Energy & Fuels 2006, 20, 2063-2066
2063
Upgrading the Solvent Used for the Thermal Extraction of Sub-Bituminous Coal Nao Kashimura, Toshimasa Takanohashi,* and Ikuo Saito Energy Technology Research Institute, National Institute of AdVanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba 305-8569, Japan ReceiVed March 7, 2006. ReVised Manuscript ReceiVed June 5, 2006
HyperCoal is an ash-free coal produced by extraction with an industrial solvent at temperatures around 360 °C, which can be fed directly to gas turbines. We searched for more powerful solvents to extract low-rank coals, such as sub-bituminous coal. Polar materials were successfully concentrated from a polar industrial solvent, crude methylnaphthalene oil (CMNO), by extraction with a mixture of methanol and water or aqueous HCl. The soluble fraction obtained using the former (MW-S) extracted 73 wt % (daf) of Wyodak Anderson sub-bituminous coal at 360 °C, while that obtained using the latter (AC-S) extracted 63 wt %. These extraction yields were much higher than those with CMNO (43 wt %), indicating that fractionation concentrated the materials that dissolve the constituents of coal. MW-S contained more indole than AC-S. The results of addition tests suggested that indole had a greater ability to extract coal constituents than quinoline. In addition, the addition of 5 wt % methanol to a mixture of 1-methylnaphthalene, indole, and quinoline (30/20/50 wt %) increased the extraction yield from 58 to 69%, which was close to the yield of MW-S (73%). Therefore, the high extraction yield of MW-S can be explained by not only the composition of the nitrogen-containing polar materials in MW-S but also the presence of methanol.
Introduction Coal is an important energy resource with immense reserves in many parts of the world. The combustion of coal emits much carbon dioxide, and its emissions per unit joule are higher than for other fossil fuels, such as oil and natural gas, because of its high carbon content. Therefore, more efficient coal utilization is required to minimize the emission of carbon dioxide. In power generation systems, the direct combustion of coal in gas turbines is more efficient than combustion in a conventional pulverized coal-fired boiler. However, when coal is combusted in a gas turbine, there are serious problems, such as turbine blade erosion, corrosion, and fouling due to the deposition of coal ash. To resolve these problems, it is necessary to remove the mineral matter from coal. In Australia, an ultraclean coal process was investigated, in which mineral matter was removed from coal using acids and alkalis under hydrothermal conditions.1-3 The treated coal contains 1000-5000 ppm of ash, which exceeds the level required for the direct injection of coal into gas turbines. When coal is extracted with organic solvents, only the organic portion of coal is dissolved in the solvents. However, the amount of extract (the extraction yield) is usually quite low at room temperature. Iino et al. obtained extraction yields higher than 60 wt % (on a dry-ash-free coal basis, daf) using a 1:1 (by volume) mixture of carbon sulfide and N-methyl-2-pyrrolidinone (NMP) for some bituminous coals at room temperature. In addition, no ash was detected in the extract.4,5 The authors stated * Author to whom correspondence should be addressed. Fax: 81-29861-8408. E-mail:
[email protected]. (1) Steel, K. M.; Besida, J.; O’Donnell, T. A.; Wood, D. G. Fuel Process. Technol. 2001, 70, 171. (2) Steel, K. M.; Besida, J.; O’Donnell, T. A.; Wood, D. G. Fuel Process. Technol. 2001, 70, 193. (3) Steel, K. M.; Patrick, J. W. Fuel 2001, 80, 2019.
that the solvent relaxed the interactions among the molecules constituting the organic portion of the coal. An increase in the extraction temperature also relaxes these interactions, resulting in higher extraction yields than at room temperature. Miura et al. developed a method for producing clean fuel from coal using a solvent-flow extractor at 350 °C.6,7 They obtained extraction yields of 65-80 wt % (daf) for bituminous coal when a nonpolar solvent was used as a solvent and 80 wt % for sub-bituminous coal and lignite when the polar solvent carbol oil was used. Thermal extraction using industrial solvents has been applied to produce ash-free coal, called “HyperCoal”.8-15 The target in this process is an extraction yield exceeding 60 wt %, and an ash content in HyperCoal lower than 200 ppm, to allow direct injection into a gas turbine. Previously, extraction yields exceeding 60 wt % were obtained for several bituminous coals (4) Iino, M.; Kumagai, J.; Ito, O. Fuel Soc. Jpn. 1985, 64, 210. (5) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639. (6) Miura, K.; Shimada, M.; Mae, K. Proceedings of the 15th Annual International Pittsburgh Coal Conference; PCC: Pittsburg, 1998. (7) Miura, K.; Shimada, M.; Huan, H. Fuel 2001, 80, 1573. (8) Okuyama, N.; Deguchi, T.; Shigehisa, T.; Shimasaki, S. Proceedings of the 17th Annual International Pittsburgh Coal Conference; PCC: Pittsburg, 2000. (9) Okuyama, N.; Deguchi, T.; Shigehisa, T.; Shinozaki, S. International Conference on Clean Coal Technologies for Our Future, IEA: Sardinia, 2002. (10) Yoshida, T.; Takanohashi, T.; Sakanishi, K.; Saito, I.; Fujita, M.; Mashimo, K. Energy Fuels 2002, 16, 1006. (11) Yoshida, T.; Li, C.; Matsumuya, A.; Sato, S.; Saito, I. G. Fuel Process. Technol. 2004, 86, 61. (12) Li, C.; Takanohashi, T.; Saito, I.; Iino, M.; Aoki, H.; Mashimo, K. Energy Fuels 2004, 18, 97. (13) Li, C.; Takanohashi, T.; Yoshida, T.; Saito, I.; Aoki, H.; Mashimo, K. Fuel 2004, 83, 727. (14) Masaki, K.; Takanohashi, T.; Yoshida, T.; Li, C.; Saito, I. Energy Fuels 2004, 18, 995. (15) Takanohashi, T.; Li, C.; Saito, I.; Aoki, H.; Mashimo, K. Proceedings of the 12th International Conference on Coal Science; IEA: Cairns, 2003 [CD-ROM].
10.1021/ef0601014 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/12/2006
2064 Energy & Fuels, Vol. 20, No. 5, 2006
using a flowing solvent extractor with the nonpolar solvents 1-methylnaphthalene and light cycle oil (LCO),10 which is a cost-effective industrial solvent. When low-rank coals, such as sub-bituminous and brown coals, were subjected to extraction, the yields were generally below 60 wt %. Sub-bituminous coals are thought to form molecular aggregates via noncovalent interactions among oxygen-containing functional groups, such as carboxylic and phenolic hydroxyl groups. In previous reports, the extraction of sub-bituminous coals with polar solvents such as NMP and crude methylnaphthalene oil (CMNO) gave higher yields than that with nonpolar solvents, such as 1-methylnaphthalene and LCO. In particular, the extraction yield with NMP attained 60%. Furthermore, polar additives in nonpolar solvents enhanced the extraction yield.13,14 These results were caused by the polar materials being able to relax the aggregates. Therefore, the concentration of polar materials might be important for producing an upgraded solvent to dissolve large quantities of coal. In industrial solvents derived from coals, such as CMNO, the main polar materials are nitrogen-containing materials, such as quinoline. Acid extraction has been used to remove the nitrogen-containing materials from the distillate of fossil fuels.16 Kodera et al. reported that mixtures of methanol and water extracted nitrogen-containing materials from coal-derived liquid effectively.17,18 Acid extracts mainly basic materials, like quinoline and its derivatives, while methanol-water mixtures extract polar and neutral materials such as indole, in addition to basic materials. In this study, polar materials were concentrated from CMNO using acid or a methanol-water mixture to obtain a powerful solvent for extracting low-rank coals. Wyodak Anderson subbituminous coal was extracted with the CMNO-derived materials at 360 °C. Various effective polar constituents were explored. Experimental Section Materials. We studied Wyodak Anderson sub-bituminous coal. Coal samples were ground to
