United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-86/006 July 1986
&EPA Project Summary
PIC Formation Under Pyrolytic
and Starved Air Conditions
Barry Dellinger, Douglas L. Hall, John L. Graham,
Sueann L. Mazer, Wayne A. Rubey, and M. Malanchuk
A comprehensive program of laboratory
studies based on the non-flame mode of
thermal decomposition produced much
data on Products of Incomplete Combus-
tion (PIC) formation, primarily under
pyrolytic and starved air conditions.
Most significantly, laboratory results
from non-flame studies were compared to
those from various field tests to evaluate
incinerabilrty relationships. Measurement
of gas-phase thermal stability in an at-
mosphere of low oxygen concentration
yielded results of incinerabilrty ranking that
were far more consistent with the findings
from field tests than any one of several
common methods applied in the past such
as those that employed heat of combus-
tion, autoignition temperature, etc.
The results of four experimental studies
were presented as significant contribu-
tions to developing/expanding the data
base on Principal Organic Hazardous Con-
stituent (POHC) stability and PIC forma-
tion for pure compounds and mixtures.
This Project Summary was developed
by EPA's Hazardous Waste Engineering
Research Laboratory, Cincinnati, OH, to
announce key findings of the research pro-
ject that Is fully documented In a separate
report of the same true (see Project Report
ordering information at back).
Introduction
The ultimate goal of incineration
research is to understand the process of
incineration to the extent that one can ac-
curately predict incinerator emissions and
to determine the effect of changing design
and operational parameters upon pollutant
emission rates.
Emissions of hazardous organic com-
pounds fall into two general categories,
those compounds in the waste feed that
are not totally destroyed and those com-
pounds formed from the partial deg-
radation of the waste compounds.
Designations for these classes are Prin-
cipal Organic Hazardous Constituents
(POHCs) and Products of Incomplete
Combustion (PICs), respectively. Since
regulation of incineration will always re-
quire some type of testing or monitoring
of the actual incinerator, a desirable pro-
duct of research would be information that
can be used to simplify the testing pro-
cedure and ensure that the proper emis-
sions and operating parameters are being
monitored that can provide environmental-
ly safe waste disposal.
The complexity of the incineration
process, the differences in incinerator
designs, and the difficulties in monitoring
changing operating conditions make the
accurate prediction of absolute incinera-
tion performance an essentially impossible
task. A more reasonable goal is to be able
to predict the relative destruction efficien-
cy of POHCs and the relative emission rate
of PICs for a given incinerator. This goal
is consistent with that of reducing the
need for incinerator testing, since one
could then simply conduct tests focusing
on the least "incinerable" POHCs and the
PICs of greatest yield as predicted by
laboratory testing and research. If these
compounds are found to meet regulatory
requirements then presumably so would
the other POHCs and PICs. Of course, one
must have sufficient knowledge of the ef-
fect of incineration parameters on POHC
and PIC emissions to correctly define the
conditions for the laboratory and field
studies and allow for subsequent changes
in these conditions on the incinerator.
Laboratory and field testing under
"worst" case conditions would appear to
be the best means of assuring continuing
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incinerator compliance. Once initial com-
pliance has been established, a method of
monitoring for continuing compliance is
also necessary. This defines a second goal
of the research program, which is to iden-
tify appropriate species or operating
parameters for continuous compliance
monitoring.
Both of the above-mentioned goals can
gain considerable support from develop-
ment of a simple, qualitative incineration
model for determining the major effects
upon emissions from changing incinerator
conditions.
In determining the destruction efficien-
cy (DE) of hazardous organic materials by
incineration, primary emphasis is put on
the gas-phase chemistry, although the
nature of the physical change of material
from the condensed phase into the gas-
phase may be important. The overall
gas-phase reactions and interactions de-
pend upon both direct flame and thermal
decomposition modes of the combustion
process.
Flame-mode and also thermal decompo-
sition mode studies indicate that any
known organic waste can be destroyed in
an incinerator to greater than 99.99% DE
if it is operating under theoretically op-
timum conditions. Excursions from the
optimum (fault modes) are probably the
controlling phenomena for incineration ef-
ficiency. Only a very small fraction of the
total volume of the waste needs to exper-
ience these less than optimum conditions
to result in significant deviations from the
targeted destruction efficiencies.
The two modes are found in a two-zone
incineration model such as a simple one-
stage combustor where a waste feed mix-
ture is fed directly into a turbulent flame
and the hot gases evolving from the flame
zone pass on through a relatively long,
high temperature hold-up zone prior to
exiting from the system. Because of
various destruction failure modes in the
flame zone, it is assumed in this model that
about 1% of the waste feed escapes the
bulk reaction conditions in the flame. This
1 % enters the post-flame zona The overall
measured DE at the stack is the weighted
averages of the DEs of the flame and the
post-flame zones.
Calculated DEs for representative haz-
ardous organic compounds are presented
in Table 1. The table shows that each com-
pound is destroyed to essentially the same
efficiency in the flame, i.e., greater than
99.99%. It is the significant differences
in thermal stability of the organic com-
pounds in the post-flame zone that can af-
fect the overall DE as adversely as shown.
Table 1. Calculated Destruction Efficiency for Representative
Hazardous Organics
Calculated Destruction Efficiencies
Compound
Acetonitrile
Benzene
Chloroform
Tetrachlorobenzene
Tetrachloroethylene
Trichlorobenzene
A
Is'')
4.7x107
2.8 xlO8
2.9 xlO'2
1.9 xlO6
2.6X106
2.2x10"
Ea
(kcal/mole)
40
38
49
30
33
38
DE
(Flame)
99.999+
99.999+
99.999+
99.999+
99.999+
99.999+
DE
(Post-Flame)
66.357
99.999+
99.999+
98.556
77.127
99.968
DE
(Overall)
99.664
99.999
99.999
99.986
99.771
99.999
• The applicability of this qualitative
model has been confirmed by a more com-
plex model of hazardous waste incin-
eration developed by the Energy and
Environmental Research Corporation
(EERC). Pseudo-first order thermal decom-
position kinetics developed by the Univer-
sity of Dayton Research Institute (UDRI)
were used as inputs for the EERC model.
Modeling results for three pilot-scale
hazardous waste thermal destruction
systems have been obtained; in each case
the predicted relative destruction efficien-
cies correlated almost perfectly with the
values for T9999(1) (temperature for
99.99% destruction at 1.0 sec residence
time) of the test compounds that were
developed from the UDRI results using
pseudo-first order kinetics. The excellent
agreement between the ranking according
to T9999(1) and the EERC model was as
predicted by the two-zone incineration
model, thus illustrating the importance of
post-flame reaction kinetics.
Comparisons of UDRI generated labor-
atory flow reaction (non-flame) data with
laboratory flame-mode data indicate the
similarity in the reaction mechanisms for
both zones, i.e., a free-radical degradation
mechanism. These results suggest that
many PICs can be formed from simple
feed mixtures and the POHC DEs and the
PIC yields may depend much upon the
waste composition and oxygen level of.the
reaction atmosphere. A detailed compar-
ison of field and laboratory studies further
reveals the importance of PIC emissions
in determining incinerator performance
and how laboratory data can be used to
predict PIC fofmation.
Experimental Procedure
The bulk of the experimental laboratory
data presented throughout the full report
was generated at UDRI by use of the Ther-
mal Decomposition Analytical System
(TDAS) and the Thermal Decomposition
Unit-Gas Chromatograph (TDU-GC)
system. A block diagram. Figure 1, illus-
trates the general arrangement of equip-
ment in the TDAS. The same arrangemer
applies to the TDU-GC, except for th
coupled Mass Spectrometer (MS) and h
accessory equipment that was used in th
TDAS for identification of the emission
components. Each system is a closed it
line combination of two basic units, the
mal reactor and analyzer.
The thermal reactor incorporates
capillary quartz tube within a furnace wil
three heating zones that are independent
controlled to produce temperatures up 1
1150°C in the central zone.
The sample insertion chamber is fitte
with any one of several probes adapted 1
handle gas, liquid or solid samples. In ar
test, the sample enters the thermal reai
tor in the gas phase. A heating jacketi
the insertion chamber tube provides eli
vated temperatures programmed by a coi
troller to convert liquid and solid samplt
to the vapor phase. The vapor is com/eye
to the reactor by a measured flow of ca
rier gas which is selected according to tr
nature of the atmosphere required in th
high-temperature zone of the reactor. Ai
cording to the temperature and pressu
measured in the reactor tube, the carrii
gas is regulated at the instrument conso
to result in a precise residence time of tr
vaporized/gaseous sample in the closel
controlled high-temperature zone. Tr
gaseous emissions from the reactor pa:
to a cryogenic trap at the head of tr
chromatographic column.
The analyzer, a gas chromatograph (G<
which may be coupled with a mass spe
trometer, is fitted with a fused silk
capillary column leading to a flame ioniz
tion detector (FID) in the case of tl
TDU-GC system, or a coupled mass spe
trometer in the case of the TDAS syster
A 30:1 splitter between the furnace ar
the GC directs only a small portion (~3^
of the effluent sample to the capillary c<
umn that is needed for high-resoluti<
analysis.
The auxiliary units of computer ai
recorder provide a means of storing tl
output from the analyzer detector ai
depicting it in a chromatogram or spe
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High Temperature Transfer
Capture
of
Effluent
Products
Controlled
High
Temperature
Exposure
Sample
Insertion
and
Vaporization
Pressure and
Flow Regulation
Compressed Gas
and Purification
In-Line
Gas Chromatograph
(High Resolution)
Coupled
Mass
Spectrometer
(Magnetic)
Computer
System
NIH-EPA
Chemical
Information
System
Figure 1. Block diagrams of the TDAS.
Analysis of Effluent Products
trum according to the nature of the data
processor.
Each test run yields a single point on the
thermal decomposition plot (quantity of
compound in the effluent vs. temperature
of the thermal reactor, with the residence
time kept constant) for the POHC material.
Corresponding single points for each PIC
material formed during the thermal treat-
ment run are simultaneously obtained.
Runs made over a series of temperatures
can produce a thermal decomposition
profile of the POHC and formation-
decomposition profiles for various PICs,
within the temperature limits investigated.
Runs made over a series of residence
times provide data that can be used to
determine the Arrhenius equation values
of the constant, A, and the so-called ac-
tivation energy, Ea.
Results and Discussion
Results are reported for three major
studies, (1) comparison of flame-mode and
thermal decomposition (non-flame) mode
results, (2) correlation of laboratory-based
predictions and field study results, and (3)
expansion of data base on POHC stability
and PIC formation for pure compounds
and mixtures.
Comparison of Flame and
Thermal Decomposition Results
For the first study, 13 compounds of
wide interest to hazardous waste inciner-
ation were combusted in a laboratory dif-
fusion flame, from which the relative
burning rates of the compounds were
determined based on their flame front
velocities. For the six compounds for
which thermal decomposition data are
available from the UDRI flow reactor
systems, the non-flame ranking obtained
from T9999(2) (temperature for 99.99%
destruction at 2.0 seconds residence time)
was determined. Comparison of flame
with non-flame data showed that the rank-
ing of those six compounds was in the
same order for all six as listed among the
relative burning rates that were deter-
mined in the diffusion flame experiment.
In a second flame experiment, various
combinations of dichlorobenzene, benzene
and hydrogen chloride were combusted at
40% of stoichiometric air. Most (n = 23)
of the observed PICs (n = 33), as com-
plex as some of them were, were also
found as PICs from the thermal decompo-
sition of a mixture of carbon tetrachloride,
toluene, chlorobenzene, trichloroethylene
and Freon 113. The production of like PICs
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from dissimilar molecular mixtures but
similar atomic ratios of carbon, hydrogen
and chlorine, in flame and in thermal
decomposition environments suggests
that similar reaction mechanisms are
operative in both processes. At temper-
atures above 450°C, a free-radical
mechanism appears to predominate the
attack of molecular species, such as
peroxide, that has been observed at lower
temperatures.
The correlation between flame-mode
and non-flame flow reactor POHC and PIC
data indicates free radical decomposition
reactions taking effect in both instances.
The good agreement in identical PICs,
even for dissimilar feed mixtures, supports
the case of free-radical mechanisms. The
majority of the products are due to recom-
bination of free-radical fragments or of
radical addition to aromatic substrates.
The lack of oxygen-containing products
even under oxidative conditions suggests
that abstraction of H by OH and 0
dominates over addition reactions.
Correlation of Laboratory
Predictions and Field Results
A comparison of various proposed
scales of incinerability with recently
available field test results was developed
that included "thermal stability" values
determined from the UDRI flow reactor
studies.
Six methods of ranking the relative in-
cinerability of hazardous organic com-
pounds were considered in this specific
evaluation:
1. Heat of Combustion (Hc/g)
2. Auto-Ignition Temperature (AIT)
3. Theoretical Flame-Mode Kinetics
(TFMK)
4. Experimental Flame Failure Modes
(EFFM)
5. Ignition Delay Time (IDT)
6. Gas-Phase Thermal Stability [T99
(2) (99% destruction at 2.0 seconds
residence time), TSHi02 (high ox-
ygen concentration), TSLoO2 (low
oxygen concentration)].
The TSHiO2 (oxygen-rich) and TSLo02
(oxygen-deficient) data were considered
when recently generated data showed
that relative thermal stability varied with
the waste feed/oxygen ratio.
To ensure a valid comparison of
predicted results as obtained from the
laboratory study with its precisely con-
trolled, well-defined conditions and of the
more general results of field study reports,
the following data validation and reduction
criteria were used:
• Compare only POHC Destruction and
Removal Efficiencies (DREs) for a
given incinerator.
• Compare only POHC DREs when
they are fed to the system at a com-
mon point.
• Use averages of DREs when no
significant run-to-run variation in
relative POHC ORE is observed.
• Use only data where the majority of
the POHC DREs are less than
99.995%.
• Include data from non-concurrently
fed POHCs if other key parameters
are held constant.
• Conduct the correlation of observed
field vs. predicted results on
rank/order basis with a minimum c
four data points.
The observed incinerability rankings c
the test compounds at each source wei
compared with the prediction of each pr<
posed ranking method using a rank/ord<
correlation approach. The rank-correlatic
coefficient, rs, was used to judge if a co
relation existed at the 90% confidenc
level for a number of test compounds, f
Table 2 summarizes the results of sue
analysis for ten studies judged to meet th
data validation criteria.
Of all the ranking methods propose
above, only Hc/g, AIT, T99(2), TSHi02 an
TSLoO2 had a sufficient data base 1
make predictions for a significant numb<
of sources. Of these, only the experimei
tally predicted order under low oxyge
conditions, TSLoO2, had a reasonab
success, i.e., 70%. The other four methoc
only correlated with field observatior
10-20% of the time.
Observed deviations from laborator
predicted rankings of incinerability migl
be attributed to product formation <
"contamination" of the stack effluent t
volatile POHCs that did not pass throug
the destruction zones of the incinerate
or even perhaps to volatile POHCs strippc
from scrubber waters at the time of me.
surement of stack gas concentrations.
The following conclusions are offere
• Measured POHC DREs and relati\
stabilities of all but the most stab
compounds are influenced by the
formation as products from oth
components of the waste feed and/
fuel.
Table 2. Results of Statistical Analyses of Observed Versus
Predicted Thermal Stability Rankings
Heirarchy
Study
A
B
C
D
E
F
G
H
1
J
# Of Successes
# Of Failures
% Success*
Hc/g
-0.300/5'
-0.190/8
-0.500/5
-0.100/9
0.589/7"
0.343/15
0.400/4
-0.333/7
-0.077/10
-0.291/10
1
9
10
AIT
-0.200/4
0.200/4
—
-0.060/
0.428/6
0.571/7*
—
0.457/6
-0.262/8
0.147/8
1
8
11
TFMK
—
—
—
_
_
-o.roo/5
_
—
0.600/4
0.800/4*
1
2
33
EFFM
_
_
—
_
_
—
_
—
0.600/4
0.600/4
0
2
0
IDT
—
—
_
_
—
—
—
-0.300/4
-0. 100/5
-0.100/5
0
3
0
T99<2)
—
-0.057/6
0.500/5
-0.800/4
-0.300/5
-0.425/9
0.800/4*
-0. 16 1/7
-O.217/9
-0.202/9
1
8
11
TSHiO2
0.000/5
0.533/10*
0.400/5
0.386/9
0.425/8
0.041/15
0.800/4*
-0.036
-0.318/11
-0.114/11
2
8
20
TSLoO
0.900/5
0.529/1<
0.600/5
0.493/9
0.429/6
0.073/1
0.900/4
0.655/6
0.536/1
0.523/1
7
3
70
1rs/N
*Correlation was statistically significant at the 90% confidence level.
4
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• Only DREs for very stable POHCs or
POHCs difficult to form as reaction
products lag., acetonitrile) are ex-
pected to be unaffected by PIC for-
mation and these stabilities are
predictable from pure compound
thermal decomposition kinetics.
• The stack emissions and observed
DREs of the very volatile compounds
lag., methylene chloride, chloroform,
di- and trichloroethanes) may be in-
fluenced by fugitive emissions in the
ambient air or stripping of these com-
pounds from contaminated scrubber
water.
• Thermal decomposition, not in-flame
destruction, determines relative
POHC DREs and the identity and
yield of products of incomplete
combustion.
• Pyrolytic conditions in the incinerator
are responsible for most emissions
and control the relative DREs of
POHCs and the formation of
products.
• Results from laboratory thermal
decomposition testing of pure com-
pounds and mixtures can be effec-
tively used to predict relative POHC
DREs.
• Laboratory testing under pyrolytic
conditions on actual waste streams
or closely simulated waste streams
is a potentially effective and reliable
method for predicting relative POHC
stabilities and PIC emissions.
Expansion of Data Base on POHC
Stability and PIC Formation for
Pure Compounds and Mixtures
Contributing to an expanding data base
that can be used to predict the effect of
changing reaction atmosphere and waste
composition, the results of experimental
studies like the following can be quite
useful.
1. Thermal Decomposition of "CRF
Soup-1"*
Thermal degradation of a mixture of
Freon 113, carbon tetrachloride,
trichloroethylene, chlorobenzene and
toluene was conducted in three reaction
* Simulated Hazardous Waste Mixture #1 tested
recently at EPA's Combustion Research Facility (CRF)
in Pine Bluff, Arkansas.
atmospheres: oxygen-rich (<)> = 0.06),
stoichiometric oxygen (<(> = 1.0) and
oxygen-starved (pyrolysis). The T99(2) °C
(temperature for 99% destruction at 2.0
seconds residence time) thermal stability
values for each compound in mixture and
as pure compounds were compared. It
was clear that carbon tetrachloride and
Freon 113 were little affected by the dif-
ferences in oxygen concentration in the
test atmosphere. The other three com-
pounds (trichloroethylene, chlorobenzene
and toluene), however, showed con-
siderable differences in absolute and in
relative thermal stabilities as a function of
both oxygen concentration and of feed
composition (pure compounds vs.
mixture).
Unimolecular decomposition by simple
bond ruptures would explain the in-
dependence of the thermal stability of car-
bon tetrachloride and Freon 113 from
changes in the reaction atmosphere. In
contrast, the degradation of trichloro-
ethylene, toluene and chlorobenzene is
more likely to be caused by their interac-
tion with free-radicals such as OH, 0 and
H. Since the absolute and relative concen-
trations of these species will vary with the
oxygen concentration and waste composi-
tion, the measured thermal stabilities will
also vary with changing reaction
atmosphere.
2. Formation of PCDFs and other PICs
from PCBs
Thermal degradation of 2,3',4,4',5-
pentachlorobiphenyl (2,3',4,4',5-PCB) was
conducted in various reaction at-
mospheres (oxygen equivalence ratio <|> =
3.0, 1.0, 0.2, and 0.05, oxygen-starved to
oxygen-rich concentration, respectively) at
a gas-phase residence time of 2.0
seconds, and at temperatures ranging
from 500 to 1000 °C.
With the increase in oxygen concentra-
tion through the range shown, the yield of
total PCDFs increased by a factor of 7.
Thermal decomposition profiles based
on the yield data showed that the degrada-
tion rate of the feed 2,3', 4,4',5-PCB rapid-
ly increased above approximately 750°C.
With the degradation of the PCB, there
was an increasing production of the
PCDFs. According to pseudo-equilibrium
calculations of the concentration of small
reactive species, the concentration of
reactive radicals such as OH and 0 in-
creases rapidly between 700 °C and
900 °C. Since incorporation of oxygen is
needed to form PCDFs from PCBs, it is
likely that the OH and 0 radicals are the
predominant reactive species responsible
for PCDF formation. The subsequent
degradation of the formed PCDFs, starting
at 850-875 °C and undergoing total
elimination along with the parent PCB at
1000 °C indicates still some other reaction
mechanism taking hold to destroy the
PCDFs at the relatively high temperature
range of 850-1000 °C.
3. Formation of PICs from Chloroform
Thermal decomposition of chloroform
(CHCI3) at an equivalence ratio <(> = 0.76
and at 2.0 seconds residence time showed
formation of pentachloroethane (C2HCI5)
at lower temperature levels and shortly
afterwards the formation of tetrachloro-
ethylene (C2CI4). However, with increas-
ing temperature the pentachloroethane
then decomposed at a rate approaching
that for the chloroform and in the same
temperature range, indicating comparable
thermal stability for these two com-
pounds. The tetrachloroethylene, on the
other hand, increased to yields almost
equaling the initial quantity of parent
chloroform. It is likely that the product
pentachloroethane plays an intermediate
role in the chloroform thermal decomposi-
tion pathway that eventually produces the
tetrachloroethylene.
In the initial step it is indicated that the
biradical dichlorocarbene (:CCI2) is form-
ed along with HCI. The dichlorocarbene
may then react with chloroform through
insertion in the C-H bond to form pen-
tachloroethane. Another proposed reac-
tion of dichlorocarbene is its combination
with another such radical to form
tetrachloroethylene.
4. Formation of PICs from Polychlori-
nated Phenols
The thermal decomposition of
chlorophenols is of intense interest
because of the potential formation of
polychlorinated dibenzodioxins (PCDDs)
as PICs. Thermal decomposition profiles
were developed for pentachlorophenol
(PCP) in nitrogen, pentachlorophenol in air,
and 2,4,5-trichlorophenol (2,4,5-TCP) in
nitrogen.
While there were various major PICs
observed at selected reaction temper-
atures on the TDAS, the production of
octachlorodibenzo-p-dioxin (OCDD) was
determined by retention time on the TDU-
GC, after which identification was con-
firmed by examining low-level peaks on the
TDAS. The maximum yield in air (~1%)
was seen at 500 °C, while the maximum
-------
yield in nitrogen (~1.5%) was seen at
550 °C.
5. Expansion of Pure Compound
Kinetic and Thermal Stability Data
Base
Pseudo-first order oxidation kinetic data
was generated for various pure com-
pounds. The Ea and A values were
calculated for 23 compounds ranked by
the T99(2) calculated from those values.
Barry Dellinger, Douglas L Hall. JohnL Graham, SueannL Mazer, and Wayne A.
Rubey are with University of Dayton Research Institute, Dayton, OH 45469.
Robert E. Mournighan is the EPA Project Officer (see below).
The complete report, entitled "PIC Formation Under Pyrolytic and Starved Air
Conditions," (Order No. PB 86-145 422/AS; Cost: $11.95, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
-------
United States Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-86/006
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