Modeling of CO 2 sorption on coal P. Dutta 1 , S. Harpalani * , B. Prusty 2 Southern Illinois University, Carbondale, IL 62901, USA Received 5 June 2007; received in revised form 13 November 2007; accepted 14 December 2007 Available online 24 January 2008 Abstract This paper discusses moderate pressure CO 2 sorption behavior of Illinois coals. The results fit the Langmuir and Dubinin–Astakhov (D–A) sorption models satisfactorily although the fit is better for D–A equation. Since factors like swelling of coal with CO 2 sorption and CO 2 dissolution in coal matrix contribute to uncertainties in estimating the void volume in and around the sample, an attempt was made to account for these by modifying the conventional adsorption equation. Re-fitting the experimental data using the modified equation results in improved fit for both models. The adsorption capacities of coals tested, as predicted by the equations, also reduce by 7% to 32%. The effect of volumetric uncertainty is more in lower rank coals than the higher rank ones. Furthermore, it explains the excess sorp- tion behavior observed by others when extrapolated beyond the experimental pressure range. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Sorption isotherm; Langmuir; Dubinin–Astakhov; Sequestration; Swelling 1. Introduction Global warming, resulting from increasing amounts of greenhouse gases, is regarded as one of the most important environmental issues facing mankind. Of all the greenhouse gases, anthropogenic emission of carbon dioxide (CO 2 ) is considered the main contributor to global warming. The primary source of atmospheric CO 2 emission is the burning of fossil fuels, which continues to grow. Estimates of eco- nomic growth and associated emissions from the usage of fossil fuels as provider of primary energy suggest that the concentration of CO 2 in the atmosphere will continue to increase during this century, unless significant steps are taken to reduce the release of CO 2 to atmosphere. There- fore, in addition to other avenues of emission reduction through fuel switching, conservation, and efficiency improvements in the existing processes, stabilization of atmospheric CO 2 would require large-scale and low-cost carbon sequestration. Of the various options for CO 2 sequestration currently being considered, geological sequestration in deep and unmineable coal seams is a par- ticularly promising one since it has the potential of leading to enhancement in the production of coalbed methane (CBM), partially offsetting the cost of sequestration. Primarily, CO 2 is stored in coal in an adsorbed state rather than compressed or liquefied state, as in the case of other conventional gas reservoirs. When injected into coal, CO 2 molecules get competitively adsorbed on the coal micropores and replace the adsorbed methane molecules, releasing additional methane in the process. Coals are known to exhibit much higher sorption affinity to CO 2 than methane due to the higher adsorption energy and smaller molecular diameter of CO 2 [1], with a preferential sorption ratio of CO 2 to methane of 2:1. However, results of a few recent studies indicate widely varying ratios, all the way from 10:1 for low rank coals to less than 2:1 for low to medium volatile bituminous coals [2–5]. It is, therefore, critical to determine the adsorption capacity of coal and its ability to retain CO 2, both of which are affected by the nature of coal and other environmental factors. The adsorption capacity is determined by establishing adsorption isotherms in the laboratory, either by gravimet- ric or by volumetric methods. The laboratory adsorption 0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.12.015 * Corresponding author. Tel.: +1 6184537918; fax: +1 6184537455. E-mail address: satya@engr.siu.edu (S. Harpalani). 1 Currently at Bengal Engineering and Science University, India. 2 Currently at Central Institute of Mining and Fuel Research, India. www.fuelfirst.com Available online at www.sciencedirect.com Fuel 87 (2008) 2023–2036