水分子对初始碳烟颗粒形成过程影响的分子动力学模拟

采用LAMMPS软件,基于ReaxFF,以十氢化萘、萘、2-甲基蒽、1-乙基芘为催化油浆的模型化合物研究了600～2500 K温度下催化油浆形成初始碳烟颗粒的过程,考察了2500 K时水分子对初始碳烟颗粒形成过程的影响。研究表明温度在600 K时模型化合物分子主要是物理聚集成核。温度在900～1700 K时模型化合物分子处于聚集和分离的动态过程,无法从单体向碳烟颗粒转变。温度高于2100 K时主要是化学成核,模型化合物分子碳氢键先断裂,然后碳碳键断裂产生大量短碳链,碳链经成键和环化形成初始碳烟颗粒。温度在2500 K时水分子抑制模型化合物分子化学成核,随着体系氢碳比的增加,抑制初始碳烟颗粒形成的作用增强。水分子产生氢自由基和氢氧自由基,这些基团会直接导致模型化合物分子的侧链断裂和碳碳键断裂形成大量短碳链。碳链继续与氢自由基和氢氧自由基作用形成一氧化碳、二氧化碳、氢气、甲烷等而被消耗,水分子的作用为促进短碳链形成和抑制短碳链向形成初始碳烟颗粒的方向进行。

Molecular dynamics simulation of influence of water molecules on formation process of nascent soot particles

By using ReaxFF software, the process of forming the initial soot particles in the catalytic slurry at 600－2500 K was studied based on ReaxFF and the model compound of decalin, naphthalene, 2-methylindole and 1-ethylindole as catalytic oil slurry. The effect of water molecules on the formation of NSPs at 2500 K has been investigated. The study shows that the formation of the NSPs at 600 K is mainly due to the physical aggregation. At the temperature range of 1100－1700 K, model compound molecules are in a dynamic process of gathering and separating, and cannot transform from monomers to soot particles. At the temperature above 2100 K, the formation of NSPs is mainly due to the chemical nucleation. The carbon-hydrogen bonds of the model compound are broken firstly, then the carbon-carbon bonds are broken into a large number of short carbon chains, and the short carbon chains are bonded and cyclized to form the initial soot particles. At 2500 K, water molecules inhibit the chemical nucleation process of the model compound molecules, and the inhibition effect to the formation of the NSPs is enhanced with the increase of the hydrogen-to-carbon ratio. Water molecules produce hydrogen radicals and hydroxyl radicals, which directly lead to the formation of a large number of short carbon chains due to the cleavage of side-chains and carbon-carbon bonds. These carbon chains continue to be consumed by hydrogen radicals and hydroxyl radicals to form carbon monoxide, carbon dioxide, hydrogen, methane, etc. The water molecules can promote the formation of short carbon and inhibit the transition of short carbon chains toward the formation of NSPs.