Bai Dongsheng, Zhang Jie
Department of Chemistry, Beijing Technology and Business University, Beijing 100048, People's Republic of China.
J Chem Phys. 2024 Mar 7;160(9). doi: 10.1063/5.0189469.
Understanding the ignition and dynamic processes for the combustion of hydrate is crucial for efficient energy utilization. Through reactive force field molecular dynamics simulations, we studied the high-temperature decomposition and combustion processes of methane hydrates in a pure oxygen environment. We found that at an ignition temperature of 2800 K, hydrates decomposed from the interface to the interior, but the layer-by-layer manner was no longer strictly satisfied. At the beginning of combustion, water molecules reacted first to generate OH•, followed by methane oxidation. The combustion pathway of methane is CH4→CH3•→CH3O•→CH2O→HC•O→HCOO•→CO(CO2). During the combustion process, a liquid water layer was formed between melted methane and oxygen, which hindered the reaction's progress. When there is no heat resistance, oxygen will transform into radicals such as OH• and O•, which have faster diffusion rates, allowing oxygen to conveniently cross the mass transfer barrier of the liquid water layer and participate in the combustion process. Increasing the amount of OH• may cause a surge in the reaction. On the other hand, when significant heat resistance exists, OH• is difficult to react with low-temperature hydrate components, but it can transform into O• to trigger the oxidation of methane. The H• generated has a sufficient lifetime to contact high-temperature oxygen molecules, converting oxygen into radicals that easily cross the water layer to achieve mass transfer. Therefore, finding ways to convert oxygen into various radicals is the key to solving the incomplete combustion of hydrates. Finally, the reaction pathways and microscopic reaction mechanisms of each species are proposed.
了解水合物燃烧的点火和动态过程对于高效能源利用至关重要。通过反应力场分子动力学模拟，我们研究了甲烷水合物在纯氧环境中的高温分解和燃烧过程。我们发现，在2800 K的点火温度下，水合物从界面向内部分解，但不再严格满足逐层分解的方式。燃烧开始时，水分子首先反应生成OH•，随后甲烷发生氧化。甲烷的燃烧途径为CH4→CH3•→CH3O•→CH2O→HC•O→HCOO•→CO(CO2)。在燃烧过程中，液态水层在熔化的甲烷和氧气之间形成，阻碍了反应的进行。当没有耐热性时，氧气会转化为扩散速率更快的OH•和O•等自由基，使氧气能够方便地跨越液态水层的传质屏障并参与燃烧过程。增加OH•的量可能会导致反应激增。另一方面，当存在显著的耐热性时，OH•难以与低温水合物成分反应，但它可以转化为O•以引发甲烷的氧化。生成的H•具有足够长的寿命与高温氧分子接触，将氧气转化为易于跨越水层实现传质的自由基。因此，找到将氧气转化为各种自由基的方法是解决水合物不完全燃烧问题的关键。最后，提出了各物种的反应途径和微观反应机理。
