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Energy & Fuels 2000, 14, 904-909
Mechanistic Study on the Plastic Phenomena of Coal Masakatsu Nomura,* Koh Kidena, Masataka Hiro, and Satoru Murata Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Received December 17, 1999. Revised Manuscript Received April 17, 2000
Highly coking coal, Goonyella coal, and slightly coking coal, Witbank coal, were submitted to ruthenium ion catalyzed oxidation (RICO) reaction to clarify the structural features of the two coals. Expected structural differences between the two coking coals were meager but interesting: Witbank coal has a little bit larger amount of longer methylene bridges (C15-C25) and longer alkyl chains (C15-C30) than Goonyella coal. Combined use of 13C NMR and 1H NMR spectra gave valuable information of the average aromatic ring size: Goonyella coal has larger aromatic rings than Witbank coal on average. To obtain the information about the evolution of volatile materials during heating which are supposed to be significant for the appearance of coal plasticity, two steps of heat treatment of the coals were performed: the first step is heating to the softening temperature, and the second step is heating the resulting sample to resolidification temperature. Witbank coal gave a relatively larger amount of tar in the first step of the heating than in the case of Goonyella coal, while Goonyella coal gave a larger amount of tar in the second step of the heating compared with the corresponding fraction of Witbank coal. These strongly suggest that a larger amount of metaplast, which is indispensable for the appearance of fluidity, could be produced during the plastic range (the temperature range from the softening temperature to the resolidification temperature) in the case of Goonyella coal, this leading to its higher Gieseler maximum fluidity. We previously reported that these two coking coals have almost the same amount of transferable hydrogen. Therefore, Witbank coal is supposed to consume relatively large amounts of transferable hydrogen for the formation of tar during the heating to softening temperature, probably via oxygen functional group related reactions and carbon-carbon bond breaking reactions. Due to a lower amount of transferable hydrogen in the char, subsequent bond cleavage reactions at the plastic range lead to recombination to show a very low value of its Gieseler maximum fluidity, although the three-dimensional structure framework of Witbank coal collapses to a small extent.
Introduction Plastic phenomena of coal are essential in making a high quality of metallurgical coke. The origin of these phenomena has not been explained yet at the molecular level; however, several mechanisms for the appearance and development of plasticity have been suggested.1-8 One explanation is called as metaplast theory.2-4 Another theory is that the amounts of the inherent solvent soluble fraction in coal (γ-compounds) determine the coal plasticity.5,6 The role of the hydrogen transfer reaction was also discussed in view of coal plasticity;9-11 however, no investigation can explain why noncoking * To whom correspondence should be addressed. Fax: +81-6-68797362. E-mail:
[email protected]. (1) Elliot, M. A. Chemistry of Coal Utilization, 2nd Suppl. Vol.; Wiley-Interscience: New York, 1981; Chapter 1. (2) Fitzgerald, D. Trans. Faraday Soc. 1956, 362. (3) Solomon, P. R.; Best, P. E.; Yu, Z. Z.; Charpenay, S. Energy Fuels 1992, 6, 143. (4) van Krevelen, D. W. Coal; Elsevier: Amsterdam, 1993. (5) Ouchi, K. Fuel 1961, 40, 485. (6) Ouchi, K.; Itoh, H.; Itoh, S.; Makabe, M. Fuel 1989, 68, 735. (7) Fortin, F.; Rouzaud, J. N. Fuel 1993, 72, 245. (8) Fortin, F.; Rouzaud, J. N. Fuel 1994, 73, 795. (9) Neavel, R. C. Coal Science I; Academic Press: London, 1982. (10) Clemens, A. H.; Matheson, T. W. Fuel 1992, 71, 193. (11) Yokono, T.; Obara, T.; Iyama, S.; Yamada, J.; Sanada, Y. Nenryo Kyokaishi 1984, 63, 239.
or slightly coking coals cannot reach the fluid state during heating. In previous work,12,13 we have found that the amount of transferable hydrogen evaluated by the reaction of coal with anthracene can correlate well with values of the Gieseler maximum fluidity of six coking coals and the parameter of Rmax/WL (maximum rate of weight loss divided by total weight loss up to 1000 °C) derived from thermogravimetric analysis also shows correlation similar to the values of Gieseler maximum fluidity. Those studies suggested that both transferable hydrogen in coal and volatile material in the plastic range play important roles in the appearance of coal plasticity. Since 1998, the project concerning mechanistic clarification and modeling of the appearance of plastic properties of coal particles had started in Japan with the support of the Iron and Steel Institute of Japan (ISIJ). We are determined to conduct chemical structural analysis of highly coking coal and slightly coking coal to compare the structural differences and clarify the mechanistic aspects of the goal of this project based on the structural studies. (12) Kidena, K.; Murata, S.; Nomura, M. Energy Fuels 1996, 10, 672. (13) Kidena, K.; Murata, S.; Nomura, M. Energy Fuels 1998, 12, 782.
10.1021/ef990257m CCC: $19.00 © 2000 American Chemical Society Published on Web 05/27/2000
Mechanistic Study on the Plastic Phenomena of Coal
Our position is that the bond breaking reaction in the coal molecule takes place during heating as one of the key reactions; therefore, we consider the ruthenium ion catalyzed oxidation (RICO) reaction14,15 as a strong tool because this reaction can clarify the chemical structures of bridge bonds and alkyl side chains, and besides we have succeeded in developing this reaction as a reliable method for this purpose. We have also paid careful attention to the tar fraction evolved during the heating of coals. We are convinced that measurement of the tar fraction up to the softening temperature and during the plastic range (the temperature range from the softening temperature to the resolidification temperature) will give us valuable information for the present purpose. Recently, we have also accomplished the method of evaluating the average aromatic ring size in coal by the combination use of SPE/MAS 13C NMR and 1H CRAMPS NMR spectra.16 All these results were employed to explain the appearance or development of coal plasticity in terms of structural features and the reactivity of coal. Experimental Section Coal Samples. Goonyella (GNY) and Witbank (WIT) coals were provided by ISIJ. Their ultimate analyses, ash contents, and Gieseler properties are as follows: For GNY: C, 87.3; H, 5.3; N, 1.9; O + S, 5.5 wt %, dry and ash free basis; H/C, 0.72; ash, 9.5 wt %, dry basis. Gieseler maximum fluidity (MF), 2.99 log(ddpm); softening temperature 397 °C, MF temperature 456 °C, and resolidification temperature 498 °C. For WIT: C, 82.5; H, 5.0; N, 2.0; O + S, 10.5 wt %; daf. H/C 0.72; ash 8.1 wt %, db. Gieseler MF, 0.95 log(ddpm); softening temperature 412 °C, MF temperature 432 °C, and resolidification temperature 446 °C. These coal samples were ground under 100 mesh and dried at 60 °C in vacuo for a night prior to use. Measurement of SPE/MAS 13C NMR Spectra. All spectra were recorded on a Chemagnetics CMX-300 spectrometer with a 13C frequency of 75.55 MHz. The sample was put into a rotor with 5-mm diameter, and its size was 80-120 mg. Magic angle rotation was performed with the rate of 10.5 kHz. The pulse width was 1.5 µs (45° pulse), and the delay time for the next pulse was 100 s. The FIDs were accumulated above 3000 scans for each sample. The resulting spectra were treated with Spinsight ver. 3.5.2 (attached with the spectrometer) and GRAMS/32 (Galactic Industries Corp.). Deconvolution of the spectra was performed with the latter software by using the following assignment and parameters:17-19 carbonyl carbon (CdO, CHO; peak center, >200 ppm; full width at halfmaximum, 12-15 ppm), carboxyl carbon (COOH, COOR; 187, 178; 12-15), oxygen-bonded aromatic carbon (Ar-O; 167, 153; 15-16), carbon-bonded aromatic carbon (Ar-C; 140; 16-17), bridgehead aromatic carbon and protonated aromatic carbon (bridgehead, Ar-H; 126, 113, 100; 17-18), oxygen-bonded aliphatic carbon (aliphatic-O; 93, 70, 56; 16-18), methylene and methyne carbon (CH2; 40, 31; 16-17, 11-13), methyl carbon (CH3; 20,13; 10-12). The function of all curves was Gaussian. (14) Stock, L. M.; Tse, K. Fuel 1983, 62, 974. (15) Artok, L.; Murata, S.; Nomura, M.; Satoh, T. Energy Fuels 1998, 12, 391. (16) Kidena, K.; Murata, S.; Artok, L.; Nomura, M. J. Jpn. Inst. Energy 1999, 78, 869. (17) Snape, C. E.; Ladner, W. R.; Burtle, K. D. Anal. Chem. 1979, 51, 2189. (18) Hayamizu, K.; Yanagisawa, M.; Yabe, A.; Sugimoto, Y.; Yamamoto, O. J. Jpn. Inst. Energy 1994, 73, 267. (19) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187.
Energy & Fuels, Vol. 14, No. 4, 2000 905 Measurement of 1H CRAMPS NMR Spectra. 1H NMR spectra with combined rotation and multiple pulse spectroscopy (CRAMPS)16,20-23 were recorded on the same spectrometer as the 13C measurements. A glass rotor was used as the sample container and sealed in a glove box by using resin to avoid absorbing moisture. The sample size was 16-20 mg. The BR24 pulse sequence was used in this measurement. The pulse width was 1.3 µs, while a 108 µs cycle time and 256 cycles were applied. The rotation speed of the sample rotor was 2.5 kHz. The offset frequency from the resonance position was 3.5 kHz. RICO Reaction of Coal. Dried GNY and WIT coals were submitted to RICO reaction.14,15 The coal sample (1 g), deionized water (30 mL), carbon tetrachloride (20 mL), acetonitrile (20 mL), sodium periodate (20 g), and ruthenium(III) chloride n-hydrate (40 mg) were stirred in a glass flask at 40 °C for 48 h. Nitrogen was passed through the glass vessel during the reaction period. Evolved CO2 was trapped by ascarite, which was put at the end of nitrogen flow, and its yield was determined by the weight change of ascarite. As to analysis of lower carboxylic acids, 50 mL of aqueous sodium hydroxide (5%) was added to the product mixture, followed by the filtration. The filtrate was analyzed by ion chromatography. With regard to the other acids, the product mixture from another run was filtered and separated into insoluble fraction, organic phase, and aqueous phase. After methyl esterification by diazomethane, the soluble parts were analyzed by GC and GC-MS to get the information about mono-, di-, and polycarboxylic acids with both aliphatic and aromatic carbons. The insoluble fractions were submitted to elemental analyses and measurements of FT-IR and solid-state 13C NMR spectra. Heat Treatment of Coal. Coal samples (ca. 3 g) were heattreated in a quartz tube (60-mm diameter) by using an Isuzu DKRO-14K electric furnace. The temperature regime was wellcontrolled to the heating rate of 3 K/min. The first stage heat treatment was conducted up to the softening temperature of each sample coal (GNY, 397 °C; WIT, 412 °C). The evolved gaseous products were collected in the aluminum gas bag for analyses by GC, and the tar fractions condensed inside the quartz tube were recovered by washing with acetone. The resulting char was used for the second stage of heat treatment and heated to the resolidification temperature (GNY 498 °C; WIT 446 °C). The gaseous products and tar and char fractions were recovered in a way similar to the first stage of heat treatment. This is called two step heat treatment. The single step heat treatment was also performed. The procedure was the same as the first step heat treatment of the above method except for the temperature (up to resolidication temperature). The yields of gaseous products and tar and char fractions were determined by their weights, respectively. The fraction that remained in a cold trap was a mixture of compounds with low molecular weight such as benzene or naphthalene analogues; however, we could not determine the yield of this light fraction directly because of the difficulty in weighing. Measurement of FT-IR Spectra. FT-IR spectra were recorded on a Shimadzu FTIR-8100M spectrometer with or without DRS-8000. The insoluble fraction of the RICO reaction was mixed with dried KBr in the ratio of 1:10 by weight, being submitted to FT-IR measurement by the diffuse reflectance method. The tar fraction in acetone solution obtained from the heat treatment of the coal was mixed with dried KBr fine particles in the ratio of 1:80 by weight.24 The tar-coated KBr (20) Bronnimann, C. E.; Hawkins, B. L.; Zhang, M.; Maciel, G. E. Anal. Chem. 1988, 60, 1743. (21) Jurkiewicz, A.; Bronnimann, C. E.; Maciel, G. E. Fuel 1989, 68, 872. (22) Rosa, L.; Pruski, M.; Lang, D.; Gerstein, B. Energy Fuels 1992, 6, 460. (23) Xiong, J.; Maciel, G. E. Energy Fuels 1997, 11, 856. (24) Ledesma, E. B.; Li, C.-Z.; Nelson, P. F.; Mackie, J. C. Energy Fuels 1998, 12, 536.
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Table 1. Carbon Distribution of Two Coking Coals carbon typea CdO, CHO COOH, COOR Ar-O Ar-C bridgehead Ar-H aliphatic-O CH2 CH3 fa a
GNY ∼0
