Influence of Coal Slit Pore Dimensions on the Adsorption-Diffusion Properties of CH ₄ and CO ₂ Based on GCMC and MD

In order to quantify the regulatory pattern of coal pore diameter on the thermodynamics of CH4/CO2 adsorption, the energy and diffusion response mechanisms of pore diameter were elucidated. To examine the adsorption and diffusion behaviors of CH4 and CO2 in coal slit pores, this study utilizes Grand Canonical Monte Carlo and Molecular Dynamics methods to analyze these characteristics across slit pores of various sizes and analyzes the influences of slit pore size on the thermodynamic properties, energy distribution, interaction energy, and the diffusion coefficients of CH4 and CO2 adsorption to determine the adsorption configurations at various pressures and slit pore sizes. Research indicates that the isosteric adsorption heat of CO2 decreases rapidly to a minimum as the absorption capacity increases, then rises continuously with further increases in absorption capacity. When the coal slit pore diameter increases from 0.4 to 8.0 nm, the average isosteric adsorption heat for CH4 and CO2 decreases by 59.27 and 35.19%, respectively. At a pressure of 10 MPa, the change of Gibbs free energy of CH4 (ΔG) was found to increase from −4.44 to −2.45 kJ/mol as the pore size of the slit expanded from 0.4 to 8.0 nm, and ΔG of CO2 increased from −8.37 to −2.46 kJ/mol. An increase in coal slit pore diameter is detrimental to gas adsorption. As the slit pore diameter increases, the pressure effect on Gibbs free energy diminishes, with the pressure influence acting only on micropores and having a negligible effect on pores larger than 6.0 nm. At 10 MPa, the interaction energy of CO2 in a 1.6 nm slit pore is 5.68 times that of CH4. When the slit pore diameter exceeds 3.0 nm, the proportion of electrostatic forces increases rapidly, leading to capillary condensation of CO2.As the slit aperture expanded from 0.4 to 8.0 nm, the diffusion coefficient of CH4 increased from 0.58 × 10–10 to 2.8842 × 10–8 m2/s, and the diffusion coefficient of CO2 increased from 0.65 × 10–10 to 0.2795 × 10–8 m2/s.