Modeling and experiments for transient diffusion coefficients in the desorption of methane through coal powders

Diffusion is widely thought to be the key factor that controls the desorption of gas through porous media. However, the classical diffusion model with a constant Fick diffusion coefficient is inadequate for describing desorption from coal over the entire timescale. In this study, a new model to describe the time-dependent diffusion coefficients during desorption was developed. Experiments including proximate analyses, isothermal adsorptions, mercury intrusions, N2 adsorptions and methane desorptions were conducted to verify its validity. The results demonstrated that the Fick diffusion coefficients decreased over the desorption period first sharply and then slowly. By introducing the time-dependent relationship into the general unipore model, we obtained a unipore diffusion model with D(t) (UDMD), which had a better degree of fit with the laboratory desorption than that of the original. Moreover, the simplification for the UDMD was acquired for engineering applications.