THE EVOLUTION OF SOOT PRECURSOR PARTICLES IN A DIFFUSION FLAME

The chemical evolution of soot precursor particles on the centerline o f the laminar ethene diffusion flame has been analyzed using laser mic roprobe mass spectrometry (LMMS) as they undergo the transition to car bonaceous aggregates. LMMS is a reliable microanalytical technique for the detection of intermediate and heavy polycyclic aromatic hydrocarb ons (PAHs) in particulate material. The analyses show that many of the masses present within the precursor particles coincide with those pre dicted by Stein and Fahr (1985) to be most thermodynamically stable (s tabilomers). The stabilomer PAHs that consist solely of six-membered r ings, the benzenoid PAHs, prove to be the most important members of th e stabilomer grid. Pericondensed PAHs as large 472 amu, which is attri buted to the molecule C38H16 with 12 hexagonal rings, are found to be constituents of the precursor particles. The PAH mass distribution div erges to the larger sizes with increasing height in the flame, and inc ludes many of the species identified by others as gas-phase PAH consti tuents in hydrocarbon flames. Carbonization on the centerline of the f lame occurs abruptly between 35 and 40 mm above the burner where the p article metamorphosis (from single precursor liquid-like particles to fused aggregates) and the decrease in hydrogen mole fraction (from 0.3 5 to 0.15) simultaneously occur. The presence of stabilomer PAHs repor ted by others in the particulate combustion product of a variety of fu els-aliphatic and aromatic gases, diesel fuel, elude oil, kerogen, car bon black feed stuck, cigarette tobacco, and biomass-suggests that the stabilomer grid represents the common path for the growth of PAHs whi ch contribute to the formation of carbonaceous soot in these diverse i nstances. This observation can account for the previously noted invari ance of the soot product of combustion from diverse fuels and devices. (C) 1998 by The Combustion Institute.