Accepted Manuscript

Title: Production and Utilization of Biochar: A Review

Author: Jin Sun Cha Sung Hoon Park Sang-Chul Jung

Changkook Ryu Jong-Ki Jeon Min-Chul Shin Young-Kwon
Park

PII: S1226-086X(16)30147-2
DOI: http://dx.doi.org/doi:10.1016/j.jiec.2016.06.002
Reference: JIEC 2954

To appear in:

Received date: 20-4-2016

Revised date: 5-6-2016
Accepted date: 6-6-2016

Please cite this article as: J.S. Cha, S.H. Park, S.-C. Jung, C. Ryu, J.-K. Jeon, M.-C. Shin,
Y.-K. Park, Production and Utilization of Biochar: A Review, Journal of Industrial and
Engineering Chemistry (2016), http://dx.doi.org/10.1016/j.jiec.2016.06.002

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Production and Utilization of Biochar: A Review

Jin Sun Chaa,b, Sung Hoon Parkc, Sang-Chul Jungc,

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Changkook Ryud, Jong-Ki Jeone, Min-Chul Shinb, Young-Kwon Parka,*

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a
School of Environmental Engineering, University of Seoul, Seoul 02504, Republic of Korea

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b
Korea Testing Laboratory, Seoul 08389, Republic of Korea

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c
Department of Environmental Engineering, Sunchon National University, Suncheon 57922,

Republic of Korea
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d
School of Mechanical Engineering, Sungkyunkwan University, Suwon 16419, Republic of
d
Korea
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e
Department of Chemical Engineering, Kongju National University, Cheonan 31080, Korea
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*Corresponding author

Tel:+82-2-6490-2870, Fax:+82-2-6490-2859, e-mail: catalica@uos.ac.kr

Page 1 of 65
 Graphical abstract

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Page 2 of 65
Abstract

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Biochar produced during the thermochemical decomposition of biomass not only reduces the

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amount of carbon emitted into the atmosphere, but it is also an environment-friendly

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replacement for activated carbon and other carbon materials. In this review paper, researches

on biochar are discussed in terms of production method and application. Different processes

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for biochar production, such as pyrolysis, gasification, hydrothermal carbonization, etc., are

compared. Physical and chemical activation methods used to improve the physicochemical

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properties of biochar and their effects are also compared. Various environmental application

fields of biochar including adsorption (for water pollutants and for air pollutants), catalysis
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(for syngas upgrading, for biodiesel production, and for air pollutant treatment), and soil

conditioning are discussed. Recent research trend of biochar in other applications, such as
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fuel cell, supercapacitor, and hydrogen storage, is also reviewed.

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Keywords : Biochar, Biochar activation, Biochar application, Adsorbent, Catalyst, Soil

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amendment
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Page 3 of 65
 Contents

Introduction

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Production of Biochar

Pyrolysis

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Gasification

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(1) Drying

(2) Pyrolysis

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(3) Oxidation/Combustion

(4) Gasification
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Hydrothermal carbonization (HTC)

Other thermochemical technologies

Modification of biochar
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Physical activation
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Chemical activation
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Application of biochar
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Adsorbent

(1) Adsorbent for water pollutants

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(2) Adsorbent for air pollutants

Catalysts

(1) Catalysts for syngas cleaning

(2) Catalyst for biodiesel production

(3) Catalyst for air pollution control

Soil amendment

Other applications

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Conclusion

Acknowledgement

References

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Introduction

The combustion of fossil fuels emits CO2, which causes global warming and climate change

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as well as the production of air pollutants, such as the oxides of sulfur and nitrogen [1]. Fossil

fuel exhaustion, increasing oil price, and the rise of global environmental problems are

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calling for the development of alternative energy resources to replace conventional fossil

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fuels. Organic materials originating from living matter or complexes of organic and inorganic

materials from said sources are referred to collectively as biomass. Biomass includes not only

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living organisms, such as plants and animals, but also the excrement of animals, sludge, and

waste wood [2]. Thermochemical decomposition processes convert biomass materials to

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syngas, bio-oil, and biochar. The gas product syngas and the liquid product bio-oil are

regarded as alternative fuels to fossil fuels, and extensive research is being conducted on their
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formation, upgrading, and applications [3-7]. Biomass is considered carbon neural because
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the carbon dioxide emitted from biomass is compensated for by the carbon assimilation
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occurring during the photosynthesis of biomass. Biomass is also known to have fewer
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adverse effects on the atmosphere because it contains less S and N, resulting in lower SOx

and NOx emissions than fossil fuels [1].

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Biochar, the solid material formed during the thermochemical decomposition of biomass, is

defined, by the International Biochar Initiative (http://www.biochar-international.org/biochar),

as a solid material obtained from the carbonization of biomass. As biochar is inexpensive,

environment-friendly, and can be used for a variety of purposes, such as soil remediation,

waste management, greenhouse gas reduction, and energy production, several studies have

been conducted to develop new applications of biomass [8]. Although the main element of

biochar is carbon (C), it also contains hydrogen (H), oxygen (O), ash, and trace amounts of

Page 6 of 65
nitrogen (N) and sulfur (S) [9]. The elemental composition of biochar varies according to the

raw biomass material from which the biochar was produced and the characteristics of the

carbonization process [10-12]. Because of its large specific surface area, porous structure,

surface functional groups, and high mineral content, biochar has been used as an adsorbent

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for water and air pollutants [12,13], a catalyst to remove tar or produce biodiesel [14,15], and

as a soil amendment [12,16]. Recently, the applications of biochar to fuel cells [17,18] and

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supercapacitors [19,20] have also been reported.

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In this article, before looking into the application instances of biochar, the thermochemical

decomposition processes for the production of biochar and previous studies on the biochar

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modification to improve its properties are reviewed to enhance the understanding of biochar

formation. Most previous review articles on biochar application discussed the removal of pollutants
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in water [13,21,22] and soil [23-27] using biochar. Only recent research trend in those fields is

briefly presented in this article, whereas the applications of biochar to the removal of hazardous
d

pollutants (including air pollutants and tar) and other recently developed application fields, such as
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fuel cell, supercapacitor, and hydrogen storage, are discussed in detail.

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Production of Biochar
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This section summarizes several different carbonization processes to produce biochar along

with their characteristics.

Pyrolysis

Pyrolysis is a process for decomposing organic materials thermally under oxygen-free

conditions in the temperature range, 300~900 [28-31]. During thermal decomposition,

cellulose, hemicellulose and lignin that comprise biomass undergo their own reaction

Page 7 of 65
pathways, including cross-linking, depolymerization and fragmentation at their own

temperature, producing solid, liquid and gaseous products. The solid and liquid products are

referred to as char and bio-oil, respectively, whereas the gaseous mixture containing CO, CO2,

H2, and C1-C2 hydrocarbons are called syngas. The yields of the pyrolysis products depend on

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the characteristics of the raw biomass materials and the pyrolysis processes adapted.

The parameters that influence the products of the pyrolysis processes include the reaction

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temperature, heating rate, and residence time. In general, the biochar yield decreases and the

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syngas yield increases with increasing pyrolysis temperature [32-35]. Mohammad et al. [35]

and Zhang [36] reported that the yields of biochar and acidic functional groups decreased

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with increasing pyrolysis temperature, whereas those of the basic functional groups, ash

content, pH, and carbon stability increased. The increase in pH with increasing pyrolysis
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temperature was attributed to the reduction of organic functional groups, such as COOH and

-OH. The bio-oil yield was reportedly highest at approximately 500 because cracking
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takes place at higher temperatures [34]. Fig. 1 shows the elemental compositions of biochar
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produced at different pyrolysis temperatures [37]. Pyrolysis processes are divided into slow
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pyrolysis and fast pyrolysis depending on the rate of the increase in temperature. In a slow
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pyrolysis, in which the pyrolyzed vapors reside for a long time in the reactor at low

temperatures, continuing vapor-phase reactions increase the char yield [32,34,38]. On the
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other hand, Inguanzo et al. [32] reported that the increased in char yield with decreasing

temperature rising rate only occurred when the pyrolysis temperature was lower than 650.

They attributed this to cracking reactions occurring at temperatures below 650, whereas

decarbonylation reactions of oxygenated hydrocarbons become dominant above 650.

Inguanzo et al. [39] evaluated the characteristics of the chars produced at two different

temperature rising rates of 5/min and 60/min. They reported that the char produced at

Page 8 of 65
the higher temperature rising rate had a lower volatile matter content and a higher ash

(including fixed carbon) yield and concluded that a high temperature rising rate is preferable

in terms of the quality of the product biochar. This effect of the temperature rising rate was

not observed at high pyrolysis temperatures [40]. Therefore, fast pyrolysis is generally aimed

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at producing a liquid product in high yield [41,42]. To suppress the gas production due to

secondary cracking, the vapor residence time is controlled short and rapid cooling is used to

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maximize the liquid product yield.

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Regarding the effects of the residence time on the product composition in a pyrolysis process,

Zhang et al. [36] reported that the biochar yield decreased with increasing residence time at

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the same pyrolysis temperature. In a study of the effects of the residence time on the specific

surface area and pore characteristics of biochar, Lu et al. [43] reported that the specific
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surface area and pore area increased with increasing residence time up to 2 h at 500900

but they decreased when the residence time exceeded 2 h. In particular, the specific surface
d

area and pore area decreased rapidly when the residence time exceeded 2 h at high
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temperatures. Bandosz et al. observed in a study of the pyrolysis of sewage sludge that the
p

specific surface area decreased from 141 m2/g to 125 m2/g and the pore volume decreased
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from 0.209 cm3/g to 0.187 cm3/g when the residence time was increased from 30 min to 1 h

at a pyrolysis temperature of 950 [44]. Lu et al. attributed the results of Bandosz et al. to a
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narrowing and closure of the pore entrances owing to the sintering of char, resulting in a

reduction of the specific surface area [43]. Table 1 summarizes the characteristics of biochar

produced under various pyrolysis conditions.

Gasification

Gasification is a thermochemical partial oxidation process converting carbonaceous materials,

such as biomass, coals, and plastic materials, to gaseous products using gasification agents

Page 9 of 65
(air, steam, oxygen, CO2 or gas mixture). In a gasification process, gaseous products (H2, CO,

CO2, N2, etc.), liquid products (tar and oil), and solid products (char and ash) are formed.

Because gasification is aimed at producing gaseous products, the yield of biochar is only

approximately 5~10% of the raw biomass material mass, which is lower than that of fast

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pyrolysis (15~20%) [13,45]. Fig. 2 compares the product yields of slow pyrolysis, fast

pyrolysis, and gasification.

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The mechanism of gasification can be divided into several steps, as shown below, but each

step cannot be separated from the others clearly in terms of temperature and pressure.

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Drying Pyrolysis Oxidation/Combustion Gasification
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(1) Drying
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The moisture in biomass is evaporated in this step and the energy used for drying is not
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recovered. The moisture content of biomass varies according to the biomass material gasified.
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A separate drying step is required before the biomass is introduced into the gasifier when the
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moisture content is too high.

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(2) Pyrolysis

Pyrolysis takes place over the temperature range, 150~400, decomposing the thermally

weak constituents of biomass, such as lignin, and producing char, gases and liquids.

Biomass + Heat Char + Gases + Liquids

Gaseous products formed during the pyrolysis step include H2, CO, CO2, H2O, and small-

Page 10 of 65
molecular-mass hydrocarbons, such as CH4. The liquid product is mostly mucous tar

composed of small-molecular-mass organic matter. The product composition depends on the

reaction temperature, pressure, and temperature rising rate.

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(3) Oxidation/Combustion

Oxidation and combustion of some gas species and char are important sources of energy

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required for gasification reactions. The gasification agent provided into the gasifier reacts

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with combustible species, producing CO, CO2 and H2O, and with the char produced during

the pyrolysis step, producing CO2.

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C + O2 CO2
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H2 + 1/2O2 H2O
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(4) Gasification
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The char produced during the pyrolysis step is converted to CO, CH4, and H2 through various
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gasification reactions. The gasifiers, in which the gasification reactions take place are divided
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into fixed bed reactors, moving bed reactors, fluidized bed reactors, and entrained bed

reactors depending on the way of contact between gasification agent and biomass. The
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parameters affecting the gasification reactions include the reaction temperature, gasification

agent type, gasification agentbiomass ratio, and pressure. Taba et al. [47] suggested that the

temperature is the most important among the parameters directly affecting the gasification

reactions. They reported that increasing temperature enhanced the production of H2, CO, and

carbon, while reducing the contents of CO2, CH4, hydrocarbons and tar. Gomez-Barea et al.

[48] also reported that the reduction of the gasification temperature in fluidized bed

gasification resulted in reduced carbon conversion and an increased tar concentration in gas.

Page 11 of 65
Carbon conversion as well as the composition and heating value of the product gas also

varies according to the gasification agent used in the gasification process. Tay et al. [49]

examined the effects of gasification agents on the characteristics of the char using a reducing

gasification agent (H2O) and oxidizing gasification agents (CO2 and O2). The ratio of the

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small and large aromatic ring structure of the aromatic compounds composing the char was

lower and the contents of alkaline metals, such as Mg and Ca, in char were higher when H2O

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was used.

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Hydrothermal carbonization (HTC)

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Dry processes, such as pyrolysis and gasification can achieve high product yields with low

energy loss when the moisture content of the biomass is low. On the other hand, most
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biomass materials have high moisture contents and thus a separate drying step is required to

obtain high product yields and reduce the process energy. Hydrothermal processes are
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anticipated to be able to remedy this shortcoming of dry processes. Although the char
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produced from a hydrothermal process is often called hydrochar to distinguish it from the
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biochar produced from dry processes, the general term biochar is used for hydrochar in this
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article. In a hydrothermal process, biomass mixed with water is placed in a closed reactor and

the temperature is raised after a certain time for stabilization. The pressure is also raised for
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water to maintain a liquid state above 100. Depending on the temperature under saturated

pressure, biochar [50,51], bio-oil [52], and gaseous products, such as CO, CO2, CH4, and H2

[53], are the main products of a hydrothermal process below 250, at 250~400, and above

400, respectively. Therefore, the hydrothermal process at each temperature range is called

hydrothermal carbonization (HTC), hydrothermal liquefaction (HTL), and hydrothermal

gasification (HTG), respectively. The char produced from the HTC process has a higher C

Page 12 of 65
content than the char produced from dry processes [54], and reaction temperature, pressure,

residence time, and water-biomass ratio are reportedly the main parameters determining the

characteristics of the products [23,50,54]. The yield and characteristics of biochar produced

under different conditions (HTC temperature, time, biomass/water ratio) are summarized in

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Table 2.

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Other thermochemical technologies

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Besides pyrolysis, gasification, and hydrothermal carbonization discussed in the foregoing,
flash carbonization [55-57] and torrefaction [58-60] are another thermochemical biomass

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conversion processes. In the flash carbonization, flash fire is ignited on a packed bed of
biomass at a high pressure (1-2 MPa) to convert biomass into gas-phase and solid-phase
products. Temperature and reaction time used generally for flash carbonization are
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300~600 and 30 min, respectively. It has been reported that about 40% of biomass is

converted into solid-phase product (biochar) at 1 MPa [55] and that the carbonization time
d

decreases with increasing pressure [56]. Compared to other processes for biochar production,
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however, reports on flash carbonization found in the literature are limited. Torrefaction is a
process in which moisture, carbon dioxide, and oxygen contained in biomass are removed
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under an inert condition at 200~300 and long polysaccharide chains are depolymerized to
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produce hydrophobic solid product with a low O/C ratio. This process is generally operated
with a slow heating rate and hence is referred to as mild pyrolysis. Several researchers
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[61,62] have reported that torrefaction temperature is a more important parameter, than
residence time, that influences the biomass weight loss and the chemical and thermophysical
properties of the product. Torrefaction is studied usually to improve the thermochemical
properties of biomass used for combustion, gasification, and co-combustion with coal; the
focus is on the gas production. Table 3 compares the typical operation conditions and product
properties of various processes for biochar production.

Modification of biochar

Page 13 of 65
The biochar produced from thermal treat95ment processes is often activated to increase its

specific surface area and pore fraction or to form functional groups in it prior to use. The

activation methods are divided into physical activation and chemical activation [64-66]. The

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pore fraction and pore size distribution of the activated biochar vary according to the type and

quantity of biomass and activating gas.

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Physical activation

Physical activation, also called gas activation, uses gas, such as steam [67-73], CO2 [74-78], and

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ozone [79,80], to activate biochar at temperatures above 700. Physical activation can be divided

into two steps. In the first step, the unstructured parts of the carbonized material are
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decomposed selectively while fine pores enclosed in the carbon structure are opened, thereby

increasing the internal surface area. In the second step, crystalloid carbon comprising
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carbonized material or carbon containing fine pores are depleted by the activation reactions,
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forming larger pores. The procedure of pore formation in the physical activation is closely
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related to the depletion of carbon by reactions. The reaction mechanism of carbon activation
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using steam is as follows:

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Cf + H2O C(O) + H2 (a)

C(O) CO (b)

2Cf + H2 2C(H) (c)

Cf + 2H2O CO2 + 2H2 (d)

Cf : carbon-free active site

Page 14 of 65
 C(O) : oxygen surface complex

C(H) : chemisorbed hydrogen

The rate of formation of pores by physical activation depends on the ash content in the

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carbonized material and the status of the pores.

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C + H2O C(H2O) (e)

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C(H2O) C(O) + H2 (f)

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The parentheses appearing in Eqs. (e) and (f) represent the attachment to the solid carbon
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surface. Water vapor attached to the carbon surface is decomposed to emit hydrogen gas,

whereas the oxygen attached to the carbon surface reacts with carbon forming CO. The
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hydrogen gas formed may remain in the vicinity of the carbon surface and retard the progress
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of the reactions as follows:

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C + H2 C(H2) (g)
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Under this condition, the rate of activation by H2O can be represented by the following
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equation:

(h)

The reaction equation of carbon activation using CO2 is given as follows:

Page 15 of 65
 C + CO2 2CO (i)

As in the activation process using steam, the CO formed by the reaction shown in Eqs. (j)~(l)

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may remain in the vicinity of the carbon surface to retard further reactions.

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C + CO2 C(O) + CO (j)

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C(O) CO (k)

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C + CO C(CO)
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The CO formed during CO2 activation removes the oxide complex from the carbon surface

and causes non-uniform gasification, retarding the C-CO2 reaction. Under the assumption of
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uniform gasification, however, the rate of the C-CO2 reaction can be expressed as
p te

(m)
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The characteristics of physical activation depend on the biochar activated, activating gas, and

reaction conditions. Nabais et al. [73] activated the biochar obtained from the pyrolysis of

coffee endocarp using steam and CO2. They reported that the activation rate is higher when

steam was used, whereas the specific surface area and pore size were higher when CO2 was

used.

In a study of the carbonization of olive seed to produce activated carbon, Reinoso et al. [81]

used CO2 and steam as the activating gas and compared the results. The activated carbon

Page 16 of 65
produced using pure steam had a smaller micropore volume than that activated using CO2

(CO2-activated carbon). When diluted steam was used at high temperatures, however, a

microporosity similar to that of CO2-activated carbon developed. They also reported that activation

using steam was superior to CO2-activation in developing meso- and macro-porosity, leading to

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a higher pore size.

Zhang et al. [77] examined the effects of temperature and duration of CO2-activation on the

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characteristics of the biochar produced from the fast pyrolysis of oak wood waste, corn hulls,

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and corn stover. All three types of chars showed an increasing specific surface area and

micropore volume with increasing activation temperature. The effects of the activation

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duration, however, was different depending on the raw biomass material. The specific surface

area increased with increasing activation duration in the case of oak wood waste-derived char,
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whereas the other two chars showed a decreasing specific surface area with increasing

activation duration. Guo et al. [78] generated char by pyrolyzing coconut shell at 600 and
d

activated it using CO2 in the temperature range, 750~950, to examine the effects of the
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activation temperature and duration and CO2 flow rate on the specific surface area, total pore
p

volume, micropore volume, and carbon fraction of char. When the activation temperature was
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lower than 900, the specific surface area and micropore volume of the char increased with

increasing activation temperature. When the activation temperature was higher than 900,
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however, the specific surface area and micopore volume decreased with increasing activation

temperature above 900 C, whereas the mesopore volume increased. The specific surface area,

micropore volume, and mesopore volume increased with increasing activation duration up to

5 h. Activation for more than 5 h, however, caused the pores to collapse, leading to a

decrease in specific surface area and pore volume. The char yield decreased with increasing

CO2 flow rate due to the burn-out of carbon. A high CO2 flowrate below a certain threshold

Page 17 of 65
promoted the formation of pores but an excessive flow above a certain threshold decreased

the specific surface area and pore volume, indicating that char activation is affected by the

process parameters. Table 4 compares the characteristics of biochars produced from various

feedstock biomass materials using different physical activation methods.

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Chemical activation`

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In chemical activation, char is doped with a chemical agent and micropores are formed by

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subsequent dehydration and oxidation. Although chemical activation has several drawbacks,

such as corrosion of the apparatus by chemicals, difficult recovery of chemicals, and high

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cost of chemicals, its activation efficiency is higher than that of physical activation.

Basic chemicals, such as KOH [8,67,69,75,83-85], NaOH [8,75,86], NH3 [76], K2CO3 [83,87],
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and ZnCl2 [83,84,88] as well as acids, such as H3PO4 [8,68,75,89], H2SO4 [89], and HCl [90],

are representative chemical agents used for the chemical activation of char. Eqs. (n) and (o)
d

present the reaction of carbon with alkali metal activating agents [83].
p te

6NaOH + 2C 2Na + 3H2 + 2Na2CO3 (n)

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6KOH + 2C 2K + 3H2 + 2K2CO3 (o)

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The parameters through which the chemical activation of biochar is influenced include the

type and dose of activating agent and activation temperature. Ros et al. [75] used CO2 as the

activating agent for physical activation, and H3PO4, NaOH, and KOH as the activating agents

for chemical activation to examine the effects of the activating agents on the activation of

sewage sludge-derived char. According to their study, H3PO4 did not increase the specific

surface area and pore volume of sludge char and activation using CO2 also provided much

smaller increase in specific surface area and pore volume than the alkali metal activating

Page 18 of 65
agents. When NaOH or KOH was used, the specific surface area and pore volume increased

with increasing dose of activating agent; the specific surface area and pore volume increased

from 689 m2/g to 1,224 m2/g and from 0.29 cm3/g to 0.44 cm3/g, respectively, when the

NaOH:char ratio was increased from 1:1 to 1:3. In contrast, an increase in the KOH:char ratio

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from 1:1 to 1:3 resulted in an increase in specific surface area and pore volume from 853

m2/g to 1,686 m2/g and from 0.34 cm3/g to 0.64 cm3/g, respectively. KOH was more effective

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in improving the characteristics of sludge char than NaOH.

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Tay et al. [83] investigated the effects of the activation temperature (600~800) and

activating agents (K2CO3 and KOH) on the generation of activated carbon using soybean oil

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cake. They reported that K2CO3 led to a larger activated carbon yield, a higher pore fraction,

and lower ash and sulfur contents than KOH. Although both activating agents produced a
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larger increase in specific surface area at a higher activation temperatures, the effects of the

activation temperature was much larger for K2CO3 than for KOH; the pore volume,
d

microporosity, and mesoporosity increased with increasing activation temperature when K2CO3 was
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used, whereas when KOH was used, the microporosity decreased with increasing activation
p

temperature. The maximum specific surface area of the activated carbon (1,352.86 m2/g) was
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achieved using K2CO3 at 800, which was comparable to those of commercial activated

carbon.
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Angn et al. [88] activated the biochar produced from the pyrolysis of safflower seed press

cake using ZnCl2 as the activating agent to examine the effects of the activation temperature

and impregnation ratio on the characteristics of the char and its adsorption of methylene blue

dye. In that study, the porosity, surface area, and adsorption capacity of the activated char

increased with increasing activation temperature (600~900) and impregnation ratio

(1:1~4:1).

Page 19 of 65
Zhang et al. [76] activated the pyrolysis char produced from cotton stalk using two different

activating agents, CO2 and NH3, and compared the CO2 adsorption capacities of the two

activated chars. CO2 adsorption increased with increasing activation temperature at an

adsorption temperature of 20 and the CO2-activated char adsorbed more CO2 than the

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NH3- or CO2+NH3-activated chars. This was attributed to the ability of CO2 to upgrade the

micropore structure and form heteroatom groups, such as C-O and C=O, particularly at high

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activation temperatures, which are stronger active sites than nitrogen functional groups.

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When the adsorption temperature was 120, however, the char activated at 500~700

using CO2+NH3 showed the highest adsorption capacity, which was attributed to the nitrogen

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functional groups formed by the reaction of the C-matrix of biochar with ammonia. When the

activation temperature was higher than 700, the formation of nitrogen functional groups
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was suppressed, resulting in lower CO2 adsorption capacity. Table 5 compares the

characteristics of biochars produced from various feedstock biomass materials using different
d

chemical activation methods.

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Application of biochar
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Adsorbent
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Activated carbon generally has a high specific surface area and pore fraction and contains

oxygen functional groups and aromatic compounds on its surface [91,92]. These

characteristics of activated carbon have been utilized to adsorb various kinds of pollutants

[93-97]. As mentioned above, biochars with various specific surface areas, pore structures,

and functional groups have been developed and used for the adsorption of a variety of

pollutants by taking advantage of these physical and chemical characteristics [98-101].

Page 20 of 65
The adsorption characteristics of biochar depend on the raw material used for the production

of char. Xu et al. [102] generated biochar from 4 different biomass materials (canola straw

char, peanut straw char, soybean straw char, and rice hull char) and applied them to the

adsorption of methyl violet. The methyl violet adsorption capacities of the chars were in the

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order of canola straw char, peanut straw char, soybean straw char, and rice hull char. They

suggested that the adsorption characteristics of biochar were influenced by the quantity of

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negative charges that it contained. Xu et al. [103] produced biochar from rice husk and dairy

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manure under the same conditions and used them for the adsorption of Pb, Cu, Zn, and Cd in

water. The adsorption capacity of biochar was strongly dependent on the biomass material

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used for the production of biochar because the quantity of the mineral component, such as

CO32- and PO42-, in the biochar varied according the raw biomass material.
M
The adsorption capacity of biochar also depends on the process conditions under which the

biochar is produced. The effects of the pyrolysis temperature on the adsorption characteristics
d

of biochar have been investigated by a number of researchers [104-106]. An increase in

te

pyrolysis temperature results in an increase in carbon content, specific surface area, and
p

porosity of biochar and a decrease in the number of oxygen functional groups on the char
ce

surface. Ahmad et al. [104] reported that an increase in pyrolysis temperature led to an

increase in specific surface area, micro-porosity, and C content, leading to an increase in TCE
Ac

removal efficiency. Zhou et al. [105] also reported that the benzene and nitrobenzene

adsorption capacity of pine char increased with increasing pyrolysis temperature. On the

other hand, Ding et al. [106] reported a decrease in Pb adsorption with an increase in

pyrolysis temperature. Sun et al. [107] produced biochar by pyrolyzing grass and wood at

200600 and applied them to the adsorption of fluridone and norflurazon. In their

experiments, the adsorption capacity of the biochars increased with increasing pyrolysis

temperature below 400; further increases in pyrolysis temperature above 400 rather

Page 21 of 65
decreased the adsorption capacity. This was attributed to the polarity of the pollutants

adsorbed; polar compounds, such as fluridone and norflurazon, are adsorbed on H-bonding

between the adsorbed substances or on O-groups of biochar, whereas non-polar compounds,

such as TCE are adsorbed on the hydrophobic sites of the biochar surface.

t
ip
The solution pH is another important parameter affecting the adsorption process. Tan et al.

[108] used the biochar produced from municipal sewage sludge for the adsorption of Cd. The

cr
Cd adsorption capacity was 20 mg/g when the solution pH was 2 or lower, whereas it was 40

us
mg/g when the pH was 3 or higher. Xu et al. [102] also reported that the methyl violet

adsorption capacity of the biochars produced from crop residue increased to a large extent

an
when the solution pH was increased from 7.7 to 8.7. They suggested that an increase in pH

enhanced the dissociation of phenolicOH groups on the biochar surface, which in turn
M
increased the electrostatic attraction between adsorbent and adsorbate.
d

(1) Adsorbent for water pollutants

te

Heavy metals are important toxic water pollutants with adverse effects on the metabolism of
p

humans, animals and plants [109]. A range of biochars have been applied to the removal of
ce

heavy metals [71,103,110-116]. Xu et al. [103] used the chars produced from rice husk and

feces to remove Pb, Cu, Zn, and Cd from aqueous solutions. Feces-char had higher metal
Ac

removal efficiencies than rice husk-char for all the 4 heavy metals examined. When feces-

char was used, heavy metals were not only adsorbed on the ionized hydroxyl-O- groups but

also precipitated due to the reaction with CO32- or PO43, whereas the adsorption on ionized

phenolic O- groups is the only heavy metal removal mechanism of rice husk-char. Their study

indicated that oxygen-functional groups and mineral compounds, such as CO32- and PO43-,

play important roles in the sorption of heavy metals on biochar. Kim et al. [110] produced

biochars from the slow pyrolysis of giant Miscanthus over the temperature range of

Page 22 of 65
300~600 with an interval of 100. They reported that a higher pyrolysis temperature

resulted in a higher aromatic structure and a smaller quantity of polar functional groups in

biochar. In particular, when the pyrolysis temperature was higher than 500, the pH and

specific surface area of the biochar increased considerably and the Cd sorption capacity also

t
ip
increased. Based on the results obtained, the authors suggested that the main mechanisms for

cr
the removal of aqueous Cd were sorption due to large specific surface area of char and

precipitation due to the high pH. Cho et al. [111] and Kim et al. [117] generated chars from

us
marine macroalgae and applied them to the removal of aqueous copper. They reported

different results from the above-mentioned previous studies. Cho et al. [111] pyrolyzed

an
Undaria pinnatifida at 500 to produce biochar (U) and activated the char physically with

40 wt. % steam (UW) or chemically using a 1.0 M KOH solution (UK). UW exhibited the
M
largest copper adsorption capacity, followed by U and UK, which was not in agreement with

the order of the specific surface area of the chars; the specific surface area of UK was quite
d

large (1,287.0 m2/g), whereas that of UW was only 57.9 m2/g and that of U was not even
te

measurable (see Fig. 3 and Table 6). The authors measured the quantity of ion-exchangeable
p

cations in each char and concluded that copper removal by biochar is dominated by ion
ce

exchange rather than by adsorption. Despite its large specific surface area, the KOH-activated

char (UK) showed the lowest copper removal efficiency because it lost most of its ion-
Ac

exchangeable cations during the activation process.

Kim et al. [119] produced biochar from the pyrolysis of the green macroalga, Enteromorpha

compressa. Steam activation increased both the specific surface area and cation exchange

capacity (CEC), resulting in a huge increase in the copper removal efficiency of the char,

whereas KOH activation, despite increasing the specific surface area and pore fraction greatly,

reduced the quantity of CEC and hence the copper removal efficiency. Ion exchange was also

Page 23 of 65
shown to be the most important mechanism for copper removal in their study. Shim et al. [73]

produced giant Miscanthus char (BC) from slow pyrolysis at 500 and activated it with

steam (ABC) to increase its specific surface area from 181 m2/g to 322 m2/g. On the other

hand, activation did not improve the copper removal by fast sorption significantly and rather

t
ip
reduced the copper removal by slow sorption. They concluded that the specific surface area

was not an important factor for copper ion sorption. They also found that ABC exhibited high

cr
toxicity to Daphnia magna. Therefore, they concluded that BC is a more adequate sorbent for

us
toxic pollutant removal than ABC. Samsuri et al. [112] produced biochar from oil palm

empty fruit bunches (EFB) and compared it with commercial rice husk char (RHB) in terms

an
of their adsorption capacities for Zn, Cu, and Pb. The two chars had similar carbon contents

but EFB had higher O, H, S, N and K contents than RHB. Despite its lower specific surface
M
area, EFB exhibited higher Zn, Cu and Pb adsorption efficiencies than RHB. The authors

attributed the higher metal removal efficiency of EFB to its large content of oxygen-
d

containing functional groups, high O/C ratio, and high polarity index [(O+N)/C]. They
te

concluded that the chemical properties of biochar were more important factors in the
p

adsorption of heavy metals than specific surface area. Table 7 lists the maximum Cu
ce

adsorption quantities of pyrolytic biochars found in the literature.

Organic matter is another important target water pollutants to remove using biochar [118-
Ac

122]. Many studies applied biochar to the adsorption of tetracycline (TC), which is used as

antibiotic for animals and plants. Zhu et al. [118] activated char thermally over the temperature

range of 300~700 by injecting N2 gas to a hydrochar produced from 300 HTC. They

reported that the pH, ash content, specific surface area, pore volume, and aromaticity

increased with increasing activation temperature, whereas the polarity decreased. A high

activation temperature led to a large TC adsorption capacity. The authors concluded that the

organic matter adsorption on biochar is correlated closely with aromaticity index (H/C),

Page 24 of 65
polarity index (O/C and (O+N)/C), and porosity (specific surface area, total pore volume, and

micropore volume). Liu et al. [119] activated crude bio-char with acid and alkali and used it for

the adsorption of tetracycline. The alkali-activated char had a larger specific surface area and

exerted a larger adsorption capacity (58.8 mg/g) than acid-activated char. They suggested that

t
ip
the specific surface area and O-functional groups are important factors affecting the

adsorption of TC, whereas the effect of solution pH on TC adsorption was not large. Sun et al.

cr
[120] produced biochar from eucalyptus saw dust and activated it using citric acids, tartaric

us
acids, and acetic acids. When the activated chars were used for the removal of aqueous

methylene blue (MB), the citric acid-activated char (BC-CA) showed the largest adsorption

an
capacity despite having the lowest specific surface area. Fourier transform infrared spectroscopy

showed that BC-CA contained the largest quantity of carboxyl groups (COOH), indicating that
M
the quantity of oxygen functional groups is another important factor for the adsorption of organic

pollutants.
d
te

(2) Adsorbent for air pollutants

p

Air pollutants have adverse acute or chronic impacts on human health depending on their
ce

type, concentration, and exposure duration [123]. Lee et al. [124] activated sludge char using

20% steam gas, KOH and ammonia, and used it for the adsorption of an indoor air pollutant,
Ac

formaldehyde, at low concentrations. As shown in Fig. 4, the biochar showed a higher

adsorption capacity than the commercial activated carbon even before activation, which was

attributed to the basicity of the char surface due to the abundant metal species. KOH- and

ammonia-activation increased the formaldehyde adsorption capacity even further by

increasing the number of oxygen and nitrogen functional groups as well as the specific

surface area. They concluded that the basicity due to metal species and oxygen and nitrogen

functional groups are important factors that improve the formaldehyde adsorption capacity of

Page 25 of 65
the biochar.

CO2 is the most important global warming gas, even though its direct health impact is not

large. Adsorption is one of the most widely used methods to capture and store CO2. The

adsorbents used widely to capture CO2 include carbon materials (e.g. activated-carbon,

t
ip
carbon molecular sieve, and carbon nanotube), zeolites, and MOFs (metal-organic

frameworks) [125]. Extensive studies have been conducted to develop new carbon adsorbents

cr
from biochar. Creamer et al. [126] produced biochar from the pyrolysis of sugarcane bagasse

us
(BG) and hickory wood (HW) at 300, 450 and 600 and used them for the adsorption of

CO2 at 25 and 75. The biochar produced at high temperatures showed a very large CO2

an
adsorption capacity (73.55 mg/g at 25), which was attributed to the physisorption of CO2

of which the efficiency is proportional to the specific surface area of the char. When the char
M
specific surface area was sufficiently large, however, the quantity of nitrogenous groups

played a more important role in the adsorption of CO2 on the biochar surface. Huang et al.
d

[127] pyrolyzed rice straw using microwaves to produce biochar and used it to adsorb CO2.
te

They reported that the specific surface area is the most important characteristic of biochar in
p

CO2 adsorption. In general, the CO2 adsorption capacity increased with increasing pyrolysis
ce

temperature. Compared to the biochar produced using a conventional pyrolysis process at the

same temperature, the biochar produced using microwave pyrolysis showed a higher CO2
Ac

adsorption capacity when the pyrolysis temperature was 400 or lower, demonstrating the

merits of microwave pyrolysis in terms of cost and time. Creamer et al. [128] pyrolyzed

cottonwood at 600 after pretreating it with various concentrations of metal ions (Al, Mg

and Fe) to generate metal oxyhydroxidebiochar composites. The modified biochar showed a

higher CO2 adsorption capacity than the unmodified biochar. The Fe2O3biochar composites

showed the largest specific surface area but the AlOOHbiochar composite showed the

Page 26 of 65
largest CO2 adsorption capacity, indicating that the CO2 adsorption capacity is affected not

only by the specific surface area, but also by metal oxyhydroxides. The maximum adsorption

capacity was 71 mg/g by AlOOHbiochar at 25, which was similar to those of commercial

adsorbents. The desorption efficiency at 120 was 90~99%, showing that the biochar-based

t
ip
composites are very efficient and economical CO2 capturing adsorbents.

cr
Catalysts

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(1) Catalysts for syngas cleaning

an
As mentioned above, mucous tar produced during the pyrolysis step of the gasification

processes is composed of organic components of the raw material. Being a mixture including
M
1~5 ring aromatic organic compounds, oxygenated hydrocarbons, and polycyclic aromatic

hydrocarbons (PAHs), tar exhibits toxicity due to aromatic compounds, such as PAHs and
d

benzene, and produces aerosols and soot that may block the filter, reactor and line of
te

gasification processes [129]. Therefore, the removal of tar is important in gasification. A

p

range of physical and chemical tar reduction methods are summarized in Table 8 [130-134].
ce

Catalytic reforming is a method to convert tar catalytically to H2 and CO. In a review paper

by Han et al. [135] the catalysts used for tar cracking were divided into Ni-based catalysts,
Ac

alkali metal catalysts, dolomite catalysts, and novel metal catalysts. In recent studies, char has

also been used as a support material for tar-removing catalysts [14].

El-Rub et al. [40] compared biomass char with other catalysts (calcined dolomite, olivine,

fluid catalytic cracking (FCC) catalyst, biomass ash, and commercial nickel catalyst) with

respect to their tar removal capability. Among the tar components, the inlet and outlet

concentrations of phenol and naphthalene were measured to determine the tar conversion.

Page 27 of 65
The results are summarized in Table 9. Most phenol (98%) was converted at 900 even

without a catalyst (with only thermal cracking), whereas naphthalene was converted only

when the catalysts were used. With the exception of a commercialized nickel catalyst,

biomass char showed the highest naphthalene conversion. The authors argued that

t
ip
gasification-derived biomass char is a good candidate catalyst for tar reduction because char

can be produced continuously from gasification and the by-products, CO2 and steam, can be

cr
used to activate the char.

us
Chen et al. [136] used rice straw char, corn straw char, and fir sawdust char particles with a

size range of 100~150 to reduce tar. They reported that the type of char had little effects

an
on tar reduction, whereas the tar reduction efficiency decreased with increasing char particle
M
size. They attributed this to a reduction of the specific surface area by the increase in particle

size. On the other hand, the effects of char particle size decreased when the reaction
d

temperature was increased from 700 to 1,000.

te

Qian et al. [137] examined the effects of various factors on tar cracking using the char

produced from the gasification of red cedar as a tar cracking catalyst. They reported that the
p

char-based catalyst was quite effective in removing lignin tar. The reaction temperature,
ce

injection of moisture, and pressure influenced tar removal. The removal efficiencies of most
Ac

tar components except for naphthalene increased with increasing reaction temperature. The

high pressure also enhanced the catalytic conversion of tar. Excess moisture, however, was

reported to reduce the tar removal efficiency.

(2) Catalyst for biodiesel production

Biodiesel is a mixture of methyl ester compounds produced from the reaction

(transesterification or esterification) of alcohol with vegetable oil or animal fat composed of

Page 28 of 65
triglycerides and free fatty acids. As shown in Fig. 5, transesterification is a reaction, in

which the main component of fat and oil, triglyceride, reacts with alcohol over a catalyst

producing fatty acid methyl ester and glycerol [138,139].

In esterification, free fatty acids (FFAs) and alcohol react to produce biodiesel and water (Fig.

t
ip
6).

Although solid-phase acid catalysts have been used widely as the catalyst for biodiesel

cr
production, biochar has recently been applied to biodiesel production.

us
Dehkhoda et al. [140] sulfonated hardwood char using concentrated sulfuric acid and fuming

sulfuric acid, activated it chemically with 10 M KOH to increase the specific surface area,

an
and applied it to the transesterification of vegetable oil and the esterification of FFA.

Concentrated sulfonated biochar exhibited high activity only for the esterification of free fatty
M
acid, whereas fuming sulfonated char showed a higher transesterification activity than

concentrated sulfonated biochar. The authors reported that a large specific surface area and
d

high acid density led to a high biodiesel production yield, whereas high transesterification
te

activity was accompanied by a high specific surface area when the acid density was similar.
p

They also reported that the FFA conversion increased with increasing alcohol to oil ratio,
ce

reaction duration, and catalyst dose. Kastner et al. [141] pyrolyzed pelletized peanut hulls,

pine logging residues, and wood chips at 400~500 to produce biochar. They sulfonated
Ac

those biochars and commercial activated carbon using H2SO4 at 100, 150, and 200 for 12

h and using SO3 gas at 23. They applied the sulfonated carbon to the esterification of

vegetable oil and animal fat (515 wt.% FFA) with methanol. The esterification of fatty acids

for 30~60 min resulted in 90~100% conversion at 55~60. When the catalysts were reused,

the char sulfonated at 100 exhibited the highest reaction rate and the lowest conversion

reduction, demonstrating its potential as an acid catalyst for biodiesel production.

Page 29 of 65
(3) Catalyst for air pollution control

Several studies have investigated the application of activated biochars as low-temperature

SCR (selective catalytic reduction) catalysts [67,68,90,142]. Cha et al. [67] pyrolyzed rice

t
ip
straw and sewage sludge, whereas Jo et al. [142] pyrolyzed sewage sludge, to generate

biochar and use them as a low-temperature SCR catalyst. Ammonia was used as a reducing

cr
agent. Each char was activated physically or chemically and their characteristics and NOx

us
removal efficiencies were examined. The chemically activated char exhibited a higher NOx

removal efficiency than the physically activated char, indicating that the chemical properties,

an
such as oxygen functional groups and NH3 adsorption sites, were more important factors for

NOx removal than the physical properties, such as specific surface area and pore size. Cha et
M
al. also reported that the impregnation of 3 wt. % manganese on chemically activated char

enhanced its NOx removal activity even further. Ko et al. [90] activated municipal waste char
d

and RDF (refuse derived fuel) char physically using steam or chemically using HCl and KOH,
te

and used them as low-temperature SCR catalysts. The NOx removal efficiency of KOH-
p

activated char was higher than those of the other chars, suggesting that the increased oxygen
ce

functional groups, specific surface area, and pore size increased the quantity of active sites on

the char surface, leading to improved NH3 adsorption ability. The impregnation of MnOx on
Ac

the KOH-activated catalyst resulted in an even higher adsorption efficiency than the

commercial carbon SCR catalyst, demonstrating the high feasibility of municipal waste char

and RDF char as de-NOx catalysts (Fig. 7). Shen et al. [68] impregnated a Mn-CeOx base

catalyst on modified cotton biochar and activated carbon and used them for the low-

temperature SCR reaction. The modified cotton biochar exhibited a higher potential as a low-

temperature SCR catalyst support material than the commercial activated carbon. In

particular, the biochar activated physically and chemically and impregnated with Mn-CeOx

Page 30 of 65
had the highest specific surface area, surface acidity, and Mn4+/Mn3+ ratio. The Mn-CeOx

dispersion was good and there were large quantities of adsorbed oxygen and oxygen

functional groups, which promoted the oxidation of NO to NO2 and hence improved the SCR

activity.

t
ip
Kastner et al. [69] used the pyrolysis char produced from pellet-type peanut hull and wood fly

ash as the catalyst for the catalytic ozonation of gas-phase ammonia. Wood fly ash showed

cr
the highest ammonia conversion at low ozone concentrations, whereas the steam-activated

us
char showed a higher ammonia conversion when the ozone concentration was higher than

500 ppm. Ammonia conversion by char was very low at low air humidity, whereas it

an
increased considerably at high humidity. In addition, the steam-activated char produced little

NO2 by-product, which was another merit.

M
Soil amendment
d

Application of biochar to soil can not only isolate carbon in soil, but also improve the soil
te

quality by neutralizing acidic soil, enhancing the CEC of soil, and increasing the activity of
p

soil microorganisms. As shown in Table 10, biochars are basic. Therefore, the use of biochar
ce

as a soil amendment can neutralize acidic soils [143, 144].

In addition, biochar has phenolic, carboxyl and hydroxyl functional groups, which react with
Ac

H+ ions in the soil to reduce the H+ ion concentration and increase the soil pH. Silicates,

carbonates and bicarbonates in biochar can also combine with H+, controlling the soil pH [23].

Yuan et al. [144] applied 10 g/kg of 9 different kinds of biochars to soil and found that in 60

days, the soil pH increased by 0.59~1.05 depending on the biochar applied

Biomass materials, which are used as raw materials to produce biochar, contain many basic

cations. As shown in Table 11, the cations in the biomass reside in biochar after the biochar

production processes [145,146]. When biochar is applied to soil, the basic cations contained

Page 31 of 65
in the biochar are discharged into the soil, replacing Al and H+ and enhancing the CEC of soil.

The CEC of the soil increases with increasing charge density in soil (per unit surface area)

and with increasing soil surface area that can adsorb cations [147]. In general, the CEC

increases with increasing pH. Lee et al. [148] reported that the CEC of char increased when

t
ip
the pH was increased from 5.0 to 8.5. In particular, CEC was negative when the pH was low,

which was attributed to the discharge of bound cations. Liang et al. [147] reported a 1.9-fold

cr
increase in CEC by the injection of black carbon in soil.

us
Biochar contains high concentrations of N, P, Ca, and K, which may provide soil with

nutrients directly or may be used as nutrients of microorganisms. When biochar is used as a

an
soil amendment, the pore fraction of soil is increased by biochar. Each pore provides the

space in which the microorganisms can grow and increases the quantity of air and moisture
M
and the residence time of nutrients, resulting in the enhanced activity of microorganisms and

increased growth rate of plants in that soil.

d

The use of biochar as a soil amendment can also reduce the global warming gas emissions.
te

Although the direct combustion of biomass emits carbon in biomass to the atmosphere as
p

CO2, the pyrolysis or gasification of biomass converts C to biochar, which can be stored in
ce

soil (Fig. 8). The isolation of biochar in soil can reduce the carbon emissions because biochar

is barely decomposed by microorganisms or through mineralization.

Ac

Lehmann et al. [24] reported that the conversion of biomass carbon to biochar can isolate

50% of the initial carbon quantity in soil. This is much higher than those obtained from

combustion (3%) or from biological decomposition (10~20% in 5~10 years). The effects of

reducing the global warming gas emissions using biochar as a soil amendment have been

reported [150-152]. Spokas et al. [151] reported that the amendment of soil using biochar

suppressed the decomposition activity of microorganisms, resulting in reduced CO2

emissions by 2~60%, depending on the area of biochar application, reduced N2O emission by

Page 32 of 65
60%, and reduced CH4 oxidation. They also reported that the application of 5% w/w of

biochar to soil enhanced the sorption of atrazine and acetochlor and reduced the dissipation

rate of herbicides. Overall, the authors concluded that the application of biochar to soil is an

effective carbon isolation strategy and an effective way of reducing global warming gas

t
ip
emissions if the continuous reduction of N2O is possible. Yanai et al. [150] also reported up

to 89% reductions in N2O emissions by adding 10% biochar to re-wetted soil.

cr
us
Other applications

In addition to the fields discussed in the previous sections, biochar has been used in other

an
applications. A number of different types of fuel cells exist, depending on the kinds of fuels

and electrolytes used and operation temperature, even though their principles are the same.
M
Among others, the fuel cells that use the largest quantity of carbon materials are the

phosphoric acid fuel cell and direct carbon fuel cell (DCFC) [153]. Ahn et al. [154] examined
d

the effects of the type of fuel on the cell performance and the characteristics of
te

electrochemical reactions between the fuel and electrolyte using wood biomass char as the
p

fuel for DCFC. The maximum power density obtained using biomass char was 60~70% of
ce

that obtained using coal, which was attributed to the low carbon content and high ash content

of biomass char. Nevertheless, they argued that biomass char has potential as a DCFC fuel
Ac

because the porous characteristics of biomass char and the functional groups on the char

surface promote the electrochemical reactions in the low current density region.

Supercapacitors are used for energy storage for portable electrical devices and hybrid

vehicles. Supercapacitors have been reported to have higher power, higher energy density,

higher chemical stability, and longer life cycle than secondary batteries [155]. Various carbon

materials have recently been used as the electrodes of supercapacitors because carbon

materials exert high electric conductivity and a large pseudo capacitance. A number of

Page 33 of 65
studies reported biochar as an electrode in supercapacitors [156,157]. Those studies reported

that the capacitive performance of biochar electrode was influenced by surface functional

groups, electric conductivity, and pore structure and distribution as well as the specific

surface area [158].

t
ip
Hydrogen has attracted significant attention as a replacement for fossil fuels because of its

high energy efficiency and lack of pollutants. The most critical problem of hydrogen as an

cr
energy source is the difficulty in storage, which is one of the most important research topics

us
on hydrogen energy [159]. Among the various hydrogen storage methods, adsorption on

carbon materials has several advantages in terms of chemical stability, reversibility, and price,

an
leading to many studies on various carbon materials for hydrogen storage [160-164]. Choi et

al. [163] produced watermelon flesh char using a hydrothermal reaction and activated it with
M
KOH at different temperatures. The activated char with the largest specific surface area and

micropore volume exhibited the highest hydrogen storage capacity (2.7 wt. %) and it was
d

expected that the impregnation of metals or the formation of surface functional groups may
te

increase the hydrogen storage capacity even further. Ramesh et al. [165] produced biochar
p

from tamarind seed using heat and microwave treatments, activated it with KOH, and
ce

measured its hydrogen storage capacity. The hydrogen adsorption capacity, which was

measured at room temperature and 4 MPa, was 4.73 wt. %, which is 80% of the target
Ac

hydrogen storage capacity of the U.S. DOE (Department of Energy). The microwave

treatment produced carbon with a larger pore fraction than thermal treatment. They

concluded that micropores and a large specific surface area were important parameters for

hydrogen storage.

Biomass is a unique resource because it is renewable and can be converted into various forms

of chemical feedstock and energy products. Among various products from biomass, biochar

Page 34 of 65
would be the most valuable because of its interesting physical and chemical properties. As

was reviewed thus far, biochar has various conventional and emerging applications with its

potential not only as cheap fuel but also as expensive carbon material. The importance of

biochar is expected to increase further with increasing severity of global warming. In this

t
ip
regard, wider applications and further quality improvement of biochar will contribute to the

enhancement of global sustainability in terms of carbon cycle. To realize the full potential of

cr
biochar, it is also essential to develop optimized processes for biochar production with

us
minimized energy input and costs combined with efficient use of byproducts (bio-oil and

gases) at large scales.

Conclusion
an
M
This article reviewed the processes to produce biochar together with different methods to
d

modify the produced biochar and its applications. According to previous studies, the
te

characteristics of biochar are influenced by the raw biomass material and the process used to
p

produce it as well as the process parameters. Biochar has been utilized in a variety of
ce

applications, such as adsorbents, catalysts, and soil amendments, depending on their

characteristics. Modification of char, e.g. by increasing the specific surface area and pore
Ac

fraction or by forming functional groups, depending on the application purpose, is important

for increasing the reaction activity of biochar, even though mineral components in biochar

itself are sometimes used. Biochars are being used in an increasing number of fields. The

establishment of a continuous supply system will be needed to promote the application of

biochar to higher value-added areas.

Page 35 of 65
Acknowledgement

This work was supported by the National Research Foundation of Korea(NRF) grant funded

by the Korea government(MSIP) (No. 2015R1A2A2A11001193).

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References

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[1] Q. Zhang, J. Chang, T. Wang, Y. Xu, Energy Convers. Manage. 48 (2007) 87.

us
[2] M. Tripathi, J. N. Sahu, P. Ganesan, Renew. Sustain. Energy Rev. 55 (2016) 467.

[3] C. H. Ko, S. H. Park, J. K. Jeon, D. J. Suh, K. E. Jeong, Y. K. Park, Korean J. Chem. Eng.

29 (2012) 1657.

an
[4] S. Y. No, Renew. Sust. Energ. Rev. 40 (2014) 1108.
M
[5] L. Zhang, R. Liu, R. Yin, Y. Mei, Renew. Sust. Energ. Rev. 24 (2013) 66.
d

[6] N. Abdoulmoumine, S. Adhikari, A. Kulkarni, S. Chattanathan, Appl. Energy 155 (2015)

te

294.
p

[7] Y. Richardson, J. Blin, A. Julbe, Prog. Energy Combust. Sci. 38 (2012) 765.
ce

[8] J. Lehmann, S. Joseph, Biochar for Environmental Management : An Introduction, Biochar

for Environmental Management Science and Technology, Earthscans, 2009, 1.

Ac

[9] N. Liu, A. B. Charrua, C. H. Weng, X. Yuan, F. Ding, Bioresour. Technol. 198 (2015) 55.

[10] M. H. Duku, S. Gu, E. B. Hagan, Renew. Sust. Energ. Rev. 15 (2011) 3539.

[11] H. S. Kambo, A. Dutta, Renew. Sust. Energ. Rev. 45 (2015) 359.

[12] M. Ahmad, A. U. Rajapaksha, J. E. Lim, M. Zhang, N. Bolan, D. Mohan, M. Vithanage,

S. S. Lee, Y. S. Ok, Chemosphere 99 (2014) 19.

[13] D. Mohan, A. Sarswat, Y. S. Ok, C. U, P. Jr, Bioresour. Technol. 160 (2014) 191.

Page 36 of 65
[14] Y. Shen, Renew. Sust. Energ. Rev. 43 (2015) 281.

[15] L. J. Konwar, J. Boro, D. Deka, Renew. Sust. Energ. Rev. 29 (2014) 546.

[16] A. H. Lone, G. R. Najar, M. A. Ganie, J. A. Sofi, A. L, I. Tahir, Pedosphere 25 (2015)

t
639.

ip
[17] J. Yu, Y. Zhao, Y. Li, J. Power Sources 270 (2014) 312.

cr
[18] A. Elleuch, A. Boussetta, J. Yu, K. Halouani, Y. Li, Int. J. Hydrog. Energy 38 (2013)

us
16590.

[19] R. K. Gupta, M. Dubey, P. Kharel, Z. Gu, Q. H. Fan, J. Power Sources. 274 (2015) 1300.

an
[20] J. Jiang, L. Zhang, X. Wang, N. Holm, K. Rajagopalan, F. Chen, S. Ma, Electrochim.

Acta 113 (2013) 481.

M
[21] M. Inyang, E. Dickenson, Chemosphere 134 (2015) 232.
d

[22] X. Tan, Y. Liu, G. Zeng, X. Wang, X. Hu, Y. Gu, Z. Yang, Chemosphere 125 (2015) 70.
te

[23] S. Gul, J. K. Whalen, B. W. Thomas, V. Sachdeva, H. Deng, Agric. Ecosyst. Environ.

206 (2015) 46.

p

[24] J. Lehmann, J. Gaunt, M. Rondon, Mitig. Adapt. Strateg. Glob. Chang. 11 (2006) 403.
ce

[25] L. Beesley, E. M. Jimnez, J. L. G. Eyles, E. Harris, B. LRobinson, T. Sizmur, Environ.

Ac

Pollut. 159 (2011) 3269.

[26] S. Jeffery, F.G.A. Verheijen, M. van der Velde, A.C. Bastos, Agric. Ecosyst. Environ. 144

(2011) 175.

[27] J. Lehmann, M. C. Rillig, J. Thies, C. A. Masiello, W. C. Hockaday, D. Crowley, Soil

Biol. Biochem. 43 (2011) 1812.

[28] E. H. Lee, R. Park, H. Kim, S. H. Park, S. C. Jung, J. K. Jeon, S. C. Kim, Y. K. Park, J.

Page 37 of 65
 Ind. Eng. Chem. 37 (2016) 18-21.

[29] T. U. Han, Y. M. Kim, C. Watanabe, N. Teramae, Y. K. Park, S. Kim, Y. Lee, J. Ind.

Eng. Chem. 32 (2016) 345.

t
[30] A. Heidari, R. Stahl, H. Younesi, A. Rashidi, N. Troeger, A. A. Ghoreyshi, J. Ind. Eng.

ip
Chem. 20 (2014) 2594.

cr
[31] H. Shafaghat, P. S. Rezaei, W. M. A. W. Daud, J. Ind. Eng. Chem. 35 (2016) 268.

us
[32] M. Inguanzo, A. Dominguez, J.A. Menendez, C.G. Blanco, J.J. Pis, J. Anal. Appl.

Pyrolysis 63 (2001) 209.

an
[33] A. Demirbas, J. Anal. Appl. Pyrolysis 72 (2004) 243.

[34] G. Chen, J. Andries, Z. Luo, H. Spliethoff, Energy Conv. Manag. 44 (2003) 1875.
M
[35] A. Mohammad I, A. Abdulrasoul, E . Ahmed H, N. Mahmoud, U. Adel R.A., Bioresour.

Technol. 131 (2013) 374.

d
te

[36] J. Zhang, J. Liu, R. Liu, Bioresour. Technol. 176 (2015) 288.

[37] Y. Lee, P. R. B. Eum, C. Ryu, Y. K. Park, J. H. Jung, S. Hyun, Bioresour. Technol. 130
p

(2013) 345.
ce

[38] M. Ayllon, M. Aznar, J.L. Sanchez, G. Gea and J. Arauzo, Chem. Eng. J. 121 (2006) 85.
Ac

[39] M. Inguanzo, J.A. Menendez, E. Fuente, J.J. Pis, J. Anal. Appl. Pyrolysis 5859 (2001)

943.

[40] Z. A. El-Rub, E. A. Bramer, G. Brem, Fuel 87 (2008) 2243.

[41] M.E. Boucher, Biomass Bioenerg. 19 (2000) 351.

[42] A.V. Bridgwater, Research in Thermochemical biomass Conversion, Elsevier Science

Publishers: London, 1991.

Page 38 of 65
[43] G. Q. Lu, J. C. F. Low, C. Y. Liu, A. C. Lua, Fuel 74 (1995) 344.

[44] T. J. Bandosz, K. Block, Appl. Catal. B: Environ. 67 (2006) 77.

[45] Y. W. Lee, J. J. Park, C. K. Ryu, K. S. Gang, W. Yang, Y. K. Park, J. H. Jung, S. H.

Hyun, Bioresour. Technol. 148(2013) 196.

t
ip
[46] C. E. Brewer, K. Schmidt-Rohr, J. A. Satrio, R. C. Brown, Environ. Prog. Sustain.

cr
Energy 28 (2009) 386.

[47] L. E. Taba, M. F. Irfan, W. A. M. W. Daud, M. H. Chakrabarti, Renew. Sust. Energ. Rev.

us
16 (2012) 5584.

an
[48] A. Gmez-Barea, P. Ollero, B. Leckner, Fuel 103 (2013) 42.

[49] H. L. Tay, S. Kajitani, S. Zhang, C. Z. Li, Fuel 103 (2013) 22.

M
[50] S. Romn, J. M. V. Nabais, C. Laginhas, B. Ledesma, J. F. Gonzlez, Fuel Process.

Technol. 103 (2012) 78.

d

[51] L. P. Xiao, Z. J. Shi, F. Xu, R. C. Sun, Bioresour. Technol. 118 (2012) 619.
te

[52] Y. H. Chana, S. Yusupa, A. T. Quitainb, Y. Uemura, M. Sasaki, J. Supercrit. Fluids 95

p

(2014) 407.
ce

[53] A. Kruse, J. Supercrit. Fluids 47 (2009) 391.

Ac

[54] E. Sabio, A. Alvarez-Murillo, S. Roman, B. Ledesma, Waste Manage. 47 (2016) 122.

[55] M. J. Antal, Jr. K. Mochidzuki, L. S. Paredes, Ind. Eng. Chem. Res. 42(2003) 3690.

[56] T. Nunoura, S. R. Wade, J. P. Bourke, M. J. Antal, Jr. Ind. Eng. Chem. Res. 45(2006)

585.

[57] S. R. Wade, T. Nunoura, M. J. Antal, Ind. Eng. Chem. Res. 45(2006) 3512.

[58] W. H. Chen, Y. Q. Zhuang, S. H. Liu, T. T Juang, C. M. Ysai, Bioresour. Technol. 199

(2016) 367.

Page 39 of 65
[59] V. Benavente, A. Fullana, Biomass Bioenerg. 73 (2015) 186.

[60] B. S. Chioua, D. V. Medina, C. B. Sainz, A. P. Klamczynski, R. J. A. Bustillos, R. R.

Milczarek, W. X. Du, G. M. Glenn, W. J. Orts, Ind. Crop. Prod. 86 (2016) 40.

[61] M. N. Nimlos, E. Brooking. M. J. Looker, R. J. Evans, Prepr. Pap.-Am. Chem. Soc., Div.

t
ip
Fuel Chem. 48(2003) 590.

[62] S. Meyer, B. Glaser, P. Quicker, Environ. Sci. Technol. 22(2011) 9473.

cr
[63] S. H. Jung, J. S. Kim, J. Anal. Appl. Pyrolysis 107 (2014) 116.

us
[64] S. M. Manocha, H. Patel, L. M. Manocha, Carbon Lett. 11 (2010) 201.

an
[65] D. Agarwal, D. Lal, V. S. Tripathi, G. N. Mathur, Carbon Lett. 4 (2003) 126.

[66] T. S. Hui, M. A. A. Zaini, Carbon Lett. 16 (2015) 275.

M
[67] J. S. Cha, J. C. Choi, J. H. Ko, Y. K. Park, S. H. Park, K. E. Jeong, S. S. Kim, J. K. Jeon,

Chem. Eng. J. 156 (2010) 321.

d

[68] B. Shen, J. Chen, S. Y. G. Li, Fuel 156 (2015) 47.

te

[69] J. R. Kastner, J. Miller, P. Kolar, K. C. Das, Chemosphere 75 (2009) 739.

p

[70] J. F. Gonzalez, S. Roman, J. M. Encinar, G. Martnez, J. Anal. Appl. Pyrolysis 85

ce

(2009) 134.

[71] T. Y. Shim, J. S. Yoo, C. K. Ryu, Y. K. Park, J. H. Jung, Bioresour. Technol. 197 (2015)
Ac

85.

[72] R. Azargohar, A. K. Dalai, Microporous Mesoporous Mat. 110 (2008) 413.

[73] J. M. V. Nabais, P. Nunes, P. J. M. Carrott, M. M. L. R. Carrott, A. M. Garca, M. A. D.

Dez, Fuel Process. Technol. 89 (2008) 262.

[74] C. A. Toles, W. E. Marshall, L. H. Wartelle, A. McAloon, Bioresour. Technol. 75 (2000)

197.

Page 40 of 65
[75] A. Ros, M. A. L. Rodenas, E. Fuente, M. A. M. Moran, M. J. Martn, A. L. Solano,

Chemosphere 65 (2006) 132.

[76] X. Zhang, S. H. Zhang, H. P. Yang, Y. Feng, Y. Q. Chen, X. H. Wang, H. P. Chen,

Chem. Eng. J. 257 (2014) 20.

t
ip
[77] T. Y. Zhang, W. P. Walawender, L. T. Fan, M. H. Fan, D. Daugaardb, R. C. Brown,

cr
Chem. Eng. J. 105 (2004) 53.

[78] S. H. Guo, J. H. Peng, W. Li, K. B. Yang, L. Zhang, S. M. Zhang, H. Xia, Appl. Surf.

us
Sci. 255 (2009) 8443.

an
[79] J. R. Kastner, J. Miller, K. C. Das, J. Hazard. Mater. 164 (2009) 1420.

[80] D. Jimenez-Cordero, F. Heras, N. Alonso-Morales, M. A. Gilarranz, J. J. Rodriguez,

M
Fuel Process. Technol. 139 (2015) 42.

[81] F. R. Reinoso, M. M. Sabio, M. T. Gonzalez, Carbon 33 (1995) 15.

d

[82] M. C. Ncibi, V. J. Rose, B. Mahjoub, C. Jean-Marius, J. Lambert, J. J. Ehrhardt, Y.

te

Bercion, M. Seffen, S. Gaspard, J. Hazard. Mater. 165(2009) 204

p

[83] T. Tay, S. Ucar, S. Karagoz, J. Hazard. Mater. 165 (2009) 481.

ce

[84] F. C. Wu, R. L. Tseng, R. S. Juang, Sep. Purif. Technol. 47 (2005) 10.

[85] K. Y. Foo, B. H. Hameed, Microporous Mesoporous Mat. 148 (2012) 191.

Ac

[86] J. Y. Park, I. Hung, Z. H Gan, O. J. Rojas, K. H. Lim, S. K. Park, Bioresour. Technol.

149 (2013) 383.

[87] J. Hayashi, T. Horikawa, I. Takeda, K. Muroyama, F. N. Ani, Carbon 40 (2002) 2381.

[88] D. Angn, E. Altintig, T. E. Kse, Bioresour. Technol. 148 (2013) 542.

[89] F. S. Zhang, J. O. Nriagu, H. Itoh, Water Res. 39 (2005) 389.

Page 41 of 65
[90] J. H. Ko, Y. H. Kwak, K. S. Yoo, J. K. Jeon, S. H. Park, Y. K. Park, J. Mater. Cycles

Waste Manag. 13 (2011) 173.

[91] J. H. Kim, D. Y. Lee, T. S. Bae, Y. S. Lee, J. Ind. Eng. Chem. 25 (2015) 192.

t
[92] M. S. Park, S. H. Cho, E. G. Jeong, Y. S. Lee, J. Ind. Eng. Chem. 23 (2015) 27.

ip
[93] H. S. Moon, I. S. Kim, S. J. Kang, S. K. Ryu, Carbon Lett. 15 (2014) 203.

cr
[94] J. H. Ko, R. S. Park, J. K. Jeon, D. H. Kim, S. C. Jung, S. C. Kim, Y. K. Park, J. Ind.

us
Eng. Chem. 32 (2015) 109.

[95] X. Xiao, F. Tian, Y. Yan, Z. Wu, G. Cravotto, Korean J. Chem. Eng. 32 (2015) 1129.

an
[96] K. Mahmoudi, K. Hosni, N. Hamdi, E. Srasra, Korean J. Chem. Eng. 32 (2015) 274.

[97] M. S. Park, S. E. Lee, M. I. Kim, Y. S. Lee, Carbon Lett. 16 (2015) 45.

M
[98] M. E. Mahmoud, G. M. Nabil, N. M. E. Mallah, H. I. Bassiouny, S. Kumar, T. M. A.
d

Fattah, J. Ind. Eng. Chem. 37 (2016) 156-167.

te

[99] S. H. Park, H. J. Cho, C. K. Ryu, Y. K. Park, J. Ind. Eng. Chem. 36 (2016) 314-399.

[100] Z. Ding, Y. Wan, X. Hu, S. Wang, A. R. Zimmerman, B. Gao, J. Ind. Eng. Chem. 37
p

(2016) 261-267.
ce

[101] M. Ruthiraan, N. M. Mubarak, R. K. Thines, E. C. Abdullah, J. N. Sahu, N. S.

Ac

Jayakumar, P. Ganesan, Korean J. Chem. Eng. 32 (2015) 446.

[102] R. K. Xu, S. C. Xiao, J. H. Yuan, A. Z. Zhao, Bioresour. Technol. 102 (2011) 10293.

[103] X. Xu, X. Cao, L. Zhao, Chemosphere 92 (2013) 955.

[104] M. Ahmad, S. S. Lee, X. Dou, D. Mohan, J. K. Sung, J. E. Yang, Y. S. Ok, Bioresour.

Technol. 118 (2012) 536.

[105] Z. Zhou, D. Shi, Y. Qiu, G. D. Sheng, Environ. Pollut. 158 (2010) 201.

Page 42 of 65
[106] W. Ding, X. Dong, I. M. Ime, B. Gao, L. Q. Ma, Chemosphere 105 (2014) 68.

[107] K. Sun, M. Keiluweit, M. Kleber, Z. Pan, B. Xing, Bioresour. Technol. 102 (2011)

9897.

t
[108] C. Tan, Z. Zeyu, H. Rong, M. Ruihong, W. Hongtao, L. Wenjing, Chemosphere 134

ip
(2015) 286.

cr
[109] J. Y. Lee, M. Y. Han, Int, J. Urban Sci. 16 (2014) 115.

us
[110] W. K. Kim, T. Y. Shim, Y. S. Kim, S. H. Hyun, C. K. Ryu, Y. K. Park, J. H. Jung,

Bioresour. Technol. 138 (2013) 266.

an
[111] H. J. Cho, K. T. Baek, J. K. Jeon, S. H. Park, D. J. Suh, Y. K. Park, Chem. Eng. J. 217

(2013) 205.
M
[112] A. W. Samsuri, F. S. Zadeh, B. J. S. Bardan, Int. J. Environ. Sci. Technol. 11 (2014)

967.
d

[113] Z. Ding, X. Hu, Y. S. Wan, S. G. Wang, B. Gao, J. Ind. Eng. Chem. 33 (2015) 239.
te

[114] H. Roh, M. R. Yu, K. Yakkala, J. R. Koduru, J. K. Yang, Y. Y. Chang, J. Ind. Eng.

p

Chem. 26 (2015) 226.

ce

[115] T. M. A. Fattah, M. E. Mahmoud, S. B. Ahmed, M. D. Huff, J. W. Lee, S. Kumar, J.

Ind. Eng. Chem. 22 (2015) 103.

Ac

[116] S. E. Elaigwu, V. Rocher, G. Kyriakou, G. M. Greenway, J. Ind. Eng. Chem. 20 (2014)

3467.

[117] B. S. Kim, H. W. Lee, S. H. Park, K. T. Baek, J. K. Jeon, H. J. Cho, S. C. Jung, S. C.

Kim, Y. K. Park, Environ. Sci. Pollut. Res. 23 (2016) 985.

[118] X. Zhu, Y. Liu, C. Zhou, G. Luo, S. Zhang, J. Chen, Carbon 77 (2014) 627.

Page 43 of 65
[119] P. Liu, W. J. Liu, H. Jiang, J. J. Chen, W. W. Li, H. Q. Yu, Bioresour. Technol. 121

(2012) 235.

[120] L. Sun, D. Chen, S. Wan, Z. Yu, Bioresour. Technol. 198 (2015) 300.

t
[121] S. Mubarik, A. Saeed, M. M. Athar, M. Iqbal, J. Ind. Eng. Chem. 33 (2016) 115.

ip
[122] Z. Zhang, X. Feng, X. X. Yue, F. Q. An, W. X. Zhou, J. F. Gao, T. P. Hu, C. C. Wei,

cr
Korean J. Chem. Eng. 32 (2015) 1564.

us
[123] A. Kumar, A. C. Pandey, Int, J. Urban Sci. 17 (2013) 117.

[124] J. Y. Lee, S. H. Park, J. K. Jeon, K. S. Yoo, S. S. Kim, Y. K. Park, Korean J. Chem.

an
Eng. 28 (2011) 1556.

[125] A. Samanta, A. Zhao, G. K. H. Shimizu, P. Sarkar, R. Gupta, Ind. Eng. Chem. Res. 51
M
(2012) 1438.

[126] A. E. Creamer, B. Gao, M. Zhang, Chem. Eng. J. 249 (2014) 174.

d
te

[127] Y. F. Huang, P. T. Chiueh, C. H. Shih, S. L. Lo, L. Sun, Y. Zhong, C. Qiu, Energy 84

(2015) 75.
p

[128] A. E. Creamer, B. Gao, S. Wang, Chem. Eng. J. 283 (2016) 826.

ce

[129] Y. Shen, M. Chen, T. Sun, J. Jia, Fuel 159 (2015) 570.

Ac

[130] P. Hasler, Th. Nussbaumer, Biomass Bioenerg. 16 (1999) 385.

[131] A. Paethanom, S. Nakahara, M. Kobayashi, P. Prawisudha, K. Yoshikawa, Fuel

Process. Technol. 104 (2012) 144.

[132] T. Phuphuakrat, T. Namioka, K. Yoshikawa, Appl. Energy 87 (2010) 2203.

[133] T. Phuphuakrat, T. Namioka, K. Yoshikawa, Bioresour. Technol. 102 (2011) 543.

[134] S. A. Nair, A. J. M. Pemen, K. Yan, F. M. V. Gompel, H. E. M. V. Leuken, E. J. M. V.

Page 44 of 65
 Heesch, K. J. Ptasinski, A. A. H. Drinkenburg, Fuel Process. Technol. 84 (2003) 161.

[135] J. Han, H. J. Kim, Renew. Sust. Energ. Rev. 12 (2008) 397.

[136] Y. Chen, Y. H. Luo, W. G. Wu, Y. Su, Energy Fuels 23 (2009) 4659.

t
[137] K. Qian, A. Kumar, Fuel 162 (2015) 47.

ip
[138] Y. B. Jo, S. H. Park, J. K. Jeon, Y. K. Park, Appl. Chem. Eng. 23 (2012) 604.

cr
[139] E. Lotero, Y. J. Liu, D. E. Lopez, K. Suwannakarn, D. A. Bruce, J. G. Goodwin, Ind.

us
Eng. Chem. Res. 44 (2005) 5353.

[140] A. M. Dehkhoda, A. H. West, N. Ellis, Appl. Catal. A: Gen. 382 (2010) 197.

an
[141] J. R. Kastner, J. Miller, D. P. Geller, J. Locklin, L. H. Keith, T. Johnson, Catal. Today

90 (2012) 122.
M
[142] Y. B. Jo, J. S. Cha, J. H. Ko, M. C. Shin, S. H. Park, J. K. Jeon, S. S. Kim, Y. K. Park,
d

Korean J. Chem. Eng. 28 (2011) 106.

te

[143] J. H. Yuan, R. K. Xu, Soil Use Manage. 27 (2011) 110.

[144] J. H. Yuan, R. K. Xu, N. Wang, J. Y. Li, Pedosphere 21 (2011) 302.

p
ce

[145] N. B. Klinghoffer, M. J. Castaldi, A. Nzihou, Fuel 157 (2015) 37.

[146] R. Azargohar, S. Nanda, J. A. Kozinski, A. K. Dalai, R. Sutarto, Fuel 125 (2014) 90.
Ac

[147] B. Liang, J. Lehmann, D. Solomon, J. Kinyangi, J. Grossman, B. ONeill, J. O.

Skjemstad, J. Thies, F. J. Luizao, J. Petersen, E. G. Neves, Soil Sci. Soc. Am. J. 70 (2006)

1719.

[148] J. W. Lee, M. Kidder, B. R. Evans, S. Paik, A. C. Buchanan, C. T. Garten, R. C. Brown,

Environ. Sci. Technol. 44 (2010) 7970.

[149] C. L. Sabine, M. Heimann, P. Artaxo, D. C. E. Bakker, C. T. A. Chen, C. B. Field, N.

Page 45 of 65
 Gruber, C. L. Qur, R. G. Prinn, J. E. Richey, P. R. Lankao, J.A. Sathaye, R. Valentini,

in: C. B. Field, M. R. Raupach (Eds.), Integrating Humans, Climate,and the Natural

World, Island Press,Washington, DC, 2004, pp. 1744.

[150] Y. Yanai, K. Toyata, M. Okazaki, Soil Sci. Plant Nutr. 53 (2007) 181.

t
ip
[151] K. A. Spokas, W. C. Koskinen, J. M. Baker, D. C. Reicosky, Chemosphere 77 (2009)

cr
574.

[152] A. Thomazini, K. Spokas, K. Hall, J. Ippolito, R. Lentz, J. Novak, Agric. Ecosyst.

us
Environ. 207 (2015) 183.

an
[153] D. H. Jung, Carbon Lett. 1 (2000) 98.

[154] S. Y. Ahn, S. Y. Eom, Y. H. Rhie, Y. M. Sung, C. E. Moon, G. M. Choi, D. J. Kim,

M
Appl. Energy. 105 (2013) 207.

[155] I. I. G. Inal, S. M. Holmes, A. Banford, Z. Aktas, Appl. Surf. Sci. 357 (2015) 696.
d

[156] E. A. Cho, S. Y. Lee, S. J. Park, Carbon Lett. 15 (2014) 210.

te

[157] K. Wang, N. Zhao, S. Lei, R. Yan, X. Tian, J. Wang, Y. Song, D. Xu, Q. Guo, L. Liu,
p

Electrochim. Acta 166 (2015) 1.

ce

[158] A. M. Abioye, F. N. Ani, Renew. Sust. Energ. Rev. 52 (2015) 1282.

Ac

[159] S. Niaz, T. Manzoor, A. H. Pandith, Renew. Sust. Energ. Rev. 50 (2015) 457.

[160] S. H. Hwang, W. M. Choi, S. K. Lim, Mater. Lett. 167 (2015) 18.

[161] G. Krishnamurthy, R. Namitha, S. Agarwal, Proc. Mater. Sci. 5 (2014) 1056.

[162] A. G. Klechikov, G. Mercier, P. Merino, S. Blanco, C. Merino, A. V. Talyzin,

Microporous Mesoporous Mat. 210 (2015) 46.

[163] Y. K. Choi, S. J. Park, Carbon Lett. 16 (2015) 127.

Page 46 of 65
[164] Y. J. Heo, S. J. Park, J. Ind. Eng. Chem. 31 (2015) 330.

[165] T. Ramesh, N. Rajalakshmi, K. S. Dhathathreyan, Energy Stor. 4 (2015) 89.

t
ip
cr
us
an
M
d
p te
ce
Ac

Page 47 of 65
 t
ip
cr
us
an
M
d
te

Fig. 1. Van Krevelen diagram for raw materials and biochars produced at different pyrolysis
p

temperatures [37]
ce
Ac

Page 48 of 65
 t
ip
cr
us
an
Fig. 2. Comparison of the product distributions of pyrolysis and gasification of biomass [13]
M
d
p te
ce
Ac

Page 49 of 65
 t
ip
cr
us
an
M
Fig. 3. Comparison of the Cu removal capacities of seaweed biochars [111]
d
p te
ce
Ac

Page 50 of 65
 t
ip
cr
us
an
M
d
te

Fig. 4. Adsorption of formaldehyde over chars treated using various methods [124]
p
ce
Ac

Page 51 of 65
 t
ip
cr
Fig. 5. Production of biodiesel by transesterification reaction

us
an
Fig. 6. Production of biodiesel by esterification reaction
M
d
p te
ce
Ac

Page 52 of 65
 t
ip
cr
us
an
M
Fig. 7. NOx conversion as a function of reaction temperature over municipal solid waste char
d

(YC) and RDF char (RC). W: H2O activation, H: HCl activation, K: KOH activation
te

[90]
p
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Ac

Page 53 of 65
 t
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us
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Fig. 8. The global carbon cycle of net primary productivity and release to the

atmosphere from soil [149]

Page 54 of 65
Table 1. Characteristics of biochar produced using different feedstock materials under

different pyrolysis conditions.

t
Heating Residence Yield of Ash Fixed
Temperature

ip
Raw biomass rate time biochar pH contents carbon C C/H C/O Ref
()
(/min) (h) (%) (%) (%)

cr
450 5 - 53.0 80 50.7 - 36.0 1.15 7.50
450 60 - 47.0 8.2 58.0 - 29.9 1.38 6.13
650 5 - 47.0 9.0 60.3 - 30.8 2.13 9.78
Sewage sludge [32]

us
650 60 - 45.0 8.6 62.2 - 29.2 2.02 9.49
850 5 - 45.0 11.7 62.3 - 33.0 3.93 25.89
850 60 - 38.0 11.9 66.3 - 29.6 3.52 26.31
200 - 4 51.33 7.37 4.53 - 64.19 16.21 2.42

an
Conocarpus 400 - 4 31.86 9.67 5.27 - 76.83 27.15 5.43
[35]
wastes 600 - 4 27.22 12.21 8.56 - 82.93 64.79 12.66
800 - 4 23.19 12.38 8.64 - 84.97 137.05 17.45
200 - 1 84.95 5.34 11.90 33.63 45.57 7.90 1.19
M
200 - 2 81.23 5.91 12.29 34.47 46.33 8.24 1.26
200 - 4 78.24 6.11 12.78 34.72 46.52 8.46 1.29
400 - 1 37.30 10.82 25.74 40.41 57.07 17.14 3.46
Straw 400 - 2 35.91 10.86 27.54 42.66 57.59 17.89 3.67
d

400 - 4 36.65 10.78 28.40 42.76 57.92 19.05 3.77

te

600 - 1 32.48 10.93 32.33 49.87 59.17 38.42 5.36

600 - 2 32.48 10.96 33.57 54.34 59.09 41.03 5.54
600 - 4 30.89 10.99 34.31 55.37 60.80 45.71 5.73
[36]
p

200 - 1 81.99 4.18 35.86 29.06 30.33 7.96 0.87

200 - 2 78.93 4.56 36.15 28.79 31.07 8.61 0.95
ce

200 - 4 74.97 4.34 36.93 28.64 31.94 10.34 1.00

400 - 1 58.51 9.75 45.61 33.56 33.58 18.45 1.20
Lignosulfonate 400 - 2 57.80 9.65 47.61 34.13 33.67 19.24 1.23
400 - 4 57.24 9.35 49.83 32.28 33.60 21.27 1.25
Ac

600 - 1 52.99 10.68 56.13 36.83 34.52 31.67 1.35

600 2 48.12 12.5 58.12 35.56 34.60 31.45 1.24
600 4 43.85 12.95 59.69 35.05 36.81 29.21 1.24
Bagasse 43.7 9.3 8.57 80.97 85.59 30.35 8.17
Cocopeat 62.9 10.3 15.90 67.25 84.44 29.32 7.24
Paddy straw 49.6 10.5 52.37 39.10 86.28 27.65 11.74
500 10 1 [45]
palm kernel shell 53.5 6.9 6.86 80.85 87.85 30.19 10.79
Wood stem 42.6 9.5 2.28 83.47 89.31 34.75 12.17
Wood bark 50.3 9.6 12.84 68.66 84.84 27.11 8.32

Page 55 of 65
Table 2. Yield and characteristics of biochar produced under different HTC operation

conditions [54].

t
ip
Temperature Biomass/Water HHV
Time (h) Biochar yield (%)
() ratio (MJ/kg)

cr
170 3.3 5.0 64.6 26.4
170 3.3 15.0 60.8 27.4
170 10.0 5.0 68.0 26.7

us
170 10.0 15.0 61.3 29.0
200 1.1 10.0 49.8 29.1

an
200 6.7 10.0 62.0 29.2
200 12.3 10.0 61.5 31.1
230 3.3 5.0 49.6 31.2
230 3.3 15.0 27.6 32.9
M
230 10.0 5.0 62.2 28.3
230 10.0 15.0 35.4 34.8
250 6.7 10.0 29.4 34.6
d
p te
ce
Ac

Page 56 of 65
Table 3. Comparison of the typical operation conditions and product properties of various
processes for biochar production [62]

Solid product Carbon content Carbon

Process yield yield
temperature of the solid
Process Residence time on a dry wood (mass carbon,
product
() feedstock product/mass carbon,
basis (mass%) (mass%)
feedstock)

t
Slow pyrolysis ~400 Minutes to days 30 95 0.58

ip
Fast pyrolysis ~500 ~1s 12-26 74 0.2-0.26
Gasification ~800 ~10 to 20s 10 - -

cr
HTC ~180-250 1-12h < 66% < 70% 0.88
Flash
~300-600 < 30min 37 85 0.65
carbonization

us
Torrefaction ~290 10-60min 61-84 51-55 0.67-0.85

an
M
d
p te
ce
Ac

Page 57 of 65
Table 4. Characteristics of biochar activated using physical activation methods.

Ave. pore
BET surface Pore volume
Raw material Activation method diameter Ref.
area (m2/g) (cm3/g)
(nm)
Rice straw 363 0.164 1.81
40% H2O for 1hr at 700 [67]
Sewage sludge 64 0.039 2.45

t
Almond shell 601 0.375 -

ip
Walnut shell 792 0.524 -
Almond tree steam for 0.5hr at 850 [70]
1080 0.95 -
pruning

cr
Olive stone 813 0.545 -
carbonization 600 for 30min,
820 0.4 -

us
CO2 activation at 700

carbonization 600 for 60min,

554 0.28 -
Coffee CO2 activation at 700

an
[73]
endocarp carbonization 600 for 120min,
919 0.49 -
CO2 activation at 700

carbonization 600 for 60min,

M
630 0.35 -
steam at 700

CO2 at 670 12 0.01 -

d

Sewage sludge CO2 at 750 62 0.03 - [75]

CO2 at 800 7 - -
te

Palm oil shell O3 for 30min at 33g/m3 at 23 1 ND - [79]

Pyrolyzed at 600 for 1hr,
p

steam activation at 600 for 375 0.056 13.03

20min
ce

Pyrolyzed at 600 for 1hr,

steam activation at 600 for 496 0.086 13.26
Posidonia
120min
oceanica (L.) [82]
Ac

fibres Pyrolyzed at 600 for 1hr,

steam activation at 600 for 615 0.160 13.09
300min
Pyrolyzed at 600 for 1hr,
steam activation at 600 for 313 0.707 17.16
720min

Page 58 of 65
Table 5. Characteristics of biochar activated using chemical activation methods.
Raw BET surface Pore volume Ave. pore
Activation method Ref.
material area (m2/g) (cm3/g) diameter (nm)
Rice straw 772.3 0.422 2.185
char/KOH=1.0 for 2hr at 60,
[67]
Sewage heat treatment at 700 782.6 0.606 3.096
sludge

t
30% H3PO4 at 450 6 - -

ip
Sewage 50% H3PO4 at 450 17 - -
[72]
sludge char/NaOH=1.0 60 and heat
689 0.29 -
treatment at 700

cr
Mixed with K2CO3 for 24hr, heat
643.54 0.336 1.04
treatment at 600 for 1hr

us
Mixed with K2CO3 for 24hr, heat
1352.86 0.680 1.01
Soybean oil treatment at 800 for 1hr
[83]
cake Mixed with KOH for 24hr, heat
600.05 0.299 0.99
treatment at 600 for 1hr

an
Mixed with KOH for 24hr, heat
618.54 0.291 0.94
treatment at 800 for 1hr
Pyrolyzed at 500(50/min),
ZnCl2/biochar=4, heat treatment 249.3 0.151 2.42
M
at 600 for 1hr
Pyrolyzed at 500(50/min),
ZnCl2/biochar=4, heat treatment 491.9 0.249 2.02
d
at 700 for 1hr
Pyrolyzed at 500(50/min),
te

ZnCl2/biochar=4, heat treatment 772.0 0.358 1.85

at 800 for 1hr
Safflower Pyrolyzed at 500(50/min),
p

seed press ZnCl2/biochar=4, heat treatment 801.5 0.393 1.96 [88]

cake at 900 for 1hr
ce

Pyrolyzed at 500(50/min),
ZnCl2/biochar=3, heat treatment 719.3 0.371 2.23
at 900 for 1hr
Ac

Pyrolyzed at 500(50/min),
ZnCl2/biochar=2, heat treatment 703.0 0.344 1.94
at 900 for 1hr
Pyrolyzed at 500(50/min),
ZnCl2/biochar=1, heat treatment 619.8 0.330 2.13
at 900 for 1hr
Organic 3M H2SO4, 650 for 60min 408 0.523 5.21
sewage 3M H3PO4, 650 for 60min 289 0.436 2.65 [89]
sludge 5M ZnCl2, 650 for 60min 555 0.752 2.26

Page 59 of 65
Table 6. Physical characteristics of seaweed chars [111]

Ave. Pore
BET Surface Area Pore Volume
Diameter

t
2 3
(m /g) (cm /g)

ip
(nm)

cr
U ND ND ND

us
UW 57.9 0.02 1.25

UK 1287.0 0.64 2.00

an
M
d
p te
ce
Ac

Page 60 of 65
Table 7. Maximum Cu adsorption quantities of pyrolytic biochars [117].

Pyrolytic biochar source Maximum Cu adsorption quantity (mg/g)

t
ip
Hardwood 7.44

cr
Switch grass 31.0

Corn straw 12.52

us
Amino-modified biochar 17.01

an
Spartina alterniflora 48.49

Peanut straw 88.9

M
Soybean straw 50.2

Composted swine manure 20.11

d

Sewage sludge 10.56

te

Commercial activated carbon 11.4

p

Commercial powdered activated carbon 1.80

ce

Undaria pinnatifida 125.8

Enteromorpha compressa 75.1

Ac

Page 61 of 65
Table 8. Reduction of tars in various tar reduction systems

t
Method Temperature () Tar reduction (%)

ip
Sand bed filter 10-20 50-97

cr
Fabric filter 130 0-50

us
Wash tower 50-60 10-25

Oil scrubber132,133 27-28 50-90

an
Physical Water scrubber134 2828.5 20-40

Wet electrostatic
M
40-50 0-60
precipitator

800 78
d

Thermal cracking 40,133

800-900 >98
te

Catalytic tar cracker 900 >95, >98

p

Chemical
Pulse corona discharge 135 200 8-44
ce
Ac

Page 62 of 65
 t
Table 9. Thermal and catalytic conversion of phenol and naphthalene [40]

ip
Conversion (%)

cr
Catalyst T()
Phenol Naphthalene

us
700 6.0
Empty reactor
800 98.2

an
(Thermal cracking)
900 98.4 2

700 34.5
M
Olivine
900 100 55.0

700 42.7
d

biomass Char
900 100 94.4
te

700 81.6
FCC
p

900 100 60.3

ce

700 90.0
Dolomite
900 100 61.0
Ac

700 91.0
Nickel
900 100 100

Page 63 of 65
 t
Table 10. pH and base cation content of biochar [144]

ip
Base cation content (cmol/kg)

cr
Source of Biochar pH
Ca2+ Mg2+ K+ Na+

us
Rice 7.69 32.8 11.4 92.2 5

Corn 9.24 26.5 10.5 188.4 2.6

Peanut

Faba bean
8.88

10.33
90.8

35.7 an 46.9

8.7
21.3

111.1
1.5

64.4
M
Mung bean 10.35 74.9 12.3 178.2 1.5
d
p te
ce
Ac

Page 64 of 65
 t
ip
Table 11. Concentrations of metal elements in biochar [143, 144]

(unit : ppm)

cr
Poplar Poplar wood Wheat straw Poultry litter Sawdust Flax straw

us
wood char-750 char-475 char-475 char-475 char-475

Ca 5,200 10,900 42,293 161,395 11,423 41,479

K 1,200 7,200 33,899

an 67,798 3,928 13,384

M
Na <50 64 264 14,624 238 2,027

Mg 340 1,200 2,418 19,255 1,419 5,846

d
p te
ce
Ac

Page 65 of 65