H2 promotes the premature replacement of CH4–CO2 hydrate even when the CH4 gas-phase pressure exceeds the phase equilibrium pressure of CH4 hydrate

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Submarine CO2 sequestration via solid hydrate has been considered a promising strategy for controlling greenhouse gas emissions. The simultaneous replacement of CH4 from subsea natural gas hydrates through buried CO2 offers a win-win approach. Utilizing H2 as an enhancer to promote CH4–CO2 hydrate replacement, coupled with integrating the replacement products into CH4 steam reforming for cyclic H2 production, presents a potential pathway for CO2 geological storage and eventual H2 recovery. However, the enhancement mechanism of H2 in the hydrate replacement process remains unclear. Focusing on the critical replacement condition, this study investigates the effects of gas-phase partial pressures on hydrate replacement in CH4-rich systems to elucidate the H2 influence mechanism. Remarkably, the results indicate, for the first time, that H2 induces premature CH4 hydrate dissociation under conditions exceeding the corresponding CH4 gas equilibrium pressure in CO2/CH4/H2 ternary gas systems. The critical condition for hydrate replacement depends on the total system pressure, rather than CO2 gas partial pressure. Additionally, in the CH4-rich systems, the driving force for CH4 hydrate decomposition takes precedence over that for hydrate reformation in controlling replacement. The replacement ratio increases from 16.7 % to 24.5 % as the CH4 partial pressure decreases from 2.01 MPa to 1.11 MPa at 274.15 K, despite simultaneous declines in both CO2 partial pressure and total system pressure from 1.70 MPa to 0.97 MPa and from 6.10 MPa to 3.39 MPa, respectively. These findings can provide insights into the improvement of CH4–CO2/H2 hydrate replacement efficiency and the development of oceanic CO2 sequestration.