Effects of NH3 addition on polycyclic aromatic hydrocarbon and soot formation in C2H4 co-flow diffusion flames

摘要

? 2021As a promising carbon-free alternative fuel, ammonia (NH3) cofiring with hydrocarbon fuels can improve its own combustion properties and suppress the soot emission. In this work, the chemical effects of NH3 on soot volume fraction (SVF) and polycyclic aromatic hydrocarbons (PAHs) spatial distributions and the soot morphological evolution were experimentally investigated in five C2H4 diffusion flames with and without NH3/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 NH3. Meanwhile, a numerical study was carried out to reveal the chemical effects of NH3 on the PAHs formation and its main pathways. Both the experimental and numerical results showed that NH3 delays and suppresses the formation of PAHs and NH3 has greater reduction influence than Ar. In addition, NH3 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 C2H4 and C2H4[sbnd]NH3 flames revealed that as NH3 content increases, the region of maximum SVF is transformed from the flame wings to flame centerline. Moreover, with NH3 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 NH3 addition provides new pathways for C2H4 and NH3 through the nitrogen-hydrocarbon interactions. The existence of NH3 suppresses the key formation intermediates (C2H2, C3H3, PC3H4 and C4H4) 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 NH3 on the whole soot generation and oxidation process will be further revealed in the future work.