Enhancing hydrate-based natural gas storage capacity via optimal concentrations of epoxycyclopentane

With the growing demand for energy, natural gas has gained attention as a lower-emission option compared to the other conventional fossil fuels such as oil and coal. Consequently, there is significant interest in developing energy-efficient natural gas storage technologies. Gas hydrate-based storage offers a promising solution due to its relatively high capacity at moderate operating pressure and temperature conditions. Epoxycyclopentane (ECP) has emerged as an effective thermodynamic promoter, but its limited miscibility in water can influence storage capacity depending on its concentration. This study investigated the effect of varying ECP concentrations (4.0, 5.0, 5.6, 6.0, 6.5, and 7.0 mol%) on the gas storage capacity of hydrates formed from pure CH₄ and a natural gas mixture (CH₄ (90%) + C₂H₆ (7%) + C₃H₈ (3%)). We systematically examined thermodynamic stability, formation kinetics, crystalline structure, and guest distribution in ECP hydrates. The results revealed that ECP significantly promoted hydrate thermodynamic formation across all concentrations, with the optimal concentration for natural gas storage identified as 6.5 mol%, which achieved the highest gas storage capacity. Synchrotron XRD confirmed the formation of sII hydrates in both ECP + natural gas and ECP + CH₄. Raman and ¹³C solid-state NMR analyses showed no CH₄ occupancy in sII-L, which suggests the absence of pure natural gas hydrates. Cage occupancy estimation revealed that the highest CH₄ occupancy in sII-S cages occurred at 6.5 mol% ECP, which contributed to an enhanced gas uptake. Time-dependent in-situ Raman and NMR analyses demonstrated that excess ECP can induce a two-step formation process, which further enhances storage capacity. Hence, we believe that these findings offer valuable insights into optimizing ECP concentration for the development of hydrate-based natural gas storage.