Variation of Surface Free Energy in the Process of Methane Adsorption in the Nanopores of Tectonically Deformed Coals: A Case Study of Middle-Rank Tectonically Deformed Coals in the Huaibei Coalfield

Methane adsorption of coal is essentially a process of energy and material state transformation. It is a complex process especially for tectonically deformed coals (TDCs). In order to clarify this process, 12 middle-rank TDCs were screened for the experimental and theoretical study. On the basis of the liquid nitrogen adsorption experiment and methane isothermal adsorption experiment, the nanopore structure and the methane adsorption capacity of the TDC samples were analyzed. Using the theories of Polanyi adsorption potential and surface free energy reduction (SFER), the variation of energy in the process of methane adsorption in nanopores was explored. The following results were obtained. (1) The Langmuir volume has a stronger positive relationship with the specific surface area of the ultra-micropore, mild-micropore, and micropore than with those of the transitional pore. The adsorption potential decreases as the adsorption space grows; however, it rises as tectonic deformation enhances. The increase in the ratio of specific surface area for the mild-micropore and ultra-micropore indicates that the tectonic deformation can enhance both the available adsorption area and adsorption potential for methane in TDCs, eventually resulting in the increase of the Langmuir volume and the SFER. (2) The methane molecules preferentially occupy the sites with strong adsorption potential on the coal surface during the strong adsorption stage, resulting in the sharp increase of the methane adsorption volume and SFER. Then, the number of sites with strong adsorption potential decreases and the methane molecules have to settle at the energetically suboptimal sites during the weak adsorption stage, leading to the slight increase of methane adsorption volume and SFER. The above results may help to improve the accuracy of coalbed methane resource estimations and gas outburst prediction in the area where TDCs are developed.