Study on the microstructure and soot formation mechanism of hydrogen addition ethylene inverse diffusion flame.

Hydrogen has a strong effect on the tendency of fuels to produce soot. The aim of this study is to investigate the effect of hydrogen addition on the formation of soot and its structural evolution in ethylene/oxygen inverse diffusion flames. Laser spectroscopy was used to measure the spatial distribution of soot and Raman spectroscopy was utilized to characterize the nanostructure of soot. Additionally, the influence of hydrogen addition on the soot formation pathway was analyzed using ReaxFF molecular dynamics simulations. The results indicate that increasing hydrogen addition (α) progressively delays soot formation and narrows the flame radius. At 40% hydrogen, soot peak reduction was approximately 64.7%. Raman spectra revealed an increase in both graphite particle size and amorphous carbon content during soot growth. During the evolution of soot nanostructures, the initial particles dominated by sp carbon chains are gradually reorganized to form sp2 aromatic carbon structure, which is consistent with the simultaneous increase of soot concentration and graphite crystal size. It is worth noting that the crystal size of these graphite structure particles is concentrated in the range of 1.6–2.3 nm, indicating that they are in the transition stage from amorphous to microcrystalline. However, at the flame's end, both particle size and amorphous carbon content decreased simultaneously, attributable to oxidation by OH radicals. The study confirms the influence of soot formation and oxidation processes on nanostructures within flames. Raman spectroscopy also indicated a nonlinear change in soot oxidation reactivity along the axial direction of the flame, initially governed by gradual graphitization as flame temperature rises. Molecular dynamics simulations elucidated the nucleation and growth mechanisms of initial soot from polycyclic aromatic hydrocarbons (PAHs). These simulation results show that the increase of hydrogen content in the gas phase reduces the aggregation ability of polycyclic aromatic hydrocarbons and delays the formation of initial soot particles (C41+). With the growth of soot, its morphology gradually evolved from the initial dense, curved, and rotating spindle to a planar structure. The main chemotactic effect of this hydrogen participation is to hinder the dehydrogenation reaction of unsaturated hydrocarbons (H + C m H n → C m H n-1 + H 2), thereby inhibiting the formation of soot precursors, resulting in a decrease in the overall soot concentration. • The microstructure changes of soot in different regions of flame were obtained. • The effect of increasing hydrogen concentration on initial soot generation was obtained. • The effect of hydrogen doping on the morphology change and reaction path of soot formation was obtained. • The main factors affecting soot formation by hydrogen mixing were obtained. [ABSTRACT FROM AUTHOR]