Thermodynamic and 13C NMR spectroscopic verification of methane–carbon dioxide replacement in natural gas hydrates

Abstract

The conversion of CH4 hydrate to CO2 hydrate with net recovery of CH4 is regarded as an attractive method of both CO2 sequestration and CH4 production. In this study, the CH4–CO2 swapping phenomenon occurring in gas hydrates and its potential application to CO2 sequestration was examined through thermodynamic equilibrium studies and a 13C NMR spectroscopic analysis. It was found that the CO2 composition in the hydrate phase and the expected recovery level for CH4 after swapping can be easily estimated from thermodynamic equilibrium studies linked to 13C NMR spectroscopic results. The experimental results showed that approximately 67% of CH4 is recoverable after replacement by CO2, which was also confirmed by direct dissociation. The corresponding chemical formula for the mixed gas hydrate after CO2 replacement is 5.03CO2·2.51CH4·46H2O. The results presented in this study further build the thermodynamic and physicochemical background required for understanding the simultaneously occurring dual mechanism of CO2 sequestration and CH4 recovery.