FORMATION OF PCDDS AND PCDFS IN MUNICIPAL WASTE INCINERATION AND ITS INHIBITION MECHANISMS - A REVIEW

The reaction mechanisms by which polychlorinated dibenzo-p-dioxins (PC DDs) and dibenzofurans (PCDFs) form in municipal solid waste (MSW) inc ineration are reviewed. Some of the possible reaction pathways are exp lored in detail: e.g. high-temperature pyrosynthesis, low-temperature de novo formation from macromolecular carbon and organic or inorganic chlorine present in the fly ash matrix, and formation from different o rganic precursors such as chlorophenols. Although all these mechanisms have been known for a number of years, few detailed reaction mechanis ms can yet be offered for PCDD/F formation from macromolecular carbon or even structurally closely related precursors. The corresponding rea ctions are always only minor side-reactions with low yields. This is d ue in particular to the extreme complexity of the fly ash matrix, whic h effectively obscures the identity of the active catalytic sites and the overall processes. This work discusses the production of PCDD/Fs a nd the interactions between transition metals and aromatic precursors in relation to the amount of PCDD/F emitted in MSW incineration. It th en goes on to discuss the correlation between transition metals and PC DD/F emissions in MSW incinerators. A literature survey and some therm odynamic considerations allow conclusions to be drawn on the relations between the loading of transition metals in flue gases and particles, PCDD/F concentrations, and operating temperature. Some possible inhib ition mechanisms and their thermodynamics are also discussed briefly. (C) 1998 Elsevier Science Ltd.