Additive influence on polycyclic aromatic hydrocarbon formation

The effects of HBr and Fe(CO)(5) flame radical scavengers on the concentration of polycyclic aromatic hydrocarbon (PAH) molecules and their precursors have been determined on the basis of simulations of experiments for premixed flames and in a jet-stirred reactor. Results are derived from a mechanism that combines a soot model and a kinetic model with the reactions that define the flame inhibition capability of iron pentacarbonyl and HBr. Near stoichiometric conditions, radical scavenger effects are very large. An increase of equivalence ratio decreases the relative effect of these additives. For a relatively large equivalence ratio, the gas-phase influence of radical scavenger additives disappears. Kinetic models demonstrate that the gas-phase effect of additives is to enhance PAH formation. The results of this work support the suggestion of earlier works that experimentally observed decrease of particulate formation is possibly the result of heterogeneous reactions of iron oxides by increase of oxidation rate of formed soot. The simulations show that contributions from propargyl combination and C-4 mechanisms to benzene formation are dependent on the equivalence ratio. The former predominates in a rich system where propargyl combination is responsible for benzene formation, while the latter is more important at lower values of the equivalence ratio.