NaCl-induced enhancement of thermodynamic and kinetic CO2 selectivity in CO2 + N2 hydrate formation and its significance for CO2 sequestration.

[Display omitted] • The effects of NaCl on thermodynamic and kinetic CO 2 selectivity were investigated. • The presence of NaCl enhanced thermodynamic CO 2 selectivity in CO 2 + N 2 hydrate. • The conversion into CO 2 + N 2 hydrate was lower in the NaCl solution system. • The kinetic CO 2 selectivity in the hydrate phase was more pronounced in the NaCl solution system. The importance of carbon capture and storage (CCS) has recently been emphasized owing to the ever-increasing global warming. Direct CO 2 sequestration in marine sediments is an attractive option for CO 2 storage, and some amount of injected CO 2 can be stored in the form of solid gas hydrates. In this study, the effects of NaCl on the hydrate phase equilibria, thermodynamic and kinetic CO 2 selectivity, and time-dependent growth behaviors of CO 2 + N 2 hydrates were experimentally investigated to elucidate their implications for hydrate-based CO 2 sequestration. The presence of NaCl shifted the equilibrium conditions of CO 2 + N 2 hydrates to the higher-pressure or lower-temperature region, whereas it increased thermodynamic CO 2 selectivity at a specified temperature and pressure, which was also confirmed by the pressure-composition diagram. As revealed by powder X-ray diffraction analysis, the conversion into CO 2 + N 2 hydrate was lower in the saline water system because of the lower initial driving force and gradual salt enrichment in the residual solution during hydrate growth. In situ Raman spectroscopic measurements demonstrated that CO 2 was kinetically selective at the early stage of CO 2 + N 2 hydrate formation and that kinetic CO 2 selectivity was more noticeable in the saline water system. The overall results provide an in-depth understanding of the role of salts in CO 2 + N 2 hydrate formation and thus offer valuable insights into hydrate-based CO 2 storage and geological CO 2 sequestration. [ABSTRACT FROM AUTHOR]