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CO 2 and steam-gasification in a high-pressure wire-mesh reactor: the reactivity of Daw Mill coal and combustion reactivity of its chars R.C. Messenbo ¨ck, D.R. Dugwell, R. Kandiyoti * Department of Chemical Engineering and Chemical Technology, Imperial College (University of London), London SW7 2BY, UK Received 24 August 1998; accepted 26 November 1998 Abstract Interrelationships between extents of coal gasification, char gasification and combustion reactivities have been examined as a function of CO 2 and steam pressure and holding time. Experiments have been carried out in a high-pressure wire-mesh reactor equipped with a steam injection facility. Evidence has been presented linking minima in weight loss vs. reactive gas pressure curves with deactivation as a result of secondary char deposition. At longer times, extents of gasification were about 2–3 times higher in steam compared to CO 2 . With increasing reactivity of the ambient gas, the minimum in the weight loss vs. pressure curve appears only at the shortest reaction times. At higher pressures, reactivities between zero and 10 s were lower than those between 10 and 20 s. These data are novel and support the suggestion that a relatively unreactive layer of re-polymerised tar tends to slow down the gasification of the main body of the char in the initial stages – until it is itself consumed. Combustion reactivities of pyrolysis and gasification chars decrease with increasing pressure. When exposed to a temperature of 10008C, combustion reactivities of chars were found drop rapidly within about 10 s to relatively low and stable values. The end values were independent of pressure and composition of the reactive gas. The results are relevant to the design of pilot and commercial scale reactors. On the basis of limited available evidence, it appears that secondary char deposition caused by tar re-polymerisation plays an increasingly significant effect with increasing particle size. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: Coal gasification; Char combustion reactivities; Tar re-polymerisation; Steam gasification 1. Introduction In combined cycle processes, with low temperature air/ steam gasification ( , 9508C–10508C) to generate fuel gas for the gas turbine, 25%–35% of the feed coal is normally recovered as residual char. The gasification reactivity of this solid residue is minimal; these chars are usually sent to a combustor, to raise steam for additional power generation in a second (steam) turbine. However, calculation shows that overall cycle efficiencies could be improved by increasing the proportion of coal consumed in the gasifier, i.e. by increasing the amount of fuel gas fed to the turbine per unit of feed coal [1]. There appear to be good reasons for linking the formation of relatively unreactive residues in these pilot-scale gasifiers to the use of relatively large feed coal particle sizes. The challenge would appear to be the formulation of alternative fluidised bed gasifier operating strategies which would allow the use of smaller particle sizes. Within this frame- work, rapid and inexpensive bench scale experiments are normally expected to provide background information on fuel behaviour. The utility of bench scale data to provide the necessary data, however, depends on the ability of laboratory reactors to mimic essential reaction parameters of larger scale plant. In the case of gasification, the usual methodology of attempting to relate laboratory work to pilot scale operation has in the past faced a number of difficult issues. Numerous investigations on the gasification of coal with carbon dioxide and/or steam have been conducted in ther- mogravimetric (TGA) balances (cf. e.g. Refs. [2,3]). TGA instruments are convenient for continuously recording changes of weight loss with time and usually give good repeatability. However, (i) heating rates are lower than those encountered in fluidised bed reactors and (ii) particle stacking on the TGA-pan affects the outcome of experi- ments; furthermore, (iii) carrier gas is usually allowed to flow around the TGA-furnace and not through the sample stack. Compared to fast heating rate experiments conducted using (more realistically) segregated particles [4,5], all three effects lead to tar loss by re-condensation on heated fuel solids. The proportion of volatile loss appears to be Fuel 78 (1999) 781–793 0016-2361/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0016-2361(98)00221-X * Corresponding author. Tel.: 1 44 171 594 5581; fax: 1 44 171 594 5604. E-mail address: [email protected] (R. Kandiyoti)

CO2 and steam-gasification in a high-pressure wire-mesh reactor: the reactivity of Daw Mill coal and combustion reactivity of its chars

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Page 1: CO2 and steam-gasification in a high-pressure wire-mesh reactor: the reactivity of Daw Mill coal and combustion reactivity of its chars

CO2 and steam-gasification in a high-pressure wire-mesh reactor: thereactivity of Daw Mill coal and combustion reactivity of its chars

R.C. Messenbo¨ck, D.R. Dugwell, R. Kandiyoti*

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

Received 24 August 1998; accepted 26 November 1998

Abstract

Interrelationships between extents of coal gasification, char gasification and combustion reactivities have been examined as a function ofCO2 and steam pressure and holding time. Experiments have been carried out in a high-pressure wire-mesh reactor equipped with a steaminjection facility. Evidence has been presented linking minima in weight loss vs. reactive gas pressure curves with deactivation as a result ofsecondary char deposition. At longer times, extents of gasification were about 2–3 times higher in steam compared to CO2. With increasingreactivity of the ambient gas, the minimum in the weight loss vs. pressure curve appears only at the shortest reaction times. At higherpressures, reactivities between zero and 10 s were lower than those between 10 and 20 s. These data are novel and support the suggestion thata relatively unreactive layer of re-polymerised tar tends to slow down the gasification of the main body of the char in the initial stages – untilit is itself consumed. Combustion reactivities of pyrolysis and gasification chars decrease with increasing pressure. When exposed to atemperature of 10008C, combustion reactivities of chars were found drop rapidly within about 10 s to relatively low and stable values. Theend values were independent of pressure and composition of the reactive gas. The results are relevant to the design of pilot and commercialscale reactors. On the basis of limited available evidence, it appears that secondary char deposition caused by tar re-polymerisation plays anincreasingly significant effect with increasing particle size.q 1999 Elsevier Science Ltd. All rights reserved.

Keywords:Coal gasification; Char combustion reactivities; Tar re-polymerisation; Steam gasification

1. Introduction

In combined cycle processes, with low temperature air/steam gasification (, 9508C–10508C) to generate fuel gasfor the gas turbine, 25%–35% of the feed coal is normallyrecovered as residual char. The gasification reactivity of thissolid residue is minimal; these chars are usually sent to acombustor, to raise steam for additional power generation ina second (steam) turbine. However, calculation shows thatoverall cycle efficiencies could be improved by increasingthe proportion of coal consumed in the gasifier, i.e. byincreasing the amount of fuel gas fed to the turbine perunit of feed coal [1].

There appear to be good reasons for linking the formationof relatively unreactive residues in these pilot-scale gasifiersto the use of relatively large feed coal particle sizes. Thechallenge would appear to be the formulation of alternativefluidised bed gasifier operating strategies which wouldallow the use of smaller particle sizes. Within this frame-

work, rapid and inexpensive bench scale experiments arenormally expected to provide background information onfuel behaviour. The utility of bench scale data to providethe necessary data, however, depends on the ability oflaboratory reactors to mimic essential reaction parametersof larger scale plant. In the case of gasification, the usualmethodology of attempting to relate laboratory work to pilotscale operation has in the past faced a number of difficultissues.

Numerous investigations on the gasification of coal withcarbon dioxide and/or steam have been conducted in ther-mogravimetric (TGA) balances (cf. e.g. Refs. [2,3]). TGAinstruments are convenient for continuously recordingchanges of weight loss with time and usually give goodrepeatability. However, (i) heating rates are lower thanthose encountered in fluidised bed reactors and (ii) particlestacking on the TGA-pan affects the outcome of experi-ments; furthermore, (iii) carrier gas is usually allowed toflow around the TGA-furnace and notthrough the samplestack. Compared to fast heating rate experiments conductedusing (more realistically) segregated particles [4,5], all threeeffects lead to tar loss by re-condensation on heated fuelsolids. The proportion of volatile loss appears to be

Fuel 78 (1999) 781–793

0016-2361/99/$ - see front matterq 1999 Elsevier Science Ltd. All rights reserved.PII: S0016-2361(98)00221-X

* Corresponding author. Tel.:1 44 171 594 5581; fax:1 44 171 5945604.

E-mail address:[email protected] (R. Kandiyoti)

Page 2: CO2 and steam-gasification in a high-pressure wire-mesh reactor: the reactivity of Daw Mill coal and combustion reactivity of its chars

relatively small, usually of the order of several percent andnever more than 8%–10%. However, there is evidencestrongly suggesting that re-condensed tars tend to form arelatively unreactive residue, overlaying chars evolved fromthe main body of coal particles [6,7] (also see followingtext). Gasification and combustion reactivities of chars areknown to change rapidly and significantly with changes inpyrolysis related parameters [8,9]. Meticulous scrutiny ofmodes of sample devolatilisation is therefore essential ifbench scale experiments are to be used in visualising andeventually predicting the outcome of pilot and commercialscale gasifier operation.

Relatively rapid heating has in fact been attempted inTGA instruments, by lowering a sample basket into theheated zone [2]. However, this configuration does notappear to avoid tar re-condensation caused by particle stack-ing. Our own experiments show that very high gas velocities(several meters per second) are requiredthrough shallowstacked sample beds for removing volatiles with an effi-ciency nearly matching experiments where particles aresegregated [4]. Rapid heating of TGA-pans would also belikely to give rise to temperature gradients across the samplebasket; inevitably, not all particles would be heated at thesame high rates.

To avoid these pyrolysis related effects, numerous bench-

scale gasification experiments have been carried out bysuccessively pyrolysing thecoal and gasifying the resultantchar in different vessels [10–22]. Typically, this is done bypyrolysing the coal sample in a small fluidised bed, followedby TGA-based experiments conducted to determine thereactivities of the chars. However, early work [16] clearlysuggested that steam gasification rates could change (by upto 30% in their case), depending on the conditions of thepreceding pyrolysis step. There is also more recent evidence[8,16,21,23–26] showing that different gasification reactiv-ities are obtained from coal chars prepared from the samecoals, but under different pyrolysis conditions. In particular,soaking samples at high temperatures during char prepara-tion leads to thermal ‘annealing’ of the char and tends todistort results [8]. Chitsera et al. [27] have observed thatlonger soak times during pyrolysis at elevated temperaturesand pressures, as well as sample ageing under ambientconditions, tend to lower steam reactivities of chars. In thesame vein, using a TGA-instrument for gasification in steambetween 10008C–13008C, Peng et.al. [2] have reportedreactivities of in-situ generated chars up to six times greaterthan those of the corresponding ex situ generated samples.

We have shown elsewhere [8,28] how deliberate manip-ulation of reaction parameters during char preparation canaffect the CO2-gasification reactivity of coals and chars. We

R.C. Messenbo¨ck et al. / Fuel 78 (1999) 781–793782

Fig. 1. Steam injection unit and the wire-mesh reactor: (1) water reservoir; (2) filter; (3) displacement pump; (4) steam heater; (5), (6) on/off valves; (7) steamby-pass collector; (8) sweep gas; (9) safety valve; (10) flow control valve; (11) gas heater; (12) non-return valve; (13) mixing point of steam and gas;(14) wiremesh reactor; (15) countercurrent condenser; (16) flow control valve; (17) water collector; (18) cold trap; (19), (20) flowmeter; (21) transformer; (22)watchdog. P-pressure, T-temperature, C-controller, I-indicator; full lines stand for tubing, dash-dot for thermocouple lines, dash-dot-dot forelectric currentlines. (Published by permission of Energy and Fuels).

Page 3: CO2 and steam-gasification in a high-pressure wire-mesh reactor: the reactivity of Daw Mill coal and combustion reactivity of its chars

have also shown that nearly identical CO2-gasificationconversionsand char reactivities could be obtained in twobench-scale reactors of very different configuration: a wire-mesh instrument (see following text) and a fluidised bedreactor [29]. The two reactors were both operated at highheating rates and under regimes requiring particle segrega-tion (as opposed to stacking). In these experiments, extentsof gasification have been evaluated by subtracting weightloss during pyrolysis from total conversion during gasifica-tion, carried out under similar reaction conditions. Morerecently, we have reported on the development of a steaminjection facility for carrying out steam-gasification experi-ments in the high-pressure wire-mesh reactor [6]. Thepresent paper examines interrelationships between extentsof gasification and char reactivities as a function of reactionpressure and holding time. Experiments were carried out inthe high-pressure wire-mesh reactor operated with steam orCO2 injection. Following earlier reports [30] suggesting thatthe steam not only increases weight loss and tar yield, butalso enhances the reactivity of resultant chars, morphologiesand combustion reactivities of chars prepared during steamand CO2-gasification were examined in some detail.

2. Experimental

The wire-mesh reactor: The wire-mesh reactor equippedwith steam injection used in this study has been describedelsewhere [6,31,32]. Briefly, a folded wire-mesh is placedbetween two water cooled electrodes, one of them spring-loaded. The mesh serves as resistance heater for the sample(typically 5–7 mg), spread as a monolayer. Two thermocou-ples are placed in the reaction zone, to monitor lateraltemperature variations in the sample holding part of themesh. The temperature is monitored 100 times a secondand the heating current across the mesh adjusted using on-line three-term control. During an experiment, a stream ofgas is passed through the sample holding part of the mesh toremove evolving volatiles and to suppress secondary reac-tions of the volatiles. Product gases are passed through acooled tar trap for capture and recovery.

A diagram of the reactor system and ancillaries, includingthe new steam injection facility is presented in Fig. 1. Modi-fications for operation in CO2 and steam have been recentlydescribed in Ref. [6], and will only be summarised here.

Operation with steam injection ordinarily leads tocondensation on internal surfaces of the reactor, as allcomponents except the mesh itself are normally kept atambient temperature. In the present design, such condensa-tion has been avoided by directing the steam through apreheated flow path, which includes the mesh itself. Beforethe run, steam was raised continuously and discardedthrough a bypass line (Fig. 1: (6) and (7)). During operation,the steam and helium heaters were set at 3008C, with typicalsteam and helium delivery temperatures of about 2808C and2508C, respectively. The temperature of the mixture varied

between 2608C–2708C; the steam condensation temperatureat 20 bar (maximum steam pressure used) is 212.48C.Before triggering a run, the mesh was electrically preheatedto 3008C. To carry out a run, steam was switched into thewire-mesh cell and contacted with the coal sample onlyinstants before the temperature of the sample was ramped.Time–temperature plots from experiments with steaminjection have shown levels of temperature stabilitycomparable to those observed during experiments with‘dry’ gases [6].

In the present study, samples were heated at 10008C s21

to 10008C and held at peak temperature between 0 and 60 s.The effect of pressure was investigated in a range between1–30 bar. During pyrolysis (in helium) and CO2 gasifica-tion, the gas stream flowed through the mesh at a velocity of0.1 m s21. In ‘steam-gasification’, a steam: helium (80 : 20)mixture was used; the He-flow rate was set at 0.02 m s21 toachieve 0.1 m s21 for the total flow of gas.

Sample: Daw Mill (a UK noncaking) coal was used: vitri-nite 66% (v/v, mmf), exinite 13% (v/v, mmf), inertinite 21%(v/v, mmf); vitrinite reflectance: 0.6. Proximate analysis:moisture 6.1% (a.d.), ash 4.4% (a.d.), fixed carbon 53.8%(a.d.), volatile matter 39.9% (d.a.f.). Elemental composition(% w/w d.a.f.): carbon 80.1, hydrogen 4.7, nitrogen 1.3,oxygen (by difference) 11.5; organic sulphur 1.12% (d.b.),sulphate sulphur,0.1% (d.b.), pyritic sulphur 0.28% (d.b.).The sample (particle size: 106–150mm) was dried for 18 hat 508C under vacuum and stored under nitrogen untilrequired. Results have been calculated on a dry ash free(d.a.f.) basis.

Gasification reactivities: In this work, ‘extents of gasifi-cation’ were calculated by subtracting sample weight lossrecorded during apyrolysisexperiment from the weight lossobserved during a gasification run, performed under other-wise 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 time.‘Gasification reactivities’ were calculated, from this data, bydetermining the extent of gasificationper secondduringspecific hold-time intervals (e.g. 0–10 s or 20–60 s):

dG%t2 � TV�Gasification�t2 2 TV�Gasification�t1t2 2 t1

:

Relative char combustion reactivity determinations: Anisothermal method was used. The pan of a Perkin-ElmerTGA7 thermogravimetric analyser was charged with about1.5 mg of char and heated at 258C min21 to 5008C in nitro-gen flowing at 40 ml min21. Once a constant weight wasestablished at 5008C, the flow was switched to air; followingvirtually complete combustion at 5008C, the temperaturewas raised to 8508C to determine the ash content of thesample. The maximum value of the first derivative of theweight vs. time curve was taken as the relative combustion

R.C. Messenbo¨ck et al. / Fuel 78 (1999) 781–793 783

Page 4: CO2 and steam-gasification in a high-pressure wire-mesh reactor: the reactivity of Daw Mill coal and combustion reactivity of its chars

reactivity of the char:

Rmax� 21

W0

dWdt

� �umax

wheret is time,W0 is the initial andW the time dependentchar weight.

In this work, only single determinations have beenreported; repeatabilities are usually high and well within^ 5% of the measured reactivity value (one repeated resultof the 10 s, 10 bar run is given in Fig. 7).

Scanning electron microscopy: Samples were mountedon brass stubs and gold plated. Scanning electron

photo-micrographs were recorded in a JEOL JSM-T220Ascanning electron microscope operated at 15 kV.

3. Results and discussion

Figs. 2 and 3 present the effect of pressure and of hold-time at peak temperature, on conversions of Daw Mill coalduring gasification with CO2 and steam, respectively [6].Samples were heated at 10008C s21 to 10008C.

Underpyrolysisconditions (not shown; cf. Ref. [6]), theweight loss was observed to decline steadily with increasingpressure; the overall change over the 1–30 bar range was

R.C. Messenbo¨ck et al. / Fuel 78 (1999) 781–793784

Fig. 2. Total volatile yields in CO2 gasification (10008C s21, 10008C, CO2).

Fig. 3. Total volatile yields in steam gasification (10008C s21, 10008C, Steam).

Page 5: CO2 and steam-gasification in a high-pressure wire-mesh reactor: the reactivity of Daw Mill coal and combustion reactivity of its chars

about eight percent. Extensive particle melting could beobserved at pressures above 10 bar. At all pressures, asmall increase in weight loss from zero to 10 s hold-timecould be observed, but the 60 s hold-time runs gave virtuallythe same results as runs with 10 s holding.

The CO2-gasification experiments over the pressure range(1–30 bar; Fig. 2) were carried out with ‘zero’ seconds

holding (i.e. heatup only) and with 10, 20 and 60 s holdingat the peak temperature (10008C). In CO2, relatively littlegasification took place during heatup and the curve for‘zero’ holding was qualitatively similar to data frompyrolysis experiments.

Fig. 2 shows that, with 10 s holding at 10008C, the sampleweight loss curve as a function of increasing pressure traced

R.C. Messenbo¨ck et al. / Fuel 78 (1999) 781–793 785

Fig. 4. Extent of CO2 gasification (10008C s21, 10008C, CO2).

Fig. 5. Extent of steam gasification (10008C s21, 10008C, Steam).

Page 6: CO2 and steam-gasification in a high-pressure wire-mesh reactor: the reactivity of Daw Mill coal and combustion reactivity of its chars

a shallow minimum near 10 bar. Consistent with earlierfindings from hydropyrolysis, the initial decline is thoughtto reflect the suppression of devolatilisation by the physicaleffect of pressure, followed at higher pressures by the even-tual increase in weight loss through the greater reactivity ofthe ambient gas [32,33]. However, recent work has enabledextension of this explanation to include the role of second-ary char deposition [6] as follows.

In the presence of both H2 and CO2, the decline in volatilerelease observed with increasing pressure at high heatingrates has been accompanied by loss of tar release[32,34,35]. The tars appear mostly to re-polymerise to achar, with partial cracking leading to some additional gasrelease. It is thought likely that the minimum in weight lossdiscussed earlier is also associated with loss of sample reac-tivity owing to this secondary char deposition. Combustionreactivities ofhydropyrolysischars have been found to gothrough a minimum [34], and those of chars prepared in CO2

and steam steadily decline (over a smaller pressure range).Accordingly, the rising edge of the weight loss vs. pressure

curve after the minimum (e.g. in the 10 s curve in Fig. 2) islikely to reflect both an increase in the reactivity of the gasand the (at least) partial elimination of the layer of re-poly-merised tar deposit. Between 20–60 s, these effects are nolonger observable; the monotonically rising weight lossprofile indicates that longer reaction times override effectsowing to low reactivity of the ambient gas and of the re-polymerised char.

In contrast to the CO2-gasification results, the steam-gasi-fication data in Fig. 3 showed no evidence of a minimum inweight loss with increasing pressure. At longer times($ 10 s) trends were analogous to those observed for CO2-gasification at longer times – reflecting the faster reactionbetween the carbonaceous material and steam. At 20 bar,the sample was almost completely consumed after 20 s,leaving nothing but a few ash particles in the sample holder.

Extents of gasification: In order to distinguish betweenpyrolytic weight loss and fuel consumption by the reactivegas in these experiments, parallel tests were carried out inhelium under otherwise similar conditions. Figs. 4 and 5present the ‘extent of gasification’ in CO2 and steam, respec-tively, (weight loss in pyrolysis subtracted from weight lossin gasification) as a function of pressure and hold-time at10008C. The initial delay of gasification owing to low reac-tivities can be observed for CO2 at 10 and 20 bar (the curvessomewhat disguise the effect). At longer times, the initialincrease in the ‘extent of gasification’ with pressure wasmarked but appeared to slow down as total sampleconsumption was approached.

Table 1 shows the pressure dependence of the extent ofgasification at 0-second hold-time (i.e. heatup only). Bothsets of data (steam and CO2) showed a minimum around

R.C. Messenbo¨ck et al. / Fuel 78 (1999) 781–793786

Table 1Extent of gasification after heatup (10008C s21, 10008C, 0 s)

Gas Pressure (bar) Extent of gasification (%, d.a.f)

CO2 1 4.5CO2 10 2.0CO2 20 3.3CO2 30 3.3Steam 1 2.8Steam 10 1.7Steam 20 4.8

Fig. 6. CO2 gasification reactivity (10008C s21, 10008C, CO2).

Page 7: CO2 and steam-gasification in a high-pressure wire-mesh reactor: the reactivity of Daw Mill coal and combustion reactivity of its chars

10 bar at relatively small extents of gasification; clearly, forvery short times at high temperature, the greater reactivityof steam cannot be fully observed. This trend would havebeen difficult to depict in weight-loss vs. pressure diagrams(as in Figs. 2 and 3). As expected, the minimum is notobserved at longer times (Table 2), where extents of gasifi-cation were about 2–3 times higher in steam compared toCO2. The extent of gasification increased from 20.6% to54.4% with steam pressure increasing from 1 to 20 bar.

Char gasification reactivities: Fig. 6 presents ‘gasifica-tion reactivities’ (defined earlier), reflecting average rates ofweight loss by gasification during specified time intervals:e.g. the point shown at 10 s indicates the average reactivitybetween 0–10 s. At atmospheric pressure, the gasificationreactivity was observed to decrease monotonically withtime. At intermediate pressures, reactivities between zeroand 10 s were lower than those between 10 and 20 s,while at high pressure a continuous decrease was observed.These data are novel and support the suggestion that a rela-tively unreactive layer of re-polymerised tar tends to slow

down the gasification of the main body of the char in theinitial stages – until it is itself consumed. Above 20 s thereactivity decreases slowly – reflecting the relatively smallamount of sample left behind. Various ways to calculategasification reactivities are possible. While the usualconcept is based on the sample weight loss per originalsample mass and therefore decreases monotonically, basicmathematical considerations show that the rate of gasifica-tion based on mass loss per actually existing mass at thattime tends to flatten out and to become independent of hold-time after 10 s.

In the case of experiments at 1 bar, gasification reactiv-ities in both steam and CO2 were observed to decrease stea-dily with holding time at 10008C (Table 3). At twenty bar(Table 4), gasification reactivities look slightly different:The carbon dioxide reactivity appears to go through a maxi-mum at 10 bar, the low value at 10 s reflecting the lowreactivity of residual tars, whilst in steam, this less reactivelayer appears to have been removed much more quickly,leaving almost no char to be gasified after 10 s; the gasifica-tion reactivities after 10 s appear rather small.

Relative char combustion reactivities: Relative combus-tion reactivities of the chars were determined in a TGA-instrument by the standard ‘isothermal’ method describedin the ‘Experimental’ section. Fig. 7 presents combustionreactivities of Daw Mill coal chars recovered from CO2-gasification experiments, as a function of pressure andhold-time.

Chars from the 0-second holding experiments (at 10008C)showed a rapid drop with increasing pressure; as outlinedpreviously, the effect appears linked to tar re-deposition andre-polymerisation within pyrolysing particles during heatup.

R.C. Messenbo¨ck et al. / Fuel 78 (1999) 781–793 787

Table 2Extent of gasification after 20 s hold-time (10008C s21, 10008C, 20 s)

Gas Pressure (bar) Extent of gasification (%, d.a.f)

CO2 1 12.1CO2 10 24.4CO2 20 34.3CO2 30 41.9Steam 1 20.6Steam 10 48.3Steam 20 54.4

Fig. 7. Combustion reactivity of CO2-chars (chars: 10008C s21, 10008C, CO2).

Page 8: CO2 and steam-gasification in a high-pressure wire-mesh reactor: the reactivity of Daw Mill coal and combustion reactivity of its chars

A wider range of pressures (up to 80 bar) had been used inprevious work where a full minimum had been observed (asa function of pressure at constant holding time) in thecombustion reactivity of hydropyrolysis chars. Differencesbetween reactivities of chars other than those from 0-secondholding experiments were small and only marginally greaterthan experimental repeatability: above 10 s holding thereactivities appeared to be almost independent of holdingtime and pressure.

The observed link between deactivation of chars andincreasing temperatures is well established (cf. e.g. Ref.[36]). Combustion reactivities of two chars that were gasi-fied for 10 s at 8508C, under otherwise identical conditionswere considerably higher. However, the often used descrip-tions of the phenomenon in terms of ‘annealing’ of thechars, or ‘loss of active sites’ are admittedly non-specific.More detailed work in terms of dominant molecularmechanisms may be both interesting and useful.

These data are useful in indicating the time required forreaching a point beyond which chars do not significantlydeactivate further; the results are relevant to the design ofpilot and commercial scale reactors. It is also relevant topoint out that combustion reactivities of chars recoveredfrom the ABGC reactor (3 mm top-size coal feed) werefound to behalf (or less) as reactive again as chars preparedfrom the present study starting with sample size (106–150mm) [9,37]. Judging on the basis of the availableevidence, it would appear that secondary char depositionowing to tar re-polymerisation plays an increasingly signif-icant effect with increasing particle size. However, thereappears to be a clear need to investigate further the relation-ship between starting sample particle size and rapid dropsobserved in the reactivity of the resultant chars.

Combustion reactivities of chars prepared bypyrolysisin

helium are presented in Table 5, showing a clear decrease incombustion reactivity with time. Comparison with Fig. 7suggests this drop to be – apparently – independent of thechemical composition of the ambient gaseous medium. Thereactivities of the chars pyrolysed for 10 s at 8508C confirmearlier findings in respect of the effect of temperature onreactivities.

Table 6 presents relative combustion reactivities of charsfrom the steam-gasification experiments. At 20 bar, notenough char was left after the 20 s steam-gasificationexperiments, to conduct combustion reactivity determina-tions. The effect of pressure (causing secondary char deposi-tion) and time at 10008C in reducing reactivities may beclearly observed. After 10 s holding at 10008C, the 10 barchars gave relative reactivities of the order of 0.5% min21,by far the lowest values measured for any of these chars. Itmay be noted that 0-second hold-time reactivities of thesteam-gasification chars were comparatively high, andconsiderably higher compared to 0-second hold-time chars

R.C. Messenbo¨ck et al. / Fuel 78 (1999) 781–793788

Table 3Gasification reactivity at 1 bar (10008C s21, 10008C, 1 bar)

Gas Hold-time (s) Gasification reactivity (%, d.a.f)

CO2 10 0.39CO2 20 0.23CO2 60 0.06Steam 10 1.12Steam 20 0.53Steam 60 0.31

Table 4Gasification reactivity at 20 bar (10008C s21, 10008C, 20 bar)

Gas Hold-time (s) Gasification reactivity (%, d.a.f)

CO2 10 1.37CO2 20 1.60CO2 60 0.22Steam 10 4.41Steam 20 0.72Steam 60 0.00

Table 5Relative combustion reactivities of pyrolysis chars determined isothermallyin TGA at 5008C (chars: 10008C s21, 8508C and 10008C, He)

Temperature (8C) Pressure (bar) Hold-time (s) Combustionreactivity (%min21, d.a.f)

850 1 10 3.93850 30 10 1.471000 1 0 2.531000 10 0 2.351000 20 0 2.351000 30 0 2.291000 1 10 1.901000 10 10 1.181000 20 10 1.031000 30 10 1.011000 1 60 1.261000 10 60 1.001000 20 60 0.771000 30 60 0.71

Table 6Relative combustion reactivities of steam gasification chars determinedisothermally in TGA at 5008C (chars: 10008C s21, 10008C, Steam)

Pressure (bar) Hold-time (s) Combustionreactivity (%min21, d.a.f)

1 0 4.101 10 2.321 20 1.501 60 1.3610 0 3.6510 10 0.4710 20 0.3510 60 0.3020 0 3.2020 10 0.92

Page 9: CO2 and steam-gasification in a high-pressure wire-mesh reactor: the reactivity of Daw Mill coal and combustion reactivity of its chars

from pyrolysis and CO2-gasification (Table 7) – in somecases by a factor of two. These findings are consistentwith our previous work showing greater reactivities ofchars prepared in helium compared to gasification in CO2

– as well as mirroring the effect observed with increasingpressure [28]. They also indicate that the higher conversionsin steam-gasification were not only owing to the more reac-tive gasification medium itself but also to the greater reac-tivity of the char gasified in steam – as previously suggestedin Ref. [30].

Table 8 presents the effect of hold-time at 10008C onrelative char reactivity in isolation from pressure effects:all chars were prepared at atmospheric pressure. Asexpected, the steam-gasification chars were the most reac-tive at up to 10 s and chars from carbon dioxide gasificationalways least reactive. At this pressure, all char reactivitiesreached relatively stable values after about 10–20 s at10008C. Relatively little difference could be found betweenchar reactivities after 60 s, irrespective of the compositionof the gaseous medium and levels of conversion.

Scanning electron microscopy of the chars: The follow-ing figures present SEM photomicrographs of some of the

chars. For each gas, four photographs are presented, a shortand long hold-time run for a high and a low pressure experi-ment.

Figs. 8 and 9 show the effect of hold-time during pyro-lysis at atmospheric pressure. The structure of the char wasnot found to change significantly between 0-second holdingand 60 s at 10008C. Particles tended to form a network, withtheir original size (106–150mm) still recognisable. Differ-ences between short and long holding at 30 bar (Figs. 10 and11, respectively) were also relatively small. Howevercomparing the morphologies of the high pressure pyrolysischars with those from atmospheric pressure pyrolysisrevealed striking differences. Chars from the higher pressureexperiments clearly show evidence of greater fluidity duringheatup, with the sample no longer showing evidence of theoriginal particle size distribution; the imprint of the mesh onthese larger agglomerated particles is may be clearlyobserved. Increased fluidity during heating at higher pres-sures is well known and is thought to be directly related tolower levels of tar release and the deposition of secondarychar. As a result of the small amounts of char left afterexperiments, surface area determinations were impossibleto conduct.

Figs. 12 and 13 show the effect of hold-time on charsgasified under CO2 at atmospheric pressure. Whilst origi-nal particle sizes were still discernible, the char from the60 s experiment showed a more porous surface, owing tothe increased conversion in gasification (conversions:53.9% after 0 s and 62.7% after 60 s). In the case ofthe thirty bar CO2-experiments (Figs. 14 and 15, respec-tively) hold-time effects were easier to observe than inpyrolysis. The sixty second char was found to have avery porous, spongy structure. Again, the structure ofthe residual agglomerated mass did not reflect the sizesof the original particles at all.

The chars from steam-gasification at atmospheric pres-sure (Figs. 16 and 17), did not appear to agglomerate atall. Even after 60 s some of the particles showed somerough surfaces, indicating that they did not soften or meltto the same degree as their pyrolysis or CO2-gasificationcounterparts. Fig. 17 shows, in the top right corner, thatalthough the particles look smooth and unchanged insize, a structure with large pores has evolved. However,at twenty bar (Figs. 18 and 19) the original sizes andstructures of the coal particles could not be discerned:the char held for ten seconds (no 20 s char was left forcharacterisation) appears to have more rough edges andpores than the zero second char. On the whole the twochars appear rather similar. This is not very surprising, asin steam gasification at twenty bar (Table 1), the gasifi-cation conversion was quite high.

4. Summary and conclusions

Interrelationships between extents of gasification, char

R.C. Messenbo¨ck et al. / Fuel 78 (1999) 781–793 789

Table 7Relative combustion reactivities of 0 s hold-time chars determined isother-mally in TGA at 5008C (chars: 10008C s21, 10008C, 0 s)

Gas Pressure (bar) Combustionreactivity (%min21, d.a.f)

He 1 2.53He 10 2.35He 20 2.35He 30 2.29CO2 1 2.29CO2 10 2.20CO2 20 1.85CO2 30 1.08Steam 1 4.10Steam 10 3.65Steam 20 3.20

Table 8Relative combustion reactivities of 1 bar chars determined isothermally inTGA at 5008C (chars: 10008C s21, 10008C, 1 bar)

Gas Hold-time (s) Combustionreactivity (%min21, d.a.f)

He 0 2.53He 10 1.90He 60 1.26CO2 0 2.29CO2 10 1.09CO2 20 0.91CO2 60 1.02Steam 0 4.10Steam 10 2.32Steam 20 1.50Steam 60 1.36

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R.C. Messenbo¨ck et al. / Fuel 78 (1999) 781–793790

Fig. 8. Daw Mill (10008C s21, 10008C, 0 s, He, 1 bar).

Fig. 9. Daw Mill (10008C s21, 10008C, 60 s, He, 1 bar).

Fig. 10. Daw Mill (10008C s21, 10008C, 0 s, He, 30 bar).

Fig. 11. Daw Mill (10008C s21, 10008C, 60 s, He, 30 bar).

Fig. 12. Daw Mill (10008C s21, 10008C, 0 s, CO2, 1 bar).

Fig. 13. Daw Mill (10008C s21, 10008C, 60 s, CO2, 1 bar).

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R.C. Messenbo¨ck et al. / Fuel 78 (1999) 781–793 791

Fig. 14. Daw Mill (10008C s21, 10008C, 0 s, CO2, 30 bar).

Fig. 15. Daw Mill (10008C s21, 10008C, 60 s, CO2, 30 bar).

Fig. 16. Daw Mill (10008C s21, 10008C, 0 s, steam, 1 bar).

Fig. 17. Daw Mill (10008C s21, 10008C, 60 s, steam, 1 bar).

Fig. 18. Daw Mill (10008C s21, 10008C, 0 s, steam, 20 bar).

Fig. 19. Daw Mill (10008C s21, 10008C, 10 s, steam, 20 bar).

Page 12: CO2 and steam-gasification in a high-pressure wire-mesh reactor: the reactivity of Daw Mill coal and combustion reactivity of its chars

gasification and combustion reactivities have been exam-ined as a function of CO2 and steam pressure and hold-ing time. Experiments have been carried out in a high-pressure wire-mesh reactor equipped with a steam injec-tion facility.

1. The suppression of tar release under pressure is known tolead to re-deposition and re-polymerisation in the form ofrelatively unreactive chars. Evidence has been presentedlinking minima in weight loss vs. reactive gas pressurecurves with deactivation caused by secondary chardeposition. At longer reaction times, these effects areno longer observable; monotonically rising weight lossprofiles suggest that that longer reaction times overrideeffects caused by low reactivity of the ambient gas and ofthe re-polymerised char.At short times, extents of gasification were about 2–3times higher in steam compared to CO2. With increasingreactivity of the ambient gas, the minimum in the weightloss vs. pressure curve appears only at the shortest reac-tion times.

2. At atmospheric pressure, the gasification reactivity wasobserved to decrease monotonically with time. At higherpressures, reactivities between zero and 10 s were lowerthan those between ten and twenty seconds. These dataare novel and support the suggestion that a relativelyunreactive layer of re-polymerised tar tends to slowdown the gasification of the main body of the char inthe initial stages – until it is itself consumed. Above20 s the reactivity decreases slowly – reflecting the rela-tively small amount of sample left behind.

3. Combustion reactivities of pyrolysis and gasificationchars decrease with increasing pressure. Secondarychar deposition also appears to affect the combustionreactivities of chars prepared under high pressure gasi-fication conditions. When exposed to a temperature of10008C, combustion reactivities of chars were founddrop rapidly within about 10 s to relatively low andstable values. The end values were independent ofpressure and composition of the reactive gas. Theresults are relevant to the design of pilot and commer-cial scale reactors. Combustion reactivities of charsrecovered from a pilot-scale reactor using 3 mm top-size coal feed were found to behalf (or less) asreactive as chars prepared from the present startingsample size (106–152mm). On the basis of limitedavailable evidence, it appears that secondary chardeposition caused by tar re-polymerisation plays anincreasingly significant effect with increasing particlesize.

4. Steam-gasification chars were the most reactive at up to10 s and chars from carbon dioxide gasification alwaysleast reactive. The higher combustion reactivities ofchars recovered from steam gasification experimentsindicate that the higher conversions were not only causedby the more reactive gasification medium itself but also

to the greater reactivity of the char gasified in steam – aspreviously suggested in Ref. [30].

SEM photomicrographs showed clear correspondencebetween char porosity and char reactivity; at high pressures,fluidity, low tar yields and low char reactivities all happenedat the same time.

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. MitsuiBabcock Energy Ltd are thanked for providing the samples.

References

[1] Paterson N. (private communications), 1998.[2] Peng FF, Lee IC, Yang RYK. Fuel Proc. Tech. 1995;41:233.[3] Muhlen H-J, van Heek KH, Ju¨ntgen H. Fuel 1985;64:944.[4] Gonenc ZS, Gibbins JR, Katheklakis IE, Kandiyoti R. Fuel

1990;69:383.[5] Gibbins JR, Gonenc ZS, Kandiyoti R. Fuel 1991;70:621.[6] Messenbo¨ck, RC, Dugwell, DR, Kandiyoti R. Energy and Fuels 1999

(accepted for publication).[7] Guell AJ, Cai H-Y, Dugwell DR, Kandiyoti R. Fuel Proc. Tech.

1993;36:259.[8] Megaritis A, Messenbo¨ck RC, Collot A-G, Zhuo Y, Dugwell DR,

Kandiyoti R. Fuel 1998;77:1411.[9] Messenbo¨ck, RC. PhD Thesis, University of London, 1998.

[10] Nozaki T, Adschiri T, Fujimoto K. Energy and Fuels 1991;5:610.[11] Guo C, Zhang L. Fuel 1986;65:1364.[12] Haga T, Nishiyama Y. Fuel 1988;67:743.[13] Gadsby J, Hinshelwood C N, Sykes KW. Proc. R. Soc. (London)

1946;A189:129.[14] Long FJ, Sykes KW. Proc. R. Soc. (London) 1948;A193:377.[15] Goring GE, Curran GP, Zielke CW, Gorin E. Ind. Eng. Chem.

1953;45:2586.[16] Goring GE, Curran GP, Tarbox RP, Gorin E. Ind. Eng. Chem.

1953;44:1051.[17] Goring GE, Curran GP, Tarbox RP, Gorin E. Ind. Eng. Chem.

1952;44:1057.[18] Yang Y, Watkinson AP. Fuel 1994;73:1786.[19] Zielke CW, Gorin E. Ind. Eng. Chem. 1957;49:396.[20] Meijer R, Kapteijn F, Moulijn JA. Fuel 1994;73:723.[21] Ginter DM, Somorjai GA, Heinemann H. Energy and Fuels

1993;7:393.[22] Haga T, Sato M, Nishiyama Y. Energy and Fuels 1991;5:317.[23] Sha X, Chen Y, Cao J, Yang Y, Ren D. Fuel 1990;69:656.[24] Muhlen H-J, van Heek K H, Ju¨ntgen H. Fuel 1986;65:591.[25] Alvarez T, Fuertes AB, Pis JJ, Parra JB, Pajares J, Menendez R. Fuel

1994;73:1358.[26] Silveston PL. Energy and Fuels 1991;5:933.[27] Chitsora CT, Mu¨hlen H-J, van Heek KH, Ju¨ntgen H. Fuel Proc. Tech.

1987;15:17.[28] Lim J-Y, Chatzakis IN, Megaritis A, Cai H-Y, Dugwell DR,

Kandiyoti R. Fuel 1997;76:1327.[29] Megaritis A, Zhuo Y-Q, Messenbo¨ck R, Dugwell DR, Kandiyoti R.

Energy and Fuels 1998;12:144.

R.C. Messenbo¨ck et al. / Fuel 78 (1999) 781–793792

Page 13: CO2 and steam-gasification in a high-pressure wire-mesh reactor: the reactivity of Daw Mill coal and combustion reactivity of its chars

[30] Khan MR, Hshieh FY. Prepr. Am. Chem. Soc.: Div. Fuel Chem.1989;34:1245.

[31] Gibbins JR, Kandiyoti R. Energy and Fuels 1989;3:670.[32] Guell AJ, Kandiyoti R. Energy and Fuels 1993;7:943.[33] Howard JB. In: Elliot MA, editor. Chemistry of coal utilization:

supplementary volume II, John Wiley and Sons, 1981. pp. 665.[34] Suuberg EM, Peters WA, Howard JB. Fuel 1980;59:45.

[35] Messenbo¨ck RC, Megaritis A, Lazaro M, Herod A, Dugwell DR,Kandiyoti R. in preparation.

[36] Cai H-Y, Megaritis A, Messenbock R, Dix M, Dugwell DR,Kandiyoti R. Fuel 1998;77:1273.

[37] Messenbo¨ck RC, Paterson N, Dugwell DR, Kandiyoti R. Fuel 1999(submitted).

R.C. Messenbo¨ck et al. / Fuel 78 (1999) 781–793 793