Experimental and kinetic studies of soot formation in methanol-gasoline coflow diffusion flames

Methanol has been considered to be a potential alternative fuel to reduce soot emissions from GDI engine. In order to fully understand the effect of methanol addition on soot formation, the 2-D distribution of soot volume fraction in methanol/gasoline laminar diffusion flames was measured quantitatively with two-color laser induced incandescence (TC-LII) technique. In addition, the Methanol-TRF-PAH mechanism is constructed and used to analyze the formation pathways of soot precursors based on the CHEMKIN PRO 0-D constant pressure reactor. In this experiment, the blending ratio of methanol/gasoline was set as M0/20/40/60/80. Considering the carbon content decreasing due to methanol addition, carbon mass flow rate was remained constant. The experimental results showed that methanol is able to decrease the soot significantly, while the effect of methanol on soot reduction is weakened with the increasing methanol ratio. Compared with pure gasoline, the average soot volume fraction in the M20, M40, M60, and M80 flames decrease by 48.2%, 70.4%, 83.8%, and 97.7%, and the peak soot volume fraction decrease by 41.5%, 64.1%, 75.8% and 91.8% respectively. There is little soot formation in the M80 flame, inferring the pure methanol hardly forms soot. The kinetic analysis showed that mole fraction of A1-A4 decrease with the increasing methanol ratio. For the toluene-containing fuel M0-M80, A1 is mainly formed by C₆H₅CH₃ + H = A1 + CH₃ and oxidized by A1 + OH = A1- + H₂O. A4 is mainly produced by C₆H₅CH₂ + C₉H₇ = A4 + 2H₂ and oxidized by H-abstraction reaction with H or OH radical. The major reaction pathways of A1 and A4 formation are consistent under different methanol blending ratios. The soot reduction as methanol added mainly attributes to aromatics dilution effect. In addition, the formation process of soot precursors is largely affected by chemical processes of OH, CH₃, HO₂ radicals.