Heteroatom distribution in pyrolysis products as a function of heating rate and pressure

Heteroatom distribution in pyrolysis products as a function of heating rate and pressure H.-Y.

Cai, A. J. Giiell,

D. R. Dugwell

and R. Kandiyoti

Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, University of London, Prince Consort Road, London SW7 2BY, UK (Received 16 April 1992; revised 70 September 1992)

Nitrogen and sulphur partitioning between product phases during the pyrolysis of Illinois No. 6 and Tilmanstone (high-rank UK) coals was monitored as a function of heating rate and pressure. An atmospheric-pressure wire-mesh pyrolysis reactor was used at heating rates of 555OOOKs-’ to 950°C. High-pressure experiments were performed at lOOOK s-l to 700°C in a wire-mesh reactor redesigned to allow tar capture by continuous sweeping of volatile products away from the sample holder into an externally cooled trap. The reactor configurations made it possible to determine yields and recover tars in the relative absence of secondary reactions. Generally, the net transfer into the tar phase of both nitrogen and sulphur was enhanced by increasing heating rate and suppressed by increasing pressure. At 950°C and 0.1 MPa, depending on coal type and heating rate, between 25 and 50% of the evolved nitrogen was found in the tar. At 7OO”C, the split of evolved N between tar and gas as a function of pressure differed sharply between the two coals. Despite comparable organic S contents of the two coals (56% of total S in Illinois No. 6, 51% for Tilmanstone), the fractions of evolved S at 950°C (0.1 MPa) were different, rising from 60 to 70% for Illinois No. 6 and from 35 to 45% for Tilmanstone with increasing heating rate. With increasing pressure, the distribution of coal-S between product phases differed considerably between the coals: for Illinois No. 6 the volatilized S remained roughly constant over the pressure range, whilst tar-S rapidly shifted (52 to 9%) to the gas phase. For Tilmanstone, volatilized coal-S increased by N 10% between 0.1 and 7.0 MPa but the tar-S content also shifted to the gas, suggesting the presence of tar-S within structures thermally more sensitive than those associated with tar-N. Considerably less coal-S in Tilmanstone volatilized. In view of the apparent differences between the combustion chemistries of the different phases formed during devolatilization, the results appear to be of direct relevance to the prediction of NO, and SO, formation during combustion and gasification (Keywords: pyrolysis; heteroatom: distribution)

The development of methods for the control and abatement of coal-related SO, and NO, emissions requires precise information on the modes of formation of these pollutants. The outcome of pyrolytic processes, which generally precede the conversion or combustion steps, usually has a direct bearing on subsequent stages of the process. Information on the devolatilization step is therefore necessary for understanding the formation of pollutant species in order to control overall levels of emission. This paper presents pyrolysis product yields and data on the distribution of coal sulphur and nitrogen between the tar, char and gas phases as functions of heating rate and pressure. Despite the similarities observed between the pyrolytic steps in many different applications, the modes of release of SO, and NO, during the utilization of coal are determined primarily by the particularities of the chosen

Presented at ‘Environmental Aspects of Coal Utilization and Carbon Science Conference’, upon Tyne. UK

31 March-2

OQ16-2361/93/03032147 (‘ 1993 ButterworthbHeinemann

April 1992. University

Ltd

of Newcastle

process route. It is known that during p.f. combustion, under conventional power station firing conditions, 15-30% of fuel-N is converted to nitrogen oxides, principally NO (with - 10% NO, and small amounts of N,O), and that this fuel-derived NO, accounts for the majority (7~80% of the total NO, formed’.‘). The contribution from hydrocarbon fragments reacting with nitrogen is considered minor, being ~5% of the NO, produced3. Pershing and Wendt4 have shown that although - 50% of coal-N remains within the char under normal combustion conditions, some 6(X30% of the NO, produced is derived from the volatiles-N. This difference in conversion efficiency to NO, appears to be mainly due to the greater time-scale of the combustion reactions which permit heterogeneous reduction of newly formed NO at the char surface. The relatively recent introduction of staged combustion, combining a fuel-rich primary region with secondary (and in some applications tertiary’) aeration, has been instrumental in reducing both fuel and thermal NO, emissions, to the extent that 6&95% of NO formed in ‘low-NO,’ burners is now derived from the char-N component6. However, in contrast to

FUEL, 1993, Vol 72, March

321

Heteroatom

distribution

in pyrolysis

products:

H.-Y. Cai et al.

volatile+N, the conversion of char-nitrogen to NO, appears to be relatively insensitive to modifications in combustor design, and at present appears to represent a limit to NO, emission reduction. In contrast to coal-N, the conversion of coal-S to SO, normally exceeds 95% during the p.f. combustion of bituminous coals, the balance being retained as sulphates in the ash. A small fraction of the SO, is oxidized to sulphur trioxide, a further fraction of which is in turn converted to sulphuric acid. Apart from limiting excess air levels to reduce SO, formation, there appears little that can be done with combustor design to prevent almost complete conversion of the coal-S to SO,. Pollution abatement strategy for p.f. combustion has consequently mainly focused on flue gas desulphurization (FGD) despite the attendant increase in cost. Although the maturity of modern p.f. combustor design may therefore appear to obviate the need for further investigation of pyrolytic processes, knowledge of the tar-char-gas split during devolatilization appears necessary for refining existing p.f. combustor modelling. For example, whereas it is commonly assumed that all pyrolysis volatiles burn at the rate of light hydrocarbons’, the determination of tar yields is thought necessary for refining assumptions related to the rate of combustion of pyrolysis volatiles in order to increase the precision of near-burner-zone calculations. Thus the large difference in the rates of combustion of heavy hydrocarbon and tar molecules compared with light hydrocarbon gases would appear to necessitate reassessment of the effect of the actual slower combustion of tars on the outcome of burner simulation calculations. It is known furthermore that the chemical structure of the burning volatiles affects the outcome as well as the rate of volatiles combustion. For example, the propensity to soot formation increases in the order: paraffins, olefins, naphthenes, aromatic$. In view of the relative abundance of coal pyrolysis tars (some two-thirds by mass of coal volatiles) and their predominantly aromatic character, accurate determination of tar release before combustion appears necessary for improved prediction of p.f. combustor behaviour. Further improvements in NO, and SO, emission control in p.f.-fired power plant appear to require costly downstream cleanup. However, the configurations of combined-cycle power plant allow reductions of up to 90% in the discharge of sulphur compounds by means of sorbents, typically limestone or dolomite, injected into fluidized-bed gasification and combustion reactors’. In the gasification stage of combined-cycle systems, the discharge of H,S has been found to be in near-equilibrium with sulphided sorbent’. As will be seen below, the relative amounts of product phases formed and the partitioning of nitrogen and sulphur between these product phases may be influenced by the heating rate imposed on injected particles. Moreover, when the pressure is raised above atmospheric, the pyrolytic distribution of the original coal mass into product phases and the partitioning of sulphur and nitrogen between these phases is further modified. Pyrolysis tars are generally considered as a nuisance product during gasification operations: in the gasification step of a combined-cycle operation, 60-80% of the coal charged is normally gasified in the first stage, where it is desired to maximize destruction of evolving tars. Unreacted char and elutriated fines (both with negligible volatiles content), additional quantities of fresh coal and sorbent

322

FUEL, 1993, Vol 72, March

are then charged to the combustor as required, in order to raise additional steam’. Pyrolysis tars are not normally expected to survive the oxidizing atmosphere of the combustor. As thermal NO, formation is not expected below 1300°C the relatively low combustor temperatures (- 1000°C) allow lower overall levels of NO, formation5. Clearly, however, the NO, reduction strategy for the whole plant would depend on the split of fuel nitrogen between the volatiles (evolved in the reducing atmosphere of the gasifier and carried into the combustion chamber of the gas turbine) and the residual char transferred to the combustor. In coal hydrogenation, the initial split into pyrolysis tar, char and gaseous product, with the attendant partitioning of coal sulphur and nitrogen between product phases, similarly plays a major role in determining the downstream product cleanup. Published process schemes usually feature fixed- or entrained-bed hydrogasification, with the tars normally considered as valuable by-product; char gasification is used for hydrogen production, and in more advanced systems the gas stream is recirculated” . In addition to the initial split of the coal into the usual product phases during the hydropyrolysis stage, the tar-gas balance within the reactor is further modified in entrained-flow reactors by (mostly) cracking reactions of the initially formed tars. Major factors affecting the extent of these secondary reactions are pressure, temperature, and residence time in the reaction zone1 I. Bench-scale data on both the initial partitioning of nitrogen and sulphur into the primary tar, char and gas phases and the modification of the distribution of sulphur- and nitrogen-bearing species by subsequent secondary reactions of the tars are thus relevant to refining the design and operation of coal hydrogenation reactors. The present study was undertaken in order to investigate the effects of heating rate and pressure on the distribution of nitrogen and sulphur in the tar and char phases during pyrolysis. Two coals of widely different rank, Tilmanstone and Illinois No. 6, were used for experiments with atmospheric-pressure and high-pressure wire-mesh pyrolysis reactors. Both reactors featured recently redesigned tar traps, and were provided with streams of sweep gas through the sample holders for removing volatiles from the reaction zone into externally cooled traps for subsequent recovery. The new configuration of the high-pressure reactori2*i3 and the redesigned tar trap of the atmospheric-pressure reactor14 thus make it possible to determine yields and recover pyrolysis tars in the relative absence of secondary reactions, and allow the characterization of tar samples expected to be similar in structure to primary tars.

EXPERIMENTAL Sawzples

Compositions of the two coals used are presented in The Illinois No. 6 sample was obtained from the collection of the European Centre for Coal Specimens (SBN Sample No. 5 11 US 43) and is different in character (see below) from the coal sample with the same name supplied by the Argonne (APCS) Program”. Samples were ground and sieved under nitrogen; the 106150 ,um fraction was dried under vacuum at 35°C for 3 h and stored in sample vials under nitrogen until required. Table 1.

Heteroatom Table 1 Proximate and elemental and Tilmanstone coals

Proximate analysis Volatile matter Fixed carbon Ash” Ultimate analysis Carbon Hydrogen Nitrogen Sulphur Oxyger?

analyses

of Illinois

No. 6 (SBN)

Illinois No. 6

Tilmanstone

47.0 53.0 10.4

17.2 82.8 5.4

15.6 5.8 1.4 4.4 12.8

87.9 4.0 I .4 1.3 5.4

(wt% daf)

(wt% daf)

Sulphur forms (wt% db) Organic Sulphate Pyritic

2.46 0.38

1.08

0.67 0.02 0.54

“Dry basis b By difference

The atmospheric-pressure wire-mesh pyrolysis reactor, its temperature measurement and control system and the experimental procedure for determination of tar and total volatile yields have been described16-18. The reactor consists of a wire-mesh sample-holder stretched between two electrodes, the sample-holder also serving as the resistance heater. During operation, helium (0.1 m s-‘) is swept through the sample-holder in order to remove volatiles from the reaction zone and into the tar trap. Tar and char yields are then determined by weight difference of the trap and sample-holder respectively. Recently the trap configuration has been altered to allow recovery of captured tars for analysis and characterization14. The high-pressure wire-mesh reactor used in the present study was based on an apparatus previously developed in this laboratory, in which uneven temperature fluctuations across the surface of the mesh prevented use of the sweep stream through the sample holder; determinations of tar yield at elevated pressure had not therefore been undertaken’7,‘9 before the present study. Recent work has shown that the fluctuating temperature distributions were caused by turbulence within the stream of sweep gas flowing through the sample-holder at pressures at which the heat capacity of the gas would be sufficiently high to conduct heat away from the wire mesh in uneven fashion. A full description of the redesigned reactor allowing the smooth passage of a stream of gas through the sample holder during high pressure experiments is in preparation’2*‘3. During the present study, a helium flow of 0.3 m s-l was used at pressures up to 2.0 MPa and 0.1 m s- ’ at pressures > 2.0 MPa. In both reactors the coal sample mass was 5-7 mg. In the atmospheric-pressure reactor the heating rate was varied from 5 to 5000 K s-l and the final temperature was 950°C with a 5s hold. In the high-pressure reactor the heating rate was 1000 K s- ‘, the pressure was varied from 0.1 to 7.0 MPa (1 to 70 bar), and the final temperature was 700°C with a 10s hold. Elenlentul

analyses

Tars collected were washed from the traps using a mixture of 20% methanol in chloroform. The solutions were concentrated, transferred into metal capsules used

distribution

in pyrolysis

products:

H.-Y.

Cai et al.

for elemental analysis and dried at 5O”‘C,such that each capsule contained between 0.5 and 0.9mg of sample. A Carlo Erba 1106 elemental analyser was used for CHN determinations on tar and char. Sulphur determinations were undertaken by Butterworth Laboratories Ltd: samples were burnt in an oxygen-rich atmosphere at - 1050°C and the evolved SO, was dissolved in alkaline solution; sulphur contents were determined by ion chromatography. The reproducibilities of the determinations were within f 0.2 wt% for CHN and about ?O.l wt% for sulphur. RESULTS AND DISCUSSION Eflkct

of heating

rate and pressure on yields

shows the variation of atmosphericpressure tar and total volatile yields from Illinois No. 6 and Tilmanstone coals with heating rate. The observed increases in yield with heating rate are broadly in line with previous findings’8-21, with the lower-rank Illinois No. 6 showing greater sensitivity to changes in heating rate than the higher-rank Tilmanstone. Pyrolysis yields and the sensitivities of yields to changes in heating rate for a set of coals have previously been noted to go through a maximum with increasing rank”, with the APCS Illinois No. 6 sample placed near the maximum of the yield vs. rank (expressed as wt% daf carbon) series. The present sample of SBN Illinois No. 6 coal is significantly different from the APCS sample of the same name. Table 2 compares yields from the two I samples under similar pyrolysis conditions. Comparison Figure

60

I(a)

,

1

10

100

Heating

1OooO

1000

Rate,

K/s

_I 10

20

30

40

50

Pressure,

60

70

00

bar

Figure 1 Effect of (a) heating rate at 0.1 MPa, (b) pressure at 1000 K s- ‘, on pyrolysis yields of Illinois No. 6 and Tilmanstone coals. Illinois No. 6: A, tar; 0. total volatiles. Tilmanstone: A, tar; 0, total volatiles

FUEL, 1993, Vol 72, March

323

Heteroatom

distribution in pyrolysis products:

H.-Y. Cai et al.

Table 2 Comparison of pyrolysis yields (wt% daf) from Illinois No. 6 samplessupplied by the APCS Program and the European Centre for Coal Specimens (SBN) Sample

SBN APCS

Tar

Total volatiles

lOOOKs-’

lKs-’

lOOOKs-’

1 KS-’

28.9 45.0

17.1

48.7

40.1

n.d.b

59.2 __

n.d.

’ Pyrolysis at 700°C with ‘Not determined

30 s hold at final temperature

of pyrolysis chars by scanning electron microscopy showed moreover that the APCS Illinois No. 6 passed through a considerably more fluid plastic phase than that of the SBN Illinois No. 6. Figure l(a) also shows that for SBN Illinois No. 6 the total volatile yield continues to increase above 1OOOK s-l, whereas the tar yield remains relatively stable with increasing heating rate; similar results were observed using a rank-ordered set of two UK coals (Taff Methyr, Gedling) and two German coals (Emil Mayrisch, Heinrich Robert)“. Figure I(b) shows the dependence of tar and total volatile yields on pressure. The monotonic decreases in these yields are qualitatively in line with most reports on pyrolysis under increasing pressure of inert gas (see refs 11 and 23 for brief reviews), although, owing to the method of tar capture and recovery described above, the tar samples from the present study are expected to reflect the structures and compositions of primary tars closely. It can also be seen that the difference between Illinois No. 6 tar and total volatile yields increases from 18 wt% at atmospheric pressure to 26-27 wt% at 7.0MPa, suggesting that the suppression of volatilization by increasing pressure of inert gas also leads to cracking of some tar precursors to gaseous hydrocarbons.

EfSrct of heating rate on elementul compositions qf tur and char Elemental analyses indicated that for the Illinois No. 6 sample, the conversion of carbon to total volatiles increased from 30 to 42.5% and the conversion to tar from 14.6 to 22.6 wt%, as the heating rate increased from 5 to 5000 K s-l. As expected from Figure Z(a), both the absolute level and the rate of increase with increasing heating rate of carbon conversion were markedly less for Tilmanstone. Hydrogen conversion to total volatiles was higher for Illinois No. 6 (-92%) than for Tilmanstone (84%); -70% of the original hydrogen content of both coals appeared to find its way into the gas phase (evaluated by difference). The hydrogen contents of the tars from atmosphericpressure pyrolyses between 5 and 5000 K s- 1 showed a clear downward trend with increasing heating rate, from 6.8 to 5.1 wt%. Recent evidence from size-exclusion chromatography of tars from the pyrolysis of vitrinite and liptinite concentrates I4 has suggested that tars which are more aliphatic in character are likely to crack more rapidly as a function of increasing heating rate. The effect appears to be due to increased likelihood of intraparticle reactions caused by rapid heating of the pyrolysing mass, leading to the destruction of the more thermally sensitive components of the tars before volatilization. Furthermore, the present sample of Illinois No. 6 appears to be

324

FUEL, 1993, Vol 72, March

considerably less aromatic in character than Tilmanstone: aromaticities, f,, derived from 13C n.m.r. for the two samples were found to be 0.67 and 0.79 respectively, and the 24-27 ppm band, thought to indicate the long-chain aliphatic content, gave 13% for Illinois No. 6 and 5.5% for Tilmanstone. Together, these data suggest that the gradual drop in the hydrogen contents of Illinois No. 6 tars is due to intraparticle (partial) cracking of aliphatic side-chains from these tars, which are expected to be more aliphatic and therefore more thermally sensitive than Tilmanstone tars. It is thought that the lack of increase in tar yield between 1000 and 5000 K s-’ may be due to competition between enhanced intraparticle cracking of tars at 5000Ks-’ and the positive effect of the increase in heating rate on tar yield. The effect of heating rate on coal-N partitioning between product phases at 950°C is shown in Figure Z(a). Although the method of calculation of these results tends to amplify experimental errors inherent in determination of both the yield and the elemental composition, the trends may be observed without great difficulty. Conversion to total volatiles of nitrogen from both coals increasesfrom -4O%at5Ks-‘to -45%at5000Ks-‘. Conversion of coal-N to tar is more strongly marked for Illinois No, 6, increasing from 14 to 27% when the heating rate increases from 5 to 5000 K s- ’ ; the corresponding conversion for Tilmanstone is 1 l-12%. The Illinois No. 6 data in Figure ,?(a) mirror closely the char-total volatiles split observed by Blair et ~1.‘~ using (nominally) the same coal on a heated carbon ribbon: at 95O”C, the coal-N was split as 64% to char and 36% to volatiles, compared with 60 and 40% respectively for

10

1

100

Heating

1000

10000

K/s

Rate,

100

m

80 ‘;;i G ‘& .L-4 g

b

90 -

‘0. 60

-

50-

w

40.

0

30-

R

h

2010

_--.

_----

___--*_-----

A

n

t,

-----_--________A

-

d

0 1

1000

100

10

Heating

Rate,

10000

K/s

Figure 2 Effect of heating rate on (a) nitrogen distribution and (b) sulphur distribution between pyrolysis products from Illinois No. 6 and Tilmanstone coals at 0.1 MPa. Symbols as in F@ure I

Heteroatom the same temperature at 5 KS-’ in the present study, with similar figures of 58 and 42% for Tilmanstone. The analogous results at the lower temperature of 700°C and 0.1 MPa shown in Figure 3(a), at 1000 K s-l, were 67% nitrogen to char and 33% nitrogen to volatiles for Illinois No. 6, and 63 and 37% respectively for Tilmanstone. Comparable results on the char-volatiles partitioning of coal nitrogen have been reported by a number of researchers6.21~25~26; one related early study reporting tar as well as total volatile yields appears to have suffered from experimental problems involving secondary tar cracking2’. Results from the present study suggest that at 95o”C, between 25 and 50% of the evolved nitrogen may be found in the tar, and that the exact proportions depend on the heating rate and type of coal; the evolved nitrogen in the tars does not appear therefore to constitute a predominant fraction of total devolatilized nitrogen. Figure 2(b) shows the effect of heating rate on the sulphur split between product phases: at 95O”C, the fraction of coal sulphur evolving with the volatiles depends on both coal type and heating rate. Compared with coal nitrogen, the fraction of sulphur in the tars is less sensitive to both these parameters. Comparison of the partition of sulphur between char, tar and gas at 700°C and 1000 KS-‘-Figure 3(b)-and the 95O”C, 1000 K s- ’ result for Illinois No. 6 shows a marked shift of coal sulphur to lighter gas at the higher temperature. The splits are 33:50:17 and 30:20:50 respectively. A similar trend is observed for Tilmanstone, although the levels of sulphur retained in the char by the higher-rank coal are much higher than for Illinois No. 6: 76:18:6 at

70 a =

60

distribution in pyrolysis products:

H.-Y. Cai et al.

7OO”C, and 55:12:33 at 950°C. Similar trends of substantial sulphur retention by the char have been reported for another relatively high-rank coal (Pittsburgh No. 8): 61% to char, 31% to gas for pyrolysis to 500°C in a split in a fixed bed2’, and a 45:55 char-volatiles wire-mesh reactor operated at lOOOK s-’ to 1000”CZh. As a function of heating rate, the distribution of sulphur between char, tar and gas for Illinois No. 6 at 950°C is 40:18:42 at lOOKs-’ and 30:20:50 at lOOOKs_‘, indicating an increased conversion of coal-S primarily to gas. Elemental analyses of the Illinois No. 6 tars show small but systematic decreases in both sulphur (4.5-2.7% of tar) and nitrogen (1.5--l .2%) contents with increasing heating rate, suggesting particular thermal sensitivity of sulphur- and nitrogen-bearing molecules. Comparison of the coal-type and heating-rate sensitivities of nitrogen and sulphur release during pyrolysis, Figure 2, suggests the following. Whereas the proportion of total nitrogen released is similar for the two coals, the Illinois No. 6 tars contain an increasing proportion of total nitrogen with increasing heating rate, and since little appears to be known about the combustion rates of coal pyrolysis tars, the rates at which tar-related nitrogen might be converted to NO, remain unclear. Despite comparable proportions of organic sulphur (56% of total sulphur in Illinois No. 6, 51% for Tilmanstone), the proportions of sulphur released into the tars show relatively small differences, whereas the total sulphur released from the two coals is substantially different. The differences between the sulphur release patterns of the two coals appear relevant to possible shifts in sorbent load between gasifier and combustor in combined-cycle operations, and would alter significantly the partitioning of sulphur-related pollution in coal hydrogenation processes between product gas and char. However, the validity of some of these findings for both combined-cycle and hydrogenation routes depends on an investigation of the effect of pressure on nitrogen and sulphur distributions during initial thermal decomposition.

0

70

20

30

40

50

Pressure,

60

70

00

bar

100

cn

90

‘;;

00

b -

G 7Ob .r( e0 60rrl

Q

0

.

0

10

20

30

40

Pressure,

50

60

70

00

bar

Figure 3 Effect of pressure on (a) nitrogen distribution and (b) sulphur distribution between pyrolysis products from Illinois No. 6 and Tilmanstone coals heated at lOOOK s-l. Symbols as in Figure I

Eflkct of pressure on elemental compositions char

qf tar ant!

Figure 3 shows the distribution of coal nitrogen and sulphur between product phases as a function of pressure. The change in carbon conversion figures for this set of experiments reflects the drop in total volatile and tar yields with increasing pressure shown in Figure I(b): from 42% at atmospheric pressure to 28% at 7.0MPa for Illinois No. 6, and from 19% to 16% for Tilmanstone. Elemental analyses (not reported) suggest that one effect of increased pressure is a net transfer of hydrogen from the tar to the gas phase, consistent with intraparticle tar cracking as discussed above. At 700°C. the proportions of nitrogen released at all pressures are lower than those reported for 950°C. As before, however, the split of evolved nitrogen between tar and gas appears to depend on coal type. At atmospheric pressure, - 70% of the nitrogen released by Illinois No. 6 is found in the tar, whereas at 7.0 MPa the proportion falls to - 50%; corresponding figures for the higher-rank Tilmanstone are - 32 and 22% respectively.

FUEL, 1993, Vol 72, March

325

Heteroatom

distribution

in pyrolysis

products:

H.-Y.

Cai et al.

For both coals, tar yields decrease with increasing pressure, whereas elemental analyses indicate that the nitrogen content of tars increases smoothly, from 1.1 to 1.5 wt% for Illinois No. 6 and from 1.3 to 1.8 wt% for Tilmanstone. It would appear that the greater part of the tar-N is embedded in (presumably polycyclic aromatic) structures which are relatively unaffected by the intraparticle cracking that is thought to take place (before release from the particle) with increasing intensity as the pressure is raised. The distribution of coal sulphur between product phases, Figure 3(b), shows a different character from that of coal nitrogen, and differs considerably between Illinois No. 6 and Tilmanstone. Total sulphur release from Illinois No. 6 remains roughly constant at 65-70% over the pressure range, whereas the distribution of sulphur between the product phases shifts markedly from tar to gas: the proportion of original coal-S in the tar decreases rapidly from 52.2 to 9.2% between 0.1 and 7.0MPa. Analogous experiments under hydrogen are necessary for a definitive assessment of the fate of coal-S during fine-particle coal hydrogenation. In the case of Tilmanstone, the total sulphur released increases by _ 10% over the pressure range, although the tar-S content decreases significantly, suggesting the presence of sulphur within thermally more sensitive structures, compared with those associated with tar-N. Furthermore, a considerably smaller proportion of coal-S in Tilmanstone is volatilized, presumably reflecting the greater stability of the sulphurbearing structures within the higher-rank coal than in Illinois No. 6.

CONCLUSIONS A method developed for the capture and recovery of tars during high-pressure wire-mesh pyrolysis experiments has enabled a set of tars to be prepared which can be expected to reflect closely the structures and compositions of primary tars. Heteroatom release from both low- and high-rank coals is sensitive to heating rate and pressure. A systematic decrease in the hydrogen contents of Illinois No. 6 tars with increasing heating rate is thought to be due to intraparticle (partial) cracking of aliphatic side-chains. The absence of any increase in tar yield between 1000 and 5000 K s- ’ is attributed to competition between enhanced intraparticle cracking of tars at 5000Ks-’ and the effect of increased heating rate on tar yield. The proportion of volatilized N during atmospheric pressure pyrolysis is similar for the two coals; however, tars from the lower-rank Illinois No. 6 coal contain an increasing proportion of coal-N with increasing heating rate. At 950°C and 0.1 MPa between 25 and 50% of evolved N is found in the tar, depending on the coal type and the heating rate. The proportion of evolved coal-N found in the tar diminishes rapidly with increasing temperature. Under elevated pressures, the split of evolved nitrogen between tar and gas appears to depend on coal type. At atmospheric pressure (700C), 70% of the Illinois No. 6 nitrogen is released into the tar, whereas at 7.0MPa the proportion decreases to -50%; corresponding figures for Tilmanstone are -32 and 22% respectively.

326

FUEL, 1993, Vol 72, March

Overall, considerably less of the coal-S in Tilmanstone volatilizes compared with Illinois No. 6. Despite comparable organic-S contents of the two coals (56% of total S in Illinois No. 6 against 51% in Tilmanstone), the fractions of evolved S at 950°C and 0.1 MPa differ, rising from 60 to 70% for Illinois No. 6 and from 35 to 45% for Tilmanstone with increasing heating rate. As a function of increasing pressure at 700°C the distribution of coal-S between product phases again differs considerably between the coals: for Illinois No. 6, volatilized S remains roughly constant over the pressure range, whereas tar-S rapidly shifts (52 to 9% of total sulphur) to the gas phase. In the case of Tilmanstone, volatilized coal-S increases by - 10% (to -35%) between 0.1 and 7.0 MPa, but the tar-S content also rapidly shifts to the gas, suggesting the presence of tar-S within structures thermally more sensitive than those associated with tar-N. The differences between the sulphur release patterns of the two coals appear to have direct relevance to changes in sorbent load between gasifier and combustor in combined-cycle operations, and to alter significantly the partitioning of sulphur-related pollution in coal hydrogenation processes between product gas and char. ACKNOWLEDGEMENTS The authors thank the European Community for supporting this work under EC Contract No. JOUF-0050C(TT) and ECSC Contract No. 7220-EC/849. They also thank the Coal Research Establishment, British Coal Corporation, for the supply of samples, D. J. A. McCaffrey and P. Burchill of British Coal for encouragement and useful discussions, and the Ministry of Planning and Co-operation of the Republic of Chile (MIDEPLAN-CHILE) for a Fellowship to AJG. REFERENCES

9 10

11 12 13 14 15 16 17

Pershing, D. W. and Wendt, J. 0. L. Comhust. Sci. Technol. 1977, 16. 111 Pershing, D. W. and Wendt, J. 0. L. In 16th Symp. (Int.) on Combustion, The Combustion Institute, Pittsburgh, 1977, p. 389 Hayhurst. A. N. and Vince, I. M. Prog. Energy Corn/w/. Sci. 1980, 6, 35 Pershing, D. W. and Wendt, J. 0. L. Ind. Eng. Chem. Process Des. Des. 1979, 18, 60 Cooke, M. J. and Ford, N. Enriron. Conlrol Bull. 1990, (9). 3 Phong-Anant, D., Wibberley, L. J. and Wall, T. F. Comhusr. Flame 1985, 62, 21 Lockwood, F. C. personal communication Lawn, C. J., Cunningham, A. T. S., Street, P. J., Matthews, K. J., Sarjeant, M. and Godridge, A. M. In ‘Principles of Combustion Engineering for Boilers’ (Ed. C. J. Lawn), Academic Press, New York, 1987, p. 61 Dawes, S. G., Reed, G. P., Gale, J. and Clark. R. K. Institution of Chemical Engineers, Symposium Series No. 123, 1991 Stroud, H. F. J., Matsui, H. and Noguchi, F. Paper to 17th World Gas Conf., Int. Gas Union, Washington DC, June 5-9, 1988 Gibbins, J. R., Gonenc, Z. S. and Kandiyoti, R. Fuel 1991, 70, 621 Giiell, A. J. Ph.D. Thesis, University of London, 1993 (in preparation) Giiell, A. J. and Kandiyoti, R. in preparation Li. C.-Z.. Bartle. K. D. and Kandiyoti, R. Fuel submitted Vorres, K. S. Energy Fuels 1990, 47 420 Gibbins-Matham. J. R. and Kandivoti, R. Energy . Fuels 1988. 2, 505 Gibbins-Matham, J. R., King, R. A. V., Wood, R. J. and Kandiyoti, R. Rev. SC;. Instrum. 1989, 60. 1129

Heteroatom 18 19 20 21 22 23

Gibbins. J. R. and Kandiyoti, R. Fuel 1989,68, 895 Gibbins, J. R. and Kandiyoti, R. Energy Fuels 1989, 3, 670 Gibbins. J. R., Khogali, K. and Kandiyoti, R. Fuel Process. Technol. 1990. 24, 3 Fine. D. H., Slater, S. M., Sarofim, A. F. and Williams, G. C. Fuel 1974, 53, 120 Periodic Report No. 3, EC Contract No: JOUF-@X0-C(TT), December 1991 Unger, P. E. and Suuberg, E. M. Am. Chem. Sot. Dir. Fuel Chem. Preprints 1983, 28(4), 278

24

25 26

27 28

distribution

in pyrolysis

products:

H.-Y. Cai et al.

Blair, D. W., Wendt, J. 0. L. and Bartok. W. In 16th Symp. (Int.) on Combustion, The Combustion Institute. Pittsburgh. 1977, p.475 Pohl, J. H. and Sarofim, A. F. In 16th Symp. (Int.) on Combustion, The Combustion Institute, Pittsburgh. 1977, p.491 Howard. J. B. In ‘Chemistry of Coal Utilization’, Second Suppl. Vol. (Ed. M. A. Elliott), Wiley-Interscience, New York, 1981. Ch. 12 Solomon, P. R. and Colket. M. B. Furl 1978. 57, 749 Khan. M. R. Fuel 1989,68, 1439

FUEL,

1993,

Vol 72, March

327