Adsorption characteristics of CO 2 and CH 4 on dry and wet coal from subcritical to supercritical conditions

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.