Supercritical Pure Methane and CO2 Adsorption on Various Rank Coals of China: Experiments and Modeling

Carbon dioxide sequestration in coal seams with enhanced coal-bed methane recovery (CO2-ECBM) is considered as a promising option for permanent carbon dioxide storage. As one of the most :important factors to the success of the CO2-ECBM process, adsorption of methane and CO2 on coal helps to assess the amount of recoverable methane as well as the storage capacity of. CO2 of the targeted coal seam. In this work, the methane and CO2 adsorption isotherms were measured with a volumetric technique at temperatures of 35, 50, and 65 C and pressures up to 1:6 and 12 MPa (CO2 adsorption at 35 C is limited below 6 MPa), respectively. Four coals of various rank exploited from four main coal seams in China were tested. The isotherms fit well to the Ono-Kondo lattice model, which confirms the applicability of this model in describing the adsorption behaviors of methane and CO2 on coal under the supercritical conditions. In addition, the experimental results show that the excess adsorption of CO2 reaches the maximum level between 7 and 9 MPa, while the excess adsorption of methane exhibits a less pronounced maximum. The maximum adsorption capacities of the coals for methane and CO2 decrease slightly with temperature increase. Additionally, the maximum adsorption capacities of methane and CO2 are also dependent on coal rank (indicated by vitrinite reflectance coefficient, R-o max) and present a U-shaped trend with coal rank. The preferential adsorption ratio of CO2 to methane on a basis of absolute adsorption obtainer, from Ono-Kondo lattice model is in the range of 1.13-3.52 under test conditions. This ratio of bituminous coals (R-o max ranging from 0.47% to 1.35%) decreases significantly with increasing pressure; however, the pressure dependence of the preferential adsorption ratio is less pronounced for the anthracite (R-o max = 4.06%). The preferential adsorption ratio decreases with an increase in coal rink with the only exception of anthracite.