Thermal driving force as an operating lever in CH4 hydrate-based SNG: Mapping kinetics and conversion trade-off

Abstract

Hydrate-based solidified natural gas (SNG) has emerged as a promising route for natural gas storage and transportation due to its intrinsic safety and water-based host matrix. To enable CH4 hydrate formation under milder hydrate conditions using THF-alternative promoters, this study investigated 1,3-dioxolane (DIOX) and 1,3-dioxane (Dioxane) as thermodynamic promoters for sII hydrates. A key challenge for practical deployment is to achieve rapid hydrate formation while maintaining high final water-to-hydrate conversion. For hydrate formation, the degree of subcooling (ΔT) is commonly used as a practical measure of the thermal driving force. Still, quantitative guidelines for selecting ΔT to balance the trade-off between rapid formation and high water-to-hydrate conversion remain limited. Here, we examined three representative subcooling conditions to delineate practical operating windows for promoter-assisted CH4 hydrate-based SNG. Time-resolved in-situ Raman revealed subcooling-dependent guest enclathration behavior, with higher ΔT increasing the early-time ratio, suggesting faster initial CH4 enclathration relative to promoter enclathration. Solid-state 13C NMR showed CH4 occupancy in sII-S increased with ΔT, whereas overall gas uptake decreased in parallel with reduced water-to-hydrate conversion. These results indicate that macroscopic methane storage is governed primarily by conversion yield rather than incremental changes in cage occupancy. Overall, these results provide practical guidance for selecting ΔT by considering the trade-off between faster initial hydrate formation and higher final water-to-hydrate conversion for hydrate-based SNG technology.