Polychlorinated Dibenzo-p-dioxins and Dibenzofurans Formation in Incineration:  Effects of Fly Ash and Carbon Source

Research aimed at understanding polychlorinated dibenzo- p -dioxin (PCDD) and dibenzofuran (PCDF) formation in combustion and incineration processes constitutes an important component in developing strategies for controlling their emission. Incinerator ashes from six different process sources have been examined and characterized in terms of their behavior with respect to PCDD/F formation in laboratory experiments. The effects of varying the carbon content of one ash has been investigated by replacing the native carbon content with activated charcoals and pyrocarbons from paper and PVC. Analysis of homologue totals and full isomer profiles indicate that PCDDs tend to form in preferential isomer groups, while PCDF isomers are distributed more broadly. PCDD formation is consistent with condensation of chlorophenols, together with stepwise chlorination and dechlorination. Experiments with a pentachlorophenol precursor show that fly ashes are more reactive dechlorinators than model systems, probably due to the presence of alkali elements. PCDF formation probably occurs via condensation of nonchlorinated phenol followed by chlorination of the dibenzofuran skeleton; this gives schemes of formation for both PCDDs and PCDFs that start with phenol. Demonstration of the formation of phenols in ashes and simulates is not currently satisfactory, but aromatization of residual aliphatic oils is believed to be the most probable source. A relationship was found between the amounts of such oils present on the original carbons/ashes and the quantities of PCDD and PCDF formed.