Comparisons of Methane Adsorption/Desorption, Diffusion Behaviors on Intact Coals and Deformed Coals: Based on Experimental Analysis and Isosteric Heat of Adsorption

Abstract

Isosteric heat of adsorption is a key parameter to assess the thermodynamic properties in coals for gas adsorption and migration applications. To explore the relationships between isosteric heat of adsorption and gas storage and migration in intact and deformed coals, this paper carried out the isotherms experiments of CH4, low-temperature N2, and low-pressure CO2 on four coal samples with different deformation intensities. The results show that Langmuir VL in coal was dependent on temperature and coal structures. From 25 to 45 °C, Langmuir VL in coals decreased with the increased temperatures. Adsorption heat in intact coals was higher than deformed coals, illustrating that interaction actions between gas molecules and coal atoms were more intense in intact coal, which led to the hard desorption of gas molecules from intact coal surfaces. The specific area and pore volume of ultramicropore (<2 nm) and larger pores (>2 nm) positively increased with coal deformation intensities. The diffusion coefficients of four coal structures decreased as the depressurization processes, and the diffusion coefficient of tectonic coals was higher than intact coal in the whole diffusion stages. At a pressure zone of higher than 5 MPa, 1.5–5 MPa, and lower than 1.5 MPa, gas diffusion mainly occurred in macropores, mesopres, and micropores, respectively, corresponding to Fick’s, transitional, and Knudsen diffusion. Additionally, it is found that the isosteric heat of adsorption can be effectively used to predict adsorption isotherms in coals, and the high initial diffusion coefficient in deformed coals is related to the low isosteric heat of adsorption.