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 NH 3 blending fuels. However, this strategy introduces challenges for soot prediction due to the competing effects of high OI (promoting soot formation) and NH 3 (suppressing soot formation). In this context, it remains unclear whether OI can alter the influence of NH 3 on the soot formation. This study experimentally and numerically investigated the effects of NH 3 on soot nucleation, condensation, HACA (hydrogen-abstraction-carbon-addition) surface growth, and oxidation processes in C 2 H 4 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 NH 3 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 NH 3 addition at 27% OI compared with 21% OI. Additionally, compared to the C 2 H 4 /21%OI flame, the C 2 H 4 /27%OI flame exhibited higher peak flame temperatures following NH 3 addition. This led to an enhanced decomposition rate of C 5 H 5 through the reaction of C 5 H 5 =C 3 H 3 +C 2 H 2 , partially counteracting the chemical suppression of NH 3 on the formation of C 2 H 2 . As a result, the inhibitory effect of NH 3 on the conversion of C 2 H 2 to A1 (benzene) was attenuated in the C 2 H 4 /27% OI flame relative to the C 2 H 4 /21% OI flame, leading to a smaller reduction in soot nucleation and condensation rates. In contrast, in the flame wing region, the NH 3 addition led to a more substantial increase in the forward rates of H + O 2 OH+ O and O + H 2 O=2OH under the 27% OI condition, thereby enhancing the oxidation rate via OH radical and amplifying the suppression effect of NH 3 on the soot formation in the flame wing region.