chem. 2015,
168, 92–101. 87, 160.
(10) Rodriguez Correa, C.; Bernardo, M.; Ribeiro, R. (23) Quantachrome Instruments. Powder Tech Note 35:
P.P.L.; Esteves, I. A.A.C.; Kruse, A. Evaluation of Micropore size analysis of porous carbons using CO2
hydrothermal carbonization as a preliminary step adsorption at 273.15 K (0 °C), Boynton Beach, USA.
for the production of functional materials from 2010.
biogas digestate. J. Anal. Appl. Pyrolysis 2017, 124, (24) Quantachrome Instruments. autosorb iQ Brochure:
461–474. Characterization Porous Materials and Powders:
(11) Jain, A.; Balasubramanian, R.; Srinivasan, M. P. Autosorb iQ and ASiQwin. Gas sorption system
Hydrothermal conversion of biomass waste to operation manual Version 2.0, Boynton Beach, USA.
activated carbon with high porosity: A review. 2009–2011.
Chem. Eng. J. 2016, 283, 789–805. (25) Suhas; Carrott, P. J. M.; Ribeiro Carrott, M. M. L.
(12) Funke, A.; Ziegler, F. Hydrothermal carbonization of Lignin--from natural adsorbent to activated carbon:
biomass: A summary and discussion of chemical A review. Bioresour. Technol. 2007, 98, 2301–2312.
mechanisms for process engineering. Biofuels, (26) Oh, G. H.; Park, C. R. Preparation and characteristics
Bioprod. Bioref. 2010, 4, 160–177. of rice-straw-based porous carbons with high
(13) Reiche, S.; Kowalew, N.; Schlögl, R. Influence of adsorption capacity. Fuel 2002, 81, 327–336.
synthesis pH and oxidative strength of the catalyzing (27) Boehm, H.P. Surface oxides on carbon and their
acid on the morphology and chemical structure of analysis: A critical assessment. Carbon 2002, 40,
hydrothermal carbon. Chemphyschem 2015, 16, 145–149.
579–587. (28) Goertzen, S. L.; Thériault, K. D.; Oickle, A. M.;
(14) Lynam, J. G.; Coronella, C. J.; Yan, W.; Reza, M. T.; Tarasuk, A. C.; Andreas, H. A. Standardization of the
Vasquez, V. R. Acetic acid and lithium chloride Boehm titration. Part I. CO 2 expulsion and endpoint
effects on hydrothermal carbonization of determination. Carbon 2010, 48, 1252–1261.
lignocellulosic biomass. Bioresour. Technol. 2011, (29) Alcañiz-Monge, J.; Illán-Gómez, M. J. Insight into
102, 6192–6199. hydroxides-activated coals: Chemical or physical
(15) Suwelack, K.; Wüst, D.; Zeller, M.; Kruse, A.; activation? J. Colloid Interface Sci. 2008, 318, 35–41.
Krümpel, J. Hydrothermal carbonization of wheat (30) Lillo-Ródenas, M.A.; Cazorla-Amorós, D.; Linares-
straw—prediction of product mass yields and Solano, A. Understanding chemical reactions
degree of carbonization by severity parameter. between carbons and NaOH and KOH. Carbon 2003,
Biomass Conv. Bioref. 2016, 6, 347–354. 41, 267–275.
(16) May-Britt Hagg; Xuezhong He. Carbon Molecular (31) Ahmadpour, A.; Do, D. D. The preparation of active
Sieve Membranes for Gas Separation, Chapter 15, carbons from coal by chemical and physical
2011. activation. Carbon 1996, 34, 471–479.
(17) Qu, W.-H.; Xu, Y.-Y.; Lu, A.-H.; Zhang, X.-Q.; Li, W.-C. (32) Hayashi, J.; Kazehaya, A.; Muroyama, K.; Watkinson,
Converting biowaste corncob residue into high A.P. Preparation of activated carbon from lignin by
value added porous carbon for supercapacitor chemical activation. Carbon 2000, 38, 1873–1878.
electrodes. Bioresour. Technol. 2015, 189, 285–291. (33) Maciá-Agulló, J. A.; Moore, B. C.; Cazorla-Amorós,
(18) Wang, J.; Kaskel, S. KOH activation of carbon-based D.; Linares-Solano, A. Activation of coal tar pitch
materials for energy storage. J. Mater. Chem. 2012, carbon fibres: Physical activation vs. chemical
22, 23710–23725. activation. Carbon 2004, 42, 1367–1370.
(19) Aygün, A.; Yenisoy-Karakaş, S.; Duman, I. Production (34) Otowa, T. Activation Mechanism, Surface Properties
of granular activated carbon from fruit stones and and Adsorption Characteristics of KOH Activated
nutshells and evaluation of their physical, chemical High Surface Area Carbon. In: Fundamentals of
and adsorption properties. Microporous Adsorption: Proceedings of the Fifth International
Mesoporous Mat. 2003, 66, 189–195. Conference on Fundamentals of Adsorption, 1996, p.
709–716.
80
(35) Lillo-Ródenas, M. A.; Juan-Juan, J.; Cazorla-Amorós, (48) Libra, J. A.; Ro, K. S.; Kammann, C.; Funke, A.; Berge,
D.; Linares-Solano, A. About reactions occurring N. D.; Neubauer, Y.; Titirici, M.-M.; Fühner, C.; Bens,
during chemical activation with hydroxides. Carbon O.; Kern, J. et al. Hydrothermal carbonization of
2004, 42, 1371–1375. biomass residuals: A comparative review of the
(36) Stavropoulos, G. G. Precursor materials suitability chemistry, processes and applications of wet and
for super activated carbons production. Fuel Proces. dry pyrolysis. Biofuels 2014, 2, 71–106.
Technol. 2005, 86, 1165–1173. (49) Bergius, F. Beitrge zur Theorie der Kohleentstehung.
(37) Teng, H.; Yeh, T.-S.; Hsu, L.-Y. Preparation of Naturwissenschaften 1928, 16, 104.
activated carbon from bituminous coal with (50) Berl, E.; Schmidt, A. Über das Verhalten der
phosphoric acid activation. Carbon 1998, 36, 1387– Cellulose bei der Druckerhitzung mit Wasser. Justus
1395. Liebigs Ann. Chem. 1928, 461, 192–220.
(38) Hu, Z.; Srinivasan, M.P. Preparation of high-surface- (51) Titirici, M. M.; Thomas, A.; Yu, S.-H.; Müller, J.-O.;
area activated carbons from coconut shell. Antonietti, M. A Direct Synthesis of Mesoporous
Microporous Mesoporous Mat. 1999, 27, 11–18. Carbons with Bicontinuous Pore Morphology from
(39) Khezami, L.; Chetouani, A.; Taouk, B.; Capart, R. Crude Plant Material by Hydrothermal
Production and characterisation of activated carbon Carbonization. Chem. Mater. 2007, 19, 4205–4212.
from wood components in powder: Cellulose, lignin, (52) Titirici, M.-M.; Thomas, A.; Antonietti, M. Back in the
xylan. Powder Technol. 2005, 157, 48–56. black: Hydrothermal carbonization of plant material
(40) Fierro, V.; Torné-Fernández, V.; Celzard, A.; as an efficient chemical process to treat the CO2
Montané, D. Influence of the demineralisation on problem? New J. Chem. 2007, 31, 787.
the chemical activation of Kraft lignin with (53) Tsukashima, H. The Infrared Spectra of Artificial Coal
orthophosphoric acid. J. Hazardous Mat. 2007, 149, made from Submerged Wood at Uozu, Toyama
126–133. Prefecture, Japan. BCSJ 1966, 39, 460–465.
(41) Sudaryanto, Y.; Hartono, S. B.; Irawaty, W.; (54) Ruyter, H. P. Coalification model. Fuel 1982, 61,
Hindarso, H.; Ismadji, S. High surface area activated 1182–1187.
carbon prepared from cassava peel by chemical (55) Sevilla, M.; Fuertes, A. B. The production of carbon
activation. Bioresour. Technol. 2006, 97, 734–739. materials by hydrothermal carbonization of
(42) Tiryaki, B.; Yagmur, E.; Banford, A.; Aktas, Z. cellulose. Carbon 2009, 47, 2281–2289.
Comparison of activated carbon produced from (56) Buttmann, M. Klimafreundliche Kohle durch
natural biomass and equivalent chemical Hydrothermale Karbonisierung von Biomasse.
compositions. J. Anal. Appl. Pyrolysis 2014, 105, Chemie Ingenieur Technik 2011, 83, 1890–1896.
276–283. (57) Hoekman, S. K.; Broch, A.; Robbins, C. Hydrothermal
(43) Hameed, B. H.; Din, A. T. M.; Ahmad, A. L. Carbonization (HTC) of Lignocellulosic Biomass.
Adsorption of methylene blue onto bamboo-based Energy Fuels 2011, 25, 1802–1810.
activated carbon: Kinetics and equilibrium studies. J. (58) Mumme, J.; Eckervogt, L.; Pielert, J.; Diakité, M.;
Hazardous Mat. 2007, 141, 819–825. Rupp, F.; Kern, J. Hydrothermal carbonization of
(44) Yagmur, E.; Ozmak, M.; Aktas, Z. A novel method for anaerobically digested maize silage. Bioresour.
production of activated carbon from waste tea by Technol. 2011, 102, 9255–9260.
chemical activation with microwave energy. Fuel (59) Chen, W.-H.; Ye, S.-C.; Sheen, H.-K. Hydrothermal
2008, 87, 3278–3285. carbonization of sugarcane bagasse via wet
(45) Li, M.; Li, W.; Liu, S. Hydrothermal synthesis, torrefaction in association with microwave heating.
characterization, and KOH activation of carbon Bioresour. Technol. 2012, 118, 195–203.
spheres from glucose. Carbohydrate Res. 2011, 346, (60) Escala, M.; Zumbühl, T.; Koller, C.; Junge, R.; Krebs,
999–1004. R. Hydrothermal Carbonization as an Energy-
(46) Jain, A.; Balasubramanian, R.; Srinivasan, M. P. Efficient Alternative to Established Drying
Production of high surface area mesoporous Technologies for Sewage Sludge: A Feasibility Study
activated carbons from waste biomass using on a Laboratory Scale. Energy Fuels 2012, 27, 454–
hydrogen peroxide-mediated hydrothermal 460.
treatment for adsorption applications. Chem. Eng. J. (61) Kruse, A.; Badoux, F.; Grandl, R.; Wüst, D.
2015, 273, 622–629. Hydrothermale Karbonisierung: 2. Kinetik der
(47) Jain, A.; Balasubramanian, R.; Srinivasan, M. P. Biertreber-Umwandlung. Chemie Ingenieur Technik
Tuning hydrochar properties for enhanced 2012, 84, 509–512.
mesopore development in activated carbon by (62) Funke, A.; Reebs, F.; Kruse, A. Experimental
hydrothermal carbonization. Microporous comparison of hydrothermal and vapothermal
Mesoporous Mat. 2015, 203, 178–185. carbonization. Fuel Proces. Technol. 2013, 115, 261–
269.
(63) Oliveira, I.; Blöhse, D.; Ramke, H.-G. Hydrothermal
carbonization of agricultural residues. Bioresour.
Technol. 2013, 142, 138–146.
81
(64) He, C.; Giannis, A.; Wang, J.-Y. Conversion of sewage

sludge to clean solid fuel using hydrothermal
carbonization: Hydrochar fuel characteristics and
combustion behavior. Appl. Energy 2013, 111, 257–
266.
(65) Stemann, J.; Erlach, B.; Ziegler, F. Hydrothermal
Carbonisation of Empty Palm Oil Fruit Bunches:
Laboratory Trials, Plant Simulation, Carbon
Avoidance, and Economic Feasibility. Waste
Biomass Valor. 2013, 4, 441–454.
(66) Hu, J.; Shen, D.; Wu, S.; Zhang, H.; Xiao, R. Effect of
temperature on structure evolution in char from
hydrothermal degradation of lignin. J. Anal. Appl.
Pyrolysis 2014, 106, 118–124.
(67) Yan, W.; Hoekman, S. K.; Broch, A.; Coronella, C. J.
Effect of hydrothermal carbonization reaction
parameters on the properties of hydrochar and
pellets. Environ. Prog. Sustain. Energy 2014, 33,
676–680.
(68) Suwelack, K. U.; Wüst, D.; Fleischmann, P.; Kruse, A.
Prediction of gaseous, liquid and solid mass yields
from hydrothermal carbonization of biogas
digestate by severity parameter. Biomass Conv.
Bioref. 2016, 6, 151–160.
(69) Wikberg, H.; Ohra-aho, T.; Pileidis, F.; Titirici, M.-M.
Structural and Morphological Changes in Kraft
Lignin during Hydrothermal Carbonization. ACS
Sustain. Chem. Eng. 2015, 3, 2737–2745.
(70) Akhtar, J.; Amin, N. A. S. A review on process
conditions for optimum bio-oil yield in
hydrothermal liquefaction of biomass. Renewable
and Sustainable Energy Rev. 2011, 15, 1615–1624.
82
Refined biocarbons for gas adsorption and separation

Sabina A. Nicolaea, Xia Wangb, Niklas Hedinb, Maria-Magdalena Titiricia
a
School of Engineering and Material Science, Queen Mary University of London, Mile End Road, London E1 4NS, UK
b
Department of Material and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden
Abstract
Nowadays challenges presented by the growing population and excessive consumption of fossil fuels can be overcome by
developments of renewable and green technologies for energy production and storage, air depollution and water treatment. For
diminution of gaseous pollutants CO2, CH4, and H2S; adsorption processes are in several cases bringing both economic and environment
advantages, over other technologies. Zeolites and porous carbon materials are the most popular adsorbents studied for gas capture.
Porous carbon materials are studied and used because of affordability, availability, high hydrophobicity, and reusability. They have
been popular in applications since long time. The first recorded case dates back to 3750 BC, when Egyptians and Sumerians used wood
char for the reduction of copper, zinc and tin ores in the manufacture of bronze. Later on, around 1550 BC, the Egyptians used such
carbon materials for medicinal purposes. Present day applications use porous carbons for environmental remediation, gas storage,
catalysis and energy storage.
it also has a high poisoning effect on noble metal catalysts, even

1. Introduction in very small amounts. Unfortunately, H2S is largely emitted
from chemical processes, like natural gas processing and
Climate change is a matter of great concern and primarily
utilization, hydrodesulphurisation of crude oil, and coal
related to the increasing concentration of CO2 in the
chemistry.5 Moreover, CO2 and H2S, are the major impurities in
atmosphere, contributing to the global warming. Studies have
natural refinery gases, synthesis gas for ammonia production,
shown that an increase in the global average temperature with
synthetic natural gas, and hydrogen manufacturing.6
more than 2 °C, above preindustrial levels, poses severe risks
Because of all the damages and inconvenience created by the
for ecosystems, human health and overall well-being.1 CO2
gas emissions, many technologies have been developed for
absorbs and emits infrared radiation at its two infrared-active
their capture and removal. The main options for reducing CO2
vibrational frequencies and contributes to surface warming in
emissions are: (i) increasing efficiency to reduce energy
the lower part of the atmosphere and cooling in the upper part.
consumption; (ii) reducing the quantity of carbons from fuels;
The increased CO2 concentration does not only influence the
(iii) improving the use of nuclear and renewable energy sources;
global surface temperature, but in turn, the increased global
(iv) to strengths the biological absorption capacity of forest and
temperature also contributes to increase the very
soils to capture CO2; and (v) to involve chemically and physically
concentrations of CO2 in the atmosphere. As stated in IPPCC’s
methods to capture CO2.7
Third Assessment Report,2 human activities, like deforestation,
With respect to the technologies for H2S removal, we can point
combustion of fossil fuels, biomass burning, and cement
out the following: (i) absorption via conventional absorption
production represent the main cause of global warming, since
process or membrane contactor (for the improvement of
1950. Several other gases are also important for the enhanced
absorption performance); (ii) adsorption using porous carbon
greenhouse effect.
materials, metal oxides adsorbents; (iii) conversion via Claus
Another important aspect of a more sustainable world is to
process (conversion of gaseous H2S into elemental sulphur),
counteract air pollution. Civilizations have always been
selective catalytic oxidation, liquid redox sulphur recovery; and
accompanied by a degree of air pollution. It started from
(iv) catalytic membrane reactor via H2S decomposition into H2
prehistoric times when people created the first fires. It has been
and S, H2S desulfurisation.8
described that “soot” was found on ceilings of prehistoric caves
Among all the methodologies mentioned above for both CO2
and related to inadequate ventilation of open fires.3 Nowadays,
and H2S removal, adsorption is either highly promising (CO2
major sources of air pollution come from emissions from motor
capture) or one of the most used (H2S capture). The reason for
vehicles, power plants, and industries. China, US, Russia, India,
this preference can be related to some advantages such as
Mexico, and Japan are the world leaders when it comes to air
reversibility and recyclability of the processes. The adsorbents
pollution,4 and these emissions are severely reducing the
are used usually in the form of spherical pellets, rods, monoliths
average life spans of numerous humans.
or powders. Important adsorbents are: (i) oxygen-containing
The most frequent contaminants are chlorinated hydrocarbons
compounds (hydrophilic and polar, including materials such as
(CHF), heavy metals, zinc, arsenic, benzene, H2S, etc. H2S has an
silica gel and zeolites); (ii) carbon-based compounds
unpleasant smell and is toxic, even in small amounts. The
(hydrophobic and non-polar), including materials such as
human nose can sense H2S at very low concentrations (0.4 ppb),
activated carbon (AC) and graphite; (iii) amine modified
and the toxicity threshold is about 10 ppmv. Apart from its
materials; and (iv) polymer based compounds (polar and non-
toxicity, H2S is highly corrosive to piping and production facility
polar functional groups in a porous polymer matrix).9
in both gaseous and solution state; it can provoke acid rain
There are clear evidence of a long history of use of AC, but the
when is oxidized to sulphur oxide (SO2), and many industrial
exact time and date for the first use has not been determined.
processes (hydrogenation, ammonia synthesis, and fuel cells);
The Ancient Egyptians mentioned the charcoal in a papyrus

83
Chapter 7 Nicolae, Wang, et al.
from 1550 BC as a way to adsorb unpleasant odours, and in 400 separation/absorption membranes, and separation through
BC, the Old Hindus and Phoenicians had started using charcoal adsorption.
to clean water, because of its antiseptic properties. The earliest
time in which active carbon was mentioned as an adsorptive 2.2.1. Separation with chemical/physical solvents
material was in 1773, when Scheele reported experiments with (absorption)
gases. In 1862, Frederick Lipscombe introduced AC for Absorption is one of the most used techniques for treating high
commercial applications, by using the material to purify potable volume gas streams containing both H2S and CO2, and it
water, and in 1881 Heinrich Kayser mentioned the ability of involves the usage of many solvents, like primary, secondary or
charcoal to uptake gases. AC was implemented at industrial ternary amines, activators, propylene carbonate, sulfinol, etc.16
scale in the beginning of twentieth century, by Chemische However, the related scrubbing processes require high energy
Werke and, during the First World War, it was used in gas masks input and is accompanied by corrosion and solvent
works by American soldiers to protect them from poison gas. degradation.16
Today, the applications of ACs are on the rise and they are being For example, H2S removal takes place in aqueous solutions of
used in numerous applications in medicine, water cleaning, gas alkanolamines. These are used in columns which can be either
cleaning, etc.10 packed or plate columns, with working temperatures for the
sorption step of 35–50 °C and absolute pressures of 5–205
atm.16 Regeneration is performed at an elevated temperature.
2. Present technologies for gas capture Another option is to use hollow fibre membrane contactors for
2.1. Carbon dioxide chemistry the scrubbing of acid gas mixtures, which can operate at
reduced temperatures, between 25 and 50 °C.16
CO2 is a non-polar, colorless and odorless gas (at low
For CO2 capture, the same scrubbing approach with absorption
concentration) with a density about 60% higher than that
using, for example monoethanolamine (MEA), is a commonly
of dry air. It is relatively nontoxic and noncombustible, and
used process.17 The efficiency of this removal is based on the
C is double bonded to O. The C=O bonds are equivalent and
acidic properties of CO2 and the base properties of the amine.
its length is 1.163 Å (0.1163 nm). As mentioned, it has two
As an effect, CO2 will be absorbed from the flue gas stream and
IR active bands: an antisymmetric stretching mode at
reacted with the amine in the scrubbing liquid. After the
2350 cm−1 and a degenerate pair of bending modes at
absorption, heat is applied to the absorbent liquid to release
667 cm−1.11 The two equal electric dipoles in CO2 molecular
the CO2 for storage and simultaneously the solvent is
fall on the same line and are oppositely directed, so the
regenerated.18 Besides the amines, there are other solvents
total dipole moment of CO2 is zero. However, because of
which could be used, including sodium hydroxide (NaOH),
the strong and oppositely directed dipole moments, the
sodium hypochlorite (NaClO), sodium or potassium carbonate
CO2 molecule has a strong electric quadrupole moment,
(Na2CO3 and K2CO3), which can be used to dissolve H2S and
which interacts with electrical field gradients on porous
CO2.5
materials allowing the gas molecule to adsorb on pore
surfaces.12 Other flue gas components such as N2, H2, O2 2.2.2. Conversion of H2S and CO2
and methane have either zero (methane) or much smaller
Both H2S and CO2 can be converted into other molecules and
quadrupole moments, which then give them less
solids. One of the most significant processes for H2S conversion
tendencies to adsorption.13 Table 1 provides some physical
is the Claus process, which was patented in 1883 by the chemist
properties of CO2.14,15
Carl Friedrich Claus. This process consists of recovering
elemental sulphur by oxidation of gaseous H2S, which is found
Table 1. Physical properties of CO2
in raw natural gas, biogas, and by-product gases in refineries
Physical Properties of CO2 and other industrial processes. The process involves thermal
Molar mass 44.009 g·mol−1 oxidation and catalytic reaction. Because of thermodynamic
Molecular shape Linear limitations of this process, a few per cent of H2S remains in the
Kinetic diameter 0.330 nm treated gas stream, which is removed in a second step via
Critical pressure (bar) 73.9 selective catalytic oxidation using a fixed bed reactor.19
Critical temperature (°C) 31.1 Conversion of CO2 can be performed via numerous approaches
Acidity (pKa) 6.35, 10.33 and is a very active research field and relates to the concept of
Specific volume at STP 0.506 m3 kg–1 CO2 reduction obtaining formic acid (HCOOH), methanol
Gas density at STP 1.976 kg m–3 (CH3OH), ethylene (C2H4), methane (CH4) and carbon
Dynamic viscosity at STP 13.72 μN s m–2 monoxide.20
Normal boiling point (°C) –78.5
2.2.3. Gas separation/adsorption membranes
pH of saturated CO2 solutions 3.7 at 101 kPa, 3.2 at 2370 kPa
A highly explored set of technologies for the removal of acidic
gases relies on separation via polymeric membranes. Many
2.2. Gas capture and storage techniques studies have been devoted to new materials and manufacturing
Various technologies for capture and storage of CO2 and H2S pathways for the development of permeable membrane,
have been developed, which can be classified in separation with appropriate for efficient CO2 and H2S separation from natural
chemical/physical solvents (absorption), conversion, gas gas streams.21 Despite of that focus, the technologies based on
84
membrane separation have some drawbacks. These drawbacks molecules and condensed state. Type IV(a) appears when
include the easiness of alteration of the separation efficiency capillary condensation is accompanied by hysteresis and the
because of different contaminants, concentration polarisation pore width exceeds a certain critical width. Type IV(b) is
phenomena, when adsorbed gases are accumulated in the common for closed conical and cylindrical mesopores,25 with
boundary layer of the membrane without diffusion and also, pore sizes smaller than the dimension being characteristic of
the balance between selectivity and permeability.22 the cavitation of the adsorbate in its liquid form. Type V is
obtained for hydrophobic microporous and mesoporous
2.2.4. Adsorption adsorbents, and type VI describes a highly uniform nonporous
2.2.4.1. Mechanism of adsorption surface.25 Chemisorption isotherms are usually described by a
In adsorption, atoms, ions or molecules are concentrated at an plateau being formed at lower pressures than the micropore-
interface. Adsorbate denotes the adsorbing molecule and filling plateau. The adsorption is limited in this case by the
adsorbent the material containing the interfaces, often a number of available sites for chemisorption. The shape of
porous solid. When a gas mixture is contacted with an chemisorption isotherms may be similar to that of Langmuir
adsorbent with a high internal surface and small pores, the isotherms but the related adsorption mechanisms are
components adsorb at different extent. The adsorption capacity different.23
depends on the specific surface of the adsorbent and the gas-
solid interactions. We distinguish physical adsorption
(physisorption) from chemical adsorption (chemisorption)23
with respect to the nature of the interactions.12 The collections
of intermolecular physical interactions grouped in the term
“van der Waals interactions” are dominant in physisorption. It
has a relatively low degree of specificity, taking place, initially in
a monolayer and then usually in multilayer, and the process is
reversible. Physical adsorption is always exothermic, and the
energy involved is larger than the energy of the condensation
of the molecules.23 Chemisorption, on the other hand, implies
that covalent bonds form between the molecules and
adsorbent, which are usually much stronger than van der Waals
interactions.24 It is sometimes a non-reversible process. For
chemisorption to take place, sometimes a significant activation
energy is involved.23
Both physisorption and chemisorption are described by
adsorption isotherms. They describe the pressure dependency
of the amount of gas that adsorb or desorb on the adsorbent at
a given temperature. Experimental adsorption and desorption
isotherms present a variety of shapes and forms. But, the
majority of the isotherms resulted from physical adsorption
have been classified in six types, according to IUPAC (see Fig.
1).25 Figure 1. Physical adsorption isotherms according to
Type I or Langmuir isotherms describe the adsorption on IUPAC (reproduced from25).
microporous solids with relatively small external surfaces, for 2.2.4.2. Types of adsorbents
example zeolites or certain porous oxides. In this case, the
The sorbents used for gas separation applications are diverse
amount adsorbed approaches a limiting value, governed by the
and include low-temperature (< 200 °C) solid adsorbents
accessible micropore volume. Type I(a) are usually
(carbon based adsorbents, zeolite based adsorbents, metal
characteristic of the adsorption on microporous materials with
organic frameworks, alkali metal carbonate based adsorbents,
mainly narrow micropores (< 1nm) and type I(b) describes the
amine-based solid adsorbents); intermediate-temperature
adsorption on materials having wider micropores and possibly
(200–400 °C) solid adsorbents (layered double hydroxides,
small mesopores (< 2.5nm).
magnesium oxide) and high-temperature (> 400 °C) solid
Isotherms of type II are usually obtained from the physisorption
adsorbents (calcium based adsorbents, alkali ceramic based
of most gases on nonporous or macroporous adsorbents. Their
adsorbents).26
shape is related to the multilayer adsorption, the completion of
the monolayer is marked by the point B.25 Type III isotherms are Non-carbonaceous adsorbents
characteristic of nonporous or macroporous solids. The Zeolites are used in several industrial gas separation processes
monolayer is not identifiable in this case, and the molecules and many studies have been performed. Most of them have
tend to form clusters around the most favourable adsorption used as adsorbents the materials with high crystallinity and high
sites because of weak interactions between adsorbent and surface area, like zeolite X, Y, and A. For example, Siriwardane
adsorbate.25 The mesoporous adsorbents are described by type et al.27 studied the CO2 adsorption capacity for zeolite 13X and
IV isotherms. The adsorption in mesopores is influenced by 4A at 25 °C and 1 atm of CO2 partial pressure, concluding that
both adsorptive interactions and the interactions between the both were performing similar, with highest adsorption
85
capacities of 3.64 mmol g–1 for 13X and 3.07 mmol g–1 for 4A. and reversible, so the energy management is a problem to take
Zeolites have also been studied for H2S adsorption. Ducom et into account in possible applications. Amine based solid
al. reported on the adsorption of H2S on zeolite 13X. In that adsorbents, like polymers or organic molecules containing NHx
study, 10 g of adsorbent was subjected to 80 ppmv of H2S and a groups, showed high potential for CO2 and H2S capture, because
capacity of 133 mgH2S g–1 was observed. Hernandez et al.28 of the possibility to bind chemically with acidic molecules.40
conducted a comparative study of different adsorbents for Carbon based adsorbents
biogas desulphurisation using Cu and Cr salts-impregnated on ACs are very popular for their adsorption properties. They
AC (RGM-3), zeolite 13X, Sylobead 522 and Sylobead 534 present several advantages over the other adsorbents, such as:
molecular sieve and two metal oxides, commercially known as (i) the low price of the raw materials; (ii) the fact that they can
ST and Sulfcatch ECN. They reported the metal-impregnated be produced from coals (coal, lignite), industrial by-products
carbon as the most efficient adsorbent, followed by zeolite 13X, (scraps of polymeric materials, petroleum) and sustainable
which performed better than the molecular sieves and the resources (wood or biomass sources); 40 and (iii) their relatively
metal oxides. Certain metal organic frameworks are very good high degree of microporosity and high surface areas. Usually,
adsorbents for the adsorption of CO2 at low pressures because carbonaceous adsorbents are prepared via carbonisation from
of a strong interaction. different carbon sources, like wood, coal, sugars, etc. at high
Metal organic frameworks (MOFs) have been reported as good temperatures. To create porosity and increase the specific
adsorbents for CO229–31 and desulphurisation.32,33 For example, surface area, activation processes can be employed. Physical
Hamon et al.34 studied H2S adsorption on several MOFs such as activation can be conducted by partial gasification of chars with
MIL-53 (AlIII, CrIII, FeIII), MIL-47 (VIV), MIL-100(Cr) and MIL- CO2, steam, air, or a mixture of them; at high temperatures
101(Cr). The first two materials were small-pore, about 11 Å, (800–1000 °C). Alternatively, chemical activation, which takes
and the last ones were large pore, of about 25 and 30 Å. The place at lower temperatures and is based on the introduction
maximum H2S uptake was obtained on MIL-101 at 30 °C and 20 of chemicals (like KOH, NaOH, K2CO3, and H3PO4) with the
bar purpose to open the pores and create a well-developed carbons
(38.4 mmol g–1). MIL-53 (Al, Cr), MIL-47 and MIL-100 adsorbed structure.41 Due to all these possibilities, a lot of research
in between 11.8 and 16.7 mmol g–1. Yang et al.33 studied the H2S studies were carried out using carbon-based adsorbents with
adsorption on MIL-68(Al) at room temperature. They reported engineered textural properties.
that the material was completely regenerable for five cycles,
retaining about 12 mmol g–1 of H2S at 30 °C and 12 bar. a) CO2 adsorption on ACs
Kumar et al.35 studied the CO2 adsorption on Mg-MOF-74. In The use of ACs and other carbon-based adsorbents is also
this way, they reported 165 cm3 g–1 CO2 at RTP on the pristine studied for potential use in CCS.42 The prospect of CO2 removal
Mg-MOF-74, followed by a decrease once the material has been using specifically carbonaceous materials obtained by using
exposed to moisture. They observed that one day exposure to through HTC of biomass as a pre-treatment step has been of a
humidity reduced the adsorption capacity to 127 cm3 g–1 CO2; 7 great interest for many research groups. For example, Sevilla et.
days to about 79 cm3 g–1 CO2 and 14 days to 53 cm3 g–1 CO2. Cho al43 reported the synthesis of highly microporous carbon
et al.36 studied the adsorptive properties of Co-MOF-74 for CO2. materials by direct activation of glucose with potassium oxalate
Adsorption and desorption cycles at RTP conditions showed a at high temperatures (> 800°C) under N2 flow. The micropores
positive affinity of CO2 on Co-MOF-74, retaining up to 288 mg volume was in between 0.49 and 0.67 cm3 g–1 (Dubinin-
g–1 of CO2. The selectivity towards CO2 over N2 in Co-MOF-74 Radushkevich model), and CO2 uptake at 25 °C and 1 bar was
was high as well (CO2:N2 = 25:1); the authors concluded that CO2 about 4.5 mmol g–1. By adding small amounts of melamine, they
was more strongly adsorbed than N2 because of a stronger tuned the structure more towards mesopores regime,
quadrupole moment. There are countless studies on CO2 underlined by a decrease in CO2 adsorption capacities with
adsorption on MOFs and to highlight one of them, Nandi et al.37 almost 50%, maximum CO2 uptake for the melamine containing
reported the synthesis of a 2D Cu based ultramicroporous MOF samples being 2.5 mmol g–1, in the same conditions. Sevilla et
(IISERP-MOF20), which showed moderate CO2 uptake as well as al.44 reported important adsorption capacities (both for H2 and
CO2:N2 selectivity for flue gas composition at room CO2) using porous carbon materials derived from three
temperature. At 1 bar and 25°C, IISERP-MOF20 adsorbed different biomass sources (starch, cellulose and sawdust) by
3.5 mmol g–1 of CO2, reaching a saturation capacity of about means of hydrothermal carbonisation (HTC) and subsequent
9 mmol g–1 at 1 bar and –78°C. chemical activation. The high surface areas (2690–3540 m2 g–1)
Different studies have been performed on alkali metal combined with bimodal porosity in the micromesopore range
carbonate-based adsorbents. Liang et al. and Chao et al.38,39 led to high CO2 adsorption capacities, such as 20–21 mmol g–1,
studied the mechanism of the CO2 capture on Na2CO3. They at 20 bars and 25 °C. By extending the pressure range, up to 40
proposed the follow reactions: bars, the CO2 uptake was considerably increased to
Na2CO3(s) + CO2(g) + H2O(g) 2NaHCO3(s) 30–31 mmol g–1. Xiao et al.45 reported adsorption capacities for
Na2CO3(s) + 0.6CO2(g) + 0.6H2O(g) 0.4[Na2CO3*3NaHCO3](s) CO2 of about 4.7 mmol g–1, at 0°C and 1 atm, using nitrogen-
doped carbon materials obtained from glucosamine via HTC-
The theoretical adsorption capacities for CO2, calculated from soft templating approach. CO2 adsorption was performed also
the first and second reaction were 9.43 and 5.66 mmol g–1, by Boyjoo et al.,46 who reported an adsorption capacity of about
respectively. K2O3-based adsorbents behave similar to Na2CO3- 5.22 mmol g–1 at RTP conditions, using as adsorbents activated
based ones. However, these reactions are highly exothermic hydrochars produced from CocaColaTM-derived wastes. The
86
value was even higher, at 0 °C and 1 atm, reaching about 6.27 pore volume were highly influenced by the amount of activating
mmol g–1. In their study, they prepared three ACs, starting from agent. When the amount of KOH was increased, both surface
the same biomass precursor and changing either the mass ratio area and pore volume increased up to 1122 m2 g–1 and 0.59 cm3
between the activator and hydrochar or the activation agent: g–1, respectively. By increasing further the amount of activating
(1) activation with ZnCl2, denoted as CMC_1 and CMC_2 (at agent, the textural properties were affected, decreasing up to
ZnCl2/HTC carbon mass ratios of 1 and 3, respectively), and (2) 849 m2 g–1 and 0.5 cm3 g–1, respectively; probably because of
activation with KOH, denoted as CMC_3 (KOH/HTC carbon = 4). the over-oxidation phenomenon of carbon walls with the
The adsorption capacities at 0°C and 1 atm were approximately generated CO2 and CO gases from the activating agent, and
4.8 mmol g–1 for ZnCl2 activated samples and 6.27 mmol g–1 for formation of insoluble potassium residues or minerals. The ACs
KOH activated sample. Bellemare et al.47 reported a high CO2 were tested for CO2 adsorption, and the uptake values followed
uptake by using sucrose-derived carbon materials. The the same trend as the surface area. First, it increased with the
adsorbents had been synthesised by combining ice and hard KOH concentration reaching a highest CO2 uptake of 6.3 mmol
templating and physical activation (via CO2). The obtained g–1 at 0 °C and 1 bar. Further increases in the KOH concentration
carbon materials possessed high surface areas with values caused a decrease in the CO2 uptake (up to 3.7 mmol g–1). The
ranging from 2560 and 2770 m2 g–1. The micropore surface above-mentioned highest adsorption capacity was observed for
areas were calculated via the t-plot method fom N2 adsorption the 1:2 biomass/KOH mass ratio, which had the highest surface
data and had values of 260–500 m2 g–1. The samples exhibited area and micropore volume together with a flat morphology of
a CO2 uptake of 3.8 mmol g–1 at 25 °C and 1 bar. Sevilla et al.44 the carbon particles. The CO2 uptake at 30 bar and 0°C was
prepared mesoporous and microporous ACs materials about 15.4 mmol g–1. The value obtained at STP (6.3 mmol g–1)
containing high concentration of functional groups. The ACs was similar to that measured for African palm shell-based
have been prepared by dry chemical activation through carbons,50 and slightly higher than those derived from rice husk
carbonisation of biomass, (starch and gelatine), at different (6.2 mmol g–1),51 sawdust (6.1 mmol g–1),52 celtuce leaves
ratios (gelatine: starch = 1:1, 2:1, 1:2). After carbonisation, the (6.0 mmol g–1),53 and wheat flour (5.7 mmol g–1).54
powders were further chemically activated with KOH, at 1/4 b) H2S adsorption on ACs
carbon/KOH mass ratio. The materials have been characterised
and tested for CO2 adsorption at 0 °C and 25°C at 1 bar, using a Considering all the harmful properties of H2S, many studies
volumetric method. The highest CO2 uptake was 7.49 mmol g–1 have been focused towards development of new, efficient and
at 0°C and 1 bar, decreasing with approximately 50% at room sustainable adsorbents for its removal.55–57 Among the present
temperature. Pure gelatine-derived carbon exhibited about technologies for desulphurisation5, removal of H2S with carbon
4.25 mmol g–1 at STP conditions and 3.30 mmol g–1 at 25 °C (RTP based adsorbents has been recently reported.58–61 For example,
conditions). Starch-derived carbon exhibited the lowest CO2 Olalere et al.62 recently studied the H2S removal from waste
uptake among all the samples: 3.02 mmol g–1 (at STP) and water using coconut shell-based ACs. The carbon-based
2.81 mmol g–1 (at RTP). The improved adsorption capacity for adsorbents were prepared by pyrolysis combined with chemical
the gelatine-starch samples was tentatively ascribed to N- activation using KOH. The obtained powders were
doping. Moreover, the ACs showed a good CO2/N2 selectivity, characterised by a porous structure composed of about 76% C
up to 98/1 at 0 °C and 1 bar. The power of N-doping to enhance and 19% O, the rest was traces of Al, Si, and K. During the
the adsorption capacity of ACs has though been convincingly adsorption experiments, the influence of initial concentration
challenged.48 In another study, Yaumi et al.49 studied the CO2 of H2S was tested. The adsorption was measured under stirring,
capture using rice husk derived carbon materials. The authors with a gas flow starting from 100 mg L–1 up to 500 mg L–1. The
prepared carbon adsorbents starting from rice husk by chemical H2S adsorption capacity increased by increasing the initial gas
activation with H3PO4 and melamine, at different mass ratios of concentration, varying from 2.52 to 13 mmol g–1, after 14 h
acid impregnated rice husk to melamine. The obtained carbons contact time, using 1 g of adsorbent. Shang et al. 63 reported the
performed well for CO2 uptake, the adsorption capacities being adsorption of H2S on biochars derived from agricultural and
in between 1.08 and 4.41 mmol g–1, at 30 °C and 1 bar. The best forestry waste, such as camphor tree, rice hull and bamboo. The
performance was achieved for the sample impregnated with obtained adsorption capacities were smaller than those
phosphoric acid at a mass ratio of 1:2, and subsequently reported by Habeeb et al.62 with values of 0.05–11.2 mmol g–1.
modified with melamine at a ratio of 5:1 (rice husk containing The maximum adsorption capacity was achieved for the biochar
H3PO4 : melamine). An increase in melamine concentration with the higher specific surface area (115 m2/g) and the higher
caused a decrease in CO2 sorption capacity, probably because carbon content (rice hull, 26% C), which was expected as the
of a high number of nitrogen species, which could block part of number of adsorption sites for H2S increases. The experiments
the pores and decrease the accessible surface area. Also, by were performed at room temperature, using 1 g of adsorbent
means of chemical activation, Ello et al.50 prepared activated for each test. Mochizuki et al.64 studied the adsorption
biocarbons with high surface area and adsorption capacity behaviour of H2S on ACs prepared from petroleum coke by KOH
towards CO2. They used Arundo donax or giant cane as carbon chemical activation. The H2S uptake was tested at 30 °C at H2S
precursor and KOH as chemical activator. The dried material partial pressure 0.2 kPa for different times (5, 15, 24 and 48 h).
was crushed and sieved into fine particles and mixed with KOH. The H2S uptake increased with increasing the weight ratio of
To optimise the effect of activation, different KOH/biomass KOH-petcoke (the carbon source). The maximum adsorption
mass ratios were used. Subsequently, the mixtures were capacity for H2S obtained was about 3 mmol g–1. Zhang et al.65
calcined at 600 °C, for 2h, under N2 flow. The surface area and reported on H2S removal with ACs prepared from black liquor.
87
They used a gravimetric method with a spring balance to record All of the studies mentioned above seem to support that
the mass at room temperature, 35 °C and 45°C. The gas biomass represents a very good precursor for ACs. Beside of the
concentration was varied from 200 to 800 ppm. It was observed advantageous of a low cost and sustainable activation
that the amount of gas adsorbed decreased as the adsorption processes, the availability and recyclability of carbon are
temperature was increased, consistent with the exothermal environmentally sound. There are plenty resources of biomass,
nature of adsorption. which can be exploited, such as wood and waste wood, leaves
Bagreev and Bandosz et al.8 studied the influence of alkali of the plants, agricultural waste, waste paper, garbage, and
compound on the carbon surface, such as NaOH and K2CO3. human waste. Recent articles reported that the globally food
Their results showed an improvement in the sorption capacity waste each year is 1.3 billion tonnes, including about 45% of all
for H2S. The highest uptake were observed for a moderate fruits and vegetables, 35% of fish and seafood, 30% of cereals,
amount of impregnated NaOH as an excess resulted in pore 20% of dairy products and 20% of meat. In UK, 15 million tonnes
blocking. Castrillon et al.57 also studied the influence of carbon of food is wasted each year from which 4.2 million tonnes of
modified with caustic materials and iron oxide (Fe2O3 for CO2 edible food each year, meaning that a family throws away £700
and H2S removal from CH4 streams). Several studies were worth of perfectly good food every year.68 Biomass can be
dedicated to proving the benefits on sorption capacity for H2S further processed and used in different applications, as energy
when combining physical adsorption with a preceding chemical storage, electrocatalysis and photocatalysis, heterogeneous
reaction. Based on the same idea, Fe2O3 was reported as an able catalysis, gas storage and conversion. One of the processes with
inorganic compound for the removal of H2S. Since H2S is a the smallest ecological foot print is HTC through which the
diprotic acid, it might react with impregnated surfaces, biomass can be further converted to carbon. As a general
according to the following reactions: principle, HTC allows the production of coal in similar way like
• Impregnation with NaOH 55 in nature, but in shorter time in the laboratory.
H2S + NaOH NaHS + H2O (1) 2.2.4.3. Post-combustion CO2 capture
H2S + 2NaOH Na2S + 2H2O (1a)
In relation to the carbon materials studied for CCS (see
• Impregnation with K2CO3 66 above) it is important to reflect on the different
(2)
H2S + K2CO3 KHS + KHCO3 approaches to CCS. There are mainly three different
(2a)
H2S + K2CO3 K2S + H2CO3 configurations of technologies applied to capture CO2 from
• Impregnation with Fe2O361 flue gas: post-combustion, pre-combustion and oxyfuel
Fe2O3
+ 3H
FeS + FeS2
+ 3H
2O (3)
combustion.69 In post-combustion CO2 capture, the main
2S
+ 3H Fe2S3
+ 3H challenge is to separate CO2 in a dilute mixture at close to
Fe2O3 2S 2O (3a)
+ 3O + 6S (3b) ambient pressure of mainly N2.70 The emitted flue gas is
2Fe2O3 2Fe2S3
2 typically at a temperature of 45–180 °C, a pressure close to
1 atm,71 and has a partial pressure of 0.05–0.15 atm of CO2.
In the study conducted by Castrillon et al.57, three commercial
A low temperature is typical for a modern combustion unit
ACs produced by Donau Carbon GmbH (Germany), with specific
with effective heat integration. Fig. 2 shows a diagram of
surface areas in the range 815–1005 m2 g–1, were tested for
carbon capture and compression process in a coal-fired
their CO2 and H2S uptake capacities. Co-adsorption of H2O and
power plant.70
H2S enhanced the adsorption of H2S. Under dry conditions, the
adsorption capacity of H2S was 8.2 mmol g–1, meanwhile under
humid conditions, it was almost four times higher
(30.9 mmol g–1). The CO2 adsorption capacity was considerably
smaller with a value of 1.67 mmol g–1.
Sethupathi et al. 58 conducted a study involving biochars as
potentials absorbents of CH4, CO2 and H2S. They investigated
four types of optimised biochars derived from perilla leaf,
soybean stover, Korean oak and Japanese oak. When adsorbed
together, CO2 and H2S are competing. In order to study in more
detail this competition phenomenon, the authors conducted a
single-gas study for CO2, H2S and CH4. During the measurement,
it was observed that without the CH4 and CO2, all the biochars Figure 2. Process diagram of carbon capture and
performed better for H2S adsorption compared to the compression in a coal-fired power plant.70
simultaneous adsorption experiments, and a single-gas
adsorption measurement for H2S revealed about 294 mg g–1 In adsorption processes, adsorbents are typically
adsorption capacity, which was significantly higher than that granulated and put in two or more interconnected
obtained in a similar study (73 mg g–1) and also higher than the columns. In a case of two columns, the flue gas first is
adsorption capacity obtained during simultaneous tests. It was contacted with column 1 where the more interacting CO2
also observed that all biochars showed higher preference for is adsorbing preferably and a N2 rich gas is flowing out of
H2S than CO2. This was based on that CO2 adsorbs mainly the column 1 outlet. When the column 1 capacity is
through physisorption, and the mode of H2S adsorption exhausted, the flue gas stream is switched to column 2
depends on the local pH within the pores.67 where CO2 continues to adsorb; meanwhile, the CO2 in
88
column 1 is recovered/desorbed by either a reduced total microns; the porosity can be changed by using natural
pressure (pressure swing or vacuum swing adsorption, templates or activation procedures; their chemical and physical
PSA, VSA) or by an increased temperature (temperature properties can be easily changed by the combination of the
swing adsorption, TSA). Due to the massive flows of flue carbon materials with different components, such as inorganic
gas, it becomes critical that the PSA/VSA/TSA have much compounds, and they can be further functionalised because of
shorter cycle times than what is usual in adsorption driven the highly oxygenated surface of the carbons.
technologies, which puts additional demands on the 3.2. Templating methods
adsorbent and the structuring of the adsorbent.
Gas separation with adsorption can be either under One of the first attempts using templating methods to develop
thermodynamic control or under kinetic control, where the porous carbon materials was carried out by Gilbert et. al.74 in
different diffusion rates of the different gases within the 1982. They synthesised porous glassy carbons by
micropores can contribute to the selectivity.72 Almost all impregnation of the silica template with phenol-
commercialised PSA/VSA/TSA processes operates under formaldehyde resin mixture. This methodology reflects the
equilibrium or close to equilibrium processes with notable “hard(exo) templating” approach (Fig. 3b), and, in general,
exceptions including the purification of N2 from air with is based on the mixture of a carbon precursor (usually
carbon molecular sieves. For AC, which is a nonpolar phenolic resins) with a hard template. In this way, the
adsorbent, the van der Waals interaction of the dispersion carbon precursor infiltrates in the structure of the template
plays a significant role. The size and the polarizability of the and is carbonised within the pores (at high temperatures, >
adsorbate and the pore size distribution of adsorbent are 700 °C). Finally, the template is removed, leaving behind a
influential, while the surface chemistry of the adsorbent is well-defined structure. Later on, Dai et al.75 developed a new
less important. According to the International Union of strategy for creating porous carbon materials based on the
Pure and Applied Chemistry (IUPAC),72,73 the pore size self-assembly properties of block copolymers and aromatic
classifications is shown in Tab. 2. So, if the flue gas is not to resins, such as phloroglucinol, or resorcinol/formaldehyde.
be compressed it will be the effective uptake capacity of This method is known as “soft (endo) templating” and is a
the ACs in the domain of about 0.01–0.1 bar of CO2 at classical way to produce inorganic porous materials (Fig. 3a).
about The classical carbonisation techniques, such as pyrolysis,
50 °C that will be important in a VSA process. present many drawbacks when compared to HTC. Since HTC
takes place in aqueous phase, it can be easily combined with
Table 2. The pore size classification in IUPAC these templating methods. The use of carbohydrates as
carbon precursors together with different templates might
Type Pore size (nm) improve textural properties. In addition, the surface
Ultramicro-pores < 0.7 chemistry of the resulting hydrothermal carbons is highly
Supermicro-pores 0.7–2 populated oxygenated groups and can be further modified.
Micro-pores <2
Meso-pores 2–50
Macro-pores  50
pH of saturated CO2 solutions 3.7 at 101 kPa, 3.2 at 2370 kPa
3. Carbon materials and HTC

This section describes several synthetic routes used to get a
well-defined porosity in hydrothermal carbons. Different
methods to create and develop porosity based on the usage of
structure-directing agents (organic or inorganic templates) is
discussed and explained here. The methods introduced below Figure 3. Graphical representation of the templating methods:
allow customising the carbon structure via the tools of colloid soft templating (a), and hard templating (b).76
and polymer science, leading to well-developed morphologies
3.2.1. Soft templated carbons and its interfacing with HTC
for a broad range of applications.
Nanostructured HTC-derived materials can be synthesised
3.1. Introduction
using polymeric templates of defined size and shape. In soft
Porous hydrothermal carbon materials have attracted templating, an amphiphilic molecule such as a surfactant or
considerable attention, over the time, because of their wide block-copolymer is self-assembling with the carbon precursor
range of applications. HTC is a methodology developed 100 into an organised mesophase, which is stabilised by the thermal
years ago, but it is one of the most used strategies for the treatment. The process is influenced by several parameters, like
production of carbon materials. This method proceeds at low concentration, temperature, hydrophilic or hydrophobic
carbonisation temperatures (180–250 °C), and self-generated reaction, pH, etc. The most common polymers used are
pressure; the solvent normally used is water; the particles poly(ethylene oxide)-b-poly(propylene oxide)-b-PEO) from the
present spherical shape and small sizes in the domain of Pluronic family,77 polystyrene-b-poly (ethylene oxide)
89
(PS-b-PEO)78 or polystyrene –b-poly (4-vinlpiridine) (PS-b-poly The specific surface areas were between 640 and 857 m2 g–1,
(ethylene oxide), triblockcoplymers (PEO-b-PPO-b-P4VP); and the total pore volumes between 0.34 and 0.45 cm3 g–1,
carbon precursors are small clusters of phenol-formaldehyde, depending on the high temperature treatment.
or “resol” or phloroglucinol-formaldehyde resins. The main
reason for using resins as carbon precursor is their high number
of hydroxyl groups that can form very strong hydrogen bonds
with the miscible segment of the block copolymer, which
creates micelles, responsible for producing mesoporosity in the
resulted carbon. Liang et al.75 prepared highly ordered porous
carbon films by using block copolymers (PS-P4VP) and
resorcinol. The synthesis was performed sequentially by
monomer-blockcopolymer film casting, solvent annealing,
polymerisation, and finally carbonisation. The highly ordered
carbon film possessed a thickness from several tens of
nanometeres up to about 1 µm and sizes up to 6 cm2. In another
study, Tanaka et al.79 prepared mesoporous carbon membranes
using soft-templating route. They started from resorcinol,
phloroglucinol and formaldehyde together with Pluronic F127
as structuring agent. The reason to use a combination of
phloroglucinol-resorcinol-formaldehyde, instead of the Figure 5. (a,d) HRSEM images, scale bar, 100nm; (b,e)
standard mixture of resorcinol-formaldehyde, was to improve TEM images, b – scale bar, 50nm, e – scale bar, 100nm
on the thermal stability and to reduce the weight loss. The (reproduced from81).
resulted membrane was porous (Fig. 4c), having a surface area
of 670 m2 g–1 and a pore volume of 0.58 cm3 g–1. The average
pore diameter was approximately 4 nm as estimated from BET
and TEM measurements (Figs. 4a,b). The stability of the
membrane was also studied. The membrane was subjected to
a hydrothermal treatment at 90 °C and the structure
characteristics and gas permeation of the carbon material
remained unchanged.
Figure 6. N2 sorption isotherms for mesoporous
carbonaceous nanospheres carbonised at 400 ˚C (left
side) and 600 ˚C (right side) (reproduced from81).
The soft templating method has been shown to be efficient

for the development of carbon structures and studies have
used different carbon precursors, such as carbohydrates,82
nitrogen and carbon rich compounds, like melamine,83
glucosamine,84 and natural polymers (lignin, chitin,
Figure 4. (a) Top-view SEM, (b) TEM micrographs and (c)
cellulose).85 The synthesis of ordered porous carbon
N2 adsorption/desorption isotherm of the mesoporous
materials derived from fructose has been reported. Kubo
carbon membrane carbonised at 600°C (reproduced
from79). et al.82 used the advantages of HTC together with the soft-
templating method. By performing the HTC at 130 ˚C, they
Starting from phloroglucinol-formaldehyde system, Chengdu et ensured the stability of the polymer and created the
al.80 prepared composites of carbon-carbon nanotubes for Li- necessary conditions for HTC of fructose. In their first
ion batteries. They used Pluronic F127 as template and the final attempt, the HTC of fructose was performed with the F127
products were carbonised at 550 °C to preserve a high number block copolymer surfactant. The final composite presented
of defects. The samples showed porous structure with surface ordered porosity with pores diameter around 10 nm and
areas between 378 and 570 m2g–1. wall thickness of 6 nm, indicating that the self-assembly of
Qiao et al.81 reported on the synthesis of mesoporous carbon the polymer together with fructose was successful. SEM
nanospheres starting from resorcinol and formaldehyde. Fig. 5 shows the presence of spherical particles derived from
shows the spherical morphology of the carbon spheres. The fructose (Fig. 7a), and TEM presents the ordered porosity
porosity was analysed by N2 adsorption-desorption isotherms. of the carbonaceous material (Figs. 7b-c). The N2 sorption
The isotherms showed a multimodal distribution of pores, it isotherms show a non-reversible microporous type I
had an intermediate shape in between type I, characteristic for isotherm, with a pore size distribution composed of a sharp
micropores, and type IV characteristic for mesopores (Fig. 6). peak at 0.9 nm and a shoulder around 2 nm. The material
The pore sizes from the adsorption analyses were in good had a surface area of 257 m2 g–1 and a pore volume of 0.14
agreement with TEM indicating a small pore size of about 3 nm. cm3 g–1.
90
Figure 7. (a) SEM, (b) TEM, and (c) HRTEM micrographs of

ordered mesoporous carbons derived from fructose.
Figure 9. Nitrogen sorption isotherms (a) and pore size
(reproduced from82).
distribution (b) of the soft templating materials.
Yan et al.83 reported on the synthesis of mesoporous (reproduced from86).
graphitic carbon nitrides, which are potential photo Overall, soft-templating method can be easily used in the
catalysts for the H2 evolution reaction. Melamine was used synthesis of nanostructured HTC-derived materials. By
as both the carbon precursor and nitrogen source, and using block copolymers during HTC treatment, we can tune
Pluronic P123 as soft template for the preparation of the shape and size of the materials. Moreover, the
graphitic-C3N4. This solid had a high surface area and template can easily be removed by treating the material at
extended light absorption towards the maximum of the high temperature under inert atmosphere.
solar spectrum (800 nm), which is promising for the use of
this class of materials for the photocatalytic preparation of 3.2.2. Hard templated carbons
H2 using visible light. The mesopores could also be Carbon materials with ordered porous structures can be
observed from TEM analysis, as shown in Fig. 8. The prepared from hard-templating processes. Starting from a
graphitic-C3N4 had a plate-like surface morphology with a porous silica mold, a porous carbon negative replica can be
worm-like porous structure. Without the soft template, produced by first infiltrating the silica with a carbon
only few worm-like pores were observed and it was clear precursor, followed by subsequent pyrolysis and final
from that study that the template played an important role dissolution of the silica template.87–90 For instance,
in controlling both the morphology and pores. similarly to the production of CMK-type materials
pioneered by Jun et al,91 hexagonally-ordered mesoporous
SBA-15 silica has also successfully been used as the
sacrificial inorganic template for the preparation of
ordered carbonaceous replicas, using furfural as carbon
precursor, synthesized under hydrothermal conditions.92
Significantly, it was found that it is important to match the
template surface polarity (e.g., hydroxylated vs. calcined
silica) with that of the carbon precursor, so that HTC
decomposition products can successfully penetrate the
pores of the template. Demir-Cakan et al.100 reported on
Figure 8. TEM images of g-C3N4 prepared with Pluronic the synthesis of mesoporous carbon materials with
P123: (a) low magnification, (b) high magnification. increased and versatile functionality. This replication was
(Reproduced from83). achieved by performing the HTC of glucose in the presence
of small amounts of water-soluble vinyl monomers.
Another study involving the soft-templating method was
conducted by Xiao et al.84 HTC coupled with soft 3.2.2.1. Zeolite-templated microporous carbons
templating was used to prepare porous carbon materials Although carbon mesostructures can be prepared by filling
using glucosamine as the carbon precursor and the triblock the ordered mesopores of silica scaffolds, and removing
copolymer P123 as a soft template. They considered the the silica afterwards, preparing well-defined ordered
influence of pH and amount of template. By moving from microporous (pore diameter < 2 nm) carbon materials has
neutral to acidic conditions during synthesis, the specific been a long-standing challenge. A variety of inorganic
surface area of the final carbon product increased from templates (e.g., mesoporous silicas) and various carbon
550 to 980 m2 g–1. As was expected, an increased amount sources (including sucrose, furfuryl alcohol, acrylonitrile,
of template in the synthesis led to that the pore volume of propylene, pyrene, vinyl acetate, and acetonitrile) have
the carbon material increased. The N2 sorption isotherms been used to prepare porous carbons.95 However, these
revealed that the materials had both micro and mesopores have consisted of not only micropores but also a large
with Type I and IV isotherms characteristics, and H4 amount of mesopores. Based on the hard template
hysteresis loops (Fig. 9). Due to these specific textural techniques, carbon replicas from microporous materials
properties, the carbon materials displayed adsorption such as zeolites have also been synthesised. Typically,
capacities of CO2 between 3.7 and 4.7 mmol g–1, at 25 °C carbon can be introduced into the nanochannels of the
and 100 kPa. microporous materials by means of polymer impregnation
(e.g., furfuryl alcohol-impregnation at a reduced pressure)
followed by carbonisation and/or chemical vapour
91
deposition (CVD) using small molecules, such as ethylene lanthanum, yttrium or calcium catalysts lowers the
and acetylene as a carbon source for achieving successful decomposition temperature of ethylene by 200 °C.
carbonisation within the zeolite pores. After removal of the Although the exact catalytic mechanism remains unclear, it
zeolite, a zeolite-templated carbon (ZTC) can be obtained results in carbon formation exclusively inside of the zeolite
as a reverse replica of the parent zeolite. Ma. et. al. pores in which the catalyst is present. After zeolite
reported on certain carbon replicas prepared from zeolite removal, a well-defined, ordered and interconnected
Y (with a framework type code of FAU96) that possessed a microporous carbon structure was obtained. The same
periodic ordering structure, an impressively high surface strategy was applied to several different zeolite structures,
area and large micropore volume. The framework type FAU with pores down to 0.71 nm. Three-dimensional open
is three-dimensional and belongs to the 12-ring zeolites, zeolite porosity leads to interconnected carbon networks.
which have the smallest interconnecting pore openings Interestingly, also well-defined carbon quantum dots with
construed of 12 oxygen-linked aluminum or silicon atoms. intriguing optical properties can be formed.100
In this experiment, zeolite channels were first filled with In terms of these hard templated carbons, a stoichiometric
furfuryl alcohol (FA) by impregnation and then additional C removal of the zeolite framework is promoted by using HF
was deposited into zeolite channels by a propylene CVD or NaOH under hydrothermal conditions101; however, as
process. The carbon/zeolite Y composite was further heat- HF imposes safety hazards, it limits the options for
treated at 900 °C under a N2 flow, and the resultant carbon industrialisation. For the hydrothermal treatment with
was liberated from the zeolite framework by a reactive NaOH, zeolites are sometimes dissolved and recrystallised.
treatment with hydrogen fluoride (HF). In other For instance, van Tendeloo et al.102 showed that a
experiments, zeolites with large pore materials (e.g., those commercial NH4+-zeolite Y zeolite transformed into five
with framework type codes of *BEA, LTL and MOR, which different frameworks types depending on the cations of
have interconnected pore channels with sizes up to 12- the base but with NaOH, the zeolite-Y framework largely
rings) allow more carbon precursor loading. There are persisted.
different parameters influencing the results of the carbon 3.2.2.2. (Silico)aluminophosphate templated carbons
replication. The disorder or the symmetry of the structure,
which is the case for zeolite beta (*BEA framework type Aluminophosphate molecular sieves (AlPO4s) have
code) and zeolite Y (FAU framework type code), while attracted considerable interest since they were first
zeolite L (LTL framework type code) that possesses a one- synthesised a few decades ago at Union Carbide
dimensional channel system could not produce a three- Laboratories.103 They are isoelectronic representations of
dimensional carbon replica. Moreover, the intersecting microporous silicates (SiO2), with alternating and oxygen-
pore channel systems of mordenite (MOR structure code) linked aluminum and phosphorous atoms. Their
with 12- and 8-rings did not result in a carbon network frameworks are mainly covalently bonded. The chemistry
because of poor carbon infiltration throughout the small 8- of AlPO4s and microporous silicates (SiO2) are different and
ring pore openings. Gaslain et.al.97 used the zeolite EMC-2 AlPO4s can be dissolved in aqueous solutions of
(EMT structure code) and achieved a microporous carbon HCl.104,105,106 Even though this chemical difference is
with three well-resolved XRD peaks. Nishihara and Kyotani promising for the use of AlPO4s to derive ACs, only a limited
summarised in a recent article98 that ZTC with a 3D amount of work has been performed. AlPO4-5 (AFI), with
graphene framework is free from graphene the space group of P6cc2 or P6/mmc3 was the first member
aggregation/stacking, therefore, unlike representative of the family studied for carbon replication. It is a material
nanocarbons C60, SWCNT, graphene of 0D, 1D, and 2D with potential application in catalysis, separation
structures, which inevitably cause unfavourable surface technology and nonlinear optics, but it is particularly
loss, the 3D ZTC with a self-standing open framework can relevant in model systems as the pore walls are smooth.103
almost fully expose the entire surface. The high geometric Tang et.al107 synthesised hexagonal carbon micro and
surface area of ZTC (3710 m2 g–1) can exceed the value of submicro tubes within the channels of an AlPO4-5 material;
graphene (2630 m2g-1), and the experimentally measured however, its one dimensional pore system makes it less
highest surface area of ZTC (3730 m2g–1) is almost the same useful for the synthesis of ACs. We expect that the pore
as the geometric one. systems of suitable AlPO4 templates should be three or at
Zeolite-type materials represent an interesting and least two dimensional, have sufficiently large pore window
extreme test for replication strategies, because the apertures and high thermal stability.108 A related class of
dimensions of their channels and cages are quite similar to suitable templates is the microporous
those of the infiltrated materials that constitute the silicoaluminophosphates (SAPOs). Similar to zeolites
replica. For instance, Kim et al99 described a new strategy (aluminosilicates), SAPOs have a negatively charged
to prepare these materials with unprecedented precision. framework. Si (formal charge of +4) replaces P (formal
However, common carbon precursors such as sucrose or charge of +5) in SAPOs.109 H-SAPOs can be used for acid
furfuryl alcohol are too large to easily diffuse in the 0.5–1.3 catalysis,110 which is relevant for promoting the CVD of
nm-sized pores of zeolites. Smaller precursor molecules carbon precursors.
like ethylene decompose only at high temperatures, For the reasons mentioned above, synthesis of ACs derived
resulting in incomplete pore filling and carbon structures from a SAPO/AlPO/MAPO whose framework can be
being formed outside the zeolite pores, while using the removed under mild conditions without using HF would be
92
meaningful. We initially started the studies from SAPO-37-

templated carbon synthesis, because of several reasons. Table 3. Structure codes and properties of selected
Similar with zeolite Y, SAPO-37 has FAU framework.96 In SAPO/AlPO/MAPOs from zeolite database.
addition to having a 3D structure, large pores and pore
Structure Largest Accessible
window apertures, H-SAPO-37 is highly acidic and its -OH Name and reference Dimension
code ring volume (%)
groups are thermally stable (up to 1000 °C)111. AFR SAPO-40115 2 12 19.60
Nevertheless, after removing the organic template, it MAPSO-46116
AFS 3 12 21.06
becomes amorphous in an ambient atmosphere when MAPO-46117
being exposed to water vapour.112 The high thermal CoAPO-50116
stability and acidity of H-SAPO-37 are advantageous for AFY MgAPO-50118
3 12 21.88
infiltration and deposition of carbon precursors at high MnAPO-50119
temperature. On the other hand, its low hydrothermal ZnAPO-50120
stability makes it promising for dissolution under mild ZnAPSO-64121
conditions. Details of the templating of the H-SAPO-37 will BPH [Co-P-O]-CAN122 3 12 21.65
[Zn-P-O]-CAN123
be shown in the PhD Thesis of GreenCarbon’s ESR 8 (Xia
[Mn-Ga-P-O]--
Wang) and the associated publications, and be compared
CLO124
with templating using zeolite Y. The comparison will -CLO 3 20 33.76
[Zn-Ga-P-O]--CLO124
involve detailed characterization with methods that [Al-P-O]--CLO125
include TEM/SEM, infrared and Raman spectroscopy, SAPO-37109
adsorption studies and various digestions studies [Co-Al-P-O]-FAU126
comparing the ease of removing the SAPO-37 with that of Beryllophosphate
FAU 3 12 27.42
zeolite Y from carbon-zeotype composites. Table 3 X127
provides the list of already synthesised SAPO/AlPO/MAPOs Zincophosphate
with pore window apertures ≥ 10- rings and at least two X127
dimensional pores systems, as derived from the current LTL [Al-P-O]-LTL128 3 12 15.37
zeolite database.96 These molecular sieves could be used MEI ECR-40129 3 10 21.11
as templates for hydrothermal carbon synthesis. Recently,
Braun et al.113 developed a theoretical framework to
generate a ZTC model from any given zeolite structure,
predicting the structure of known ZTCs, which would be a
reference for future SAPO/AlPO/MAPO-templated carbon
studies. In this work, HTC technique will be used to fill the
carbon precursor into the SAPO/AlPO/MAPO micropores
using a modification of the protocol by Demir-Cakan et
al.100 followed by CVD of selected precursors.
3.2.3. Salt templating

Another approach to generate porosity in the structure
of the carbon materials is called “salt templating” (see
Fig. 10). This method uses inorganic salts as templates
during the synthesis at elevated temperatures. The
melting points of the salts are reduced by addition in Figure 10. Graphical representation of the salt-templating
eutectic compositions and at high temperatures, ion method (reproduced from 130).
pair or clusters in the carbon structure are form. Finally,
the salts are removed by washing with water, leaving
pores behind114,139. The advantage of this process is the
possibility to improve the ratio of micropores to
macropores, obtaining a structure with multimodal pore
size distribution. Fechler et al.114 showed that, by changing
the components of the eutectic mixture, the porosity of
the resulting carbon could be controlled. For example,
moving from LiCl/ZnCl2 to KCl/ZnCl2, the porosity varied
from microporous structure to mesoporous and then Figure 11. N2 sorption isotherms of carbons templated
macroporous structure (Fig. 11). The specific surface areas with LiCl/ZnCl2, NaCl/ZnCl2, KCl/ZnCl2 (upper part) and
of the carbons ranged from 1100 to 2000 m2 g–1 and they schematic representation of pore formation (lower part).
had different morphologies. With mixtures with LiCl, the Each left image depictures the carbon (grey)/salt(red)
materials exhibited mainly micropores, maybe because in composite and each right image the carbon after washing;
this case, the salts act as molecular templates forming ion reproduced from114.
pairs or clusters of free minimal energy.
93
Using NaCl, mesopores were formed, which was ascribed to the had been developed, it was realised that there are many
lower melting point of NaCl and tentatively to the formation of other decent reasons for using electrons, most of which are
bigger salt clusters. Finally, when KCl was used as a component used to some extent in a modern TEM instrument.
of the eutectic mixture(the salt with the lowest melting point), 4.1. The instrument
sorption isotherms typical for macropores were obtained.114
Another attempt using the salt templating approach has been Max Knoll and Ernst Ruska invented the first TEM
described by Kumar et al.131, where they used a eutectic instrument in 1931 at the Technische Hochschule Berlin,
mixture of LiCl/ZnCl2 together with a biomass derived which was capable of displaying magnified images of a thin
precursor. They studied the influence of the salt concentration specimen with a magnification of 103–106. In a TEM
and biomass precursor type. Their materials had surface areas instrument, electrons are emitted by the electron source
from 520–1000 m2 g–1. An increase in the salts concentration situated at the top of the microscope column and are
seemed to have altered the dimension of the pores in such a accelerated towards the specimen with the positive
way that by using small amount of salts, the final product electric potential. Then, the beam of electrons is
exhibited multimodal structure with large micropores, condensed by the first and second condenser lenses on the
approximately 1.3 nm, and mesopores with dimensions of way to the specimen. The fast electrons are transmitted
3–4 nm. By increasing the amount of salt, the samples tended through the sample, which are normally prepared to be
to have lost some micropores, possessing more mesopores or thin enough so that the electron beam can pass through
with even more salt only mesopores. This switch could have without losing too much intensity. After interacting with
occurred that by increasing the salt concentration, the clusters the sample, the electrons are focused by the objective lens
that formed at a high temperature tended to increase in size to form an image. There is an objective aperture just above
and in turn formed large pores. In that study, the highest the objective lens, which can be used to select either direct
specific surface areas were achieved when a mixed precursor or scattered electrons to contribute an image. Below the
consisting of algae and arginine was used. These surface areas specimen, series of lenses are used to magnify the images
were tentatively rationalised by the ability of arginine to or electron diffraction (ED) patterns, which are very useful
carbonise within the pores of carbonised algae. TEM images of for analysing crystalline matter. Fig. 13 shows the
the carbon materials derived from different precursors are schematic diagrams of projecting the image and the ED
shown in Fig. 12. They confirm the presence of ribbon-like pattern onto the screen by both image diffraction modes.
morphology in the case of samples prepared from arginine and Modern TEM instruments are able to achieve a very high
mixture of arginine and algae. The carbon materials derived resolution, and usually consist of a beam column as tall as
from pure algae showed neither oriented porosity nor 2.5 m with a diameter of about 30 cm, while the addition
distinctive morphologies. Overall, the salt templating method of aberration correctors compensates the spherical
provides control over textural properties, porosity and surface aberration.
areas. Because of the possibility to create multimodal pore
structures, the final materials are targeted in plenty of
applications such as gas storage and conversion, water
treatment and catalysis.
Figure 12. TEM micrographs of carbon materials derived from

(a) pure algae, (b) algae and arginine mixture, and (c) pure
arginine, at molar ratio 1:4, biomass to salts mixture.
(reproduced from131).
4. Application of Transmission Electron

Microscopy for studies of porous material
The lowest point resolution of our eyes is 100-200 µm, and
microscopy is a large set of techniques used to study Figure 13. Schematic diagram of (A) diffraction mode:
objects that are too small to be examined by the naked projecting the diffraction pattern onto the viewing screen, and
eye. For example, the distance between atoms in a solid is (B) image mode: projecting the image onto the screen.132
generally in the range of 2–3 Å. Here we discuss TEM,
which was developed because of the limitation of image
resolution in light microscopes. After electron microscopes
94
4.2. TEM images excited by the incident beam, creating an electron hole
TEM images are magnified images of the electron intensity where the electron was initially. Another electron from an
on the bottom surface of the specimen. The contrast of outer, higher-energy shell will fill the hole, and the
images arises if the intensity varies significantly from one difference of energy between the higher-energy shell and
region to another. There are several types of contrast in the lower energy shell may be released, forming a
TEM imaging: mass-thickness contrast, and diffraction characteristic X-ray, which can be measured by EDS.
contrast make the most contribution to the images. TEM 4.5. Electron energy-loss spectroscopy (EELS)
imaging is extensively applied to both amorphous and When a high-energy electron passes through a thin
crystalline specimens. In crystalline samples, the contrast specimen, either it penetrates it without losing any energy
mostly contributes to the diffraction contrast. or it may suffer inelastic scattering, losing energy via a
4.3. Electron diffraction (ED) variety of processes. EELS can separate these inelastically
The elastic electron scattering in a crystalline material is scattered electrons into a spectrum and be used to analyse
called diffraction. The regularity of the atomic nuclei the energy distribution of these electrons to form images
spacing leads to a redistribution of the angular scattering, or DPs from electrons of specific energy, which can be
which can be displayed on the TEM instrument’s screen by interpreted and quantified. EELS offers more information
weakening the intermediate lens. Then, the intermediate than mere elemental identification, and it is well suited to
and projector lenses magnify the initially formed small detect light elements, which are difficult to analyse with
diffraction pattern at the back-focal plane of the objective EDS. In amorphous carbon films, C-C bonds with sp2- and
lens. sp3-type hybridisations occur, which can be analysed by
The ED patterns can be interpreted with the reciprocal studying the K- edge region in an EELS. A common method
lattice concept. Every crystalline material can be described for quantifying the sp2 bonding fraction in an amorphous
with two lattices, one is a real lattice, and the other is a carbon film was described by Berger et al.133
reciprocal lattice. The reciprocal lattice is related to the In addition, there are numerous types of TEMs and they
real one via the Fourier transformation. Comparing it with include HRTEMs, STEMs, AEMs, etc., but normally a new
X-ray diffraction, which is the atom plane diffraction of X- 200 or 300 keV TEM can combine aspects of all the above
rays in a crystal, is helpful to understand the electronic microscope types of TEMs. Fig. 15 shows a picture of a
diffraction mechanism. X-ray diffraction can be explained given TEM instrument (JEOL JEM-2100F) with field
by the Bragg’s reflection law: emission gun (FEG), used for the TEM related work in this
chapter. Also, sample preparation is a key factor for
𝑛𝑛 λ = 2 sin 𝜃𝜃 (4)
successful TEM experimentation. Using or developing a
where λ is the X-ray wavelength, d is the spacing between preparation technique that does not change the properties
atomic planes measured in a direction perpendicular to the of the samples under study is of great significance.
planes, and n is an integer that represents the order of
reflection. Fig.14 shows an ED pattern of an amorphous carbon
material.
Figure 14. Diffraction pattern of an amorphous carbon

material.
4.4. X-ray energy-dispersive spectrometry (EDS)

EDS can be used as an elemental composition analysis
technique by measuring the number and energy of the X-
rays emitted by the sample. The main principle here is that Figure 15. Photograph of the Transmission Electron
each element has its own atomic structure, allowing a Microscopy used in this chapter, JEOL JEM-2100F, with
unique set of characteristic X-ray peaks. When a high- field emission gun (FEG), situated at the Arrhenius
energy beam of charged particles such as electrons or Laboratory, Stockholm University (Sept. 2018).
protons or a beam of X-rays is focused into the sample, an
atom within the sample contains unexcited electrons
bound to the nucleus. Then, an electron in an inner shell is
95
5. Conclusions (2) J.T. Houghton, Y.Ding, D.J. Griggs, M.Noguer, P.J. van
der Linden, X. Dai, K. Maskell, C. A. J. Climate Change
This chapter summarises technologies for gas separation, 2001: The Scientific Basis; Cambridge University
mainly focused on adsorption process together with some Press: Cambridge, 2001.
examples of adsorbents used for CO2 and H2S storage. The (3) Spengler, J. D.; Sexton, K. Indoor Air Pollution: A
second part of this section describes the HTC coupled with Public Health Perspective. Science 1983, 221 (4605),
templating methods, and the main possibilities to improve the 9–17.
textural properties of the final product. Among all the (4) Barnett, J. Security and Climate Change. Glob.
technologies for gas separation, adsorption processes are of Environ. Chang. 2003, 13 (1), 7–17.
(5) Wiheeb, A. D.; Shamsudin, I. K.; Ahmad, M. A.; Murat,
great interest because of the variety of adsorbents that can be
M. N.; Kim, J.; Othman, M. R. Present Technologies
used, as well as the flexibility of the process conditions to
for Hydrogen Sulfide Removal from Gaseous
achieve the adsorption capacity as highest as possible. Carbon- Mixtures. Rev. Chem. Eng. 2013, 29 (6), 449–470.
based adsorbents are known for their gas separation (6) Mandal, B. P.; Bandyopadhyay, S. S. Absorption of
properties, since the ancient times and nowadays they are Carbon Dioxide into Aqueous Blends of 2-Amino-2-
among the most used materials for these kinds of applications. Methyl-1-Propanol and Monoethanolamine. Chem.
HTC is an advantageous method for the synthesis of carbon- Eng. Sci. 2006, 61 (16), 5440–5447.
based adsorbents. Due to the possibility to combine the HTC (7) Plaza, M. G.; Pevida, C.; Arenillas, A.; Rubiera, F.; Pis,
with templating techniques, the structure of the final carbons J. J. CO2 capture by Adsorption with Nitrogen
can be easily improved. Besides, their chemical and physical Enriched Carbons. Fuel 2007, 86 (14), 2204–2212.
properties can be modified by the combination of carbon with (8) Bagreev, A.; Bandosz, T. J. A Role of Sodium
Hydroxide in the Process of Hydrogen Sulfide
different compounds and thanks to the high number of
Adsorption / Oxidation on Caustic-Impregnated
oxygenated groups from the carbon surface, further
Activated Carbons. Ind. Eng. Chem. Res. 2002, 41 (4),
functionalisation is achieved. When HTC is combined with soft- 672–679.
templating methods, the possibilities to obtain an ordered (9) D’Alessandro, D. M.; Smit, B.; Long, J. R. Carbon
porous material increase. This occurs because of direct Dioxide Capture: Prospects for New Materials.
arrangements of the block copolymer aggregates. By using a Angew. Chem. Int. Ed. 2010, 49 (35), 6058–6082.
hard template, the advantage comes with the possibility to (10) The history of activated carbon
obtain nanostructured materials with different surface https://www.jurassiccarbon.com/(accessed Feb 25,
functionalities, and easiness in controlling the final structure, 2019).
but the removal of the template is more challenging than in the (11) Ngoy, J. M. A CO2 Capture Technology Using Carbon
case of soft template approach. SAPO-templated carbons have Nanotubes with Polyaspartamide Surfactant. Ph.D.
Thesis, University of the Witwatersrand,
industrial potential for CO2 separation not only because of its
Johannesburg. 2016, 287.
high surface area, well-ordered ultramicropore structure as
(12) Wilcox, J. Carbon Capture; Springer-Verlag: New
other hard-templated carbons, but also because of the the non- York, 2012.
toxic prepapring procedure when it is compared with zeolite- (13) Liu, Y.; Wilcox, J. Molecular Simulation Studies of CO2
templated carbons. To ensure the presence of a multimodal Adsorption by Carbon Model Compounds for Carbon
pore structure, the salt templating technique is promising; a Capture and Sequestration Applications. Environ. Sci.
change of the salt composition and concentration makes that Technol. 2013, 47 (1), 95–101.
the size of the pores can be shifted from micropores to (14) Carbon Dioxide. Wikipedia (accessed Feb 25, 2019).
mesopores, or even macropores. Furthermore, the surface area (15) Pubchem. Carbon dioxide
can be easily increased by modification in the salt mixtures https://pubchem.ncbi.nlm.nih.gov/compound/280
concentration. TEM is an important tool to deeply charecterise (accessed Feb 25, 2019).
(16) Wiheeb, A. D.; Shamsudin, I. K.; Ahmad, M. A.; Murat,
engineered porous carbons.
M. N.; Kim, J.; Othman, M. R. Present Technologies
for Hydrogen Sulfide Removal from Gaseous
Acknowledgements Mixtures. Rev. Chem. Eng. 2013, 29 (6).
(17) Mandal, B.; Bandyopadhyay, S. S. Simultaneous
This project has received funding from the European Absorption of CO 2 and H 2 S Into Aqueous Blends of
Union’s Horizon 2020 research and innovation programme N -Methyldiethanolamine and Diethanolamine.
under the Marie Skłodowska-Curie grant agreement No Environ. Sci. Technol. 2006, 40 (19), 6076–6084.
721991. (18) Yagi, T.; Shibuya, H.; Sasaki, T. Application of
Chemical Absorption Process to CO2 Recovery from
Flue-Gas Generated in Power-Plants. Energy Convers.
References Manage. 1992, 33 (5–8), 349–355.
(19) McIntyre, G.; Lyddon, L. Claus Sulphur Recovery
(1) Annual Report 2010-2011 Academic Year –
Options. Pet. Technol. Quarterly 1997, 1–8.
Universitat Oberta de Catalunya (UOC)
(20) Zhao, C.; Wang, J. Electrochemical Reduction of
https://www.uoc.edu/memories/memoria1011/eng
CO2to Formate in Aqueous Solution Using Electro-
lish/teaching/departments-schools-chairs/ecs.html
Deposited Sn Catalysts. Chem. Eng. J. 2016, 293, 161–
(accessed Jan 31, 2019).
170.
96
(21) Hao, J.; Rice, P. A.; Stern, S. A. Upgrading Low-Quality (35) Kumar, A.; Madden, D. G.; Lusi, M.; Chen, K.-J.;
Natural Gas with H2S- and CO2-Selective Polymer Daniels, E. A.; Curtin, T.; Perry, J. J.; Zaworotko, M. J.
Membranes Part I. Process Design and Economics of Direct Air Capture of CO2 by Physisorbent Materials.
Membrane Stages without Recycle Streams. J. Angew. Chem. Int. Ed. 2015, 54 (48), 14372–14377.
Membr. Sci. 2002, 177-206. (36) Cho, H.-Y.; Yang, D.-A.; Kim, J.; Jeong, S.-Y.; Ahn, W.-
(22) Ahmad, F.; Keong, L. L.; Shariff, A.; Removal of CO2 S. CO2 Adsorption and Catalytic Application of Co-
from Natural Gas Using Membrane Process: Design, MOF-74 Synthesized by Microwave Heating. Catal.
Fabrication and Parametric Study. In: EMS Summer Today 2012, 185 (1), 35–40.
School, Germany, 2010. (37) Nandi, S.; Maity, R.; Chakraborty, D.; Ballav, H.;
(23) Rouquerol, F.; Rouquerol, J.; Sing, K. S. W.; Llewellyn, Vaidhyanathan, R. Preferential Adsorption of CO2 in
P.; Maurin, G. Adsorption by Powders and Porous an Ultramicroporous MOF with Cavities Lined by
Solids; 2nd ed.; Academic Press: France, 2014. Basic Groups and Open-Metal Sites. Inorg. Chem.
(24) Burwell, R. L. Manual of Symbols and Terminology for 2018, 57 (9), 5267–5272.
Physicochemical Quantities and Units–Appendix II. (38) Liang, Y.; Harrison, D. P.; Gupta, R. P.; Green, D. a.;
Pure Appl. Chem. 1976, 46, 71–90. McMichael, W. J. Carbon Dioxide Capture Using Dry
(25) Thommes, M.; Kaneko, K.; Neimark, A. V.; Olivier, J. Sodium-Based Sorbents. Energy Fuels 2004, 18 (2),
P.; Rodriguez-Reinoso, F.; Rouquerol, J.; Sing, K. S. W. 569–575.
Physisorption of Gases, with Special Reference to the (39) Zhao, C.; Chen, X.; Zhao, C. Study on CO2 Capture
Evaluation of Surface Area and Pore Size Distribution Using Dry Potassium-Based Sorbents through
(IUPAC Technical Report). Pure Appl. Chem. 2015, 87 Orthogonal Test Method. Int. J. Greenhouse Gas
(9–10), 1051–1069. Control 2010, 4 (4), 655–658.
(26) Wang, Q.; Luo, J.; Zhong, Z.; Borgna, A. CO2 Capture (40) Choi, S.; Drese, J. H.; Jones, C. W. Adsorbent Materials
by Solid Adsorbents and Their Applications: Current for Carbon Dioxide Capture from Large
Status and New Trends. Energy Environ. Sci. 2011, 4 Anthropogenic Point Sources. ChemSusChem 2009, 2
(1), 42–55. (9), 796–854.
(27) Siriwardane, R. V.; Shen, M.-S.; Fisher, E. P.; Poston, (41) Ahmadpour, A.; Do, D. D. The Preparation of Active
J. A. Adsorption of CO2 on Molecular Sieves and Carbons from Coal by Chemical and Physical
Activated Carbon. Energy Fuels 2001, 15 (2), 279– Activation. Carbon 1996, 34 (4), 471–479.
284. (42) Creamer, A. E.; Gao, B. Carbon-Based Adsorbents for
(28) Hernández, S. P.; Scarpa, F.; Fino, D.; Conti, R. Biogas Postcombustion CO2 Capture: A Critical Review.
Purification for MCFC Application. Int. J. Hydrogen Environ. Sci. Technol. 2016, 50 (14), 7276–7289.
Energy 2011, 36 (13), 8112–8118. (43) Sevilla, M.; Al-Jumialy, A. S. M.; Fuertes, A. B.;
(29) Yu, J.; Xie, L.-H.; Li, J.-R.; Ma, Y.; Seminario, J. M.; Mokaya, R. Optimization of the Pore Structure of
Balbuena, P. B. CO2 Capture and Separations Using Biomass-Based Carbons in Relation to Their Use for
MOFs: Computational and Experimental Studies. CO2 Capture under Low- and High-Pressure Regimes.
Chem. Rev. 2017, 117 (14), 9674–9754. ACS Appl. Mater. Interfaces 2018, 10 (2), 1623–1633.
(30) Britt, D.; Furukawa, H.; Wang, B.; Glover, T. G.; Yaghi, (44) Sevilla, M.; Sangchoom, W.; Balahmar, N.; Fuertes, A.
O. M. Highly Efficient Separation of Carbon Dioxide B.; Mokaya, R. Highly Porous Renewable Carbons for
by a Metal-Organic Framework Replete with Open Enhanced Storage of Energy-Related Gases (H2 and
Metal Sites. Proc. Natl. Acad. Sci. U.S.A. 2009, 106 CO2) at High Pressures. ACS Sustain. Chem. Eng.
(49), 20637–20640. 2016, 4 (9), 4710–4716.
(31) Rosi, N. L.; Kim, J.; Eddaoudi, M.; Chen, B.; O’Keeffe, (45) Xiao, P.-W.; Guo, D.; Zhao, L.; Han, B.-H. Soft
M.; Yaghi, O. M. Rod Packings and Metal−Organic Templating Synthesis of Nitrogen-Doped Porous
Frameworks Constructed from Rod-Shaped Hydrothermal Carbons and Their Applications in
Secondary Building Units. J. Am. Chem. Soc. 2005, 127 Carbon Dioxide and Hydrogen Adsorption.
(5), 1504–1518. Microporous Mesoporous Mater. 2016, 220, 129–
(32) Han, L.; Budge, M.; Alex Greaney, P. Relationship 135.
between Thermal Conductivity and Framework (46) Boyjoo, Y.; Cheng, Y.; Zhong, H.; Tian, H.; Pan, J.;
Architecture in MOF-5. Comput. Mater. Sci. 2014, 94, Pareek, V. K.; Jiang, S. P.; Lamonier, J.-F.; Jaroniec, M.;
292–297. Liu, J. From Waste Coca Cola® to Activated Carbons
(33) Yang, Q.; Vaesen, S.; Vishnuvarthan, M.; Ragon, F.; with Impressive Capabilities for CO2 Adsorption and
Serre, C.; Vimont, A.; Daturi, M.; De Weireld, G.; Supercapacitors. Carbon 2017, 116, 490–499.
Maurin, G. Probing the Adsorption Performance of (47) Bellemare, M. F.; Çakir, M.; Peterson, H. H.; Novak, L.;
the Hybrid Porous MIL-68(Al): A Synergic Rudi, J. On the Measurement of Food Waste. Am. J.
Combination of Experimental and Modelling Tools. J. Agricultural Economics 2017, 99 (5), 1148–1158.
Mater. Chem. 2012, 22 (20), 10210 - 10220. (48) Sevilla, M.; Parra, J. B.; Fuertes, A. B. Assessment of
(34) Hamon, L.; Serre, C.; Devic, T.; Loiseau, T.; Millange, the Role of Micropore Size and N-Doping in CO2
F.; Férey, G.; De Weireld, G. Comparative Study of Capture by Porous Carbons. ACS Appl. Mater.
Hydrogen Sulfide Adsorption in the MIL-53(Al, Cr, Fe), Interfaces 2013, 5 (13), 6360–6368.
MIL-47(V), MIL-100(Cr), and MIL-101(Cr) Metal- (49) Yaumi, A. L.; Bakar, M. Z. A.; Hameed, B. H.
Organic Frameworks at Room Temperature. J. Am. Melamine-Nitrogenated Mesoporous Activated
Chem. Soc. 2009, 131 (25), 8775–8777. Carbon Derived from Rice Husk for Carbon Dioxide
Adsorption in Fixed-Bed. Energy 2018, 155, 46–55.
97
(50) Ello, A. S.; de Souza, L. K. C.; Trokourey, A.; Jaroniec, (65) Zhang, J. ping; Sun, Y.; Woo, M. W.; Zhang, L.; Xu, K.
M. Development of Microporous Carbons for CO2 Z. Preparation of Steam Activated Carbon from Black
Capture by KOH Activation of African Palm Shells. J. Liquor by Flue Gas Precipitation and Its Performance
CO2 Util. 2013, 2, 35–38. in Hydrogen Sulfide Removal: Experimental and
(51) Li, D.; Ma, T.; Zhang, R.; Tian, Y.; Qiao, Y. Preparation Simulation Works. J. Taiwan Inst. Chem. Eng. 2016,
of Porous Carbons with High Low-Pressure CO2 59, 395–404.
Uptake by KOH Activation of Rice Husk Char. Fuel (66) Przepiorski, J.; Oya, A. K2CO3-Loaded Deodorizing
2015, 139, 68–70. Activated Carbon Fibre Against H2S Gas: Factors
(52) Sevilla, M.; Fuertes, A. B. Sustainable Porous Carbons Influencing the Deodorizing Efficiency and the
with a Superior Performance for CO2 Capture. Energy Regeneration Method. J. Mater. Sci. Lett. 1998, 17
Environ. Sci. 2011, 4 (5), 1765 - 1771. (8), 679–682.
(53) Wang, R.; Wang, P.; Yan, X.; Lang, J.; Peng, C.; Xue, Q. (67) Sethupathi, S.; Zhang, M.; Rajapaksha, A. U.; Lee, S.
Promising Porous Carbon Derived from Celtuce R.; Nor, N. M.; Mohamed, A. R.; Al-Wabel, M.; Lee, S.
Leaves with Outstanding Supercapacitance and CO2 S.; Ok, Y. S. Biochars as Potential Adsorbers of CH4,
Capture Performance. ACS Appl. Mater. Interfaces CO2 and H2S. Sustainability (Switzerland) 2017, 9 (1),
2012, 4 (11), 5800–5806. 1–10.
(54) Hong, S.-M.; Jang, E.; Dysart, A. D.; Pol, V. G.; Lee, K. (68) Lyons, K.; Swann, G.; Levett, C. Produced but Never
B. CO2 Capture in the Sustainable Wheat-Derived Eaten: A Visual Guide to Food Waste. The Guardian.
Activated Microporous Carbon Compartments. Sci. Aug. 12, 2015.
Reports 2016, 6 (1). (69) Global CCS Institute. Data & knowledge.
(55) Tsai, J.-H.; Jeng, F.-T.; Chiang, H.-L. Removal of H2S https://www.globalccsinstitute.com/consultancy/ou
from Exhaust Gas by Use of Alkaline Activated r-services/data-knowledge/ (accessed Feb 18, 2019).
Carbon. Adsorption 2001, 7 (4), 357–366. (70) National Energy Technology Laboratory. Post-
(56) Guo, J.; Luo, Y.; Lua, A. C.; Chi, R.; Chen, Y.; Bao, X.; combustion CO2 Capture
Xiang, S. Adsorption of Hydrogen Sulphide (H2S) by https://www.netl.doe.gov/research/coal/carbon-
Activated Carbons Derived from Oil-Palm Shell. capture/post-combustion (accessed Aug 30, 2018).
Carbon 2007, 45 (2), 330–336. (71) Zevenhoven, R.; Kilpinen, P. Control of Pollutants in
(57) Castrillon, M. C.; Moura, K. O.; Alves, C. A.; Bastos- Flue Gases and Fuel Gases; Helsinki University of
Neto, M.; Azevedo, D. C. S.; Hofmann, J.; Möllmer, J.; Technology: Espoo, 2001.
Einicke, W.-D.; Gläser, R. CO2 and H2S Removal from (72) Burwell, R. L. Manual of Symbols and Terminology for
CH4 -Rich Streams by Adsorption on Activated Physicochemical Quantities and Units–Appendix II.
Carbons Modified with K2CO3, NaOH, or Fe2O3. Pure and Appl. Chem. 1976, 46, 71–90.
Energy Fuels 2016, 30 (11), 9596–9604. (73) Zdravkov, B.; Čermák, J.; Šefara, M.; Janků, J. Pore
(58) Sethupathi, S.; Zhang, M.; Rajapaksha, A.; Lee, S.; Classification in the Characterization of Porous
Mohamad Nor, N.; Mohamed, A.; Al-Wabel, M.; Lee, Materials: A Perspective. Open Chem. 2007, 5 (2),
S.; Ok, Y. Biochars as Potential Adsorbers of CH4, CO2 385-395.
and H2S. Sustainability 2017, 9 (1), 121-131. (74) Gilbert, M. T.; Knox, J. H.; Kaur, B. Porous Glassy
(59) Bagreev, A.; Bandosz, T. J. Study of Hydrogen Sulfide Carbon, a New Columns Packing Material for Gas
Adsorption on Activated Carbons Using Inverse Gas Chromatography and High-Performance Liquid
Chromatography at Infinite Dilution. J. Phys. Chem. B Chromatography. Chromatographia 1982, 16 (1),
2000, 104 (37), 8841–8847. 138–146.
(60) Bandosz, T. J.; Bagreev, A.; Adib, F.; Turk, A. (75) Liang, C.; Hong, K.; Guiochon, G. A.; Mays, J. W.; Dai,
Unmodified versus Caustics- Impregnated Carbons S. Synthesis of a Large-Scale Highly Ordered Porous
for Control of Hydrogen Sulfide Emissions from Carbon Film by Self-Assembly of Block Copolymers.
Sewage Treatment Plants. Environ. Sci. Technol. Angew. Chem. Int. Ed. 2004, 43 (43), 5785–5789.
2000, 34 (6), 1069–1074. (76) Zhang, Y.; Liu, L.; Van der Bruggen, B.; Yang, F.
(61) Bagreev, A.; Bashkova, S.; Locke, D. C.; Bandosz, T. J. Nanocarbon Based Composite Electrodes and Their
Sewage Sludge-Derived Materials as Efficient Application in Microbial Fuel Cells. J. Mater. Chem. A
Adsorbents for Removal of Hydrogen Sulfide. 2017, 5 (25), 12673–12698.
Environ. Sci. Technol. 2001, 35 (7), 1537–1543. (77) Wang, L.; Yang, R. T. Significantly Increased CO2
(62) Habeeb, O. A.; Ramesh, K.; Ali, G. A. M.; Yunus, R. M.; Adsorption Performance of Nanostructured
Thanusha, T. K.; Olalere, O. A. Modeling and Templated Carbon by Tuning Surface Area and
Optimization for H2S Adsorption from Wastewater Nitrogen Doping. J. Phys. Chem. C. 2012, 116 (1),
Using Coconut Shell Based Activated Carbon. Aust. J. 1099–1106.
Basic Applied Sci. 2016, 10 (17), 136-147. (78) Bagshaw, S. a; Prouzet, E.; Pinnavaia, T. J. Templating
(63) Shang, G.; Li, Q.; Liu, L.; Chen, P.; Huang, X. of Mesoporous Molecular Sieves by Nonionic
Adsorption of Hydrogen Sulfide by Biochars Derived Polyethylene Oxide Surfactants. Science (New York,
from Pyrolysis of Different Agricultural/Forestry N.Y.) 1995, 269 (5228), 1242–1244.
Wastes. J. Air Waste Manage Assoc. 2016, 66 (1), 8– (79) Tanaka, S.; Nakatani, N.; Doi, A.; Miyake, Y.
16. Preparation of Ordered Mesoporous Carbon
(64) Mochizuki, T.; Kubota, M.; Matsuda, H.; D’Elia Membranes by a Soft-Templating Method. Carbon
Camacho, L. F. Adsorption Behaviors of Ammonia and 2011, 49 (10), 3184–3189.
Hydrogen Sulfide on Activated Carbon Prepared from
Petroleum Coke by KOH Chemical Activation. Fuel
Process. Technol. 2016, 144, 164–169.
98
(80) Liang, C.; Dai, S. Synthesis of Mesoporous Carbon (95) Yang, Z.; Xia, Y.; Sun, X.; Mokaya, R. Preparation and
Materials via Enhanced Hydrogen-Bonding Hydrogen Storage Properties of Zeolite-Templated
Interaction. J. Am. Chem. Soc. 2006, 128 (16), 5316– Carbon Materials Nanocast via Chemical Vapor
5317. Deposition: Effect of the Zeolite Template and
(81) Liu, J.; Yang, T.; Wang, D. W.; Lu, G. Q.; Zhao, D.; Qiao, Nitrogen Doping. J. Phys. Chem. B 2006, 110 (37),
S. Z. A Facile Soft-Template Synthesis of Mesoporous 18424–18431.
Polymeric and Carbonaceous Nanospheres. Nat. (96) Baerlocher Ch.; McCusker, L.B. Database of Zeolite
Commun. 2013, 4, 1–7. Structures http://www.iza-structure.org/databases/
(82) Kubo, S.; White, R. J.; Yoshizawa, N.; Antonietti, M.; (accessed Jul 6, 2018).
Titirici, M. M. Ordered Carbohydrate-Derived Porous (97) Gaslain, F. O. M.; Parmentier, J.; Valtchev, V. P.;
Carbons. Chem. Mater. 2011, 23 (22), 4882–4885. Patarin, J. First Zeolite Carbon Replica with a Well
(83) Yan, H. Soft-Templating Synthesis of Mesoporous Resolved X-Ray Diffraction Pattern. Chem. Commun.
Graphitic Carbon Nitride with Enhanced 2006, 0 (9), 991–993.
Photocatalytic H2 Evolution under Visible Light. (98) Nishihara, H.; Kyotani, T. Zeolite-Templated Carbons-
Chem. Commun. 2012, 48 (28), 3430-3432. Three-Dimensional Microporous Graphene
(84) Xiao, P. W.; Guo, D.; Zhao, L.; Han, B. H. Soft Frameworks. Chem. Commun. 2018, 54 (45), 5648–
Templating Synthesis of Nitrogen-Doped Porous 5673.
Hydrothermal Carbons and Their Applications in (99) Kim, K.; Lee, T.; Kwon, Y.; Seo, Y.; Song, J.; Park, J. K.;
Carbon Dioxide and Hydrogen Adsorption. Lee, H.; Park, J. Y.; Ihee, H.; Cho, S. J.; et al.
Microporous Mesoporous Mater. 2016, 220, 129– Lanthanum-Catalysed Synthesis of Microporous 3D
135. Graphene-like Carbons in a Zeolite Template. Nature
(85) Deng, J.; Li, M.; Wang, Y. Biomass-Derived Carbon: 2016, 535 (7610), 131–135.
Synthesis and Applications in Energy Storage and (100) PE De Jongh, P. E. Miniature Structures Meticulously
Conversion. Green Chem. 2016, 18 (18), 4824–4854. Replicated in Carbon. NPG Asia Materials 2017, 9 (1),
(86) Xiao, P. W.; Guo, D.; Zhao, L.; Han, B. H. Soft 339–339.
Templating Synthesis of Nitrogen-Doped Porous (101) Inagaki, M.; Kang, F.; Toyoda, M.; Konno, H.
Hydrothermal Carbons and Their Applications in Advanced Materials Science and Engineering of
Carbon Dioxide and Hydrogen Adsorption. Carbon. Chapter 7 - Template Carbonization:
Microporous Mesoporous Mater. 2016, 220, 129– Morphology and Pore Control. In Adv. Mater. Sci.
135. Eng. Carbon, Boston, 2014, 133–163.
(87) Li, R.; Shahbazi, A. A Review of Hydrothermal (102) Van Tendeloo, L.; Gobechiya, E.; Breynaert, E.;
Carbonization of Carbohydrates for Carbon Spheres Martens, J. a; Kirschhock, C. E. a. Alkaline Cations
Preparation. Trends Renew. Energ. 2015, 1 (1), 43– Directing the Transformation of FAU Zeolites into
56. Five Different Framework Types. Chem. Commun.
(88) Titirici, M.-M. Sustainable Carbon Materials from 2013, 49 (100), 11737–11739.
Hydrothermal Processes; John Wiley & Sons: London, (103) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.;
2013. Flanigen, E. M. Aluminophosphate Molecular Sieves:
(89) Yu, L.; Brun, N.; Sakaushi, K.; Eckert, J.; Titirici, M. M. A New Class of Microporous Crystalline Inorganic
Hydrothermal Nanocasting: Synthesis of Solids. J. Am. Chem. Soc. 1982, 104 (4), 1146–1147.
Hierarchically Porous Carbon Monoliths and Their (104) Davis, M. E. Ordered Porous Materials for Emerging
Application in Lithium–Sulfur Batteries. Carbon 2013, Applications. Nature 2002, 417 (6891), 813–821.
61, 245–253. (105) Winiecki, A. M.; Suib, S. L. Chemical Stability of
(90) Titirici, M.-M.; Thomas, A.; Antonietti, M. Replication Aluminophosphate Molecular Sieves during HCl
and Coating of Silica Templates by Hydrothermal Treatment. Langmuir 1989, 5, 333–338.
Carbonization. Adv. Funct. Mater. 2007, 17 (6), 1010– (106) Liu, G.; Liu, Y.; Zhang, X.; Yuan, X.; Zhang, M.; Zhang,
1018. W.; Jia, M. Characterization and Catalytic
(91) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Performance of Porous Carbon Prepared Using in
Z.; Ohsuna, T.; Terasaki, O. Synthesis of New, Situ-Formed Aluminophosphate Framework as
Nanoporous Carbon with Hexagonally Ordered Template. J. Colloid Interface Sci. 2010, 342 (2), 467–
Mesostructure. J. Am. Chem. Soc. 2000, 122 (43), 473.
10712–10713. (107) Tang, Z. K.; Wang, N.; Zhang, X. X.; Wang, J. N.; Chan,
(92) Titirici, M.-M.; Thomas, A.; Antonietti, M. Aminated C. T.; Sheng, P. Novel Properties of 0.4 Nm Single-
Hydrophilic Ordered Mesoporous Carbons. J. Mater. Walled Carbon Nanotubes Templated in the Channels
Chem. 2007, 17 (32), 3412–3418. of AlPO4 -5 Single Crystals. New J. Phys. 2003, 5, 146–
(93) Demir-Cakan, R.; Baccile, N.; Antonietti, M.; Titirici, 146.
M.-M. Carboxylate-Rich Carbonaceous Materials via (108) Kyotani, T.; Ma, Z.; Tomita, A. Template Synthesis of
One-Step Hydrothermal Carbonization of Glucose in Novel Porous Carbons Using Various Types of
the Presence of Acrylic Acid. Chem. Mater. 2009, 21 Zeolites. Carbon 2003, 41 (7), 1451–1459.
(3), 484–490. (109) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, R. T.;
(94) Demir-Cakan, R.; Makowski, P.; Antonietti, M.; Cannan, T. R.; Flanigen, E. M. Silicoaluminophosphate
Goettmann, F.; Titirici, M.-M. Hydrothermal Molecular Sieves: Another New Class of Microporous
Synthesis of Imidazole Functionalized Carbon Crystalline Inorganic Solids. J. Am. Chem. Soc. 1984, 0
Spheres and Their Application in Catalysis. Catal. (8), 6092–6093.
Today 2010, 150 (1), 115–118.
99
(110) Chen, J.; Wright, P. A.; Thomas, J. M.; Natarajan, S.; (124) Yoshino, M.; Matsuda, M.; Miyake, M. Effect of
Marchese, L.; Bradley, S. M.; Sankar, G.; Catlow, C. R. Transition Metal Doping on Crystallization of
A.; Gai-Boyes, P. L. SAPO-18 Catalysts and Their Cloverite. Solid State Ionics. 2002, 151 (1), 269–274.
Broensted Acid Sites. J. Phys. Chem. 1994, 98 (40), (125) Wei, Y.; Tian, Z.; Gies, H.; Xu, R.; Ma, H.; Pei, R.; Zhang,
10216–10224. W.; Xu, Y.; Wang, L.; Li, K.; et al. Ionothermal
(111) Dzwigaj, S.; Briend, M.; Shikholeslami, A.; Peltre, M. Synthesis of an Aluminophosphate Molecular Sieve
J.; Barthomeuf, D. The Acidic Properties of SAPO-37 with 20-Ring Pore Openings. Angew. Chem. Int. Ed.
Compared to Faujasites and SAPO-5. Zeolites 1990, 2010, 49 (31), 5367–5370.
10 (3), 157–162. (126) Feng, P.; Bu, X.; Stucky, G. D. Hydrothermal Syntheses
(112) Briend, M.; Shikholeslami, A.; Peltre, M.-J.; Delafosse, and Structural Characterization of Zeolite Analogue
D.; Barthomeuf, D. Thermal and Hydrothermal Compounds Based on Cobalt Phosphate. Nature
Stability of SAPO-5 and SAPO-37 Molecular Sieves. J. 1997, 388 (6644), 735–741.
Chem. Soc., Dalton Trans. 1989, 7, 1361-1362. (127) Harrison, W. T. A.; Gier, T. E.; Moran, K. L.; Nicol, J.
(113) Braun, E.; Lee, Y.; Moosavi, S. M.; Barthel, S.; M.; Eckert, H.; Stucky, G. D. Structures and Properties
Mercado, R.; Baburin, I. A.; Proserpio, D. M.; Smit, B. of New Zeolite X-Type Zincophosphate and
Generating Carbon Schwarzites via Zeolite- Beryllophosphate Molecular Sieves. Chem. Mater.
Templating. Proc. Natl. Acad. Sci. U.S.A. 2018, 115 1991, 3 (1), 27–29.
(35), 8116-8124. (128) Venkatathri, N. Synthesis and Characterization of
(114) Fechler, N.; Fellinger, T. P.; Antonietti, M. “salt AlPO4-n Molecular Sieves from Hexamethyleneimine
Templating”: A Simple and Sustainable Pathway Template. Indian J. Chem. 2002, 41A, 2223–2230.
toward Highly Porous Functional Carbons from Ionic (129) Afeworki, M.; Dorset, D. L.; Kennedy, G. J.;
Liquids. Adv. Mater. 2013, 25 (1), 75–79. Strohmaier, K. G. Synthesis and Structure of ECR-40:
(115) Estermann, M. A.; McCusker, L. B.; Baerlocher, C. Ab An Ordered Sapo Having the MEI Framework. In
Initio Structure Determination from Severely Studies in Surface Science and Catalysis; van Steen,
Overlapping Powder Diffraction Data. J. Appl. E., Claeys, M., Callanan, L. H., Eds.; Recent Advances
Crystallogr. 1992, 25 (4), 539–543. in the Science and Technology of Zeolites and Related
(116) Bennett, J. M.; Marcus, B. K. The Crystal Structures of Materials Part B; Elsevier, 2004; Vol. 154, pp 1274–
Several Metal Aluminophosphate Molecular Sieves. 1281.
In Studies in Surface Science and Catalysis; Grobet, P. (130) Zhang, Z.; Feng, J.; Jiang, Y.; Feng, J. High-Pressure
J., Mortier, W. J., Vansant, E. F., Schulz-Ekloff, G., Eds.; Salt Templating Strategy toward Intact Isochoric
Innovation in Zeolite Materials Science; Elsevier: Hierarchically Porous Carbon Monoliths from Ionic
1988; Vol. 37, pp 269–279. Liquids. RSC Adv. 2017, 7 (81), 51096–51103.
(117) Akolekar, B.; Kaliaguine, S. Synthesis, (131) Kumar, K. V.; Gadipelli, S.; Preuss, K.; Porwal, H.;
Characterization, Thermal Stability, Acidity and Zhao, T.; Guo, Z. X.; Titirici, M. M. Salt Templating
Catalytic Properties of Large-Pore MAPO-46. J. Chem. with Pore Padding: Hierarchical Pore Tailoring
Soc., Faraday Trans. 1993, 89, 4141-4147. towards Functionalised Porous Carbons.
(118) Akolekar, D. B. Novel, Crystalline, Large-Pore ChemSusChem 2017, 10 (1), 199–209.
Magnesium Aluminophosphate Molecular Sieve of (132) Williams, D. B.; Carter, C. B. Transmission Electron
Type 50: Preparation, Characterization, and Microscopy: A Textbook for Materials Science, 2nd
Structural Stability. Zeolites 1995, 15 (7), 583–590. ed.; Springer: New York, 2008.
(119) Novak Tušar, N.; Ristić, A.; Meden, A.; Kaučič, V. (133) Berger, S. D.; McKenzie, D. R.; Martin, P. J. EELS
Large-Pore Molecular Sieve MnAPO-50: Synthesis, Analysis of Vacuum Arc-Deposited Diamond-like
Single-Crystal Structure Analysis and Thermal Films. Philos. Mag. Lett. 1988, 57 (6), 285–290.
Stability. Microporous Mesoporous Mater. 2000, 37
(3), 303–311.
(120) Arcon, I.; Tusar, N. N.; Ristić, A.; Kaucic, V.; Kodre, A.;
Helliwell, M. Incorporation of Mn, Co and Zn Cations
into Large-Pore Aluminophosphate Molecular Sieves
MeAPO-50. J Synchrotron Rad. 2001, 8, 590–592.
(121) Broach, R. W.; Greenlay, N.; Jakubczak, P.; Knight, L.
M.; Miller, S. R.; Mowat, J. P. S.; Stanczyk, J.; Lewis, G.
J. New ABC-6 Net Molecular Sieves ZnAPO-57 and
ZnAPO-59: Framework Charge Density-Induced
Transition from Two- to Three-Dimensional Porosity.
Microporous Mesoporous Mater. 2014, 189, 49–63.
(122) Bieniok, A.; Brendel, U.; Paulus, E. F.; Amthauer, G.
Microporous cobalto- and zincophosphates with the
framework-type of cancrinite. Eur. J. Mineral. 2005,
17 (6), 813-818.
(123) Yakubovich, O.; Karimova, O.; Melnikov, O. A new
representative of the cancrinite family
(Cs,K)0.33[Na0.18Fe0.16(H2O)1.05]{ZnPO4}:
Preparation and crystal structure. Crystallogr. Rep.
1994, 39 (4), 630–634.
100
Biochar as sustainable platform for pyrolysis vapours upgrading

Christian Di Stasi, Gianluca Greco, Belén Gonzalez, Joan J. Manyà
Aragón Institute of Engineering Research (I3A), University of Zaragoza, crta. Cuarte s/n, Huesca E-22071, Spain
Abstract
Thanks to their potential applications in catalysis, adsorption and energy storage, porous carbon materials are considered promising
candidates to address environmental issues related to global warming and pollution. The main problem is that their production
frequently involves carbon fossil feedstocks and high energy demand synthesis procedures. An interesting alternative pathway to
obtain such materials is the slow pyrolysis of biomass with production of a carbon-rich solid product, called “biochar”. Acccording to
the different final applications, biochar may need a post-production treatment (activation) aimed at improving some properties (i.e.,
the specific surface area or the surface functional groups ). Slow pyrolysis not only produce biochar but also a gas stream and a high
oxygenated liquid called “bio-oil”. The presence of a liquid phase leads to low yields of gas and solid and ,moreover, it can condensate
inside the pipelines, causing clogging problems. One of the most interesting application of the biochar is the production of a biochar-
based metal catalyst for upgrading of pyrolytic vapours through catalytic steam/dry reforming. The specific aim of this chapter is to
give an overview on the different types of activated biochars and on their application in bio-oil upgrading processes.
lignin. Due to the complexity of the whole reaction system

1. Introduction and to the possibility of interactions between the
intermediate reaction products, in literature, the
Carbon materials, such as carbon nanotubes, amorphous
degradation pathways of the three components are
carbons and activated carbons, are considerate good
studied separately. Each reaction is classified depending on
candidates for adsorption and/or catalytic applications.
when it takes place. Following this criterion, they are
The main drawback is that their production can involve
labelled as primary or secondary reactions.
electrochemical treatment or organic solvents, which
Primary reactions involve the degradation and rupture of
make difficult the commercialisation of such materials.
the bonds within the biomass constituents. Collard and
Furthermore, they often require carbon fossil precursors,
Blin3,4, in their review, categorised the primary reactions in
which are not environmentally sustainable options.
three different type: reactions that leads to char
An alternative to the above-mentioned route is to produce
formation, depolymerisation and fragmentation.
carbon materials starting from biomass and, through a
The char formation is due to the production of benzene
thermal process, which leads to the carbonisation of the
molecules and other aromatic compounds with
organic matter, producing biochar. The word biochar refers
subsequent rearrangement of these molecules in a
to a carbon-rich product when biomass such as wood,
polycyclic structure, followed by the release of water and
manure or leaves is heated in a closed container with little
permanent gases5.
or unavailable air1. It represents a suitable way for the
Depolymerisation reactions occur when the bonds
long-term storage of carbon matter. The production of
between the monomers of the original biopolymer are
biochar can be obtained through different processes such
broken. These reactions lead to a decrease of the
as fast and slow pyrolysis, which were already discussed in
polymerisation degree. The obtained products are smaller
Chapter 1, and hydrothermal carbonisation, described in
molecules if compared with the original ones, which are
Chapter 6.
recovered in the pyrolysis liquid. The formation of
permanent gases is due to the rupture of covalent bonds
2. Biochar production within the polymers (i.e. fragmentation reactions). Besides
permanent gases, condensable molecules like acids and
Pyrolysis operating conditions have a key role in the
alcohols are produced too.
products distribution. Choosing the best combination of
When the products of primary reactions are not chemically
peak temperature, heating rate, pressure and inert gas
stable under pyrolysis conditions, they undergo further
used, it is possible to address the process to the production
transformations. The reactions in series of all the above-
of more bio-oil, gases, or biochar. In many studies, it is
mentioned are called “secondary reactions”. Such
accepted that high temperatures and high heating rates
reactions mainly involve thermal cracking and other
promote the formation of volatile compounds, whereas
recombination reactions. Cracking essentially consists on
low temperatures and low heating rates increase the yield
the rupture of the bonds within the molecules, obtaining
of biochar. On the other hand, also the composition of
compounds with a lower molar weight. On the other hand,
original biomass is responsible for the final distribution of
recombination reactions between volatile molecules lead
the products. For example, pyrolysis of biomass with a high
to the formation of higher molecular weight products
lignin content offers high yields of biochar.2
which are likely to condense above the char structure,
The whole pyrolysis process involves decomposition
resulting in the so-called “secondary char formation”.
reactions of the three main components of the
lignocellulosic feedstock: cellulose, hemicelluloses and

101
Chapter 8 Di Stasi et al.
This section is only aimed to give a brief overview about Table 1. Most used techniques for the characterisation of
the main reactions involved in biochar production. More bulk and surface of biochar.
detailed information is available in Chapter 4.
Proximate analysis
Moisture ASTM standard E1756-08
3. Biochar Characterisation Volatile matter ASTM standard E872-82
Ash content ASTM standard E1755-01
A deep characterisation of biochar is an essential step to
Fixed carbon Determined by difference
evaluate its suitability for further applications. The first
Structure
analysis that is usually performed is the assessment of the
Macro analysis SEM, TEM
amount of moisture, ash, volatile matter and fixed-carbon.
Micro analysis XRD, Raman, NMR
These values are obtained by means of a proximate
Surface area BET (N2 at 77K and CO2 at 273
analysis. The measurement can be performed in
K)
accordance with the American Society for Testing and
Functional groups
Materials (ASTM) standards. However, for a detailed
Surface functional XPS, FTIR, TPD, Boehm titration
description of the procedures, Cai et al.6 provided a
groups
complete review on this topic.
Elemental Composition
Proximate analysis is usually used to evaluate, in a
Bulk inorganics ICP-AES, XRF
preliminary way, the characteristics of the biochar;
CHNS-O Combustion analysis
however, it should be necessary to perform additional
characterisations for a given application. For example,
when the biochar is produced for catalytic purposes, it is
very important to investigate both bulk and surface 4. Biochar Activation
properties of the solid. For the morphological
Generally, raw biochar has only a limited number of
characterisation of biochar, the most used techniques are
functional groups, such as C=O, OH and COOH, and a
the transmission electron microscopy (TEM) and scanning
relativity specific surface area mainly dominated by narrow
electron microscopy (SEM)7,8. Bulk chemical
micropores. These poor properties hinder the direct
characterisation can be done using Raman spectroscopy,
application of biochar in different fields such as catalysis
solid-state 13C and 1H nuclear magnetic resonance (NMR)9
and adsorption, in which the presence of specific
and X-ray diffraction (XRD)10.
functional groups on the surface and of a hierarchical pore
The textural properties (i.e., specific surface area and pore
size distribution is mandatory.
size distribution) can be evaluated from the adsorption
Nevertheless, biochar can be easily activated by means of
isotherm of N2 at –196 °C, using BET and DFT models.
chemical or physical processes aimed at introducing some
However, nitrogen at cryogenic temperatures cannot
specific functional groups and increase the solid textural
access the small micropores. Thus, the volume of narrower
properties. Fig. 1 illustrates the different pathways to
micropores (pore size below 0.7 nm) should be estimated
produce a biochar-derived activated carbon depending on
from the CO2 adsorption isotherm at 0 °C11,12.
its final application.
Bulk composition of inorganics is assessed using
inductively coupled plasma atomic emission spectroscopy
(ICP-AES)13 or X-ray fluorescence (XRF)14.
The elemental analysis for carbon, hydrogen, oxygen,
nitrogen and sulphur (CHNS analysis) are usually
determined by oxidation methods (combustion analysis)15.
Pristine biochar can contain different surface functional
groups, depending on the production conditions. Such
functional groups can interact with the reactants/products
of a specific reaction, leading to an increase in the
selectivity of the process. In other words, it is possible to
produce a high selective catalyst by adding chemical
functional groups on the surface of the biochar. The Figure 1. Main pathways and applications of raw biochar.
identification and quantification of these groups is
obtained using X-ray photoemission spectroscopy (XPS)16, 4.1. Development of surface area
Boehm titration8, Fourier transform infrared spectroscopy As stated before, biochar is a good platform to produce
(FTIR)17, and temperature-programmed desorption activated carbons via physical or chemical activations. In
(TPD)18. Table 1 summarises all the techniques that can be order to avoid structural and chemical modification during
employed in biochar characterisation. its utilisation, it is extremely important that the activation
procedure has to be carried out under temperatures and
pressures not lower than those used in the final application
of the activated biochar.
102
4.1.1. Physical activation produce and activate the biochar at the same time in a
Physical activation is the process in which the development single-step process as well as recycling CO2 from residual
of porosity is obtained by a controlled gasification of the flue gases and therefore avoiding the use of expensive nert
solid through an oxidising agent such as CO2 or steam. In gases (like N2). The biochar obtained from their
practice, carbon atoms present on the surface are experiments had a moderate high surface area
converted into gaseous species, leading to the formation (SBET = 197 m2 g–1) and a microporous structure.
of a porous structure. Generally, the physical activation is 4.1.2. Chemical Activation
performed exposing the biochar to the activating agent
under high temperatures (700–900 °C) 19,20. The main When the activation of biochar is conducted using a
reactions involved in the physical activation process are: chemical agent, which promotes the gasification of the
solid (i.e., chemical activation), two different pathways are
C + H2 O ⇆ CO + H2 (1)
possible:
C + CO2 ⇆ 2CO (2) (1) Mixing the raw biomass with an activating agent
C + O2 ⇆ CO2 (3) followed by a heating step (under inert atmosphere)
resulting in the production of activated biochar (see Fig.
The extent of the gasification process, which is a function
2a).
of different variables such as soaking time, temperature
(2) The biochar produced via pyrolysis is then mixed with
and activating agent, is evaluated through the “burn-off
the activating agent and heated again under inert
degree”, which is defined as the relative difference
atmosphere (see Fig. 2b).
between the initial and the final mass of biochar.
Furthermore, it is also possible to distinguish between wet
Table 2 reports some results of physical activations
and dry mixing, depending on how the activating agent is
reported in literature, which clearly show that, through
added (as a solid or as an aqueous solution, respectively).
physical activation, it is possible to produce activated
The most used chemical agents are H3PO4, ZnCl2, KOH and
carbons with relatively high specific surface areas.
NaOH. These agents, besides the benefits on the
Table 2. Some examples from literature of physically activated gasification step, have also dehydrating properties that can
biochars (initial and final SBET values, in m2 g–1, correspond to influence the pyrolysis vapours during the carbonisation
the SBET measured for raw and activated biochar, respectively). process, increasing the carbon yield28. Despite the fact that
these chemicals are the most studied, their employment at
Temp. Time Initial Final industrial scale has different drawbacks. Residual ZnCl2 is
Biomass Agent Ref.
(°C) (min) SBET SBET not easily separated from the activated biochar, and this
Olive
CO2 800 120 43 1079 20
leads to pollution and waste disposal problems. H3PO4 can
stones cause an increased eutrophication if the char is finally
Almond applied for soil remediation purposes29. Furthermore, also
CO2 700 – 21 1090 21
shell the application of hydroxides, such as KOH and NaOH, is
Oak problematic because of corrosion issues30.
CO2 900 120 107 1126 22
wood
Coconut
CO2 800 60 – 720 23
shell
Peanut
CO2 900 120 7 1308 19
hull
Palm
H2O 500 10 111 571 24
Shell
Wood 900/20 Figure 2. The two different ways to chemically activate a
H2O/O2 60/30 50 1025 25
waste 0 biochar (wet mixing): (a) starting from the impregnation
of biomass or (b) starting from the raw biochar.
Since the reaction rate of water gasification (Reaction 1) is An interesting alternative is using K2CO3, which is a cheap
faster than CO2 gasification (Reaction 2), the activation and non-hazardous compound and already studied in
process with steam is usually faster. Chang and co- literature31,32.
workers26 compared the results of the activation of a corn The first phase of the chemical activation process is the
cob biochar with CO2 and H2O. Their results showed that impregnation of the biomass/biochar that is usually carried
water activation led to the formation of a microporous out at 50 °C in order to enhance the diffusion of the agent
structure, whereas with the employment of CO2 as inside the inner structure of the biochar. The following step
activating agent, it was possible to produce an activated is the separation of the impregnated biomass/biochar from
carbon with a higher fraction of mesopores. the solution and its drying to remove the remaining water.
As stated before, the production of an activated carbon Dehkhoda et al.33 found that the ratio between mesopores
involves two different phases: the pyrolysis of biomass, and micropores can be modulated by changing the drying
and the partial gasification of the resulting biochar. Azuara atmosphere and duration. If air is used in the drying step,
et al.27 studied the application of CO2 as carrier gas during the carbon solid is characterised by the presence of both
the pyrolysis of biomass. In this way, it is possible to
103
micro- and mesopores. Furthermore, using N2 it is possible Table 3. Some results reported in literature of biochar
to obtain a solid with a narrower pore size distribution in chemical activation via wet impregnation (ratio is given in
mass basis chemical/precursor; SBET in m2 g–1).
the range of micropores.
Chemical activation mechanism is not yet completely
Temp. Time
understood34. In the case of the activation with KOH, the Biomass Agent Ratio SBET Ref.
(°C) (min)
porosity development can be ascribed to three different
Palm shell K2CO3 1 800 120 1170 36
phenomena:
Soybean oil
(1) KOH can react with the carbon matrix, releasing CO KOH 1 800 60 618 32
cake
and H2.
Soybean oil
(2) CO2 and H2O resulting from the thermal degradation K2CO3 1 800 60 1352 32
cake
of biochar can react with KOH producing K2CO3, which can
Safflower
react with carbon to release more gaseous species. ZnCl2 4 900 60 802 37
seed
(3) The metallic K produced in situ can intercalate in the
matrix of the biochar, leading to the formation of larger Peach
H3PO4 0.43 800 60 1393 38
pores (after the removal of K species through an acidic stones
solution). Apricot shell NaOH 2 400 60 2335 39
These effects are the results of the Reactions 4–735. Vine shoots KOH 2 700 60 1671 12
2 KOH + CO2 → K2 CO3 + H2 O (4)

2 C + 2KOH → 2CO + H2 +2K (5)
C + K2 CO3 → CO + K2 O (6) 4.2. Biochar functionalisation
C + K2 O→CO + 2K (7) Some functional groups containing S, N, and O can be

available in the raw biochar. These groups make the
The most important parameter in this kind of activation is biochar suitable for adsorption and catalytic applications40.
the amount of chemical agent used for the impregnation, If the aim is to enhance the activity of the biochar, it is
the so-called impregnation ratio, which corresponds to the possible to add some interesting surface functional groups.
mass ratio between the chemical and precursor (biochar). For example, groups such as C=O, OH, COOH are important
The effects of using different impregnation ratios depend if the final purpose of the biochar is to be used as
on the feedstock and the chemical agent. After the adsorbent for heavy-metal removal or when it has to be
carbonisation step, the resulting activated carbon can still applied in aqueous systems.
contain part of the activating agent or related compounds, The functionalisation can be done through surface
which can clog the pores of the material and/or alter its pH. oxidation, which is the most used method of carbon
Thus, a cleaning step is required. For this purpose, a diluted surface modification, using reactants such as air, O2, H2O241
acid solution is commonly used. However, in the case of or O342. Usually, the process is carried out at high
water-soluble compounds like K2CO3, a rinse performed temperature (200‒350 °C) using a continuous flow of
with hot water could be enough. The process temperature oxidant agent followed by a rinse step with water to
is a relevant parameter, since depending on the activation remove water-soluble products43.
agent, it is possible to promote some reactions over others. During the production of a biochar-supported catalyst by
Hayashi et al.30 studied the influence of the carbonisation impregnation, low dispersion of the active phase is usually
temperature on the pore size distribution of the produced obtained. This is mainly due to the fact that biochar surface
activated carbon using K2CO3.The results showed that has a hydrophobic behaviour, which hinder the
higher temperatures led to an increase in the total and homogeneous distribution of the metal particles. It was
mesopore volumes. Results from several previous studies demonstrated that the addition of acidic groups on biochar
(using different precursors and activating agents) are given surface leads to an increase in the hydrophilicity, making
in Table 3. the surface more accessible to the aqueous solution of the
active phase precursor44 and resulting in a better
dispersion of the metal on the biochar.
The introduction of sulphur functionalities is usually done
by heating the carbonaceous material in presence of
elemental sulphur or hydrogen sulphide45. SO3H group
could be added using a concentrated sulphuric acid and the
so-called “solid acid” can find applications in esterification
reactions46,47, such as the production of biodiesel and
hydrolysis of biomass.
The presence of amino groups on the surface of the
biochar, on the other hand, can enhance the performance
of CO2 capture in gas-cleaning processes. The methods
used to provide these groups to a carbon solid is the
104
nitration, using HNO3, or a procedure that involves amino- alkaline compounds are present in the feedstock, leading,
containing reagents48. in some cases, to the formation of a double-phase bio-oil.
Finally, halogen groups can also be added to the surface of Likewise, also pyrolysis temperature is an influent
biochar and the stability of the complex carbon-halogen parameter that has to take into account. Higher
decreases in the order chlorine > bromine > iodine43. The temperatures result in a higher production of aromatic
treatment can be carried out employing a halogen vapour compounds, which, through condensation reactions, tend
or an aqueous solution of halogens. Chlorine, for example, to create polyaromatic structures (see Fig. 352).
can be incorporated into the surface of a carbon material With regards to the influence of the particles size of the
by contact with a continuous flow of Cl2 at high feedstock, it was observed that using large particle sizes
temperature49. results in a decreased bio-oil yield. It is assumed that the
“actual heating rate” experienced by biomass, which
decreases with increasing particle size, is the major factor
4.3. Biochar-supported metal catalyst
contributing to the decrease in the yield of pyrolysis
As stated before, biochar and carbon material are, in liquid53.
general, a good platform to produce catalyst supports.
Carbonaceous supports have different advantages over
the traditional supports such as alumina and silica. For
example, carbon surface is resistant to acid and basic
agents and its structure is stable at high temperatures.
Furthermore, the combustion of the spent catalyst allows
the recovery of the active phase in an easy way43.
The active phase can be loaded on the support in two
different ways:
(1) By impregnation of the starting material (i.e., raw
biomass) with a solution containing the active phase and
subsequent carbonisation step, where the metal ions are
reduced to zero oxidation state. In this case, the metal
can also promote further char gasification (leading to a
more porous product).
(2) By impregnation of the prostine biochar. In this case,
the catalyst must be activated by heating under a
Figure 3. Influence of temperature on the distribution of
reducing atmosphere.
pyrolysis products.
5.1. Pyrolysis liquid characterisation

5. Pyrolysis Vapours
Bio-oil is a multicomponent mixture of hundreds of organic
Bio-oil (also called as pyrolysis liquid) is a dark brown compounds derived from the depolymerisation and
viscous liquid with a high concentration of oxygen- fragmentation of the three main “bricks” of biomass:
containing compounds, the presence of which is the main lignin, cellulose and hemicelluloses. Therefore, the
difference between a bio-oil and a conventional liquid fuel composition strictly depends on the nature of the
derived from fossil sources. Depending on its properties, feedstock (in terms of its composition) and, to a lesser
the bio-oil can be used directly as low-grade fuel for diesel extent, of the operating conditions of pyrolysis.
engines or gas turbines, transportation fuel or platform for The main compounds present in the mixture are alkanes,
the production of other chemicals. All these applications aromatic hydrocarbons, phenol derivatives and ketones,
make the bio-oil feasible to replace the fossil oil in different esters, ethers, sugars, amines, and alcohols. The collection
fields or, at least, to reduce the world dependence on non- of the liquid in lab-scale is usually conducted by
renewable resources. condensation of the outlet stream from the reactor and
Bio-oil is not the product of thermodynamic equilibrium, the simpler configuration consists of one or more ice-
since it is produced at a relatively short residence time and cooled traps. An alternative method is the fractional
is usually recovered through a quick quenching after the condensation system, which consists on a series of
exit of the pyrolysis system. Therefore, the chemical condensers operating at different temperatures in which,
composition of the bio-oil tends to change during its typically, the first bottle is operating at a higher
storage50. temperature in order to condensate only the heaviest
The chemical composition of the biomass feedstock is an fraction. In this way, it is possible to obtain a more accurate
important factor, which has influence on the pyrolysis separation of the compounds based on the different
liquid yield. Furthermore, the ash content of the feedstock boiling points. The condensers can also be filled with a
also effects the yield of bio-oil. In fact, the liquid yield is solvent, or a mixture of them, in order to avoid further
higher when the used biomass has a low content in ashes51. reactions amoung the compounds of the liquid. The most
Moreover, also the liquid water-fraction increases when used solvents in literature are isopropanol, methanol and
dichloromethane.14,54–56
105
Mass spectrometry coupled to gas chromatography These processes can be classified in two categories,
(MS/GC) is commonly used to identify and quantify volatile depending on where they take place (see Fig. 5). If the
substances present in organic liquids. The main problem of upgrading process is performed within the pyrolysis
this kind of analysis is that they can only detect about process system, using as reagents the CO2 and water
40 wt. % of conventional pyrolysis oil components because produced during the overall process, it is called “primary
of the low volatility resulting from the high molecular process”. On the other hand, the liquid can be fed in
weight of the mixture. Therefore, complementary analysis downstream processes where water, CO2 or both can be
techniques are necessary57. Oasmaa and co-workers have supplied externally. In this case, it is called “secondary
developed a solvent fractionation method, which can be process”.
able to characterise all the chemical families present in the
mixture58 (see Fig. 4).
Figure 5. Primary and secondary upgrading processes of

the pyrolysis liquid.
Figure 4. Bio-oil characterisation method suggested by
Oasmaa et al.58 (DCM: dichloromethane).
6.1. Cracking
Addition of water aliquots to the bio-oil leads to a phase Cracking of bio-oil aims at producing lighter compounds by
separation, dividing the liquid into a water-soluble and breaking the bonds of the longer molecules. The main
water-insoluble fraction. The latter undergoes further reaction involved in this process is the thermal degradation
separation with dicchlorometane (DCM) in order to obtain (Reaction 8).
two solutions with different molecular size distribution. Cn Hm Ok → Cx Hy Oz + gases (H2 , H2 O, CO2 , CO, CH4 , C2 Hx ,
The analysis of the different fractions is carried out with C3 Hy ,…) + coke (8)
different analytical techniques, as mentioned by Oasmaa56.
This process is carried out feeding the pyrolysis vapours to
The water-insoluble fraction contains mainly lignin-derived
a secondary reactor at high temperature (700–1000 °C)
compounds, extractives and solids. Meanwhile, in the
without the addition of external reagents. There are two
aqueous phase, it is possible to find sugar-type
kinds of cracking processes: (1) catalytic cracking, where
compounds, acids, aldehydes, ketones, pyrans, and furans.
bio-oil is converted using a metal-based catalyst such as
The content of water of the mixture, which is the result of
nickel; and (2) thermal cracking by partial oxidation or
the original moisture of the feedstock as well as
direct thermal contact.
dehydration reactions occurring during the pyrolysis, is
Both processes have drawbacks: catalytic cracking is more
within the range of 35–85 wt. %. The presence of water has
effective and leads to higher conversion, but it is also more
advantages and disadvantages. For example, it reduces the
expensive because of the catalyst. On the other hand,
viscosity of the whole liquid, enhancing the fluidity59 but,
thermal cracking is cheaper, because no catalyst is
on the other hand, water lowers the heating value of the
required, but, due to the fact that the pyrolytic liquid is a
mixture making necessary subsequent separation
very refractory substance61, higher temperatures (up to
processes (i.e., more expensive process). Water in bio-oil
1000 °C) are required to achieve an appropriate
can be found dissolved in the pyrolytic liquid or can be
conversion.
present as an emulsion. Generally, it cannot be removed
by physical separation methods such as centrifugation60. 6.2. Steam reforming
The percentage of water is commonly measured using Karl Among the existing several methods for pyrolytic vapours
Fischer titration, according to ASTM E 203-96 standard. upgrading, catalytic reforming is considered to be the most
promising for large-scale applications, because of its high
rate of reaction and high reliability14. Reforming process is
6. Upgrading of pyrolysis vapours generally used when the objective is the production of an
Slow pyrolysis is mainly aimed at producing biochar; thus, H2-rich syngas. However, besides hydrogen, also other
the liquid fraction is considered a problem, since it reduces permanent gases such as CO, CO2 and light hydrocarbons
the yield of the main product and can also condense inside are produced and, depending on the products distribution,
the pipelines or filters, causing clogging and the the effluent gas can be used as fuel or as a platform for the
subsequent breakdown of the entire system. To avoid synthesis of chemicals. The main reactions involved in the
these problems and upgrade the composition of the steam reforming of oxygenated compounds are:
pyrolysis gas, it is important to reduce the yield of liquid by CnHm Ok + (2n – k) H2 O ⇄ nCO2 + �2n +
m
– k� H2 (9)
choosing the optimal operating conditions and/or remove 2
(10)
m
such fraction via downstream processes (e.g., thermal CnHm Ok + (n – k) H2 O ⇄ nCO + �n + – k� H2
2
cracking or catalytic reforming). CO + H2 O ⇄ CO2 + H2 (11)
106
Reforming is usually carried out using a supported metal modify the performance of the catalyst, as well as the
catalyst to improve the conversion and the selectivity of above-mentioned doping compounds. Feng et al.71
the process (catalytic steam reforming). compared the effects of K and Ca on the catalyst
Besides the steam reforming (Reactions 10 and 11), and performance in steam reforming of a model compounds
water gas shift (Reaction 11), also thermal cracking (naphthalene and toluene), finding that a high
(Reaction 8) can take place, leading to the formation of concentration of K in the catalyst led to a higher liquid
coke deposition on the catalyst, which is the main cause of conversion.
deactivation. It was reported by Czernik et al.62 that, using Despite the high conversions usually obtained in steam
a fixed bed reactor operating at 850 °C with a commercial reforming, the outlet syngas still contains a small
nickel catalyst, it was possible to achieve a yield of percentage of unreacted bio-oil or some condensable
hydrogen equal to 80%. Conversely, after 4 hours, the subproducts. When a high purity syngas is required, the
activity of the catalyst decreased markedly as a result of steam reforming process can be coupled in series by an
the coke deposition on the catalyst surface, making a adsorption step in a fixed-bed reactor, using activated
generation step necessary. carbons as adsorbent material.72
An important parameter of steam reforming is the molar As stated at the beginning of this section, the produced
proportion between water and carbon, called steam-to- syngas can find application in different fields. The most
carbon ratio. The excess of water promotes coke important parameter is the molar ratio H2/CO and every
gasification and the subsequent in-situ regeneration of the process that employs syngas as reactant has an ideal value
catalyst, but it also leads to a dilution of the reagents and of this ratio. The production of hydrogen and carbon
products. monoxide is in a first-place influenced by the composition
The catalyst needs to be supported, commonly by alumina of bio-oil and then by the reforming conditions. A way to
monoliths, to enhance its mechanical and textural modify such ratio is to simultaneously feed CO2 and steam
properties. The support is usually doped with a chemical in order to displace the water gas shift reaction towards
agent which promotes specific reactions during the the formation of CO.73
process. For example, the acidity of alumina promotes
cracking reactions that lead to coke formation with
subsequent deactivation of the catalyst. Therefore, the 7. Conclusions
support is doped with basic oxides like CaO, MgO and K2O Pyrolysis-derived biochar represents a sustainable
that help to avoid such coke formation. A study carried out precursor for advanced carbon materials. Its easy
by Adrados et al.63 has shown that, through the doping functionalisation makes the biochar to be a versatile
with CeO2 of a Ni/Al2O3 catalyst, it is possible to obtain material, which can be used in different fields such as
better performances, in terms of produced hydrogen, than catalyst and adsorption. To be employed in such fields, a
that those obtained using a zirconia-doped support. deep characterisation of the biochar structure and
Another important parameter is the pore size distribution chemical composition is needed. Depending on the
of the catalyst. The pores responsible for the adsorption of production condition and its final application, the biochar
the organic compounds are the micropores64, but also could not have the appropriate textural properties or the
larger pores are essential for making the micropores required functional groups. Thus, an activation step aimed
accessible to the aromatic molecules. at expanding the biochar porosity and/or at introducing
The employment of specific catalysts in steam reforming some specific functional group, is mandatory. Besides its
process can also allow to work at lower temperatures, direct application, biochar can also be used to create metal
maintaining a reasonable level of reactant conversion and composites to be used in catalyst. In fact, carbon surface
product selectivity. In fact, using a commercial nickel has some interesting inherent properties, such as the high
catalyst (C11-PR) at 500 °C in a second stage reformer, an temperature stability and the acid/base resistance, which
increase in the total gas yield of 58% was obtained65. made biochar a good platform for the production of
Recent studies suggested that a biochar-based catalyst can catalyst supports.
represent a valid and sustainable alternative to the One of the most interesting application of biochar-based
commercial products66–68. Iron-supported biochar, for catalysts is their use in bio-oil upgrading processes, in
example, was successfully used in gas-cleaning which the condensable phase resulting from pyrolysis is
applications. At 800 °C, iron-supported biochar was able to converted, through catalytic cracking and/or steam
convert 100% of toluene in lighter compounds, with 0% of reforming, to permanent gases such as CO2, CO, CH4, H2
benzene selectivity69. Shen et al.70 studied the application and light hydrocarbons. In this way, it is possible to
of different metal catalysts supported by a biochar enhance the efficiency of the pyrolysis process, increasing
produced from slow pyrolysis of rice husk. They found that the heating value of the gaseous products. Furthermore,
a char-supported Ni-Fe catalyst, used in pyrolysis the spent biochar-based catalyst can be regenerated or
processes, was able to reduce liquid yield by 92.3%.14 burnt/gasified to recover more energy.
The metal active phase is not the only factor that The specific aim of the GreenCarbon’s ESR 10 (Christian Di
influences the activity and the efficiency of the biochar- Stasi) is to produce metallic catalysts supported by
supported catalyst. Inherent alkali and alkaline earth chemically and physically activated biochars. The produced
metallic species, present in the original biomass, can also catalysts are addressed to be used in bio-oil upgrading
107
processes with focus on hydrogen production through (10) Ghani, W. A. W. A. K.; Mohd, A.; da Silva, G.;
steam and dry reforming. This study is carried out in close Bachmann, R. T.; Taufiq-Yap, Y. H.; Rashid, U.; Al-
collaboration with the ERS 1 (Gianluca Greco), who will Muhtaseb, A. H. Biochar Production from Waste
provide the ESR 10 with all the raw biochars needed for Rubber-Wood-Sawdust and Its Potential Use in C
further activation and metal dispersion. Sequestration: Chemical and Physical
Characterization. Ind. Crops Prod. 2013, 44, 18–24.
(11) Jagiello, J.; Thommes, M. Comparison of DFT
Acknowledgments Characterization Methods Based on N2, Ar, CO2, and
This project has received funding from the European H2 Adsorption Applied to Carbons with Various Pore
Union’s Horizon 2020 research and innovation programme Size Distributions. Carbon 2004, 42 (7), 1227–1232.
under the Marie Skłodowska-Curie grant agreement No (12) Manyà, J. J.; González, B.; Azuara, M.; Arner, G. Ultra-
721991. Microporous Adsorbents Prepared from Vine Shoots-
Derived Biochar with High CO2 Uptake and CO2/N2
Selectivity. Chem. Eng. J. 2018, 345, 631–639.
References
(13) Mohanty, P.; Nanda, S.; Pant, K. K.; Naik, S.; Kozinski, J.
(1) Shackley, S.; Carter, S.; Knowles, T.; Middelink, E.; A.; Dalai, A. K. Evaluation of the Physiochemical
Haefele, S.; Sohi, S.; Cross, A.; Haszeldine, S. Development of Biochars Obtained from Pyrolysis of
Sustainable Gasification–biochar Systems? A Case- Wheat Straw, Timothy Grass and Pinewood: Effects of
Study of Rice-Husk Gasification in Cambodia, Part I: Heating Rate. J. Anal. Appl. Pyrolysis 2013, 104, 485–
Context, Chemical Properties, Environmental and 493.
Health and Safety Issues. Energy Policy 2011, 42, 49– (14) Shen, Y.; Zhao, P.; Shao, Q.; Ma, D.; Takahashi, F.;
58. Yoshikawa, K. In-Situ Catalytic Conversion of Tar Using
(2) Antal, M. J.; Grønli, M. The Art, Science, and Rice Husk Char-Supported Nickel-Iron Catalysts for
Technology of Charcoal Production. Ind. Eng. Chem. Biomass Pyrolysis/Gasification. Appl. Catal. B Environ.
Res. 2003, 42 (8), 1619–1640. 2014, 152–153 (1), 140–151.
(3) Collard, F. X.; Blin, J. A Review on Pyrolysis of Biomass (15) Bahng, M. K.; Mukarakate, C.; Robichaud, D. J.; Nimlos,
Constituents: Mechanisms and Composition of the M. R. Current Technologies for Analysis of Biomass
Products Obtained from the Conversion of Cellulose, Thermochemical Processing: A Review. Anal. Chim.
Hemicelluloses and Lignin. Renew. Sustain. Energy Rev. Acta 2009, 651 (2), 117–138.
2014, 38, 594–608. (16) Singh, B.; Fang, Y.; Cowie, B. C. C.; Thomsen, L. NEXAFS
(4) Van de Velden, M.; Baeyens, J.; Brems, A.; Janssens, B.; and XPS Characterisation of Carbon Functional Groups
Dewil, R. Fundamentals, Kinetics and Endothermicity of Fresh and Aged Biochars. Org. Geochem. 2014, 77,
of the Biomass Pyrolysis Reaction. Renew. Energy 1–10.
2010, 35 (1), 232–242. (17) Jensen, A.; Dam-Johansen, K.; Wójtowicz, M. A.; Serio,
(5) McGrath, T. E.; Geoffrey Chan, W.; Hajaligol, M. R. Low M. A. TG-FTIR Study of the Influence of Potassium
Temperature Mechanism for the Formation of Chloride on Wheat Straw Pyrolysis. Energy Fuels 1998,
Polycyclic Aromatic Hydrocarbons from the Pyrolysis 12 (5), 929–938.
of Cellulose. J. Anal. Appl. Pyrolysis 2003, 66 (1), 51– (18) Lillo-Ródenas, M. A.; Cazorla-Amorós, D.; Linares-
70. Solano, A. Behaviour of Activated Carbons with
(6) Cai, J.; He, Y.; Yu, X.; Banks, S. W.; Yang, Y.; Zhang, X.; Different Pore Size Distributions and Surface Oxygen
Yu, Y.; Liu, R.; Bridgwater, A. V. Review of Groups for Benzene and Toluene Adsorption at Low
Physicochemical Properties and Analytical Concentrations. Carbon 2005, 43 (8), 1758–1767.
Characterization of Lignocellulosic Biomass. Renew. (19) Fang, J.; Gao, B.; Zimmerman, A. R.; Ro, K. S.; Chen, J.
Sustain. Energy Rev. 2017, 76, 309–322. Physically (CO2) Activated Hydrochars from Hickory
(7) Zhu, J.; Jia, J.; Kwong, F. L.; Ng, D. H. L.; Tjong, S. C. and Peanut Hull: Preparation, Characterization, and
Synthesis of Multiwalled Carbon Nanotubes from Sorption of Methylene Blue, Lead, Copper, and
Bamboo Charcoal and the Roles of Minerals on Their Cadmium. RSC Adv. 2016, 6, 24906–24911.
Growth. Biomass Bioenergy 2012, 36, 12–19. (20) Plaza, M. G.; Pevida, C.; Arias, B.; Fermoso, J.; Casal, M.
(8) Suliman, W.; Harsh, J. B.; Abu-Lail, N. I.; Fortuna, A. M.; D.; Martín, C. F.; Rubiera, F.; Pis, J. J. Development of
Dallmeyer, I.; Garcia-Perez, M. Influence of Feedstock Low-Cost Biomass-Based Adsorbents for
Source and Pyrolysis Temperature on Biochar Bulk and Postcombustion CO2 Capture. Fuel 2009, 88, 2442–
Surface Properties. Biomass Bioenergy 2016, 84, 37– 2447.
48. (21) Plaza, M. G.; Pevida, C.; Martín, C. F.; Fermoso, J.; Pis,
(9) Cao, X.; Pignatello, J. J.; Li, Y.; Lattao, C.; Chappell, M. J. J.; Rubiera, F. Developing Almond Shell-Derived
A.; Chen, N.; Miller, L. F.; Mao, J. Characterization of Activated Carbons as CO2 Adsorbents. Sep. Purif.
Wood Chars Produced at Different Temperatures Technol. 2010, 71 (1), 102–106.
Using Advanced Solid-State 13C NMR Spectroscopic
Techniques. Energy Fuels 2012, 26 (9), 5983–5991.
108
(22) Jung, S. H.; Kim, J. S. Production of Biochars by (36) Adinata, D.; Wan Daud, W. M. A.; Aroua, M. K.
Intermediate Pyrolysis and Activated Carbons from Preparation and Characterization of Activated Carbon
Oak by Three Activation Methods Using CO2. J. Anal. from Palm Shell by Chemical Activation with K2CO3.
Appl. Pyrolysis 2014, 107, 116–122. Bioresour. Technol. 2007, 98 (1), 145–149.
(23) Laine, J.; Calafat, A. Factors Affecting the Preparation (37) Angin, D.; Altintig, E.; Köse, T. E. Influence of Process
of Activated Carbons from Coconut Shell Catalized by Parameters on the Surface and Chemical Properties of
Potassium. Carbon 1991, 29 (7), 949–953. Activated Carbon Obtained from Biochar by Chemical
(24) Yek, P. N. Y.; Liew, R. K.; Osman, M. S.; Lee, C. L.; Chuah, Activation. Bioresour. Technol. 2013, 148, 542–549.
J. H.; Park, Y.-K.; Lam, S. S. Microwave Steam (38) Attia, A. A.; Girgis, B. S.; Fathy, N. A. Removal of
Activation, an Innovative Pyrolysis Approach to Methylene Blue by Carbons Derived from Peach Stones
Convert Waste Palm Shell into Highly Microporous by H3PO4 Activation: Batch and Column Studies. Dyes
Activated Carbon. J. Environ. Manage. 2019, 236, 245– Pigments 2008, 76 (1), 282–289.
253. (39) Xu, B.; Chen, Y.; Wei, G.; Cao, G.; Zhang, H.; Yang, Y.
(25) Bardestani, R.; Kaliaguine, S. Steam Activation and Activated Carbon with High Capacitance Prepared by
Mild Air Oxidation of Vacuum Pyrolysis Biochar. NaOH Activation for Supercapacitors. Mater. Chem.
Biomass Bioenergy 2018, 108, 101–112. Phys. 2010, 124 (1), 504–509.
(26) Chang, C. F.; Chang, C. Y.; Tsai, W. T. Effects of Burn-off (40) Su, D. S.; Zhang, J.; Frank, B.; Thomas, A.; Wang, X.;
and Activation Temperature on Preparation of Paraknowitsch, J.; Schlögl, R. Metal-Free
Activated Carbon from Corn Cob Agrowaste by CO2 Heterogeneous Catalysis for Sustainable Chemistry.
and Steam. J. Colloid Interface Sci. 2000, 232 (1), 45– ChemSusChem 2010, 3 (2), 169–180.
49. (41) Xue, Y.; Gao, B.; Yao, Y.; Inyang, M.; Zimmerman, A. R.;
(27) Azuara, M.; Sáiz, E.; Manso, J. A.; García-Ramos, F. J.; Zhang, M.; Ro, K. S. Hydrogen Peroxide Modification
Manyà, J. J. Study on the Effects of Using a Carbon Enhances the Ability of Biochar (Hydrochar) Produced
Dioxide Atmosphere on the Properties of Vine Shoots- from Hydrothermal Carbonization of Peanut Hull to
Derived Biochar. J. Anal. Appl. Pyrolysis 2017, 124, Remove Aqueous Heavy Metals: Batch and Column
719–725. Tests. Chem. Eng. J. 2012, 200–202, 673–680.
(28) Kandiyoti, R.; John I. Lazaridis; Bo Dyrvold; C.Ravindra (42) Jimenez-Cordero, D.; Heras, F.; Alonso-Morales, N.;
Weerasinghe. Pyrolysis of a ZnC12-Impregnated Inert Gilarranz, M. A.; Rodriguez, J. J. Ozone as Oxidation
Atmosphere. Fuel 1984, 63 (11), 1583–1587. Agent in Cyclic Activation of Biochar. Fuel Process.
(29) Tsai, W. T.; Chang, C. Y.; Wang, S. Y.; Chang, C. F.; Chien, Technol. 2015, 139, 42–48.
S. F.; Sun, H. F. Preparation of Activated Carbons from (43) Serp, P.; Figueiredo, J. L. Carbon Materials for
Corn Cob Catalyzed by Potassium Salts and Subsequent Catalysis; John Wiley & Sons, Inc.: Hoboken, NJ, USA,
Gasification with CO2. Bioresour. Technol. 2001, 78 (2), 2008.
203–208. (44) Prado-Burguete, C.; Linares-Solano, A.; Rodriguez-
(30) Hayashi, J.; Horikawa, T.; Takeda, I.; Muroyama, K.; Reinoso, F.; De Lecea, C. S. M. Effect of Carbon Support
Nasir Ani, F. Preparing Activated Carbon from Various and Mean Pt Particle Size on Hydrogen Chemisorption
Nutshells by Chemical Activation with K2CO3. Carbon by Carbon-Supported Pt Ctalysts. J. Catal. 1991, 128
2002, 40 (13), 2381–2386. (2), 397–404.
(31) Di Blasi, C.; Galgano, A.; Branca, C. Influences of the (45) Feng, W.; Kwon, S.; Borguet, E.; Vidic, R. Adsorption of
Chemical State of Alkaline Compounds and the Nature Hydrogen Sulfide onto Activated Carbon Fibers: Effect
of Alkali Metal on Wood Pyrolysis. Ind. Eng. Chem. Res. of Pore Structure and Surface Chemistry. Environ. Sci.
2009, 48 (7), 3359–3369. Technol. 2005, 39 (24), 9744–9749.
(32) Tay, T.; Ucar, S.; Karagöz, S. Preparation and (46) Kastner, J. R.; Miller, J.; Geller, D. P.; Locklin, J.; Keith,
Characterization of Activated Carbon from Waste L. H.; Johnson, T. Catalytic Esterification of Fatty Acids
Biomass. J. Hazard. Mater. 2009, 165 (1–3), 481–485. Using Solid Acid Catalysts Generated from Biochar and
(33) Dehkhoda, A. M.; Gyenge, E.; Ellis, N. A Novel Method Activated Carbon. Catal. Today 2012, 190 (1), 122–132.
to Tailor the Porous Structure of KOH-Activated (47) Dehkhoda, A. M.; Ellis, N. Biochar-Based Catalyst for
Biochar and Its Application in Capacitive Deionization Simultaneous Reactions of Esterification and
and Energy Storage. Biomass Bioenergy 2016, 87, 107– Transesterification. Catal. Today 2013, 207, 86–92.
121. (48) Titirici, M. M.; Thomas, A.; Antonietti, M. Aminated
(34) Lillo-Ródenas, M. A.; Lozano-Castelló, D.; Cazorla- Hydrophilic Ordered Mesoporous Carbons. J. Mater.
Amorós, D.; Linares-Solano, A. Preparation of Chem. 2007, 17, 3412–3418.
Activated Carbons from Spanish Anthracite - II. (49) Pérez-Cadenas, A. F.; Maldonado-Hódar, F. J.; Moreno-
Activation by NaOH. Carbon 2001, 39 (5), 751–759. Castilla, C. On the Nature of Surface Acid Sites of
(35) Wang, J.; Kaskel, S. KOH Activation of Carbon-Based Chlorinated Activated Carbons. Carbon 2003, 41 (3),
Materials for Energy Storage. J. Mater. Chem. 2012, 22, 473–478.
23710–23725. (50) Zhang, Q.; Chang, J.; Wang, T.; Xu, Y. Review of Biomass
Pyrolysis Oil Properties and Upgrading Research.
Energy Convers. Manage. 2007, 48 (1), 87–92.
109
(51) Oasmaa, A.; Solantausta, Y.; Arpiainen, V.; Kuoppala, (67) Bhandari, P. N.; Kumar, A.; Bellmer, D. D.; Huhnke, R.
E.; Sipilä, K. Fast Pyrolysis Bio-Oils from Wood and L. Synthesis and Evaluation of Biochar-Derived
Agricultural Residues. Energy Fuels 2010, 24 (2), 1380– Catalysts for Removal of Toluene (Model Tar) from
1388. Biomass-Generated Producer Gas. Renew. Energy
(52) Blanco, P. H.; Wu, C.; Onwudili, J. A.; Williams, P. T. 2014, 66, 346–353.
Characterization of Tar from the Pyrolysis/Gasification (68) Gilbert, P.; Ryu, C.; Sharifi, V.; Swithenbank, J. Tar
of Refuse Derived Fuel: Influence of Process Reduction in Pyrolysis Vapours from Biomass over a
Parameters and Catalysis. Energy Fuels 2012, 26 (4), Hot Char Bed. Bioresour. Technol. 2009, 100 (23),
2107–2115. 6045–6051.
(53) Shen, J.; Wang, X.-S.; Garcia-Perez, M.; Mourant, D.; (69) Kastner, J. R.; Mani, S.; Juneja, A. Catalytic
Rhodes, M. J.; Li, C.-Z. Effects of Particle Size on the Decomposition of Tar Using Iron Supported Biochar.
Fast Pyrolysis of Oil Mallee Woody Biomass. Fuel 2009, Fuel Process. Technol. 2015, 130, 31–37.
88 (10), 1810–1817. (70) Hosokai, S.; Norinaga, K.; Kimura, T.; Nakano, M.; Li, C.
(54) Volpe, M.; D’Anna, C.; Messineo, S.; Volpe, R.; Z.; Hayashi, J. I. Reforming of Volatiles from the
Messineo, A. Sustainable Production of Bio- Biomass Pyrolysis over Charcoal in a Sequence of Coke
Combustibles from Pyrolysis of Agro-Industrial Wastes. Deposition and Steam Gasification of Coke. Energy
Sustainability 2014, 6 (11), 7866–7882. Fuels 2011, 25 (11), 5387–5393.
(55) Volpe, M.; Panno, D.; Volpe, R.; Messineo, A. Upgrade (71) Feng, D.; Zhao, Y.; Zhang, Y.; Sun, S.; Meng, S.; Guo, Y.;
of Citrus Waste as a Biofuel via Slow Pyrolysis. J. Anal. Huang, Y. Effects of K and Ca on Reforming of Model
Appl. Pyrolysis 2015, 115, 66–76. Tar Compounds with Pyrolysis Biochars under H2O or
(56) Oasmaa, A, Leppaemaeki, E, Koponen, P, Levander, J, CO2. Chem. Eng. J. 2016, 306, 422–432.
and Tapola, E. Physical characterization of biomass- (72) Phuphuakrat, T.; Namioka, T.; Yoshikawa, K. Tar
based pyrolysis liquids. Application of standard fuel oil Removal from Biomass Pyrolysis Gas in Two-Step
analyses. Finland: N. p., 1997. Web. Function of Decomposition and Adsorption. Appl.
(57) Scholze, B.; Meier, D. Characterization of the Water- Energy 2010, 87 (7), 2203–2211.
Insoluble Fraction from Pyrolysis Oil (Pyrolytic Lignin). (73) Choudhary, V. R.; Rajput, A. M. Simultaneous Carbon
Part I. PY-GC/MS, FTIR, and Functional Groups. J. Anal. Dioxide and Steam Reforming of Methane to Syngas
Appl. Pyrolysis 2001, 60 (1), 41–54. over NiO-CaO Catalyst. Ind. Eng. Chem. Res. 1996, 35
(58) Oasmaa, A.; Kuoppala, E. Fast Pyrolysis of Forestry (11), 3934–3939.
Residue. 3. Storage Stability of Liquid Fuel. Energy
Fuels 2003, 17 (4), 1075–1084.
(59) Sipilä, K.; Kuoppala, E.; Fagernäs, L.; Oasmaa, A.
Characterization of Biomass-Based Flash Pyrolysis Oils.
Biomass Bioenergy 1998, 14 (2), 103–113.
(60) Elliott, D. C. Water, Alkali and Char in Flash Pyrolysis
Oils. Biomass Bioenergy 1994, 7 (1–6), 179–185.
(61) Bridgwater, A. V. The Technical and Economic
Feasibility of Biomass Gasification for Power
Generation. Fuel 1995, 74 (5), 631–653.
(62) Czernik, S.; French, R.; Feik, C.; Chornet, E. Hydrogen
by Catalytic Steam Reforming of Liquid Byproducts
from Biomass Thermoconversion Processes. Ind. Eng.
Chem. Res. 2002, 41 (17), 4209–4215.
(63) Adrados, A.; Lopez-Urionabarrenechea, A.; Solar, J.;
Requies, J.; De Marco, I.; Cambra, J. F. Upgrading of
Pyrolysis Vapours from Biomass Carbonization. J. Anal.
Appl. Pyrolysis 2013, 103, 293–299.
(64) Hosokai, S.; Kumabe, K.; Ohshita, M.; Norinaga, K.; Li,
C. Z.; Hayashi, J. ichiro. Mechanism of Decomposition
of Aromatics over Charcoal and Necessary Condition
for Maintaining Its Activity. Fuel 2008, 87 (13–14),
2914–2922.
(65) Hornung, U.; Schneider, D.; Hornung, A.; Tumiatti, V.;
Seifert, H. Sequential Pyrolysis and Catalytic Low
Temperature Reforming of Wheat Straw. J. Anal. Appl.
Pyrolysis 2009, 85 (1–2), 145–150.
(66) Abu El-Rub, Z.; Bramer, E. A.; Brem, G. Experimental
Comparison of Biomass Chars with Other Catalysts for
Tar Reduction. Fuel 2008, 87 (10–11), 2243–2252.
110
Hydrothermal carbonisation and its role in catalysis

Pierpaolo Modugnoa, Anthony E. Szegob, Maria-Magdalena Titiricia, Niklas Hedinb
a
School of Engineering and Material Science, Queen Mary University of London, Mile End Road, London E1 4NS, UK
b
Department of Material and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden
Abstract
This chapter provides an overview of the most recent advances in the mechanistic study of hydrothermal carbonisation (HTC) and the
strategies to improve the conversion by using carbon-based catalysts. HTC, although not a recent discovery, has lately been receiving
increasingly attention by both academic and industrial sectors due to the possibility to exploit this process to perform a simple, green
and inexpensive conversion of bio-derived waste material into valuable chemicals and advanced materials and, as such, this chapter
will also look into the use of hydrochars formed in HTC and their application in catalysis, more specifically heterogeneous catalysis
with a mention on electrocatalysis. The versatility and tuneability of these solids give rise to the great range of applicability in different
fields. A detailed overview of the HTC process is presented and the main uses of hydrochars in catalysis is then shown, highlighting
their use as solid acid catalysts, as pristine solid catalysts, as sacrificial agents in synthesis, since their removal through combustion is
easy, and the niche application of these solids in electrocatalysis for future research perspective.
conditions13. Coupled with these advantages is the ability

1. Introduction to tune or manipulate the surface of the resulting
hydrochar during the HTC process such as increasing
The term hydrothermal carbonisation (HTC) is referred to
acidity (using different acids in a one-pot synthesis).14
as a process in which an aqueous solution or dispersion of
The most common way hydrochars might be used in
carbon containing compounds, typically lignocellulosic
catalysis is as support for metal or metal oxide active sites.
biomass or saccharides, are heated under water at
However, since the surface of hydrochar involves many
subcritical temperatures, at moderate to high pressures.
oxygen functionalities such as furanic, phenolic, or
HTC was first studied about a century ago1,2 as a way to
carboxylic oxygen atoms, catalytic activity can also be
mimic natural conversion process of biomass into fossil
achieved through surface functionalities alone.
fuel. However, in the last decades3–9, it has become object
Modification of the conditions during the synthesis or post-
of intense research because it potentially allows the
treatment of the material can allow the addition of metallic
conversion of ubiquitous and inexpensive cellulose –
or metal oxide particles on the structure. Another useful
derived from agricultural waste– into: (a) other chemicals,
function of hydrochars is that they can be eliminated
like furfural, 5-hydroxymethylfurfural (5-HMF), levulinic
through combustion in air. Taking all these properties into
acid (LA), formic acid, acetic acid and lactic acid; some of
account, this book chapter analyses the use of hydrochars
which being recognised as platform chemicals for the
in heterogeneous catalysis in three groups: sulfonated
transition to a greener chemistry10; and (b) carbonaceous
(solid acid catalysts) and pristine hydrochars, as support,
materials (i.e., hydrochar). Through a sequence of
and as a sacrificial agent. Another important subject of
dehydration, polymerisation, and aromatisation reactions,
analysis is the process through which these materials are
this previously mentioned hydrochar is formed, mostly
obtained: HTC. The use of hydrochars in electrocatalysis isl
composed of condensed furan rings bridged by aliphatic
also mentioned, as there is a growing interest in the use of
regions with terminal hydroxyl and carbonyl functional
these solids in such processes.
groups. Upon the “polymerisation” of 5-HMF or furfural,
nucleation takes place followed by growth of the particles
upon further incorporation of 5-HMF-derived monomers, 2. Hydrothermal carbonisation:
which leads to the formation of spherically shaped mechanism
particles.
A deep and thorough understanding of the mechanisms of
One of the main disadvantages of hydrochars is that they
carbonisation of cellulosic biomass is the key to control the
present limited porosity and surface area. For certain
process and, therefore, make it scalable for industrial
applications, a nanoscale porosity is highly desirable. There
conversion of biowaste. Although the complex network of
are many well established technologies to produce porous
transformations and decomposition of cellulosic materials
carbons among which the most common ones are chemical
under hydrothermal conditions is still not fully understood
activation11 and templating methodologies.12
and subject of debate, it can be summarised in a few basic
Nonetheless, there are advantages to using this method.
steps: (i) hydrolysis of long cellulose and hemicellulose
HTC has been considered as energy- and atom-economical
chains into their constituting monomers (mainly glucose
process because only one-third of the combustion energy
and other C6 and C5 sugars); (ii) dehydration of C6 sugars
is released via dehydration; pre-drying process is
to 5-HMF and C5 sugars to furfural; (iii) decomposition to
unnecessary due to the aqueous reaction media and the
lower molecular weight compounds or, alternatively,
carbon efficiency is close to one after suitable operational

111
Chapter 9 Modugno, Szego et al.
condensation and aromatization reaction, to produce Jin et al.19, who studied the production of lactic acid during
hydrochar (see Fig. 1). hydrothermal treatment of cellulose and glucose. In fact,
although lactic acid is traditionally known to be a product
of alkaline degradation of sugars, it has been detected in
large amounts even in neutral condition. Its production has
been explained by the benzylic acid rearrangement of
pyruvaldehyde, which in turn is a product of a retro-aldol
condensation of fructose. This rearrangement proved that
Figure 1. General hydrothermal reaction scheme. water under sub-critical condition can act as a Brønsted
acid catalyst when catalysing isomerizsation of glucose to
Many studies in the last twenty years have contributed to fructose and also as an effective Brønsted base catalyst in
put together this general scheme, by focusing on different catalysing rearrangement of pyruvaldehyde to lactic acid.
aspects of the whole process. Since the dawn of the
“rediscover” of HTC , it is known that the decomposition of
glucose under neutral hydrothermal treatment produces
fructose, dihydroxyacetone, glyceraldehyde, erythrose,
glycolaldehyde, pyruvaldehyde, 1,6-anhydroglucose, 5-
HMF, acetic acid, formic acid and hydrothermal carbon15.
In 2007, Asghari and Yoshida16 reported the results from a
study on the reactivity of fructose under hydrothermal
conditions in presence of HCl as an acid catalyst. This study
proved the dehydration path of fructose under
hydrothermal conditions to 5-HMF, which can easily
undergo hydration reaction, with loss of a C atom in form
formic acid (FA) and subsequent ring opening to form LA.
While the latter two do not undergo any further reactions,
fructose and 5-HMF can also form soluble polymers
through distinct paths. 2-furaldehyde (2FA), originated
only from fructose, was also detected. Finally, the study
proved that pressure had a minor impact on products yield,
Figure 2. Scheme of reaction pathways of cellulose
compared to other reaction parameters such as decomposition as proposed by Buendia-Kandia17.
temperature, initial pH and composition of feedstocks.
Recently, Buendia-Kandia et al.17 have provided a quite More recently, some studies have emphasised some
exhaustive scheme of the dehydration, decomposition and discrepancies between experimental observations and the
condensation pathways that take place during HTC (see common belief of formic acid and levulinic being present in
Fig. 2). The scheme has been elaborated by studying the equal concentration20. Yang et al.21 have explained this
decomposition of microcrystalline cellulose in excess of FA by a sequence of reactions that entail a retro-
hydrothermal conditions, testing three temperatures (180, aldol pathway; furthermore, they have demonstrated,
220, and 260 °C), sampling the liquid phase every 20 min using computational methods (in particular Density
on an overall reaction time of 120 min. Their results functional theory, DFT, calculations) the possibility of two
confirmed the pathway proposed by Matsumura18: long alternative dehydration pathways of glucose and fructose:
chains of cellulose are firstly hydrolysed to smaller the former produces 5-HMF and dominates at low
oligomers or monomers of glucose. Glucose can undergo temperatures and low rates of conversion; the latter does
isomerizsation to fructose via Lobry de Bruyn–Alberda–van not involves 5-HMF and proceeds through retro-aldol
Ekenstein transformation or epimerisation to mannose. reaction and secondary furfuryl alcohol chemistry,
Dehydration reaction of fructose produces 5-HMF, which is contributing to the FA excess. Latest studies, nonetheless,
readily decomposed to levulinic and formic acid; glucose tend to assume that dehydration of glucose to 5-HMF
oligomers can undergo dehydration before complete mainly happen via an isomerisation step to fructose, which
hydrolysis, producing cellobiosan and subsequently is also a rate limiting step.22
levoglucosan. Retro-aldol condensation of glucose
2.1 Organic compounds in the liquid phase
produces erythrose and glycolaldehyde from which lactic
and acetic acid are derived. Decarbonylation and A large variety of chemical compounds can be found in the
decarboxylation reactions account for the production of liquid phase after a hydrothermal decomposition of
CO and CO2. Finally, due to limited access of water cellulosic biomass. Among them, 5-HMF is a very
molecules on the inner cellulose fibres, pyrolysis of promising, highly functionalised, bio-based chemical
cellulose can occur, leading to direct formation of building block, produced from the dehydration of hexoses,
hydrochar. which can play a key role not only as intermediate for the
It is worth noting how water can serve as both acid and production of the biofuel dimethylfuran (DMF), but also for
basic catalyst, in sub-critical condition, as pointed out by other biomass-derived intermediates, such as 2,5-furan-
112
dicarboxylic acid23, 2,5-dimethylfuran24,25, adipic acid and suggests that 5-HMF and furfural are the main building
LA26. 5-HMF is synthesized mainly by the dehydration of blocks of hydrochar. More recent investigations by van
monosaccharides such as fructose27 and glucose28,29, Zandvoort et al.38 has provided further details about the
through the loss of three water molecules. Disaccharides chemical structure of hydrochar (see Fig. 3). In fact, a
or polysaccharides, such as sucrose22 or cellulose30,31, can better characterisation of the linkages between furanic
be used as starting materials, but hydrolysis is necessary units has been achieved by means of 1D and 2D solid-state
for depolymerisation. Sucrose hydrolysis is more efficiently NMR spectra of 13C-labeled humins. The most abundant
catalysed by a base; however, dehydration of the are Cα–Caliphatic and Cα –Cα linkages, whereas other ones
monomers is catalysed by acids. This difference introduces such as Cβ–Cβ and Cβ–Caliphatic cross-links appear to have
a problem, namely that the formation of 5-HMF by smaller contribution to the overall structure. This
dehydration is a very complex process because of the difference shows that furan rings are mostly linked to each
possibility of side-reactions. Moreover, with respect to the other by short aliphatic chains; LA is also included in the
dehydration reaction which leads to the formation of 5- structure through covalent bonds. A chemical structure
HMF, glucose (an aldose) reactivity is lower than that of (see Fig. 2) has been proposed.
fructose (a ketose). Since the ultimate aim is to convert
cellulosic biomass into 5-HMF, hydrolysis of the cellulose
polymer to give glucose must be followed by a further step
of isomerisation to fructose in order to enhance the final
yield. Finally, it is important not only to optimise the
synthesis of this compound, but also to develop an efficient
isolation method. 5-HMF is not easy to extract from an
aqueous phase, since the distribution coefficient between
the organic and the aqueous phase is not favourable32.
LA, also named 4-oxopentanoic acid, is derived from 5-
HMF which, under hydrothermal conditions, is rehydrated
to form LA along with formic acid. LA is considered as one
of the most promising platform chemicals derived from
lignocellulosic biomass for the synthesis of fuels and
chemicals10,33. It is regarded as a platform chemical that
finds applications for several purposes, such as source of Figure 3. Chemical structure of humins proposed by van
monomers for the synthesis of polymer resins as well as Zandvoort24.
components in flavouring and fragrance industries34,
textile dyes, additives, extenders for fuels, antimicrobial DFT calculations have also proved to be helpful in supplying
agents, herbicides and plasticisers35,36. Many of the more information about the chemical structure of
processes for the highly selective production and hydrochar. A study by Brown et al.39 proposed two possible
separation of LA are still early in their development stage. structures: (1) a structure consisting of arene domains
Thus, finding an economically viable process for converting comprised of 6-8 rings connected via aliphatic chains; (2) a
more complex biomass feedstock to fuels and to chemical furan/arene structure consisting primarily of single furans
precursors for industrial production would be attractive to and 2 or 3 ring arenes. These two structures have been
reduce the release of atmospheric CO2, without inferred by simulating Raman spectra of model
compromising food supply. constituents of hydrochar by means of DFT and
subsequently fitting the experimental Raman hydrochar
2.2 Hydrochar: morphology and chemistry
spectrum with a 12-peak fit. Following NMR and IR
Synthesis of carbonaceous spheres via HTC of sugars was analysis, however, suggested that the latter model is more
first reported in 2001, in a study by Wang et al.3 In this consistent with the experimental observations.
study, it was noted that the diameter of these particles Formation and growth mechanism of carbonaceous
grew proportionally with both dwell time and initial spheres (such as those in Fig. 4) is still object of debate.
concentration of the precursor. The spherical appearance Based on their observations regarding the dissolution
of these carbon particles was attributed to the separation behaviour of cellulose under hydrothermal conditions,
of early sugar dehydration products from the aqueous Sevilla and Fuertes9 proposed a nucleation pathway to
solution and subsequent formation of an emulsion, from explain the formation of the typical carbonaceous spheres.
which first nuclei originates. This pathway encompasses six steps: (1) hydrolysis of
Better insights on the chemical structure of these carbon cellulose chains; (2) dehydration and fragmentation into
spheres were later provided by Baccile et al.37, who used soluble products of the monomers that come from the
different solid-state 13C NMR techniques to characterise hydrolysis of cellulose; (3) polymerisation or condensation
the carbon product of HTC of glucose. Their results of the soluble products; (4) aromatisation of the polymers
indicated that the core of the carbonaceous scaffold thus formed; (5) appearance of a short burst of nucleation;
consists on condensed furan rings linked either via the α - (6) growth of the nuclei formed by diffusion and linkage of
carbon or via sp2- or sp3- type carbon37. This structure species from the solution to the surface of the nuclei.
113
2.3 Carbon dots

The name “carbon dots” encompasses different carbon-
based nanomaterials whose properties resembles those of
well-known metal-based quantum dots. Since their
fortuitous discover in 2004 by Xu et al.43, this new class of
material has been object of intense study and research
because of their promising and sometimes outstanding
properties. Carbon dots are fluorescent and have tuneable
emission wavelengths, like traditional metal quantum
dots, but, unlike their inorganic cognate, they show better
aqueous solubility, photo-stability, ease to be
functionalised, low toxicity and, therefore, good bio-
compatibility. They also have a much lower cost compared
to the heavy metal based quantum dots, as they can be
synthetised using biomass waste as raw carbon source.
Carbon dots can be divided in two sub-groups, based on
their morphology: graphene quantum dots (GQDs) and
Figure 4. SEM image of carbonaceous spheres obtained carbon quantum dots (CQDs). Graphene quantum dots are
by hydrothermal carbonization of sugars. small, single- or multi-layered graphene discs with
diameters ranging from 3 to 20 nm and carboxylic
5-HMF obviously plays a major part in the carbon spheres functionalities on the edges44. It can be argued that this
formation, as highlighted by their chemical structure of kind of nanoparticles, commonly referred to as graphene
linked furan rings. Acid catalysed degradation of 5-HMF quantum dots, are in fact graphene oxide quantum dots,
has been studied by Tsilomelekis et al.40 by means of ATR- due to the high amount of oxygen-containing groups on
FTIR spectroscopy, Scanning Electron Microscopy (SEM) their surface and edges. In fact, true oxygen-free graphene
and Dynamic Light Scattering (DLS) to understand the quantum dots have been produced by Fei et al.45 through
growth mechanism of 5-HMF derived humins. The solvent exfoliation of graphite nanoparticles.
proposed mechanism involves an initial nucleophilic attack Carbon quantum dots are quasi-spherical carbon
of a nanoparticles, with diameters around or below 10 nm46;
5-HMF carbonyl group to the α- or β-position of the furanic they can have an amorphous or nanocrystalline structure
ring, along with aldolic condensation and etherification. with sp2 carbon clusters47 with high amount of oxygen
This condensation leads to the formation of small, soluble atoms and carboxylic groups on their surface48. First
oligomers that grow heavier until they form small solid chemical route to synthesis of CQDs was developed by Pan
nuclei through phase separation. Further growth is the et al. in 2010, through hydrothermal treatment of
result of both smaller particles aggregation and continuous graphene sheets.49
5-HMF Addition. It is unclear if aggregation of constituting The approaches to the synthesis of carbon dots are two:
small particles is based on chemical reactions or physical “top-down” and “bottom-up”. Bottom-up approach relies
interaction. Cheng et al.41 have noted that humins, which on small molecular precursors as seeds for the carbon dots
are related to hydochars, can be partially dissolved by to grow, whereas top-down method consists in the
multistage dissolution in organic solvent to oligomers that breaking down of macromolecules derived from larger
have mass numbers ranging from 200 to 600 Da, as carbon structures, biomass included, into small carbon
detected by LC-MS. This observation may, in fact, suggest dots. Given the purpose of this chapter, only examples of
that humins are actually aggregates of oligomeric species top-down approaches via HTC are relevant and therefore
rather than macromolecules. How that relates to will be mentioned. A vast variety of biomass has been used
hydrochars is still not fully resolved. as carbon source to successfully synthetize CDs in one-pot
Formic acid and LA are abundant in the reaction medium reactions: papaya powder50, coconut water51, peach
during HTC, where formation and growth of hydrochar gum52, prunus avium53, salep flour54, unripe prunus
based spherical particles take place. It is reasonable to mume55 and grape skin56 just to name a few. CQDs can be
think that these acids might play a role in the process. In doped with different elements, in order to enhance their
fact, it has been shown that formic acid, due to its rather properties through the addition of dopants (boron57,
high pKa, significantly increases the rate of conversion of nitrogen52,53,57) or carefully choosing biomass which
C6 sugar to 5-HMF in an autocatalytic fashion, therefore naturally contains heteroatoms (N- and S-doped CQDs
speeding up the growth of the spherical particles of from garlic58,59). These materials have proven to be
hydrochar. LA, on the other hand, has a lower pKa and effective as probes for the detection of metal ions52,53,57,58
therefore does not have a strong Brønsted catalytic effect, (due to the quenching effect of these species on the
but it does effect the growth of the spherical particles fluorescence of the CDs) and other chemicals54,56,59, as
taking part in the process as building units and slowing the catalyst54 or for imaging of cells55.
growth by reducing the surface density of hydroxyl groups
of carbonaceous spheres.42
114
HTC for the synthesis of carbon dots is a sustainable, low add sulfonic groups to the carbon structure. The MOF-
cost and relatively easy strategy to turn biomass waste into derived sulfonated carbon was macro/mesoporous and
a valuable material. However, this hydrothermal route active towards the dehydration of fructose to 5-HMF in
produces high amounts of by-products, mostly isopropanol/DMSO with a yield of 89.6% in the optimal
hydrothermal carbon. Although hydrothermal carbon is conditions. An interesting alternative consists on the use of
itself a valuable, versatile and promising material, as it will carbonaceous materials derived from biomass waste. In
be mentioned later on, the whole process of HTC still needs fact, effective carbon-based catalysts can be prepared
a deeper, insightful comprehension of its mechanisms, in from a variety of bio-derived carbon sources. Moreover,
order to selectively drive the reaction towards the desired their preparation usually requires a reduced number of
products (soluble chemicals, hydrothermal carbon or steps. Hu et al.69 have reported the preparation of
nanoparticles). magnetic lignin-derived carbon catalyst from enzymatic
2.4 Carbon catalysts for hydrothermal conversion hydrolysis lignin (EHL), a residue of enzymatic hydrolysis of
of biomass lignocellulosic biomass to separate cellulose from lignin.
EHL was impregnated with an aqueous solution of FeCl3 10
Nowadays, industrial processes for the production of mmol·L–1, then dried and carbonised at 400 °C and finally
5-HMF and LA from cellulosic biomass rely on mineral acids treated with H2SO4 for sulfonisation. FeIII salts are found to
as homogeneous catalysts of fructose dehydration60,61. be reduced to Fe3O4 during carbonisation. The catalysts
Despite satisfactory yields of conversion and selectivity demonstrated high performance in fructose conversion in
achieved, the use of mineral acids as homogeneous DMSO and 5-HMF yield, good recyclability and excellent
catalysts poses some challenges, namely uneasy acid recovery due to its magnetic properties.
recovery and high maintenance costs due to pipe Amongst the biomass-derived carbonaceous materials,
corrosion. These difficulties could easily be overcome by humins are promising starting material for the synthesis of
means of heterogeneous catalysis. A large variety of solid catalysts. Hydrochars, which are usually regarded to
heterogeneous acid catalysts have been developed to as by-products of the hydrothermal conversion of biomass,
perform catalytic conversion of biomass or biomass- are rich in oxygen functionalities on their surface such as
derived sugars, such as mixed metal oxides62, hydroxyl, carbonyl and carboxylic groups9 which provide
phosphates63, zeolites64,65 to name a few. acid sites for catalytic purposes or further
Carbonaceous materials can also serve as a starting functionalisation. Moreover, hydrochars porosity can be
material for the synthesis of solid acid catalysts. Shen et enhanced by activation70. All these features make
al.66 have prepared a bi-functional carbon based acid hydrochars promising for the development of carbon-
hetero-catalyst through hard-templating using sucralose as based heterogeneous catalysts, as it will be further
carbon source. Due to the use of this synthetic chlorinated discussed in this chapter.
sugar, the resulting carbon material possessed –Cl groups
able to bind cellulose and –SO3H to catalyse
depolymerisation and dehydration. With this catalyst, LA 3. Hydrochar applied for heterogeneous
was formed from untreated cellulose in pure water with
catalysis
yields as high as 51.5%. Ball-milling pretreatment of
cellulose improved the performance of the solid acid After establishing the fundamentals of the HTC process,
catalyst. However, a downside of this preparation resides this next section will deal with the use of these formed
in the use of a synthetic sugar, not readily available from hydrochars as heterogeneous catalysts. Carbon-based
natural sources, which requires previous treatment and materials have long been used in heterogeneous catalysis
possibly higher costs. Recently, Zhang et al.67 developed a reactions because of their desired properties for catalyst
macro-/mesoporous carbonaceous catalyst with hybrid support and carbon-based materials act as direct catalysts
Brønsted-Lewis acid sites (sulfonic groups and ZrIV in many industrial applications71. This kind of materials are
respectively). This catalyst was tested for the thermal especially suitable for catalysis because of their resistance
conversion of fructose and glucose in a water/DMSO to acid/base media, porosity and surface chemistry control
biphasic solvent and cellulose in a 1-butyl-3-methyl as well as for environmental aspects.
imidazolium chloride ([BMIM]·Cl) ionic liquid. Conversion Along with the use of hydrochars as-produced, a lot of
and 5-HMF yield was high, with a remarkable 5-HMF yield research has been applied to devise various modification
(43.1%) and selectivity (57.7%) from cellulose. Although approaches to further expand its activation capacities72.
this synthetic route allows excellent control of porosity and Given that hydrochars can be a highly porous and carbon-
very fine surface functionalisation, it requires non- rich, they are promising alternatives to replace
renewable precursors and transition metals and it involves conventional solid carbon-based catalysts with some
several preparation steps. Metal organic frameworks known demerits (e.g., high costs and environmentally
(MOFs) can also serve as starting material for the synthesis unfriendliness).
of carbonaceous catalysts, as reported by Jin et al.68, who Since catalytic activity is highly dependent on accessibility
fabricated a MOF starting from zinc nitrate hexahydrate to catalytic active sites dispersed throughout the internal
and terephthalic acid. The MOF was subsequently pores, the morphology and porosity of hydrochars without
carbonised and treated with concentrated sulphuric acid to activation exhibit very poor catalytic properties. To
overcome these issues, many studies have been directed
115
to modifying the morphology and porosity of hydrochars sulfonated resins75. Turnover numbers of glycerol and the
via various treatments73. acetic acid were in the same range for both the commercial
Hydrochars and related compounds have been used in and the hydrochar solid catalysts. To reuse the catalysts,
catalysis in many ways. On their own they can be used as these were first treated with acid to cleave the esterified
catalysts, mainly as solid acid catalysts. This functionality is sulfonic groups, which could have formed regenerating the
secured by introducing strong Brønsted acidity, mainly by acidity of the surface of the catalyst.
sulfonated groups on the surface of the hydrochar Pileidis et al.77 also prepared hydrochars (HTC conditions
particles. 230 °C, 24 h), turned them into solid catalysts, and studied
Another widely spread use of hydrochar particles is as them for the esterification of LA. In this case, not only
catalyst support. The tuneability of their surface polarity glucose was used. Cellulose and rice straw were used as
and area facilitate the anchoring of metal nanoparticles, carbon sources as well. These sources led to hydrochars
which can then be used in different reactions. with 80, 76, and 70% carbon (for glucose, cellulose and rice
A last and least studied use for these hydrochars is the use straw, respectively) and were then sulfonated (80 °C, 4 h)
of them as templates. Since hydrochar can be eliminated introducing 5%–%6 sulphur. Esterification was carried out
by combustion in air at temperatures that are not too at 60 °C and after 3 h almost full conversion was achieved
drastic, they can be used as structural directing agents. using the glucose-derived catalyst and a 97% selectivity
3.1 Sulfonated hydrochar catalysts toward the ester. The second best in performance was the
catalyst prepared from rice straw with 92% of both
Addition of sulfonated groups to HTC synthesised carbon conversion and selectivity. With carbonised and sulfonated
materials give rise to introduction of sulfonic acid groups cellulose, 89% conversion and selectivity were observed.
leading to the formation of a solid acid catalyst that can be It is worthy to note that it has been reported that
used in catalytic reactions such as those tested by Roldán sulfonation at high temperature (150 °C) induces changes
et al.74 (see Fig. 5). These catalysts can be recovered in the structure of hydrochar. This change is due to
through simple filtration methods and are generally made decreasing the abundance of furanic groups and increasing
by treatment of porous carbon in concentrated sulphuric the presence of benzenic rings78.
acid at high temperatures. Roldán et al. prepared catalysts Sulfonated hydrochar has been used for the production of
in this way and tested them in esterification reactions of biomass-derived platform molecules, such as
palmitic acid with methanol, observing in the end a change monosaccharides or 5-HMF by hydrolysis and dehydration
in the deactivation of the catalyst depending on the reactions. In this way, glucose or sucrose were
activation temperature employed for each catalyst. When transformed into hydrochar at 180 °C for 10–15 h and
temperatures lower than 500 °C were used, the sulfonated with concentrated sulphuric acid at 200 °C for
deactivation of the catalyst was attributed to formation of 15 h.79
surface sulfonate esters on the surface of the carbon Alternative methods of sulfonation have been proposed,
particles, and while for those treated at higher such as direct HTC with sulfonic precursors (mainly
temperature it is thought that accumulation of reactants hydroxyethylsulfonic acid)80. The catalysts prepared using
and products in the pores of the particles is the main cause this method presented very high stability and reusability,
of deactivation. enabling future applications.
3.2 Pristine hydrochar as catalysts
Sulfonation is a simple method to introduce strong acidic
sites into hydrochar. However, the pristine surface also
possesses catalytic properties because of the high density
Figure 5. Schematic view of sulfonation of porous carbon.
of hydroxyl and carboxylic groups. This property has been
Similar catalysts have been used for the esterification of demonstrated in the application of such catalysts for the
glycerol75 and oleic acid76. In this case, the glucose used as 5-HMF production from fructose in ionic liquid81.
carbon source was treated for 19 h at 195 °C, leading to the Hydrochar was produced from glucose at 180 °C and 10 h
formation of a carbonaceous material, which contained and used after oven-drying without any further treatment.
67.9% carbon and 27.5% oxygen. It was observed that after The results showed that fructose was converted into
sulfonation (150 °C, 15 h), the carbon content decreased to 5-HMF with a maximum yield of 80% after 120 min of
55.8% and oxygen content increased to 40.5%. From this reaction time. The stability of these catalysts was not
change in values and taking into account the amount of properly evaluated, hence, clarification would be needed;
sulphur added, one conclusion is that additional oxygen however, this study showed that even sulphur-free
other than that of the sulfonated groups was added during surfaces of hydrochars presented catalytic activity in
the treatment with sulphuric acid. This inclusion of O may reactions such as the dehydration of fructose.
have happened by water addition to double bonds, Hydrochar also permits alkaline functionalisation of
hydrolysis of furan groups, or ether functionalities. The surfaces and their use in catalysis82. Hence, spherical
sulfonated catalyst was tested in the esterification of particles of hydrochar were synthesised from glucose at
glycerol with acetic, butyric, and caprylic acid and the 160 °C maintaining the temperature for 12 h. Thereafter,
catalytic performance compared to the one of commercial acidic functionalities such as carboxylic and hydroxyl
116
groups were neutralised with sodium hydroxide at room surface area significantly. This adsorbent was then loaded
temperature. The sodium-hydroxide-treated hydrochar with gold NPs and tested for the hydrogenation of
was active for the base-catalysed aldol condensation82. 4-nitrophenol to 4-aminophenol with sodium borohydride,
Benzaldehyde was reacted with acetaldehyde to form resulting in high catalytic activity.
cinnamaldehyde. High selectivity (94%) was achieved at As mentioned before, hydrochars can be treated under
34% conversion based on benzaldehyde consumption. At basic condition neutralising any acid surface functionality.
this conversion value, the cinnamaldehyde also started to This neutralisation can be interesting when using them as
react with acetaldehyde to produce the higher weight supports such as when loading palladium NPs and using
homolog. In comparison with sodium hydroxide solutions, them in oxidation reactions82. The absence of acid sites
the modified hydrochar is less active but more selective lowers the number of side reactions that can occur
and can be used in three catalytic runs with the same augmenting selectivity, and the high dispersion of the
activity. metal allows high activities to be achieved. This high
In summary, it can be said that hydrochar possesses a dispersion is aided by this basic pre-treatment of the
promising potential as a metal-free catalyst for industrial material as evidenced when smaller palladium NPs were
application. Introduction of strong Brønsted acid sites is observed for those samples treated with higher
straightforward by sulfonation, for example, by treatment concentration of basic solution (2.7 nm versus 7.5 nm).
with sulphuric acid. However, oxygen functionalities of The hydrothermal process can also be performed together
pristine hydrochar can also be used for catalytic with metal oxide particles90–92. Hence, using magnetic
transformation. metal oxide cores, a magnetically active material can be
3.3 Hydrochar as catalyst support obtained. In this way, active catalysts for the Suzuki–
Miyaura cross-coupling reaction have been prepared with
Active carbons are classical supports for numerous palladium and platinum NPs as active sites90. In this work,
catalysts found in commercial processes. This widespread Fe3O4 particles (magnetite) were introduced during the
use is due to their high stability and surface area. In carbonisation of glucose at 180 °C for 4 h. After this,
general, activated carbons possess a high surface area of palladium or platinum NPs were deposited on the carbon
1000–1500 m2g–1, and they can be made nonpolar and shell and the whole material protected by a further layer
hydrophobic if they have a low oxygen content. In contrast, of approx. 35 nm thickness of mesoporous silica. The silica
hydrochar has a very polar surface and a much lower was added to prevent sintering of the metal NPs while its
surface area. By reduction of oxygen content and porosity allowed the organic molecules tested to pass
increasing that of carbon, the properties of hydrochars can through it. The magnetite particles had a uniform diameter
become closer to those of nonpolar activated carbon74. By of approx. 180 nm composed of nanocrystals of approx.
tuning these parameters (surface area and polarity) the 8 nm sizes. The carbon shell thickness was measured to be
deposition of metal precursors can be enhanced. approx. 15 nm. The size of the supported palladium and
Hydrochars with or without further modification have platinum particles was determined by HRTEM and mean
been employed as carbon support for metals83–86. The diameters of 14 and 25 nm were obtained, respectively.
process is, in most cases, the same. A hydrochar is The hydrophilic surface of the hydrochar facilitated
synthesised and then activated (thermally or chemically) to dispersion of the particles in water during the mesoporous
support the metal precursor, which is then reduced by silica synthesis and allowed a regular coverage of the
addition of a reducing agent (NaBH4 for example). In some particles. At all stages of the synthesis, the particles
cases, the reduction step can be avoided when using showed high superparamagnetic properties, which
pristine carbon surfaces87,88, as C is a common reduction facilitated the retrieval of the catalyst. High conversions
agent. Glucose-derived and modified hydrochars support were shown by the catalysts (77%–99%) depending on the
and stabilise the metal nanoparticles (NPs) and keep them nature of the reactants.
active for prolonged time under reaction conditions. It can be seen from this overview that carbonaceous
Palladium NPs supported onto hydrochar were employed materials synthesised through the HTC process have a very
for the Suzuki–Miyaura coupling83. The catalyst wide application as catalysts support thanks to their
demonstrated high catalytic activity for the reaction of physical and chemical properties.
many aryl halides and boronic acids. It could be recycled
for up to five times through simple centrifugation. In liquid- 3.4 Hydrochar as sacrificial component
state reactions, leaching of the supported metal is usually The defined structure and geometry of the spherical
a problem but the properties of hydrochar favours the particles of hydrochar can be used as a template, since it
redeposition of leached particles during cool-down of the can be easily removed with a thermal treatment in air at
reaction. about 500 °C.93–95 With this in mind, very effective catalysts
More elaborated supports can be designed by combining for the low-temperature oxidation of CO were produced93.
HTC with a porous polymer as template. Such was done by In his work, Zhao et al. produced gold NPs by bringing in
Cheng at al.89 where, in this case, a polymer was contact the gold precursor and glucose in water. Glucose
introduced during the HTC process serving as a template, has two main roles in this synthesis, it acts as reducing
which was then treated at 700 °C in a reducing agent and it is also a carbon source for generating the
atmosphere. This thermal treatment increased the BET hydrochar. Once the gold NPs were formed, the cerium
117
precursor was added and the mixture heated to 180 °C for nitrogen contained) modifies the energies of adjacent
different periods of time (1, 6, 10, and 20 h). Afterward, the carbon atoms, generating active sites for the ORR reaction.
solid was collected and calcined at different temperatures At the same time, the delocalisation of donated electrons
in the range of 300–600 °C for 6 h, eliminating all hydrochar within the π-system translates into an increase in the
(see Fig. 6). TEM images (of the calcined samples) showed n-type conductivity of the material102. Examples of these
gold NPs of an average size of 11 nm after 1 h hydrothermal nitrogen-doped carbon materials have been synthesised
treatment and the whole diameter of the core–shell using natural halloysite as a template and urea as the
structure was about 40 nm. When the hydrothermal nitrogen source98; a flaky morphology was obtained with
treatment was prolonged to 10 h, gold NPs grew to 16 nm glucose as a carbon source, whereas using furfural resulted
and the shells to 100 nm. The best catalytic performance in in rod-like structures. The metal-free electrocatalysts were
carbon monoxide oxidation was that shown by the catalyst tested for ORR in alkaline aqueous electrolytes, and the
that was prepared by hydrothermal treatment for 10 h and rod-like catalyst demonstrated a better performance than
was subsequently calcined at 600 °C. This catalyst allowed the flaky material. Due to both of them containing a similar
the reduction of the reaction temperature from 300 to amount of N and graphitisation degrees, the high
155 °C for full conversion and it was also tested on stream performance of the rod-like catalysts was attributed to the
for 70 h without any deactivation being evident. high surface area and large pore volume (which provided
more active sites), the great complexity in pore size
distribution, and the rod-like morphology, which facilitated
electron transport. Compared to a commercial Pt/C (20 wt.
%) catalyst, the carbon catalysts demonstrated a higher
retention in diffusion limiting current density (after 3000
cycles) and enhanced methanol tolerances. When tested
Figure 6. Schematic representation of the synthesis of as cathodes in a single cell fuel cell of the H2/O2 anion
core–shell distribution of gold particles and cerium oxide. exchange membrane kind, the rod-like catalyst delivered a
peak power density as high as 703 mW cm–2 (vs
Similarly, cobalt NPs protected within hollow mesoporous 1100 mW cm–2 with the commercial Pt/C cathode catalyst).
silica spheres were synthesized94. Starting from spherical Time and resources have also been invested in researching
particles of hydrochar with diameters of 100–150 nm and doping with different heteroatoms such as B and S99,103,104.
synthesised from glucose at 180 °C for 4.5 h. After In one study of carbogels derived from glucose and
impregnation with cobalt nitrate providing NPs of approx. ovalbumin, the synergistic effect of boron and nitrogen
4 nm and the synthesis of a mesoporous silica shell was thought to augment the electron transfer numbers
(thickness approx. 20 nm) with cetyltrimethylammonium and lower hydrogen peroxide yields when compared to
bromide-based structures as soft template, all organic those observed in purely N-doped systems, whereas the
material was removed by calcination at 430 °C. This presence of S decreased the surface area and nitrogen
synthesis procedure provided a catalyst with interesting content resulting in diminished ORR performance99. In
performance in the epoxidation of alkenes. When the contrast, sulphur doping of 5.5 wt. % in SiO2-templated
cobalt/silica hollow spheres were employed in the mesoporous ordered carbons was found to enhance the
epoxidation of styrene with oxygen, a 94% selectivity electrocatalytic activity in the ORR in alkaline solution,
toward the epoxide was achieved at almost complete likely because the mesoporous structure was retained
conversion. from the templating method.103
Heteroatom-doped systems are not only used for ORR,
they have also been tested for the HER. 2D crystalline
4. Hydrochars in electrocatalysis carbons were obtained from hydrochars of glucose,
Electrocatalysis plays a crucial role in many energy storage fructose or cellulose with guanine, which played an
and conversion technologies such as the oxygen reduction important role in producing the 2D-morphology of the
reaction (ORR) at the cathode of metal-air batteries or fuel resultant carbon materials105. The porous N-doped carbons
cells, the oxygen evolution reaction (OER) and hydrogen were not only found to be highly active towards ORR, but
evolution reaction (HER) at either electrode of water also showed efficiency for HER with a very low
electrolysers, and CO2 reduction in liquid fuel conversion overpotential of 0.35 V to achieve 10 mA cm–2 in alkaline
devices. The problem of these electrocatalytic reactions is medium.
that they generally display slow kinetics and as such In the other half of the water splitting reaction, suitable
requires the development of a catalyst that can improve catalysts are also required for the ORR; in the past, the
and give future perspective to these technologies96. development of fuel cells has been held back by the slow
An important reaction in sustainable energy systems is the kinetics of the OER. Non-noble metal alternatives for OER
oxygen reduction reaction (ORR). Out of the metal-free are often based on transition metal oxides, while carbon-
ORR electrocatalysts tested, significant research has been based materials have generally been underexplored
focused on nitrogen-doped carbon materials97–101. Inside because of their relatively poor performance. However,
the graphitic matrix of carbon materials, polarised C-N one approach used activated carbon cloth by creating
bonds (whose polarisation strength depends on the type of oxygen-containing functional groups on its surface using
118
peroxovanadium complexes, which results in a higher Another important aspect shown here is the use of these
specific surface area and faster electron transfer rate, hydrochars in heterogeneous catalytic applications.
when compared to a pristine carbon cloth106. The Pristine hydrochar surfaces that do not require addition of
overpotential (310 mV) at 10 mA cm–2 of the activated any extra species contain a wide variety of oxygen
carbon cloth is much lower than the pristine material and functionalities that can be exploited for catalytic purposes.
comparable to that of RuO2/C (280 mV), making the carbon Further than this, modification of the surface through
cloth a competitive non-metal catalyst for OER. The growth thermal treatment reduces the oxygen content and shifts
of transition metals on a carbon fabric can also improve hydrophilicity of the materials as well as increases the
electrocatalytic performance. In one study, highly porous surface area.
and granular Ni-Co nanowires were grown hydrothermally We can find similarities between hydrochar and silica with
on a carbon fibre woven fabric and then coated with a the exception that hydrochar can be easily combusted.
conductive shell by glucose carbonisation107. The structure Making use of these similarities, various complex
of the nanowires greatly increased the catalytic surface structured materials can be prepared.
area delivering an overpotential of 302 mV at a current Although many of the works cited in this chapter used
density of 10 mA cm–2. The conductive carbon layer not glucose as a carbon source due to it being more
only enabled facile electron transport throughout the scientifically reproducible, the outlook of all experiments is
entire electrode, but also prevented fragmentation of the the use of real raw biomass as this carbon source.
nanowires during reaction, resulting in greater structural In conclusion, from the information recapped, it seems
integrity and more reliable performance. proven that HTC is a valuable asset both in refinement of
Lastly, HTC has been used to prepare a 3D hierarchical bio-waste as well as in synthesis of catalysts. The
structure of mesoporous SnO2 nano-sheets supported on preparation of hydrochar, and its chemical reactivity,
flexible carbon cloth, which could efficiently and selectively allows the incorporation of heteroatoms other than O. This
electrochemically reduce CO2 to formate in aqueous flexibility has been exploited in material synthesis, but for
conditions108. The electrode exhibited a partial current catalysis, further research is needed. Many impulses from
density of 45 mA cm–2 at a moderate overpotential (0.88 V) HTC for catalysis can be expected in the future that might
with high Faradaic efficiency (87%), even larger than most also lead to industrial applications following a sustainable
gas diffusion electrodes. The performance was attributed alternative to already established processes.
to the presence of SnO2 particles, which showed high
selectivity in the reduction of CO2. The highly porous
structure provided a large surface area increasing the Acknowledgements
contact surface between electrode and electrolyte and This project has received funding from the European
facilitating mass and charge transfer, and the robust Union’s Horizon 2020 research and innovation programme
structure maintained the high stability of the under the Marie Skłodowska-Curie grant agreement No
electrocatalyst during long-term operation. 721991.
In summary, hydrochars and related compounds have a
wide range of applicability, going beyond heterogeneous
References
catalysis and going into the niche field of electrocatalysis.
(1) Marinovic, A.; Pileidis, F. D.; Titirici, M.-M. CHAPTER 5.
Hydrothermal Carbonisation (HTC): History, State-of-
5. Conclusions the-Art and Chemistry. In Porous Carbon Materials
In this chapter, the HTC process has been explained and we from Sustainable Precursors; The Royal Society of
can conclude that it has established itself as a very Chemistry, 2015; pp 129–155.
promising strategy to emancipate from fossil fuel by usage (2) Funke, A.; Ziegler, F. Hydrothermal Carbonization of
of bio-based waste. This conceptually simple process Biomass: A Summary and Discussion of Chemical
allows to obtain valuable platform chemicals (5-HMF, LA), Mechanisms for Process Engineering. Biofuels,
hydrochar and carbon dots. Although many advances have Bioprod. Biorefining 2010, 4 (2), 160–177.
been achieved in the last two decades in the (3) Wang, Q.; Li, H.; Chen, L.; Huang, X.; Qing, W.; Hong, L.;
comprehension of the mechanisms of this HTC process, Liquan, C.; Xuejie, H. Monodispersed Hard Carbon
many efforts are still needed to shed light on the complex Spherules with Uniform Nanopores. Carbon 2001, 39,
networks of reaction and interaction pathways that lead to 2211–2214.
the formation of the aforementioned products. Moreover, (4) Falco, C.; Baccile, N.; Titirici, M. M. Morphological and
there is still a strong need for an optimisation of the Structural Differences between Glucose, Cellulose and
process through development of catalysts, which can drive Lignocellulosic Biomass Derived Hydrothermal
selectively the reaction towards the desired product, Carbons. Green Chem. 2011, 13 (11), 3273–3281.
avoiding loss of material and synthesis of by-products. (5) Falco, C.; Perez Caballero, F.; Babonneau, F.; Gervais,
Carbon-based catalyst can potentially fulfil this C.; Laurent, G.; Titirici, M. M.; Baccile, N. Hydrothermal
requirement because of their great versatility, the Carbon from Biomass: Structural Differences between
abundance of functionalities on their surface and the easy Hydrothermal and Pyrolyzed Carbons via 13C Solid
tuneability of their physical properties such as porosity. State NMR. Langmuir 2011, 27 (23), 14460–14471.
119
(6) Camillo, F.; Manuel, S. J.; Nicolas, B.; Marta, S.; (19) Jin, F.; Zhou, Z.; Enomoto, H.; Moriya, T.; Higashijima,
Torbjorn, van der M.; Emilia, M.; Diego, C.; Maria‐ H. Conversion Mechanism of Cellulosic Biomass to
Magdalena, T. Hydrothermal Carbons from Lactic Acid in Subcritical Water and Acid–Base Catalytic
Hemicellulose‐Derived Aqueous Hydrolysis Products Effect of Subcritical Water. Chem. Lett. 2004, 33 (2),
as Electrode Materials for Supercapacitors. 126–127.
ChemSusChem 2013, 6 (2), 374–382. (20) Flannelly, T.; Lopes, M.; Kupiainen, L.; Dooley, S.;
(7) Arne, T.; Markus, A.; Titirici, M.-M.; Thomas, A.; Leahy, J. J. RSC Advances Levulinic Acids from the
Antonietti, M. Back in the Black: Hydrothermal Hydrolysis of Biomass Derived Hexose Carbohydrates.
Carbonization of Plant Material as an Efficient RSC Adv. 2016, 6 (7), 5797–5804.
Chemical Process to Treat the CO2 Problem? New J. (21) Yang, L.; Tsilomelekis, G.; Caratzoulas, S.; Vlachos, D. G.
Chem. 2007, 31 (6), 787–789. Cover Picture: Mechanism of Brønsted Acid-Catalyzed
(8) Titirici, M. M.; Thomas, A.; Antonietti, M. Aminated Glucose Dehydration (ChemSusChem 8/2015).
Hydrophilic Ordered Mesoporous Carbons. J. Mater. ChemSusChem 2015, 8, 1289–1289.
Chem. 2007, 17 (32), 3412–3418. (22) Steinbach, D.; Kruse, A.; Sauer, J.; Vetter, P. Sucrose Is
(9) Fuertes, A. B.; Sevilla, M. The Production of Carbon a Promising Feedstock for the Synthesis of the
Materials by Hydrothermal Carbonization of Cellulose. Platform Chemical Hydroxymethylfurfural. Energies
Carbon 2009, 47 (9), 2281–2289. 2018, 11 (3), 645.
(10) Werpy, T.; Petersen, G. Top Value Added Chemicals (23) Pacheco, J. J.; Davis, M. E. Synthesis of Terephthalic
from Biomass, Vol. I. National Renewable Energy Acid via Diels-Alder Reactions with Ethylene and
Laboratory (NREL), report DOE/GO-102004-1992, Oxidized Variants of 5-Hydroxymethylfurfural. Proc.
2004. Natl. Acad. Sci. 2014, 111 (23), 8363–8367.
(11) Yahya, M. A.; Al-Qodah, Z.; Ngah, C. W. Z. Agricultural (24) Liu, Z. Y.; Dumesic, J. A.; Barrett, C. J.; Román-Leshkov,
Bio-Waste Materials as Potential Sustainable Y. Production of Dimethylfuran for Liquid Fuels from
Precursors Used for Activated Carbon Production: A Biomass-Derived Carbohydrates. Nature 2007, 447
Review. Renew. Sustain. Energy Rev. 2015, 46, 218– (7147), 982–985.
235. (25) Jae, J.; Zheng, W.; Lobo, R. F.; Vlachos, D. G. Production
(12) Lu, A. H.; Schüth, F. Nanocasting: A Versatile Strategy of Dimethylfuran from Hydroxymethylfurfural through
for Creating Nanostructured Porous Materials. Adv. Catalytic Transfer Hydrogenation with Ruthenium
Mater. 2006, 18 (14), 1793–1805. Supported on Carbon. ChemSusChem 2013, 6 (7),
(13) Kubo, S.; Demir-Cakan, R.; Zhao, L.; White, R. J.; Titirici, 1158–1162.
M. M. Porous Carbohydrate-Based Materials via Hard (26) González Maldonado, G. M.; Assary, R. S.; Dumesic, J.;
Templating. ChemSusChem 2010, 3 (2), 188–194. Curtiss, L. A. Experimental and Theoretical Studies of
(14) Demir-Cakan, R.; Baccile, N.; Antonietti, M.; Titirici, M.- the Acid-Catalyzed Conversion of Furfuryl Alcohol to
M. Carboxylate-Rich Carbonaceous Materials via One- Levulinic Acid in Aqueous Solution. Energy Environ. Sci.
Step Hydrothermal Carbonization of Glucose in the 2012, 5 (5), 6981–6989.
Presence of Acrylic Acid. Chem. Mater. 2009, 21 (3), (27) Asghari, F. S.; Yoshida, H. Acid-Catalyzed Production of
484–490. 5-Hydroxymethyl Furfural from D-Fructose in
(15) Kabyemela, B. M.; Adschiri, T.; Malaluan, R. M.; Arai, Subcritical Water. Ind. Eng. Chem. Res. 2006, 45 (7),
K. Glucose and Fructose Decomposition in Subcritical 2163–2173.
and Supercritical Water: Detailed Reaction Pathway, (28) Haibo Zhao; Johnathan E. Holladay; Heather Brown; Z.
Mechanisms, and Kinetics. Ind. Eng. Chem. Res. 1999, Conrad Zhang. Metal Chlorides in Ionic Liquid Solvents
38 (8), 2888–2895. Convert Sugars to 5-Hydroxymethylfurfural. Science
(16) Asghari, F. S.; Yoshida, H. Kinetics of the 2007, 316 (15), 1597–1600.
Decomposition of Fructose Catalyzed by Hydrochloric (29) Alipour, S. High Yield 5-(Hydroxymethyl)Furfural
Acid in Subcritical Water: Formation of 5- Production from Biomass Sugars under Facile Reaction
Hydroxymethylfurfural, Levulinic, and Formic Acids. Conditions: A Hybrid Enzyme- and Chemo-Catalytic
Ind. Eng. Chem. Res. 2007, 46 (23), 7703–7710. Technology. Green Chem. 2016, 18 (18), 4990–4998.
(17) Buendia-Kandia, F.; Mauviel, G.; Guedon, E.; Rondags, (30) Li, C.; Zhang, Z.; Zhao, Z. K. Direct Conversion of
E.; Petitjean, D.; Dufour, A. Decomposition of Cellulose Glucose and Cellulose to 5-Hydroxymethylfurfural in
in Hot-Compressed Water: Detailed Analysis of the Ionic Liquid under Microwave Irradiation. Tetrahedron
Products and Effect of Operating Conditions. Energy Lett. 2009, 50 (38), 5403–5405.
Fuels 2018, 32 (4), 4127–4138. (31) Eminov, S.; Filippousi, P.; Brandt, A.; Wilton-Ely, J.;
(18) Matsumura, Y.; Sasaki, M.; Okuda, K.; Takami, S.; Hallett, J. Direct Catalytic Conversion of Cellulose to 5-
Ohara, S.; Umetsu, M.; Adschiri, T. Supercritical Water Hydroxymethylfurfural Using Ionic Liquids. Inorganics
Treatment of Biomass for Energy and Material 2016, 4 (4), 32.
Recovery. Combust. Sci. Technol. 2007, 178, 509–536. (32) Kuster, B. F. M. 5-Hydroxymethylfurfural (HMF). A
Review Focussing on Its Manufacture. Starch-starke
1990, 42 (8), 314–321.
120
(33) Bozell, J. J.; Petersen, G. R. Technology Development (46) Zheng, X. T.; Ananthanarayanan, A.; Luo, K. Q.; Chen,
for the Production of Biobased Products from P. Glowing Graphene Quantum Dots and Carbon Dots:
Biorefinery Carbohydrates - The US Department of Properties, Syntheses, and Biological Applications.
Energy’s “Top 10” Revisited. Green Chem. 2010, 12 (4), Small 2015, 11 (14), 1620–1636.
539–554. (47) Ray, S. C.; Saha, A.; Jana, N. R.; Sarkar, R. Fluorescent
(34) Ghorpade, V.; Hanna, M. Industrial Applications for Carbon Nanoparticles: Synthesis, Characterization,
Levulinic Acid. In Cereals; Springer US: Boston, MA, and Bioimaging Application. J. Phys. Chem. C 2009, 113
2013; pp 49–55. (43), 18546–18551.
(35) Fang, Q.; Hanna, M. A. Experimental Studies for (48) Baker, S. N.; Baker, G. A. Luminescent Carbon
Levulinic Acid Production from Whole Kernel Grain Nanodots: Emergent Nanolights. Angew. Chemie - Int.
Sorghum. Bioresour. Technol. 2002, 81 (3), 187–192. Ed. 2010, 49 (38), 6726–6744.
(36) Suganuma, S.; Nakajima, K.; Kitano, M.; Yamaguchi, D.; (49) Pan, D.; Zhang, J.; Li, Z.; Wu, M. Hydrothermal Route
Kato, H.; Hayashi, S.; Hara, M. Hydrolysis of Cellulose for Cutting Graphene Sheets into Blue-Luminescent
by Amorphous Carbon Bearing SO3H, COOH, and OH Graphene Quantum Dots. Adv. Mater. 2010, 22 (6),
Groups. J. Am. Chem. Soc. 2008, 130 (38), 12787– 734–738.
12793. (50) Wang, N.; Wang, Y.; Guo, T.; Yang, T.; Chen, M.; Wang,
(37) Baccile, N.; Laurent, G.; Babonneau, F.; Fayon, F.; J. Green Preparation of Carbon Dots with Papaya as
Titirici, M. M.; Antonietti, M. Structural Carbon Source for Effective Fluorescent Sensing of Iron
Characterization of Hydrothermal Carbon Spheres by (III) and Escherichia Coli. Biosens. Bioelectron. 2016,
Advanced Solid-State MAS 13C NMR Investigations. J. 85, 68–75.
Phys. Chem. C 2009, 113 (22), 9644–9654. (51) Purbia, R.; Paria, S. Green Synthesis of Single-
(38) Van Zandvoort, I.; Koers, E. J.; Weingarth, M.; Crystalline Akaganeite Nanorods for Peroxidase Mimic
Bruijnincx, P. C. A.; Baldus, M.; Weckhuysen, B. M. Colorimetric Sensing of Ultralow-Level Vitamin B1 and
Structural Characterization of 13C-Enriched Humins Sulfide Ions. ACS Appl. Nano Mater. 2018, 1 (3), 1236–
and Alkali-Treated 13C Humins by 2D Solid-State NMR. 1246.
Green Chem. 2015, 17 (8), 4383–4392. (52) Liao, J.; Cheng, Z.; Zhou, L. Nitrogen-Doping Enhanced
(39) Brown, A. B.; McKeogh, B. J.; Tompsett, G. A.; Lewis, R.; Fluorescent Carbon Dots: Green Synthesis and Their
Deskins, N. A.; Timko, M. T. Structural Analysis of Applications for Bioimaging and Label-Free Detection
Hydrothermal Char and Its Models by Density of Au3+ Ions. ACS Sustain. Chem. Eng. 2016, 4 (6),
Functional Theory Simulation of Vibrational 3053–3061.
Spectroscopy. Carbon 2017, 125, 614–629. (53) Edison, T. N. J. I.; Atchudan, R.; Shim, J. J.; Kalimuthu,
(40) Tsilomelekis, G.; Orella, M. J.; Lin, Z.; Cheng, Z.; Zheng, S.; Ahn, B. C.; Lee, Y. R. Turn-off Fluorescence Sensor
W.; Nikolakis, V.; Vlachos, D. G. Molecular Structure, for the Detection of Ferric Ion in Water Using Green
Morphology and Growth Mechanisms and Rates of 5- Synthesized N-Doped Carbon Dots and Its Bio-Imaging.
Hydroxymethyl Furfural (HMF) Derived Humins. Green J. Photochem. Photobiol. B Biol. 2016, 158, 235–242.
Chem. 2016, 18 (7), 1983–1993. (54) Jahanbakhshi, M.; Habibi, B. A Novel and Facile
(41) Cheng, Z.; Everhart, J. L.; Tsilomelekis, G.; Nikolakis, V.; Synthesis of Carbon Quantum Dots via Salep
Saha, B.; Vlachos, D. G. Structural Analysis of Humins Hydrothermal Treatment as the Silver Nanoparticles
Formed in the Brønsted Acid Catalyzed Dehydration of Support: Application to Electroanalytical
Fructose. Green Chem. 2018, 20 (5), 997–1006. Determination of H2O2 in Fetal Bovine Serum. Biosens.
(42) Qi, Y.; Song, B.; Qi, Y. The Roles of Formic Acid and Bioelectron. 2016, 81, 143–150.
Levulinic Acid on the Formation and Growth of (55) Atchudan, R.; Edison, T. N. J. I.; Sethuraman, M. G.; Lee,
Carbonaceous Spheres by Hydrothermal Y. R. Efficient Synthesis of Highly Fluorescent Nitrogen-
Carbonization. RSC Adv. 2016, 6 (104), 102428– Doped Carbon Dots for Cell Imaging Using Unripe Fruit
102435. Extract of Prunus Mume. Appl. Surf. Sci. 2016, 384,
(43) Xu, X.; Ray, R.; Gu, Y.; Ploehn, H. J.; Gearheart, L.; 432–441.
Raker, K.; Scrivens, W. A. Electrophoretic Analysis and (56) Li, J.; Zhang, L.; Li, P.; Zhang, Y.; Dong, C. One Step
Purification of Fluorescent Single-Walled Carbon Hydrothermal Synthesis of Carbon Nanodots to Realize
Nanotube Fragments. J. Am. Chem. Soc. 2004, 126 (40), the Fluorescence Detection of Picric Acid in Real
12736–12737. Samples. Sensors Actuators, B Chem. 2018, 258, 580–
(44) Shen, J.; Zhu, Y.; Yang, X.; Li, C. Graphene Quantum 588.
Dots: Emergent Nanolights for Bioimaging, Sensors, (57) Wang, F.; Hao, Q.; Zhang, Y.; Xu, Y.; Lei, W.
Catalysis and Photovoltaic Devices. Chem. Commun. Fluorescence Quenchometric Method for
2012, 48 (31), 3686–3699. Determination of Ferric Ion Using Boron-Doped
(45) Liu, F.; Jang, M. H.; Ha, H. D.; Kim, J. H.; Cho, Y. H.; Seo, Carbon Dots. Microchim. Acta 2016, 183 (1), 273–279.
T. S. Facile Synthetic Method for Pristine Graphene
Quantum Dots and Graphene Oxide Quantum Dots:
Origin of Blue and Green Luminescence. Adv. Mater.
2013, 25 (27), 3657–3662.
121
(58) Chen, Y.; Wu, Y.; Weng, B.; Wang, B.; Li, C. Facile (71) Rodríguez-Reinoso, F. The Role of Carbon Materials in
Synthesis of Nitrogen and Sulfur Co-Doped Carbon Heterogeneous Catalysis. Carbon 1998, 36 (3), 159–
Dots and Application for Fe(III) Ions Detection and Cell 175.
Imaging. Sensors Actuators, B Chem. 2016, 223, 689– (72) Qian, K.; Kumar, A.; Zhang, H.; Bellmer, D.; Huhnke, R.
696. Recent Advances in Utilization of Biochar. Renew.
(59) Guo, Y.; Yang, L.; Li, W.; Wang, X.; Shang, Y.; Li, B. Sustain. Energy Rev. 2015, 42, 1055–1064.
Carbon Dots Doped with Nitrogen and Sulfur and (73) He, X.; Zhang, Y.; Zhu, C.; Huang, H.; Hu, H.; Liu, Y.;
Loaded with Copper(II) as a “Turn-on” Fluorescent Kang, Z. Mesoporous Carbon Nanoparticles: A Super
Probe for Cystein, Glutathione and Homocysteine. Catalyst Support for Fuel Cells. New J. Chem. 2015, 39
Microchim. Acta 2016, 183 (4), 1409–1416. (11), 8667–8672.
(60) Krawielitzki, S.; Kläusli, T. M. Modified Hydrothermal (74) Roldán, L.; Pires, E.; Fraile, J. M.; García-Bordejé, E.
Carbonization Process for Producing Biobased 5-HMF Impact of Sulfonated Hydrothermal Carbon Texture
Platform Chemical. Ind. Biotechnol. 2015, 11 (1), 6–8. and Surface Chemistry on Its Catalytic Performance in
(61) Hayes, D. J.; Fitzpatrick, S.; Hayes, M. H. B.; Ross, J. R. Esterification Reaction. Catal. Today 2015, 249, 153–
H. The Biofine Process – Production of Levulinic Acid, 160.
Furfural, and Formic Acid from Lignocellulosic (75) De La Calle, C.; Fraile, J. M.; García-Bordejé, E.; Pires,
Feedstocks, in Biorefineries-Industrial Processes and E.; Roldán, L. Biobased Catalyst in Biorefinery
Products: Status Quo and Future Directions. Processes: Sulphonated Hydrothermal Carbon for
Biorefineries - Ind. Process. Prod. 2006, 1 (1), 139–164. Glycerol Esterification. Catal. Sci. Technol. 2015, 5 (5),
(62) Wang, P.; Yu, H.; Zhan, S.; Wang, S. Catalytic Hydrolysis 2897–2903.
of Lignocellulosic Biomass into 5- (76) Laohapornchaiphan, J.; Smith, C. B.; Smith, S. M. One-
Hydroxymethylfurfural in Ionic Liquid. Bioresour. Step Preparation of Carbon-Based Solid Acid Catalyst
Technol. 2011, 102 (5), 4179–4183. from Water Hyacinth Leaves for Esterification of Oleic
(63) Asghari, F. S.; Yoshida, H. Dehydration of Fructose to 5- Acid and Dehydration of Xylose. Chem. Asian J. 2017,
Hydroxymethylfurfural in Sub-Critical Water over 12 (24), 3178–3186.
Heterogeneous Zirconium Phosphate Catalysts. (77) Pileidis, F. D.; Tabassum, M.; Coutts, S.; Ttitirici, M. M.;
Carbohydr. Res. 2006, 341 (14), 2379–2387. Titirici, M.-M. Esterification of Levulinic Acid into Ethyl
(64) Nandiwale, K. Y.; Galande, N. D.; Thakur, P.; Sawant, S. Levulinate Catalysed by Sulfonated Hydrothermal
D.; Zambre, V. P.; Bokade, V. V. One-Pot Synthesis of 5- Carbons. Chinese J. Catal. 2014, 35 (6), 929–936.
Hydroxymethylfurfural by Cellulose Hydrolysis over (78) Fraile, J. M.; García-Bordejé, E.; Pires, E.; Roldán, L.
Highly Active Bimodal Micro/Mesoporous H-ZSM-5 New Insights into the Strength and Accessibility of Acid
Catalyst. ACS Sustain. Chem. Eng. 2014, 2 (7), 1928– Sites of Sulfonated Hydrothermal Carbon. Carbon
1932. 2014, 77, 1157–1167.
(65) Ramli, N. A. S.; Amin, N. A. S. Kinetic Study of Glucose (79) Liu, M.; Jia, S.; Gong, Y.; Song, C.; Guo, X. Effective
Conversion to Levulinic Acid over Fe/HY Zeolite Hydrolysis of Cellulose into Glucose over Sulfonated
Catalyst. Chem. Eng. J. 2016, 283, 150–159. Sugar-Derived Carbon in an Ionic Liquid. Ind. Eng.
(66) Shen, F.; Smith, R. L.; Li, L.; Yan, L.; Qi, X. Eco-Friendly Chem. Res. 2013, 52 (24), 8167–8173.
Method for Efficient Conversion of Cellulose into (80) Nata, I. F.; Putra, M. D.; Irawan, C.; Lee, C.-K. Catalytic
Levulinic Acid in Pure Water with Cellulase-Mimetic Performance of Sulfonated Carbon-Based Solid Acid
Solid Acid Catalyst. ACS Sustain. Chem. Eng. 2017, 5 (3), Catalyst on Esterification of Waste Cooking Oil for
2421–2427. Biodiesel Production. J. Environ. Chem. Eng. 2017, 5
(67) Zhang, T.; Li, W.; Xu, Z.; Liu, Q.; Ma, Q.; Jameel, H.; (3), 2171–2175.
Chang, H. min; Ma, L. Catalytic Conversion of Xylose (81) Qi, X.; Liu, N.; Lian, Y. Carbonaceous Microspheres
and Corn Stalk into Furfural over Carbon Solid Acid Prepared by Hydrothermal Carbonization of Glucose
Catalyst in γ-Valerolactone. Bioresour. Technol. 2016, for Direct Use in Catalytic Dehydration of Fructose. RSC
209, 108–114. Adv. 2015, 5 (23), 17526–17531.
(68) Jin, P.; Zhang, Y.; Chen, Y.; Pan, J.; Dai, X.; Liu, M.; Yan, (82) Yan, Y.; Dai, Y.; Wang, S.; Jia, X.; Yu, H.; Yang, Y.
Y.; Li, C. Facile Synthesis of Hierarchical Porous Catalytic Applications of Alkali-Functionalized Carbon
Catalysts for Enhanced Conversion of Fructose to 5- Nanospheres and Their Supported Pd Nanoparticles.
Hydroxymethylfurfural. J. Taiwan Institute Chem. Eng. Appl. Catal. B Environ. 2016, 184, 104–118.
2017, 75, 59–69. (83) Dong, W.; Cheng, S.; Feng, C.; Shang, N.; Gao, S.; Wang,
(69) Hu, L.; Tang, X.; Wu, Z.; Lin, L.; Xu, J.; Xu, N.; Dai, B. C.; Wang, Z. Carbon Nanospheres with Well-Controlled
Magnetic Lignin-Derived Carbonaceous Catalyst for Nano-Morphologies as Support for Palladium-
the Dehydration of Fructose into 5- Catalyzed Suzuki Coupling Reaction. Appl. Organomet.
Hydroxymethylfurfural in Dimethylsulfoxide. Chem. Chem. 2017, 31 (10), e3741.
Eng. J. 2015, 263, 299–308.
(70) Sevilla, M.; Fuertes, A. B. Sustainable Porous Carbons
with a Superior Performance for CO2capture. Energy
Environ. Sci. 2011, 4 (5), 1765–1771.
122
(84) Eiad-Ua, A.; Jomhataikool, B.; Gunpum, W.; Viriya- (96) Tang, C.; Titirici, M.-M.; Zhang, Q.; Cheng, T.; Maria-
Empikul, N.; Faungnawakij, K. Synthesis of Magdalena, T.; Qiang, Z. A Review of Nanocarbons in
Copper/Carbon Support Catalyst from Cattail Flower Energy Electrocatalysis: Multifunctional Substrates
by Calcination with Hydrothermal Carbonization. and Highly Active Sites. J. Energy Chem. 2017, 26 (6),
Mater. Today Proc. 2017, 4 (5), 6153–6158. 1077–1093.
(85) Liu, J.; Wickramaratne, N. P.; Qiao, S. Z.; Jaroniec, M. (97) Alatalo, S.-M. M.; Qiu, K.; Preuss, K.; Marinovic, A.;
Molecular-Based Design and Emerging Applications of Sevilla, M.; Sillanpää, M.; Guo, X.; Titirici, M.-M. M. Soy
Nanoporous Carbon Spheres. Nat. Mater. 2015, 14 (8), Protein Directed Hydrothermal Synthesis of Porous
763–774. Carbon Aerogels for Electrocatalytic Oxygen
(86) Gai, C.; Zhang, F.; Yang, T.; Liu, Z.; Jiao, W.; Peng, N.; Reduction. Carbon 2016, 96, 622–630.
Liu, T.; Lang, Q.; Xia, Y. Hydrochar Supported Bimetallic (98) Lu, Y.; Wang, L.; Preuß, K.; Qiao, M.; Titirici, M.-M. M.;
Ni–Fe Nanocatalysts with Tailored Composition, Size Varcoe, J.; Cai, Q. Halloysite-Derived Nitrogen Doped
and Shape for Improved Biomass Steam Reforming Carbon Electrocatalysts for Anion Exchange
Performance. Green Chem. 2018, 20 (12), 2788–2800. Membrane Fuel Cells. J. Power Sources 2017, 372, 82–
(87) Lu, Y. M.; Zhu, H. Z.; Li, W. G.; Hu, B.; Yu, S. H. Size- 90.
Controllable Palladium Nanoparticles Immobilized on (99) Preuss, K.; Tǎnase, L. C.; Teodorescu, C. M.; Abrahams,
Carbon Nanospheres for Nitroaromatic I.; Titirici, M. M.; Tănase, L. C.; Teodorescu, C. M.;
Hydrogenation. J. Mater. Chem. A 2013, 1 (11), 3783– Abrahams, I.; Titirici, M. M. Sustainable Metal-Free
3788. Carbogels as Oxygen Reduction Electrocatalysts. J.
(88) Dubey, S. P.; Dwivedi, A. D.; Kim, I.-C. C.; Sillanpaa, M.; Mater. Chem. A 2017, 5 (31), 16336–16343.
Kwon, Y.-N. N.; Lee, C. Synthesis of Graphene-Carbon (100) Huijun, Y.; Lu, S.; Tong, B.; Run, S.; N., W. G. I.; Yufei, Z.;
Sphere Hybrid Aerogel with Silver Nanoparticles and Chao, Z.; Li-Zhu, W.; Chen-Ho, T.; Tierui, Z. Nitrogen-
Its Catalytic and Adsorption Applications. Chem. Eng. J. Doped Porous Carbon Nanosheets Templated from g-
2014, 244, 160–167. C3N4 as Metal-Free Electrocatalysts for Efficient
(89) Cheng, J.; Wang, Y.; Teng, C.; Shang, Y.; Ren, L.; Jiang, Oxygen Reduction Reaction. Adv. Mater. 2016, 28 (25),
B. Preparation and Characterization of Monodisperse, 5080–5086.
Micrometer-Sized, Hierarchically Porous Carbon (101) Qiao, M.; Tang, C.; He, G.; Qiu, K.; Binions, R.; Parkin, I.
Spheres as Catalyst Support. Chem. Eng. J. 2013, 242, P.; Zhang, Q.; Guo, Z.; Titirici, M. M.
285–293. Graphene/Nitrogen-Doped Porous Carbon
(90) Sun, Z.; Yang, J.; Wang, J.; Li, W.; Kaliaguine, S.; Hou, X.; Sandwiches for the Metal-Free Oxygen Reduction
Deng, Y.; Zhao, D. A Versatile Designed Synthesis of Reaction: Conductivity: Versus Active Sites. J. Mater.
Magnetically Separable Nano-Catalysts with Well- Chem. A 2016, 4 (32), 12658–12666.
Defined Core-Shell Nanostructures. J. Mater. Chem. A (102) Terrones, M.; Ajayan, P. M.; Banhart, F.; Blase, X.;
2014, 2 (17), 6071–6074. Carroll, D. L.; Charlier, J. C.; Czerw, R.; Foley, B.;
(91) Yu, J.; Yan, L.; Tu, G.; Xu, C.; Ye, X.; Zhong, Y.; Zhu, W.; Grobert, N.; Kamalakaran, R.; et al. N-Doping and
Xiao, Q. Magnetically Responsive Core–Shell Coalescence of Carbon Nanotubes: Synthesis and
Pd/Fe3O4@C Composite Catalysts for the Electronic Properties. Appl. Phys. A Mater. Sci. Process.
Hydrogenation of Cinnamaldehyde. Catal. Letters 2002, 74 (3), 355–361.
2014, 144 (12), 2065–2070. (103) Wang, L.; Jia, W.; Liu, X.; Li, J.; Titirici, M. M. Sulphur-
(92) Zhang, F.; Hu, H.; Zhong, H.; Yan, N.; Chen, Q. Doped Ordered Mesoporous Carbon with Enhanced
Preparation of γ-Fe2O3@C@MoO3 Core/Shell Electrocatalytic Activity for the Oxygen Reduction
Nanocomposites as Magnetically Recyclable Catalysts Reaction. J. Energy Chem. 2016, 25 (4), 566–570.
for Efficient and Selective Epoxidation of Olefins. Dalt. (104) Wan, W.; Wang, Q.; Zhang, L.; Liang, H. W.; Chen, P.;
Trans. 2014, 43 (16), 6041–6049. Yu, S. H. N-, P- and Fe-Tridoped Nanoporous Carbon
(93) He, B.; Zhao, Q.; Zeng, Z.; Wang, X.; Han, S. Effect of Derived from Plant Biomass: An Excellent Oxygen
Hydrothermal Reaction Time and Calcination Reduction Electrocatalyst for Zinc-Air Batteries. J.
Temperature on Properties of Au@CeO2core–Shell Mater. Chem. A 2016, 4 (22), 8602–8609.
Catalyst for CO Oxidation at Low Temperature. J. (105) Liu, Y.; Huang, B.; Lin, X.; Xie, Z. Biomass-Derived
Mater. Sci. 2015, 50 (19), 6339–6348. Hierarchical Porous Carbons: Boosting the Energy
(94) Shi, Z. Q.; Jiao, L. X.; Sun, J.; Chen, Z. B.; Chen, Y. Z.; Zhu, Density of Supercapacitors via an Ionothermal
X. H.; Zhou, J. H.; Zhou, X. C.; Li, X. Z.; Li, R. Cobalt Approach. J. Mater. Chem. A 2017, 5 (25), 13009–
Nanoparticles in Hollow Mesoporous Spheres as a 13018.
Highly Efficient and Rapid Magnetically Separable (106) Huang, D.; Li, S.; Zhang, X.; Luo, Y.; Xiao, J.; Chen, H. A
Catalyst for Selective Epoxidation of Styrene with Novel Method to Significantly Boost the
Molecular Oxygen. RSC Adv. 2014, 4 (1), 47–53. Electrocatalytic Activity of Carbon Cloth for Oxygen
(95) Sun, Y.; Zhang, L.; Wang, Y.; Chen, P.; Xin, S.; Jiu, H.; Liu, Evolution Reaction. Carbon 2018, 129, 468–475.
J. Hollow and Hollow Core/Shell CeO2/SiO2@CeO2
Spheres: Synthesis, Structure Evolution and Catalytic
Properties. J. Alloys Compd. 2014, 586, 441–447.
123
(107) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.;
Balakrishnan, J.; Lei, T.; Ri Kim, H.; Song, Y. Il; et al. Roll-
to-Roll Production of 30-Inch Graphene Films for
Transparent Electrodes. Nat. Nanotechnol. 2010, 5 (8),
574–578.
(108) Zhang, J.; MacFarlane, D. R.; Li, F.; Chen, L.; Knowles,
G. P.; MacFarlane, D. R.; Zhang, J.; Fengwang, L.; Lu, C.;
P., K. G.; et al. Hierarchical Mesoporous SnO2
Nanosheets on Carbon Cloth: A Robust and Flexible
Electrocatalyst for CO2 Reduction with High Efficiency
and Selectivity. Angew. Chemie - Int. Ed. 2016, 56 (2),
505–509.
124
Bio-based carbon materials for a direct-carbon fuel cell

Maciej P. Olszewski, Pablo J. Arauzo, Joscha Zimmermann, Dennis Jung, Viola Hoffmann, Andrea Kruse
Department of Conversion Technologies of Biobased Resources, Institute of Agricultural Engineering, University
of Hohenheim, Garbenstrasse 9, 70599 Stuttgart, Germany
Abstract
Due to the rapid and constantly developing society, there is an increase in the demand for electricity. Most of the electricity generated
in the world is produced by burning fossil fuels (mostly coal, natural gas, and oil). Also, all type of transportation including land, maritime
and air are powered by engines using processed crude oil. This leads to the depletion of fossil fuel reserves and increasing the price of
fuels. Moreover, combustion releases enormous amounts of CO2, particulate matter and other pollutants into the atmosphere, which
leads to the environmental pollution as well as global warming. To reduce the consumption of fossil fuels and related emissions,
biomass can be used in specific applications. Bio-based carbon materials can be used as electrodes in supercapacitors as well as in
batteries. Furthermore, bio-based carbons can be used as fuel for high-efficiency electricity production in electrochemical reaction
occurring in a direct-carbon fuel cell, which represents a promising technology for electricity generation. In this review, the theoretical
background and principles of operation of different types of direct-carbon fuel cells are presented. Besides, the required properties of
carbon materials used as a fuel are listed.
2.1. Hydrothermal process

1. Introduction HTC is a thermochemical process, converting biomass in a
The depletion and increasing price of fossil fuels, as well as coal-like product. The feedstock is a favourable wet
the climate change and the need to decrease CO2 emission, biomass7, in principle any lignocellulosic material, as well
are currently a major interest on a global scale. However, as pure lignin8,9. The process temperatures are in general
rapid economic growth and increasing population on the relatively low, around 150–350 °C10–16. The selected
Earth results in high demand for modern electronic temperature is primary depending on the type of starting
devices. It requires continuous improvement of energy material and the favoured product. Because the process
storage systems including faster charging, longer lifetime, runs in a closed system, a high autogenous pressure is
higher conversion efficiency and maintaining the lowest generated. Furthermore, the temperature is determining
possible price. This focused the researchers' attention on the pressure by the vapour pressure curve and is limited by
enhancing the use of biomass for more efficient production the critical point in the phase diagram of water. Increasing
of electricity (direct-carbon fuel cell) and carbon materials. the temperature and crossing this, these limits will lead to
According to recently published research, bio-based a higher liquid and gas fraction of the product (i.e.,
carbon materials achieve similar or even better properties liquefaction and gasification).
than fossil equivalent. These materials are used as The hydrothermal conversion of biomass into a carbon
electrode material in energy storage devices, including material occurs in water, which acts as a catalyst by
supercapacitors, lithium-ion batteries (LIBs), sodium ion facilitating the hydrolysis of organic polymers. In general,
batteries (SIBs), and as a fuel in direct-carbon fuel cells the process of HTC involves several mechanisms, including
(DCFCs)1–3. Biomass can be converted into carbon-rich hydrolysis, dehydration, polymerisation, aromatisation,
material using several conversion technologies; herein, and decarboxylation. The three essential mechanisms for
hydrothermal carbonisation process are described as well the production of hydrochar are hydrolysis, dehydration,
as direct carbon fuel cells. and polymerisation. Aromatisation and decarboxylation
usually occur at higher operating temperatures and
extended reaction times.
2. Hydrothermal carbonisation The HTC-process starts with the hydrolysis of the
The fundamental process of hydrothermal carbonisation carbohydrates in the biomass structure. During this step,
(HTC) has been known for a century. In 1913, the process the polymer chains of cellulose and hemicellulose are split
was firstly reported when Friedrich Bergius was up into monomers (e.g., glucose from cellulose). Hydrolysis
investigating the production of a synthetic coal by of hemicellulose produces acetic acid, D-xylose,
subjecting a material to heating water and pressure4. D-mannose, D-galactose and D-glucose. It is deduced that
Bergius compared the hydrothermal carbonisation with lignin dissolved slowly, and exposed fragments of non-
the natural coalification process that takes place during dissolved lignin in water are decomposed into phenolics17.
millions of years, which he accomplished in a much more Hydrolysis of hemicellulose begins approx. at 180 °C, while
rapid reaction. However, the idea to hydrothermally cellulose and lignin hydrolysis occur above 200 °C8,18. A
convert biomass is relatively young5 and according to complete hydrolysis of cellulose and lignin will not occur,
Titirici et al6, the somewhat forgotten “synthetic rather two reaction pathways will lead to the solid char.
coalification” has a new renaissance of interest since the One through a (1) solid-solid conversion and (2) aqueous
turn of the twenty-first century. phase degradation7,19. It is assumed that in reaction
pathway (1), the cellulose-bound in the lignin structure is

125
Chapter 10 Olszewski et al.
unable to hydrolyse and thus maintains structural cannot be analysed separately. Another influencing
elements of the precursor biomass20. The product is called parameter is the concentration of precursor. According to
‘primary char’. Final knowledge about the molecular Jung et al.27, the reaction order of the hydrochar formation
structure is still missing and researchers are debating is higher than one and hence, a lower precursor
about it. Previous statements assumed that the structure concentration or additional water results in more carbon
is polyaromatic21,22, but Brown et al.23 suggested a polymer in the liquid phase.
of furans and arene with carbonyl-substituted compounds. As previously referenced, protons enhance the hydrolysis
In reaction pathway (2), the liquid state reacts further by and have a catalytic effect on the production of HMF28.
polymerisation namely. The solid product ends up as a However, a lowered pH-value can also increase the
fraction of the HTC-char and is called ‘secondary char’, a production of levulinic acid, which is also dependent on the
polymerised hydrochar. present acid29. Reza et al. 30 refer to that hemicellulose and
During dehydration, single sugars in the liquid phase are cellulose are less reactive within a basic environment,
converted into furfurals, namely 5-Hydroxymethylfurfural contrarily to lignin’s enhanced reactivity.
(hereafter ’HMF’) and organic acids, namely levulinic and
2.3. Hydrochar
formic acid8,24. This intramolecular step significantly
carbonises the biomass since the hydrogen-to-carbon ratio The resulting hydrochar is a coal-like product with a quite
(H/C) and the oxygen-to-carbon ratio (O/C) is reduced by similar higher heating value (hereafter ‘HHV’). During the
seceding H2O. Dehydration with the preceding hydrolysis is process, the HHV is increased by ca. 40% of the starting
shown within the example of cellulose in Fig. 1. biomass precursor, by rising the carbon content6.
Furthermore, carbonic organic compounds are lost to the
solvent and a minor amount is gasified (up to 5%)31.
Compared to biochars produced via pyrolysis, he
carbonisation of hydrochar is lower, as well the
crosslinking and polymerisation and thus the thermal
stability is. This also implies a higher O/C and H/C ratio and
hence a richer surface chemistry with high content of OFG
(oxygenated functional group). These OFGs make them an
Figure 1. Mechanism of acid-catalysed cellulose hydrolysis
to glucose and conversion to hydroxymethylfurfural interesting feedstock to produce activated carbon32.
(HMF), and levulinic acid (LA), and formic acid (FA) Depending on the precursor biomass and treatment
(adapted from 79). method, a hydrochar with special characteristics can be
created9. Nanospheres, nanocables, nanofibers,
2.2. Process parameters and their influence submicrocables, submicrotubes, carbon gels and porous
The final structure and composition of the product is structures are reported by Titirici et al. 6 and Hu et al. 33,
mainly determined by the feedstock6,8. But by controlling which are expected to be advanced carbon materials for
the temperature, residence time, solid load and pH-value, applications in supercapacitors, membranes or in the
the characteristics of the hydrochar can be influenced. production of fuels, fertilisers or energy storage systems.
The temperature is an important process parameter by An interesting application of hydrochar is its use in a direct-
determining the fate of degradation reactions and reaction carbon fuel cell to generate electricity with very high
rate according to the thermodynamic equilibrium. Ionic efficiency.
reactions occur at lower temperatures, since an acidic
environment improves the hydrolysis. At higher
temperatures, homolytic cleavage dominates, due to a 3. Direct carbon fuel cell background
reduction of the acidic dissociation constant (Ka-value)25. The first direct-carbon fuel cell (DCFC) was built by William
Elevating the temperature can also lead to the formation W. Jacques and patented in 189634. However, his results
of a wider range of products and higher gas yield26. were not reproducible until the seventies. For a long time,
Additionally, a higher temperature extends the his concept was suspected to generate electricity by a
dehydration and increases the degree of condensation of thermoelectric effect (conversion of temperature into
the hydrochar. Sevilla et al.18 reported that especially the electric voltage) not by an electrochemical reaction. In the
O/C and H/C ratios are decreasing during the hydrothermal 1970s, the US Stanford Research Institute finally showed
process at 230–250 °C. However, Kang et al.17 reported a that such a system was feasible2. Since the nineties, the
small decrease in the yield of hydrochar based on cellulose fuel cell technology has been significantly developed.
with an increasing temperature of 225–265 °C. The DCFC technology has the potential to utilise an
A longer residence time increases the carbonisation of the abundant and relatively easily accessible primary energy
resulting char while reducing the mass yield. In terms of sources as fuel. Various carbon materials can be utilised,
energy yield, a longer residence time is positive, since such as fossil bituminous coal or lignite, charred and
dehydration and subsequent condensation polymerisation untreated biomass or even waste products like plastic2.
processes in the liquid state enhance the production of the Furthermore, the carbon fuel is easy to handle and
secondary char. Since operation temperature and compared to hydrogen, which is often used in fuel cells,
residence time interact and affect the product, both easy to store. The substantial advantage of a fuel cell is the
126
− 2−
direct electrochemical conversion of chemical energy into 𝑂𝑂2 (𝑔𝑔) + 4𝑒𝑒(𝑐𝑐𝑐𝑐𝑐𝑐ℎ𝑜𝑜𝑜𝑜𝑜𝑜) = 2𝑂𝑂(𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒) (1)
electricity. This conversion is highly efficient, since 2−
𝐶𝐶(𝑠𝑠) + 2𝑂𝑂(𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒) −
= 𝐶𝐶𝐶𝐶2 (𝑔𝑔) + 4𝑒𝑒(𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎) (2)
intermediary processes are eliminated. For example, the
direct conversion of carbon into carbon dioxide has a Combination of Eq. (1) and Eq. (2) gives the overall
theoretical thermodynamic efficiency of 100%; from reaction, running in a DCFC:
thermal conversion, less than the half is typical obtained35. 𝐶𝐶(𝑠𝑠) + 𝑂𝑂2 (𝑔𝑔) → 𝐶𝐶𝐶𝐶2 (𝑔𝑔) (3)
The practical efficiency is stated with 80%36,37, which is The desired anode reaction is referred in Eq. (2) as a full
much higher than that of a coal-fired power plant with oxidation, but in most cases, a partial oxidation of carbon
30%–40% (without cogeneration).35,38 occurs by releasing only two electrons and carbon
The fuel cells exhaust, in principle, zero harmful emissions monoxide.38
to the environment. A hydrogen fuel cell produces water 2−
𝐶𝐶(𝑠𝑠) + 𝑂𝑂(𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒) −
= 𝐶𝐶𝐶𝐶 (𝑔𝑔) + 2𝑒𝑒(𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎) (4)
as a reaction product and a DCFC almost pure CO2, which
can be easily separated and used in a further process. By This gives an undesired overall reaction:
1
utilising biomass, this gives the opportunity to sequester 𝐶𝐶(𝑠𝑠) + 𝑂𝑂2 (𝑔𝑔) = 𝐶𝐶𝐶𝐶 (𝑔𝑔) (5)
2
CO2 from the atmosphere.
In a galvanic cell, the chemical energy is converted directly
Despite the stated advantages, the fuel cell cannot be seen
into electrical energy. The proportion of maximum
as the ‘Holy Grail’ for the prospective energy supply by
electrical energy is referred to as a change in Gibbs free
means of fuels, since it comes also with many
energy of reaction. The change of Gibbs free energy of the
disadvantages, especially those related to the conversion
electrochemical reaction gives the maximum of electrical
of carbon. The following section summarises the
work obtained in a fuel cell40. Therefore, Eq. (6) is applied.
fundamentals in electrochemistry and thermodynamics,
𝑊𝑊 = ∆𝑟𝑟 𝐺𝐺 = −𝑛𝑛𝑛𝑛∆𝐸𝐸0 (6)
showing advantages and disadvantages, as well as
different concepts and the state of the art. In Eq. (6), W is the electrical work, ΔrG is the change in
Gibbs free energy, n the number electrons transferred in
3.1. Fundamentals in electrochemistry and the reaction, F the Faraday constant, and ΔE0 is the
thermodynamics equilibrium potential of the cell, which is defined by Eq. (7).
A fuel cell operating on carbon fuel is named carbon fuel ∆𝐸𝐸0 = 𝐸𝐸0 𝐶𝐶𝐶𝐶𝐶𝐶ℎ𝑜𝑜𝑜𝑜𝑜𝑜 − 𝐸𝐸0 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 (7)
cells and is related to the concept of a galvanic cell. An
At standard conditions (pressure of 105 Pa)41, the
idealized DCFC system is schematically shown in Fig. 2. It
temperature dependency is introduced to indicate the
consists of an electrolyte and two electrodes, namely
operation temperature of the fuel cell. Accordingly, the
cathode and anode. The electrolyte can mobilise ions, but
value of the thermodynamic equilibrium potential can be
at the same time, it is electronically insulating (not
calculated from Eq. (8).
transferring electrons).35
∆𝑟𝑟 𝐺𝐺(𝑇𝑇)
∆𝐸𝐸0∗ (𝑇𝑇) = − (8)
𝑛𝑛𝑛𝑛
This theoretical value makes it possible to compare
different fuels at different operation temperatures. The
electrochemical conversion offers inherently higher
thermodynamic efficiency than incineration into electrical
energy. The theoretical efficiency of a common
incineration power plant is defined by Eq. (9)38,42, in which
T1 and T2 are the lowest and highest temperatures of the
process, respectively.
𝑇𝑇1
𝜂𝜂𝑡𝑡ℎ 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 = 1 − (9)
𝑇𝑇2
By contrast, the electrochemical oxidation of carbon inside

a DCFC runs isothermal and isobaric. Additionally, the
reactants and product streams are separated by the
Fig. 2. Illustration of an idealized direct-carbon fuel cell) electrolyte and physical states. Further entropic losses due
(adapted from39). to mixing are eliminated. The theoretical efficiency is
defined by the thermodynamic state functions shown in
The type of electrolyte is classifying the DCFCs into Eq. (10).38,40
hydroxide, molten carbonate, and solid oxide types. The ∆ 𝐺𝐺 𝑇𝑇∆ 𝑆𝑆
𝜂𝜂𝑡𝑡ℎ 𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷 = 𝑟𝑟 = 1 − 𝑟𝑟 (10)
∆𝑟𝑟 𝐻𝐻 ∆𝑟𝑟 𝐻𝐻
oxygen (O2) is reduced with four electrons at the cathode
and then transported through the electrolyte. The anode The common approach to calculate the cell efficiency
consisting of the fuel in form of solid carbon is oxidised to would add further efficiency factors for cell polarisation
carbon dioxide by releasing four electrons. Accordingly, the losses, current (or Coulombic) efficiency at the electrodes
desired cathode and anode reactions for a DCFC are: and the efficiency of fuel utilisation. Since DCFC runs on a
solid fuel, Brentan et al.43 proposed the following equation:
127
𝐸𝐸 ∫ 𝑖𝑖𝑖𝑖𝑖𝑖 However, at increasing temperature, the formation of CO

𝜂𝜂𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = (11)
𝑚𝑚̇𝑐𝑐ℎ𝑎𝑎𝑎𝑎 𝐻𝐻𝐻𝐻𝐻𝐻𝑐𝑐ℎ𝑎𝑎𝑎𝑎
is preferred over CO2. This phenomenon is related again to
where the term in the numerator represents the total the Boudouard equilibrium and is discussed below.
power of the cell (in W), whereas mchar is the mass
consumption rate (in kg s–1) and HHVchar the higher heating
value of the char (in J kg–1).
3.2. Efficiency and equilibrium cell potential

In the case of the full oxidation shown in Eq. (3), the
thermodynamic data is given in Table 1. The reaction is
exergonic and exothermic since the Change in Gibbs free
energy and enthalpy is negative. The change in entropy is
for the case of full oxidation positive, but nearly zero since
the amount of substance in the gas phase is unchanging.
One mole of O2 reacts to one mole of CO2. This is also the
compensatory advantage in contrast to other fuel cell
system44. Additionally, the disarray of the system
increases, as CO2 has a higher degree of freedom
compared to O2. Consequently, the theoretical cell Figure 3. Temperature dependence of the
conversion efficiency (𝜂𝜂𝑡𝑡ℎ ) is practically independent of thermodynamic efficiency for different fuel cell systems.
temperature. Most importantly, the small value for the
entropy change results in yielding a conversion efficiency
of 100% over the whole temperature spectrum. This
provides a major advantage for electrochemical
conversion of carbon over chemical processes at favoured
temperatures, although it is usually difficult to achieve
direct conversion.
Table 1. Total and partial oxidation of carbon.

Thermodynamic data at Temperatures of 300, 600, 900
and 1200 K (calculated with data from Ref.45).
Full oxidation 𝐶𝐶(𝑠𝑠) + 𝑂𝑂2 (𝑔𝑔) → 𝐶𝐶𝐶𝐶2 (𝑔𝑔)

T [K] 300 600 900 1200
∆𝑟𝑟 𝐺𝐺
–394.37 –395.14 –395.68 –396.01
[𝑘𝑘𝑘𝑘 𝑚𝑚𝑚𝑚𝑚𝑚−1 ]
∆𝑟𝑟 𝐻𝐻 Figure 4. Temperature dependence of the equilibrium
–393.51 –393.80 –394.41 –395.04
[𝑘𝑘𝑘𝑘 𝑚𝑚𝑚𝑚𝑚𝑚−1 ]
potential for different fuel cell systems.
∆𝑟𝑟 𝑆𝑆
2.88 2.22 1.41 0.80
[𝐽𝐽 𝐾𝐾 −1 𝑚𝑚𝑚𝑚𝑚𝑚−1 ]
3.3. Efficiency and equilibrium cell potential
In Fig. 3, the temperature dependency of different fuel cell Another way to define the ideal performance of a fuel cell
systems is shown on their theoretical efficiency, calculated is the Nernst potential, represented again as a cell voltage.
using literature data45 and Eq. (10). It is shown that the The Nernst equation describes the relationship between
efficiency level of a hydrogen or carbon monoxide cell is the ideal standard potential ∆𝐸𝐸0∗ (𝑇𝑇) at standard
decreasing with an increasing temperature. However, in temperature and pressure for the fuel cell reaction and the
DCFC systems the oxidation reactions of carbon are in ideal equilibrium potential ∆𝐸𝐸0 (𝑇𝑇) at changing
competition with the Boudouard reaction, as released CO2 temperatures and pressures of reactants and products.
reacts with the solid carbon fuel to CO. This reaction is Once the ideal potential at standard conditions is known,
endothermic and lowers the fuel utilisation. Eq. (12) can be used to determine the ideal potential at
The previous paragraph introduced the equilibrium cell changing temperatures and pressures.
𝑅𝑅𝑅𝑅
potential and its temperature dependency, according to ∆𝐸𝐸0 (𝑇𝑇) = ∆𝐸𝐸0∗ (𝑇𝑇) − 𝑙𝑙𝑙𝑙𝑄𝑄 (12)
𝑛𝑛𝑛𝑛
Eq. (8). However, for the case of full carbon oxidation, the
To calculate the reaction quotient (Q) of the
potential behaves similar to the efficiency and can be seen
thermodynamic activity of each half-cell, Eq. (13) is
as constant. Fig. 4 shows the temperature dependency of
applied.
various oxidation reactions. It can be noticed that the
[𝑃𝑃 ]υ𝑝𝑝1 [𝑃𝑃 ]υ𝑝𝑝2
equilibrium cell potential of oxidation of H2 and CO are 𝑄𝑄 = [𝑅𝑅1 ]υ𝑟𝑟1 [𝑅𝑅2 ]υ𝑟𝑟2 (13)
1 2
decreasing with an increasing temperature. Nonetheless, it
where [Pi] is the concentration (dissolved form) or pressure
must be considered that the curve of the oxidation of CO
(gaseous form) of product, [Ri] the concentration
and the partial oxidation of C is mirrored. The result of
(dissolved form) or pressure (gaseous form) of reactant,
adding these two reactions again gives the full oxidation.
128
whereas υpi and υri correspond to the stoichiometric

coefficients of products and reactants, respectively.
A further and deeper explanation and derivation of the
Nernst equation is referend in a previous study.41
3.4. Boudouard equilibrium

In addition to the electrochemical reactions given in Eq. (2)
and Eq. (4), CO and CO2 react in contact with carbon in a
redox reaction according to a chemical equilibrium, the so-
called Boudouard equilibrium.2
𝐶𝐶(𝑠𝑠) + 𝐶𝐶𝐶𝐶2 (𝑔𝑔) → 𝐶𝐶𝐶𝐶 (𝑔𝑔) (14)
The equilibrium constant of this reaction (KB) is defined by Figure 5. Boudouard equilibrium at 1 atm (adapted
Eq. (15), which can be calculated, as the partial pressures from80).
of the respective gasses are known or with the operation
temperature and the relative Gibbs free energy.
𝑝𝑝𝐶𝐶𝐶𝐶 2 ∆𝑟𝑟 𝐺𝐺(𝑇𝑇) 4. DCFC concepts and reaction mechanism
𝐾𝐾𝐵𝐵 = = exp( ) (15)
𝑝𝑝𝐶𝐶𝐶𝐶2 𝑅𝑅𝑅𝑅
State of the art, different DCFC concepts with three
The thermodynamic equilibrium of the reaction is strongly possible types of electrolyte have been developed. Namely
dependent on the operating temperature as well as the a liquid hydroxide molten electrolyte or alkaline fuel cell
applied pressure in the system. The corresponding values (hereafter AFC)46,47, a liquid molten carbonate electrolyte
provided in Table 2 are calculated at 1 atm. As expected, fuel cell (MCFC)48,49 and a solid oxide ceramic electrolyte
the formation of CO2 is thermodynamically favoured at low (SOFC)50,51. All concepts have their advantages and
temperatures. In contrast, elevated temperatures favour disadvantages, further explained in this section. To
the formation of CO. The resulting carbon monoxide can overcome specific disadvantages, recently, hybrid
be further electrochemically oxidised to CO2 in a second technologies have also been developed, representing a
step, but isn’t efficient as the direct conversion of solid combination of different concepts. For example, a solid
carbon38, as it yields only two electrons. oxide system with a molten carbonate52 or a molten
hydroxide with a molten carbonate.53
Table 2. Temperature dependency of Boudouard constant
(KB) and the respective equilibrium partial pressures for 4.1. Hydroxide fuel cell
CO and CO2 at 1 atm. (calculated from Thermochemical
Data45). This type of DCFC uses a molten hydroxide as the
electrolyte. The electrolyte, mainly KOH or NaOH, is
contained within a metallic container, which also acts as
𝑇𝑇 [𝐾𝐾] 800 1000 1200
the cathode. The anode is made with carbon and is dipped
𝐾𝐾𝐵𝐵 0.01 1.76 53.75 into the electrolyte.
The advantages of using hydroxide as electrolyte come
𝑝𝑝𝐶𝐶𝐶𝐶 0.09 0.71 0.98 with a high ionic conductivity54, especially when mixed
with water55 and a high reactivity on the carbon fuel56.
𝑝𝑝𝐶𝐶𝐶𝐶2 0.91 0.29 0.02 Additionally, hydroxides have a relatively low melting point
and thus the operating temperatures of the fuel cells are
The CO oxidation is described by Eq. (16). Such chemical 400–650 °C46. Since temperatures below 700 °C favour the
losses due to the Boudouard consumption highly decrease oxidation of carbon to CO2 rather than CO, according to the
the carbon conversion efficiency. This concludes a Boudouard equilibrium, the fuel utilisation is higher, which
reduction of the operating temperatures in the DCFC, to result in a higher system efficiency. Despite the
avoid this phenomenon shown in Fig. 5, without advantages, the fundamental problem of a hydroxide
decreasing the kinetics of ongoing reactions. electrolyte is the formation of carbonate, already
experienced by Jacques34. Further technical challenges are
𝐶𝐶𝐶𝐶 + 2𝑂𝑂2− → 𝐶𝐶𝐶𝐶2 + 2𝑒𝑒 − (16)
the high corrosion rates of metals used in the cell and fuel
pre-treatment. Since volatile hydrocarbons and mineral
ash reduce the performance of the electrolyte57, they must
be removed. Because of referenced issues, this fuel cell
concept will not be discussed any further in this work.
4.2. Molten carbonate fuel cell

Replacing the hydroxide melt by a carbonate melt as an
electrolyte distinguish or minimise the previously
mentioned problems. So far, the most frequently used
concept of a DCFC is based on molten carbonate fuel cell
(MCFC). The carbon is distributed and surrounded in the
129
liquid electrolyte, which consists of a eutectic carbonate

mixture, typically Li2CO3, K2CO3 and/or Na2CO3.
The advantages of carbonates are unlike hydroxides, a
good long-term stability of the electrolyte in CO258,59.
Additionally, carbonates show a high ionic conductivity and
have sufficient contact area between the carbon and the
electrolyte60,61. Furthermore, the operating temperatures Figure 6. Mechanism of electrochemical oxidation of
are in a wide range and usually defined by the composition carbon in a molten carbonate: (a), first adsorption of O2-
of the electrolyte; typically between 500 and 900 °C48,49,62.. ion; (b), second adsorption of O2- ion and CO2 formation.
In the system, carbonate ions (CO32-) are involved in the
4.3. Solid oxide fuel cell
electrochemical oxidation of carbon. Therefore, oxygen
and carbon dioxide are needed on the cathode side. A The concept of a Solid Oxide Fuel Cell (SOFC) reduces or
recycle of 2/3 of generated CO2 is required38. The lifetime even removes previous issues. Especially corrosion and
is the main issue for a MCFC, since the cathode suffers material stability, coming with hydroxide and carbonate
degradation due to carbonate attacks. melts, are eliminated. Additionally, kinetic and transport
A reaction mechanism for the oxidation of carbon in a limitations due to low viscosity of the melt shortened. The
molten carbonate was suggested by Cherepy et al.60. At a conventional electrolyte is a ceramic material made from
temperature of 700 °C, the molten carbonate electrolyte yttrium stabilised zirconia (YSZ), providing a good ion
shows a strong dissociation, leading to the formation of conductivity and stable structure at low to high-
oxygen ions. temperature conditions, in general in the range of
2𝐶𝐶𝐶𝐶3 2− → 2𝐶𝐶𝐶𝐶2 + 2𝑂𝑂2− (17) 700–1000 °C.35
The requirements for an anode material in an SOFC include
The electrochemical oxidation of carbon is then divided
a high electronic conductive material with a catalytic
into seven further steps. Firstly, the oxygen ions yielded in
activity to accelerate the kinetics of electrochemical
Eq. (17) are adsorbed on the carbon atoms of the fuel.
oxidation of carbon. The common anodes are nickel and
Therefore, two separate adsorptions take place. The first
platinum, since they are extensively studied and have
adsorption of an oxygen ion is described by Eq. (18),
excellent catalytic activities. Also, carbides and vanadium
followed with two fast electronic discharges, given in Eq.
found to be good catalysts. The anodic half reactions occur
(19) and (20). The process is summarised in Fig. 6a. The
at the triple phase boundary (TPB), where fuel, electron-
resulting functional group decomposes very slowly to form
conducting material (anode) and ion-conducting material
free CO; thus, a second mechanism step can take place.
(electrolyte) meet. The simplified interactions on the
𝐶𝐶 + 𝑂𝑂2− → 𝐶𝐶𝐶𝐶2− 𝑎𝑎𝑎𝑎𝑎𝑎 (18) reactive surface (no liquid phase) avoid phenomena like
𝐶𝐶𝐶𝐶2− 𝑎𝑎𝑎𝑎𝑎𝑎 → 𝐶𝐶𝐶𝐶 − 𝑎𝑎𝑎𝑎𝑎𝑎 + 𝑒𝑒 − (19) bubbling and flooding, which can lower again the
𝐶𝐶𝐶𝐶 − 𝑎𝑎𝑎𝑎𝑎𝑎 → 𝐶𝐶𝐶𝐶𝑎𝑎𝑎𝑎𝑎𝑎 + 𝑒𝑒 − (20) conversion efficiency38. On the other hand, this solid/solid
In the second step, another oxygen ion is adsorbed, as interface provides a poor contact between the fuel anode
shown in Eq. (21). This adsorption is considered to be the surfaces, which hinder the electrons to flow63. To promote
rate-determining step for the whole carbon oxidation, electrochemical oxidation and electron flow and surface
since it is kinetically hindered and requires a considerable contact, molten metals are used. Ju et al. 64 developed a
overpotential. Analogous to the previous step, a rapid Ni-YSZ anode supported with Sn to build a favourable
discharge takes place and two electrons are yielded, as bridge between the solid carbon and anode. Additionally,
described by Eq. (22) and (23). carbon particles in the liquid metal phase are gasified to CO
via a 2-electron transfer. The CO can then further be
𝐶𝐶𝐶𝐶 − 𝑎𝑎𝑎𝑎𝑎𝑎 → 𝐶𝐶𝐶𝐶𝐶𝐶2− 𝑎𝑎𝑎𝑎𝑎𝑎 (21)
utilised in the DCFC as a gaseous fuel.
𝐶𝐶𝐶𝐶𝐶𝐶 2− 𝑎𝑎𝑎𝑎𝑎𝑎 → 𝐶𝐶𝐶𝐶𝐶𝐶 − 𝑎𝑎𝑎𝑎𝑎𝑎 + 𝑒𝑒 − (22) It is assumed that the electrochemical oxidation in an SOFC
𝐶𝐶𝐶𝐶𝐶𝐶 − 𝑎𝑎𝑎𝑎𝑎𝑎 → 𝐶𝐶𝐶𝐶2 𝑎𝑎𝑎𝑎𝑎𝑎 + 𝑒𝑒 − (23) is proceeding similar to the carbonate mechanism2. Unlike
Finally, the carbon dioxide desorbs from the surface. This an MCFC, the O2- ions are delivered directly from the
reaction involves the two adsorbed O2- ions and the active electrolyte through bulk transfer. Y3+ ions replace Zr4+ on
carbon on the surface, as described in Eq. (24) and shown the cationic sub-lattice, thus generating oxygen
in Fig. 6b. vacancies65 and providing ion conductivity at elevated
𝐶𝐶𝐶𝐶2 𝑎𝑎𝑎𝑎𝑎𝑎 → 𝐶𝐶𝐶𝐶2 (24) temperatures, while electrical conductivity is prevented.
This can be described by the so-called Kröger-Vink
notation. The reaction is described in Eq. (25), the oxygen
adsorbed by the crystal lattice of a particle is indicated as
𝑂𝑂𝑂𝑂 𝑥𝑥 , whereas the vacancies for an oxygen ion are designed
as 𝑉𝑉𝑂𝑂 ∙∙ .
𝑂𝑂𝑂𝑂 𝑥𝑥 ↔ 𝑂𝑂𝑎𝑎𝑎𝑎𝑎𝑎 2− + 𝑉𝑉𝑂𝑂 ∙∙ (25)
This mechanism was proposed by Li et The first
al.66
adsorption of an O2- ion followed by the discharging in two
steps is the same as stated in Eqs. (18), (19) and (20).
130
Subsequently, carbon monoxide is desorbed from the 4.5. Fuel properties

carbon surface, as shown in Fig. 7a and Eq. (26). The basic principle of a DCFC is the utilisation of carbon as
𝐶𝐶𝐶𝐶𝑎𝑎𝑎𝑎𝑎𝑎 → 𝐶𝐶𝐶𝐶 (26) fuel. Therefore, various materials are available, ranging
The mechanism can react further by adsorbing again an O2- from biomass waste residues70, biochars71, lignite and
ion (see Fig. 7b), which leads to the formation of CO2, as bituminous coal and highly carbonised, pure materials like
described in Eqs. (21), (22), (23) and (24). carbon blacks. Since the different sources influence the
fuels chemical and physical properties, improving or
impairing further utilisation, this chapter gives an
overview.
According to Kinoshita et al.72, a crucial parameter for the
reactivity in the oxidation of a carbon material is the
structure and its surface, depending on the dimensions of
the specific surface area and crystallinity. Amorphous
Figure 7. Mechanism of electrochemical oxidation of carbon materials show a higher reactivity, since the
carbon on the electrolyte surface: (a), first O2- ion number of defects (e.g., edges and steps) on the surface
adsorption and CO formation; (b), second O2- ion act as an active layer. A bigger surface provides more active
adsorption and CO2 formation.
sites, leading to a higher current density in the DCFC.51
4.4. Hybrid technology Another important parameter is the conductivity, since the
carbon material performs as fuel and anode in the same
The concept of a hybrid DCFC merges the electrolyte of a moment. Thus, resulting electrons need to be transferred.
SOFC and MCFC, which result in a binary electrolyte. Due The intrinsic conductivity in carbon particles strongly
to the solid oxide electrolyte, the cathode can be separated depends again on the surface and structure73. However,
from the anode compartment. Additionally, the molten the conductivity is in general improved by a crystalline
carbonate is utilised in the anode compartment. structure and so by a graphite-like structure in the carbon
This concept brings some advantages. Firstly, recirculation particle. In general, different groups can be recognised on
of CO2, which is necessary for an MCFC, is not needed the surface of a carbon particle, which may contain oxygen,
anymore. Secondly, the cathode material is not exposed to hydrogen, nitrogen, sulphur or possibly halogens. These
the carbonate, which normally performs corrosive. Thus, elements can be introduced during the various production
several oxygen reducing materials that are already steps of carbon fuel. Nevertheless, the functional groups
developed for SOFCs are accessible. Thirdly, the contact that can most affect the physicochemical properties on the
between fuel and electrolyte is improved, since carbonate surface are the different complexes of carbon and oxygen,
is a slurry. This result in an enhanced oxidation, especially the oxygen functional group (hereafter ‘OFG’)72. Typical
at lower temperatures67. Nabae et al68 presented two kinds structures on a particle surface are shown in Fig. 9.
of possible reaction schemes, which are illustrated in Fig.
8.
Figure 8. Possible reaction schemes of the direct Fig. 9. OFG on a carbon particle: (a) phenol, (b) carbonyl,
oxidation of solid carbon with (a) sufficient supply of O2-, (c) carboxyl, (d) quinone, and (e) lactone.
(b) less amount of O2- (adapted from68).
As reported in the literature, simple aliphatic groups (C-H)
In general, a supply of O2- ions is needed, which will be increase the distance between polyromantic regions from
dissolved in the carbonate slurry up to a certain different particles74. This result in an increase in the
concentration (0.5 mol. % at 400–700 °C69). This supply is contact resistance. Additionally, it is known that an
provided by the solid oxide electrolyte. Mechanism (a) increase in functional groups with oxygen and sulphur
describes a sufficient supply of O2- into the carbonate, content on the surface decreases the conductivity75.
which works as an oxidiser of carbon to CO2 or even Chemical composition of the fuel also has a notable effect
convert CO2 into CO32-, keeping the concentration low and on the lifetime of a fuel cell. As previously stated, it was
thus the effect of the Boudouard reaction. Mechanism (b) observed that various mineral impurities in the carbon
describes a reduced supply of O2-, which would result in a material lead to deterioration of DCFC performance57, 76.
decomposition of the carbonate. Consequently, CO32- Additionally, Cherepy et al.60 and Gong et al.77 observed a
needs to be regenerated. degradation of cell performance caused by sulphur in the
fuel.
131
Several references reported an enhancement of the (6) Titirici, M.-M.; White, R. J.; Brun, N.; Budarin, V. L.; Su,
performance of a DCFC by volatile matter67,70,78. Especially, D. S.; del Monte, F.; Clark, J. H.; MacLachlan, M. J.
carbon materials, formally hydrochars and biochars from Sustainable Carbon Materials. Chem. Soc. Rev. 2015,
refuse biomass, have a high content of volatile matter that 44 (1), 250–290.
can contribute to the cell performance. (7) Kruse, A.; Funke, A.; Titirici, M.-M. Hydrothermal
Conversion of Biomass to Fuels and Energetic
Materials. Curr. Opin. Chem. Biol. 2013, 17 (3), 515–
5. Conclusion 521.
Direct carbon fuel cell (DCFC) is a promising technology for (8) Funke, A.; Ziegler, F. Hydrothermal Carbonization of
the production of electricity using solid fuels. Its advantage Biomass: A Summary and Discussion of Chemical
is the production of electricity directly from the Mechanisms for Process Engineering. Biofuels,
electrochemical reactions. It results in a much higher Bioprod. Biorefining 2010, 4 (2), 160–177.
electrical efficiency (80%) for direct carbon fuel cells (9) Dinjus, E.; Kruse, A.; Tröger, N. Hydrothermal
compared to coal combustion plants (up to 45%). DCFC can Carbonization - 1. Influence of Lignin in
be divided into three types depending on the electrolyte Lignocelluloses. Chem. Eng. Technol. 2011, 34 (12),
used: solid oxide (ceramic material), molten carbonate 2037–2043.
(Li2CO3, Na2CO3, and K2CO3), and molten hydroxide (LiOH, (10) Titirici, M.-M.; White, R. J.; Falco, C.; Sevilla, M. Black
NaOH, and KOH). Historically, in this type of fuel cell, the Perspectives for a Green Future: Hydrothermal
carbon material was used both as anode and fuel. Carbons for Environment Protection and Energy
However, due to technical problems such as the Storage. Energy Environ. Sci. 2012, 5 (5), 6796.
penetration of impurities from the carbonaceous anode to (11) Sevilla, M.; Maciá-Agulló, J. A.; Fuertes, A. B.
the electrolyte, a separate anode is currently used, and the Hydrothermal Carbonization of Biomass as a Route for
carbon material is used as fuel. To increase the contact the Sequestration of CO 2: Chemical and Structural
between the fuel and the anode, the anode compartment Properties of the Carbonized Products. Biomass
is operating in flow mode (e.g., fluidised bed). For this Bioenergy 2011, 35 (7), 3152–3159.
reason, the fuel should have a fine grain size. Direct carbon (12) Falco, C.; Sevilla, M.; White, R. J.; Rothe, R.; Titirici, M.-
fuel cells operate at high temperatures (600–1000 °C), M. Renewable Nitrogen-Doped Hydrothermal Carbons
where corrosion of cell-forming materials is severe. Derived from Microalgae. ChemSusChem 2012, 5 (9),
Therefore, it is necessary to use fuel with low sulphur and 1834–1840.
ash content. It makes bio-based carbon materials an (13) Berge, N. D.; Ro, K. S.; Mao, J.; Flora, J. R. V; Chappell,
attractive feedstock for direct carbon fuel cells. M. A.; Bae, S. Hydrothermal Carbonization of
Municipal Waste Streams. Environ. Sci. Technol. 2011,
45 (13), 5696–5703.
Acknowledgements (14) Liu, Z.; Quek, A.; Parshetti, G.; Jain, A.; Srinivasan, M.
This project has received funding from the European P.; Hoekman, S. K.; Balasubramanian, R. A Study of
Union’s Horizon 2020 research and innovation programme Nitrogen Conversion and Polycyclic Aromatic
under the Marie Skłodowska-Curie grant agreement No Hydrocarbon (PAH) Emissions during Hydrochar-
721991. Lignite Co-Pyrolysis. Appl. Energy 2013, 108, 74–81.
This work was partially financed by the ERANET-MED (15) Parshetti, G. K.; Liu, Z.; Jain, A.; Srinivasan, M. P.;
initiative (ERANETMED 2-72-251/MEDWASTE). Balasubramanian, R. Hydrothermal Carbonization of
Sewage Sludge for Energy Production with Coal. Fuel
2013, 111, 201–210.
References (16) Liu, Z.; Zhang, F. S.; Wu, J. Characterization and
(1) Paraknowitsch, J. P.; Thomas, A.; Antonietti, M. Carbon Application of Chars Produced from Pinewood
Colloids Prepared by Hydrothermal Carbonization as Pyrolysis and Hydrothermal Treatment. Fuel 2010, 89
Efficient Fuel for Indirect Carbon Fuel Cells. Chem. (2), 510–514.
Mater. 2009, 21 (7), 1170–1172. (17) Kang, S.; Li, X.; Fan, J.; Chang, J. Characterization of
(2) Cao, D.; Sun, Y.; Wang, G. Direct Carbon Fuel Cell: Hydrochars Produced by Hydrothermal Carbonization
Fundamentals and Recent Developments. J. Power of Lignin, Cellulose, d -Xylose, and Wood Meal. Ind.
Sources 2007, 167 (2), 250–257. Eng. Chem. Res. 2012, 51 (26), 9023–9031.
(3) WeiZi, C.; Qian, Z.; YongMin, X.; Jiang, L.; GuoHui, L.; (18) Sevilla, M.; Fuertes, A. B. The Production of Carbon
Shuang, C.; MeiLin, L. A Direct Carbon Solid Oxide Fuel Materials by Hydrothermal Carbonization of Cellulose.
Cell Operated on a Plant Derived Biofuel with Natural Carbon 2009, 47 (9), 2281–2289.
Catalyst. Appl. Energy 2016, 179, 1232–1241. (19) Kruse, A.; Koch, F.; Stelzl, K.; Wüst, D.; Zeller, M. Fate
(4) Bergius, F. Die Anwendung Hoher Drucke Bei of Nitrogen during Hydrothermal Carbonization.
Chemischen Vorgängen Und Eine Nachbildung Des Energy Fuels 2016, 30 (10), 8037–8042.
Entstehungsprozesses Der Steinkohle. W. Knapp 1913.
(5) Ruyter, P. Coalification Model *. Fuel 1982, 61 (12),
1182–1187.
132
(20) Kruse, A.; Kirchherr, M.; Gaag, S.; Zevaco, T. A. (34) William W. Jacques. Method of Converting Potential
Hydrothermale Karbonisierung. 4. Thermische Energy of Carbon into Electrical Energy. US patent
Eigenschaften Der Produkte. Chemie Ing. Tech. 2015, 555511A, 1896.
87 (12), 1707–1712. (35) Jiang, C.; Ma, J.; Corre, G.; Jain, S. L.; Irvine, J. T. S.
(21) Sevilla, M.; Fuertes, A. B. The Production of Carbon Challenges in Developing Direct Carbon Fuel Cells.
Materials by Hydrothermal Carbonization of Cellulose. Chem. Soc. Rev. 2017, 46 (10), 2889–2912.
Carbon 2009, 47 (9), 2281–2289. (36) Cooper, J. F. Direct Conversion of Coal and Coal-
(22) Chuntanapum, A.; Matsumura, Y. Formation of Tarry Derived Carbon in Fuel Cells, in: Second International
Material from 5-HMF in Subcritical and Supercritical Conference on Fuel Cell Science, Engineering and
Water. Ind. Eng. Chem. Res. 2009, 48 (22), 9837–9846. Technology, ASME, Rochester, NY, June 14-16. In 2nd
(23) Brown, A. B.; McKeogh, B. J.; Tompsett, G. A.; Lewis, R.; International Conference on Fuel Cell Science,
Deskins, N. A.; Timko, M. T. Structural Analysis of Engineering and Technology; ASME, 2004; pp 375–385.
Hydrothermal Char and Its Models by Density (37) Lee, A. C.; Mitchell, R. E.; Gür, T. M. Thermodynamic
Functional Theory Simulation of Vibrational Analysis of Gasification-Driven Direct Carbon Fuel
Spectroscopy. Carbon 2017, 125, 614–629. Cells. J. Power Sources 2009, 194 (2), 774–785.
(24) Peterson, A. A.; Vogel, F.; Lachance, R. P.; Fröling, M.; (38) Gür, T. M. Critical Review of Carbon Conversion in
Antal, M. J.; Tester, J. W. Thermochemical Biofuel “Carbon Fuel Cells.” Chem. Rev. 2013, 113, 6179–6206.
Production in Hydrothermal Media: A Review of Sub- (39) Werhahn M.; Schneider O.; Stimming U. Direct Carbon
and Supercritical Water Technologies. Energy Environ. Fuel Cell Global Energy Trends Motivation, (2012).
Sci. 2008, pp 32–65. https://mediatum.ub.tum.de/doc/1110007/file.pdf
(25) Bobleter, O. Hydrothermal Degradation of Polymers (accessed December 12, 2018).
Derived from Plants. Progress in Polymer Science. (40) Carrette, L.; Friedrich, K. A.; Stimming, U. Fuel Cells -
1994, pp 797–841. Fundamentals and Applications. Fuel Cells 2001, 1 (1),
(26) Möller , M.; Nilges , P.; Harnisch, F.; Schröder, U. 5–39.
Subcritical Water as Reaction Environment: (41) Atkins, P. W.; Paula, J. de. Physical Chemistry; John
Fundamentals of Hydrothermal Biomass Wiley & Sons, 2006.
Transformation. ChemSusChem 2011, 4 (5), 566–579. (42) Kurzweil, P. Brennstoffzellentechnik; Springer, 2016.
(27) Jung, D.; Zimmermann, M.; Kruse, A. Hydrothermal (43) Alexander, B. R.; Mitchell, R. E.; Gür, T. M. Oxy-
Carbonization of Fructose: Growth Mechanism and Combustion of Solid Fuels in a Carbon Fuel Cell. Proc.
Kinetic Model. ACS Sustain. Chem. Eng. 2018, Combust. Inst. 2013, 34 (2), 3445–3452.
acssuschemeng.8b02118. (44) Hemmes, K.; Dijkema, G. P. J.; van der Kooi, H. J. From
(28) Weingarten, R.; Conner, W. C.; Huber, G. W. Chemical Processes to Electrochemical Processes: The
Production of Levulinic Acid from Cellulose by Key to Minimal Entropy Production. Russ. J.
Hydrothermal Decomposition Combined with Electrochem. 2004, 40 (11), 1100–1104.
Aqueous Phase Dehydration with a Solid Acid Catalyst. (45) Ihsan Barin. Thermochemical Data of Pure Substances;
Energy Environ. Sci. 2012, 5 (6), 7559–7574. 1997; Vol. 55.
(29) Körner, P.; Jung, D.; Kruse, A. The Effect of Different (46) Zecevic, S.; Patton, E. M.; Parhami, P. Carbon–air Fuel
Brønsted Acids on the Hydrothermal Conversion of Cell without a Reforming Process. Carbon N. Y. 2004,
Fructose to HMF. Green Chem. 2018, 20, 2231–2241 42 (10), 1983–1993.
(30) Reza, M. T.; Rottler, E.; Herklotz, L.; Wirth, B. (47) Guo, L.; Calo, J. M.; DiCocco, E.; Bain, E. J. Development
Hydrothermal Carbonization (HTC) of Wheat Straw: of a Low Temperature, Molten Hydroxide Direct
Influence of Feedwater PH Prepared by Acetic Acid and Carbon Fuel Cell. Energy Fuels 2013, 27 (3), 1712–
Potassium Hydroxide. Bioresour. Technol. 2015, 182, 1719.
336–344. (48) Vutetakis, D. G.; Skidmore, D. R.; Byker, H. J.
(31) Libra, J. A.; Ro, K. S.; Kammann, C.; Funke, A.; Berge, N. Electrochemical Oxidation of Molten Carbonate-Coal
D.; Neubauer, Y.; Titirici, M.-M.; Fühner, C.; Bens, O.; Slurries. J. Electrochem. Soc. 1987, 134 (12), 3027.
Kern, J.; et al. Hydrothermal Carbonization of Biomass (49) Wu, W.; Zhang, Y.; Ding, D.; He, T. A High-Performing
Residuals: A Comparative Review of the Chemistry, Direct Carbon Fuel Cell with a 3D Architectured Anode
Processes and Applications of Wet and Dry Pyrolysis. Operated Below 600 °C. Adv. Mater. 2018, 30 (4),
Biofuels 2011, 2 (1), 71–106. 1704745.
(32) Sevilla, M.; Fuertes, A. B.; Mokaya, R. High Density (50) Jain, S. L.; Nabae, Y.; Lakeman, B. J.; Pointon, K. D.;
Hydrogen Storage in Superactivated Carbons from Irvine, J. T. S. Solid State Electrochemistry of Direct
Hydrothermally Carbonized Renewable Organic Carbon/Air Fuel Cells. Fuel Cells Bull. 2008, 11 (10), 10–
Materials. Energy Environ. Sci. 2011, 4 (4), 1400. 13.
(33) Hu, B.; Yu, S. H.; Wang, K.; Liu, L.; Xu, X. W. Functional (51) Nürnberger, S.; Bußar, R.; Desclaux, P.; Franke, B.;
Carbonaceous Materials from Hydrothermal Rzepka, M.; Stimming, U. Direct Carbon Conversion in
Carbonization of Biomass: An Effective Chemical a SOFC-System with a Non-Porous Anode. Energy
Process. Dalt. Trans. 2008, 40, 5414–5423. Environ. Sci. 2010, 3 (1), 150–153.
133
(52) Elleuch, A.; Boussetta, A.; Yu, J.; Halouani, K.; Li, Y. (65) Hund, F. Anomale Mischkristalle Im System
Experimental Investigation of Direct Carbon Fuel Cell ZrO2Y2O3 Kristallbau Der Nernst‐Stifte. Zeitschrift für
Fueled by Almond Shell Biochar: Part I. Physico- Elektrochemie und Angew. Phys. Chemie 1951, 55 (5),
Chemical Characterization of the Biochar Fuel and Cell 363–366.
Performance Examination. Int. J. Hydrogen Energy (66) Li, C.; Shi, Y.; Cai, N. Mechanism for Carbon Direct
2013, 38 (36), 16590–16604. Electrochemical Reactions in a Solid Oxide Electrolyte
(53) Hemmes, K.; Cassir, M. A Theoretical Study of the Direct Carbon Fuel Cell. J. Power Sources 2011, 196 (2),
Carbon/Carbonate/Hydroxide (Electro-) Chemical 754–763.
System in a Direct Carbon Fuel Cell. J. Fuel Cell Sci. (67) Kaklidis, N.; Kyriakou, V.; Garagounis, I.; Arenillas, A.;
Technol. 2011, 8 (5), 051005. Menéndez, J. A.; Marnellos, G. E.; Konsolakis, M. Effect
(54) Pesavento P. V. Carbon-air fuel cell, US6200697B1, of Carbon Type on the Performance of a Direct or
1999. Hybrid Carbon Solid Oxide Fuel Cell. RSC Adv. 2014, 4
https://patents.google.com/patent/US6200697B1/en (36), 18792–18800.
(55) Eberz, A.; Franck, E. U. High Pressure Electrolyte (68) Nabae, Y.; Pointon, K. D.; Irvine, J. T. S. Electrochemical
Conductivity of the Homogeneous, Fluid Water- Oxidation of Solid Carbon in Hybrid DCFC with Solid
Sodium Hydroxide System to 400°C and 3000 Bar. Oxide and Molten Carbonate Binary Electrolyte.
Berichte der Bunsengesellschaft für Phys. Chemie 1995, Energy Environ. Sci. 2008, 1 (1), 148–155.
99 (9), 1091–1103. (69) White, S. H.; Twardoch, U. M. The Solubility and
(56) Hérold, C.; Hérold, A.; Lagrange, P. Ternary Graphite Electrochemistry of Alkali Metal Oxides in the Molten
Intercalation Compounds Associating an Alkali Metal Eutectic Mixture of Lithium Carbonate-Sodium
and an Electronegative Element or Radical. Solid State Carbonate-Potassium Carbonate. J. Appl. Electrochem.
Sci. 2004, 6 (1), 125–138. 1989, 19 (6), 901–910.
(57) Zecevic, S.; Patton, E. M.; Parhami, P. Direct Carbon (70) Jang, H.; Ocon, J. D.; Lee, S.; Lee, J. K.; Lee, J. Direct
Fuel Cell With Hydroxide Electrolyte: Cell Performance Power Generation from Waste Coffee Grounds in a
During Initial Stage of a Long Term Operation. In 3rd Biomass Fuel Cell. J. Power Sources 2015, 296, 433–
International Conference on Fuel Cell Science, 439.
Engineering and Technology; ASME, 2005; Vol. 2005, (71) Ahn, S. Y.; Eom, S. Y.; Rhie, Y. H.; Sung, Y. M.; Moon, C.
pp 507–514. E.; Choi, G. M.; Kim, D. J. Utilization of Wood Biomass
(58) Tanimoto, K.; Yanagida, M.; Kojima, T.; Tamiya, Y.; Char in a Direct Carbon Fuel Cell (DCFC) System. Appl.
Matsumoto, H.; Miyazaki, Y. Long-Term Operation of Energy 2013, 105, 207–216.
Small-Sized Single Molten Carbonate Fuel Cells. J. (72) Kinoshita K. Carbon: Electrochemical and
Power Sources 1998, 72 (1), 77–82. physicochemical properties, Wiley, 1988.
(59) Morita, H.; Kawase, M.; Mugikura, Y.; Asano, K. (73) Donnet, J.-B.; Bansal, R. C.; Wang, M.-J. Carbon Black :
Degradation Mechanism of Molten Carbonate Fuel Cell Science and Technology; Dekker, 1993.
(74) Pantea, D.; Darmstadt, H.; Kaliaguine, S.; Sümmchen,
Based on Long-Term Performance: Long-Term
L.; Roy, C. Electrical Conductivity of Thermal Carbon
Operation by Using Bench-Scale Cell and Post-Test
Blacks: Influence of Surface Chemistry. Carbon 2001,
Analysis of the Cell. J. Power Sources 2010, 195 (20), 39 (8), 1147–1158.
6988–6996. (75) Verhelst, W. F.; Wolthuis, K. G.; Voet, A.; Ehrburger, P.;
(60) Cherepy, N. J.; Krueger, R.; Fiet, K. J.; Jankowski, A. F.; Donnet, J. B. The Role of Morphology and Structure of
Cooper, J. F. Direct Conversion of Carbon Fuels in a Carbon Blacks in the Electrical Conductance of
Molten Carbonate Fuel Cell. J. Electrochem. Soc. 2005, Vulcanizates. Rubber Chem. Technol. 1977, 50 (4),
152 (1), A80. 735–746.
(61) Li, X.; Zhu, Z. H.; Marco, R. De; Dicks, A.; Bradley, J.; Liu, (76) Cooper, J. F. Direct Conversion of Coal Derived Carbon
S.; Lu, G. Q. Factors That Determine the Performance in Fuel Cells BT - Recent Trends in Fuel Cell Science and
of Carbon Fuels in the Direct Carbon Fuel Cell. Ind. Eng. Technology, In: S. Basu (Ed.), Springer New York, New
York, NY, 2007, pp. 248–266.
Chem. Res. 2008, 47 (23), 9670–9677.
(77) Gong, M.; Liu, X.; Trembly, J.; Johnson, C. Sulfur-
(62) Cooper, J. F.; Selman, R. Electrochemical Oxidation of
Tolerant Anode Materials for Solid Oxide Fuel Cell
Carbon for Electric Power Generation: A Review. ECS Application. J. Power Sources 2007, 168 (2), 289–298.
Trans. 2009, 19 (14), 15–25. (78) Ahn, S. Y.; Eom, S. Y.; Rhie, Y. H.; Sung, Y. M.; Moon, C.
(63) Lee, A. C.; Li, S.; Mitchell, R. E.; Gür, T. M. Conversion E.; Choi, G. M.; Kim, D. J. Application of Refuse Fuels in
of Solid Carbonaceous Fuels in a Fluidized Bed Fuel a Direct Carbon Fuel Cell System. Energy 2013, 51,
Cell. Electrochem. Solid State Lett. 2008, 11 (2), B20. 447–456.
(64) Ju, H.; Uhm, S.; Kim, J. W.; Song, R.-H.; Choi, H.; Lee, S.- (79) Kupiainen, L.; Ahola, J.; Tanskanen, J. Kinetics of Formic
H.; Lee, J. Enhanced Anode Interface for Acid-Catalyzed Cellulose Hydrolysis. BioResources
Electrochemical Oxidation of Solid Fuel in Direct 2014, 9 (2), 2645–2658.
Carbon Fuel Cells: The Role of Liquid Sn in Mixed State. (80) Penchini, D.; Cinti, G.; Discepoli, G.; Sisani, E.; Desideri,
U. Characterization of a 100 W SOFC Stack Fed by
J. Power Sources 2012, 198, 36–41.
Carbon Monoxide Rich Fuels. Int. J. Hydrogen Energy
2013, 38 (1), 525–531.
134
An overview of the methods used in the assessment of biochar carbon stability

R.M.D. Chathurika, Frederik Ronsse
Department of Green Chemistry and Technology, Ghent University, Coupure links 653, 9000 Gent, Belgium
Abstract
Soil application of biochar has become popular worldwide due to its number of benefits as a soil amendment. High porosity, improving
soil moisture retention, higher cation exchange capacity, liming effect, ability to bind organic pollutants and heavy metals, supplying
habitats for microorganisms, high stability in soil due to high aromaticity, priming effect and carbon sequestration potential, reducing
greenhouse gas emissions contribute to act as a potential soil amendment. However, all these mentioned beneficial outcomes depend
on the quality of biochar. Biochar quality and the stability mainly depend on the feedstock type and biochar production conditions. The
stability of biochar is fundamental to the knowledge of determining the residence time of biochar derived carbon within soil, further
contributing to the mitigation of climate change and additional benefits to the environment. Biochar stability test methods are the key
to understand the long term behaviour of biochar after its application to the soil. Those methods vary from simple to most sophisticated
methods based on the simplicity, cost effectiveness and the technology involved during the analysis. For the better assessment of
biochar stability, calibrating simple and cost effective methods from most sophisticated ones provides a great approach.
approach for that. Application of biochar in to soil enhances

1. Importance of biochar as a soil amendment water holding capacity. It facilitates the moisture retention in
the soils in arid climates. High porosity improves aeration and
“Biochar is the porous, carbonaceous material produced by
moisture retention thus increase the plant shoot and root
thermochemical treatment of organic materials in an oxygen-
growth in soil. Also, biochar has high surface area which creates
limited environment.”1,2 Pyrolysis of biomass has been used to
a better cation exchange capacity in nutrient low unfertile soils.
produce biochar for several thousand years in history. During
Also, it helps to reduce the leaching losses of ions by binding
the charring process, biomass is subjected to several
with active sites. Indirectly, this phenomenon could help to
physicochemical breakdowns to produce a charred by-product.
reduce the eutrophication in surface waterbodies. As a carbon
This solid by-product has diverse applications due to its special
rich source, biochar contains a lot of aromatic compounds
beneficial characteristics. Biochar plays an important role in the
which are resistant to weathering process. Therefore, it lowers
global carbon budget and carbon cycle due to its potential to
the mineralisation of soil organic matter and increases the soil
act as a significant sink for atmospheric carbon dioxide3. In
organic carbon content.5,6
order to evaluate the influence of biochar on the global carbon
The use of manure and compost directly as fertilisers may
cycle, a clear understanding about the stability of biochar is a
create problems such as the scarcity of them in long-term use,
necessity4. As a stable compound, biochar supplies several
serious ground water and stream nutrient pollution,
services to eco systems.
containment of pathogens and heavy metals, and can increase
The main uses of biochar can be noted as soil improvers, a
the emission of greenhouse gases. But, the production and
waste management strategy, as an energy source, gas
application as biochar could be a better way of disposing
absorbing advanced carbon material, microbial carriers in
agricultural and industrial wastes, livestock manure, etc. with
composting, and to help in rehabilitation of degraded unfertile
less greenhouse gas emission while reducing pollution and
soils and mitigating climate change. Biochar can enhance plant
rehabilitating degraded land and bringing poor soils into
growth and increase crop yield by improving soil physical,
production7. Previous studies reported that the addition of
chemical and biological properties due to its high porous nature
biochar to soils resulted, on average, in increased above ground
and aromatic carbon compounds. Also, biochar acts on carbon
productivity, crop yield, soil microbial biomass, rhizobia
sequestration in soil to mitigate the greenhouse gas emissions
nodulation, plant K tissue concentration, soil phosphorus (P),
and it has the ability to retain pollutants and reduce pollution
soil potassium (K), total soil nitrogen (N), and total soil carbon
due to agrochemicals. Moreover, biochar has good nutrient
(C) compared with the controlled conditions8–10. Soil pH also
retention capacity, therefore improves the soil fertility and
tended to increase, becoming less acidic, following the addition
decrease the demand for mineral fertilisers. Biochar inherently
of biochar. Also, higher nutrient retention and nutrient
has alkaline pH, which facilitates the ameliorating of soil acidity.
availability were observed after charcoal addition to soil,
Due to high porosity of biochar, it will improve soil aeration and
related to higher cation exchange capacity, surface area and
supply habitats for microbial growth. Biochar provides a unique
direct nutrient additions4. Soil water retention, saturated
opportunity to improve soil fertility and nutrient-use efficiency
hydraulic conductivity and nutrient availability increased with
of locally available, inexpensive and renewable materials for a
the application of biochar11. Rather than applying easily
sustainable soil fertility management with the minimum use of
expensive fertiliser additions. mineralisable organic material into soil, application of biochar
Reduction of soil organic carbon is a major constraint that limits as a recalcitrant material provides above advantages (as shown
the productivity of agricultural lands. In order to increase the in Fig. 1). The addition of straw to soil resulted in high CO2
productivity of a land, the chemical, physical and biological emissions, indicating lower soil carbon sequestration potential
resilience should increase. Biochar is identified as a feasible compared with the straw biochar amended soils. Furthermore,
biochar amendment showed increasing effects on some of the

135
Chapter 11 Chathurika and Ronsse
investigated soil parameters, such as CEC and soil pH. Soil production method, and this has consequences towards how
respiration was lower in biochar amended soils than in residue- biochar behaves once added to soil.15,16 MRT of biochar varies
amended soils12,13. Biochar has been used to rehabilitate acidic with factors and processes as summarised in Table 1.
and unfertile soils to ameliorate soil acidity and improve the
nutrient retention capacity and soil physiochemical properties Table 1. Mean Residence Time (MRT) of biochar reported in
such as soil structure and soil aggregation. Also, biochar is used different studies.
to remediate soils contaminated with poly aromatic
Source Estimated MRT (Years)
hydrocarbons and heavy metals where its’ special binding
Rosa et al. (2018)17 Decadal
ability will reduce the bio availability of certain compounds.
Hamer et al.
Decadal
(2004)18
Decadal to Centennial (44 to 610
Fang et al. (2014)19
years)
Murray et al.
Decadal to Centennial
(2015)20
Santos et al.
centennial (390 to 600 years)
(2012)21
Centennial to millennial (244 to
Peng et al. (2011)22
1700 years)
Figure 1. Decomposition of biochar and un-charred organic Singh et al. Centennial to millennial ( 90 to
matter in soil.14 (2012)23 1600 years)
Fang et al. (2015)24 Centennial to millennial
Highly aromatic nature of biochar facilitates reduction of the
Kuzyakov et al.
soil organic carbon mineralisation. Soil is the largest terrestrial Millennial (2000 years)
(2009)25
carbon source and it releases a significant amount of carbon
dioxide through explosion of soil organic matter by intensive Liard et al. (2008)26 Millennial (2000 years)
agriculture related activities and excavation of peat lands. Schmidt et al.
Millennial (1160 – 5040 years)
Application of biochar into soil as a carbon negative approach (2002)27
will reduce the emissions of greenhouse gases from the soil. Cheng et al.
Millennial (1000 years)
Also, biochar is used to mitigate the emissions during (2006)28
composting and used as a scrubber to adsorb syngas. Due to Cheng et al.
Millennial (1335 years)
higher number of promising benefits after applying biochar into (2008)29
the soil, application of biochar is identified as a globally
recognised approach to improve soil fertility and rehabilitation 2.1. Effects of feedstock type and pyrolysis conditions
of other soil degradation related issues and mitigating climate on biochar stability in soil
change. Properties of biochar play an important role when it is used as
a soil amendment. They basically depend on the feedstock type
and the pyrolysis conditions used for biochar production.
2. Factors affecting the biochar stability in
Different substrates like agricultural wastes, manure, food
soil
wastes and urban solid wastes can be used as feedstock for
The use of biochar varies with its characteristics and has a biochar production. Depending on the chemical composition of
diverse range of applications. Stability of biochar is important in feedstock, the chemical composition and the stability of the
supplying the above mentioned advantages over long-term. resulted biochar could be varying. Especially, the moisture
The stability of biochar is fundamental to the knowledge of content, ash content and inorganic matter content could be
determining the residence time of biochar derived carbon varying according to the feedstock material. Even for the same
within soil, further contributing to the mitigation of climate type of feedstock, the composition could be varying with the
change and additional benefits to the environment15. After maturity of the plant or the particular component used for the
addition of biochar into soil, its stability mainly depends on its biochar production (e.g., leaves, stems, etc.) and with the pre-
decomposition rate in soil. Biochar decomposition in soil and its process conditions30. This is mainly determined by the carbon
effect on soil organic carbon mineralisation depends on various content of the biochar, which could be divided into two pools:
factors under different environmental conditions. as recalcitrant (no-labile) pool and labile (easily accessible)
Characteristics of biochar itself also effects on the stability in pool. According to the recent review done by Wang et al., MRT
soil. Different studies conducted under various environmental of the labile and recalcitrant biochar C pools were estimated to
conditions have resulted different Mean Residence Times be about 108 days and 556 years with pool sizes of 3% and 97%,
(MRT) for the same biochar. There are several factors and respectively31. Also, depending on the ash content of the
processes which affect the biochar stability in soil such as biochar, their MRT can vary20. On the other hand, the oxidation
properties of biochar, abiotic and biotic environmental factors, resistance ability of a biochar increases with the increase in
abiotic and biotic processes and indirect processes. Biochar and aromatic C and endogenous mineral content32. Pyrolysis
its properties can be significantly altered according to the temperature heavily affects the stability of biochar and its
136
carbon sequestration potential in different soils33. The biochar– Labile carbon content of biochar is very low and due to high
mineral interactions that affect the stability of biochar in soil recalcitrant carbon content, this microbial-mediated carbon
vary with the biochar production temperature, since it has been mineralisation get lower with the time41,42. However, this
proved that the wood biochar produced at high temperature is carbon assimilation in soil microorganisms is also important
more stable than low temperature biochar in different types of because it is another way of soil carbon sequestration potential
soils and different incubation temperatures34. With the of biochar25. The available substrates in soil determine the
increase in temperature, main oxygen-containing functional activity of the microbes present. Biochar has extremely low
groups decrease gradually while the degree of aromatisation availability for microbial consumption43. So biochar will
increases. Due to these high aromatic compounds, biochar decompose mainly through co-metabolism and it has a very low
produced in higher pyrolysis temperatures showed higher significance as a carbon source for microorganisms. The
stability. With the increasing pyrolysis temperature, the assimilation of biochar carbon in microorganisms varied
amount of oxidised carbon with respect to the total carbon of between 1.5% and 2.6% of total application.25
the biochar decreases. As a result of that, the H:C and O:C molar Abiotic factors determine the decomposition rate of the
ratios will decrease.35 microbial decomposition. When a soil receives an input,
The yield of biochar decreased considerably with pyrolysis bacteria are the first group to trap it and metabolise. Fungi,
temperature and the yield of stable carbon showed only minor which can withstand low nutrient contents, get involved in the
dependence on the pyrolysis temperature, despite the increase decomposition with the rest of poorly available substrates for
in concentration of the stable carbon in the biochar produced bacteria44–46. In addition, the compounds released by
with increasing temperature. The non-stable fraction of biochar microorganisms affect the biochar degradation. The plant rhizo
consists predominantly of semi-labile carbon with the labile depositions also facilitate microorganism growth (increase their
carbon presenting only in a minor fraction. Despite its low yield, growth) and those microorganisms and soil fauna facilitate to
the labile fraction can play an important role in application of the overall process.47
biochar, as it is important in soil processes36. Most of the Abiotic and biotic processes are also important in initial
inherent properties of the biochar are determined by the oxidation of fresh biochar. Due to these processes, rapid
feedstock type and the pyrolysis conditions of biochar oxidation of biochar happens. This may facilitate further
production. But, as a soil amendment, its’ stability and other decomposition by microorganisms. Therefore, this aspect has a
soil interactions depends on the soil environmental conditions. significant effect on biochar stability. As a result of the biochar
2.2. Effect of abiotic and biotic environmental factors decomposition, soil fertility will improve by increasing surface
and processes on biochar stability in soil cation retention28,48. Scanning electronic microscopy (SEM)
images of biochar particles aged with soil showed colonisation
With the growing interest of biochar as a soil amendment, the by microbes and widespread organic matter coatings. Thus,
behaviour of biochar in different soil environments took a sorption of both microbially produced organic matter and soil
special attention in the biochar research community. Due to the organic matter are likely processes that enhance biochar
slow decomposition rate in soil, biochar has been identified as aging49. Due to highly aromatic nature of the biochar, it is
having a carbon sequestration potential. There are numerous resistant to microbial degradation. However, it is added to the
abiotic and biotic soil factors and processes that affect the matrix, which is more susceptible to the microbial degradation.
stability of biochar in soil. Temperature of the soil environment With the surface mineralisation, biochar carbon is exposed to
is crucial for the microbial activity in soil. Also, chemically the environment. Physical mechanisms such as abrasion,
mediated oxidations could be accelerated through the increase erosion due to rain splash and trampling will accelerate this
in temperature. According to several studies conducted with process. After mineralisation of the labile carbon content in the
elevated soil incubation temperatures (> 30 °C), MRT of biochar biochar, the rest of the recalcitrant carbon content exists in the
was decreased with higher incubation temperatures37,38. environment with different lifespans, which may vary from few
Moreover, high temperatures in many tropical and subtropical decades to millennia.38
regions occur in the same months as the highest precipitations,
providing ideal conditions for microbial degradation of 2.3. Effect of soil properties on biochar stability in soil
pyrogenic carbon on the soil surface38. Global warming and The stability of biochar in soils may not be solely attributable to
tropical climates may lower the C sequestration potential of its chemical characteristics, but also to its reduced accessibility
biochar, by reducing its capacity to decline the mineralisation of when it is involved in organo-mineral associations. Biochar can
labile organic matter carbon content, while increasing the be stabilised through chemical interactions with soil minerals
mineralisation of native soil organic carbon.39 and subsequent physical occlusions in organo-mineral
Biochar decomposition in soil is mainly a biotic mediated fractions, thereby limiting its spatial accessibility to soil
process, despite that several abiotic processes can microorganisms and their enzymes.42 Both the biochar-mineral
simultaneously occur. Due to the biotic oxidation and interactions and the intrinsic chemical recalcitrance of biochar
decomposition formation of oxygen-containing functional are important in determining the long-term C sequestration
groups, loss of aryl carbon content in biochar can happen40. potential of biochar in soils19. Soils containing high minerals
Since biochar has a high porosity, it can increase water holding seem to favour long-term stability of biochar due to the
capacity in soil and alter soil pH. It will create favourable enhanced oxidation resistance. Clayey soils, which are rich in
conditions for inhabitant microbes of biochar. Also, the labile minerals, such as kaolinite, Fe- and Al- oxides, could be a
carbon content in biochar provides food for the microbes. beneficial environment for biochar in terms of long-term
137
carbon sequestration50. The combined spectroscopy- biochar stability. Accordingly, Gamma methods can be used to
microscopy approach revealed the accumulation of aromatic-C calibrate the Alpha and Beta methods. The following
in discrete spots in the solid-phase of micro aggregates and its paragraphs give the short description about major biochar
co-localisation with clay minerals in soils amended with stability testing methods/indicators used in different studies.
biochar.13 Enhanced oxidation resistance of biochar surface was 3.1. Elemental ratios
likely due to the physical isolation from newly formed minerals,
while organometallic complex formation was probably Hydrogen to organic carbon molar ratio (H:Corg) and oxygen to
responsible for the increase in oxidation resistance of entire carbon molar ratio (O:C) are identified as the mostly used
biochar particles. As a result, mineral-rich soils seemed to be a element basis biochar stability indicators. Especially, organic
beneficial environment for biochar, since soil minerals could carbon content (Corg) is used to assign the biochar material to a
increase biochar stability, which displays an important class that is dependent on the percentage of Corg in the material.
environmental significance of biochar for long-term carbon Carbon stability is indicated by the molar ratio of hydrogen to
sequestration50. Soil mineral attachment may occur directly on organic carbon. Lower values of this ratio are correlated with
the biochar surface because of the formation of carboxylic and greater carbon stability54. Elemental composition of the biochar
phenolic functional groups on the aged biochar surface by is assessed through elemental analysis. At present, the dry
oxidation reactions. For biochar, adsorption of organic matter combustion method is used for the CHNS total elemental
from soil facilitate the interactions between biochar and analysis. According to the literature, in most of the studies
minerals in the soil. Calcium is believed to be important in this these elemental ratios reported a good correlation with
process.51 pyrolysis conditions, since indicating a good prediction of
Moisture content of the soil has an important role in the biochar stability1. Especially, the O:C ratio depicted a very good
degradation of biochar stability in soil. With the altered correlation with biochar production temperature and the
conditions of saturated and unsaturated moisture, it will volatile matter content, biochar carbon loss and volatile
increase biochar carbon mineralisation compared to the matter/fixed-carbon ratio in the biochar and other elemental
saturated conditions, probably due to the increased carboxylic ratio like H:Corg 1,55–57 (see Fig. 2). In addition, O:C ratio showed
and hydroxide functional groups while decreasing aliphatic a good correlation with the stable carbon content measured
groups. Carbon loss through this process strongly correlated through the Edinburgh stability tool2. Lower O:C molar ratios
with changes in O:C values of biochar indicating that oxidation correlated with longer predicted biochar half-life. O:C ratios are
of biochar was most likely the main mechanism to control its typically higher near the surface than at the interior of the black
stability52. In contrast, an increase in greenhouse gas emissions C particles58. Therefore, the O:C ratio could be used as an
(CH4 and CO2 efflux) was recorded in moist soils compared to indicator of black C oxidation. Depending on the O:C ratio,
the dry soils. Authors argued that this may be due to an additive biochar could be categorised into three stability classes, as
effect, due to the decomposition of biochar or an interactive reported in Table 2.
synergistic effect originating both from the decomposition of
the biochar and priming effects stimulating the decomposition
of the soil organic matter.53
3. Methods to determine biochar carbon

stability
Biochar stability test methods are the key to understand the
long term behaviour of biochar after its applications to the soil.
Several methods have been developed to measure it. In most
of the test methods, elemental composition, ash content,
volatile matter content, organic and inorganic carbon content,
carbon functional groups and aromatic moieties were assessed.
Those test methods can be categorised into three groups based
Figure 2. Van Krevelen plot for rice straw-derived biochar
on the feasibility, knowledge, technology and cost effectiveness produced at different temperatures.57
involved during the testing procedure15. According to the
guidelines of the International Biochar Initiative, simple and Table 2. Predicted half-life based on the Biochar O:C ratio.1
reliable measures of the relative stability of carbon in biochar
that are readily available at a low cost are categorised as alpha O:C molar Predicted half
methods. Some of alpha methods are the hydrogen to organic ratio life
carbon molar ratio (H:Corg), oxygen to carbon molar ratio (O:C) < 0.2 > 1000 years
and volatile matter content. Methods focused on directly 0.2–0.6 100–1000 years
quantify the biochar loss over a period of time are grouped as > 0.6 over 100 years
beta methods15. Laboratory and field-based incubations as well
as field chronosequence measurements are examples of
currently used Beta methods. A third category is the gamma
methods, which measure the biomolecules regarding to the
138
As an element based ratio, hydrogen to organic carbon molar methods improve the applicability of them as biochar stability
ratio (H:Corg) also has an important role. It also has a very strong measurements to depict the actual biochar stability. Chemical
correlation with biochar production temperature and other oxidation methods can supply low cost determinations on
elemental ratios like O:C ratio and proximate analysis results, biochar stability while correlating good with higher and
such as volatile matter content/fixed carbon mass fraction advanced biochar stability measures. Nonetheless, these
ratio. H/Corg was selected as the preferred alpha method for methods should be validated with different types of biochar
being cost effective and simple measure to predict the biochar produced with different type of feedstock and biochar
degradation in soil15. However, the O:C molar ratio showed a production conditions.
weak correlation (r = 0.73) with biochar stability determined by 3.3. Fixed-carbon, volatile matter and ash contents of
Edinburgh stability tool58. As the upper limit of H/Corg, 0.7 is biochar
used to distinguish biochars from biomass that does not have
the fused aromatic structure that is the source of C stability in Fixed-carbon content, volatile matter content and ash content
biochar materials.54 are the results of proximate analysis of biochar. As thermal
Most often, C and H contents of the biochar are directly decomposition methods, manual proximate analysis and
analysed in the lab and the O content is determined by thermogravimetric analysis decompose thermally labile
difference. During the analysis of elemental composition, component of the biochar, which is called as “volatile matter”.
precautions should be taken to prevent any over or lower This is done by heating the biochar sample under an inert
estimation of elements. By increasing intensity of oxidative atmosphere to avoid combustion, while the ash content is
treatment, it is possible to eliminate more carbon, resulting in determined by heating in an oxidative atmosphere59. The most
an increased surface O:C and H:Corg ratios55. Since some biochar resistant part, which is neither ash nor volatile, is called as fixed-
contain high carbon content, the sample amount needed to carbon content. As shown in Fig. 3, fixed-carbon content of the
measure should be very small and that small sample weight biochar showed a good correlation with the baseline
should be measured with high accuracy. attenuated total reflectance–Fourier transmission Infrared
Furthermore, as a measure of stability, organic carbon content (ATR-FTIR) absorbance (4393 cm−1), which gives the
of the biochar is mostly important than its total carbon content. quantitative benchmarks for estimating the degree of
Therefore, the pre-treatments to remove inorganic carbon carbonisation of biochar.61 In most of the studies, proximate
content of the biochar are essential and the method to remove analysis of biochar is used as a biochar characterisation method
inorganic carbon in biochar should not do excessive removal of to predict biochar stability. However, practical drawbacks can
carbon in biochar. Biochar contains a highly aromatic carbon lead to less accurate estimate of fixed-carbon by
structure; the fully destruction or combustion of the aromatic underestimation of ash content. Proximate analysis relies on
structure cannot be assured in some cases. Also, relative to the thermal decomposition for calculation of products, which does
higher carbon content, the relatively small peaks of other not provide an analogue for the degradative processes that
elements such as H, N and S should be carefully assessed to not exist in soil.2
to underestimate their quantity during total elemental Greater pyrolysis temperature for low-ash biochars increased
analyses59. Therefore, using of a combination of volatile matter, fixed-carbon content, but decreased it for biochars with more
and O:C or H:Corg ratios to classify the stability of biochar would than 20% ash1,55. H/Corg and O/C molar ratios correlated best
be the best option.55 with the fixed-carbon content61, volatile matter/fixed-carbon
mass fraction ratio. C and O mass fractions correlated best with
3.2. Chemical oxidation methods mass fractions of volatile matter (VM), fixed carbon (FC), and
Resistant to chemical oxidation is another measure to assess ash; whereas H mass fraction correlated best with VM, FC and
the biochar stability. Few studies reported the stability of VM/FC56. Evaluation of volatile matter (VM) in biochar samples
biochar carbon content with different chemical oxidation was proposed as the simplest method for the evaluation of
methods such as hydrogen peroxide oxidation, potassium biochar stability62. A volatile matter above 80% (w/w biochar
dichromate oxidation and potassium permanganate oxidation. ash-free mass) may indicate biochars with no carbon
Those methods provide very good idea on labile biochar sequestration value. A volatile matter below 80% (w/w biochar
fraction and the chemically resistant stable biochar fraction. ash-free mass) and an O:C ratio above 0.2 or H:Corg above 0.4
Wet oxidation with potassium dichromate and potassium may indicate moderate sequestration ability, and a volatile
permanganate revealed the strong correlation with H:C atomic matter below 80% (w/w biochar ash-free mass) and an O:C ratio
ratio, O:C atomic ratio and thermo-degradable fraction of below 0.2 or H:Corg below 0.4 may indicate high C sequestration
biochar obtained through TG/DTG analyses60. Stable carbon potential1. For an accurate and reliable prediction of biochar
content of the biochar, which is assessed after eliminating the stability, a combination of methods is a good approach55, since
less stable portion of biochar by oxidation with hydrogen single parameter itself do not predict the biochar stability
peroxide (H2O2), is used as an analogue for the accumulated correctly.
effect of oxidation over extended periods of time in soil.
Comparison of results from direct oxidation of biochar with
stability indicators derived from proximate and ultimate
analysis showed a strong correlation between the approaches
across feedstock and production conditions (pyrolysis
temperature and heating rate)2,58. Such modifications of alpha
139
though the resulted MRT of biochar is vary from the actual

conditions74. Different models such as single exponential
model, double exponential model and triple exponential
models were used to extrapolate biochar carbon stability.
Single and double exponential models can underestimate the
stability of biochar carbon compared to the three pool model,
since with the greater number of pools added the prediction
ability will be greater15. Therefore, precautions should take
Figure 3. Linear regression for (a) baseline ATR-FTIR before extrapolating the data from incubation studies or field
absorbance (4393 cm−1) and (b) atomic H:C ratio as a function studies. Percentage of carbon in biochar that can remain stable
of fixed-carbon content in biochar.61 for more than 100 years predicted from MRT has a good
correlation with the proximate results (H/Corg)15, as shown in
3.4. Recalcitrance index
Fig. 4.
Temperature programmed oxidation (TPO) is used to calculate
the recalcitrance index. Based on the thermograms (which were
corrected for ash and water content) obtained through
thermogravimetric or coupled TGA-DSC analysers, the mass loss
resulting from oxidation of carbon is continuously monitored
and used to calculate the recalcitrance index (R50) according to
the following equation63:
𝑹𝑹𝑹𝑹𝑹𝑹 = 𝑻𝑻𝟓𝟓𝟓𝟓 𝑿𝑿 / 𝑻𝑻𝑻𝑻𝑻𝑻 𝒈𝒈𝒈𝒈𝒈𝒈𝒈𝒈𝒈𝒈𝒈𝒈𝒈𝒈𝒈𝒈 (1)
where T50 X and T50 graphite are the temperatures
corresponding to the 50% mass loss as a result of oxidation
/volatilisation of biochar (X) and graphite, respectively. The
resulting R50 value depends on the nature of the carbon stability
of the sample. With the higher stability, the value for the T50 will
Figure 4. The correlation between H/Corg and biochar C
increase. Depending on the value obtained for the recalcitrance
remained after 100 years as predicted by a two component
index, biochar can be categorised into three different stability model.15
classes in terms of their carbon sequestration ability: class A
biochar (R50 ≤ 0.70), Class B biochar (0.50 ≤ R50 < 0.70) and Class Both increase75,76 and decrease5,6,77 priming has been recorded
C biochar (R50 < 0.50). Class A and Class C biochars would have after addition of biochar into the soil. Higher carbon released
carbon sequestration potential comparable to soot/graphite from biochar amended soils at the beginning of the incubation
and uncharred plant biomass, respectively; whereas Class B than the non-amended soils was observed with biochar
biochars would have intermediate carbon sequestration produced at lower temperature from grass in lower organic
potential58. As a result of the refinement of the recalcitrance matter content soils. However, opposite pattern was observed
index, “the gained stability“ with washed biochar prior to with hardwood-derived biochar produced at higher
analyse with TGA is also used for assessing the biochar stability. temperatures and in higher organic matter containing soils.
These values showed a strong correlation with the values of Initial increase in carbon mineralisation after biochar addition
accelerated aging test.64,65 was decreased in the later stages of the incubations
3.5. Laboratory incubation and field based methods and observed5,24,25,31,47. Adsorption (sorption of native soil organic
chronosequence to assess biochar stability in soil matter to the surface of the pyrogenic organic matter) and
dilution were identified as the two major mechanisms for the
Both laboratory incubation studies19,24,37,68–72 and field negative priming observed after additions of the biochar into
application68,69,73 of biochar are using in the context of assessing the soil. These mechanisms heavily contribute to the long term
the temporal and spatial biochar decomposition patterns. carbon storage in soils. Other than that, substrate switching and
Studies based on the field application of biochar depict the co-metabolism also affect to the negative priming effect but in
more realistic conditions compared to the laboratory very lower contribution71. For the better understanding of
incubation studies of soil and biochar. But field based methods pyrogenic carbon decomposition in soil, the long-term field and
constraint with different mineralisation pathways and losses. In laboratory incubation studies are important, since very few
addition, it will restrict the use and comparison of various types long-term field and laboratory incubation studies are reported
of biochar in field studies. Compared to the field studies, in the literature.
laboratory incubation studies do with the more controlled Chronosquences, a set of sites that share many similar soil
environmental conditions such as moisture and temperature forming factors (climate, organisms, relief, and parent
etc. Also, constraint with the limited time durations and materials) but with soils of different ages78, can be used to
absence of additions of litter and manure, real fluctuations of determine the biochar carbon loss. According to the results
moisture and temperature and other soil management reported from the measurements of biochar distribution from
practices related to the agriculture. But, laboratory incubations sites that vary in time interval since biochar was applied, several
allow to do the comparison between numbers of biochar factors governing the stability of biochar in soil were identified.
samples and provide overall outlook within short time period Oxidation, erosion and biochar particles transport (physical
140
transport) into the depth in the soil were identified as factors

that accelerate the biochar decomposition. Also, adsorption of
organic and inorganic materials was identified as a factor that
protect biochar from mineralisation79–81. Especially, the biochar
carbon in more stable organo mineral fraction and lower carbon
mineralisation in biochar rich Anthrosols compared to the
biochar poor adjacent soils were observed.4
3.6. Nuclear magnetic resonance (NMR) spectroscopy
NMR spectroscopy has been used in several studies to assess Figure 6. Relationships between mean residence time (MRT) of
the degree of aromatic condensation of several compounds82. biochar carbon (using 5-year mineralisation data set) and
In the context of biochar, solid state NMR spectroscopy was nonaromatic C proportion (a) and degree of aromatic
used to identify the distribution of major biomolecules in condensation (b) determined by 13C CP-NMR.23
biochar83,84. Aromatic condensation of biochar is measured
using the NMR spectroscopy. The upfield shift in the peak 3.7. Pyrolysis Gas Chromatography Mass Spectrometry
position of a probe molecule (13C- benzene) which is sorbed (Py-GC-MS)
into a biochar that contains aromatic structures capable of Pyrolysis gas chromatography is a chemical analysis method
sustaining ring currents is used in this technique. That shift in where the sample is heated, under an inert atmosphere, for
the peak position (Δδ) can be expressed according to Eq. (2)85. decomposition to produce smaller molecules that can be
More condensed aromatic structures, Δδ becomes increasingly separated by gas chromatography and detected using mass
negative as the ring current increases in strength. spectrometry.
𝜟𝜟𝜟𝜟 = 𝜹𝜹 𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔 𝒃𝒃𝒃𝒃𝒃𝒃𝒃𝒃𝒃𝒃𝒃𝒃𝒃𝒃 – 𝜹𝜹 𝒏𝒏𝒏𝒏𝒏𝒏𝒏𝒏 𝒃𝒃𝒃𝒃𝒃𝒃𝒃𝒃𝒃𝒃𝒃𝒃𝒃𝒃 (2) The resulting pyrolysis products can be categorised into
hundreds of groups based on their chemical composition. The
The aromaticity and the condensation derived from the NMR
sum of most released compounds can be used to assess the
spectroscopic analysis showed a clear influence of production
degree of charring/aromatic carbon content in biochar. This
temperature and the type of feedstock. Degree of aromaticity
technique has been used in resent studies with different
of biochar increased with increasing pyrolysis temperature57.
purposes, such as identifying the molecular composition of
Higher amount of aromatic condensation was observed in
biochars to assess the biochar stability60,90,91, as well as
biochar derived from woody materials compared to biochar
identifying poly aromatic hydrocarbons in biochar92. The effect
from mineral-rich feedstocks produced at same pyrolysis
of feedstock type and biochar production conditions on biochar
temperature85. According to the 13C NMR spectroscopy results,
stability correlated well with the analytical pyrolysis results.
aged biochar showed increased amounts of O-alkyl C and alkyl
Also, Py-GC-MS results correlated well with the results from
C at the expense of aromatic-C, revealing that aged biochars are
thermogravimetric tests, 13C NMR spectroscopy results and
less aromatic and more functionalised than fresh ones.17,86
biochar wet oxidation tests60. Therefore, analytical pyrolysis
Therefore, 13C NMR spectra can be used as a marker to detect
can be identified as a gamma method which provides a good
black carbon in soil48. The fraction of aromatic C in the biochar
idea about biomolecules present in biochar. Hence, it is a very
strongly increases with peak temperature, fixed-carbon yield,
good proxy to calibrate low cost alpha methods.
and decrease with increased H:Corg and O:C ratios88. Also, 13 C
The molecular ratio toluene/naphthalene is governed by the
NMR analyses can be used to assess the
extent of carbonisation and the presence of proteins in the
O-containing functional groups, including substituted aryl,
original substrate. This was well correlated with the volatile
carboxyl and carbonyl C, and losses of O-alkyl groups in aged
matter content of biochar91. Pyrolysis products resulting from
biochar49. Furthermore, the aromaticity and aromatic
the analytical pyrolysis act as a pyrolysis fingerprint of a
condensation derived from NMR analysis have a good
particular substance. Therefore, thermal stability index derived
correlation with fixed-carbon content from proximate analysis
from these pyrolysis products could be used to assess the
and MRT of biochar, as can be observed in Figs. 5 and 6.
stability of biochar, since it is considered as a more reliable
indicator than other pyrolytic proxies, such as benzene/toluene
or napthalene/C1-napthalene ratios93. A decrease in the H:Corg
molar ratio was observed when the intensity and the variety of
pyrolysis products in the resulted pyrolysis products decreased.
In highly carbonised biochar (i.e., H:Corg < 0.4), the released
compounds were mainly highly aromatic. Their relative
abundance (% charred) with respect to other pyrolysis products
was linearly correlated with the H:Corg ratio90, as shown in Fig.
7.
Figure 5. Biochar aromaticity from quantitative NMR analysis

as a function of fixed-carbon fraction from proximate
analysis.89
141
Table 3. Advantages and disadvantages of different methods

focused on assessing bichar carbon stability.12,15,97
Method Advantages Disadvantages

Elemental ratios Low cost, simple and Qualitative
(H/Corg and O/C reliable. indicator.
molar ratios) High robustness and Conservative.
repeatability.
Strongly correlated
with the data from
incubation studies,
thermogravimetry
and Biochar
production
conditions.
Chemical Simple and low cost. Hydrophobic
oxidation method Quantitative. nature of biochar
can make
disturbances.
Need to take
precautions while
doing measuring.
Proximate analysis Simple and low cost. Conservative.
(Ash, Fixed-carbon Repeatability is high. Qualitative
Figure 7. Py–GC–MS yield ratios of selected pyrolysis products and volatile
matter contents)
with naphthalene in the whole (a) and in a selected H:Corg
molar ratio interval (b).90 Recalcitrance Simple and low cost. Qualitative.
index Higher accuracy.
Field application More realistic data Constraint with
3.8. Benzene poly carboxylic acid (BPCA)
of biochar on temporal and different
Condensed aromatic moieties were the most stable fraction spatial pattern of mineralisation
compared to all other biochar compounds and the high portion biochar pathways and
decomposition. losses.
of BPCA in biochar explains its very high stability and its Quantitative. Difficult for using
contribution to long-term C sequestration in soil94. Therefore, different types of
BPCA can be used as a molecular marker to assess black carbon biochar.
in soils and sediments. The analytical procedure to determine Laboratory Able to quantify Most of the
incubation studies biochar carbon studies
BPCA includes acid digestion, oxidation, sample clean-up,
mineralisation constrained with
derivatisation, and gas chromatography95. The degree of without any other the limited time
polymerisation in individual monomers varies with their degree losses. duration and
of carboxylation. This can be used to determine the stability of Quantitative. actual field
biochar via assessing the degree of aromatisation. Single-ring environmental
conditions.
aromatic molecules with different degree of carboxylation (i.e.,
NMR Quantitative. Higher cost
B3-, B4-, B5- and B6-CA) represent BPCA with three, four, five spectroscopy It gives better idea comparatively.
and six carboxylic groups. These BPCA monomers reflect the about aromaticity Need of a good
size of the original polycyclic structure. The proportion of BPCA and aromatic analytical
condensation. knowledge.
normalised to total organic carbon content and the proportion
High accuracy. Large variation
of individual BPCA monomers to the total BPCA were used as between
indicators to depict the degree of aromatic condensation and different NMR
the aromaticity of the black carbon.96 equipment.
Pyrolysis Gas High accuracy. High cost.
Chromatography Not much
4. Future perspectives Mass reliable for
Spectrometry highly
Biochar application into soil as a soil amendment has become aromatised
popular among globally due to its beneficial effects on soil biochar.
fertility, remediation of contaminated soils and degraded lands Excessive
charring can
and mitigating climate change. However, the use of biochar is happen.
still under experimental investigations. As a highly stable Benzene poly Quantitative. It needs several
recalcitrant substance, biochar has a long-term persistence in carboxylic Acid High accuracy. sample
soil. Therefore, before using it as a soil amendment, (BPCA) treatment
procedures and
precautions should be taken to avoid the introduction of
precise analysis.
contaminants and harmful substances through biochar into soil.
Accordingly, proper characterisation of biochar is a key to
identify and predicting its behaviour in soil environment.
142
Biochar stability assessment methods play an important role in (11) Jeffery, S.; Verheijen, F. G. A.; Van Der Velde, M.;
predicting biochar stability. Both advantages and disadvantages Bastos, A. C. A quantitative review of the effects of
involved in those analysis methods are summarised in Table 3. biochar application to soils on crop productivity using
Among the prevailing biochar stability indicators, laboratory soil meta-analysis. Agric. Ecosyst. Environ. 2011, 144, 175–
incubation studies and field-based studies provide more 187.
realistic insight about biochar decomposition patterns and its (12) Hansen, V.; Müller-Stöver, D.; Munkholm, L. J.; Peltre,
effect on soil chemical, physical and biological properties. C.; et al. The effect of straw and wood gasification
However, these methods are constrained by time and cost biochar on carbon sequestration, selected soil fertility
involved during the stability assessments. As a result, lab-based indicators and functional groups in soil: an incubation
simple and low-cost proxies can be used for assessing biochar study. Geoderma 2016, 269, 99–107.
stability such as elemental ratios, proximate analysis, etc. These (13) Hernandez-Soriano M. C.; Kerré, B.; Kopittke, P. M.;
proxies have strong correlation with biochar mineralisation in Horemans, B.; Smolders, E. Biochar affects carbon
soil. composition and stability in soil: a combined
spectroscopy-microscopy study. Scientific Reports
2016, 6, 25127.
Acknowledgements (14) Lehmann, J.; Gaunt, J.; Rondon, M. Bio-char
“This project has received funding from the European sequestration in terrestrial ecosystems–a review.
Union’s Horizon 2020 research and innovation programme Mitigation and adaptation strategies for global change.
under the Marie Skłodowska-Curie grant agreement No Mitig. Adapt. Strat. Glob. Chang. 2006, 11, 403–427.
721991”. (15) Budai, A.; Zimmerman, A.; Cowie, A. L.; Webber, J. B.;
et al. Biochar Carbon Stability Test Method: An
assessment of methods to determine biochar carbon
References stability. Techical report, International Biochar
(1) Spokas, K. A. Review of the stability of biochar in soils: Initiative: 2013, pp. 1–10.
predictability of O:C molar ratios. Carbon Manage., (16) Ameloot, N.; Graber, E. R.; Verheijen, F. G.; De Neve, S.
2010, 1, 289–303. Interactions between biochar stability and soil
(2) Crombie, K.; Mašek, O.; Sohi, S. P.; Brownsort, P.; organisms: review and research needs. Eur. J. Soil Sci.
Cross, A. The effect of pyrolysis conditions on biochar 2013, 64, 379–390.
stability as determined by three methods. GCB (17) De la Rosa, J. M.; Rosado, M.; Paneque, M.; Miller, A.
Bioenergy, 2013, 5, 122–131. Z.; Knicker, H. Effects of aging under field conditions on
(3) Woolf, D.; Lehmann, J. Modelling the long-term biochar structure and composition: Implications for
response to positive and negative priming of soil biochar stability in soils. Sci. Total Environ. 2018, 613–
organic carbon by black carbon. Biogeochemistry 2012, 614, 969–976.
111, 83–95. (18) Hamer, U.; Marschner, B.; Brodowski, S.; Amelung, W.
(4) Liang, B.; Lehmann, J.; Solomon, D.; Sohi, S.; et al. Interactive priming of black carbon and glucose
Stability of biomass-derived black carbon in soils. mineralisation. Org. Geochem. 2004, 35, 823–830.
Geochim. Cosmochim. Acta, 2008, 72, 6069–6078 (19) Hilscher, A.; Knicker, H. Carbon and nitrogen
(5) Zimmerman, A. R.; Gao, B; Ahn, M. Y. Positive and degradation on molecular scale of grass-derived
negative carbon mineralization priming effects among pyrogenic organic material during 28 months of
a variety of biochar-amended soils. Soil Biol. Biochem. incubation in soil. Soil Biol. Biochem. 2011, 43, 261–
2011, 43, 1169–1179. 270.
(6) Whitman, T.; Enders, A.; Lehmann, J. Pyrogenic carbon (20) Murray, J.; Keith, A.; Singh, B.The stability of low-and
additions to soil counteract positive priming of soil high-ash biochars in acidic soils of contrasting
carbon mineralization by plants. Soil Biol. Biochem. mineralogy. Soil Biol. Biochem. 2015, 89, 217–225.
2014, 73, 33–41. (21) Santos, F.; Torn, M. S.; Bird, J. A. Biological degradation
(7) Barrow, C. J. Biochar: potential for countering land of pyrogenic organic matter in temperate forest soils.
degradation and for improving agriculture. Appl. Soil Biol. Biochem. 2012, 51,115–214.
Geography 2012, 34, 21–28. (22) Peng, X. Y.; Ye, L. L.; Wang, C. H.; Zhou, H.; Sun, B.
(8) Mukherjee, S.; Weihermueller, L.; Tappe, W.; Temperature-and duration-dependent rice straw-
Vereecken, H.; Burauel, P. Microbial respiration of derived biochar: Characteristics and its effects on soil
biochar-and digestate-based mixtures. Biol. Fert. Soils properties of an Ultisol in southern China. Soil Tillage
2016, 52, 151–164. Res. 2011, 112, 159–166.
(9) Biederman, L. A.; Harpole, W. S. Biochar and its effects (23) Singh, B. P., Cowie, A. L.; Smernik, R. J. Biochar carbon
on plant productivity and nutrient cycling: a meta‐ stability in a clayey soil as a function of feedstock and
analysis. GCB Bioenergy 2013, 5, 202–214. pyrolysis temperature. Environ. Sci. Technol. 2012, 46,
(10) Glaser, B.; Lehmann, J.; Zech, W. Ameliorating physical 11770–11778.
and chemical properties of highly weathered soils in (24) Fang, Y.; Singh, B.; Singh, B. P. Effect of temperature on
the tropics with charcoal–a review. Biol. Fert. Soils biochar priming effects and its stability in soils. Soil
2002, 35, 219–230. Biol. Biochem. 2015, 80, 136–145.
143
(25) Kuzyakov, Y.; Subbotina, I.; Chen, H.; Bogomolova, I.; (40) Hilscher, A.; Heister, K., Siewert, C.; Knicker, H.
Xu, X. Decomposition of 14C labeled pyrogenic carbon Mineralisation and structural changes during the initial
and its incorporation into soil microbial biomass phase of microbial degradation of pyrogenic plant
estimated during 4 years incubation. In: EGU General residues in soil. Org. Geochem. 2009, 40, 332–342.
Assembly Conference Abstracts 2009, p. 7254. (41) Lehmann, J.; Kinyangi, J.; Solomon, D. Organic matter
(26) Laird, D. A. The charcoal vision: a win–win–win stabilization in soil microaggregates: implications from
scenario for simultaneously producing bioenergy, spatial heterogeneity of organic carbon contents and
permanently sequestering carbon, while improving carbon forms. Biogeochemistry 2007, 85, 45–57.
soil and water quality. Agronomy J. 2008, 100,178– (42) Lehmann, J.; Rillig, M. C.; Thies, J.; Masiello, C. A.; et al.
181. Biochar effects on soil biota–a review. Soil Biol
(27) Schmidt, M. W.; Skjemstad, J. O.; Jäger, C. Carbon Biochem. 2011, 43, 1812–1836.
isotope geochemistry and nanomorphology of soil (43) Kemmitt, S. J.; Lanyon, C. V.; Waite, I. S.; Wen, Q.; et al.
black carbon: Black chernozemic soils in central Europe Mineralization of native soil organic matter is not
originate from ancient biomass burning. Glob. regulated by the size, activity or composition of the soil
Biogeochem. Cycles 2002, 16,70–71. microbial biomass—a new perspective. Soil Biol.
(28) Cheng, C. H.; Lehmann, J.; Thies, J. E.; Burton, S. D.; Biochem. 2008, 40, 61–73.
Engelhard, M. H. Oxidation of black carbon by biotic (44) Paterson, E.; Gebbing, T.; Abel, C.; Sim, A.; Telfer, G.
and abiotic processes. Org. Geochem. 2006, 37, 1477– Rhizodeposition shapes rhizosphere microbial
1488. community structure in organic soil. New Phytologist.
(29) Cheng, C. H.; Lehmann, J.; Thies, J. E.; Burton, S. D. 2007, 173, 600–610.
Stability of black carbon in soils across a climatic (45) Blagodatskaya, E. V.; Blagodatsky, S. A.; Anderson, T.
gradient. J. Geophys. Res. 2008, 113, G02027. H.; Kuzyakov, Y. Priming effects in Chernozem induced
(30) Vassilev, S. V.; Baxter, D.; Andersen, L. K.; Vassileva, C. by glucose and N in relation to microbial growth
G. An overview of the chemical composition of strategies. Appl. Soil Ecology 2007, 37, 95–105.
biomass. Fuel 2010, 89, 913–933. (46) Otten, W.; Hall, D.; Harris, K.; Ritz, K.; Young, I. M.;
(31) Wang, J.; Xiong, Z.; Kuzyakov, Y. Biochar stability in soil: Gilligan, C. A. Soil physics, fungal epidemiology and the
meta‐analysis of decomposition and priming effects. spread of Rhizoctonia solani. New Phytologist 2001,
GCB Bioenergy 2016, 8, 512–523. 151, 459–468.
(32) Yang, Y.; Sun, K.; Han, L.; Jin, J.; et al. Effect of minerals (47) Kuzyakov, Y.; Friedel, J. K.; Stahr, K. Review of
on the stability of biochar. Chemosphere 2018, 204, mechanisms and quantification of priming effects. Soil
310–317. Biol Biochem. 2000, 32, 1485–1498.
(33) Purakayastha, T. J.; Das, K. C.; Gaskin, J.; Harris, K.; et (48) Bird, M. I.; McBeath, A. V.; Ascough, P. L.; et al. Loss
al. Effect of pyrolysis temperatures on stability and and gain of carbon during char degradation. Soil Biol
priming effects of C3 and C4 biochars applied to two Biochem. 2017, 106 ,80–89.
different soils. Soil Tillage Res. 2016, 155, 107–115. (49) Mukherjee, A.; Zimmerman, A. R.; Hamdan, R.; Cooper,
(34) Fang, Y.; Singh, B.; Singh, B. P.; Krull, E. Biochar carbon W. T. Physicochemical changes in pyrogenic organic
stability in four contrasting soils. Eur. J. Soil Sci. 2014, matter (biochar) after 15 months of field aging. Solid
65,60–71. Earth 2014, 5,693–704.
(35) Chen, D.; Yu, X.; Song, C.; Pang, X.; et al. Effect of (50) Yang, F.; Zhao, L.; Gao, B; Xu, X.; Cao, X. The interfacial
pyrolysis temperature on the chemical oxidation behavior between biochar and soil minerals and its
stability of bamboo biochar. Bioresour. Technol. 2016 , effect on biochar stability. Environ. Sci. Technol. 2016,
218,1303–1306. 50, 2264–2271.
(36) Mašek, O; Brownsort, P.; Cross, A.; Sohi, S. Influence of (51) Lin, Y.; Munroe, P.; Joseph, S.; Kimber, S.; Van Zwieten,
production conditions on the yield and environmental L. Nanoscale organo-mineral reactions of biochars in
stability of biochar. Fuel 2013, 103, 151–155. ferrosol: an investigation using microscopy. Plant Soil
(37) Dempster, D. N.; Gleeson, D. B.; Solaiman, Z. I.; Jones, 2012, 357, 369–380.
D. L.; Murphy, D. V. Decreased soil microbial biomass (52) Nguyen, B. T.; Lehmann, J. Black carbon decomposition
and nitrogen mineralisation with Eucalyptus biochar under varying water regimes. Org. Geochem. 2009, 40,
addition to a coarse textured soil. Plant Soil 2012, 846–853.
354,311–324. (53) Rittl, T. F.; Butterbach-Bahl, K.; Basile, C. M.; Pereira, L.
(38) Zimmermann, M.; Bird, M. I.; Wurster, C.; Saiz, G.; et A.; et al. Greenhouse gas emissions from soil amended
al. Rapid degradation of pyrogenic carbon. Glob. with agricultural residue biochars: Effects of feedstock
Chang. Biol. 2012, 18, 3306–3316. type, production temperature and soil moisture.
(39) Fang, Y.; Singh, B. P.; Matta, P.; Cowie, A. L.; Van Biomass Bioenergy 2018, 117, 1–9.
Zwieten, L. Temperature sensitivity and priming of (54) International Biochar Initiative. Guidelines for
organic matter with different stabilities in a Vertisol specifications of biochars for use in soils, 2012.
with aged biochar. Soil Biol. Biochem. 2017, 115,346–
356.
144
(55) Enders, A.; Hanley, K.; Whitman, T.; Joseph, S.; (70) Cui, J.; Ge, T.; Kuzyakov, Y.; Nie, M.; et al. Interactions
Lehmann, J. Characterization of biochars to evaluate between biochar and litter priming: a three-source 14C
recalcitrance and agronomic performance. Bioresour. and δ13C partitioning study. Soil Biol. Biochem. 2017,
Technol. 2012, 114, 644-–653. 104, 49–58.
(56) Klasson, K. T. Biochar characterization and a method (71) DeCiucies, S.; Whitman, T; Woolf, D.; Enders, A.;
for estimating biochar quality from proximate analysis Lehmann, J. Priming mechanisms with additions of
results. Biomass Bioenergy 2017, 96, 50–58. pyrogenic organic matter to soil. Geochim.
(57) Wu, W.; Yang, M.; Feng, Q.; McGrouther, K.; et al. Cosmochim. Acta. 2018, 238, 329–342.
Chemical characterization of rice straw-derived (72) Dharmakeerthi, R. S.; Hanley, K.; Whitman, T.; Woolf,
biochar for soil amendment. Biomass Bioenergy 2012, D.; Lehmann, J. Organic carbon dynamics in soils with
47, 268–276. pyrogenic organic matter that received plant residue
(58) Cross, A.; Sohi, S. P. A method for screening the relative additions over seven years. Soil Biol. Biochem. 2015,
long‐term stability of biochar. GCB Bioenergy 2013, 5, 88, 268–274.
215–220. (73) Munda, S.; Bhaduri, D.; Mohanty, S.; Chatterjee, D.; et
(59) Singh, B.; Camps-Arbestain, M.; Lehmann, J.; Eds. al. Dynamics of soil organic carbon mineralization and
Biochar: a guide to analytical methods. Csiro C fractions in paddy soil on application of rice husk
Publishing, 2017. biochar. Biomass Bioenergy 2018, 115, 1–9.
(60) Pereira R. C.; Kaal, J.; Camps-Arbestain, M.; Lorenzo, R. (74) Lehmann, J.; Joseph, S.; Eds. Biochar for environmental
P.; et al. Contribution to characterisation of biochar to management: science, technology and
estimate the labile fraction of carbon. Org. Geochem. implementation. Routledge, 2015.
2011, 42, 1331–1342. (75) Luo, Y.; Durenkamp, M.; De Nobili, M.; Lin, Q.; Brookes,
(61) Uchimiya, M.; Orlov, A.; Ramakrishnan, G; Sistani, K. In P. C. Short term soil priming effects and the
situ and ex situ spectroscopic monitoring of biochar's mineralisation of biochar following its incorporation to
surface functional groups. J. Anal. Appl. Pyrolysis 2013, soils of different pH. Soil Biol. Biochem. 2011, 43,
102, 53–59. 2304–2314.
(62) Zimmerman, A. R. Abiotic and microbial oxidation of (76) Wardle D. A.; Nilsson, M. C.; Zackrisson, O. Fire-derived
laboratory-produced black carbon (biochar). Environ. charcoal causes loss of forest humus. Science 2008,
Sci.Technol. 2010, 44, 1295–1301. 320, 629–629.
(63) Harvey, O. R.; Kuo, L. J.; Zimmerman, A. R.; (77) Whitman, T.; Zhu, Z.; Lehmann, J. Carbon
Louchouarn, P.; et al. An index-based approach to mineralizability determines interactive effects on
assessing recalcitrance and soil carbon sequestration mineralization of pyrogenic organic matter and soil
potential of engineered black carbons (biochars). organic carbon. Environ. Sci. Technol. 2014, 48, 13727–
Environ. Sci.Technol. 2012, 46, 1415–1421. 13734.
(64) Gómez, N.; Rosas, J. G.; Singh, S.; Ross, A. B.; et al. (78) Sauer, D. Pedological concepts to be considered in soil
Development of a gained stability index for describing chronosequence studies. Soil Res. 2015, 53, 577–591.
biochar stability: Relation of high recalcitrance index (79) Nguyen, B. T.; Lehmann, J.; Kinyangi, J.; Smernik, R.; et
(R50) with accelerated ageing tests. J. Anal. Appl. al. Long-term black carbon dynamics in cultivated soil.
Pyrolysis 2016, 120, 37–44. Biogeochemistry 2009, 92, 163–176.
(65) Carrier, M.; Loppinet-Serani, A.; Denux, D.; Lasnier, J. (80) Major, J.; Lehmann, J.; Rondon, M.; Goodale, C. Fate of
M.; et al. Thermogravimetric analysis as a new method soil‐applied black carbon: downward migratioºn,
to determine the lignocellulosic composition of leaching and soil respiration. Glob. Chang. Biol. 2010,
biomass. Biomass Bioenergy 2011, 35, 298–307. 16, 1366–1379.
(66) Keith, A.; Singh, B.; Dijkstra, F. A. Biochar reduces the (81) Foereid, B.; Lehmann, J.; Major, J. Modeling black
rhizosphere priming effect on soil organic carbon. Soil carbon degradation and movement in soil. Plant Soil
Biol. Biochem. 2015, 88, 372–379. 2011, 345, 223–236.
(67) Ventura, M.; Alberti, G.; Viger, M.; Jenkins, J. R.; et al. (82) Duer, M. J.; Ed. Solid state NMR spectroscopy:
Biochar mineralization and priming effect on SOM principles and applications, John Wiley & Sons, 2008.
decomposition in two European short rotation (83) Krull, E. S.; Baldock, J. A.; Skjemstad, J. O.; Smernik, R.
coppices. GCB Bioenergy 2015, 7, 1150–1160. J. Characteristics of biochar: organo-chemical
(68) Weng, Z. H.; Van Zwieten, L.; Singh, B. P.; Kimber, S.; et properties. In: Biochar for environmental
al. Plant-biochar interactions drive the negative management: Science and technology. Earthscan,
priming of soil organic carbon in an annual ryegrass 2009, p. 2053.
field system. Soil Biol. Biochem. 2015, 90, 111–121. (84) Smernik, R. J. 14 Analysis of biochars by 13C nuclear
(69) Rasse, D. P.; Budai, A, O’Toole, A.; Ma, X.; et al. magnetic resonance spectroscopy. In: Biochar: A Guide
Persistence in soil of Miscanthus biochar in laboratory to Analytical Methods. Csiro Publishing, 2017.
and field conditions. Plos One 2017, 12, e0184383.
145
(85) McBeath, A. V.; Smernik, R. J.; Krull, E. S.; Lehmann, J.

The influence of feedstock and production
temperature on biochar carbon chemistry: a solid-
state 13C NMR study. Biomass Bioenergy 2014, 60,
121–129.
(86) Joseph, S. D.; Camps-Arbestain, M.; Lin, Y.; Munroe, P.;
et al. An investigation into the reactions of biochar in
soil. Soil Res. 2010, 48, 501–515.
(87) De la Rosa, J. M.; Knicker, H.; López-Capel, E., Manning,
D. A.; et al. Direct detection of black carbon in soils by
Py-GC/MS, carbon-13 NMR spectroscopy and
thermogravimetric techniques. Soil Sci Soc Am. J. 2008,
72, 258–267.
(88) Manyà, J. J.; Laguarta, S.; Ortigosa M. A.; Manso, J. A.
Biochar from slow pyrolysis of two-phase olive mill
waste: effect of pressure and peak temperature on its
potential stability. Energy Fuels 2014, 28, 3271–3280.
(89) Brewer, C. E.; Unger, R.; Schmidt-Rohr, K.; Brown, R. C.
Criteria to select biochars for field studies based on
biochar chemical properties. Bioenergy Res. 2011, 4,
312–323.
(90) Conti, R.; Rombolà, A. G.; Modelli, A.; Torri, C.; Fabbri,
D. Evaluation of the thermal and environmental
stability of switchgrass biochars by Py–GC–MS. J. Anal.
Appl. Pyrolysis 2014, 110, 239–247.
(91) Conti, R.; Fabbri, D.; Vassura, I.; Ferroni L. Comparison
of chemical and physical indices of thermal stability of
biochars from different biomass by analytical pyrolysis
and thermogravimetry. J. Anal. Appl. Pyrolysis 2016,
122, 160–168.
(92) Fabbri, D.; Rombolà, A. G.; Torri, C.; Spokas, K. A.
Determination of polycyclic aromatic hydrocarbons in
biochar and biochar amended soil. J. Anal. Appl.
Pyrolysis 2013, 103,60–67.
(93) Suárez-Abelenda, M.; Kaal, J.; McBeath, A. V.
Translating analytical pyrolysis fingerprints to Thermal
Stability Indices (TSI) to improve biochar
characterization by pyrolysis-GC-MS. Biomass
Bioenergy 2017, 98, 306–320.
(94) Rombolà, A. G.; Meredith, W.; Snape, C. E.; Baronti, S.;
et al. Fate of soil organic carbon and polycyclic
aromatic hydrocarbons in a vineyard soil treated with
biochar. Environ. Sci. Technol. 2015, 49, 11037–11044.
(95) Glaser, B.; Haumaier, L.; Guggenberger, G.; Zech, W.
Black carbon in soils: the use of benzenecarboxylic
acids as specific markers. Org. Geochem. 1998, 29,
811–819.
(96) Camps-Arbestain, M.; Shen, Q.; Wang, T.; van Zwieten,
L.; Novak, J. 10 Available nutrients in biochar. In:
Biochar: A Guide to Analytical Methods. Csiro
Publishing, 2017, p. 109.
(97) Leng, L.; Huang, H.; Li, H.; Li, J.; Zhou, W. Biochar
stability assessment methods: a review. Sci. Total
Environ. 2019, 647, 210–222.
146
Synergies in sequential biochar systems

Christian Wurzer, Saran Sohi, Ondrej Mašek
UK Biochar Research Institute, School of Geoscience, University of Edinburgh, Edinburgh EH9 3FF, UK
Abstract
Biochar still lacks widespread and large-scale applications. Past concepts have failed to define systems that are economically attractive,
new ways to deliver biochar’s environmental benefits are needed. This chapter starts by evaluating the original idea of biochar as a
simple soil amendment and the implications of depending on a price for carbon stabilisation. Developments of biochar systems are
discussed with a special emphasis on the potential competition for pyrolysed biomass between biochar and activated carbon. The
concept of a sequential use system is introduced. Based on the diffusion-of-innovation theory, the advantage of a centralised biochar
system is highlighted and compared to alternative systems. A brief description of potential synergies in sequential biochar systems
outlines opportunities for further research. Examples of applications relevant to these systems are briefly discussed.
1. Introduction 2. Biochar systems

The continuing rise of atmospheric CO2 concentrations 2.1. Pyrolysis-biochar-soil system
poses the greatest environmental challenge in the 21st
According to the original definition of biochar, the main
century. While considerable efforts are underway to cut
intended use is in building soil fertility, inspired by the terra
CO2 emissions, it is now acknowledged that even an
preta example.7
immediate halt in emissions would not avoid continuing
The first concept for a biochar system was based on a
temperature rise within this century1. Relying on emission
simple biomass-pyrolysis-soil scheme8. Waste or marginal
reductions alone will not be sufficient. Carbon negative
biomass pyrolysed to produce biochar and applied to
technologies are geoengineering options that have
agricultural land could be used to improve crop yields. The
potential to actively removing CO2 from the atmosphere at
condensable and gaseous by-products of the pyrolysis
globally relevant scale. These may become essential to
process, bio-oil and pyrolysis gas, would be burned to
limit global warming and stabilise atmospheric greenhouse
provide heat and/or electricity9. The concept was built on
gas emissions during societal decarbonisation2. The most
the premise of a win-win-win situation whereby waste
prominent technologies range from bioenergy with carbon
streams could be utilised to improve soils and
capture and storage to direct air capture as well as biochar.
simultaneously sequester carbon into land.10
Each technology has its own limitations regarding their
From an economic point of view, the concept rests on two
scalability and energy requirements. Biochar is one of the
main sources of return. These concern increased revenue
few technologies that offer additional benefits beside the
from higher crop yield and a new revenue stream for
reduction of atmospheric carbon.2
carbon sequestration12 (see Fig. 1).
Despite these beneficial effects for a number of
environmental management applications3, commercial
production is still a niche activity in most parts of the world.
Although biochar research has increased to over 1300
publications per year within a decade4, economic studies
of biochar have focused on established use concepts rather
than re-specifying more efficient meta-concepts for the
technology itself5,6. Due to this limitation, only a few
concepts for biochar systems exist today. To become
“Green Carbon”, economic as well as social and
Figure 1. Original concept of a biochar system.
environmental sustainability must be incorporated.
In the first part of this chapter we review the established Early studies showed promising results with regards to
concepts for biochar use and the major obstacles that crop yields as well as the long-term stability of the carbon
these present. The second part of the chapter defines a within the soil13. However, the largest yield responses were
new concept that we describe as a sequential biochar observed in marginal soils with less impact on productive
system. This is explored by populating the concept with land. Field and pot experiments have also been based on
specific examples of how this alternative concept might be experimental doses of 1–140 tonnes per hectare14. Due to
applied. the considerable cost of biochar production and high
dosage15, the additional revenue of a higher crop yield of
13 % (grand mean) 16 is not sufficient to make biochar
application attractive, especially on marginal land with an
extensive agricultural system17. Although biochar is
expected to show long-term benefits, a pay-back time of

147
Chapter 12 Wurzer et al.
decades makes market driven application of biochar fertiliser or by doping fertilising compost with biochar17.
currently unrealistic in agricultural systems.18 Another way to minimise area dosage is targeting biochar
Shackley et al.8 highlighted the impact of feedstock costs towards plant roots in row crops, rather than simple
on the overall biochar price. While virgin biomass is broadcasting or blanket application.29
becoming increasingly expansive due to increasing A profitable model for biomass pyrolysis that depends on
demand from bioenergy, non-virgin biomass potentially direct soil amendment has yet to be demonstrated.
offers additional revenue to biochar systems through Successful examples may emerge in limited circumstances
avoided costs for waste disposal (such as gate fees). such as converting marginal feedstock or where soil
However, non-virgin waste or marginal biomass are among physical improvements are exceptional. General
the most challenging feedstocks for subsequent use due to application will likely depend on a change in governmental
their general heterogeneity, moisture content as well as policy with an increased focus on carbon abatement rather
potential contaminants and regulatory hurdles.19 than simply emissions reduction.20
Figure 3: Value-cascade concept.
Figure 2.Breakdown of cost for biochar production at 2.2. Value-cascade system

large scale.20 A concept put forward to add value using the
multifunctional nature of biochar can be found in the value
The current price of synthetic chemical fertiliser is between
cascade system proposed by Schmidt30. It is based on the
270-380 € t–1.21 A comparison with the feedstock price for
idea that the full value of a given biochar can only be
biochar production (see Fig. 2) indicates that biochar has
extracted incrementally (i.e., through repeated contrasting
to be equally efficient as chemical fertilisers on a mass
use). The residual value diminishes through the value-
basis, which is highly unrealistic22,23. It is therefore clear
cascade, as shown in Fig. 3.
that an accepted carbon accounting methodology for
The value in the cascade constitutes an economic unit
biochar systems and a higher carbon price is a necessary
obtained from the sum of uses, either in series or in
part of the concept to make biochar competitive in normal
parallel31. The original example placed biochar in a farm
agricultural practice.12,24
setting, involving use of biochar in applications such as
In the largest (regional) scheme for emissions trading, the
silage management, as an additive to animal feed and
European Trading Scheme, the current price for carbon
bedding, for liquid manure management, or as an aid to
emissions sits at 20 € t–1 CO2.25 National schemes like the
composting32. The concept was captured in the phrase
carbon price support in the United Kingdom exist, but the
“use it nine times – pay for it only once”.32
combined price in the United Kingdom does not exceed
The concept of integrating several properties of biochar for
38 € t–1CO2.26 Even when assuming that pyrolysis would be
several applications within an economic unit assumes that
acknowledged within such trading schemes, it would not
one biochar can be defined to deliver a range of
be sufficient to drive carbon markets as large scale effects
applications without interim diminution of relevant
are expected to start around 200 € per ton CO2.21 Due to
functionality33. Biochar is certainly a multifunctional
these economic problems, only a handful of companies are
material but it is unlikely that one biochar will be
commercially successful to date such as Sonnenerde in
adequately effective in all these different applications. A
Austria or Soil Reef in the USA.
simultaneous optimisation for multiple uses will be difficult
Research increasingly focusses on areas where biochar can
to achieve, if not impossible. It is simpler to define a type
have economic benefits besides a simple crop yield
of biochar that works well as a feed additive, but which
increase. These might be through improvements to the
might, for example, be less effective in aiding the
dynamics in soil water to mitigate drought stress and
composting process. The concept clearly has potential to
control erosion, or to interface or slowly release fertiliser
increase the economic efficiency of the whole biochar life-
to avoid groundwater contamination and increase fertiliser
cycle, but relaying on initial functionality to deliver all
use efficiency27,28. Another way to address the high cost of
cascaded uses likely fall short in maximising its sum
biochar is to decrease the dose. Biochar usually has a low
potential.
to negligible native nutrient content, but adding biochar to
traditional fertilisers can increase the overall nutrient
efficiency. This can be done either by doping biochar with
148
The idea of cascading use can be seen as part of a unsuitable for soil applications should be called biochar is
“paradigm shift”17 towards applications beyond those yet to be decided43,44. Definitions of biochar may or may
traditionally seen as within the domain of biochar31. not preclude engineered biochars unsuitable for soil
Experimental studies have gradually aligned with this new applications; however, with regard to a sustainable biochar
paradigm. In one example, a filter to fertiliser scheme has concept, the potential for long term carbon sequestration
been explored, where phosphorus-rich wastewater sludge is likely to be lost without a final application into soil43.
is used to make biochar and then used to recover Much of the development to engineer biochar is focussed
phosphorus from wastewater aqueous phase, with on the production of advanced materials based on a
subsequent transfer of the phosphorus enriched biochar to renewable carbon structure rather than the idea of biochar
land.34 as a tool for carbon sequestration in soils 38,40,45 (see Fig. 5).
2.3. Engineered biochar
Initial research concentrated on qualitative assessments of
biochar properties for a wide variety of feedstock, with
incremental understanding of biomass pyrolysis. This has
translated into quantitative relationships between
feedstock properties, pyrolysis parameters and biochar
function35. The capacity to produce biochars with
prescribed function by manipulation of production
parameters and feedstock selection leads to the principle Figure 5. Concept of an engineered biochar system.
of engineered biochar35–37. While not a standalone concept
for a biochar system, the potential to engineer biochar The concept of targeting biochar properties to specific
clearly changes the perception of biochar as a material. higher-value applications in comparison to soil
Biochar is seen less as a multi-functional material and amendment is one step forward to obtain economically
increasingly as a platform structure with predictable and sustainable biochar systems. However, in most
tuneable properties. With the possibility of manipulating applications biochar will have to compete with other
or adding native function by methods such as surface materials already in place. In wastewater or gas filtration,
doping, surface modification or pore structure tailoring, activated carbon has a long tradition of being a highly
the range of potential biochar applications is considerably effective adsorbent and will be the benchmark to compete
increased (see Fig.4).38 with.46–48
3. Biochar versus activated carbon

Activated carbon (AC) is a common adsorbent and catalyst
in a wide variety of industrial processes49. Biochar and
activated carbon are both classes of carbon materials with
a wide variety of different properties. The materials are
commonly distinguished despite no commonly accepted
differentiation in functional terms44. Their main
characteristics are essentially similar, namely a high carbon
content, a recalcitrant carbon skeleton, a highly porous
structure and surface activity. Biochar is by definition made
from renewable biomass. Activated carbon in comparison
can also be produced from fossil resources such as
Figure 4. Overview of biochar applications. petroleum coke, coal or derived wastes such as used road
tires50. The production of activated carbon also utilises
Conception of biochar as a platform material is similar pyrolysis techniques to biochar, albeit with some
accompanied by a shift of focus towards the efficiency of modifications. The main difference lies in an explicit
engineered biochars for specific singular applications activation step. Activated carbons are commonly
rather than life-cycle efficiency39. Reflecting this differentiated according to (1) chemical activation or (2)
development are the use of environmentally problematic physical activation (see chapter 8 for more details).
materials such as toxic chemicals40, nanomaterials 39 or the Chemical activation utilises an activation agent such as
addition of rare elements38. Studies that assess the ZnCl, H3PO4 or KOH to partially degrade the feedstock prior
potential environmental concerns of engineered biochar in to pyrolysis at medium to high temperatures. Subsequent
the manner put forward for traditional biochar are few19,41. washing of activated carbons results in considerably
In particular, the use of different nanomaterials will be increased micro-porosity. Major drawbacks of this method
difficult to assess and could impose long-term risks in are the need for a chemical activation agent as well as a
relation to handling, soil application, or even post-pyrolysis washing step, resulting in considerable
incineration39,42. If these novel engineered biochars environmental impacts. A large consumption of acidic
activation agents, a problematic wastewater production
149
and an additional post-production drying step are Despite it being often produced from fossil material, its
obstacles to a widespread use of this activation high energy demand and overall low carbon conversion
method.51,52 efficiency and abatement potential, the exceptionally
Physical activation is more common52. The feedstock is efficient performance of activated carbon in many
partially oxidised using an oxidising gas such as CO2, water industrial applications and the maturity of the production
steam or air, by applying higher temperatures than in technology make it highly attractive for the removal of a
chemical activation. A major disadvantage of physical variety of contaminants54. Filtration using activated carbon
activation is the higher loss of carbon associated with the as an adsorbent is of increasing interest due to demands of
high temperatures and the oxidising effect of the activation more stringent environmental protection and expanding
agents. This results in a yield of activated carbon from raw environmental pressure on surface and groundwaters
feedstock as low as 10%–15% of the original biomass worldwide.
mass.50 Biochar as a low-cost and high-volume material for
The high reactivity of oxygen typically limits use of air to adsorption applications will most likely not be as effective
post-activation at low to medium temperatures. Although as high-cost low-volume activated carbons. This means
high temperature activation is possible at low partial that other properties must be targeted in order for it to
oxygen pressures.53 become directly competitive56. In general, adsorbent
Biochar is generally produced using lower pyrolysis materials can be divided into those of high-removal
temperatures than activated carbon. Several studies of capacity per mass and considerably higher production
biochar draw on techniques from the vast accumulated costs such as activated carbon, carbon nanotubes or resins
literature on activated carbon to adapt production and low-cost low capacity materials such as clay, sand or
methods. The biochar literature therefore encompasses biochar. Beside the traditional characteristic of removal
research in which biochars have been activated by similar capacity, a comparison of low-cost and high-cost materials
methods to activated carbon, leading to materials often have to be based on a more comprehensive foundation5.
termed activated biochars54. In these examples, The price of adsorbent per removed contaminant can be
differentiation based on production is no longer useful. A used for the comparison of different materials57. The
distinction based solely on the feedstock is also not economic assessment of adsorbent costs can extend to
sufficient, since both materials can be produced from potential regeneration pathways and regeneration losses
renewable resources. As biochar research expands to to provide a more realistic comparison of adsorbent costs.
consider new applications such as wastewater filtration An important difference between low-cost and high-cost
with almost no mention of post-sorption utilisation, the adsorbents is often neglected based on a knowledge gap in
differentiation of biochar from activated carbon based on current biochar research. Literature seldom reports the
the targeted application also becomes increasingly properties of post-use biochar. Potential secondary
indistinct and overlapping55. Hagemann et al.44 proposed a applications are therefore usually neglected. The absence
characterisation based on feedstock and potential for use of a second phase use option for biochars can be seen in
in carbon sequestration as more promising (see Fig.6). several comparative life-cycle assessments of biochar and
AC, where different disposal or recycling routes have been
omitted6,58. Only a few studies have emphasised the need
to further examine the potential post-use value of sorbent
materials so far.39,56
The potential to recycle biochar for further use could be a
key advantage. Recycling spent biochars seems a logical
route for a material originally conceived as a soil
amendment39. Cleaning and recycling might not be
possible for all kinds of adsorbed contaminants, but low-
cost adsorbents could have an additional competitive
advantage where suitable post-sorption options can be
identified.56
4. The idea of sequential biochar systems

As outlined above, past concepts for biochar systems are
not economically efficient and compromise the full
environmental potential of biochar use. The original
conception of biochar as one of few carbon negative
Figure 6. Overview on feedstock (orange), treatments for technologies has become lost2. Alternative biochar
production (blue) and applications (grey) of pyrogenic systems synthesising the benefits of engineering biochar
carbonaceous materials.44 and utilising biochar as a carbon negative technology are
needed. The proposed concept of a sequential biochar
system balances the cost of biochar production with a
150
sequence of uses values, realising the potential to

sequester carbon as a side-benefit. Focus can be placed on
establishing recycling and reuse opportunities for biochar
that increase its life-time and maximise environmental
gain.
A sequential biochar system can be defined as a stepwise
sequence of different applications. A recycling intervention
between each step allows the properties of the material to
be manipulated and optimised for the subsequent
application. This essentially reverses the idea of
Figure 8. Ownership rights in a sequential biochar system.
engineering biochar for certain applications, each
application phase requires tailoring of a certain specific The benefit to the biochar producer in this system is the
characteristic. In contrast to the value-cascade, the lowering of price for the first user in expectation of
limitation for biochar to be used multiple times within the additional revenue through reselling to subsequent users.
same economic unit is abolished to deliver the most Flexibility to changing market demands is another
effective use of biochar’s changed properties in recycled attractive advantage, as the price for biochar can be
biochars. individually adjusted between each step in the sequence.
The main advantage of this stepwise combination of post- This will ensure that not only the initial production costs
use biochars with subsequent applications can be based on will be shared by several users, but also the costs of
simple economic considerations. When post-use biochar recycling steps can be shared. The biochar producer has
can be valued as an input material into a subsequent full information over the cost distribution of the whole
application, initial feedstock, transport, and production sequence and this knowledge can be utilised to adjust
costs can be distributed over different use phases and prices individually and distribute costs to applications that
therefore reduced for each single application (see Fig. 7). are more competitive on the market.
The main driver for the biochar producer will be an
optimised revenue from the whole sequence. Lowering the
price for one application below its maximum profit can
therefore still be economically attractive. As it is the case if
a lower profit in one application ensures that mass flows of
biochar can be adjusted between different applications
and the additional subsequent revenues offset the lower
profit of the previous step. The flexibility in individually
Figure 7. Concept of a sequential biochar system.
adjusted prices for each step of the sequence can not only
The benefit to the first customer in the sequential biochar increase the resilience for demand changes, but
system will be solely on the buyer’s site. The ownership additionally adjust mass flows within the system.
rights are transferred, and the first user can sell the biochar Users of sequential biochar system will benefit from
to the next user. This means that the price to the first user purchasing a service rather than a product. This will lower
would still be the same as in a single use transaction, the transaction costs associated with waste disposal as well as
seller covering the full cost of production to the first buyer. the overall benefit of a reduced price in comparison to
Feedstock, production and transport costs will be fully traditional systems.
accounted for this first transaction. The only difference From an environmental perspective, the use of biochar for
would lie in the potential for the first user to sell the multiple applications potentially lowers energy and
biochar after its use. material input through a focus on recycling of the material.
To facilitate the potential benefits of cost-distribution over The lower price will make biochar more competitive as a
several use phases the ownership of the biochar within a soil amendment, potentially abolishing the need for a
sequential system would be kept constant. This means that carbon price. However, opportunity costs from
a biochar producer in a sequential biochar system acts as a incineration for energy generation would still be the
service provider, presenting biochar as a carrier of competing price to match.59
environmental services rather than a product (see Fig. 8). 4.1. Biochar integration into existing markets
This means the producer may sell a function such as Activated carbon suppliers can provide an example for
wastewater filtration to a user, but would remain the future sequential biochar operators, as some already
owner of the char and the responsibility for its recovery. operate partly as service companies selling not only the
After a first use, the biochar is recycled and provided to the activated carbon but organising the transport, application
next user in sale of an alternate service. Such systems are equipment and regeneration of the material60. The
already established in other sectors such as car sharing, activated carbon industry could play a key role for the
where a service rather than a car is sold, alongside a wide organisation and the initial implementation of sequential
variety of industries leasing capital-intensive equipment. biochar systems. The production technology of this
industry can act as an analogue for biochar production59.
151
Synergies can be found in the similarities and overlapping be considered a gradual break with an existing
applications of biochar and activated carbon as already system. In contrast, biochar in wastewater filtration
outlined. AC companies may act as an entry point for the to substitute AC is more compatible due to the
biochar concept into a variety of adsorption related similarities between the two materials. Compatibility
markets. The existing production facilities, market also relates to the organisational structure of the
knowledge and producer-user networks would lower initial innovation user65. If biochar production is based on
costs for commercial trials of biochar in applications where small-scale on-farm production, for example, the
activated carbon is less competitive due to its higher price. technology will less likely be acknowledged by
The market for activated carbon is growing at an expected companies producing chemical fertilisers as it does
annual growth rate of 13% until 202061. However, it is not fit to the centralised nature of their business.
subject to the availability of traditional biomass feedstock, • Complexity of the innovation. Complex innovations
such as coconut shells, which are already becoming scarce are less likely to be adopted. Biochar may provide an
or it is subject to growing environmental concerns in the example of how complexity can inhibit adaption.
case of fossil coal or petroleum coke52,61. As a While the concept of amending soil with biochar is
complementary material, biochar can act as a strategic simple, biochar as a heterogenous class of materials
investment to secure future applications, such as can be more difficult to understand. Matching of
micropollutant removal from wastewaters as well as a biochar with a specific soil type and crop species
general greening of the activated carbon industry.62 requires sufficient expert knowledge to avoid
The advantage of the incorporation of biochar into existing negative results.68
established markets can be explained by examining the • Trialability defines the degree to which adopters can
problems of existing concepts, which often assume test the innovation. If an innovation is trialable, it is
decentralised and small production as more efficient.63,64 more likely to be adapted. Biochar is a relatively easy
4.2. Diffusion of innovations theory material to trial if the material is available in small
quantities from qualitative producers.
The success or failure of new technologies such as biochar • Observability of the idea to other potential users. The
depend on a process of uncertainty reduction among more visible an innovation is to current non-users,
potential users, which if successful, lead to the adoption. the higher the rate of their adoption. Although the
The speed or rate of adoption can be estimated by five influence of biochar application can be visible within
attributes 65: a crop season, additional benefits such as a higher
• Relative Advantage —the degree to which the drought resilience of the soil or increased soil carbon
innovation is better than the status quo. The higher stocks are unlikely to be seen in a short trial.
relative advantage, the higher the rate of adaption.
This attribute is not only understood in economic 4.3. Rate of biochar adoption
turns, such as profitability or initial costs, but also as According to these innovation attributes, the adoption rate
social advantage. Green technologies such as biochar of biochar in developed countries can be assessed. Biochar
can have significant social benefits in the form of for soil amendment lacks relative economic advantage in
social status. Sustainability became a selling point for the short term in competition with chemical fertilisers
a variety of products and industries, not only in based on the generally lower crop responses to biochar23.
relation to customers and stakeholders but also to Due to the absence of widespread large-scale production,
governments62. Governments tend to improve current users must invest considerable time and know-
environmental protection over time, and industries how in the production of specific biochar, adding further
move forward self-organising their own regulation in costs to the technology69. Self-production of biochar might
the face of new laws.66 Another aspect of relative correlate with a production-oriented role of farmers, but it
advantage is time-related. Short-term benefits are is often only partially compatible with current agricultural
more influential than long-term. Considering biochar, practices due to a further addition of workload to the
the relative economic advantage due to high farmers70. High complexity in matching soil properties,
transaction costs and moderate yield increases is crop species and biochars requires expert knowledge and
often minimal in the short term67. While soil can impose entry barriers for agricultural adopters.
application of biochar will eventually lead to a long- Therefore, adoption of biochar is still concentrated on
term increase in soil fertility, end-users will be entrepreneurs with a higher risk affinity rather than the
focussed on short-term benefits such as crop yield or average agricultural farm and overall adoption rate
lower fertiliser requirements as compared with the remains small.63,64,70
additional cost of biochar.63 For other applications of biochar, especially in the context
• Compatibility can be understood as the consistency of a sequential biochar system, there is still a relative
of the innovation with current values and beliefs. If advantage in terms of social status, but also an additional
an innovation is diverting from an existing system, it short-term economic advantage, as wastewater filtration
is less likely to be adapted. In relation to biochar for costs are almost directly measurable. Furthermore,
soil application, the change of chemical fertilisation biochar will be highly compatible to current systems
to a slow and complex fertiliser such as biochar can operating with activated carbon as these materials are
152
almost similar and already used in industry. In contrast to • “Contaminant to constituent” —if a contaminant has
agricultural farms, the end users of wastewater treatment to be decomposed due to environmental concerns,
plants or industrial plants are used to complexity. It seems (e.g., pharmaceuticals) or has to be desorbed like in
promising for sequential biochar systems to use the the case of heavy metals. In contrast to a
similarity with activated carbon as an entry point to “contaminant to nutrient” type of recycling,
existing markets and users of this industry. decomposing or desorbing a contaminant from the
Therefore, the choice of applications is of importance to adsorbent will require more elaborate treatment
increase not only the speed of adaption of the biochar technologies similar to regeneration methods
technology, but also to enable additional synergies of a currently applied in activated carbon regeneration.
sequential biochar system. 5.1. Regeneration methods for activated carbon
Two general indicators for analysing the efficiency of these
5. Synergies in sequential biochar systems methods are : (1) desorption and extraction efficiency
—the amount of contaminant recovered as a proportion of
The concept of sequencing different applications implies
that initially adsorbed; and (2) regeneration efficiency
that the output material from one application will be the
—the adsorption capacity of the regenerated adsorbent
input material of the subsequent application with
relative to its original capacity71. Regeneration efficiency is
minimum treatment in-between. Therefore, one of the
the more commonly used indicator as the general aim is to
determining processes of the efficiency of the system will
prolong the lifetime of the adsorbent within the same
be the necessary recycling of the spent biochar between
application. In the case of decontaminating within a
two applications. As the recycling process, the transport
sequential system, a modified desorption efficiency will be
and handling of spent biochar are additional costs for the
of interest. This would describe the removal (either by
sequence, it is crucial to find a combination of applications,
desorption or decomposition) of the contaminant to the
which make recycling superior to disposal or incineration
level required for a safe application of the decontaminated
(see Fig. 9).
product in a subsequent use step.
Figure 9. Recycling step between different applications.
Two types of a recycling step could be differentiated in

sequential biochar systems:
• “Contaminant to nutrient” —where a contaminant Figure 10. General classification of regeneration methods
adsorbed or loaded on biochar in one application of adsorbent carbonaceous materials.71
constitutes a valuable nutrient in the subsequent
According to one scheme71, regeneration methods for AC
application. Examples of this context-specified
can be classified according to four main approaches:
contaminant/nutrient scenario are various filter-to-
thermal, chemical, microbiological and vacuum (see
fertiliser sequences. Prominent contaminants in this
Fig.10). Only thermal and chemical regeneration reflect
context include phosphorus, nitrogen and sulphur
current industrial practice and are discussed in more detail.
species, which may pose environmental concerns in
Thermal regeneration is the dominant technique to
aquatic environments. On the other side, in an
desorb or decontaminate the adsorbent. In general,
agricultural setting these species are essential plant
thermal regeneration applies enough thermal energy to
nutrients. For recycling based on a “contaminant to
the adsorbent, either by inert gas, steam or an electrical
nutrient” type, almost no relevant treatment must be
current to desorb or decompose the contaminant. If a
conducted. The recycling step could consist only of
contaminant desorbs or decomposes depends on the
drying or other necessary conditioning treatments as
adsorbate and its interaction with the adsorbent. Liu et
well as delivery to the next user. Due to these low
al.72 classified organic compounds into three classes: type
treatment requirements and the added nutritional
1: vaporisation; type 2: decomposition and type 3: char
value for subsequent applications, this type of
formation. Classification depends on their reaction to
recycling step represents the best-case scenario for a
thermal energy input in regeneration as well as their
synergistic combination of applications.
binding energy with the adsorbent. As regeneration
efficiency is the most important indicator in activated
153
carbon regeneration, the dominant techniques are method, which adds an oxidising agent into the
gasification methods utilising a hot gas stream of H2O or hydrothermal treatment79. Wet oxidation is already
CO2 to desorb the contaminants and gasify charred conducted in industrial applications to completely destroy
residues. Typical temperatures are above 800 °C, the organic matter in polluted wastewaters80. This process is
temperature range to enable mild oxidation of the rather aimed at decomposing organic carbon instead of
adsorbent. A drawback of this techniques is the partial adsorbent regeneration, making it a promising technique
oxidation of the adsorbent leading to relevant mass loss of for the decontamination of biochar in a sequential system.
5%–15% during regeneration as well as a gradual However, the production of intermediate products is not
degradation of the adsorption capacity49. While still being precluded by wet-oxidation methods and problematic
the only large-scale regeneration method for activated intermediates can be formed75,80, making additional
carbon, regeneration of spent activated carbon is research necessary.
expensive73. Costs are seldom reported in literature, but a The regeneration methods mentioned so far focus on
model calculation in Switzerland for the upgrade of a public organic contaminants. Only a few studies have focused on
wastewater treatment using a granular activated carbon inorganic pollutants such as heavy metals81. Desorption of
filter reported €1300 t–1 for fresh granular activated carbon heavy metals from adsorbents such as activated carbon is
and €900 t–1 for regenerated carbon.74 conducted by using alkalis, acids or chelating agents82.
Chemical regeneration has been developed in response to Another alternative technique includes the use of
the high energy demands of traditional thermal ultrasound to desorb metals or decompose organic matter
regeneration methods75. The main aim is to lower the from activated carbon.49
energy requirements as well as the mass loss of the Regeneration versus recycling. Although much research
adsorbent during regeneration. A conceptual problem of has been done in the field of carbon regeneration, the
chemical regeneration is the production of a considerable nature of activated carbon as an expensive material and
solvent waste stream. When looking at the whole life cycle the aim to reuse the material limits regeneration research
of water pollutant removal using chemical regeneration, to a focus on adsorption capacity. However, within the
the pollutant is transferred from a low concentration concept of sequential biochar systems, decontamination of
stream into a higher concentrated liquid solvent stream. In the spent adsorbent is more important than conserving its
some cases, the solvent itself can be regarded a pollutant adsorptive properties. While biochar research is still in its
adding additional treatment costs49. infancy and many important research questions are still
Promising methods include sub- and supercritical unanswered, the integration of biochar’s recycling
extraction using water or CO2 as solvents. At elevated potential and post-sorption utilisation into research
pressure and temperature, CO2 and H2O change their designs can open opportunities for novel treatment
solvent properties. Water near or above its critical point methods. While current regeneration methods might be
(Pc = 221 bar, Tc = 374 °C) has a drastically decreased sufficient to decontaminate biochar, it can be
dielectric constant and an increased dissociation hypothesised that pure decontamination techniques can
constant75. At these elevated conditions, water solubility increase the efficiency of the process and open a new
of organic compounds is elevated, making this extraction research field focussed on low-cost adsorbents.
methods highly effective. Additionally to a high extraction Additionally, several applications for biochar itself can act
efficiency, regeneration efficiency sometimes even as decontamination methods with anaerobic digestion and
exceeds 100% by opening up additional adsorption sites49. composting being the most prominent ones. Both
While most studies exposed the adsorbent to a constant applications can destroy pathogens and decompose a
stream of water, a few studies highlighted the possibility to variety of different contaminants through elevated
use a hydrothermal batch process instead76,77. Using a temperatures, acidic conditions as well as a high
closed system can substantially reduce the amount of microbiological activity83,84. However, the non-separability
polluted solvent in the regeneration process, making the of post-use biochar from neither digestate nor compost
regeneration environmentally favourable. Similar to will determine that these applications will only be relevant
current hydrothermal carbonisation techniques, the as the last application before the use as a soil amendment.
reactor is heated to mild temperatures between 180 and
240 °C and autogenic pressure76. In contrast to stream-
based extraction methods, the process is targeted at 6. Suitable applications for sequential
decomposing the pollutant rather than extraction. biochar systems
However, this also means that the closed system does not
Not every potential application of biochar is suitable for
allow to remove the dissolved contaminant from the
regeneration or decontamination. The characteristic of
system during the treatment. As the system eventually has
separability of post-use biochar is an additional
to cool down, re-adsorption can occur if the pollutant is not
characteristic that must be fulfilled by an application to be
decomposed. Another problematic implication of this
suitable within a sequential biochar system. Separability
technique can be the production and adsorption of stable
can be defined as the ability to obtain biochar particles
intermediate decomposition products, which might be
contaminated or non-contaminated with homogenous
even less favourable than the original pollutant78. An
properties. This means that any application which mixes
adaption of this method can be found in the wet-oxidation
biochar with other solids or non-water liquids is most likely
154
not promising as a first sequence. This includes indicating the high costs of current removal techniques 92–
applications such as the use in anaerobic digestion, as a 94.
litter additive in agriculture, as a building material or any While a variety of different techniques are currently being
soil or composting application. researched, adsorption is seen as one of the most
While a variety of different applications fulfil the criteria of promising techniques due to its technical simplicity and
separability, ranging from the use of biochar as a catalyst high removal efficiency94. Additionally, no metabolites are
in biodiesel production to its use as an electrode, not all formed during this wastewater treatment. The use of
these applications are at a stage of development where a biochar for the adsorption of antibiotics and pollutants of
realistic assessment of biochars post-application emerging concern are increasingly studied and showed
properties can be conducted. However, two examples of promising results. In this context, biochar is often
promising applications for a sequential biochar system are highlighted as a low-cost adsorbent. Although no studies
described briefly. While both resembling existing exist on decontamination of the spent adsorbent to be
applications currently utilising activated carbon, they are used in subsequent applications, regeneration studies on
also based on the before mentioned principles of activated carbon show promising removal rates for these
“contaminant to nutrient” and “contaminant to organic contaminants. The use and subsequent recycling of
constituent”. biochar could potentially enable a wider application of
6.1. Adsorption of hydrogen sulphide tertiary wastewater treatment to areas currently not
targeting micropollutants due to its low-cost nature.
Hydrogen sulphide is a common contaminant of biogas However, research still has to proof that full
from different sources such as anaerobic biogas plant or decontamination of micropollutants on biochar is possible.
landfills. It is an acidic and corrosive gas, toxic to humans.
The removal of hydrogen sulphide prior to combustion is a
necessary step to prevent engine damage. It is well known 7. Conclusion
that H2S can be removed by carbon surfaces, with activated
Based on the analysis of past biochar concepts, we
carbon currently used as a commercially available material
introduced the idea of a sequential biochar system to
suitable for H2S removal due to its high carbonaceous
synthesise the recent advancements of biochar research
surface area and porosity.85
with its carbon sequestration potential. We showed that
Biochar has gained increasing interest in substituting the
the perception of biochar solely as a material is a rather
use of activated carbon. While not fully understood yet, it
narrow perspective on its multiple characteristics. In
is postulated that although having a lower surface area, the
contrast, the view of biochar as a carrier for environmental
alkaline nature of biochar and a high mineral matter
services enables a more flexible conceptualisation of
content are determining the adsorption performance of
biochar systems.
biochar85,86, beside a general influence of moisture
The economic focus of our concept is based on distributing
content. Therefore, mineral-rich biochar seems to be a
initial production costs over several users, and associated
promising and sustainable biochar additive for enhanced
risks over several applications. We showed that continuous
H2S removal87. H2S adsorbs on biochar in the form of
ownership rights for biochar within a sequence are a
elemental sulphur within pores or SO4- on the surface of
necessity for this cost and risk distribution. While the
biochar. While regeneration seems to be possible, spent
activated carbon industry can act as an analogue and
S-enriched biochar could potentially be further used as
promising entry point to large-scale application of biochar,
sulphur is an essential plant nutrient and sulphur contents
a sequential biochar system will ultimately have to develop
of soils are gradually declining due to decreasing
a distinct organisational form based on the different cost
S-depositions from atmosphere through emissions as well
structure of biochar being a low-cost material. At present,
as insufficient fertilisation.88
research is often focused on simultaneously boosting the
6.2. Pollutants of emerging concern in wastewater efficiency as well as the costs of biochar into the range of
Emerging micropollutants such as pharmaceuticals and activated carbons. However, we think that the nature of
personal care products are of increasing concern in biochar as a high volume and low-cost material is a
wastewaters and their treatment89. The release of prerequisite to retain a globally relevant carbon
pharmaceuticals such as human and veterinary antibiotics sequestration potential.
or hormone active substances can not only lead to We highlighted the need to find synergies between value-
unpredictable changes in ecosystems, but also to an adding and cost-adding steps as essential to enable
increase in antibiotic resistance genes, a potential threat to efficient sequential biochar systems. At present, potential
human health90. Current wastewater treatment plants are sequences can only be hypothesised due to the lack of
generally not equipped to remove these pollutants91. practical knowledge with most biochar applications.
Additional treatment techniques must be added to remove However, our analysis suggests that recycling of used
or decompose these contaminants. Although the problem biochars will be a key advantage for economic
of emerging pollutants is known, Switzerland is the only competitiveness. As literature generally neglected the
European country with existing legislation for the removal utilisation of a second use phase so far, recycling
of micropollutants in wastewater treatment plants treatments for biochar seem a promising area for further
research.
155
Acknowledgements (13) Lehmann, J.; Gaunt, J.; Rondon, M. Bio-Char

“This project has received funding from the European Sequestration in Terrestrial Ecosystems - A Review.
Union’s Horizon 2020 research and innovation programme Mitig. Adapt. Strateg. Glob. Chang. 2006, 11 (2),
under the Marie Skłodowska-Curie grant agreement No 403–427.
721991”. (14) Jeffery, S.; Verheijen, F. G. A.; van der Velde, M.;
Bastos, A. C. A Quantitative Review of the Effects of
Biochar Application to Soils on Crop Productivity
References Using Meta-Analysis. Agric. Ecosyst. Environ. 2011,
(1) Frölicher, T. L.; Winton, M.; Sarmiento, J. L. 144 (1), 175–187.
Continued Global Warming after CO2 Emissions (15) Co, T.; Preta, T.; Preta, T.; The, C.; Preta, T.;
Stoppage. Nat. Clim. Chang. 2014, 4 (1), 40–44. Technique, C. Ithaka J. 2012, 289, 286–289.
(2) Smith, P. Soil Carbon Sequestration and Biochar as (16) Jeffery, S.; Abalos, D.; Prodana, M.; Bastos, A. C.;
Negative Emission Technologies. Glob. Chang. Biol. Van Groenigen, J. W.; Hungate, B. A.; Verheijen, F.
2016, 22 (3), 1315–1324. Biochar Boosts Tropical but Not Temperate Crop
(3) Verheijen, F. G. A.; Montanarella, L.; Bastos, A. C. Yields. Environ. Res. Lett. 2017, 12 (5).
Sustainability, Certification, and Regulation of (17) Joseph, S.; Graber, E. R.; Chia, C.; Munroe, P.;
Biochar. Pesqui. Agropecu. Bras. 2012, 47 (5), 649– Donne, S.; Thomas, T.; Nielsen, S.; Marjo, C.;
653. Rutlidge, H.; Pan, G. X.; et al. Shifting Paradigms:
(4) Rosales, E.; Meijide, J.; Pazos, M.; Sanromán, M. A. Development of High-Efficiency Biochar Fertilizers
Challenges and Recent Advances in Biochar as Low- Based on Nano-Structures and Soluble Components.
Cost Biosorbent: From Batch Assays to Continuous- Carbon Manage. 2013, 4 (3), 323–343.
Flow Systems. Bioresour. Technol. 2017, 246, 176– (18) Vochozka, M.; Marouškova, A.; Váchal, J.; Straková,
192. J. Biochar Pricing Hampers Biochar Farming. Clean
(5) Alhashimi, H. A.; Aktas, C. B. Life Cycle Technol. Environ. Policy 2016, 18 (4), 1225–1231.
Environmental and Economic Performance of (19) Buss, W.; Graham, M. C.; Shepherd, J. G.; Mašek, O.
Biochar Compared with Activated Carbon: A Meta- Risks and Benefits of Marginal Biomass-Derived
Analysis. Resour. Conserv. Recycl. 2017, 118, 13–26. Biochars for Plant Growth. Sci. Total Environ. 2016,
(6) Thompson, K. A.; Shimabuku, K. K.; Kearns, J. P.; 569–570, 496–506.
Knappe, D. R. U.; Summers, R. S.; Cook, S. M. (20) Shackley, S.; Hammond, J.; Gaunt, J.; Ibarrola, R. The
Environmental Comparison of Biochar and Activated Feasibility and Costs of Biochar Deployment in the
Carbon for Tertiary Wastewater Treatment. Environ. UK. Carbon Manage. 2011, 2 (3), 335–356.
Sci. Technol. 2016, 50 (20), 11253–11262. (21) Shackley, S. The Economic Viability and Prospects
(7) Glaser, B.; Haumaier, L.; Guggenberger, G.; Zech, W. for Biochar in Europe - Shifting Paradigms in
The “Terra Preta” Phenomenon: A Model for Uncertain Times. In Biochar in European Soils and
Sustainable Agriculture in the Humid Tropics. Agriculture: Science and Practice; Shackley, S., Ed.;
Naturwissenschaften 2001, 88 (1), 37–41. Routledge: London; New York, 2016; p 301.
(8) Shackley, S.; Hammond, J.; Gaunt, J.; Ibarrola, R. The (22) Crombie, K.; Mašek, O.; Cross, A.; Sohi, S. Biochar -
Feasibility and Costs of Biochar Deployment in the Synergies and Trade-Offs between Soil Enhancing
UK. Carbon Manage. 2011, 2 (3), 335–356. Properties and C Sequestration Potential. GCB
(9) Crombie, K.; Mašek, O. Pyrolysis Biochar Systems, Bioenergy 2015, 7 (5), 1161–1175.
Balance between Bioenergy and Carbon (23) Chan, K. Y.; Van Zwieten, L.; Meszaros, I.; Downie,
Sequestration. GCB Bioenergy 2015, 7 (2), 349–361. A.; Joseph, S. Agronomic Values of Greenwaste
(10) Laird, D. A. The Charcoal Vision: A Win–Win–Win Biochar as a Soil Amendment. Soil Res. 2007, 45 (8),
Scenario for Simultaneously Producing Bioenergy, 629–634.
Permanently Sequestering Carbon, While Improving (24) Roberts, K. G.; Gloy, B. A.; Joseph, S.; Scott, N. R.;
Soil and Water Quality. Agronomy J. 2008, 100 (1), Lehmann, J. Life Cycle Assessment of Biochar
178-181. Systems: Estimating the Energetic, Economic, and
(11) Verheijen, F.; Jeffery, S.; Bastos, a C.; Van Der Velde, Climate Change Potential. Environ. Sci. Technol.
M.; Diafas, I. Biochar Application to Soils: A Critical 2010, 44 (2), 827–833.
Scientific Review of Effects on Soil Properties, (25) Finanzen.net. CO2 European Emission Allowances
Processes and Functions. JRC Scientific and http://www.finanzen.net/rohstoffe/co2-
Technical Reports 2010, EUR 24099, 151. emissionsrechte/historisch (accessed Aug 25, 2018).
(12) Galinato, S. P.; Yoder, J. K.; Granatstein, D. The (26) Hirst, D. Carbon Price Floor (CPF) and the Price
Economic Value of Biochar in Crop Production and Support Mechanism. Brief. Pap. House Commons
Carbon Sequestration. Energy Policy 2011, 39 (10), 2018, No. 05927.
6344–6350. (27) Liang, C.; Zhu, X.; Fu, S.; Méndez, A.; Gascó, G.; Paz-
Ferreiro, J. Biochar Alters the Resistance and
Resilience to Drought in a Tropical Soil. Environ. Res.
Lett. 2014, 9 (6), 064013.
156
(28) Ding, Y.; Liu, Y.; Liu, S.; Li, Z.; Tan, X.; Huang, X.; Zeng, (41) Mumme, J.; Getz, J.; Prasad, M.; Lüder, U.; Kern, J.;
G.; Zhou, L.; Zheng, B. Biochar to Improve Soil Mašek, O.; Buss, W. Toxicity Screening of Biochar-
Fertility. A Review. Agron. Sustain. Dev. 2016, 36 (2). Mineral Composites Using Germination Tests.
(29) Schmidt, H.P.; Pandit, H.; Cornelisson, G.; Kammann, Chemosphere 2018, 207, 91–100.
C. Organic Biochar Based Fertilization. Geophysical (42) Wang, B.; Gao, B.; Fang, J. Recent Advances in
Research Abstracts 2017, 19, 5683. Engineered Biochar Productions and Applications.
(30) Schmidt, H.P.; Wilson, K. The 55 Uses of Biochar. Crit. Rev. Environ. Sci. Technol. 2017, 47 (22), 2158–
biochar J. 2014, 1–8. 2207.
(31) Shackley, S. Shifting Chars? Aligning Climate Change, (43) European Biochar Foundation (EBC). Guidelines for
Carbon Abatement, Agriculture, Land Use and Food a Sustainable Production of Biochar. Eur. Biochar
Safety and Security Policies. Carbon Manage. 2014, Found. 2016, No. August, 1–22.
5 (2), 119–121. (44) Hagemann, N.; Spokas, K.; Schmidt, H. P.; Kägi, R.;
(32) Schmidt, H.P. Novel uses of biochar. 2013, Proc., Böhler, M. A.; Bucheli, T. D. Activated Carbon,
USBI North American Biochar Symp., Center for Biochar and Charcoal: Linkages and Synergies across
Agriculture, Univ. of Massachusetts, Amherst, MA. Pyrogenic Carbon’s ABCs. Water (Switzerland) 2018,
(33) Simon Shackley, Abbie Clare, Stephen Joseph, Bruce 10 (2), 1–19.
A McCarl, H.-P. S. Economic Evaluation of Biochar (45) Gao, Z.; Zhang, Y.; Song, N.; Li, X. Biomass-Derived
Systems: Current Evidence and Challenges. In Renewable Carbon Materials for Electrochemical
Biochar for Environmental Management—Science Energy Storage. Mater. Res. Lett. 2017, 5 (2), 69–88.
and Technology (2nd Ed) Earthscan, London; (46) Huggins, T. M.; Haeger, A.; Biffinger, J. C.; Ren, Z. J.
Lehmann, J., Joseph, S., Eds.; Routledge, 2015; pp Granular Biochar Compared with Activated Carbon
813–852. for Wastewater Treatment and Resource Recovery.
(34) Shepherd, J. G.; Sohi, S. P.; Heal, K. V. Optimising the Water Res. 2016, 94, 225–232.
Recovery and Re-Use of Phosphorus from (47) Christina Berger. Biochar and Activated Carbon
Wastewater Effluent for Sustainable Fertiliser Filters for Grey Water Treatment - Comparidson of
Development. Water Res. 2016, 94, 155–165. Organiv Matter and Nutrient Removal. Swedish
(35) Morales, V. L.; Pérez-Reche, F. J.; Hapca, S. M.; Univ. Agric. Sci. 2012, 1–36.
Hanley, K. L.; Lehmann, J.; Zhang, W. Reverse (48) Ahmed, M. B.; Zhou, J. L.; Ngo, H. H.; Guo, W. Insight
Engineering of Biochar. Bioresour. Technol. 2015, into Biochar Properties and Its Cost Analysis.
183, 163–174. Biomass Bioenergy 2016, 84, 76–86.
(36) Rajapaksha, A. U.; Chen, S. S.; Tsang, D. C. W.; Zhang, (49) Zanella, O.; Tessaro, I. C.; Féris, L. A. Desorption- and
M.; Vithanage, M.; Mandal, S.; Gao, B.; Bolan, N. S.; Decomposition-Based Techniques for the
Ok, Y. S. Engineered/Designer Biochar for Regeneration of Activated Carbon. Chem. Eng.
Contaminant Removal/Immobilization from Soil and Technol. 2014, 37 (9), 1447–1459.
Water: Potential and Implication of Biochar (50) Stavropoulos, G. G.; Zabaniotou, A. A. Minimizing
Modification. Chemosphere 2016, 148, 276–291. Activated Carbons Production Cost. Fuel Process.
(37) Dieguez-Alonso, A.; Funke, A.; Anca-Couce, A.; Technol. 2009, 90 (7–8), 952–957.
Rombolà, A.; Ojeda, G.; Bachmann, J.; Behrendt, F. (51) Hjaila, K.; Baccar, R.; Sarrà, M.; Gasol, C. M.;
Towards Biochar and Hydrochar Engineering— Blánquez, P. Environmental Impact Associated with
Influence of Process Conditions on Surface Physical Activated Carbon Preparation from Olive-Waste
and Chemical Properties, Thermal Stability, Nutrient Cake via Life Cycle Assessment. J. Environ. Manage.
Availability, Toxicity and Wettability. Energies 2018, 2013, 130, 242–247.
11 (3), 496. (52) Correa, C.; Kruse, A. Biobased Functional Carbon
(38) Liu, W. J.; Jiang, H.; Yu, H. Q. Development of Materials: Production, Characterization, and
Biochar-Based Functional Materials: Toward a Applications—A Review. Materials (Basel). 2018, 11
Sustainable Platform Carbon Material. Chem. Rev. (9), 1568.
2015, 115 (22), 12251–12285. (53) Plaza, M. G.; González, A. S.; Pis, J. J.; Rubiera, F.;
(39) Harikishore Kumar Reddy, D.; Vijayaraghavan, K.; Pevida, C. Production of Microporous Biochars by
Kim, J. A.; Yun, Y. S. Valorisation of Post-Sorption Single-Step Oxidation: Effect of Activation
Materials: Opportunities, Strategies, and Conditions on CO2 Capture. Appl. Energy 2014, 114,
Challenges. Adv. Colloid Interface Sci. 2017, 242, 35– 551–562.
58. (54) Ahmad, M.; Rajapaksha, A. U.; Lim, J. E.; Zhang, M.;
(40) Xiong, X.; Yu, I. K. M.; Cao, L.; Tsang, D. C. W.; Zhang, Bolan, N.; Mohan, D.; Vithanage, M.; Lee, S. S.; Ok,
S.; Ok, Y. S. A Review of Biochar-Based Catalysts for Y. S. Biochar as a Sorbent for Contaminant
Chemical Synthesis, Biofuel Production, and Management in Soil and Water: A Review.
Pollution Control. Bioresour. Technol. 2017, 246, Chemosphere 2014, 99, 19–23.
254–270. (55) Cha, J. S.; Park, S. H.; Jung, S. C.; Ryu, C.; Jeon, J. K.;
Shin, M. C.; Park, Y. K. Production and Utilization of
Biochar: A Review. J. Ind. Eng. Chem. 2016, 40, 1–15.
157
(56) De Gisi, S.; Lofrano, G.; Grassi, M.; Notarnicola, M. (71) Salvador, F.; Martin-Sanchez, N.; Sanchez-
Characteristics and Adsorption Capacities of Low- Hernandez, R.; Sanchez-Montero, M. J.; Izquierdo,
Cost Sorbents for Wastewater Treatment: A Review. C. Regeneration of Carbonaceous Adsorbents. Part
Sustain. Mater. Technol. 2016, 9, 10–40. I: Thermal Regeneration. Microporous Mesoporous
(57) Ahmed, M. B.; Zhou, J. L.; Ngo, H. H.; Guo, W. Mater. 2015, 202 (C), 259–276.
Adsorptive Removal of Antibiotics from Water and (72) Liu, P. K. T.; Feltch, S. M.; Wagner, N. J. Thermal
Wastewater: Progress and Challenges. Sci. Total Desorption Behavior of Aliphatic and Aromatic
Environ. 2015, 532, 112–126. Hydrocarbons Loaded on Activated Carbon. Ind.
(58) Moreira, M. T.; Noya, I.; Feijoo, G. The Prospective Eng. Chem. Res. 1987, 26 (8), 1540–1545.
Use of Biochar as Adsorption Matrix - A Review from (73) Santadkha, T.; Skolpap, W. Economic Comparative
a Lifecycle Perspective. Bioresour. Technol. 2017, Evaluation of Combination of Activated Carbon
246, 135–141. Generation and Spent Activated Carbon
(59) Shackley, S.; Clare, A.; Joseph, S.; A McCarl, B.; Regeneration Plants. J. Eng. Sci. Technol. 2017, 12
Schmidt, H.-P. Economic Evaluation of Biochar (12), 3329–3343.
Systems: Current Evidence and Challenges; 2015. (74) Wermter, P.; Herbst, H.; Türk, J. Ertüchtigung von
(60) Jacobi Carbon Group. Services. Kläranlagen, Investitionen & Kosten in NRW, BW &
https://www.jacobi.net/services (accessed Aug 21, CH. Presented at Maßnahmenprogramm WRRL
2018). 2015, Bezirksregierung Münster, 10.10.2013.
(61) Radiant Insights. Global activated carbon market. Flussgebietsmanagement, https://www.fiw.rwth-
https://www.radiantinsights.com/press- aachen.de/neo/index.php?id=192 (accessed August
release/global-activated-carbon-market (accessed 25, 2018).
August 25, 2018). (75) Salvador, F.; Martin-Sanchez, N.; Sanchez-
(62) Tariq, A.; Badir, Y. F.; Tariq, W.; Bhutta, U. S. Drivers Hernandez, R.; Sanchez-Montero, M. J.; Izquierdo,
and Consequences of Green Product and Process C. Regeneration of Carbonaceous Adsorbents. Part
Innovation: A Systematic Review, Conceptual II: Chemical, Microbiological and Vacuum
Framework, and Future Outlook. Technol. Soc. 2017, Regeneration. Microporous Mesoporous Mater.
51, 8–23. 2015, 202, 277–296.
(63) Bach, M.; Wilske, B.; Breuer, L. Current Economic (76) Sühnholz, S.; Kopinke, F. D.; Weiner, B.
Obstacles to Biochar Use in Agriculture and Climate Hydrothermal Treatment for Regeneration of
Change Mitigation. Carbon Manage. 2016, 7 (3–4), Activated Carbon Loaded with Organic
183–190. Micropollutants. Sci. Total Environ. 2018, 644, 854–
(64) Ahmed, S.; Hammond, J.; Ibarrola, R.; Shackley, S.; 861.
Haszeldine, S. The Potential Role of Biochar in (77) Weiner, B.; Suhnholz, S.; Kopinke, F. D.
Combating Climate Change in Scotland: An Analysis Hydrothermal Conversion of Triclosan-The Role of
of Feedstocks, Life Cycle Assessment and Spatial Activated Carbon as Sorbent and Reactant. Environ.
Dimensions. J. Environ. Plan. Manag. 2012, 55 (4), Sci. Technol. 2017, 51 (3), 1649–1653.
487–505. (78) Weiner, B.; Baskyr, I.; Poerschmann, J.; Kopinke, F.
(65) Rogers, E. Diffusion of Innovations, Fifth edit.; Free D. Potential of the Hydrothermal Carbonization
Press: New York, 2003. Process for the Degradation of Organic Pollutants.
(66) Gunningham, N. Environment Law, Regulation and Chemosphere 2013, 92 (6), 674–680.
Governance: Shifting Architectures. J. Environ. Law (79) Ledesma, B.; Román, S.; Sabio, E.; Álvarez-Murillo,
2009, 21 (2), 179–212. A. Improvement of Spent Activated Carbon
(67) Vochozka, M.; Maroušková, A.; Váchal, J.; Straková, Regeneration by Wet Oxidation Processes. J.
J. The Economic Impact of Biochar Use in Central Supercrit. Fluids 2015, 104, 1–10.
Europe. Energy Sources, Part A Recover. Util. (80) Riedel, G.; Koehler, R.; Poerschmann, J.; Kopinke, F.
Environ. Eff. 2016, 38 (16), 2390–2396. D.; Weiner, B. Combination of Hydrothermal
(68) Borchard, N.; Siemens, J.; Ladd, B.; Möller, A.; Carbonization and Wet Oxidation of Various
Amelung, W. Application of Biochars to Sandy and Biomasses. Chem. Eng. J. 2015, 279, 715–724.
Silty Soil Failed to Increase Maize Yield under (81) Da’Na, E.; Awad, A. Regeneration of Spent Activated
Common Agricultural Practice. Soil Tillage Res. Carbon Obtained from Home Filtration System and
2014, 144, 184–194. Applying It for Heavy Metals Adsorption. J. Environ.
(69) Salo, E. Current state and future perspectives of Chem. Eng. 2017, 5 (4), 3091–3099.
biochar applications in Finland. M.Sc. Thesis, (82) Lata, S.; Singh, P. K.; Samadder, S. R. Regeneration
Jyväskylä University School of Business and of Adsorbents and Recovery of Heavy Metals: A
Economics, Jyväskylä, Finland, 2018. Review. Int. J. Environ. Sci. Technol. 2015, 12 (4),
(70) Otte, P. P.; Vik, J. Biochar Systems: Developing a 1461–1478.
Socio-Technical System Framework for Biochar
Production in Norway. Technol. Soc. 2017, 51, 34–
45.
158
(83) Godlewska, P.; Schmidt, H. P.; Ok, Y. S.; Oleszczuk, P.

Biochar for Composting Improvement and
Contaminants Reduction. A Review. Bioresour.
Technol. 2017, 246, 193-202.
(84) Youngquist, C. P.; Mitchell, S. M.; Cogger, C. G. Fate
of Antibiotics and Antibiotic Resistance during
Digestion and Composting: A Review. J. Environ.
Qual. 2016, 45 (2), 537.
(85) Xu, X.; Cao, X.; Zhao, L.; Sun, T. Comparison of
Sewage Sludge- and Pig Manure-Derived Biochars
for Hydrogen Sulfide Removal. Chemosphere 2014,
111, 296–303.
(86) Bamdad, H.; Hawboldt, K.; MacQuarrie, S. A Review
on Common Adsorbents for Acid Gases Removal:
Focus on Biochar. Renew. Sustain. Energy Rev. 2016,
91(2), 1705-1720.
(87) Xu, X.; Zhao, Y.; Sima, J.; Zhao, L.; Mašek, O.; Cao, X.
Indispensable Role of Biochar-Inherent Mineral
Constituents in Its Environmental Applications: A
Review. Bioresour. Technol. 2017, 241, 887–899.
(88) Webb, J.; Jephcote, C.; Fraser, A.; Wiltshire, J.;
Aston, S.; Rose, R.; Vincent, K.; Roth, B. Do UK Crops
and Grassland Require Greater Inputs of Sulphur
Fertilizer in Response to Recent and Forecast
Reductions in Sulphur Emissions and Deposition?
Soil Use Manag. 2016, 32 (1), 3–16.
(89) Bo, L.; Shengen, Z.; Chang, C.-C.; Zhanfeng, D.;
Hongxiang, L. Emerging Pollutants - Part II:
Treatment. Water Environ. Res. 2015, 87 (10), 1873–
1900.
(90) Dulio, V.; van Bavel, B.; Brorström-Lundén, E.;
Harmsen, J.; Hollender, J.; Schlabach, M.; Slobodnik,
J.; Thomas, K.; Koschorreck, J. Emerging Pollutants
in the EU: 10 Years of NORMAN in Support of
Environmental Policies and Regulations. Environ.
Sci. Eur. 2018, 30 (1), No. 5.
(91) Méndez, E.; González-Fuentes, M. A.; Rebollar-
Perez, G.; Méndez-Albores, A.; Torres, E. Emerging
Pollutant Treatments in Wastewater: Cases of
Antibiotics and Hormones. J. Environ. Sci. Heal. Part
A 2016, 52 (3), 1–19.
(92) Müller, S. Micropollutants in municipial waste water
- the Swiss strategy; Reference: N222-2504; Federal
Office for the Environment: Bern, Switzerland, 2015.
(93) Abegglen, C.; Siegrist, H. Mikroverunreinigungen
aus kommunalem Abwasser - Verfahren zur
weitergehenden Elimination aus Kläranlagen;
Bundesamt für Umwelt: Bern, Switzerland, 2012:
210.
(94) Eggen, R. I. L.; Hollender, J.; Joss, A.; Scha, M.
Reducing the Discharge of Micropollutants in the
Aquatic Environment: The Bene Fi Ts of Upgrading
Wastewater Treatment Plants. Environ. Sci. Technol.
2014, 48, 7683–7689.
159
160
View publication stats

GreenCarbon ETN Book

Uploaded by

Copyright:

Available Formats

GreenCarbon ETN Book

Uploaded by

Document Information

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

GreenCarbon ETN Book

Uploaded by

Copyright:

Available Formats

See discussions, stats, and author profiles for this publication at: https://www.researchgate.

Reﬁned biocarbons for gas adsorption and separation

Chapter · May 2019

Sabina Alexandra Nicolae Xia Wang

SEE PROFILE SEE PROFILE

Pyrolysis View project

Postdoc UCSB View project

The user has requested enhancement of the downloaded file.

Edited by Joan J. Manyà

Copyright © GreenCarbon Project and Consortium

The main objective of the GreenCarbon European Training Network (MSCA-­‐ITN-­‐ETN-­‐721991) is to

Associate Professor of Chemical Engineering, Universidad de Zaragoza

Coordinator of the GreenCarbon ETN

Chapter 2 ........................................................................................................................................... 21

Chapter 3 ........................................................................................................................................... 33

Chapter 4 ........................................................................................................................................... 45

Chapter 5 ........................................................................................................................................... 57

Chapter 6 ........................................................................................................................................... 71

Chapter 7 ........................................................................................................................................... 83

Chapter 8 ......................................................................................................................................... 101

Chapter 9 ......................................................................................................................................... 111

Chapter 10 ....................................................................................................................................... 125

Chapter 11 ....................................................................................................................................... 135

Chapter 12 ....................................................................................................................................... 147

The Intergovernmental Panel on Climate Change (IPCC)

This book is published under a CC BY-NC 4.0 license

2. Biomass and char b)

2.2. The biochar concept • Energy industry to produce electricity.

Pyrolysis of biomass and wastes in a continuous screw reactor for char

sugarcane, and many others, compose the herbaceous

This book is published under a CC BY-NC 4.0 license

References (19) Oasmaa, A.; Czernik, S. Fuel Oil Quality of Biomass

(64) Yang, Y.; Brammer, J. G.; Mahmood, A. S. N.; Hornung,

Industrial technologies for slow pyrolysis

1. Introduction 2. Batch Processes

Feeding Drying Pyrolysis Cooling Collection

Figure 1. Main process steps.

One of the characteristics mentioned in pyrolysis is the

This book is published under a CC BY-NC 4.0 license

4.1. Lambiotte 4.2.Reichert

Figure 6. Reichert system.

Figure 8. Herreshoff multiple-hearth furnace.

4.5.Herreshoff multiple-hearth furnace

Figure 12. Rotary kiln reactor.

Garcia-Nunez et al.3 highlight the good balance between

Figure 2. Block diagram.

5.3.2. Economic evaluation (𝑁𝑁𝑁𝑁𝑁𝑁 𝐶𝐶𝐶𝐶𝐶𝐶ℎ 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹)𝑛𝑛

Table 1. Comparison of the reactor technologies available for slow pyrolysis.

Direct Direct Direct Direct Direct Direct Direct Direct Direct

Heating with an Volatile external Heating with an

Acknowledgements (16) Dhyani, V.; Bhaskar, T., A comprehensive review on the

Matter of modelling in thermochemical conversion processes of biomass

improvements of technologies. In the last four decades,

This book is published under a CC BY-NC 4.0 license

3. Idea behind the CFD-DEM method

equations). The accuracy and precision of the result from a

Figure 4. Visualization of meshing based on finite volume

3.4. Structure of a comprehensive model

Figure 9. Ranzi’s kinetic scheme, adapted from Anca-

valid to obtain the value of superficial gas velocity: = ∇�𝜆𝜆𝑒𝑒𝑒𝑒𝑒𝑒 ∇𝑇𝑇� + 𝑄𝑄

The main objective of the GreenCarbon European Training Network (MSCA-‐ITN-‐ETN-‐721991) is to