No abstract

Conversion of benzene to chlorobenzenes and monochlorophenols by reaction with chlorine radicals (Cl*) in the cool-down zone of a plug-flow combustor has been studied, and a mechanistic analysis of the initial steps of the oxy-chlorination process is proposed. Superequilibrium concentrations of Cl* are formed during combustion of chlorocarbon species and persist at significant concentration levels even after a substantial reduction in the flue gas temperature (T = 500-700 degrees C). At these temperatures, Cl* attack on benzene present in trace concentrations (initial benzene concentration of 300 ppmv or 1080 ppmv were used for the experiments) in the post-flame gas is shown to result in stable chlorinated products (chlorobenzenes and chlorophenols) and loss of benzene. These results suggest that Cl* attack on trace level aromatics and possibly other organic species may be the initial step in the formation of a broad class of chlorinated and oxy-chlorinated pollutants in the post combustion zone.

This study is concerned with the formation and persistence of atomic and molecular chlorine in the late stages of chlorocarbon combustion under fuel-lean conditions as well as in postcombustion cooling stages, since chlorinecontaining hazardous organic pollutants including dioxin precursors can be formed through reactions involving these active forms of chlorine. In bench-scale experiments with a chlorine-containing fuel mixture (C 2 H 4 /CH 3 Cl or CH 4 / CH 3 Cl, with Cl/C in the range of 0 to 2.2%) the fuel is oxidized in a relatively short time (10-20 ms). The major chlorine-containing product is HCl; however, the cooled combustion gases contain significant concentrations of Cl 2 (up to 18% of the total chlorine load), with the exact amount depending on the fuel equivalence ratio, the residence time in the combustor, the H/C ratio of the fuel, and the rate of cooling of the gas products. Calculations indicate that the Cl 2 measured in the cold exhaust gas is formed by recombination during the quenching of Cl radicals present at high temperature. The model predicts that, under conditions found in practice, Cl radicals can likewise be present in significant concentrations (1 to 5% of the total chlorine) in combustion products at the exhaust. Due to kinetic constraints, Cl radicals can then persist at surprisingly low temperatures (down to ca. 500 K) before recombining to form Cl 2 .