PAH AND HIGH-MOLECULAR-WEIGHT SPECIES FORMED IN A PREMIXED METHANE FLAME

The concentration profiles of CO, CO2, O-2, light hydrocarbons, condensed species and soot have been measured in the soot inception region of a slightly-sooting premixed methane flame. Condensed species (CS) are early detected within the main oxidation zone of the flame and their concentration increases rapidly reaching a maximum at 4.5 mm together with the maximum formation of acetylene. Soot begins to appear just in correspondence of the CS decrease at 5 mm, thereafter both soot and CS concentration rise in the post oxidation region of the flame. Polycyclic aromatic hydrocarbons (PAH) up to coronene have been easily identified in the condensed species and their absorption and fluorescence strongly contribute to the absorption and fluorescence of the condensed species in the ultraviolet, but do not justify the noteworthy absorption and fluorescence in the visible. By extraction and high pressure liquid chromatography, other aromatic species, more absorbing and fluorescent in the visible than light PAH, have been isolated in the condensed species. The visible absorption and fluorescence of these compounds, as well as their different solubility in hexane, suggest a higher molecular weight of these species. These compounds have an absorption in the uv not so large as that shown by strongly-condensed aromatic structures suggesting a low degree of ring condensation and/or a certain degree of aliphatic functionality for these high molecular weight species.