Chemistry of Acetylene Flames

Abstract

Acetylene constitutes one of the major intermediates in hydrocarbon flames and is important through its links to soot inception and mass growth processes. In the present study a detailed kinetic mechanism is developed and tested against experimental data for six lean (φ = 0.12) to sooting (φ = 2.50) laminar, premixed, low-pressure acetylene flames. Generally the agreement between computations and experiments is acceptable. It is suggested that OH attack competes with O attack as the major acetylene breakdown path in rich flames. It is further shown that the balancing of the ketyl radical destruction chemistry to a significant extent determines important flame features such as CO/C0₂ ratio and H radical concentrations. The balancing of the methylene and methyne radical chemistry in both lean and rich environments is discussed in detail and the importance of molecular oxygen attack on ³CH₂ is outlined. It is further shown that the present mechanism accurately predicts the qualitative evolution of methylene and methyne radical levels as a fucntion of stoichiometry. The present study incorporates the benzene oxidation mechanism by Lindstedt and Skevis (1994) and the benzene formation steps, involving isometri-zation reactions between linear and cyclic C₆ intermediates, reported by Leung and Lindstedt (1995). The results obtained show that for rich acetylene flames the primary path for benzene formation passes via propargyl radical recombination and that benzene levels are generally satisfactorily predicted. However, computations also indicate that for leaner flames paths involving acetylene addition to n-C₄H₃ and 1,3-C₃H₅ radicals become increasingly important. This study also identifies reactions where further experimental investigations are required.

Keywords:

• Acetylene oxidation
• benzene formation
• soot formation
• reburn chemistry
• detailed kinetics
• laminar flames