Molecular simulation of superheated gas injection for CH4 desorption behavior

To investigate the effects of superheated N 2 , CO 2 , and H 2 O injection on CH 4 desorption behavior in coal, this study employs the grand canonical Monte Carlo method to determine, for the first time, the lowest energy adsorption configuration of CH 4 on coal under conditions of 373.15–773.15 K and 1–10 MPa. Based on this, systems with adsorbed N 2 , CO 2 , and H 2 O were constructed, and molecular dynamics properties were analyzed using molecular dynamics simulations. The results show that desorption is endothermic, with the Coal/CH 4 /H 2 O system being the most stable. The average relative concentrations and average velocities of gases in the different systems follow the order: CH 4 >H 2 O > CO 2 >N 2 . As the temperature increases, the average relative concentration and velocity of CH 4 in the vacuum layer of each system increase. In the Coal/CH 4 /H 2 O system, the relative concentration of CH 4 increases from 1.411 to 1.453, while its velocity increases from 2.057 Å ps −1 to 2.283 Å ps −1 . Moreover, as temperature increases, the MSD and diffusion coefficients of CH 4 in all systems show an increasing trend. The MSD-temperature slopes for CH 4 are 1.094, 1.124, and 1.159 Å 2 /K. In the Coal/CH 4 /H 2 O system, the diffusion coefficient of CH 4 experiences an increase, rising from 1.231 × 10 −20 m 2 ps −1 to 2.985 × 10 −20 m 2 ps −1 . Compared with the injection of N 2 and CO 2 , the injection of H 2 O reduces the diffusion activation energy for CH 4 by 1.64 % and 0.83 % respectively. The interplay of these factors facilitates CH 4 desorption, offering quantitative perspectives for enhancing the optimization of CH 4 extraction. • The lowest energy adsorption configuration of CH 4 on coal under conditions of 373.15–773.15 K and 1–10 MPa was determined for the first time using the Grand Canonical Monte Carlo method. •In the Coal/CH 4 /H 2 O system, CH 4 has the steepest MSD-temperature slope (1.59 Å 2 /K) and highest diffusion coefficient-temperature slope (0.0043 × 10 −20 m 2 ps −1 ·K −1 ). •In the Coal/CH 4 /H 2 O system, the diffusion activation energy for CH 4 showing a decrease of 1.64 % and 0.83 % compared to the N 2 and CO 2 systems.