Effects of NH 3 addition on polycyclic aromatic hydrocarbon and soot formation in C 2 H 4 co-flow diffusion flames

As a promising carbon-free alternative fuel, ammonia (NH 3 ) cofiring with hydrocarbon fuels can improve its own combustion properties and suppress the soot emission. In this work, the chemical effects of NH 3 on soot volume fraction (SVF) and polycyclic aromatic hydrocarbons (PAHs) spatial distributions and the soot morphological evolution were experimentally investigated in five C 2 H 4 diffusion flames with and without NH 3 /Ar addition using nonintrusive and intrusive methods (planar laser induced incandescence, PLII, planar laser induced fluorescence, PLIF and thermophoretic sampling particle diagnostic method combined with transmission electron microscope, TSPD-TEM). In order to highlight the chemical effects of ammonia on soot and PAHs formation, taking argon-doping flame as reference, the chemical effects are defined as the disparity of soot reduction between the flames with doping Ar and NH 3 . Meanwhile, a numerical study was carried out to reveal the chemical effects of NH 3 on the PAHs formation and its main pathways. Both the experimental and numerical results showed that NH 3 delays and suppresses the formation of PAHs and NH 3 has greater reduction influence than Ar. In addition, NH 3 and Ar decrease the flame temperature at the early stage of flame and increase the temperature at the post stage. The soot distribution in pure C 2 H 4 and C 2 H 4 -NH 3 flames revealed that as NH 3 content increases, the region of maximum SVF is transformed from the flame wings to flame centerline. Moreover, with NH 3 addition, the soot loading zone is shrunk and shifted to the downstream due to the lift-off of flame height and decrease of flame temperature. Sensitivity and reaction pathways analysis of A1 formation indicate that NH 3 addition provides new pathways for C 2 H 4 and NH 3 through the nitrogen-hydrocarbon interactions. The existence of NH 3 suppresses the key formation intermediates (C 2 H 2 , C 3 H 3 , PC 3 H 4 and C 4 H 4 ) responsible for the first aromatic benzene ring formation and larger PAH growth. In conclusion, the chemical effects of ammonia not only reduce the formation of soot and PAHs precursors, but significantly affect the evolution of soot morphology. The underlying mechanism of NH 3 on the whole soot generation and oxidation process will be further revealed in the future work. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.