Formation mechanism of polycyclic aromatic hydrocarbons and fullerenes in premixed benzene flames

Abstract

A better understanding of the formation of polycyclic aromatic hydrocarbons (PAH) and fullerenes is of practical interest due to the apparent environmental health effects of many PAH and potential industrial applications of fullerenes. In the present work, a kinetic model describing the growth of PAH up to coronene (C24H12) and of C60 and C70 fullerenes is developed. Comparison of the model predictions with concentration profiles in a nearly sooting low-pressure premixed, laminar, one-dimensional benzene/oxygen/argon flame (equivalence ratio φ = 1.8, pressure = 2.67 kPa) measured by Bittner using a molecular beam system coupled to mass spectrometry shows reasonably good predictive capability for stable and radical intermediates and growth species up to C16H10 isomers. Cyclopentadienyl is found to be a key species for naphthalene formation. The further growth process is based on H abstraction and acetylene addition but also the contribution of small PAH is considered. Good to fair agreement between model predictions and experimental data for larger PAH including the different C16H10 isomers obtained by gas chromatography coupled to mass spectrometry and high performance liquid chromatography could be achieved for PAH in a sooting low-pressure premixed, laminar, one-dimensional benzene/oxygen/argon flame (φ = 2.4, 5.33 kPa). C60 and C70 fullerenes are underpredicted, and possible reasons such as uncertainties in rate coefficients or the existence of other formation pathways are discussed. PAH depletion in the burnt gas is not reproduced by the model and is believed to involve supplementary sinks such as reactions involving PAH and growing soot particles.