Condensed Phases in Soot Formation Process

The condensed phases sampled in a premixed laminar flame burning ethylene in fuel-rich conditions were separated by extraction and characterized by SEC, mass spectrometry and spectroscopic analysis. The condensed phases were extracted by DCM and subsequently by NMP (in order to detach all the low MW species adsorbed on sampled soot particles. SEC chromatography was used to fractionate the DCM-soluble species and NMP-soluble species into well-defined MW classes to be further analysed by using uv-visible absorption and fluorescence spectroscopy and by AP-LDI mass spectrometry. Overall, the SEC analysis coupled with the spectroscopic and mass spectrometric put in evidence the presence in condensed phases of two different classes of large MW aromatics, namely PAH's oligomers and pericondensed PAH, the first one deriving from chemical growth by aromatics and the second one by chemical growth by acetylene. The pericondensed PAH's are in the 100-1000 u MW range with the lower MW pericondensed PAH's presumably existing in gas phase and the higher MW pericondensed PAH's present in a tarry phase.The PAH'oligomers are constituted of species from about 400 u to one or even more thousands of mass units. These aromatic classes could be formed to an extent that depends on the combustion conditions and fuel characteristics, mainly aromaticity, and can participate to soot inception.