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.