Soot formation in a laminar co-flow C₂H₄-NH₃ diffusion flame at different oxygen indices

Increasing oxygen indices (OI) provides a feasible solution to enhance the intermediate soot loading, which can improve the radiative heat transfer and combustion efficiencies in the combustion of NH₃ blending fuels. However, this strategy introduces challenges for soot prediction due to the competing effects of high OI (promoting soot formation) and NH₃ (suppressing soot formation). In this context, it remains unclear whether OI can alter the influence of NH₃ on the soot formation. This study experimentally and numerically investigated the effects of NH₃ on soot nucleation, condensation, HACA (hydrogen-abstraction-carbon-addition) surface growth, and oxidation processes in C₂H₄ co-flow diffusion flame under various OI conditions ranging from 21% to 27%. The results showed that increasing OI can weaken the suppression impact of NH₃ on the soot formation in the flame centerline region but enhance that in the flame wing region. The former was attributed to a larger decrease in the soot nucleation and condensation processes caused by NH₃ addition at 27% OI compared with 21% OI. Additionally, compared to the C₂H₄/21%OI flame, the C₂H₄/27%OI flame exhibited higher peak flame temperatures following NH₃ addition. This led to an enhanced decomposition rate of C₅H₅ through the reaction of C₅H₅=C₃H₃+C₂H₂, partially counteracting the chemical suppression of NH₃ on the formation of C₂H₂. As a result, the inhibitory effect of NH₃ on the conversion of C₂H₂ to A1 (benzene) was attenuated in the C₂H₄/27% OI flame relative to the C₂H₄/21% OI flame, leading to a smaller reduction in soot nucleation and condensation rates. In contrast, in the flame wing region, the NH₃ addition led to a more substantial increase in the forward rates of H + O₂=OH+O and O + H₂O=2OH under the 27% OI condition, thereby enhancing the oxidation rate via OH radical and amplifying the suppression effect of NH₃ on the soot formation in the flame wing region.