Effect of MgCl2 on CO2 sequestration as hydrates in marine environment: A thermodynamic and kinetic investigation with morphology insights

Abstract

Oceanic hydrate-based CO2 sequestration (HBCS) holds great promise for achieving carbon neutrality. However, the presence of inorganic salts, particularly MgCl2 besides NaCl, in seawater can significantly impact the formation rate and the stability of CO2 hydrate. In this study, experimental investigations were conducted to examine the thermodynamics, kinetics, and the resulting morphological features of CO2 hydrate in the presence of MgCl2, covering mass fractions ranging from 0 to 5.0 wt%. The experimental findings reveal that MgCl2 exerts a thermodynamic inhibitory effect with its inhibitory capacity increasing with higher mass fractions. The solubility model of CO2 in MgCl2 solution was modified, demonstrating a gradual weakening of CO2 solubility as MgCl2 mass fraction increases. Additionally, the growth kinetics of CO2 hydrate decreases with increasing MgCl2 mass fraction. Regarding CO2 hydrate morphology, it was observed that at low mass fractions of MgCl2 (<1.0 wt%), a dense hydrate film rapidly formed at the gas-liquid interface after CO2 hydrate nucleation, hindering the further conversion of CO2 into hydrate. Conversely, at higher mass fractions (>3.0 wt%), CO2 hydrate exhibits a more porous and slurry-like structure, facilitating more gas-liquid contact and mass transfer, thereby enhancing the conversion of CO2 into hydrate. During hydrate dissociation, a salt-removal effect associated with CO2 hydrate formation was observed, leading to the accumulation of concentrated electrolyte (MgCl2) and facilitating CO2 hydrate dissociation. These findings have implications for understanding the CO2 hydrate formation and dissociation in the presence of MgCl2 relevant in the subsea environment and can contribute to the development of effective hydrate-based CO2 sequestration strategies.