Modeling the rich combustion of aliphatic hydrocarbons

A new kinetic mechanism has been developed for the formation of benzene and high-molecular-mass aromatic compounds in rich flames of aliphatic hydrocarbons. The kinetic scheme emphasizes both the role of resonantly stabilized radicals in the growth of aromatics and the standard acetylene addition mechanism. The model has been used to simulate premixed flames of acetylene and ethylene where the concentrations of radicals and high-molecular-mass compounds are known. The kinetic scheme accurately reproduces the concentrations and trends of radicals and stable species including benzene and total aromatic compounds. Formation of benzene is controlled by propargyl radical combination. The model reproduces well the profiles of benzene for the different hydrocarbons and in the different operating conditions. Key reactions in the formation of high-molecular-mass aromatics are the combinations of resonantly stabilized radicals, including cyclopentadienyl self-combination, propargyl addition to benzyl radicals, and the sequential addition of propargyl radicals to aromatic rings. The predicted amounts of total aromatic compounds increase at the flame front and remain constant in the postoxidation zone of the flames, attaining the final concentrations of soot, in slightly-sooting conditions. As a consequence, the carbonaceous species which contribute to soot formation are already present at the flame front as high-molecular-mass structures. Soot is formed through dehydrogenation and aromatization of the high-molecular-mass compounds, rather than by surface growth. (C) 2000 by The Combustion Institute.