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 CH 4 , CO 2 , and N 2 single-component gases and various CH 4 /CO 2 and CH 4 /N 2 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 CO 2 and N 2 replaced CH 4 in the coal seam. The results indicated that CO 2 exhibited a significantly higher inhibitory effect on CH 4 than on N 2 at the same ratio. As the ratio of N 2 increased, the inhibition of CH 4 became more pronounced. When the CH 4 /N 2 ratio exceeded 1:3, CH 4 was at a disadvantage in terms of competitive adsorption, with the coal’s adsorption capacity of N 2 surpassing that of CH 4 . This suggested that the amount of N 2 injected was critical for effectively displacing CH 4 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 CO 2 adsorption over CH 4 , and CH 4 adsorption over N 2 . The van der Waals energy generated during competitive adsorption of CH 4 /CO 2 was twice that of CH 4 /N 2 . Based on these findings, an optimized scheme incorporating CO 2 -ECBM and N 2 -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. • The effects of N 2 and CO 2 on CH 4 were analysed in terms of molecular dynamics. • The gas mixture ratio doesn’t affect the adsorption sequence of single gases by coal. • Gas injection strategies for CO 2 -ECBM and N 2 -ECBM have been proposed.