The role of precursors in formation of polychloro-dibenzo-p-dioxins and polychloro-dibenzofurans during heterogeneous combustion

Abstract

Although by now literature reports suggest that compounds containing almost any combination of C, H, O, and Cl can form PCDD/PCDF's under suitable conditions, questions of the relevancy of such findings to the formation of these compounds in MSW-incinerators remain. To address these questions the formation and destruction of plausible precursors (intermediates) over time/temperature scales found in incinerators must be better understood. In our laboratory we have attempted to simulate MSW-incineration using a systems approach, i.e., a spouted bed combustor coupled to two post-combustion stages. In this system, formation of PCDD/PCDF's and potential precursors has been investigated in the bed (pre-flame), the post flame regions, and over fly ash (post flame combustion gases). From the lean combustion of chlorobenzenes (mono, 1,2-di-, and 1,2,4-tri-) hexachlorobenzene (HCB) and pentachlorophenol (PCP) are major PICs (products of incomplete combustion) formed in characteristic times <<0.1 s. While initial results found the PCP/HCB ratio >1, recent results suggest that this ratio may be very sensitive to local conditions. These two compounds are certainly possible precursors on the relevant time/temperature scales, but it has not been proven that they are required intermediates. In these combustions OCDD is the major dioxin congener, lower chlorinated congeners are only formed at longer residence times or over fly ash (at ∼300°C). A mechanism accommodating these observations will be proposed and the relevancy of the findings to recent measurements from incinerators will be discussed.