The objective of this study is to investigate the effects of aliphatic and aromatic halogenated additives on soot and PAH formation (polycyclic aromatic hydrocarbon) using a laminar co-flow ethylene-air diffusion flame. Both aliphatic surrogates (bromobutane, chlorobutane and bromo/chlorobutane mix) and aromatic surrogates (benzene, bromobenzene, chlorobenzene and bromo/chlorobenzene mix) were added to the base fuel for investigation. The soot particles were collected on quartz filters and their mass was determined using a Leco carbon burn-off method. The gaseous effluents were collected in an Amberlite XAD-2 sorbent trap and extracted using the Soxhlet technique. Gas chromatography / mass spectrometry (GC/MS) was used to identify and quantify the extracted gaseous effluents. Significant amounts of enols were detected along with PAHs, indicating that enols are also important intermediate species in hydrocarbon combustion.
The results showed that all the halogenated additives reduced the temperature of the sampling system, indicating flame suppression properties. All the additives gave increased soot yields when compared to the baseline experiment. The brominated fuel additives displayed a higher propensity to soot than the corresponding chlorinated fuel additives. The aromatic additives showed a greater tendency to soot than their aliphatic counterparts. All the additives increased the total PAH and enol yields, with benzene yielding the highest. The halogenated form of benzene decreased the total PAH and enol yields when compared to benzene, suggesting that halogens accelerate the conversion of PAHs to soot. The effect of bromine in increasing the total PAH and enol yields is more than that of chlorine.