Soot formation has become a subject of interest in the scientific community because of its application to the efficient use of hydrocarbon fuels and to health and environmental issues associated with its emissions to the atmosphere. Since this is a complex process that involves a great number of chemical and physical steps, different analytical techniques and theoretical models need to be coupled to gain a better understanding of the inception and growth of nanoparticles in flames.

The main purpose of this study was to investigate the particle size distributions (PSD) from a flat-flame burner fueled with benzene and ethylene and compare these results to a simulation. Transmission electron microscopy (TEM) measurements are also reported to show the characteristics of the particles. These experimental results are targeted at characterizing differences in size, structures and morphologies of soot extracted from aromatic and aliphatic flames. The experimental data results support findings presented in previous theoretical and experimental studies that benzene flames lead to a very early nucleation of particles. This is then responsible for the observed differences in the particle size distribution and morphologies in benzene versus aliphatic flames.

A detailed kinetic mechanism was used to model the experimental data. The model includes reaction pathways leading to the formation of nano-sized particles and their coagulation to larger soot particles by using a discrete-sectional approach for the gas-to-particle process. Good predictions of particle-phase concentrations and particle sizes in the two flames are obtained without any change to the kinetic scheme. In agreement with experimental data, the model predicts a higher formation of particulate in the benzene flame respect to the ethylene flame. Furthermore the model predicts that in the ethylene flame small precursor particles dominate the particulate loading in the whole flame whereas soot is the major component in the benzene flame.

The driving factor in the model responsible for the quite different soot predictions is the large concentration of PAHs in the flame front of the benzene flame respect to the ethylene flame. In the benzene flame, particles are formed immediately downstream of the flame front and their subsequent coagulation leads to the formation of soot. In the ethylene flame, PAHs are formed more gradually and late in the flame respect to the benzene flame hence decreasing the rate of particle inception and consequently the coagulation to soot.