Understanding the formation and growth of polycyclic aromatic hydrocarbons (PAHs) and young soot from n-dodecane in a sooting laminar coflow diffusion flame

Abstract

The mechanistic pathways of PAH formation and growth remain uncertain. In addition, our current understanding of the transformation of PAHs into young soot is limited. The PAHs participating in this transformation remain ambiguous. Simultaneous measurements of PAHs and particles are necessary to better understand how particulates are formed in flames. A comprehensive analysis has been performed on a n-dodecane doped methane coflow diffusion flame. PAHs have been analyzed with GC/MS while the particulates have been collected from the flame centreline for studying under a Transmission Electron Microscope (TEM). The soot measurements from a previous study have been used to complete the comprehensive analysis. The novelty of the current study lies in the fact that it provides quantitative information on the growth of PAHs ranging from naphthalene (A2) to pyrene and fluoranthene (together they are referred to as A4 in this study) and how these species participate in the formation, growth and aging of the young soot. The experimental results show that before the commencement of the young soot, the gaseous phase is dominated by species equal to and smaller than A4. PAHs larger than A4 were not detected. With the growth of the young soot, the mole fraction of A4 decreases. The results suggest that small and medium-sized PAHs (A4 and smaller) are responsible for the formation and growth of the young soot. As the young soot transform into mature solid soot, A4 increases abruptly. The study shows that PAH growth and soot maturity are not mutually exclusive. The target flame has also been simulated numerically to identify the problems associated with the state-of-the-art model. The numerical model with irreversible nucleation can not capture the formation of young soot even though it considers nucleation from A4. A comprehensive database of PAHs and soot has been created for future numerical model validations.