Analysis of Inhibitory Mechanisms of Ammonia Addition on Soot Formation: A Combined ReaxFF MD Simulations and Experimental Study

Abstract

This paper demonstrates the effect of ammonia addition on soot formation co-firing with ethylene combined with reactive force field molecular dynamics simulations and experimental study. The effects of ammonia addition on soot propensity and polycyclic aromatic hydrocarbons (PAHS) evaluated from simulation results were consistent with the laser-induced incandescence/laser-induced fluorescence (LII/LIF) findings. By tracing the evolution of species and the visualization of reactions, the inhibitory mechanisms that occurred with the increased proportion of ammonia were revealed and embodied in the fuel decomposition stage and growth stage of PAHs and soot. In the fuel decomposition stage, the pools of aromatic hydrocarbon precursors weakened with the increase in the proportion of NH3 and generated more nitrogen–carbon species, which continuously showed rising trends until the end of simulations. The overlap between the consumption of acetylene and the incipience of C13+ and C16+ species indicated that the strong inhibition on the growth of heavy PAHs was due in part to less acetylene generated. The snapshots of acetylene addition reactions with the PAH molecules in simulations provided further evidence for this result. The morphology of nanoparticles formed at 3 ns of simulations for 15 and 30% ammonia blending ratios was presented. The nitrogen atoms were mostly distributed at the edge sites of particles. The nitrogen–carbon molecules reacted with PAH-like molecules and mainly formed the edge sites of nanoparticles rather than becoming involved in establishing the internal skeleton of nanoparticles. In addition, with the increased proportion of ammonia, the nitrogen atom sites grabbed more active sites originally meant for hydrogen atoms, thus further inhibiting the growth of PAHs and soot.