On the mechanisms of dioxin formation in combustion processes

The experimental observations on dioxin formation from various combustion sources, from detailed incinerator measurements and from laboratory simulation studies including de novo synthesis and precursor formation pathways are discussed in this paper. The de novo synthesis seems to be the dominant mechanism of dioxin formation in actual combustion systems. As de novo synthesis experiments indicate that carbon morphology of certain degenerated graphitic structure is essential for dioxin formation, the morphologies of the particulate emissions from actual combustion sources are examined and it appears that soot particles formed in gas phase combustion reactions consisting of degenerated graphitic structures are the plausible source for de novo synthesis of dioxins. With this understanding dioxin formation in combustion systems is described as a two-stage process: (1) the formation of the graphitic structure of soot particles in the combustion zone; and (2) the conversion of the graphitic structure of soot particles to aromatic compounds including PCDD/Fs in the postcombustion zone. New explanations are given for some seemingly unrelated experimental observations including the high dioxin emissions from municipal waste incineration, the low dioxin emissions from coal combustion, the similar dioxin “fingerprint” from all combustion sources and the discrepancy between the dioxin formation rates observed in laboratory experiments and incinerator measurements.