Effect of Moisture on Adsorption Isotherms and Adsorption Capacities of CO 2 on Coals Ekrem Ozdemir* ,†,‡ and Karl Schroeder † National Energy Technology Laboratory, U.S. Department of Energy, P.O. Box 10940, Pittsburgh, PennsylVania 15236 and Department of Chemical Engineering, Izmir Institute of Technology, Gulbahce Campus, Urla, Izmir, Turkey ReceiVed December 24, 2008. ReVised Manuscript ReceiVed February 20, 2009 The effect of moisture on the adsorption isotherms and adsorption capacities of CO 2 on Argonne Premium coals has been investigated. In some experiments a small hysteresis was observed between the adsorption and desorption isotherms. The hysteresis was absent or negligible for high-rank and as-received coals but was discernible for lower rank and dried coals. An equation that accounted for the volumetric changes when an adsorbate alters the structure of an adsorbent was employed to interpret the data. The best-fit solutions indicate that the coal volume decreases upon drying. The microscopic shrinkage estimated using helium expansion was greater than the shrinkage reported using the bed-height technique. The microscopic shrinkage was 5-10% for low-moisture medium and high-rank coals and up to 40% for low-rank coals having higher moisture contents. The CO 2 swelling of coals during adsorption isotherm measurements was estimated to be about the same as the shrinkage that occurred during the moisture loss. The adsorption capacity, isosteric heat of adsorption, average pore size, and surface area of the as-received (moist) and dried Argonne coals were estimated after accounting for the volume changes. The isosteric heat of adsorption of CO 2 was found to be between 23 and 25 kJ/mol for as-received coals and between 25 and 27 kJ/mol for dried coals, regardless of the rank. The degree of drying was shown to affect the adsorption capacity and the calculated surface area. For dried coals, the adsorption capacity showed the typical ‘U-shape’ dependence on rank whereas the as-received coals displayed a more linear dependence. A relationship is proposed to quantify the effect of moisture on the adsorption capacity. The mechanism of CO 2 adsorption on moist coals and the implications of the lower adsorption capacity of wet coals to coal seam sequestration of CO 2 are presented. 1. Introduction Sequestration of carbon dioxide (CO 2 ) in unmineable coal seams is a possible strategy for mitigating greenhouse gas emissions. 1 Adsorption on a solid, immobile matrix that is stable over geologic times is an important advantage of coal seam sequestration of CO 2 . The enhancement of the production of coalbed methane (CBM) is another advantage that reduces the sequestration cost, and in most cases, it is profitable. 2 However, the adsorption capacity and stability of the adsorbed CO 2 are affected by the nature of the coal and the environment in which it is placed. Pressure, temperature, coal rank, coal moisture content, and changes in the pH of the coalbed water are factors that may limit or enhance the extent of adsorption. 3 Safety issues associated with the mining of gassy coal seams have prompted studies of methane(CH 4 ) adsorption and the effect of moisture on the CH 4 adsorption capacity of coal. 4-6 There have been a few studies of the CO 2 adsorption on coal and of the parameters that affect the stability of the adsorbed CO 2 under in-seam conditions. 7-14 One of these parameters is the moisture content of the coal. The fact that the CH 4 adsorption capacity of coal decreases with increasing moisture content is well established; 4,15,16 the effect of moisture on the CO 2 adsorption isotherms and adsorption capacity of coals with various ranks is of special interest. Moisture plays an important role in the adsorption of gases on coal, and there have been some efforts to quantify the effect of moisture on the adsorption capacity. 17,18 Joubert et al. 19 showed a linear decrease in CH 4 adsorption with increasing moisture content up to a critical value, which appeared to be the equilibrium moisture content as estimated at 30 °C and 96% relative humidity. For moisture contents greater than the critical * To whom correspondence should be addressed. Telephone: +90(232)750 6685. Fax: +90(232)750 6645. E-mail: ekremozdemir@iyte.edu.tr. † U.S. Department of Energy. ‡ Izmir Institute of Technology. (1) White, C. M.; Smith, D. H.; Jones, K. L.; Goodman, A. L.; Jikich, S. A.; LaCount, R. B.; Dubose, S. B.; Ozdemir, E.; Morsi, B. I.; Schroeder, K. T. Energy Fuels 2005, 19, 659–724. (2) Stevens, S. H.; Gale, J. Oil Gas J. 2000, 98, 40–44. (3) Schroeder, K.; Ozdemir, E.; Morsi, B. I. Sequestration of Carbon Dioxide in Coal Seams. First National Carbon Sequestration Conference Proceedings. DOE/NETL, 2001, 1–10. (4) Levy, J. H.; Day, S. J.; Killingley, J. S. Fuel 1997, 76, 813–819. (5) Bustin, R. M.; Clarkson, C. R. Int. J. Coal Geol. 1998, 38, 3–26. (6) Moffat, D. H.; Weale, K. E. Fuel 1955, 34, 449–462. (7) Dutta, P.; Harpalani, S.; Prusty, B. Fuel 2008, 87, 2023–2036. (8) Day, S.; Fry, R.; Sakurovs, R. Int. J. Coal Geol. 2008, 74, 41–52. (9) Romanov, V.; Soong, Y. Energy Fuels 2008, 22, 1167–1169. (10) Romanov, V. N.; Goodman, A. L.; Larsen, J. W. Energy Fuels 2006, 20, 415–416. (11) Karacan, C. O. Energy Fuels 2003, 17, 1595–1608. (12) Sakurovs, R.; Day, S.; Weir, S.; Duffy, G. Energy Fuels 2007, 21, 992–997. (13) Siemons, N.; Busch, A. Int. J. Coal Geol. 2007, 69, 229–242. (14) Goodman, A. L.; Campus, L. A.; Schroeder, K. T. Energy Fuels 2005, 19, 471–476. (15) Joubert, J. I.; Grein, C. T.; Bienstock, D. Fuel 1972, 52, 185. (16) Yee, D.; Seidle, J. P.; Hanson, W. P. Gas Sorption on Coal and Measurement of Gas Content. Law, B. E., Rice, D. D., Eds., Hydrocarbons from Coal. Am. Assoc. Pet. Geol.: Tulsa, OK, 1993, 159, 184. Energy & Fuels 2009, 23, 2821–2831 2821 10.1021/ef801126a CCC: $40.75  2009 American Chemical Society Published on Web 04/01/2009