Simulation study of competitive adsorption of CH4/CO2 and CH4/N2 in anthracite coal

This study investigated the adsorption properties of gas molecules in coal seams, which are crucial for gas injection in enhanced coalbed methane recovery (ECBM). Competitive adsorption simulations of CH4, CO2, and N2 single-component gases and various CH4/CO2 and CH4/N2 mixing ratios were conducted using an anthracite coal model from the Zhao Zhuang coal mine. Grand Canonical Monte Carlo (GCMC) simulations were performed at a system temperature of 298 K and pressures ranging from 0.1 to 5 MPa to explore the mechanism by which injected CO2 and N2 replaced CH4 in the coal seam. The results indicated that CO2 exhibited a significantly higher inhibitory effect on CH4 than on N2 at the same ratio. As the ratio of N2 increased, the inhibition of CH4 became more pronounced. When the CH4/N2 ratio exceeded 1:3, CH4 was at a disadvantage in terms of competitive adsorption, with the coal's adsorption capacity of N2 surpassing that of CH4. This suggested that the amount of N2 injected was critical for effectively displacing CH4 from the coal seam. During competitive adsorption, the gas mixture ratio had a minimal impact on the potential energy distribution and adsorption selectivity coefficients, which consistently favored CO2 adsorption over CH4, and CH4 adsorption over N2. The van der Waals energy generated during competitive adsorption of CH4/CO2 was twice that of CH4/N2. Based on these findings, an optimized scheme incorporating CO2-ECBM and N2-ECBM technologies was proposed to provide theoretical guidance for gas injection in Enhanced Coalbed Methane Recovery (ECBM) and reduce the cost of coalbed methane (CBM) recovery.