hno

elle

ouse

ised f

ne 9

Global electricity demand is increasing at about

capture and sequestration as CO2 can be removed

from the pressurised syngas (pre-combustion) rather

processes is feasible, but extremely expensive as this

coal fired plants as well as the reduction of emissions

of greenhouse gases and particulates to the atmo-

sphere (Clayton et al., 2002; Collot, 2003, 2004).

International Journal of Coal Geolothree times the rate of total energy while the industry

is expected to reduce CO2 emissions because of global

warming. As a consequence, there is pressure to

improve the efficiency of energy use through changes

in technology and to produce energy vectors such as

H2 with near zero emissions of greenhouse gases.

Oxygen-blown gasification may be the most attractive

route for the production of H2 from coal with CO2

is carried out at atmospheric pressure and implies the

treatment of a much larger volume of gas (10 times

the volume of syngas). Another attraction of gasifica-

tion technologies and, in particular, of Integrated Coal

Gasification Combined Cycle (IGCC) is the possibi-

lity of cogeneration of electricity, H2 and chemicals.

This contributes to the improvement of power genera-

tion efficiency compared with conventional pulverisedThe gasification of coal to produce hydrogen for use either in power generation or/and for synthesis applications and

transport is attracting considerable interest worldwide. Three types of generic gasifiers (entrained flow, fluidised bed and fixed

bed gasifiers) presently in use in commercial gasification plants or under development worldwide are described. Their

suitability for processing all types of coals is discussed. This includes an assessment of the impact of some of the major

properties of coal on the design, performance and maintenance of gasification processes.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Hydrogen; Coal gasification; Gasifiers; Coal properties

1. Introduction than the exhaust gas (post-combustion). Removing

CO2 from the exhaust gas in conventional combustionMatching gasification tec

Anne-Ga

International Energy Agency-Clean Coal Centre, Gemini H

Received 23 January 2004; received in rev

Available onli

Abstract0166-5162/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.coal.2005.05.003

* Fax: +44 20 8780 17 46.

E-mail address: collota@cia.org.uk.logies to coal properties

Collot *

, 10-18 Putney Hill, London SW15 6AA, United Kingdom

orm 18 May 2005; accepted 22 May 2005

August 2005

gy 65 (2006) 191212

www.elsevier.com/locate/ijcoalgeooal for use as anH2 is currently produced from cintermediate for the synthesis of chemicals such as

methanol, ammonia/urea, FischerTropsch products

al ofand substitute natural gas (SNG) all over the world.

However IGCC technology for commercial-scale

plants is relatively recent. A summary of the

main existing gasification processes is given in

the following.

There are presently sixty five Chevron Texaco

owned or licensed gasification facilities worldwide

that produce power, chemicals and H2 from coal (6

plants), oil derivatives and natural gas. Three of the

coal gasification facilities produce ammonia, one pro-

duces town gas and electricity, one is an IGCC plant

and one is producing methanol and chemicals. There

are also other gasification projects in development or

engineering for the production of diesel, H2/steam,

syngas or electricity from either coal, natural gas or

oil derivatives (Preston, 2003).

Sasol, which was established in 1950 with the

prime objective to convert low grade coal into petro-

leum products and chemical feedstocks currently

operates three major coal to liquid (CTL) complexes

based on the former Lurgi gasification process (now

known as SasolLurgi dry bottom gasifier) in South

Africa for the gasification of coal into Fischer

Tropsch (FT) products (Van Dyk et al., 2001). The

Great Plains Synfuels plant (Dakota Gasification)

located in North Dakota (USA) has been producing

substitute natural gas (SNG) from lignite using the

same technology since 1984 (Lukes and Wallach,

2003).

There are presently five gasification plants using

the Shell gasification technology. Only one of them,

the Nuon Power Buggenum IGCC plant in the

Netherlands (formerly named Demkolec) which

was started up in 1994, is fed with coal for the

production of electricity. All the other gasification

plants are fed with petroleum wastes to produce

chemicals and/or H2 (Postuma et al., 2002). Eight

other coal gasification plants using the Shell Coal

Gasification Process (SCGP) for the production of

chemicals are planned to be built in China and one

in the USA (the Waste Management and Processors

Inc project). The plants will all produce syngas for

ammonia/urea, FischerTropsch liquids production

or H2 for other chemical plants (methanol, oxo),

replacing naphtha reformers, oil gasifiers or out-

dated coal gasifiers (Ploeg, 2001; Zuideveld,

A.-G. Collot / International Journ1922003). It is expected that the same technologies

as the ones developed at the Nuon Power Bug-genum IGGC facility, based in the Netherlands,

will be used for the construction of future SCGP

plants with power/H2.

New projects based on demonstrated or commer-

cial technologies for the production of hydrogen from

coal for power generation are presently under devel-

opment all over the world.

The EAGLE project (coal Energy Application for

Gas, Liquid and Electricity) is one of two Clean Coal

Technologies (CCT) projects sponsored by the Japa-

nese New Energy and Industrial Technology Devel-

opment Organisation (NEDO) and the Ministry of

Economy, Trade and Industry (METI) as part of a

new strategy called the dDeployment of Coal Utiliza-tion Technology Development Strategy for the 21st

centuryT. The objective of the EAGLE project is thedevelopment of an Integrated coal Gasification Fuel

Cell combined cycle (IGFC) (Wasaka and Kubota,

2003).

In Italy, a partnership of Sotacarbo, Ansaldo

Ricerche, Enea and the University of Cagliari is

presently developing a pilot scale gasifier for the

production of H2 and power from coal/biomass and

waste mixtures. The process, based on a 5 MW

(thermal) gasifier combined with an internal combus-

tion engine, will generate 0.2 MW power (Pratola et

al., 2002).

The New Zealand government through its science

funding agency, the Foundation for Research, Science

and Technology, has approved funding to develop a

dtechnology platformT for hydrogen energy. CRLEnergy Ltd is presently constructing a small scale

atmospheric air blown fluidised bed gasifier pilot

plant fed with local lignites for the production of an

equivalent of 50 kW hydrogen energy (Pearce, 2003;

S. Pearce, pers. comm., 2003).

The FutureGen project in the USA is a 10 year,

US$ 1 billion, demonstration project which was

launched by the US government in February 2003

for the production of H2 from coal. The 275 MW

prototype plant known as FutureGen will serve as a

large-scale engineering laboratory for testing new

clean power, carbon capture and coal to hydrogen

technologies. Every aspect of the prototype plant

will be based on cutting edge technologies (US

DOE, 2003).

Coal Geology 65 (2006) 191212Two new IGCC projects, the Kentucky Pioneer

Energy project (Kentucky) and the Lima Energy pro-

! plant configuration which includes: the coal feed-ing system (fed as a dry powder or as a slurry with

l of Cject (Ohio) and one existing IGCC power plant, the

Wabash River IGCC (Indiana), are being developed

by Global Energy Inc in the USA. The objective of the

Kentucky Pioneer Energy project co-sponsored by the

US DOE, is to demonstrate the reliability, availability

and maintainability of a utility-scale IGCC system

using a high sulphur bituminous coal, coal fines and

pelletized refuse-derived fuel (RDF) blend in a BGL

(British Gas/Lurgi) gasifier (Bailey, 2001). The Lima

Energy 580 MW gasification plant project is based on

the use of the E-GASk technology, for the co-gen-eration of H2 and electricity from petcoke. The

Wabash River IGCC power plant also designed with

an E-GASk entrained flow gasifier has been operat-ing with a range of local coals since 1995. A molten

carbonate fuel cell is currently being installed at the

Wabash River IGCC plant instead of as originally

planned, at the Kentucky Pioneer Energy plant. It is

expected that operation of the integrated IGCC-fuel

cell will start in spring 2004 first with natural gas

followed soon after with coal syngas. The Wabash

River IGCC power plant and the Lima Energy pro-

jects are owned by Global Energy although ConoPhil-

lips recently acquired the patents and intellectual

property associated with the E-GASk Technologyfor Gasification (P. Amick, pers. comm., 2004).

There are also three projects of cogeneration plants

(power and chemicals) projects sponsored by the US

DOE as Early Entrance Coproduction Plants (EECP)

(Amick et al., 2003; Rich et al., 2003; Shah and

Schrader, 2003; Strickland and Tsang, 2003) and

one project for the production of 100 t/d of dimethyl

ether (DME) from coal in Japan (Ohno and Omiya,

2003). More details on these projects can be found in

Collot (2004).

New concepts based on the gasification of coal for

the production of hydrogen are presently under devel-

opment. Some of the concepts are based on the com-

bination of three steps which include the gasification

of coal (either steam gasification or hydrogasifica-

tion), the shift reaction and carbon dioxide removal.

Examples of this type of concept are the Absorption

Enhanced Reforming (AER) process developed in

Germany (Weimer et al., 2002), the Advanced Gasi-

fication-Combustion (AGC) project (Rizeq et al.,

2002) and the Zero Emission Coal Alliance (ZECA)

A.-G. Collot / International Journaprocess (Ziock et al., 2002, 2003) developed in the

USA, and the Hydrogen Production Reaction Inte-water); the way by which contact between the fuel

and the gasification agents is established (flow

geometry); as to whether the minerals are removed

as dry ash or molten ash (slag); the way heat is

produced and transferred and finally, the way syn-

gas is cleaned (sulphur removal, nitrogen removal,

other pollutants removal).

A large number of gasification technologiesgrated Novel Gasification (HyPr-RING) process

developed in Japan (Lin et al., 2002, 2003). Other

concepts under development include membrane reac-

tors (Sammells and Barton, 2003) and molten bath

processes (HydroMaxR, HyMeltR) adapted frommetal smelting processes existing in the iron making

industry (Alchemix Corporation, 2003; Trowbridge et

al., 2002). More details on these new concepts can be

found in Collot (2003).

2. Coal gasification and its applications

Gasification is defined as the reaction of solid

fuels with air, oxygen, steam, carbon dioxide, or a

mixture of these gases at a temperature exceeding

700 8C, to yield a gaseous product suitable for useeither as a source of energy or as a raw material for

the synthesis of chemicals, liquid fuels or other

gaseous fuels. More details concerning the mechan-

isms of these reactions and their kinetics can be

found in Kristiansen (1996).

Common gasifying agents used in industrial gasi-

fiers include a mixture of steam and air or oxygen

with the amount of oxygen being generally one-fifth

to one-third the amount theoretically required for

complete combustion. The chemical composition

and future use of the gas produced (syngas) varies

depending on the following parameters:

! coal composition and rank! coal preparation (particle size)! gasification agents employed (oxygen or air)! gasification conditions: temperature, pressure,heating rate and residence time in the gasifier

oal Geology 65 (2006) 191212 193exist (see Section 1) and are detailed by Collot

(2002). They can however be classified into three

particles and gases flow concurrently at high speed.

They are the most commonly used gasifiers for

al ofcoal gasification.

! fluidised bed gasifiers, in which coal particles aresuspended in the gas flow; coal feed particles are

mixed with the particles undergoing gasification,

! moving bed (also called fixed bed) gasifiers, inwhich gases flow relatively slowly upward through

the bed of coal feed. Both concurrent and counter

concurrent technologies are available but the latter

is more common.

Other gasifier types have been developed based on

rotary kilns or molten baths, but no gasifiers of these

types are near to commercialisation. Gasification may

also be carried out in situ in coal deposits (also known

as underground gasification).

The choice of a gasification technology is difficult

as it depends on diverse factors such as (Vamvuka,

1999):

! coal availability, type and cost;! gasifier end use locations and interactions;! size constraints;! production rate of energy;! turndown requirements;! heating value of the gas;! allowed gas purity (S, CO2, etc) and cleanliness(tars, soot, ash) for meeting international regula-

tions, plant requirements and further use of the gas

products.

Coal choice maybe the least flexible factor for

economic, geographical and political reasons and it

is thus necessary to adapt the gasification technology

to the base coal to be processed.

3. Entrained flow gasifiers

In entrained flow gasifiers, coal particles concur-

rently react at high speed with steam and oxygen orcategories of gasifier configurations according to their

flow geometry:

! entrained flow gasifiers, in which pulverised coal

A.-G. Collot / International Journ194air in a suspension mode called entrained fluid flow.

Short gas residence times (seconds) give them a highload capacity but also require coal to be pulverised.

Coal can either be fed dry (commonly using nitrogen

as a transport gas) or wet (carried in a slurry water)

into the gasifier. They usually operate at high tem-

peratures of 12001600 8C and pressures in the rangeof 28 MPa. Entrained flow gasifiers are all slagging

gasifiers which are either lined with a refractory or a

slag self-coating system. Raw gas exiting the gasifier

usually requires significant cooling before being

cleaned. There are two main methods of cleaning

the gas by: using a high temperature syngas cooler,

this can also include recycling of cooled gas to the

gasifier, or quenching the gas with water. In entrained

flow gasifiers flexible load operation is more difficult

to handle than with the other types of gasifiers (flui-

dised and moving bed gasifiers). As entrained flow

gasifiers have a small heat capacity and no inventory

of process feedstocks, it is critical to control the

coal:oxidant ratio within narrow limits through the

entire operation in order to maintain a stable flame

close to the injector tip.

Entrained flow gasifiers are the most widely used

gasifiers with seven different gasification technologies

based on entrained flow gasifiers presently used at

industrial scale or under development worldwide.

Tables 1 and 2 give a summary of the main character-

istics of seven entrained flow gasification technolo-

gies. More detail on each gasification process can be

found in Collot (2002).

3.1. Slurryability and grindability

In entrained flow gasifiers, coal is pulverised to

ensure high carbon conversion during gasification.

The grindability of a coal is measured by the Hard-

grove Grindability Index (HGI). The HGI is used as a

comparison basis against experience with other coals

that have had satisfactory size distribution from grind-

ing operations. Slurryability is another important coal

property to take into account for slurry-fed gasifiers.

These two properties are very much interrelated as the

grind size distribution from a grinding mill affects the

slurry properties of a coal and then the conversion in

the gasifier. If a coarse grind size is used, a high solid

concentration of slurry can be produced but the larger

coal particles will not gasify as well as smaller parti-

Coal Geology 65 (2006) 191212cles that will tend to form a slurry with a lower

concentration. A coal with a high HGI favours the

roces

led t

led ta

allowi

en fro

e and

h lon

s: com

jected

g sec

ded. A

ed in

ined

ncorp

steel

n cha

l of CTable 1

Main characteristics of existing dry-fed entrained flow gasification p

Technology Operating conditions Gasifier

Hitachi Operate under slagging

temperatures at pressure

of 2.5 MPa

Water coo

are instal

sidewall

and oxyg

lower stag

time muc

stream.

Mitsubishi heavy

industries

Air blown Two-stage

coal is in

a reducin

coal is ad

is contain

Prenflo Four burners at the

bottom

Gasifier l

cooler is i

shell.

Shell coal gasification

process

Operation at 24 MPa, at

1500 8C and aboveA carbon

gasificatio

A.-G. Collot / International Journaproduction of slurry with a high concentration for use

in slurry-fed gasifiers. Important points to consider for

the slurry concentration are whether or not it is pump-

able, stable with particle settling and its rheology. A

considerable amount of research has been dedicated to

the development of techniques for the production of

coal/water mixtures in the last 2030 years with a

view to replacing oil by coal slurries (Thambimuthu,

1994). Results are also relevant to the preparation of

coal slurry for gasification.

Dooher et al. (1990) studied the slurryability of six

bituminous coals and one subbituminous coal to

develop a methodology for assessing the suitability

of coals for slurry fed gasifiers. They reported that the

most important coal properties affecting coal slurry-

ability were: equilibrium moisture, fixed carbon, sur-

face carbon/oxygen bonding as determined by

electron microscopy and free swelling index. The

equilibrium moisture (ASTM test D1412) of the

coal is a measure of the inherent moisture rather

than the coal surface moisture. According to Curran

non-refractory m

into the gasifier

opposed burnerses

Cooling and cleaning modes and

ash removal system

ube. Two sets of burners

ngentially to the gasifier

ng a spiral flow of coal

m the upper stage to the

making particle residence

ger than those of a gas

Gas is cooled in a syngas cooler

(4508C) followed by a cycloneand a filter. Char and fly ash are

recycled to the gasifier. Slag is

water quenched and removed

through a lock hopper.

bustion zone where O. Gas produced diffuses to

tion where the remaining

welded water tube vessel

a pressure shell.

Char particles are collected in a

syngas cooler and cyclones and

recycled to the gasifier. Slag is

water quenched and removed

through a lock hopper.

with a refractory. Syngas

orated into the gasifier

Syngas is quenched with recycled

cleaned cooled syngas. Raw gas

is dedusted in ceramic candle

filters. Slag is water quenched and

removed through a lock hopper.

vessel, which contains a

mber, is enclosed by a

Syngas is quenched with cooled

recycled product gas and further

oal Geology 65 (2006) 191212 195(1989), experience showed that the equilibrium moist-

ure correlates with pumpable slurry concentrations.

Kanamori et al. (1990) performed tests on twenty

coals and found that it was possible to predict the

slurryability and stability of coal slurries with the

oxygen to carbon ratio in coal, the oxygen-containing

groups and the clay minerals.

All coals that can be pulverised (high HGI) can be

processed in dry-fed entrained flow gasifier systems.

Bituminous coals with their low inherent moisture

content and hydrophobic nature are the coals of

choice for the commercial preparation of high solid

content coal/water fuels. However higher and lower

rank coals can also be processed in slurry fed gasifiers

provided the coal is pre-treated or an additive is used

(Thambimuthu, 1994).

3.2. Coal reactivity

Highly reactive coals provide high carbon conver-

sion at moderate gasifier temperatures improving

embrane wall. Coal is fed

through horizontally

s.

cooled in a syngas cooler. Raw

gas is cleaned in ceramic filters.

50% gas is recycled to act as a

quenching medium. Molten slag

is removed through a slag tap and

water quenched.

asifie

ither

creen

re mo

uppor

ith a

cooli

han co

wo-st

ith re

ressu

ining.

al ofTable 2

Main characteristics of existing slurry fed entrained flow gasifiers

Technology Operating conditions G

Babcok borsig

power (Noell)

Dry-fed or slurry-fed gasifier.

Operate in the range 13501600 8Cat pressure of 2.6 MPa

E

s

a

s

w

a

t

E-GASk Slurry is preheated prior to injection.80% of the coal is injected through 2

burners and is partially combusted at

temperatures of 13501400 8C and 3MPa pressure. The remaining coal slurry

is injected in the upper stage and reacts

with the fuel gas produced in the lower

stage.

T

w

Texaco Slurry-fed through burners at the top of

the gasifier. Operate at temperatures in

the range 12501450 8C and 38 MPa

P

l

A.-G. Collot / International Journ196overall system efficiency. The reactivity of coal chars

under gasification conditions is a major determinant of

the gasifier size and design. Char reactivity has a

significant influence on the degree of char recycle

and on the volume of oxidant required for the gasifier.

Boyd and Benyon (1999) studied the reactivity of

six Australian coals in a laboratory-scale pressurised

Drop Tube Furnace (DTF) and a Pressurised Thermo-

gravimetric Apparatus (PTGA). Chars were produced

in the DTF and their reactivities were measured with

the PTGA. Their results were then extrapolated to

full-scale entrained flow gasifiers (Shell, Texaco and

Mitsubishi Heavy Industries) using a dsensitivitymethod of analysisT. They reported that the effectsof change in char reactivity (coal) were the largest in

the air blown MHI process and the lowest in the Shell

process. They concluded that even though the rank of

the six coals studied was very similar, they had sig-

nificant differences in reactivity. These differences

were due to physical characteristics as well as the

chemical characteristics defining ASTM rank. A cor-

relation with fixed carbon, volatile matter or specific

pressures.r Cooling and cleaning modes

and ash removal system

covered by a cooling

(refractory on which

unted cooling tubes

ted by pins) for coals

ash content N1%wt orng wall (other feedstocks

al).

Raw gas is cooled with water spray

and further cooled before being

recycled to the gasifier. The molten

slag is water quenched. The granular

slag is removed from the gasifier

through a lock hopper.

age pressure shell lined

fractory.

Crude gas is cooled in a firetube

syngas cooler, which is a boiler

system with the hot gas circulating

on the boiler side as opposed to a

water gas cooler. Syngas is cleaned

in metal filters and ash and char

particles are re-injected into the

gasifier. Molten slag is removed

through a tap hole into a water

quench.

re vessel with refractory Raw gas can either be cooled and

cleaned from slag by water

quenching or radiant cooler. The raw

Coal Geology 65 (2006) 191212energy was not found. They also pointed out that

inorganic constituents may have a small effect on

coal reactivity by acting as catalysts.

More fundamental studies (Kelly et al., 2001;

Roberts et al., 2000) to better understand the process

of coal conversion focused on the effect of operating

pressure, temperatures and heating rate on coal reac-

tion behaviour under conditions relevant to entrained

flow gasification systems. The work reported by Kelly

et al. (2001) provided comparative data on 13 Aus-

tralian coals usually processed in entrained flow gasi-

fiers worldwide. Volatile yields of the coals were

determined in a wire mesh reactor and results showed

that coal volatile yield decreases with increasing reac-

tion pressure despite the enhancing effect of heating

rates. The greatest reduction in volatile yield with

pressure was observed for the higher coal ranks.

Char gasification of the same coals was studied in a

PTGA. Results indicated that char gasification in CO2atmospheres was strongly influenced by coal rank,

whereas steam and O2 gasification did not show

much of a coal rank effect on apparent reaction

gas and slag flow out towards the

bottom of the gasifier. The molten

slag is water quenched and removed

through a lock hopper.

l of Crates. These results suggest that other coal character-

istics, such as crystallinity and coal mineral matter are

playing a greater role under these conditions. The

conversion profiles of each char were also different

with the different gases tested (CO2, H2O, O2). This

fundamental study claims to be useful for predicting

coal performance in practical high-pressure reaction

systems as well as contributing to the resolution of

problems associated with Australian coals in interna-

tional demonstration and pilot-scale facilities. The

same authors are now expanding their study to inter-

national coals (Harris et al., 2003; Roberts and Harris,

2003).

In practice, reactive coals can be gasified at lower

temperatures and hence at higher cold gas efficiency,

whereas less reactive coals may need higher gasifi-

cation temperatures in order to achieve adequate

conversion efficiencies. At the same time, the tem-

perature must be high enough to yield a tappable

slag. Thus the preferred operating strategy for a coal

is always a balance between reactivity and slag

tapping considerations.

3.3. Ash/slag properties

Entrained flow gasifiers are usually recom-

mended for coals with a low ash content for both

economical and technical reasons. Considering that

gasifier operating conditions are kept constant, an

increase in coal ash content will lead to a decrease

in gasification efficiency and an increase in slag

production and disposal. These three factors contri-

bute to an increase in the overall cost of the

process. The decrease in gasification efficiency is

mainly due to an increase in oxygen consumption

which is necessary to melt the minerals as well as a

thermodynamic penaltythe heat in the slag exiting

the gasifier cannot be fully recovered. An increase

of the slag quantity can also cause blockage of the

slag removal and cleaning devices. It is neverthe-

less difficult to draw an optimum ash content value

for coals processed in entrained flow gasifiers. This

is because some of the technologies require a mini-

mum ash content. These technologies (BBP, Hitachi

and SCGP, see Tables 1 and 2) use a slag self

coating system that has to be covered by slag to

A.-G. Collot / International Journafunction and minimise heat loss through the wall of

the gasifiers.3.3.1. Ash chemical composition

In entrained flow gasifiers, the high temperature of

gasification (usually up to 1600 8C) combined withpressures of up to 3 MPa can accelerate the deteriora-

tion of refractory lining existing on the walls of some

gasifiers. Current refractory materials, which are

expensive pieces of gasifier equipment, have typically

a service life of no more than two years and the study of

their service life is an important parameter for the

selection of base coals for a gasifier. Texaco developed

amethod to predict gasifier refractory life based on coal

ashASTM fluid temperature which depends directly on

the composition of the ash/slag (see also Section 3.3.2).

Dogan et al. (2001) performed an extensive analysis

of spent refractories from commercial gasifiers and

concluded that some compounds present in coal slag

(SiO2, CaO, iron oxides) can penetrate deeply into

high chrome refractory materials and eventually give

rise to cracks that lead to material loss.

3.3.2. Ash fusion temperature (AFT) or ash melting

point

In slagging gasifiers, the ash flows down the gasifier

walls and drains from the gasifier as a molten slag.

Coals selected for slagging gasifiers should thus have

an ash fusion temperature (AFT) below the operating

temperature of the gasifier (14001600 8C). In practice,AFTcan be lowered by either the addition of flux, such

as limestone, or by blending with low ash fusion coals.

AFT is widely used as a guide to slag behaviour and can

be simply and quickly determined by laboratory stan-

dard methods, which consist of coal ashing by slow

heating in air. However this method does not reproduce

real commercial gasification conditions and other tests

have been developed. For example, engineers at the

Polk power station (Texaco technology) in the USA use

the difference between the ASTM ash fluid tempera-

ture, determined under reducing conditions and the

gasifier operating temperature plotted versus refractory

liner life, to estimate the optimum operating tempera-

ture of the gasifier for successful tapping (McDaniel

and Shelnut, 1998; McDaniel et al., 1998). Shell uses

the ash melting point as a preliminary indication for a

new coal and to check if the addition of flux would be

required (Ploeg, 1997). The temperature of 1400 8C isconsidered as a breaking point with higher values of ash

oal Geology 65 (2006) 191212 197fusion temperatures requiring flux. For an actual

design/operation a complete analysis of the ash should

al ofbe done to confirm the first approach and it is then

necessary to either perform further measurements on

slags produced in a gasifier or to rely on other methods

which can predict ash fusion temperature based on the

chemical composition of the ash.

Ashizawa et al. (1993) developed a method to

determine the ash fusion temperature of 26 coals as

a function of the chemical composition of their ash

(acidity). The acidity is defined as follows:

Acidity = (SiO2+Al2O3)/ (Fe2O3 + CaO + MgO

+ Na2O+K2O).

To validate the dacidityT method they producedslags under oxidising conditions in a 2 t/d pilot

plant gasifier developed by CRIEPI (MHI technol-

ogy). The slags produced were analysed and com-

pared to results obtained by the dacidityT method.They found a correlation between the ash composition

(acidity) and the ash fusion temperature and the equa-

tion giving the relationship between Tf (fusion tem-

perature) and X (acidity) is as follows:

Tf 27:7931TX 1236:89:This dacidityT method was then used to predict the

ash fusion temperature of coals in the presence of flux

and coal blends. Results obtained showed that the

prediction was even more precise when flux was

added to the coals. This could be explained by the

lowering of the importance of the influence that minor

oxides have on the ash fusion temperature when flux

is used.

3.3.3. Slag viscosity and temperature of critical visc-

osity (Tcv)

As the ash melts, its viscosity has to be low enough

to enable it to flow down and drain from the gasifier.

The viscosity of the slag, which depends on the slag

composition, is one of the most critical factors in the

operation of slagging gasifiers.

There are two types of coal slag behaviours:

! Type I exhibits a glassy behaviour, which meansthat when the slag cools down the increase in the

viscosity of the slag, is predictable and continuous.

! Type II exhibits a crystalline behaviour when cool-

A.-G. Collot / International Journ198ing down and, as a consequence, the flow becomes

non-Newtonian and the viscosity of the slagincreases sharply below a temperature called the

temperature of critical viscosity (Tcv). This type of

slag will behave the same as Type I (Newtonian

flow) at temperatures well above the Tcv. However

in the Tcv region crystallisation begins to have a

significant effect on the viscosity of the slag and

induces the possibility of blockage of the tapping

system with crystalline deposits.

Tcv is the minimum temperature required for safe

operation with slags that exhibit crystalline behaviour

rather than glassy behaviour. In practice the operating

temperature of the gasifier must be high enough to

maintain the slag in the Newtonian flow region (Scott

and Carpenter, 1996). Tcv depends on slag composi-

tion and, in particular, on the ratio of SiO2/Al2O3 in

the slag.

It has been established that slag viscosity must be

low enough (2515 Pa.s), with an optimum value of

15 Pa.s at temperatures of 14001500 8C, to achievesuccessful slag tapping (Browning et al., 1999). As a

consequence, viscosity models have been developed

to predict coal ash slag behaviour in coal slagging

gasification processes; this is necessary for optimisa-

tion of the operating parameters (coal selection, blend-

ing and flux) in order to achieve stable process

conditions and to reduce operating costs.

Shell uses a model (bSLAGSQ program) developedby Mills and Broabent (1994) to predict ash behaviour

for the range of coals in the SCGP (Ploeg, 1997,

2000). The model gives an estimation of the physical

properties of the slags from their chemical composi-

tions. This model was originally developed for metal-

lurgical slags but was later successfully applied to

coal slags, provided that all components present at

concentrations above 5%wt were included for the

calculations. The bSLAGSQ model contains routinesfor the estimation of ash fusion temperatures, viscos-

ities, densities and surface tensions. Fluxing agents

(mainly limestone) are then used to balance the slag

viscosity of the different coals fed.

Eastman Chemicals developed an in-house test in

order to determine the quality of their feedstocks prior

to gasification in a Texaco gasifier (Trapp, 2001,

2002). Considering that the typical standard viscosity

or melt point tests usually used to estimate slag visc-

Coal Geology 65 (2006) 191212osity are inadequate to fully predict slag behaviour in

the gasifier, they determined a continuous curve of

l of Cviscosity as a function of temperature. Eastman Che-

micals also reported, without giving more details, that

they co-fed slag modifiers (probably limestone) to

positively affect certain ash properties. As a result,

dbetter qualityT coals were gasified leading to a reduc-tion in operation difficulties and shutdowns.

Oh et al. (1995) studied the slag characteristics of

four US coals used in a Texaco gasifier. They stated

that all the empirical models used to determine slag

viscosity as a function of temperature and composi-

tion can only be applied with success for slags having

a dglassyT behaviour and that the models often fail topredict correct slag viscosity behaviour when a crys-

talline phase appears during cooling.

Patterson et al. (2001) studied the slag character-

istics of 68 Australian coals for their utilisation in

slagging gasifiers. They established an extensive

slag viscosity database versus ash composition, tem-

perature and flux addition (Patterson and Hurst,

2000). They concluded that the slag viscosities of

Australian coals can be properly predicted from their

models for tappings at 1500 8C, however these pre-dictions were not validated for all the coals at 1400 8Cdue to slag crystallisation. The same authors (Patter-

son and Harris, 1998) also reported that after addition

of limestone flux (up to 20% CaO) to some Australian

coals having a very low iron content (b2.5% Fe3O2 inash) and a very high ash flow temperature (N16008C), the slag viscosities of those coals were indepen-dent of SiO2/Al2O3 in the range 1.25 at the optimum

viscosity of 15 Pa.s. They then concluded that varia-

bility in ash composition of those coals was not a

problem and that the addition of a fixed amount of

limestone flux by weight of coal should be effective at

all times.

The effects of slag composition on the Tcv still

remain poorly understood and need more investiga-

tion. Nevertheless, three solutions can be implemen-

ted to tackle the problem arising in slagging processes

when using coals having a high ash fusion tempera-

ture and slags with high critical viscosity temperature

(Tcv). The most common solution implemented is

flux addition (either CaO or FeO) in order to decrease

ash melting points and slag viscosity. The viscosity

models will give the optimum quantity of flux to add

to the coal in order to have a continuous flow. The

A.-G. Collot / International Journasecond solution is blending with a low ash fusion coal

to provide the necessary CaO and FeO. Blendingpresents two advantages: The first one is that it

could help to overcome limitations arising from slag

crystallisation at the slagging temperature by choosing

the appropriate coal. The second one is economic as

blending can minimise or even avoid the use of flux.

The third solution would be to increase the gasifier

operating temperature above 15001600 8C toachieve the necessary slag tap viscosity or to reduce

the rate of flux addition. However this solution would

necessitate an additional oxidant requirement that

would lead to a reduced cold gas efficiency and a

decrease in refractory life for gasifiers lined with a

refractory layer.

The optimum ash fusion temperature (AFT) and

critical temperature viscosity (Tcv) recommended for

smooth slag tapping in entrained flow gasification

processes differ depending on the operating tempera-

ture of the gasifier. In principle, the AFT of a coal

should be below the operating temperature of the

gasifier (14001600 8C). Tcv is the minimum gasifieroperating temperature required for safe operation with

slags that exhibit crystalline behaviour. Tcv depends

on slag composition (SiO2/Al2O3). A solution com-

monly applied to widen the range of coals that can be

processed in entrained flow slagging systems is to

either blend them with flux or with a coal having a

low AFT.

3.3.4. Ash fouling

Some of the minerals present in coals are entrained

in the gas stream and can lead to fouling at different

locations downstream of the gasifier. Fouling pro-

blems can often be characterised as the impaction of

particles into a sticky layer of material in piping that

could occur in the quench zone of the gasifier and/or

in the cooling sections downstream of the gasifier. As

an example, during the first commercial operating

year of the Wabash River repowering plant (E-

GASk technology) the cleaning device of the plantexperienced problems related to ash deposition at the

inlet of the firetube boiler and particulate break-

through in the particulate filter system. These pro-

blems necessitated a large-scale capital improvement

programme. The Polk Power station (Texaco Technol-

ogy) also experienced several convective syngas

cooler pluggings that led to serious damage of the

oal Geology 65 (2006) 191212 199combustion turbine during the second and third years

of commercial operation (US DOE, 2000). This was

al ofparticularly due to the ash constituents of some of the

fuels tested.

Two major research and development programmes

on Coal Ash Behaviour in Reducing Environments

(CABRE I and II) were initiated by a consortium of

industries in partnership with the US DOE, the Neth-

erlands Energy Research Foundation (ECN) and the

Netherlands Agency for Energy and the Environment

(NOVEM) (Kiel and Bos, 1999). A series of tests was

performed with seven coals and a coal mixture cur-

rently used in industrial IGCC power plants. The fuels

were subjected to an initial particle temperature of

more than 2000 8C followed by a gasification tem-perature of up to 1400 8C under 1.0 MPa pressure.These conditions were considered as a realistic gas-

eous environment for entrained flow gasification

simulation. It was reported that in the presence of

H2S in the gas phase there were small deposits of

FeS on the surface of the ash (solidified Ca/Fe-con-

taining aluminosilicate spheres) collected in the

cyclone sample of the PEFG-simulator.

The authors explained that FeS formed in the

vapour-phase condensed on the ash sample during

quenching in the collection probe. All the experiments

and observations undertaken on the different ash sam-

ples led to the building of an ash deposition model

that is claimed to predict ash partitioning, ash fouling

and also ash slagging under gasification conditions in

entrained flow gasifiers (Kiel et al., 2000).

3.4. Sulphur and chlorine contents

During gasification sulphur originally present in

coal is converted to H2S which is highly corrosive

to syngas coolers and causes serious damage to heat

exchange systems as well as having an impact on the

cost of sulphur removal and recovery units. Sulphur

content in coals is reported by Shell (Ploeg, 1997) and

Texaco (McDaniel and Shelnut, 1998; McDaniel et

al., 1998) to be one of the major coal properties to

take into account for the design of a gasification plant.

Most coals also contain minor quantities of chlorine

with concentrations ranging from 0.05 wt.% to 0.5

wt.% with 0.10.2 wt.% being the most typical. As an

example, 0.1% Cl in coal will result in 200400 ppm

of HCl in the syngas. HCl can react with available

A.-G. Collot / International Journ200metal compounds present on flyash, which has accu-

mulated on syngas coolers. This results in local depos-its containing up to 15% chlorides such as FeCl2,

NaCl, CaCl and causes other problems downstream

such as those at the Wabash River repowering plant

(E-GASk technology) which experienced during itsfirst commercial year of operation, the poisoning of

the COS catalyst by chloride vapours present in the

syngas (US DOE, 2002).

Alloy composition is one of the main factors influ-

encing corrosion behaviour and most of the research

in this area has focused on the development of new

materials with enhanced resistance to corrosion. Most

of the high temperature alloys used for syngas coolers

are made of Cr, Ni and Al with some minor elements

influencing their properties. Bakker (1997, 1998,

1999, 2000) studied mixed oxidant corrosion in gas

simulating conditions under a non-equilibrium state in

syngas coolers of coal gasifiers and reported that

corrosion losses were a function of the PS2/PO2ratio and defined three types of corrosion depending

on this ratio and the concentration of HCl in the

syngas.

3.5. Comments

All entrained flow gasifiers are slagging gasifiers

and each technology has slightly different require-

ments for coal properties depending on the design.

There is a minimum ash content required for gasifiers

with slag self-coating walls which have to be covered

by slag to function and minimise heat loss through the

wall. A maximum ash content is also usually fixed for

each type of entrained flow gasifier as the tolerance to

ash content depends on economic and technical fac-

tors. Gasifiers lined with a refractory are susceptible

to some of the compounds present in coal slag (SiO2,

CaO, iron oxides) which can penetrate deep into the

refractory and eventually give rise to cracks that lead

to material loss. The optimum ash fusion temperature

(AFT) and critical temperature viscosity (Tcv) recom-

mended for smooth slag tapping in entrained flow

gasification processes differ depending on the operat-

ing temperature of the gasifier. In principle, the AFT

of a coal should be below the operating temperature of

the gasifier (14001600 8C). Tcv is the minimumgasifier operating temperature required for safe opera-

tion with slags that exhibit crystalline behaviour. Tcv

Coal Geology 65 (2006) 191212depends on slag composition (SiO2/Al2O3). A solu-

tion commonly applied to widen the range of coals

that can be processed in entrained flow slagging sys-

tems is to either blend them with flux or with a coal

having a low AFT. The tolerance of entrained flow

gasifiers to sulphur and halogens depends on the

composition and resistance of the material used in

the cooling, cleaning and tapping systems but also

on the operating conditions of the gasification process

(gasifier temperature), as well as the processing capa-

city of the downstream equipment, such as the sulphur

plant.

carbon conversion in a single stage and it is therefore

common for the residual char to be either removed

and burnt in a separate combustion unit (hybrid

cycle) or recirculated into the gasifier. Fluidised

bed gasifiers may differ in ash discharge conditions,

being dry or agglomerated. One of the main advan-

tages of this type of gasifier is that they can operate

at variable loads which gives them a high turndown

flexibility.

There are six types of gasification processes using

fluidised bed gasifiers. Only two of them have been

operated at industrial scale. Their main characteristics

charg

nular

wn fro

of th

a wa

xtract

ed th

A.-G. Collot / International Journal of Coal Geology 65 (2006) 191212 201Table 3

The BHEL fluidised bed gasifier

Feeding system and operating

conditions

Gasifier Ash dis

mode

Crushed coal injected

through lock hoppers.

Operates at a temperature

of 1000 8C and 1.3 MPapressure.

Refractory lined

reactor with 1.4 m

inside diameter

expanding to 2 m at

the upper section of

the gasifier.

Dry gra

withdra

bottom

through

screw e

discharg4. Fluidised bed gasifiers

Fluidised bed gasifiers, with the exception of the

Transport Reactor Gasifier which is midway between

a fluidised bed and an entrained flow gasifier, can

only operate with solid crushed fuels (coal: 0.55

mm, less than 50 Am for the Transport ReactorGasifier) that are introduced into an upward flow

of gas (either air or oxygen/steam) that fluidises

the bed of fuel while the reaction is taking place.

The bed is either formed of sand/coke/char/sorbent

or ash. Residence time of the feed in the gasifier is

typically in the order of 10100 s but can also be

much longer, with the feed experiencing a high

heating rate from the entry in the gasifier. High

levels of back-mixing ensure a uniform temperature

distribution in the gasifier. Fluidised bed gasifiers

usually operate at temperatures well below the ash

fusion temperatures of the fuels (9001050 8C) toavoid ash melting, thereby avoiding clinker forma-

tion and loss of fluidity of the bed. A consequence

of the low operating temperatures is the incompletelock hopper.are summarised in Tables 38.

4.1. Coal reactivity

As fluidised bed gasification is a low temperature

process (8001050 8C), the reactivity of the coal-derived char must be sufficiently high. The reaction

which plays the biggest role in the coal conversion

rate is the endothermic carbon-steam reaction that

results from coal devolatilisation. The rate of this

reaction determines whether or not a coal is suffi-

ciently reactive for fluidised bed gasification. Some

examples of studies of coal reactivity impact on

fluidised bed gasification processes are given in the

following.

Clemens et al. (2000) have studied the applicability

of clean coal gasification technologies to New Zeal-

and coals using a laboratory-scale fluidised bed gasi-

fier and a pressurised thermogravimetric analyser

(PTGA). They determined the reactivities of New

Zealand coals, which include lignites and sub-bitumi-

nous coals, and compared them with those of overseas

coals known to have sufficient reactivity to be gasified

in fluidised bed gasifiers. A peculiarity of New Zeal-

and coals is their high content in alkaline elements

which can probably act as a catalyst and hence

e Cooling and cleaning

modes

Comments

ash

m the

e gasifier

ter-cooled

or and

rough a

Fines collected in three

cyclones can be recycled

in the gasifier.

168 t/d air-blown

gasifier designed for

the gasification of

Indian coals with high

ash content.

Table 4

The High Temperature Winkler (HTW) gasifier

Feeding system and

operating conditions

Gasifier Ash discharge mode Cooling and cleaning

modes

Comments

Coal dropped from a

bin via a gravity pipe

into the gasifier. Gasifier

is fluidised from the

bottom. Additional

gasification agent is

introduced at the

freeboard (900950 8C).

Bed is formed of

particles of ash,

semi-coke and coal

and is maintained at

800 8C.

Dry ash is removed

at the bottom of the

gasifier via a

discharge screw.

Raw gas is passed

through a cyclone

to remove particulates

that are recycled to the

gasifier. Either water

cooled or fire tube

syngas cooling system.

Successfully applied

for the synthesis of

methanol from lignites

between 1986 and

1997. Wide range of

coals tested. Plan

to replace old Lurgi

dry ash reactors by

A.-G. Collot / International Journal of Coal Geology 65 (2006) 191212202increase char gasification reactivity. Calcium, which is

present in significant proportions in New Zealand coal

ashes is one of the most efficient catalysts. Although

the overseas coals they studied had very different

characteristics (very high ash content in German

brown coals and very high water content in Australian

lignite), the authors concluded that New Zealand coals

were readily able to meet the reactivity criterion for

being processed successfully in future fluidised bed

IGCC plants.

A laboratory at Imperial College London (UK),

studied the impact of several coal characteristics on

the gasification reactivity of some internationally

traded coals in bench scale reactors that could

mimic the behaviour of single coal particles in the

ABGC (Megaritis et al., 1998; Collot, 1999; Messen-

bock et al., 2000; Zhuo et al., 2000a; Lemaignen et al.,

Operating pressure is

13 MPa2002). The characteristics of coal studied included

coal maceral composition and coal mineral matter

composition. CO2-gasification experiments with

coals and their macerals revealed that it was difficult

Table 5

The Integrated Drying Gasification Combined Cycle (IDGC)

Feeding system and

operating conditions

Ash discharge

mode

Feed coal is pressurised in

a lock hopper and fed into

a dryer where it is mixed

with the hot gas leaving

the gasifier. The gasifier

operates at 900 8C and2.5 MPa pressure.

Char and ash are

collected at the

bottom of the

gasifier and burnt

in a separate boiler.to predict coal gasification reactivity in the ABGC

only from coal maceral composition, although predic-

tion of coal pyrolysis reactivity matched quite well

results obtained in the bench scale reactors. However

the authors (Zhuo et al., 2000b) concluded that the

nature and reactivity of the chars depend on a number

of factors which include not only the maceral content

of the coals but also the conditions of char formation,

such as temperature, pressure, residence time and

parent coal. These affect the two main processes that

seem to govern the reactivity, which are the deposition

of secondary carbon (by the intraparticle decomposi-

tion of volatiles) and change in the base char structure

(caused by the development of fluidity and escape of

volatiles from the melt and its re-solidification). Their

results showed that under conditions relevant to the

ABGC, vitrinite is a maceral that melts and swells,

HTW technology at

Vresova IGCC plant

(Czech republic)liptinite also melts but does not swell or agglomerate

and loses a large proportion of its mass by pyrolysis

and the third maceral, inertite, does not melt, but only

loses a small proportion of its mass under pyrolysis

Cooling and cleaning

modes

Comments

The heat of the gas

produced is used to

dry the coal whilst

the evaporation of

water from the coal

cools down the gas:

there is no need for

a syngas cooler.

Air-blown gasification

system specially

developed for the

gasification of high

moisture (up to 62%)

low rank coals.

Table 6

The Kellog Rust Westinghouse (KRW) gasifier

Feeding system and

operating conditions

Gasifier Ash discharge mode Cooling and cleaning

modes

Comments

Crushed coal is fed

at the bottom

through lock

hoppers. Operates

at pressure up to

2 MPa.

Combustion of a portion

of the coal and

agglomeration of the ash

occurring at temperatures

of 11501260 8C aroundthe tip of the feed nozzle.

Large agglomerated

particles formed are

removed at the bottom of

the gasifier while finer

particles flow upwards to

The separation of the

large agglomerated

particles formed of char,

ash and sorbent is done

through a char-ash separator

at a minimum fluidisation

velocity between that of the

char and ash so that the char

is kept in the bed while ash

and sorbent are removed

from the gasifier via an

er.

Raw gas (900 8C) passesthrough a cyclone and

particles collected are

recycled to the gasifier.

The gas is cooled to

600 8C and enters a hotgas cleaning system. A

portion of the gas is

re-circulated to the

gasifier to control the

temperature of the

Air-blown system.

The Pinon Pine IGCC

plant was designed for

bituminous coal but

other coals were tested.

A.-G. Collot / International Journal of Coal Geology 65 (2006) 191212 203and is unreactive towards the gasification agent, CO2.

The suite of coals tested were rich in vitrinite and the

authors claim that this maceral seemed to dominate

the morphological changes that occurred during char

formation. The second part of their study was on the

influence of mineral matter composition in coals.

Experiments consisted of the pyrolysis and CO2-gasi-

fication of two coals, which were first demineralised

and then impregnated with different salts, in a wire-

mesh reactor in which gasification conditions were

relevant to the ABGC. Results from their work

showed that although mineral matter contents clearly

the upper section where

gasification and sulphur

capture occur.

ash feed hoppaffect coal conversion under pyrolysis and gasification

conditions, it was difficult to find systematic patterns

for the effect of specific inorganic components on

different coals. The authors also concluded that it

Table 7

The transport reactor gasifier

Feeding system and

operating conditions

Gasifier Ash discharg

Coal is ground close to

50 Am. Coal and sorbentare fed separately through

lock hoppers into the

mixing zone. Operates at

temperatures between

8701000 8C and pressuresof up to 1.5 Mpa

Operates with a much

higher circulation rate

and higher velocities than

conventional fluidised

bed gasifiers. Gas and

entrained particles move

up from the mixing zone

into the riser and enter a

disengager.

Both the cha

extracted fro

gasifiers, an

in the barrie

are cooled, d

in lock hopp

combined pr

combustion

atmospheric

bed combuswas almost impossible to develop a predictive tool

linking catalytic activity to amounts and composition

of particular inorganic components (Lemaignen et al.,

2002).

4.2. Bed agglomeration and in bed desulphurisation

In fluidised bed gasifiers, mineral matter is a major

constituent of the bed and, as a consequence, the

characteristics of coal ash can have a major impact

on the operation of the gasifier. Ash fusion tempera-

ture is, in particular, a parameter to study as some

agglomeration zone.components of the mineral matter can soften at the

bed temperature usually leading to agglomeration and

uneven fluidisation. Disturbances will thereby result

in problems of blockage in the bottom product dis-

e mode Cooling and cleaning

modes

Comments

r and ash

m the

d collected

r filter,

epressurised

ers and

ior to

in an

fluidised

tor.

The disengager removes

the larger particles by

gravity separation and the

remaining particles are

removed in a cyclone.

Solids collected are

recycled to the mixing

zone.

Designed to operate

as a gasifier or a

combustor. The reactor

is under commission

and is being tested

with several fluxes.

The multiple passes of

the coal/char through

the gasification zone

leads to a high carbon

conversion of 95%.

e mo

ified c

esidue

an

press

uidise

perat

al ofcharge and also in the re-circulating system if there is

one.

West et al. (1994) studied the agglomeration prop-

erties of eight UK coals in a pressurised spouted

fluidised bed and an atmospheric fluidised bed

pilot plant. The authors observed the formation of

a FeSO coating on large clay-derived particles

present in coal and claimed that these particles

could be the precursors to agglomerate formation.

They concluded that iron present in coal as Fe2O3(pyrite) plays a crucial role in the formation of

agglomerates in fluidised bed gasifiers. Uemiya et

al. (1997), who studied the agglomeration formation

of coals with an Fe2O3 content of 16% and 6.2% in

a jetting fluidised bed gasifier, reported that iron

compounds were concentrated near the surface of

the agglomerated particles.

Holden and Hodges (1997) studied ash clinker

formation during the gasification of Australian

brown coals in a fluidised bed gasifier (0.3 t/d) prior

to the development of the IDGCC. Their results

showed that the rate of clinker formation in the bed

Table 8

The Air Blown Gasification Cycle (ABGC)

Feeding system and

operating conditions

Gasifier Ash discharg

Coal is injected with

sorbent to retain sulphur

in bed. Operates at

temperatures up to

1000 8C and pressuresup to 2.5 MPa

Gasifier based on a

spouted bed. Only

7080% of the coal

is gasified.

Partially gas

and other r

transferred to

atmospheric

circulating fl

combustor o

1000 8C.

A.-G. Collot / International Journ204and near the air nozzles was related to the bed tem-

perature as well as the amount of sodium and silica

(quartz and clay) in the feed coal. They claimed that

the reaction of gaseous sodium species with silica

particles could result in the formation of sodium

silicate phases, which melt at low temperature

(b1000 8C). When the operating temperature exceedsthe melting point of sodium silicate, particles become

sticky and can agglomerate on impact, resulting in the

formation of porous clinkers. More drastic gasifica-

tion conditions, such as higher gasification tempera-

ture or longer particle residence time, will produce a

molten or fused material, which can capture and flow

around ash particles, forming a consolidated clinker.4.2.1. In bed-desulphurisation

In most of the fluidised bed gasification processes

sulphur released during gasification (essentially H2S

and COS) is retained in the bed in the form of

sulphides of calcium and/or iron when using lime-

stone or dolomite. Retention efficiency is usually

around 90%. Although gasifiers are usually insensi-

tive to sulphur coals with a higher sulphur content will

require a higher addition of sorbents which will con-

sequently increase the quantities of solids discharged

by the process and hence its overall cost. Alternatively

in bed-sulphur capture can also be adversely affected

by coal ash chemistry and particularly by the presence

of alkalis in coal that promote bed agglomeration.

Sulphur capture on limestone requires a bed tempera-

ture of 870 8C or higher for sorbent activation bycalcination. At this temperature alkalis are likely to

cause agglomeration in the fluidised bed. Thus the

gasification of high alkali coals requires careful con-

trol of the temperature in a range at which carbon

conversion can be maximised (Sondreal et al., 1997;

Rousaki and Couch, 2000).

de Cooling and cleaning

modes

Comments

har

s are

ure

d bed

ing at

Syngas is first cleaned

in a cyclone then cooled

to 400 8C and cleaned bya ceramic filter.

The preferred fuels are

high-volatile bituminous

coals or possibly lower

rank coals (subbituminous,

lignites/brown coals)

Coal Geology 65 (2006) 1912124.2.2. High free swelling index

The caking and swelling characteristics of a coal

can be described by the high free swelling index

which is related to ash composition. During the heat-

ing phase (360450 8C), coal particles pass through aplastic state and swollen particles can then combine to

form agglomerates. Agglomerate formation can be

reduced by passing the temperature range of the plas-

tic state rapidly and by mixing the swollen particles

with non swelling particles, such as demonstrated at

the HTW Wesseling gasification plant (Germany) in

which Pittsburgh No 8, a high swelling index coal,

could be processed successfully in the plant only if it

when processing coals with high alkali content. The

use of coals with a low swelling index (low caking

l of Cwas mixed with a coked (non-swelling) fluidised bed

material prior to injection into the gasifier. However

test operations (air-blown and oxygen-blown) per-

formed at Wesseling showed that, even with a pneu-

matic feeding system, Pittsburgh No 8 input could not

be more than 1.5 t/h. Up to this coal feed rate, the

formation of agglomerates was controlled by strong

base fluidisation and stepped-bottom product dis-

charge. Changing the injection pipe into the flow

direction of the gas would have improved rapid mix-

ing of the feed coal with the fluidised bed material

(Adlhoch et al., 1993). The spouted bed designed for

the ABGC plant in the UK nearly eliminated the

agglomeration problem by introducing the coal

through the spout in a dilute phase hence limiting

coal particles interactions during the plastic phase.

However the rapid heating rate of the coal in the

spout reduces its swelling propensity implying a pre-

ference for the processing of low swelling coals in the

ABGC (Welford et al., 2000).

4.3. Gas cleaning system and corrosion

Most of the studies on corrosion have been per-

formed by simulation of syngas from dry fed

entrained flow gasifiers (see Section 3.4). However

Norton et al. (2000) recently studied the effect of a

CO-based gas mixture containing 0.1% H2S under

non-equilibrium conditions at 550 8C on a series offive alloys used in syngas coolers. The low percentage

of H2S present in the syngas represented the average

concentration of H2S that can be found in syngas

produced in fluidised bed gasifiers, where coal is

gasified in the presence of limestone, which can cap-

ture up to 90% of the sulphur as solid CaS. The alloys

tested, although cheaper than the ones used in syngas

coolers of dry fed entrained flow gasifiers, resisted

corrosion well. According to Norton et al. (2000),

their corrosion resistance was improved by having

an ash deposit on their surface. The same authors

(Bakker, 1999, 2000) did some corrosion tests of

syngas with an even lower concentration of H2S

(0.05% instead of 0.8% for entrained flow gasifiers)

and HCl (0.01% instead of 0.04%) to simulate syngas

produced in hybrid coal gasification systems, such as

the ABGC which also uses dolomite and limestone for

A.-G. Collot / International Journain-bed desulphurisation. Their preliminary results

indicated that in-bed desulphurisation could signifi-coals) are preferred to avoid bed agglomeration. Flui-

dised bed gasifiers are more tolerant to coals with high

sulphur content as sulphur can be partly retained in

the bed (up to 90%) by the use of sorbents.

5. Moving bed gasifiers

Moving bed gasifiers are only suitable for solid

fuels with a particle size in the range of 580 mm. A

mixture of steam and oxygen is introduced at the

bottom of the reactor and runs counter-flow to the

coal. Coal residence times in moving bed gasifiers

are of the order of 15 to 60 min for high pressure

steam/oxygen gasifiers and can be several hours for

atmospheric steam/air gasifiers. The pressure in the

bed is typically of the order of 3 MPa for commer-

cial gasifiers with tests realised at up to 10 MPa.

Coal enters the top of the gasifier and it is sequen-

tially preheated, dried, devolatilised/pyrolysed, gasi-

fied and combusted while moving towards the

bottom of the gasifier. Moisture is first driven off

in the drying zone then the coal is further heated and

devolatilised by the hotter product gas while moving

down to the gasification zone where it is gasified bycantly reduce corrosion and may permit the use of less

expensive alloy (ferritic 12 Cr steels) for the building

of cleaning devices and heat exchangers. Corrosion at

low H2S levels is being further investigated by EPRI

and the EEC Institute for Advanced Material in the

Netherlands.

4.4. Comments

In fluidised bed processes it is necessary to process

coals with a higher ash fusion temperature (AFT) than

the operating temperature (N1000 8C) of the gasifierto avoid ash agglomeration (which causes uneven

fluidisation in dry ash, fluidised bed gasifiers). The

presence of pyrite (Fe2O3) in coal as well as sodium

silicates formed during gasification are believed to be

among the factors that can cause agglomeration in

fluidised bed systems. Very careful control of the

gasifier operating temperature is therefore required

oal Geology 65 (2006) 191212 205reacting with steam and carbon dioxide. The remain-

ing char is finally completely burnt in the combus-

tion zone where the bed reaches its highest tempera-

ture. Maximum temperatures in the combustion zone

are typically in the range of 15001800 8C forslagging gasifiers and 1300 8C for dry ash gasifiers.As the flow is counter-current, the gas leaving the

gasifier is cooled against the incoming feed and

typical gas exit temperatures are of the order of

400500 8C. Ash is removed either as a dry ash oras a slag, depending on the gasifier type. Although

moving bed gasifiers are presently less used than

fines with less than 6 mm in size for the Lurgi dry ash

gasifier. A high concentration of fines in coal will lead

to unstable operation of the gasifier as it will cause

pressure drops in the bed. Pressure drop problems

induce the possibility of grate traction loss (due to

bed fluidisation), channel burning (leading to unac-

ceptable gas outlet temperatures) and solid elutriation.

At Sasol (Van Dyk et al., 2001), pressure drop is

estimated by the Ergun equation as a function of

bed voidage (e), viscosity (l), density (q), superficial

emov

emov

e.

A.-G. Collot / International Journal of Coal Geology 65 (2006) 191212206entrained flow gasifiers for the construction of new

power plants, these moving bed gasifiers present the

advantage of being a mature technology. Three gasi-

fication processes based on moving bed gasifiers are

detailed in Tables 9 and 10.

5.1. Bed permeability

The main requirements for moving bed gasifiers

are efficient heat and mass transfer between solids and

gases within the bed. This involves good bed perme-

ability and consequently the control of coal particle

size.

5.1.1. Coal particle size

Modern mining methods and mechanical cutting of

coal as well as wet coal, plugged coal screens and

aged/brittle coal, tend to reduce coal particle size to

the extent that run of mine coal can contain up to 40

50% fines by weight. Dittus and Johnson (2001)

pointed out that a high concentration of fines entering

the Lurgi dry ash gasifier with the coal feed will lead

to a disruption of the entire plant operation as the

operator will have to reduce the gas production by

decreasing coal loading at the top of the gasifier. The

authors fixed a design limit at a maximum of 5% coal

Table 9

The BHEL moving bed gasifier

Feeding mode and

operating conditions

Gasifier Ash r

Crushed coal (540 mm).

Operates at 1 MPa

High jacketed gasifier.

Air and steam are fed

through a grate, which

also enables ash removal.

Ash r

a gratvelocity Us and particle size diameter (dp).

P=L 150 1 e 2lUs=e3d2p 1:75 1 e qU2s =e3dpAs it is a particle size distribution rather than a

uniform particle, dp is replaced by Udp, where U isthe particle sphericity and dp, the average particle size

reflecting the mean surface area (also referred to as the

Sauter mean diameter). The Sauter mean diameter of a

coal sample with a specific particle size distribution is

calculated as follows:

dp 1=Uixi=dp;iwhere i is the screen number xi the fraction (mass %)

on screen idp,i is the diameter (mm) of screen i.

According to Van Dyk et al. (2001), experience on

the Lurgi dry ash gasifiers at Sasol has shown that dpis a useful parameter to predict which particle size

distributions are more likely to result in gasifier

instability. They also reported that an inefficient

screening due to screen overload causes misplacement

of coal fines that can easily reduce the Sauter diameter

to unacceptably low values resulting in highly

unstable gasifier operation.

Lacey et al. (1992) reported a study on coal fines in

BGL gasifiers. Their results showed that the BGL

al system Cooling and cleaning

modes

Comments

al through A gas cooler is used to

produce steam for the

gasifier. Further gas

cooling and tar

condensation are done

by water quenching.

The development

was stopped due to

poor performance of

the gas cooler and the

gasifier was replaced

by a fluidised bedParticulates are removed

by Venturi scrubber

(see Table 3)

is d

a seq

to en

of th

conn

rating

surro

dded

es. T

asifier

apped

arbon

tion

d in t

) is c

to rem

from

l of CTable 10

Lurgi moving bed gasification processes

Technology British Gas Lurgi (BGL)

Feeding mode and

operating conditions

Lumped coal together with a flux

at the top of the gasifier as

A distributor plate slowly rotates

distribution of the coal at the top

caking coals, the distributor is

to prevent the bed from agglome

Gasifier Double-walled cylindrical reactor

steam jacket. O2 and steam are a

bottom of the bed through tuyer

internal temperature within the g

Ash removal system Slagging gasifier. Molten ash is t

quenched with water.

Cooling and cleaning

modes

Tars, high boiling points hydroc

released during the devolatilisa

in a quench vessel and re-injecte

the tuyeres. The gas (450500 8Cby a water quench and scrubbed

Comments It is a slagging gasifier modified

gasifier.

A.-G. Collot / International Journagasifier can accommodate a reasonable quantity of

fines (b6 mm) in the lump feed. The authors per-formed tests in the BGL gasifier from the Westfield

facility (500 t coal/d, 2.3 m diameter) with coal fines

at a pressure of 2.5 MPa. Two UK power plant coals,

one weakly caking (Kellingley) and the other medium

caking (Coventry) and two American coals (Pitts-

burgh No 8 and Illinois No 6) were tested. When an

excess of fines was injected at the top of the gasifier,

the free flow of coal and gas within the bed was

disturbed leading to unstable gasifier operation. This

was characterised by the fluctuation of the outlet

temperatures and a product gas with varying compo-

sitions. It was however possible to process at standard

load, Pittsburgh No 8 coal, a caking coal, with fine

contents of up to 30 to 40%, without adversely affect-

ing the stability of the gasifier. Gasification of Illinois

No 6 was also found to be satisfactory but the amount

of fines that could be tolerated in the coal feed was

lower than with Pittsburgh No 8. Other tests with the

two coals included the injection of their slurries (30%

coal/70% water) through the tuyeres while simulta-

neously feeding the top of the gasifier with 30% to

40% of their fines. This enabled the processing ofSasolLurgi dry bottom gasifier

ischarged

uence of batches.

sure even

e bed. For

ected to a stirrer

.

Similar to BGL.

unded by a

towards the

his results in high

(2000 8C).

Gasifier is surrounded by a water

jacket that raises steam for use in

the gasifier. A high ratio of steam

to O2 (55 :1) is blown up through

a grate at the bottom of the gasifier.

The combustion zone is at a

temperature (1000 8C) just belowthe AFT.

off and Ash removed by a revolving grate

and depressurised in a lock hopper.

s and particulates

step are removed

he bed near

ooled and cleaned

ove H2S.

The gas (300500 C) is water

quenched.

the Lurgi dry ash More suitable than BGL for use

with the more highly reactive coals.

oal Geology 65 (2006) 191212 207coals with up to 50% fines in the gasifier. Briquetting

is also an alternative to be considered for processing

coal fines but it is not a cheap process and can

introduce a cost penalty to the process. A joint pro-

gramme, supported by amongst others the UK Depart-

ment of Energy (now UK DTI) and the EC, tested the

performance of the BGL gasifier with oversize coals

collected from a screening plant and briquettes made

from the fines of British power plant coals (Coventry

and Kellingley). Stable operation of the gasifier was

achieved at standard load with the two coals (British

Gas/Lurgi slagging gasifier, 1993).

5.1.2. Thermal fragmentation

High concentrations of fines in the coal feed can

also cause carry over of fines to downstream equip-

ment. To estimate the quantity of fines carried over

downstream of the gasifier Van Dyk et al. (2001)

defined the term dthermal fragmentationT, as the dif-ference between average particle size before and after

pyrolysis at a temperature of 700 8C. Thermal frag-mentation is largely affected by the moisture in coal

but also by a complex interaction with other factors,

such as oxidation and weathering (Van Dyk, 1999).

(1996) that an optimum slag viscosity at tapping

temperature should be less than 5 Pa.s in the BGL

al of5.1.3. Caking properties

Caking of coal within the gasifier can also cause

pressure drop fluctuations and channel burning, result-

ing again in unstable gasifier operations. In severe

cases oxygen break-through can occur, which can

cause a safety hazard because of the probability of

downstream explosions. Caking of coal particles can

be defined as the softening or plasticity property of

coal, which causes particles to melt together to form

larger particles when heated. In order to process cak-

ing coals, a stirrer connected to the coal plate distri-

butor has been added to the BGL gasifier. It ensures

that strongly caking coals are completely carbonised

and converted to free-flowing solids that pass to the

lower gasification bed (Lacey et al., 1992). Sasol has

developed an in-house technique to characterise cak-

ing propensity of coal processed under gasification

conditions existing in their plants. Details of the tech-

nique are given by Van Dyk et al. (2001). A solution

used at Sasol to avoid caking (Van Dyk et al., 2001),

is the blending of high caking coals with low caking

ones. Normal blends used for gasification at Secunda

have a caking property that can vary within +/20%and coal blends used in the Sasolburg plant have no or

little caking.

5.1.4. Ash fusion temperature

Low ash fusion temperature (AFT) can result in the

formation of a large amount of fused ash or clinkers in

the ash bed of the dry ash Lurgi gasifiers. Ash clin-

kering can also cause channel burning, pressure drop

problems and unstable gasifier operation in both the

slagging and non slagging gasifiers. Ash composition,

especially calcium and iron content give an indication

of the expected ash fusion temperature (see Section

3.3). A coal rich in Fe or Ca has usually a low ash

fusion temperature due to the fluxing properties of the

Ca and Fe minerals. The ash fusion temperature gives

an indication of the extent of ash agglomeration and

clinkering within the gasifier and is used by Sasol to

estimate the risk of ash clinkering. The Dakota Gasi-

fication Co has experienced the formation of clinkers

that filled 20% of the gasifier. The gasifier had to be

shutdown and jack hammers used to break up the

clinker so it could be removed. Dakota Gasification

believe that sodium in the coals they process in their

A.-G. Collot / International Journ208gasifiers, is mainly responsible for ash clinkering and

they fixed the limit of sodium content at which clin-gasifier in comparison to less than 15 Pa.s in entrained

flow gasifiers. The same author also suggested a

tapping temperature of 1400 8C which is lower thanin most of the major entrained flow gasifiers. That

means that more flux may need to be used in order to

comply with those limits and as a consequence the

total cost of the system might be increased.

5.3. Comments

The main requirement of moving bed gasifiers is

good bed permeability to avoid pressure drops and

channel burning that can lead to unstable gas outlet

temperatures and composition as well as risk of a

downstream explosion. Depending on the gasifier

design and other characteristics of coal, such as cak-

ing propensity, the tolerance of the different gasifiers

to coal fines varies from 5% in the Lurgi dry ash

gasifier to up to 50% fines in the BGL gasifier. As

caking causes particles to melt and sinter together to

form larger particles when heated, caking coals can be

processed in Lurgi dry ash gasifiers only if they are

blended with non-caking coals. However, BGL gasi-

fiers are equipped with a stirrer connected to the coal

plate distributor to allow the processing of strongly

caking coals.

6. Conclusions

Hydrogen is currently and mainly used as an inter-

mediate for the synthesis of chemicals and clean fuels.

However with the move towards the dhydrogen econ-omyT there is an incentive to use Hydrogen itself as anenergy carrier itself. New programmes and research

projects, which are particularly dedicated to the pro-kering can be avoided. Coal blending is also a solu-

tion to keep the sodium content constant.

5.2. Slag mobility in the BGL gasifier

The most recent published work on slagging prop-

erties in gasifiers concerned entrained flow gasifiers.

However it has been reported by Patterson and Hurst

Coal Geology 65 (2006) 191212duction of Hydrogen from coal, are presently under-

way worldwide. In this paper some coal properties

Coproduction of F-T liquids and power by the gasification of

EW92004. Central Research Institute of Electric Power Industry

(CRIEPI), Tokyo, Japan.

A.-G. Collot / International Journal of Coal Geology 65 (2006) 191212 209Bailey, R.A., 2001. Projects in Development Kentucky Pioneer

Energy, Lima Energy. Gasification Technologies Conference.

Gasification Technologies Council, Arlington, VA, USA 11 pp.

Bakker, W.T., 1997. The effect of chlorine on corrosion of stainless

steels in gasifiers. Corrosion/97: Conference Papers. NACE

International, Houston, TX, USA. paper 134, 21 pp.

Bakker, W.T., 1998. Corrosion of iron aluminides in HCl contain-

ing coal gasification environments. Corrosion 98: Conference

Proceedings. NACE International, Houston, TX, USA. paper

185, 16 pp.

Bakker, W.T., 1999. Variables affecting mixed oxidant corrosion of

stainless steels in gasifiers. Corrosion 99: Conference Pro-

ceedings. NACE International, Houston, TX, USA. paper 65,petroleum coke and coal. Twentieth Annual International Pitts-

burgh Conference. Pittsburgh Coal Conference, Pittsburgh, PA,

USA. paper 13-3, 23 pp.

Ashizawa, M., Hara, S., Ichikawa, K., Mimaki, T., Inumaru, J.,

Hamamatsu, T., 1993. Development of high-performance coal

gasification technology for high ash fusion temperature coals

evaluation of flux addition method and coal blending method.that have an influence on gasifier design, operation

and performance have been reviewed. All types of

coal can be gasified and gasification appears to have a

promising future for the production of hydrogen from

coal. Gasification plants are also in the best position,

compared to other coal-based alternatives, to capture

CO2. The main technical challenge presently faced in

the production of hydrogen from coal is the separation

of hydrogen from the syngas and the capture and

sequestration of CO2.

Acknowledgements

The author would like to thank Dr Geoffrey Mor-

rison, head of publication at the IEA Clean Coal

Centre for his help in preparing the manuscript.

References

Adlhoch, W., Bellin, A., Dehms, G., Shumacher, H.J., Schuma-

cher, I., 1993. Gasification of Pittsburgh No 8 in Rheinbrauns

Pressurized High-Temperature Winkler Pilot Plant. EPRI TR-

103367. EPRI Distribution Center, Pleasant Hill, CA, USA.

Alchemix Corporation, 2003. HydroMaxR: bridge to a hydro-gen economy (downloaded from). http://www.alchemix.us/

TechnologyDescriptionweb710.pdf (Carefree, AZ, USA).

Amick, P., Breton, D., Geosits, R., Kramer, S., Tam, S., 2003.14 pp.Bakker, W.T., 2000. Variables affecting mixed oxidant corrosion of

stainless steels in gasifiers. Materials and Corrosion 51 (4),

219223.

Boyd, R.K., Benyon, P.J., 1999. Coal property impacts on gasifica-

tion, Research symposium on entrained-flow gasification: pro-

ceedings. CRC Black Coal Utilisation, Advanced technology

Centre, Callaghan, NSW, Australia. 8 pp.

British Gas/Lurgi slagging gasifier, 1993. British Gas/Lurgi slag-

ging gasifier, final report. EU-14399-EN. Commission of the

European communities, Luxembourg. 31 pp.

Browning, G.J., Bryant, G.W., Lucas, J.A., Wall, T.F., 1999. The

effects of heterogeneous slag character on viscosit