Calculation of gas concentration-dependent diffusion coefficient in coal particles: Influencing mechanism of gas pressure and desorption time on diffusion behavior

Abstract

To investigate the effects of equilibrium pressure and desorption time on gas diffusion in coal particles, we described the time-varying diffusion process by introducing an indeterminate function of desorption time. Using gas desorption data, we established an analytical model for calculating the gas diffusion coefficient under the different desorption times. The analytical solution of the concentration-dependent diffusion model is equivalent to that of the unipore diffusion model at the initial desorption time. Still, the diffusion coefficient decreased with the decrease of gas equilibrium pressure and the extension of desorption time. Additionally, the numerical method compared adsorbed and free gas effects on gas desorption. The results demonstrated that the diffusion coefficient considered adsorbed gas was more significant than the analytical solution's. The diffusion coefficient in the free gas system was overestimated due to the change of diffusion distance and decreased to the actual value over desorption time. However, the attenuation rate of diffusion coefficients in the adsorbed gas system positively correlates with gas pressure. The concentration gradient of free gas is the driving force for gas diffusion. The simulation and experimental results show that the free to total gas ratio has a strong linear relationship with the diffusion coefficient, which explains the influencing mechanism of gas pressure and desorption time on the gas diffusion.