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Energy & Fuels 1997, 11, 236-244
In-Situ 1H NMR Investigation of Particle Size, Mild Oxidation, and Heating Regime Effects on Plasticity Development during Coal Carbonization† M. Mercedes Maroto-Valer, John M. Andre´sen, and Colin E. Snape* Department of Pure and Applied Chemistry, Thomas Graham Building, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, United Kingdom Received August 19, 1996. Revised Manuscript Received October 24, 1996X
High-temperature in-situ 1H NMR with a probe operating at a frequency of 100 MHz has been used to quantify the effects of particle size, mild oxidation, and different heating regimes on plasticity development for a low-volatile Australian bituminous coal in terms of the proportions of rigid and fluid material present. At the temperature of maximum fluidity, the fluid phase accounts for 35% of the hydrogen remaining, with both its concentration and mobility increasing up to this temperature. Reducing the particle size below ca. 150 µm suppresses plasticity through a reduction in the mobility of the fluid phase with the concentration of rigid material remaining constant. This effect is considerably more pronounced with slow heating than it is with fast heating (3-4 cf. 30 °C min-1). In contrast, suppressing the fluidity by mild oxidation reduces primarily the concentration of the fluid phase. Isothermal treatments give rise to a loss of fluidity due to reductions in both the proportion and mobility of the fluid component. The in-situ measurements have confirmed that plasticity development is a reversible phenomenon provided that relatively fast quenching rates (ca. 75 °C min-1) are used. These results are discussed in relation to estimating the contribution to fluidity development from the non-solvent-extractable material in coals. Heating coking coal in a tube furnace to the temperature of maximum fluidity followed by fairly rapid cooling is shown to be a simple procedure for recovering relatively large amounts of partially carbonized coal with the structural features responsible for maximum fluidity preserved.
Introduction The production of blast furnace coke continues to be a major process for coal utilization, with 11% of the total coal consumed worldwide in 1990 being used for coke making.1 The thermoplasticity of coals is a key phenomenon in high-temperature carbonization but, despite extensive investigation over the past 50 years, it cannot yet be rationalized quantitatively in structural terms. Essentially, two distinct hypotheses have emerged from the early classical and more recent studies summarized below to explain the phenomenon. The first considers that molecular constituents, which are mainly clathrated or occluded within coking coals and cannot readily be extracted prior to heating, are largely responsible for thermoplasticity. The second, which generally finds acceptance today among coal scientists, considers that there is a significant contribution to the fluid phase or “metaplast” from molecular species released by pyrolysis at relatively low temperatures (ca. 400 °C). As early as 1914, Harger2 found that for certain coals, after being shock heated to temperatures between 200 and 400 °C and cooled, the yields of extractable material in pyridine and benzene were higher than in the untreated coal. Illingworth3 observed that this phe†
This paper received the R. A. Glenn Award for the best paper presented in the Division of Fuel Chemistry program at the National Meeting of the American Chemical Society, Orlando, FL, Aug 1996. X Abstract published in Advance ACS Abstracts, December 15, 1996. (1) Scott, D. H. Developments Affecting Metallurgical Uses of Coal; IEACR/74; IEA Coal Research: London, 1994. (2) Harger, J. J. Soc Chem. Ind. 1914, 33, 389.
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nomenon only occurred for coking coals, and the highest extraction yield was reached at the temperature of maximum fluidity. In 1955, Dryden and Pankhurst4 found that chloroform was the solvent that gave the greatest increases in extraction yields upon shock heating and attributed the formation of the additional extract or metaplast to thermal decomposition. Fitzgerald5 and van Krevelen and co-workers6-8 studied the kinetics of the carbonization and also ascribed the formation of metaplast to primarily be responsible for the softening of the coal. In 1961, Dryden9 provided further support for the metaplast hypothesis from his study of chloroform extraction after shock heating and volatilization during vacuum carbonization, concluding that only relatively small amounts of chloroformextractable material were present as such in coking coals with a much larger amount formed by thermal decomposition. Roughly at the same time as the metaplast hypothesis was emerging, Ouchi formulated his “γ compound” hypothesis.10 After coal is preheated, low molecular weight or γ compounds, occluded in the network, are released and can be extracted in nonpolar solvents, particularly chloroform. These, rather than the low(3) Illingworth, S. R. Fuel 1922, 1, 213. (4) Dryden, I. G. C.; Pankhurst, K. S. Fuel 1955, 34, 363. (5) Fitzgerald, D. Fuel 1956, 35, 178. (6) van Krevelen, D. W.; Huntjensm F. J.; Dormans, H. N. M. Fuel 1956, 35, 462. (7) Chermin, H. A. G.; van Krevelen, D. W. Fuel 1957, 36, 85. (8) Fitzgerald, D.; van Krevelen Fuel 1959, 38, 17. (9) Dryden, I. G. C.; Joy, W. K. Fuel 1961, 40, 473. (10) Ouchi, K. Fuel 1961, 40, 485.
© 1997 American Chemical Society
Plasticity Development during Coal Carbonization
temperature pyrolysis products, were chiefly responsible for fluidity development of the plasticity and were an inherent component of the coal structure, instead of a primary. In 1965, Brown and Waters11,12 supported Ouchi’s theory with their studies on the characterization of the chloroform-extractable material. A close relationship was found between the extract yields and the accessibility of pores to fluids, and they concluded that the chloroform extracts were held physically in the coal. There is no disputing that solvent-extractable material plays a key role in fluidity development with many studies dating back to that by de Marsilly in 186013 having demonstrated that its removal destroys fluid properties. By the same token, aromatic additives, such as coal extracts, pitch, and individual polycyclic aromatic compounds, can improve fluid properties considerably. For example, using Gieseler fluidity measurements for a series of New Zealand coals, Clemens and Matheson14,15 reported that both decacyclene and solvent extracts can improve plasticity development. Fortin and Rouzaud used TEM to view the beneficial effects of N-methyl-2-pyrrolidone and boiling anthracene oil extracts on the formation of coke microtexture16,17 and related their findings to the plastic properties. To rationalize the role of solvent-extractable material present on the development of a potentially much larger pool of fluid material, Neavel and Marsh18,19 proposed a parallel between coal liquefaction and carbonization. The development of plasticity can be considered as a pseudoliquefaction process, in the sense that the extractable material acts as a hydrogen donor and transfer agent, stabilizing the unstable radical species produced by pyrolytic reaction pathways. The loss of fluidity in the resolidification temperature range was ascribed to a reduction in the concentration of transferable hydrogen. A similar picture has emerged for the carbonization of petroleum residues from the high-temperature electron spin resonance (ESR) studies by Yokono et al.,20 who found the stabilization of the free radical species formed by hydrogen transfer determine the formation of mesophase and anisotropic coke. In the absence of stabilization, the free radical species combine more readily and an isotropic coke structure develops. Coals have been cocarbonized with anthracene, and the amount of 9,10-dihydroanthracene generated has been used to give an indication of hydrogen transfer ability of the extractable phase.21 However, at the same time, Ouchi22 further developed his γ compound hypothesis, establishing a linear correlation between the amount of quinoline extractables (corrected for inertinite content) and the log of maximum fluidity measured by Gieseler plastometer. Contrary to the results reported by Neavel and Marsh,18,19 Larsen,23 and Seki,24 Ouchi (11) Brown, H. R.; Waters, P. L. Fuel 1966, 45, 17. (12) Brown, H. R.; Waters, P. L. Fuel 1966, 45, 41. (13) de Marsilly, C. Ann. Chim. Phys. 1860, 3, 66 and 167. (14) Clemens, A. H.; Matheson, T. W. Fuel 1992, 71, 193. (15) Clemens, A. H.; Matheson, T. W. Fuel 1995, 74, 57. (16) Fortin, F.; Rouzaud, J. N. Fuel 1993, 72, 245. (17) Fortin, F.; Rouzaud, J. N. Fuel 1994, 73, 795. (18) Marsh, H.; Neavel, R. C. Fuel 1980, 59, 511. (19) Neavel, R. C. In Coal Science; Academic Press: New York, 1982; Vol. 1, p 9. (20) Yokono, T.; Obara, T.; Sanada, Y.; Shimomura, S.; Imamura, T. Carbon 1986, 24, 29. (21) Yokono, T.; Marsh, H.; Yokono, M. Fuel 1981, 60, 607. (22) Ouchi, K.; Tanimoto, K.; Makabe, M.; Itoh, H. Fuel 1983, 62, 1227. (23) Larsen, J. W.; Sams, F. L.; Rodgers, B. R. Fuel 1980, 59, 666.
Energy & Fuels, Vol. 11, No. 1, 1997 237
contended that the pyridine and quinoline extracts have no hydrogen transfer properties.25,26 Thomas and co-workers studied the changes occurring in the macromolecular structure of coals during the carbonization process by the solvent swelling technique and constant shear rate plastometer.27-29 They presented evidence that the decomposition of the macromolecular structure close to the softening point was related to the rupture of cross-links. However, the relationship between thermoplasticity and changes in the macromolecular structure was found to be complex. Marzec and co-workers carried out field ionization mass spectrometry on a wide range of coals30-32 and classified the pyrolysates into two groups of compounds that primarily consisted of aromatic plus partially hydrogenated aromatic compounds and oxygen plus nitrogen species that gave positive and negative correlations, respectively, with both fluidity development and the amount of anisotropic coke formed. Although phenomenological models have been developed to rationalize plasticity, such as Spiro’s space-filling model33 and the empirical model of Solomon et al.34 based on van Krevelen’s hypothesis of metaplast generation,6-8 these do not provide an overall quantitative picture on the origin and generation of the fluid phase. The obvious problem with standard empirical tests, such as the Gieseler plastometer and Audibert-Arnu dilatometer to study the plasticity behavior of coking coals,35 is that they do not relate to the actual structural changes that occur. Of the in-situ analytical techniques available, high-temperature in-situ 1H NMR has proved to be the most successful for investigating the molecular motions of coals during carbonization. There are usually two contributions to the free induction decays (FIDs) of coals arising from mobile (faster relaxing) and rigid (slower relaxing) components, which display Lorentzian and Gaussian decays, respectively.36 Coal as a cross-linked macromolecular network37 gives rise to a substantial inert component that does not soften, and this produces the broad Gaussian peak in the 1H NMR spectra with a much narrower Lorentzian peak from the mobile material superimposed.36,38 Indeed, the peak width or spin-spin relaxation time (T2) of the fluid phase is highly responsive to changes in mobility.38 In the late 1970s, Sanada and co-workers conducted a high-temperature 1H NMR study on coal and pitches (24) Seki, H.; Kumagai, J.; Matsuda, M.; Ito, O.; Iino, M. Fuel 1990, 68, 978. (25) Ouchi, K.; Itoh, S.; Makabe, M.; Itoh, I. Proceedings, 1987 International Conference on Coal Science, Maastricht; Elsevier: Amsterdam, 1987; p 363. (26) Ouchi, K.; Itoh, S.; Makabe, M.; Itoh, I. Fuel 1989, 68, 735. (27) Chan, M.; Parkyns, N. D.; Thomas, K. M. Fuel 1991, 70, 447. (28) Butterfield, I. M.; Thomas, K. M. Fuel 1995, 74, 1780. (29) Nomura, S.; Thomas, K. M. Fuel 1996, 75, 801. (30) Schulten, H.-R.; Marzec, A.; Simmleit, N.; Muller, R. Energy Fuels 1989, 3, 481. (31) Schulten, H.-R.; Marzec, A.; Czajkowska, S. Energy Fuels 1992, 6, 103. (32) Marzec, A.; Czajkowska, S.; Schulten, H.-R. Energy Fuels 1994, 8, 360. (33) Spiro, C. L.; Kosky, P. G. Fuel 1982, 61, 1080. (34) Solomon, P. R.; Best, P. E.; Yu, Z. Z.; Charpenay, S. Energy Fuels 1992, 6, 143. (35) van Krevelen, D. W. Coal, 3rd revised ed.; Elsevier: Amsterdam, 1993; Chapters 23, 24. (36) Sakurovs, R.; Lynch, L. J.; Maher, T. P.; Banerjee, R. N. Energy Fuels 1987, 1, 167. (37) Green, T. K.; Larsen, J. W. Fuel 1984, 63, 138. (38) Parks, T. J.; Cross, L. F.; Lynch, L. J. Carbon 1991, 7, 921.
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at the early stages of carbonization.39 The development of plasticity in coals was monitored by changes in the half-width of the spectra. However, experimental factors prevented the spectra being deconvoluted to derive the proportion of inert material present. This limitation was overcome from about 1980 onward by Lynch and co-workers, who have referred to the technique as “proton magnetic resonance thermal analysis” (PMRTA).40-44 They have used mainly the empirical parameter, M2T16, corresponding to the spectrum truncated at a width of 16 kHz, for gauging changes in fluidity,42,44 where 16 kHz was found to be the optimum frequency range for this purpose. A number of facets of carbonization have been investigated by PMRTA. Of particular note is the quantification of interactive effects between different components in blends, with both pitch and decacyclene being found to have a much greater effect than predicted on improving fluidity.45 They have also studied the effects of oxidation on high-fluidity coals43 and the dichloromethane extracts and residues of a series of brown coals.46 In most of the studies with PMRTA, the overall concentrations of fluid and inert material over the thermoplastic range have not often being reported,36 possibly due to a combination of convenience for handling large data sets and the likelihood of truncating broad Gaussian signals at high temperatures where sensitivity is lowest. This has been achieved here using a high-temperature Doty NMR probe operating at a frequency of 100 MHz. The effects of particle size, mild oxidation, and different heating regimes on plasticity development for a low-volatile Australian bituminous coal are addressed in terms of the relative proportions of inert and fluid phases and the mobility of the latter. Surprisingly, very little attention has been paid to rationalizing the effect of particle size in fluidity development, which was recognized in earlier work as having a significant bearing.47,48 It is well-known that mild oxidation gives rise to a loss of fluidity. Huggins showed that at temperatures about 50 °C, the free swelling index and Gieseler fluidity both decreased.49 Larsen reported reductions in pyridine extraction yields and swelling ratios for oxidized coals,50 with the loss of fluidity being ascribed to the selective oxidation of transferable hydrogen groups, such as benzylic into carbonyl and carboxylic groups, a reaction that has been identified by FTIR.51 Previous work52,53 has shown that partially carbonized coals need to be quenched rapidly to regain (39) Miyazawa, K.; Yokono, T.; Sanada, Y. Carbon 1979, 17, 223. (40) Lynch L. J.; Webster, D. S. Am. Chem. Soc. Symp. Ser. 1983, No. 230, 353. (41) Lynch, L. J.; Webster, D. S.; Sakurovs, R.; Barton, W. A.; Maher, T. P. Fuel 1988, 67, 579. (42) Lynch, L. J.; Webster, D. S.; Barton, W. A. Adv. Magn. Reson. 1988, 12, 385. (43) Clemens, A. H.; Matheson, T. W.; Lynch, L. J.; Sakurovs, R. Fuel 1989, 68, 1162. (44) Sakurovs, R.; Lynch, L. J.; Barton, W. A. Adv. Chem. Ser. 1993, No. 229, 229. (45) Sakurovs, R.; Lynch, L. J. Fuel 1993, 72, 743. (46) Lynch, L. J.; Sakurovs, R.; Webster, D. S.; Redlich, P. J. Fuel 1988, 67, 1036. (47) Kreulen, D. J. W. Fuel 1950, 29, 112. (48) Ghosh, S. R.; Das Gupta, N. N.; Lahiri, A. J. Sci. Ind. Res. 1957, 16, 89. (49) Huggins, F. E.; Huffman, G. P.; Dunmyre, M. J.; Nardossi, M. J.; Lin, M. C. Fuel Process. Technol. 1987, 15, 233. (50) Larsen, J. W.; Doyoung, L.; Schmidt, Y.; Grint, A. Fuel 1986, 65, 595.
Maroto-Valer et al. Table 1. Elemental and Proximate Analyses, Vitrinite Reflectance, and Gieseler Plastometer Data for the Coals Investigated AUS-1 % C (daf) % H (daf) % N (daf) % O (daf)a VM (db) ash (db) R0 (max) Gieseler data Tsoftening (°C) Tmax fluid.b (°C) Tresolidif (°C) max fluid. (ddpm)
AUS-4
AUS-8
AUS-9
87.8 4.4 1.7 6.1 23.1 9.4 1.20
90.3 4.6 2.1 3.0 19.6 9.5 1.46
85.1 5.3 2.4 7.2 33.9 7.5 0.86
82.1 5.2 1.8 10.9 38.4 8.6 0.69
422 451 (453) 474 392
442 472 (470) 485 41
399 460 (460) 487 1545
408 432 (425) 447 19
aDetermined by difference. bValues in parentheses were determined by 1H NMR.
maximum fluidity after cooling. Experiments here demonstrate that with the relatively fast cooling rate achievable in the NMR probe (75 °C min-1), the loss of fluidity is relatively minor. It is also shown that heating coking coal in a tube furnace to the temperature of maximum fluidity and cooling very rapidly is a simple procedure for recovering relatively large amounts of representative material from partially carbonized coal for characterization. The results obtained are discussed in relation to the longer term goal of this investigation of being able to quantify the separate contributions to fluidity development from the extractable material present in coking coal and the low molecular mass species or metaplast generated during low-temperature carbonization. Experimental Section Table 1 lists the elemental, proximate and reflectance analysis, and Gieseler plastometer results for the principal coal investigated, a low-volatile Australian coking coal (AUS-1), together with those of three other Australian coals that have been used primarily to demonstrate the ability of the hightemperature 1H NMR probe to identify differences in plasticity development in terms of the amount and the mobility of the fluid phase. Coals were received from British Steel with a particle size of
