CO₂ and CH₄ Hydrates: Replacement or Cogrowth?

Abstract

Replacement of CH4 with CO2 in gas hydrates is a promising option for producing CH4 and sequestrating CO2 simultaneously, but it remains debatable if the CH4 hydrate dissociates or converts to the CH4–CO2 mixed hydrates during the replacement. To gain insights into the underlying micromechanisms, molecular dynamics simulations were performed in this study at six temperatures (250–275 K) and three pressures (20, 50, and 100 bar) by systematic analysis of the hydrate cages. Results demonstrated that it was possible to destabilize the CH4 hydrates by decreasing the adsorption of CO2 on the CH4 hydrate surface or increasing the interfacial area for the contact between CO2 and CH4 hydrates. However, the simultaneous dissociation of CH4 hydrates and formation of CO2 hydrates were not observed or only lasted for a very short time, while the released CH4 would form CH4–CO2 mixed hydrates even though the driving force was suitable for breaking up CH4 hydrate cages and forming CO2 hydrate cages. Moreover, this study revealed that the coalescence of CH4 hydrate particles was mainly contributed by the linkage from 512 cages in the region between the two CH4 hydrate particles, which was further stabilized by the 51263 cages.