Effects of CO/H2/N2 addition on the soot morphology and nanostructure in laminar co-flow ethylene diffusion flame

Abstract

This study investigated the effect of CO/H2/N2 addition on the morphological evolutions and nanostructures of soot generated from a laminar co-flow ethylene diffusion flame through utilizing the method of thermophoretic sampling and transmission electron microscopy (TEM). The additions were introduced into the inner tube along with the fuel stream, and the volume fractions of the additions are 10%, 20% and 30%. Specifically, the parameters of soot production, primary particle diameter (Dp), fractal dimension (Df), fractal pre-factor (kg) and the nanostructures were determined and analyzed. A reduction in soot production was observed with increasing proportion of the additives, demonstrating the addition of these diluent gases could inhibit the soot formation. Among the three additives, N2 is the most effective in reducing soot yield. The size analysis of soot particles and aggregates indicated the addition of CO or N2 could diminish the primary particle diameter and fractal parameters. The addition of H2 could reduce the primary particle diameter at the bottom flame while yields greater primary particles at 40 mm HAB. This result demonstrated that H2 addition may weaken the oxidation occurring on soot particles. High resolution transmission electron microscopy (HRTEM) analysis was also developed and the results showed that the addition of CO or H2 results in greater fringe length, smaller fringe tortuosity and inter-fringe spacing for soot particles, demonstrating the more orderly and compact lattice structure. In particular, the nanostructure parameters of soot particles in H2 enriched flame vary more significantly than those in CO enriched flame, demonstrating the soot nanostructure is more graphitized in H2 enriched flame. Different from the case of CO or H2 addition, the lattice characteristics for soot in the flames with N2 addition present inverse variation tendency.