Molecular simulation of the adsorption and diffusion properties of CH4 and CO2 in the microporous system of coal

Abstract

Micropores are the main adsorption sites for gases in coal. The research of the adsorption and diffusion pattern of gases in micropores is important for CO2-ECBM engineering and other measures to improve production. In this study, a multiscale microporous system with intramolecular pores, intermolecular pores, and slit pores is proposed. Adsorption and diffusion characteristics of CH4 and CO2 in different microporous systems were investigated. The results show that the adsorption of different microporous systems is basically consistent at low pressures (<2 MPa). Upon further increase in pressure, adsorption increases with increasing slit pore size. Gas molecules in coal are preferentially adsorbed in the intramolecular and intermolecular pores, and gas molecules in the slit pores are preferentially adsorbed in the slit pore walls. The increase in gas pressure and slit pore size decreases the proportion of gases adsorbed in the intramolecular and intermolecular pores and also increases the proportion adsorbed in the slit pores. Increase in size of the slit pores and decrease in adsorption will result in an increase in self-diffusion coefficients of gases. The results of self-diffusion coefficients of pores with different scales indicate that slit pores have the largest self-diffusion coefficients, followed by the intermolecular pores, and the smallest for the intramolecular pores. The isosteric heat of gases tends to decrease and then increase with increasing adsorption. The increase of slit pore size will decrease interaction energy. This study gives a reference for adsorption and diffusion mechanisms of gases in microporous systems, as well as a basis for gas injection displacement studies.