Molecular simulation study on multicomponent competitive adsorption of CH₄, CO₂, and H₂O in coal

Coal exhibits heterogeneous pore networks and chemically diverse surfaces, resulting in complex competitive adsorption among CH₄, CO₂, and H₂O. The underlying molecular mechanisms remain unclear. In this work, molecular simulation methods were applied to investigate the adsorption behavior and interaction characteristics of CH₄/CO₂/H₂O mixtures on two typical coal components (inertinite and vitrinite) under different CO₂ enrichment levels, corresponding to gas-phase CO₂ mole fractions of 4.8%, 9.1%, and 16.7%. The results demonstrate that CH₄ dominates surface occupation in all cases, maintaining 30-70 adsorbed molecules, whereas CO₂ adsorption is significantly weaker, remaining below 5 at low loading and increasing to only 10-17 at high loading. This indicates a limited competitive capability of CO₂ for adsorption sites. From an interaction perspective, water governs the electrostatic environment, with surface-water Coulombic energies consistently distributed around - 600 to - 750 kJ mol^-1. In contrast, CO₂-water interactions decrease from - 220 to - 360 kJ mol^-1 to - 100 to - 170 kJ mol^-1 as CO₂ loading increases, reflecting a pronounced screening effect. Meanwhile, direct CO₂-surface interactions remain weak (typically - 5 to - 15 kJ mol^-1). Overall, CH₄ adsorption is primarily controlled by dispersion interactions, while CO₂ is constrained by weak surface affinity and reduced hydration strength, resulting in a secondary role in multicomponent competitive adsorption within coal systems.