Chemical Mechanism of Exhaust Gas Recirculation on Polycyclic Aromatic Hydrocarbons Formation Based on Laser-Induced Fluorescence Measurement

Exhaust gas recirculation (EGR) has been widely used in engines to meet current emission regulations. Investigating the chemical mechanism of EGR on polycyclic aromatic hydrocarbons (PAHs, the precursor of soot) formation in premixed flames contributes to understanding the EGR dependence on soot formation in engines. In this study, the influence of flame temperature, equivalence ratio, and CO2 addition on the formation of PAHs was systematically investigated in premixed C2H4/O-2/Ar/CO2 flames using laser-induced fluorescence (LIF) technology. The temperature dependence of PAHs formation was studied at a fixed equivalence ratio and dilution ratio. It was found that the LIF signal of PAH reaches the maximum value around 1730 K and decreases at a lower or higher temperature in this study. The LIF signal of PAHs almost increases linearly with the equivalence ratio, as the maximum flame temperature and dilution ratio are kept constant. The experimental results show that the CO2 addition in the inlet gas suppresses PAHs formation due to the chemical inhibition effect. The thermal effect of CO2 addition on PAHs formation is highly sensitive to flame temperature. The PAHs reaction mechanisms proposed by Appel et al. and Wang et al. are used to clarify the experimental results. The first-order temperature sensitivity analysis showed that the hydrogen-abstraction-carbon-addition pathway with high reaction reversibility should account for temperature effects on PAHs formation. The pathway sensitivity analysis showed that the CO2 inhibition chemical effect is realized through the route CO2 (+H) -> OH -> C3H3 (C2H2) -> A(1) -> PAHs with the assistance of the entrance reaction CO2 + H = CO + OH.