The homogeneous, gas-phase formation of chlorinated and brominated dibenzo-p-dioxin from 2,4,6-trichloro- and 2,4,6-tribromophenols

The homogeneous, gas-phase formation of polychlorinated dibenzo-p-dioxins (PCDD) and polybrominated dibenzo-p-dioxins (PBDD) has been observed from the high-temperature thermal decomposition of 2,4,6-trichlorophenol (2,4,6-TCP) and 2,4,6-tribromophenol (2,4,6-TBP), respectively. Experiments were conducted in a 1.0-cm-i.d. flow reactor over a temperature range of 300°–800°C with reactant concentrations of not, vert, similar 3.0 × 10−7 mol/L in a reaction atmosphere of dry air. The 1,3,6,8- and 1,3,7,9-tetra chlorinated isomers were the dominant PCDDs observed from the thermal oxidation of 2,4,6-TCP with maximum yields of 0.05% each. The corresponding tetrabrominated isomers were observed from the thermal oxidation of 2,4,6-TBP; however, the maximum yields were approximately 500 times higher. The observed PCDD/PBDD yields and the temperature of their formation can be readily accounted for using a modified form of the original gas-phase formation model of Shaub and Tsang, if the activation energy for the formation of diphenyl ether by displacement of Cl/Br from halophenol by phenoxy is decreased from 26 to 19.5 and 8.8 kcal/mol, for the chlorinated and brominated systems, respectively. This suggests that gas-phase formation reactions make a significant contribution to observed dioxin and furan yields in full-scale incinerator.