Experimental and numerical investigation of soot growth and inception in an ammonia-ethylene flame

The effect of ammonia (NH3) addition on soot growth and inception is investigated in laminar co-flow NH3-ethylene (C2H4) diffusion flames. By comparing C2H4 flames with increasing% of NH3 by volume for the same carbon flow and flame height, the impact of NH3 addition on soot formation is identified. Experimental measurements of flame temperature, soot volume fraction, and primary particle sizes and number densities are compared with a numerical two-dimensional co-flow flame model to evaluate the contribution of NH3 to soot reduction in C2H4 flames. Experimental and numerical results show a significant reduction in the amount of soot formed and the diameter and the number of the primary particles suggesting reduced rates of soot growth and inception due to NH3 addition. The numerical model provides the understanding of how NH3 inhibits soot formation in the flame by analyzing the soot formation pathways and investigating the concentration of relevant gas phase species in the flame. While the impact of NH3 addition on the growth of soot particles is not fully captured by the model, the NH3’s reductive effect on the number of soot particles formed is well predicted. The results suggest that NH3 reduces soot by suppressing the concentration of methyl radicals which are responsible for forming odd numbered carbon species such as propargyl and cyclopentadiene. These species are found to be important contributors to the formation of large polycyclic aromatic compounds which are precursors of soot inception.