Microscopic mechanisms of competitive gas adsorption and diffusion during high volatile bituminous coal CO2-ECBM process

In order to explore the microscopic mechanisms of competitive adsorption and diffusion between methane (CH₄) and carbon dioxide (CO₂) within coal reservoirs, high-volatile bituminous coals from the Ehuo (EH) and Wudong (WD) coal mines in Xinjiang were analysed. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and ¹³C nuclear magnetic resonance (¹³C NMR) spectroscopy were employed for characterization. Subsequently, a combined coal molecular and micropore structure model was constructed using Material Studio software. Grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations were conducted to calculate the adsorption capacity, heat of adsorption, adsorption potential energy distribution, and diffusion coefficients of CO₂ and CH₄, under both single-component and binary gas mixture conditions, across a range of temperatures and pressures. The findings indicated that CO₂ exhibited a significantly higher adsorption capacity than CH₄ in the high-volatile bituminous coal. Under all investigated adsorption conditions, the heat of adsorption and adsorption potential energy of CO₂ were greater than those of CH₄, indicating a stronger affinity of the coal matrix surface for CO₂ during the competitive adsorption phase. This observation highlights the dominant role of CO2 in the adsorption process. Additionally, the diffusion coefficients of both gases decreased with increasing pressure, but increased with higher temperatures. Upon reaching adsorption equilibrium, the diffusion coefficient of CO₂ molecules consistently remained lower than that of CH₄, and in binary adsorption, the diffusion coefficient of CH₄ molecules gradually decreased with increasing CO₂ volume fraction. CO₂, through strong van der Waals forces and micropore confinement effects, forms a stable adsorbed state, thereby inhibiting CH₄ adsorption sites, and exhibiting superior long-term storage capabilities. These findings provide valuable insights for CO₂-ECBM and CO₂ sequestration strategies.