Incinerator Design and Operation:
The Robust Approach to PCDD/F Minimization
William F. Carroll, Jr., Ph.D.
Thousands of experiments have been conducted over the past
twenty-five years exploring the relationship between combustion
conditions--including the amount of chloride in incinerator
feed--and the generation and emission of dioxins and furans
(PCDD/F). Two sources of chloride have been studied in great
detail: salt and polyvinyl chloride (PVC) plastic. The data
are diverse because there is a virtually infinite combination
of fuels, combustion conditions, and designs of experiments
ranging from one gram samples burned in a quartz tube1,
to full-scale experiments where PVC and salt are purposely
added and removed2,3.
Recently, Pat Costner of Greenpeace published a report on
the Greenpeace website entitled "Chlorine, Combustion and
Dioxins: Does Reducing Chlorine in Wastes Decrease Dioxin
Formation in Waste Incinerators?"4 In it, she asserts:
1) Small-scale combustion is well-controlled and reproducible
and clearly shows a correlation between the chloride content
of waste and dioxins generated and emitted from combustion.
2) Data from experiments performed in large-scale combustors
is prone to experimental error and is irreproducible; as a
result, no clear conclusions can be drawn about the relationship
between chloride and dioxin in those devices. 3) Despite the
equivocal nature of data from large-scale combustors, small-scale
results should be considered indicative of the overall mechanism
of combustion and a material policy (elimination of PVC) should
be put into place. She claims such a policy would result in
reduction or elimination of PCDD/PCDF from combustion emissions.
In response to literature cited and analysis presented in
Costner4, this paper makes four points: 1) Small- and intermediate-scale
combustors do not provide clear evidence that chloride concentration
is the controlling influence on PCDD/F generation. 2) Full-scale
combustors, because they represent real-world operation and
not a simulation, provide the best information. 3) The known
catalytic formation of PCDD/F in combustion gases argues against
a strong impact of the amount of chloride present in the fuel.
4) The best policy option for incinerators is to utilize combustion
and emission technology that is robust with respect to any
Chloride and the PCDD/F Molecule It is literally true that
in order to make chlorinated dioxins and furans-or chlorinated
anything, for that matter-the chemical element chlorine is
required. On the other hand, there is a more than a million-fold
oversupply of chloride already present in normal wastei
above what is needed to make the PCDD/F emitted. In fact,
it would be impossible to "starve" the chloride out of the
feed, since even ambient air alone can contain nearly 20 times
as much chlorideii as is necessary to make that
same PCDD/F5. Given these facts, it is difficult
to imagine how micromanaging the chloride or PVC in the waste
could have a major effect on PCDD/F generation.
1) Small- and Intermediate-scale Combustion: Scale Matters.
Many experiments but results are not consistent.
Laboratory experiments In general, small-scale experiments
are easier to conduct and to control than experiments conducted
in full-scale apparatus. However, experimental accessibility
and diverse experimental conditions and results provide important
evidence of the complexity of the PCDD/F formation mechanism
and the importance of design and operation. Costner cites
the following articles:
Wikstrom, et al.,19966 This experiment uses
a pelletized, artificial fuel containing cellulosics, known
catalytic metals, and polyethylene, varying chloride as a
salt or PVC from 0-2%. They find no difference in effect between
salt and PVC, and no effect of chloride amount below a threshold
of about 1%.
This laboratory has expanded upon these results in later
experiments7. In testing the effects of good,
intermediate and poor combustion in addition to fuel composition,
these authors find:
".the most important variable for changes in the PCDDs/Fs
and PCBs formation was disturbance in the combustion condition
and not the variation in chloride content of the fuel. Furthermore,
no differences in formation between the chloride sources
could be seen."
Four of the seventeen PCDD/F species measured in the article
correlate with chloride content. By far, the majority do not,
and the overall conclusion of the authors is that chloride
is not a leading variable.
Hatanaka, et al., 20008 Hatanaka tests
artificial wastes containing salt, PVC and copper chloride
as a catalyst for PCDD/F formation. The types and amounts
of PCDD/F produced in this experiment differ from those
of a normal combustor, and the authors note this, calling
into question the applicability of this experiment to full-scale
PCDD/F also correlates strongly with carbon monoxide (CO),
and increasing CO concentration is usually indicative of
poorer combustion. "As Cl content of the waste was increased,
CO concentration in the flue gas became higher and more
PCDD/Fs were formed."
These experiments are simply telling us that combustion
conditions are changing as the feedstock is changing; thus,
combustion conditions may, in fact, be the most important
variable. At the very least, the two potential causes are
indistinguishable. This experiment was not set up to maintain
good combustion regardless of fuel as is the case for most
full-scale combustors. If it had been the results may well
Hatanaka, et al. added to their interpretation of the work
in a later publication not cited in Costner9.
".CO concentration in the gas became higher and more PCDD/Fs
were formed as chlorine content in waste increased. This means
that combustion conditions indicated by CO are strongly related
to the PCDD/Fs formation during incineration."
Later experiments were conducted at a different combustion
temperature, holding the fuel (and Cl concentration) constant.
The researchers reported:
".the PCDD/Fs concentration at 700°C is significantly lower
than at 900°C in the experiment of the waste with the same
Thus, a change in combustion temperature had a significant
effect on PCDD/F in the absence of change in chloride concentration
in the fuel.
Gullett, et al., 200010 This is the least
applicable study to full-scale combustion. Gullett's experiments
were conducted as uncontrolled combustion in an open industrial
metal drum. This condition is diametrically opposed to the
controlled, closed operation of a full-scale combustor.
While experiments run with 7.5% added PVC yielded the highest
PCDD/F TEQ values, these experiments also demonstrated poor
combustion. Statistical models constructed by the authors
show that the most important variables relate to combustion
rather than chloride. Gullett reports in a later article:11
"The most significant one-predictor model actually contains
an interactive term, CO*TC6, which is the product of CO emissions
and the temperature at the uppermost portion of the barrel..The
logarithm of Cu emissions or waste Cl content is also significant
(based on its P<0.05), but not as effective at predicting
emissions as the aforementioned models."
As another indicator of the importance of good combustion,
when wet waste of the control composition (0.2% PVC) was burned,
dioxin generation increased five-fold beyond that generated
from waste containing 1% added PVC, although these two results
were statistically indistinguishable.
2) Full-scale Incineration: Data for full-scale incineration
exists and should be used. Scale-up, design and operation
are the most important variables.
Hundreds of full-scale incinerator tests have been conducted,
and robust statistical methods-the most reliable approach--have
been used to interpret the data. In its landmark study of
data from 170 combustors, the American Society of Mechanical
"While some laboratory experiments show there is a functional
relationship between chlorine input and PCDD/F concentrations
in the products of combustion under certain conditions, the
effect is much smaller than the effect of confounders like
combustor design, operating practices and the normal variability
found in emission measurements made at commercial scale systems.
Whatever effect chlorine has on stack gas PCDD/F concentrations
from waste combustors is masked by these other variables12."
This study, reviewed by over 25 experts in the field of combustion,
found that for about 10 percent of commercial combustors,
PCDD/F increases significantly with increasing chloride; for
about 10 percent, PCDD/F decreases with increasing
chloride, and for about 80 percent there is no significant
effect observed. Yet, the ASME researchers are not the only
ones who believe that changes in chloride concentration have
a relatively minor effect, if any, on full-scale combustors.
Gullett13, et al note:
"There is poor correlation between total chlorine in waste
streams and formation of polychlorinated dibenzodioxin and
polychlorinated dibenzofuran (PCDD/F) during waste combustion."
Everaert and Bayens14 carried out a similar review
of large-scale combustors in order to determine mechanism
of formation. They found positive correlation between electrostatic
precipitator (ESP) temperature and PCDD/F. When values were
normalized for ESP temperature:
"The scatter and/or dependency of results disappears completely
and PCDD/F levels achieve nearly constant values. It might
therefore be concluded that the main correlating parameter
for the PCDD/F emission level is the temperature of the ESP."
The authors also report "no correlation of PCDD/F with water,
oxygen, hydrogen chloride (HCl), sulfur dioxide (SO2) and
polycyclic aromatic hydrocarbons (PAHs)"
Accurate transcription of the EPA document15 cited by Costner
agrees, stating that small-scale incineration is at best a
poor indicator of full-scale performance. Section 2.4.2, "Review
of Full Scale Combustion Systems" begins:
"The review of experimental data clearly indicates an association
between chlorine content of feed/fuels and potential
synthesis of CDDs and CDFs." ("potential" omitted in Costner;
So as not to be misunderstood, however, the document continues
"Paradoxically, the review of full-scale operating incineration
processes does not yield such unequivocal results indicating
that complex kinetic events make strong associations difficult
in full-scale systems."
EPA's opinion is clear: there are differences in chemistry
between large- and small-scale operations. In the section
"Potential Prevention of CDD/CDF Formation in Combustion Systems,"
the EPA authors cite experiments adding sulfur compounds or
calcium oxide to inhibit catalytic formation of PCDD/Fs. While
EPA is very well aware of chloride reduction as a "potential"
solution, no mention is made of removing chloride or PVC.
Moreover, in the "Integrated Summary and Risk Characterization"
of the Dioxin Reassessment, the question is answered directly:
"Although chlorine is an essential component for the formation
of CDD/CDFs in combustion systems, the empirical evidence
indicates that for commercial scale incinerators, chlorine
levels in feed are not the dominant controlling factor for
rates of CDD/CDF stack emissions. Important factors which
can affect the rate of CDD/CDF formation include the overall
combustion efficiency, post-combustion flue gas temperatures
and residence times, and the availability of surface catalytic
sites to support CDD/CDF synthesis"
3) Dioxin Formation Mechanisms: PCDD/Fs are formed under
the influence of catalytic metals and largely at well-known
temperatures. Fly ash surfaces are also involved.
Much of the work done on PCDD/F and combustion in the past
twenty-five years has been oriented toward understanding the
mechanism of formation. The sheer number of studies stands
in testament to the complexity of this question; still, much
is known and has been translated from the laboratory to operational
a) Formation temperature: The temperature for maximum formation
of PCDD/F is between about 250 and 400 degrees C16,17,18.
Combustion gas systems such as "hot-side electrostatic precipitators"
hold combustion gases at these temperatures and will tend
to be incubators for PCDD/F19. On the other hand,
so-called "fast-quench" systems that cool the gases quickly
through these temperatures discriminate against formation.
b) Surfaces and metals: It is well-known that active fly
ash containing elemental carbon as soot or carbon black
can yield PCDD/F when simulated combustion gases are passed
over it 14,20,21. This seems to be due to the
action of small amounts of metal chlorides-usually copper-on
the carbon black attached to the fly ash. The metal chlorinates
the carbon, and eventually, aromatic structures detach from
the carbon particle. These can act as precursors to PCDD/F
or be PCDD/F themselves.
c) Further chlorination and dechlorination: Experiments
have demonstrated the possibility of active exchange of
chlorine atoms on and off the basic skeletons of dioxins
and furans. This exchange helps to determine the mix of
species found in the final emissions22.
It appears not to be true, however, that simply increasing
the amount of chloride in the combustion gas impacts these
mechanisms. Catalyst is the scarcest material present in these
incineration systems, and there is already a huge excess of
Combustors fitted with apparatus designed with knowledge
of these mechanisms perform significantly better than those
of earlier design. US EPA estimates that implementation of
its Maximum Achievable Control Technology (MACT) will be responsible
for reducing PCDD/F generation from municipal and medical
waste combustors from about 1700 g TEQ to about 20 g TEQ in
the period 1995-200423. This 99% reduction comes without modifying
the material entering the incinerator; e.g., no restriction
4) Incineration Policy: The best control option for incinerators
is combustion and emission technology that is robust in the
face of any reasonable fuel.
If there is any general conclusion to be drawn from twenty-five
years of research into dioxin generation in combustion, it
is that combustors vary. Similarly, emissions from combustors
vary, even with respect to dioxin emissions and correlation
with chloride. Carefully chosen literature can demonstrate
that there is a positive correlation between input chloride
and PCDD/F (e.g, more chloride = more dioxin); similar care
can find experiments showing negative correlation (e.g., more
chloride = less dioxin). Conversely, if the correlation
is correct, reducing chloride from the input to these two
sets of combustors would yield two different results: less
dioxin emission from the first set, but more dioxin
emission from the second. Adjusting the chloride content of
the combustor fuel is clearly not a robust PCDD/F reduction
|Table 1: PCDD/F by effluent, before and
after application of technology
(pct of total)
(pct of total)
|Fly ash/Treatment solids
On the other hand, there is scientific knowledge regarding
the temperature of formation and impact of metal catalyst
on the reactions that lead to dioxin. This knowledge leads
to technology that reduces both the generation and emission
of PCDD/F--technology which has been specified by governments
in Europe, Japan and the United States over the past decade.
It is the application of this technology that has, and will,
lead to the 99% reduction in dioxin emissions EPA foresees.
Similarly, it is patently not true that newer technology
simply shifts PCDD/F from one medium to another. EPA's observation
that a change in temperature of gases entering the ESP greatly
reduces dioxin generation in that device is simply one example19.
Less dioxin is made in modern systems, and less is emitted.
|"Scrub" indicated use of
a Ca(OH)2 semi-dry scrubber
A direct comparisons of "before" and "after" application
of modern technology have been published24,25.
In this paper, sequential application of technology shows
the impact of different strategies (Figure 1). Reduction of
greater than 99% in flue gas emissions is obtained without
management of combustor fuel. Generation-and not just emission--of
dioxin is decreased markedly as shown in Table 1.
Contrary to the conclusions of Costner's paper, the key
to reduction of generation and emissions of PCDD/Fs is proper
design and operation of the combustor. This means specification
and operation of pollution control equipment and utilization
of good combustion practices like the "Three T's": Time in
the combustion zone, Temperature of combustion and combustion
gases, and good Turbulence. Successful governmental policy
reflects this and requires it as a condition for operation
of combustors. Once this is done correctly little, if anything,
is to be gained by other measures like removal of chloride.
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*The ASME study was partially funded by The Vinyl
Institute and the Chlorine Chemistry Division of the American
Chemistry Council, industry associations.
iAssume 10 ng TEQ/cubic meter of air emissions
(dscm)-50-100 times the currently allowed emissions. This
is ca. 400 ng total/dscm. At 6 chlorides average per D/F,
this is approximately 1 nanomole PCDD/F per dscm or 6 nanomoles
chloride/dscm. At 7000 m3 air/ton waste, 42 micromoles
(1.5 mg) chloride is emitted as dioxin per ton of waste. Normal
MSW contains approximately 0.5% chloride, so 5000 g chloride
enters the combustor per ton of waste. This means that even
under these poor combustion conditions there is well over
3,000,000 times as much chloride present in the waste as is
required to make the PCDD/F.
ii Grosjean measures up to 3.9 micrograms/ m3
chloride in ambient air. At 7000 m3 air/ton waste,
27 mg chloride is introduced with the air, an 18-fold excess
over what would be needed to synthesize PCDD/F.