Energy & Fuels 2004, 18, 97-101
97
Elucidation of Mechanisms Involved in Acid Pretreatment and Thermal Extraction during Ashless Coal Production Chunqi Li, Toshimasa Takanohashi,* and Ikuo Saito Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba 3058569, Japan
Hideki Aoki and Kiyoshi Mashimo College of Science and Technology, Nihon University, 1-8 Kanda-Surugadai, Chiyoda-ku, Tokyo 1018308, Japan Received May 15, 2003. Revised Manuscript Received October 8, 2003
Some low-rank coals, i.e., Wyodak coal (C%, 75.0%), acid-treated in aqueous methoxyethoxy acetic acid (MEAA) and acetic acid (AA), and extracted in polar N-methyl-2-pyrrolidinone (NMP), showed a considerable increase in thermal extraction yield at 360 °C, as the acid concentration increased from 0.01 to 0.1 M. No significant changes were seen with a further increase in acid concentration to 1.0 M. A corresponding decrease occurred in the intensity of FT-IR spectral bands near 1555 and 1400 cm-1, assigned to metal carboxylate groups, as acid concentrations increased from 0.01 to 0.1 M, while bands assigned to carboxyl groups at about 1720 cm-1 increased over the same range. Furthermore, most Mg2+ and Ca2+ ions could be removed from the coals with acids between 0.01 and 0.1 M. Thermogravimetric analyses showed that the acidtreated coal yielded a weight loss similar to that of raw coal. Thermal decomposition of acidtreated coals cannot play a significant role in the increase in extraction yield that is obtained with polar solvent. A mechanism is proposed for the processes involved in the acid treatment and thermal extraction of some low-rank coals: cation-bridging cross-links existing among metal carboxylate groups in the low-rank raw coals are released upon removal of Mg2+ and Ca2+ ions by acid treatment. The resulting carboxyl groups then form new hydrogen bonds among themselves, which can become released when polar NMP solvent is introduced. Thus, both the disruption of cation-bridging cross-links by acid treatment and the release of the hydrogen bonds by NMP are involved in the enhancement of extraction yields upon acid treatment of some lowrank coals.
Introduction Thermal extractions that involve organic solvents are commonly used to facilitate clean, value-added utilization of coals.1-8 For example, extraction of bituminous coal with polar N-methyl-2-pyrrolidinone (NMP) at its boiling point of 202 °C gives a 74% of extract that contains as little as 1000 ppm ash.4 Yields of 65-80% have been obtained among bituminous coals extracted with the nonpolar solvent tetralin, and of ∼80% in subbituminous and lignite coals where the polar solvent * Corresponding author. E-mail:
[email protected]. (1) Ouchi, K. Fuel 1961, 40, 485. (2) Stiller, A. H.; Sears, J. T.; Hammack, R. W. U. S. Patent SN4, 37, 356, 1981. (3) Stiller, A. H.; Zondlo, J. W.; Mintz, E. A.; Jagodzinski, P.; Leong, C. W. Proceedings of the 2nd Annual International Pittsburgh Coal Conference 1985, 413. (4) Renganathan, K.; Zondlo, J. W.; Mintz, E. A.; Kneisl, P.; Stiller, A. H. Fuel Process. Technol. 1988, 18, 273. (5) Cai, M. F.; Smart, R. B. Energy Fuels 1994, 8, 369. (6) Miura, K.; Shimada, M.; Mae, K. Proceedings of the 15th Annual International Pittsburgh Coal Conference 1998, 1988. (7) Miura, K.; Shimada, M.; Mae, K.; Huan, H. Fuel 2001, 80, 1573. (8) Ihara, T.; Ashida, R.; Nakagawa, H.; Miura, K. Proceedings of the Japan Conference of Coal Science (in Japanese) 2002, 95.
carbol oil has been used at ∼350 °C.6-8 Thermal extraction of coals with organic solvents has proved particularly successful in the production of ashless coals (HyperCoal),9-14 which can be fired directly in gas turbines to achieve a higher net power output and a lowering of CO2 emissions. In practice, a thermal extraction yield of at less 60% is required. In a HyperCoal gas turbine system, a higher net power output of HHV 48% is estimated, leading to 20% CO2 reduction. In addition, HyperCoals with an ash content of less than 200 ppm, and with sodium and potassium contents less (9) Okuyama, N.; Deguchi, T.; Shigehisa, T.; Shimasaki, K. Proceedings of the 17th Annual International Pittsburgh Coal Conference 2000. (10) Okuyama, N.; Deguchi, T.; Shigehisa, T.; Shinozaki, S. International Conference on Clean Coal Technologies for Our Future (CCT2002) 2002, Sardinia, Italy. (11) Saito, I.; Shinozaki, S. J. Mining Mater. Process. Inst. Jpn. (in Japanese) 2002, 118, 115. (12) Yoshida, T.; Takanohashi, T.; Sakanishi, K.; Saito, I.; Fujita, M.; Mashimo, K. Energy Fuels 2002, 16, 1006. (13) Yoshida, T.; Li, C.; Takanohashi, T.; Matsumura, A.; Sato, S.; Saito, I. Proceedings of 9th APCChE Congress and CHEMECA 2002. (14) Li, C.; Takanohashi, T.; Yoshida, T.; Saito, I.; Iino, M. Proceedings of 2002 China International Hi-tech Symposium on Coal Chemical Industry and Conversion 2002, 170.
10.1021/ef0340054 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/22/2003
98
Energy & Fuels, Vol. 18, No. 1, 2004
than 0.5 ppm are also required in order to avoid several problems such as turbine blade erosion, corrosion, and fouling due to ash deposition from HyperCoal during firing.11 In our earlier studies,12-14 we achieved extraction yields greater than 60% from several bituminous coals by employing a flow-type extractor at 360 °C and the nonpolar solvents 1-methylnaphthalene (1-MN) and light cycle oil. Yields in excess of 80% were obtained when the polar solvents NMP and crude methylnaphthalene oil were used. In the case of 1-MN, the elevated extraction yields were attributed mainly to the heatinduced relaxation of the coal aggregates, while those obtained with NMP were considered the result of both heat-induced and solvent-induced relaxation.14 However, some low-rank coals gave relatively low extraction yields under both sets of extraction conditions.14,15 In low-rank coals, considerable oxygen is present in the form of carboxylic and phenolic functional groups. The carboxylic groups in coal are present in both the acid (carboxyl group) and salt (metal carboxylate group) forms, where they are associated with cations such as Mg2+, Ca2+, and Na+.16-18 Therefore, significant cationbridging cross-links ((COO-)2 M2+) can form between the divalent cations, such as Mg2+ and Ca2+, and the carboxylate anions. The disruption of these cross-links via cation exchange upon treatment with strong acids considerably enhances extraction yields in coals.19,20 For example, Opaprakasit et al.20 reported that the pyridine yield from a Soxhlet extraction at 115 °C of subbituminous Wyodak coal was increased from 15% in the raw coal, to 39% for coal treated with 0.1 M HCl. Organic acids, such as methoxyethoxy acetic acid (MEAA) and acetic acid (AA), have also been found to be effective in promoting cation exchange of metal carboxylate groups in coals.21,22 In our previous work,15 low-rank coals were treated with the organic acids MEAA and AA, and inorganic HCl, prior to extraction, and in all cases the thermal extraction yields were enhanced. Although the thermal extraction yields for these acid-treated low-rank coals were greatly increased in polar NMP, the changes in the yield in nonpolar 1-MN proved small. It was found that the type and concentration of acid had a profound effect on the yield, with increased yields, as reflected in the de-ashing ratios. However, it has not yet been ascertained how a coal’s structure changes during acid treatment, nor what types of coal aggregates are released during thermal extraction. The present study reports the results of changes in the chemical structure of coals undergoing acid treatment under a variety of conditions, along with quantitative analysis of the metal ions eluted under these conditions. The results provide a basis for deducing the probable mechanisms involved in both acid treatment and thermal extraction. (15) Li, C.; Takanohashi, T.; Yoshida, T.; Saito, I.; Aoki, S.; Mashimo, K. Fuel, in press. (16) Murray, J. B. Fuel 1973, 52, 105. (17) Schafter, H. N. S. Fuel 1970, 49, 197. (18) Huttinger, K. J.; Michenfelder, A. W. Fuel 1987, 66, 1164. (19) Nishioka, M. Fuel 1991, 70, 1413. (20) Opaprakasit, P.; Scaroni, A. W.; Painter, P. C. Energy Fuels 2002, 16, 543. (21) Shimohara, T.; Mochida, I. J. Fuel Soc. Jpn. 1987, 66, 134. (22) Sakanishi, K.; Watanabe, I.; Nonaka, T.; Kishino, M.; Mochida, I. Fuel 2001, 80, 273.
Li et al. Table 1. Ultimate Analysis and Ash Content of Coal Samples Used ultimate analysis (wt %, daf) coal sample
C
H
N
S
Oa
ash (wt %, db)
Wyodak(WY) Adaro(AD) Pasir(PA) Beulah-Zap(BZ) Banko 97(BA)
75.0 73.0 73.5 71.6 70.0
5.4 5.1 5.3 4.8 5.3
1.1 1.1 1.9 1.0 1.3
0.5 0.0 0.2 0.9 0.3
18.0 20.8 19.1 21.7 23.1
8.8 1.8 4.9 9.6 2.4
a
By difference.
Experimental Section Materials. Samples of two Argonne Premium coals (Wyodak (WY), and Beulah-Zap (BZ)) ground to
