Emissions of polychlorinated dibenzo-p-dioxins and dibenzofurans from catalytic and thermal oxidizers burning dilute chlorinated vapors.

Emissions of polychlorinated dibenzo-p-dioxins and dibenzofurans (dioxins) have been found from 57 field tests on the oxidation of low (a few to a few hundred) parts per million levels of chlorinated and non-chlorinated volatile organic compounds (VOCs). The oxidation occurs in catalytic oxidizers with platinum, platinum/palladium or chromium(IV) oxide combustion catalysts, or in thermal oxidizers (without a catalyst). The catalyst inlet temperatures ranged from 293 to 573 degrees C. The thermal oxidizer operating temperatures (post-flame) were from 773 to 927 degrees C. Data of the toxic dioxin and furan isomers are reported and also weighted and expressed as international toxic equivalents (TEQ) of 2,3,7,8-tetrachlorodibenzo-p-dioxin. The maximum stack emissions, 1.07 ng/m3 TEQ, occurred at 293 degrees C. Salient results of this field study are: (1) TEQ levels in the stack exponentially increase with a decrease in operating temperature, an empirical equation is TEQ (ng/dscm)=8.4 exp(-0.0084T degrees C); (2) dioxin/furan production occurs at the combustion catalyst; (3) small variations in temperature cause large changes in the congener distribution of the dioxin and furan isomers; (4) molar TEQ yields from the parent compounds fed to the oxidizers are very small (10(-9)-10(-13)); (5) catalytic and thermal oxidizers may destroy dioxins fed from the ambient air; and (6) the oxidation of chlorinated VOCs with non-chlorinated VOCs reduces emissions of dioxins, likely due to the consumption of Cl in producing HCl. Laboratory investigations are needed to understand how dioxins are formed (and emitted) under conditions of this study.