Chemical Engineering Journal 171 (2011) 45–53 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Adsorption characteristics of CO 2 and CH 4 on dry and wet coal from subcritical to supercritical conditions Hae Jung Kim, Yao Shi, Junwei He, Hyeon-Hui Lee, Chang-Ha Lee ∗ Department of Chemical and Biomolecular Engineering, Yonsei University, 262 SeongSanno, Seodaemun-gu, Seoul 120-749, South Korea article info Article history: Received 3 December 2010 Received in revised form 5 March 2011 Accepted 8 March 2011 Keywords: Carbon dioxide Methane Coal Adsorption Desorption Hysteresis abstract Enhanced coal bed methane (ECBM) recovery has been proposed as an attractive way to store captured CO 2 while recovering CH 4 . The adsorption and desorption behaviors of CO 2 and CH 4 on dry and wet coal (anthracite) were studied at 318 and 338 K and up to 150 atm. The sorption capacity of CO 2 and CH 4 on anthracite coal was higher at lower temperatures and dry coal condition, but smaller than those on bituminous coals at a similar condition. In wet coal, the sorption capacity and stability of high pressure CO 2 stored at 318 K could be lower than those at 338 K in the supercritical region because higher density of CO 2 at 318 K could lead to the structural change of wet coal. The difference in the excess adsorbed amount between dry and wet coal was only noticeable under the subcritical conditions at 338 K but became more significant under the supercritical conditions with pressure at 318 K. In dry and wet coal, the CO 2 desorption isotherms had different shapes, depending on temperature, but all the CH 4 desorption isotherms showed a weak positive hysteresis. The mutual solubility between the CO 2 -rich (or CH 4 -rich) phase and aqueous phase as well as coal swelling should be considered in evaluating the sorption capacity of a wet coal seam. Fluid density in free volume was the important variable to estimate the CO 2 storage capacity or ECBM recovery because the density variation significantly influenced the isotherm shape. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Due to greenhouse gas emissions, earth is facing the issue of global warming. Geological CO 2 storage is a promising method to treat captured CO 2 . Possible sites for geological sequestration of CO 2 include unminable coal seams, depleted oil and gas reservoirs, abandoned and sealed mines, and saline aquifers [1]. Storing CO 2 in coal seams among potential storage sites mainly depends on the adsorption properties of the porous coal structures [2]. In addition, this technology can contribute to CH 4 production in a process called enhanced coal bed methane (ECBM) recovery [3,4]. Because high pressure CO 2 is injected and CO 2 concentration finally becomes higher than CH 4 in coal seam, CO 2 molecules adsorb onto coal micropores, replacing and releasing adsorbed CH 4 . Coal seams for CO 2 sequestration will probably be at a depth where the temperature and pressure are above the critical point of CO 2 [2,5]. Therefore, ECBM recovery is accomplished by adsorp- tion/desorption processes under supercritical conditions (CO 2 : T c = 304.2 K, P c = 72.8 atm; CH 4 : T c = 190.6 K, P c = 45.4 atm) in natu- ral underground coal formations. Compared to CH 4 , many physical properties of CO 2 , such as density, diffusivity and viscosity, change when the phase changes from subcritical to supercritical condi- ∗ Corresponding author. Tel.: +82 2 2123 2762; fax: +82 2 312 6401. E-mail address: leech@yonsei.ac.kr (C.-H. Lee). tions. And it was reported that CO 2 adsorption behaviors on an activated carbon under supercritical conditions differed from CH 4 adsorption [6–8]. The adsorption of pure CO 2 and CH 4 onto bituminous coals has been widely studied [9–15]. Many studies reported that CO 2 adsorption isotherms first increase with pressure and then decrease in a non-linear way at high pressure [9–11,16]. Increas- ing moisture usually decreases the sorption capacity of coal up to a limiting moisture content [12] but the extent of the reduced capacity depends on the rank of coal [17]. Bituminous coal also shows a non-linear relationship between moisture content and CH 4 adsorption [18]. It has been reported that coal swelling induced by CO 2 sorption is greater than CH 4 [19–21]. A positive correlation between swelling by CO 2 adsorption and coal rank was observed, and a decrease in CO 2 permeability to coal seam could be caused by coal swelling [19]. Therefore, permeability decreases with increas- ing effective pressure on the coal, and this effect is even larger when swelling of the coal by gas adsorption occurs [22]. And a theoretical model was derived to describe adsorption-induced coal swelling at adsorption and strain equilibrium [23]. Also, CO 2 diffu- sivity does not show any discernable trend with coal depth or rank, and porosity of coals decreases with an increase in coal depth and rank [24]. Moreover, swelling occurs less in moisturized coals than in dry coals because adsorption and swelling are related [25]. In other studies, attempts were made to find a relationship between coal properties and CO 2 sorption capacity (on dry, ash free basis 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.03.035