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Energy & Fuels 2004, 18, 1414-1418
Increase in Extraction Yields of Coals by Water Treatment Masashi Iino,* Toshimasa Takanohashi, and Chunqi Li Institute for Energy Utilization,National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba 305-8569, Japan
Haruo Kumagai Center for Advanced Research of Energy Technology, Hokkaido University, Sapporo 060-8628, Japan Received October 28, 2003. Revised Manuscript Received May 25, 2004
The effect of water treatment at 500 and 600 K on solvent extractions of Pocahontas No. 3 (PO), Upper Freeport (UF), and Illinois No. 6 (IL) coals was investigated. All the coals used show that the water treatments at 600 K increased the extraction yields greatly in the extractions with a 1:1 carbon disulfide/N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent, NMP, or 1-methylnaphthalene (1-MN). However, the water treatments at 500 K and the heat treatments at 600 K without water gave only a slight increase in the yields. Characterizations of the watertreated coals were performed using ultimate and proximate compositions, Fourier transform infrared analysis, solvent swelling, nuclear magnetic resonance relaxation time, and viscoelasticity behavior. The swelling degree in methanol and toluene was increased by the water treatment at 600 K, suggesting that crosslinks become loosened by the treatment. The results of infrared analysis and the extraction temperature dependency of the extraction yields with NMP and 1-MN suggest that the loosening of π-π interactions, and of both π-π interactions and hydrogen bonds, are responsible for the yield enhancements for PO and UF coals, respectively. However, for IL coal, which exhibited a decrease in oxygen content and the amount of hydrogen-bonded OH, suggesting the occurrence of some chemical reactions, the yield enhancements may be due to the relaxation of hydrogen bonds and the removal of oxygen functional groups, such as the breaking of ether bonds.
Introduction The effect of water on coal properties and reactivity is one of the most important issues in coal science and industry. Thus, many papers have been writen on the effect of water addition on coal extraction and liquefaction. For example, Graff and Brandes1,2 reported that the treatment of Illinois No. 6 coal with 50 atm of steam at 340-350 °C increased the pyridine extraction yields from 17% for the raw coal to ∼30%. If the steam-treated sample is exposed to air, the extraction yields decrease to the levels observed for untreated coal. Steam treatment of Wyodak coal at 200 °C increased the conversion of toluene solubles in liquefaction at 400 °C from 27.3% to 38.4%.3 Various mechanisms were proposed for the enhancements of extraction and liquefaction yields. The rupture of weak covalent bonds such as ether bonds,2,4 the breaking of hydrogen bonds,3 the removal of minerals * Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Graff, R. A.; Brandes, S. D. Energy Fuels 1987, 1, 84. (2) Brandes, S. D.; Graff, R. A.; Gorbaty, M. L.; Siskin, M. Energy Fuels 1989, 3, 494. (3) Bienkowski, P. R.; Narayan, R.; Greenkorn, R. A.; Chao, K. Ind. Eng. Chem. Res. 1987, 26, 202. (4) Mapstone, J. O. Energy Fuels 1991, 5, 695.
by sulfuric acid formed during the treatment,5 and the decrease in oxygen functional groups, which are thought to be responsible for cross-linking reactons,6 have been proposed. The suppression of metal carboxylate-induced retrogressive reactions in liquefaction by the association of metal cation with water,7,8 and the effect of dihydroxy aromatics formed by water treatment and their reactions with coal,4,8 were also considered. However, several papers have reported that water treatment gave no benefits. Khan et al.9 reported that a steam treatment of Illinois No. 6 coal at 330 °C with careful protection from oxygen exposure gave only a very small increase in total volatiles of rapid steam pyrolysis, unlike the great increase in total volatiles and tar obtained by Graff and Brandes.1,2 Ross and Hirschon reported that water treatment of Illinois No. 6 coal at 250 °C did not increase toluene solubles in liquefaction in tetralin, although the S/C atomic ratio and ash (5) Mochida, I.; Iwamoto, K.; Tahara, T.; Korai, Y.; Fujitsu, H.; Takeshita, K. Fuel 1982, 61, 603. (6) Serio, M.; Kroo, E.; Solomon, P. R. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 1992, 37 (1), 432. (7) Serio, M. A.; Kroo, E.; Teng, H.; Solomon, P. R. Prepr. Pap.Am. Chem. Soc., Div. Fuel Chem. 1993, 38 (2), 577. (8) Serio, M. A.; Kroo, E.; Chapeney, S.; Solomon, P. R. Prepr. Pap.Am. Chem. Soc., Div. Fuel Chem. 1993, 38 (3), 1021. (9) Khan, M. R.; Chen, W.; Suuberg, E. Energy Fuels 1989, 3, 223.
10.1021/ef034078n CCC: $27.50 © 2004 American Chemical Society Published on Web 07/09/2004
Extraction Yields of Coals by Water Treatment
content decreased greatly, because of the conversion of pyrite to water-soluble sulfates.10 In the pyrolysis and liquefaction of low-rank coals, low-temperature crosslinking reactions have been correlated with the loss of carboxyl groups and the evolution of CO2 and H2O. Eskay et al.11 recently found that decarboxylation reactions of carboxylic acids occurred primarily via an acid-promoted ionic mechanism that does not lead to crosslinking. However, pyrolysis in a nondonor solvent produced a small amount of cross-linked products, of which the precursor was anhydrides formed from carboxylic acids, and the addition of water decreased the cross-linking reaction by the decomposition of the anhydrides. The use of a 1:1 carbon disulfide/N-methyl-2-pyrrolidinone (CS2/NMP) mixed solvent gave high extraction yields (>50 wt %) for several bituminous coals at room temperature.12,13 CS2/NMP extraction is unique in that a pyridine-insoluble extract fraction that is a heavier fraction than pre-asphaltene is obtained via solvent fractionation of the extracts. In this paper, the effect of water treatment on coal extraction yields with CS2/NMP and other single solvents was investigated, and the mechanism for the extraction yield enhancement by the water treatment was discussed. Experimental Section Coal Samples. Argonne Premium coalssPocahontas No. 3 (PO), Upper Freeport (UF), and Illinois No. 6 (IL) coalss with particle sizes of 620 K, the tan δ value of the treated coal is less than that of the raw coal for IL coal and is only slightly different from that of the raw coal for UF coal, suggesting that, at high temperatures, the benefit for the water treatment was lost. The result is in agreement to the extraction results, although characteristic temperature ranges are different, probably because of the difference in the heating rate. Mechanisms of the Water Treatment of Coal. The extraction yields increased by the water treatment at 600 K, although no special precautions were made to protect from exposure to oxygen in the workup of the treated samples and in their extractions, unlike the results by Graff and Brandes,1,2 in which the extraction yields fall to the levels observed for untreated coal if the steam-treated sample is exposed to air. However,
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the temperature range where the treatment is effective is consistent with their result, in that the treatment at 613-623 K was effective.1 Treatment of the IL coal resulted in a decrease in oxygen content and the IR peaks that are due to clay minerals, indicating that some chemical reactions occurred. For UF and PO coals, little chemical reactions occur, from their IR spectra and elementary compositions. An increase in the swelling degree by the water treatment suggests that the cross-linking network becomes relaxed by the treatment. For UF coal, the increment in the NMP and 1-MN extraction yields by the water treatment was the highest for the extraction at room temperature; the increment decreased as the extraction temperatures increased, and the benefit of the treatment was lost at and above 473 and 573 K for NMP and 1-MN extractions, respectively. This can be explained from that the treatment relaxed or broke noncovalent crosslinks of UF and PO coals; however, for extractions at high temperature, this merit was lost, because almost all the noncovalent bonds were broken, resulting in little difference between the treated and raw coals. For PO coal, which showed little change in the hydrogen bonds by the treatment, it may be π-π interactions between aromatic ring layers that are responsible for the loosening of noncovalent crosslinking bonds. For UF coal, which showed a small decrease in hydrogen-bonded OH, the relaxations of π-π interactions and hydrogen bonds seem responsible. The results of the dynamic viscoelasticitysthat, at high temperatures, the mobility of the treated coal was losts also support the aforementioned mechanism. The water treatment of IL coal is accompanied by chemical reactions, i.e., a loss of oxygen functional
Iino et al.
groups, as indicated by the decrease in oxygen content, and this may be responsible for the enhancements in extraction yields. Depolymerization by scissions of ether and ester groups is considered to occur to increase the yield. The decrease in OH groups also contributes to the yield enhancement, because of the decrease in the amount of hydrogen bonds. The result that the benefit of the treatment was not lost, even for extraction at high temperature with NMP, unlike PO and UF coals, can be explained by this loss of oxygen functional groups. In addition, hydrogen bond loosening and/or breaking, such as that in UF coal, is also responsible for the extraction yield enhancement for IL coal, as indicated by the large decrease in hydrogen bonds shown by IR. The hydrogen bond decrease is smaller for weaker hydrogen bonds, such as OH-π (3530 cm-1) and OHOH (3400 cm-1), than stronger ones, such as OH-ether (3280 cm-1), OH-N (2940 cm-1), and COOH-COOH (2640 cm-1).17 This can be explained by the fact that π electrons of aromatic rings and phenolic OH, which are abundant in coal, are preferentially used when new hydrogen bonds form during the water removal process (drying) for the treated coal. The effect of the chemical change in clay minerals on the extraction yields is not clear at present and will be studied further. Acknowledgment. This work was supported by “Research for the Future ” project of the Japan Society for the Promotion of Science (JSPS). The authors are grateful to the other members of the project for their collaboration in this work. EF034078N (17) Miura, K.; Mae, K.; Li, W.; Kusakawa, T.; Morozumi, F.; Kumano, A. Energy Fuels 2001, 15, 599.
