Molecular dynamic simulations of selective self-diffusion of CH4/CO2/H2O/N2 in coal

Abstract

The self-diffusion coefficients (D) of CH4/CO2/H2O/N2 at a relatively broad range of temperatures(298.15∼ 458.15K)and pressures (1∼6MPa) under the NPT, NPH, NVE, and NVT ensembles were obtained after the calculations of molecular mechanics(MM), annealing kinetics(AK), giant canonical Monte Carlo(GCMC), and molecular dynamics (MD) based on Wiser bituminous coal model (WM). The Ds of the adsorbates at the saturated adsorption configurations are DCH4<DCO2<DH2O<DN2(NPT, 298.15K, 0.1MPa). The diffusion activation energy (E) is EH2O (1.07kJ/mol)<EN2(1.82kJ/mol)<ECO2 (2.94kJ/mol)<ECH4(10.88kJ/mol), indicating that CH4 diffusion requires to overcome the highest adsorption energy barrier and N2 and H2O to the lowest. The order of different ensembles is DN2 (NVE)< DN2 (NVT)≈DN2 (NPH)≈DN2 (NPT) (T<418K) and DN2 (NVE) is remarkable higher than other ensembles when T>418K. The average swelling ratios manifest as H2O (14.7∼35.18%)>CO2 (13.38∼32.25%)>CH4 (15.35∼23.71%)> N2 (11.47∼22.14%) (NPH, 1∼6MPa). There exits differences in D, swelling ratios and E among various ensembles, indicating that the selection of ensembles has an important influence on the MD calculations for self-diffusion coefficients.