Dynamic Evolution and Governing Mechanisms of the CH4 Diffusion Coefficient during Desorption from Anthracite Particles under Thermo–Hydro–Mechanical Coupling

The diffusion coefficient governs the microscopic transport rate of gas in coal and is thus critical to evaluating coalbed methane extraction. To reveal its evolution and quantify the effects of individual factors and their interactions, a fully coupled thermal–gas diffusion–mechanical model of methane desorption and diffusion in coal particles was developed, incorporating the dependence of the diffusion coefficient on adsorption equilibrium pressure, temperature, and porosity. Furthermore, four multiphysics coupling scenarios were developed to quantitatively elucidate the effects of coal deformation and thermal effects on the gas diffusion coefficient. The model was solved in COMSOL Multiphysics and validated against methane desorption–diffusion experiments on anthracite particles, showing good reliability. Simulations show that CH4 desorption content relates to the diffusion coefficient exponentially, and that the coefficient increases with initial temperature and adsorption equilibrium pressure but decreases with initial porosity. Among the four multiphysics scenarios, its magnitude follows: gas diffusion–coal deformation > gas diffusion alone > gas diffusion–coal deformation–temperature > gas diffusion–temperature; relative to gas diffusion alone, coal deformation raises the coefficient by 1.11% on average, whereas gas–temperature and gas–coal deformation–temperature coupling reduce it by 9.32% and 8.06%, respectively. Range analysis and ANOVA indicate that adsorption equilibrium pressure and initial porosity are both significant, with influence ranked as follows: adsorption equilibrium pressure > initial porosity > initial temperature. These findings support methane disaster prevention and efficient coal mine methane utilization.