A Comprehensive Review of Biomass Gasification Process

A.S. El-Shafaya,b, A.A. Hegazia, F. S. H. El-Emama, M. Okashaa

a
Department of Mechanical Engineering, Mansoura University, Egypt
b
Department of Mechanical Engineering, Prince Sattam Bin Abdulaziz University, KSA
Abstract
Gasification can be considered as one of the most conventional techniques in the conversion of biomass because
of some crucial factors, such as the consideration on space, reduction of the substantial waste volume, the flexibility of the
fuel used as well as the recovery of energy. The overall process of Gasification tends to involve the partial combustion of
the organic part of the fossil fuels, which forms an abundant combustible gas in the presence of carbon monoxide,
hydrogen, and some saturated hydrocarbon gases, or methane. When we consider the process of Gasification, the
parameters deemed to affect its performance, including the catalysts, Gasifying Agents or the Biomass ration and
temperatures, as well as the type of raw materials are reviewed and discussed in this study. The primary purpose of this
paper is to address the Gasification Process, including the Types of Gasifiers System Designs like the Fluidized bed,
Downdrafts, and the Updrafts. It’s also responsible for addressing the production of tar from the process of Gasification.
The paper explores and discusses all the primary, secondary and tertiary types of tar while touching on the Gasification
Modeling Methods and Equations used in the Modelling.

Keywords: Biomass, Sawdust, Gasification, hydrogen production and Renewable Energy

(Edit the Table of Contents)

(Edit the Nomenclature for all paper)

1. Introduction
Gasification is a thermochemical process that occurs at high temperatures usually more than 700C, responsible for
converting carbonaceous materials including fossil fuels, biomass, plastics, and coal into syngas which is consists of H2,
CH4, CO, and CO2. A specific amount of oxygen (air) and steam is used as Gasifying agent and heat carrier agent. The
syngas yield can be burnt directly to produce heat at temperatures that are higher compared to the combustion products.
Also, the syngas, if well cleaned, may be applied to different pathways to yield useful outputs including that:

1. Production of methanol,
2. Purified syngas used in the gas engine, gas turbines, and fuel cells to generate electrical energy,
3. Methane through Sabatier reaction,
4. Production of dimethyl by methanol dehydration,
5. Production of hydrogen, and
6. Production of fuels like gasoline and diesel [32].

This chapter presents a literature summary which involves of six parts: Fluidized bed, an overview of biomass used in
Gasification, tar reduction, Gasification agent, finally influence of Gasifier operating conditions and Mathematical
modeling. Particular attention is paid to literature regarding in tar formation, and mitigation and enhancing of operational
conditions during biomass Gasification are introduced. Finally, refined research requirements for this research work based
on the literature overview are determined.

2. Fluidized Bed Gasifier

In the Gasification process, the biomass is heated to a high temperature producing a series of chemical and
physical changes, which result in the development of Volatile Products plus solid carbonaceous residues. The amount of
the Volatile Products depends on: the heating rate, temperature height, and type of biomass fuel. It is acceptable that the
Char Gasification Stage is the rate-limiting factor in the Gasification of Harvest Residues because of the quick
Devolatilization Stage. The composition of the syngas produced typically depends on the degree of equilibrium attained by
various gas phase reactions especially water gas shift reaction [32, 33].

In the absence of a catalyst, Char Gasification with reactive gases such as oxygen are produced at high
temperatures. Otherwise, when Char is Gasified in the presence of the steam, the gas produced is composed mainly of CO,
CO2, H2, and CH4. In reactor operating at low temperatures, low heating rates and very high-pressure, secondary reactions
are very significant because of long residence time. At low pressure, high temperature and high heating rates, most of the

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Volatile Products escape from the biomass particle due to the pyrolysis process. The process, on the other hand, reduces the
chance of Solid Char reaction with the gas.

In fluidized bed gasifiers, the latter prevails, but because of the mixing nature of the bed, secondary reactions in
the gas-solid and gas phase takes place. Biomass gasification process is considered to be occurring in four stages: drying of
the feedstock pyrolysis to produce volatile matters and char, gasification of the char with reactive gases such as O2, H2,
H2O and secondary reactions of primary gasses and tars [50]. The advantages of the Fluidized bed reactors are good gas-
solid contact, better temperature control, excellent heat transfer characteristics, and high volumetric capacity. The
temperature can be controlled by both of varying the feed rate or the agent rate. Moreover, low operating temperature leads
to produced more slag and clinker. Fluidized bed reactors have the more extensive adaptability to handle different types of
fuel. High ash or moisture content of the feedstock pointing to no problems to the fluidized gasifiers such as those
ordinarily encountered with moving bed Gasifiers.

Figure 1.0: Gasification In Fluidized Beds.

The tar contents of the syngas obtained from Fluidized bed are less than that in the syngas obtained in the updraft.
These desirable features of the Fluidized bed Gasifiers make it more appropriate for large-scale operation than downdraft
Gasifier [35, 50]. The disadvantages of the Fluidized reactors are a significant pressure drop and corrosion of the reactor
body. Because of Fluidized bed reactors operate at pressures little above atmospheric, so must be designed to prevent
leakage. Other disadvantages of the Fluidized bed Gasifier are higher tar content of the product gas and the incomplete
combustion of carbon atoms [37]. Fluidized bed Gasification has been widely used for coal Gasification for many years;
uniform temperature distribution achieved in the Gasification zone also is an advantage over the fixed bed. The uniformity
of the temperature is obtained using a bed of fine material into which air is introduced, fluidizing the bed materials and
guarantee intimate mixing of the hotbed materials, the hot combustion gas, and the biomass feed. Fluidized bed Gasifiers
can be classified by their configuration and the Gasifying agent velocity into bubbling Fluidized bed, circulating Fluidized
bed and spouted Fluidized bed as present in the following sections.

2.1. Fluidization and fluidization velocity

The flow rate of oxidizing agent supplied to the fluidized-bed gasifier is a crucial parameter to maintain effective
fluidization of bed materials. The fluidization condition is usually described using a superficial gas velocity which is the

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ratio of volumetric gas flow rate and bed cross-sectional area. The superficial gas velocity, at which the drag force on the
bed materials equals the gravitational force, is defined as the minimum fluidization velocity (Umf) of the bed materials. At
minimum fluidization condition (shown in Fig. 3.3)., the bed materials lift upward and remain in suspension; bed pressure
drop (dTmf) reaches to a maximum and remains constant with further increase in the superficial gas velocity. Fluidization
characteristics, such as Umf and dPmf, depend upon the particle size and composition of the bed materials [137].

Umf and dPmf are also influenced by segregation and mixing behaviors of bed materials. Segregation is a process during
which a bed material with higher particle density, such as sand, moves downwards in the bed while a material with lower
particle density, such as biomass, floats upwards [138,139]. This, in turn, causes separation of biomass from sand and
results in a localized accumulation of biomass particles as smaller and more significant sized lumps throughout the bed.
These lumps further lead to channel formation, called in bed channelization, that give rise to larger void space and a shorter
path to the gas flow [140]. As a result, the gas quickly escapes through in-bed channels, which affect bubble formation, and
thus turbulence level in the bed resulting in ineffective fluidization. Segregation occurs due to differences in densities or
sizes of the bed materials such as sand and biomass [141, 142]. Genially, when a packed bed of particles is subjected to a
sufficient high upward flow of fluid the weight of the particles is supported by the drag force exerted by the fluid on the
particles and the particles become freely suspended or fluidized. The behavior of fluidized suspension is similar in many
aspects to that of a pure liquid. Mass transfer and heat transfer rates between particles and submerged objects (e.g., heat
exchanger tubes) are significantly enhanced in fluidized beds, to avoid or reduce carryover of particles from the fluidized
bed, keep the gas velocity between minimum fluidization velocity (Umf) and terminal velocity (Ut). Also, rapid particle
mixing allows uniformity in bed. As a result, the fluidized bed is widely used for conducting gas-solid reactions (coal
combustion), gas-solid Catalytic Reactions (catalytic cracking of petroleum), etc. [142].

Therefore, the quantity of biomass in the mixture plays a crucial role in segregation behavior of bed materials. Also, a bed
consisting of a material, such as particulate matters, that has adhesive or cohesive properties may enhance segregation
tendency and suppress fluidization [143]. Chok et al. [144] indicated improved mixing with a decrease in the particle size
ratio from 30 to 20 of palm shell and sand mixture. The author also reported that segregation and channelization were
predominant at higher particle size ratio and biomass weight fraction (10% and 15%) in the mixture. Correlations to
determine Umf and bed expansion of coal particles were suggested for coal gasification [145]. These methods are derived
from pressure drop method, dimensional analysis, drag force method, and terminal velocity method. At the onset
fluidization, drag force by upward moving gas on the whole system of particles must be equal to the weight of particles of
the bed. The range of fluidizing velocity, Umf, in a fluidized bed should be within the minimum fluidization and terminal
velocities of the mean bed particles. In the following subsection the Minimum fluidization velocity, the Terminal velocity
of the particle, Fluidization velocity during the gasification and agent flow rate to meet the fluidization condition
calculation procedures are described as following.

Fig. 1.1 Fluidization characteristics of sand

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2.2. Bubbling Fluidized Bed Gasifier

The bed is termed a bubbling fluidized bed when the granular material (e.g., sand) is lifted in a reactor through
which an upward flow of gas is passing through it at a flow rate where the pressure drop across the particles is enough to
support their weight. In bubbling fluidization, low fluidization velocity just above the minimum agent velocity passes
through the bed in the form of bubbles. Bubbling bed gasifiers consist of a vessel with distributor plate at the bottom
through which the air is introduced. Above the distributor plate the moving bed of fine-grained material into which the
prepared biomass comes in. The setting of the bed temperature to 600-1000C is maintained by controlling the equivalence
ratio. The biomass is pyrolyzed in the hotbed of char form and gaseous compounds, the high compounds weight being
cracked by contact with the hotbed materials, giving a product gas with low tar content. The bubbling bed gasifiers can be
classified according to the number of the bed, such as single fluidized bed and multi fluidized bed [38].

Fig 1.2: Bubbling Fluidized Bed Gasifier.

2.2.1. Single Fluidized Bed Gasifiers

This system consists of one bed, into which the feedstock and the gasifying agent enter and out of which the
produced syngas and char exit. The advantage of this gasifier is, low operating and initial cost compared to the multi-bed,
less maintenance, and maintenance cost due to uses of one bed only, the produced syngas is ready to direct use. On the
other hand, the disadvantages of single bed as generated syngas heating value is lower than that provided by the double
bed; pyrolysis process occurs in the combustion zone at the bottom of the gasifier which leads to non-uniform temperature
distribution and the separation of the inorganic materials in the feedstock is hopeless.

Figure 1.3: Single Fluidized Bed Gasifiers

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2.2.2. Dual and Multi-Fluidized Bed Gasifiers
These gasifiers consist of more than one bed commonly first bed used to burn some of the char to produce the
energy for the second bed in which the pyrolysis process occurs. The advantage of the dual bed system was reported by
many authors [37]. These include the syngas heating value more significant than that produced by the single bed due to the
combustion of the char occurs in separate reactors and hence the combustion syngas does not dilute the pyrolysis gases and
separation of the inorganic material in the feed is possible. But, the disadvantages are higher construction cost and more
maintenance requirement as compared to the single bed. Keiichi Tomishige et al. [37] studied the gasification of the
biomass by using the Single bed and Dual bed gasifier. It was found that in the Dual bed gasifier combined with a suitable
catalyst; some the tar can be converted to syngas at a lower temperature than that needed by the traditional methods with
high energy efficiency.

Figure 1.4: Dual Fluidized Bed Gasifiers

2.3 Circulating Fluidized Bed Gasifiers

If the gas velocity in a bubbling bed is too increased, more particles will be entrained in the syngas stream and
leave the gasifier. Finally, the transport velocity for the most of the particle is reached, and the vessel can be quickly empty
of the solid except new particles are fed to the base of the gasifier. If the particles leaving the vessel return through an outer
collection system, then the gasifier now called a circulating fluidized bed. The Circulating Fluidized Bed has high
processing capacity compared to the conventional gasifiers, better gas-solid contact and the ability to handle coherent solids
that otherwise be difficult to fluidize in a bubbling bed. Regardless of these advantages, the circulating fluidized bed
gasifiers are yet less conventional than the bubbling ones because of their height which restricts their application regarding
cost analysis [39].

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Figure 1.5: Circulating Fluidized Bed Gasifiers

2.4 Spouted bed gasifier

A spouted fluidized bed gasifier consists of a bed of a solid particle partially filling the reactor which is provided
with a relatively large control opening at its base. Gas is injected through this aperture. With sufficient flow of the gas, the
particle in the gas can be made to rise in a jet at the center of the bed and develop periodic motion on the bed as a whole.
The bed motion can be assisted by the extra air at the base to produce a spouted bed. Spouted bed gasifiers have been used
to gasifying the coal of various ranks. Abdul Salam [40] has studied the gasification of biomass in a spouted bed, which has
specific possible advantages over the fluid bed configurations. They found that at a higher agent velocity, the gasification
efficiency of circular slit spouted bed was slightly high compared with that of central jet spouted.

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Figure 1.6: Spouted bed gasifier

3. Overview of Biomass Used in Gasification

In this section, several results from the gasification studies are summarized to present the biomass gasification
process utilization and its performance.

3.1. Crop Residues Gasification (insert the summary of last 3 researches related to this title done in 2016, 2017 and
2018)
Chen et al. [41] perform some experiments involving the gasification of biomass blends in a bench scale fluidized
bed gasifier at atmospheric pressure. Two types of mixtures were prepared, which mixing of pine chips with low-grade
black coal and Sabero coal in the ratio range of 0:100-100:0. The experiments were conducted using gasification agent as
mixtures of air and steam at gasification temperatures of 840C-910C and fluidized gas velocities of 0.7-1.4m/s. The results
are showing that efficiently blending improved the performance of fluidized bed gasification of the low-grade coal, and the
possibility of converting the refuse coal to a low heating value syngas. Their study indicated that a blend ratio with no less
than 20% chip pellets for the low-grade coal and 40% chip pellets for the refuse coal are the most suitable. The low heating
value of the Dry Product Gas increase with increasing blend ratio from 3.7 to 4.5 MJ/Nm3 for chip pellets /low-grade coal
and from 4.0 to 4.7 MJ/Nm3 for chip pellets/refuse coal. The syngas yield increase with the increase of the blend ratio from
1.80 to 3.20 Nm3/kg (chip pellets /low-grade coal), and from 0.75 to 1.75 Nm3/kg (chip pellets /refuse coal), respectively.
Also, about 50% thermal efficiency was achieved for the two types of the blend.

3.2. Straw Gasification (insert the summary of last 3 researches related to this title done in 2016, 2017 and 2018)
Sadaka et al. [42] develop a fluidized bed gasifier to gasify wheat straw in an air/steam mixture as an agent. Their
study showed that the performance of the fluidized bed gasifier (gasifier temperature, pressure drop, heating value and
syngas flow rate) was influenced by fluidization velocity, steam (agent) flow rate, and biomass/ steam ratio. The
temperature of the gasification bed reached its maximum at of 891C, indicating that the agglomeration problem was
overcome by injecting more air/steam mixture. The heating value of the syngas yield and the syngas flow rate reached their
maximum of 10.63MJ/Nm3 and 2.47Nm3/min, respectively. The bed pressure drop occurs ranged from 37.2 to 51.6
cmH2O, gives a good fluidization quality. Risen's et al. [43] studied the influence of calcium addition in straw gasification.
The effect of calcium addition as calcium sugar/molasses solutions to straw significantly influenced the ash chemistry and
the ash sintering tendency but much less the char reactivity. Walawender et al. [44] studied the straw gasification with
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steam as an agent in a 0.23m diameter fluidized bed gasifier over a temperature range of 552 to 757C. The fraction of the
biomass was converted to syngas ranged from 32% at the temperature of 552C to 73% at the temperature of 757C. The
heating value of the syngas appeared at parabolic variation with temperature with a maximum value of 12.3MJ/Nm3
obtained at 672C.

3.3. Rice Husk Gasification (insert the summary of last 3 researches related to this title done in 2016, 2017 and 2018)
Yin et al. [45] tested rice husk gasification in circulating fluidized bed for biomass gasification and power
generation system to provide power for a rice mill. The system consists of a circulating fluidized bed gasifier, a gas cleaner
(including an inertial separator, a cyclone, filter, a venture and two water scrubbers). It was found that the gasifier can be
operated stably within the temperature range of 700C to 850C; it's optimal condition. The main performance indices at
capacity, 1500 kg/hr., are gasification efficiency 65%; rice husk consumption, 1.7-1.9kg/Whr; total efficiency, about 17%.
The gasification of rice husk was also studied by Chen and Rei [41] within a temperature range of 600C to 700C. They
used electrical heaters to provide the heat required to gasification in a 0.05m internal diameter fluidized bed reactor; the bed
consisted of alumina sand and agent was super-heated steam. The syngas yield increased from 0.38 to 0.55m3/kg and the
heating value varied from 12.8 to 18.5MJ/m3. Over this temperature range H2, CH4, CO, and CO2 concentrations in the
syngas produced changed from 3.6 to 13.1%, from 14.4 to 13.5%, from 52.2 to 51.1% and from 23.0 to 14.6%,
respectively.

3.4. Cotton Gin Trash Gasification (insert the summary of last 3 researches related to this title done in 2016, 2017
and 2018)
Some agricultural waste as biomass samples such as rapeseed, cotton refuse, sunflower shell, pinecone, and olive
refuse was the first pyrolysis in air. Their chars were then gasified in a gas mixture of steam and air. Pyrolysis of the
biomass samples was performed at a heating rate of 20K/min from ambient to 1273K in a dynamic air atmosphere of
40cm3/min. It was concluded that gasification characteristics of biomass chars were somewhat dependent on the biomass
properties such as ash and fixed carbon contents and the constituents present in the ash. Singh et al. [46] performed the
steam gasification of cottonwood branches in a fluidized bed gasifier and compared the produced syngas characteristics
with those for pure cellulose. The results showed that the syngas heating value, the energy recovery, carbon conversion and
mass yield of the syngas obtained from cottonwood were found to be lower than those derived from pure cellulose. Groves
et al. [47] studied the gasification of cotton gin trash in a fluidized bed with air over the temperatures range of 649C and
87C in a 0.3m internal diameter gasifier. Its results showed the syngas heating value and the energy recovery increased
from 3.4 to 4.3MJ/Nm3 and from 27 to 53%, respectively.

3.5. Corn Cobs Gasification (insert the summary of last three types of research related to this title done in 2016,
2017 and 2018)
Results for the gasification of corncobs over a temperature range of 500C to 1000C were presented by Epstein et al. [48].
Produced syngas yield (reported as a mass fraction of the biomass fuel feed) increased from 0.17 to 0.60kg/kg. The syngas
contains large amounts of CO and H2 and the heating value varied from 1.4 to 10.9 MJ/m3.

3.6. Sawdust Gasification (insert the summary of last three types of research related to this title done in 2016, 2017
and 2018)
Wander et al. [49] observed that, the technology of wood gasification could produce a syngas’s qualified of being
combusted in an internal combustion engine, as long as it is appropriately cleaned enough from tar and ashes. In order to
evaluate the performance of the woody biomass gasification process, a small downdraft, fixed bed, stratified and open top
gasifier was constructing. This gasifier, whose capacities were around 12kg/hr., has an internal gas recirculation, new to
this type of gasifier, which can burn part of the syngas produced to raise the gasification reaction temperature. The syngas
yields ranged from 1.1m3/kg at 600C to 1.3m3/kg at 800C, and the heating value was more than 11.2MJ/Nm3 for all
temperatures. Lian and Findley [34] tested the air gasification of oak sawdust and found that the tar and char yields
decreased linearly with temperature from 6% of the dry wood weight at 650C to 0.5% at 800C. They tested the effect of
equivalence ratio on the gasification performance and concluded that the total carbon-to-air ratio in the dry syngas gave the
better correlation with the concentrations of carbon, hydrogen and the higher heating value.

3.7. Cellulose Material Gasification (insert the summary of last three types of research related to this title done in
2016, 2017 and 2018)
Walwender et al. [45] performed alpha cellulose gasification in a bench scale fluidized bed gasifier with steam as an
agent within a temperature range of 600C to 800C. The significant components of the produced syngas were H2, CO2, CO

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and CH4 and the volumetric syngas yields were 0.5-1.4m3/kg, the average gas heating value was 11.8MJ/Nm3. The energy
recovery and carbon conversion were within the range of 32-90% range. Hoveland et al. [50] studied the steam gasification
of alpha cellulose at high temperatures and confirmed an earlier assumption that proposed the existence of two temperature
regimes for cellulose gasification, one below 567C dominated by volatile cracking and the other above 567C dominated by
the water gas shift reaction.

3.8. Manure Gasification (insert the summary of last three types of research related to this title done in 2016, 2017
and 2018)
Sweeten et al. [51] reported that, feedlot manure has approximately half the heating value of coal, twice the volatile
matter of coal, four times the N content of coal on heat basis. Due to soil pollution during collection, the ash content is
roughly 9-10 times that of low ash (5%) coal. The addition of 5% biomass residues had a little apparent effect on heating
value. Based on heating values and alkaline oxides, partial composting seems favorite to a full composting cycle. Even
though the percentage of alkaline oxides is reduced in the ash, the increased total ash percentage results in an increase of
total alkaline oxides per unit mass of fuel. Raman et al. [36] tested the gasification of feedlot manure with different
fluidization velocities. They found that the fluidization velocities did not have a significant influence on produced syngas
yield, heating value or composition. Walawender and Fan [52] studied the feedlot manure gasification using air as an agent,
found that the generated syngas yield, the energy recovery and the higher heating value increased by 131%, 244% and 77%
when the temperature increased from 627 to 827C. Halligan et al. [53] gasified feedlot manure in a 0.05m internal diameter
fluidized bed gasifier. The agent was a mixture of air and steam, and the bed consisted of the feed material only. Over the
temperature range of 693C to 796C, the syngas yield flow rate increased from 0.6 to 1.3m3/kg, and the gas heating value
increased from 8.7 to 9.8MJ/Nm3. The energy recovery and carbon conversion also risen from 23 to 49% and from 20 to
50%, respectively.

4. Tar Reduction
Tar is one of the most complex compounds produced from biomass gasification, and its formation is strongly dependent
on the operating conditions such as gasification temperature, a gasifying agent used and equivalence ratio. Therefore, to
avoid various problems related to tar condensation and formation of tar dust, tar reduction is necessary before the final use
of the product syngas. In the following, detailed presentation about tar definition, tar measuring as well as how to reduce it.

4.1. Tar Definition

Due to its complication and different definitions have been given by various researchers working on biomass
gasification. In Milne’s review report, tar is defined as the organics produced under thermal or partial oxidation regimes of
any organic material and assumed to be hugely aromatic [54]. The European Committee agreed on unanimity on the
definition of tar for Standardization (CEN), and tar is defined as all organic compounds existing in the gasification product
gases with molecular weight more than benzene [55, 56]. During biomass gasification process, tar is formed in a series of
complex reactions and its formation is strongly dependent on the process conditions. The tar formation layout proposed by
Elliott and summarized by Milne [57], which shows the transition of tar as a function of process temperature from primary
products to phenolic compounds to aromatic hydrocarbons. Evans and Milne [54] reported that the tar could be classified
into four product classes which are identified as a result of gas-phase thermal cracking reactions:

1. Primary products which are characterized by cellulose-derived, hemicellulose-derived and lignin-derived products;
2. Secondary outcomes which are described by phenolics and olefins;
3. Alkyl tertiary products which are mainly methyl derivatives of aromatic compounds; and
4. Condensed tertiary products which are PAH series without a substituent.

The composition, quantity, and properties of tar in product syngas vary depending upon the biomass feedstock,
gasification agent, gasifier type and gasification conditions. Among them, the type of gasifier is one of the fundamental
parameters influence the tar content. The downdraft fixed bed (DFB) was the favored effective method in suppressing the
formation of tar during biomass gasification. For different utilization applications, the maximum allowable tar content is
different. For instance, Boerrigter et al. [58] reported that for Fischer-Tropsch (FT) Diesel synthesis the naphthalene
concentration in product syngas should be less than 2ppm to avoid condensation during the compression stroke before
catalytic conversion. Bui et al. [59] mentioned that the preferable tar and dust loads in syngas’s for engines must be lower
than 10 mg/Nm3.

4.2. Tar Reduction (insert the summary of last 3 researches related to this title done in 2016, 2017 and 2018)

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 Devi et al. [60] reported that tar removal technologies could be divided into two types: primary methods and secondary
methods. The primary purpose includes (a) the proper selection of the operating conditions; (b) the use of a suitable bed
additive or a catalyst during gasification; and (c) an appropriate gasifier design. Secondary methods consist of chemical
(catalytic cracking) or physical (mechanical separation) treatment [61]. Concerning this two methods, Corella et al. [62]
perform a comparison between them; Corella found no significant difference in their influence concerning tar reduction.
Sutton et al. [63] reported that a suitable combination of different primary and secondary treatments is likely to improve
gasifier performance and produced syngas with minimum tar concentration. For both methods, tar decomposition mainly
occurs due to a chain of complex, multiple and simultaneous reactions such as steam and dry reforming responses,
cracking. Although secondary methods are reported to be very functional in tar reduction, in some cases, they are not
economically viable [62]. Since they are out of the scope of this research, detailed information is available in the excellent
reports by Milne et al. [57] and Neeft et al. [64]. Primary methods are gaining a lot of attention as they may eliminate or
profoundly reduce the need for downstream cleanup. Widely studies have been conducted by different researchers
regarding effects of operating conditions (e.g., steam to fuel ratio (SFR), temperature, pressure and residence time) and
active materials (e.g., bed material, additive) on tar formation during biomass gasification. A proper selection of operating
conditions can strong reduce the amount of tar produced. Moreover, the optimized operating conditions using catalyst
during biomass gasification can also promote char reaction, reduce the tar yield.

4.2.1. Effect of Temperature on Tar Reduction (insert the summary of last 3 researches related to this title done in
2016, 2017 and 2018)
The temperature is one of the most critical factors affecting the overall biomass gasification process. Temperature can
influence the amount of tar formed as well as the composition of tar. Kinoshita et al. [65] reported that the total number of
detectable tar species produced from sawdust gasification decreased as well increasing of temperature. Lower temperatures
favored the formation of more aromatic tar species with diversified substituent groups, while higher temperatures supported
the establishment of fewer aromatic tar species without substituent groups. Li et al. [66] reported that the amount of tar gain
from biomass gasification strong decreased from 15 to 0.54 gram/Nm3 as the average temperature increased from 700C to
820C. Van Paasen and Kiel [67] observed that tar concentration decreased within a temperature range from 750C to 950C,
at the same time the tar compositions shifted from alkyl substituted poly-aromatic hydrocarbons (PAHs) to non-substituted
PAHs. The effect of gasification temperature on tar concentration is shown in figure (2.1), from which it can be seen that
gasification temperature mainly affects the formation and composition of tar. Kurkela et al. [68] and Simell and Leppälahti
[69] studied the effects of operating conditions on the creation of tar produced from different feedstocks ranging from hard
coals to wood gasification in a pressurized fluidized bed gasifier. They found that, the total tar concentration in the
pressurized fluidized bed product syngas appears to depend mainly on the feedstock and on the gasification temperature,
which can be clearly seen in figure (2.2) which shows the tar content produced from for wood, peat and brown coal
gasification in the pressurized fluidized bed gasifier at different freeboard temperatures.

4.2.2. Effects of Equivalence Ratio and Steam to Fuel Ratio on Tar Formation (insert the summary of last 3
researches related to this title done in 2016, 2017 and 2018)
Similar to temperature, an increase in equivalence ratio (ER) also has a useful effect on reducing tar yield. Narvaez et al.
[71] studied operating conditions on the product syngas produced from sawdust pellets gasification with air in an
atmospheric bubbling fluidized bed gasifier. The effect of ER on the tar yield in the syngas is shown in figure (2.3) for two
H/C ratios in the bed. The tar content produced from sawdust pellets gasification at 800°C decreased with increasing ER,
the tar content was found at about 2-7 gram/Nm3 obtained at an ER value of 0.45. Also, the H/C ratio is also almost
critical, and the tar content decreases with increasing H/C ratios. Lv et al. [72] reported that the lower heating value (LHV)
of the product gas decreased with an ER increase due to intensification oxidization reactions of product syngases. Steam to
fuel ratio (SFR) also influences tar formation due to more(less) tar steam reforming reactions. Herguido et al. [73] observed
that the amount of tar decrease in sharply trending from 8% wt. to minimal content with an increasing SFR range from 0.5
to 2.5. Aznar et al. [74] reported that with varying the ratio of ((steam/oxygen) / biomass mass) from 0.7 to 1.2 more than
85% reduction in the total tar was recorded.

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Figure (2.1) Effect of gasification temperature on tar Figure (2.2) Effect of free-board temperature on the
concentration and dew-point [67] total amount of tar in the pressurized fluidized bed
gasification with different feed-stocks [68]

5. Effect of Agent Type on Gasification

Biomass gasification using fluidized bed has been performed in various gasifying agents such as air, steam, pure
oxygen, carbon dioxide and hydrogen [75]. In the following subsections, a detailed discussion of agent types and their
applications are presented.

5.1. Air Gasification (insert the summary of last three researches related to this title done in 2016, 2017 and 2018)
Gasification process using air as a gasifying agent is most straightforward and widely used method. Excess char formed
by the pyrolysis process within the gasifier is burned with a limited supply of air (usually at ER=0.25). The syngas has a
lower value of the heating value due to the dilution effect of nitrogen from the air. The heating value of the produced
syngas is in the range of 3.5-7.8MJ/Nm3, which makes it suitable for boiler and engine applications. Due to its simplicity,
air gasification technology is being studied by many researchers for various types of biomasses. Because air is the gasifying
agent, the reactor temperature is dependent on the air flow rate and/or biomass feed rate. Shallow inlet air to the system
results in superficial bed temperature, which produces lower syngas, and higher tar content. Groves et al. [47] tested the
fluidized bed air partial oxidation Figure (2.3) Tar
of the cotton ginconcentration at different equivalence
trash within temperatures range of 922K and 1144K in a 0.3m internal
diameter reactor. The syngas yield heating valueratio values from
increased at 800°C
3.4 [70]
to 4.3MJ/m3. Lian and Findley [34] studied the oak
sawdust gasification with air as agent; the results showed that the tar and char yields decreased as temperature increased by
6%. The researchers tested the effect of ER on the gasifier performance and concluded that the total carbon/nitrogen ratio in
the dry syngas gave the better correlation with the concentrations of carbon, hydrogen and the higher heating value.
Walawender and Fan [50] studied the air gasification of feedlot manure, were observed that the syngas yield, heating value,

11
and energy recovery increased by 131%, 77%, and 244%, respectively; when the temperature increased from 900K to
1100K.

Ergudenler [75] tested the effect of ER on the syngas quality and its flow rate in the air as an agent of wheat straw through
a fluidized bed gasifier. The results showed that, at ER of 0.25, the mole fraction of the combustible syngas achieved their
maximum. Cao et al. [91] studied the sawdust gasification in a fluidized bed using air. They combined two individual zones
of pyrolysis, gasification, and combustion of biomass fuel in one gasifier. The primary air stream and the biomass fuel were
interfering to the gasifier from the bottom and the top, respectively. Secondary air was injected into the upper zone of the
reactor to maintain a high temperature. The study indicated that under optimum operating conditions; syngas produced at a
rate of 3.0 Nm3/kg and heating value at about 5.0 MJ/Nm3. The concentration of carbon monoxide, hydrogen, and methane
in the fuel gas produced were 9.25%, 9.27%, and 4.21%, respectively.

5.2. Steam Gasification (insert the summary of last three researches related to this title done in 2016, 2017 and 2018)
Steam gasification requires an external heat source mainly when used as a gasifying agent, unlike of air gasification.
Using a mixture of steam/air as a gasifying agent is not unique technology; in fact, this mixture has been studied by many
researchers. The oxygen present in the air helps to provide the required energy due to the exothermic nature of biomass fuel
burning. Also, the high temperature assists the devolatilization process of biomass to produce various gases. Steam reacts
with carbon monoxide to produce hydrogen and carbon dioxide, this reaction namely water-gas-shift reaction. By
comparison to air gasification, steam gasification generates a higher energy content producer syngas.

Boateng et al. [77] studied the effects of reactor temperature and SFR on product syngas composition and heating value.
The syngas yield contains a high concentration of hydrogen; have heating value ranged from 11.1MJ/Nm3 at 700C to
12.1MJ/Nm3 at 800C. Also, energy recovery varied from 35%-59% within the same temperature range. Walwender et al.
[50] studied corn grain-dust gasification in a 0.05m internal diameter fluidized bed gasifier by steam as agent and a mixture
of sand/limestone as the bed material. The produced syngas increased from 0.13m3/kg at 867K to 0.73m3/kg at 1033K.
The syngas heating value rose from 9.4MJ/Nm3 to 11.5MJ/Nm3 in the examined temperature range. The main components
of the syngas were H2, CO2, CO, and CH4 and the syngas flow rate yield was 0.5-1.4 m3/kg, heating value was
11.8MJ/m3and the carbon conversion within range of 32-90%.

According to Slapak et al. [78], steam gasification is one of the possibilities for recycling waste in fluidized bed reactor.
The primary product is syngas, applicable for energy recovery. The produced syngas has a heat value of 8.6MJ/Nm3.
Mermoud et al. [79] perform experimental gasification of charcoal using steam of beech charcoal spheres of different
diameters (10-30mm) at different temperatures (830C-1030C). The results showed that, a prolonged reaction at 830C. A
difference in gasification rate as high 6.5 to 1 was observed between temperatures at 1030C and 830C. Experiments carried
out with mixtures of H2O/N2 at 10%, 20%, and 40% mole of steam proven that, oxidant partial pressure influences
gasification. A gasification rate of 1.9 was gain for H2O partial pressure varying from 0.4 to 0.1 bar.

Corella et al. [73] observed on steam gasification of four different biomasses (wood chips, thistle, sawdust, and straw) in a
0.15m internal diameter fluidized bed gasifier. They determined the syngas produced, char and tar yield at temperatures
ranged from 650C-780C for each type of biomasses. Straw and sawdust showed higher syngas and lower tar yields
compared to wood chips and thistle. Wei et al. [80] tested the legume straw and sawdust pellets as biomass in downdraft
gasifier by varying the operating conditions during experiments; the examined factors affecting the hydrogen
concentrations are detected.

5.3. Oxygen Gasification (insert the summary of last 3 researches related to this title done in 2016, 2017 and 2018)
If the amount of nitrogen supplied to the gasification process is restricted, the product syngas will not contain nitrogen
thus, has average energy content (about 12-21MJ/Nm3). Such the syngas can be distributed in pipeline network systems at
lower cost, therefore, be appropriately used for process heat or possibly as synthesis gas fuels. In this case, oxygen plant or
nearer sources of oxygen are required, this maybe elevates the initial cost needed for the plant installation.

Bailie [81] recoding the relationship between the heating value of a syngas and oxygen concentrations in the agent. At
oxygen concentration of 20% and 100%, the heating values of the syngas yield were 6 and 11.2 MJ/Nm3, respectively. At
the level of oxygen in the agent increased from 20% to 100%, the methane, hydrogen, and carbon monoxide mole fraction
increased from 4 to 6%, from 13 to 19% and from 25 to 55%, respectively. Tillman [82] studied the municipal solid waste
gasification using oxygen. The biomass fuel (shredded and magnetically sorted) was fed into the top of the gasifier, and the
oxygen was supplied at the bottom. Char was burned by the assist of oxygen at the bottom of the gasifier to produce

12
enough energy at temperatures in the range of 1593C-1704C and to yield a molten slag from all noncombustible materials.
The maximum concentrations of the produced syngas for CO, H2, CO2, and CH4 detected were 44%, 31%, 13% and 4%,
respectively. The maximum low heating value was 10.6MJ/Nm3.

Watkinson et al. [83] gasified coal to study the effect of an oxygen/steam mixture on carbon conversion and syngas low
heating value during the gasification in a spouted bed. They record that, by increasing the ratio of oxygen/biomass ratio
from 0.5 to 1.1kg/kg, the low heating value of the syngas yield increased sharply from 5MJ/kg to 16MJ/kg. Biogas dry
reforming is a catalytic process that operates at 500C-800C to produce gas that simultaneously consumes two main
greenhouse gases, CH4 and CO2, therefore supporting environmental preservation efforts [84]. The ratio of the generated
H2/CO is unity in stoichiometric reaction, which is suitable for further use in hydro-formulation and carboxylation process
to synthesize liquid fuels [85]. Therefore, dry reforming has become a better option for renewable biogas energy, although,
several problems might prevent its further industrial application. The primary attention is energy consumption [86].
Because dry biogas reforming is a highly endothermic reaction, for the dynamic thermal significance, a higher temperature
is necessary to gain significant conversions. Another attention is the active catalyst suppression, mainly caused by the
carbon sedimentation and active metal sintering [87], mainly when using base catalysts, which will also increase the
operating cost of biogas reforming process. Finally, biogas is consisting of some gas species, and the effects of components
besides CH4 and CO2, such as O2 have not been enough study on biogas reforming. Studies have observed that two
primary reactions can be caused by carbon formation which including of CH4 decomposition and CO disproportionation
[88]. On another hand, a reaction that often occurs during dry biogas reforming is reverse water gas shift (RWGS) reaction
which consumes more H2 and produces more CO. Therefore, the decreasing in the H2/CO ratio occurs [89]. In the
presence of oxygen, partial CH4 oxidation and total CH4 oxidation, carbon oxidation, CO oxidation and H2 oxidation may
occur, where carbon gasification can help to remove carbon sedimentation.

The process of combining of dry and partial oxidative reforming of methane, also known as auto-thermal reforming of
methane, have been recorded to be useful in recover energy consumption of dry reforming and adjusting the H2/CO ratio in
syngas yield. The use of pure oxygen and steam required a significant amount of input energy and expensive initial
investments [90]. Butterman and Castaldi studied the production of more active chars formed by the heat treatment of
woody biomass as fuel with CO2, compared to those developed using steam [90]. Seiler et al. suggested that it’s possible to
modify the economics of the biomass to liquid process through the sale of N2 which formed as a by-product from the
production of oxygen enriched air [90]. The gasification process with oxygen enriched air and CO2 is considered as one of
the most favorable. In this case, it’s needed to enhance the boundary reaction (C+CO2→2CO) using biomass chars
produced under an N2/CO2/O2 atmosphere with CO2. If a CO-rich gas is yields during the gasification process, so, the
feed gas with desirable H2/CO ratio appropriate for the Fischer-Tropsch synthesis reaction can be results without new H2
by promoting the water gas shift reaction (CO+H2O → CO2+H2) in the downstream process at a lower temperature
compared to that in the gasification step. In practice, CO2 gasification of biomass char produced under an N2/CO2/O2
atmosphere has not been fully studied [92].

5.4. Hydrogen Gasification (insert the summary of last 3 researches related to this title done in 2016, 2017 and 2018)

Through this method of gasification, the biomass fuel is converted to gaseous fuels in the presence of hydrogen under
high pressures. This gasification process is critical that strict reaction conditions are maintained since the majority of the
products usually are in the gaseous phase. This process is undesirable, because of the strongly of control which necessary
as well as the fact that hydrogen must be readily obtainable. Weil et al. [93] used preheated hydrogen mixed with peat at
the entrance of fluidized bed gasifier. The reactor was operated as an entrained flow reactor in an isothermal or a constant
heat up mode. They reported that increase in temperature from 426C to 760C would increase the carbon monoxide and
hydrocarbon gases from 8% to 18% and 41% to 63%, respectively.

5.5. Carbon Dioxide Gasification (insert the summary of last three types of research related to this title done in 2016,
2017 and 2018)
Gasification is a thermo-chemical process, in which a solid or liquid fuel is converted at higher temperatures and in the
presence of gasification agent (air, H2O, O2 or CO2) to a mixture of combustible gases (H2, CO, CH4 and others minor
gases), accompanying gases (CO2, N2) and undesired components (tars, dust and others). Several attempts have been made
for the production of carbon monoxide from biomass with CO2 as the gasifying agent as follows.

Zhang, et al. [94] studied the effect of the pyrolysis mediums composition in the biomass fluidized bed gasification process
at 550C. their pyrolysis mediums were CO2, air, CO, H2, and CH4. The authors report that the liquid yield, gas

13
composition, and syngas heating value depend on the gasification agent composition. The gasification with CO2 at
atmospheric pressure was producing less char than in the other agent. In case of CO2 as an agent, the CO2 in gas yield
decreased compared to the CO2 yield obtained in air. Furthermore, the CO2 led to the highest yield of acetic acid compared
to the other atmospheres. Zhang et al. illustrate these observations by two possible mechanisms; the CO2 reacts with the
active volatiles or with the char. newly, in their study on macro-algae pyrolysis/gasification, Kwon et al. [95] observed that
uses of CO2 in the pyrolysis process at 550C in a cylindrical gasifier minimized of the generation of pyrolysis oil. They
detected that, a decrease in the pyrolytic oil by the ratio of 24.3% and promoted pyrolysis syngas production. This
observation means that hydrocarbons yields from the pyrolysis process can be decomposed in the presence of CO2. The
same authors did another study about the pyrolysis of styrene butadiene rubber [96] they reported that the CO2 increases
C4 hydrocarbons cracking in addition to impeding the gas phase addition reaction by which is formed benzene. Their
pyrolysis experiments performed at 650 C in a free and CO2/ air mixture. The authors showed that the number of
condensable hydrocarbons decreased by the ratio of 30 to 50% when adding CO2 to a modification of the syngas. The
authors studied the effect of CO2 on a volatile matter during the thermal dissolution of cellulose [96] at low and high
heating rates of 500C/min. The authors reported that there is a substantial increase in H2 and CO production in the presence
of CO2. The authors performed experimental tests on cellulose gasification in a cylindrical gasifier. The concentrations of
H2, CO, and CH4 in the presence of CO2 increased by ratios of 4%, 10%, and 7%, respectively.

As the boundary reaction is only favorable at temperatures values higher than 700C, this increase of gaseous compounds is
related to the fact that CO2 likely accelerate the thermal cracking of volatile compounds release biomass fuel.
Approximately 67% decreased the condensable hydrocarbons collected during the experiments based on the mass balance.
The author evidenced that the primary influence of CO2 during biomass thermal degradation is perceived in the gas phase
was this latter found to participate in the hydrocarbons cracking leading to more permanent gases. The same results were
found using real biomass from corn stover. The authors did not observe noticeable differences in the thermo-gravimetric
analysis of the different biomasses degradation under CO2 and N2 respectively.

Kwon, et al. [96] performed macro-algae gasification tests with steam and variable concentrations of CO2 in a tubular
reactor with temperatures ranging from 600C to 1000C. The authors observed that the CO concentration increased by a
factor of 2 even at 600C and 700C with a CO2 level of 30% compared to a reference state of a CO2-free atmosphere. The
yield of C2 hydrocarbons also increased, and the amount of tar was reduced by 51.2% which can be directly correlated with
the gas yield augmentation. In their paper on CO2 as a carbon neutral fuel source via enhanced biomass gasification, the
authors studied the gasification of several kinds of wood, grasses, and agricultural residues with steam and CO2 via
thermo-gravimetric analysis and gas chromatography. The CO2 concentrations were varied between 0% and 100% with
steam as a co-reactant. The authors observed that, when only injecting 5% of CO2 with steam, CO concentration increased
by the ratio of 10% and H2 decreased by the rate of 3.3% at 900C.

Rising of CO2 from 5% to 50% resulted in continued CO increases and H2 decrease by the ratio of 3% at 900 C. A high
CO2 fraction in gasifying agent resulted in low H2 yield and high CO yield. The CO2 can potentially react in the gas phase
with hydrocarbons, such as methane, through dry reforming reaction. The CO2 can also react with hydrogen atoms
according to the reverse water gas shift reaction (RWGS). As well, in biomass gasifier; CO2 can react with the carbon in
the char form after the pyrolysis step through the heterogeneous boundary reaction. The primary results of these reactions
are the enhanced CO production. The authors’ conclusion, the introduction of CO2 decrease the energy yield of the
producer syngas, but on the other hand, reduce the preheating energy of gasifying agent resulting in a higher thermal
efficiency under testing conditions ranges. The most elevated thermal efficiency of the process without O2 was 52% under
N2 (40%), CO2 (60 %) at 850C. For the operation using O2, where the part of gasifying agent preheating energy supplied
by the partial biomass combustion the higher thermal efficiency was 60% under the CO2 (60%), O2 (8.3 %) and N2
(31.7%) atmosphere at 950C.

Beside excellent thermal efficiency, the authors discussed if the use of CO2 in biomass gasifies as an agent can provide an
N2-free syngas which is more suitable for the synthesis of liquid fuels and chemicals. M. Pohorel et al. [92] found that the
use of CO2 as an agent in the process of catalytic biomass gasification had a significant effect on the conversion of biomass
fuel into gaseous and on decreasing the tar yields. The authors performed their study on beech wood pellets in a fluidized
bed gasifier. The bed material was dolomitic limestone preheated to 500C with the gasification agent. The gasification
reactions were performed at 850C, respectively in H2O/O2, air/O2 and CO2/O2/air mixtures. The higher cold gas
efficiency was observed when gasifying biomass with a CO2 containing air.

14
6. Factors Affecting Gasification Performance (insert the summary of last 3 researches related to this title done in
2016, 2017 and 2018)
Many variables appear effect on the gasification performance, syngas composition and tar yields, including bed pressure,
bed temperature, fluidization velocity, bed height, gasifying medium, ER, feed material moisture content, biomass particle
size, SFR and presence of catalysts. In the following, a brief discussion about more factors influencing the gasification
process.

6.1. Effect of Gasification Temperature

The gasifier bed temperature affects all of the chemical reactions occurs in the gasification process. In an experiment
conducted by Narvaez et al. [71], an increase in gasifier bed temperature from 700C to 850C, at a constant ER of 0.30, the
composition of the syngas produced was changed as; H2 concentration increased from 5% to 10%, CO increased from 12%
to 18%, CO2 decreased from 16% to 14%. Methane and C2H2 decreased minimally. The results from a similar experiment
conducted by Radmanesh et al. [98] validated the conclusions drawn from Narvaez et al. [71] in that when the temperature
of the gasifier bed is increased, and the ER is held constant, the concentrations of H2 and CO in the syngas increased.
From Petersen and Werther’s [99] study on a circulating fluidized bed gasifier using sewage sludge as the fuel, a significant
change in syngas composition was observed with an increase in bed temperature from 530C to 730C at an ER of 0.30. The
H2 to CO ratio doubled over this temperature range. In a sewage sludge gasification experiment from de Andrés et al.
[100], it was shown that higher bed temperatures favor hydrogen production and that the concentration of CH4 increases
slightly with increasing bed temperatures. Higher temperatures produce more intense volatilizations and cracking reactions
instead of creating more intense reforming reactions. Thus at higher temperatures, there is an increase in H2 and light
hydrocarbons in the syngas [100]. This pattern is shown for temperature ranges of 750C-850C at constant ERs of 0.20,
0.30, and 0.40. Tar content also decreases as temperature increases. These changes in composition affect the overall heating
value and quality of the gas. An increase in bed temperature, up to a point, increases the heating value and decreases the tar
content which makes for a better quality syngas. Note that increased bed temperature could have positive effects on the
syngas quality (lower tar content) but could also be detrimental to the amount of energy produced (lower heating value).
Experimental literature has shown that increases in temperature in sewage sludge gasifier-bed improve the heating value of
the gas until the temperature reaches 1470°F for a bubbling fluidized bed gasifier and 1340°F for a circulating fluidized bed
gasifier [99, 94].

The FLETGAS system used in the laboratory experiment conducted by Gómez-Barea, et al. [101] consisted of a (1)
devolatilizer, a (2) reformer, and (3) a moving bed. The devolatilizer is where most of the volatile gases are released from
the biomass, and was operated at temperatures in the range of 700C-750C. These are high enough to release the volatile
gases from the sewage sludge but not high enough to cause any tar cracking, so a significant amount of tar was released
from the fuel in the devolatilizer. The tar and syngas were directed to another stage downstream of the volatilization zone
called the reformer. Oxygen-enriched air (40%O2) and high-temperature steam were blown into the reformer to reduce the
number of tars significantly by raising the temperature in the reformer to 1200C which partially combusted the tars. The
solids (char and ash) produced in the fluidized bed were then transferred to the third stage, the moving bed. The reformer
gases flowed into and transferred heat to the moving bed. The bed acted as a catalytic filter in promoting tar decomposition
reactions while steam introduced into the bed supported endothermic char gasification reactions. Finally, the ashes exited
the bottom of the moving bed containing very little carbon and the syngas exited near the bottom of the moving bed
containing negligible amounts of tar due to the two reduction steps encountered in the process. The final stage of the
system, the moving bed, cooled the ash and syngas streams which increased the chemical energy of the syngas and thus the
overall gasification efficiency.

6.3. Effect of Equivalence Ratio

The equivalence ratio (ER) is defined as; the ratio between the air flow rate introduced into the gasifier and
stoichiometric air flow rate that required for complete combustion/pyrolysis of the biomass fuel. It has also been stated in
the literature that it is one of the most significant operating conditions in biomass gasification process [71, 99].
O 2 Supplie d
ER= (2.15)
Theor .O2 Required
The O2 Supplied is the oxygen mass flow rate introduced into the gasifier (kg/hr.). Theoretical O2 required is the
notional amount of oxygen needed for complete combustion/pyrolysis of biomass (kg/hr.). ER dramatically affects the
syngas composition. As ER increases, the concentrations of combustible gases (H2, CO, CH4, and tars) decrease while the
levels of CO2 and H2O increase. Increases in ER provide a significant amount of Oxygen to the gasifier which then goes to
oxidize CO, H2, CH4, and tars. The methanation and oxidation reactions use O2 to oxidize CH4 to CO and H2; hydrogen

15
is oxidized to H2O. This trend can be seen in the literature over temperature ranges of 700-850C for both biomass and
sewage sludge fuel [71, 99]. Recommendations in the literature for an optimal ER vary based on feedstock and type of
gasifier. In Petersen and Werther’s [99] experiment using sewage sludge as a fuel and a circulating bed gasifier, the optimal
ER was found to be 0.30.

Narvaez et al. [71] recommend values between 0.18 and 0.45 for the ER in their experiment using biomass as a fuel and a
bubbling fluidized bed gasifier. A lower ER is not practical because too much tar is produced and a higher ER produces
syngas with a little heating value. In Manyà, et al. [102] experiments with a dried sewage sludge fed fluidized bed gasifier,
an optimal ER was found at 0.35; the optimum qualification was determined by the highest concentration of H2 in the
syngas. Also, a high degree of combustion occurs at a higher value of ER; which supplies more amount of air into the
gasifier and promote char burning to produce more CO2 instead of combustible gases as H2, CO, and CH4. Also, the ER
increase results in a decrease in the low heating value (LHV) of the syngas because it hinders the CH4 production other
light hydrocarbons which have relatively large LHV. In additional to, at higher ER, extra nitrogen provided by air dilutes
the producer syngas which in turn results in its low energy content [102]. Studies have shown that too small ER is also
hostile to biomass gasification as it lowers the reaction temperature [71]. Therefore, an optimum value for ER in biomass
gasification exists in the range of 0.2-0.4 which differs according to various operating parameters. Selection of the suitable
ER is somehow depended on the producer gas subsequent application. When the raw producer gas is going to be burnt in
downstream furnaces, tar is not a severe issue. Also, the gas should have a high heating value, and therefore the gasifier can
be operated at the minimum ER of about 0.2. In the case of temperatures lower than 850C, tar yield is high, and ER should
be increased to about 0.3-0.4 to compensate such adverse effects. Lv et al. [72] studied the effect of ER on gas yield and
LHV. They varied the ER from 0.19 to 0.27 and realized that the variation of ER could be divided into two stages of 0.19-
0.23 and 0.23-0.27. In the first stage, the gas yield increased from 2.13 to 2.37 m3/kg, and the gas LHV rose from 8.82 to
8.84MJ/Nm3. It was observed that in the second stage, the LHV and gas yield decreased due to the improvement of the
oxidation reactions which also reduced the concentration of CO, CH4, and CnHm and increased the CO2 level. So, the
value of 0.23 was selected as the optimum ER. In another set of experiments conducted by Narvaez et al. [71], ER was
varied in the range of 0.25-0.45 to find the optimum ER. It was observed that increasing the ER reduced the amount of H2,
CO, CH4, and C2H2. The maximum H2 concentration of 10% was obtained at ER of 0.26. They also realized that while
the ER was increased the tar content of the producer gas was gradually decreased and at ER of 0.45, minimum tar
concentration of 27g/m3 was achieved. They obtained LHV of 5.2-7MJ/m3 and 3.5-4.5MJ/m3 at ERs of 0.25 and 0.45,
respectively. It was also concluded that the gas yield was in a direct relationship with ER.

Similar trends were obtained by Li et al. [94] who investigated the co-gasification of biomass and coal while the ER was in
the range of 0.31-0.47. They also explained that as ER increased, more oxygen was introduced into the gasifier which
enhanced the combustion and increased the bed temperature from 948 to 1026C. Skoulou et al. [105] also studied the effect
of ER variation (0.2-0.4) as one of the most critical operation parameters on the quality of the producer gas. They reported
the favored concentration of CO at low ER of 0.2 and its hindered production at ER of 0.4 because of complete oxidation of
carbon to CO2. Also, H2 production peaked at ER of 0.2. Lower heating value of the producer gas was obtained at high ER
which was due to the promotion of the oxidation reaction and dilution of the producer gas with N2.

6.4. Effect of Steam to Fuel Ratio (insert the summary of last 3 researches related to this title done in 2016, 2017 and
2018)
Steam to fuel ratio (SFR) is defined as the flow rate of the steam fed into the gasifier divided by the biomass flow rate is
one of the critical process parameters involved in steam gasification [75]. An experimental study on biomass air-steam
gasification was conducted by Lv et al. [72]. They investigated the effect of SFR on the quality of the producer gas in the
range of 0-4.4. It was observed that the introduction of steam to the system improved the gas yield, LHV and carbon
conversion efficiency. They reported the SFR range of 1.35-4 as the optimum SFR in which the CO, CH4 and C2H2
content of the producer syngas decreased, whereas the CO2 and H2 concentration gradually increased. It was explained that
in this SFR range, more steam reforming reactions of CO, CH4, and C2H2 occurred in the presence of vapor which
resulted in high concentrations of H2 and CO2. Over the optimum range, a decreasing trend was observed in the syngas
yield, LHV and carbon conversion efficiency due to the low reaction temperature affected by low-temperature steam.

Qin et al. [104] examined the effect of SFR on tar formation and the resultant tar properties. In their experiments, SFR was
varied in the range of 0.49-2.66 at 900C. The results showed that, as the SFR was increased, the tar yield slowly decreased
from 3.87 to 1.71%. It was also concluded that high SFR values lower the aromaticity of the tar contents. Another set of
experiments was conducted by Gil et al. [106] who studied the effect of steam/oxygen gasification on product
concentrations. Their results showed steam to oxygen ratio, and steam/oxygen to biomass ratios was varied in the range of

16
2-3 mol/mol and 0.6-1.6 kg/kg, respectively. The achieved results shown that the H2 content of the syngas was in the field
of 14-30% and decreased as the steam/oxygen to biomass ratio was increased, or the steam/oxygen ratio was steadily
reduced. As the O2 introduced into the system was improved, more H2 was combusted in the gasifier and less was found in
the discharge stream. A similar trend was observed for CO while changing the distinct ratios and its concentration in the
producer syngas was seen in the range of 30-50%. The tar content of the syngas naturally reduced to less than 10 gram /m3
as the steam/oxygen to biomass ratio was increased to 1.0-1.1kg/kg. The char produce also decreased to 10% while the
gasifying agent to biomass ratio was expanded to the values higher than 1.0.

As mentioned previous, steam gasification can provide a gas stream with a high content of H2, but the concentration of the
unattractive products, for example, CO2 is also increased. Therefore, to improve the efficiency of the steam gasification
process, considerable efforts have been dedicated to the production of producer gas with a high yield of H2 with the
instantaneous capture of CO2.

6.5. Effect of Biomass Size (insert the summary of last 3 researches related to this title done in 2016, 2017 and 2018)
It has been known that small particle size biomass significantly increases the overall energy efficiency of the gasification
process, but it also increases the gasification plant cost. It has been estimated that for a 5-10MW gasification plant, about
10% of the output energy is required for the biomass particle size reduction [107]. On the other side, an increase in biomass
particle size reduces the pretreatment costs, but the devolatilization time increases, and thus for a defined throughput the
gasifier size increases [107]. Hence, a balance should be considered while investigating the effect of biomass particle size
on the gasification efficiency. Lv et al. [39] studied the impact of biomass size on the quality of the producer syngas in four
ranges as of 0.6-0.9, 0.45-0.6, 0.3-0.45 and 0.2-0.3mm. They detected that small particle size biomass created more
amounts of CH4, CO and C2H4 and less CO2 in comparison to large particles. Consequently, the producer syngas yield,
LHV, and carbon conversion were enhanced as the biomass particle size decreased. It was clarified that small biomass
particles contribute to the large surface area and high heating rate which in turn produce more light gases and less char and
condensate. Thus, the flow rate and composition of the producer syngas improved while using the small particle biomass.

6.6. Effect of Bed Materials (Catalyst) (insert the summary of last 3 researches related to this title done in 2016, 2017
and 2018)
Bed materials are of great significance in fluidized bed gasifiers. They turn as heat transfer medium, but their crucial role
involves in tar cracking which avoids complex downstream tar removal process [108]. The presence of a catalyst in the bed
material during biomass gasification promotes several chemical reactions which influence the composition and heating
value of the producer gas. It also reduces the tar yield and prevents stable agglomeration tendency of the bed [89]. The
catalytic reforming reactions through which tar is converted into valuable gaseous compounds are summarized as follows
[89]:
CnHm+ nH2O ↔ (n+0.5 m) H2+ n CO
CnHm+ nCO2 ↔ 0.5m H2+2n CO
CnHm+ (0.5n+0.25m) O2 ↔ 0.5m H2O+ n CO
CnHm↔ 0.5m H2+ n C
The three main groups of catalysts are implemented to eliminate tar from the producer syngas [108]: (1) natural
catalysts such as dolomite and olivine; (2) alkali-based catalysts such as (Li, Na, K, Rb, Cs and Fr) and (3) metal-based
catalyst such as nickel catalysts. Dolomite is the most usually used catalyst which effectively removes heavy hydrocarbons
from the syngas stream [89,108,109]. It also decreases accumulation in fluidized bed while using biomass with high alkali
content. But, the undesired property of dolomite is its rapid calcination in the gasifier which subsequently results in a
syngas with high particulate. Alkali-based catalysts (Li, Na, K, Rb, Cs, and Fr) are capable of increasing the gasification
rate and decreasing the tar content of the producer syngas. However, effort in recovery, high cost, and accumulation at high
temperatures are some of the difficulties of the alkali-based catalysts [109]. Metal-based catalysts are also significantly
effective in eliminating tar and increase the superiority of the producer syngas. The main challenges associated with this
type of catalysts are carbon deposition and nickel particle growth, which cause catalyst deactivation [108].

Asadullah et al. [108] studied the performance of the heterogeneous catalyst of Rh/CeO2/SiO2 in fluidized bed gasification
to that of dolomite, steam reforming catalyst and inert bed materials whereas the ER was set at 0.31 and the bed
temperature within the range of 823K-973K. It was detected that the tar content of the producer was utterly negligible while
using Rh/CeO2/SiO2 as the bed material. However, the tar concentration at about of 113 and 139gram/m3 was achieved
with dolomite and inert bed materials, respectively. It was also concluded that in the case of Rh/CeO2/SiO2 catalysts the
efficiency of cold gas was about 71% more than others cases. Also, little char and coke were observed in the experiments

17
with the Rh/CeO2/SiO2 as catalysts. Skoulou et al. [105] used quartz sand and olivine as bed materials in a bubbling
fluidized bed gasifier at ER of 0.2-0.4 and bed temperature as 750C-850C. They reported that, while quartz sand is an
economy and abundant element, it produced severe de-fluidization due to its tendency to tar formation at temperatures
below 800C. They replaced quartz sand with olivine and observed that, at low gasification temperature of 750C and ER of
0.2, components of tar were pyrolysis and released H2 and CO, under the catalytic effect of iron-based olivine.

Li et al. [94] examined the influence of bed material on tar elimination efficiency in a circulating fluidized bed. They used
silica sand and a commercial Ni-alumina catalyst as bed material. At the bed temperature of 800C, the quantity of tar
reduced from 0.4 gram/m3 to 0.15 mg/m3, as silica sand was substituted with Ni-alumina catalyst.

7. Modeling of Biomass Gasification Process (insert the summary of last 3 researches related to this title done in
2016, 2017 and 2018)
The primary goals of models are to study the thermo-chemical processes during the gasification of the biomass and to
evaluate the influence of the primary input variables, such as steam to fuel ratio, equivalence ratio and gasification
temperature on syngas composition and its heating value. In the present section, the literature survey has been carried out
for mathematical models, ASPEN PLUS models and neural network models.

7.1. Simulation and Mathematical Modeling Studies

Mathematical simulation is one of the most critical aspects of research and development work such as the development
of gasification technology. Though it may not provide a very accurate prediction of systems performance, it may provide
qualitative guidance on the effect of design, input variables and operating conditions. Moreover, modeling may provide a
less expensive means of evaluating the benefits and the associated risk in the real-time scenario [110]. Gasifier simulation
models may be classified into thermodynamic equilibrium model, kinetic model, computational fluid dynamics (CFD)
model and artificial neural network. All these models approach different methods to assess the prediction of the one’s
model and have separate utility and limitations. However, modeling of gasification using different approach may consider
the Boudouard reaction following reaction as necessary gasification reaction [112, 113].

Boudouard reaction (R1) C+CO2  2CO (2.1)

Water-gas reaction (R2) C+H2O H2+CO (2.2)
Methane formation (R3) C+2H2  CH4 (2.3)
Steam reforming reaction (R4): CH4 +2H2O  4H2 + CO2 (2.4)
7.1.1. Thermodynamic Equilibrium Model [114]
Thermodynamic equilibrium models are based on the chemical and thermodynamic equilibrium, which is determined by
implication of equilibrium constants and minimization of Gibbs free energy. At chemical equilibrium, the system is
considered to be at its most stable composition, which means the entropy of a system is maximized, while its Gibbs free
energy is minimized. Though chemical or thermodynamic equilibrium may not be reached within the gasifier, equilibrium
models provide a designer with a reasonable prediction for the final composition and monitor the process parameter like
temperature [112]. Some significant assumptions of thermodynamic equilibrium are presented below:

1. The reactor is considered as zero-dimensional [115]

2. There is perfect mixing of materials and uniform temperature in the gasifier although different hydrodynamics are
observed in practice [112]
3. The reaction rates are fast enough and residence time is long enough to reach the equilibrium state [112].

Equilibrium models are independent of gasifier design and cannot predict the influence of hydrodynamics or geometric
parameters like fluidizing velocity, design variables (gasifier height). However, these models are quite convenient to study
the impact of fuel and the process parameters and can predict the temperature of the system [111]. Thermodynamic
equilibrium models can be approached by either stoichiometric or non-stoichiometric methods as follows.

7.1.1.1. Stoichiometric Equilibrium Models [117]

Stoichiometric equilibrium models incorporate the thermodynamic and chemical equilibrium of chemical reactions and
the species involved. The model can be designed either for a global gasification reaction or can be divided into sub-model
for drying, pyrolysis, oxidation, and reduction.

7.1.1.1.1. Single step stoichiometric equilibrium model [116]

18
 This model embodies the several complex reactions of gasification into one general reaction as mentioned in Eq. (2.5). It
assumes that one mole of biomass CHxOy, based on a single atom of carbon that is being gasified with w mole of
water/steam in the presence of a mole of air as the following general equation.

(2.5)

In the above equation, w and a, are the variables and changed in order to get desired amount of product. There are six

unknowns are nc, , , and . Based on stoichiometric balance of carbon, hydrogen and oxygen,
following equations are obtained:

Carbon balance: (2.6)

Hydrogen balance: (2.7)

Oxygen balance: (2.8)

As Boudouard reaction, water-gas reaction, methane formation and steam reforming reaction are considered as the
major reaction of gasification, the equilibrium constants (k eq) for reactions R1, R2 R3 and R4 are given as Perry et al.,
[142].

(2.9)

(2.10)

(2.11)

(2.12)
If the gasification process is assumed to be adiabatic, then the energy balance of the gasification reaction results to a new
set of equation, which can determine the final temperature of the system [116].

(2.13)
Modifying Eq. (2.13) on the basis of Eq. (2.5), we get:

(2.14)
In this equation,h f , C p ,C , h vap represents heat of formation of corresponding chemical species, specific heat capacity and
0

enthalpy of vaporization of water respectively and ΔT=T gasification-Tambient refers to temperature difference between the
gasification temperature and the ambient or the initial temperature of biomass feedstock [110,114]. Single step
stoichiometric equilibrium model may be formulated by the application of the chemical equilibrium state and the reaction
stoichiometric condition.

7.1.1.1.2. Sub-models for stoichiometric equilibrium model [117]

This model incorporates modeling of separate sub-model for drying, pyrolysis, oxidation and reduction, the output
from one sub-model becomes input for the successive sub-model. This model has more utility than the single step
stoichiometric equilibrium model as the composition and temperature at different zone can be assessed with the aid of sub-
model. Several combinations of sub-models can be achieved and can be selected as per the requirement of the model and its
feasibility. For the sake of convenience and clarity of sub- model, sub-models for drying and pyrolysis, oxidation and
19
reduction zone have been proposed for the current paper. However, the modeling approach follows similar principle as that
of single step stoichiometric equilibrium model regarding the mathematical formulation. One of the uncertainties of such
sub-model lies in their assumption for final product. For example, the assumptions implied in pyrolysis sub-model indicate
that that the product composition mainly includes CO, CO 2 H2 ,H2O, CH4 and tar with higher concentration of lighter
component as in Eq. (2.14) [118]. The pyrolysis products undergo partial oxidation in presence of non-stoichiometric
oxygen supply, and the reaction in oxidation sub-model may be proposed as in Eq. (2.15) [117, 119].

7.1.1.2. Non-stoichiometric Equilibrium Model [120]

The non-stoichiometric equilibrium model is solely based on minimizing Gibbs free energy of the system and
there is not any specification for particular reaction mechanisms. However, moisture content and elemental composition of
the feed is needed which can be obtained from the ultimate analysis data of feed. Therefore, this method is particularly
suitable for fuels like biomass whose exact chemical formula is not distinctly known [110]. The Gibbs free energy, G total for
the gasification product which consists of N species (i=5…N) is represented as in Eq. (2.19) [110].

(2.19)
0
Where,ni ∆ G f ,i is the standard Gibbs energy of i species, R is gas constant. The solution of Eq. (2.19) for
unknown values of ni is approached to minimize Gtotal of the overall reaction considering the overall mass balance,
Though, non-stoichiometric equilibrium model does not specify the reaction path, type or chemical formula of the fuel, the
amount of total carbon obtained from the ultimate analysis must be equal to sum of total of all carbon distributed among the
gas mixtures (Eq. (2.20)) [120].

(2.20)
Where a i , j is the number of atoms of the j element and A j is the total number of atoms of jth element in reaction
mixture. The objective of this approach is to find the values of ni such that the Gtotal will be minimum. Lagrange multiplier
[120]. Thus, the Lagrange function (L) can be defined as:

(2.21)
Where λ is Lagrangian multipliers. The equilibrium is achieved when the partial derivatives of Lagrange function are zero.
i.e.,

(2.22)
Dividing Eq. (2.21) by RT and substituting the value of Gtotal from Eq. (2.19,) then taking its partial derivate results to Eq.

(2.23)
The standard Gibbs free energy of each chemical species can be obtained by subtracting the standard enthalpy form the
standard entropy multiplied by a specific temperature of the system as in Eq. (2.24) [110].

(2.24)
0
Where ∆ S is the standard entropy of i species According to first law of thermodynamics, the energy balance of the non-
f ,i
stoichiometric equilibrium model can be achieved by Eq. (2.25) [110, 120].

(2.25)
Thus, the final compositions of the product gas can be determined via non-stoichiometric equilibrium approach.
Moreover, this model gives the utility to examine the effect on product gas composition and temperature by changing the
20
moisture content and biomass feed. However, such models have plenty of limitations. Srinivas et al. [122] did
thermodynamic modeling studies to predict the performance characteristics of a rice husk based integrated gasification
combined cycle plant at the variable operating conditions of gasifier. They applied for wet fuel (fuel with moisture) for
predicting the gas composition, gas generation per kg of fuel, plant efficiency and power generation capacity, and NOx and
CO2 emissions.

7.2. ASPEN PLUS Gasification Models

ASPEN Plus is a problem-oriented input program that is used to facilitate the calculation of physical, chemical and
biological processes. It can be used to describe processes involving solids in addition to vapor and liquid streams. ASPEN
Plus makes the model creation and updating easier since small sections of complex and integrated the systems can be
created and tested as separate modules before they are combined. This process simulator is equipped with a large property
data bank containing the various stream properties required to model the material streams in a gasification plant, with an
allowance for the addition of in-house property data.

Mathieu and Dubuisson [123] modeled wood gasification in a fluidized bed using ASPEN Plus. The model was based on
the minimization of the Gibbs free energy, and the process was uncoupled in pyrolysis, combustion, Boudouard reaction
and gasification. They performed a sensitivity analysis and concluded that there is a critical air temperature above which
preheating is no longer efficient, that there is an optimum oxygen factor, that the oxygen enrichment of air plays an active
role under a particular value, and that the operating pressure has only a slightly positive effect on process efficiency. Mitta
et al. [124] modeled a fluidized-bed type gasification plant with air and steam using ASPEN Plus and validated their results
with the gasification pilot plant located at the Chemical Engineering Department of the Technical University of Catalonia.
Their gasification model was divided into three different stages: drying, devolatilization pyrolysis, and gasification-
combustion. Nikoo and Mahinpey [125] developed a model capable of predicting the steady-state performance of an
atmospheric fluidized-ban ed gasifier by considering the hydrodynamic and reaction kinetics simultaneously. They used
four ASPEN Plus reactor models and external FORTRAN subroutines for hydrodynamics and kinetics nested to simulate
the gasification process.

The ASPEN Plus yield reactor, “RYIELD,” was used to affect the decomposition of the feed. They validated their model
using different sets of operating conditions for a lab-scale pine gasifier with air and steam. Paviet et al. [128] proposed, a
thermochemical equilibrium model using ASPEN Plus for downdraft and staged gasifiers. They claimed that proposed
model is easy to build and predicted with accuracy the composition of the flaming pyrolysis gas and the producer gas. The
reaction temperature is the parameter that controls the whole gasification process. It influences the final producer gas
directly. The models developed in this work can be used as input parameters in the design of a gasifier. Concentrations
given by the flaming pyrolysis gas model can be used as input parameters to the char gasification model. Levels provided
by the producer gas model used as input data to the combustion model of an SI engine, to design or to predict the overall
performance, of a gasification unit. Naveed et al. [131] developed steady-state simulation model for gasification using
ASPEN Plus. The model can be used as a predictive tool for optimization of the gasifier performance. The authors of the
study modeled the gasifier in three stages. In first stage moisture content of biomass, the feed is reduced. In second stage
biomass is decomposed into its elements by specifying yield distribution. In third stage gasification reactions have been
modeled using Gibbs free energy minimization approach. The simulation results were compared with the experimental
results obtained through hybrid biomass gasifier. They concluded that higher temperature improves gasifier performance. It
increases production of carbon monoxide and hydrogen in syngas which ultimately results in higher heating value and gas
conversion efficiency. Equivalence ratio controls the production of syngas by controlling carbon conversion of fuel and
extent of gasification reactions.

Damaris et al. [132] carried out an assessment of combined heat and power (CHP) biomass bubbling fluidized bed
gasification unit coupled with an internal combustion engine (ICE) by using a comprehensive mathematical model based on
the ASPEN PLUS process simulator. The proposed model is based on a combination of modules that ASPEN PLUS
simulator provides representing the three steps of gasification process (drying, pyrolysis, and oxidation), gas cleaning and
the ICE. The model is also based on mass and energy balances and reaction kinetics.

7.3. Neural Network Models

Since the invention of the digital computer, researchers attempted to create machines which directly interact with the real
world without their intervention. In this sense, the Artificial Intelligence, in general, and particularly the Artificial Neural
Networks (ANN’s) represent an alternative for endowing to the computers one of the characteristics that make the

21
difference between humans and live beings, the intelligence. An artificial neural network is an abstract of a real nervous
system, and its study corresponds to growing interdisciplinary fields which consider the systems as adaptive, distributed
and mostly nonlinear, three of elements found in the real applications. Artificial neural networks (ANN) have been
extensively used in the fields of pattern recognition, signal processing, biomedical instrumentation, function approximation
and process simulation. The Neural Networks package supports different types of training or learning algorithms. Pavlas et
al. [134] have proposed a biomass gasification system for complex design interactions as many streams requiring heating
and cooling in the energy recovery. The conceptual understanding gained from the case study provides systematic design
guidelines for further process development and industrial implementation in practice.

Sipöcz et al. [135] present the development and validation of an ANN model of a CO2 capture plant. An evaluation of the
concept is made of the usefulness of the ANN model as well as a discussion of its feasibility for further integration into a
conventional heat and mass balance programme. It is shown that the trained ANN model can reproduce the results of a
rigorous process simulator in a fraction of the simulation time. A multilayer feed-forward form of Artificial Neural
Network was used to capture and model the non-linear relationship between inputs and outputs of the CO2 capture process.
They concluded that Artificial Neural Networks are found to be useful tools for predicting complex processes such as CO2
capture processes, which, when simulating the closed process network, yields a challenging solution pathway which is
computationally demanding and challenging to reproduce using traditional methods. The average value of the errors for the
prediction of specific re-boiler duty is well below 0.2%, and the maximum error does not exceed 3.1%. The prediction of
solvent rich load and amount CO2 captured are even better, with the maximum error below 2.8% and 0.17% respectively.
Puig et al. [136] applied Artificial neural networks (ANNs) used for modeling biomass gasification process in fluidized bed
reactors. Two architectures of ANNs models are presented; one for circulating fluidized bed gasifiers (CFB) and the other
for bubbling fluidized bed gasifiers (BFB). Both models determine the producer gas composition (CO, CO2, H2, and CH4)
and gas yield. Published experimental data from other authors has been used to train the ANNs. The obtained results show
that the percentage composition of the primary four gas species in producer gas (CO, CO2, H2, and CH4) and producer gas
yield for a biomass fluidized bed gasifier can be successfully predicted by applying neural networks. ANNs models use in
the input layer the biomass composition and few operating parameters, two neurons in the hidden layer and the
backpropagation algorithm. The results obtained by these ANNs show high agreement with published experimental data
used R2 > 0.98.

8. Summary of Literature Review

In this chapter, an overview of biomass gasification and research activities around the world, as well as literature study
regarding the central research questions related to biomass gasification has been performed. The literature review includes
fluidized bed and their type, an overview of biomass used in gasification, tar formation and tar reduction and tar measuring
techniques, the effect of the agent on gasification and factors affect of gasifiers performance and different gasification
modeling methods. Significant topics of biomass gasification are outlined to identify opportunities and potential challenges
facing biomass gasification. Particular attention is paid to review the different approaches to model biomass gasification
process, i.e., thermodynamic equilibrium models and other models. Observations and gaps have been listed at the end along
with the objectives for the present work. Several important points can be concluded from this literature study:

1. The biomass was observed to be sustainable energy resource for agriculture dependent economic countries.
2. Thermo-chemical conversion or gasification is a perspective and viable route to make use of biomass in the energy
generation in a decentralized manner.
3. Performance of gasification is dependent on reaction zone temperature, type of fuel and fuel prosperities, equivalence
ratio, steam to fuel ratio and agent type.
4. The gasification agent, strongly affecting on tar yield and syngas quality.
5. A few studies have been performed on a carbon dioxide and pure oxygen as gasification agent, also limited work is
reported on sawdust as biomass fuel.
6. More studies were conducted on tar minimization by different researchers. However, the formation and distribution
behavior of tar produced from the gasification of various biomass fuels still lacks in the literature. Despite the fact that an
experimental study of tar yield during biomass gasification is critical, the process could be time-consuming as well as a
challenge due to limitations and availabilities of tar measuring techniques. Thus, the thermodynamic equilibrium
simulations of the distribution performance of the variation of tar yields during biomass gasification will be helpful to
optimize gasifier operating conditions gas cleaning systems.
7. Using primary tar reduction methods is very attractive since working conditions have a significant influence tar yield and
formation. Therefore, to investigate the fate of tar during biomass gasification, tar measurement and analysis is primarily
required.

22
 8. There is a significant variation among kinetic parameters derived from combustion and gasification of different biomass
chars reported by various researchers depending on biomass fuel types and fuel properties.
9. A few researchers have developed thermodynamic based equilibrium and ANN-based models to predict the gas
composition and gasifier performance.
10. None of the researchers reported the development of GA found ANN models to predict producer gas composition from
downdraft gasifier.

Conclusion
Research and development on new process of gasification is directed towards production of hydrogen-rich gas from
biomass the parameters has affected on the gasification of biomass such as (temperature, gasifying agent/biomass ratio,
pressure) and of the materials, type of biomass, type of gasifier) on the performance of the gasification system was clarified
which allow to increase yield of hydrogen gas. Critical parameters for success are the feedstock properties and the
feedstock pretreatment. For the third world, the use of a robust and straightforward technology represented by gasification
can assist the development of the rural economies by providing the electricity produced from local sources of biomass.

References

[32] Elif Kirtay “Recencnt advance in production of hydrogen from biomass” Energy conversion and management, vol.52,
2011, pp.1778-1789.
[33] Maria Aznar P., Jose Corella, Jesus Delgado and Joaquin Lahoz, “Improved steam gasification of lingo-cellulosic residues
in a fluidized bed with commercial steam reforming catalysts” Ind. Eng. Chem. Res. vol. 32,1993.
[34] Lian, C.K. and M.E. Findley. “Air blown wood gasification in large fluidized bed reactor” Ind. Eng. Chem. Process Dec ,
vol.21, 1982, pp.699-701.
[35] FAO. Food and Agriculture Organization of the United Nation, 1986.
[36] Raman, K.P., W.P. Walawender, L.T. Fan and C.C. Chang. “Mathematical model for fluid bed gasification of biomass
material” Application to feedlot manure. Ind. Eng. Chem. Process Des, vol.20, 1981, pp.686-692.
[37] Keiichi T., M. Asadullah, T. K. Kunmori, “Syngas production by biomass gasification using Rh/CeO2/SiO2 catalysts and
fluidized bed reactor” Catalysis Today. vol.89, 2004. pp.389-403.
[38] Khater E. M. K., N. N. El-Ibiary, I. A. Khatab and M. A. Hamad “Gasification of rice hulls” Biomass and Bioenergy,
vol.3,1992, pp.329-333.
[39] Midilli A., M. Dogru, Gr. Howarth, T., Ayhan, “Hydrogen production from hazelnut shell by applying air blown
downdraft gasification technique” International Journal of hydrogen energy, vol.26, 2001, pp.29-37.
[40] Abdul Salam P., S.C.Bhattacharya “A comparative study of char coal gasification in two types of spouted bed reactors”
Energy, vol.31, 2006, pp.228-243.
[41] Chen, C.C. and M.H. Rei. “Gasification of rice husk” Presented at Bio-Energy, World Congress and Exposition, Atlanta,
GA, vol.80, April 21, 1980.
[42] Sadaka, S. S., A. E. Ghaly and M. A. Sabbah. “Two phase biomass air-steam gasification model for fluidized bed reactor:
Part I, II, III” Biomass and Bioenergy, vol.22, 2002, pp. 439-487.
[43] Risnes, H., J. Fjellerup, U. Henriksen, A. Moilanen, P. Norby, K. Papadakis, D. Posselt, L. Sorensen. “Calcium addition in
straw gasification” Fuel, vol.82, 2003, pp. 641-651.
[44] Walawender, W.P., S. Ganesan and L.T. Fan. “Steam gasification of manure in a fluid bed: Influence of limestone as a bed
additive” IGT Symposium on Energy from Biomass and Wastes V, Lake Buena Vista, FA, Jan. 26-30, 1981.
[45] Yin, X., C. Zhi, S. Zheng and Y. Chen. “Design and operation of a CFB gasification and power generation system for rice
husk Biomass” Bioenergy, vol.23, 2002, pp.181-187.
[46] Singh, S.K., W.P. Walawender, L.T. Fan and W.A. Geyer. “Steam gasification of cotton wood (branches) in a fluidized
bed” Wood Fiber Sci., vol.18, 1986, pp.327-344.
[47] Groves, J.D., J.D. Craig, W.A. Le Pori and R. G. Anthony. “Fluidized bed gasification of cotton gin waste” ASAE, 1979,
pp.79-4547.
[48] Epstein, E., H. Kosstrin and J. Alpert. “Potential energy production in rural communities from biomass and wastes using a
fluidized-bed pyrolysis system” IGT Symposium on Energy from Biomass and Wastes, Washington, D. C., Aug. 14-18,
1978.
[49] Wander, P., C. Altafini and R. Barreto. “Assessment of a small sawdust gasification unit” Biomass and Bioenergy , vol.27,
2004, pp.467-476.
[50] Hoveland, A.D., W.P. Walawender, L.T. Fan and F.S. Lai. “Steam gasification of grain dust in a fluidized bed reactor”
Transactions of ASAE, vol.25, 1982, pp.1074-1080.

23
[51] Sweeten, J., K. Annamalai, B. Thien, L. McDonald. “Co-firing of coal and cattle feedlot biomass (FB) fuels. Part I.
Feedlot biomass (cattle manure) fuel quality and characteristics” Fuel, vol.82, 2003, pp.1167-1182.
[52] Walawender, W.P., D.A. Hoveland and L.T. Fan. “Steam gasification of Alpha- cellulose in a fluid bed reactor” presented
at Fundamentals of Thermochemical Biomass Conversion Conference, Estes Park, CO, 1982.
[53] Halligan, J.E., K.L. Herzog and H. W. Parker. “Synthesis gas from bovine wastes” Ind. Eng. Chem. Process Des , vol.14,
1971, pp.64-69.
[54] Evans, R., Milne, T. “Chemistry of tar formation and maturation in the thermochemical conversion of biomass” Fuel and
Energy, vol.39, 1998, pp.197-198.
[55] Li, C., Suzuki, K. “Tar property, analysis, reforming mechanism and model for biomass gasification-An overview”
Renewable and Sustainable Energy Reviews, vol.13, 2009, pp.594-604.
[56] Li, J., Wang, Z., Yang, X., Hu, L., Liu, Y., Wang, C. “Evaluate the pyrolysis pathway of glycine and glycylglycine by TG-
FTIR” Analytical and Applied Pyrolysis, vol.80, 2007, pp. 247-253.
[57] Milne, T., Abatzoglou, N., Evans, R. “Biomass gasifier “tars”: their nature, formation, and conversion” National
Renewable Energy Laboratory, NREL/TP-570-25357, 1998.
[58] Boerrigter, H., Calis, H.P., Slort, D.J., Bodenstaff, H., Kaandorp, A.J., den Uil, H., Rabou, L.P.L.M. “Gas cleaning for
integrated biomass gasification (BG) and Fischer-Tropsch (FT) systems; experimental demonstration of two BG-FT
systems” ECN, 2004.
[59] Bui, T., Loof, R., Bhattacharya, S. “Multi-stage reactor for thermal gasification of wood” Energy, vol.19, 1994, pp.397-
404.
[60] Devi, L., Ptasinski, K.J., Janssen, F.J.J.G. “A review of the primary measures for tar elimination in biomass gasification
processes” Biomass and Bioenergy, vol.24, 2003, pp.125-140.
[61] Campoy, M., Go mez-Barea, A., Fuentes-Cano, D., Ollero, P. “Tar Reduction by Primary Measures in an Autothermal
Air-Blown Fluidized Bed Biomass Gasifier” Industrial & engineering chemistry research, vol.49, 2010, pp.11294-11301.
[62] Corella, J., Aznar, M.P., Gil, J., Caballero, M.A. “Biomass gasification in fluidized bed: where to locate the dolomite to
improve gasification” Energy & Fuels, vol.13, 1999, pp.1122-1127.
[63] Sutton, D., Kelleher, B., Ross, J.R.H. “Review of literature on catalysts for biomass gasification” Fuel Processing
Technology, vol.73, 2001, pp. 155-173.
[64] Neeft, J., Knoef, H., Nederland, S.E.C., Group, B.B.T. “Behavior of Tar in Biomass Gasification Systems: Tar Related
Problems and Their Solutions” Novem, 1999.
[65] Kinoshita, C., Wang, Y., Zhou, J. “Tar formation under different biomass gasification conditions” Analytical and Applied
Pyrolysis, vol.29, 1994, pp.169-181.
[66] Li, X., Grace, J., Lim, C., Watkinson, A., Chen, H., Kim, J. “Biomass gasification in a circulating fluidized bed” Biomass
and Bioenergy, vol.26, 2004, pp.171-193.
[67] Van Paasen, S.V.B., Kiel, J.H.A. “Tar formation in fluidized-bed gasification-impact of gasifier operating conditions” The
2nd world conference and technology exhibition on biomass for energy, industry and climate protection, ECN-RX-04-037,
2004.
[68] Kurkela, E., Ståhlberg, P., Laatikainen, J., Simell, P. “Development of simplified IGCC-processes for biofuels: supporting
gasification research at VTT” Bioresource technology, vol.46, 1993, pp.37-47.
[69] Simell, P.A., Leppälahti, J.K. “Catalytic purification of tarry fuel gas with carbonate rocks and ferrous materials” Fuel ,
vol.71, 1992, pp.211-218.
[70] D.L Brink, Goldstein, I.S., “Organic Chemicals from Biomass” CRC press, Florida, Chapter 4, 1981, pp.45.
[71] Narvaez, I., Orio, A., Aznar, M.P., Corella, J. “Biomass gasification with air in an atmospheric bubbling fluidized bed.
Effect of six operational variables on the quality of the produced raw gas” Industrial & engineering chemistry research,
vol.35, 1996, pp.2110-2120.
[72] Lv P., Z. Yuan, C. Wu, L. Ma, Y. Chen and N. Tsubaki “Bio-syngas production from biomass catalytic gasification”
Energy Conversion and Management, 2007, pp.1132-1139.
[73] Herguido, J., Corella, J., Gonzalez-Saiz, J. “Steam gasification of lignocellulosic residues in a fluidized bed at a small pilot
scale. Effect of the type of feedstock” Industrial & engineering chemistry research, vol.31, 1992, pp.1274-1282.
[74] Aznar, M.P., Caballero, M.A., Gil, J., Martin, J.A., Corella, J. “Commercial steam reforming catalysts to improve biomass
gasification with steam-oxygen mixtures. Part 2. Catalytic tar removal” Industrial & engineering chemistry research,
vol.37, 1998, pp. 2668-2680.
[75] Ergudenler, A. “Gasification of Wheat Straw in a Dual-Distributor Type Fluidized Bed Reactor” Unpublished Ph.D.
Thesis. Technical University of Nova Scotia. Nova Scotia. Canada, 1993.
[76] Cao, Y., Y.Wang, J. Rieley and W. “A novel biomass air gasification process for producing tar-free higher heating value
fuel gas” Fuel Processing Technology, vol.87, 2006, pp.343-353.

24
 [77] Boateng, A.A., W.P. Walawender, L.T. Fan and C.S. Chee. “Fluidized bed gasification of rice hull” Bioresource
Technology, vol.40, 1992, pp.235-239.
[78] Slapak, M. J., J. M. van Kasteren, A.A.Drinkenburg. “Design of a process for steam gasification of PVC waste”
Resources, Conservation and Recycling, vol.30, 2000, pp.81-93.
[79] Mermoud, F., F. Golfier, S. Salvador,Van de Steene and J Dirion. “Experimental and numerical study of steam
gasification of a single charcoal particle” Combustion and Flame, vol.145, 2006, pp.59-79.
[80] Wei L, Xu S, Zhang L, Liu C, Zhu H, Liu S. “Steam gasification of biomass for hydrogen-rich gas in a free-fall reactor”
Int. J Hydrogen Energy, vol. 32, 2007, pp.24-31.
[81] Bailie, R.C. “Hessleman Gas Generator Testing” Solar Energy Research Institute. Contract No. AH-8-1077-1, 1979.
[82] Tillman, D.A. “Biomass Combustion. Biomass: Regenerable Energy” Hall, D.O. and Overened, R. P. (eds). John Wiley
and Sons, 1987, pp.203-219.
[83] Watkinson, A., C. Cheng and C. Lim. “Oxygen-steam gasification of coals in a spouted bed” Canadian. Journal of
Chemical Engineers, vol.65, 1987, pp.791-798.
[84] Estephane J, Aouad S, Hany S, El Khoury B, Gennequin C, El Zakhem H, et al. “CO 2 reforming of methane over
Ni-CO/ZSM5 catalysts” Aging and carbon deposition study. Int. J. Hydrogen Energ., vol.40, 2015, pp.9201-8.
[85] T. Hanaoka, T. Miyazawa, M. Nurunnabi, S. Hirata, K. Sakanishi “Liquid Fuel Production from woody biomass via
oxygen-enriched air/CO2 gasification on a bench scale” J. Jpn. Inst. Energy, vol.90, 2011, pp.1071-1080.
[86] Baratieri, M., Baggio, P., Fiori, L., Grigiante, M. “Biomass as an energy source: Thermodynamic constraints on the
performance of the conversion process” Bio resource Technology, vol.99, 2008, pp.7063-7073.
[87] Anjireddy Bhavanam and R. C. Sastry “Biomass Gasification Processes in Downdraft Fixed Bed Reactors: A Review”
International Journal of Chemical Engineering and Applications, vol.2, 2011, pp.6.
[88] Tae-Young Mun, Pyeong-Gi Seon, Joo-Sik Kim “Production of a producer gas from woody waste via air gasification
using activated carbon and a two-stage gasifier and characterization of tar” Fuel, vol.89, 2010, pp.3226-3234.
[89] Miccio F, Piriou B, Ruoppolo G, Chirone R. “Biomass gasification in a catalytic fluidized reactor with beds of different
materials” Chem. Eng J., vol.154, 2009, pp. 369-74.
[90] J.M. Seiler, C. Hohwiller, J. Imbach, J.F. Luciani, “Technical and economical evaluation of enhanced biomass to liquid
fuel process” Energy, vol.35, 2010, pp.3587-3592.
[91] Heidi C Butterman and Marco J Castaldi. “CO 2 as a carbon neutral fuel source via enhanced biomass gasification”
Environmental science & technology, Vol.43, 2009, pp.9030-7.
[92] M. Pohorel´y, M. Jeremi´aˇs, K. Svoboda, P. Kamen´ıkov´a, S. Skoblia, and Z. “Beno. CO 2 as moderator for biomass
gasification” Fuel, vol.117, 2014, pp.198-205.
[93] Weil, S.A., S.P. Nandi, D.V. Punwani and J.L. Johnson. “Peat hydrogasification” Presented at 176 th National Meeting of
ACS, Maiami, 1978.
[94] Li K, Zhang R, Bi J “Experimental study on syngas production by co-gasification of coal and biomass in a fluidized bed”
Int. Hydrogen Energy, vol.35, 2009, pp.2722-6.
[95] Eilhann E Kwon, Haakrho Yi, and Marco J Castaldi. “Utilizing Carbon Dioxide as a Reaction Medium to Mitigate
Production of Polycyclic Aromatic Hydrocarbons from the Thermal Decomposition of Styrene Butadiene Rubber”
Environmental science & technology, vol.46, 2012, pp.10752-10757.
[96] Eilhann E Kwon, Eui-Chan Jeon, Marco J Castaldi, and Young Jae Jeon. “Effect of carbon dioxide on the thermal
degradation of lignocellulosic biomass” Environmental science & technology, vol.47, 2013, pp.10541-7.
[97] M. Pohorel´y, M. Jeremi´aˇs, K. Svoboda, P. Kamen´ıkov´a, S. Skoblia, and Z. Beno. “CO 2 as moderator for biomass
gasification” Fuel, vol.117, 2014, pp.198-205.
[98] Radmanesh, R., Chaouki, J., & Guy, C. “Biomass Gasification in a Bubbling Fluidized Reactor: Experiments and
Modeling” American Institute of Chemical Engineers Journal, vol.52, 2006, pp.4258-4272.
[99] Petersen, I., & Werther, J. “Experimental Investigation and modeling of gasification of sewage sludge in the circulating
fluidized bed” Chemical Engineering and Processing, vol.44, 2005, pp.717-736.
[100] de Andrés, J., Narros, A., & Rodríguez, M. “Air-steam gasification of sewage sludge in a bubbling bed reactor: Effect of
alumina as a primary catalyst” Fuel Processing Technology, vol.92, 2011, pp.433-440.
[101] Gómez-Barea, A., Leckner, B., Perales, A., Nilsson, S., & Cano, D. “Improving the performance of fluidized bed
biomass/waste gasifiers for distributed electricity: A new three-stage gasification system” Applied Thermal Energy,
vol.50, 2013, pp.1453-1462.
[102] Manyà, J.J., Aznar, M., Sánchez, J.L., Arauzo, J., & Murilla, M.B. “Further Experiments on Sewage Sludge Air
Gasification: Influence of the Nonstationary Period on the Overall Results” Industrial & Engineering Chemistry, vol.45,
2006, pp.7313-7320.
[103] Mansaray K, Ghaly A, Al-Taweel A, Hamdullahpur F, Ugursal V. “Air gasification of rice husk in a dual distributor type
fluidized bed gasifier” Biomass Bioenergy, vol.17, 1999, pp.315-32.

25
[104] Qin Y, Feng J, Li W. “Formation of tar and its characterization during air-steam gasification of sawdust in a fluidized bed
reactor”. Fuel, vol.89, 2009, pp.1344-7.
[105] Skoulou V, Koufodimos G, Samaras Z, Zabaniotou A. “Low temperature gasification of olive kernels in a 5-kW fluidized
bed reactor for H2-rich producer gas” Int. J. Hydrogen Energy, vol.33, 2008, pp.6515-24.
[106] Gil J, Aznar M, Caballero M, Frances E, Corellas J. “Biomass gasification in fluidized bed at pilot scale with steam-
oxygen mixtures. Product distribution for very different operating conditions” Energy Fuels, vol.11, 1997, pp.1109-18.
[107] Huiyan Zhang, Rui Xiao, Denghui Wang, Guangying He, Shanshan Shao, Jubing Zhang, and Zhaoping Zhong. “Biomass
fast pyrolysis in a fluidized bed reactor under N 2, CO2, CO, CH4 and H2 atmospheres” Bioresource technology, vol.102,
2011, pp. 4258-64.
[108] Asadullah M, Miyazawa T, Ito S, Kunimori K, Koyama S, Tomishige K. “A comparison of Rh/CeO 2/SiO2 catalysts with
steam reforming catalysts, dolomite and inert materials as bed materials in low throughput fluidized bed gasification
systems” Biomass Bioenergy, vol.26, 2004, pp.269-79.
[109] Miccio F, Piriou B, Ruoppolo G, Chirone R. “Biomass gasification in a catalytic fluidized reactor with beds of different
materials” Chem. Eng J., vol.154, 2009, pp. 369-74.
[110] Basu P (2010) , Chapter 5 - gasification theory and modeling of gasifiers, in Biomass Gasification and Pyrolysis
Anonymous Boston: Academic Press, , pp. 117-165.
[111] Puig-Arnavat M,. Bruno J. C , Coronas A. (2010)., Review and analysis of biomass gasification models, Renewable and
Sustainable Energy Reviews, 12, pp. 2841-2851
[112] Melgar A., Pérez J. F., Laget H.,Horillo A. (2007) , Thermochemical equilibrium modelling of a gasifying process,
Energy Conversion and Management, 1, pp. 59-67.
[113] Jarungthammachote S. Dutta A. (2007), Thermodynamic equilibrium model and second law analysis of a downdraft
waste gasifier, Energy, 9, pp. 1660-1669.
[114] Zainal Z. A., Ali R., Lean C. H., Seetharamu K. N. (2001) Prediction of performance of a downdraft modeling for
different biomass materials, Energy conversion and Management, 42 , pp.1499-1515.
[115] Li X., Grace J. R., Watkinson A. P., Lim C. J. Ergüdenler A. (2001), Equilibrium modeling of gasification: a free
energy minimization approach and its application to a circulating fluidized bed coal gasifier, Fuel, 1 , pp. 195-207
[116] Koroneous C., Lykidou S. (2011), Equilibrium modeling for a downdraft biomass gasifier for cotton stalks biomass in
comparison with experimental data, Journal of Chemical Engineering and Materials Science, 2, pp. 61-68.
[117] Ratnadhariya J. K., Channiwala S. A. (2009), Three zone equilibrium and kinetic free modeling of biomass gasifier – a
novel approach, Renewable Energy, 4 , pp. 1050-1058.
[118] Bridgwater A. V. (2012), Review of fast pyrolysis of biomass and product upgrading, Biomass Bioenergy, 3, pp. 68-94.
[119] Centeno F., Mahkamov K., Silva Lora E. E., Andrade R. V(2012), Theoretical and experimental investigations of a
downdraft biomass gasifier-spark ignition engine power system, Renewable Energy, 1, pp. 97-108
[120] Koukkari P, Pajarre R. (2006), Introducing mechanistic kinetics to the Lagrangian Gibbs energy calculation, Comput.
Chem. Eng., 30, pp. 1189-1196.
[121] Jarungthammachote S., Dutta A. (2008), Equilibrium modeling of gasification: Gibbs free energy minimization
approach and its application to spouted bed and spout-fluid bed gasifiers, Energy Conversion and Management, 6, pp.
1345-1356.
[122] Antonopoulos I., Karagiannidis, A. Gkouletsos A., Perkoulidis G.(2012), Modelling of a downdraft gasifier fed by
agricultural residues, Waste Manage., 4, pp. 710-718.
[123] Srinivas T., Reddy B.V., Gupta A. V. S. S. K. S.(2012) Thermal Performance Prediction of a Biomass Based Integrated
Gasification Combined Cycle Plant - Journal of Energy Resources Technology-ASME Transactions 134, pp.1-9
[124] Mathieu P., Dubuisson R. (2002) Performance analysis of a biomass gasifier, Energy Conversion and Management, 43,
pp. 1291-1299.
[125] Mitta N.,R.,Ferrer-Nadal S.,Lazovic A.,M., Perales J.F., Velo E.,Puigjaner L. (2006) Modelling and simulation of a
tyre gasification plant for synthesis gas production, Proceedings of 1 6th European Symposium on Computed Aided Process
Engineering and 9th International Symposium on Process Systems Engineering, Garmisch-Partenkirchen Germany, pp.
1771-76.
[126] NikooM. B., Mahinpey N. (2008) Simulation of Biomass gasification in fluidized bed reactor using ASPEN PLUS,
Biomass and Bioenergy, 32, pp. 1245-1254.
[127] Doherty W., Reynolds A., Kennedy D. (2008) Simulation of a Circulating Fluidised Bed Biomass Gasifier using ASPEN
Plus a Performance Analysis, Proc. 21 st International Conference on Efficiency, Cost, Optimization, Simulation and
Environmental Impact of Energy Systems, Krakow, Poland. Pp.1241-1248.
[128] Fr´ed´eric Paviet Florent Chazarenc Mohand Tazerout (2009) Thermo Chemical Equilibrium Modelling of a Biomass
Gasifying Process Using ASPEN PLUS, International Journal Of Chemical Reactor Engineering, 7, Article A 40, pp.1-16.

26
[129] Hannula I.,Kurkela E. (2010) A semi-empirical model for pressurised air-blown fluidised-bed gasification of biomass,
Bioresource Technology, 101, pp. 4608-15.
[130] Atnaw S. M., Sulaiman S. A., and yusup S. (2011) A Simulation Study of Downdraft Gasification of Oil-palm Fronds
using ASPEN PLUS , Journal of Applied science 11(11), pp 1913-1920.
[131] Ramzan Naveed,Asma Ashraf, Shahid Naveed,Malik Abdullah (2011) Simulation of hybrid biomass gasification using
ASPEN PLUS, A comparative performance analysis for food, municipal solid and poultry waste, Biomass and
Bioenergy,35(9) pp.3962-3969.
[132] Damartzis T., Michailos S., Zabaniotou A. (2012) Energetic assessment of a combined heat and power integrated
biomass gasification–internal combustion engine system by using Aspen Plus , Fuel Processing Technology, 95, pp. 37-
44.
[133] Chen C., Jin Y. Q., YanJ. H., Chi Y. (2013) Simulation of municipal solid waste gasification in two different types of
fixed bed reactors, Fuel, 103, pp. 58-63.
[134] Pavlas M., Stehlík P., Oral J., Klemeš J., Kim J. K., and Firth B. (2010) Heat Integrated Heat Pumping for Biomass
Gasification Processing, Applied Thermal Engineering , 30, No. 1, pp. 30-35.
[135] Sipöcz Nikolett, Finn Andrew Tobiesen, Assadi Mohsen (2011), Applied Energy, 88, 2368-2376.
[136] Maria Puig-Arnavat , Alfredo J. Herna´ndez , Carles Bruno Joan, Alberto Coronas (2013) Artificial neural network
models for biomass gasification in fluidized bed gasifiers , Biomass and Bioenergy, 49, pp. 279-289.

Sources for the Figures

Figure 1.0
Ptasinski, K. J. (2015). Efficiency of biomass energy: An exergy approach. John Wiley.

Figure 1.2:
Basu, P. (2009). Combustion and Gasification in Fluidized Beds. New York.

Figure 1.3:
Li, X., Grace, J. R., Lim, C. J., Watkinson, A. P., Chen, H. P., and Kim, J. R.,(2004) Biomass
gasification in a circulating fluidized bed, Biomass Bioenergy, 26, 171–193.

Figure 1.4:
Kaiser, S., Loffler, G., Bosch, K., and Hofbauer, H., (2003). Hydrodynamics of a dual fluidized bed
gasifier, Part II: simulation of solid circulation rate, pressure loop and stability, Chem. Eng. Sci., 58,
4215–4223.

Figure 1.5:
Source: http://www.hauserman-engineering.com/Gasification.html

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