A computational study of hydrocarbon growth and the formation of aromaticsin coflowing laminar diffusion flames of ethylene

A kinetic mechanism, previously developed and successfully applied to predi ct the formation of benzene and larger aromatics in pre-mixed flames is app lied to co-flowing diffusion flames. The mechanism emphasizes the role of r esonantly stabilized radicals, in addition to the acetylene addition mechan ism (HACA mechanism). The flames modeled are atmospheric pressure, axisymme tric, co-flowing laminar diffusion flames with varying amounts of oxygen ad ded to the fuel, as studied by McEnally and Pfefferle. The model predicts, with a good level of accuracy, the growth of hydrocarbons and the formation of benzene and aromatic species. The results show that in diffusion-contro lled conditions. as in pre-mixed flames, benzene formation is controlled by propargyl radical combination. Key reactions leading to the formation of l arger aromatics are the combination of resonantly stabilized radicals, incl uding propargyl addition to benzyl radicals and to a lesser extent cyclopen tadienyl radical combination. The mechanism of acetylene addition to aromat ic rings (HACA mechanism) contributes negligibly to the formation of larger aromatics. The model also explains the effect on the formation of non-fuel hydrocarbons of pre-mixing oxygen with the fuel. In particular, the model shows that partial pre-mixing shifts the pyrolysis mechanism of the fuel to ward odd-carbon hydrocarbons. This in turns leads to enhanced production of benzene and larger aromatics, because of the importance of reaction mechan isms involving resonantly stabilized radicals with an odd number of carbon atoms, such as propargyl (C3H3), cyclopentadienyl (cC(5)H(5)) and benzyl (C 7H7) radicals, in forming aromatics. (C) 2001 by The Combustion Institute.