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Objective Energy is one of the most significant inputs for development and economic growth. Jordan faces big internal and regional challenges concerns. One of these challenges is the growing Electricity demand, which accompanied by a shortage of available natural resources. Locally, Jordan is generating very limited Electrical energy that contributes only 2.4% of total energy consumption. Therefore, providing reliable and affordable Electricity in Jordan is considered one of the National Energy Strategy. The off-grid energy generating technologies can provide a more reliable supply and has a great potential to supply power to remote and rural areas. It is more environmentally friendly, cost-efficient, and operates independently without relying on multiple public utilities. The purpose of this research is to study gasification technology as one of a renewable energy source that can provide a more reliable supply and has a great potential to supply power to remote and rural areas. The gasification of the carbonaceous material is a method to produce syngas. Such technology is a process used to converts carbonaceous materials to synthetic gas to use as energy. In the gasification process, the most common materials used are Biomass. This technology has many challenges, such as low energy density, low heating value, higher tar content, and unstable supply. To overcome these disadvantages, Biomass and coal have been employed in a single process called the co-gasification. Although this method improved the process of co-gasification various factors influenced such a process. These factors include flow geometry, where the gasifier is classified for several types: entrained flow gasifier, moving bed gasifier, and fluidized bed gasifier. Other factors are gasification agent, operation conditions (temperature, pressure), heating rate, feedstock composition, fuel blending ratio, and particle size, where it is influenced by the percentage of the gases and ratio between produced (CO, CO2, CH4, H2). Methods Previous works and research. of the gases and ratio between produced (CO, CO2, CH4, H2). Results Compared the production of synthesis gas by co-gasification process. of the gases and ratio between produced (CO, CO2, CH4, H2). Conclusion This paper presented the co-gasification process from the literature. Then, the comparison was made between the co-gasification process and normal gasification to determine the main factors that impact these processes, which will attend to future improving gasification. The gasification agents is one of factors that influence the gasification process, which depends on the gasifier design and operation. The other factor that can affect the co-gasification is temperature.
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Current Alternative Energy, 2020, 4, 1-000 1
2405-4631/20 $65.00+.00 © 2020 Bentham Science Publishers
A State!of!the!art Review on a Thermochemical Conversion of Carbona-
ceous Materials: Production of Synthesis Gas by Co-gasification Process-
Part I
Mohamed R. Gomaa1,2,*, Ghayda’ A. Matarneh1, Mohammad Shalby1 and Hani A. AL-Rawashdeh1
1Mechanical Engineering Department, Faculty of Engineering, Al-Hussein Bin Talal University, Ma’an, Jordan;
2Mechanical Engineering Department, Benha Faculty of Engineering, Benha University, Benha, Egypt
Abstract: Presently, one of the biggest predicaments in developing countries is the ever-growing local
demand for electrical energy in the face of limited availability of locally derived natural resources. The
Middle Eastern country of Jordan provides for an apt example of this. Domestically, Jordan generates a
very limited amount of its own electrical energy output. Contributing 2.4% of its total energy consump-
tion, Jordan has been driven by the need to diversify its reliance on alternative energy sources. One
such alternative is that of renewable energy with its potential to cater to local supply and demand for
electricity. Off-grid energy generating technologies can provide a more reliable supply and extending
its reach into remote and rural areas. These technologies provide the added benefits of being more en-
vironmentally sustainable, cost-efficient, and can operate independently, not reliant on multiple public
utilities. Against this backdrop, this study evaluates the benefits of gasification technology, providing
for a renewable energy source that can meet the needs for a reliable supply whilst simultaneously dis-
tributing power to remote rural areas. It does this by scrutinizing existing investigative works and ex-
perimentations premised on the gasification of carbonaceous material for the purpose of producing
syngas that can then be used as an energy source. In this gasification process, the most common mate-
rial typically used is biomass. However, such technologies and their accompanying processes are not
without their challenges. These include, but are not limited to, low energy density, low heating value,
higher tar content, and an unstable supply. In an attempt to overcome these associated issues, biomass
and coal are often synergized in a singular process referred to as ‘co-gasification’. While the combina-
tion of biomass and coal vastly improved the process of co-gasification, various other factors aid this
process. These include flow geometry, where the gasifier can be categorized into several forms: an en-
trained flow gasifier, a moving bed gasifier, and a fluidized bed gasifier. Further factors included a gas-
ification agent, operating conditions (i.e. temperature, pressure), heating rate, feedstock composition,
fuel blending ratio, and particle size, influenced by the percentage of gases and ratio produced between
CO, CO2, CH4, and H2. This study therefore provides a comparative analysis between a co-gasification
process and normal gasification to determine not only the elements that impact these processes, but al-
so what can be improved for ultimately optimizing gasification.
Received: June 08, 2020
Revised: July 23, 2020
Accepted: August 11, 2020
Keywords: Energy, environment, solar energy, gasification process, synthesis gas, co-gasification.
Contemporary urbanization with accompanied improve-
ment in living standards in developing countries has seen a
surge in energy demands . In response, many states have
begun to move towards alternative and more sustainable en-
ergy sources with a particular focus on renewable energy [1-
3]. In this context, the middle eastern country of Jordan is an
interesting case study. Jordan, like many of its developing
counterparts, has seen an equal surge in demand for energy
*Address correspondence to this author at the Mechanical Engineering
Department, Faculty of Engineering, Al-Hussein Bin Talal University,
Ma’an, Jordan; E-mails:
whilst also suffering from a shortage of readily available
natural resources, coupled with an increase in public debt. At
present, Jordan’s domestic energy resources contribute 2.4%
to the country’s total energy consumption. Yet local efforts
predict this to increase rapidly in the near future following
local government plans and strategy to diversify and develop
renewable energy capacities for the foreseeable future.
These include several support mechanisms that have only
recently been introduced into domestic policy and civic
planning. The Renewable Energy and Energy Efficiency Act
adopted in 2010 in Jordan is one significant step toward this
target. The law provides incentives to encourage the use of
renewable energy in Jordan. Further attempts for incentivi-
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2 Current Alternative Energy, 2020, Vol. 4, No. 1 Gomaa et al.
zation resulted in the establishment of the Jordan Renewa-
ble Energy and Energy Efficiency Fund (JREEEF) [4].
Amongst Jordan’s natural reserves, solar energy is the
most abundant, placing it under the internationally endowed
category titled the ‘Sun Belt’ rubric. Jordan’s annual solar
radiation ranges from 3.8 kWh/m2 in winter to 8.0 kWh/m2
in summer, with its annual direct solar radiation totaling abe-
tween 2400 kWh/m2 to more than 2700 kW h/m2. These
figures leave Jordan well-endowed to fulfill its own domes-
tic, economic and environmental investments [5-8].
Biomass is typically considered an energy source that can
substitute fossil energy in the production of electricity, heat,
and transport fuels. Therefore, Jordan has encouraging bio-
mass energy resources to exploit solid wastes, agricultural
residues, animal manure, and organic industrial wastes [9].
Yet the advantages of biomass have not been realized in Jor-
dan’s industrial sector despite its energy value. Therefore,
biomass conversion technologies require further examination
in order to diversify its usage and expand its reach within the
commercial sector [4, 10].
In principle, synthetic gas can be produced from any hy-
drocarbon raw material by a gasification process [11]. It is a
clean technology that converts carbonaceous materials (e.g.,
natural gas, coal, oil, coke, biomass, and municipal solid
wastes) under a controlled amount of air and high tempera-
ture inside a gasifier to obtain syngas. In other words, this
technology helps convert solid fuels into combustible gas or
syngas vis-vis a partial oxidation process [12].
Recently, the gasification process has been improved by
a technology named ‘Co-gasification’. This technology com-
bines biomass gasification and coal gasification in one pro-
cess. A co-gasification process has several advantages that
include, higher efficiencies of carbon conversion into syngas
and reduction in the costs of feedstock. Biomass also con-
tributes to treating the limitation of coal in resource reserves,
reducing its pollutant emissions, product distribution, and
reactivity. Moreover, the sufficient Alkali and Alkaline Earth
Metals (AAEM) and minerals!in biomass act as a natural
catalyst in the coal gasification process, thereby, reducing
the catalyst cost requirement [13-15]. Solar gasification is a
promising technology for thermochemical conversion, which
can produce clean chemical fuels by using high-temperature
solar heat.
This paper covers the theoretical premise of gasification
technology as one of the energy processing techniques used
to extract valuable gases. It then presents a review of co-
gasification methods that have been applied and their ex-
pected outcomes, in an attempt to derive the best conditions
that can induce optimal performance in co-gasification tech-
nologies and ultimately cater to commercialized chains of
supply and demand within the global energy sector.
Syngas mainly consists of carbon monoxide (CO), hy-
drogen (H2), water vapor (H2O), methane (CH4), nitrogen
(N2), and hydrocarbons with low levels of pollutants, such as
carbon particles, tar, and ash (de Souza-Santos, 2010). The
combustion is a high-temperature exothermic redox chemical
reaction between a fuel and an oxidant that produces oxi-
dized products where the oxidant is usually atmospheric ox-
ygen. Fuel combustion can be described as a complete reac-
tion if all the fuel's carbon is burned into carbon dioxide
(CO2), all hydrogen is transformed into water vapor (H2O),
and all the sulfur, if present, burns into sulfur dioxide (SO2).
This means that all combustible components of the fuel burn
to mark the completion of the process. On the other hand, if
the fuel combustion process is incomplete, the combustion
products contain non-burnt components such as carbon (C),
hydrogen (H2 (g)), carbon monoxide (CO (g)) or HO [16].
Gasification is described as an incomplete combustion pro-
cess because the products are carbon monoxide and hydro-
gen gases (CO (g)) and (H2 (g)). Controlling the air and fuel
ratio is the key to have an incomplete combustion process, or
gasification [17].
When the carbon materials are heated at a low to medium
heating value with some gasifying agents to produce a syn-
thetic gas or industrial gas, the resulting mixture will be con-
sisting mainly of the carbon monoxide, hydrogen, and some
by-products (such as char, ash, tar, and oils) [18]. This defi-
nition excludes complete combustion because the produced
flue gas has no residual heating value due to the complete
combustion of the fuel. In the partial oxidation process, oxi-
dation (also called a gasifying agent) can occur with air
/oxygen, steam, carbon dioxide, or a mixture of two or more
of these gaseous agents [18].
The gasification agent is selected, and the agent ratio to
the carbon raw materials is adjusted to meet the required
chemical composition for syngas [19,20]. At the same time,
the by-products are removed to produce a clean industrial
gas. This gas can be used as a fuel to generate electricity or
steam where the steam system is considered fundamental in
the chemical industry, petrochemical refining, and hydrogen
production industries. Gasification adds value to low- or
negative-value stocks by converting them into fuels and
marketable products [18]. The main reactions involved in the
gasification process are combustion (interaction with O2),
boudouard reaction (reaction with CO2) and steam gasifica-
tion (reaction with steam), which are as follows [21]:
Gasification with oxygen:
C+ 1/2O2 ! CO (H°298 = -110.5KJ/mol) (1)
Combustion with oxygen:
C+ O2 ! CO2 (H°298 = -393KJ/mol) (2)
Gasification with carbon dioxide:
C+ CO2 !2CO (H°298= +172.0KJ/mol) (3)
Gasification with steam
C + H2O ! CO+ H2 (H°298 = +113.4KJ/mol) (4)
It is worth noting here that the quality and quantity of
synthetic gas produced by the gasification process depend on
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A State
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the properties of the raw materials and the type of the gasifi-
er [22].
Numerous variants of carbonaceous materials such as
coal, petcock, heavy petroleum residues/fuel oil, natural gas,
biomass, and municipal solid waste have previously been
used as forms of feedstocks for gasification [20,23].
Amongst them, ~ 49% has been composed of coal, and ~
40% petroleum residues and coal [21]. The remaining 11%
of feedstock usually includes natural gas, and bio-
mass/municipal solid waste[22].
3.1. Coal
Coal is the most overused source of energy in the world,
to the extent that, a total of 40% of power plants worldwide
are fueled by coal [23]. Analysis suggests that this overcon-
sumption of coal as an energy resource is resultant of the
immediate fluctuations in the price of crude oil from renew-
able sources. The abundance of global coal reserves means
that it can recover its role as a significant energy source by
2030 owing to the detrimental impact of oil prices and other
energy sources. The attractiveness of coal lies in its abundant
reserves and fixed prices when compared to both oil and
natural gas [24], thus making coal a material of importance
owing to its vast abundance, wide geographical distribution
and low cost .
Coal gasification is a promising technology to produce
hydrogen gas (H2) and electricity. In the gasification process,
the raw materials interact with steam and oxygen under high
temperature and pressure. The synthetic gas mainly consists
of H2, CO, and some amounts of methane and carbon diox-
ide. This gas then undergoes additional processing to sepa-
rate and purify H2 by trapping CO2 [25].
Coal gasification mainly consists of coal pyrolysis and
char gasification [26]. In pyrolysis, the moistures are evapo-
rated beside the devolatilization of condensed hydrocarbons.
Hence, there are no condensable gases at temperatures rang-
ing from 350 to 800 °C. In the process of gasification, char
reacts with gasification agents to produce syngas. This is a
very complicated heterogeneous gas-solid reaction due to the
change in the pore char structure. In general, the rate of gasi-
fication char is much slower than evaporation and volatiliza-
tion rates. Therefore, the gasification of char will determine
the rate of the process in the coal gasifier [27]. Charcoal gas-
ification properties largely depend on coal type and operat-
ing conditions such as the reaction temperature, the partial
pressure of the gas reactor, total system pressure, and particle
size. The effects of coal type and operating conditions on the
gasification reaction have been widely investigated in recent
years. These studies have found that the most critical param-
eter controlling the gasification rate is temperature, suggest-
ing that the chemical reaction taking place under low tem-
peratures controls the gasification rate. Alternatively, under
high temperatures, the gasification rate is governed by the
diffusion inside the pores [27,28]. All variants of coal can be
gasified. However, from an economic point of view, low-ash
coal provides the most appropriate variant. There also exist
critical criteria for different coal properties depending on the
technology used and implemented [29]. During the coal gasi-
fication process, the coal is mixed with oxidation (O2 or air)
and steam in a reactor operating under a pressure of 24-70
atm and at a temperature of 500-1800 °C to produce synthet-
ic gas containing CO2, H2, H2O, methane, and nitrogen.
Trace amounts of elements are formed naturally on the coal
rank or its geographical location [19,20].
Common gasification agents used in industrial gasifiers
include a mixture of steam, air, or oxygen. Theoretically, the
amount required for complete combustion of oxygen content
is from one-fifth to one-third. According to Collot [30], the
chemical composition and future use of the syngas can be
changed in accordance with the following parameters :
1. Coal composition and rank.
2. Coal preparation (particle size).
3. Gasifying agents (oxygen or air).
4. Gasification conditions: temperature, pressure, heating
rate, and gasification time.
5. Plant composition which includes: coal feed system
(feed as dry powder or clay with water), the contact be-
tween the fuel and gasifying agents (flow geometry);
concerning whether the metal is removed dry ash or
molten ash (slag); the method of production and transfer
of heat, and finally, how synthetic gas is cleaned (sulfur
removal, nitrogen removal, removal of contaminants).
Coal can be ranked as low and high due to its properties,
including the heating value, moisture content, composition
time, impurities, etc. [31]. Commonly, the rank of coal is
determined by the stage reached via coalification.
Low-rank coal accounts for about 45% of the total coal
reserves, potentially making it a preferreable fossil fuel in
many countries, in spite of the high humidity involved in its
mining, that ranges from 30% to 66%. Low-rank coal is pri-
marily used to generate electricity, classified as low-grade
fuel and characterized by the low heat value content, high
water content (25-65%), and low sulfur content. Its ad-
vantages, particularly over black coal, include low mining
costs, high reactivity, high amount of volatiles, and low pol-
lution-forming impurities such as sulfur, nitrogen, and heavy
metals [32, 33].
3.2. Biomass
Biomass is an organic matter derived from plants, mak-
ing it one of the most abundant organic materials on earth
[34] and a primary renewable energy source [35]. It is an
environmentally friendly fuel that reduces greenhouse gas
emissions because of the less sulfur and ash contents and a
high volatile matter. Biomass is composed of plant materials
such as wood, crops, and their residues as well as industrial
and municipal wastes Fig. (1) Fundamentally, biomass is an
energy source that can supply up to 14% of the world’s ener-
gy consumption after coal, petroleum, and natural gas [36].
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4 Current Alternative Energy, 2020, Vol. 4, No. 1 Gomaa et al.
The energy in biomass comes from the chemical energy
that is produced by a photosynthesis reaction in a green plant
exposed to sunlight where this solar energy is stored in its
chemical bonds in the form of chemical energy [35]. This, in
turn, can further create a bond, as a chemical evolved by
breaking down the bonds [37].
Biomass can be converted into a useful product by em-
ploying several technologies that depend on its active char-
acteristic properties, the end product requirements and its
applications [38]. Various forms of liquid and gaseous fuels
can be derived from biomass. These technologies are sum-
marized in Fig. (2).
Table 1 summarizes the main advantages and disad-
vantages of biomass as an energy source [40, 41].
3.3. Petcoke
Petroleum coke (petcoke) is the by-product of refined
crude oil. Over the past two decades, pet-coke production
has increased with the development of crude oil refining.
Petcoke is a product from the Coker unit in refineries having
a high calorific value (15000 Btu/lb), and high carbon con-
Fig. (1). Classification of Biomass materials. (A higher resolution / colour version of this figure is available in the electronic copy of the
Fig. (2). Different types of fuels obtainable from biomass [39]. (A higher resolution / colour version of this figure is available in the elec-
tronic copy of the article).
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A State
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tent. It is cheaper than coal and is available in abundance.
Petcoke can be utilized as an alternative fuel for energy pro-
duction due to its high fixed carbon and low ash content
[42,43]. The main barriers to burning petcoke are sulfur di-
oxide emissions. They contain large amounts of sulfur, and
vanadium (5-7 wt % and up to 500 ppm), respectively, with
a considerable amount of nickel, creating corrosion that is
not environmentally approved, especially for direct combus-
tion in boilers [42,44]. It is also challenging to ignite petcoke
because of the low content of volatile matters [43]. The reac-
tivity of petcoke is still low in the gasification process as
compared to coal and biomass. Therefore, to increase the
reactivity of petcoke and reduce CO2 emissions, blending of
coal/biomass is necessary as proposed in the existing studies
[45, 42].
Typically, gasification is implied for the thermochemical
conversion of solid or liquid fuels into combustible gas or
syngas by partial oxidation and heat [12]. The process in-
volves the breakdown of large, heavy molecules of the hy-
drocarbons into simpler and lighter ones [46]. The process
further involves several steps that are shown in Fig. (3).
The process of gasification generally follows sequential
steps that begin with preheating and drying, followed by
pyrolysis and then char gasification and char oxidation [46].
Although these steps are modeled in series, there are no
sharp boundaries to divide each one. The quality of the gas
produced from gasification varies according to the gasifying
agent used. A lower quality gas is produced in terms of the
heating value (LHV~ 4-7 MJ/nm3) if the gasifying agent is
oxygen coming directly from the air. This process is termed
air-gasification. In contrast, a relatively high-quality gas
(LHV~ 10 to 18 MJ/nm3) without nitrogen is produced when
the gasification medium is pure oxygen or steam. This pro-
cess is referred to as ‘oxygen or steam gasification'. It is suit-
able for use as synthesis gas for conversion into either meth-
anol or gasoline [47].
Drying, pyrolysis, and char gasification are the sequential
phases of biomass gasification. These phases are generally
applied to all carbonaceous materials processing during gasi-
4.1. Drying
For generating gas with a reasonably high heating value,
most gasification systems use dry biomass with a moisture
content of 10-20%. In comparison, fresh wood's typical
moisture content ranges from 30-60%; as the feed is added ,
the gasification creates heat, where water is released at
around 200 °C. The bounded water in biomass is removed at
above 100 °C. As the temperature rises, the low molecular
weight extractives are volatilized [48]. For gasification pro-
cess modeling, it is common to combine drying and pyroly-
sis [46, 49].
Table 1. Biomass: Advantages vs Disadvantages
The insecurity of biomass feedstock supply, indefinite availability of sus-
tainable biomass.
Great growing, harvesting, collection, transportation, storage and pre-
treatment costs, use of extra water, fertilizers and pesticides.
Technological problems during processing (agglomeration, deposit for-
mation, slagging, fouling, corrosion, erosion)
Miss of accepted terminology, methodologies, standards and classification
and certification systems.
Lack of accepted terminology, methodologies, standards and classification
and certification systems Regional and seasonal availability and local ener-
gy supply.
The insecurity of biomass feedstock supply, Indefinite availability of sus-
tainable biomass resources for the production of biofuels and chemicals
High contents of moisture, water-soluble fraction, Cl, K, Na, O and some
trace elements (Ag, Br, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Se, Tl, Zn, others)
The low energy density (bulk density and calorific value)
Lack of developed biomass markets, high investment cost
Insufficient knowledge and variability of composition, properties and
quality for assessment and validation.
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6 Current Alternative Energy, 2020, Vol. 4, No. 1 Gomaa et al.
4.2. Pyrolysis
The first phase of thermal degradation of the feedstock is
pyrolysis. The dried fuel is heated in the absence of oxygen
under 200 500°C, and turns into solid char, volatiles (con-
densable hydrocarbon or tar), and gases. The yield gas, liq-
uid, and solid depend on the final pyrolysis temperature and
the feedstock's heating rate. Those products react with the
gasifying agent (steam, air, or O2) to produce CO, CO2, H2,
and lighter hydrocarbon. Heat is transferred to the feedstock
during pyrolysis, first to the particle surface by radiation and
convection, and then into the pyrolysing particles through
conduction, convection, and radiation [46]. The pyrolysis
process during gasification and combustion is generally simi-
lar. However, as many gasification processes operate at a
higher pressure than that of combustion, the pressure effect
during gasification has to be considered in this instance [50].
4.3. Char gasification
This step is the most significant, as it is the final element
in thermal conversion. The char may not form the bulk of the
fuel. However, its conversion (kinetics) controls the gasifi-
er’s performance. This is because the gasification of char is
the slowest of all three gasification steps. It takes longer than
that for pyrolysis or drying. Char gasification is the rate-
limiting step determining the residence time in the reactor,
besides the reactor size and the gasification efficiency [51].
Therefore, kinetic data related to char gasification is neces-
sary for the proper design and operation of the gasifier. It is
essential to highlight here that the gasifier often turns out to
be a char gasification model [52].
Several gasification technologies have been developed to
convert the carbonaceous material by thermochemical con-
version to meet the demand for power and heat production
from syngas. The main differences determining the classifi-
cation of the gasifiers are:
1. The way feedstock is fed into the gasifier and the way it
then moves.
2. The oxidant or the gasifying agent that is used, i.e.
whether this is oxygen, air or steam
3. The temperature range in which the gasifier is operated
4. The way heat is provided to the gasifier whether it is
provided by partially combusting some of the feedstock
in the gasifier (directly heated), or from an external
source (indirectly heated)
5. The operating pressure that the gasifier undergoes,
whether it is above the atmospheric pressure "pressur-
ised gasification" or not.
The efficiency of the gasifier depends on the feedstock
type, its size, moisture content, air flow rate (or other gasify-
ing agents), and temperature in all zones inside the gasifier
Gasification technologies are categorized into three cate-
gories of gasification and in accordance to their flow geome-
try [54]:
1. Entrained flow gasifier, which is the most commonly
used technology for coal gasification, with pulverised
coal particles where gases flow concurrently at a high
2. Fluidized bed gasifiers; in this technology, the coal par-
ticles are pendent in the gas flow; coal particles are
mixed with the particles that undergo gasification.
3. Moving bed (also called fixed bed) gasifiers; in this
technology, the gases flow upward at a slow speed
through the bed of coal. Both concurrent and counter
concurrent technologies are available, but the latter is
more commonly used.
5.1. Entrained Flow Gasifiers
In the entrained flow gasifier process shown in Fig. 4,
feedstock particles and the gasification agent are at the same
stream inside the gasifier where the solid particles or liquid
droplets of feedstock have been entrained or “trapped” inside
the gas stream. This forms what is referred to as ‘entrain-
Fig. (3). Steps involved in a coal and biomass gasification process [46]. (A higher resolution / colour version of this figure is available in the
electronic copy of the article).
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Fig. (4). Entrained flow gasifier [55]. (A higher resolution / colour
version of this figure is available in the electronic copy of the arti-
This process has numerous advantages, including:
1. Temperature distribution and a steadier reaction rate. It
also forms a slag, which creates a protective coating along
the side of the gasifier that protects the walls from more cor-
rosive substances that may form during gasification.
2. It produces no tar inside the gasifier because of its ca-
pability of operating at a very high temperature (> 2000).
Even with all the advantages that the entrained flow gasi-
fier has, it has a few disadvantages that must take into con-
sideration while using this technology;
1. The average particle size of the feedstock used in the
entrained flow gasifier is extremely small; it may reach
tenths to hundreds of millimeters in diameter [56]. Apparent-
ly, it is not a problem for liquid feedstocks. However, due to
this drawback of the entrained flow gasifier features, solid
feedstocks like coal and biomass must be pretreated before
they can be used in the gasifier.
2. The gas obtained has a low heating value of (~5
MJ/Nm3), besides its dilution in nitrogen [57]. The high tem-
perature and pressure in the entrained flow gasifier during
syngas production with air (presence of nitrogen) produces
large amounts of NOX compounds, making the resulting
syngas mixture unusable for power applications. Conse-
quently, the majority of entrained flow gasifiers typically
require the use of oxygen, instead of air, as the gasifying
agent. Here, the use of oxygen in the process increases the
calorific value of the gas. As there is need for an air separa-
tion unit, making the process more expensive, this option is
realistically feasible only in the context of large-scale facili-
3. The syngas that leaves the gasifier has an extremely
high temperature compared to other gasifier types, and there
is a resulting energy loss during the cooling stage before it
enters the gas cleanup system [57,58].
This type of gasifier is common in large power plants (>
200 MW), providing higher syngas mass flow rates than any
other gasifier type [58].
To ensure efficient operation of the gasification process
in the entrained flow gasifier, particular factors need to be
taken in to consideration. Specifically, these are:
1. The time where the fuel must be converted entirely into
gas and by-products; this period takes between 0.5 and 4
seconds in these types of gasifiers [59].
2. The temperature values, as mentioned earlier, have a
significant effect on the gasification process. This is de-
pendent on the gasification agent chosen, gas preheat-
ing, and air-to-fuel ratio [59,60].
5.2. Fixed Bed Gasifiers
Fixed bed gasifiers support the solid fuel and maintain a
stable reaction zone. These types satisfy the small or medium
power applications (below 1 MW); these gasifiers are also
relatively easy to operate. However, there is some difficulty
in maintaining uniform temperatures and appropriate mix-
tures in the reaction area. Therefore, the obtained gas fuel
and income are variable. As the fuel makes its way to gasifi-
cation, four different processes occur within the gasifier,
these include; the drying zone, pyrolysis zone, combustion
zone, and reduction zone. The drying process strongly de-
pends on the amount and thermodynamic state of water in
the fuel (moisture), as well as water vapor [61]. In the com-
bustion zone, the combustible fuel usually contains carbon,
hydrogen, and oxygen wherein the complete combustion
process, CO2 and H2O are obtained. This oxidation reaction
is exothermic that yields a theoretical temperature of
1450°C. The main combustion reaction of the fuel is as fol-
lows [62]:
C + O2 ! CO2 (+ 393 MJ/Kg mole) (5)
2H2 + O2 ! 2H2O (- 242MJ/Kg mole) (6)
In the reduction zone, when the partially cracked pyroly-
sis and uncombusted products pass through a hot charcoal
bed, the following reaction takes place [63]:
C+ CO2 !2CO2 (-164.9 MJ/kg mole) (7)
C+ H2O !CO+H2 (-122.6 MJ/kg mole) (8)
CO+ H2O !CO+ H2 (-42 MJ/kg mole) (9)
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8 Current Alternative Energy, 2020, Vol. 4, No. 1 Gomaa et al.
C+ 2H2 !CH4 (+ 75 MJ/kg mole) (10)
CO2+ H2 !CO + H2O (-42.3 MJ/kg mole) (11)
The main reduction reactions are the endothermic reac-
tions in Eqs. 7 and 8 that reduce the gas temperature where
the temperature in the reduction zone is generally between
800-1000 °C. This temperature affects the heating value of
the product gas, where the lower the reduction zone tempera-
ture (~700-800 °C), the lower the calorific value of the gas
Pyrolysis is a complicated process where the products
depend on numerous factors such as temperature, pressure,
residence time, and heat losses. In general, during pyrolysis
of up to 200 °C, only water is extracted. Carbon dioxide,
acetic acid, and water release between 200-2800 °C, and
between 280-5000 °C, real pyrolysis occurs, producing large
quantities of tar and gases that contain carbon dioxide. Aside
from light tars, some methyl alcohol is also formed; at 500 -
7000 °C, gas production is marginal and contains hydrogen.
This is why much more tar is produced in the updraft gasifier
than the downdraft. In the latter instance, tar is partially bro-
ken down while passing through combustion and reduction
zones [63].
The main types of moving bed gasifiers include counter-
current (updraft) and co-current (downdraft), and cross draft
gasifier [64].
5.2.1. Updraft Gasifier
The oldest and the simplest type of gasifier is the counter
current or updraft gasifier [65]. This type of gasifier is shown
in Fig. (5). When the carbonaceous materials are fed into
gasification, sub-processes occur owing to the hot air intro-
duced into the gasification. Here, four sub-processes inter-
fere with the grate; these are; dehydration, pyrolysis, oxida-
tion (combustion), and reduction (gasification of char).
The gases emit upwards from the solids and escape from
the gasifier [66]. In the updraft gasifier, the flowing hot
product gas dries the downward moving feedstock. Solid fuel
such as pyrolysied biomass, is heated in the absence of oxy-
gen to thermally decompose the chemical composition com-
pounds of those materials (i.e., cellulose, hemicelluloses, and
lignin) [64].
After the decomposition of the solid material, char moves
down to be gasified. The pyrolysis vapors are carried upward
by the up flowing hot product gas. These vapors contain tar
(up to 20% of the pyrolysis products), which condenses on
the cold descending fuel to be recycled and sent back to the
reaction zone where they further decompose into gas and
char. In the bottom of the gasifier, the incoming air and oxy-
gen partially oxidize the solid char and tar cracking. The
second part of tar may be carried upward with the product
gas out of the gasifier contributing to its high tar content.
The steam is added to provide a higher proportion of hydro-
gen in the product. Since the heat of the product gas is used
for preheating, drying, and pyrolysis of the incoming fuel,
the product gas temperature is relatively low (300-600 °C)
Fig. (5). Updraft gasifier [55]. (A higher resolution / colour version
of this figure is available in the electronic copy of the article).
The product gas from an updraft gasifier has a high heat-
ing value referring to the significant proportion of tars and
hydrocarbons it contains. For processing, the fuel gas re-
quires a substantial clean up before it can be used.
5.2.1. Downdraft (Co-current) Gasifier
A downdraft gasifier is intended to overcome the prob-
lem of tar content in the product gas in the updraft gasifier.
In downdraft gasifiers, the product gas is extracted from the
gasifier's bottom near the hottest zone and the gasification.
The agent (air) is fed at or above the gasifier's oxidation
zone, as shown in Fig. (6). Hence, the fuel and the gasifying
agent moves in the same direction downward toward the
acid. Tar products from the fuel must pass through a hotbed
of charcoal to be converted into gases such as hydrogen,
carbon dioxide, carbon monoxide, and methane. In this way,
the tar concentration will decrease much lower than in the
updraft gasifier. This type of gasifier is used typically suited
to small scale applications where the maximum size of these
units is limited to a few MW fuel power [64,68].
Table 2 summarizes the advantages and disadvantages of
a downdraft gasifier. The main design parameters for gasifier
include a high heating value of gas, moisture in feedstock
ranging between 10-30 %, low tar content, low-pressure
drop, and smooth flow of the feed [69]. The physical limita-
tions of a downdraft gasifier in the diameter and particle size
of the fuel are: the fuel should have low ash content (< 1
wt%) and moisture (up to 30 wt% moisture) besides a low
proportion of fine and coarse particles (not smaller than 1 cm
and not larger than 30 cm in the longest dimension) which
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leads to low tar content in the gas. This configuration is fa-
vorable to small scale power generation with an internal
combustion engine with an upper capacity of 500 kW [65].
5.2.2. Cross Draft Gasifier
A cross draft gasifier, shown in Fig. (7), differs from both
updraft and downdraft gasifiers, possessing certain ad-
vantages over both, but none of which are an ideal alterna-
tive owing to its design. The design, where the fire and the
reduction zone are separate, the producer gas exists in high
temperature and velocity with inadequate CO2 reduction.
These design characteristics severely limit the type of feed-
stock that can be used. These are either low ash and moisture
fuels such as wood, charcoal, and coke. The cross-draft gasi-
fier's concentrated zone operates at a high temperature value
of 1200 °C, which improves the load following ability.
Startup time in the cross-draft gasifier is faster than both
updraft and downdraft by up to 5-10 minutes. The relatively
higher temperature in a cross-draft gas producer has a no-
ticeable effect on the exit gas composition such as high car-
bon monoxide and low hydrogen and methane content when
dry fuel such as charcoal is used. Cross draft gasifiers oper-
ate well on dry air blast and dry fuel [70]. Table 2 below
summarizes the advantage and disadvantages of each class of
fixed bed gasifiers.
5.3. Fluidized Bed Gasifiers
Fluidized bed gasification is considered one of the oldest
technologies used to produce energy from coal. This is not
the same for biomass, where its application faces many oper-
ational problems, including the accumulation of the bed ma-
terials during the process [71].
Fig. (6). Downdraft gasifier [55]. (A higher resolution / colour version of this figure is available in the electronic copy of the article).
Table 2. Advantages and disadvantages of fixed bed gasifiers.
Gasifier type
- Small pressure drops
- Good thermal efficiency
- little tendency toward slag formation
- Sensitivity to tar and moisture in the fuel
- Long start-up time of IC engines
- Poor reaction capability with heavy gas load
- Flexible adaption of gas production to load
- Low sensitivity to charcoal dust and tar in fuel
- Tall design
- Not feasible for the very small particle size of the fuel
Cross draft
- Short design height
- Very fast response time to load
- Flexible gas production
- Very high sensitivity to slag formation
- High-pressure drop
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10 Current Alternative Energy, 2020, Vol. 4, No. 1 Gomaa et al.
Fig. (7). Cross draft gasifier [55]. (A higher resolution / colour
version of this figure is available in the electronic copy of the arti-
Fluidized bed gasification has been used extensively for
coal gasification for many years. In this technology, the uni-
form temperature distribution during gasification provides
this technology an advantage over fixed bed gasifiers [72]. A
bed of fine-grained material is fluidized and well mixed with
the hot combustion gas and carbon feedstock to achieve a
consistent temperature in the fluidized bed gasifier. This
mixture is then introduced to the uniform air distribution of
temperature in the gasifier [73]. Fluidized bed gasifiers have
certain advantages over fixed bed gasifiers. These are; the
good mixing of bed materials and the gas-solid contact im-
proves the rate of reaction and conversion efficiency [74-76].
Also, the implemented bed materials have great importance
in fluidized bed gasifiers. They act as a heat transfer medium
that stabilizes the process as well as have a significant tar
cracking role and preventing solid agglomeration tendency.
There is no complexity in the downstream tar removal pro-
cess [77]. Furthermore, the presence of a catalyst in the bed
material during fuel gasification promotes several chemical
reactions that improve the composition and the heating value
of the producer gas [78]. A fluidized bed gasifier can be used
here, e.g. air, steam, O2-steam, airsteam, O2-enriched air,
and oxygenairsteam [79].
Amongst all gasification technologies, the fluidized bed
has often been the preferred option. It is used in conjunction
with fuel mixtures, where the hot, inert bed material (usually
silica sand and dolomite), constitutes 90-98% of the total
solid mix (fuel + bed material). In comparison, the fuel (2-
10%) [80] provides high thermal inertia and stabilizes the
process. This feature guarantees a good mixing of the fuel so
that a fluidized bed can accept various fuel mixtures while
taking reasonable control of particle size in the mix. Another
main advantage is that the relatively low operating tempera-
ture (around 800 °C or below) of the biomass fluidized bed
gasifier prevents ash sintering that can cause de-fluidization
in the bed [81].
Generally, two main types of FB gasifiers are used: cir-
culating fluidized bed and bubbling fluidized bed.
5.3.1. Bubbling Fluidized Bed Gasifiers
These are considered as the simplest and the most cost-
effective concept of continuous fuel gasification. There are
many advantages such as these gasifiers consist of a vessel
with a grate at the bottom where the gasifying agent is intro-
duced. The schematic representation of this particular type is
shown in Fig. (8). The prepared fuel is fed into a fluidized
bed of fine-grained materials above the grate. The gasifier
temperature is regulated between 700900º C, and can be
maintained by controlling the air/fuel ratio. Once the fuel is
introduced to the gasifier, it undergoes pyrolysis in the hot-
bed to form a char with gaseous compounds where the high
molecular weight compounds are cracked by contact with the
hotbed material, giving a product gas with a low tar content,
typically <13 g/Nm3 [16, 64].
Fig. (8). Schematic of bubbling fluidized bed gasifier [55]. (A high-
er resolution / colour version of this figure is available in the elec-
tronic copy of the article).
Bubbling fluidized bed gasifiers have a high degree of
mixing fuel, which improves the gasification process. How-
ever, the intimate mixing of fully and partially gasified fuels
results in a low solid conversion where the solid stream con-
tains partly gasified particles that reduce the solid conver-
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sions [46, 82, 83]. Another problem associated with BFB is
the reduction in gasification efficiency caused by the slow
oxygen diffusion rate, which creates an oxidizing condition
in the whole bed [46]. Bubbling fluidized bed gasifiers can
process a wide range of fuels, with the main concern of the
quantity of fuel being used. With surrounding conditions
being common variables that affect the moisture and con-
densing vapors on biomass, the moisture content varies. The
BFB can handle such varying fuels due to the presence of
inert bed material, which makes bubbles and mixes turbu-
lently under the buoyancy force of the fluidizing agent
[56,84]. Under such poor bed conditions, fuel particles can
react fully to release volatiles as a result of high solids con-
tact rate [84].
5.3.2. Circulating Fluidized-bed Gasifiers
Circulating fluidized bed gasifiers (CFB) are primarily
suited to bark and other forestry residue gasification in paper
industries because of their ability to withstand high capacity
throughputs [64]. In CFB, gasifier bed material is circulated
through a circulating loop, between the reaction vessel and a
cyclone separator, in which ash is removed [46]. The bed
material and char are then returned to the reaction vessel,
refer to Fig. (9).
Circulating fluidized bed gasifiers are a proven and relia-
ble technology that can handle a wide variety of feedstocks.
They are the preferred system for large-scale applications
and are used by most industries where it is relatively easy to
scale up from a 10 MW up to 100 MW. Even for capacities
above 100 MW, CFB gasifiers provide reliable operation in
industries. These systems, therefore, have a high market val-
ue and are technically well proven [85].
Table 3 below summarized the advantages and the disad-
vantages of the fluidized bed gasifier types.
Co-gasification has several advantages. Mitigating the
undesired effects of carbon-intensive utilization of coal and
the low efficient and troublesome operation of bio-
mass/waste-fed gasification systems [86] are amongst these
added advantages. On the other hand, coal improves biomass
deficiencies and endorses their gasification industrialization.
Biomass also contributes to treating coal limitation in re-
source reserves, reducing its harmful emissions, improving
product distribution, and reactivity.
Throughout the co-gasification of biomass and coal, bi-
omass's volatile matter immediately decomposes to form free
radicals, which increase the conversion rate by reacting with
the organic matter in coal. At the same time, it will reduce
the CO2, SO2, and NOx emissions and other air pollutants
that are responsible for air poisoning and acidic rain [87-89].
Gasification of coal and biomass is an effective technology
Table 3. Advantages and disadvantages of fluidized bed gasification technologies.
Gasifier Type
Bubbling fluid-
ized bed gasifi-
Produces a uniform syngas output;
The temperature throughout the reactor is uniform;
Ability to accept a wide range of fuel particles sizes;
High rates of heat transfer between the bed material, fuel and gas;
High conversion is possible with low tar and unconverted carbon.
Poor solid conversion
Slow oxygen diffusion
Gas bypass throughout the bed caused by large bubble size
fluidized bed
Suitable for rapid reactions;
High heat transport rates possible due to the high heat capacity of bed
High conversion rates possible with low tar and unconverted carbon
Temperature gradients occur in the direction of the solid
The size of fuel particles determine minimum transport
velocity; high velocities may result in equipment erosion;
Heat transfer less efficient than bubbling fluidized bed
Fig. (9). Schematic of circulating fluidized bed gasifier [55]. (A
higher resolution / colour version of this figure is available in the
electronic copy of the article).
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12 Current Alternative Energy, 2020, Vol. 4, No. 1 Gomaa et al.
possessing many environmental advantages. Moreover, this
technology is considered an economically profitable process
that manages large amounts of industrial wastes such as for-
estry, agriculture, and food processing wastes by using them
in power production. Co-gasification further reduces the cost
of establishing a new system of a biomass power plant, to
prepare an existing coal power plant to co-fire biomass with
coal [90, 91]. In other words, co-gasification of biomass and
coal provides low-cost energy, low-risk of emissions and
pollutants, low-cost operation, and implementation, therefore
making it a highly efficient and inexpensive procedure for
using biomass and for overall development of sustainable
energy [92, 93].
Co-gasification of biomass and coal is affected by several
parameters: (1) the composition of biomass and coal; (2)
blending ratio of coal and biomass in the feed; (3) final tem-
perature in the TGA/reactor and the heating rate; (4) the gas-
ification agent (air, air/steam mixture, CO2, etc.,); and (5)
particle size and packing density of biomass and coal parti-
cles [94], where kinetic analysis and product distribution
evaluations are essential to the optimization of the gasifier’s
design and operation [89].
There many significant technical challenges that are re-
lated to biomass co-gasification that include [93]: 1. fuel
preparation, storage, and delivery; 2. ash deposition, 3. fuel
conversion, 4. pollutant formation, 5. corrosion, 6. fly ash
utilization, and 7. formation of striated flow.
Tables 4, 5, and 6 below review numerous investigative
studies in the co-gasification of fluidized bed gasifiers, fixed
bed gasifiers, and entrained flow gasifiers.
Table 4. Summary of selected studies of the co-gasification in fluidized bed gasifiers.
Operation Conditions/Main Fea-
Aigner et
al. [95]
In this experiment, the co-
gasification of coal and
Biomass was performed in
various ratios. The result-
ing changes of the produc-
er gas composition were
conducted in the 100kW
dual fluidized bed gasifier
that consists of two reac-
- The gasification zone was placed
in a bubbling fluidized bed reactor
- The combustion zone was placed
in a transporting fluidized bed reac-
- The gasifying agent was steam
(1) There is a linear relationship between producer gas composition
changes with linear changing fuel ratios (coal/wood).
(2) The concentration of H2 increases with increasing wood ratio, while
the concentrations of CO decrease.
(3) Decreasing the coal ratio in the feedstock decreases the levels of
the impurities NH3 and H2S since the sulfur and nitrogen content in
wood is low and high in coal.
(4) No further cleaning of the flue gas is needed since most of the sulfur
is released in the gasification zone.
(5) The linear relationship between producer gas components during
co-gasification leads to the opportunity of being able to choose the
producer gas composition according to the desired gas utilization.
(6) No synergetic effects of co-gasification between coal and Biomass
were found.
(7) Tar content decreases with increasing wood ratios, but it is generally
low due to the presence of the catalytically active bed material olivine.
(8) The carbon conversion increases with an increasing wood ratio.
Jeong et
al. [14]
The co-gasification of
Shinhwa coal and Pine
sawdust in a Fluidized bed
gasifier with 40% CO2 and
60% N2 as a gasification
-Three mass ratios of coal and Bio-
mass, 4:1, 1:1, and 1:4 (25%, 75%,
and 100% of Biomass) were tested.
- Each resulted char was co-gasified
with CO2 also tested under three
isothermal conditions of 900, 1000,
and 1100 °C. The ratio of fuel/CO2
was 0.20, 0.21, 0.21, and 0.23.
(1) The reactivity of char was improved with an increasing amount of
Biomass due to the catalytic effect of the alkaline minerals content in
the Biomass.
(2) Synergy with Biomass blended char was increased with the amount
of Biomass used.
(3) The random pore model (RPM) could be used to take the carbon
conversion data.
(4) For each coalbiomass ratio in the mixture, the activation energy
and pre-exponential factor were determined using the Arrhenius equa-
!=!"#$ !!" (12)
Where k is the rate constant based on the reaction temperature T. A, R,
and E are the pre-exponential factor (1/s), universal gas constant (8.314
J/mol K), and activation energy (kJ/mol), respectively.
(Table 4) Contd
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Operation Conditions/Main Fea-
Aznar et
al. [96]
In the experiment, the
treatment was performed
to the plastic waste by the
co-gasification of pine
saw-dust, plastic, and coal
in a fluidized bed gasifier.
-The dolomite was a catalyst and air
was a gasifying agent with equiva-
lence ratio ranging between 0.3-
0.46 (indicates the amount of air
which is introduced),
- The feed blend tested was 60%
coal, 20% pine, and 20% plastic
under the temperature of 750- 880
(1) The optimal condition of the gas yield that composed of (H2, CO,
CO2, CH4, light hydrocarbons) was under the temperature of 850 °C and
0.36 equivalent ratios.
(2) An increase of equivalent ratio causes a decrease of all contents in
flue gas composition (H2, CO, CO2, CH4, C2Hn) that refers to the in-
crease of nitrogen content when a higher amount of air is introduced.
(3) Resulted in gas contained medium hydrogen content (up to 15% dry
basis) and low tar content
Tursun et
al. [88]
The co-gasification of pine
sawdust and bituminous
coal in lab-scale external
circulating radial-flow
moving bed (ECRMB)
consists of three decoupled
reactors a gas-solid coun-
tercurrent moving bed
pyrolyzer, a radial-flow
moving bed gasifier and a
riser-type combustor with
olivine as a catalyst.
-Steam was used as a gasifying
agent and a circulating hear carrier.
-The tested biomass ratios were
(0%, 25%, 50%, 75% and 100%) of
Biomass under the temperature of
600 °C and 800 °C and 1.3 steam to
carbon mass ratio S/C.
(1) By increasing in biomass ratio, the gas and tar yields increased.
(2) The synergetic effect based on gas composition was found during
(3) At the S/C range of (0 to 1.3), the gas yield and H2 content in prod-
uct gas increased, but CO2 decreased with the increase of S/C.
(4) optimal gasification conditions were at the gasifier temperature of
850 °C, BR of 50% and S/C of 1.3.
(5) Higher gasifier temperature promoted the gas yield, (H2+CO) in the
product gas, carbon conversion, and chemical efficiency of the process.
(6) Pyrolysis temperature at the range of 500 to 700 °C had no remark-
able influence on the product gas composition.
Pan et al.
Fluidized bed gasifier was
utilized to co-gasification
of Pine chips and poor
-In this experiment, air-steam was a
gasifying agent
-Three mass ratios of Biomass were
tested (0%, 25%,40%, and 100%)
under the gasification temperature
of 840910°C.
(1) Firstly, H2 increased up to 25% of Biomass and then decreased.
(2) Overall thermal efficiency was increased (40% to 68%).
(3) Carbon conversion efficiency was increased (63% to 83.4%).
(4) About 50% co-gasification process, the overall thermal efficiency
can be achieved for the two types of the blend.
et al.
This experiment allowed
using various types of
Biomass in the same co-
gasification equipment in
biomass-coal co-
gasification to provide
higher operational flexibil-
ity and to minimize the
effect of seasonal biomass
supply variation; besides
the impact of the main
gasifier operating condi-
tion, including the relative
biomass ratio and the
reaction temperature to
determine the conditions
that allow higher gasifica-
tion efficiency, carbon
conversion and/or fuel
constituents (CO, H2, and
CH4) concentration and
-The experiments were carried in a
circulating flow gasifier with air as
a gasification agent,
- Various types of Biomass have
been used including, forestry and
agricultural wastes as well as indus-
try wastes (0%, 10%, 30%, 50%,
70%, 90%),
- the temperature was elevated from
ambient to 950 °C at a constant
heating rate of 5 °C/min.
(1) Different biomass fuels can be used without significant modification
in the system installation.
(2) The relative biomass/air ratio is the main parameter affecting the gas
yield and the gasification efficiency
(3) Agricultural wastes led to higher gasification efficiency and higher
gas yield, while sawdust is favorable in producing H2-rich gas due to its
highest reactivity.
(4) H2 concentration increased with increasing temperature.
(5) The catalyst effect of the K2O content of the biomass ash improved
the producer gas energy content.
(6) Synergic effects between Biomass and coal have been observed.
(Table 4) Contd
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14 Current Alternative Energy, 2020, Vol. 4, No. 1 Gomaa et al.
Operation Conditions/Main
Pinto et al.
In this process, the co-
gasification of high ash coal
(Puertolla-no coal) with
different biomass wastes
(pine based wastes, petcoke
wastes, and polyethylene
(PE)) was performed to
study the effect of several
types of catalysts (minerals
either natural or calcined,
like dolomite and olivine,
two commercial catalysts,
G-72D and C49 TRX, and
two catalysts synthesized in
the laboratory, NiMg [100]
and Nidolomite [101,102].
-The co-gasification was carried in
a bubbling fluidized bed gasifier,
- operating at atmospheric pressure,
with oxygen and steam as a gasify-
ing agent.
- The operating conditions of the
process were as follows : reaction
temperature (850 °C 900 °C),
steam flow rate 5.0 g/min, O2 flow
rate 2.9 g/min, feedstock flow rate
6.0 g daf (dry and ash free)/min,
with varying coal content (55%
and 100%) , feedstock mixture
particle size (12502000 µm); the
bed material was silica sand and
the amount of the catalyst used was
about 25% (w/w).
(1) Co-gasification (smaller amounts of wastes in existing coal gasification
installation) allowed the utilization of these wastes with-out significant
alterations and led to the improvement of the gasification process.
(2) The presence of wastes improves the gasification of low-grade coals, as
these wastes could counteract against some harmful properties of low-grade
coals, such as low volatile matter and high ash and Sulphur contents.
(3) Even at high temperatures, mainly (900 C), co-gasification of coal and
PE, produced high enough content of tar and hydrocarbons in the gas yield
during the process, which may require an efficient gasification gas treatment.
(4) The most active catalysts in tars removal were nickel-based ones. These
catalysts led to the highest increase in hydrogen release, the lowest hydro-
carbons contents, the highest gas yields and reducing NH3 contents. (5)
Dolomite and olivine natural catalysts were less effective in tars removal.
(6) The use of G-72D and of C49 TRX did not produce any significant
improvement in tars reductions where their effects on tars reduction and on
gas yields were in opposite directions.
(7) Regarding NH3 destruction, the effectiveness of different catalysts
during both gasification of coal alone and mixed with wastes was as fol-
lows: NiMg > Nidolomite > C49 TRX > G-72-D > olivine > dolomite.
Saw and
Pang [103]
The co-gasification of
lignite and radiata pine
sawdust performed in a 100-
kW dual fluidized bed
gasifier consist of a bubbling
fluidized bed reactor (BFB)
and fast fluidized bed com-
bustion reactor (FFB).
-The gasification agent was a steam
- The steam to fuel ratio (S/F) of
(0.9 to 1.0 kg/kg dry).
- Both sawdust and lignite were
grounded to smaller particles
around 450 µm and then they were
mixed and pelletised in the dimen-
sions of 7 mm (diameter) by 20
mm (length) in order to be tested at
lignite mass ratio of (0%, 40%,
70%, 80% and 100%),
-under a controlled temperature of
800 °C at the (BFB) and a tempera-
ture of 850 °C at the (FFB), which
has been maintained by the addi-
tion of supplementary liquefied
petroleum gas (LPG), the used bed
material was silica sand
(1) The producer gas yield and producer gas compositions were non-
linearly correlated to the lignite to wood ratio, which shows synergy with
the pelletising blending.
(2) Due to the presence of catalytic metals such as (Ca and Fe) and the
cracks in the blended chars, the synergetic effect was observed on the tar
concentration and the tar yield, where tar concentration in the gas yield
decreased when increasing the L/W ratio.
(3) The optimum H2/CO ratio can be achieved by blending 40% lignite and
60% wood as feedstock; therefore, it is not necessary to use expensive
catalytic bed material or increase of gasification temperature over 800 °C to
achieve this target.
(4) Premixing the fuels (coal and Biomass) and then pelletize it into pellets
ensures uniform mixing, to minimize any segregation of the two fuels to
achieve significant synergetic effect.
(5) The producer gas yield and the cold gas efficiency increased with
increasing lignite/wood mass ratio, due to the high carbon content in the
lignite and higher char conversion.
(6) With the increasing of L/W ratio from 0% to 100%, both CO and CH4
yields had been decreased, while the yields of H2 and CO2 increased.
André et
al. [104]
In this experiment, the co-
gasification of poor-quality
and high-ash content lignite
and olive oil industry wastes
(bagasse) was tested.
-In a fluidized bed gasifier with a
mixture of steam, air was the
gasification agent,
- The tested bagasse mass ratio
varied between (0% to 70%)
-Under a gasification temperature
of (730 °C to 900 °C),
-The silica sand is a bed material
with a particle size of 350 µm,
mixed with 25% dolomite.
- Steam flow rate (4.9 to 7.8 g/min),
- Oxygen/fuel ratio (0 to 0.6 g/g daf),
- Feedstock flow rate (5.2 to 7.5
- Feedstock particle size (1250 to
2000 µm).
(1) Increasing the temperature from (770 to 890 °C) led to an increase in
gas yield, H2 content, H2/CO and H2/CO2 ratios, while a decrease of CO2
concentration and the HHV value, due to the decrease in both CH4 and
heavy hydrocarbons concentrations.
(2) The increase in bagasse content up to 70% led to an increase in gas
yield, methane, heavy hydrocarbons, and tar while decreasing H2 content.
(3) The increase of O2/fuel ratio led to an increase in gas yield while de-
creasing the heavy hydrocarbons and the HHV of the gas due to the diluting
effect of nitrogen, which may be overtaken by substituting air with pure
oxygen. However, this would lead to higher operational costs.
(4) The presence of dolomite in the fluidized bed had the benefit of decreas-
ing tars content and rising gas yield, with the gas richer in hydrogen content.
(5) Bagasse content incorporation should not exceed 40% (w/w) to guaran-
tee gasification stabilization and to prevent the formation of high amounts
of tars and heavier hydrocarbons. Besides the high contents of silica, calci-
um and potassium in bagasse may demand purging the bed more frequently
to prevent bed sintering.
(Table 4) Contd
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Operation Conditions/Main
et al.
The co-gasification of
Biomass (spring
switchgrass and fall
switchgrass) and low ash
coal, to evaluate the cata-
lytic/synergistic effects of
the blended fuel in a bub-
bling fluidized bed gasifier
In this test, superheated steam
under 525 °C was the gasification
agent. Silica sand as the bed
material. The tested samples were
50-50 wt% coal/switchgrass
mixtures under the gasification
temperature of (800 °C and 860
°C) and a pressure of 1 atm.
(1) With increasing the steam/fuel ratio, both H2 and CO concentrations
increased, while decreasing the concentration of CO2 and CH4 due to
more methane steam reforming and more water gas shift reaction.
(2) With increasing the flow rate of steam, both carbon and cold gas effi-
ciencies increased due to the enhanced water-gas shift, steam-carbon and
steam-methane reforming reactions.
(3) Hydrogen efficiency and HHV value of the product gas were in-
creased with increasing the fuel feed rate, due to the increase in CH4 con-
centration at lower steam/fuel ratio.
(4) With increasing the temperature, H2 concentration increased, while the
concentrations of CO, CH4, and CO2 slightly decreased, whereas the HHV
remained almost constant.
(5) The gas yield was increased with increasing the temperature due to the
increase of gas production during the initial pyrolysis stage, besides steam
cracking and reforming of the heavier hydrocarbons and tars [106].
(6) H2 yield, carbon efficiency, cold gas efficiency and HHV of the prod-
uct gas were much higher during the co-gasification of fall switchgrass
than co-gasification of spring switchgrass due to the higher ash, the potas-
sium content of fall switchgrass.
(7) At lower gasification temperature, biomass ash alkali metals act as
natural catalysts for steam gasification. In contrast, reducing tar yield and
enhancing the thermal efficiency.
(8) Synergy was observed between coal and Biomass based on the result
of experiment by Kumabe et al., [107] showing that the extent of the
water-gas shift reaction is maximized at a SF ratio of 0.5.
Table 5. Summary of selected studies of co-gasification in fixed bed gasifiers.
Operation conditions/features
Qin et al.
In this experiment, the
effect of the organic struc-
ture and mineral matter in
the coal-biomass mixture
during the co-gasification
of anthracite and rice straw
in a fixed bed gasifier was
In this test, CO2 was a gasifying
agent. The tested biomass ratios
were 20%, 33%, 43% and 50%
under isothermally gasification
temperature of 1100 °C
(1) The organic structure in char became less ordered with the addition of
Biomass during coal and biomass co-gasification than only coal gasification.
(2) The reactivity of coal gasification increased when the bio-mass ratio
addition was more significant than 20%.
(3) The bulk concentrations of K and Na and their bearing minerals and
phases in char increased with the addition of Biomass during the gasification
(4) The transformation of mineral matter played a significant role in pro-
moting the coal gasification by the addition of Biomass and the extension
of gasification time.
et al.
The co-gasification of
woody Biomass (Japanese
cedar) and coal in a
downdraft bed gasifier
In this test, air, and steam were a
gasifying agent. The tested bio-
mass ratio ranged from 0% to
100% under a gasification tem-
perature of 900 °C.
(1) The conversion rate and H2 increased with increasing the biomass ratio
while CO2 and conversions to char and tar decreased.
(2) CO and hydrocarbon composition were not affected by the biomass
ratio addition.
(3) Based on the product gas compositions (H2/CO ratio), a low biomass
ratio led to the production of a gas favorable for the syntheses of methanol
and hydrocarbon fuel. In contrast, a high biomass ratio facilitated the pro-
duction of a gas profitable for DME (Direct Dimethyl Ether) synthesis.
(4) A cold gas efficiency ranging between (65%-85%) was found in the
study co-gasification condition.
(5) No apparent synergy was observed due to the mixture of Japanese
cedar and Mulia coal in terms of the carbon distribution of products.
(Table 5) Contd
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16 Current Alternative Energy, 2020, Vol. 4, No. 1 Gomaa et al.
Operation conditions/features
Li et al.
The co-gasification of coal
and Biomass was studied
through investigating the
Co-pyrolysis behaviors of
rice straw and bituminous
coal in a fixed bed gasifier
In this test, nitrogen was a gasify-
ing agent. The tested biomass
ratios were 0%, 20%, 40%, 60%,
80% and 100% under the temper-
atures of 700 ºC, 800 ºC and 900
(1) Tar and gas yields increased with the increasing biomass ratio due to
the high volatile matter in Rice Straw.
(2) The addition of Biomass changed the atmosphere during the pyrolysis
process and promoted tar decomposition.
(3) At higher temperatures, tar cracked and led to a higher gas yield. (4)
No significant differences were observed between coal/biomass blend char
and their char individually.
(5) No significant synergetic effect was observed between the two kinds
of particles.
The co-gasification of
agriculture biowaste and
coal (lignite and bitumi-
nous coal) in an updraft
fixed bed gasifier under
the atmospheric pressure
The tested fuel blends were (20%
and 40%w/w) of biomass content
added to both lignite and bitumi-
nous coal separately. In this test,
steam was the gasification agent,
with a flow rate of 0.053 cm3/s
for one hour. The gasification
temperature varied between (700
°C to 900 °C), with a heating rate
of 1.3 °C/s.
(1) The highest carbon conversion rate and the highest total gas and hy-
drogen yields were observed in the processing of coal blends of 20%w/w
biomass content at 900 °C.
(2) The carbon conversion rate in co-gasification increased with increasing
biomass content in the fuel.
(3) The synergy observed in the process of co-gasification of bituminous
coal/lignite and biowaste may be attributed to the catalytic effects of bio-
mass ash components.
Patel et
al. [111]
The co-gasification of coal
(lignite) and Biomass
(waste wood mixtures)
downdraft fixed bed gasi-
fier under atmospheric
To overcome the clinker for-
mation, bridging or channel burn-
ing, erosion, and corrosion due to
high ash content of lignite in an
existing lignite gasifier system by
used air as the gasification agent.
The tested samples were (10%,
20%, and 30%) waste wood mass
ratio; wood cubical particle size
was (50×50×5mm), while lignite
particle size was (22-25mm).
(1) Co-gasification of lignite and wood eliminates the problems associated
with the gasification of lignite alone, such as clinker formation, whereas
the wood mass ratio in the mixture clinker formation was reduced until it
completely disappeared at 30% wood mass ratio.
(2) With increasing the wood content in the mixture from 0% to 30%, the
temperature in the reduction zone and oxidation zone increased due to the
higher reactivity of wood. In contrast, the temperature in the drying zone
was not affected by the wood content; a reduction in heavy tar content
increased the gas yield. The concentration of both H2 and CO2 increased,
while CO and CH4 were not much affected. The heating value (LHV &
HHV) was increased due to a higher concentration of H2 and CO in the
producer gas, and cold gas efficacy was increased.
(3) The synergistic effect between lignite and wood has been observed.
Z. Zhang
et al.
The co-gasification of coal
(lignite) and Biomass
(radiata pine wood) to
evaluate the effect of
blending ratio and alkali
and alkaline earth metals
(AAEM) species on char
reactivity and producer gas
composition in a fixed bed
The tested samples were 100%,
80%, 50%, 20%, 0% of lignite in
the blend with steam as the gasi-
fication agent. The blended fuel
was pelletized into cylindrical
pellets with about 20mm length
and 8mm diameter. Then they
were charred at 900 °C, part of
the lignite was washed by HCl
acid to investigate the effect of
AAEM in the lignite on gasifica-
tion performance, where they
were effectively removed, the co-
gasification temperature was 950
(1) With the acid-washed lignite, producer gas yield, H2, and CO2 de-
creased, while increasing the CO yield.
(2) The presence of AAEM in lignite is essential to promote the producer
gas yields and to enhance the gasification reaction rate.
(3) With increasing the proportion of lignite in the blend, producer gas
yield, H2, and CO2 yields decreased, while increasing CO yield and the
time for complete gasification.
(4) The ratios of H2/CO, H2/CO2, and CO/CO2 in the producer gas were
nonlinearly related to lignite/pine ratio in the lignite blended chars, which
match the findings of Saw & Pang [103]. In contrast, these gaseous ratios
were linearly correlated to the lignite/Pine ratio in co-gasification of acid-
washed lignite blended chars, and that confirmed the synergetic effect in
the co-gasification of blended chars of lignite and pine when the AAEM is
present in the lignite.
(5) The synergetic effect has been observed in the co-gasification of blend-
ed chars of lignite and radiate pine.
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Table 6. Summary of selected studies pertaining to co-gasification in an entrained flow gasifier.
Operation conditions / features
Kajitani et al.
The co-gasification of
bituminous coals and
cedar bark added into
flow gasifiers with car-
bon dioxide (CO2) as a
gasifying agent under
high temperature using a
drop tube furnace, was
Tow mass ratios of Biomass in fuel
blend were tested: 0%-30% under the
temperature of 1200 and 1300 °C, and
0.5 MPa of furnace pressure, the ratio of
fuel/CO2 was 0.20, 0.21, 0.21, and 0.23
(1) At 1200 °C or lower, the mixture of biomass and coal reactivi-
ty was higher than the reactivity of coal alone due to the catalysis
behavior of alkaline and alkaline-earth metal species in Biomass.
(2) At 1400°C, the reactivity of the fuel blends and the reactivity
of coal alone was almost the same.
(3) Distinguished synergy to improve the gasification reactivity
was not observed.
Feng et al.
The gasification of
coal/bio-oil slurry
(CBS), the gasification
of coal/water slurry
(CWS) and the co-
gasification of coal/bio-
oil slurry and coal/water
slurry in an atmospheric
entrained flow gasifier
In this test, steam was a gasification
agent under a gasification temperature
of (1200 C to 1400 ºC) at a
steam/carbon ratio of 5
(1) Through the comparison between coal/bio-oil slurry gasifica-
tion and coal/water gasification at the same conditions, at 1300 ºC
and steam/carbon ratio of 5, Coal/bio-oil slurry and bio-oil carbon
conversions reaching up to 99% and 95%, respectively, which is
higher than coal/water slurry carbon conversion 61.76%.
(2) Coal/bio-oil slurry and bio-oil have a higher gasification reac-
tion rate than coal/water slurry leading to higher syngas products.
(3) Solids and residual carbon produced from coal/bio-oil slurry
gasification are much lower than that produced from coal/water
(4) A synergistic effect exists between coal and bio-oil in
coal/bio-oil slurry gasification caused by the catalysis effect of
alkali metals and alkaline earth metals in bio-oil.
(5) A high steam/carbon ratio is useful for reducing solid residual
production and enhancing the H2/CO molar ratio.
Hernández et
al. [57]
The effect of the relative
fuel/air ratio, and the
gasification performance
with Biomass blended
with coal through the co-
gasification of coal-coke
mixture and Biomass
(grape marc) in entrained
flow gasifier
In this test, air was a gasification agent.
The tested samples were 0%, 10%, 30%,
50%, 80%, and 100% biomass, under
the gasification temperature ranging
between 750 °C and 1150 °C and
air/fuel ratio raining between 2.5 and
7.5, and a constant residence time of 1.4
s for all cases.
(1) An increase of the fuel blend's biomass content upgrades the
producer gas quality and improves the cold gas efficiency.
(2) At low fuel/air ratios and low reaction temperatures (750- 850
°C) and 50%-50% blended fuel, some hints of synergy were
observed due to the catalyst effect of biomass ash and minerals
coal/coke ash.
(3) The synergy diminished as the temperature (above 950 °C)
and/or the relative fuel/air ratio increased.
Chen, & Hung
The co-gasification of
torrefied Biomass (euca-
lyptus) and coal (bitumi-
nous coal) using the
Taguchi method to ap-
proach the optimum co-
gasification operation in
entrained flow gasifier
In this test, steam and oxygen were the
gasification agent. The fuel blend's mass
flow rate was 0.023 kg/s, the tempera-
tures of oxygen and steam were 27 °C
and 127 °C, respectively. Air was used
as a carrier gas to enhance the transport-
ing the fuel particles with a mass flow
rate of 0.025 kg/s. The torrefaction of
Biomass was carried at temperatures of
250, 275, 300, and 325 C for 1 h. The
fuel particles' sizes were in the range of
44250 µm, with an average particle
size of 103 µm.
(1) The optimum operation condition was at oxygen to fuel ratio
of 0.7, the biomass blending ratio of 5 wt%, biomass Torre fac-
tion temperature of 300°C, gasification pressure of 2MPa, the
inlet temperature of the carrier gas of 427 °C, and steam- fuel
ration of 0.07.
(2) Oxygen to fuel ratio was an essential factor in determining the
performance of the co-gasification process.
(3) At the optimum operating conditions of co-gasification, the
value of cold gas efficiency was80.99%, and carbon conversion
was 94.51%.
(4) The performance of the co-gasification process was signifi-
cantly affected by biomass torrefaction.
In the face of ever-growing urbanization and improved
living standards, the world's present energy systems,! with
their overreliance on the burning of fossil fuels, have proven
to be unsustainable in the longer term. Their growing inade-
quacy and harmful environmental implications have diverted
scientific and commercial attention toward alternative
sources of energy. Recently, various countries, including
Jordan, have shifted their focus towards such sources of re-
newable energy alternatives. Jordan has begun investing in
high-energy efficiency areas, better management and opera-
tion, high-efficiency applications, and machinery utilization
of renewable energy sources.
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18 Current Alternative Energy, 2020, Vol. 4, No. 1 Gomaa et al.
Gasification is one particular promising clean energy
technology that converts various carbon raw materials such
as natural gas, coal, oil, coke, biomass, and municipal solid
waste into a positive energy source. This paper scrutinized
the co-gasification process, via an analysis of of existing
experimentations. A comparison was then made between a
co-gasification process and normal gasification in identifying
the main influential factors determining the outcomes these
processes. These were found to be primarily reliant upon the
gasification agents and temperature values. This study’s
overarching objective was to further best practice and per-
formance optimization of gasification processes within the
industry through a thorough evaluation of operating condi-
tions and outcomes.
AAEM = Alkali and Alkaline Earth Metals
BFB = Bubbling Fluidized bed
CFB = Circulating fluidized bed
CH4 = Methane
CO = Carbon monoxide
FFB = Fast fluidized bed
GOJ = Government of Jordan
H2 = Hydrogen
H2O = Water vapor
HHV = High Heating Value
JREEEF = Jordan Renewable Energy and Energy Ef-
ficiency Fund
LPG = Liquefied petroleum gas
LHV = Low Heating Value
Conceptualization, M.R.G., G.A.M., M.S., and H.A.A.;
methodology, M.R.G., G.A.M., and M.S.; formal analysis,
M.R.G., G.A.M., M.S., and H.A.A.; resources, M.R.G., and
G.A.M.,; data curation, M.R.G., and G.A.M.; writing
original draft preparation, M.R.G., G.A.M., and H.A.A.;
writingreview and editing, M.R.G., G.A.M., M.S., and
H.A.A. All authors have read and agreed to the published
version of the manuscript
Not applicable.!
The authors declare no conflict of interest, financial or
Declared none.!
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In contrast to traditional combustion, gasification technologies offer the potential for converting coal and low or negative-value feedstocks, such as petroleum coke and various waste materials into usable energy sources or chemicals. With a growing number of companies operating and marketing systems based on gasification concepts worldwide, this book combines the latest information and real-world experience in developing gasification technologies. Gasification Technologies: A Primer for Engineers and Scientists discusses gasification techniques and the benefits of each technology, including gas clean-up technologies and those used in hybrid systems and fuel cells. It also accounts for the primary products that are recovered and explains how these products are purified and can be used as fuel or for applications in petrochemical processes. The book describes the conditions in which optimal value intermediate products can be recovered, focusing on key factors such as oxygen or air blown reactor, operating temperature, internal and external heating, and reactor design. The authors also establish how gasification can help meet renewable energy targets, address concerns about global warming, and contribute to a better carbon management or achieving Kyoto Protocol commitments. Gasification Technologies provide a multidimensional and well-rounded examination of current technology, research, applications, and development challenges for the commercialization of this increasingly popular technology.
Energy services have a profound effect on productivity, health, education, climate change, food and water security, and communication services1; lack of access to clean, affordable and reliable energy hinders human, social and economic development and is a major impediment to achieving the United Nations’ Millennium Development Goals. Today, 1.4 billion people still do not have access to modern energy, while 3 billion rely on traditional biomass and coal as their main fuel sources. Through resolution 65/158, the United Nations General Assembly has designated the year 2012 as InternationalYear of Sustainable Energy for All to encourage increasing sustainable access to energy, energy efficiency, and renewable energy at the local, national, regional and international levels. This initiative will engage governments, the private sector, and civil society partners globally to achieve three major goals by 2030: •Ensure universal access to modern energy services.•Reduce global energy intensity by 40%.•Increase renewable energy use globally to 30%.
Carbonaceous solid materials are converted into gaseous fuel through the gasification process. A limited supply of steam, air, oxygen, or a combination of these serves as gasifying agent. Depending upon the gasifying agent used, the fuel gas will contain mainly hydrogen, carbon monoxide, carbon dioxide, methane, higher hydrocarbons, and nitrogen (if air is used). In gasification, different technologies are used depending upon the requirement. Technologies used for gasification can broadly be classified into four groups; fixed bed or moving bed gasification, fluidized bed gasification, entrained bed gasification, and plasma gasification. In the present chapter, a detail discussion on the design, working principle, merits and demerits of different types of gasifiers are presented. Some of the important commercial gasifiers installed worldwide are also discussed.