Technical
Technical
Technical
1.1, BACKGROUND
Since the 1970s energy crisis, numerous nations have expressed interest in developing biomass
as a fuel source. Because of the recent technological advancement that has made fossil energy
relatively affordable, interest in biomass energy has decreased. However, the substantial research
efforts in the development of bioenergy have been stimulated by the high greenhouse emissions,
fatal air pollution, unstable prices for fossil-based energy, and rapid expansion of the world's
transportation fuel consumption. Bioenergy is power produced from fuels sourced from biomass.
Since biomass is a renewable resource, it has been taken into consideration as a substitute
feedstock for the production of sustainable energy in the future. Direct combustion of biomass,
such as firewood, has long been a method of supplying humans with energy.
Biomass is a renewable energy resource which comprises carbon, hydrogen, oxygen, traces of
nitrogen and some minerals. Biomass utilization has an advantage over other renewable sources
such as solar energy, wind energy and hydro-electric power because of its low dependence on
1
site and climate as diverse biomasses can grow in varied conditions. Moreover, biomass can be
easily stored and transported (albeit with a lower energy density than fossil fuels). Rural areas in
under-developed countries are dependent up on bio-mass for essential activities such as cooking
and heating. For example, India has considerable coal reserves of around 223 billion tonnes, but
they are located in specific areas such as north-east India, unlike biomass, which is evenly and
broadly spread over the whole country. Furthermore, easy availability of waste biomass as a low-
cost fuel make it a promising global energy source. Developed nations are also focusing on
biomass as a sustainable energy option because of these benefits.
Plant biomass was the first fuel used by humans. The nineteenth century saw fossil fuels allow
industrialization and biomass was to a significant extent displaced. On the other hand, fossil fuels
have created grave environmental issues such as climate change, due to CO2 emissions, and
major pollution problems worldwide. In the light of depleting easily accessible and cheap coal
resources and oil reservoirs, it is imperative to shift our focus back to biomass, although
underground gasification might help extend the use of coal. Currently biomass provides more
than 10% of the global energy supply making it one of the leading potential viable renewable
energy resources.
The carbon cycle associated with biomass production and end use must match up the longer time
scales (annual for agricultural residues and grassy energy crops and of the order of three years
for woody short rotation energy crops) of carbon absorption in the growing phase with the rapid
production of CO2 during the combustion phase. This is essentially the same requirement as
ensuring a secure and sustainable feedstock supply. Much can be learnt from the sustainable
timber and pulp and paper industries which have been managing exactly this problem from any
decades. This issue is elaborated on further in Section11.4 on life cycle assessment.
An extensively explored research area is the development of clean and sustainable technologies
to utilize biomass feed stocks to produce biofuels. Biofuels are liquid or gaseous fuels produced
from biomasses which are predominantly employed in the transportation sector. They are also
used to generate heat and electricity or can be used as the feed stock to synthesis important
chemicals. Gaseous biofuels are normally used for heat and power production, where as liquid
biofuels are generally employed in the transporta-tionsector.Biofuels,ingeneral,includebio-
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methanol(MeOH),bio-ethanol(EtOH),bio-dimethylether(DME),synthetic natural gas(bio-
methane),Fischer Tropsch (FT) fuels and H2.
Biofuels can be classified as first generation, second generation, third generation and fourth
generation biofuels. Their composition and calorific content are dependent up on the type of
biomass and process employed. First generation biofuels such as bio-methanol, bio-ethanol, bio-
propanol, bio-butanol, fatty acid esters,etc., are derived from simple sugars, starch, fats and
vegetable oils. Inderwildiet al. stated that second generation biofuels, such as EtOH are produced
by the‘biomass-to-liquid’(BtL) route employing lingo cellulosic biomass. The third and fourth
generation of biofuels products use the ‘algae-to-biofuel’ strategy. In third generation
technologies, algal biomass is treated for biofuel production, where as the fourth generation
approach utilizes metabolic engineering of algae for generating biofuels from oxygenic
photosynthetic microbe sand creating artificial carbon sinks.
Gasification technology is almost 100 years old. In the 1920s, cars in Sweden were powered by
wood gasifiers owing to ample wood biomass and lack of petroleum resources. During the
Second World War (19391945), numerous studies were conducted to optimize the design of
wood gasifiers and enhance their performance. In the 1970s and 80s, about 40 companies around
the globe proposed to build gasification plants based on biomass, to generate heat and power.
Advances in gasification technologies and multiple uses of syngas have permitted gasification to
integrate with several industrial processes to produce chemical feed stocks and generate power.
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1.2, STATEMENT OF THE PROBLEM
The present review has revealed that many waste-to-bioenergy technological routes are made
available to produce bioenergy from waste feedstock/substrates. Waste utilization is supposedly
to be the most economical process for renewable energy production, coupled with its
complementary benefit that is to clean the environment. Significant amounts of biomass residues
and waste are produced inevitably from different sectors across world, and the waste could be a
promising feedstock for bioenergy if efficient and economically viable technologies were
developed. Still, there are several limitations to the development of biomass residues and waste
as an immediate energy resource. First and foremost, waste-to-bioenergy production is still not
as cost-competitive as fossil-based fuels, based on the current technologies developed. The
utilization of MSW for bioenergy production is not economically profitable due to the high cost
of technologies for incineration, gasification and pyrolysis. Gasification has relatively high
4
operational cost (250,400 USD/d), and, in terms of decreasing cost, is followed by incinerator,
landfill gas recovery system and lastly anaerobic digestion. The high energy required for waste
pre-treatment process, purification of the biofuels produced, plant equipment set up, and reactor
operation and maintenance, could limit the commercialization of waste-to-bioenergy
technologies. Therefore, the process optimization is being the research focus nowadays to
increase the production yield and process efficiency.
To some extents, the implementation of waste-to-bioenergy approach should aid to improve the
environment by lessening the amount of waste that must be landfilled. However, the processing
of waste-to-bioenergy might lead to the release of undesirable and harmful by-products to
atmosphere. For an example, the emissions of trace organics such as furans, polychlorinated
dioxins, lead, mercury, and cadmium could be attributed to the inadequate design and/or poorly
operated of MSW combustion systems for the generation of electricity . The volatile elements
such as mercury might get vaporized during the combustion process of MSW and might not able
to be removed effectively using a particulate removal device. Some measurements have been
taken to prevent the emission of harmful compounds, include the adequate control of mixing and
temperature of air/fuel and the avoidance of “quench” zones in the furnace. However, a
satisfactory control technology to prevent the emission of harmful volatiles during waste-to-
bioenergy processes should be continuously developed. The hazardous gas emission should be
taken into consideration when selecting waste-to-bioenergy technology. For instances, an
incinerator generally produces higher amounts of pollutants compare to anaerobic digestion.
Anaerobic digestion could be an optimum choice for converting MSW that contains high
moisture content if low demand for heat energy and cleaner technology are needed. Lastly,
proper waste classification is equally important to ensure higher energy recovery efficiency in
power generation and minimize environmental impacts.
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1.3, OBJECTIVE
1.4, LIMITATION
The waste biomass is good and bright for energy production but certain factors pose a threat
which calls for day to day development of waste biomass for energy purpose. At the moment
there are numerous barriers hindering the implementation. When a large resource base is
available, the rate at which waste biomass can be used to generate energy commercially differs
between countries.
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The most important criteria for the further development on the technology is the availability and
costs. This development leads to greater efficiencies, low investment and maintenance costs
which will pave way for feasible conversion of biomass fuels.
There is diverse opinion regarding the degree of availability of agricultural land for energy crops.
It was an accepted principle that production of biomass for energy purposes should not have a
conflicting effect with production food.
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2, LITERATURE REVIEW
2.1, Introduction
Biomass gasification represents a significant way to produce energy from biomass. It features
renewable properties and offers great potential for utilization. The application of biomass
gasification products, design of the gasifier, type of biomass feedstock, gasification agents, and
gasification parameters are key for the biomass gasification process.
Gasification converts the carbonaceous feedstock into a gaseous fuel or chemical feedstock
which can be further burned to release energy or can be used to produce value-added chemicals,
e.g. hydrogen. Despite the relative similarity between gasification and combustion, they differ in
the treatment of the product gas and the energy available within the chemical bonds in the
product gas. The difference of combustion is in the release of the energy by breaking those
chemical bonds. The primary function of gasification is to produce gases with a high ratio of
hydrogen to carbon by stripping carbon away from the hydrocarbon feedstock and by adding
hydrogen.
(1) Drying,
(2) Pyrolysis,
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Gasification, as one step after combustion, uses the reduction zone to make H2 and CO from the
combustion products. A clear understanding of each step of the gasification process is needed to
model the thermochemical process accurately. In an ideal complete gasification process, the
syngas only consists of CO and H2. However, the ideal scenarios never take place due to
reactions of the gasifying agent (air) and by-products (H2O, CO2, and CH4), which exist from
the pyrolysis and combustion processes.
The heating value and the composition of the gas products in a gasifier are dependent on the type
and amount of the gasifying agent. A ternary diagram (Fig.2 ) of carbon, hydrogen, and oxygen
demonstrates the conversion path towards the formation of different gaseous products in a
gasifier. Each corner of the triangle represents 100% of the element, and each point within the
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triangle is a mixture of the C, O, and H. Thus, each side of triangle is divided into six parts to
show the percentage available of elements. If oxygen is the gasifying agent, the conversion path
moves toward the oxygen-driven reactions. In this case, the gasification products include CO
with low amounts of oxygen and CO2 for the high amount of oxygen. When the amount of
oxygen exceeds the stoichiometric level, the process moves from gasification to combustion. The
excessive air during the combustion results in producing flue gas which contains no residual
heating value compared to fuel gas (or synthesis gas). If steam is the gasification agent, the
process moves toward the hydrogen-dominate reaction in Fig. 2. The gas product contains more
hydrogen per unit of carbon, resulting in a higher H/C ratio.
Fig. 3 shows the broad range of gasification products while using various gasifying agents. As
shown in Fig. 3, steam, as the gasifying agent, produces syngas with the highest H2 content as
well as a high content of CO and CO2. Using air as the agent results in a higher production of
CO and a lower amount of H2 and CO2, compared to a case when steam is used as the agent.
The gasifying agent can also affect tar content in the char, syngas and equivalence ratio (ER).
ER or the exact ratio of fuel to agent indicates the oxygen feed during gasification and presents a
crucial parameter for the gasifier performance. The gasifier performance determines the range of
syngas products. For example, increasing ER decreases the molar fraction of nitrogen to a
minimum value; afterward, an increase can be observed. In their experiment, the fraction of
carbon monoxide and hydrogen shows an opposite trend to nitrogen when ER is increased.
Carbon dioxide, however, monotonically increase by the increase of ER.
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Figure 3, Effect of gasifying agents on the combustion of gas product.
12
Figure 4, illustrates the development of biofuel generation and highlights the second generation
biofuels produced by biomass residues and waste, and their conversion pathways to produce
different kinds of bioenergy, including syngas, bio-oil, biochar, electricity, biogas, bioethanol,
bio-hydrogen, and biodiesel. Amongst the biomass residues and waste, wood and agricultural
residues (primary and secondary biomass residues), waste cooking oils (tertiary biomass
residues) and microalgae biomass have demonstrated their promising potentials.
Figure 4, Diagram of the development of biofuel generation with highlights on the second
generation biofuels produced by biomass residues and waste and their conversion pathways to
produce a wide variety of bioenergy.
Wood and agricultural residues
Wood processing wastes like sawdust, wood chips and discarded logs that are generated through
sawmill and lumber processing activities can be used as feedstocks for biofuels. For instances,
the wood residues and sawdust generated from saw and paper mills industry can be applied as
boiler fuels and feed stocks for ethanol production. Besides, Zeng et al. Reported that straw has
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accounted for 72.2% of the biomass energy resources in China. The straw is referred to the
residues or by-products of the harvesting food crops such as rice, wheat, corn, beans, cotton and
sugar crops. Corn stover such as stalks, cobs, and leaves, has been also reported to show
potential to be converted into fermentable sugars for bio-butanol production. While in tropical
countries, sugarcane residues, particularly sugarcane bagasse and leaves, can be a good candidate
for the economic utilization of residual substrates for the production
of bioethanol and other biofuels such as biochar. Palm kernel press cake, a residue obtained
from palm oil extraction, demonstrated its use to produce bioethanol via fermentation process.
Algae biomass
In a first approximation, algae can be categorized into two major groups, which are macro algae
(or known as seaweeds) and microalgae. Macro algae are generally referred to large multi-
cellular algae that commonly seen growing in ponds. Whereas, microalgae are unicellular and
tiny algae that often grow in a suspension within water-bodies. Macro algae contain a wide
variety of bioactive compounds, and however, lower margins with regard to biofuels can be
obtained from macro algae than microalgae. Therefore, microalgae represent another promising
source of oil owing to their high lipids accumulation and fast growth rates. Additionally,
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microalgae do neither compete for purely agricultural land nor large freshwater resources.
Similar to biomass residues and waste, the spent microalgae biomass can be converted to
biofuels after the extraction processing of target products such as oils or/and other high value
compounds from microalgae biomass.
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principles, financial circumstances and project precise aspects. Based on several research studies,
it was reported that thermal conversion technologies have gained extra attention due to the
availability of industrial infrastructure to supply thermochemical transformation equipment that
is highly developed, short processing time, reduced water usage and added advantage of
producing energy from plastics wastes which cannot be digested by microbial activity.
Additionally, thermochemical conversion is essentially independent of environmental
circumstances for production purposes. Thus, it is vital to comprehend the different
thermochemical process options to assess their future potential.
Gasification
The gasification technique comprises chemical reaction in an environment which is oxygen-
deficient. This process involves biomass heating at extreme temperatures (500–1400 °C), from
atmospheric pressures up to 33 bar and with low/absent oxygen content to yield combustible gas
mixtures. with low/absent oxygen content to yield combustible gas mixtures. Gasification
process transforms carbonaceous constituents into syngas comprising hydrogen, carbon
monoxide, carbon dioxide, methane, higher hydrocarbons, and nitrogen with the presence of a
gasification agent and catalyst. By utilizing this syngas, various types of energy/energy carriers
are supplied for examples biofuel, hydrogen gas, biomethane gas, heat, power and chemicals.
It is reported that gasification process is the most efficient technique in the production of
hydrogen gas from biomass. Contrasting to additional thermochemical conversion techniques,
gasification technique is considered to be independent auto thermic route based on energy
balance. It is revealed that biomass gasification able to recover more energy and higher heat
capacity compared to combustion and pyrolysis. This is attributed to optimal exploitation of
existing biomass feedstock for heat and power production. Conversion of carbon monoxide and
hydrogen by means of pyrolysis and liquefaction is poor due to their complexity process, greatly
reliant on operating conditions and the presence of secondary reaction resulting from hot solid
particles and volatiles. Additional benefit of gasification process is the simple conversion by
means of catalytic methanation of carbon monoxide and carbon dioxide of syngas to synthetic
natural gas. Thus, gasification of biowaste is deliberated to be ideal route for the conversion of
diverse biomass feedstocks varying from wastes of agriculture, industrial, kitchen, food, and
farm.
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Gas composition produced from gasification process varies according to type of gasifier,
gasification agent, catalyst type and size of particle. Generally, high amount of CO2 and CO is
generated via gasification process of feedstock that contains high amount of carbon and oxygen.
It is found that among all the waste feed stocks, MSW and agricultural residue have greater CO
and CO2 content. During gasification process, sulphur is emitted as H2S form that causes
complexity in gas separation and treatment. That is the reason that gas treatment methods are
required for feed stocks that contain high amount of sulphur. Normally, bio-waste feed stocks
comprise < 1.5 wt% of sulphur. Among which, sewage sludge and animal waste comprises
highest quantity of sulphur with 1 wt% and 0.5 wt% correspondingly.
Typically, in biowaste gasification, there are four types of gasifier that are used which are
1, Fixed bed
2, Fluidized bed
3, Entrained flow and
4, Plasma gasifiers.
As for the fixed bed gasifier there are two dissimilar forms known as downdraft gasifier and
updraft gasifier. The downdraft gasifier is more popular due to its ability to yield high good
quality gas quickly and the utilization of flexible moisture content of the biomass. At present
small scale gasifiers are practically utilised for electric power generation and power heat
cogeneration.
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Figure 5, Schematic Diagram of (a) Up draft gasifier; and (b) Downdraft gasifier showing
different stages of gasification.
Table 2 outlines the generally employed types of gasification in recent studies for the gasification
of bio-waste.
Recent study by Salimi and colleague on the energy generation from lignocellulosic wastes of
canola stalks discovered the use of novel bimetallic catalysts supported on activated carbon and
graphene nano sheets in the hydrothermal gasification process. It was found that the addition of
metal such as Nickle (Ni), Rudium (Ru), Copper (Cu) and Cobalt (Co) based catalyst able to
accelerate the reforming reaction that eventually results in the enhanced hydrogen and methane
production. From the study, Ni (20%)/activated carbon, and Ni (20%) – Cu (2%)/ activated
carbon catalysts resulted in greater generation of H2, CO2 and CO yields, high catalytic activity
and stability. The feasibility and behaviour of fuel gas produced by the gasification process of
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coffee waste. The feedstocks were exposed to gasification in an open-source and low-cost
downdraft gasifier, via gasifying agent in this case air. From this experiment, the fuel gas
produced via eucalyptus chips contributed average higher heating value of 6.81 ± 0.34 MJ·Nm-3,
with pre-dominance of carbon monoxide (20.24 ± 0.93%).
Plasma gasification is a rather novel thermochemical technique that is applicable for harmful
biomass wastes. Plasma gasification method is an allothermal method that uses exterior power to
heat up and maintain the elevated temperatures. The products that are produced from this process
are mostly syngas, slug and ash. Since this process uses high temperature, plasma gasification
process able to break down nearly all the materials including medical basis such as bandages,
infusion kits, biomedical waste containing cytotoxic drugs, antibiotics and also laboratory waste
that comprises biomolecules or organisms that are harmful to be released to the environment.
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Mazzoni and colleague investigated on plasma co-gasification to evaluate the possibility of
plasma gasification in recovering energy from MSW and waste of plastic solid. From the study,
they found the process consumes oxygen rich air as plasma forming gas and result in the increase
in the plant efficiency beyond 26%. This performance has been recognised as the best point of
reference for conventional grounded combustionof waste-to-energy technique [74]. Latest study
on plasma gasification of biomedical waste (bonny tissue) and household waste, exhibited that
the overall concentration of gas synthesised was 69.6 and 71.1 vol.%, correspondingly.
A recent promising technology for the conversion of biomass to electricity is the use of MFC.
MFC technology involves the bioelectricity generation through the conversion of organic
substrates by electrogenic bacteria under anaerobic conditions. MFC consist of two chambers
with a biotic anode and abiotic cathode that are separated by a proton exchange membrane. MFC
can simultaneously treat various waste (food waste, household food waste and MSW) while
generating electricity and represents a new source of renewable energy process. The current and
power density produced through MFC can be altered by the operational conditions, including
temperature, substrate concentration, pH, loading rate, microorganisms activity, hydraulic
retention time and static magnetic field. In addition, many parameters relating to the electrode
materials, architectures, cost effectiveness and also the membrane characteristics have been
investigated to evaluate the improvement on electricity generation of MFC.Tthe utilization of Ti-
TiO2 electrode showed around 4 times higher power density compared to Pt electrode. Apart
from that, the usage of food residue biomass as a substrate in the MFC process achieved the
highest power density as well, indicating that the hydrolysis of food residue biomass could
significantly enhance the performance of MFC. MFC shows great potential as a green and
sustainable process; its implementation can provide new insights for bioelectricity generation.
2.3, Summary
Biomass energy has an important role to play in preventing global warming and improving the
security of European energy supply. Property of biomass is different from other conventional
solid fuels such as coal.
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We hope to see a strong and viral competition about the use of biomass and the land to grow
such biomass.
The future utilization of biomass for heat energy will depend on how available biomass is for
distribution among uses for heating, electricity and other necessary applications.
Biomass gasification promise to pave way for more effective and efficient use of wood chips,
gasifier is expected to take up the responsibility of increasing the efficiency of burning wood to
produce electricity.
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3, Methodology
Gasification is a process that converts organic or fossil-based carbonaceous materials at high
temperatures (>700°C), without combustion, with a controlled amount of oxygen and/or steam
into carbon monoxide, hydrogen, and carbon dioxide. The carbon monoxide then reacts with
water to form carbon dioxide and more hydrogen via a water-gas shift reaction. Adsorbers or
special membranes can separate the hydrogen from this gas stream.
Table 1 shows the overview of recent studies that are conducted related to bioenergy conversion
using gasification methods.
Eucalyptus chip
Higher heating value
Fuel gas Eucalyptus chips Temperature: 22.1 °C
(MJ·N− 1m− 3):
and Air input flow: 182.7
Eucalyptus chips: 6.81
coffee husk Nm3·s-2
Coffee husk: 7.76
Air consumption: 38.2
Nm-3 Coffee husk
Temperature: 26.3 °C
Air input flow: 124
Nm3·s-1
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Air consumption:13.4
Nm-3
Temperature: 600–800
°C
Efficiency: 33.78%
Oxygen ratio: 33%
Fuel gas Rice straw CO gas: 2.01%
Air flow: 0.6 Nm3·h− 1
H2 gas: 5.48%
Feed rate: 1.12 kg·h− 1
CH4 gas: 0.51%
Equivalence ratio: 0.2
Co-gasification using
downdraft fixed
Cold gas efficiency:
gasifier at atmosphere
70.68%
Fuel gas Acid hydrolysis pressure.
residues Temperature: 800 °C
and sewage sludge Catalyst: CaO
Sewage sludge
composition: 50 wt%
CaO/C (molar ratio):1.0
Equivalence ratio: 0.22
23
Figure 6, Source of Biomass
Table 2, Syngas conversion condition and syngas composition (H2/CO and CO2)
24
When biomass-derived syngas is used for biofuel production, the cleaning of the raw gas is
needed strictly in order to remove contaminants and potential catalyst poisons as well as to
achieve the qualitative composition required by the biofuel production process.
3.1.1 Methanol
Methanol has been produced via a catalytic process using natural gas and steam as feeding.
This is a two-step process;
In the first step, methane is reformed by using steam at about 600–650 OC and nickel-based
catalysts in order to increase the CO + H2 yield. These catalysts are often doped with potassium
in order to avoid char formation which could reduce the active metal surface, reducing the
catalytic effect on the reaction. The product of steam reforming reaction is syngas, which is
composed of a mixture of hydrogen and carbon monoxide with a stoichiometric ratio of 3:1, as
reported below:
CH4 +H2O ↔ CO + 3H2 ∆ HOR = - 191.7 kJ/mol (1)
In the second step, syngas is converted to methanol by using predominantly copper-based on
25
alumina support catalysts through an exothermic equilibrium limited synthesis process at
pressures in the range of 50–100 bar and temperatures in the range 200–300 OC, according to the
following reactions.
CO + 2H2 ↔ CH4OH ∆ HOR = 94.1 kJ/mol (2)
CO2 + 3H2 ↔ CH3OH + H2O ∆HOR = 52.8 kJ/mol (3)
CO + H2O ↔ CO2 +H2 ∆HOR = 41.5 kJ/mol (4)
A ratio (H2 - CO2)/(CO + CO2) slightly above two is usually used in order to favour kinetics
and to control by-products. The main reaction for methanol production is the reaction based on
CO and H2 (Equation 2); however, the effect of a methanol production promoter by carbon
dioxide. Thus, in presence of CO2, the rate of the reaction between CO and H2 increased
approximately by a factor of 100.
As a result of the exothermic nature of the reactions, a low temperature helps to increase the
conversion. Furthermore, this is a reaction where there is a decreasing amount of mole numbers
and by increasing the pressure, the reaction yield also increases. The choice of process
temperature close to 250 O C is not attributable to the thermodynamics of reaction (preferred at
lower temperature); it is a result of the higher performance of the catalysts in these operating
conditions.When syngas is used as feed stream, methanol production starts from the second step.
3.1.2. Ethanol
Ethanol from syngas is directly obtained by employing ad hoc catalysts such as
Mo, Rh, K, Cu, Zn, and Fe and this process is facilitated at pressure in the range of 1–100 bar
and temperatures of about 230–300 OC. The predominant reaction for ethanol production from
syngas consists in CO hydrogenation (Equation (5)); moreover, ethanol also can be produced by
CO2 hydrogenation (Equation (6)), which are both processes exothermic.
Beginning with syngas, ethanol production also can be carried out through methanol synthesis
26
followed by methanol homologation, according to the following exothermic reactions catalysed
by Cu/Co catalysts.
Clearly, the main issue of ethanol synthesis from syngas is the H2/CO ratio. This ratio in the
syngas may be closer to one, resulting from an occurrence of side reactions, such as WGS, which
reduce the H2/CO ratio from 2 to ≅ 1.0.
DME production also can be carried out in a single-step synthesis starting from syngas through
the use of bi functional catalysts (CuO–ZnO–MnO and zeolite) operated at 30–70 bar and 200–
300◦C, according to the following reaction.
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biofuels such as gasoline, kerosene, and diesel fuel. Accordingly, it is possible to produce fuels
with linear chains and with a high grade of purity [192] and simultaneously without sulphur,
nitrogen, or aromatics [193,194]. At present, it is considered to be the most complete technology
for transportation biofuel production .FT Synthesis produces several hydrocarbons, paraffin, and
olefins such as methane, ethane, ethylene, LPG (C3–C5), fuel (C5–C12), gasoline (C13–C22),
and waxes (C23–C33). Their distribution depends on the type of the catalyst used as well as by
the process parameters, such as temperature, pressure, feed gas composition, and residence time.
The set of reactions is described below.
where n is the number of carbon atoms and m is the same for hydrogen atoms contained in the
produced hydrocarbon.
Co and Fe catalysts are often used for these reactions in the range of temperatures between 475
K and 625 K at pressure in the range 15–40 bar. In particular, cobalt catalysts improve
performance in terms of conversion when compared with iron catalysts; however, iron catalysts
guarantee a higher production in terms of olefin and alcohols than Co catalysts which give more
paraffinic molecules. C20+ linear HCs, C5+ paraffins and medium weight olefins, which are
further processed to generate usable liquid transportation fuels, are the most desired products
obtained via FTS .
3.1.5. Hydrogen
Hydrogen is a component of syngas, from a minimum of ∼=5–10% v/v to a maximum of ∼=40–
50% v/v, depending on gasifier type, biomass feed, and operating conditions. Biomass
gasification using steam as a GA results in syngas with H2 content higher than 40% v/v,
reducing tar production ]. In order to increase H2 concentration, syngas is reformed via catalysed
reactions such as the steam reforming of methane and higher hydrocarbons as well as the WGS
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reaction, several catalysts, such as Ni, Fe, and Mo catalysts at temperature in the range 200–1100
◦C and pressure between 1 and 30 bar :
An example of platform of hydrogen from biomass is the project “Hydrogen from biomass for
Industry” , according to which the production of hydrogen was carried out by several steps,
beginning with syngas produced via steam gasification of biomass; this was followed by steam
gasification, CO-shift stage, CO2-separation with a press rized water scrubber, a PSA system, a
steam reformer, and advanced gas cleaning components] with H2 purity > 98–99% v/v.
Gasification via water in supercritical condition (SWC = 22.1 MPa and 374 ◦C) is a valuable
way to process wet biomass, producing hydrogen-rich syngas . Demirbas investigated the effect
of operating temperatures (650–700 K) on hydrogen production from biomass gasification in
supercritical water condition, observing an increase of hydrogen content from 6.6% to 9.4% with
the temperature increasing from 650 to 700 k.
Both carbon monoxide hydrogenation and carbon dioxide hydrogenation are exothermic
reactions; therefore, continuous cooling of the reactor is necessary to guarantee a temperature of
29
250–300 OC, i.e., the activation temperature of the catalysts. In order to increase the performance
of these reactions, the operative pressure must be in the range between 15 and 25 bars.
Another issue for SNG production is char formation, in particular because of the low process
temperature:
2CO ↔ CO2 + C(S) ∆ H = -172 kJ/mol
CO +H2 ↔ C(S) +H2O ∆ H = -131 kJ/mol
Char formation could cause deactivation of Ni-based catalysts, thus decreasing the performance
of methane production.
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4 Conclusion
Biomass residues and waste can be converted into different types of bioenergy using gasification
method. The second-generation biofuels produced by biomass gasification from syngas, such as
methanol, ethanol, dimethyl ether, hydrogen, and synthetic natural gas, are critically reviewed.
Gasification technologies are also discussed. The production of biofuels from syngas is a
practical and efficient solution to address both the global energy shortage and GHG emissions.
The syngas cleaning and conditioning process, specifically the feed biomass, gasifier type, and
operating circumstances. Gasification process optimization must be connected to the type of
biofuel production process in terms of catalyst and operating conditions in order to define the
notion of a full synthesis chain in terms of the correct ratio of syngas components and of
contaminant removal. Notably, parameters must be determined and established in accordance
with the intended application, such as the working pressure of gasifiers in order to have syngas at
the right downstream pressure, the required syngas purity, and the specified composition (such as
H2/CO and CO2).
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5, Summary
Currently, only MSW and agricultural residues may be gasified at a large-scale commercial
level. The gate charge, the ability to generate power, and the effectiveness of the gasifier all have
a significant impact on the economics of biowaste gasification.
In a number of environmental and social effect categories, gasification shows net environmental
and social advantages. To lessen air and aquatic ecosystem pollution, it is necessary to address
how to dispose of coke and heavy metals in the solid residue.
The abundance of tar in the resultant gas, the challenge of separating different gaseous
components, and the deactivation of gasification catalysts due to nitrogen- and sulfur-containing
compounds are the key issues that the economic potential of biowaste gasification faces. For the
future of large-scale biowaste conversion, it is essential to resolve these three particular
problems.
6, REFERENCE
[1] Sansaniwal, S.K., Pal, K., Rosen, M.A. and Tyagi, S.K., 2017. Recent advances in the
development of biomass gasification technology: A comprehensive review. Renewable and
sustainable energy reviews, 72, pp.363-384.
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[2] Mazaheri, N., Akbarzadeh, A.H., Madadian, E. and Lefsrud, M., 2019. Systematic review of
research guidelines for numerical simulation of biomass gasification for bioenergy
production. Energy conversion and management, 183, pp.671-688.
[3] Watson, J., Zhang, Y., Si, B., Chen, W.T. and de Souza, R., 2018. Gasification of biowaste:
A critical review and outlooks. Renewable and Sustainable Energy Reviews, 83, pp.1-17.
[3] .Lee, S.Y., Sankaran, R., Chew, K.W., Tan, C.H., Krishnamoorthy, R., Chu, D.T. and Show,
P.L., 2019. Waste to bioenergy: a review on the recent conversion technologies. Bmc
Energy, 1(1), pp.1-22.
[4] Molino, A., Larocca, V., Chianese, S. and Musmarra, D., 2018. Biofuels production by
biomass gasification: A review. Energies, 11(4), p.811.
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