Experimental Investigation on Diffusion Coefficients of CH₄ in Coal under High-Temperature and -Pressure Conditions

Here, a self-developed experimental system of diffusion at high temperatures and pressures was utilized to clarify the impacts of high pressure and temperature (303, 323, 343, and 363 K; 2, 6, 12, and 20 MPa) and coal ranks on diffusion properties in coal. At constant temperature and pressure, the diffusion coefficients of CH₄ gradually decrease with increasing particle size and increase gradually with the increasing pressure and temperature. However, the diffusion coefficients first decrease and then increase with the increasing coal rank. The minimum diffusion coefficients occur at about R _o,max = 1.7%. The temperature and pressure sensitivity of CH₄ diffusion was quantified via the experimental data. The increase extent first increases and then decreases with the increasing pressure. The peak pressure for the increase extent was 6 MPa for all coals. The increments of diffusion rate per pressure unit were 1.50, 0.90, and 0.61%, indicating that the impact of pressure at the low-pressure stage (2-6 MPa) was more significant than that at the high-pressure stage (6-20 MPa). At the same pressure, the diffusion rates overall increase with the increasing temperature. At the low-pressure stage (2-6 MPa), the impacts of pressure were more significant than at the high-pressure stage. However, as for the temperature dependence, the impacts of temperature were more significant at the high-pressure stage than at the low-pressure stage.