Influence of potassium and iron chloride on the early stages of soot formation studied using imaging LII/ELS and TEM techniques

Laser-induced incandescence (LII), elastic light scattering (ELS) and extinction were used to investigate the early stages of soot formation when seeding metal salts at various concentrations into a premixed ethylene/air flame on a PerkinElmer burner. The investigation contained seven salts, where we in this work mainly focus on two of these salts, potassium chloride (KCl) and iron chloride (FeCl₃), as these for mature soot resulted in the largest reduction and highest increase in soot concentration, respectively. The optical measurements show that the potassium chloride significantly reduces the build-up of larger soot precursors and also decreases the sizes of the primary soot particles both at the nucleation stage as well as for more mature soot later in the formation process. This is also confirmed by analysis of transmission electron microscopy (TEM) images, which clearly show smaller primary soot particles for the potassium chloride-seeded flame. The iron chloride-seeded flames were found to have marginal influence on the primary particle sizes based on the optical data, although somewhat more irregular shapes could be observed in TEM images.