Effects of moisture on diffusion kinetics in Chinese coals during methane desorption.

Many experimental measurements have been conducted to evaluate the effect of moisture on gas sorption and diffusion kinetics in coal but these works normally focus on the adsorption process that takes place during carbon dioxide injection. While for coalbed methane recovery, methane desorption dominates. Therefore, the conclusions on adsorption kinetics may be not always valid for the desorption-dominated coalbed methane recovery. In this paper, a series of methane desorption measurements were conducted on pulverized Chinese coal samples. Then the diffusion kinetics were modeled using the unipore model and the bidisperse model. The results show that the bidisperse model is more suitable than the unipore model for modeling the diffusion kinetics in coals used in this study. The diffusivity value of the anthracite is lower compared with the bituminous coal. The moisture effects on diffusion kinetics are different between the two rank coals used in this work. For the anthracite, diffusivity decreases with increasing moisture content, while for the bituminous coal, it varies with the moisture content according to a U-shape function. The U-shape relationship between diffusivity and moisture content for bituminous coal during methane desorption process has not been observed in experimental measurements on adsorption kinetics, which normally show that the diffusivity varies monotonically with moisture content. This difference may be due to the different moisture effects on adsorption and desorption. This indicates that the diffusion kinetics are different between adsorption and desorption, and the conclusions based on adsorption kinetics may be invalid for coalbed methane recovery because it is the desorption that takes place during methane production. [ABSTRACT FROM AUTHOR]