Further details on particle inception and growth in premixed flames

Soot inception in flames remains mostly unknown. What are, if any, the differences between inception particles formed in non-sooting and sooting flames, and to what extent soot formation is governed primarily by coagulation or by a different composition/aromatization of inception particles are open questions. In this work, the initial particle growth occurring in premixed flames of ethylene has been investigated. On line size-selected photoionization efficiency of inception particles produced in various flames has been measured. Structural/chemical differences of both nucleating and growing particles have been additionally investigated by off-line cyclic voltammetry, Raman spectroscopy, and light absorption. Results show that the size of aromatic domains within particles slightly increases when moving from flames in which the particle size distribution remains nearly constant and mono-modal in the nanometer range along the flame height, to flames in which particle coagulation gives rise to the formation of a second mode constituted by larger but still nanometric particles. Only in particles of this latter mode, d ≈ 10 nm, the first appearance of stacking of polyaromatic units is observed. On this basis, it has been hypothesized that two graphitization processes occur during the first stages of soot inception: a slight increase of in plane aromatic islands in primary particles and the formation of aromatic plane stacks in coagulated primary particles. This second type of coagulating-graphitization process explains the closure of the band gap observed in the grown particles respect to the primary ones, and hence the change of optical properties, towards the typical values observed in soot. © 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.