Complementary and competitive dynamics of CO2 and N2 in CH4 – Flue gas replacement within natural gas hydrates

To mitigate global warming, the paramount imperative lies in curbing the emission of CO₂. The guest replacement method is a prominent carbon-neutral technological advancement that involves injecting CO₂ into natural gas hydrate layers to accomplish the dual objectives of energy production and carbon storage. In this study, the guest dynamics in the CH₄ – flue gas replacement process were examined, and the impacts of the N₂ concentration of the injected gas were systematically analyzed. A powder X-ray diffraction analysis of the cage-specific guest distributions after CH₄ − CO₂ (20 %) + N₂ (80 %) replacement revealed that CH₄ production increased in both the large and small cages compared to the CH₄ – CO₂ replacement. This enhancement was attributed to the N₂ molecules participating in both cages. However, this simultaneously led to a decrease in CO₂ storage potential, indicating a ‘complementary’ relationship for CH₄ production and a ‘competitive’ one for CO₂ storage with respect to CO₂ and N₂. In situ Raman spectroscopy revealed that the introduction of N₂ resulted in a deceleration of CO₂ storage kinetics. Guest composition measurements after replacement showed an upward trend in CH₄ production and a simultaneous decline in CO₂ storage as the N₂ composition increased. Notably, an intriguing correlation was established between the CO₂/N₂ ratios for the injected gas and the replaced hydrates, exhibiting a strong alignment with a simple first-order equation. The findings not only contribute to a deeper understanding of the CH₄ − CO₂ + N₂ replacement technique but provide practical insights for its application in real-world scenarios.