| dc.description.abstract | Clathrate hydrates are crystalline compounds where water forms cage-like structures around gas molecules at low temperatures and very high pressures. Due to the widespread presence of methane (CH4) hydrates in the continental shelves and seabed sediments, they are regarded as a highly promising emerging energy source. A replacement of CH4 by carbon dioxide (CO2) in hydrates, allowing CO2 to be stored as hydrates on the seafloor while the released CH4 can be used as energy, was proposed to both mitigate greenhouse gas effects and address energy challenges. In addition, a novel method involves using flue gas to extract CH4 hydrates was suggested. Flue gas, the exhaust gas from power generation, primarily consists of CO2 and nitrogen (N2). Using flue gas to replace CH4 hydrates can reduce the cost of separating CO2, and N2 can replace hydrate structures that are difficult for CO2 to replace, thus enhancing CH4 recovery rates. However, the current understanding of the nucleation process of mixed gas molecules in hydrate formation is still limited.
In this study, molecular dynamics (MD) simulations were used to investigate the nucleation competition of hydrate formation in systems of liquid water and gas molecules (CH4, CO2, and N2) and the effect of gas types and gas ratio on nucleation rates. The results showed that the nucleation rate of pure CO2 hydrates is faster than that of pure CH4 and pure N2 hydrates. Adding CO2 significantly promotes the nucleation of CH4 hydrates compared to N2 hydrates. In a binary CH4/CO2 gas system, the initial nucleation 512 cage (with 12 pentagonal faces formed by water molecules) always encapsulates CH4. In a ternary CH4/CO2/N2 gas system, N2 can also be encapsulated in the initial 512 cage. The proportion of hydrates formed shows a linear correlation with the ratio of mixed gases. CH4 and N2 form 512 cages, while CO2 forms 4151062 cages (with 1 square face, 10 pentagonal face, and 2 hexagonal faces formed by water molecules). Additionally, simulation results indicated that without solid hydrates, hydrate growth only accelerates after the first stable hydrate forms somewhere in the system. In contrast, with solid hydrates present, growth begins at the interface between solid hydrates and the liquid, and the structure of the growing hydrate resembles that of the solid hydrate, demonstrating the feasibility of storing flue gas. | en_US |