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Energy & Fuels 1996, 10, 1128-1132
Extraction and Swelling of Low-Rank Coals with Various Solvents at Room Temperature Toshimasa Takanohashi,* Takayuki Yanagida, and Masashi Iino Institute for Chemical Reaction Science, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-77, Japan
David E. Mainwaring Department of Applied Chemistry, Royal Melbourne Institute of Technology, Melbourne, Victoria 3001, Australia Received March 6, 1996. Revised Manuscript Received June 10, 1996X
Low-rank coals were extracted with various solvents under ultrasonication at room temperature. In the case of single solvent extraction, N-methyl-2-pyrrolidinone (NMP) gave the highest extraction yield for Loy Yang coal, 14.3 wt % (daf). Among the mixed solvents used in this study, NMP-methanol (8:2) and NMP-water (3:1) mixed solvents gave the high yields, 15.3 and 15.1 wt % (daf), respectively. The NMP-carbon disulfide mixed solvent was not effective, although it gave very high extraction yields for bituminous coals. FT-IR measurements for Loy Yang coal, its extract, and residue showed that they have similar chemical structures. In polar solvents, Loy Yang coal swelled more highly than subbituminous coals, and the highest swelling ratio was also obtained in NMP, indicating high affinity between Loy Yang coal and NMP. The mixture of Loy Yang coal with NMP forms a gel structure at the composition of about 83 wt % NMP and 17 wt % coal.
Introduction Low-rank coals such as lignites and subbituminous coals contain a significant amount of moisture, in some cases 60-80 wt %, and it is often necessary to dewater before their use. Water in lignites was reported1,2 to form hydrogen bonds with hydroxyl or carboxylic groups of coal, and drying makes lignites less reactive due to the formation of new coal-coal interactions, especially hydrogen bond. Solvent extractions with organic solvents have been used to investigate chemical structure of coals. Dryden found that a good solvent for coals has a nitrogen or oxygen atom with lone pair.3 Iino et al.4-6 found that a carbon disulfide-N-methyl-2-pyrrolidinone (CS2-NMP) mixed solvent was very effective for the extraction for bituminous coals; i.e., high extraction yields (45-78 wt % (daf)) were obtained at room temperature. One of the reasons for the large extraction of the mixed solvent was suggested4 to be large swelling caused by the disruption of noncovalent bonds in bituminous coals. The addition of a small amount of tetracyanoethylene (TCNE) to the CS2-NMP mixed solvent increased greatly the extraction yields of bituminous coals,7,8 probably because TCNE strongly interacts with coal by charge transfer * To whom correspondence should be addressed. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, July 15, 1996. (1) Schafer, H. N. S. Fuel 1972, 51, 4. (2) Suuberg, E. M.; Otake, Y.; Yun, Y. Energy Fuels 1993, 7, 384. (3) Dryden, I. G. C. Fuel 1951, 30, 39, 145, 217. (4) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639. (5) Takanohashi, T.; Iino, M. Energy Fuels 1990, 4, 452. (6) Iino, M.; Takanohashi, T.; Ohkawa, T.; Yanagida, T. Fuel 1991, 70, 1236.
interactions and dissociates the association among coal molecules. Such specific interactions between solvents or additives are also possible with lignites and subbituminous coals. Hall and Larsen have reported from DSC and TGA measurements that Illinois No. 6 coal interact with NMP strongly to form a gel structure.9 An investigation on the interaction with solvents will help understand chemical structure of coals, especially their macromolecular structure. The macromolecular structure of coals has been also studied10 by solvent swelling theory developed for crosslinked polymer system, i.e., Flory-Rehner theory.11-13 One problem applying Flory-Rehner theory to coal is that it does not include specific interactions, such as hydrogen bonds, and charge-transfer and π-π interactions, which play important roles in the formation of cross-links of coals, i.e., physical cross-links.14,15 Painter et al. proposed16 a new model for that swelling of coal by combination of c* model with an association model for hydrogen-bonding interactions. The mechanisms of solvent penetration into coals and solvent swelling are not still understood. (7) Ishizuka, T.; Takanohashi, T.; Ito, O.; Iino, M. Fuel 1993, 72, 579. (8) Liu, H.-T.; Ishizuka, T.; Takanohashi, T.; Iino, M. Energy Fuels 1993, 7, 1108. (9) Hall, P. J.; Larsen, J. W. Energy Fuels 1993, 7, 47. (10) Hombach, H.-P. Fuel 1980, 59, 465. (11) Sanada, Y.; Honda, H. Fuel 1966, 45, 295. (12) Kirov, N. Y.; O’Shea, J. M.; Sergeant, G. D. Fuel 1968, 47, 415. (13) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729. (14) Fujiwara, M.; Ohsuga, H.; Takanohashi, T.; Iino, M. Energy Fuels 1992, 6, 859. (15) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247. (16) Painter, P.; Shenoy, S. Energy Fuels 1995, 9, 364.
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Extraction and Swelling of Low-Rank Coals
Energy & Fuels, Vol. 10, No. 5, 1996 1129
Table 1. Ultimate Analysis of Coals coal
ranka
Collie mvsb Leigh Creek mvsb Loy Yang lignite c
ultimate analysis, wt % (daf) C H N S Ob 73.6 72.7 65.5
4.3 4.3 4.8
1.3 1.6 0.5
0.6 0.3 0.3
20.2 21.1 28.9
ash,c wt % (db) 3.3 11.7 2.0
a mvsb ) medium-volatile subbituminous coal. b By difference. Acid-washed coal.
In the present paper, extraction and swelling of lowrank coals were investigated using various solvents at room temperature and interactions of the solvents with the coals are discussed. Experimental Section Coal Samples. Three Australian coals, Collie, Leigh Creek, and Loy Yang coals, were used and their ultimate analysis is shown in Table 1. Coal was ground to
