Study on gas desorption-diffusion behavior from coal particles using triple-pore structure models: Experimental, mathematical models, and numerical solutions

Abstract

To accurately describe gas emissions from coal particles, a conceptual model comprising macropores, mesopores, and micropores has been proposed. On the basis of the conceptual model, two mathematical models were developed for gas diffusion from coal particles: Model Ⅰ includes the effect of desorption, while Model Ⅱ does not. The models were then validated by comparing them to experimental data obtained from gas desorption diffusion kinetic tests. Meanwhile, the effective diffusion coefficients for pores of varying scales were gained and analyzed. The results indicated that both models were capable of providing a reasonable fit to the experimental data; however, Model Ⅱ was unable to provide a good fit to the experimental data from the initial stage, which Model Ⅰ captured well. This indicates that gas desorption could accelerate gas emission from coal particles in their initial state; the increase of the porosity of micropore or macropore has a significant effect on total gas emission and a moderate effect on the initial gas emission volume, whereas the variation of the porosity of mesosphere could have a remarkable influence. This demonstrated that the intensity of gas desorption in coal with abundant mesopores is greater than that of coal with fewer mesopores in the initial desorption phase. In Model Ⅰ, the effective coefficients for pores of different scales varied significantly with increasing pressure, whereas in Model Ⅱ, this was not the case, with the exception of the effective diffusivity of macropores. The results of the current study have significant implications for the determination of gas contents for gas resources calculations, gas flow in the coal matrix, and the prediction of coal seams outburst.