[doi 10.1002%2f9783527628148.hoc063] lackner, maximilian; winter, franz; agarwal, avinash k. --...

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

. 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


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.


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


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


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


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].


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].


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.


9.4.1.2 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].


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


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].


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].


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].


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].


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].


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


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].


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].


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.


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].


9.4.3.3 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


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].


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,


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].


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].


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


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].


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


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.


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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[doi 10.1002%2f9783527628148.hoc063] lackner, maximilian; winter, franz; agarwal, avinash k. --...

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