Stability and structure of multiply occupied sII CO2 clathrate hydrates: A possibility for carbon capturing

Abstract

The hydrate method is an important technology that has been widely studied for carbon dioxide (CO2) capture, utilization and storage (CCUS). Improving the gas reserves per unit volume is the key goal for this technology. Therefore, the occurrence behaviors of large CO2 hydrate cages, the structure II, occupied with 1–5 gas molecules were investigated via molecular dynamics (MD) in this study. The occupancy rate, stability and decomposition laws of CO2 hydrate were systematically analyzed. The results indicate that the large hydrate cages occupied with 4–5 molecules were directly broken due to excessively large intermolecular repulsion. In contrast, the large hydrate cages occupied with 3 molecules showed an obvious four-rupture-stage: (1) a small part of cages rupturing; (2) metastable cages formation along with the escape of CO2 molecules, and the cages do not continue to break until all the CO2 molecules escape; (3) pure residual cages rupturing, with no CO2 gas existing in cages in this stage; and (4) complete decomposition of the CO2 hydrate. It is interesting that the large hydrate cages occupied with 2 gas molecules are stable, and 2 is the maximum occupancy number. The pressure is more important for improving the stability of the double-occupancy hydrate, and a higher pressure (20 MPa) strengthens the stability of the CO2 hydrate structure. The findings in this study indicate that the sII CO2 hydrate under sufficiently high pressure can maximally increase the storage efficiency by 36.5 wt%.