On the growth of Polycyclic Aromatic Hydrocarbons (PAHs) in a coflow diffusion flame of ethylene

Abstract

In the last two decades several mechanisms have been proposed for the growth of PAHs during combustion. Studies have suggested that the growth of PAHs of different structures should be analyzed carefully to improve our understanding of PAH formation during combustion. In this study, a wide of range of PAHs of different structures have been measured along the centreline of a coflow diffusion flame of ethylene using GC/MS, allowing for a previously not possible detailed analysis of potential formation routes. The discussed potential global empirical pathways can help in the development of accurate and detailed chemical mechanisms in the future. The experimental results show that the difference in reactivity of the sites of a PAH where growth takes place limits the formation of benzenoid PAHs at high temperatures. This difference in reactivity also leads to the formation of PAHs with a five-membered ring at high temperatures. The recombination of PAH radicals may not be favoured in this flame which explains the small amounts of alkyl-bridged PAHs at high temperatures. PAHs with an alkyl sidechain are favoured only along the free edge. Alkyl substituted PAHs are formed in low amounts and decrease with an increase in temperature. Methylene-bridged PAHs can lead to faster growth of large PAHs and become significant at intermediate temperatures. The target flame has also been simulated using state-of-the-art models. The simultaneous measurements of both PAHs and soot allowed for a comprehensive assessment of the chemical mechanisms and soot aerosol dynamics. The assessment suggests that both the chemical mechanisms and the soot aerosol dynamics need further improvements. The results confirm the importance of accounting for PAH structure when developing kinetic mechanisms and their coupled soot formation models. A complete database comprising of PAHs, soot and temperature is generated for future model validations.