A Kinetic Model of Particulate Carbon Formation in Rich Premixed Flames of Ethylene and Benzene

A detailed kinetic model has been developed and used to simulate aromatic growth in premixed benzene and ethylene flames. The model considers the role of resonantly stabilized radicals in the growth of aromatic species, in addition to the hydrogen abstraction carbon addition (HACA) mechanism, which involves hydrogen abstraction to activate aromatics followed by subsequent acetylene addition. Model results show that the self-combination of resonantly stabilized radicals in particular, the combination of cyclopentadienyl radicals is the controlling pathway for the aromatic-ring growth. The kinetic model reproduces the experimental trends of these compounds already below the flame front and their decrease within the flame front, which has been observed experimentally but never predicted numerically. The reaction mechanism has been used to identify the different behaviors of aromatic growth in ethylene and benzene flames. Benzene formation is the rate-limiting step for aromatic growth in ethylene flames. C6H6 is formed across the flame front and carbon growth continues through the formation of multi-ring aromatics in the postflame region. In benzene oxidation, the aromatic ring is already present in the main oxidation zone and it is mainly oxidized to cyclopentadienyl radicals. As a consequence, a large amount of cyclopentadienyl radicals are available for recombination reactions, leading to multi-ring formation already in the main oxidation region. The increase of the temperature at the flame front reduces their concentration due to pyrolysis and/or oxidation resulting in a peak value in the main oxidation region and a leveling-off in the post-oxidation region of the flame.