Thermal formation of polychlorinated dibenzo-p-dioxins and -furans: Investigations on relevant pathways

The recent status of the relevant formation of polychlorinated dibenzo- p-dioxins and -furans (PCDD/F) in technical incineration processes is discussed. Within this discussion, the competition experiments for the alternative pathways, de novo, and precursor reactions are critically evaluated. The results suggest that the precursor mechanism is dominant under conditions of municipal waste incineration (MWI). Results of field observations support this primary hypothesis. Chloroaliphatic compounds such as dichloroacetylene are responsible for the formation of higher or perchlorinated aliphatic and aromatic compounds and precursors in thermal PCDD/F formation. The meaning and implications of indicator compounds for PCDD/F are also evaluated. Observed correlation among interrelated classes of chloroaromatic compounds can be explained by stationary reaction equilibrium in the postcombustion zone of incinerators in the presence of fly ash surfaces (i.e., by heterogeneous catalysis). Recent findings on the selective formation of PCDD/F obtained from typical pattern of isomers suggest a subtle balance between thermodynamically controlled formation and kinetically controlled destruction reactions.