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

Abstract

Thermal Reactions of Chi oro and Dichlorobenzene in H, and Chlorobenzene in H,jO, mixtures were studied using tubular flow reactors between 835 to 1275 K, I atm pressure. Complete conversion of ehlorobenzenes in hydrogen required temperatures above I lOOK. while in mixtures containing small quantities of O 2 equivalent conversion occurred at 893 K. Major products in both systems were benzene, carbon solids and Hel, with CH 4 and C 2 H 6 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 chlorocyclohexadienyl radical) was responsible for conversion of the chlorobenzene. O 2 can initiate the chain mechanism by H 2 + O 2 .-H0 2 + 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.

Presence of Chlorine Radicals and Formation of Molecular Chlorine in the Post-Flame Region of Chlorocarbon Combustion

“…Chlorine chemistry is particularly important in the late stages of combustion by its impact on the final oxidation of CO and of trace byproducts which may have escaped the primary reactor zone. Chlorine is thought to inhibit these processes by consuming HO2 and O radicals (8) and to participate in the formation of chlorinated and oxychlorinated pollutants if the temperature is such as to favor recombination reactions (9)(10)(11)(12)(13). Relatively little research has focused on the persistence of Cl radicals and Cl2 to the temperature of importance for dioxin formation (500 to 650 K) or at the combust or exhaust, since the general perception was that HCl was the dominant product of the chlorine present in fuels or wastes.…”