Importance of Volume Effects to Adsorption Isotherms of Carbon Dioxide on Coals Ekrem Ozdemir, † Badie I. Morsi, † and Karl Schroeder* U.S. Department of Energy (DOE), National Energy Technology Laboratory (NETL), P.O. Box 10940, Pittsburgh, Pennsylvania 15236-0940 Received April 25, 2002. In Final Form: May 15, 2003 Attempts to describe high-pressure carbon dioxide (CO2) adsorption isotherm data using conventional adsorption equations to model the coal behavior have been only partially successful. Because swelling of the coal organic matrix in the presence of adsorbing gases is a well-known phenomenon and because traditional isotherm models assume a rigid structure, an adsorption isotherm equation was derived to account for the volume effects which may occur when an adsorbate alters the structure of an adsorbent. The equation, which accounts for volume change in general, was applied to the particular example of CO2 adsorption on coal. In some cases, significantly better fits were obtained when the adsorption data were fit to the swelling-modified model. The modified model partitions the experimentally determined adsorption into a surface adsorption term, which is important at lower pressure, and a rectilinear term, related to volume effects, which is important at higher pressures. This is particularly significant to the problem of CO2 sequestration in coal seams where high pressures of CO2 will be used. 1. Introduction Sequestration of CO 2 in coal seams is one of the strategies being considered to mitigate the increasing atmospheric concentration of CO 2 . 1 One of the most important advantages of coal seam sequestration is that CO 2 is stored in an adsorbed state rather than as a compressed or liquefied gas. However, the adsorption capacity and the stability of the adsorbed CO 2 , which can be affected by the nature of the coal itself as well as by environmental factors, need to be accurately known. The adsorption isotherm is one of the most important tools for estimating adsorption capacity. Historically, the adsorp- tion of CO 2 on coals has been used to estimate surface areas 2,3 and micropore structures of coals. 4 Usually, these measurements have been conducted at low pressures (usually below atmospheric) and low temperatures (-78 °C). 3 Although information obtained from measurements such as these is important to current sequestration efforts, low-pressure, low-temperature adsorption isotherm data do not represent geologic, in-seam conditions. High- pressure, moderate-temperature CO 2 adsorption data for coal/CO 2 systems are sparse and often have been reported to fit the conventional adsorption equations poorly. 5,6 We propose that volumetric effects, such as those related to coal swelling, are important factors in these experiments and that they need to be addressed in order to adequately describe the adsorption process. Coal swelling during the sorption of liquids, 7 gases, 8,9 and vapors 10,11 is a well-known phenomenon. Reucroft et al. 8,12 measured the swelling and shrinkage of various ranks of Kentucky coal due to the adsorption of gases, such as helium (He), nitrogen (N 2 ), CO 2 , and xenon (Xe), by directly observing directional (length) changes at pressures between 0 and 1.5 MPa. They found that under a vacuum the coal samples shrank, presumably due to moisture loss because the extent of shrinkage was dependent on the amount of moisture removed from the coal. Pressurization with He or N 2 compressed the coal and resulted in shrinkage, whereas CO 2 caused consider- able swelling. The volume increase observed in going from 0 to 1.5 MPa was between 0.36% and 4.18%. They also found that swelling in a CO 2 atmosphere was rank dependent with lower-rank coals swelling more than higher-rank coals. Walker et al. 13 measured the expansion of powdered coals and macerals induced by gaseous CO 2 and methanol (CH 3 OH). They found that expansion to an equilibrium value was faster in CO 2 than in CH 3 OH although significantly greater expansion was observed in CH 3 OH. They concluded that most of the CO 2 uptake was due to CO 2 uptake in open and closed (to He) micropores at the temperatures and pressures used to measure surface areas. Recently, St. George and Barakat 9 have investigated volumetric changes of the coal matrix upon the adsorption and desorption of CH 4 , CO 2 ,N 2 , and He. Again, compres- sion, rather than swelling, was observed in the presence of high-pressure He. Expansion was observed for the other gases. Expansion in the presence of CO 2 was 12 times larger than in N 2 and 8 times larger than in CH 4 . In addition, the volumetric strains (ΔV/V) resulting from the desorption of CO 2 , CH 4 , and He for ΔP ) 4 MPa were -4.5%, -2.2%, and +0.1%, respectively. The negative sign indicates that shrinkage occurs upon loss of CO 2 and CH 4 , * Corresponding author. E-mail: Karl.Schroeder@netl.doe.gov. Phone: (412) 386 5910. Fax: (412) 386 6004. † Permanent address: Chemical and Petroleum Engineering Department, University of Pittsburgh, 1249 Benedum Hall, Pitts- burgh, PA 15261. Ekrem Ozdemir: e-mail, ekost4@pitt.edu; phone, (412) 624 9692; fax, (412) 624 9639. Badie I. Morsi: e-mail, morsi@engrng.pitt.edu; phone, (412) 624 9650; fax, (412) 624 9639. (1) Gentzis, T. Int. J. Coal Geol. 2000, 43, 287-305. (2) Gan, H.; Nandi, S. P.; Walker, P. L., Jr. Fuel 1972, 51, 272-277. (3) Walker, P. L., Jr.; Kini, K. A. Fuel 1965, 44, 453. (4) Medek, J. Fuel 1977, 56, 131-133. (5) Milewska-Duda, J.; Duda, J.; Nodzenski, A.; Lakatos, J. Langmuir 2000, 16, 5458-5466. (6) Clarkson, C. R.; Bustin, R. M. Fuel 1999, 78, 1345-1362. (7) Walker, P. L. J.; Mahajan, O. P. Energy Fuels 1993, 7, 559-560. (8) Reucroft, P. J.; Sethuraman, A. R. 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