Effects of simultaneous CO2 addition to the fuel and oxidizer streams on soot formation in co-flow diffusion ethylene flame

Abstract

Soot formation in a co-flow diffusion ethylene flame with the addition of CO2 to the fuel (the CO2-F), oxidizer (the CO2-O), and fuel/oxidizer (the CO2-F/O) streams was numerically and experimentally investigated in this study. The effects of different CO2 addition ways on soot inception, soot condensation, H-abstraction-C2H2-addition (HACA) and oxidation by O2/OH processes, were quantitatively analyzed by introducing the integrated reaction rates over the whole computational domain. The simulated and experimental results showed that the CO2-F/O was the most effective in inhibiting soot formation and flame temperature, followed by the CO2-O, and the CO2-F. Compared with the CO2-F, the suppression effect of the CO2-O on soot inception was weaker due to the higher concentration of benzo(ghi) fluoranthene (BGHIF). Since the rate of C4H2 formation via C2H4 → C2H3 → C2H2 → C4H2 was inhibited by the CO2-O, lowering the consumption rate of acenaphthalene (A2R5) via C4H2 + A2R5=>A4, more A2R5 converted to BGHIF via A2R5 → A2- → A2 → BGHIF. The suppression effects of different ways of CO2 addition on HACA surface growth and soot condensation were identical: CO2-F < CO2-O < CO2-F/O. The decrease of benzo(a)pyrene (BAPYR) mole fraction accounted for the decline of soot condensation rate, and the decreases of H and OH mole fractions were responsible for the drop of HACA surface growth rate. Compared with the CO2-F, the CO2-O and the CO2-F/O had stronger suppression effects on the soot oxidation by O2 process due to the lower concentration of O2 in the oxidizer stream. Whichever CO2 addition ways were adopted, the soot oxidation by O2 process was more sensitive than the soot oxidation by OH process with the CO2 addition.