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Energy & Fuels 2004, 18, 1140-1148
Effect of Rapid Preheating on the Coking Behavior of Bituminous Coals: Retrogressive Reactions and Extract Yields Koichi Fukuda,† Denis D. Dugwell, Alan A. Herod, and Rafael Kandiyoti* Department of Chemical Engineering and Chemical Technology, Imperial College London, London SW7 2AZ, United Kingdom Received March 21, 2004
Mechanisms underlying observed changes in the plastic properties and swelling ability of weakly coking coals, induced by rapid heating to 400-450 °C, have been investigated. The enhanced softening would allow the shifting of coke blend compositions toward weakly coking coals. A surge in extractables content was observed at heating rates of >500 °C/s. After rapid heating, 25%28% more of the mass of the weakly coking coal could be extracted. At these low temperatures (∼400-450 °C), repolymerization rates of solvent-extractables seem to be negligible. Increases in the extractables content that were determined by rapid heating clearly improve coke strengths via the related increase in thermoplasticity. However, when a “prime coking coal” was tested under similar conditions, no measurable effect of heating rate on the extractable contents was observed. A third “coking” coal showed behavior that was intermediate between these two cases. Taken together, the data strongly suggest the sequential occurrence of (i) fast retrogressive recombinations between reactive free radicals formed during thermally induced covalent bond cleavage reactions during the heat-up process, and (ii) slower repolymerization/recombination reactions occurring between the more-stable free radicals, observable at >450 °C. The example of the prime coking coal suggests that more of the reactive free radicals might be stabilized (quenched) by locally available hydrogen, irrespective of the heating rate. Liquefaction in the presence of hydrogen donors shows that high conversions to extractables are possible during slow heating. However, for transitional coals (between weakly coking coal and prime coking coals), pyrolysis yields are clearly sensitive to changes in heating rate. Transitional coals thus seem to be those that are marginally deficient in donatable hydrogen.
1. Introduction The plastic properties of heated coals seem to be closely related to their instantaneous extractable contents. Early work by Brown and Waters1,2 showed how chloroform extractable inventories of coal particles related to their softening and agglomerating behavior. Using a melting coal (Pittsburgh No. 8), Howard and co-workers observed relationships between the duration of plasticity (at a given temperature), the extractable contents of the particles (at that temperature), and the temperature itself. They correlated the plasticity of a rapidly heated coal with the generation-destruction kinetics of (pyridine-soluble) extract within the particles.3,4 On the other hand, some bituminous coals that remain morphologically unchanged when heated slowly (∼1 °C/s) have nonetheless been observed to soften and melt under rapid heating conditions (e.g., 1000 °C/s).5-7 * Author to whom correspondence should be addressed. E-mail address:
[email protected]. † Present address: Ironmaking Research Lab., Nippon Steel Corporation, 20-1 Shintomi Futtsu, Chiba 293-8511, Japan. (1) Brown, H. R.; Waters, P. L. Fuel 1966, 45, 17-39. (2) Brown, H. R.; Waters, P. L. Fuel 1966, 45, 41-59. (3) Fong, W. S.; Peters, W. A.; Howard, J. B. Fuel 1986, 65, 251. (4) Fong, W. S.; Khalil, Y. F.; Peters, W. A.; Howard, J. B. Fuel 1986, 65, 195. (5) Gibbins-Matham, J. R.; Kandiyoti, R. Energy Fuels 1988, 2, 505. (6) Hamilton, L. H. Fuel 1980, 59, 112.
Research on the making of coke is now attempting to take advantage of such observations, to reduce the proportion of “prime coking coals” in coke blends. These coals are usually more expensive and are becoming less available. Therefore, the shifting the coke blend composition toward weakly coking coals is commercially attractive. One process concept involves the rapid heating of coal particles to ∼400 °C prior to their injection into modified coke ovens, for conventional treatment from that point onward. More recently, Aramaki et al.8 confirmed the increased strength of resultant cokes, when weakly coking coals are “preheated” rapidly to temperatures of 450 °C. It is not difficult to visualize that reactive free radicals form during bond scission processes, at temperatures upward of 310-320 °C, and that where possible, these free radicals would undergo rapid recombination reactions. These reactions seem to be completed by the time the sample reaches 400 °C. We can thus explain the results in Figures 4-6 and 8 in terms of rapid recombination reactions, which are better able to proceed during slow heating (1 °C/s), compared to fast heating (∼1000 °C/s). However, we still need to explain why rapid retrogressive reactions of extractable materials are expected to be less intense during rapid heating. One likely set of answers requires a review of the ideas on chemical mechanisms underlying plasticity in coals. However, it would be useful to examine, first, the (26) Gibbins, J. R.; Kimber, G.; Gaines, A. F.; Kandiyoti, R. Fuel 1991, 70, 380-385
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Figure 9. Effect of hold time on volatile yields for heating at a rate of (O) 1 and (×) 1000 °C/s to 400 °C (Coal B).
Figure 10. Comparison of NMP-extractable yield between rapid400+slow and slow heating of Coal B.
behavior of a prime coking coal (Coal B) under the same experimental conditions. 3.2. The Pyrolytic Behavior of Strongly Coking Coal B. Coal B was a prime coking coal with a volatile matter (VM) content of 24.1% and a maximum Gieseler fluidity of 3.0 (see Table 1). Overall, tar and volatile yields were lower for this higher rank coal, and the effect of changes in heating rate were barely discernible against a background of (1%-1.5% experimental scatter (Figure 9). For a holding time of 0 s at 400 °C, volatile yields from slow and rapid heating were low and similar (∼1%). Figure 9 also shows that devolatilization from Coal B is completed within 30 s. The difference in tar yields due to changes in the heating rate (not shown) was less than experimental scatter: ∼1%. Figure 10 presents extract yields from Coal B chars, obtained using the slow, rapid350+slow, and rapid400+slow heating programs (Table 4). As expected, extractables yields increased as the temperature increased, up to a maximum and then declined, reaching almost-total resolidification at ∼600 °C. Over 85% of the coal mass could be dissolved in NMP after heating to 400 °C. However, no measurable effect of heating rate on the extract yields was observed. Clearly, the internal processes of the coking coal did not allow the reduction of solvent-soluble material in the coal mass, observed during slow-heating experiments with Coal A. In terms of our evaluation of data on Coal A, the effects of rapid recombination reactions identified in the 320-400 °C interval were not observed. To attempt an explanation of differences in behavior between the two coals, we need to review some ideas on chemical mechanisms thought to underlie plasticity in coals. In developing ideas by Wiser27 and Brown and
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Table 4. Description of the Applied Heating Patterns for Coal B heating pattern “slow” heating “rapid400+slow” heating “rapid350” heating
description heating at a rate of 1 °C/s to peak temperatures of 400-500 °C, followed by a 120 s hold time at the peak temperature heating at a rate of 1000 °C/s to 400 °C, followed by a 120 s hold time, followed by heating at a rate of 1 °C/s to peak temperatures of 400-500 °C, followed by a 120 s hold time at the peak temperature heating at a rate of 1 °C/s to 350 °C, followed by a 120 s hold time at 350 °C
Waters,1,2 Neavel28 has summarized a consensus view on plasticity in coals as representing a transient hydrogen donor process. The coal itself was considered to supply the solvating and hydrogen-donating vehicle. In this view, hydrogen-donor ability is considered to reside in the hydroaromatic component, which is already present in (i.e., native to) the coal. During pyrolysis, the extent of softening and the magnitude of tar yields are perceived to be dependent on local hydrogen availability; this is considered to be “directly proportional” to hydroaromatic hydrogen content. (Presumably, the hydrogen would be in a more-reactive state than simple molecular hydrogen.) In this description, the pyrolytic process prior to the evaporation of tars may thus be viewed as a softening stage, almost an internal liquefaction step, where the image of hydrogen donation by hydroaromatic groups must owe something to what we know of liquefaction in tetralin. The plastic phase is thus considered to serve as a medium that is transmitting hydrogen, which, in turn, may quench and stabilize free radicals formed by covalent bond scission. We will thus assume, what now seems to be a reasonable, but unproven, propositionsthat internal hydrogen transfer from hydroaromatic species contributes significantly to the softening behavior of coals. This sequence can help explain the differences in behavior between Coal A and Coal B. Support for this line of thinking may be found in differences between the behavior of maceral concentrates. Table 5 compares changes in extract yields of chars from a vitrinite, an inertinite, and a liptinite concentrate, as a function of the heating rate. The properties of these samples have been discussed elsewhere.29 For present purposes, it suffices to note that only extractables yields from the liptinite concentrate are insensitive to changes in the heating rate. What we know of pyrolyzing liptinites suggests that they contain larger proportions of donatable hydrogen, in comparison to that of vitrinites and inertinites from the same coals.28,30,31 Fast heating has a tendency to telescope the sequence of events into a narrower time frame and shift the temperature scale upward. It seem consistent with the data in hand to argue that hydrogen (which is known to be released from pyrolyzing solids from ∼300 °C (e.g., cf. ref 21) would remain in contact with the pyrolyzing mass up to higher temperatures during fast heating. In particular, it is probable that this evolving hydrogen reacts with some of the internally released free radicals and blocks some of the char-forming recombination (27) Wiser, W. Fuel 1968, 47, 475. (28) Neavel, R. C. Coal Science, Vol. I; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic Press: New York, 1981; pp 1-19. (29) Fukuda, K.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Effect of Preheating on Pyrolytic Behaviours of Coal Macerals, paper in preparation. (30) Gaines, A. F.; Li, C.-Z.; Bartle, K. D.; Madrali, E. S.; Kandiyoti, R. Proc. Int. Conf. Coal Sci. 1991, 830-833. (31) Li, C.-Z.; Gaines, A. F.; Kandiyoti, R. Proc. Intl. Conf. Coal Sci. 1991, 508-511.
Table 5. NMP-Extractable Yields of Maceral Concentrates when Heated to 400°C heating rate (°C/s)
NMP-extractable yield (mass %) Vitrinite Concentrate
1 1000
8.4 13.7 Inertinite Concentrate
1 1000
7.2 13.1 Liptinite Concentrate
1 1000
17.3 16.0
reactions. Consistent with this view, we have seen how Coal A internally retained more extractable material during heating at a rate of 1000 °C/s, compared to 1 °C/s: ∼25%-28% more of the mass of coal seems to have survived as “extractables” in the case of fast heating. Within this postulated framework, reactive free radicals internally released by the coal would undergo immediate recombination reactions, unless quenched by whatever happens during fast heating. As explained, we believe that they are quenched by internally generated hydrogen and perhaps by light hydrocarbon free radicals. According to this model, the small but critical amount of hydrogen released would escape during slow heating. However, it may be noted from Figure 5 that the difference in extracts is observable from a holding time of 0 s onward. In other words, quenching reactions that have involved ∼25%-28% of the coal mass, and caused these extractables to remain solvent-soluble during the heat-up stage, must have been relatively rapid. Comparison of results from pyrolysis and liquefaction experiments on the same coals tends to support these ideas. Linby and Point of Ayr are among the coals that have shown the greatest sensitivity to heating rate during pyrolysis experiments in this laboratory.5,32 However, neither coal showed sensitivity to the heating rate during liquefaction in the hydrogen-donor solvent tetralin.33 Upon heating to 450 °C at a rate of 5 °C/s in the presence of an excess of hydrogen-donor solvent (tetralin), ∼80% of the “dry-ash-free” mass of Linby coal could be dissolved in tetralin.32 The corresponding figure for Point of Ayr coal25 was 82.5%. Retrogressive reactions observed during coal liquefaction in the presence of a non-hydrogen donor (1-methylnaphthalene) have been described elsewhere.26 The foregoing suggests that thermally released free radicals might be stabilized (quenched) by locally available hydrogen, irrespective of the heating rate. However, for transitional coals, pyrolysis yields are clearly sensitive to changes in heating rate. Transitional coals thus (32) Li, C.-Z.; Madrali, E. S.; Wu, F.; Xu, B.; Cai, H.-Y.; Gu¨ell, A. J.; Kandiyoti, R. Fuel 1994, 73, 851-865. (33) Gibbins, J. R.; Kandiyoti, R. Fuel Proc. Technol. 1990, 24, 237242.
Coking Behavior of Bituminous Coals
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Table 6. Description of the Applied Heating Patterns for Coal C heating pattern 1 °C/s heating 1000 °C/s heating “rapid400+slow” heating “slow” heating
description heating at a rate of 1 °C/s to peak temperatures of 400-600 °C, followed by a 120 s hold time at the peak temperature heating at a rate of 1000 °C/s to peak temperatures of 300-600 °C, followed by a 120 s hold time at the peak temperature heating at a rate of 1000 °C/s to 400 °C, followed by a 120 s hold time, followed by heating at a rate of 1 °C/s to peak temperatures of 400-600 °C, followed by a 120 s hold time at the peak temperature same as that for 1 °C/s heating described above
seem to be those that are marginally deficient in donatable hydrogen. However, it is well-known that coals do not always fall into neat categories. The third sample examined in this study is useful in highlighting how the behavior of coals conforms to simple rules only in extreme cases. 3.3. The Pyrolytic Behavior of Coking Coal C. Despite a VM content of 17.9% and a maximum Gieseler fluidity of 1.3, Coal C is considered to be a “good” coking coal, albeit with somewhat unusual properties. Its elemental carbon content (90.7%) was high, and its sulfur (0.15%) and oxygen (3.3%) contents were low. The hydrogen content (4.6%) was sufficient to cause melting and agglomerating behavior. Coal C was also observed to have a relatively high softening temperature (∼450 °C), compared to ∼400 °C for the two other samples in this study. The effect is possibly due to a high level of crosslinking in this high rank coal. The pyrolytic behavior of Coal C has been presented in detail elsewhere.12 Up to 400 °C, much of the volatiles seemed to be composed of tar; yields (∼4%) were not observed to change as a function of heating rate. Given that the onset of plasticity for Coal C occurred at the higher temperature of 450 °C, these low yields at 400 °C are not surprising. Volatile and tar yields were enhanced by fast heating when the coal was heated directly to 450 °C: ∼14% volatiles and ∼6% tar, compared to 6% and 3.5%, respectively, for heating at a rate of 1 °C/s. However, hold times as long as 400 s were required during fast heating experiments for volatile evolution to dissipate at 450 °C. Yields stabilized after ∼30 s during slow heating experiments. Overall, a small but measurable effect of heating rate could be observed for this unusual coal, from approximately its softening point onward. The long (∼400 s) holding times required at 450 °C for these effects to become apparent are consistent with slow depolymerization of a highly cross-linked coal. These effects were reflected in results from the extraction of chars from Coal C. Figure 11 presents extractable yields from heating at rates of 1 and 1000 °C/s directly to the target temperature. For 1 °C/s heating, the extractable yield at 400 °C was similar to that from untreated Coal C. Above 400 °C, the extractable yield traced a sharp maximum at ∼450 °C, with resolidification reaching completion somewhat above 600 °C. By contrast, when samples were heated at a rate of 1000 °C/s, the extractable yield remained similar to that of untreated Coal C up to 300 °C but increased sharply from 350 °C onward, reflecting the behavior of Coal A in Figure 6. The shape of the curve was similar to that obtained for heating at a rate of 1 °C/s in Figure 11, tracing a sharp maximum at ∼450 °C and declining rapidly at higher temperatures. However, extractable yields were enhanced by ∼30% of the coal mass, because of rapid heating. Almost-identical results were obtained12 when NMP-extractable yields
Figure 11. NMP-extractable yields of Coal C heated at a rate of (O) 1 and (×) 1000 °C/s.
from the rapid400+slow and slow heating programs (Table 6) were investigated. Taken together, the pyrolytic behavior of Coal C seemed to be intermediate between that of Coal A and Coal B, with the added peculiarity of a high softening temperature and the slow development of pyrolytic reactions at 450 °C. 4. Conclusions Mechanisms underlying observed changes in the plastic properties and swelling ability of weakly coking coals, induced by rapid heating, have been investigated. The effect would allow shifting the coke blend compositions toward weakly coking coals. The response of a weakly coking sample to changes in heating rate has been compared with those of two coking coals, under similar conditions. (1) A surge in extractables content was observed at heating rates above 500 °C/s. After rapid heating, 25%28% more of the mass of the weakly coking coal could be extracted. Up to ∼400 °C, weight loss by volatilization was far smaller than fractions of extractable material within the coal mass. At these low temperatures (∼400-450 °C), repolymerization rates of solventextractables seem negligible. Increases in extractables content, determined by rapid heating, clearly improve coke strengths via the related increase in thermoplasticity. (2) However, when a “prime coking coal” was tested under similar conditions, no measurable effect of heating rate on the extractables content was observed. A third “coking” coal exhibited behavior that was intermediate between these two cases. (3) Some bituminous coals remain morphologically unchanged when heated slowly, yet show melting behavior when heated rapidly. These coals also give greater tar yields during fast heating. In view of past work, it seems easier to visualize how excess extract-
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ables formed during rapid heating can be “explosively” ejected from coal particles at temperatures of >500 °C. The key problem is: how was excess extractable material formed and/or how did it survive during fast heating in the first place? (4) Populations of free radicals formed when a coal is heated are dependent on the temperature, irrespective of the heating rate. To explain the greater survival of extractables during fast heating, it is necessary to invoke the concept that internal hydrogen transfer from hydroaromatic species contributes significantly to the softening behavior of coals. The more-reactive free radicals internally released by the coal would undergo faster recombination reactions, unless quenched by internally generated hydrogen and perhaps by light hydrocarbon free radicals. Fast heating has a tendency to telescope the sequence of events into a narrower time frame and shift the temperature scale upward. Hydrogen that is known to be released from pyrolyzing solids from ∼300 °C would remain in more intimate contact with the pyrolyzing mass up to higher temperatures during fast heating. It would react with some of the internally released free radicals and block some of the char-forming recombination reactions. According to this model, more of the small but critical amount of hydrogen released by the pyrolyzing mass would escape during slow heating. For the weakly coking coal, the difference observed in extractables content seems to be due to
Fukuda et al.
more-intense recombination reactions during slow heating. Taken together, the data strongly suggest the sequential occurrence of (i) fast retrogressive recombinations between reactive free radicals formed during thermally induced covalent bond cleavage reactions during the heat-up stage, and (ii) slower repolymerization/recombination reactions occurring between the more-stable free radicals, observable at >450 °C. (5) The example of the prime coking coal suggests that more of the reactive free radicals might be stabilized (quenched) by locally available hydrogen, irrespective of the heating rate. Liquefaction in the presence of the good hydrogen donor tetralin shows that high conversions to extractables are possible during slow heating. However, for transitional coals, pyrolysis yields are clearly sensitive to changes in heating rate. Thus, transitional coals seem to be those that are marginally deficient in donatable hydrogen. Acknowledgment. The authors would like to thank Nippon Steel Corporation for funding this work and supplying the coal samples. K.F. would like to express his appreciation to Nippon Steel Corporation for giving him the opportunity to perform this study as a postgraduate student at Imperial College. EF0499271
