Reactions of Chlorinated Benzenes in H2 and in H2/O2Mixtures: Thermodynamic Implications on Pathways to Dioxin

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Abstract
Thermal Reactions Chloro and Dichlorobenzene in H2 and Chlorobenzene in H2/O2 mixtures were studied using tubular flow reactors between 835 to 1275 K, 1 atm pressure. Complete conversion of chlorobenzenes in hydrogen required temperatures above 1100K. while in mixtures containing small quantities of O2, equivalent conversion occurred at 893 K. Major products in both systems were benzene, carbon solids and HC1, with CH2 and C2H6 also present in low concentrations. Results from detailed kinetic mechanisms showed that displacement of the chlorine by atomic hydrogen (H addition to the ring with subsequent beta scission of the chlorine from the activated chlorocyclohexadie-nyl radical) was responsible for conversion of the chlorobenzene. O2, can initiate the chain mechanism by H2 + O2 → HO2 + H, which explains the lower required temperature for conversion when Oxygen is present. The molecule and radical thermodynamic properties required for the kinetic models led to development of thermodynamic parameters for chlorinated aromatics including dioxins and dibenzofurans. These properties demonstrate that it is thermodynamically more favorable to form chlorinated dioxins and dibenzofurans over the analogous non-chlorinated species. The pathways presented demonstrate plausible elementary reactions to chlorinated dibenzofuran and dioxin formation in incineration and combustion processes. We suggest two regimes in incineration or high temperature reaction systems, where homogeneous dioxin formation may occur. The first is a fuel rich (pyrolysis) region where molecular weight growth may occur. The second is a region where poly- phenyls and aromatics may be present with oxygen and oxy radicals at temperatures which favor addition reactions over oxidation of the ring systems.

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