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 is among the most effective methods for biomass conversion due to some important
factors such as flexibility of fuel used, space consideration, energy recovery, and reduction of solid
waste. Gasification process involves partial burning down or combustion of fuel into flammable gas
which consist of hydrogen, carbon monoxide, and methane which is a saturated hydrocarbon. This
paper aims at discussing the gasification process and the affecting parameters on the performance
such as catalyst, temperature, gasifying agent and the biomass ratio. The paper also addresses
intensively on different types of gasifer system design such as downdraft, updraft, and fluidized bed. It
also deals with Tar production and the type which include primary, secondary, and tertiary. Besides,
the gasification modelling techniques and equations are also discussed.
1. Introduction
Gasification is a thermochemical process which occurs at high temperatures
usually more than 700C to convert carbonaceous materials such as fossil fuels,
biomass, plastics, and coal into syngas like H 2, CH4, CO, and CO2. Oxygen (air)
and/or steam is used as gasifying agent and heat carrier agent. The syngas yield
can be burnt directly to produce heat at temperatures higher than the
combustion products. Also, the syngas if well cleaned, can be employed
throughout different pathways to yield useful outputs such as;
1. Production of methanol,
2. Purified syngas used in gas engine, gas turbines, and/or 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 generally involves six
parts: fluidized bed, overview of biomass used in gasification, tar reduction,
gasification agent, finally influence of gasifier operating conditions and
Mathematical modelling. Special attention is paid to literature regarding tar
formation and reduction 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
,http://www.enggcyclopedia.com/2012/01/types-gasifier/
In the gasification process, the biomass is heated to a high temperature
producing a series of chemical and physical changes resulting in development of
volatile products and carbonaceous solid residues. The amount of the volatile
products depends on: the heating rate, the temperature height and the type of
biomass fuel. It is generally accepted that the char gasification stage is the rate
limiting factor in the gasification of harvest residues because of the quickly
devolatilization stage. The composition of the syngas produced depend on the
degree of equilibrium attained by various gas phase reactions specially water gas
shift reaction [32, 33]. In absence of catalyst, char gasification with reactive
gases such as oxygen occurs 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 volatile products escape from the biomass particle during the
pyrolysis process, hence reducing the chances of solid char gas reaction. 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 take place. Biomass
gasification process occurs 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, good heat transfer characteristics, and high volumetric
capacity. The temperature can be controlled by varying the feed rate or the
agent rate. Moreover, low operating temperatures produces more slag and
clinker. Fluidized bed reactors have wider adaptability to handle different types
of fuel. High ash or moisture content of the feedstock points no problem to the
fluidized gasifiers such as those ordinarily encountered with moving bed
gasifiers. 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 large
pressure drop and corrosion of the reactor body. Because 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 is an
advantage over 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 hot bed materials, the hot combustion gas, and
the biomass feed. Fluidized bed gasifiers can be classified on the basis of 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 key
parameter to maintain effective fluidization of bed materials. The fluidization
condition is usually described using a gas superficial velocity which is the ratio of
volumetric gas flow rate and bed cross sectional area. The gas superficial
velocity, at which the drag force on the bed materials equals the gravitational
force, is defined as the minimum fluidization velocity (U mf) of the bed materials.
At minimum fluidization condition (shown in Fig. 3.3)., the bed materials lift
upward and remain in suspension; bed pressure drops (dP mf) reaching maximum
and remains constant with further increase in the gas superficial velocity.
Fluidization characteristics, such as U mf and dPmf, depend upon the particle size
and composition of the bed materials [137]. U mf 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 the 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/or bigger 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 easily 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 a sand and biomass
[141, 142]. Generally, 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) is greatly enhanced
in fluidized beds, to avoid or reduce carryover of particles form the fluidized bed,
keep the gas velocity between minimum fluidization velocity (Umf) and terminal
velocity (Ut). In addition, rapid particle mixing allows uniformity in bed. As a
result, fluidized bed are widely used for conducting gas solid reactions (coal
combustion), gas solid catalytic reactions (catalytic cracking of petroleum), etc
[142]. Thus, 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 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 U mf 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, U mf, 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, Terminal velocity of the particle, Fluidization velocity during the
gasification, and agent flow rate to meet the fluidization condition calculation
procedures are described as follows
Fig. 1. Fluidization characteristics of sand
2.2. Bubbling Fluidized Bed Gasifier
The bed is termed as bubbling fluidized bed when granular material (e.g.,
sand) are lifted in a reactor through which an upward flow of gas is passing
through it at a flow rate where the pressure drops 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 consists of a vessel with distributor plate at the
bottom through which the air is introduced. Above the distributor plate there’s
moving bed of fine grained material into which the prepared biomass come in.
Setting of the bed temperature to 600-1000C is maintained by controlling the
equivalence ratio. The biomass is pyrolysed in the hot bed to char form and
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 is among the most effective methods for biomass conversion due to some important
factors such as flexibility of fuel used, space consideration, energy recovery, and reduction of solid
waste. Gasification process involves partial burning down or combustion of fuel into flammable gas
which consist of hydrogen, carbon monoxide, and methane which is a saturated hydrocarbon. This
paper aims at discussing the gasification process and the affecting parameters on the performance
such as catalyst, temperature, gasifying agent and the biomass ratio. The paper also addresses
intensively on different types of gasifer system design such as downdraft, updraft, and fluidized bed. It
also deals with Tar production and the type which include primary, secondary, and tertiary. Besides,
the gasification modelling techniques and equations are also discussed.
1. Introduction
Gasification is a thermochemical process which occurs at high temperatures
usually more than 700C to convert carbonaceous materials such as fossil fuels,
biomass, plastics, and coal into syngas like H 2, CH4, CO, and CO2. Oxygen (air)
and/or steam is used as gasifying agent and heat carrier agent. The syngas yield
can be burnt directly to produce heat at temperatures higher than the
combustion products. Also, the syngas if well cleaned, can be employed
throughout different pathways to yield useful outputs such as;
1. Production of methanol,
2. Purified syngas used in gas engine, gas turbines, and/or 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 generally involves six
parts: fluidized bed, overview of biomass used in gasification, tar reduction,
gasification agent, finally influence of gasifier operating conditions and
Mathematical modelling. Special attention is paid to literature regarding tar
formation and reduction 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
,http://www.enggcyclopedia.com/2012/01/types-gasifier/
In the gasification process, the biomass is heated to a high temperature
producing a series of chemical and physical changes resulting in development of
volatile products and carbonaceous solid residues. The amount of the volatile
products depends on: the heating rate, the temperature height and the type of
biomass fuel. It is generally accepted that the char gasification stage is the rate
limiting factor in the gasification of harvest residues because of the quickly
devolatilization stage. The composition of the syngas produced depend on the
degree of equilibrium attained by various gas phase reactions specially water gas
shift reaction [32, 33]. In absence of catalyst, char gasification with reactive
gases such as oxygen occurs 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 volatile products escape from the biomass particle during the
pyrolysis process, hence reducing the chances of solid char gas reaction. 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 take place. Biomass
gasification process occurs 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, good heat transfer characteristics, and high volumetric
capacity. The temperature can be controlled by varying the feed rate or the
agent rate. Moreover, low operating temperatures produces more slag and
clinker. Fluidized bed reactors have wider adaptability to handle different types
of fuel. High ash or moisture content of the feedstock points no problem to the
fluidized gasifiers such as those ordinarily encountered with moving bed
gasifiers. 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 large
pressure drop and corrosion of the reactor body. Because 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 is an
advantage over 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 hot bed materials, the hot combustion gas, and
the biomass feed. Fluidized bed gasifiers can be classified on the basis of 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 key
parameter to maintain effective fluidization of bed materials. The fluidization
condition is usually described using a gas superficial velocity which is the ratio of
volumetric gas flow rate and bed cross sectional area. The gas superficial
velocity, at which the drag force on the bed materials equals the gravitational
force, is defined as the minimum fluidization velocity (U mf) of the bed materials.
At minimum fluidization condition (shown in Fig. 3.3)., the bed materials lift
upward and remain in suspension; bed pressure drops (dP mf) reaching maximum
and remains constant with further increase in the gas superficial velocity.
Fluidization characteristics, such as U mf and dPmf, depend upon the particle size
and composition of the bed materials [137]. U mf 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 the 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/or bigger 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 easily 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 a sand and biomass
[141, 142]. Generally, 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) is greatly enhanced
in fluidized beds, to avoid or reduce carryover of particles form the fluidized bed,
keep the gas velocity between minimum fluidization velocity (Umf) and terminal
velocity (Ut). In addition, rapid particle mixing allows uniformity in bed. As a
result, fluidized bed are widely used for conducting gas solid reactions (coal
combustion), gas solid catalytic reactions (catalytic cracking of petroleum), etc
[142]. Thus, 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 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 U mf 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, U mf, 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, Terminal velocity of the particle, Fluidization velocity during the
gasification, and agent flow rate to meet the fluidization condition calculation
procedures are described as follows
Fig. 1. Fluidization characteristics of sand
2.2. Bubbling Fluidized Bed Gasifier
The bed is termed as bubbling fluidized bed when granular material (e.g.,
sand) are lifted in a reactor through which an upward flow of gas is passing
through it at a flow rate where the pressure drops 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 consists of a vessel with distributor plate at the
bottom through which the air is introduced. Above the distributor plate there’s
moving bed of fine grained material into which the prepared biomass come in.
Setting of the bed temperature to 600-1000C is maintained by controlling the
equivalence ratio. The biomass is pyrolysed in the hot bed to char form and
