Molecular simulation of superheated gas injection for CH4 desorption behavior

To investigate the effects of superheated N2, CO2, and H2O injection on CH4 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 CH4 on coal under conditions of 373.15–773.15 K and 1–10 MPa. Based on this, systems with adsorbed N2, CO2, and H2O were constructed, and molecular dynamics properties were analyzed using molecular dynamics simulations. The results show that desorption is endothermic, with the Coal/CH4/H2O system being the most stable. The average relative concentrations and average velocities of gases in the different systems follow the order: CH4>H2O > CO2>N2. As the temperature increases, the average relative concentration and velocity of CH4 in the vacuum layer of each system increase. In the Coal/CH4/H2O system, the relative concentration of CH4 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 CH4 in all systems show an increasing trend. The MSD-temperature slopes for CH4 are 1.094, 1.124, and 1.159 Å2/K. In the Coal/CH4/H2O system, the diffusion coefficient of CH4 experiences an increase, rising from 1.231 × 10−20 m2 ps−1 to 2.985 × 10−20 m2 ps−1. Compared with the injection of N2 and CO2, the injection of H2O reduces the diffusion activation energy for CH4 by 1.64 % and 0.83 % respectively. The interplay of these factors facilitates CH4 desorption, offering quantitative perspectives for enhancing the optimization of CH4 extraction.