-
9Gasification and Pyrolysis of CoalAdam Luckos, Mohammed N.
Shaik, and Johan C. van Dyk
9.1Introduction
Gasication is a process that converts solid or liquid fuels into
a clean combustiblegas (called synthesis gas or syngas) consisting
of carbon monoxide, hydrogen, andsomemethane and carbon dioxide.
Proportions of these components depend on thechemical composition
of the fuel, reactants used in the process, and on the
operatingconditions. Clean syngas can have many applications; it
can be used for synthesis oftransportation fuels and chemicals,
production of hydrogen, direct reduction ofmetal ores, generation
of electric power, or a combination of these products [17].Coal
gasication was invented in 1792 and it was extensively used to
produce town
gas in the nineteenth century [8]. For more than 100 years, coal
gasicationtechnologies have been commercially used for the
production of liquid fuels andchemicals. The development of
large-scale processes began in the late 1930s inGermany. After
World War II, interest in coal gasication waned because of
theincreasing availability of relatively cheap oil and natural gas
from the Middle East.Interest in coal gasication was renewed after
the rst oil crisis in 1973 when gas andoil prices increased
dramatically.The development of high ring-temperature gas turbines
in the late 1980s created
a new, potentially large, market for coal gas as a fuel for
electric power generation inthe integrated gasication combined
cycle (IGCC) plants. IGCC integrates twocommercially proven
technologies: the manufacture of a clean-burning fuel gasfrom coal
and the highly efcient use of that gas to produce electricity in a
combinedcycle power generation system. The combination of the gas
turbine and steamturbine cycles gives IGCC systems a coal-to-power
efciency of 4045%, comparedwith about 3540% achieved by
conventional coal-red steam-cycle power plants[8, 9]. Conventional
plants can produce electricity having heat rates of about 10MJkWh1
and 90% SO2 removal. The heat rates for IGCC plants are from 8.0
to9.5MJ kWh1with 99%SO2 removal [10]. Additional efciency gains
canbe achieved
Handbook of Combustion Vol. 4: Solid FuelsEdited by Maximilian
Lackner, Franz Winter, and Avinash K. AgarwalCopyright 2010
WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN:
978-3-527-32449-1
j325
-
through hot-gas cleaning, improved design of gas turbines, and
application of fuelcells. It is anticipated that with these
innovations in place, IGCC systems can reachthermal efciencies
close to 60% [8, 9, 11].Gasication-based energy conversion systems
are capable of providing stable,
high-efciency energy supply with reduced environmental impact
compared withconventional technologies. Existing commercial IGCC
power plants have met themost stringent pollutant emission limits
currently applicable to combustion-basedpower plants. They have
achieved the lowest levels of NOx, SOx, CO, and PM10(particulate
matter smaller than 10mm) of any coal-red power plants in theworld
[12, 13].The high coal-to-power efciency of IGCC systems also
provides a signicant
advantage in responding toCO2 emissions and thus to global
warming concerns. Themost advanced IGCCunits can reduceCO2
emissions by about 1520%relative to theemissions from conventional
coal-red power plants with typical emission controlsystems.
Moreover, they can be readily adapted to concentrate CO2 for
subsequentstorage and sequestration. Carbon dioxide capture from
IGCC plants requires lessenergy and is signicantly cheaper than in
the case any other fossil fuel-based powerplant [9].Gasication is
the cleanest of all commercial coal-based technologies [14] and
provides a feasible and economical route to produce hydrogen
from abundant coalreserves. In the twenty-rst century, coal
gasication will be at the heart of a newgeneration of energy plants
possessing product exibility, near-zero emissions, lowproduction of
solid wastes and waste water, high thermal efciency, and ability
tocapture CO2. IGCC systems can be congured to generate electric
power and toproduce, at the same time, ultra-clean transportation
fuels, chemicals, andhydrogen for fuel cells. In developing
countries with no oil and natural gasdeposits, coal gasication
systems can increase domestic energy security andimprove foreign
trade balance. With the development of CO2 capture andsequestration
technologies and the large reserves of coal,
gasication-basedsystems are poised to be the technology of choice
during the transition to ahydrogen-based economy.
9.2Fundamentals of Coal Gasification Technology
Gasication is a commercially proven technology for the
conversion of solid andliquid carbonaceous feedstocks to
combustible gases. It also provides the lowest-costapproach for
capturing carbon dioxide [1].Common gasifying agents used in
commercial gasiers include mixtures of air or
oxygen with steam. The chemical composition and caloric value
(CV) of the gasproduced depend on coal composition, gasifying
agent, gasication conditions, andplant conguration [7]. Depending
on the intended application three types of syngascan be
produced:
326j 9 Gasification and Pyrolysis of Coal
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. low heating value gas with CV< 10.0MJm3;
. medium heating value gas with CV in the range
10.020.0MJm3;
. high heating value gas with CV> 20.0MJm3.
A low-heating value gas is produced in air-blowngasiers and is
primarily used as afuel gas for gas turbines (IGCC) and industrial
furnaces. A medium-heating valuegas, consisting essentially of CO
andH2, is produced by gasication with oxygen andsteam for use as a
fuel gas or for chemical synthesis.The choice of appropriate
gasication technology depends on many diverse
factors, which include coal availability, type and cost, the
syngas production rateand its end use, and turndown requirements
[7]. Among them, the physical andchemical properties of the coal
such as char reactivity, volatile matter, ash andmoisture contents,
and swelling propensity are the most important factors thatinuence
the design and performance of a gasication plant [1517].High
reactivity is a desirable property for gasication as it increases
carbon
conversion and reduces oxygen and steam consumption, thus
improving gasicationefciency. The reactivity of coals depends
strongly on their mineral matter contentand composition. Low-rank
chars can be up to 100more reactive than chars fromhigher rank
coals [16]. Gasication of low-rank coals can be efciently conducted
atlower temperatures and higher supercial gas velocities, resulting
in smallerequipment and lower capital costs.However, the high
moisture content of low-rank coals acts as a dilutent in the
gasication process and causes efciency losses when too much
water is fed into thegasier. To achieve the same energy output as
in the case of high-rank coals, coal feedrates have to be greater,
resulting in larger gasication plant. The high oxygen contentof
low-rank coals affects the gasication process in the similar way
[16].Gasication of coals with high volatile matter contents in
xed-bed gasiers
produce large quantities of condensable tars and oils that have
to be separated fromthe syngas stream. In the case of uidized-bed
gasiers and entrained-ow gasiersoperated at temperatures above 800
C, the need for separation facilities can beeliminated because
heavy organic compounds are cracked to light gases.Gasication of
bituminous coals in xed-bed gasiers can be problematic due to
their propensity to caking and swelling. Low-rank coals do not
have caking propertiesand can be processed in all types of gasiers
[16].Ash content is an issue for all types of gasiers as it reduces
the overall process
efciency. Air-blown xed-bed and uidized-bed gasiers with dry ash
removal aremore suitable for high ash coals.The most signicant ash
property is its fusion temperature. The ash fusion
temperature determines the maximum operating temperature for
gasiers with dryash removal systems, and theminimumoperating
temperature for slagging gasiersthat discharge the ash as a molten
slag [9].Physical and chemical properties of the coal mineral
matter can also affect the
operation of a gasier and downstream equipment through the
erosive and corrosiveaction on metal and refractory surfaces
[9].
9.2 Fundamentals of Coal Gasification Technology j327
-
9.3Pyrolysis and Gasification Chemistry
9.3.1Pyrolysis
Pyrolysis refers to the decomposition of organicmatter by heat
in the absence of air. Acommon synonym for pyrolysis is
devolatilization. Pyrolysis is an importantprocess in all coal
conversion technologies. When coal is pyrolyzed,
hydrogen-richvolatile matter (gases, oil, and tar vapors) is
released and a carbon-rich solid residue(char or coke) is left
behind. The volatiles released,which can account for up to 70%ofthe
coals mass loss, control the ignition, temperature, and stability
of ame incombustion and the temperature and product distribution in
gasication. Moreover,the pyrolysis process controls softening,
swelling, particle agglomeration, charreactivity, and char physical
structure.The chemistry of pyrolysis includes the thermal
decomposition of individual
functional groups in the coal to produce light gaseous species,
and the decompositionof macromolecular network to produce smaller
fragments that can evolve as tar. Thenetwork decomposition is a
complex set of bridge breaking, crosslinking, hydrogentransfer,
substitution reactions, and concerted reactions and others
[1822].Upon heating, the bridges having lower bond energies would
rst decompose.
Hydrocarbon gases released during pyrolysis come from the
decomposition ofaliphatic structures of coal molecule, while gases
such as CO, CO2 and H2O areproduced through the decomposition of
oxygen-containing functional groups, andhydrogen comesmainly from
condensation reactions of aromatic nuclei [23, 24].
Thedecomposition of CH2CH2 bridges is responsible for tar formation
in the case ofhigh rank coals, while the decomposition of CH2O
bridges is more important fortar formation in low rank coals [25,
26].The coal pyrolysis process can be divided into three principal
stages [27]. In therst
stage, which occurs below 300 C, thermal decomposition is slow
and the yield ofvolatiles small. The primary products of this stage
are oxides of carbon (CO, CO2),water, and hydrogen sulde. The
second stage occurs in the temperature range350550 C. The
decomposition reactions are fast and approximately 75% of all
theultimate volatile matter is released. The main products of this
stage are lighthydrocarbon gases and a great variety of organic
condensable compounds that formtar. The third stage occurs at
temperatures above 550 C and involves the secondarydevolatilization
associated with the transformation of the char. Typical products
ofthis stage are hydrogen (main product) and non-condensable gases
such as CO, CO2,CH4, C2H6, C2H4, and NH3.The quantity of volatiles
released is highly dependant on pyrolysis conditions
and can exceed 150% of the proximate analysis value [28]. The
yield of volatilesincreases with increasing heating rate,
temperature, and residence time at naltemperature. The reduction in
coal particle size and gas pressure also helps toincrease volatile
yields. At low temperatures (550 C), the total yield of volatiles
islow because thermal decomposition is incomplete. At these
temperatures high
328j 9 Gasification and Pyrolysis of Coal
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molecular mass species are stable, and the product distribution
favors liquids (oiland tar). At high temperatures (1000 C),
devolatilization is complete and theoverall volatile yield high.
However, elevated temperatures cause substantialcracking of oil and
tar components and give light gases as the principal productof
devolatilization.The effect of heating rate on the amount and
composition of volatiles released
during pyrolysis is an important question [27]. Rapidly heated
ne coal particlesusually produce larger volatile yields than those
predicted by the proximate analysis.In slow pyrolysis (heating
rates< 102 K s1), the decomposition reactions are alwaysin or
close to the local equilibrium. In this case, the ultimate volatile
yield and productdistribution depend on the temperature history. In
contrast, in fast pyrolysis (heatingrates> 104 K s1), the
decomposition reactions proceed at the nal temperatureunder almost
isothermal conditions.The effect of particle size on pyrolysis
yield is related to heating rate. Large particles
heat up slowly, their average particle temperature is lower, and
hence volatile yieldsmay be less. Experimental data indicate that
particle size does not affect volatile yieldsfor particles smaller
than 50 mm [28].Pressure affects volatile yields,with higher
pressures reducing the yields and lower
ones increasing them. However, at higher pressures cracking
reactions occur thatproduce larger volumes of light gases, whereas
at lower pressures larger tar and oilfractions are produced
[27].The prediction of yield and product distribution in coal
pyrolysis is difcult
because the extreme complexity of the process precludes accurate
thermodynamicanalysis. Earlier experimental and modeling studies
have been reviewed by Anthonyand Howard [29], Juntgen and van Heek
[30], Kobayashi et al. [31], Howard [28] andSolomon and Hamblen
[32], and more recent ones by Serio et al. [33], Saxena
[34],Solomon et al. [35] andNiksa et al. [36]. The results of
pyrolysis experiments have beeninterpreted with several assumptions
on the heat transfer and coal particle temper-ature using various
kinetic models.The simplest and the most often employed model is
the single rst-order model,
which has been applied for overall mass loss and for individual
species evolution. Inthis model, the rate of devolatilization is
expressed as:
dVdt kVV 9:1
where
k k0exp E=RT 9:2
The experimental values of rate constant, k, for different coals
and reactionconditions have been reported by Solomon and Hamblen
[32]. The large discre-pancies in the values of k can be attributed
to differences in reaction conditions andexperimental techniques
employed, variation in rank of tested coals, and to the factthat
some of the constants may have not been compared at the same level
ofconversion.
9.3 Pyrolysis and Gasification Chemistry j329
-
Themodeling of the complex pyrolysis process by a single
rst-order reaction is anoversimplication. A more advanced model has
been proposed by Anthony andHoward [29]. In this model, the
pyrolysis is assumed to consist of a large number ofindependent
rst-order chemical reactions that represent the rupture of
variousbonds within the coal structure:
dVidt
kiVi V 9:3
The rate constants, ki, are assumed to differ only in the values
of their activationenergies Ei. The distribution of Ei values is
expressed as a continuous Gaussiandistribution function:
f E 1s
2p
p exp EEo2
2s2
" #9:4
and:
10
f E dE 1 9:5
The relative amount of volatiles remaining in the coal is
obtained by integratingEquation 9.3 over all values of E using
Equation 9.4:
VVV
10
exp k0t0
expE=RT dt24
35f E dE 9:6
This model, called the distributed activation energy model
(DAEM), represents asignicant improvement over the single rst-order
reaction model. The DAEM hasbeen successful in predicting the
temperature response and the residual volatilemater content for
different coals.Pyrolysis of large coal particles, for example in
xed-bed and uidized-bed
combustion and gasication, may be controlled by heat transfer to
and within thecoal particle, chemical kinetics, and mass transfer
of volatiles within the porestructure of the coal particle. Several
mathematical models have been developed todescribe the
devolatilization of large coal particles and all these models
haveconrmed that the effect of mass transfer within pores is small
and can beneglected [3742]. These results support the assumption
that heat transfer andchemical kinetics dominate the overall
reaction mechanism.The DAEM does not provide information on the
volatile product distribution. In
1980s and 1990s detailed species evolution models and even more
sophisticatedgeneral mechanistic models, which include chemistry,
heat transfer, and masstransport, were developed [4357]. The
product distribution predictions generatedby these models are,
generally, in good agreement with experimental data. However,the
main disadvantage is that they require detailed information on the
internalstructure of the coal, which is not readily available.
330j 9 Gasification and Pyrolysis of Coal
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9.3.2Stoichiometry and Thermodynamics of Gasification
A gasication process must satisfy chemical constraints based on
the stoichiometryof the coal gasication reactions and the energy
requirements to sustain thesereactions. Gasication of char produced
by the devolatilization process involveschemical reactions between
primary reactants, that is, carbon in the char, oxygen, andsteam,
as well as several reactions between primary and secondary
reactants that is,CO, CO2, andH2. Table 9.1 shows themost important
gasication reactions and theirstandard enthalpies, DH, at 25 C
(298K).Char combustion reactions (9.7) and (9.8) take place
simultaneously. These
reactions can be combined and written in the form:
C 1wO2! 2 2
w
CO 2
w1
CO2 9:19
Table 9.1 Gasification reactions and their standard enthalpies
at 25 C (298 K).
Reaction DH298 (kJ mol
1)
C 1=2O2!CO 9:7 110.51
CO2!CO2 9:8 393.51
CO 1=2O2!CO2 9:9 283.00
H2 1=2O2!H2O 9:10 242.00
CH2O!COH2 9:11 131.49
C 2H2O!CO2 2H2 9:12 90.49
CCO2! 2CO 9:13 172.49
C 2H2!CH4 9:14 75.19
2C 2H2O!CH4CO2 9:15 7.65
3C 2H2O!CH4 2CO 9:16 62.60
CO 3H2!CH4H2O 9:17 206.68
COH2O!CO2H2 9:18 41.00
9.3 Pyrolysis and Gasification Chemistry j331
-
where w is the mechanism factor based on stoichiometric relation
of CO and CO2.The value of w can be estimated by the following
equations [58]:
w 2Z 2Z 2 for dp 0:00005m 9:20
w 2Z 2Zdp0:00005=0:00095Z 2 for 0:00005m dp 0:001m
9:21
w 1:0 for dp > 0:001m 9:22
Z 2500 exp 6249=T 9:23
T TsTg2
9:24
The overall char conversion rate is determined by the slowest
reactions, which arethe heterogeneous reactions of carbon with
steam, CO2, and H2.In contrast to combustion processes, gasication
processes operate at sub-stoi-
chiometric conditions with the oxygen supply controlled in the
range 2070%(usually 35%) of the amount required for complete
combustion. Reactions (9.7)and (9.8) consumemost of the oxygen fed
to the gasier. These reactions provide thethermal energy necessary
to dry the coal, to heat up the products to reactiontemperature,
and to drive the endothermic gasication reactions.The Boudouard
reaction, (9.13), is several orders of magnitude slower than
the
CO2 reaction at the same temperature. Below 1000K and in the
absence of catalyst,this reaction proceeds very slowly.The
water-gas reaction, (9.11), is slightly faster than the Boudouard
reaction under
the same conditions. Both the Boudouard reaction and the water
gas reaction arefavored by lower pressures and are inhibited by
their products.The slightly exothermic CH2 reaction, (9.14), is
very slow except at high pressures.Two homogeneous gas-phase
reactions the water-gas shift reaction, (9.18), and
the methanation reaction, (9.17), are important from the nal gas
quality point ofview. The water-gas shift reaction determines the
H2/CO ratio, which is important ifthe gas is used for the synthesis
of hydrocarbons. The methanation reaction, (9.17),increases the
caloric value of the gas, which is desirable if the gas is used as
a fuel(e.g., in an IGCC plant).Stoichiometric analysis of a xed
carbonsteamoxygen system shows that not all
of the reactions, (9.7)(9.18), in Table 9.1 are linearly
independent [59]. Any coalgasication reaction can be constructed by
non-negative linear combinations ofequations describing reactions
(9.7), (9.8), (9.11), (9.12), (9.15), and (9.16). Compar-ison with
full scale and pilot-plant data shows that these six reactions are
sufcient topredict the overall change and product distribution [27,
59, 60].It is very difcult to predict the exact composition of the
product gas from a
gasier [8]. Product gas composition depends on the chemical
composition of the
332j 9 Gasification and Pyrolysis of Coal
-
coal and gasifying agent, the feed coal-to-gasifying agent
ratio, and on the operatingtemperature and pressure. However,
because the rates of chemical reactions attemperatures prevailing
in gasication processes (8001800 C) are sufciently high,predictions
based on the assumption of chemical equilibrium give results that
areclose enough to reality [9, 27]. Both stoichiometric models
(based on equilibriumconstants) and non-stoichiometric models
(based on Gibbs free energy minimiza-tion) have been developed to
predict the performance of gasiers [6165]. All thesemodels give
good estimates of the gas composition from uidized-bed,
spouted-bed,and entrained-ow gasiers.
9.3.3Kinetics of Gasification Reactions
The reactivity of coal chars depends on several factors such as
pyrolysis conditionswhich convert coal into char, the properties of
coal minerals, and gasicationconditions which convert char into
gases [66]. The process of char gasicationinvolves external mass
transfer of reactants and products, diffusion of reactants
andproducts through the pores of the char particle, and surface
chemical reactions.Temperature is one of the most important factors
for the stable operation of a
gasier. It affects both transport processes (diffusion) and
rates of gasicationreactions. At low temperatures chemical
reactions are slower and therefore theirkinetics control the carbon
conversion process. In contrast, at higher temperaturesdiffusion is
slower than chemical reactions and becomes the rate-controlling
step.The rates of heterogeneous gasication reactions can be
estimated from the
unreacted shrinking-core model proposed by Wen [6769]. The model
takes intoaccount gas lm diffusion, ash layer diffusion, and
chemical reaction effects.According to this model, the overall
reaction rate is expressed as [70]:
r dp6
PjPjPdp6kg
d2pRT12Da1jj 1gj3CCks
9:25
For the CO2 reactions [(9.7) and (9.8)] the equilibrium oxygen
partial pressure,PO2 , is practically zero. The equilibrium partial
pressure of reactant j, P
j , can be
calculated from the expression for the relevant equilibrium
constant [70].Specic reviews on the mechanisms involved in
heterogeneous coaloxygen and
related reactions have been given by Laurendeau [71], Wendt
[72], Johnson [73], andSmith [74, 75]. The intrinsic reaction rate
coefcient, ks, varies widely with the type ofcarbon investigated.
For different carbons at a given temperature, differences
inreactivity up to four orders of magnitude have been found [75].
Correlations forkinetics constants of homogeneous andheterogeneous
reactions can also be found inpapers dealing with mathematical
modeling of combustion and gasication sys-tems [58, 6870,
7686].There is a considerable variation in the proposed mechanisms
and kinetic
expressions used to correlate experimental data. This variation
in kinetic datareported in literature is due to the type of feed
material used, the range of
9.3 Pyrolysis and Gasification Chemistry j333
-
experimental conditions employed, the differences in the
techniques used to evaluateexperimental results, and the reactor
system design. The kinetics of gasication arestill the subject of
intensive investigations because existing kinetic models are
oflimited value in designing commercial gasiers [9, 11].
9.4Coal Gasification Technologies
A recent database compiled by theUnited StatesDepartment of
Energy (USDoE) andthe National Energy Technology Laboratory (NETL)
[87] summarized the industrystatus of gasication technologies.
According to this database 193 coal gasiers are inoperation, and
more coal gasiers would be started up in the near future.
Thisdatabase includes only commercial applications with a capacity
exceeding anequivalent of 100 MWe (megawatt electrical).Coal
gasication technologies can be classied in several different ways.
Themost
popular way of classifying coal gasication technologies is
according to the gasierconguration or, in other terms, the type of
reactor bed. According to Simbecket al. [88] most gasiers can be
classied into three generic types based on the reactorbed, which
are commonly referred to as:
. xed or moving-bed gasiers
. uidized-bed gasiers
. entrained-ow gasiers.
In xed or moving-bed gasiers, the coal particles are moved
either mechanicallyor by gravity, and not by the oxidant or
reaction gases [89]. In the case of a uidized-bed gasier, the coal
particles are suspended in the gasifying agent or reaction
gases,whereas for entrained-ow gasiers the gas velocity is high
enough to accelerate andpneumatically lift the coal particles
[89].Rotary kiln and molten bath gasiers are examples of gasiers
that cannot be
classied into the three generic types mentioned above. According
to a report byCollot [7] the rotary kiln and molten bath
technologies are far from commerciali-zation. These technologies
will not be reviewed here.There also exists a gasication technology
inwhich the coal is not gasied in a steel
reactor vessel but, rather, it is gasied in situ. This process
is also called undergroundcoal gasication (UCG). Furthermore, in
most of the gasication processes theenergy required to gasify the
coal is supplied by combustion of part of the coal. Theseprocesses
are referred to as autothermal gasication processes. There are
someprocesses inwhich the energy required is supplied by an
external source, for example,a plasma arc. In situ coal gasication
and a non-autothermal gasication process arebriey reviewed in
Section 9.4.4.This chapter is by nomeans extensive. There is a
plethora of information regarding
coal gasication technologies in the open literature. Selected
reviews of gasicationtechnologies are the texts by Collot [7],
Higman and van der Burgt [9], Rezaiyan andCheremisinoff [11], and
Supp [89].
334j 9 Gasification and Pyrolysis of Coal
-
9.4.1Fixed-Bed Gasifiers
Fixed-bed gasiers are also referred to as moving-bed gasiers.
Some authorsdifferentiate between xed-bed and moving-bed gasiers,
saying that moving-bedgasiers are equipped with a mechanical
stirrer, which moves or stirs the coalbed [90]. These stirrers are
installed to prevent the agglomeration of coal particles,which is
the case for caking coals. The terms xed- andmoving-bed gasier are
usedsynonymously in this chapter.In xed-bed gasiers, the coal and
oxidant may either ow co-currently or counter-
currently. All major or commercial applications of xed-bed
gasiers are of thecounter-current type [1].In
counter-currentxed-bed gasiers, coal is introduced at the top of
the gasier via
lock hoppers and is consumed as it moves slowly downwards. The
ash is dischargedfrom the bottom of the gasier either as a dry ash
or a slag. The residence time of coalparticles in xed-bed gasiers
is 1560min for pressurized steam and oxygen-blowngasiers. For
atmospheric air and steam-blown xed-bed gasiers, the coal
residencetime may be several hours [7].The oxidant is injected at
the bottom of the gasier and is distributed either by an
ash grate or tuyeres, depending on the manner in which the ash
is discharged fromthe gasier. The raw gas exits at the top of the
gasier and is laden with pyrolysisproducts such as tar, oil, and
light hydrocarbon gases. The raw gas outlet temperatureof a xed-bed
gasier is substantially lower than the corresponding raw gas
outlettemperatures of both uidized-bed and entrained-ow
gasiers.Figure 9.1 shows a generic representation of a
counter-current dry-ash xed-bed
gasier. Typical coal and gas temperature proles are also
illustrated.The coal that is fed to the top of xed-bed gasiers is
graded lump coal particles
(580mm) [7]. Fixed-bed gasiers are not suited for processing of
a ne coal onlystream. Fine coal particles introduced at the top of
the gasier via the lock hopper will
Figure 9.1 Generic representation of a counter-current dry-ash
fixed-bed gasifier and typical coaland gas temperature profiles
[91].
9.4 Coal Gasification Technologies j335
-
be entrained in the raw gas and can block the passage of the raw
gas. However, theslagging version of the xed-bed gasier can process
a limited amount of ne coal,which is injected via the
tuyeres.According to the survey by the US DoE and NETL [87],
xed-bed coal gasiers
account for approximately 75% of the actual number of operating
commercial-scalecoal gasiers. A description of the major xed-bed
coal gasication technologiesfollows below.
9.4.1.1 Fixed-Bed Dry-Bottom (FBDB) ProcessThe Sasol Synthetic
Fuels complex in Secunda, South Africa is currently the
worldslargest gasication centre. A total of 80 Sasol FBDB gasiers
are installed in thiscomplex [92], where approximately 79.2 million
m3n d
1 of syngas is produced fromthe gasication of sub-bituminous
coal [87]. Other major installations of the FBDB(xed-bed dry
bottom) gasication technology are at
theDakotaGasicationCompanyplant located in North Dakota, USA, and
the Vresova IGCC plant located in the CzechRepublic. Both of these
complexes gasify lignite and produce approximately 13.9 and4.7
million m3n d
1 of syngas, respectively [87].Figure 9.2 shows a diagram of a
FBDB gasier. In this gasier the coal and
oxidant ow counter-currently to each other and the ash is
discharged as a dry ash.The oxidant is a mixture of high-pressure
oxygen and steam.Atmospheric coal bunkers are situated above the
coal lock, which is in turn situated
above the gasier. Coal is introduced into the gasier via these
coal lock hoppers. Thecoal lock is isolated from the atmospheric
bunker and pressurized reaction vessel via
Figure 9.2 Representation of a FBDBTM gasifier (reprinted with
permission from Elsevier) [9].
336j 9 Gasification and Pyrolysis of Coal
-
hydraulically operated cone valves. The coal lock is
periodically charged with coalfrom the atmospheric bunker and coal
is thus periodically charged into the gasier.The coal lock is
usually pressurizedwith rawproduct gas takendownstreamof the
gascooling train. Other gases such as nitrogen, carbon dioxide,
andmethanemay also beused as a coal lock pressurizing gas [93].The
thin-walled gasier is contained within a pressure bearing
thick-walled outer
shell. Water is circulated in the annular space between the
gasier and the outer shell.Heat is transferred from thegasier to
the circulatingwater,which results in coolingofthe gasier shell and
the formation of steam that is termed jacket steam. Jacket steamis
recycled back in the gasier, which decreases the overall steam
consumption.The gasiermay be equipped with both a coal distributor
and amechanical stirrer,
collectively referred to as a Coal Stirrer Distributor or CSD,
as illustrated inFigure 9.2. The mechanical stirrer is essentially
a set of rotating blades. The stirreris installed only if caking
coals are to be processed. A revolving ash grate is installed atthe
bottom of the gasier.According to Rudolph [93], the ash grate
serves several purposes: support of the
fuel bed, extraction of the dry ash, and distribution of the
oxidant. The dry ash fallsinto a pressurized chamber called the ash
lock. Ash is periodically discharged intoeither an underground ash
sluiceway or a dry removal system. In sluiceway systemsthe ash lock
is connected to theunderground ash sluiceway via a chute. The ash
lock isisolated from the gasier and chute via hydraulically
operated cone valves. Theoperation of the ash lock is comparable to
the coal lock operation.The temperature of the raw gas at the
gasier exit is dependent on the type of coal
being processed. For coals with low ash fusion temperatures and
for coals with highinherent moisture content (such as lignite), a
typical raw gas exit temperature is250 C. For bituminous coals this
exit temperaturemay be approximately 550 C [93].The low raw gas
exit temperature that is characteristic of counter-current
xed-bedgasiers nullies the need for expensive heat recovery
equipment, which is the casefor uidized-bed and entrained-ow
gasiers. The raw gas exiting the gasier isimmediately quenched with
gas condensate in the wash cooler. Heavy hydrocarbons(which are
condensed as the gas is cooled) and entrained ne coal particles
arescrubbed out by the gas condensate. Both the de-dusted raw gas
and gas condensateare then routed to the waste heat boiler
sump.Coals of all ranks, ranging from peat to anthracite, have been
successfully gasied
at either commercial or pilot-plant scale using the FBDB
gasication technology.Furthermore, coals with high ash content
(>50% ash) have also been successfullygasied in commercial
plants [93]. The FBDB gasier has low oxygen consumptioncompared
touidized-bed and entrained-owgasiers. Steam is used as
amoderatorin the FBDB gasier and the steam-to-oxygen ratio is
adjusted to ensure that thetemperature within the gasier never
exceeds the ash fusion temperature. Depend-ing on the type of coal
being processed, the steam-to-oxygen ratiomay range from 4.5to more
than 6:0 kgm3n [89].The FBDB technology is amature technology and
is in commercial use in several
locations across the globe. The syngas derived from the FBDB
process is used forseveral applications, which include power
generation, substitute natural gas, syn-thetic fuels, and chemicals
production.
9.4 Coal Gasification Technologies j337
-
9.4.1.2 British Gas/Lurgi ProcessThe British Gas/Lurgi (BGL)
gasication technology is currently licensed by Ad-vantica/Allied
Syngas Corporation and Envirotherm. The history of the
developmentof this gasication technology is well documented by
Brooks et al. [94], and itsdevelopment can be traced back to the
1950s. The only commercial application of thistechnology was at the
Schwarze Pumpe complex in Germany, where approximately2.3 million
m3n d
1 of syngas was produced from the gasication of biomass andwaste
to produce electricity and methanol [87].The BGL gasier is a
counter-current xed-bed gasier; essentially, it is a slagging
version of the FBDB gasier. Figure 9.3 shows a diagram of the
BGL gasier. Thisgasier has a different ash extraction and oxidant
injection systems compared to theFBDB gasier. Apart from the
differences in the gasier, the BGL process is verysimilar to the
FBDB process; hence only the differences between the two
xed-bedtechnologies will be discussed.The gasier is contained
within an outer pressure bearing vessel and it may be
refractory lined. The gasier is cooled by means of generating
jacket steam as is thecase for the FBDB gasier.There is no rotating
ash grate in the BGL gasier because of the high temperatures
that prevail in the lower section of the gasier. These high
temperatures are required
Figure 9.3 Representationof aBritishGas/Lurgi gasifier
(reprintedwithpermission fromElsevier) [9].
338j 9 Gasification and Pyrolysis of Coal
-
to melt the ash and to ensure continuous discharge of slag. The
slag is dischargedthrough a tap located in the center of the
gasier. It then ows into a quench chamberand solidies when it comes
into contact with the quenchwater. To ensure successfulslag tapping
and low oxygen consumption, a certain slag viscosity at a certain
slagtapping temperature is desired. For the BGLprocess this
viscosity is less than 5Pa s ata tapping temperature of 1400 C [7].
A ux is added to the coal feedstock to achievethe desired viscosity
at the tapping temperature. The solidied slag forms an inertglassy
frit. The frit and quench water are periodically discharged into a
slag lockhopper. The contents of the slag lock hopper are further
processed in a slag handlingsystem. The slag lock hopper is
isolated from the quench chamber and slag handlingsystem via cone
valves. In the slag handling system the solid particles are
separatedfrom the quench water. The quench water is cooled and
recycled to the quenchchamber and slag lock hopper.The oxidant,
which is amixture of steamand oxygen, is injected into the gasier
via
water cooled tubes called tuyeres. Additionally, ne coal and
liquid feedstock (tar andoil)may be injected into the gasier via
these tuyeres. It is claimed that when the tar isinjected via the
tuyeres it is completely gasied and it may be recycled toextinction
[94].TheBGLgasierhas lower steamconsumption than theFBDBgasier.
Supp [89]
states that the steam consumption and gas condensate production
of a BGL gasierare approximately one-sixth and one-fth,
respectively, of a FBDB gasier. A BGLgasier typically operates with
a steam to oxygen volumetric ratio that varies between1.0 and 2.0
[94], to achieve the high temperatures that are required.Various
coal feedstocks have been gasied in the BGL gasier [94];
however,
mention is made that the BGL gasier is suited for the gasication
of low reactivity,high rank coals. Furthermore, the ash content
should be kept below 20% and the ashshould have a melting
temperature below 1200 C [89].
9.4.2Fluidized-Bed Gasifiers
Fluidized-bed gasiers are gasiers in which a mixture of freshly
introduced coal,partly gasied coal, and ash particles is suspended
in a oating bed by the gasifyingagent. The ow pattern inside the
reactor is best described as well mixed orcontinuously stirred
[95]. As the coal particles react, they shrink in size,
becomelighter and the ne coal and ash particles are entrained in
the raw gas. To improve theoverall carbon conversion the entrained
particles are recovered from the raw gas andrecycled back to the
gasier.Coal that is fed to uidized-bed gasiers is crushed coal with
sizes usually between
0.5 and 5.0mm. Feedstocks with a high percentage of nes can be
problematic toprocess, and should be avoided. The residence time of
the feed in uidized-bedgasiers is in the range 10100 s
[7].Fluidized-bed gasiers have a moderate oxygen and steam
consumption and are
usually operated at temperatures below the ash fusion
temperature. There are certainuidized-bed processes in which local
temperatures are higher than the ash fusion
9.4 Coal Gasification Technologies j339
-
temperature. In these cases ash is intentionally sintered to
enable its easier removal.However, in all uidized-bed processes
excessive ash sintering is undesirable as thiscould cause
instabilities in the gasier.The raw gas exit temperature of
uidized-bed gasiers is on the order of 1000 C
and the raw gas contains small amounts of hydrocarbons. The ash
particles entrainedin the raw gas are non-sticky and hence no
quench is required before entering thesyngas cooler, which reduces
the complexity and cost of the heat recovery equipment.Figure 9.4
shows a generic representation of a uidized-bed gasier and the
corresponding gas and solid temperature prole. Compared to
xed-bed andentrained-ow gasication technologies, uidized-bed
gasication technologieshave limited commercial application [1,
90].
9.4.2.1 High Temperature Winkler ProcessThis technology is an
extension of the originalWinkler process. The
originalWinklerprocess was designed for atmospheric pressure
operation. The rst Winkler gasierwas built in 1925 and a total of
70 atmospheric pressureWinkler units were built [9].The combined
syngas production capacity from the atmospheric Winkler
processespeaked at about 20 million m3n d
1. Currently there is only one application of theatmospheric
pressure Winkler process, which is in the former Yugoslavia.
Thesyngas production capacity of this unit is approximately 120 000
m3n d
1 [87]. Thehigh temperature Winkler (HTW) process is actually a
high-temperature and high-pressure version of the original Winkler
process [96]. The high temperature oper-ation was desired to
increase the carbon conversion, and reduce the gaseous andliquid
hydrocarbon production, whilst the high pressure operation was
desired toincrease the throughput [9].The HTW process was developed
by Rheinbraun. An atmospheric process
development unit (1.0 t d1) was rst operated, then a pressurized
pilot plant unitoperated from 1978 to 1985, followed by a
pressurized (9.0 bar) demonstration scaleunit, which was started up
in 1986 [96]. This demonstration-scale unit was designedfor
synthesis gas applications and therefore the pressure was limited
to 9.0 bar to
Figure 9.4 Generic representation of a fluidized-bed gasifier
and typical coal and gas temperatureprofiles [91].
340j 9 Gasification and Pyrolysis of Coal
-
reduce the formation of methane, since methane is undesired in
certain synthesisprocesses. However, Rheinbraun also developed a
version of the HTW process thatoperated at approximately 24.0 bar.
This process was designed for IGCC applicationsand the oxidant was
either air or oxygen [96]. According to the US DoE and NETLsurvey
[87] there were no applications of the HTWprocess on a scale large
enough tobe included in the database. Figure 9.5 shows a schematic
of the HTW process.The HTW gasier is lined with a refractory and is
cooled by circulating boiling
water inside a jacket [1]. Coal with amaximum size of 8.0mm is
pressurized in a coallockhopper, and then it is fed into the gasier
using a variable speed screw feeder [95].The gasifying agent, which
is a mixture of steam and oxygen or steam and air, isintroduced
into the gasier at two different points. Conceptually this gasier
may beseen as consisting of two reaction zones, namely a primary
and secondary reactionzone. The primary reaction zone is located
towards the bottom of the gasier. In theprimary reaction zone the
gasifying agent is injected at the bottom of the gasier touidize
the solid particles. The temperature in the primary reaction zone
is typicallybetween 850 and 950 C [89, 90], and is maintained at
these levels, to preventagglomeration of the ash particles. Any ash
agglomeration would cause instabilitiesin the gasier and even
de-uidization of the bed. Fine coal and ash particles areentrained
into the upper secondary reaction zone of the gasier, whereas the
heavierash particles tend to accumulate in the bottomsection of the
gasier. The ash particleseventually move downwards into an ash
lock, and are removed using screw feeders.The ash product contains
about 20% carbon, and is typically used as a feed for coalboilers
[89].To increase the overall carbon conversion and to decrease the
hydrocarbon yield,
gasifying agent is injected into the secondary reaction zone.
The temperature in thiszone is approximately 150230 C higher than
the temperature in the primaryreaction zone. To ensure that the
temperature in the secondary reaction zone does
Figure 9.5 Representation of a HTW gasifier [90].
9.4 Coal Gasification Technologies j341
-
not exceed the ash softening temperature, a small amount of
quench water isinjected [96].Unreacted char and ash are entrained
in the raw gas that leaves the gasier. Some
of these solid particles are recovered in a hot cyclone and are
returned to the bottomof the gasier via a hot dipleg. The raw gas
from the hot cyclone is then cooled toabout 300 C in a syngas
cooler, which may either be a re tube or water tubedesign [9]. The
cooled raw gas then passes through a candle lter to recover
theentrained y ash.It has been mentioned that this process is
restricted to the processing of low
rank, reactive, non-caking coals [89]. If higher rank, low
reactivity coals are gasied inthe HTW process, the carbon content
in the bottom ash product could increase tosuch a level that direct
combustion of this ash product in an auxiliary device isnecessary.
Furthermore, coals with high ash melting temperatures are
preferred,since this allows operation at higher temperatures, which
would increase the carbonconversion.
9.4.2.2 Kellogg Rust Westinghouse ProcessThis technology was
developed by Westinghouse Electric Corporation, and wassubsequently
renamed the Kellogg Rust Westinghouse (KRW) gasication processafter
Westinghouse was succeeded by Kellogg Rust in 1984 [97]. A pilot
plant wasoperated byWestinghouse between themid-1970s and 1980s
[96]. Ownership of thisprocess was later transferred to the M.W.
Kellogg Company in 1986 [97]. Ademonstration-scale unit was
proposed for the Sasol Synfuels site in Secunda, SouthAfrica, but
this unit was never built [1, 96]. This gasication technology was
used inthe Pinon Pine Power Project. Start-up of the gasier began
in 1996. Owing to theserious problems that were experienced in the
solids removal, and hot ltrationsections, this gasier was never
successfully started up and was mothballed in2001 [90].In the
KRWprocess, coal is pneumatically transportedwith air and is
introduced at
the bottomof the gasier via a central coaxial burner, which
extends into the uidizedbed [97]. Additional air is fed downstream
of the coal injection location. The coal andair mix at the outlet
of the burner and the coal is immediately devolatilized [1].
Thevolatile material is combusted, and the remaining char is gasied
with steam. Coolrecycled raw gas is injected at several locations
to uidize the bed and to control thetemperature in the ash
agglomerating zone [96]. The bulk operating temperature ofthe
gasier is about 982 C [97], but the temperature in the
agglomerating zone ishigher. Limestone is injected with the coal
for the purpose of in situ sulfur capture.The up owing gases form
large bubbles that force the solid particles to recirculatetowards
the central burner [1]. These particles are internally circulated
until all thecarbon has been consumed. Eventually, they contain
only ash, which then agglom-erates. These agglomerates, which are
heavier than the coal particles, drop out of theuidized bed and are
removed from the bottom of the gasier.Some ne ash and char
particles are entrained in the raw gas. Most of these
particles are captured by a cyclone and returned to the gasier
via a hot dipleg. The de-dusted raw gas is cooled in a syngas
cooler to approximately 538 C [97].
342j 9 Gasification and Pyrolysis of Coal
-
Figure 9.6 shows a schematic of the KRWgasier. Although this
gasier has littlecommercial success, it has been shown that
theKRWgasierwas capable of gasifyingcoals of all ranks [97].
9.4.2.3 Kellogg Brown Root Transport GasifierThis gasier was
developed by Kellogg Brown Root, and the demonstration scalegasier
is located at the Power System Development Facility (PSDF) in
Alabama,USA. PSDF is a clean coal technology test facility, which
has been operated bySouthern Company Services since 1996.
Construction of the apparatus was com-pleted in 1996, when the
device was originally operated as a combustor. After thecombustion
tests were concluded, modications were performed in 1999 in order
tooperate the device as an air blown gasier, and up until 2007 the
device was operatedas a gasier using both air and oxygen [9].This
gasier operates in a high velocity regime, where solids are carried
upwards
and then recovered and recirculated to the gasier. This gasier
has characteristics ofboth a uidized-bed and entrained-ow gasier,
but is classied as a uidized-bedgasier since the entrained solids
are recycled back to the gasier [7]. It is claimed that
Figure 9.6 Representation of a KRW gasifier (reprinted with
permission from Elsevier) [1].
9.4 Coal Gasification Technologies j343
-
the high velocities and high circulation rates result in higher
throughput and higherheat and mass transfer rates compared to
conventional circulating uidized-bedgasiers [7, 95]. Figure 9.7
shows a schematic of the KBR (Kellogg Brown Root)transport
gasier.The gasier consists of two sections, which are the reaction
zone and solids
recovery section. The reaction zone consists of the mixing zone
and riser, whilst thesolids recovery section consists of the
disengager, cyclone, loop seal, standpipe, andnon-mechanical
J-valve.Coal and limestone, which are fed from separate lock
hoppers, are mixed in the
mixing zone with the gasifying agent and recovered solids. The
solids and gas owinto the riser, which has a smaller diameter than
the mixing zone. Owing to thechange in diameter, the velocity in
the riser increases. The gasication and devo-latilization reactions
occur primarily in the riser, where the temperature is main-tained
below the fusion temperature of the ash. Although the temperature
in the riseris lower than the ash fusion temperature, the riser is
designed such that the residencetime is sufciently long to allow
cracking of the devolatilization products [95].The entrained solids
and gas ow into the disengager where separation of large
particles takes place by the action of gravity. These large
solids ow down into thestandpipe. Fine solid particles are
separated from the raw gas by means of a cyclone.
Figure 9.7 Representation of a KBR transport gasifier [95].
344j 9 Gasification and Pyrolysis of Coal
-
These ne particles also ow into the standpipe, and along with
the large solidparticles are introduced into the mixing zone via
the J-valve. To keep the solids in asuspended state, the standpipe
is aerated. The de-dusted raw gas is then cooled in are-tube syngas
cooler to approximately 400500 C, after which ne particulateremoval
takes place in a candle lter [91]. Typical operating temperature
and pressureare 8701000 C and 15 bar, respectively [7].Coals of
ranks varying from lignite to bituminous have been tested in a
pilot plant
unit. This gasier is suited for processing of lower rank, high
reactivity and highmoisture coals [91, 95]. Carbon conversions
greater than 95% have been achievedwhen processing these types of
coals. Since the gasier is operated below the ashfusion
temperature, this type of gasier would also be better suited to
process coalswith high ash melting temperature [90].
9.4.3Entrained-Flow Gasifiers
Characteristics of entrained-ow gasiers are the co-current ow of
coal andgasication agent, pulverized coal feed, short residence
time and operation in ahigh temperature regime where the ash is
melted and discharged as a liquid slag.Entrained-ow gasiers may be
congured for upward or downward co-current owof coal and oxidant.
Most upward ow co-current gasiers are side-red, whereasmost
downward ow gasiers are top-red. The pulverized coal is transported
to thegasier pneumatically using a carrier gas (dry feed system) or
by means of a coal/water slurry. Owing to the high temperature
operation of entrained-ow gasiers, theraw gas contains a
substantial portion of sensible heat. Some technology suppliersmake
use of a syngas cooler where high-pressure steam is raised, whilst
othersemploy a combination of a second stage and a syngas cooler to
recover this heat.There are options and designs available, in which
the heat in the raw gas is notrecovered, and the syngas and slag
are cooled bymeans of quenching. Note that sometechnology suppliers
offer both heat recovery and quench options for the
sameprocess.Figure 9.8 shows generic representations of (a)
downward ow top-red and (b)
upward ow side-red single stage gasiers and their respective gas
and solidtemperature proles.Numerous entrained-ow gasication
technologies exist and most of the emerg-
ing gasication technologies are of this type. Only a selected
few entrained-owtechnologies will be reviewed below.
9.4.3.1 Shell Coal Gasification ProcessShell has developed two
distinct gasication processes, which are called the ShellGasication
Process (SGP) and the Shell Coal Gasication Process (SCGP).
SGPwasdeveloped for gasication of liquid and gaseous feedstocks.
The SCGP is currentlylicensed by Shell Global Solutions
International. It originates from the Koppers-Totzek technology,
where Shell and Koppers collaboratively developed a
pressurizedversion of the atmospheric Koppers-Totzek Process. In
1981 Shell and Koppers
9.4 Coal Gasification Technologies j345
-
decided to pursue separate development of their own gasication
processes [7]. Shelloperated a 250 t d1 demonstration unit
inHouston, Texas, USA. Shellmatured theirtechnology to industrial
scale, when in 1994 the Nuon Power IGCC plant, situated inBuggenum,
The Netherlands, was started up. At the time of compilation of the
USDoE and NETL gasication database, four gasiers based on SCGP were
currentlyoperating, whereas a further seven more were currently
under start-up. The com-bined syngas capacity of the operating
gasiers is approximately 10 million m3n d
1.Furthermore, several Shell coal gasiers are either in the
construction, engineering,or development phase, with the last of
these gasiers due to start-up in 2011 [87].Figure 9.9 shows a block
ow diagram of the SCGP [90]. The process consists of
the following sections: integrated milling and drying, coal
pressurization andfeeding, gasier and syngas cooler, dry solids
removal, wet scrubbing, y ash lockhopper system, and efuent water
treatment.The gasier itself is a water-cooled membrane wall vessel.
The membrane wall is
covered with a castable refractory, which reduces the heat loss
and protects the steelfrom the high temperature inside the gasier.
Highpressure saturated steam isgenerated in the membrane wall. The
coal and oxidant are side-red; the gases owupwards, whilst the slag
is discharged from the bottom of the gasier. The gasier is
Figure 9.8 Generic representations of dry feed (a) downward flow
top-fired and (b) upward flowside-fired single-stage gasifiers and
their respective gas and coal temperature profiles (reprintedwith
permission from Elsevier) [9].
346j 9 Gasification and Pyrolysis of Coal
-
usually equipped with four diametrically opposed burners. The
coal is conveyed tothe gasier pneumatically with a carrier gas.
Depending on the application of thesyngas, the carrier gasmay be
nitrogen or carbon dioxide. Since the ash is dischargedas a slag,
the gasier operating temperature should be higher than the ash
meltingtemperature. The ame temperature may be as high as 2000 C,
and the gasieroutlet temperature is typically 1500 C [98]. For
coals with high ash meltingtemperature, a ux, like limestone, is
usually added to lower the ash meltingtemperature. In contrast to
slagging moving-bed gasiers, the slag viscosity maybe higher (1525
Pa s) to ensure successful slag tapping [7]. The gasier is
usuallyoperated at a pressure of 3040 bar and the gas residence
time is typically 0.54.0 s [9].Since the raw gas exits the gasier
at a high temperature, the only hydrocarbonpresent is methane and
this only in trace quantities (ppm range) [89].The hot raw gas and
entrained slag droplets are quenched with cold recycled
de-dusted raw gas in the upper section of the gasier, called the
quench chamber.The rawgas is quenched to a temperaturewhere the
slag droplets solidify andbecomenon-sticky. The raw gas and y ash
particles are then further cooled in the syngascooler from 900 to
approximately 280 C [9]. High-pressure steam is raised in thesyngas
cooler. Depending on the application either saturated or
superheated steammay be generated in the syngas cooler. The y ash
particles are separated from theraw gas by means of a
high-pressure, high-temperature candle lter. In some casesa cyclone
is used in combination with the candle lter to remove the y
ashparticles. They ash particlesmay be recycled to the gasier, or
theymay be sold as aby-product.
Figure 9.9 Block flow diagram of the Shell Coal Gasification
Process (SCGP) (syngas cooleroption) [90].
9.4 Coal Gasification Technologies j347
-
Approximately half of the de-dusted gas is recycled via a
compressor to the quenchchamber. The remainder of the gas passes to
a wet scrubber where solids are washedout. The residual solids
content in the raw gas after wet scrubbing is approximately1.0mgm3
[98].Shell Global Solutions are also marketing a quench version of
the SCGP [99, 100].
In this version of the process, the syngas cooler is replaced by
a water quench vessel,whereas the gasier itself and the remainder
of the process are left unchanged. Theraw gas is rst quenched with
cold de-dusted recycled raw gas in the upper section ofthe gasier
and then it is further quenched with raw gas and water in the
waterquench vessel [99]. The quench version of the SCGP is suited
for applications wherehydrogen or a syngas with high syngas ratio
is required. Shell Global Solutions haslicensed its rst quench
version of the process to Powerfuel Plc in the UK [99, 100].It is
claimed that the SCGP is a versatile process and that any rank of
coal may be
gasied [101], provided it is milled to the correct size and can
be pneumaticallyconveyed to the gasier. However, it has been
mentioned that the quench version issuited for coals, with high
sodium or chlorine content, which would foul the syngascooler [99].
The ash content of the coal affects the efciency and the cost of
theprocess [7]. During operation of the gasier, a slag layer coats
themembranewall andprovides insulation against heat loss. For a
coal with ash content lower than 8%, thisslag layer is expected to
be thinner, thus increasing the heat loss and reducingefciency.
Shell has claimed that coals with ash content up to 40% may be
gasied,without negatively impacting the gasier efciency [101].
9.4.3.2 Prenflo Gasification ProcessThe name of this technology
is derived from the words pressurized entrainedow [102]. It is
currently licensed by Uhde, a Thyssen-Krupp company. Just like
theShell technology, this technology originates from the
Koppers-Totzek process, andwas independently developed by
Krupp-Koppers after Shell and Koppers split [102].The Preno process
is also marketed with two different options for syngas and
slagcooling, that is, a heat recovery and quench option [103].
These are marketed by therespective names Preno with Steam
Generation (PSG) and Preno with DirectQuench (PDQ) [103]. Owing to
the KoppersShell collaboration between 1976 and1981, the heat
recovery option of the Preno and Shell technologies are very
similar,and as such discussion on this technology is intended to be
brief.A 48 t d1 pilot plant, located in Furstenhausen, Germany was
operated for many
years before a 3000 t d1 commercial unit was commissioned in
1997 in Puertollano,Spain. The Preno process forms part of an IGCC
scheme in which amixture of coaland petroleum coke is gasied. The
syngas production capacity of this unit isapproximately 4.3 million
m3n d
1; this installation in Spain is the only commercialinstallation
of this technology [87].In the case of the heat recovery option,
the major difference between the Preno
and Shell technologies lies in the gasier and syngas cooler
arrangement. For thePreno technology, the gasier is integrated with
a radiant syngas cooler [96], andthe raw gas is cooled further in a
waste heat boiler. This is illustrated in Figure 9.10.The coal
milling and drying section is the same as for the SCGP.
348j 9 Gasification and Pyrolysis of Coal
-
The Preno gasier, like the Shell gasier vessel, is a
water-cooledmembrane wallvessel, which is linedwith a refractory,
andwith the burners located on the sides of thegasier. The up owing
hot gases are immediately quenched with cold de-dustedrecycled raw
gas in the central or quench pipe. These gases ow through the
centralpipe and reverse ow direction in a gas reversal chamber,
after which they owdownwards through the high-pressure steam
boiler. The gases are then furthercooled in awaste heat boiler. The
cooled gas is de-dusted and recycled in a very similarmanner as for
the SCGP.The quench version of the Preno process is substantially
different to the quench
version of the SCGP. In the PDQprocess, the gasier vessel itself
is unchanged, butnow the ow direction of the gas is downward. It is
benecial to congure thegasier in a manner such that both the hot
raw gas and slag ow in the samedirection, since this arrangement
reduces the risk of slag tap blockages. The hotraw gas and slag
exiting the gasier are quenchedwith water and the raw gas exits ata
temperature of approximately 200250 C. The solidied slag is then
removed vialock hoppers. Uhde claim that, compared to the PSG
design, the PDQ designreduces the engineering, procurement, and
construction costs by approximately30% [103].It is claimed that the
Preno process is exible in terms of the solid feedstocks
that may be gasied and that a syngas with a nearly constant
heating value isobtained, regardless of the feedstock [103].
Commercial scale tests have beenperformed in which mixtures of high
ash coal and high sulfur petroleum cokewere gasied [7].
Figure 9.10 Representation of a Prenflo gasifier and heat
recovery equipment [103].
9.4 Coal Gasification Technologies j349
-
9.4.3.3 General Electric Coal Gasification ProcessThis
technology was developed by Texaco, and is now licensed by General
Electric(GE). Just like Shell, Texaco developed processes for the
gasication of gaseous andliquid feedstocks, as well as a process
for the gasication of solid feedstocks. Texacohad extensive
commercial experience on the gasication of gaseous and
liquidfeedstocks, and decided to retain the fundamental principles
of these processes inthe coal gasication process. As such, this
resulted in a slurry feed, non-cooled,refractory insulated gasier
vessel that is congured for top-re downward ow. Therst work on coal
gasication on a pilot plant scale began as early as 1948 [104],
andthe rst instance of coal gasication on a commercial scale using
this processoccurred in 1983 [90]. According to the survey
performed by the US DoE andNETL [87] the current number of
operating commercial scale GE coal gasiers is 33,with the
corresponding syngas production equal to approximately 30millionm3n
d
1.Several GE coal gasiers are either under start-up,
construction, or development andthe combined production of these
gasiers is approximately 47 million m3n d
1.There are currently three versions of theGeneral Electric
CoalGasication Process
(GECGP) being licensed [9]; they differ in the manner in which
the hot raw gas andslag are cooled. These are the heat recovery,
quench version, and a version that is acombination of the rst two
options, that is, a radiant cooler followed by a waterquench. In
all versions the coal is conveyed to the gasier as a coal/water
slurry. Thecoal ismilled to a certain specication before
beingmixedwithwater. Thewater in theslurry replaces the steam that
is needed for gasication. The slurry is pressurized togasication
pressure via a membrane pump. The coal and oxidant (usually
oxygen)are injected into the gasier via a centrallymountedburner.
The reaction temperatureis lower than the melting temperature of
the refractory, but higher than the ashmelting temperature. The
material for the refractory is determined by the compo-sition of
the ash and the gasication temperature, and it has been generally
found thathigh chromium oxide content refractories perform
satisfactorily [104]. For coals withhigh ash melting temperature, a
ux is usually added. The gasier pressure isdependent on the syngas
application, and could be as high as 70 or 80 bar [9].In the heat
recovery version, the hot gas and slag ow downwards through a
constriction into a radiant syngas cooler, where the hot raw gas
and slag are cooled to atemperature (approximately 760 C) [9] where
the slag becomes non-sticky. High-pressure steam is raised in the
radiant syngas cooler. The sintered ash particles dropinto a quench
bath located at the bottom of the radiant syngas cooler, where
theysolidify and are removed via lock hoppers. The syngas is
further cooled in two parallelre-tube convective coolers to a
temperature less than 450 C [7]. Thereafter, hot wetscrubbing of
the raw gas takes place. The scrubber bottoms are processed in
aclarier, and the recovered solids are recycled to the feed
preparation step.In the quench version of this process the hot gas
and slag ow downwards, and are
immediately quenched with water at the exit of the gasier. The
raw gas is saturatedwith water vapor and is cooled to 200300 C [9].
As was for the heat recovery version,the ash particles are removed
via lock hoppers. Any entrained solid particles arewashed out in
the wet scrubber and after clarication they are recycled to
increase theoverall conversion. Someof the clariedwater is also
recycled to the quench section to
350j 9 Gasification and Pyrolysis of Coal
-
minimize water consumption. Figure 9.11 shows a diagram of the
heat recoveryversion of this process.Conveying coal to the gasier
as slurry is a much simpler operation than
pneumatic feeding of pulverized coal. However, the simplicity is
achieved with aloss in efciency. Thewater that is present in the
slurry has to be vaporized andheatedto the reaction temperature,
and usually the water in the slurry is in excess of what isrequired
to achieve complete gasication of the coal. Therefore, this
technology issuited for low inherent moisture content coals. The
reactivity of the coal plays animportant role in the overall
conversion. When processing low reactivity coals, hightemperatures
are required to achieve satisfactory conversions. For this
technology, toprolong the refractory life, the temperature should
not be excessively high. Hencecoals with high reactivity are suited
for this process.
9.4.3.4 Conoco-Phillips E-Gas Gasification ProcessThis
technology was developed by Dow Chemical Company and is nowmarketed
byConoco-Phillips (CP). Dow operated a pilot plant from 1978 until
1983, where it wasrst used in an air-blownmode. The capacity in the
air-blownmodewas about 12 t d1
(dry lignite basis) and it reached 36 t d1 (dry lignite basis)
when the oxidant waschanged to oxygen [96]. Following the success
of the pilot scale tests, a demonstrationscale gasier was built,
which was operated with both air and oxygen as the oxidant.Two IGCC
facilities have been built that utilize the CP gasication
technology. Thesewere the Louisiana Gasication Technology
Incorporated (LGTI) plant and theWabash River Coal Gasication
Repowering Project. The LGTI plant operated from1987upuntil 1995
[105]. The gasier situated atWabashRiver IGCCplant, which hasa
production capacity of about 4.3 millionm3n d
1, was started up in 1995 and is still
Figure 9.11 Representation of the GE Coal Gasification Process
(GECGP) (heat recoveryoption) [90].
9.4 Coal Gasification Technologies j351
-
operating [87]. There are numerous projects underway that intend
to utilize the CPgasication technology. The combined syngas
production of these intended projectsis approximately 77 million
m3n d
1. Most of them are intended to start up in2013 [87].TheCPgasier
is a slurry fed, entrained-ow, two-stage gasier. The gasier
vessel
is un-cooled, and a refractory insulation is used to protect the
steel vessel from thehigh temperatures. The coal iswetmilled to a
certain specication to create the slurry.The slurry and oxygen are
injected into the rst stage of the gasier, which operates
attemperatures between 1370 and 1540 C [105]. Approximately 75% of
the total coalfeed is injected into the rst stage [96]. All the
oxygen is injected into the rst stageonly. Analogous to the
operation of theGE gasier, therst stage of this gasier has tobe
operated at a temperature that is high enough to allow smooth slag
tapping, but lowenough to prolong the refractory liner life. Unlike
any of the other slagginggasication technologies, there is no slag
lock hopper [7]; the slag from the CPgasier is continuously
removed. The coal ashows down the sides of the gasier as aslag,
through a tap into a slag bath quench. After solidication, it is
milled underpressure and then let down to atmospheric pressure
using an innovative system [89].The hot entrained gas from therst
stageows into the second stagewhere the rest
of the coal slurry is injected. This hot gas provides the energy
required to vaporize theslurry water and devolatilize or gasify the
coal injected in the second stage. Theseendothermic processes
reduce the gas temperature to about 1000 C. This mode ofoperation
results in the formation of unreacted char. It is claimed that the
charabsorbs hydrocarbon liquids that are formed in the second stage
[96]. The unreactedchar and raw gas are then cooled in a re tube
syngas cooler, where high-pressuresteam is generated. It is
mentioned that the re tube syngas cooler design is lessexpensive
than a water tube boiler of equivalent duty, and no soot blowers
ormechanical devices are required to dislodge soot from the heat
transfer surface[96, 105]. The raw gas is cooled to about 370 C in
the syngas cooler and thereafter theunreacted char particles are
separated from the cooled raw gas in a high temperaturelter. The
unreacted char particles are re-injected into the rst stage of the
gasier, toincrease the overall carbon conversion. Since all the
recovered solid particles arerecycled to the rst stage, there is
only a single stream where the inorganiccomponent (ash) of the coal
is discharged, which is the inert glassy slag stream.It has been
mentioned that the CP process was originally designed for the
gasication of reactive lignite and sub-bituminous coals;
however, bituminous coalsand petcoke have been successfully gasied
in the commercial scale gasier. Aminormodication to themixer nozzle
was required when bituminous coals were gasied,which increased the
burner life to satisfactory times and also increased the
overallcarbon conversion [105]. Figure 9.12 shows a representation
of the CP gasicationprocess.
9.4.3.5 Mitsubishi Heavy Industries Coal Gasification
ProcessThis gasication technology was developed by Mitsubishi Heavy
Industries (MHI)and the Japanese Central Research Institute of
Electric Power Industry [9]. A 2 t d1
process demonstration unit and a 200 t d1 pilot plant have been
operated in Nakoso,
352j 9 Gasification and Pyrolysis of Coal
-
Japan, where several different coals have been gasied [7]. The
MHI technology hasnow achieved commercial status, after the
start-up of the 250 MWe Nakoso IGCCplant, and, as of 17 September
2008, the gasier had accumulated 2660 operatinghours [106]. The
Nakoso IGCC plant is the only commercial application of
thetechnology, and the syngas production capacity of this unit is
approximately 3.4million m3n d
1 [87].The conceptual principles of the MHI gasier are very
similar to that of the
Combustion Engineering (CE) gasier, the primary difference being
the operatingpressure. TheMHIgasierwas designed to be operated at
elevatedpressure,whilst theCEgasierwasdesigned for atmospheric
pressureoperation [107]. It is no surprise thatthese two
technologies are very similar, since CE collaborated with MHI in
the1980s [96]. The MHI gasier (Figure 9.13) is a single chamber,
two-stage, membranewall gasier, in which the fuel and oxidant are
red from the side. The oxidant used inthis process is enriched air.
The coal is pneumatically conveyed to the gasier usingnitrogen.
Therst stage is referred to as the combustor,whilst the second
stage is calledthe reductor. The combustor and reductor are
connected via a diffuser, the purpose ofwhich is to minimize the ow
of gas back into the combustor [107].Approximately half of the
total coal feed is supplied to the combustorwhere it reacts
with the oxidant, resulting in a temperature of approximately
1700 C [90]. The slagdrips down the sides of the gasier wall into a
bath, where it is quenched and
Figure 9.12 Representation of Conoco-Phillips (CP) gasification
process (reprinted withpermission from Elsevier) [9].
9.4 Coal Gasification Technologies j353
-
solidied. The remainder of the coal feed is injected into the
reductor; no oxidant isinjected into this stage. The hot gas from
the combustor ows upward through thediffuser into the reductor, and
the energy in the gas is utilized to drive theendothermic reactions
that take place in the reductor, bringing about a quench ofthe hot
gas and entrained slag. The temperature of the gas at the exit of
the reductor isapproximately 1000 C [9]. This temperature is
intentionally chosen, since at thistemperature gasication reactions
have terminated and the ash is generally below itssoftening point,
hence it is non-sticky [107]. This mode of quenching the raw
gasobviates the need for a recycle quench gas system and reduces
the size of thedownstreamheat recovery equipment. The downside of
thismethod is the formationof unconverted char and pyrolysis
products.The gas and char ow from the reductor into a syngas cooler
where the raw gas is
cooled by raising high-pressure steam. The unconverted char is
recovered from thegas by means of a high-temperature candle lter
and cyclone. This is recycled to the
Figure 9.13 Representation of a MHI gasifier [109].
354j 9 Gasification and Pyrolysis of Coal
-
rst stage. There is no y ash discharge in this process, since
the unconverted char isrecycled to the rst stage; hence all the ash
is discharged from the combustor as aninert glassy slag.Since this
gasier is operated with a very high temperature in the
combustion
section, it is particularly suited for gasifying high ash
melting temperature coalswithout the addition ofux [7]. The syngas
produced by theMHI process is not suitedfor synthesis applications
since it contains a large amount of nitrogen.Conversely, forpower
applications, it is claimed that air-blown IGCC plants have a
higher netefciency than oxygen-blown IGCCs [108].
9.4.3.6 Siemens Fuel Gasification TechnologyDevelopment of this
technology started in 1975 and was initiated by
DeutschesBrennstofnstitut [9, 110]. This technology was acquired by
Noell in 1991, then byBabcock Borsig Power in the early 2000s, and
later by Future Energy. Finally, in 2006Siemens Power Generation
Group took ownership and now licenses the technologythrough Siemens
Fuel Gasication Technology (SFGT) GmbH. There are currentlyplans to
construct two coal gasiers in North America and ve in China, each
gasierwith a capacity of 500 MWth (megawatt thermal), or 2.9
million m3n d
1 of syngas.These gasiers are expected to be started up in 2009.
There are also plans to increasethe size of the gasier, so that the
syngas production rate is approximately 5.9millionm3n d
1 [110].The SFGTgasier is a top-red, downward ow, single stage,
dry feed entrained-
ow gasier. The design and conguration of the SFGT gasier
presents somebenets and advantages. A top-red, downward ow gasier
is less complex andcheaper to construct than a side-red gasier.
Since both the gas and slag ow in thesame direction, the risk of
slag tap blockages is minimized, and this sort ofconguration allows
for exibility in terms of the gas quenching [9].There are several
versions of the SFGT available to the market. The feedstock
properties and syngas application dictate which version would be
best suited. Forfeedstocks with an ash content above 1.0% a gasier
vessel with a cooling screen isrecommended. The cooling screen
consists of spirally wound, studded water-cooledtubes, which are
pressed into a SiC refractory. A layer of solidied slag forms on
thehot face of the refractory. During operation themolten slag ows
down the side of thesolidied slag layer. The solidied slag layer
and SiC refractory protects the walls ofthe water-cooled tubes from
the high temperature inside the gasier. The coolingscreen design of
the SFGT is analogous to the membrane wall designs of the
othergasication technologies.For fuels with an ash content less
than 1.0% the solidied slag layer does not
continuously regenerate, and hence a gasier design with a
refractory lining andcooling jacket is recommended.The fuel and
oxidant are injected at the top of the gasier through a
centrally
mounted burner. The hot raw gas and molten slag ow downwards
into a quenchchamber. There are currently two options available to
cool the raw gas and moltenslag, which are partial and total
quenches using water sprays. The quenched slag is
9.4 Coal Gasification Technologies j355
-
solidied by cooling in a slag water bath. The solidied slag is
then removed via lockhoppers. The quenched raw gas is then further
cooled and cleaned. Siemens iscurrently working on a design that
will incorporate a partial gas quench, followed byhigh temperature
heat recovery [110].Virtually all ranks of coal may be gasied with
the SFGT, and various coals have
been gasied at a pilot scale in the SFGT gasier since the 1980s.
Siemens havecapped the maximum ash content of a coal feedstock at
15%. Coals with higher ashcontent are blended with low ash or no
ash content material to ensure successfulgasication [7]. Figure
9.14 shows a schematic of a SFGT gasier equipped with acooling
screen and the quench chamber.
9.4.4Other Gasification Technologies
9.4.4.1 Opposed Multi-Burner (OMB) Gasification
TechnologyDevelopment of this gasication technology began in 1995
at the Institute of CleanCoal Technology at the East
ChinaUniversity of Science andTechnology in Shanghai,China [9,
111]. Pilot scale tests were performed in 2000 in which the coal
was fed as aslurry to a gasier with a coal throughput of 22 t d1
[112]. Pneumatic feeding of coal
Figure 9.14 Representation of a SFGT gasifier with cooling
screen and quench chamber [90].
356j 9 Gasification and Pyrolysis of Coal
-
to the pilot scale gasier was performed in 2004 and 2005 where
nitrogen and thencarbon dioxide was used as the carrier gas. Two
demonstration scale plants werestarted up at separate locations in
China in the second half of 2005 [111, 112]. InChina, there seems
to be tremendous interest in this gasication technology, since
ithas been announced that this technology would be used in 14
projects [113]. Thelatest development of this technology involves
replacement of the opposed multi-burners with a single, centrally
mounted, top-red burner. The developers of thistechnology also
provide different options for cooling of the hot raw gas and slag,
sinceboth a quench design and radiant boiler design are
available.
9.4.4.2 Pratt and Whitney Rocketdyne (PWR) Gasification
TechnologyThis gasication technology is derived from the rocket
engine technology of Rock-etdyne, andwas later acquired by Pratt
andWhitney in 2005 [91]. Development of thistechnology began in the
1970s and several feedstocks such as coal, petroleum coke,and
biomass were gasied [114].The PWR (Pratt andWhitney Rocketdyne)
gasier is a top-red, down ow gasier.
Thecoalisdryfedtothegasierasadensephasebymeansofadrysolidspump.Thecoalis
gasied in a mixture of oxygen and steam. It is claimed that the ame
temperaturecould be as high as 2760 C [114], and the heating rate
as fast as 1.1million C s1 [90],andasa result thecoal
isgasiedwithin3 feetof theburner [114].Therapidheatingandsubsequent
rapid gasication of the coal particles results in a gasier that is
approx-imately 90% smaller than the currently available gasiers
[114, 115].PWR has partnered with Exxon Mobil to develop,
commercialize, and license the
PWR gasier technology. Plans are under way to start up a 18 t d1
pilot scale gasierat the Gas Technology Institute in Des Plaines,
Illinois [116].
9.4.4.3 Plasma GasificationOne of the technologies on offer that
utilizes a plasma arc to supply the energyrequired for gasication
is that licensed by Westinghouse Plasma Corporation(WPC). WPC is a
subsidiary of AlterNRG (AlterEnergy). Plasma torches offered byWPC
have been in commercial operation since 1989 at a foundry in the
UnitedStates [117]. There is currently no large-scale application
of plasma gasication ofcoal; however, there are several
applications of plasma gasication of waste. Theseapplications
include the waste processing facilities in Japan, where at two
separatelocations 280 and 22 t d1 are gasied. At the location where
280 t d1 of waste isprocessed, approximately 4 MWof electricity is
exported to the grid [117]. There arecurrently several projects
under development that intend utilizing the WPC tech-nology. One
such project is theGeoplasmawaste-to-energy project. Once
constructedthe Geoplasma plant located in Florida, USA will be the
largest plasma gasicationsite in the world, and will export 120 MW
of electricity. This plant, however, willprocess municipal solid
waste and not coal [117].
9.4.4.4 Underground Coal GasificationUnderground coal gasication
(UCG) is also referred to as in situ coal gasication.Gasication of
coal in situ is usually applied to coal seams that cannot be
economically
9.4 Coal Gasification Technologies j357
-
mined. In situ gasication of coal involves drilling of injection
and productionboreholes from the surface to the coal seam. The coal
seammay be located as shallowas 30mor as deep as 800m [118]. The
oxidant, which could be steam and air or steamand oxygen, is
injected into the coal seam via the injection borehole and the
productgas exits via the production borehole. The injection and
production boreholes areconnected by means of directional drilling
techniques.UCGhas been practiced in several locations around
theworld. In the former Soviet
Union, UCGwas practiced on a commercial scale from 1940 to the
late 1970s. UCGtrials also took place in the United States and
Europe in the 1980s [9].There seems to be renewed interest in this
technology. Several UCG projects have
been announced around the world. The most prominent UCG project
is theChinchilla UCG project, which is based in Australia. Gas was
produced in 1999from a pilot scale plant at this location [119].
The operations were later expanded andup until 2002 approximately
32 000 tons of coal was gasied. Pilot trials have alsobeen recently
conducted in South Africa by the electricity utility Eskom [120].
AUCGpilot plant located in Inner Mongolia, China was also
commissioned, and gas wasproduced in October 2007 [121].
9.5Outlook
Today, the gasication of coal to produce a syngas for power
generation (IGCC) orchemical synthesis (CTL) is attracting
considerable interest worldwide. In the last 20years, electricity
generation has emerged as a large new market for
gasicationtechnologies because gasication is seen as ameans of
enhancing the environmentalacceptability of coal. Gasication
technologies are being developed to provide thepower industry with
efcient, clean and economically competitive alternatives
toconventional power generation technologies.From the environmental
point of view, gasication offers several advantages over
the combustion of coal. First, emissions of sulfur and nitrogen
oxides, furans anddioxins, particulates, volatile organic
compounds, polycyclic aromatic hydrocarbons,and heavy metals are
signicantly reduced due to the cleanup of syngas. The secondmajor
advantage is that gasication provides the best option for producing
aconcentrated carbon dioxide stream (pre-combustion capture) that
may be storedor sequestrated to reduce the emission of greenhouse
gases.The future of coal gasication is closely associated with the
future of energy and
energy policy. In the transition between fossil fuels and fully
renewable energy, coalgasication can play an important role. It is
anticipated that during this transitionperiod, hydrogenwill be
produced directly from fossil fuels rather than by electrolysisof
water.Gasication is a key technology for more efcient and
environmentally friendly
utilization of abundant coal resources. It will have an
important role to play in thecoming decades both for power
generation and for the production of transportationfuels and
chemicals.
358j 9 Gasification and Pyrolysis of Coal
-
9.6Summary
This chapter provides an introduction and overview of the
chemistry, thermody-namics, and kinetics of coal pyrolysis and
gasication processes, and currenttechnology developments. It
reviews research and development work carried outin the past three
decades.Gasication is the cleanest, commercially proven technology
for conversion of coal
into fuel gas or syngas. It also provides the lowest-cost
approach for capturing carbondioxide.Current trends support the
observation that advanced gasication processes will
continue to be used for the production of clean combustible
gases with an increasingnumber of applications in power generation
and manufacturing of high gradetransportation fuels, hydrogen, and
chemicals. It is anticipated that, in the twen-ty-rst century, coal
gasication will be widely used in a new generation of energyplants
possessing product exibility, near-zero emissions, high thermal
efciency,and ability to capture carbon dioxide.
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