Large molecules, radicals ions, and small soot particles in fuel-rich hydrocarbon flames

A fuel-rich, near-sooting, low-pressure, premixed haphthalene/oxygen/argon flame has been analyzed for condensible raction intermediates, mainly mono- and polycyclic aromatic hydrocarbons (PAHs), and for free radicals. Samples were taken through a nozzle beam that was expanded and condensed together with the radical scavenger dimethyldisulfide in a cold trap. Product separation was done with gas chromatography (GC) equipped with a mass spectrometer and a special detector for radical scavenging products. More than 310 different nonradicalic compounds and 23 mainly aromatic radicals could be detected. Among them were partially oxidized aromatics such as 1- and 2-naphthol phenol, benzofuran, inden-1-one, and benzaldehyde. Aromatic radicals with an even number of C and σ-radicals, for example, phenyl and two naphthyls: those with an odd number are π-radicals, such as indenyl and cycloheptatrienyl, the latter being detected for the first time in a flame. An important initiation of growth reactions toward larger PAHs is the formation of various biaryls by the reaction of aromatic radicals with the fuel molecule and aromatic degradation products from it. Biaryls are themselves highly reactive intermediates that first undergo intramolecular cyclodehydrogenations, if they posses four-or five-sided carbon bays, and then grow to highly condensed PAHs by addition of acetylene.