Energy & Fuels 2004, 18, 1463-1471
1463
Electron-Transfer-Induced Desulfurization of Organic Sulfur from Sub-bituminous Coal Dipu Borah* Department of Chemistry, Pragjyotika J. College, Titabar 785630, Assam, India Received September 11, 2003
The intent of this paper is to investigate the effects of leaching time and temperature on the level of desulfurization of organic sulfur by an electron-transfer process involving the transitionmetal ion Ni2+ from a sub-bituminous Indian coal. The electron-transfer process was accomplished both in the presence and the absence of naphthalene with unoxidized and oxidized samples. Desulfurization has been determined to be greater in oxidized samples. The presence of naphthalene enhanced the amount of desulfurization both in unoxidized and oxidized samples, revealing that it serves as an excellent electron-transfer agent. Increases in reaction time and temperature increased the level of desulfurization. The electron-transfer process, in the presence of naphthalene, succeeded in removing a maximum of 16.3 wt % organic sulfur from unoxidized coal and 18.7 wt % from oxidized coal at 50 °C and 4 h. Study of a model sulfur compound indicated that the desulfurization is primarily due to aliphatic-type compounds, such as dibenzothiophene, that is unable to release sulfur under these conditions. A higher level of desulfurization in oxidized coals is consistent with the formation of oxidized sulfur compounds, as revealed by the infrared study, where it is observed that band intensities due to -SdO and -SO2 units have decreased in their respective regions in the desulfurized coals. The sulfur removal process is continuous. Application of a pseudo-first-order kinetic model produced overall rate constants for the desulfurization reaction that consistently fell in the range of 5.2 × 10-6-2.1 × 10-5 s-1, implying a slow and steady process. The frequency factor (ln A) for the desulfurization reaction in different systems was in the range of 10.8-11.0 s-1, which is in support of predicting an associated type of reaction that envisages the formation of an activated complex. The Arrhenius activation energy of the sulfur loss reaction in different systems has been observed to be in the range of 10.9-23.0 kJ/mol. A semiquantitative thermodynamic approach of transition-state theory revealed that the desulfurization reaction is nonspontaneous in nature and proceeded with the absorption of heat, accompanied by the reduction in the degree of disorder in the systems, irrespective of the leaching time and temperature.
Introduction Coal is the world’s most abundant fossil fuel. The coal industry is of considerable size and is spread over fifty countries. There is great demand for low-ash and lowsulfur coals, because these have excellent economic value and potential use.1 Moreover, the ever-increasing demand of coal is due to the depletion of petroleum and natural gas reserves.2 The existing accessible stock of coal resources will be able to supply power at least for three centuries.3 There are huge deposits of coal scattered throughout the world;4 however, all of them have not received efficient utilization, because of the presence of a high amount of sulfur. In various coal conversion processes, especially in combustion, high-sulfur coal releases sulfur * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Bowler, S. New Sci. 1993, 137, 32. (2) Williams, A.; Pourkashanian, M.; Bysh, P.; Norman, J. Fuel 1994, 73, 1006. (3) Barooah, P. K.; Baruah, M. K. Fuel Process. Technol. 1996, 46, 83. (4) Yurovskii, A. Z. Sulfur in Coals (in Russ.); Academy of the Sciences of the USSR: Moscow, 1960. (English Translation by INSDOC: New Delhi, India, 1974.)
predominantly in the form of SO2, which is toxic and causes acid rain. The presence of sulfur in coke beyond a certain limit makes it unsuitable for metallurgical purposes.5 In weathered coals, sulfur species percolate with groundwater, generating the problem of acid mine drainage.6 The presence of moisture and sulfur are indicators of the liability of coal to spontaneous combustion during storage.6 Other deleterious effects observed include corrosion of boilers, underground pipelines, metallic installations, mine machineries, etc.7,8 Because of these effects, the environmental protection agencies suggest the use of high-sulfur coals after the sulfur content has been reduced to the permissible limit. Sulfur in coal is found in five different forms; these forms include elemental, sulfate, pyritic, secondary, and organic sulfur.4-9 Elemental sulfur is not found in all (5) Chandra, D. J. Mines, Met., Fuels 1982, 30, 208. (6) Gupta, A.; Saroj, K. K.; Thakur, D. N. Chem. Era 1977, 17, 238. (7) Ahmed, M. Presented at the Indo-Polish Symposium on Application of Geological and Geophysical Techniques of Mining, Dhanbad, India, February 22-24, 1984. (8) Rawat, M. S. In An Approach for Neutralization of AMD in North Eastern Coal Fields (CIL), Assam, Short Term Course on Geomicrobiology, Dhanbad, India, 1981. (9) Baruah, M. K.; Gogoi, P. C. Fuel 1998, 77, 979.
10.1021/ef030157n CCC: $27.50 © 2004 American Chemical Society Published on Web 08/06/2004
1464 Energy & Fuels, Vol. 18, No. 5, 2004
the coals;10 however, a trace amount (in parts per million, ppm) is present in some coals.11 Sulfate sulfur (in the form of iron sulfate and calcium sulfate) are basically found in coals.12 However, the presence of other sulfate sources such as jarosite13 as well as sulfate bonded with a metal-coal organic complex14 has been known. The pyritic sulfur occurs in the form of either pyrite or marcasite or both, having the same molecular formula (FeS2). There are other metal sulfides present in coals, viz. HgS, PbS, ZnS, CuFeS2, etc.15 The typical secondary sulfur, which contains Fe-S and C-S bondings, is neither purely organic nor pyritic sulfur.9 This form of sulfur has only recently been reported and is hardly soluble in dilute HNO3.9 The organic sulfur directly bonded to the coal organic matter is present in the form of thiols, sulfide, disulfide, thioether, thiophenol, thioketonic, thiophene, etc.;16 however, the amount of each type is dependent on the environmental conditions of the early state of coal formation and subsequent coalification process. Various physical, microbial, and chemical methods of desulfurization are known; however, the former two are ineffective in leaching organic sulfur to the permissible level from coal.17-19 In contrast, chemical methods are promising desulfurization processes and are usually accomplished using various oxidizing agents.20-28 Almost all the sulfate and pyritic sulfur can effectively be removed by chemical methods; however, in the case of organic sulfur, removal is achieved to a reasonable extent. A literature survey reveals that chemically controlled electron-transfer processes are also used by various workers for desulfurization purposes.29-37 Sternberg et (10) Chakrabari, J. N. Analytical Methods of Coal and Coal Products, Vol. I.; Karr, C., Jr., Ed., Academic Press: New York, 1978; Chapter 9. (11) Richard, J. J.; Vick, R. D.; Junk, G. A. Environ. Sci. Technol. 1977, 11, 1084. (12) Volbroth, A., Ed. In Coal Science and Chemistry; Elsevier: Amsterdam, 1987. (13) Huffman, G. P.; Huggins, F. E. Fuel 1978, 57, 592. (14) Baruah, M. K.; Gogoi, P. C.; Kotoky, P. Fuel 2000, 79, 211. (15) Speight, J. C. The Chemistry and Technology of Coal: Marcel Dekker: New York, 1983. (16) Reviews in Coal Science. The Problems of Sulphur; Butterworths (on behalf of IEA Coal Research): London, 1989; pp 5-10. (17) Martinez, O.; Diez, C.; Miles, N.; Shah, C.; Moran, A. Fuel 2003, 782, 1085. (18) Moran, A.; Cara, J.; Miles, N.; Shah, C. Fuel 2002, 81, 299. (19) Silverman, M. P.; Rogott, M. H.; Wender, J. Fuel 1963, 42, 113. (20) Kozowski, M.; Pietrzak, R.; Wachoesska, H.; Yperman, J. Fuel 2002, 81, 2397. (21) Shah, C. L.; Abbott, J. A.; Miles, N. J.; Xuejun, Li.; Jianping, Xu. Fuel 2002, 81, 519. (22) Borah, D.; Baruah, M. K.; Haque, I. Fuel 2001, 80, 1475. (23) Borah, D.; Baruah, M. K.; Haque, I. Fuel 2001, 80, 501. (24) Borah, D.; Baruah, M. K. Fuel Process. Technol. 2001, 72, 83. (25) Mukherjee, S.; Mahiuddin, S.; Borthakur, P. C. Energy Fuels 2001, 15, 1418. (26) Palmer, S. R.; Hippo, E. J.; Dorai, X. A. Fuel 1994, 73, 161. (27) Rodrigues, R. A.; Jul, C. C.; Gomez-Limon, D. Fuel 1996, 75, 606. (28) Sain, B.; Saikia, P. C.; Baruah, B. P.; Bordoloi, C. S.; Mazumder, B. Fuel 1991, 70, 753. (29) Sternberg, H. W.; Delle Donne, C. L.; Pantags, P.; Moroni, E. C.; Markby, R. E. Fuel 1971, 50, 432. (30) Ignasiak, B. S.; Gawlak, M. Fuel 1977, 56, 216. (31) Ignasiak, T.; Kemp-Jones, A. V.; Strausz, O. P. J. Org. Chem. 1977, 42, 312. (32) Chatterjee, K.; Wolny, R.; Stock, L. M. Energy Fuels 1990, 4, 402. (33) Chatterjee, K.; Stock, L. M. Energy Fuels 1991, 5, 704. (34) Chatterjee, K.; Stock, L. M.; Gorbaty, M. L.; George, G. N.; Keleman, S. R. Energy Fuels 1991, 5, 771. (35) Stock, L. M. Energeria 1992, 3, 1.
Borah
al.29 was the first to apply an electron-transfer process to coal. They treated coal with alkali metal in tetrahydrofuran (THF) in the presence of a small amount of naphthalene, which acted as an electron transferring agent. Ignasiak et al.31 demonstrated the cleavage of the C-S bond by the electron-transfer process while studying the molecular structure of asphaltene; the potential ion radical used to accomplish C-S bond scission is potassium naphthalenide, which is prepared by treating potassium metal with naphthalene in THF. A single electron-transfer process for removing organic sulfur from coal was also investigated by Chatterjee et al.,32 and they succeeded in removing 50%-90% of the organic sulfur by treating coal with potassium naphthalenide in THF. Recently, Borah and Baruah36 reported a novel specific electron-transfer process that utilizes naphthalene as an electron transferring agent, when treated with transition-metal ions that have variable valence states resulted in the formation of metal naphthalenide. This radical ion transfers electrons to the S atom of the organic sulfur compound of coal that resulted in the effective cleavage of C-S bonds that cause desulfurization. Their work unveiled that metal ions that have high negative oxidation potentials, viz. Ni2+, Co2+, and Sb3+, formed soluble sulfur compounds in large amounts, whereas those which have low negative oxidation potentials (viz. Cu+ and Sn2+) formed insoluble metal sulfur compounds in significant proportion, limiting the level of desulfurization. There are large reserves of coal available in the northeastern region of India. These coals are subbituminous in rank and contain a high amount of sulfur, which restricts their large-scale utilization. The sulfur content is in the range of 2%-6%, and >70% of the sulfur is present in the organic form.3 Moreover, these coals are friable and perhydrous in nature.38 Works relating to desulfurization on these coals are very scarce. The present work is an attempt to desulfurize organic sulfur from a Meghalaya coal via an electron-transfer process with Ni2+ as the transition-metal ion, both in the presence and absence of naphthalene. To understand the impact of pretreatment oxidation on desulfurization, the electron-transfer process also is applied to oxidized coal. The influence of temperature on the level of desulfurization is also investigated. Kinetic and thermodynamic approaches have also been made to represent the desulfurization reaction. Experimental Section Coal Sample. The coal sample used in the present investigation is a sub-bituminous Meghalaya (India) coal. It was ground to a particle size of
