Multicomponent Competitive Adsorption of CH₄ with Injected Superheated CO₂, N₂, and H₂O in Lignite

Investigating competitive adsorption between injected gases and CH₄ is crucial for enhancing coalbed methane recovery. Two research gaps remain: the competitive adsorption of H₂O with CH₄ and the behavior of superheated gases with CH₄ have not been systematically studied. Here, a lignite supercell model was constructed, and grand canonical Monte Carlo and molecular dynamics simulations were performed at 473.15 K and 0-10 MPa. In the CH₄/H₂O binary system, the selectivity coefficient increases with pressure, indicating that low-pressure H₂O injection (<3 MPa) offers the strongest selectivity advantage, while the marginal benefit above 7 MPa approaches zero. At 90% injected gas ratio, the maximum atomic density (H₂O:0.01, CO₂:0.07, N₂:0.10) and potential energy peak (H₂O: -5.95, CO₂: -3.75, N₂: -3.35 kcal/mol) of CH₄ evaluate the displacement limits from "quantity" and "quality" perspectives, respectively. In the N₂/CH₄/CO₂ ternary system, a CH₄ molar fraction of 30% is identified as the competitive adsorption turning point: below 30%, N₂ and CO₂ jointly suppress CH₄ (favorable for displacement); above 30%, CH₄ gradually dominates. This critical composition is verified by both maximum atomic density and potential energy peaks, providing a quantitative basis for optimizing gas injection ratios. This study fills the knowledge gap in superheated gas-CH₄ competitive adsorption and supports enhanced CBM recovery.