Formation of PCDDs and PCDFs in municipal waste incineration and its inhibition mechanisms: A review

The reaction mechanisms by which polychlorinated dibenzo- p-dioxins (PCDDs) and dibenzofurans (PCDFs) form in municipal solid waste (MSW) incineration are reviewed. Some of the possible reaction pathways are explored 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 organic precursors such as chlorophenols. Although all these mechanisms have been known for a number of years, few detailed reaction mechanisms can yet be offered for PCDD/F formation from macromolecular carbon or even structurally closely related precursors. The corresponding reactions are always only minor side-reactions with low yields. This is due in particular to the extreme complexity of the fly ash matrix, which effectively obscures the identity of the active catalytic sites and the overall processes. This work discusses the production of PCDD/Fs and the interactions between transition metals and aromatic precursors in relation to the amount of PCDD/F emitted in MSW incineration. It then goes on to discuss the correlation between transition metals and PCDD/F emissions in MSW incinerators. A literature survey and some thermodynamic 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 inhibition mechanisms and their thermodynamics are also discussed briefly.