Mechanism of Soot Formation in Hydrocarbon Flames

Abstract

The analogy between a sooting hydrocarbon flame and a stoichiometric flame front seeded with alkaline metals is studied. It is found that under certain conditions they are very similar as far as their electrical behaviour is concerned. Theoretical and experimental demonstration is made that the presence of a thermal ionization zone in the post-flame region is the cause of a large decrease of the potential necessary to extract a given current from the flame front. However, the saturation current extracted from the flame is very close to the saturation current measured in the absence of any thermal ionization. The consequence of the similarity of behaviour is that an ionic mechanism of nucleation of soot initiated by the flame chemi-ions appears to be insignificant compared to a pure radical mechanism of nucleation. The presence of positively charged soot particles in the post flame region of sooting hydrocarbon flames must be explained by the thermal ionization of large hydrocarbon molecules with low ionization potentials (around 5 eV)