Effects of NO₂ on soot formation in a laminar premixed ethylene-air flame

Experiments were conducted on a laminar premixed ethylene-air flame at equivalence ratios of 2.34 and 2.64. An addition of 5% NO₂ by volume was added to the flame as part of the fuel and was observed to significantly reduce (15-30%) the soot volume fraction. Soot properties were measured using light extinction, scattering and fluorescence. The flame temperatures were measured using a spectral line reversal technique in the non-sooting region, while two-color pyrometry was used in the sooting region. Chemical kinetics modeling using the PREMIX 1-D laminar flame code was used to understand the chemical role of the NO₂ gas in the soot formation process. The modeling used kinetic mechanisms available in the literature. Experimental results indicated a delay in the onset of soot in the flame with NO₂ added with an overall reduction in the soot volume fraction. In addition, fluorescence signals - often argued to be an indicator of polycyclic aromatic hydrocarbons (PAH) - were weaker near the burner surface for the flame with NO₂ added as compared to the baseline flame. A chemical kinetics model, which included reactions for the formation of PAH species, was used to simulate these ethylene-air flames. The reduction in soot volume fraction is attributed to a decrease in the H-atom concentration due to reaction with NO₂ and a subsequent reduction in acetylene in the pre-soot inception region.