Reactive Molecular Dynamics Study on Soot Formation Mechanism in Iron-Doped Ethylene Combustion

Iron (Fe) powder is a promising zero-carbon energy carrier, and its co-combustion with hydrocarbon fuels offers a transitional route toward decarbonized power generation. Fe has nevertheless been experimentally found to aggravate early soot nucleation and growth in flames while reducing engine-out soot emissions, and the atomic-scale mechanism by which Fe regulates polycyclic aromatic hydrocarbons (PAHs) and soot formation remains unresolved. To address this gap, reactive molecular dynamics (ReaxFF MD) was employed to investigate PAHs and soot formation during the combustion of ethylene (C2H4) doped with Fe, across three reaction temperatures (2000, 2500, and 3000 K) and three Fe loading levels (0, 6, and 12 Fe atoms, corresponding to Fe/C atomic ratios of 0, 2.5%, and 5.0%). The CHOFe-2012.ff reactive force field was selected for the Fe-containing simulations because it reproduces the experimentally validated Fe-free ethylene combustion benchmarks most closely among the force fields compared, while also describing Fe-C and Fe-O interactions. At 20 ns and 3000 K, the largest soot cluster detected contains 191 carbon atoms with a C/H ratio of 7.07, favoring the formation of highly graphitized mature soot. In contrast, under 2000 K, the corresponding largest cluster possesses only 77 carbon atoms and exhibits a C/H ratio of 3.35, yielding only loosely structured young soot. It should be noted that these descriptors characterize the most evolved cluster identified in each simulation rather than the entire soot population. Within the tested conditions, Fe doping is found to promote soot formation: both the carbon-atom number and the C/H ratio of the largest soot cluster increase markedly, and this effect strengthens with increasing Fe loading. Notably, the O2 level varied by less than 2.2% across the Fe-loading series, and these descriptors refer to the largest detected cluster; within the present simulation framework, the promotion is therefore attributed predominantly to the chemical effect of Fe doping. The reaction path analysis reveals that Fe atoms form Fe-containing small carbon nuclei by combining with C2H4, dehydrogenation and addition reactions, and then gradually develop into mature soot particles through coalescence, surface growth and graphitization, indicating that Fe participates in the nucleation stage of soot formation under the tested conditions. This study provides a molecular-level understanding of the microscopic mechanism of Fe–soot interactions, offering mechanistic insight at the molecular scale that may inform the optimization of Fe-hydrocarbon dual-fuel systems and the mitigation of Fe-induced soot formation.