Small-molecule gas sorption and diffusion in coal: Molecular simulation

Injections of carbon dioxide (CO₂) into unmineable coalbeds can both enhance coalbed methane recovery (ECBM), a high-efficiency energy, and realize underground storage of CO₂. In these processes, the diffusion and sorption of methane (CH₄) and carbon dioxide are key dominant processes. In this study, the diffusion and sorption behavior of CH₄ and CO₂ in coal are investigated and compared based on molecular simulation. The calculated diffusion coefficient of CO₂ was in the order of 10⁻⁹ m²/s, which is reasonably close to the experimental result. The sorption isotherms were obtained using the grand canonical Monte Carlo method. Coal tended to adsorb more CO₂ than CH₄ at a given temperature and pressure. The sorption heat of CO₂ was larger than that of CH₄ (7.9 and 5.8 kcal/mol respectively), accounting for the fact that the CH₄ adsorbed in the coal seam could be replaced by CO₂. This presents an alternative method for directly studying the interactions between coal macromolecule and small-molecule gases under various external environments.