Modelling of multiple gas transport mechanisms through coal particle considering thermal effects

Abstract

Abstract Multiple gas diffusion mechanisms coexist in the nanopores of coal particle, including Knudsen diffusion, slip flow and transition flow of bulk gas and surface diffusion of adsorbed gas. The contribution of each mechanism to the total gas diffusion quantity varies with gas pressure and pore radius. In this study, a coupled thermal-diffused-mechanical model was constructed considering the multiple gas transport mechanisms. The complex coupling relations among gas adsorption/desorption, gas diffusion field, mechanical deformation field and thermal field during gas diffusion of coal were considered. The contribution of each diffusion mechanisms to the total gas diffusion quantity under variable pressure and pore radius was discussed. Thermal effects of gas diffusion were studied by considering the whole diffusion process as adiabatic. The results show that Knudsen diffusion is dominant in the low-pressure region, regardless of the pore size. In the high-pressure region, the slip flow is dominant when the pore radius is large, while the surface diffusion is dominant when the pore radius is relatively small. The gas diffusion velocity and total diffusion quantity under adiabatic conditions are lower than that under isothermal conditions. But the rate of pressure drop is opposite, which can be explained by the ideal gas equation of state. In reality, the gas diffusion process is neither isothermal nor adiabatic, but between the two states. The thermal effect should be considered in the study of gas diffusion properties.