Incineration and Dioxins

Review of Formation Processes

A consultancy funded by Environment Australia

Department of the Environment and Heritage

Prepared by Environmental and Safety Services

© Commonwealth of Australia 1999
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Citation
This report should be cited as follows:
Environment Australia (1999), Incineration and Dioxins: Review of Formation Processes, consultancy
report prepared by Environmental and Safety Services for Environment Australia, Commonwealth
Department of the Environment and Heritage, Canberra.
 Incineration and Dioxins:
Review of Formation Processes

This review of scientific literature on dioxin and furan formation has

been derived from a study conducted on behalf of Environment
Australia by Environmental and Safety Services.
The review investigates the possible relationship between chlorine input
to incinerators and presence of dioxins and furans in stack gases
emitted from incinerators. It also provides an overview of dioxins and
furans, their toxicity, pathways to their formation from combustion
processes, sources and rates of emissions from such processes, and
long-term trends in emissions from these processes.
1 Description of Dioxins and Furans

The polychlorinated dibenzo-para-dioxins (PCDD) and polychlorinated dibenzofurans

(PCDF) are chlorinated, planar, aromatic compounds containing two benzene rings.
The terms PCDD/F and dioxins carry the same meaning and are used interchangeably
in the literature. The structures of the most toxic forms of dioxin and furan molecules
are shown in Figure 1. A dioxin molecule is bonded by two oxygen atoms, and a
furan molecule by a single oxygen atom and a direct bond. Under standard
atmospheric conditions, all dioxins are solid and are characterised by low vapour
pressure and limited solubility in water.

Cl O Cl

Cl O Cl
2,3,7,8,-TCDD

Cl Cl
Cl
Cl O Cl
Cl
Cl
Cl O Cl O
Cl
1,2,3,7,8,-pentaCDD 2,3,4,7,8,-pentaCDF

Figure 1. Most toxic isomers of dioxins and furans.

There are 75 different forms of PCDD and 135 different forms of PCDF that are
distinguished by the position and number of chlorine atoms attached to the two
benzene rings. These different forms of dioxins are called congeners and their
number is listed in Table 1. Dioxins and furans with the same number of chlorine
atoms constitute a homologue group of isomers. Table 1 shows that there are 16
homologues groups, 8 for PCDD and 8 for PCDF. For example, there are 22 different
isomers of dioxins and 38 of furans belonging to the same tetrachlorinated homologue
groups.

For toxicity purposes, only the homologues with four or more chlorine atoms are
considered and these are called tetra (TCDD, TCDF or D4, F4), penta (PCDD, PCDF
or D5, F5), hexa (HxCDD, HxCDF or D6, F6), hepta (HpCDD, HpCDF or D7, F7)
and octa (OCDD, OCDF or D8, F8). It is these 10 PCDD/F homologues that are
normally quoted in the literature. Note the overlapping terminology as the penta
homologues have the same abbreviation (PCDD, PCDF) as all 210 dioxins and furans
(PCDD/F).
Table 1. Number of various isomers of dioxins (Rappe, 1996).

Number of Number of Number of

chlorine atoms PCDD isomers PCDF isomers
1 2 4
2 10 16
3 14 28
4 22 38
5 14 28
6 10 16
7 2 4
8 1 1
Total 75 135

Those PCDD/F that have four chlorine atoms substituted in positions 2, 3, 7, and 8 are
considered to be the most toxic. Their toxicity depends on the location and the
number of additional chlorine atoms attached to the benzene rings. The 17 most toxic
PCDD/F isomers, belong to the 2, 3, 7, 8-subsituted group. For comparison, the LD
50/30 values of some toxic (2, 3, 7, 8-substituted) and non-toxic dioxins are listed in
Table 2 (Steel Times, 1995). LD 50/30 represents a lethal dose of a chemical to 50%
of test animals after 30 days. In general, the more chlorine atoms a PCDD/F molecule
has, the less toxic it becomes.

Table 2. Toxicity of chlorinated PCDD in Guinea pigs (Steel Times, 1995); the more
toxic compounds have smaller LD 50/30 number.

Position of Guinea
chlorine atoms pigs
2,8 >300,000
2,3,7 30,000
2,3,7,8 2
1,2,3,7,8 3.1
1,2,4,7,8 1,125
1,2,3,4,7,8 72.5
1,2,3,6,7,8 70-100
1,2,3,7,8,9 60-100

Since the toxicity of a dioxin congener depends on the level of chlorination and
location of chlorine atoms on the benzene rings, three toxicity equivalence schemes,
called Eadon, Nordic and ITEQ (international toxic equivalent, also denoted as
NATO or CCMS for NATO Committee on the Challenges of Modern Society, 1988)
have been developed. Within each scheme, 2, 3, 7, 8-substituted PCDD/F have
weighting or conversion factors, as illustrated in Table 3. These factors are applied to
calculate the equivalent concentration of a dioxin congener. Note a small difference
between NATO and Nordic schemes, which differ only in the weighting factor
assigned to 1, 2, 3, 7, 8 PCDF. Recent studies indicate good correspondence between
the chemical concentration of PCDD/F, expressed in equivalent concentration, and the
biological potency of PCDD/F (eg Kopponen et al 1994, Clemons et al 1997).
The ITEQs are calculated as follows. If, for example, the concentration of 2, 3, 4, 7, 8
PCDF is 0.1µg/m3, its ITEQ value is (0.1x0.5) 0.05µg/m3, where the factor 0.5 was
extracted from Table 3. Similarly, the concentration of other dioxins needs to be
weighted according to the conversion factors listed in Table 3. The sum of the
converted concentrations is then reported as an ITEQ value.

In the dioxin literature, the ITEQ concentration is the one most often quoted, followed
by the total PCDD/F concentration, and the Nordic-equivalence concentration. As a
rule of thumb, ITEQ values are smaller by about 50 times than PCDD/F concentration
values. So, one can divide PCDD/F concentration by 50 to obtain a very approximate
but fast estimation of ITEQ concentration, if the latter values are not available.

Table 3. The weighting factors used in Eadon, Nordic and ITEQ equivalency
schemes.

Homologue group Toxic isomer Eadon Nordic ITEQ

TCDD 2,3,7,8 1 1 1

PCDD 1,2,3,7,8 1 0.5 0.5

HxCDD 1,2,3,4,7,8 0.03 0.1 0.1

1,2,3,6,7,8 0.03 0.1 0.1
1,2,3,7,8,9 0.03 0.1 0.1

HpCDD 1,2,3,4,6,7,8 - 0.01 0.01

OCDD 1,2,3,4,6,7,8,9 - 0.001 0.001

TCDF 2,3,7,8 0.33 0.1 0.1

PCDF 1,2,3,7,8 0.33 0.01 0.05

2,3,4,7,8 0.33 0.5 0.5

HxCDF 1,2,3,4,7,8 0.01 0.1 0.1

1,2,3,6,7,8 0.01 0.1 0.1
1,2,3,7,8,9 0.01 0.1 0.1
2,3,4,6,7,8 0.01 0.1 0.1

HpCDF 1,2,3,4,6,7,8 0.01 0.01

1,2,3,4,7,8,9 0.01 0.01

OCDF 1,2,3,4,6,7,8,9 0.001 0.001

It is generally accepted that the mole fractions of dioxin congeners and tetra to octa
homologues are specific to a dioxin formation process. The relative concentration of
homologues and congeners, usually plotted as a histogram, is called the dioxin
signature or dioxin fingerprint. The complete signature contains 10 homologues, up
to 17 toxic congeners (as listed in Table 3), and the relative ratio of the sum of
congeners to the total PCDD/F. Usually, only a subset of the 17 congeners is
included in the signature and even then some of the congeners may be lumped into
one number, eg all hexachlorinated congeners are reported together as one item in the
signature.

Rigo et al (1995) assembled signatures of dioxin formation for a range of industrial

processes and these signatures are reproduced here in Figure 2. The figure contains
the relative mole fractions of 2, 3, 7, 8 congener to homologue (9 bars) and the mole
fraction of homologue to the total PCDD/F (10 bars). The example of TCDD ("A"
bar in Figure 2) can be used to illustrate how the signature was obtained. The amount
of 2, 3, 7, 8 TCDD (in moles) was divided by the total amount of all 22 homologues
of TCDD, to obtain a ratio which was then multiplied by 100% and plotted in Figure
2. Similarly, the amount of all 22 tetrachlorinated dioxins was divided by the total
amount of PCDD/F in moles, multiplied by 100%, and plotted as "I" bar in Figure 2.
 Municipal waste incinerators Medical waste incinerators
80 80

60 60

40 40

20 20

0 0
A B C D E F1 F4 G H I J K L M N O P Q R
A B C D E F1 F4 G H I J K L M N O P Q R

Hazardous waste incinerators Cement kilns

80 100

80
60

60

40

40

20
20

0 0
A B C D E F1 F4 G H I J K L M N O P Q R A B C D E F1 F4 G H I J K L M N O P Q R

Biomass fired boilers Light weight kilns

80 80

60 60

40 40

20 20

0 0
A B C D E F1 F4 G H I J K L M N O P Q R A B C D E F1 F4 G H I J K L M N O P Q R

Figure 2. Dioxin signatures describing formation of dioxins in combustion

processes.

A good example of different dioxin signatures is observed in stack gas emissions from
boilers used in the pulp and paper industry (Luthe et al, 1997). Two types of boilers
are employed, recovery boilers that burn the so-called black liquor solids containing
chlorine from the Kraft process, and power boilers that burn wood waste that often
contains chlorine salts, especially in coastal areas. Emissions from recovery boilers
tend to contain mostly dioxins, whereas emissions from power boilers show
approximately equal concentrations of dioxins and furans.

Another excellent example illustrating the utility of dioxin fingerprint for the
identification of the PCDD/F source comes from the province of British Columbia in
Canada. Kraft pulp mills operating there employed a bleaching technology, which
used chlorine gas. This led to the formation of dioxins, and their subsequent emission
in discharge water. The dioxin signature from Kraft pulp mills was dominated by
three tetrachlorinated isomers. The problem was recognised and the chlorine gas was
replaced by chlorine dioxide or nonchlorinated reagents (Rappe 1996), resulting in
dioxin-free paper bleaching. However, the routine testing of the sediment in the
vicinity of effluent discharge points from the pulping mills soon disclosed evidence of
new dioxin deposition, which had a different signature (dominated by hexachlorinated
dioxins laid over a typical pulp-bleaching fingerprint) from that generated by gaseous-
chlorine bleaching.

It was speculated that the dioxin source was located in the paper plant but the source
would have to be different from the bleaching process. Careful investigation soon
revealed that on occasions the mills accepted wood chips that might have been
obtained from lumber contaminated with polychlorinated phenols (PCP). In the
lumber industry, PCP were applied to prevent staining of undried wood with sap. The
testing program that followed narrowed down the source of dioxin to polychlorinated
phenoxyphenols, an impurity which was originally present in PCP used to treat the
wood (Luther et al, 1993). At the same time, a similar investigation using the analysis
of dioxin fingerprint concluded that the recycled dioxin-contaminated corrugated
containers caused the dioxin emission problems from other Canadian pulp mills
(Berry et al, 1993).

2 Technical Details of Incinerators

This section describes the operation of incinerators, reviews their construction details
including unit operations1, and introduces the technical nomenclature normally used
in characterising the incinerators. In general, large-scale incineration units are
classified as municipal waste combustors (MWC), medical waste incinerators (MWI),
hazardous waste incinerators (HWI), boilers and industrial furnaces (BIF), cement
kilns (CK) and biomass combustors (BC). Here, the focus is on municipal waste
combustors, since they are considered to be the most important source of dioxin from
combustion processes; other types are described in detail by Rigo et al (1995).

Modern municipal waste combustors are designed to burn either the entire waste
stream (mass burn systems) or refuse-derived fuel (RDF systems), that is only the
combustible fraction of the municipal solid waste. Recent mass burn systems, such as
those built in Ireland or Portugal (Foster Wheeler Corp, 1997), incorporate furnaces,
which are exclusively equipped with mechanical grates that aid the flow of refuse
through a furnace, with ash being discharged at the furnace’s end. Older mass-burn
installations often used rotary kilns or hearths, but those were plagued by low
combustion efficiencies and large emission rates of pollutants. On the other hand,
1
Unit operations denote individual steps in industrial processing operations, such as scrubbers, dust
collectors, and furnaces.
RDF facilities use fluidised-bed technology to capitalise on technical advantages of
this technology, such as low emission rates of gaseous pollutants and complete
combustion of organic materials. Fluidised-bed furnaces contain a combustion
chamber with a bed of sand, which is fluidised by a stream of air forcing its way
through the bed.

Figure 3 illustrates a schematic diagram of a typical fluidised-bed facility burning

RDF. The RDF fuel is delivered to the fluidised bed by a waste feeder. However,
before feeding to the furnace, the municipal solid waste undergoes presorting to
remove metals, glass and masonry. This is a very important step that results in
decreased emission of heavy metals from incinerators, promotes recycling, limits
emission of other pollutants (such as CO and unburned organic volatiles) due to the
more homogenous nature of the fuel, and reduces production of ash (typically only
5% of the weight of the feed stream) because of a more complete combustion. Size of
particles in the waste stream is reduced by a series of operations that include mills
(hammer and flail mills) and shredders (rotary shredders and ring grinders).

Lime slu rry wit h St ack

act ivat ed carbon
Bag house

Wast e heat
boiler
Secondary air blower

Induced draft fan

Semi-dry or
wet spray
t ower
Freeboard
Wast e feeder

Incinerat or

Fluidisat ion air Fluidised bed

blower

Figure 3. Municipal waste combustor that uses revolving fluidised-bed furnace.

Unit operations downstream of the fluidised bed, such as scrubbers and
bag house, are common to other MWC, including mechanical grate
furnaces for mass burning and circulating fluidised-bed incinerators for
combustion of RDF.

Two types of fluidised beds are presently in use. In the revolving or twin
interchanging fluidised bed, the fluidisation air is introduced into the bed through a
triangular distributor creating intense vortex and leading to substantial combustion
turbulence (Saito et al, 1988). For combustion in the revolving fluidised-bed furnace,
the RDF requires minimal feed preparation and the furnace has excellent combustion
and emission characteristics. However, the revolving fluidised beds have low
capacity of around 150 tons/day (Ishikawa et al, 1997) and are unsuitable for large
facilities processing more than 1000 ton/day of RDF.

More common in large-scale installations are circulating fluidised-bed furnaces, as

illustrated in Figure 4. Circulating fluidised beds operate at higher fluidising
velocities, allowing partially combusted material to leave the bed and be separated
from the stream of combustion gases by a large cyclone. The collected solid materials
are then returned (circulated) to the fluidised bed. These characteristics allow feeding
of RDF that can be irregular in shape, since uncombusted material is returned to the
fluidised bed by a cyclone, in effect providing long residence time and turbulence for
complete combustion of fuel particles. Other advantages of this configuration include
low NOx formation rates (so a need to inject urea or ammonia to control NOx is often
avoided) and stable operation due to a large thermal mass circulating between the
furnace and a cyclone.

St eam dr um

Super heat er s
Ur ea i nj ect i on
(i f needed)

Economi ser
Fur nace
Fl ue gas t o
Ai r Pol l ut i on
Cont r ol Syst em
Ref ur e- der i ved
f uel
Fl yash

Cycl one

Secondar y
ai r

St r i pper cool er Fl yash

Bot t om ash

Pr i mar y combust i on
ai r

Figure 4. Details of the circulating fluidised-bed MWC constructed recently in

Robbins IL, USA for firing refuse-derived fuel (Foster Wheeler Corp,
1997). Unit operations in the air pollution control system are not shown
in the figure.

Recovery of heat generated in the furnace takes place downstream from the cyclone in
superheaters, which are marked as waste heat boiler in Figure 3. Energy recovery
from MSW and production of electric power by turbines running on superheated
steam constitute an important contribution of MWC to recycling. Note that the
combustion gases leaving the hot combustion zone of the furnace need to be cooled
rapidly to low temperature in the post-combustion zone, to minimise the production of
dioxins, which form in the temperature window of 200 to about 450oC.

After boiler banks, the gases are diverted to air pollution control system (APC). The
role of APC is to remove pollutants from the gas stream before the combustion gases
are released from the stack. APC technologies are constantly evolving to satisfy
emission control requirements imposed by environmental regulations, but usually
typical APCs contain 4 to 5 stages, that are designed to remove PCDD/F, heavy
metals such as mercury, particulates and acid gases such as HCl as well as to control
the emissions of SOx and NOx.

In the first stage, the combustion gases are cooled in a spray tower that may operate
with water or with hydrated lime slurry, to provide scrubbing of acid gases from the
flue-gas stream. In a spray tower, liquids are atomised at the top of the tower and the
droplets travel downward by gravity, evaporate and react with (absorb) acid gases. If
lime slurry is completely evaporated, the tower is operated in semi-dry mode with
solid particles collecting at the bottom of the tower and smaller ones being carried
away by the gas flow to the bag house, where they are removed from the gas by fabric
filters. The gases are cooled to about 130oC in the scrubbers before entering fabric
filter in the bag house. Small quantities of activated carbon can be mixed with lime
slurry to absorb mercury and PCDD/F and limit their emissions.

Fabric filters that operate at low temperatures have now replaced electrostatic
precipitators (EPS) in recently constructed MWC. Fabric filters possess very high
collection efficiency, due to the enhancement in trapping of small particulates by the
cake of collected dust. If lime slurry or other liquid and solid chemicals are injected
into the combustion gases before the bag house, fabric filters provide yet another
opportunity for acid gases to react with alkaline particles when the gases pass through
the filter cake.

It should be noted that electrostatic precipitators have been implicated in the

formation of PCDD/F. For example, Ruuskanen et al (1994) reported that octachloro
congeners might be formed by electrical processes in the electrostatic precipitators
(ESP). According to their measurements, the octachloro furans and dioxins increased
by 700 and 400% respectively, when the flue gases were passing through the ESP.

Figure 4 also indicates discharge points for ash removal from the process. The part of
ash that is carried by the flow of combustion gases is called fly ash. It accumulates on
heat exchangers in waste heat boilers, at the bottom of flue-gas scrubbers and on
fabric filters. Fly ash participates as a catalyst in the dioxin-formation process, and
may itself contain large quantities of PCDD/F on the surface and in the pores. On the
other hand, bottom ash is collected directly from the furnace, as illustrated in Figure
4.

From the review of the current technology for controlling the emission of air
pollutants, one must conclude that the amount of PCDD/F leaving incinerators with
the stack gases depends strongly on the efficiency of APC systems and the
combustion conditions. Any attempt to relate the amount of dioxin in stack gases
with the amount of chlorine introduced into the furnace with waste stream, in large
scale facilities, is unlikely to yield statistically significant correlations even if they
exist, because of the number of variables involved in the operating of a typical
incineration installation. From this standpoint in practical systems, it appears that the
emission of PCDD/F depends mainly on the combustion conditions and efficiency of
APC systems rather than on the amount of chlorine in the waste stream. If MWC are
designed, constructed and operated according to the most recent technological
advances, their emissions fall substantially below the present-day limits. As an
example, Table 4 illustrates emissions of various pollutants from the Robbins
Resource Recovery Facility near Chicago that operates two recirculating fluidised-bed
furnaces.

Table 4. Comparison of compliance stack test results to Illinois Environmental

Protection Agency emission permit limits for the Robbins Resource
Recovery Facility (Foster Wheeler Corp, 1997); burning 100% RDF, all
values are at 7% O2 dry and NOx are without the injection of urea.

Pollutants Units Boiler A Boiler B Permit

Particulate g/m3(*) 0.053 0.198 0.35
SO2 ppm 1.0 0.5 30
HCl ppm 4.6 6.2 25
CO ppm 4.4 2.2 100
Nox ppm 73.1 90.4 130
VOC ppm 1.5 0.6 10
Total PCDD/F ng/m3(*) 2.1 4.9 30
Arsenic µg/m3(*) 0.2 0.2 10
Cadmium µg/m3(*) <det limit 0.3 40
Chromium µg/m3(*) 4.8 5.6 120
Lead µg/m3(*) 3.6 30.9 490
Mercury µg/m3(*) 15.8 2.9 80
Nickel µg/m3(*) 3.1 3.1 100
* denotes dry standard cubic meter

Note: Both Rigo et al (1995) and Costner (1997) refer to controllable and
uncontrollable concentration of hydrogen chloride. The term uncontrollable
concentration relates to the levels of HCl in the combustion air after the furnace, but
before the scrubber, whereas controllable HCl concentration refers to loading of
hydrogen chloride in the exhaust gases after the gases pass through the air-control
system.

3 Pathways of PCDD/F Formation in Combustion Processes

There is considerable scientific coverage of issues related to PCDD/F, and several

review articles have been compiled (eg Boening 1998, Dyke et al, 1997). A cursory
examination of the literature quickly reveals information on issues related to the
analysis, sources, and health aspects of PCDD/F. In spite of the extensive literature
available, many fundamental issues related to PCDD/F remain uncertain or at least are
the subject of considerable debate.
It is now believed that the kinetic reactions responsible for formation of dioxins never
proceed to completion. These reactions do not even achieve their maximum or
equilibrium conversion that can be calculated from thermodynamic considerations2.
This means that, in spite of over-abundance of Cl and other elements that constitute
building blocks for PCDD/F (carbon, hydrogen, and oxygen), most of the available
material is not converted to dioxins.

Thermochemical kinetic analysis, combined with thermodynamic properties, has been

used to examine important pathways to gas phase formation of PCDD/F (Bozzelli et
al 1991, Ritter & Bozzelli 1994); see Figure 5. These analyses of various reaction
pathways led to the suggestion that formation of chlorinated PCDD/F involve
unimolecular HCl elimination or loss of Cl, which occurs following hydroxy radical
addition. Equilibrium constants for these reactions have been calculated, and strongly
favour formation of the PCDD/F products. The addition reactions of OH resulting in
PCDD/F formation compete with addition of OH radical at fused ring sites, which are
responsible for ring cleavage and destruction of species.

While an understanding of gas phase formation of PCDD/F is very important, the

surface catalysed formation of these species is generally believed to be the major
contributor to PCDD/F from incineration processes. Two different catalytic
processes, the precursor route, which involves the surface-catalysed reaction of
compounds such as chlorobenzenes and chlorophenols, and the de novo route, in
which carbon, oxygen, hydrogen and chlorine combine and react to (eventually) form
PCDD/F, have been proposed. The relative importance of the gas-phase (or
pyrosynthetic), precursor and de novo PCDD/F formation pathways remains the
subject of debate, although it is generally accepted that the precursor route is the
primary mechanism for PCDD/F formation at higher temperatures, followed by de
novo synthesis at lower temperatures, while the gas phase route is the least significant
reaction pathway (Konduri & Altwicker 1994). The role and importance of
uncatalysed surface reactions are not well established.

PCDD/F are emitted from high-temperature combustion processes, such as hazardous

waste incinerators, municipal waste incinerators and biofuel boilers, although dioxins
themselves may not necessarily be formed at high temperature. Both the quantity of
PCDD/F emitted and the homologue distribution spectra are important. These
emission characteristics of PCDD/F are a complex function of many operational
parameters such as operation of the furnace or air-pollution control system, and these
dependencies obscure or complicate the disclosure of any mechanistic pathways
responsible for PCDD/F formation.

In the following discussion, the mechanistic pathways to PCDD/F formation,

including gas-phase reactions, uncatalysed surfaced reactions, catalytic precursor and
de novo mechanisms, are reviewed. This review is based on the present

2
This is done by minimising the Gibbs free energy of the reacting system against composition at given
temperature and pressure, subject to the constraints of constant elemental abundances. If such
calculations are carried out for the formation of dioxins, one observes that the predicted distribution of
isomers does not correspond to that observed in the experiments (eg Addink et al 1998). This leads to
the conclusion that the distribution of PCDD/F does not attain its equilibrium value, suggesting that the
formation of PCDD/F is controlled by chemical-kinetic mechanisms rather than by equilibrium
thermodynamics.
understanding of these pathways, as extracted from the literature, giving special
attention to PCDD/F formation from large-scale combustion facilities.

Figure 5. Schematic diagram of chemical reactions leading to the formation of

dioxin in combustion processes; P and D denote precursor and dioxin
molecules, respectively, and superscripts s and g stand for surface-bound
and gaseous species.

Gas phase
Pg + Pg Dg

Pg
Precursor
De novo

Ps + Ps Dg ? Cl,O,C,H

Catalytic surface

3.1 Reactions in the gas phase

Ritter and Bozzelli (1994) modelled the formation of PCDD/F and related reaction
intermediates via the reaction of precursors including addition of hydroxyl radicals.
Specifically, these authors studied the gas-phase conversion of polychlorinated
biphenyls, chlorinated biphenyl ethers and chlorinated dibenzo furans to 2,3,7,8
TCDD. The major finding from this work and from others that followed (eg Bozzelli
& Chiang 1996) was to conceptualise the conversion of precursor species to PCDD/F
from short-chain chlorinated hydrocarbons, in the gas phase. For example, a kinetic
scheme was developed to explain mechanistically the formation of polychlorinated
benzene from relatively simple short-chained chlorinated hydrocarbons, subsequently
leading to formation of PCDD/F (Bozzelli & Chiang 1996), as shown schematically
in Figure 6.

Theoretical investigations of the mechanism and relative importance of gas-phase

reaction pathways to PCDD/F formation generally support the experimental findings.
The gas-phase formation mechanism contributes less than 10% of the total PCDD/F
measured, although some authors have placed the gas-phase contribution as high as
50% (eg Tuppurainen et al 1998). However, it should be stressed that significant rates
of PCDD/F formation are observed or predicted only at temperatures greater than
600oC, much higher than the "temperature window" for catalytic PCDD/F formation
(Konduri & Altwicker 1994).

These conclusions are further supported by the work of Hinton and Lane (1991b),
who examined the synthesis of PCDD, in a tubular reactor operated at 300oC, from
pentachlorophenol over PCDD extracted fly ashes collected in several US
incinerators. The ash acted as a catalyst in producing dioxins. However, when the
same reactor was operated with no fly ash, the analysis showed no formation of
PCDD. This observation signifies that reaction in the gas phase lead to negligible
rates of formation of PCDD in the same temperature range that promotes catalytic
reactions. Thus for practical purposes, contribution to the total dioxin formation from
the gas phase reaction is usually neglected.

Cl
Cl Cl
PCDF formation
Short-chained Cl O
chlorinated hydrocarbon Cl
feed
HCl + Cl

CH2 Cl2 HCl elimination

O2
Cl Cl Cl

CHCl 2 Oxidation
Cl

C2 HCl3 HO Cl Cl Cl

OH
Increasing carbon C4H4Cl
chain length Attack from hydroxyl

Cl Cl Cl

Cl
Cln
Cl Cl Cl Chlorinated biphenyls

Formation of chlorinated aromatic species

Figure 6. Details of possible homogenous pathways for PCDD/F formation.

3.2 Non-catalytic reactions on fly ash

It has been argued (Konduri & Altwicker 1994) that a surface can influence PCDD/F
formation but act non-catalytically if the reaction sequence is insensitive to the nature
of the surface. Konduri & Altwicker postulated that the surface acts primarily as an
adsorption site for PCDD/F precursors, and is then able to concentrate reactive
species for subsequent reaction. Differentiation of the catalytic and non-catalytic role
of surfaces during PCDD/F formation is difficult to establish and the role of non-
catalytic surface processes is not well described in the literature. It is clear, however,
that surface-catalysed reactions play a dominating role in determining both the
quantity and distribution of PCDD/F isomers in many combustion processes.

3.3 Fly-ash catalysed precursor pathway

It has been suggested that PCDD/F may form from precursor molecules, such as
chlorophenol, chlorinated benzenes or chlorinated biphenylenes. This process has
been studied extensively, and many authors claim it is the principal route for PCDD/F
formation, for example Tuppurainen et al (1998). The precursors, which are products
of incomplete combustion, are produced at high temperatures (>400oC, most
effectively around 750oC) and later react further in the lower temperature region of
the combustor (Froese & Hutzinger 1996).

The precursor molecules are believed to react catalytically with elements in the fly
ash to produce PCDD/F, where the yield of PCDD/F is dependent on the feed
concentration of precursor and reaction temperature (Milligan & Altwicker 1996a&b).
Milligan & Altwicker studied the reaction of tetrachlorophenol on fly ash as a
function of gas phase precursor concentration, reaction time and temperature.
Interestingly, they only detected PCDD, while PCDF levels were beyond the
detection limits of their analysis. They found that while the yield of PCDD increased
with increasing precursor concentration, the effect of temperature was not so
straightforward.

One of the most important categories of precursor compounds leading to PCDD/F

formation is chlorophenols. These are monocyclic compounds containing one or
more Cl atoms bound to a phenolic backbone structure. Mechanistically,
Tuppurainen et al (1998) has proposed different reaction schemes for PCDD and
PCDF formation. These authors have suggested that PCDD formation proceeds via
the surface-catalysed coupling of chlorinated phenolate anions, followed by oxidative
ring closing. The role of the catalyst is to serve as electron transfer oxidants, which
leads to the coupling of two aromatic rings. Dioxin formation is observed following
HCl and Cl elimination reactions. The process is illustrated in Figure 7, where in this
example 2,4,6-trichlorophenol reacts to form 1,3,7,9-TCDD or 1,3,6,8-TCDD. It has
been noted that more alkaline ash effectively adsorbs PCPs on its surface, promoting
the formation of PCDD (Ruuskanen et al 1994).

- Cl
O OH HCl Cl Cl
Cl Cl Cl Cl
O
-
O
Cl Cl Cl
Cl

Cl Cl Cl
Cl O Cl O
- Cl
O Cl O
Cl Cl Cl

Cl Cl
Cl
O

Cl O Cl

Figure 7. Mechanistic view of the formation of PCDD in the fly-ash catalysed

precursor pathway.

Furthermore, these authors argue that reactions illustrated in Figure 7 cannot be used
to rationalise PCDF formation, but rather precursors such as chlorobenzene and
phenoxyphenols are involved with PCDF formation. Their proposed mechanism for
PCDF involves a Pschorr-type ring closure catalysed by various metal species and is
believed to be of particular importance during municipal waste incineration,
especially where Cu and Fe metals may be present. This mechanism is illustrated in
Figure 8.
The so-called Ullmann reactions (I and II) have also been proposed to facilitate
condensation reactions leading to the formation of chlorinated biphenyls (Ullmann I)
or chlorinated diphenyl ethers (Ullmann II), which can readily decompose to produce
PCDF. The fly ash is then believed to be catalysing electrophilic chlorination, where
a number of catalytic species have been investigated and shown to promote the
reaction. The catalytic activity of fly ash for PCDD/F formation is well established.
However, as shown by Hinton and Lane (1991), the chemical composition of fly ash
often varies considerably between facilities. Table 5 lists the effect of various
elements, especially metals, on the formation of dioxins via the precursor pathway.

N2 + +e N2

O O

- N2

O
O - H+ H

Figure 8. Mechanistic view of the formation of PCDF in the fly-ash catalysed

precursor pathway.
Table 5. Examples of the effect of various elements in waste stream and in fly ash
on PCDD/F formation in precursor reactions.3

Element Effect Reference

Aluminum negative Hinton & Lane (1991b)

Carbon1 no effect Hinton & Lane (1991b)

Chlorine1 no effect Hinton & Lane (1991b)

Copper strongly positive Hinton & Lane (1991b)

Potassium positive or negative2 Hinton & Lane (1991b)

Sodium positive or negative2 Hinton & Lane (1991b)

Sulfur positive Hinton & Lane (1991b)

Zinc positive Hinton & Lane (1991b)

Surface area, no effect Hinton & Lane (1991b)

size
distributions, etc
1
From bulk analysis of the fly ash samples, after the samples were extracted with toluene.
2
Depends on statistical analysis (Pearson’s coefficients or multivariate) of the experimental data.

3.4 Fly-ash catalysed de novo pathway

If the fly ash from a municipal solid waste incinerator is exhaustively extracted from
its carbon content, a very small amount of carbon remains within the ash matrix. This
remaining carbon is intimately bound with catalytic sites within porous fly ash
particles. If air, partially depleted of its oxygen and containing no chlorine, is then
flowed through or above the fly ash bed, the unextracted carbon matrix reacts with
oxygen breaking off complete aromatic rings. At the same time, it is believed that
chlorine is transferred from the metal chloride ligands on the ash surface to aromatic
carbon rings. This mechanism of formation of PCDD/F, illustrated in Figure 9, is
called a de novo mechanism and was first observed and described by Stieglitz and
Vogg (1987). Table 6 lists the effect of various elements, especially metals, on the
formation of dioxins via the de novo pathway.

3
Catalytic reactions that produce dioxins, in both precursor and de novo mechanisms, occur on
surfaces. Thus the rate of dioxin generation is related to the surface concentration of catalytically
active sites (eg copper on the surface of fly ash particles). Unfortunately, in practice it is not possible
to link the rates of dioxin formation with the presence of catalytic sites on fly ash. This is because the
analytical techniques used to measure the surface concentration of elements, such as the x-ray
photoelectron spectroscopy (XPS), are not sensitive enough. For this reason, researchers often
correlate the production of dioxins with bulk rather then surface composition of fly ash, since the bulk
composition of fly ash can be easily determined by standard techniques of analytical chemistry.
It is also possible for HCl in the combustion gases to serve as a source of chlorine, in
addition to the metal chloride ligands. This is because HCl reacts with oxygen on
copper catalysts to form chlorine (Deacon process), according to the following
reaction:

2HCl + O2 → Cl2 + H2O (Deacon process).

It is chlorine (rather than HCl) that enters into the substitution reactions to chlorinate
the condensed aromatic structure of the embedded native carbon. Experimental
evidence tends to support the importance of the Deacon process in dioxin formation.
For example, Ruuskanen et al (1994) found a good correlation between HCl in flue
gases and PCDF content of fly ash. Incidentally, higher concentrations of HCl also
increase formation of chlorinated phenols, enhancing production of dioxins by the
precursor route (Kanters et al 1996).

With reference to Figure 9, the oxygen from air diffuses into pores of fly ash particles
where it reacts with the imbedded native carbon. The native carbon is the intimate
contact with catalytic sites that contain chlorine. Although the detailed mechanism of
the reactions between oxygen, carbon and chloride salts is not yet well understood, it
is known that the oxidation reactions are catalysed by metals in conjunction with Cl
transfer. A part of the PCDD/F formed diffuses to the gas phase, with the rest
remaining in the fly ash. This is why fly ash often contains large amounts of
PCDD/F. There are several important implications of this mechanism for the
incineration of the municipal solid waste:

• Yields of PCDD/F depend on the reaction temperature. It has been observed that
the maximum yield displays a peak around 325oC (Milligan and Altwicker, 1995).
This means that PCDD/F are produced, via this mechanism, in the colder sections
of the process, including for example electrostatic precipitators, but not in a
furnace itself.

• The mechanism relies on the presence of carbon matrix imbedded in the porous
structure of fly ash particles. A more complete combustion in the furnace limits
the amount of carbon available for the de novo synthesis, in the post combustion
zone.

• The rates of formation of PCDD/F are defined by the activity of fly ash and by the
morphology of carbon generated in the combustion process. The concentration of
chlorine in the combustion gases is not important, as the chlorine in the de novo
synthesis comes from metal chlorides on the pore surfaces of fly ash. This
signifies no relationship between the amount of chlorine present in the waste
stream and the amount of PCDD/F produced by the de novo synthesis.

• The de novo mechanism leads preferentially to the formation of PCDF in addition

to PCDD, in contrast to most of the known precursor routes that generate PCDD.
This is an important observation, which demonstrates that in practical systems
whose PCDD/F signatures contain large amounts of furans, the de novo synthesis
may be important.
• It has been shown experimentally that neither CO nor CO2 from the combustion
gases participates in the formation of PCDD/F, by the de novo route (Milligan and
Altwicker, 1995). This is a significant observation indicating again that de novo
synthesis involves reactions among materials present in the fly ash with oxygen
diffusing from the air.

• The formation of PCDD/F by the de novo route is minimised in systems burning

uniform fuel at higher temperatures, at longer residence times and at low chlorine-
to-sulfur ratios. For example, in the pulp and paper industry in Canada (Luthe et
al, 1997), waste is combusted at 950-1150oC for 8 s at very low chlorine-to-
sulphur ratios in recovery boilers and at 740-1000oC for 3 s in power boilers. This
leads to extremely small production of PCDD/F in recovery boilers (up to 5.2 pg
ITEQ/m3) in comparison to recovery boilers (up to 3 ng ITEQ/m3).

fly ash particle

pore within the ash

particle

air, no chlorine
diffusion of PCDD/F

imbedded native carbon

diffusion of oxygen
transfer of
chlorine

catalytic sites

Figure 9. Mechanistic view of the de novo mechanism of dioxin formation.

Table 6. Examples of the effect of various elements in waste stream and in fly ash
on PCDD/F formation in de novo reactions. Observe that fly ashes that
are catalytically active in the formation of PCDD/F by de novo route are
also active in the precursor route (Hinton and Lane 1991a&b).

Element Effect Reference

Aluminium Negative Hinton & Lane, 1991a

Carbon1 No effect Hinton & Lane (1991b)

Chlorine1 Positive Hinton & Lane, 1991a

Copper Strongly positive Hinton & Lane, 1991a

Potassium Positive Hinton & Lane, 1991a

Silicon Negative Hinton & Lane, 1991a

Sodium Positive Hinton & Lane, 1991a

Sulfur (low Cl/S) Strongly negative Luthe et al, 1997

Sulfur (as SO2) Insignificant Ruuskanen et al, 1994
Sulfur1 Positive Hinton & Lane, 1991a

Tin Positive Ruuskanen et al, 1994

Zinc Positive Hinton & Lane, 1991a

Surface area No effect Hinton & Lane, 1991a

1
In the fly ash.

3.5 Relative rates of formation of PCDD/F in the incineration processes

Although it is now acknowledged that gas phase and non-catalytic reactions provide
no significant contribution to the total formation of PCDD/F in the incineration
processes, it is still being argued which of the catalytic processes (de novo or
precursor pathway) predominates. Recently, Huang and Buekens (1994) and
Altwicker (1996) observed that if the laboratory results were extrapolated to typical
incinerator conditions by taking into account the concentrations of the precursors in
the incinerator product gas streams, then the ratio of PCDD/F formation rates via the
precursor and de novo pathways would vary between 0.03 and 211. This would
indicate that both the de novo and precursor mechanisms operate in typical
incineration processes.

The relative importance of each pathway depends on the operating conditions,

especially the temperature history of the combustion gases. It has been argued, for
example by Altwicker & Milligan (1993), that at higher processing temperatures the
precursor-type reactions predominate over the de novo pathway. On the other hand,
at low temperatures, the de novo-type reactions become faster than the precursor
route. These conclusions are supported by other investigators (eg Takacs et al 1993)
who measured the abundances of PCDD and PCDF at various points in an
incineration process. Low and high ratios of PCDD/PCDF point to precursor and de
novo-type reactions, respectively. Takacs et al’s data suggest that dioxins and furans
are formed in different regions in incinerators. These data show that as the
temperature decreases to around 300oC, the PCDF formation rate subsides and the
PCDD formation rate markedly increases. Clearly, this points to a changing
mechanism in dioxin formation and demonstrates that both de novo and precursor-
type reactions are important.

4 Role of Chlorine in Formation of PCDD/F

There is a considerable evidence that production of dioxin in large scale incineration

processes is strongly linked to furnace types, their operating conditions, and the type
and efficiency of air-pollution control systems. High temperatures and long residence
time in the furnace (leading to complete combustion, optimised in fluidised-bed
furnaces), fast cooling of combustion products (minimising the amount of time the
flue gases spent in 200-450oC temperature window), and use of scrubbers rather than
electrostatic precipitators (these are often operated above 200oC) result in very low
emissions of PCDD/F. If an incineration process uses a new technology and is
operated within its design specifications, regardless of chlorine input, PCDD/F
emissions can be controlled below the present emission limits set up by environmental
authorities around the world. In other words, whether an incineration process accepts
feed with small (say below 1%) or large concentration (say above 80%) of chlorine,
this concentration cannot be related to the emissions of PCDD/F, because of various
operations within incinerator plants, specifically designed to minimise dioxin
emissions.

4.1 Role of chlorine in precursor reactions

Almost all organic and part of inorganic chlorine is released during combustion of
municipal solid waste to form HCl. Combustion of typical MSW results in emission
of around 2.8 g HCl/kg wet MSW (Kanters et al 1996). During combustion, HCl
participates in reactions with products of incomplete combustion to form chlorinated
phenols and benzenes, whose generation precede the production of dioxins. These
precursor reactions occur below 850oC, but most effectively around 750oC (Section
2.3.3). A relationship between HCl concentration and chlorophenol formation is
shown in Figure 10a. Within the experimental scatter, which is illustrated in Figure
10a by vertical lines, an increase in HCl results in a higher generation of chlorinated
phenols, but only at low concentrations of HCl. At higher concentrations of HCl,
there appears to be no increase in precursor formation.

Once generated, chlorinated phenols and benzenes undergo a range of catalytic

condensation and dechlorination reactions as shown in Figures 7 and 8. These
reactions occur in colder regions of incineration plants, normally between 200 and
450oC with a peak around 300oC. The formation of dioxins depends strongly on the
concentration of the precursor molecules, as illustrated in Figure 10b for the
formation of PCDD (Milligan & Altwicker 1996a). It is clear from the work of
Milligan & Altwicker (1996a) that neither chlorine nor hydrogen chloride needs to be
present for these reactions to take place. This point is further supported by results of
other researchers. For example, Hinton and Lane (1991b) reacted pentachlorophenol
over PCDD extracted fly ash. The chlorine was present in the fly ash in the form of
metal salts (eg KCl, NaCl). The study demonstrated that the formation of dioxins was
related to the catalytic activity of fly ash and not to its chlorine content. This means
that if PCDD/F are formed via the precursor pathways, their formation is related to
the catalytic nature of various fly ashes, concentration of precursors and to the
incinerator operating conditions, but not directly to the concentration of chlorine in
the incinerator feedstock.
 4.0

CPs, as org. Cl (micromol/kg wet MSW 3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0
-1 1 3 5 7 9 11 13 15 17
(a)
HCL emission (g Cl/kg wet MSW)

60000

50000

40000
T4 CP = 7 00 ng/ ml
PCDD (ng)

30000

20000 T4 CP = 35 0 ng/ ml

10000
T4 CP = 150 ng/ ml

0
200 250 300 350 400 450

(b) T(C )

Figure 10. Illustration showing an indirect relationship between Cl in the feedstock

and dioxin formation via the precursor pathway: (a) a relationship
between HCl and chlorinated phenols (Kanters et al, 1996), (b) a
relationship between tetrachlorinated phenol and PCDD formation
(Milligan & Altwicker, 1996a).
4.2 Role of chlorine in de novo synthesis

There are a large number of papers in the literature which demonstrate a relationship
between Cl in the feed and PCDD/F formation, via the de novo pathway. It would
appear, however, that these studies are carried out outside optimal combustion
conditions and involve a very small amount of chlorine. For example,

• Sonnenberg & Nichols (1995) showed a strong correlation between Cl and dioxin
formation at low chlorine concentrations. In their study, they included up to 0.1%
of chlorine in the feed; 1% of bleach plant concentrate was added to black liquor
feed stock, the bleach plant solids contained 10.3% of organic and inorganic
chlorine in the solids, and liquor had about 65% of solids. The temperature in the
experiments was only 800oC and cooling down was not controlled, pointing to
non-optimal combustion. Small-scale studies were plagued by high emissions of
chlorine, generated on fused-quartz reactor, leading to small-scale emissions being
10 times higher than in a pilot plant.

• Thuß et al (1997) combusted, at low temperature, lignite briquettes containing

0.03 and 0.2% of chlorine salts. They observed substantial increase in the
emission of PCDD/F and change in dioxin signature for briquettes containing the
higher concentration of chlorine salts.

• Raghunathan & Gullett (1996) injected unextracted incinerator ash into the
postcombustion zone of an experimental furnace reactor. This means that the
embedded carbon material in the fly ash was not destroyed in the hot zone.
Raghunathan & Gullett introduced HCl into the apparatus to simulate operation of
MWI plants. HCl reacted with oxygen via the Deacon process to form chlorine,
which then enhanced the production of PCDD/F by the novo route, chlorinating
aromatic carbon structures of the carbon embedded in fly ash. An increase in HCl
concentration correlated with an increase in PCDD/F emissions.

On the other hand, when investigations were performed for uniform fuel, burned at
high temperature under turbulent conditions at long residence times, no relationships
were reported between chlorine in the feed and PCDD/F in the exhaust gases. For
example,

• Luthe et al (1997) observed very low emissions of PCDD/F from recovery boilers
processing less than 0.5% chlorine in the feed and operating at optimum
combustion conditions.

A question arises as to whether a correlation can be expected between PCDD/F in fly

ash and chlorine input. It appears from the available information that the difference
in dioxin signatures and in total concentration of PCDD/F in fly ash can be attributed
mostly to furnace type rather than to fuel composition (Kopponen et al 1994). In
brief, fluidised-bed combustion technologies produce ash with low PCDD/F
concentration (eg Kopponen et al 1994, Foster Wheeler Corp, 1997), and those using
grate furnaces are likely to show substantially higher concentration of dioxin in fly
ash.
Finally, on some occasions, the reported positive dependence of dioxin emissions on
chlorine input to an incineration process or on the amount of chlorine on the fly ash
particles may be an artefact, related to a statistical methodology adopted in data
analysis. For example, the Pearson correlation test between two variables does not
account for possible confounding effects, for example for chlorine being associated
with metals, such as potassium and sodium, which themselves may act as catalysts.
This means that a positive correlation diagnosed by the Pearson correlation test should
always be verified with partial correlation coefficients or multiple regression analysis
followed by the analysis of variance (ANOVA), as the two increases (chlorine and
dioxin) may have a common cause. From this perspective, a positive correlation
between chlorine on fly ash and dioxin output, as reported by Hinton and Lane
(1991a), comes only from the calculations of the Pearson correlation coefficient, and
does seem to be supported by the multiple regression analysis.4

5 Dioxin Flows in Municipal Waste Incinerators

This section carries out an analysis of the balance of dioxin flows, including dioxin
destruction and its re-formation in combustion and post-combustion zones of an
incineration process. The analysis is based on examples drawn from the operation of
old and new municipal waste incinerators. On average, the pre-1990 MWC units
emitted 2.3 times as much ITEQ dioxins as they took in with the waste stream. On
the other hand, the newly constructed and modernised units, operating within the
emission limits, are net sinks of dioxins. Approximate but conservative analysis
shows that the ratio of dioxins carried with output and input streams from modern
MWC units is one to six. This section demonstrates that, for older MWC, it is indeed
correct to say that only 12% of the total dioxin output is emitted with the stack gases,
with the remainder contained in fly ash, filter cake, bottom ash, and scrubber and slag
water (Ruchel et al, 1996). However, this conclusion cannot be extended to new and
modernised MWC installations.

Figure 11 illustrates the average magnitude of ITEQ dioxin flows in older type of
German municipal incinerators obtained from pre-1990 built installations (Johnke &
Stelzner, 1992). The data came from 11 plants with 15 incineration units and their
underlying numerical values are listed in Table 7. Note that the incinerator data
included in Johnke and Stelzner's paper have only historical significance, since they
cover the facilities that did not have to comply, when the study was conducted, with
the dioxin emission limitation of 0.1µg ITEQ/m3 of exhaust gases now enforced in
Germany. Similar results can be extracted from other European studies, such as Lahl
et al (1991) indicating the general nature of Johnke and Stelzner's data.

4
By the same token, analysis based on the Pearson correlation coefficients rather than on partial
correlation coefficients or on multiple regression analysis (Ruchel, 1996) must be viewed with some
caution, as this approach does not eliminate the confounding effects of other factors.
 Stack gas

25 µg Collected fly ash

ESP ash 120 µg

Boiler ash 1.4 µg
Waste stream Raw gas
Post
Combustion combustion
90 µg zone 7 µg zone
Filter cake

48 µg
Slag water Scrubber water
Bottm ash

9 µg 3 µg 5 µg

Figure 11. Average dioxins flows in old-type mass burn MWC (Johnke & Stelzner,
1992), all numbers are expressed per tonne of waste in ITEQ.

Table 7. Balance of dioxin flows in terms of ITEQ load per tonne of waste burned in
pre-1990 type of municipal incinerators (Johnke & Stelzner, 1992). Geometric mean
values used in plotting Figure 11 are given in column four of the table.

Output Mass/volume flow per µg ITEQ/tonne waste

tonne of waste
range average
Slag* (bottom ash) 300kg 3 – 42 9
Water from slag removal 350L 0.4 – 35 3
Boiler ash 7kg 0.7 – 6 1.4
ESP ash 30kg 30 – 840 120
ESP ash** 60kg 60 – 240 120
Scrub water 750L 0.8 – 80 5
Filter cake 8kg dry matter 15 – 250 48
Clean gas 5000m-3 1.0 – 300 25
Input 6.5 – 180 90***
* Levels below the detection limit are considered to be at the detection limit.
** ESP ash plus reaction products from dry or semi-dry flue gas treatment.
*** This number was extracted from Huang & Buekens (1995).

It is clear from Figure 11 that a relatively large amount of ITEQ dioxin (90µg
ITEQ/tonne) actually enters the incineration process. The input waste also includes
dioxin precursors such as polychlorinated benzenes and chlorophenols (Lahl et al,
1991). Most of the dioxin and its precursors are destroyed in the incinerator's
combustion zone (furnace) and a balance around the furnace shows that only 21% of
dioxin leaves the combustion zone with the raw gas, bottom ash and slag water. This
is a very important observation, which demonstrates that the combustion process itself
can lead to a net destruction of dioxin. The raw gas leaving the furnace carries a
relatively low concentration of dioxin in spite of its high ash content (around 9g/m3).
At this stage, the fly ash is essentially free of dioxins before entering the post-
combustion zone. Note, however, that the products of incomplete combustion in the
primary zone are transported to the post-combustion zone as a raw gas and on
surfaces of fly-ash particles.
Many studies have now confirmed (eg Fangmark et al, 1993) that dioxins are re-
formed in the low-temperature post combustion zone, if the gases (including fly ash)
are allowed a sufficient residence time of more than 1.6s in that zone. However, this
can only happen if combustion is not complete in the furnace and the products of
incomplete combustion enter the post-combustion zone. From a practical standpoint,
two phenomena have been found to be critically important in reducing the formation
of dioxins in MWC. Firstly, the burning in the combustion zone must be complete, as
much as practicable. This is accomplished by operating the furnace at temperatures
around 1000oC, ensuring high turbulence during combustion by adjusting geometry
and air injection in the furnace, and by allowing the waste to spend at least 2s in the
hot zone (Rappe, 1996). Secondly, very fast cooling of the raw gas must be realised
in the post-combustion zone to limit the re-formation of dioxin from polyaromatic
carbon chains and gaseous precursors. Under the conditions of efficient combustion
and for typical chlorine loads in the waste stream, reduction of chlorine concentration
in the waste does not lead to decreased formation rates of dioxins in MWC
installations.

A dioxin balance around the post-combustion zone in Figure 11 demonstrates 192.4µ

g ITEQ/tonne of net dioxin formation in that zone. Only 25µg ITEQ leaves with the
stack gases to pollute the air and the rest contributes to the land and water
contamination. This is because the fly ash collects in electrostatic precipitators (120µ
g) and on the boiler walls (1.4µg) and is subsequently disposed of in landfills. It is
interesting to observe that the concentration of dioxin in ESP ash (4ng ITEQ/g ash) is
about 20 times higher than in boiler ash (0.2ng ITEQ/g ash). The boiler is located just
outside the combustion zone and the residence time for the ash collecting on the boiler
walls is too short for significant formation of dioxins. The contaminated water (5µg)
and the filter cake (48µg) are generated in wet scrubbers which are positioned after
ESP units in MWC air purification trains and are used to control the emission of
acidic gases, such as HCl and SO2.

Again, it should be stressed that the information presented so far in this section relates
to the historical data on the older pre-1990 incinerators. These incinerators polluted
air, land and water with the dioxin emissions distributed among air, land and water
according to the approximate ratio of 3.1:22.3:1. Unfortunately, not enough data
(especially for dioxin concentration in fly and bottom ash and in slag water) are
available to conduct accurately a similar analysis for new and modernised MWC
units. New incinerators are based on efficient high-temperature burning of waste, its
long residence time in the furnace and improved turbulence in the combustion zone.
These new incinerators also ensure short residence time of fly ash and efficient
quenching in the post-combustion zone, followed by dry flue-gas cleaning devices
with the injection of lime, charcoal, and even bicarbonate before a fabric filter. These
technologies are able to bring the concentration of dioxins to 0.1ng ITEQ/m3
threshold in the stack gases and to avoid altogether the emission of contaminated filter
water (Rappe, 1996; Eduljee & Dyke, 1996).

With these technological measures in mind and using the projected values published
by Lahl et al (1991), which assume a thermal treatment of fly ash, Figure 12
illustrates approximately the proportion of dioxin in various waste streams in a
modern MWC installation. It is clear from Figure 12 that a new or modernised
incinerator acts as a sink for dioxins, providing for their net destruction. Even if no
improvement in the dioxin concentration in the bottom ash and in the slag water is
(conservatively) assumed, modern MWC units would discharge 13.6 and consume 90
µg ITEQ/tonne of waste.

This fundamental change in dioxin handling in MWC units (from net production to
net destruction) has occurred over the last ten years, and was made possible by: (i) a
better understanding of the formation mechanisms of dioxins in combustion
processes, which led to improved set of operating parameters, and (ii) the advances in
the development of air purification devices and the introduction of new air
purification technologies. From this perspective, the emission of dioxin from
incinerators depends mainly on the operating conditions in combustion and post-
combustion zones and the quality of the air-purification systems installed in the
commercial incinerators. It is also interesting to observe that the composting process
that is now being used more frequently to treat green waste merely transfers dioxins
from the waste stream to the treated compost, because of low processing
temperatures. This means that compost produced in these facilities may contain high
concentration of PCDD/F, if contaminated green waste enters the composting
facilities
(Malloy et al 1993). The data obtained from long-term field experiments on PCDD/F
persistence in sludge-amended soil support these findings, pointing to long half-lives
for disappearance of dioxin in the soil (McLachlan et al 1996).

Stack gas

0.5 µg Thermally treated fly

ash

0.9 µg
Waste stream Raw gas
Post
Combustion combustion
90 µg zone 7 µg zone

Bottm ash Slag water

9 µg 3 µg

Figure 12. Balance of dioxin flows around the post-combustion zone of a modern
MWC. All flows are expressed per tonne of waste in ITEQ (estimated
from Lahl et al, 1991).

6 Emission of Dioxins from Combustion Processes

Several countries, including the Netherlands (Bremmer et al 1993, Wormgoor 1994),

Sweden (Björndal 1996), the United Kingdom (ERM 1995) and the United States
(Thomas & Spiro 1995), have carried out an inventory of their dioxin sources. Global
production rates of PCDD/F based on average emissions have also been surveyed
(Brzuzy & Hites 1996a). From these studies, it is immediately obvious that a number
of industrial sources of dioxins need to be researched, and there are many unanswered
questions, which remain to be addressed. For example, the global mass balance of
dioxin formation and deposition, carried out by Brzuzy & Hites, indicates that
atmospheric deposition rates of dioxin (around 13,100 kg/year) greatly exceed the
known emission rates into the atmosphere (around 3000 kg/year). In agreement with
these findings, the new US EPA inventory of dioxins (Hileman 1998) shows that in
the US the dioxin deposition rates are much higher than the emission rates. From this
standpoint, the aim here is to describe the present understanding of dioxin emission
from combustion processes in general, and from municipal waste incineration in
particular.

Table 8 illustrates the sources of dioxins in the USA (Hileman 1998). The dioxins
generated in backyard trash burning, inadvertent landfill fires and iron ore sintering
are based on the order of magnitude estimate. Clearly, more precise assessment of
these dioxin sources would require further research. Table 8 demonstrates that the
municipal waste incineration accounts for about 25% of known anthropogenic dioxin
emission, and this number tends to decrease as new incinerators are put on stream and
the old ones are shut down. A similar conclusion has been reached by Rappe (Fiedler
& Van den Berg 1996) who wrote that “all European inventories claim that with
modern technology the incineration of municipal solid wastes is no longer a major
source of PCDD/PCDF to the atmosphere”.

Table 8. Estimates of anthropogenic sources of dioxins in the US, in g of TEQ per

year.

Source g of TEQ per year

Municipal waste incineration 1,100

Backyard trash burning 1,000*
Inadvertent landfill fires 1,000*
Secondary copper smelting 541
Medical waste incineration 447
Forest, brush and straw fires 208
Cement kilns 171
Iron ore sintering 100*
* Order of magnitude estimate.

There have been several suggestions about natural production of PCDD/F (eg Gribble
1994), but in spite of some research in that area, this remains mostly an unresolved
issue. From the perspective of this report, the key issue is whether the PCDD/F
observed in the deposition rates but not accounted for in the anthropogenic sources
can come from natural sources.

Another important issue is an apparent mismatch between dioxin signatures of known

PCDD/F anthropogenic sources and global deposition fingerprints5, at some distance
away from dioxin sources. The global deposition patterns are dominated by
octachlorinated dioxins (OCDD, eg Bonn 1998) whereas the known man-made
sources contain mostly lower-chlorinated dioxins and furans, with the exception of
OCDD formed from pentachlorophenol. Possible explanations for observed
deposition signatures have been suggested in the literature (eg Brubaker & Hites
1997, Trapp & Matthies 1997). Highly chlorinated PCDD/F characterised by low
5
The emission of PCDD/F from anthropogenic sources leads to very localised deposition of dioxin
patterns that correspond to the source signature, and to global deposition pattern that reflect source
signatures modified by atmospheric processes.
vapour pressure tend to accumulate on particles in the atmosphere and are removed
from the atmosphere when these particles settle on the soil surface. On the other
hand, lower chlorinated dioxins react with hydroxyl radicals (OH), which are
produced by photolysis of H2O2 and by photolysis of O3 in the presence of H2O.
From a practical perspective, this means that emission of PCDD/F from incineration
processes leads to global low-level contamination, although deposition signatures do
no longer correspond to the source signatures.

7 Long-Term Trends in Dioxin Emission from Combustion

Processes

A large number of studies have shown decreasing concentration of dioxins in

atmosphere (eg Coleman et al 1997) and their reduced deposition rates (eg Bruzy &
Hites 1995, Jüttner et al 1997) in recent years. A very comprehensive review of pre-
1996 literature has been conducted by Alcock & Jones (1996) and included dioxins
present in the environment (that is in air, sediments, vegetation and soil), sewage
sludge, milk, livestock tissue, as well as in human wildlife tissue. The compilation of
data presented by Alcock and Jones (1996) covers international sources.

No research facility exists in Australia to study dioxins and so very little data are
available in the open literature. Most of the Australian studies have been carried out
by commercial organisations that have been able to support high costs of analyses
carried out by foreign laboratories. Ruchel et al (1996) gave an overview of dioxin
emissions in NSW indicating that BHP operations in Newcastle and Port Kembla are
major dioxin sources in this country. However, at present, it is unclear whether other
sources, such as bushfires, contribute to the observed deposition rates. The analysis
of the emission from the Australian pulp and paper industry (Nelson 1994) confirmed
that new pulp bleaching technologies effectively eliminated formation of dioxins.
This is consistent with similar studies conducted in Canada (MacDonald et al 1998).

Alcock & Jones’s review paper is based on more than 90 reference publications.
Figure 13 provides an example of the changes in the concentration of PCDD/F and
ITEQ dioxins in herbage collected in Park Grass experiment at Rothamsted
Experimental Station near London. A sharp increase after 1950 followed by a rapid
decrease in 1980s and 1990s is evident in the figure. Note that there is a possibility of
contamination of the samples with modern air, since the vegetation was dried before
storage. This may explain rather high values of total and ITEQ dioxins in the data
derived from herbage collected prior to 1940.
 3000 400000

2500 I-TEQ
Σ PCDD/F 300000
2000

I-TEQ 1500 200000 Σ PCDD/F

1000
100000
500

0 0
1850 1900 1950 2000
Year

Figure 13. Trends in total and ITEQ dioxins in ng/kg in archived Park Grass
vegetation (Kjeller et al, 1993).

In general, the trend data (eg Alcock & Jones, 1996) support a conclusion that rates of
dioxin emission and deposition are linked to a few specific chemical and industrial
processes rather than to the additional production of chlorine and the increasing use of
PVC. As the concentration of PCDD/F in a wide range of environments continues to
decrease, the production of chlorine and PVC increase (Alcock & Jones, 1996; Fiedler
& Van den Berg, 1996). The observed decline is a direct result of the following
technological changes and phase-out of certain chemicals:

1. The implementation of the phase-out of pentachlorophenol (PCP), which is

considered the most potent precursor of dioxins and which itself contains dioxin
impurities. The production of PCP started in the 1940s, peaked by the 1970s and
stopped by the 1980s (Alcock & Jones, 1996). PCP had several applications such
as treating timber, and being an ingredient for certain dyes and pigments. Because
of these applications PCDD/F, originating from PCP, were observed to be present
in efflux from paper mills (Luthe et al, 1993) that processed treated wood, in
sewage sludges that might have contained dyes and pigments and in combustion
of PCP-containing wood (Alcock & Jones, 1996).

2. Processing changes implemented in new incinerators burning municipal and

medical waste and shut-down of old and polluting facilities. The new
technologies that led to the dramatic decrease in dioxin emissions included
fluidised-bed furnaces with long residence time, fast cooling of the combustion
products, injection of lime and activated carbon and replacement of electrostatic
precipitators with baghouses that are operated at lower temperatures than
electrostatic precipitators. The data from monitoring the environment around new
MSW combustors normally show no negative impact of these facilities on the
environment. For example, a biomonitoring study carried out around a new MSW
combustor in West Palm Beach (Florida, USA) found TCDD/F at pre-operational
levels during the first five years of the combustor operation (Rumbold et al, 1997).
3. Replacement of bleaching of paper pulp with chlorine by bleaching with ClO2, in
the operation of paper mills. Figure 14 demonstrates a dramatic impact this new
technology had on the deposition rates of dioxins, downstream from a pulp mill,
in Kamloops Lake, in the province of British Columbia, Canada (MacDonald et al
1998).
Concentration (pg/g)

(a)
Concentration (pg/g)

(b)

Figure 14. Reconstructed historical trends in the deposition of (I) PCDD/F formed
in the bleaching of pulp with chlorine and (II) PCDD/F from processing
wood chips contaminated with PCP. Arrows along the time scale refer
to the following mill history: 1965 mill opens, 1972 major expansion,
mill A goes to 10% ClO2, 1988 mill B goes to 30% ClO2, 1989 mill A
goes to 30% ClO2, 1991 mills A and B go to 60% ClO2, 1992 mill A
goes to 100% ClO2, 1993 mill B goes to 100% ClO2 (MacDonald et al
1998).
4. A drop in consumption of leaded petrol, due to a phase-out of leaded petrol in
some countries, led to the decrease in dioxin formation in motor vehicles. In the
past, lead was found to catalyse the formation of PCDD/F and especially the penta
congeners (Manninen et al 1996).

5. The changes that have taken place in the use of fossil fuels, and especially in the
use of coal for heating homes. This is because of the practice of burning domestic
refuse with coal, under less than optimal combustion conditions. In the UK alone,
the demand for house coal has decreased more than 13 fold, in the last 40 years
(Alcock & Jones 1996).

6. Further minimisation of dioxin formation has been realised by closure of some

open burn sites and metal recovery facilities that operated copper and alumina
smelting plants. This has occurred because of air pollution regulations in some
jurisdictions (eg in California, see Harnly et al 1995). However, it appears that
regulations that govern open burns and incineration of copper and alumina scraps
are not sufficiently widespread to have a substantial impact on dioxin emission
from these sources. For example, it has been reported that in the US, meat and
dairy products have very high level of dioxins, although most food production
takes place away from industrial centres (Hileman, 1998). This observation may
point to refuse burning as a possible cause.

If these declining trends in PCDD/F emissions are to be sustained in the long term,
then open refuse burning should become controlled and new technologies should be
developed for secondary copper smelting and iron ore sintering (Jager, 1993; Bröker
et al, 1993). Additional research is also needed to provide more accurate figures of
dioxin emissions from landfill and forest (bush) fires, to support or refute the present
estimates and to guide in managing and controlling these hazards.
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