Study on competitive adsorption of multicomponent gas in gas-fat coal based on GCMC and molecular dynamics

To investigate the occurrence characteristics of CH₄, CO₂, and H₂S in gas-fat coal, reveal their adsorption kinetic processes in gas-fat coal at different temperatures and the microscopic mechanism of competitive adsorption among the three gases under different volume fractions, the competitive adsorption of binary mixtures (CH₄/H₂S and CO₂/H₂S) in coal seams was studied using Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) methods under conditions of 273.15-313.15 K and 0-3 MPa. The influences of volume fractions on gas adsorption capacity, isosteric heat of adsorption, energy distribution, selectivity coefficient, and probability density distribution were analyzed. The results show that the order of gas adsorption capacity in coal from high to low is H₂S > CO₂ > CH₄. The adsorption kinetics of CH₄, CO₂, and H₂S agree well with the pseudo-second-order kinetic model, with average fitting degrees of 0.993 0, 0.999 7, and 0.999 8, respectively. At adsorption equilibrium, the adsorption capacity gradually decreases with increasing temperature, while the second-order adsorption rate constant k₂ first decreases and then increases with rising temperature. When the volume fraction of CH₄ increases from 10% to 90%, the saturated adsorption capacity of CH₄ rises from 1.86 cm³/g to 7.89 cm³/g, while that of H₂S decreases from 41.54 cm³/g to 28.71 cm³/g. As the volume fraction of CO₂ increases from 10% to 90%, the saturated adsorption capacity of CO₂ increases from 6.64 cm³/g to 26.66 cm³/g, whereas that of H₂S decreases from 36.17 cm³/g to 10.86 cm³/g. The adsorption selectivity coefficients of the CH₄/H₂S and CO₂/H₂S binary systems decay exponentially with increasing equilibrium pressure: they decrease rapidly at first, then slowly, and finally tend to be stable.