Numerical study of the effects of surface nitrogen-containing on soot formation process in ammonia/ethylene counterflow diffusion flames

Abstract

In this paper, a modified soot kinetic model was developed to explore the potential inhibitory effects of nitrogen-containing functional groups on the soot surface growth process. In the modified soot kinetic model, the formation process of cyano functional groups on the soot surface was simulated through the reaction of hydrogen cyanide (HCN) and the surface open sites, and the surface cyano functional groups were assumed to no longer participate in the subsequent hydrogen-abstraction-acetylene-addition (HACA) reaction. The prediction performance of soot volume fraction (SVF) and average particle size (D₆₃) in ammonia doping ethylene counterflow diffusion flames was compared between the traditional soot kinetic model and the modified kinetic soot model. The results show that both models reflected the suppression effects of ammonia doping on soot formation observed in the experiments. However, compared to the traditional soot kinetic model, the modified soot kinetic model exhibited better performances in predicting SVF and D₆₃ with prediction deviations reduced by 52.7 % and 2.3 nm, respectively. The simulation results showed the formation of surface cyano functional groups led to a nitrogen content of up to 0.84 % in the soot, which is consistent with previous experimental measurements. In-depth analysis shows that the formation of cyano functional groups on the soot surface decreased the proportion of hydrogen sites and further diminished the number of open sites, resulting in reductions in the rates of HACA surface growth and the soot mass growth by up to 22 % and 29 %, respectively. It is proposed that the nitrogen-containing functional groups on the surface of soot in ammonia-doped hydrocarbon flames inhibit the formation of soot, providing an alternative pathway to the well-known gas-phase suppression mechanism.