Molecular Simulation Study on the Competitive Adsorption and Diffusion of CH4 and CO2 in Coal Nanopores with Different Pore Sizes

Coalbed methane (CBM), mainly composed of methane (CH4) and carbon dioxide (CO2), has attracted increasing attention due to its dual significance as a clean energy resource and its role in greenhouse gas management. This research systematically examines the adsorption, desorption, diffusion, and bubble evolution dynamics of methane (CH4) and carbon dioxide (CO2) in graphene nanopores with diameters of 4 nm, 6 nm, and 8 nm by molecular dynamics simulations. Radial distribution function (RDF) analyses reveal strong solvation of both gases by water, with CO2 exhibiting slightly stronger interactions. Adsorption and desorption dynamics indicate that CO2 molecules display shorter residence times on the graphene surface (0.044–0.057 ns) compared with CH4 (0.055–0.062 ns), reflecting faster surface exchange. Gas-phase molecular number analysis demonstrates that CH4 accumulates more significantly in the vapor phase, while CO2 is more prone to adsorption and re-dissolution. Mean square displacement (MSD) results confirm enhanced molecular mobility in larger pores, with CH4 showing greater overall diffusivity. Structural evolution of the 8 nm system highlights asymmetric bubble dynamics, where large bubbles merge with the upper adsorption layer to form a thicker layer, while smaller bubbles contribute to a thinner layer near the lower surface. CH4 and CO2 follow similar pathways, though CO2 diffuses farther post-desorption due to its weaker surface retention. These results provide fundamental insights into confinement-dependent gas behavior in graphene systems, offering guidance for gas separation and storage applications.