Molecular Dynamic Simulation of Self- and Transport Diffusion for CO2/CH4/N2 in Low-Rank Coal Vitrinite

The self-, Maxwell-Stefan-, and transport diffusions of CO2, CH4, and N-2 in coal vitrinite macromolecules were simulated through molecular dynamics. Results indicated that these diffusion coefficients increase slowly when T < 340 K while rapidly at T > 340 K independent of the adsorbate numbers and types. The self- ([CO2] > [N-2] > [CH4] in order) and transport diffusion coefficients ([N-2] > [CO2] > [CH4] in order) decrease with increasing adsorbate number. The diffusion activation energy (Delta E) of vitrinite-n CO2 (5.07, 5.73, and 15.96 kcal/mol for vitrinite-5 CO2 vitrinite-10 CO2, and vitrinite-22 CO2 respectively) is lower than vitrinite-n CH4 (8.15, 8.97, and 17.09 kcal/mol for vitrinite-5 CH, vitrinite-10 CH4, and vitrinite-17 CH4 respectively). At the saturation adsorption state, the Delta E of vitrinite-7 N-2 (12.03 kcal/mol) is the lowest compared with vitrinite-22 CO2 and vitrinite-17 CH4, indicating that the diffusion process for N-2 is the easiest to inspire among these three gases. The swelling ratio ([CO2] > [CH4] > [N-2] in order) increases with the increasing temperature, indicating that high temperature is conducive for the swelling equilibrium. While the Delta E of pressure dependence first decreases with increasing pressure until the peak pressure (0.5-1.0, 1.5-2.0, and 2.5-3.5 MPa for CO2, CH4, and N-2 respectively) and then increases significantly, indicating that the diffusion energy barrier decreases with increasing pressure.