Experimental and kinetic modeling study of ammonia addition on PAH characteristics in premixed n-heptane flames

Abstract

Due to concerns over pollutant emissions and ever-increasing energy crisis, ammonia is being considered as a promising alternative fuel to control soot particles and their precursors (polycyclic aromatic hydrocarbons, PAHs) emissions in combustion for diesel engines. This paper aims to investigate the chemical effects of NH3 addition on PAH formation in n-heptane laminar premixed flames. The laser-induced fluorescence (LIF) technique was used to measure the relative PAH fluorescence intensities qualitatively. A kinetic modeling study was performed to predict PAH concentrations by comparing with the experimental results, and then clarify the underlying effects of NH3 on PAH formation. Both experimental and numerical results revealed that PAH concentrations decreased monotonically as ammonia fraction increased, suggesting a PAHs-inhibiting role for NH3 in n-heptane baseline flames. Reaction pathway analysis revealed that the suppression of NH3 on PAH formation could be attributed to the competition for H, OH and O radicals in fuels' decomposition, and chemical interactions of amine radicals with the C1-C2 species. For this reason, the NH3 addition produced a relatively decreased C2H2, thereby inhibiting the formation of propargyl. As a result, benzene formation was inhibited through C3H3 self-recombination reaction, resulting sequentially in the suppression of PAH formation and growth. Besides, the decreased C2H2 yield in the post-flame region of NH3 doping flames also inhibited the formation of PAHs through the hydrogen-abstraction-acetylene-addition (HACA) mechanism.