Pyrolysis product absorption by burning benzene droplets

Abstract

By burning droplets of benzene in a single-droplet combustor and performing phase-discriminating sampling of the liquid and gas phases of the droplet system, we have found that gas-phase pyrolysis products arise in the liquid phase of the droplet. The experiments are conducted at 1000 K and 21 mol % O₂ in the postcombustion gas from an oxygen-rich premixed methane flame. Disruptive burning, which has not previously been reported for a pure hydrocarbon in normal gravity conditions, is observed at the end of the droplet residence time (∼92 ms). Samples of the liquid phase have been taken at various times throughout the combustion lifetime and analyzed by high-pressure liquid chromatography. Compositional analysis using ultraviolet-visible absorbance spectra of the separated components of the samples reveals a wide variety of pure polycyclic aromatic hydrocarbons (PAH), substituted PAH, and cyclopenta-fused PAH. In addition, recent synthesis of new reference standards has enabled identification of cyclopenta-fused PAH—cyclopent[hi]acephenanthrylene, cyclopenta[cd]fluoranthene, and dicyclopenta[cd, jk]pyrene—which have never before been identified as benzene products. Because the droplet remains relatively cold (∼350 K) with respect to the gas phase in the oxygen-deficient zone between the droplet and the flame (∼2000 K), we conclude that these compounds are gas-phase pyrolysis products that are obsorbed into the droplet, rather than products of reactions within the droplet. These heavier species may play a role in observed terminal disruptive burning events by acting as additional droplet components that promote multicomponent effects. Analysis of species concentrations over time reveals the dominance of both ring rupture pyrolysis products such as phenylacetylene, triacetylene, and acenaphthylene, and biaryl pyrolysis products such as biphenyl. These four products in particular represent 70% of the identified mass of absorbed pyrolysis products, which accounts for up to 5% of the droplet mass at the end of its lifetime.