Molecular simulation study of adsorption-diffusion of CH₄, CO₂ and H₂O in gas-fat coal

In order to clarify the microscopic dynamics mechanism of CH₄, CO₂ and H₂O adsorption and diffusion in coal, and to reveal the mechanism of the influence of different temperatures and pressures on the adsorption and diffusion characteristics of coal adsorbed CH₄, CO₂ and H₂O molecules. In this paper, the macromolecular structure model of Jixi gas-fat coal was constructed, based on the Giant Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) methods. The adsorption-diffusion characteristics of CH₄,CO₂ and H₂O single-component gases in the gas-fat coal macromolecule model at temperatures ranging from 273.15 K to 313.15 K and pressures ranging from 0.01 MPa to 15 MPa were investigated by using Material Studio software. The research results indicated that: The adsorption of three gases, CH₄, CO₂ and H₂O, increased with the increase of equilibrium pressure, and the adsorption isotherms conformed to Langmuir type I isotherms. The amount of saturated adsorption of CH₄ ranged from 11.18 to 14.37 ml/g, the saturated adsorption of CO₂ ranged from 20.40 to 24.70 ml/g, and the saturated adsorption of H₂O ranged from 66.61 to 84.21 ml/g. With the increase of temperature, the saturated adsorption of CH₄ and CO₂ both decreased, and the saturated adsorption of H₂O firstly increased and then decreased, and the adsorption of H₂O by low temperature and high temperature had both an inhibitory effect on the adsorption of H₂O. The potential energy distributions of CH₄, CO₂ and H₂O molecules are poisson distributed. The absolute values of the most available interaction energies are, from highest to lowest: H₂O > CO₂ > CH₄; the activation energies for diffusion of CH₄, CO₂ and H₂O are 12.20 kJ/mol, 3.36 kJ/mol, and 8.47 kJ/mol, respectively, and the diffusion of CO₂ is the more likely to occur. The adsorption of CH₄ and CO₂ in coal is physical adsorption, while the adsorption process of H₂O molecules is beyond the scope of physical adsorption. The absolute value of the interaction energy is H₂O > CO₂ > CH₄ in descending order.