Multiscale Origins of Low Temperature Induced Gas Adsorption Promotion and Desorption Inhibition in Coking Coal: Insights from Pore Structure and Molecular Dynamics

Abstract

Abstract Accurate assessment of gas adsorption and desorption behavior in coal at low temperatures is critical for both deep coalbed methane recovery and the emerging freezing coring technique. This study presents a multiscale investigation integrating low temperature desorption experiments, pore structure characterization, and molecular dynamics simulations to elucidate the microscopic mechanisms underlying the pronounced CH4 adsorption promotion and desorption inhibition effects in coking coal. Macroscopic desorption tests conducted across 238.15–268.15 K reveal that low temperature strongly suppresses gas release, with the desorption inhibition ratio reaching up to 38.2% within 30 min at 238.15 K relative to ambient conditions. Complementary pore structure analyses demonstrate that freezing treatment induces significant contraction of micropores and mesopores, with the micropore volume decreasing by 2.41% and the specific surface area declining by 21.35%, trends that align closely with the 2.19% reduction in fractional free volume derived from molecular simulations. Grand Canonical Monte Carlo simulations show that the CH4 adsorption capacity at 238.15 K and 7 MPa reaches 8.417 mmol/g, representing a 135.9% increase over that at 368.15 K, while the isosteric heat of adsorption (11.525–13.767 kJ/mol) confirms the purely physical nature of the adsorption. The proportion of high-energy adsorption sites surges from 2.56% at 368.15 K to 41.02% at 238.15 K. Molecular dynamics simulations further reveal that the self-diffusion coefficient decays following a power-law relationship with temperature, a trend independently corroborated by the experimentally derived apparent diffusion coefficients. The diffusion activation energy (12.55 kJ/mol) closely matches the isosteric heat of adsorption at low coverage, indicating that desorption from pore wall adsorption sites constitutes the rate-controlling step for CH4 transport in micropores. The fundamental origin of the coupled adsorption–desorption behavior is attributed to the synergistic operation of thermodynamic and structural dynamic pathways. Low temperature attenuates molecular kinetic energy and deepens adsorption potential wells, while simultaneously inducing coal matrix thermal contraction that compresses free volume and narrows diffusion channels. These findings provide a basis for refining gas loss estimation models employed in the freezing coring technology.