Radicals in flames: Analysis via scavenging reaction

Abstract

Radicals in flames can be quantitatively analyzed by condensing flame gases along with a radical scavenger. Gas phase samples were taken from fuel-rich and sooting premixed low-pressure flames (27 mbar) by a nozzle beam. They were frozen simultaneously with an excess of dimethyl disulfide (DMDS) at liquid nitrogen temperature. High condensing efficiencies were achieved when both sample and scavenger alike had condensed on the inner surface of a hollow sphere. The scavenging reaction between a radical and the DMDS had taken place in the condensed phase and yielded a stable methylthio compound. The products were separated and identified by using the GC/MS-technique. This novel method was applied to ethyne/oxygen (=2.50) and benzene/oxygen (=1.80_ flames. Aside from the stable condensable flame products, such as polyynes and aromatic hydrocarbons, mono- and poly-methylthio compounds could be analyzed depending on the number of radical sites per molecule. Among the scavenged radicals aromatic radicals like phenyl, benzyl, indenyl, naphthyl, and the phenoxy radical, as well as a number of aliphatic radicals, including ethynyl, C 2 , C 3 H, and the carbenes CH 2 , C 3 H 2 , and C 5 H 2 , could be detected. Concentration profiles of radicals in both flames are reported and subsequently discussed. Conclusions are drawn concerning the polyynes, the formation of aromatic structures by the combustion of aliphatic fuels, and the reactions of aromatic compounds in sooting flames.