Ammonia is a compelling carbon-free fuel, and co-firing ammonia with conventional hydrocarbon fuels receives growing interest as a feasible solution to mitigate CO 2 emission. This study experimentally and numerically investigated the soot formation characteristics of n-heptane co-flow laminar diffusion flames with different contents of ammonia (up to 40% by volume, balanced using argon). The soot volume fraction (SVF) in the flames was quantitatively measured via laser-induced incandescence technique; PAHs and OH radical were qualitatively determined via laser-induced fluorescence technique. Results showed that ammonia addition reduced the SVF, i.e., a peak SVF decrease of 35% at 40% ammonia. Meanwhile, PAH content also decreased with ammonia addition, demonstrating that ammonia would effectively inhibit the formation of PAHs. Thus, the inception and growth of soot decreased. Interestingly, OH radical was reduced with ammonia addition, revealing that ammonia also chemically inhibited soot oxidation. The results confirmed that ammonia reduced the soot loading in the flames due to inhibitory effect on the inception and growth of soot rather than the enhancement on oxidation. The established mechanism effectively captured the variation of different PAHs in the experiments. Numerical analysis showed that the formation of the first aromatic ring benzene was reduced by ammonia as a result of the reduction of C 4 H 5 and R203:C 4 H 5 + C 2 H 2 reaction. In addition, the reduced hydrogen abstraction due to decrease in H, coupled with the reduced small PAH, led to the decrease in PAH growth. Nevertheless, a more comprehensive mechanism including the reactions between aromatics and nitrogen species, such as NH, CN, and NOx, is still required in future studies. (c) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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