224
Energy & Fuels 1988,2, 224-230
Coal-Zinc Chloride Reaction: An Interpretation B. K. Mazumdar,* D. D. Banerjee, and G.Ghosh Central Fuel Research Institute, Dhanbad, India Received October 20, 1986. Revised Manuscript Received October 16, 1987
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The coal-zinc chloride reaction, especially the pyrolytic behavior of zinc chloride impregnated coal (with 5 2 5 % loading),has been reinvestigated. This reinvestigation appears to provide a deeper insight into the mechanism of the reaction and hence the nature of the coal structure. One of the crucial findings which has emerged is that a specific dehydrogenation, e.g., about &12% of the total hydrogen (in the bituminous range) in coal, is caused by ZnCl, well below pyrolytic conditions (even as low as 240 "C). Furthermore, a parallelism has also been observed between the coal-zinc chloride reaction on the one hand and the coal-aluminum chloride reaction (Scholl's reaction) on the other, insofar as both the nature and the extent of "dehydrogenation" are concerned, as well as the aftereffects of the reaction on the pyrolytic behavior in both cases. Attention is also drawn to a similar change in pyrolytic behavior that was observed earlier, Le., partial (but specific) or controlled dehydrogenation with S, Se, or halogens. On the basis of the above parallelisms, an interpretation of the coal-zinc chloride reaction is advanced along with some observations on the nature of the structure of coal.
The coal-zinc chloride reaction received considerable attention since Georgiadis and Gaillard first observed' some remarkable changes in the pyrolytic behavior of some coking coals when they were impregnated with 3-4% ZnC1, and pyrolyzed at and up to 525 "C at the rate of 3 "C min-'. These were (1)a decrease in plasticity, (2) an increase in char yield with a corresponding decrease in tar yields (by about 2-3%), and (3) the evolution of a significantly greater volume of molecular hydrogen in gas. Similar findings, though quantitatively different, have since been reported by Bodily et a1.2-6while extending the same investigation to a noncoking low-rank bituminous coal (Utah coal, % C = 79.5). With 12% ZnClz impregnation and a heating rate of 5 "C min-l, volatile matter was found to decrease by about 15%, which was reflected in near suppression of tar formation and, conversely, in the increase of char yield to a similar extent. Such a remarkable (and, at the same time, drastic) alteration in pyrolytic behavior was ascribed by these authors3" to be due to in situ dehydrogenation of hydroaromatic structure caused by ZnC1, during the early stages of the pyrolysis. In fact, evidence in support of the evolution of a significant proportion of hydrogen well below 400 "C (being initiated at as low a temperature as 200 "C) was noted versus very little or no hydrogen for untreated coal in the same temperature range. In support of this interpretation, earlier work by Mazumdar et a1.+l1 on coal dehydrogenation and the observation of almost complete inhibition of tar formation dependent upon the prior dehydrogenation of coal with either sulfur, selenium, or halogens were cited. Furthermore, the conceptlOJ1of the origin of primary tar being essentially due to the hydroaromatic moieties of coal was also considered relevant to the interpretation. However, the above explanation may not appear to be adequate and convincing because S or Pd12 dehydrogenation (also cited by Bodily et al.), on the one hand, and zinc chloride dehydrogenation, on the other, may not be equivalent in regard to their nature and action. More recently a reversal of the phenomenon has been reported by Kandiyoti et al.;13914instead of a decrease in the yield of tat, a spectacular increase from 22 to 35% (with respect *Author to whom correspondence is to be addressed. Formerly of Central Fuel Research Institute. Now Emeritus Scientist, Central Glass and Ceramic Research Institute, Calcutta-700032, India. 0887-0624/88/2502-0224$01.50/0
to a low-rank bituminous coal; % C = 79.9) was obtained by flash pyrolysis of ZnC1,-impregnated coal (at 10% ZnCl, loading) at very high rates of heating (estimated to be loo0 "C s-l versus 5 "C min-' in the Gray-King (G.K.) assay retort). Thus, the entire gamut of coal-ZnC1, interaction during pyrolysis seems to be in flux. The above phenomenon would seem to indicate the direction for in depth studies on the coal-zinc chloride reaction by pyrolysis for a fuller interpretation of the mechanism, which has been attempted and presented here. Interpolation of these results with our previous work on coal dehydrogenation and pyrolysis seems to have yielded some critical information regarding the mechanism of the reaction and ita implication to the nature of coal, especially the nature and disposition of its hydroaromatic structure. In fact, a complete parallelism between the coal-zinc chloride reaction at lower temperatures (below pyrolysis conditions) and Scholl's reaction15J6(action of molten AIC13 (1) Georgiadis, C.; Gaillard, G. Chal. 2nd. 1956, 32, 247.
(2) Bodily, D. M.; Lee, H.D.; Hill,G. R. Prepr. Pap-Am. Chem. Soc., Diu. Fuel Chem. 1972, 17(1) 28. (3) Bodily, D. M.;Lee, H. D.; Wiser, W. H. Prepr. Pap.-Am. Chem. Soc., Dzu. Fuel Chem. 1974, 19(1), 163. (4) Bodily, D. M.; Lee, H. D.; Wiser, W. H. Prepr. Pap-Am. Chem. Soc., Diu. Fuel Chem. 1975, 87. ( 5 ) Bodily, D. M. Prepr. Coal Chem. Workshop 1976. (6) Mazumdar, B. K.; Choudhury, S. S.; Chakrabartty, S. K.; Lahiri, A. J. Sci. Ind. Res., Sect. B 1958, 17B, 509. (7) Mazumdar, B. K.; Chakrabartty, S. K.; Choudhury, S. S.; Lahiri, A. Proceedings of the Symposium on the Nature of Coal; Central Fuel Research Institute: Jealgora, India, 1969; p 167. (8) Mazumdar, B. K.; Choudhury, S. S.; Lahiri, A. Fuel 1960 39, 179. (9).Mazumdar, B. k., Chakrabartty, S. K.; De, N. G.; Ganguly, S.; Lahiri, A. Fuel 1962, 41, 121. (10) Mazumdar, B. K.; Chakrabartty,S. K.; Lahiri, A. Fuel 1959,38, 112.
(11)Mazumdar, B. K.; Ganguly, S.; Lahiri, A. Fuel 1964, 43, 281. (12) (a) Raymond, R.; Wender, I.; Reggel, L. Science (Washington, D.C.) 1962,137,681. (b) Reggel, L.; Wender, I.; Raymond, R. Fuel 1968, 47, 373. (13) Kandiyoti, R.; Lazaridis, J. I.; Dyrvoid, B.; Weerasinghe, C. R. Fuel 1984, 63, 1583. (14) Kandiyoti, R.; OBrien, R. J. Proc.-Znt. Conf. Cool Sci. 1985, 1986, 941. (15) Mazumdar, B. K.; Chakrabartty, S. K.; Lahiri, A. Fuel 1962, 41, 129. (16) Ghoah, G.; Mazumdar, B. K. Proceedings, Symposium on Coal Science & Technology for the Eighties; Central Fuel Research Institute: Dhanbad, India, 1979; p 127.
0 1988 American Chemical Society
Energy & F u e l s , Vol. 2, No. 2, 1988 225
Coal-Zinc Chloride Reaction
sample no. 1 2 3 4 5
Table I. Proximate Analysis and Elementary Composition of t h e Coals Studied for t h e Reaction" proximate anal.: % elem anal., % coal" M A V F % volatiles,b dmf C H Khandra, Jambad 8.0 12.3 34.1 45.6 41.9 (34.6) 80.5 5.4 Kottadih, Samla 5.8 11.6 33.7 48.9 40.0 (32.1) 82.5 5.5 E. Bhowrah, Jharia 1.6 2.2 29.8 66.4 31.0 (22.3) 88.2 5.2 Albion, Jharia 1.4 6.8 23.5 68.3 25.0 (18.7) 89.5 4.9 New South Wales (U.K.) 1.0 2.9 93.5 3.4
" Samples 3 and 4 are hand-picked vitrains and the rest are largely bright samples. Figures in parentheses represent volatile matter determined at 600 "C. CInthis and other tables M = moisture, A = ash, V = volatile matter, and F = fixed carbon.
coal sample no. 1
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4
5
Table 11. S t r u c t u r a l Parameters and Pyrolytic Features of t h e Coals C distribn" hydro% C retainedb in 600 "C coke/char rank of coal, aromaticity aromaticity normal pyrolysis pyrolysis in % C (dmf) fa fhnr f a + fha? (VM test) presence of Sc 80.5 0.67 0.25 0.92 72 (i) 91 (ii) 92 82.5 0.68 0.24 0.92 73 (i) 92 (ii) 93 88.2 0.78 0.16 0.94 0.80 (i) 93 (ii) 94 89.5 0.83 0.12 0.95 0.84 (i) 95 (ii) 96 93.5 0.97 nil 0.97 97
% C delivered in gas (600 oC)d 7.5
7.6 6.3 4.9
a Typical values in the light of previous estimatesz1made on more or less equivalent rank and type of coals. Determined' by heating 1 g of coal, in the presence or absence of sulfur, at 600 "C for 7 min in a standard VM crucible. "he first figures in each set were obtained in the presence of a minimum proportion of sulfur' whereas the second set was obtained under fully dehydrogenating conditions (Le. with excess sulfur); also see Mazumdar et al.16~zoDirectly estimated from the volume and the composition of the Gray-King assay gas.
Table 111. Gray-King Assay (600 "C)with or without Zinc Chloride Infusion" coal sample no.
la 1b 2a
2b
rank, % C (dmf) 80.5 89.5
% yield on dmf basis
% ZnCl
impregnatedb nil 20 nil 20
rate and manner of heating 5 "C mi&, inserted at 300 "C 5 "C mi&, inserted at room temp as in l a as in Ib
coke/char 67.8 (71.4) 82.1 (101.6) 84.lC(77.6) 91.6 (105.3)
tar 15.5 trace 8.0 nil
liquid gas (by diff) 7.0 9.7 7.6 10.3 0.9 7.0 1.3 7.1
% yield of extra char
14.3 7.5
"Figures in parentheses represent % dry yield of coke/char. Dmf yields were computed by correcting for mineral matter and/or residual ZnClz. Most of the ZnClz used was found to retained in the char. bThe infusion was done from an aqueous solution of ZnClZ. A wellkneaded mixture of coal and ZnClz solution was charged in the G.K. assay retort, keeping it slightly inclined with the front end up, while the mixture was dried in situ by heating from room temperature to 110 "C over the course of ll/zh in a Nzatmosphere after which the normal procedure was followed. Except for this, the rest were all char.
containing some NaC1) at 120-130 OC was observed as early aa 1978-1979; this parallelism was briefly reportedIe as unpublished work. The foregoing preliminary work, as well as the interpretation of the entire reaction and the pyrolysis behavior of the coal has only recently been extended and consolidated with additional experiments; the results are presented here.
Experimental Section Five samples of mal, includjng an anthracite ( C 80-93%,dmf), were chosen for the study. Of the four bituminous coals, two were hand-picked vitraina and the other two were overall seam samples that which were essentially bright in nature. In Table I, the proximate and elementary analyses of the samples are presented. In addition, volatile matter and fixed carbon determined at 600 O C for all the samples is also presented because such data are deemed1*Jeto be structurally more significant, especially in regard to the broad of carbon (in the form of aromatic, (17) Ghosh, G. Ph.D. Thesis, Jadavpur University, Calcutta, 1984.
(18) Mazumdar, B. K.; Chakrabartty,S. K.; Lahiri, A. Proceedings of the Syn'2pO8iUn'2on the Nature of Coal; Central Fuel Research Institute Jealgora, India, 1969; p 253. (19) van Krevelen, D. W.; Wolfs,P. M. J.; Waterman,H. I. Brennst. Chem. 1959,40, 371. (20) Mazumdar, B. K.; Sanyal, P. K.; Ganguly, 5.;Lahiri, A. Adu. Chem. Ser. 1966, No. 55,475.
hydroaromatic, and aliphatic) in three primary products of pyrolysis. Table I1 shows such broad distribution in terms of coke/char, tar,and gas yields at 600 "C. Furthermore, to complete the characterization, the values of carbon aromaticity (fa) and hydroaromaticity (fh) for the samples are also included in Table 11. These values were not actually determined but are considered the moat probable and typical values in the light of previous work and estimatesz1 on coals of similar rank and type. Earlier, it was observed7 that under partial or fully dehydrogenating conditions imposed during pyrolysis (600 "C),both aromatic and hydroaromatic carbon fractions appear to be retained together in char. As these data are also likely to be relevant to the interpretation of the coal-zinc chloride reaction, the same characterization was performed with the present samples of coal in the manner described7 earlier, and these results are also given in Table 11. With the coal samples thus characterized in detail, the following series of investigations on coal-zinc chloride reactions and/or pyrolysis were conducted by isolating the treatments for different temperature regimes with varying proportions of ZnClz (from 5 to 25% by weight of coal). A. The Gray-King assay at 600 "C and with ZnClz impregnation (20% ZnC1,) was performed on two of the five samples in the usual manner by heating a t the rate of 5 "C m i d , except for the insertion of the retort right from room temperature instead of 300 "C (as is normally done). This variation was performed
(21) Mazumdar, B. K.; Lahiri, A. Fuel Sci. Technol. 1983,2, 45.
Mazumdar et al.
226 Energy & Fuels, Vol. 2, No. 2, 1988
Table IV. Reaction between Coal and Zinc Chloride at and u p to 360 "C Indicating t h e Nature of t h e Reaction and Its Subsequent Impact on Pyrolytic Behavior
coal sample no. 1
rank, % C (dmfl
(A) Particulars of Treatment and Yield of Products particulars of treatmentb rate of heating, time of "C/min from heating at 350 room temp max temp, "C "C, h
80.5
5
350
2
5
350
2
blanka 4
89.5
blank"
sample no.
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1 4
% yieldCof treated coal (acid-washed) dmf basis
vol of gasd formed, cm3 NTP/100 g of dmf coal
97.2 99.5 98.1 99.7
2000 315 1200 70
(B) Elementarv Balance and Pvrolvtic Behavior of the Coals before and after the Reaction pyrolysis behavior elem anal. before and after proximate original coal, treated coal, % dmf % dmf original coal treated coal C H C H H removed: % V F V F 80.5 89.5
5.4 4.9
83.0 89.0
4.8 4.5
11.1 8.2
41.9 25.0
58.1 75.0
23.6 16.0
76.4 84.0
A ^^"^""
-
UCt,lCillDC
VM
:" 1'1
%
18.3 9.0
"Experiment without ZnClz. *Zinc chloride, 20% by weight of coal, impregnated as before (Table 111). cMost of the zinc chloride remained in the treated coal, which was washed with acid and water to get the ZnCl,-free yield. d A significant proportion of gas is formed in the case of the ZnC12-impregnated coals compared to the "blank" treatment; 40-50% of this gas was found to be hydrogen whereas there was little or no hydrogen in the small volume of gas derived from the "blank" experiments. 'If the material balance is taken into consideration the overall loss of hydrogen would correspond to about 13.7 and 9.0%, respectively, for the two coals as above. fThis corresponds to the respective yields of tar from the untreated coals. In fact, G . K. assay on the treated coals indicated complete suppression of tar formation.
to allow sufficient time for a reaction, if any, well below the onset of active pyrolysis. In Table 111, the comparative pyrolytic behavior of coal, with or without ZnC12, is presented. B. Another set of experiments in the Gray-King assay retort, with or without ZnClz,was also conducted on the same two coals as above, restricting the heating up to 350 "C and maintaining it at this temperature for 2 h. The same rate of heating (e.g., 5 OC min-') was adopted right from room temperature. T h e gas formed, if any, was collected as usual and analyzed for its composition. After the completion of the reaction, the treated coal was discharged and freed of zinc chloride by mild boiling with HCl (1:1), followed by thorough washing with water. In Table IVA, the yields for treated coals and the volume of gas generated in respect to the two coals studied are presented. Analyses, as well as the material and elementary balance of the samples before and after the treatment with ZnC12 at and up to 350 "C, are presented in Table IVB. C. The results of the above experiments immediately indicated that the coal-zinc chloride reaction could well be analogous to Scholl's reaction, which was performed earlier on coal^.'"'^ Accordingly, a similar experimental procedure was designed for such studies. Briefly, this experiment consists of treating about 5 g of coal in a ZnClz-NaC1 melt at about 240 "C in an open silica crucible for ca. 15 min (the melting point of ZnC12is about 283.7 OC, which is lowered by adding some NaCl). Coal was added to the melt in small amounts while thoroughly mixing it with the fused ZnC1, (+NaCl) and keeping the melt efficiently agitated throughout the treatment. After completion of this step, the treated coal was freed of ZnC12and NaCl by washing with acid and water. In Table VA, the yields and analyses (carbon and hydrogen contents, as well as voltile matter at 600 "C) of the reacted coals for all five samples (including the anthracite) are presented. The degree of "dehydrogenation" as computed from the elementary balance, as well as the decrease in volatile matter content, which is dependent upon the action, is also given in Table VA. In order to highlight the anticipated parallelism, one of the coals (e.g., coal no. 3, Table I) was specifically studied for Scholl's reaction (reaction with fused AlCl,) a t 120-130 "C in the manner described'"" earlier. The results of these comparative studies, as well as the changes in pyrolytic behavior resulting from the respective reactions (e.g., with molten ZnC12 and AlCl,) are presented in Table VB. A similar phenomenon was previously observed with S dehydrogenation.' A partial (but critical) degree of dehydrogenation using a predetermined proportion of sulfur was found to cause virtually identical changes in the subsequent pyrolytic behavior
as have now been found in the case of both ZnC1,- and AlC1,treated coals. Thus,to bring out the parallelisms among the three reactions, the same selective S dehydrogenation followed by pyrolysis was also performed on coal no. 3, and these results are also included in Table VB. D. Finally, one more set of experiments was performed by using the VM crucible as the reactor for an in situ reaction with pyrolysis of ZnC12-impregnated coal. (This simple VM test7 was earlier found to be very effective in coal-sulfur reaction2 studies). The procedure followed in such experiments was as follows: A 1-g sample of coal (accurately weighed) was thoroughly mixed with a given amount of zinc chloride (proportion was varied from 5 to 25%) in the VM crucible. The mixture was preheated at 350 "C for 30 min (in an air oven) with the lid on, following which the crucible was inserted in the Vh4 furnace,which was maintained at 600 "C for 7 min in the same manner used when volatile matter is determined for coal (either at 600 "C or 900 "C). The above studies were made with only two coals (coals no. 2 and 4, Table I). Most of the zinc chloride used was found to be retained as such along with the fixed carbon (i.e., char); the distillation temperature of ZnC12is around 700 "C. The true yield of fixed carbon (and, hence, the volatiles) was computed by correcting for the residual zinc salts via the ash balance and the determination of zinc and chlorides, in some cases. In Table VI, the reconstituted proximate analyses of ZnC12-impregnatedcoals on a dmf basis (corrected for both mineral matter and extraneous Zn salts) are presented along with the normal proximate analyses (600 "C) of the two coals studied.
Results and Discussion Before an interpretation of the coal-zinc chloride react i o n and i t s i m p a c t on coal pyrolysis is attempted, i t is useful to examine the results closely (Tables 111-VI) and highlight the major features. (i) In Table 111, the results of pyrolysis (600 "C) of ZnC&-impregnatedcoal fully confirm and supplement the previous findings and observations.1-4 The complete suppression of tar formation and the concomitant increase in char yields with both coals studied (with little or marginal effects on the yield b y weight of liquid and gas) bear a strong resemblance to what was previously observed by the present authors and c o - w o r k e r ~in~the ~ case of either coal-sulfur or coal-iodine reactions. (ii) The substantial decrease i n volatile matter after ZnC1, impregnation (e.g., but about 1 5 % ) , as originally
Coal-Zinc Chloride Reaction
Energy & Fuels, Vol. 2, No. 2, 1988 227 Table V
(A) Action of Fused Zinc Chloride (Containing Some NaCl) on Coal at 240 OC and Its Effect on Coal Composition and Pyrolytic Behavior'
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sample no. 1 2 3 4 5
rank, % C (dmfl 80.5 82.5 88.2 89.5 93.5c
coal-ZnCL reacn at 240 OC anal. treated coal," dmf % yield of VM at treated coal 600 OC C 98.1 16.5 (23.5) 82.0 83.5 98.4 14.8 98.0 11.5 (19.8) 88.3 9.6 (15.0) 88.6 99.0 92.8 99.6 2.3
%
H 4.8 4.8 4.7 4.5 3.5
anal. original coal: % dmf VM at % of total carbon 600 OC C retainedin chaP 34.6 (41.9) 80.5 5.4 32.1 82.5 5.4 22.3 (31.0) 88.2 5.2 18.7 (25.0) 89.5 4.9 93.5 3.5 2.4
decrease in VMcontent," % 18.1 (18.4) 17.3 10.8 (11.2) 9.1 (10.0) nil
% ofH removed 11.1 11.1 9.6 8.2 nil
(B) Parallelisms among Scholl's Reaction and Sulfur-Dehydrogenation (Partial) Showing the Zinc Chloride Reaction Common on Coal and Its Pyrolytic Behavior (Indicated with Reference to Sample 3-a Hand-Picked Vitrain) pyrolysis behavior elem anal. of (600 "C) % of total % yield treated coal, % proximate carbon dmf of % of H anal**% dmf retained in S removed product C H V F chare nature of treatment 88.3 4.7 0.5 9.6 11.5 88.3 92.7 (A) fused ZnClz reaction at 240 OC 98.0 87.6 4.7 0.5 9.6 (B) Scholl's reaction (AlClJ at 120-130 OC 98.7 11.1 88.9 93.0 99.4 88.5 4.8 0.8 8.0 10.8 89.1 (C) sulfur reactiond at 350 OC 94.0 (with predetermined proportions of sulfur) 'Figures within parentheses represent values for volatile matter (dmf) determined at 900 "C. Washed and freed of ZnC1, before analysis. 'Anthracite. dThisis selective dehydrogenation with the minimum proportion of sulfur for maximum retentionl6a of hydroaromatic carbon during subsequent pyrolysis. Sulfur used was 8 g/100 of g dmf coal. The reaction was conveniently done in a G.K.assay retort at and up to 350 OC by heating an intimate mixture of coal and sulfur at the rate of 5 OC/min and finally maintaining the temperature at 350 "C for 1 h. e Significantly this amounts to the s u m of aromatic and hydroaromatic carbon fractions of the coal (Table I), equivalence of which was earlier pointed" out.
Table VI. Effect of Zinc Chloride on the Pyrolytic Behavior of Coal, Shown by the Simple Volatile Matter Test at 600 OC
coal mule no. 2
expt
rank, % C (dmf) 80.5
a b C
4
89.5 a b Cd
level of ZnCl, impregnated,' %
nil 5
16 25 nil 6 15
materials charged in VM coal, g ZnC12, g 1.00 1.00 0.0486
1.00 1.00 1.00 1.00 1.00 1.00
0.1590 0.2500 0.0654 0.1500
yield of char/coke (dry),
proximate % g anal*: % decrease dmf manner of pretreatment corr. for in and pyrolysis (600"C) gross ZnClZb F V volatiles normal pyrolysis 0.6701 68.0 32 preheated at 350 "C (l/* 0.7972 0.7486 77.7 22.3 9.7 h) followed by pyrolysis 16.6 0.9044 0.8050 84.6 15.4 1.0070 0.8051 84.5 15.5 16.5 normal pyrolysis 0.8097 81.4 18.6 a s i n expta 0.9700 0.9046 91.8 8.2 10.4 1.0600 0.9100 92.4 7.6 11.0 1.0594 0.9080 92.2 7.8 10.8
% of carbon retained in fixed carbon 72.1 86.8
92.0 92.1 84.0 93.5 94.1 94.2
'Dry mixing of coal and zinc chloride powder in the volatile matter crucible itself prior to treatment and VM determination. b65-100% of the zinc chloride was found to be retained at 600 OC in the char, depending on the rank of the coal and the amount used in the experiments. Correction for such amounts was done on the basis of ash balance and/or analysis of zinc salts in some cases. Reconstituted from corrected char yield. Repeat.
observed by Bodily et al. in the case of a low-rank bituminous coal (Utahcoal), is primarily due to almost complete inhibition of tar formation. This inhibition adds to the yield of char, obviously at the expense of tar-forming bodies (see Table 111). At this stage, it is pertinent to know the stoichiometry of the reaction, if any. Bodily et al.u had found that, with an increasing infusion of ZnCla (from 5 to 25%), the maximum stabilization of volatiles (or, for that matter, 100%suppression of tar formation) had occurred only at about 20% loading of ZnClz during the thermogravimetric analysis at 10 OC mi+. Interestingly, during the G.K. assay with the same 20% ZnClz impregnation, virtually the same results seemed to have been obtained with the Indian coal of equivalent rank (Table 111, sample no. 1). Nonetheless, no such stoichiometry for the minimum proportion of ZnClz necessary for maximum effects on coal pyrolysis
would appear to exist. This fact became evident later when simple but analogous pyrolysis tests in a standard VM crucible were done with varying proportions of ZnClz (from 5 to 25%, Table VI). It will be seen that for a nearly equivalent rank of coal (sample no. 1;C, 80.5%)with 5% ZnC12-incorporation,volatile matter decreased by about lo%, which, by implication, corresponds to about 60% inhibition of tar-formation versus 100%completion of the effect with 15% ZnClz (Table VI). Thus, there does not appear to exist any firm proportionality between the reactants and their effect. The efficiency of the reaction and, hence, the subsequent effect appears to depend on a number of other factors as well: (1) the manner of mixing, (2) the rate of heating, and, more importantly, (3) the time of "soakingn below pyrolysis conditions. It appears, therefore, that the profound change in pyrolytic behavior of coal is not an in situ effect of zinc chloride on
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228 Energy & Fuels, Vol. 2, No. 2, 1988
coal pyrolsysis but is perhaps foreshadowed by what happens in the early stages of the reaction (Le., below the active pyrolysis point of a coal). In fact, a greater efficiency with a lesser proportion of ZnCl,, as observed in the present work, can well be ascribed to a longer time of reaction allowed below the active pyrolysis zone proper. In fact, this procedure was used with the simple VM tests (Table VI) wherein the crucible containing the coal-zinc chloride mixture was heated a t and up to 350 "C for 1 / 2 h, prior to subjecting the mixture to pyrolysis at 600 "C. By implication, the above contention would also appear to be borne out from a comparative study of the results of Kandiyoti et al.13 of the Gray-King assay (600 "C) performed on a British low-rank bituminous coal with 5% ZnCl,. In this case, the decrease in tar yield was found to be barley 14% of the total, whereas the present authors' results for a similar rank of coal (Table VI), with the same loading of ZnCl,, indicate about 60% suppression of tarformation. In a normal G.K. assay, retort is inserted at 300 "C (as was done by Kandiyoti et al.), whereas in the present work this was heated right from room tempeature, thus deliberately allowing a substantially longer time for reaction a t lower temperatures. (iii) The foregoing conjecture seems to have been correctly and directly borne out by having the reaction confined at and up to 350 "C (which largely avoids imposition of pyrolytic conditions, a t least in the bituminous range of coals). Some remarkable results have emerged (Table IV), as summarized below. (a) A substantial volume of gas was generated from both the lower and higher rank ZnC1,-impregnated coals (Table IVA), versus a much smaller volume from the corresponding untreated coals. Significantly, such gases were found to contain 40-50% hydrogen in sharp contrast to little or no hydrogen in gas from the untreated coals (Table IVA), which confirms earlier observations by Bodily et al. (e.g., that ZnC1, initiates some dehydrogenation well below 400 "C). (b) The analysis and material balance of ZnC12-free treated coals directly indicated a loss of hydrogen from both the higher and lower rank bituminous coal (C, 80.5 and 89.5) at 11and 8% respectively; the loss of carbon was minimal. ( c ) As a result of the above reaction, which seems to be selective, (to be discussed presently), there occurred a dramatic effect on the subsequent pyrolytic behavior, as indicated by proximate analysis. The volatility decreased by as much as 18 and 9%, respectively, for the lower and high-rank coals. The G.K. assay (600 "C), which was performed separately on the same two treated coals, indicated almost complete inhibition of tar formation. Furthermore, Table IVA shows that the increase in the yields of char, in the case of ZnC1,-treated coals, nearly corresponds to the normal yields of primary tar obtainable by a G.K. assay of the corresponding untreated coals (Table 111). The above findings indicate that the profound changes in pyrolytic behavior of coal are obviously due to some characteristic reaction initiated and completed by ZnCl, well below the pyrolysis point of coal rather than a n in situ reaction or aftereffect of the reaction on coal pyrolysis. (iv) In fact, recognition of the above phenomenon prompted a study of the coal-zinc chloride reaction at still lower temperatures (e.g., the action of fused ZnCl,, containing some NaCI, a t about 240 "C or so) as an analogue to Scholl's reaction on coal studied1"17 earlier at 120-130 "C. The comparative results (Table V) indicate a complete
Matumdar et al. parallelism between the two reactions, not only in the cause of the same critical degree of dehydrogenation (&11%of total hydrogen) but also in their effect on the pyrolytic behavior of such treated coal (e.g., complete destruction of coking property, if any, of the starting coal and almost complete inhibition of tar formation with a corresponding increase in char yield). Virtually a similar phenomenon had been originally observed with sulfur7or halogen8dehydrogenation. Complete or even partial15 dehydrogenation (ca. one-third of the total dehydrogenation possible), prior to pyrolysis, had been found to cause the same changes in pyrolytic behavior, both qualitatively and quantitatively speaking, as have now been observed in the case of metal halide reacted coals. In Table VB,such a remarkable parallelism among the effects of three reactions are illustrated with respect to one of the five coals studied in the present work. I t is worth noting that there was no impact on the pyrolytic behavior of the anthracite sample with the zinc chloride reaction (Table VA). The same result was also found17 earlier with Scholl's reaction or the S reaction. This fact is significant as anthracite does not produce any tar;neither does it have any hydroaromatic structure.1° (v) Finally, one more interesting feature of the effect of zinc chloride on coal pyrolysis should be highlighted-the retention of almost a constant proportion of carbon in char (600 "C) (e.g., about 92-95% of the total), irrespective of the rank of coal so treated, at least in the bituminous range, with prior treatment with ZnCl, (at 240-350 "C) and/or during in situ pyrolysis of ZnC1,-impregnated coal in one stroke or in two stages (see Table VI). This remarkable behavior was originally discovered in the case 4 ~ 11)and of S- or halogen-dehydrogenated c 0 a l s ~ J ~ P(Table later was also found16to be true for A1C13-reacted coals. This repetitive observation of the same phenomenon is very significant in regard to coal structure, bearing in mind that while both aromaticity and hydroaromaticity vary with rank, the total of the two, however, appears to remain constant at the level of 92-95% of the total carbon in coal (C, 80-92%, dmf).,O In a discussion of the mechanism of the coal-zinc chloride reaction and its implication an the nature of the structure of coal, the following crucial facts need to be considered: 1. The action of ZnCl, at temperatures below pyrolysis conditions remains at 8-11% loss of hydrogen, without any significant loss of carbon. 2. The reaction is a complete analogue of Scholl's reaction at 120-130 "C. 3. The above "dehydrogenation" appears to be specific and amounts to ca. one-third of the total dehydrogenation possible with dehydrogenating agents like sulfur, selenium, or halogens or by catalytic dehydrogenation using Pd. 4. The same specific dehydrogenation caused by partial dehydrogenation with a predetermined proportion of sulfur or iodine apparently precipitates the same reorganization in the aromatic-hydroaromatic skeletal structure, leading to the stabilization and retention of both types of carbon in char during subsequent pyrolysis as have been found for metal halide-reacted coals (see Figure 1, reproduced from earlier work15). In view of the remarkable findings in Figure 1 (later extended and supported by parallel studied6on a freshly created hydroaromatic structure), it was p o ~ t u l a t e d l ~ . ~ ~ earlier that a hydroaromatic structure in coal is perhaps not fused with the aromatic nuclei of coal (as believed by (22) Mazumdar, B. K.; Ghosh, G. Fuel Sci. Technol. 1984, 3,3. (23) Mazumdar, B. K.; Chatterjee, N. N. Fuel 1973, 52, 11.
Coal-Zinc Chloride Reaction A 100
Energy & Fuels, Vol. 2, No. 2, 1988 229
1
IE
I-
~
,o l
_ 0
I 10
I
I
-
I
l
_
I .A
2 0 30 40 90 60 70 BO 90 100 1 1 0 I20
SULPHUR Gm.USED/IOO Gm UNIT COAL
Figure 1. Inhibition of tar formation and fixation of additional carbon in char consequent of varying degrees of dehydrogenation. Dotted portion of the line indicates the region where dehydrogenation is complete. (Reproduced with permission from Mazumder et al.16 Copyright 1962 Butterworth & Co.)
possibly confers complete immunity to coke/char to any organic substitution rea~tions).~'Thus the differential VM cannot be explained in terms of cross-linking theory, even if it occurs to an extent by ZnC1, reactions at subpyrolysis temperatures. (b) The decrease in VM seems to be exclusively due to suppression of tar formation or, for that matter, the retention of hydroaromatic carbon in char. These appear to be quantitatively related (Tables 111-VI). (c) Thus, the condensation reaction between aromatic and hydroaromatic moieties, as visualized in the authors' concept, seems to be intra (within the structural "units") and not inter (i.e., between such "units"). (d) Additionally, the complete identity of the ZnCl, reaction with Scholl's reaction (AlCl,) at 120 "C would appear to weigh in favor of a cyclodehydrogenation/cyclization type of reaction. In fact, espousal of such a concept can also reasonably explain the systematic and progressive growth of both aromaticity and aromatic ring size during coal metamorphosis.15 Finally, to reinforce the interpretation presented here, the phenomenal increase in the yield of tar, instead of a decrease, during flash pyrolysis of ZnC1,-impregnated coal, as was observed by Kandiyoti et al., requires an explanation. The present authors have found that the inhibitory effect of ZnClz is relevant only when there occurs a sustained reaction below 350 "C (i.e., below the active decomposition temperature of coal). The progress and/or completion of the reaction (i.e., the cyclization reaction as shown in Figure 2) can only be ensured by either much slower rates of heating than 5 "C min-' or by the "soaking" of the coal-zinc chloride mixture below 350 "C for a longer period than what is obtainable with the normal Gray-King assay. No inhibitory effect on tar formation was observed during flash heating of ZnClz-impregnated coal; since the rate of heating was ca. 1000 "C s-l, the time of contact below 350 "C must have been a split second. On the other hand, it is well-knownz3that if the rates of heating are rapid or if the time of residence of the primary volatiles within the incipiently developing coke (or char) mass is minimized (to prevent interaction between the two), tar yields are substantially augmented.
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