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Energy & Fuels 2006, 20, 2088-2092
Relationship between Thermal Extraction Yield and Oxygen-Containing Functional Groups Nao Kashimura, Toshimasa Takanohashi,* Kensuke Masaki, Takahiro Shishido, Sinya Sato, Akimitsu Matsumura, and Ikuo Saito Energy Technology Research Institute, National Institute of AdVanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba 305-8569, Japan ReceiVed May 1, 2006. ReVised Manuscript ReceiVed July 27, 2006
Generating power from HyperCoal is a high-efficiency process in which the organic portion of coal is extracted with industrial solvents at a temperature around 360 °C and fed to a gas turbine directly. This study sought to establish a selection index for identifying subbituminous coals that give high extraction yields. Subbituminous coals were extracted at 360 °C with flowing industrial solvents, and we investigated the relationship between the extraction yield and the quantity of oxygen-containing functional groups in the coal. The extraction yield with a polar solvent, crude methylnaphthalene oil (CMNO), increased with the quantity of carboxylate groups bridged by metal cations, such as Ca2+ and Mg2+ (COOM). The correlation coefficient between the extraction yield and the quantity was 0.82. Acid treatment of coal before extraction released COOM cross-links, increasing the extraction yield. These results suggest that the thermal extraction of lowrank coals strongly depends on the cross-links rather than the hydrogen bonds. Therefore, the thermal extraction yields of low-rank coals can be estimated from the quantity of COOM in the original coals. The intercept of the regression line between the quantity of COOM and the extraction yield with CMNO was 57.8%. This value is the average extraction yield for low-rank coals with free COOM.
Introduction Although the direct combustion of coal in gas turbines increases the power output while reducing CO2 emissions, serious problems occur, 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 Ultra-Clean Coal process was investigated, in which mineral matter was removed from coal using acids and alkalis under hydrothermal conditions.1-3 However, the treated coal contained 1000-5000 ppm 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 the coal is dissolved in the solvents. Unfortunately, the amount extracted (the extraction yield) is usually quite low at room temperature. Iino et al. obtained extraction yields exceeding 60 wt % (on a dry-ash-free coal basis, daf) for some bituminous coals at room temperature using a 1:1 (by volume) mixture of carbon sulfide and N-methyl-2-pyrrolidinone (NMP).4,5 In addition, no ash was detected in the extracts.6 They stated that the solvent relaxed the interactions among the molecules constituting the organic portion of the coal. An increase in 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 sol* Corresponding author. 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. (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) Iino, M.; Takanohashi, T.; Obara, S.; Tsueta, H.; Sanokawa, Y. Fuel 1989, 68, 1588.
vent-flow extractor at around 350 °C.7 They obtained extraction yields of 65-80 wt % (daf) for bituminous coal when a nonpolar solvent was used as the solvent and 80 wt % for subbituminous 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 gas turbines. In previous work, extraction yields exceeding 60% were obtained for several bituminous coals using a flowing solvent extractor with the nonpolar solvent light cycle oil (LCO).9,10 Unfortunately, when low-rank coals, such as subbituminous and brown coals, were extracted, the yields were generally below 60%.13 Previously, we found a good correlation between the extraction yields of bituminous coals and the softening temperature of the raw coals.16 However, it is difficult to apply the correlation (7) Miura, K.; Shimada, M.; Huan, H. Fuel 2001, 80, 1573. (8) Okuyama, N.; Komatsu, N.; Shigehisa, T.; Kaneko, T.; Tsuruya, S. Fuel Process. Technol. 2004, 85, 947. (9) Yoshida, T.; Takanohashi, T.; Sakanishi, K.; Saito, I.; Fujita, M.; Mashimo, K. Energy Fuels 2002, 16, 1006. (10) Yoshida, T.; Li, C.; Takanohashi, T.; Matsumuya, A.; Sato, S.; Saito, I. Fuel Process. Technol. 2004, 86, 61. (11) Li, C.; Takanohashi, T.; Saito, I. Energy Fuels 2004, 18, 97. (12) Li, C.; Takanohashi, T.; Yoshida, T.; Saito, I.; Aoki, H.; Mashimo, K. Fuel 2004, 83, 727. (13) Masaki, K.; Takanohashi, T.; Yoshida, T.; Li, C.; Saito, I. Energy Fuels 2004, 18, 995. (14) Takanohashi, T.; Li, C.; Saito, I.; Aoki, H.; Mashimo, K. In Proceedings of the 12th International Conference on Coal Science, Caims, Australia, Nov 2-6, 2003; Australian Institute of Energy: Raymond Terrace, Australia, 2003; CD-ROM. (15) Masaki, K.; Kashimura, N.; Takanohashi, T.; Sato, S.; Matsumura, A.; Saito, I. Energy Fuels 2005, 19, 2021.
10.1021/ef060194p CCC: $33.50 © 2006 American Chemical Society Published on Web 08/26/2006
Extraction Yield and Functional Groups with Oxygen
to the extraction of low-rank coals because those low-rank coals soften very little. Therefore, it is important to find a parameter that identifies low-rank coals that give higher extraction yields. In subbituminous coals, noncovalent interactions, such as cross-links among carboxylate groups bridged by metal cations (ionic cross-links) and hydrogen bonds among heteroatomcontaining functional groups, and the π-π interaction may form aggregated structures.17 Takanohashi et al. showed that the π-π interaction in bituminous coals breaks down at 350-400 °C, while the ionic cross-links of metal carboxylate sites do not.14,18 It has been demonstrated that acid pretreatment enhances the extraction yield of subbituminous coal.12,19 Opaprakasit et al. reported that the extraction yields of a subbituminous coal with pyridine at 115 °C increased from 15% for the raw coal to 39% after acid-pretreatment with HCl.19 Li et al. found that pretreatment with an aqueous acid solution increased the thermal extraction yield for some subbituminous coals using the polar solvent NMP at 360 °C.12 They attributed the increment to the release of ionic cross-links by the pretreatment. Therefore, the extent of ionic cross-linking affects the extraction yield of the raw coal. Generally, a polar solvent such as NMP gives higher extraction yields for subbituminous coals than a nonpolar solvent like 1-methynaphthalene,12 which is attributable to the thermalinduced relaxation of the aggregate on extraction with the latter solvent, versus both thermal-induced and solvent-induced relaxation with the former solvent. The solvent-induced relaxation effectively releases the hydrogen bonds among oxygencontaining functional groups, and the release should increase the extraction yield of subbituminous coal. Therefore, the extraction yields of subbituminous coals depend not on only the extent of ionic cross-links but also on the hydrogen bonds in the coals. Schafer quantified the total carboxyl groups, that is, i.e., the total of the carboxyl (COOH) and metal carboxylate (COOM) groups, by ion exchange of the groups with barium acetate.20,21 The first step involved the demineralization of the coal to change all the COOM to the acid form, which then made it possible to quantify the total exchange capacity. If the demineralization step is not performed, the ion exchange only involves COOH. Therefore, the difference in the ion exchange capacities of the raw and demineralized coals corresponds to the quantity of COOM. This study investigated the correlation between the number of oxygen-containing functional groups and the thermal extraction yield to establish a good index for selecting coals that give high extraction yields. Experimental Section Material. Fifteen subbituminous and brown coals and three upgraded brown coals (UBC) were used. UBC is an upgraded coal produced using a coal-oil slurry process, which was obtained from Kobe Steel, Japan.22,23 The coal samples were ground to
