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

Abstract

The importance of carbon capture and storage (CCS) has recently been emphasized owing to the ever-increasing global warming. Direct CO2 sequestration in marine sediments is an attractive option for CO2 storage, and some amount of injected CO2 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 CO2 selectivity, and time-dependent growth behaviors of CO2 + N2 hydrates were experimentally investigated to elucidate their implications for hydrate-based CO2 sequestration. The presence of NaCl shifted the equilibrium conditions of CO2 + N2 hydrates to the higher-pressure or lower-temperature region, whereas it increased thermodynamic CO2 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 CO2 + N2 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 CO2 was kinetically selective at the early stage of CO2 + N2 hydrate formation and that kinetic CO2 selectivity was more noticeable in the saline water system. The overall results provide an in-depth understanding of the role of salts in CO2 + N2 hydrate formation and thus offer valuable insights into hydrate-based CO2 storage and geological CO2 sequestration.