A study of the effects of the ester moiety on soot formation and species concentrations in a laminar coflow diffusion flame of a surrogate for B100 biodiesel

The effects of the ester moiety on soot formation and species concentrations in a laminar coflow diffusion flame of a surrogate for a B100 biodiesel are investigated. The surrogate is a mixture of 50% n-decane/50% methyl-octanoate (molar) to represent methyl-oleate. The combustion chemistry and soot formation are solved using a mechanism with 288 species and 2073 reactions coupled with a sectional soot model, respectively. Soot volume fraction (fv) and temperature profiles are compared to the experimentally measured values for this biodiesel surrogate. In addition, the effects of the ester moiety on soot formation and flame chemistry are studied by numerically comparing the biodiesel surrogate flame with a pure n-decane flame. The model predicts both temperature and fv profiles with a good accuracy. Some discrepancies for fv on the flame centerline are observed between the model and the experiments; it is suggested that these discrepancies are because the model and the experiment cannot distinguish nascent transparent soot from mature soot and because the mechanism under-predicts PAH formation rates. Both n-decane and B100 surrogate flames have similar fv and temperature profiles when both flames have the same energy input. This suggests that the ester moiety does not have a major impact on soot formation. In addition, early production of CO and higher concentrations of some oxygenated species such as formaldehyde are observed in the predicted concentration contour plots of the B100 surrogate flame when they are compared to the n-decane flame. Reaction pathway analysis reveals that the higher peak concentrations of formaldehyde and the early production of CO from CH2CO and CH3CO2 that come directly from the ester moiety in the B100 surrogate are much more pronounced than other species in the B100 surrogate flame and are recognized as the main differentiating characteristics of the B100 surrogate flame from the n-decane flame.