The following information is a summary of an article from H. Huang and A. Buekens, Department of Chemical Engineering and Industrial Chemistry, Free University of Brussels, Belgium . This article describes the most probable routes how dioxins are formed in incinerators. Most dioxins are formed from the carbon in fly ash during cooling of the off-gases.
Cu2+ ions have a strong catalytic effect on PCDD/F formation, Fe3+, Pb2+ and Zn2+ have a minor effect, and a lot of others have no observable effect.
Molecular O2 must be present in the gas stream to form PCDD/F's. The formation rate increases with the O2 concentration by a reaction order of about 0.5.
Maximum PCDD/F formation occurs at 300-325 °C, little can be detected below 250 or above 450 °C.
Other flue gas components:
HCl, Cl2, SO2, CO2, CO and H2 present in the concentrations normally found in Municipal Solid Waste Incinerators (MSWI) flue gases have no significant influence on PCDD/F formation. The effect of H2O is less certain.
Cogeneration of other (chlorinated) compounds:
A variaty of other (chlorinated) compounds like (polychlorinated) benzenes, phenols, naphthalenes and biphenyls are generated together with PCDD/F's.
Relationship with carbon gasification:
PCDD/F formation has been shown to be closely related to low-temperature carbon gasification. This too is oxygen dependent and increases by a reaction order of about 0.5 with O2 concentration.
The basic reaction is the oxydation of microcrystallite carbon. At low temperatures (below 700 °C) this occurs mainly at the imperfect edges of the layer, which forms active sites. Without a catalyst, this is a slow process. Several metal oxydes catalyse the oxydation, including copper and iron. This forms or leaves some (oxydised) ring structures, including benzene, phenol, biphenyl, dibenzodioxins and -furans. The latter three can be formed by coupling elementary ring structures like the former two, called the Ullmann coupling reaction. Alternatively, the DD/DF structures are directly built by oxydation of the carbon layer and subsequent oxydative degradation of surplus carbon rings.
Chlorination of these elementary and condensed structures or directly at the carbon layer is happening concurrently and is strongly catalysed by some metal salts, especially copper (in all forms). Also the Ullmann reaction of chlorinated elementary rings is strongly catalysed by copper. At the same time dechlorination and decomposition reactions also occur and are also catalysed by copper. Which reactions are preffered is mainly a matter of temperature. Higher temperatures favour decomposition.
The net yield of all these reactions is:
For each app. 30-ring (100 carbon) graphite sheet of a 5-10 layer microcrystallite, app. one polychlorinated aromatic structure is formed [personal note: the rest is transformed into CO, CO2 and non-chlorinated aromatics]. For each app. 200 layers, one PCCD/F is formed. It should be noted that the oxydation is on a layer-by-layer basis. That results in the fact that regular graphite, which contains 5x10^4 regular carbon rings per layer, produces only a very little amount of aromatics and hence very little PCDD/F.
Metals like copper, which catalyses all three reactions leading to the formation of PCDD/F's will form a lot of them. Iron has a strong catalytic effect for oxydation reactions, but a weaker for chlorination and Ullmann-type reactions. Many other metals have some strong effect for one reaction but no or much weaker for the others. None of them produce as much dioxins as copper and iron do.
|Metal chloride 1%||PCDD||PCDF|
As you can see, copper is by far the best catalyst to form dioxins...
That was confirmed in another test, where different amounts of copper were added:
|Congeners||additional % Cu2+|
Again, copper plays a crucial role in dioxin formation...
In all these tests only INorganic chlorine was used, together with amorphous
carbon (active coal) and an inactive carrier. Crystalline carbon like graphite
doesn't form dioxins. Further, chlorinated organic molecules, including
dioxins/furans, are formed from inorganic chloride salts. Thus the assumption
that only organic chlorine is involved in dioxin formation is proven false.
And the amount of chlorine plays a very limited role, compared to other
factors like turbulence, time, temperature, oxygen and by far copper content...
My fellow countryman, Emmanuel De Broux, is a supporter of that process as an alternative for incineration. Unfortunately, his web site disappeared...
This explains why co-burning of PVC results in not more PCDD/F formation/emission than other materials in incinerators (or accidental fire), because most of the chlorine from PVC is split off as HCl, which has no measurable effect on PCDD/F formation at the concentrations found in MSW incinerators.
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Created: March 30, 1998.
Last update: February 21, 2002..
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