Effect of coal moisture content on gas desorption and diffusion: A theoretical model and numerical solution

Abstract

Moisture in coal plays a critical role in influencing gas desorption. This work numerically establishes a mathematical model for the gas desorption in spherical coal particles with different moisture contents based on the free gas density gradient (FGDG) concept. Importantly, the gas desorption pressure drop starting time (DPDST) was defined, and its response mechanism to moisture content was explored. Finally, the theoretical differences between the classical Fick's and FGDG models in describing the gas desorption/diffusion of water-containing coal were discussed in particular. The results show that the final stabilized dimensionless gas pressure in coal becomes closer and closer to the external initial dimensionless pressure as the coal moisture content increases. The DPDST is inversely proportional to the moisture content of the coal sample, possibly because more gas adsorption sites are occupied by water molecules at high moisture contents. Three different well-used empirical formulas for gas desorption were used for fitting, with one of the nth power of time t empirical formulas yielding the best fit. Based on the fitting results, the gas desorption amount limit is inversely proportional, and the gas desorption rate is proportional to the moisture content of the coal sample. On the basis of the diffusion coefficient inversion results, the prediction accuracy of the FGDG model is higher than that of the Fick's model. Theoretically, the diffusion coefficients of both models are inversely proportional to the coal moisture content, possibly because water molecules may occupy the adsorption sites of gas molecules and hinder the diffusion channel.