Molecular simulation of the competitive adsorption characteristics of CH4, CO2, N2, and multicomponent gases in coal

The microscopic mechanism of the competitive adsorption of CH4, CO2 and N2 in coal is the theoretical basis for enhancing coal seam gas recovery by injecting CO2 (CO2-ECBM). Based on this principle, this work used Grand Canonical Monte Carlo and Molecular Dynamics to investigate the microscopic adsorption mechanism of single, binary, and ternary component gases in Wiser bituminous coal molecules. The adsorption mechanism was explored by changing gas composition and concentration. The comparison of adsorption separation coefficients suggested that CO2 had the highest adsorption capacity, whereas N2 had the lowest capacity. When the CO2 concentration in the gas mixture was high, the adsorption amount was large and the adsorption separation coefficient was small. This finding indicated that high concentrations of CO2 had negative effects on competitive adsorption. Energy changes were also evaluated. The potential energy between CO2 and the framework was the strongest among the two other gases. The inhibition of CH4 adsorption intensified with decreasing molar fraction of CO2. This phenomenon was explained from the perspective of heat of adsorption. As the molar fraction of CO2 in the adsorption system decreased, the heat of the isotherm adsorption increased. Meanwhile, the adsorption system became unstable and the capacity of CH4 adsorption on the framework weakened. Results provide a theoretical basis for the use of CO2-ECBM.