The reaction of phenoxy radicals with mono-chlorobenzene and its meaning for gas-phase dioxin formation in incineration

• Published in: Symposium, International, on Combustion International, on Combustion, 1996, Vol.26 (2), p.2405-2411
• Main Authors: Grotheer, Horst-Henning, Louw, Robert
• Format: Article
• Language: English
• ISSN: 0082-0784
• DOI: 10.1016/S0082-0784(96)80070-1

The reaction of (chlorinated) phenoxy radicals with chlorophenols recently has been discussed as a source of gas-phase dioxin formation in incineration. As a surrogate for this reaction, we studied the analogous reaction (1) of phenoxy (ΦO) with chlorobenzene (ΦCl) in a well-stirred reactor between 728 and 984 K. The slow combustion of phenol served as a continuous ΦO source, and chlorobenzene was used as cofuel. Stable end products and reactants were analyzed by gas chromatography (GC), in particular ΦCl, dibenzofuran (DF), and diphenylether (ΦOΦ), the direct product of reaction 1. This made it possible to measure k1 in competition with ΦO recombination, reaction (2) for which an overall rate coefficient had earlier been established in our laboratory. The Arrhenius parameters for reaction (1) are A1=4.8×108 L/mol s, Ea1=24.5 kcal/mol When these parameters are adopted for the reaction of chlorinated phenoxy radicals with chlorophenol, an agreement with an earlier kinetic analysis of Shaub and Tsang is achieved that has a meaning for dioxin formation. The given parameters render this reaction at temperatures below 1000 K too slow to contribute significantly as a gas-phase dioxin source. However, since gas-phase dioxin formation recently has been detected in a flow reactor during trichlorophenol combustion with maximum yields of dioxin around 900 K. another dioxin source must be found consequently, we suggest the gas-phase recombination of chlorinated phenoxy radicals.