Energy & Fuels 1995,9, 956-961
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Effect of Changing Inertinite Concentration on Pyrolysis Yields and Char Reactivities of Two South African Coals H.-Y. Cai and R. Kandiyoti” Department of Chemical Engineering and Chemical Technology, Imperial College, University of London, Prince Consort Road, London S W 7 2BY, U.K. Received March 6, 1995@
Pyrolysis experiments have been carried out with sets of inertinite graded samples prepared from two South African coals (Durban Navigation and Vryheid Coronation Collieries). The effects on product distributions of increasing inertinite concentration, and of different heating rates, have been determined. An atmospheric pressure wire-mesh pyrolysis reactor was used during these experiments (1or 1000 K s-l t o 700 “C with 30 s holding at peak temperature) in order to minimize reactor related effects on the results. Tar and total volatile yields from the South African samples decreased with increasing inertinite concentration; the decrease in yields were in line with those observed for a set of maceral concentrate samples from a UK coal (Point of Ayr) and other Northern Hemisphere coals. Total volatile yields showed no detectable deviations when plotted alongside a correlation based on data from 24 Northern Hemisphere derived coals. Relative combustion reactivities of the chars from the pyrolysis experiments have been determined using a standard TGA-based method, as a function of inertinite contents of the original samples. The lower rank DNC set showed a rapid drop in char combustion reactivity with increasing inertinite concentration. By contrast, the reactivity of the VCC samples showed no sensitivity to changes in maceral concentration. It seems tempting to ascribe this similarity between vitrinites and inertinites of the VCC samples to progressively diminishing differences between macerals of higher rank coals. Clearly, however, many more samples need to be studied before definitive conclusions can be drawn.
Introduction Vast coal deposits in widely separated parts of the world (southern Africa, India, eastern Australia, Madagascar, South America, and Antarctica) are held to have common origins in the “lost” continent of Gondwana. The original plant material and the depositional environments of these coals are distinct: they are thought to be deposited mostly in the Permian age (270-225 million years before the present) from remains of stunted broad leafed flora of the Glossopteridae group and some broad-leafed plants not found in the Northern Hemisphere but similar to those found today in subarctic zones. Deposition of these coals is held t o have taken place in subarctic conditions in contrast t o much of the N. Hemisphere Carboniferouscoals, which appear to have been deposited when their present sites were near the equator.1,2 Expanding worldwide trade in coals for steam and power generation requires the development of a common basis for evaluating samples of different origins. However, much of the available information on commercial coals is based on studies of samples from the N. Hemisphere, mostly Carboniferous coals (350-270 million years before present). Relatively few direct comparisons with S. Hemisphere coals have appeared in the literature and disentangling reported similarities and differences appears to require some care.
* Author to whom correspondence should be addressed.
Abstract published in Advance ACS Abstracts, September 1,1995. (1) Stach’s Textbook of Coal Petrology; Stach, E., Mackowsky, M.TH., Teichmuller, M., Taylor, G. H., Chandra, D., Teichmuller, R., Eds.; Gebruder Borntraeger: Berlin, 1982; pp 177-197. (2) van Krevelen, D. W. Coal, 3rd ed.; Elsevier: Amsterdam, The Netherlands, 1993. @
Most Gondwana coals of the Permian age are bituminous coals with limited occurrence of anthracites. Due t o presumed slow subsidence, much thicker coal seams are encountered compared to N. Hemisphere Carboniferous deposits. The coals usually contain much finely dispersed clay minerals and less sulfur (mostly pyritic) than their N. Hemisphere counterparts. Petrographic compositions of Gondwana coals have been observed to vary more widely than those of Carboniferous coals: vitrinite contents rarely exceed 80%;occurrences of less than 50% vitrinite have been encountered; liptinite group maceral concentrations are normally low. The high percentage of inertinite often found in Gondwana coals is thought to be indicative of relatively dry conditions during coalification with greater extents of peat 0xidation.l Vitrinite is mostly formed under moist conditions, while fusinite and semi-fusinite are normally thought t o result from forest fires. But in Gondwana coals, concentrations of semifusinite have been found t o change in parallel with those of macrinite and inertodetrinite (inertinite group) macerals, suggesting a different formation route for these semifusinites, via alteration of vitrinite by bacterial and fungal action under mildly oxidising conditions rather than charring under vigorously oxidising c0nditions.l Among other factors, this likely different evolution route may, at least partly, help explain the perceived “different” character of Gondwana semifusinites compared to their N. Hemisphere counterparts. Insofar as the end user is concerned, coking behavior appears to provide the sharpest contrast between N. Hemisphere Carboniferous and S. Hemisphere Permian coals. Given,3 citing Roberts4 and D i e ~ s e l has , ~ sum-
0887-0624/95/2509-0956$09.00/0 @ 1995 American Chemical Society
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marized the problem as follows: in predicting the strength and reactivity of cokes made from N. Hemisphere Carboniferous coals by petrographic analysis, conventionally one-third of the semifusinite is added to the total of “reactive macerals”. Use of the same methods does not lead t o useful predictions with Australian coals, which produce better cokes than would be expected from their performance in dilatometer or plastometer tests or their petrographic analyses. The behavior of Gondwana coals also present similarities with N. Hemisphere coals of the Carboniferous age over a fairly wide spectrum of properties: (i) When vitrinite reflectances are plotted against carbon content, the plots follow the same curves as other Paleozoic, Mesozoic, and Tertiary c0als.~8~ (ii) Correlations between WC ratios of Australian coals and their pyrolysis yields appear to accommodate N. Hemisphere coals relatively e a ~ i l y . ~(iii) , ~ Forms and properties of individual macerals are said t o be indistinguishable from their carboniferous counterparts; physical and physicochemical changes which occur with increasing rank appear analogous t o those of N. Hemisphere coals. (iv) The data of Cudmorelo quoted by Duriell indicated a greater dependence of liquefaction conversions on vitrinite reflectance (i.e., rank) than on maceral composition. Similar observations have been made based on combustion reactivities of sets of Australian coals.l2-l4 However, the occasional superposition of (i) observations concerning the predominance of rank-related effects on reactivities (as above) and (ii)suggestions that Gondwana inertinites are as reactive as their accompanying vitrinites is not necessarily warranted. In the case of N. Hemisphere Carboniferous coals, the f i s t of these effects is well-known but inertinites are generally found t o be less reactive than the accompanying vitrinites during liquefaction15-17and pyrolysis18-20of the coals and combustion of their chars.21r22 Much of the uncertainty on reactivities of Gondwana macerals appears related to the relative scarcity of reported work on separated maceral concentrates. The (3) Given, P. H. In Coal Science; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic Press: New York, 1984; p 210. (4) Roberts, 0. C. Published Report 82-8, ISBN No.0 86722 224 X, Australian Coal Industry Reseach Laboratories Ltd, October 1982. (5) Diessel, C. F. K. Fuel 1983, 62, 883. (6) Chandra, D. Econ. Geol. 1965, 60, 1041. India 1966, 37, 37. (7) Chandra, D. Q.J. Geol. Min. Metal. SOC. ( 8 ) Tvler. R. J. Fuel 1979. 58. 680. (9) $der; R. J. Fuel 1980; 59; 218. (10) Cudmore, J. F. Fuel Process Technol. 1978, I , 227. (11)Dune, R. A. In Coal Liquefaction Fundamentals; Whitehurst, D. D., Ed.; ACS Symp. Ser. No. 139; American Chemical Society: p 53. (12) Phong-anant, D.; Thomas, C. G. Presented at the Washington, DC; Fourth Australian Coal Science Conference Dec. 1990, Brisbane 256. (13)Thomas, C. G.; Holcombe, D.; Shibaoka, M.; Young, B. C.; Brunckhorst, L. F.; Gawronski, E. Proc.Int. Conf: Coal Sci. 1989,257. (14) Jones, R. B.; McCourt, C. B.; King, K. Fuel 198664, 1460. (15)Li, C-2.;Gaines, A. F.; Kandiyoti, R. Final Report: “Liquefaction of coal macerals in a novel flowing-solvent reactor”,Commission of the European Communities, Contract No. EN3V.0052.UK(H). (16)Gaines, A. F.; Li, C.-Z.; Bartle, K. D.; Madrali, E. S.; Kandiyoti, R. Proc. Intl. Conf: Coal Sci., Newcastle-upon-Tyne, U.K., 1991,830833. (17) Li, C.-Z.; Madrali, E. S.; Wu, F.; Xu, B.; Cai, H.-Y.;Guell, A. J.; Kandiyoti, R. Fuel 1994, 73, 851. (18) Howard, J. B. In Chemistry of Coal Utilisation; Elliott, M. A., Ed.; John Wiley & Sons: New York, 1981; Chapter 12, p 665. (19) Li, C.-Z. Ph.D. Thesis, University of London, 1993. (20) Li, C.-2.; Bartle, K.; Kandiyoti, R. Fuel 1993, 72, 3. (21)Nandi, B. N.; Brown, T. D.; Lee, G. K. Fuel 1977, 56, 125. (22) Essenhigh, R. H. In Chemistry of Coal Utilisation; Elliott, M. A., Ed.; John Wiley & Sons: New York, 1981; Chapter 19, p 1153.
Table 1. Compositions of the Maceral Concentrate Samples Used in the Study sample
VM” ashb
Cc
He vitr.d lipt.d inertd
Maceral Concentrate Samples from the S. Hemisphere (VCC and DNC, South Africa) VCCl 23.1 3.7 87.8 4.9 73.5 0.5 26.0 0.5 40.5 VCCB 22.5 13.4 87.4 4.6 59.0 7.0 87.2 4.4 43.5 0.0 56.5 VCCB 21.1 DNC 1 34.7 3.9 83.7 5.3 86.5 1.0 12.5 3.5 25.5 DNC2 32.1 15.6 83.1 4.8 71.0 DNC3 30.6 17.8 83.4 4.7 57.0 0.5 42.5 Maceral Concentrate Samples from the N. Hemisphere (Point of Ayr, UK) 1.9 85.2 5.2 84 6 10 whole coal da 5 4 vitrinite conc d a 2.3 84.8 5.0 91 liptiniteconc da 2.8 85.7 6.5 30 61 9 3 80 11.3 84.2 4.5 17 inertiniteconc d a In vol % (daf). 6 In % (db). In % (daf). In % (dmmf).
Table 2. Averaged Pyrolysis Yields of Point of Ayr Maceral Concentrate Samples (Ref 19P tar whole coal liptinite conc vitrinite conc inertinte conc a
(wt % daD
total volatile (wt % daf)
26.1 47.1 24.6 16.1
42.4 62.0 40.1 31.3
The listed data were averaged from at least two data points.
present study has been undertaken to compare the pyrolytic behavior of inertinite graded samples prepared from two South African coals with that of maceral concentrates from a UK coal (Point of Ayr). Determinations of tar yields and recovery of chars were carried out under conditions minimizing reactor related effects, in an atmospheric pressure wire-mesh pyrolysis reactor. Combustion reactivities of chars prepared during these experiments have been determined by thermogravimetric methods, as a function of inertinite content of the original samples.
Experimental Section Coal Samples. Sets of inertinite-graded samples of Durban Navigation Colliery (DNC) and Vryheid Coronation Colliery coals were obtained through the European Centre for Coal Specimens (SBN, now part of Netherlands Meetinstitute): samples of “whole” coal from which these concentrates were prepared (CSIRO, Division of Energy Technology, Pretoria, South Africa) were not been made available. Table 1presents proximate, ultimate, and petrographic analyses for these samples. Within each set, the hydrogen and the (proximate analysis based) volatile matter content decreased in small steps with increasing inertinite concentration. While also attributable to the efficiency of maceral separation, the low liptinite contents are consistent with what is generally known of Gondwana coals. Petrographic analyses were carried out by CRE-Stoke Orchard (British Coal). Table 1 also presents data on maceral concentrates prepared from Point of Ayr (UK) coal, included in the study for comparison; inertinite contents of these samples vary over a wide range (4-80% (v/v, dmmol. Results from the pyrolysis of the set of Point of Ayr samples (Table 2) have been previously r e p ~ r t e d . ’ ~ ~ ~ ~ J ~ Pyrolysis Experiments. Experiments were carried out in an atmospheric pressure wire-mesh pyrolysis reactor, designed to minimize reactor-related effects in determining tar and total ~ ~ ~ ~ , ~are ~ heated volatile yields. In this c o n f i g ~ r a t i o n , 2samples between layers of stainless steel wire-mesh stretched between (23) Li, C.-Z.; Gaines, A. F.; Guell, A. J.; Kandiyoti, R. Int. Conf: Coal Sei., Newcastle upon Tyne, UK 1991.
Cai and Kandiyoti
958 Energy & Fuels, Vol. 9, No. 6,1995
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Figure 1. Comparison of pyrolysis yields between S. Hemisphere coal samples (DNC and VCC) and N. Hemisphere coal sample (PoA) as a function of inertinite concentration. Experito 700 ments were carried out at a heating rate of 1000 K "C and with 30 s holding time. VCC: ( A ) tar, (0)total volatile; DNC: (A)tar, (0)total volatile; PoA ( 0 )tar, (W total volatile. two electrodes, with the sample holder serving as the resistance heater; a flow of helium through the sample holding part of the mesh is used for removing volatiles away from the heated reaction zone. In this study, samples (typically 5-7 mg) were heated a t 1000 K s-I to 700 "C with 30 s holding at the peak temperature. In order to investigate the effect of heating rate on pyrolysis yields from the South African samples, some experiments were also carried out at 1K s-I t o 700 "C. Char Reactivity Determinations. Combustion reactivities of the pyrolysis chars were determined isothermally in a Perkin-Elmer TGA7 thermogravimetric balance. Char samples were heated a t 50 K min-I in nitrogen to 600 "C and held for 5 min, in order t o stabilize the chars before admitting air to initiate char combustion. The temperature, sample size, and air flow rate were chosen to ensure that combustion took place at a speed compatible with carrying out practical measurements, while ensuring that the process was kinetically controlled. Maximum weight loss rate of the char samples in the rectilinear period was taken as the index of char combustion reactivity. Reactivities of the VCCl and DNCl (see Table 1) chars were also determined a t temperatures between 500 and 650 "C; activation energies of 131.5 and 122.1 k J mol-l, respectively, were obtained. These results are broadly in line with activation energy values found in other studies.22j26
Results and Discussion Pyrolytic Behavior of the Samples. Figure 1 presents tar and total volatile yields from the pyrolysis of two sets of South African samples plotted against inertinite concentration. For the lower rank DNC samples, total volatile, tar, and gas (calculated by "difference" between sample weight loss and tar yield) yields were considerably higher than those from the VCC samples; the DNC gas yields of approximately 15% were in line with that of many low-middle rank Northern Hemisphere coals (e.g., Linby (UK)). All tar and total volatile yields were found t o decrease with increasing inertinite concentration of the samples: those from the lower rank DNC set were observed to decrease somewhat more rapidly. It may be noted that changes in yields of the two South African coals with changing (24) Gibbins, J.R.; King, R. A. V.; Wood, R. J.; Kandiyoti, R. Rev. Sci. Instrum. 1989,60,1129. (25) Gibbins-Matham,J.R.; Kandiyoti, R. Energy Fuels 1988,2,505. (26)Smith, I. W. Fuel 1978,57,409. (27) Gibbins, J. R.; Kandiyoti, R. Fuel 1989,68,895.
10
20 30 40 50 60 Total volatile (measured), (%daf)
70
Figure 2. Correlation of total volatile yields of N. Hemisphere coals with their VM,C, and H contents. Experiments were carried out at a heating rate of 1000 K to 700 "C and with 30 s holding time.
inertinite concentration were not significantly different from the trend found for the Point of Ayr coal (Table 1; the Point of Ayr liptinite concentrate is not relevant here). In fact, if small differences in slope could be taken literally, yields from both sets of South African samples could be said to have decreased more rapidly with increasing inertinite concentration. Comparable results have also been reported from experiments with a vitrinite concentrate (97%vitrinite, 3% liptinite, 0% inertinite) and an inertinite concentrate (7%vitrinite, 0% liptinite, 93%inertinite) prepared from a UK coking coal, Cortonwood Silkstone: between the vitrinite and inertinite concentrate samples, tar yields were observed to change from 22.5 to 26.5% and total volatiles from 35.8 t o 42.1% (% w/w daf).15 Observed similarities between the pyrolytic behavior of the present set of S. African samples and N. Hemisphere coals can be placed within a wider context. Recently,28 we reported a correlation between total volatile yields and three basic properties: volatile matter content (VM from proximate analysis), carbon content (C), and hydrogen content (HI. total volatile yield (wt %, daf) = 0.933 VM - 0.161 C 4.05 H (1) This correlation was based on data from 24 N. Hemisphere coals; total volatile yields were obtained in experiments where samples were heated at 1000\Ks-l to 700 "C with 30 s holding. Figure 2 presents experimental total volatile yields from the 24 N. Hemisphere coals, plotted against total volatile yields calculated from eq 1. Volatile yields from the DNC and VCC derived samples plotted in Figure 2 showed no significant differences from those of the N. Hemisphere coals. Effect of Heating Rate. For a large number of N. Hemisphere coals, previous work in this laboratory20,24,25129 has shown both total volatile and tar yields to increase with heating rate between 1and 1000 K s-l. For Linby coal, a low-rank UK bituminous coal used as the benchmark sample in this laboratory, increases of about 6-7% were observed in both tar and total volatile yields when, at 700 "C, the heating rate was increased from 1 t o 1000 K s-l (30 s hold). However, for a wide variety of coals, increases in heating rate from 1000 K s-l up t o 5000 K s-l, during experiments at temperatures between 700 and 1000 "C, have been found t o lead
+
(28) Cai, H.-Y.; Dugwell, D. R.; Kandiyoti, R. Proc. Third Int. Conf. Ind. Furnaces Boilers, 18-21 April, 1995,Lisbon, Portugal, in press. (29) Cai, H.-Y. Ph.D. Thesis, University of London, 1995.
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*
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Pyrolysis Heating r a t e (K/s)
Figure 3. Effect of heating rate on pyrolysis yields of DNCB and VCCB. Experiments were carried out at 700 "C and with 30 s holding time. VCCB: (A) tar, (0)total volatile; DNCZ: (A)tar, (0)total volatile.
to only small increases in total volatile yields while tar yields remained c o n ~ t a n t . An ~ ~ apparently ,~~ particle size related effect reported by Howard and co-worke r ~ , suggesting ~ ~ - ~ a~ drop in tar yield between 1000 and 20 000 K s-l (at 1000 "C) for particles between 106 and 125 pm, could not be observed in our work, where a particle size range of 106-150 pm has been used. It is possible that the drop in tar yield, accompanied by the monotonic behavior of the total volatiles, might have been due to insufficient speed in removing volatiles from the reaction zone at heating rates above 1000 K s-l. To investigate the effect of heating rate on the present South African samples, the DNCB and VCCB samples were pyrolyzed by heating at 1 and 1000 K s-l to 700 "C with 30 s holding. Figure 3 shows increases between 3 and 4% in tar and total volatile yields from the DNCB sample; a comparable increase in tar yield and a somewhat smaller increase in total volatiles was observed from the VCCB sample. The heating rate sensitivities of the PoA maceral concentrates were within the same order of magnitude: an increase of 4.1% was observed in the case of the vitrinite concentrate and less than 1%for the inertinite-rich sample.20 For N. Hemisphere coals, the old rule of thumb suggestingthat "only vitrinites are susceptible to changes in heating rate" must be considered to have many exceptions and the heating rate effect on yields is never very large.20,25 Overall, the Cortonwood Silkstone samples showed less sensitivity to the increase in heating rate than the South African Samples: 1.8%in the case of the vitrinite concentrate and 1.5% for the inertinite ~0ncentrate.l~ There seems to be comparable variations in the heating rate sensitivity of S. Hemisphere coals. Differences in yields due to changes in heating rate from Pecket "whole" coal (from Punta Arenas in the southern tip of Chile) and Catamutum (whole coal, Central Chile) gave somewhat larger differences in yields:34on increasing the heating rate from 1 to 1000 K s-l at 700 "C, the differences were 4.2 and 10.7%,respectively. (30) Cai, H.-Y.; Guell, A. J.; Chatzakis, I. N.; Lim, J.-Y.; Dugwell, D. R.; Kandiyoti, R Fuel, submitted for publication. (31) Griffin, T. P.; Howard, J. B.; Peters, W. A. Energy Fuels 1993, 7,297. (32) Darivakis, G. S.;Peters, W. A.; Howard, J. B. M C h E J. 1990, 36,1189. (33) KO,G. H.; Sanchez, D. M.; Peters, W. A.; Howard, J. B. TwentySecond Syposium (International) on Combustion, Proceedings; The Combustion Institute: Pittsburgh, 1988; p 115.
0 20
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Figure 4. Weight change of VCC char samples in TGA system at 600 "C. Char samples were prepared at a heating rate of 1000 K s-l to 700 "C and with 30 s holding time.
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1. DNCl 2. DNCZ 3. ONC3
80 60 40
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Figure 5. Weight change of DNC char samples in TGA system at 600 "C. Char samples were prepared at a heating rate of 1000 K s-l to 700 "C and with 30 s holding time.
To date, it has not been possible to postulate systematic relationships between heating rate sensitivities of yields and properties of N. Hemisphere Carboniferous coals. However, the broadly parallel increases in tar and total volatile yields over the 1-1000 K s-l heating rate range observed in the case of the two South African samples indicate a pattern of pyrolytic behavior which is in general agreement with our results from Northern Hemisphere c0als.20~25~29 Combustion Reactivity of Pyrolysis Chars. Figure 4 presents mass loss curves from the combustion of the three VCC coal derived samples in the TGA apparatus, showing differences between the three chars only in approximately the last 30% of total burnout. In the case of the DNC coal derived char samples, however, differences in reactivity could be observed from the outset of the combustion process (Figure 5). Burnout curves for PoA coal chars have been presented in Figure 6, indicating relative rates of char combustion in the expected order: liptinite concentrate > vitrinite concentrate > inertinite concentrate Char reactivities calculated from these experiments have been plotted in Figure 7 against the inertinite (34) Guell, A. J.; Kandiyoti, R. Energy Fuels 1993,7,943.
960 Energy & Fuels, Vol. 9,No. 6,1995
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Cui and Kandiyoti Table 3. Aromaticities of the S. Hemisphere Coal Samples (Ref 36)
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1. Liplinile Conc. ~~
2. Vitrinile Conc.
sample
3. lnerlinite Cone
VCCl vcc2 vcc3 DNCl DNC2 DNC3
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Elapsed lime (min)
Figure 6. Weight change of PoA maceral concentrate chars in TGA system at 600 "C. Char samples were prepared at a heating rate of 1000 K s-l to 700 "C and with 30 s holding time.
+
. I
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Inertinite Conc. (volZ, d m m l )
Figure 7. Comparison of combustion reactivities of chars prepared from S. Hemisphere coal samples (DNC and VCC) and N. Hemisphere coal sample (PoA) as a function of
inertinite concentration. Char samples were carried out at a heating rate of 1000 K s-l to 700 "C and with 30 s holding PoA. time. (A) VCC, (+) DNC, (0)
contents of the samples. Significant differences were found between the response of the two sets of South African chars: the DNC set clearly showed a drop in char reactivities with increasing inertinite concentration. In the case of one South African sample we seem t o have evidence that vitrinite chars are more reactive than inertinite chars. Changes in reactivity between the VCC coal derived char samples were within experimental error. In the case of the other South African sample, therefore, we seem to have evidence that vitrinite and inertinite chars are equally reactive. Combustion reactivities of the PoA maceral concentrate chars were found to be 13.18,8.70, and 7.52 w t % min-l (daf) for the liptinite, vitrinite, and inertinite concentrate chars, respectively. Extrapolating t o pure maceral chars,35 the corresponding reactivities were 24.8, 8.5, and 7.2% min-l (daf). Reactivities of PoA vitrinite and inertinite concentrate chars plotted in Figure 7 indicate less sensitivity to inertinite concentration than the DNC coal derived samples; the reactivity of PoA liptinite concentrate char has not been shown. Carbon Aromaticity and Pyrolysis Yields. Two different methods have been used to determine the 13C (35)Cai, H.-Y.; Kandiyoti, R. Fuel, submitted for publication.
CP-MAS 0.74 0.76 0.73 0.69 0.71 0.73
SPE 0.83 0.82 0.85 0.79 0.81 0.83
NMR derived carbon aromaticities of these South African sample^:^^,^' CP-MAS with a sideband suppression routine (TOSS) and single-pulse experiments. On the basis of either type of NMR determination, Table 3 shows the carbon aromaticities, fa, of the DNC set spread over a rather narrow range of 4% and those of the VCC samples over 3%. Relatively little data on N. Hemisphere samples are available for a direct comparison: using maceral concentrates of rather higher purity, derived from Lewiston Stockton coal (APCS No. 7),38 Joseph et al. reported much wider differences between carbon aromaticities of the major maceral groups: vitrinite concentrate (95% purity), 75%; liptinite concentrate (92.5% purity), 56%; inertinite concentrate (99.5%purity), 89%. At first glance, these data may suggest that vitrinites and inertinites of the South African samples have carbon aromaticities that are much closer to one another compared to differences observed between carbon aromaticities of N. Hemisphere maceral groups. Such an observation could be in line with the view that inertinites (or at least some inertinites) found in Gondwana coals of the Permian age were formed via the alteration of vitrinite by bacterial and fungal action under mildly oxidizing conditions rather than charring. However, a simple (approximate) calculation using the maceral group concentrations in Table 1,a vitrinite fa value of 73%, and an inertinite fa value of 90% gave a 4% spread for the VCC samples: 77.1%for VCCl and 81.7% for VCC3. Thus, the relatively mixed nature of the samples does not allow firm conclusions t o be drawn about maceral group carbon aromaticities and the latter's relationship with pyrolysis yields. (With individual maceral group fa values assumed to be similar t o those of the Lewiston-Stockton derived samples, tar yields from the South African coals would have fitted well in a correlation between carbon aromaticity and pyrolysis yields based on data from N. Hemisphere
Evaluation of Results. As observed from Figure 1, vitrinites and inertinites in the South African coals do not give similar yields during pyrolysis. The changes observed in yields with changes in inertinite concentration, and also with heating rate, were well in line with results from N. Hemisphere Carboniferous coals. Figure 8 shows that, indeed, these South African samples have carbon and hydrogen contents which place them relatively close to the main body of N. Hemisphere Carboniferous coals. (36)Madrali, E. S. Ph.D. Thesis, University of London, 1995. (37) Li, C. Z.; Guell, A. J.; Madrali, E. S.; Cai, H. Y.; Wu,F.; Dugwell, D. R.; Kandiyoti, R. Final Fbport, "Improved Methods of Coal Char-
acterisation and Char Classification", Commission of the European Communities, Contract No. JOUF-005O-C(TT). (38) Vorres, K. S. Energy Fuels 1990,4 , 420.
Pyrolysis of South African Coals
Energy & Fuels, Vol. 9,No. 6, 1995 961 1
8.0 4 0
7.0
-
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-
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1.0
-
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o
I
t
, I I
+ ?-
-
vcc ~
~~
loo
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IS
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YO Carbon Figure 8. Deviations from Seyler's coal band by coals of the S. Hemisphere.
However, the reactivities of chars derived from the pyrolysis of these samples did not produce results allowing of rapid generalizations. Contrary to expect a t i o n ~ , the ~ ~ lower - ~ ~ rank DNC set showed significant differences in char reactivity with changes in inertinite content, whereas the VCC set showed no experimentally noticeable differences in char reactivity with increasing inertinite content. While it is tempting to ascribe the similarity in reactivities of the VCC vitrinites and inertinites to progressively diminishing differences between macerals of higher rank coals, it seems obvious that many more samples need to be studied before definitive conclusions can be drawn.
Conclusions The pyrolytic behavior of inertinite graded samples prepared from two South African coals has been compared with those of samples from the N. Hemisphere, including maceral concentrates from a UK coal (Point of Ayr). Pyrolysis experiments were carried out in an
atmospheric-pressure wire-mesh reactor. Combustion reactivities of chars from these experiments have been determined as a function of inertinite content of the original samples by thermogravimetric methods. 1. Tar and total volatile yields from the two sets of South African samples have been found to decrease with increasing inertinite concentration. Yields from the lower rank Durban Navigation set were observed to decrease somewhat more rapidly. The trend was similar to that found for the Point of Ayr maceral concentrates. Total volatile yields from the Durban Navigation Colliery (DNC) and Vryheid Coronation Colliery (VCC) derived samples showed no detectable deviations when plotted alongside a correlation based on data from 24 N. Hemisphere derived coals. 2. The effect of heating rate on yields from the South African samples was investigated (1 and 1000 K s-l to 700 "C with 30 s holding). Increases of between 2 and 5% in tar and total volatile yields were found. The broadly parallel increases in tar and total volatile yields over this heating rate range indicate a pattern of pyrolytic behaviour in agreement with results from N. Hemisphere Carboniferous coals. 3. Combustion reactivities of chars from pyrolysis experiments outlined above have been determined in a TGA apparatus using an isothermal method. The DNC set showed a rapid drop in char reactivity with increasing inertinite concentration; by contrast the high rank VCC samples showed no change in reactivity with increase in inertinite concentration of the original samples. It seems possible to ascribe the similarity in reactivities of VCC vitrinites and inertinites to progressively diminishing differences between macerals of higher rank coals; clearly, however, many more samples need to be studied before definitive conclusions can be drawn.
Acknowledgment. The authors thank Dr. J. Vleeskens (ECN, The Netherlands) for provision of the S. Hemisphere samples, Dr. G. Hallam (CRE, Stoke Orchard, UK) for petrographic analyses, and Dr. C.-2. Li for preparing the set of Point of Ayr maceral concentrate char samples. The authors express their gratitude to the European Union for funding the work under Contract Nos. JOUF-0050-C(TT) and ECSC 7220-ED/016. EF950045Z
