Adsorption characteristics of CO2 and CH4 on dry and wet coal from subcritical to supercritical conditions

Enhanced coal bed methane (ECBM) recovery has been proposed as an attractive way to store captured CO2 while recovering CH4. The adsorption and desorption behaviors of CO2 and CH4 on dry and wet coal (anthracite) were studied at 318 and 338 K and up to 150 atm. The sorption capacity of CO2 and CH4 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 CO2 stored at 318 K could be lower than those at 338 K in the supercritical region because higher density of CO2 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 CO2 desorption isotherms had different shapes, depending on temperature, but all the CH4 desorption isotherms showed a weak positive hysteresis. The mutual solubility between the CO2-rich (or CH4-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 CO2 storage capacity or ECBM recovery because the density variation significantly influenced the isotherm shape.