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Dioxin formation in incinerators
Dear dioxin-l readers,
The following information is quite technical, but explains why the
formation and/or release of dioxins in incinerators is independent of the
carbon or chlorine/PVC input, but strongly depends of the circumstances of
It was obtained at a congress in Brussels two years ago on the incineration
topic. The information given here can be checked by any scientist in this
field of any university or any incineration specialist.
I apologise that several people of the list already has received this
information from me.
If you need more toroughly detailed information, we can give you the adress
of one of the specialists on these matters (wich has done a lot of work on
the next experiments):
Prof. A. Buekens, Vrije Universiteit Brussel (Free University of Brussels)
CHIS 2, Pleinlaan 2, B-1040 Brussels, Belgium. Telephone/fax/e-mail of
e-mail should be firstname.lastname@example.org, but I hope that is not misspelled.
You should certainly contact him and his Swedish colleague: prof. Rappe
from Umea, who is the dioxin-Guru of the world.
* Above about 250 °C, PVC releases near all chlorine under form of
hydrochloric acid, leaving only a (hydro)carbon skeleton. Maybe that is the
reason that burning or incidental incineration of PVC releases only very
low quantities of dioxins, in contrast to chlorinated aromatics.
* You will only find higher dioxin concentrations at the surface of partly
burned PVC, where the temperature was too low, comparable with
concentrations found in ashes from woodstoves.
* If the temperature is high enough and the residence time is long enough
and with enough oxygen, all organochlorines of any kind will be destroyed
at the end of the fire hearth. The destruction rate above 600 °C is much
higher than the de novo formation of dioxins, furans, chlorophenols, PCB's
and non-chlorinated stuff like butadiene-benzene-PAH's. Only some fly ash
mixture of oxydes, carbon and chlorine (under form of salt) will remain. It
is on the surface of this particles that new dioxins, furans, etc. are
formed during cooling below 600 °C: furans mainly around 500 °C, dioxins
mainly around 300 °C. Below 200 °C, most reactions are too slow to have any
* Some experiments were done with a 'synthetic' fly ash mixture. One of the
investigations was to see if dioxins were directly formed from
chlorophenols, to have an easier to analyse tracer in the incineration
train. That was not the case, at least not to such an extent that
differences in dioxin formation speed where seen before and after
chlorophenol extraction with solvents.
* All those experiments formed the base for an experiment at an incinerator
in wich they:
- reduced the amount of fly ash by controlled supply and reduced primary air
- temperature continuously high enough (above 600 °C) by secondary air
- cooling down as fast as possible
That resulted in a reduce of dioxin emissions with a factor 10 to 100...
* There is normally a quite direct relation between amount of dioxins in
fly ash and in the exhaust (before cleanup). If you find a relation between
amount of chlorinated polymers at the input and dioxins in the fly ash or
ashes from the fire hearth itself, that points to a too low incinerator
temperature or a too short residence time.
* We have the figures of how the dioxins are related to the different parts
of municipal waste incinerators. These figures are from the same 1991 TNO
investigation in The Netherlands we have repeatedly used. All figures
expressed in I-TEQ.
Stream related to emission reference factor total PCDD/F g/year
Off-gases 2,760 kton 138 mcg/ton 382
municipal waste waste
E-filter 83.2 kton 12,200 mcg/ton 1020
fly ash (dry matter) fly ash (d.m.)
slag 490 kton 17.3 mcg/ton 8.5
(dry matter) slag (d.m.)
sludge from 0.851 kton 12,200 mcg/ton 10.4
gaswasher (dry matter) sludge (d.m.)
wash water 175 km3 1.87 ng/m3 0.0003
As you can see, the remainder of dioxins in slag/ash from the hearth bottom
doesn't contain much dioxins.
The mayor problem is the de novo synthesizing of dioxins etc. on the fly
ash, especially in the e-filter, wich is/was almost always operated at 300
°C and with a relatively long residence time to allow settling, this gives
the ideal circumstances to form dioxins! Those partially split of in the
ventgases, the bulk being adhered to the fly ash.
* Another table from the congressbook (from another source: Johnke &
Stelzer) on waste incineration gives interesting figures about where it is
concentrations of dioxins in the gases at different points of the incinerator
(mg I-TEQ/ton waste):
min geom. mean max
combustion chamber 3 7 30
raw gas after boiler 20 70 190
raw gas following ESP 12 40 420
ESP ash 35 115 830
Here you can see that dioxins are mainly formed (90-95%) de novo during
cooling and that the ESP at least doubles the amount of dioxins.
* German data give even stronger differences in soot on the walls of a
multi-pass heat recovery from the off-gases
(Vogg H., Metzger M. & and Stieglitz L., "Recent findings on the formation
and decomposition of PCDD/PCDF in Municipal solid waste incineration",
Waste Management & Research, 87/5, p. 285-294):
dioxin type 800-400 °C 400-220 °C
pass 2/3 pass 4
OCDD 0.6 265
HpCDD 0.2 124
HxCDD 0.1 105
PeCDD 0.1 75
TCDD 0.1 25
PCDD 101 594
* From the same congressbook, we learn from experiments with (synthetic)
fly ash, that the formation of dioxins, furans, PCB's, chlorophenols,
chlorobenzenes is directly from the carbon in the fly ash, not from each
other as precursors. Extraction of all chlorinated organics before
reheating to 300 °C did give the same amount of all stuff in the same time
as before extraction. The type of carbon is important: soot of a domestic
open fire place, cleaned active carbon and carbonised sugar are good
precursors, graphite is not. The specific surface seems not to be
important. The amount of carbon is also important: above 2% of carbon all
stuff is rapidly formed, while the amount of carbon sinks (mostly as CO2),
below 2% the reaction stops.
* While the concentration of chloride (mainly as hydrochloric acid) in the
gaseous phase seems not to have any influence, the amount of chloride in
the fly ash itself is rather important: 0.5 - 6 % of chloride gives a
doubling of total PCDD/F. At the same time one can see that inorganic
chloride is rapidely transformed in organic chlorinated matters.
* The most important factor is the presence of heavy metals, copper being
an extremely good catalyst:
amount of extra copper: 0 0.08 0.24 0.4 (% Cu2+)
PCDD 4.5 101 770 1448 (ng/g)
PCDF 22.6 760 3640 8480 (ng/g)
* Other experiments with simple gases mixed and led over different
catalysts, on a silica bed give organochlorines at 300 °C:
10% CO2 + 500 ppm CO + 5% O2 + 250 ppm SO2 (synthetic off-gas) + HCl
CH4 + HCl
CO2 + HCl
without metal traces: no organochlorines.
with copper: chlorinated benzenes and other aromatics in ratio with copper
with more pure carbon on the bed: more aromatics.
with iron as catalyst: few chlorinated benzenes, but more chlorinated
aliphatics are formed, mainly chlorinated ethylene and butadiene.
a mixture of iron and copper gives a mixture of the two chlorinated
* * *
>From the above experiments we can learn that ever present carbon and
(inorganic) chlorine can form any amount of dioxins, if there is a catalyst
present at the right temperature.
Because only one millionst of the chlorine input and one hundredmillionst
of the carbon input of MSW's is transformed into dioxins, the ban on PVC or
the omitting of PVC in waste to incinerators or a ban on the whole chlorine
industry as 'the' dioxine source is simply scientific nonsense.