Formation of polycyclic aromatic hydrocarbons and their growth to soot - a review of chemical reaction pathways

The generation by combustion processes of airborne species of current health concern such as polycyclic aromatic hydrocarbons (PAH) and soot particles necessitates a detailed understanding of chemical reaction pathways responsible for their formation. The present review discusses a general scheme of PAH formation and sequential growth of PAH by reactions with stable and radical species, including single-ring aromatics, other PAH and acetylene, followed by the nucleation or inception of small soot particles, soot growth by coagulation and mass addition from gas phase species, and carbonization of the particulate material. Experimental and theoretical tools which have allowed the achievement of deeper insight into the corresponding chemical processes are presented. The significant roles of propargyl (C3H3) and cyclopentadienyl (C5H5) radicals in the formation of first aromatic rings in combustion of aliphatic fuels are discussed. Detailed kinetic modeling of well-defined combustion systems, such as premixed flames, for which sufficient experimental data for a quantitative understanding are available, is of increasing importance. Reliable thermodynamic and kinetic property data are also required for meaningful conclusions, and computational techniques for their determination are presented. Routes of ongoing and future research leading to mon detailed experimental data as well as computational approaches for the exploration of elementary reaction steps and the description of systems of increasing complexity are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.