Mechanism of formation of polychlorinated dibenzo-p-dioxins and dibenzofurans in the catalyzed combustion of carbon

Abstract

~~~ Combustion experiments of an activated carbon (Norit RX Extra), catalyzed by CuC12, were conducted in triplicate in a flow of moist air containing 5 vol % HC1 at 300 C to reveal the mechanism of formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). The catalyst concentration was varied from 0, 0.1,0.5,1.0, to 5.0 wt %. After partial combustion (1 h), the samples were analyzed on the presence of PCDDs and PCDFs. The total yield of these compounds amounted to 19 (0.1 wt % CuC12) and 2 (5.0 wt % CuClz) pg/g of carbon. The PCDD/PCDF ratio showed a decrease from 33 to 0.2. At low copper concentrations, predominantly PCDDs are formed with an isomer pattern that can be explained completely as a product of chlorophenol condensation reactions. At high copper concentrations, the PCDD/PCDF isomer distribution pattern shifts toward a characteristic waste incineration fly ash pattern. A proposal for the mechanism of formation of PCDDs and PCDFs in the catalyzed combustion of carbon will be presented. Condensation reactions of 2,4,6-trichlorophenol on carbon (Owt 9% CuC12) resulted in the formation of predominantly PCDDs with an identical isomer distribution pattern as observed in the catalyzed combustion of carbon. Active carbon was shown to catalyze the condensation reaction of 2,4,6trichlorophenol into PCDDs.