Rapid formation of high-energy-density methane hydrates promoted by L-cysteine: A promising approach for solidified natural gas

Abstract

The use of solidified natural gas (SNG) for natural gas storage and transportation has broad commercial prospects. However, the slow hydrate formation rate is a major drawback that must be overcome for this technology. Although surfactants are currently one of the most effective methods to address this issue, the environmental problems associated with their use are unacceptable. Therefore, using environmentally friendly amino acids as substitutes is a promising solution. This report reveals the rapid and efficient formation of high-energy-density methane hydrates using L-cysteine as a kinetic promoter at 275.2 K and 8 MPa. The effects of different cysteine concentrations on methane uptake, kinetics and morphology were investigated. The optimal concentration of L-cysteine as a promoter was found to be 1 wt%, which resulted in 144.98 mmol gas/mol water and allowed 90 % of the maximum methane uptake to be reached within 29 min. The presence of L-cysteine made the hydrate porous, and as the concentration increased, the characteristic morphology transformed from needle-like to vein-like, and finally to cluster-like. Combining this study with previous research, we found that the hydrophobicity of L-cysteine positively influences methane uptake and the formation of porous hydrates. L-cysteine can enrich methane locally by forming hydrophobic areas, thereby enhancing the interaction between methane and water, which promotes the formation of methane hydrate and its porous structure. In addition, we compared the promoting effects of L-cysteine with those of L-threonine, L-arginine, and L-valine, and found that L-cysteine exhibited the best promotional effects in all aspects. These findings provide valuable insights into the mechanism by which amino acids promote hydrate formation, thereby opening up possibilities for the commercialization of solidified natural gas.