Detailed study on self- and multicomponent diffusion of CO 2 -CH 4 gas mixture in coal by molecular simulation

Gas diffusion plays a key role in CO 2 -enhanced recovery of coal bed methane (ECBM), where more than one types of gases coexist and multicomponent gas diffusion occurs. Such process is now usually described by non-coupled two-component gas diffusion equations which exclude the interactions between gases. Self-diffusion and mutual diffusion of CO 2 –CH 4 mixture are investigated through molecular simulation for the first time. The self-diffusion coefficients of CO 2  and CH 4 decrease with gas concentration but increase with temperature. The mutual diffusion coefficients of binary gas mixture of CO 2 –CH 4  in coal are computed through Maxwell–Stefan diffusion theory. A 2D diffusivity matrix | D | (with diagonal element D i and non-diagonal element D ij ) is obtained to depict the mutual diffusion of the gas mixture. It is found that CO 2 (CH 4 ) diffusion is coupled with CH 4 (CO 2 ). The diffusion coupling strength of CO 2  and CH 4 decreases with increasing gas concentration. Temperature positively affects D ij but minimally influences D i , resulting in large ratios of D ij / D i at high temperatures. It means that CO 2 –CH 4 diffusion correlation interactions, which are not present in non-coupled pure gas diffusion equations, are necessary to analyze gas mixture diffusion in coal. In this case, non-coupled pure gas diffusion equations are inadequate for description of CO 2 –CH 4 mixture diffusion. The coupling between the gases can be ignored only at very high temperatures (T > 400 K), which means a large depth of coal bed in ECBM engineering.