Molecular simulation of CO₂/CH₄/H₂O competitive adsorption and diffusion in brown coal

Carbon dioxide enhanced coalbed methane recovery (CO₂-ECBM) has been proposed as a promising technology for the natural gas recovery enhancement as well as mitigation of CO₂ emissions into the atmosphere. Adsorption and diffusion of CO₂/CH₄ mixture play key roles in predicting the performance of CO₂-ECBM project, i.e., the production of coalbed methane as well as the geological sequestration potential of carbon dioxide. In the present work, the mechanism of competitive adsorption and diffusion of CO₂/CH₄/H₂O mixture in brown coal were investigated by employing grand canonical Monte Carlo and molecular dynamics simulation. The effects of temperature and pressure on competitive adsorption and diffusion behaviours were explored. It is found that CO₂ has much stronger adsorption ability on brown coal than CH₄. The adsorption amounts of CO₂/CH₄ increase with pressure but have a decreasing trend with temperature. High adsorption selectivity of CO₂/CH₄ is observed with pressure lower than 0.1 MPa. In addition, the effects of moisture content in brown coal on the adsorption characteristics have been examined. Simulation results show that the adsorption capacities of CO₂/CH₄ are significantly suppressed in moist brown coal. The competitive adsorption of CO₂/CH₄/H₂O follows the trend of H₂O ≫ CO₂ > CH₄. Moreover, the results reveal that moisture content has great effects on the self-coefficients of CO₂/CH₄. Compared with dry coal, the self-diffusion coefficients of CO₂ and CH₄ reduce by 78.7% and 75.4% in brown coal with moisture content of 7.59 wt%, respectively. The microscopic insights provided in this study will be helpful to understand the competitive adsorption and diffusion mechanism of CO₂/CH₄/H₂O in brown coal and offer some fundamental data for CO₂-ECBM project.