Effects of NH₃/H₂/N₂ addition on soot morphology and nanostructure in laminar co-flow ethylene diffusion flame

The present study investigated the effects of NH₃/H₂/N₂ addition on soot morphology and nanostructure in laminar co-flow diffusion flame of ethylene, by adopting the method of thermophoretic sampling combined with transmission electron microscope (TEM). The volume fraction of NH₃ varies from 10% to 30%, and the volume fraction of H₂ and N₂ are all 30%. The soot morphology evolution images were analyzed with the parameters representing size and fractal characteristics determined. Among the three diluents, the addition of NH₃ is found to have the best suppressing effect with the smallest primary particle size and least accumulated aggregates, especially at the lower flame heights. Meanwhile, H₂ addition is observed to advance the soot formation process due to the higher flame temperature. The soot particle images with a magnification time of 600,000 obtained from high-resolution transmission electron microscope (HRTEM) were processed by the processing code. The parameters describing the soot nanostructure were measured and analyzed with the fitted curves. It is found that the fringes of particles sampled from NH₃ enriched flames tend to be shorter and more curved, and the inter fringe spacing tends to be larger, indicating the nanostructure is more disordered and the particles are easier to be oxidized. Compared with the flames with NH₃ addition, particles from H₂ enriched flames show a more ordered and compact nanostructure. Finally, simulations were performed to further interpret the effects. Calculated results show that the NH₃ and H₂ addition obviously suppress the production of larger PAHs, and NH₃ is more effective than H₂, but this difference gradually narrows for greater PAHs. Simulations also indicate that the peak value of precursors in NH₃ enriched flame is delayed.