Kinetic Modeling Study of Polycyclic Aromatic Hydrocarbons and Soot Formation in Acetylene Pyrolysis

The aim of this kinetic work is a critical and detailed analysis of the acetylene pyrolysis in a wide range of conditions, especially from 900 to 2500 K, in order to further validate and refine the kinetic mechanism of C-2-C-4 acetylenic species. In particular, the successive reactions of the intermediate products, such as C4 species and polyynes, were especially investigated to better understand the primary C2H2 kinetics. The subsystem of acetylene pyrolysis reactions clearly constitutes an important portion in the overall kinetic scheme and also a crucial topic in the further extension of the model toward the formation of polycyclic aromatic hydrocarbons (PAHs) and soot particles. The analysis of acetylene experiments, often with the formation of large amount of soot, requires the coupling of the gas phase kinetic scheme with a soot kinetic model. The gas-phase kinetics involves the formation of the first polycyclic aromatic hydrocarbons up to the first components of the soot kinetic mechanism (C20H16 and C20H10). The soot kinetic model is based on a discrete sectional method with an extensive use of lumping rules. Analogy and similarity rules with gas phase kinetics are used to extend the soot mechanism up to the formation of species with more than 10(7) carbon atoms. The lumped approach, extensively applied in the kinetic modeling of large hydrocarbon species, becomes a necessity in treating chemical reacting systems of these dimensions. The analysis of the experimental data of acetylene pyrolysis obtained in a wide range of conditions permits us not only to highlight the competition between the molecular and the free radical pathways but also to analyze the successive condensation reactions for the formation of PAHs and the growing of soot particles.