Ultrafine particles from combustion sources: approaches to what we want to know

Soot formation and oxidation will be analysed with respect to the most important processes, namely particle inception, coagulation and surface growth. Time–scales of surface growth are estimated for premixed and diffusion flames and compared with time–scales for coagulation. It turns out that characteristic time–scales for soot formation and coagulation are similar and about one order of magnitude larger than the characteristic time–scales for combustion reactions and much smaller than the time–scales of molecular transport. Coagulation processes will be discussed in detail and a detailed chemistry approach for surface growth will be presented. The detailed information will be put into a soot model that reproduces a number of phenomena in sooting premixed hydrocarbon flames, for example: (i) the dependence of surface growth and oxidation rates on the chemical ‘environment’ of soot particles; and (ii) the fraction of soot formed by particle inception and surface growth reactions and addition of polyacrylic aromatic hydrocarbon (PAH). The ‘fine structure’ of soot is not resolved by this approach, and, furthermore, the predictions depend sensitively on information about the kinetics of growth of PAHlike structures, the detailed processes occurring on the surface of soot particles, and, most importantly, the pressure dependence of all these processes.