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

The self-diffusion coefficients (D) of CH4/CO2/H2O/N-2 at a relatively broad range of temperatures(298.15 similar to 458.15K) and pressures (1 similar to 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 D-CH4<D-CO2<D-H2O<D-N2(NPT, 298.15K, 0.1MPa). The diffusion activation energy (E) is E-H2O (1.07kJ/mol)<E-N2(1.82kJ/mol)<E-CO2 (2.94kJ/mol)<E-CH4(10.88kJ/mol), indicating that CH4 diffusion requires to overcome the highest adsorption energy barrier and N-2 and H2O to the lowest. The order of different ensembles is D-N2 (NVE)<D-N2 (NVT)approximate to D-N2 (NPH)approximate to D-N2 (NPT) (T<418K) and D-N2(NVE) is remarkable higher than other ensembles when T>418K. The average swelling ratios manifest as H2O (14.7 similar to 35.18%)>CO2 (13.38 similar to 32.25%)>CH4 (15.35 similar to 23.71%)> N-2 (11.47 similar to 22.14%) (NPH, 1 similar to 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.