Reaction mechanism of soot formation in flames

The formation of polycyclic aromatic hydrocarbons (PAH) and soot has become one of the central themes of research activities in the area of combustion and pyrolysis of hydrocarbon fuels. The interest in the subject stems mostly from environmental concerns on pollutant emission from combustion devices. The research activities span experimental, theoretical, and computational efforts and cover a diverse spectrum of the physics and chemistry of the process. The increasing amount of information prevents an exhaustive review in an article like this. Instead, the objective here is to focus on major elements in the present understanding of the phenomena, to clarify some key concepts (and especially those perceived as points of controversy), and present new results. In term of the underlying science, soot formation may be viewed as being comprised of four major processes: homogeneous nucleation of soot particles, particle coagulation, particle surface reactions (growth and oxidation), and particle agglomeration. We discuss each below.