Factors governing reactivity in low temperature coal gasification. Part 1.An attempt to correlate results from a suite of coals with experiments on

maceral concentrates

R.C. Messenbo¨ck, N.P. Paterson*, D.R. Dugwell, R. Kandiyoti

Department of Chemical Engineering and Chemical Technology, Imperial College, University of London, London SW7 2BY, UK

Received 29 March 1999; received in revised form 17 June 1999; accepted 18 June 1999

Abstract

This paper reports on the first stage of a study attempting to develop laboratory scale tests for the reliable determination and eventualprediction, of the effect of coal properties on the performance of coals in air blown gasifiers. The pyrolysis and gasification behaviour of asuite of coals have been matched, using a high-pressure wire-mesh reactor (WMR), with those of maceral enriched samples. Reactionconditions were selected to simulate those of the pilot-scale gasifier stage of the Air Blown Gasification Cycle operated by British Coal.Subsequent stages of the study have examined the role of mineral matter and char morphology. The suite of coals previously tested in thepilot scale reactor were originally selected to represent coals of a wide geographical spread, a limited range of properties and werecommercially available in the EU. The maceral enriched samples were obtained from different coals, but were of similar rank to thoseexamined in this study. Short hold time experiments (10 s) have been carried out in the laboratory scale reactor. Three distinct types ofbehaviour have been identified—arising from competing influences of the reactivity of the base char, the impact of secondary char depositionand the effect of melting on the reactivity of the ageing particles. The relative combustion reactivity of the residual chars was measured in anatmospheric pressure TGA test. The reactivities were all low, but differences were apparent between the different samples and wereconsistent with the observed gasification behaviour. The use of maceral analysis to predict the behaviour of the whole coals has beenexamined. Extrapolation from maceral behaviour was found to give reasonable estimates of the behaviour under pyrolysis conditions, butpredictions of gasification behaviour were not reliable.q 1999 Elsevier Science Ltd. All rights reserved.

Key words: Coal pyrolysis; Coal gasification; Maceral pyrolysis; Maceral gasification

1. Introduction

This paper describes the first part of a programme aimingto develop laboratory scale tests that can be reliably used todetermine—and eventually predict—the effect of funda-mental coal properties on the performance of particularcoals in pilot and commercial scale air blown gasifiers(,11008C). Several developments using air blown gasifica-tion technology are underway and it is important to under-stand why coals of nearly similar rank may performdifferently—as recently noted during the operation of apilot-scale spouted fluidised-bed gasifier. This work wasundertaken by the Coal Technology Development Divisionof British Coal, as part of the development of the Air BlownGasification Cycle (ABGC) [1,2], one of the clean coaltechnologies highlighted for further development in the

UK Government sponsored ‘Foresight study’ [3]. Theproblems relating to coal reactivity are also relevant toseveral similar developments, such as that of the KRWgasifier at Pinon Pine in the USA [4].

Recent work in this laboratory has examined variations infuel reactivities as determined in different bench scale reac-tors [5]. A high-pressure wire-mesh reactor (WMR) and afluidised bed reactor have been used to determine conver-sions of single coal particles; when operated under compar-able conditions results from these two instruments havebeen found to support each other [6]. The work has focusedon the development of bench scale experiments mimickingthe behaviour of single particles in pilot/PDU scale fluidisedbed reactors, with the aim of determining fuel reactivity[7,8] and arriving at valid test protocols [9,10]. Resultsfrom the study have highlighted the importance of matchingchar formation conditions (heating rate, temperature, pres-sure, particle size and residence time) with those in pilot/PDU scale equipment, for accurately determining—and

Fuel 79 (2000) 109–121

0016-2361/00/$ - see front matterq 1999 Elsevier Science Ltd. All rights reserved.PII: S0016-2361(99)00145-3

www.elsevier.com/locate/fuel

* Corresponding author.E-mail address:n.paterson@ic.ac.uk (N.P. Paterson)

eventually predicting—gasification conversions and reac-tivities. In particular, we have shown that unrealisticresults—and even misleading trends—may be obtainedwhen gasificationreactivity tests are carried out (i) underconditions unrepresentative of the actual process or (ii) byusing chars prepared in separate experimental stages—unrepresentative of the actual process. It appears thatsome caution is necessary when extracting data from theliterature in cases where these requirements have not beenscrupulously met.

The focus in the present phase of the work has shifted toexamining the effect of changes in feed coal on total volatileyields and extents of gasification—as observed in the high-pressure WMR. The coal samples correspond to thoseoriginally selected by British Coal as representative of feed-stocks available on the world market and have been tested inthe pilot scale pressurised spouted fluidised-bed gasifier[11]. Comparison with performance in the pilot scale reactorhas been instrumental in providing insight into the effect ofparameters such as temperature, pressure and heating rate;

observed trends have been explained in terms of physical(e.g. pore diffusion), morphological (e.g. changes in struc-ture caused by char ageing) and chemical effects (e.g. cata-lysis by mineral matter). A correlation between theperformances in that pilot-scale gasifier and in the WMRhas already been reported [7,9].

In this paper, high-pressure WMR data on the pyrolysisand gasification behaviour of the suite of six coals has beenpresented. The results have been matched with analogousdata from a set of maceral concentrates, in order to evaluatethe use of maceral analysis for predicting sample weightloss during pyrolysis and gasification.

2. Experimental

2.1. The coal samples

The six coal samples were obtained from British Coal.They had been selected for testing in the pilot scale gasifier

R.C. Messenbo¨ck et al. / Fuel 79 (2000) 109–121110

Table 1Proximate and ultimate analyses of the coals

Daw Mill a El Cerrejon Drayton Rietspruit Illinois No.6 Daw Millb

Volatile Matterc (%, daf) 39.7 40.7 39.5 32.8 45.5 39.9Ash (%, db) 14.1 7.2 10.7 12.5 9.5 4.4Moisture (%, ad) 4.5 5.4 3.1 3.9 7.1 6.1Swelling Number 1 1 2.5 1 6 1Carbon (%, daf) 80.6 82.4 82.6 82.5 78.2 80.1Hydrogen (%, daf) 5.4 5.8 5.7 4.9 5.6 4.7Nitrogen (%, daf) 1.5 1.7 2.0 2.1 1.4 1.3Sulphur (total) (%, daf) 2.0 0.8 0.8 0.7 4.4 1.1Vitrinite (%Vol, mmf) 67 84 75 63 92 66Inertinite (%Vol, mmf) 21 14 21 33 6 21Liptinite (%Vol, mmf) 12 2 4 4 2 13Mean Vitrinite Reflectance 0.60 0.72 0.65 0.73 0.40 0.60

a High ash.b Low ash.c Determined by a standard method.

Table 2Analysis of maceral concentrates

Maceral group Vitrinite Liptinite Inertinite

Sample No. (internalreference number)

14 11 22

Maceral type Hand picked vitrain (floatsat 1.4 s.g.)

Exinite Semi-fusinite

Parent coal (Seam) Markham Main (Barnsley) Peckfield (Beeston) Roddymoor (Ballarat)Volatile C (%, db) 35.0 54.2 16.4Fixed C (%, db) 62.3 42.7 79.6Ash (%, db) 1.4 1.8 4.1Moisture (%, db) 1.3 1.3 0C (%, daf) 81 82 93H (%, daf) 5.5 6.8 4.4N (%, daf) 1.7 1.2 0.9Vitrinite (vol%, mmf) 99 3 10Liptinite (vol%, mmf) 1 92 0Inertinite (vol%, mmf) 0.3 5 90

at CTDD and were representative of the range of the coalsthat could be expected to be used in the ABGC. It has there-fore been possible to link the performances of the pilot scalegasifier and the laboratory scale equipment at ImperialCollege. The coals were Daw Mill (power station blend),Daw Mill (washed product), El Cerrejon (Colombia), Dray-ton (Australia), Rietspruit (South Africa) and Illinois No. 6(USA). The analysis of the coals is given in Table 1.

2.2. The maceral enriched samples

Analyses of the maceral enriched samples are given inTable 2. Samples of this type are fairly rare, so the actualsamples were not derived from the coals used in this study.However, they were selected as they contained a similarcarbon content (i.e. they were of similar rank). The analysesshow that the individual samples contained more than 90%of the enriched maceral. The samples were part of a suite ofmaceral samples prepared at the former Coal ResearchEstablishment. The parent coal for each sample is noted inthe Table.

All the samples were sieved to produce a fraction in therange 106–150mm and were dried at 508C, under vacuumfor 18 h, before being stored under nitrogen.

2.3. Equipment

The WMR—Pyrolysis and gasification experiments havebeen carried out in an electrically heated high pressure

WMR which has been described in detail elsewhere[7,8,12,13]. The introduction of carbon dioxide into thereactor for gasification experiments required the use of amore resistant molybdenum mesh compared with the SS304 mesh used in pyrolysis experiments. In each test, asingle layer of coal particles is placed between a foldedwire mesh stretched between two electrodes, with thesample holder serving as the resistance heater. A continuousflow of gas through the sample holding part of the meshremoves volatiles from the heated reaction zone, through atrap cooled with liquid nitrogen (for pyrolysis tests) or asaline ice bath (for gasification tests) which collects conden-sibles. The high pressure version permits operation at heat-ing rates up to 10 000 K s21, to peak temperatures up to10008C (with stainless steel mesh) or 15008C (with molyb-denum mesh) and at pressures up to 150 bar. Use of a rangeof gases is possible, including helium, nitrogen, hydrogen,carbon dioxide and steam. In this study coal behaviour hasbeen investigated under both helium and carbon dioxideatmospheres.

Results from this reactor have been expressed as follows:

Total volatile yield: This is the weight loss measuredduring the test expressed as a percentage of the originalweight of sample, on a dry ash free basis.Extents of gasification:In this work, ‘extents of gasifica-tion’ were calculated by subtracting sample weight lossrecorded during a pyrolysis experiment from the weightloss observed during a gasification test, performed under

R.C. Messenbo¨ck et al. / Fuel 79 (2000) 109–121 111

35

40

45

50

55

60

0 5 10 15 20 25 30

Pressure, bar

Tot

al V

olat

ile Y

ield

, %, d

af

Daw Mill (psb)

Daw Mill (wash)

Illinois No6

Rietspruit

El Cerrejon

Drayton

Fig. 1. The total volatile yield of the six coals under pyrolysis conditions.

R.C. Messenbo¨ck et al. / Fuel 79 (2000) 109–121112

40

45

50

55

60

65

70

75

80

0 5 10 15 20 25 30

Pressure, bar

Tot

al V

olat

ile Y

ield

, %, d

af

Daw Mill (psb)

Daw Mill (wash)

Illinois No6

Rietspruit

El Cerrejon

Drayton

Fig. 2. The total volatile yields with the six coals under gasification conditions.

0

5

10

15

20

25

30

0 5 10 15 20 25 30

Pressure, bar

The

Ext

ent o

f Gas

ifica

tion,

%, d

af

Daw Mill (psb)

Daw Mill (wash)

Illinois No6

Rietspruit

El Cerrejon

Drayton

Fig. 3. The extent of gasification with the suite of coals.

otherwise identical experimental conditions (heating rate,temperature, hold time and pressure).

G%t � �TV�Gasification�t 2 TV�Pyrolysis�t�;where TV denotes the total volatile yield andt denotes thehold time.

Determinations of relative char combustion reactivitieshave been carried out using a Perkin–Elmer TGA7 thermo-gravimetric analyser, loaded with about 1.5 mg of char, andheated at 258C min21 to 5008C under nitrogen. Once aconstant weight was established, the flow gas was switchedfrom nitrogen to air (using a flow rate of 40 ml min21 inboth cases). After virtually complete reaction at 5008C thetemperature was raised to 8508C, to determine the ashcontent of the sample. The results from this test have beenexpressed as follows:

Relative combustion reactivity—the minimum in the firstderivative of the weight–time curve was taken as the reac-tivity of the char

Rmax� 2�1=W0��dW=dt�umax

wheret is time,W0 is the initial char weight andW the timedependent char weight.

2.4. Test programme

Tests have been done using the WMR at pressures of 1,10, 20 and 30 bar in He (pyrolysis) and CO2 (gasification)atmospheres. The mesh conditions were standardised foreach test with a temperature of 10008C, a hold time of10 s and a heating rate of 1000 K s21. Details of the samplesused are given above. Each test was conducted in duplicate.Samples of the char were collected after each test with theWMR for the measurement of the relative combustionreactivity.

3. Results and discussion

3.1. The pyrolysis and gasification of the suite of six coals

The effect of pressure (in the range 1–30 bar) on the totalvolatile yields, during thepyrolysisof the six coals in He, isshown in Fig. 1. All the trendlines show a decrease in thepyrolysis total volatile yields with increasing pressure. Theorder of the magnitude of the pyrolysis yields was IllinoisNo. 6. Daw Mill (psb). El Cerrejon. Daw Mill(washed). Drayton. Rietspruit. The decrease in totalvolatile yield with increasing pyrolysis pressure has beennoted in earlier work and is explained by the suppression ofvolatile release at the higher pressures and the deposition ofsecondary char within the particles [12,14].

The corresponding total volatile yields measured underCO2-gasification conditions are shown in Fig. 2. The magni-tude of the total volatile yields for the different coalsdepends upon the pressure chosen for the comparison and

these should be taken from Fig. 2. The ‘extent of gasifica-tion’ (the difference in weight loss between pyrolysis andgasification, in Figs. 1 and 2, respectively) for the suite ofcoals as a function of pressure is shown in Fig. 3. Threetypes of gasification behaviour may be observed:

1. An increase in extent of gasification as the pressureincreased over the lower part of the range, which tailsoff towards 30 bar. Illinois No. 6 coal showed this beha-viour and the results show that the coal was reactivetowards gasification at lower pressures and the high reac-tivity of this coal was sufficient to counteract the effect ofdeposition of secondary char (also cf. Refs. [7,8]).However, SEM studies have shown that this coal becamemore fluid at higher pressures and this would haveannealed the active sites and decreased the rate of reac-tion. The results show that complete conversion of thiscoal is unlikely to be achieved in CO2.

2. An increase in the extent of gasification over the wholepressure range, with an increase in the rate towards theupper end of the pressure range. This behaviour wasshown by Daw Mill (washed) and to a less marked extentwith Drayton coal. The behaviour suggests that increasedfluidity at higher pressures does not occur with this coaland that deposition of secondary char has a lesser effect atthe lower pressures. The rate of reaction at lower pres-sures was lower than that measured with Illinois No. 6coal. This behaviour is that of coals less reactive thanIllinois No.6, but more reactive than those showingminima as a function of pressure.

3. A decrease in the extent of gasification at pressures lessthan approximately 20 bar, followed by an increase athigher pressures. This behaviour was found with ElCerrejon, Rietspruit and Daw Mill coals (psb). It can beexplained by the deposition of secondary char having adominating effect as the pressure was increased towards20 bar and this retarded the extent of reaction. However,as 20 bar was approached, the CO2 was able to gasifyaway this less reactive material and expose the basechar, which was able to react at a faster rate, so that theoverall effect within the test time was an increasingextent of gasification.

These data graphically illustrate the differences betweenthe types of gasification behaviour displayed by differentcoals at short reaction times. At longer times, extents ofgasification for all the samples assume pressure-profilessimilar to that of the Illinois No. 6 sample.

As discussed in early studies [12,14] the data show thatthe overall performance is a balance between competinginfluences and that the importance of each of these willvary between coals and with the test conditions. The influ-ences can be summarised as the reactivity of the base char,the impact of secondary char deposition and the impact ofmelting of the coal, which has been observed to be moresignificant under gasification conditions than under pyroly-sis conditions. Any melting will have affected the pore

R.C. Messenbo¨ck et al. / Fuel 79 (2000) 109–121 113

structure and reduced the surface area available for reaction.Further work is in progress to understand what affects theimportance of these influences.

In this paper, the effect of the maceral content on theextent of gasification and the prediction of the overall

performance from the maceral composition and reactivityis assessed. In further papers in this series, the ability oftechniques such as Fourier Transform Infra Red (FTIR)spectrometry, mineral matter analysis and Scanning Elec-tron Microscopy (SEM) to identify differences between

R.C. Messenbo¨ck et al. / Fuel 79 (2000) 109–121114

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20 25 30

Pressure, bar

Rel

ativ

e C

ombu

stio

n R

eact

ivity

, % m

in-1

, daf

Daw Mill psb

Daw Mill wash

Illinois No6

Reitspruit

El Cerrejon

Drayton

Fig. 4. The relative combustion reactivity of the pyrolysis chars.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 5 10 15 20 25 30

Pressure, bar

Rel

ativ

e C

ombu

stio

n R

eact

ivity

, % m

in-1

, daf

Daw Mill psb

Daw Mill wash

Illinois No6

Reitspruit

El Cerrejon

Drayton

Fig. 5. The relative combustion reactivity of the gasification chars.

different coals and chars has been investigated. In addition,their potential as a means of predicting coal performancehas been assessed.

3.2. The relative combustion reactivity of the charsproduced by pyrolysis and gasification

The relative combustion reactivities of the charsproduced by pyrolysis of the six coals at pressures between1 and 30 bar are shown in Fig. 4. The combustion reactivitywas assessed using the TGA test, at atmospheric pressureand isothermal conditions at 5008C. The chars from all thecoals, apart from El Cerrejon, show a decrease in reactivitybetween 1 and 30 bar, the rate of decrease tails off above20 bar. The values for El Cerrejon chars suggest no realchange over the pressure range. The changes are thoughtto reflect the effects of deposition of less reactive secondarychar within the char structure during the pyrolysis at differ-ent pressures. The pyrolysis data shows that this effectincreased as the pressure was raised, with the effect increas-ing most between 1 and 20 bar. Apart from El Cerrejon char,the trends apparent in the combustion reactivity test data areconsistent with those observed in the original pyrolysis data.These trends reflect our earlier findings [5,6,15].

The relative combustion reactivities of the gasificationderived chars are shown in Fig. 5. The reactivity of thechars from the different coals showed some variation. TheIllinois chars had the highest reactivity, with the other coalchars bunched together in the range 0.4–1.0% min21 at(20 bar). The reactivity of all the chars decreased as the

pressure was raised to 20 bar, thereafter, for all chars exceptIllinois No. 6, the reactivity increased slightly at 30 bar. Thebehaviour is similar to that shown in Fig. 3 for the extent ofgasification, and the effects are thought to be the result of theconflicting influences of deposition of secondary char,which appears to reduce the reactivity at pressure up to20 bar; thereafter the increase appears due to the pre-domi-nating effect of the increased reactivity of the gasifyingmedium. This opens up the char structure and exposingthe base char and consequently leading to increases in thesubsequent combustion reactivity is observed. A similarbehaviour has been noted in combustion reactivities fromhydropyrolysis chars [16]. Illinois No. 6 has a medium tohigh swelling number and it showed a different gasificationbehaviour to the other coals tested. It is therefore notsurprising that it showed a different combustion behaviour.Its gasification reactivity tailed off at 30 bar, which wasexplained in terms of melting and this will have alsoaffected the subsequent relative combustion reactivity.

It is noted that the chars studied during this work were allof low reactivity, and the differences between the valuesmeasured were small, but significant. Other work [10] hasshown that chars produced at shorter hold times had higherreactivities. This highlights the rapid annealing of the charthat occurs when it is held at high temperature for more thana few seconds.

The relative combustion reactivities of the pyrolysisderived chars, except for Reitspruit and Drayton, werehigher than those derived under gasification conditions.This may be due to the morpholgical changes that have

R.C. Messenbo¨ck et al. / Fuel 79 (2000) 109–121 115

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30

Pressure, bar

Tot

al V

olat

ile Y

ield

, %, d

af

Vitrinite

Inertinite

Liptinite

Fig. 6. The total volatile yield with the macerals under pyrolysis conditions.

been noted under gasification atmospheres, which furtherdeactivate the char. However, differences are small, butthey are consistent.

3.3. The pyrolysis and gasification behaviour of somemaceral enriched samples

In previous studies associated with coal pyrolysis andliquefaction [17], maceral enriched samples of vitrinite,liptinite and inertinite were separated from a suite of UKcoals. Their pyrolysis volatile yields were measured in anatmospheric pressure WMR, together with that of the parentcoals. It was found that the pyrolysis total volatile yield ofthe ‘whole’ coal could be predicted from the maceral analy-sis and the volatile yields of the individual maceral concen-trates of that coal [18,19]. This showed theabsenceofsignificant synergistic effects between the different maceralsduring pyrolysis.

The work involving maceral concentrates has now beenextended to study the effect of pressure and reactive gaseousenvironment on the total volatile yields. The present studyhas been undertaken using a set of maceral enriched samplesproduced from different coals, but each chosen to contain asimilar carbon content to the suite of coals that have alsobeen investigated in the current study.

The pyrolysis total volatile yields of the three maceralenriched samples are shown in Fig. 6, at pressures between1 and 30 bar. As expected, the liptinite showed the highestloss of volatiles, in excess of 60% at all pressures. There wasa decrease in the volatile release as the pressure was raised.Volatile losses from the vitrinite were lower and nearly

parallel to those of the liptinite sample; the difference ofsome 20% at lower pressures decreased to about 15% at30 bar. The inertinite concentrate sample produced theleast volatiles, with approximately 20% weight loss at allpressures tested; this sample showed an almost total lack ofsensitivity to pressure; Refs. [18,19] show pyrolysis yieldsfrom inertinite concentrates to be also relatively insensitiveto changes in heating rate, unlike analogous samples ofvitrinites and liptinites.

In the case of the vitrinite and the liptinite samples, thedrop in volatile release with increasing pressure is thoughtto be associated to the softening behaviour of these samplesupon heating. It has long been argued (e.g. cf. Howard [14])that during the pyrolysis of softening coals, volatile mattermay be concentrated in bubbles formed in the plastic mass,which are pushed outwards by increasing internal pressure;the release of these volatiles from the coal particle frombursting bubbles appears to be suppressed when the externalpressure is increased. This mechanism goes some waytowards explaining the lack of pressure sensitivity of theinertinite sample to pressure as well as the pressure sensi-tivity of the other two maceral concentrates showed in Fig.6. The greater tar and total volatile yields expected from thepyrolysis of liptinites and vitrinites appear to be associatedwith their greater hydrogen, and in particular their hydro-aromatic content as discussed in detail elsewhere [20].

One consequence of the suppression of volatile releasefrom pyrolysing coal particles by the effect of pressure is theincreased deposition of secondary char within and on thesurfaces of coal/char particles. This deposition of secondarycarbonaceous material is thought to reduce the subsequent

R.C. Messenbo¨ck et al. / Fuel 79 (2000) 109–121116

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30

Pressure, bar

Tot

al V

olat

ile Y

ield

, %, d

af

Vitrinite

Inertinite

Liptinite

Fig. 7. The total volatile yield for the macerals under gasification conditions.

reactivity of the char as observed in earlier studies with‘whole’ coals [12,21]. The trend in the weight loss betweenthe different macerals in the WMR was the same as that forthe volatile matter content determined by the British Stan-dard method, however, the actual values with the WMR

were higher. This trend has been observed in earlier workwith ‘whole’ coal samples and is attributed to the higherheating rates normally used in this reactor as well as themore even distribution of the coal sample particles—simu-lating single particle behaviour [21].

R.C. Messenbo¨ck et al. / Fuel 79 (2000) 109–121 117

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30

Pressure, bar

The

Ext

ent o

f Gas

ifica

tion,

%, d

af

Vitrinite

Inertinite

Liptinite

Fig. 8. The extent of gasification of the macerals.

0

5

10

15

20

25

30

60 65 70 75 80 85 90 95

Vitrinite, % vol, mmf

Ext

ent o

f Gas

ifica

tion,

%, d

af

1 10

20 30

Daw Mill (psb)

Daw Mill (washed)

Illinois No 6

Reitspruit

El Cerrejon

Drayton

Pressure, bar

Fig. 9. Attempted correlation between vitrinite concentration and extent of gasification.

The weight losses measured during the gasification of themaceral samples in CO2 are shown in Fig. 7. The WMRoperating conditions were otherwise the same as for thepyrolysis experiments. The values for liptinite and inertinitedecreased between 1 and approximately 10 bar and there-

after showed an increase. The highest overall conversionwas obtained with the liptinite and this was a direct resultof its high pyrolysis yields. The vitrinite did not show theminimum and the yield increased over the pressure range,the form of the measured curve has been discussed in detail

R.C. Messenbo¨ck et al. / Fuel 79 (2000) 109–121118

30

35

40

45

50

55

60

30 35 40 45 50 55 60

Experimental Total Volatile Yield, %, daf

Pre

dict

ed to

tal V

olat

ile Y

ield

, %, d

af

Fig. 10. Correlation between experimental and predicted results for pyrolysis.

30

35

40

45

50

55

60

65

70

75

80

30 35 40 45 50 55 60 65 70 75 80

Experimental Total Volatile Yield, %, daf

Pre

dict

ed T

otal

Vol

atile

Yie

ld, %

, daf

Fig. 11. Correlation between experimental and predicted results for gasification.

elsewhere [7,8]. It appears to be associated with the greatergasification reactivity of the vitrinite.

Extents of gasification for the three macerals, as a func-tion of pressure is shown in Fig. 8, clearly showing similar‘extents ofgasification’ for the liptinite and the inertiniteconcentrates. The vitrinite concentrate showed an increaseover the whole range, although the rate of increase tails offas 30 bar is approached. Clearly, under the prevailing reac-tion conditions, the conversionby gasificationof the vitri-nite was higher than that of the liptinite and inertinite. Withthe liptinite only low conversions by gasification are possi-ble, because of the high proportion that is pyrolysed.

It must be noted that these results are from 10-s holdingtime experiments—and provide a ‘snapshot’ of the earlierstages of the process. At longer holding times (up to 200 s)under CO2-gasification conditions in a ‘hot-rod’ reactor,experiments using four distinct inertinite concentrateshave all shown four-to-five-fold increases in ‘extent of gasi-fication’. Analogous increases withtimewere not observedfor the liptinite and vitrinite samples used in the same study[22] indicating that only inertinite chars increase in gasifi-cation reactivity with increasing time exposure at tempera-ture (10008C). Closer scrutiny of inertinite behaviourappears warranted in view of our data [10] showing thatchars fromall coals tested appear to lose reactivity veryquickly (within 10 s) at 10008C.

3.4. Prediction of gasification performance from coalmaceral composition

In Northern Hemisphere coals the maceral found in

greatest abundance is usually vitrinite; the results describedabove show that this maceral had the highest gasificationreactivity (Fig. 8). Fig. 9 shows results from an attemptmade to correlate the vitrinite content of the present set ofcoals with observed extents of gasification at pressuresbetween 1 and 30 bar. However, there appears to be noclear relationship between the extent of reaction and thevitrinite content, when considered in isolation.

An alternative approach is to predict the performance ofthe suite of six coals from (i) their maceral composition and(ii) total volatile yields of their individual macerals underpyrolysis and gasification conditions.

In this study, it has not been possible to work withseparated maceral concentrates from the suit of thesesix coals; as second best, we have used the maceralreactivity data shown in Figs. 6–8. The results from thesecalculations have been compared with experimental data inFigs. 10–12.

The correlation for pyrolysis (Fig. 10) shows that thepredicted values are only slightly lower than the experimen-tal values for volatile yields below about 45%; thereafter theincrease in difference appears related to the relatively highrank of the ‘constituent’ maceral concentrates. Fig. 11shows the correlation for the gasification data; there wassignificantly more scatter in the results compared to thosefor pyrolysis. At the lower end of the volatile yield range,the predicted values were higher than the experimentalvalues, whereas the reverse effect was observed at the higherend of the range. Fig. 12 shows the correlation for theextent of gasification data and again there was muchscatter. Generally, the predicted values were higher than

R.C. Messenbo¨ck et al. / Fuel 79 (2000) 109–121 119

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30

Experimental Extent of Gasification, %, daf

Pre

dict

ed E

xten

t of g

asifi

catio

n, %

, daf

Fig. 12. Correlation between experimental and predicted extent of gasification.

the experimental values. The results suggest that thismethod of prediction gives an indication of the pyrolysisyields, particularly for volatile yields below 45%, but doesnot provide a reliable guide to the gasification volatile yieldor extent of gasification. This infers that the gasificationconversions are not dependent on the maceral analysisof the coal, whereas there is a link for the pyrolysisconversions.

This method has also been assessed to determine whetherit can predict the relative combustion reactivities. The origi-nal coal maceral analyses and the reactivity of the charsderived from the maceral enriched samples have beenused to predict the reactivities of the chars produced fromthe six coals. The derived values were compared with theactual experimental values for pyrolysis and gasificationderived chars. The correlations (not shown) were poor andshowed that the method is not valid as a means of predictingthe relative char combustion reactivity.

Further papers in this series will consider relationshipsbetween bench scale reactivities, mineral matter contents,FTIR spectra of the samples and the morphology of charsformed during pyrolysis and gasification.

4. Conclusions

The gasification performance of a suite of six coals,previously tested in a pilot scale spouted bed gasifier, hasbeen examined using a bench-scale high-pressure WMR.Conversions during pyrolysis and CO2-gasification experi-ments performed at 10008C between 1–30 bar pressure havebeen determined. In analogous experiments, pyrolysis andgasification conversions of a suit of maceral concentrateshave also been examined. The paper has described thedetails of an attempted correlation of performance withmaceral analysis.

Three types of gasification behaviour were noted with thecoals and these were:

1. An increase in extent of gasification as the pressureincreased over the lower part of the range, which tailedoff towards 30 bar. Illinois No. 6 coal showed this beha-viour and the results show that the coal was reactivetowards gasification at lower pressures and the reactivitywas sufficient to counteract the effect of deposition ofsecondary char. However, SEM studies have shownthat this coal became more fluid at higher pressures andthis would have annealed the active sites and decreasedthe rate of reaction. The results show that completeconversion of this coal is unlikely to be achieved inCO2.

2. An increase in the extent of gasification over the wholepressure range, with an increase in the rate towards theupper end of the pressure range. This behaviour wasshown by Daw Mill (washed) and to a less marked degreewith Drayton coal. The behaviour suggests that increasedfluidity at the higher pressures does not occur with this

coal and that deposition of secondary char has a lessereffect at the lower pressures. The rate of reaction at lowerpressures was lower than that measured with Illinois No.6 coal.

3. A decrease in the extent of gasification at pressures lessthan approximately 20 bar, followed by an increase athigher pressures. This behaviour was found with ElCerrejon, Rietspruit and Daw Mill coals (psb). It can beexplained by the deposition of secondary char having adominating effect as the pressure was increased towards20 bar and this reduced the extent of reaction. However,as 20 bar was approached, the CO2 was able to gasifyaway this less reactive material and expose the basechar, which was able to react at a faster rate, so that theoverall effect within the test time was an increasingextent of gasification.

The relative combustion reactivity of the pyrolysis andgasification derived chars has been measured in an atmo-spheric pressure TGA. In general, the pyrolysis-derivedchars had a higher relative combustion reactivity than thegasification derived chars and this may reflect morphologi-cal changes in the gasification atmosphere. The Illinoischars had the highest reactivity at all pressures that weretested, and this was followed by Daw Mill (washed) chars.At pressures of 10 bar and above these coals also showed thehighest extent of gasification. The reactivities of the charsfrom the other coals were bunched together over a fairlynarrow range.

Having observed a variation in the extent of gasificationbetweencoals, investigationshavebeendonetoassesswhetherthis can be predicted from properties of the coal. In this paperthe use of the maceral analysis and maceral reactivity is con-sidered. Further papers will investigate the use of the FTIRspectrum, the mineral matter and the char morphology.

Several ways of assessing the value of the maceral analy-sis in the prediction of the pyrolysis and gasification beha-viour have been assessed. Despite using a single set ofmaceral concentrates, it appears possible to estimate thepyrolytic behaviour of coals from their maceral contents.However, the method has been foundnot to enable theprediction of the gasification performance. It thereforeappears that gasification conversions are less dependent oninitial coal properties than pyrolysis.

Acknowledgements

Support for this work by the European Union underContract Nos. JOF3/CT95/0018 and ECSC 7220-ED/075,and the British Coal Utilisation Research Association(BCURA)/Department of Trade and Industry (DTI) underContract No. B38 is gratefully acknowledged. The authorswould also like to thank Mitsui Babcock Energy Ltd forproviding the samples.

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