enzene formation and that the maximum rate for this pathway in significantly greater than for the others. Concluding two main mechanisms seems to lead to the formation of the first aromatic ring: C2 + C4 and C3 + C3. However, despite the high number of works a generally accepted consensus about the dominant benzene formation pathway does not seem to have been reached. Miller and Melius also emphasized the potential importance of Resonantly Stabilized Free Radicals (RSFR) in forming aromatics and PAH in flames. Other compounds, in fact, can have relevance as precursors in PAH formations without passing through benzene as intermediate. The formation of naphthalene, a compound with two aromatic rings, via the reaction of two cyclopentadienyl radicals was firstly included by Dean  in a kinetic model describing the methane pyrolysis. Subsequently, based on combined experimental and kinetic modeling studies of PAH formation in methane and ethylene flames, Marinov et al.  and Castaldi et al.  jointly concluded that acetylene addition processes cannot account for the PAH levels observed in experimental flames. As a result of this work, they also proposed that in aliphatic hydrocarbon flames, the larger aromatics originate from resonance stabilized cyclopentadienyl radicals. On the other hand the identification of acetylene as key reactant of growth process leading to larger and larger PAH up to soot particles was firstly intuited by Jensen  in the 1974 and subsequently by Bockhorn et al.  and by Frenklach et al.  using a similar growth sequence namely: hydrogen-abstraction/acetylene-addiction (HACA) mechanism. The early modeling work of Frenklach et al.  is the decisive step toward the use of detailed kinetic model as tools for the development of a realistic understanding of PAH and soot formation. Frenklach and co-workers extended and refined their picture of PAH and soot formation in the subsequent years. 16
In particular the results obtained by Wang and Frenklach  support the important hypothesis in PAH growth that reactions of multi-ring aromatic species are in principle similar to those of benzene and phenyl. However, D’Anna and Violi  comparing numerical results against experimental data obtained in slightly sooting laminar premixed ethylene–oxygen flames concluded that HACA mechanism is not sufficient to explain the PAH concentrations observed experimentally. The kinetic model developed by D’Anna and Violi takes into account PAH containing up to three rings and three different sub-mechanisms are tested: (a) reactions involving H-abstraction/C2H2- addition to aromatic radicals; (b) the reaction of species containing five-membered rings leading to naphthalene and phenanthrene, i.e. the cyclopentadienyl pathway; and (c) by combination of benzyl and propargyl radicals. Comparison of the net formation rates showed a dominant role of the cyclopentadienyl and the propargyl pathways. Figure 1.1 shows the fundamental steps of PAH formation as described above. RSFR + + 17 C4 + C2 C3 + C3 HACA HACA Fig. 1.1 Reaction pathways diagram of the fundamental steps in PAH formation.