Formation of PCDD/F in municipal solid waste incinerators: Laboratory and modeling studies

Recent laboratory results on the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) are reviewed, with emphasis on heterogeneous, fly ash-mediated reactions. The well-studied de novo synthesis yields PCDD/PCDF ratios less than one, a value frequently found in municipal solid waste incinerators. Its apparent rate cannot, however, explain PCDD/F-formation in incinerators during flue gas residence times. Reactions of chlorophenols yield PCDD, either in the solid or gas phase, and are much faster, but generally lead to PCDD/PCDF much greater than 1 at comparable temperatures. PCDF-yields from chlorobenzenes are much lower under comparable conditions, but increase at higher temperatures. Different experimental combustion systems have proven that PCDD/F can form rapidly at rates comparable to apparent incinerator rates, i.e., formation during flue gas passage from the furnace through the air pollution control device. The properties of fly ash play an important role in determining rates and yields, but there is at present hardly a quantitative link between fly ash physical/chemical properties and chemical reactivity. Some success has been achieved in applying a modified four-step model of PCDD-formation to incinerator data. The concept of active and superactive sites on fly ash is introduced, it may be useful in future experimental designs. For future control approaches that are not add-on oriented a better understanding of the time/temperature history of fly ash formation and the intrinsic kinetics of precursor reactions is required.