The formation and destruction of aromatic compounds in a turbulent flame

Abstract

Systematic, detailed studies of the effects of temperature and fuel type on polycyclic aromatic compound (PAC) emissions from turbulent flames were carried out in a well-controlled, pilot-scale turbulent flame. Benzene and toluene were injected as a secondary fuel into the hot combustion products of a fuel-lean natural gas air flame at 1350 and 1550 K. The experimental results show a strong combined influence of fuel type and temperatureon the PAC emissions. At 1350 K, the PAC yield was approximately 10 times higher than that of soot, while at 1550 K, the soot yield was 10 times higher than the PAC yield, indicating a much faster formation and conversion to soot of PACs. At the higher temperature, the product distribution of PACs shifted toward species containing cyclopenta-fused rings. At 1550 K, the fuel type had virtually no impact on either the total yield of PACs and soot or the product distribution, while at the lower temperature, a large difference in yield and product distribution were found between the experimental fuels. At 1350 K, toluene injection produced up to 25 times more PACs and 30 times more soot than the injection of benzene. Also, the peak concentrations of soot and PACs in the benzene flame occurred later than in the toluene flame. A numerical model based on a segregated flow analysis was used to quantitatively analyze the experimental data. The treatment of the experimental flames by the model showed that the PAC emissions from the flames were affected by both chemistry and mixing. The model correctly predicted the trends of formation, and destruction of most compounds included in the model, and the absolute concentrations of individual species vary from within a factor of 5 (for major species) to within two orders of magnitude (for trace pollutants).