In-situ deformation behavior and mechanisms of fat coal nanopores during the gas adsorption process

Nanopores serve as the primary adsorption sites for gases in coal, directly influencing the gas adsorption, desorption, and diffusion capacities. This study employs a self-developed high-pressure gas adsorption-desorption apparatus based on small-angle X-ray scattering (SAXS) technology and a synchrotron radiation SAXS experimental station to investigate the effects of different gas types (CH₄, CO₂, N₂) and gas pressures on the evolution characteristics of nanopores in coal during the adsorption process. Using the grand canonical Monte Carlo simulation method in Materials Studio software, the Sorption module was utilized to simulate the adsorption of three gases. Adsorption systems with five slit pore sizes (0.5 nm, 1.0 nm, 2.0 nm, 3.0 nm, and 5.0 nm) were established to further explore the intrinsic relationships among nanopores, adsorption pressure, gas types, and interaction energy, thereby revealing the in-situ deformation mechanism of nanopores in fat coal during gas adsorption. The results indicate that under an adsorption pressure of 1.0 MPa, the maximum porosity values of coal adsorbing CH₄, CO₂, and N₂ are 19.57%, 20.24%, and 19.37%, respectively, while the maximum specific surface area values are 9.83 m²/g, 10.53 m²/g, and 8.84 m²/g, respectively. The adsorption of CO₂ has the most significant impact on coal porosity and specific surface area. When investigating CH₄ adsorption, a positive correlation is observed between gas adsorption pressure and both porosity and specific surface area of coal. As the number of nanopores increases, the interaction force between coal macromolecules and gas molecules weakens, leading to a decreasing trend in interaction energy. Moreover, in adsorption simulation systems with the same nanopore size of 2 nm, the interaction energy between CO₂ and coal macromolecules is significantly greater than that of CH₄ and N₂. The adsorption potential energy of the three gas molecules follows the order: CO₂ > CH₄ > N₂, further elucidating the intrinsic relationship between interaction energy and nanopore deformation.