A Novel Porous Diffusion Model during Gas Desorption in Coal Based on Fractal Characteristics of Pore Structure

In order to clarify the relationship between the conventional diffusion models and the time-dependent diffusion model during gas desorption, a porous diffusion model during gas desorption in coal was established by modeling the pore structure according to the porous diffusion theory and fractal theory. Gas diffusion processes were experimentally measured using a series of coal samples with different ranks and types under different gas equilibrium pressures. And the established theoretical model was furthermore verified by experimental data. The theoretical data derived from the established porous diffusion model agreed well with the experimental data with correlation coefficients greater than 99.0%. The attenuation of diffusion coefficient during gas desorption was essentially attributed to the gradual decrease of the gas diffusion amount for large pores during unsteady diffusion of gas in porous media. The effective diffusion coefficient presented a first fast and then slow decreasing trend with the prolonged diffusion time. The diffusion coefficient increased with the increase of gas equilibrium pressure as well as the metamorphic grade and fragmentation degree of coal. The pore connectivity enhanced as the surface area proportion of larger pores increased, leading to an increase in the proportion of the gas diffusion quantity in larger pores and the diffusion coefficient and meanwhile a decrease in the fractal dimension. This study will provide theoretical support for in-depth understanding of the gas diffusion mechanism in gas-containing coal.