Molecular-Scale Mechanisms of CH4 Hydrate Displacement Using Multi-Component CO2-Rich Industrial Waste Gas: Impacts of Gas Composition, Thermodynamic Conditions, and Salinity on Structural Evolution and Mass Transfer.

The CH4/CO2 displacement method is regarded as a highly promising approach for natural gas extraction, which can improve CH4 recovery efficiency while achieving carbon sequestration. However, CO2 purification is technologically complex and energy-intensive, and the replacement performance remains limited. Directly injecting CO2-rich industrial waste gases can not only improve the CH4 recovery efficiency but also achieve waste gas sealing, reducing the cost of gas separation and purification. In this study, a model for replacing CH4 hydrate with CO2-rich industrial waste gas is established based on the molecular dynamics method. The effects of temperature, pressure, concentration of industrial waste gas, and salinity on the molecular structure evolution and mass transfer behavior during the displacement process were comprehensively analyzed. The results indicate that an increase in temperature can significantly promote the release of CH4 and the formation of CO2 hydrates within the simulation range. The influence of pressure on the displacement process is relatively limited. Overall, the increase in pressure enhances the spatial correlation between CO2 and the hydrate cage, promoting the displacement and sequestration of CO2. Compared with pure CO2, the introduction of an appropriate amount of N2 and NO can promote the displacement process, indicating that a small amount of associated components have a synergistic regulatory effect in confined systems. Salinity exerts a significant inhibitory effect on the displacement process by weakening the hydrogen bond network of H2O molecules and the intermolecular spatial correlation. This study offers a molecular-scale basis for mechanism analysis and operational condition evaluation of CO2 participation in hydrate displacement within multicomponent industrial waste gas environments.