Effect of oxygenated aromatic biofuels with different functional groups on soot morphology evolution and PAHs formation in laminar diffusion flames

Oxygenated aromatics, as a promising new generation of carbon-neutral fuels, can help reduce soot emissions from engines, but the effect is related to the position of oxygen atoms relative to the benzene ring. In this work, the effect of 10 % similar to 30 % benzyl alcohol and anisole addition to n-heptane on the evolution of soot morphology and microstructural parameters in laminar diffusion flames was investigated experimentally. The above results were compared with toluene doped flame to elucidate the effect of oxygen-atom bonding position on the soot generation characteristics of oxygenated aromatic biofuels. In addition, the kinetic analysis of PAHs formation was carried out of the two oxygenated aromatic biofuels. The results showed that as the mixing ratio of oxygenated aromatics increases from 10 % to 30 %, the flame height of benzyl alcohol/n-heptane increases by 9.7 %, while the change of anisole/n-heptane is not significant, the peak SVF of benzyl alcohol/n-heptane flame and anisole/n-heptane flame increased by 203 % and 113 %, and the peak particle size increases by 35 % and 22 %. It is noticed that the peak SVF of benzyl alcohol and anisole decreases by 19.9 % and 32.8 % relative to toluene at the 20 % blending ratio. The role of the ether functional group in inhibiting soot is approximately 1.65 times that of the alcohol functional group for oxygenated aromatics, as benzyl alcohol produces higher concentration of PAHs than anisole. Benzyl alcohol directly produces benzene (A1) via A1CH(2)OH + H = A1 + CH2OH, while anisole requires a longer process of A1OCH(3) = A1O + CH3, A1O + H(HO2) = A1OH(+O-2), A1OH + H = A1 + OH. A1O serves as a bridge from oxygenated aromatic pyrolysis to PAHs growth, and its peak concentration in anisole is approximately 5.5 times that in benzyl alcohol. However, benzyl alcohol produces higher peak concentrations of A2O, A1C(2)H, A1-, A2R5 and A4-, increasing the ROP of A2, A3, and A4, which eventually results in the final peak A4 concentration twice that of anisole.