Effect of Toluene Addition on the PAH Formation in Laminar Coflow Diffusion Flames of n -Heptane and Isooctane

Characterization of soot formation is becoming more important for gasoline surrogate fuels, requiring a deeper understanding of the effect of toluene ratio on PAHs formation. In this work, the different size PAHs distribution in laminar diffusion flames was measured by PLIF technique, and the chemiluminescences of OH and CH radicals were recorded by ICCD coupled with band-pass filters. The effect of toluene addition to n-heptane and isooctane on the flame and PAHs formation was comparatively studied. Then, the chemical kinetic of PAHs was analyzed using the CHEMKIN Diffusion Flame model. The experimental results showed that the cool flame area ranks as n-heptane > isooctane > toluene may due to the NTC action. The OH intensity and flame lift-off height show a monotonous increasing trend with the toluene ratio. At the same toluene ratio, the peak OH intensity and flame lift-off height of n-heptane/toluene is lower than that of isooctane/toluene. The CH intensity in n-heptane, isooctane, and toluene flames shows completely different distribution characteristics. As the toluene ratio increases, the 320 nm-LIF in n-heptane and isooctane flames shows a weakening monotonous increasing trend with different inflection points. The 360/400/450 nm-LIF in n-heptane and isooctane flames show a nonmonotonic trend with different peak points. The peak point is 50% toluene ratio in n-heptane/toluene flames and 40% in isooctane/toluene flames, and it corresponds exactly to the smoke point of the flame. The kinetic analysis showed that the benzyl has a significant effect on the formation of A2-A4 through the generation of C₁₀H₉, C₉H₇, and C₁₄H₁₂. A1 trend is dominated by C₆H₅CH₃ + H = A1 + CH₃ and A1^- + CH₄ → A1 + CH₃. The trends of A2, A3 and A4 are mainly dominated by the reactions between their own dehydrogenation groups with H₂.