United States
Environmental Protection
Agency
Risk Reduction Engineering
Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-90/039 May 1991
&EPA Project Summary
Minimization and Control of
Hazardous Combustion
Byproducts
Barry Dellinger, Philip H. Taylor, and Debra A. Tirey
Control of emissions of toxic
organometallic compounds is one of
the major technical and sociological
issues surrounding the further
implementation of incineration as a
waste disposal alternative. A
comprehensive review of the status
of research concerning the emission
of organic products of incomplete
combustion (PICs) indicated that
available full-scale data were
insufficient in volume and quality to
make firm conclusions. As
enumerated below, several trends
were identified that appeared to
concur with scientific theory, and
several deficiencies in fundamental
knowledge and completeness of full-
scale emissions data were observed.
PIC emissions are a natural
consequence of the kinetically-
limited thermal degradation of
hazardous wastes. Comparison of PIC
formation/destruction rates based on
theory and nominal incineration
conditions indicate that PIC
emissions are always several orders
of magnitude higher than predicted.
This suggests that temporal or spatial
excursions from these nominal
conditions are occurring that are
responsible for PIC emissions. Low
temperatures due to quenching,
residence time short circuits due to
nonplug flow and/or unswept
recesses, and locally high
waste/oxygen concentration ratios
due to poor microscale mixing or
overloading are implicated as failure
modes. Relative emission rates are
controlled by failure conditions that
arise in post-flame regimes of full-
scale systems.
Consideration of waste feed
composition and the reported
emissions suggested that a
significant fraction of PICs that may
have been emitted from full-scale
facilities were not analyzed. Analysis
of PIC emissions data from 17 full-
scale facilities further suggested that
the magnitude of PIC emissions
correlated with the completeness of
the sampling and analysis
procedures.
A comprehensive rank-order
statistical analysis of full-scale
emissions data indicated that of the
design and operational parameters
evaluated, stack oxygen concentra-
tion and waste heat load exhibited
the strongest correlation with various
measures of PIC emissions.
Regulatory surrogates for PIC
emissions (CO and TUHC) and stack
benzene and toluene concentrations
exhibited only occasional correlation
with various measures of PIC
emissions.
Analysis of specifically identified
emissions indicated that simple
halogenated methanes, halogenated
ethanes; halogenated ethenes, and
nonhalogenated aromatic species
were the most frequently observed
and highest yield PICs. The high
yields of the relatively fragile
halogenated methanes and ethanes
inferred that many of these PICs were
formed downstream of the high-
temperature zones of full-scale
systems via radical-association
reactions.
Printed on Recycled Paper
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In conclusion, a series of design,
operational, and regulatory implica-
tions for PIC control were presented.
From the laboratory perspective, the
urgent need for detailed studies of
PIC characterization, yields, and
emission mechanisms at elevated
temperatures was emphasized. The
benefits of Interactions between
laboratory-scale PiC studies, PIC toxi-
cologlcal studies, and sampling and
analysis methods development for
full-scale systems were also
discussed.
This Project Summary was
developed by EPA's Risk Reduction
Engineering Laboratory, Cincinnati,
OH, to announce key findings of the
research project that is fully
documented In a separate report of
the same title (see Project Report
ordering Information at back).
Introduction
Of all the issues surrounding waste
disposal, the potential formation and
emission of PICs from the thermal
disposal of wastes is the most
controversial. Much of the controversy is
due to the lack of knowledge of PICs, the
current regulation only addresses
destruction and removal efficiency of
principal organic hazardous constituents
(POHCs) contained in the waste stream.
This has led to the unfortunate public
misconception of incineration as a
"landfill in the sky".
It would seem that this important
subject and controversy that has raged
all through the 80's would be well
researched and significant strides made
towards answering key issues. However,
we in reality know very little about PIC
emissions. The purpose of this project
was to define what we really know about
PICs, what we think we know, and what
we don't know. The reader is referred to
the final report for a comprehensive
analysis of theories of PIC formation and
a thorough statistical analysis of pilot-
and full-scale PIC emissions data. The
project culminates with numerous
implications for PIC emissions control.
Although every effort has been made
to approach this subject scientifically and
arrive at logical, factually-based
conclusions, there will undoubtedly
continue to arise nonfactual or even
clearly incorrect statements concerning
PICs. It is hoped that this report will at
least serve to establish a framework for
rational discussion of the issues
surrounding PICs.
Theories of PIC Formation
There are only a few issues for which
we have sufficient data that agree with
scientific theory that will allow us to
advance several theories about PICs.
These theories are discussed below.
Theory 0—Plps are a Natural
Consequence of Thermal
Degradation of Toxic Wastes
This theory jis so fundamental and
obvious that it is designated as the
Zeroth Theory of PIC Formation. From a
scientific point of view, any product of the
oxidative or pyfolytic degradation of a
compound, other than the thermo-
dynamically most stable end-products,
should be included in the definition of a
PIC. If we want to designate PICs of
environmental concern where toxicity and
concentration are factors, then we can
define a new term: toxic, principal,
product of incomplete combustion (ToP-
PIC). The question is not whether PICs
are emitted from hazardous waste
incinerators, because they will un-
doubtedly be formed and emitted from
any combustion (source. The true issue is
whether they |are of environmental
consequence, which is a very complex
issue.
Theory 1—PIC Emission Rates
are Kinetically, Not Thermo-
dynamically, Controlled
If an incinerator achieves thermo-
dynamic equilibrium, then one only
needs to know tfie elemental composition
of the waste/fuel/oxidizer (air) feed
system and the temperature to calculate
the emissions from the system. For
example, therrnodynamically complete
combustion or pyrolysis of a chlorinated
hydrocarbon at| 1800°F results in the
formation of a few stable products, e.g.,
C02, H2O, HCI, H2, CI2, etc., and trace
quantities of CO and organic byproducts
(Eq. 1). Numerous calculations of PIC
emission rates! have been performed
assuming an incinerator at equilibrium.
These calculations typically demonstrate
that observed PIC concentrations are at
least 10 orders of magnitude greater than
predicted based on equilibrium (Figure 1,
Table 1).
Some have argued for rather vague
concepts with
practical utility,
ittle scientific basis or
such as PIC emissions
being at "pseudo-equilibrium". Sufficient
time is not available for equilibrium to be
achieved, and I PIC emissions are a
function of tjme (i.e., kinetically
controlled). Chemical reaction kinetics
undoubtedly play a large role in PIC
formation. However, physical kinetics,
such as vaporization and waste/air
mixing, can also contribute to kinetically
limited routes of PIC formation.
Theory 2—Deviation from
Normal, Average Operating
Conditions are Responsible for
Most PIC Emissions
Modern, well-designed and managed
incinerators operate at temperatures in
excess of 18QO°F, residence times
greater than 2.0 s, and at least 50%
excess air. Detailed elementary and
global oxidation kinetic models
consistently show that destruction
efficiencies (DE's) for POHCs under
these conditions are greater than
99.9999% for even the most stable
compounds. Similar calculations for PICs
result in yields < 0.000001 (Table 2,
columns 3, 4, and 6).
Since the nominal operating range of
incinerators precludes measurable POHC
or PIC emissions, we are left with the
conclusion that temporal or spatial
excursions from the measured conditions
are responsible for the observed
emissions. These "failure modes" may
be due to excursions in temperature
(thermal), residence time (temporal), or
oxygen concentration (mixing). As
examples, a spatial, thermal-failure mode
could be a cold gas boundary layer in a
boiler whereas a temporal, thermal-failure
mode would be a simple flameout. A
spatial, residence-time failure could be
deviation from ideal plug flow such that
residence-time short circuits are possible.
A temporal, residence-time failure could
be loss of atomization pressure in a
burner such theat very large droplets are
produced that do not vaporize until they
have traversed much of the combustion
chamber. A temporal, mixing failure could
be poor waste/air micromixing on the
molecular level due to flash volatilization,
whereas a spatial failure could simply be
poor burner placement or location of
secondary air ports.
Hypotheses of PIC Formation
In addition to these theories that the
preponderance of the data support, a
body of qualitative and semi-quantitative
laboratory and field data have led to
certain observations concerning PIC
emissions. We refer to these as partially
verified hypotheses.
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oxidation:
pyrolysis:
v-z y~2
CxHyClz + (x + J__ )O2 = x CO2 + z HCI + H2O + trace organics and CO fory>z
4 4
CxHyClz + xO2 = x CO2 + y HCI + —- CI2 + trace organics and CO
2
CxHyClz = x C •* zHCt + -^- H2 + trace organics
CxHyClz = xC + yHCI + -flL C/2 + trace organics
2
foryz
for y
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Table. 1. Experimentally Measured and Equilibrium-Predicted POHC Concentrations from a Full-
Scale Trial Bum (All values are in mole fraction)
Equilibrium Calculation (STANJAN)
POHC
Measured
(Trial Burn)
2.65% O2
25% Oz
Deficient
100% O2
Deficient
2x10*
1X10-9
1X10-'8
1x10-™
5x70-23
Table 2. Kinetic Calculations of the Destruction Efficiency for PICs1 for Various Flame and
Post-Flame Reaction Conditions •
Optimal Thermal Overall Overall
Optimal Thermal Failure (Optimal) (Failure)
POHC PIC Flame DE2 DE3 DE4 D£5 D£6
1,1-Dichloroethane Chloroethene 99.9999+ 99.9999+ 99.9997 99.9999+ 99.9999 +
Pentachloroethane Tetrachloroethene 99.9999+ 99.9999+ 27.0164 99.9999+ 99.2702
Chlorobenzene Benzene 99.9999+ 99.9999+ 92.2267 99.9999+ 99.9223
1P1C destruction calculations based on analytical solution of the following reaction scheme :
POHC •>- PIC -*f Combustion Endproducts
2 DE calculated for optimal flame conditions (Tf = 2780°F, t, = 0.1 s).
3 DE calculated for optimal post-flame conditions (Tp( = 2300"F, tp( = 2.0 s).
4 DE calculated for suboptimal post-flame conditions (Tpt = 1340°'F, tpf 2.0 s).
5 Overall DE based on 99% destruction of the waste experiencing flame conditions and 1%
of the waste bypassing the flame but experiencing optimal post-flame conditions.
6 Overall DE based on 99% destruction of the waste experiencing flame conditions and 1%
of the waste experiencing failure post-flame conditions.
Hypothesis 1—Flame Reactions
Control Bulk or Absolute
Emissions of PICs While Post-
Flame (Thermal) Reactions
Control Relative Emission Rates
This hypothesis is based on the
reasonable assumption (Theory 2) that
flame temperatures are high enough that
any material experiencing nominal
oxidizing flame conditions will be totally,
destroyed (POHCs and PICs) (Table 2,
column 3). The small fraction of waste
that somehow escapes the flame is
responsible for PIC emissions. Since
compounds fed as mixtures will
experience very similar destruction
conditions, the'relative thermal stability
(oxidative or pyrolytic) of the escaping
POHCs will determine their relative
emission rates.! In a similar manner, the
propensity for formation of PICs and their
stability under post-flame conditions will
control their rjelative emission rates
(Table 2, column 7). Since the bulk of the
material is destroyed in the flame,
operating ancl design parameters
affecting the flame will control overall
waste destruction efficiency. Post-flame
conditions may, however, apparently
control the relative DEs of POHCs and
relative PIC yields.
There are, however, insufficient data on
the range of temperature, residence
times, and oxygen concentrations that
can potentially exist in an incinerator
flame to absolutely state that no PICs are
emitted from the flame. Although a two-
zone system seems to be conceptually
useful, there is also a transition zone
between the flame and post-flame zones
where the distinction is blurred. In
addition, differing vaporization rates of
compounds can, in principle, result in not
all species experiencing the exact same
combustion conditions. Preliminary
calculations suggest, however, that
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droplet vaporization does not significantly
affect relative DEs.
Hypothesis 2—Most PIC
Emissions are the Result of
Pyrolysis Pathways
This is essentially a narrowing of the
focus of the failure mode concept
presented as the second theory of PIC
formation. Theoretical calculations
concerning the oxidation of the most
stable organic PICs indicate that
temperatures below 1450°F for post-
flame residence times of 2.0 s (Table 2,
columns 5 and 7) or residence times less
than 0.1 s, for post-flame temperatures of
2300°F are necessary for measurable
PIC breakthrough. Since these
temperatures are so low, it seems
reasonable that pyrolysis pathways
(which are much slower than oxidation
reactions) may be responsible for a large
fraction of POHC and PIC emissions.
This hypothesis has been partially
supported by comparison of actual
versus predicted CO emissions. Since
CO is only destroyed under pxidative
conditions (primarily by OH), its stack
concentration may be considered an
indicator of the fraction of the incinerator
that is acting as an oxidizer. Theoretical
calculations indicate that CO levels
should be on the order of a few ppb
instead of the observed levels of 1 to 100
ppm, suggesting a significant fraction of
the CO does not experience an oxidative
environment. Full-scale emissions of
undestroyed POHCs also agree best with
a theoretical prediction of POHC stability
based on pyrolysis kinetics.
Recent bench-scale experiments
simulating the burning of plastics in a
rotary kiln suggest that rapid thermal
degradation results in a "transient puff"
of hydrocarbons and chlorocarbons that
can overload the system such that
intimate waste/air mixing is not
completed. Consequently, high
concentrations of various PICs and soot
are observed, apparently due to a
transient pyrolysis condition.
Some researchers have argued that
upset pyrplysis conditions in full-scale
incineration facilities actually result in
lower PIC remissions. Other researchers,
however, have shown that the high levels
of soot that- are formed may act as a
reservoir for the PICs by surface
absorption. Although air emissions may
be mitigated by a high efficiency
particulate control device, the total PIC
yield and environmental burden is
increased. The impact is simply shifted
from the air to the land.
A significant number of cases exist,
however, where emissions do not
correlate with pyrolysis kinetics. In these
instances, relative POHC and PIC
emission rates may be facility specific.
For example, very low temperature
excursions may exist for which slower
oxidation reactions can still contribute to
emissions. Poorly atomized aqueous
wastes could result in slow vaporization
of water soluble species, resulting in a
residence time failure.
Hypothesis 3—A Significant
Fraction of the Observed PICs
are Formed Outside the High
Temperature Zones
A detailed study of the types of PICs
emitted from full-scale hazardous waste
incinerators, boilers, and kilns suggests
that as high as 50% of observed PICs are
likely formed by radical-radical and
radical-O2 association reactions (Table 3).
These types of reactions are important as
the system temperature is lowered
because high activation energy
unimolecular and radical-molecular
reactions are not favorable. This may
explain why thermally fragile PICs such
as chloroform, 1,1,1-trichloroethane, and
carbon tetrachloride are observed in the
effluent of full-scale systems. In addition,
it has been proposed and limited full-
scale data suggests that formation of
poly chlorinated dibenzo-p-dioxins
(PCDDs) is due to surface catalyzed
reactions in the cool zones of the
incinerator (i.e., transfer ducts, air
pollution control devices).
Some studies, however, show evidence
of analytical artifact whereas others
suggest that volatile, fragile chlorocarbon
PICs may be stripped from contaminated
scrubber water. Both laboratory and full-
scale research on the mechanism of
PCDD formation is incomplete, and no
quantitative study of gas-phase formation
of PCDD from possible precursors ha's
been performed.
Hypothesis 4—A Significant
Fraction of Observed PICs are
Due to Fuel/Waste Interactions
Analysis of full-scale data reveals that
many of the observed PIC emissions are
aromatic and polynuclear aromatic
(PNAs) species, i.e., benzene, toluene,
naphthalene, etc. These compounds are
commonly observed in the effluent from
most combustion devices. Further
analysis, however, indicates that the
presence of chlorine-containing wastes
increases the emission of these species
as well as that of partially chlorinated
hydrocarbons. Although chlorination of a
stable hydrocarbon is kinetically
unfavorable, chlorinated radicals may
participate in condensation-type
molecular growth reactions resulting in
formation of chlorinated aromatic species
and PNAs. Chlorine can also increase
overall PIC emissions through the
traditionally proposed effect of flame
inhibition.
Not enough parametric full-scale
studies have been performed to confirm
the hypothesis, however. Although
preliminary laboratory studies support
this concept, the chemistry of chlorinated
hydrocarbons and other hazardous
materials is only beginning to be
addressed.
Hypothesis 5—Carbon Monoxide
and Total Unburned
Hydrocarbons are Surrogates
for PIC Emissions
From a scientific viewpoint, the
emission rate of a surrogate should
correlate (viz., have a statistically
significant relationship through a
continuous function) with the emission
rate of a specific PIC or total PICs. From
a regulatory viewpoint, it appears that a
reasonable surrogate must only have a
concentration greater than that of the PIC
of interest such that, if the CO
concentration is below a given level, the
PIC will be below a given environmentally
acceptable level.
The concept of CO as PIC surrogate
arises from the fact that CO is more
stable than any organic PIC. Total
unburned hydrocarbons (TUHC) has
been proposed as a PIC surrogate
because TUHC, in principle, is the total of
undestroyed POHC and organic PICs.
Results of some full-scale tests indicate
that CO and TUHC meet the regulatory
definition of a surrogate for some PICs at
some facilities. A statistically significant
rank/order correlation between CO,
TUHC, and various measures of PIC
emissions was observed at a few sites.
This is encouraging considering the
limited range of conditions tested and the
number of data points available.
The concept that the extreme stability
(slow destruction kinetics) of CO makes it
a suitable PIC surrogate is complicated
by the fact that the concept does not
recognize the formation kinetics of CO
versus organic PICs. Under some failure
conditions, PIC yields may be high
whereas CO formation has yet to reach
its'maximum.
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Barry Dellinger, Philip H. Taylor, and Debra A. Tirey are with the University of
Dayton Research Institute. Dayton. OH 45269.
C. C. Lee is the EPA Project Officer (see below).
The complete report, entitled "Minimization and Control of Hazardous Combustion
Byproducts," (Order No. PB 90-259 854/AS; Cost: $39.00, 'subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Spring field. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information :
Cincinnati OH 45268
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