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1
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Samy Sadaka, P
...
, P
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Associate Scientist, Center for Sustainable Environmental Technologies
Adjunct Assistant Professor, Department of Agricultural and Biosystems Engineering
Iowa State University
1521 West F
...

Nevada, IA 50201
1
...
Introduction
Gasification is the controlled partial oxidation of a carbonaceous material, and it is
achieved by supplying less oxygen than the stoichiometric requirement for complete combustion
...
Depending upon the process type and operating conditions, low- or medium-value
producer gas (which is a combination of combustible and non-combustible gases) is created
...
Current developments in the chemical manufacturing and petroleum refinery
industries show that use of gasification facilities to produce synthesis gas will continue to rise
...

In addition, it includes the ability to house a wide variety of gaseous, liquid, and solid
feedstocks
...
Biomass and crop residues also have bee gasified
successfully
...


1
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Gasification Process
During gasification of biomass, the material is heated to a high temperature, which causes
a series of physical and chemical changes that result in the evolution of volatile products and

carbonaceous solid residues
...
It is generally accepted that the
char gasification stage is the rate limiting in the gasification of biomass because the
devolatilization stage is very fast
...
In the absence of a catalyst, gasification of char with reactive gases such as O2,
H2O and CO2 occurs at higher temperatures (700oC to 1000oC) according to the following
reaction:
Char + limited oxygen → Gas +Tar + Ashes

(1
...
Steam may be added from an external source or from the dehydration
reactions of crop residues
...
On the other hand, at low pressure, high temperature, and
high heating rates, most of the volatile products escape instantaneously from the fuel particles
during pyrolysis, hence reducing the chances of a solid char-gas interaction
...

Thermal decomposition of woody products starts at temperatures as low as 160oC when
water distillate, known as "pyroligneous acid," is evolved
...
Thus, higher temperatures are required to attain
appreciable rates and levels of char conversion to a gaseous product, which is the primary
objective of biomass gasification
...

The gasification process uses an agent, either air, oxygen, hydrogen or steam (see Figure
1
...
), to convert carbonaceous materials into gaseous products
...

Engines

Steam

Gas

Process
Heat
&
Power

Synthetic
Liquids

Medium
& High
Pipeline
Energy

Methanol
Ammonia
Gasoline

Oil

Char

Pyrolysis
Oil

Figure 1
...
Gasification processes and their products
...
2
...
Air gasification
The simplest gasification process uses air as a gasifying agent
...
25)
...
The heating value of the produced gas is
in the 3
...
8 MJ/Nm3 range, which makes it suitable for boiler and engine applications but not
for uses that require its transportation through pipelines
...
Because air is the gasifying agent, the reactor temperature is dependent

on the air flow rate and biomass feed rate
...

Groves et al
...
3 m I
...
The off-gas heating value and
energy recovery increased from 3
...
3 MJ/m3 and from 27 to 53%, respectively
...
5% at 1073 K
...
Walawender and Fan (1978)
studied the air gasification of feedlot manure and found that gas yield, heating value, and energy
recovery increased by 131%, 77% and 244%, respectively, when the temperature increased from
900 to 1100 K
...
The results showed that
at equivalence ratio of 0
...

Cao et al
...

They combined two individual regions of pyrolysis, gasification, and combustion of biomass in
one reactor
...
Secondary air was injected into the upper region of the
reactor to maintain elevated temperature
...
0 Nm3/kg biomass and heating value
of about 5
...
The concentration of hydrogen, carbon monoxide and methane in the fuel
gas produced were 9
...
25% and 4
...


1
...
2
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Using a mixture of steam and air as a gasifying agent is not
uncommon technology and has, in fact, been studied by several researchers
...
The
elevated temperature will help in the devolatilization process of biomass to produce various

gases
...
The
principle gas-phase reaction in the steam gasification system is the water gas-shift reaction:
CO+H2O → CO2+H2

(1
...
Boateng et al
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The produced
gas, which is rich in hydrogen, had been found to have a heating value ranging from 11
...
1 MJ/m3 at temperature of 800oC
...
Hoveland et al
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05 m I
...
fluidized bed gasifier using steam as a fluidizing agent and a
mixture of sand and limestone as the bed material
...
13
m3/kg at 867 K to 0
...
The gas heating value increased from 9
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5 MJ/m3
at the same temperature range
...
(1981) gasified alpha cellulose in a bench-scale fluidized bed reactor
using steam over a temperature range of 873-1073K
...
5-1
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The average gas
higher heating value was 11
...
The energy recovery as well as carbon conversion were
ranged from 32-90%
...
(1982) gasified straw with steam in a 0
...
The fraction of feed converted to
gas ranged from 32% at 552oC to 73 % at 757oC
...
3 MJ/m3 obtained at 672oC
...
According to Slapak et al
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The main product is syngas, employable for
energy recovery
...
6 MJ/Nm3
...

(2005) studied charcoal steam gasification of beech charcoal spheres of different diameters (1030 mm) at different temperatures (830-1030oC)
...
A difference in gasification rate as high 6
...
Experiments carried out with mixtures of H2O/N2 at 10%, 20%, and 40% mol

of steam confirmed that oxidant partial pressure influences gasification
...
9
was obtained for H2O partial pressure varying from 0
...
1 atm
...
They used an electrically heated 0
...
D
...
The bed
consisted of fused alumina sand; the fluidizing medium was super-heated steam
...
38 to 0
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8 to 18
...
Over
this temperature range, H2, CH4, CO, and CO2 concentrations in the produced gas varied from
3
...
1%, from 14
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5%, from 52
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1% and from 23-14
...
The balance of
the product gas was comprised of higher hydrocarbons including ethane, ethylene, and
propylene
...
(1989) reported on steam gasification of four different crop residues
(wood chips, thistle, saw-dust and straw) in a 0
...
D
...
They determined
the gas, char, and tar yield at temperatures between 650-780oC for each type of crop residue
...


1
...
3
...
Such a
gas can be economically distributed in pipeline network systems and therefore, be conveniently
used for process heat or possibly as synthesis gas to produce chemical and fuels
...

Bailie (1979) reported a relationship between the heating value of a gas and oxygen
concentrations in the fluidizing gas
...
2 MJ/Nm3, respectively
...
Tillman (1987) gasified municipal solid waste in an oxygen gasifier
...
Pyrolytic char was combusted with the oxygen at the bottom of the
gasifier providing enough thermal energy to produce temperatures in the range of 1593-1704oC
and to produce a molten slag from all noncombustible materials
...

The maximum heating value was 10
...
Watkinson et al
...
By increasing the oxygen to the carbon fed ratio from 0
...
1 kg/kg ,the
heating value of the produced gas per unit weight of feedstock increased sharply from 5 MJ/kg to
16 MJ/kg
...
2
...
Hydrogen gasification
Through this route of gasification, the fuel feedstock is converted to gaseous fuels in the
presence of hydrogen under high pressures
...

This process is unfavorable because of the degree of control necessary as well as the fact that
hydrogen must be readily available
...
(1978) used preheated hydrogen mixed with peat at the entrance of fluidized
bed gasifier
...
Increasing the temperature from 426oC to 760oC increased carbon monoxide and
hydrocarbon gases from 8% to 18% and 41% to 63%, respectively
...
3
...
The location of these zones in the
gasifier depends on the relative movement of the fuel and air
...
The depth and relative importance of each zone depend on the chemical composition
of the feedstock, its moisture content and particle size, the mass flow rate of the gasifying agent,
and the temperature
...
3
...
Drying zone
The drying zone receives its energy through heat transfer from other zones
...
As the fuels enter the
drying zone, their internal temperature is increased to 100-150oC
...
No chemical reactions take place in this zone
...
3
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Devolatilization zone
Heat transfer from the adjacent hot reduction zone causes devolatilization of the feed
material
...
The rate of temperature rise is
controlled by heat transfer
...
The products from the devolatilization zone are gases, liquid
(tars and oil), and char
...
The amounts of each of these products vary depending on the zone
temperature, rate of heating, structure, and composition and size of catalysts
...
3
...
Oxidation zone
In the oxidation zone, physical and chemical changes are inhibited as the oxygen carrier,
which is mostly air, is introduced into the fuel bed material
...
Oxygen, however,
penetrates the material surface to a small extent because it more readily reacts at the surface with
the formed carbon monoxide and hydrogen gases
...
The oxidation zone has the highest temperature due to the exothermic nature of
the reactions
...
3
...
Ash cooling zone
In the ash cooling zone, the reminder particles start to cool down faster than particles
temperature in other zones
...
Practically no chemical reaction takes
place here, although in some fixed bed designs, this zone acts as a filter for the resulting

producer gas
...


1
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Gasification Reactors
The gasification process uses several reactors, which can be classified according to the
relative movement of the fuel and the gasifying medium as either fixed beds (updraft, downdraft
and crossdraft) or fluidized beds (bubbling, circulating, spouted and swirling)
...
4
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Fixed Bed Gasifiers
Fixed bed reactors are those in which solids move either countercurrent or concurrent to
the flow of a gas as reaction takes place, and the solids are converted to gases
...


1
...
1
...
2) illustrates that the flow of the fuel and gases are countercurrent to each other
in the updraft gasifier
...
The high
temperature oxidation zone is located at the bottom of the gasifier where part of the fuel is
burned
...
The fuel descends through three zones (drying, pyrolysis and oxidation) of
progressively increasing temperatures
...

The heat dispersed from the oxidation zone and the gasification zone above it is transferred by
forced convection and radiation upwards to the pyrolysis and drying zones, thus providing the
heat required for drying, pyrolysis and endothermic char gasification processes
...
The produced gases, tar and other
volatiles disperse at the top while ashes are removed at the bottom of the reactor
...

The gas produced by an updraft gasifier usually exits at low temperatures (approximately
400 oC) and thus is rich in hydrocarbons and has high tar content
...
The high tar content is not a major problem if the
producer gas is used for direct heat applications
...
The product gas from an updraft gasifier contains more
CO, but less CH4, ethane and acetylene than gas from other gasifiers
...

The use of an endothermic agent (usually steam) has been the traditional approach for controlling
the oxidation zone temperature of updraft gasifiers
...

Updraft gasifiers have several advantages, including simple design and construction, low
gas exit temperature, high charcoal burnout and high thermal efficiency
...
Other disadvantages include high tar content and unsuitability for use with fluffy, low
density feedstock
...
2
...
4
...
2
...
The most important difference is
that the pyrolysis products in the downdraft type (Figure 1
...
Hence, they undergo further decomposition
...
The
final product gases, which leave the gasifier from the bottom at a fairly high temperature
(700oC), contain substantially less tar than the updraft gasifiers
...

The benefit of the downdraft gasifier lies in its ability to produce gas with low oil and tar
contents, which means less cleaning before use in internal combustion engines
...
However,
due to slag and clinker formation problems, this type of gasifier has been found unsuitable for
fuels with high ash content and low ash fusion temperatures, such as crop residues
...


Figure 1
...
Downdraft gasifier

1
...
1
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Air or
air/steam mixtures are introduced in the side of the gasifier near the bottom while the product gas
is drawn off on the opposite side
...
4
...
The combustion
(oxidation) and reduction zones are both concentrated to a small volume around the sides of the
unit
...
They are normally simpler to
construct and more suitable for running engines than the other types of fixed bed gasifiers
...


Figure 1
...
Cross-draft gasifier

1
...
2
...
Fluidized bed reactors can be classified by configuration and the
velocity of the gasifying agent, e
...
, bubbling, circulating, spouted, and swirling fluidized beds
...
4
...
1
...
In bubbling fluidization (at relatively low
fluidization velocity just above the minimum fluidization velocity), the gas in excess of that
needed for minimum fluidization passes through the bed in the form of bubbles
...
At the bed surface, the bubbles burst causing a shower of
bed solids to leave the bed surface and enter the freeboard, at which the carryover occurs
...
Pressure drop across the bed, which is
affected by the characteristics of bed particles, is an important variable to consider in developing

a bubbling fluidized bed
...
Figure 1
...


Figure 1
...
Fluidized bed gasifier
A fluidized bed gasifier consists of an inert bed material such as sand, ash or char, which
acts as an effective heat transfer medium
...
The
bed material aid in heat transfers and may provide a catalytic or gas cleaning action
...

Fluidized bed gasifiers have several advantages over other gasification reactors, including
strong gas solids contact, excellent heat transfer characteristics, better temperature control, large
heat storage capacity, good degree of turbulence, and high volumetric capacity
...

Because fluidized bed reactors operate at pressures slightly above atmospheric levels,
their design and construction must prevent leakage
...
Because the fuel is immediately gasified as it is fed into
the bed, these gasifiers respond slowly to load changes
...
Due to their complicated and expensive
control systems, fluidized bed gasifiers appear to be commercially viable over 30 MW thermal
outputs
...


1
...
2
...
1
...
The advantages of the system include: (1) lower
cost than dual and multi-fluidized beds; (2) less maintenance; and (3) the produced gas is ready
for utilization
...
These include: (1) heating
value of the produced gas is lower than that produced by the dual bed; (2) inorganic materials in
the feedstock cannot be separated; and (3) pyrolysis occurs at the bottom of the gasifier leading
to a non-uniform temperature distribution
...
(1953)
...
051 m single
fluidized bed batch reactor
...
They obtained charcoal, liquid, and gas yields of 32%, 29% and 16% (by
weight of the initial charge) after operating for 30 minutes at 673 K
...


1
...
2
...
2
...
The first bed is usually used to burn some of
the char to produce the energy for the second bed, which is where the pyrolysis occurs
...

Higher construction costs and greater maintenance are the disadvantages of this system
...
4
...
2 Circulating fluidized bed gasifier
If the gas velocity in a bubbling fluidized bed is further increased, more particles will be
entrained in the gas stream and leave the reactor
...
If the solids leaving the vessel are returned through an
external collection system, the system is called a circulating or fast fluidized bed (CFB) system
(Figure 1
...
The streams of particles moving upward in the reactor are at solid concentrations
well above that for dilute phase transport
...
Despite these
advantages, circulating fluidized beds are still less commonly used that bubbling models,
primarily because their height restricts their applications in terms of cost analysis
...
6
...
4
...
3 Spouted fluidized bed
A spouted fluidized bed consists of a bed of coarse particles partly filling the vessel,
which also has a relatively large control opening at its base
...

With sufficient gas flow, the particles in the gas can be force to rise in a fountain at the center of
the bed and develop cyclic motion on the bed as a whole
...
Spouted fluidized bed gasifiers have
been used to gasify coal of various ranks in air-steam and/or oxygen-steam mixtures at
atmospheric and elevated pressures
...
This particle size is close to the value above which the gas-solid
contacting effectiveness of fluidized beds is seriously impaired due to bypassing of gas in the

form of large bubbles
...


1
...
2
...

Primary air is introduced at the bottom of the bed through a distributor plate at a sufficient
velocity to fluidize the bed material in a bubbling regime
...
The injection of secondary swirling air into the freeboard helps to achieve high relative
movement between the air and the fuel particles
...
This increases the gas residence time and the degree of mixing (gas-gas
and gas-solids) which increases the rate of reactions in the freeboard
...
They may be more suited to steam gasification
...
5
...
The chargas reaction (C-CO2 and C-H2O) controls the ultimate conversion of the char, thus their products
can dominate the final gas
...


1
...
1
...
4 kJ/mol

(1
...
6 kJ/mol

(1
...
0 kJ/mol

(1
...
4 kJ/mol

(1
...
0 kJ/mol

(1
...
The combustion reaction (1
...
4) provides the energy necessary to sustain the
endothermic gasification and other reactions
...


1
...
2
...

CO + H2O → CO2 +H2

- 41
...
8)

CO + 3 H2 → CH4 + H2O

-201
...
9)

CO + (½) O2 → CO2

-251
...
10)

H2 + (½) O2 → H2O

-280
...
11)

Other important secondary reactions occur under appropriate conditions (temperature,
pressure) involve decomposition of heavy hydrocarbons and tars to carbon and low molecular
gaseous products
...
12)

Tar + H2O (g) → CO + H2O +CH4 + …

Steam tar reforming

(1
...
14)
of

(1
...
6
...
These parameters are quite interrelated and each of them affects the
gasification rate, process efficiency, product gas heating value and product distribution
...
6
...
Bed temperature
The gasification rate as well as the overall performance of the gasifier is temperaturedependent
...

As part of a wider investigation, Harris et al
...
0 MPa pressure
and at temperatures up to 1773 K
...
Char yield decreased with increases in temperature
...
(1988) reported that
the product gas yield from maple sawdust (1
...
C
...
The
decreasing amount of char indicated that the conversion increased with increases in temperature
...
(1983) found the conversion of corncobs to increase from 94% at 500oC to 99% at
900oC in air gasification
...
Alves and Figueiredo (1989) reported that tar
production at low temperatures (below 500oC) was found to increase initially with increases in
temperature and then drop with further increases in temperature
...
(1989) reported
increases in hydrocarbon gases, especially CH4 and C2H4 (ethylene) with increases in
temperature
...
Other gas components
(H2 and CO), also increased with increases in temperature (Font et al
...
The heating value
of the producer gas is also influenced by temperature
...
(1987) found the calorific
value of crop residue gasification producer gas increased steadily up to 700oC and then
decreased
...
The decline at higher temperatures is probably due
to the cracking of hydrocarbons
...
, 1985)
...

Below 600oC, the gasification reaction rates are too slow
...
6
...
Bed pressure
Bed pressure has been reported to have a significant effect on the gasification process
...
However, at a constant temperature,
the first-order rate constant (k) for the char gasification increased as pressure increased
...
101, 1
...
201 min-1 at pressures of 0
...
79 and 2
...
McLendon (2003) co-gasified mixtures of coal and biomass in a jetting, ashagglomerating, fluidized-bed, pilot scale gasifier at an operating pressure of 3
...
Feed
mixtures ranged up to 35% by weight biomass
...
The bituminous coal mixture had marked differences
...
Plante et al
...
The increase was more
significant at high temperatures (900-950oC)
...
Increasing the pressure caused a significant increase in methane yield (Liinanki
et al
...
Another advantage of pressurized reactors is the high reaction rate which allows for
a smaller reactor vessel
...
(1985) found that H2/CO and CO2/CO molar ratios
increased with an increase in the total pressure of a char-steam gasification process
...
6
...
Bed height
At a given reactor temperature, a longer residence time (due to higher bed height)
increases total gas yields
...
(1998) showed that a higher bed height resulted in greater
conversion efficiency as well as a lower bed temperature due to the fly-wheel effect of the bed
material
...
Their results also reported that increasing

the bed height increases the bed pressure drop in the dense bed but resulted in no significant
changes in the freeboard region
...
(1988) reported increases in H2, CO, CO2, CH4 and
C2H4 when the residence time was increased as a result of increased bed height
...

Beaumont and Schwob (1984) found that at a temperature of 350oC, water yield increased with
increases in vapor residence time during the pyrolysis process of wood in an N2 atmosphere
...
6
...
Fluidization velocity
Fluidization velocity plays an important role in the mixing of particles in the fluidized
bed
...
Sadaka et al
...
28 m/s but remained fairly constant at
the fluidization velocities of 0
...
37 m/s
...
(1980) tested the
gasification of feedlot manure with different superficial gas velocities
...


1
...
5
...
Increasing the equivalence ratio
resulted in lower pressure drops both in the dense bed and the freeboard regions when the
gasifier operated at different fluidization velocities and bed heights
...
(1989) reported that high equivalence ratios increased the gas production
rate in air gasification
...
On the other hand, a very low
equivalence ratio results in very low bed temperatures, thus producing a lower gas and higher tar
yields
...
At the equivalence
ratios of 0
...
20 and 0
...
48, 6
...
98 MJ/Nm3, respectively
...
6
...
Moisture content of feed material
The moisture content of feed material affects reaction temperature due to the energy
required to evaporate water in the fuel
...
Elliot and Sealock (1985) reported on the conversion of lignin (of less than 10%wt
basis moisture content) at 350oC and about 50% wt moisture content at 450oC
...
They
reported a decrease in gasifier temperature with increases in fuel moisture content
...
6
...
Particle size
The feed particle size significantly affects gasification results
...
The rate of thermal diffusion within the particles
decreases with increased particle size, thus resulting in a lower heating rate
...
(1980)
...
51 m3/kg at 900 K to 0
...
For the largest size
fraction (-2 to +8 mesh), the gas yield increased from 0
...
60 m3/kg for the same
temperature range
...
0 MJ/m3 at 1010 K)
...
3 MJ/m3 at 900 K to a maximum of 19
...
0 MJ/m3 at
1010 K
...
(1985) found the gasification rate to depend on particle size
...
1 to 1
...
05 to 5
...


1
...
8
...
Tomeczek
et al
...
The results showed that the influence of steam-to-air ratio on char was particularly
strong at lower ratios due to the fact that the steam released at the devolatilization stage
contributed to the gasification process even in the case when steam was not added
...
25 kg/kg
...
(1989) investigated the effect of the air-to-steam ratio on the gasification of wood shavings
...
Halligan et al
...
Over a
temperature range of 966 to 1069 K, increasing the steam-to-air ratio increased the gas
volumetric yield and the heating value from 0
...
3 m3/kg and from 8
...
8 MJ/m3,
respectively
...


1
...
9
...

One of the main problems in the catalytic steam reforming of tar is the carbon deposition on the
catalyst from the carbon’s high aromatic character
...
Srinakruang (2005) developed Ni
on dolomite catalyst (Ni/dolomite)
...
The performance of the
Ni/dolomite catalysts gave negligible carbon deposition, while the conventionally supported Ni
catalysts gave large carbon deposition
...

Fung and Graham (1980) showed that potassium carbonate and calcium oxide increased
gas production in gasification processes
...
(1983) reported that for wood gasification at
900oC, potassium carbonate is the most efficient catalyst of all carbonates (Na2CO3, K2CO3,
CaCO3, NaHCO3 and KHCO3) tested
...
(1985) showed that adding alkali and cesium
carbonates to crop residues either by solution impregnation or dry mix increased gas production
in crop residue steam gasification
...
(1981) studied the effect of limestone as a bed additive in the steam
gasification of manure in a 0
...
D
...
Limestone was found to prevent agglomeration
and affected the produced gas composition, yield, and heating value
...
4 MJ/m3, respectively under the operating temperature used
...
Coffee husks and spruce sawdust
mixed with inorganic salts (sodium tetraborate, potassium chloride and lithium chloride) were
gasified at atmospheric pressure at temperatures ranging from 600oC to 800oC
...
All of the inorganic salts
increased the char yield but decreased the tar and moisture content of the product gas compared
to the gasification of untreated feedstock under similar conditions
...
3 g/Nm3 and 9
...

The carbon conversion ranged between 64 and 99%
...

Baker et al
...
Alkali carbonates (Na2CO3
and K2CO3) either impregnated on wood or on an alumina support produced tar-free gas at
750oC
...
The
Na2CO3 catalyst is very good in terms of H2 and CO and does not reduce CH4 and other
hydrocarbons
...
, 1984)
...
(1988) studied the gasification of biomass with steam and CO2 in the
presence of alkali carbonates at 750oC
...

Both catalysts increased the char gasification rates, although potassium carbonate was more
effective than Na2CO3
...
The catalyst increased the total gas yields, enhanced H2 and CO2 production,
and decreased CO concentration in the product gas
...
7
...
Some of these limitations are related to feedstock characteristics while others are
related to the overall design of gasifiers
...
7
...
Moisture Content
The operation of a gasifier is affected by the moisture content of the feedstock
...
The higher the moisture
content of the feedstock, the lower the bed temperature due to the energy required to evaporate
the water from the feedstock
...
152 I
...
Their results showed that the heat outputs decreased from
132
...
3 MJ/h when the moisture content was increased from 10 to 20%
...
(1980)
studied the effect of feedstock moisture content on the produced gas heating value when
gasifying wood
...
5 MJ/Nm3 at 20%
moisture content (wt
...
5 MJ/Nm3 at 50% moisture content (wt
...


1
...
2
...
Several feeding mechanisms
were developed to accommodate the wide variety of feedstock, which includes direct feeding to
the bed and over-the-bed feeding
...
Also, toxic and
combustible gas back-flow from the gasifier might cause explosions
...

Over-the-bed feed systems are usually less troublesome because there is no direct contact
between the hot bed material and the feeder
...
Due to particle emissions, over-the-bed feeding
usually results in a dirty gas, which increases gas clean-up costs and fuel loss
...
Bilbao et al
...
Ghaly et al
...

Better mixing and fuel distribution characteristics within the bed were experienced with
pneumatic transport feeding systems
...


1
...
3
...
Melted ash can clog the grate and ash handling becomes a critical problem
...
Ghaly and Al-Taweel (1990) reported
deformation temperatures of –900-1050oC, –730-987oC, –750-1043oC and 800-931oC for wheat,
barley, oats and rye straws, respectively
...
(1984) reported a significant decrease in
the melting temperature of the ash in the presence of large amounts of low melting temperature
mineral oxides such as Na2O and K2O
...
(1981) stated that the most desirable
chemical constituent of ash is Al and the most undesirable constituents are K, Ca and Fe
...
The bed fused
completely above 1000oC
...
(1988) reported that the high volatile content, which burns at high
temperatures causing severe slag formation and operational problems, cause difficulties in
keeping the reactor temperature below the ash melting temperature
...
Studies by Ergudenler and
Ghaly (1992) showed that K2O (which has low melting temperature and high weight percentage
in the ash of wheat straw) is the major contributor to the agglomeration process
...


1
...
4
...
Segregation in a fluidized bed is affected by the particle density,
shape, size, superficial gas velocity, mixture composition, bed aspect ratio and jetsam fraction
...
The vertical mixing in the fluidized bed occurred due to bubble-induced drift
...
The upper part of the bed attained a fairly uniform composition
while the lower component sank (jetsam) to form a concentrated bottom layer
...
Bilbao et al
...
Due to the high level of segregation,
sand settled at the bottom and the straw tended to rise to the top
...
Ghaly et al
...
Bilbao et al
...


1
...
5
...

Entrainment is expected to be influenced by many factors such as fluidizing gas properties
(superficial gas velocity, gas density, viscosity and relative humidity) the solid properties
(particle size, particle size distribution and particle density) and other factors (bed diameter, bed
depth, gas distribution and internal surfaces)
...
In the case of particle emission, gas-cleaning equipment must be
provided at extra capital and operating costs
...
Unfortunately, providing this information from basic principals

is difficult because of the complexity of the two phase flow in the freeboard
...
(1980)
reported that the solid concentration in the freeboard is non-uniform, with a downward moving
denser stream of solids at the wall and diluted upward in a central core
...
The height is larger than would be
expected as determined by their terminal velocities
...
8
...


1
...
1
...
(2000) conducted experiments involving the co-gasification of residual
biomass/poor coal blends and gasification of individual feedstock used in the blends in a bench
scale, continuous fluidized bed reactor working at atmospheric pressure
...
Experimental tests were conducted using as
gasification agent mixtures of air and steam at gasification temperatures of 840–910oC and
superficial fluidized gas velocities of 0
...
4 m/s
...
Their study
indicated that a blend ratio with no less than 20% pine chips for the low-grade coal and 40% pine
chips for the refuse coal are the most appropriate
...
7 to 4
...
0 to 4
...
Dry product gas yield rises with the increase
of the blend ratio from 1
...
20 Nm3/kg (pine chips/low-grade coal), and from 0
...
75
Nm3/kg (pine chips/refuse coal), respectively
...


1
...
2
...
(2002) developed a fluidized bed gasifier to gasify wheat straw in an
air/steam mixture
...
The temperature of the dense
bed reached its maximum (891oC) indicating that the agglomeration problem was overcome by
injecting an air/steam mixture
...
63 MJ/Nm3 and 2
...
The
bed pressure drop ranged between 37
...
6 cm H2O, indicating a good fluidization quality
...
(2003) studied the influence of calcium addition in straw gasification
...
Walawender et al
...
23 m diameter fluidized bed reactor over a temperature
range of 552 to 757oC
...
The heating value of the gas exhibited
a parabolic variation with temperature with a maximum value of 12
...


1
...
3
...
(2002) designed and installed a circulating fluidized bed (CFB) biomass
gasification and power generation system (BGPG) for rice husk to provide power for a rice mill
with a capacity of 150 td−1
...
It was found that the system can be operated stably within the temperature
range of 700oC to 850oC; its optimal condition was reached when the workload was increased
above 800 kW
...
7–1
...
The reliability of
the BGPG system has been proven through two years of operation (about 10,000 h)
...
032
USD kW−1, and the payback period is not more than two years
...
They used an electrically heated 0
...
D
...
The
bed consisted of fused alumina sand and the fluidizing medium was super heated steam
...
38 to 0
...
8 to 18
...


Over this temperature range, H2, CH4, CO and CO2 concentrations in the produced gas varied
from 3
...
1%, from 14
...
5%, from 52
...
1% and from 23
...
6%,
respectively
...


1
...
4
...
Their chars were then
gasified in a gas mixture of steam and nitrogen
...
It was concluded that gasification characteristics of biomass chars were fairly
dependent on the biomass properties such as ash and fixed carbon contents and the constituents
present in the ash
...

Singh et al
...
With the exception of the gas heating value, the energy recovery, carbon conversion
and mass yield of the gas obtained from cotton wood were found to be lower than those obtained
from pure cellulose
...
(1979) studied fluidized bed air partial oxidation of cotton gin
trash over the temperatures range of 649 and 871oC in a 0
...
D reactor
...
4 to 4
...


1
...
5
...
(1978)
...
17 to 0
...
30 to 0
...
The gas
contained large amounts of CO and H2 and the heating value varied from 1
...
9 MJ/m3
...
8
...
Sawdust gasification
Wander et al
...

In order to assess the performance of the wood residues gasification process, a small, fixed bed,
downdraft, stratified and open-top gasifier was built
...

Results from the gasification of oak sawdust in a pilot plant were reported by Beck and
Wang (1980)
...
1 m3/kg at 600oC to 1
...
2 MJ/m3 for all temperatures
...
5% at 800oC
...


1
...
7
...
(1981) gasified alpha cellulose in a bench-scale fluidized bed reactor
using steam over a temperature range of 600 to 800oC
...
5-1
...
The average gas
higher heating value was 11
...
The energy recovery and carbon conversion were within
the 32-90% range
...
(1985) investigated the steam gasification of alpha cellulose at
elevated free-board temperatures and confirmed an earlier hypothesis that proposed the existence
of two temperature regimes for cellulose gasification, one below 567oC dominated by volatile
cracking and the other above 567oC dominated by the water gas shift reaction
...
8
...
Manure gasification
Sweeten et al
...

And due to soil contamination during collection, the ash content is almost 9–10 times that of low
ash (5%) coal
...
The

fuel N per GJ is considerably higher than coal, which may result in increased NOx emissions
...
Based on heating values and alkaline oxides, partial
composting seems preferable to a full composting cycle
...
The adiabatic flame temperature for most of the biomass
fuels can be empirically correlated with ash and moisture percentage
...
(1981) tested the gasification of feedlot manure with different superficial
gas velocities
...
Walawender and Fan (1978) studied the air
gasification of the feedlot manure and found that the produced gas yield, the higher heating value
and the energy recovery increased by 131%, 77% and 244% when the temperature increased
from 627 to 827oC
...
(1971) gasified feedlot manure in a 0
...
D
...
The fluidizing gas was a mixture of air and steam and the bed consisted of the feed
material only
...
6 to 1
...
7 to 9
...
The
energy recovery and carbon conversion, also, increased from 23 to 49 % and from 20 to 50 %,
respectively
...
9
...
The gas can be used in industrial application areas, irrigation, vehicular
power, direct heating and production of value added products
...
9
...
Gasification Technology for Power Generation and Industrial Applications
Currently, electric utilities and various industrial firms have expressed growing interest in
finding economical and environmentally attractive methods of converting coal, wastes, and
renewable fuels into replacement energy sources for use in combined heat and power
applications
...
In urban
areas, the technology will be unattractive since grid electricity is usually a cheaper source of

energy
...
Exact quantities of these depend upon the tree species being
used the end product of the industry and, the efficiency of equipment employed
...

However, these residues would be utilized by a system whose end product is producer gas
...
The exact quantities of these crop residues depend upon the species being harvested,
the end product of the mechanism and the efficiency of equipment used
...
The gas could be used for direct burning to
produce heat for kiln drying or steam generation
...
Alternatively, the producer gas
could be purified and used in internal combustion engines for the generation of mechanical
power or on-site electricity
...
9
...
Shaft Power
Shaft power is significantly demanded in developing countries for irrigation
...
where the interruption of fossil fuel supply is
common, alternative energy sources such as producer gas are being sought
...
In such systems, the producer gas is
used as a supplement to diesel fuel
...
4
kg per kWh of shaft power
...

The possibility of using crop residues in gasifiers has also gained great attention
...

A practical example of this technology is the use of gasifier-powered irrigation pumps in
Brazil
...
In these operations, a potential gasification feedstock is produced as part of the

process
...
9
...
Vehicular Power
Inconvenience of gasifiers and the detrimental effects on engine performance are the
main constraints of the gasification technology in vehicular applications
...
During the two World Wars, wood chips and
charcoal were suitable fuel for vehicle gasifiers
...

To avoid excessive engine wear and plugging of valves, gas supply lines and carburetors,
the produced gas must be free of tars, dust, and acids
...
However, with the present technology and continuing
research, this problem should eventually be overcome
...
An engine designed to use diesel
fuel, on the other hand, is de-rated by only 10% to 20%
...

The need to improve the power output from single-cylinder gasoline engine fueled with
rice hull-produced gas was investigated by Camacho (1988)
...
2
...
3 and 8
...
The engine test data indicated about a 10 to 16% power output for the 7
...
2, respectively, over the normal compression ratio 6
...
The comparison values for gasoline
rated 3
...
42 kW vs
...
57 kW (6
...
3
compression ratio) and 1
...
66 kW (6
...
2 compression ratio)
...
9
...
Direct Heat Applications
Most commercial gasifiers currently in operation are used to produce heat rather than fuel
for internal combustion engine, primarily because the requirements for heating fuel are less
stringent
...

All gasifiers can be used to provide producer gas for combustion purposes
...
Where fuel oil is being used to generate process
heat, run furnaces or kilns, the gasification technology is a viable option
...
With more experience and better design, the range of potential uses will expand
...
Countries that have
large forest and agricultural resources are obviously suitable for the application of the
gasification technology to direct heating
...


1
...
5
...
The syngas is generally produced by
steam reforming of natural gas or in some cases by gasification of heavy oil fractions
...
The
specifications of hydrogen in the process industry are relatively mild, typical demands for hydrocracker are H2 content > 98% vol%, N2 < 2% vol% and CO, CO2 < 10-15 ppm
...
Carbon dioxide is also removed and a catalyst
is used to bring up H2O/CO molar ratio to about 2 (Brown, 2003)
...
16)

Methanol can be produced by means of the catalytic reaction of carbon monoxide and
some carbon dioxide in the percentage range is necessary to optimize the reaction
...
17)

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and J
...
Figueiredo
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pyrolysis of wet wood
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Deversa and E
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Fung, D
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Washington, D
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Ghaly, A
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Halligan, J
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Parker
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Horio, M
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Transactions of ASAE, 25(4): 1074-1080
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P
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