Influence of Competitive Inclusion of CO₂ and N₂ on sII Hydrate-Flue Gas Replacement for Energy Recovery and CO₂...

This study investigated the structural transformation, guest distributions, and the extent of replacement in CH₄ + C₃H₈-flue gas replacement occurring in sII hydrates via gas chromatography, NMR spectroscopy, and powder X-ray diffraction (PXRD). Simulated flue gas (CO₂ (20%) + N₂ (80%)) was injected into an sII CH₄ (90%) + C₃H₈ (10%) hydrate for guest exchange. The extent of replacement occurring in CH₄ + C₃H₈-flue gas replacement was much lower than that of CH₄ + C₃H₈-CO₂ replacement. Furthermore, ¹³C NMR spectra and PXRD patterns revealed that unlike CH₄ + C₃H₈-CO₂ replacement, CH₄ + C₃H₈-flue gas replacement did not undergo any structural transformation during the replacement (i.e., iso-structural replacement in the sII hydrate). Rietveld refinement of PXRD patterns of gas hydrates after replacement using flue gas injection demonstrated that CO₂ molecules occupied both the small (5¹²) and large (5¹²6⁴) cages, whereas N₂ molecules occupied only the small (5¹²) cages. CO₂ and N₂ were not complementary but competitive in replacing CH₄ in the small (5¹²) cages, which contributed to the maintenance of the cage stability of the initial sII hydrate and thus, resulted in a lower extent of replacement. The experimental results obtained in this study provide valuable insights on the accurate replacement mechanism and cage-specific guest exchange behavior of sII hydrates using flue gas injection for energy recovery and CO₂ sequestration.