Decoupling effects of C3H3/C4H5/i-C4H5/CN radicals on the formation and growth of aromatics: A ReaxFF molecular dynamics study

Abstract

This paper demonstrates the effects of C3H3/C4H5/i-C4H5/CN radicals on the formation and growth of aromatic hydrocarbons, particular types of polycyclic aromatic hydrocarbons (PAHs) such as heavier-PAHs, cyclopentafused PAHs (CP-PAHs), through reactive force field molecular dynamics simulations under 2250K and 2500K. Adding non-acetylene radicals enhanced the early-stage mass growth of aromatics to varying degrees. However, the later-stage mass growth and aromaticity degree were weakened by adding radicals except for the C3H3 radical. The reactivity of acetylene and additional radicals showed that the additional C3H3 radicals accelerated the consumption of radical pools, and the interaction reaction between C3H3 was responsible for the consumption of radical pools for the C3H3 additive case. The consumption of i-C4H5 was significantly faster than that of C4H5, whereas the consumption of acetylene in the two simulation cases was essentially the same. Thus, the i-C4H5 simulation case formed more intermediate species C6H7 than C4H5 radical. As for the formation of less-studied PAHs, i-C4H5 radicals generated more heavier PAHs and CP-PAHs than straight butyl C4H5 isomers. Finally, the effect of nitrogen-containing radical CN, which normally resides in ammonia fuel, was analyzed. Several factors explained the stronger enhancement in aromatic mass growth, but the relatively lowest aromatization degree resulted from the additional CN radicals. The reactions between CN and acetylene formed carbon–nitrogen species that participated less in the formation and growth of aromatics. The potential reactions between CN radical and aromatics led to ring-opening reactions and the formation of nitrogen-membered rings, which accelerated the mass growth of carbon clusters but inhibited the development of aromaticity degree. The least amount of CP-PAHs was found in CN cases compared with hydrocarbon radical cases.