(528g) Correlations between Fuel Structural Features and Benzene Formation

The relative importance of formation pathways for benzene, an important precursor to soot formation, was determined from the simulation of 22 premixed flames for a wide range of equivalence ratios (1.0 to 3.06), fuels (C1-C12), and pressures (20 to 760torr). The maximum benzene concentrations in 15 out of these flames were well reproduced within 30% of the experimental data. Fuel structural properties were found to be critical for higher benzene production potential. Cyclohexanes and C3 and C4 fuels, among the paraffins (normal, iso- and cyclo-), were found to be among the most productive in benzene formation; and long-chain normal paraffins produce the least amount of benzene. Other properties, such as equivalence ratio and combustion temperatures, were also found to be important in determining the amount of benzene produced in flames. The reaction pathways for benzene formation in four premixed flames, representing the different fuels of acetylene, n-decane, butadiene, and cyclohexane were critically examined. Fuel structural properties, again, were found to be critical in determining major benzene formation pathways in premixed flames. Reactions involving precursors, such as C3 and C4 species, were examined. Combination reactions of C3 species were identified to be the major benzene formation routes with the exception of the cyclohexane flame, in which benzene is formed exclusively from cascading fuel dehydrogenation via cyclohexene and cyclohexadiene intermediates. Acetylene addition made a minor contribution to benzene formation. Its importance, however, increases in the butadiene flame due to the direct formation of C4H5 radicals from the fuel, or in the n-decane flame because of the preferential formation of larger precursors.