Abstract:

To investigate CH₄ adsorption/desorption characteristics at low temperatures (particularly < 273.15 K), Grand Canonical Monte Carlo (GCMC) and MD methods were employed to systematically simulate the adsorption and diffusion behavior of CH₄ in a coking coal macromolecular model from 238.15 to 368.15 K. Additionally, the intrinsic molecular-scale relationship between the adsorption enhancement and diffusion inhibition effect of CH₄ was revealed. The results show that the CH4 adsorption capacity at low temperatures is significantly higher than that at high temperatures. At 7 MPa, the CH₄ adsorption capacity at 238.15 K (8.417 mmol/g) is 135.9% higher than that at 368.15 K (3.568 mmol/g). The isosteric heat of adsorption for CH₄ ranged from 11.365 to 13.604 kJ/mol and decreases with increasing adsorption amount, indicating physical adsorption. As temperature decreases from 368.15 to 238.15 K, the FFV of coking coal shows a linear decreasing trend. Meanwhile, the self-diffusion coefficient of CH₄ molecules decreases as a power function, with the value at 238.15 K (0.37×10^-8 m²/s) being an order of magnitude lower than at 368.15 K (4.417×10^-8 m²/s). The microscopic essence of the low-temperature promotion of adsorption and inhibition of desorption lies in the synergistic coupling of thermodynamic and structural-dynamic factors. Low temperature weakens the kinetic energy of CH₄ molecules, which leads to an increase in their average adsorption energy and significant raise in the desorption energy barrier. Simultaneously, it induces coal matrix contraction (reduced in FFV), leading to a decrease in the effective size of CH₄ diffusion pathways and an increase in diffusion resistance.

Key words: low-temperature environment, CH₄ adsorption, adsorption promotion and desorption inhibition, fractional free volume(FFV), self-diffusion coefficient, molecular dynamics(MD)