United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-138 Oct. 1984 % f\ \v
Project Summary
Determination of the Thermal
Decomposition Properties of 20
Selected Hazardous Organic
Compounds
Barry Dellinger, Juan L. Torres, V\
Graham
Laboratory-determined thermal dec
position profiles and kinetic data for a
list of 20 selected hazardous organic
compounds are reported. All data were
obtained in flowing air at mean jias-
phase, high-temperature zone reside nee
times ranging from one to six secoi
The extrapolated temperatures requ
for 99.99% destruction of the pa
compound at two seconds mean resi-
dence time, T9999(2), ranged fiom
600°C for 1,1,1-trichloroetham to
950°C for acetonitrile. The processes
and parameters potentially controlling
incineration efficiency are discussed,
and four previously proposed methods
of ranking compound incinerability are
critically reviewed.
The possible chemical mechani
for destruction of hazardous org
compounds are examined and use
explain trends in the experimen
ayne A. Rubey, Douglas L. Hall, and John L.
ed
ent
ms
nic
to
ally
determined thermal decomposition
data. It is proposed, through proper
application of the principles of organic
chemistry, kinetics, and physics that
laboratory, gas-phase thermal decompo-
sition data generated under controlled
conditions can be incorporated into
models of full-scale incineration, can
serve as a viable ranking of waste
incinerability, and can be used to predict
the formation of products of incomplete
combustion.
This Project Summary was developed
by EPA's Industrial Environmental Re-
search 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).
ds. Introduction
The ultimate goal of hazardous waste
incineration is to destroy the waste with
as high a destruction efficiency (DE) as
possible. Under the Resource Conserva-
tion and Recovery Act (RCRA) of 1976, an
incinerator operator must show that the
facility can adequately destroy those
hazardous waste constituents which are
most difficult to incinerate. In theory, the
permit writer selects compounds within
the mixture that are of sufficient con-
centration and thermal stability to be
designated as principal organic hazardous
constituents (POHCs). It must then be
shown, possibly by trial burn, that the
designated POHCs can be destroyed or
removed by the particular incineration
system to a destruction and removal
efficiency (ORE) of 99.99 percent. Further-
more, the specific operating conditions
must be established under which the
99.99 percent ORE is achieved.
The development of a ranking of the
incinerability for compounds that are
candidates for POHC selection is of
obvious utility. The U.S. Environmental
Protection Agency (EPA) is currently
using a ranking based on chemical com-
pound heat of combustion per gram. This
method has received considerable criti-
-------
cism, and the development of an alter-
native ranking scheme is of a very high
priority to the EPA.
Experimentally determined gas-phase
thermal stability under controlled labo-
ratory conditions has been proposed as
an alternative ranking method.This report
presents the results of the laboratory
determination of the gas-phase thermal
decomposition properties of twenty haz-
ardous organic compounds. The com-
pounds were selected by EPA based on
their frequency of occurrence in hazard-
ous waste streams, apparent prevalence
in the stack effluent, and representative-
ness of the spectrum of hazardous organic
waste materials.
Thermal decomposition profiles were
determined at various reactor residence
times in an atmosphere of flowing air,
which enabled the calculation of global
thermal decomposition kinetic param-
eters. The results of these laboratory
measurements are then compared with
previously proposed ranking procedures
and an attempt is made to correlate this
data with various parameters which may
relate to gas-phase thermal stability. The
chemical mechanisms potentially respons-
ible for thermal decomposition of the
twenty test compounds and their poten-
tial use in explaining trends in the data
and predicting the relative thermal stabil-
ity of untested compounds are discussed.
Experimental Procedures
All of the experimental data pre-
sented in this report were generated on
the thermal decomposition unit-gas chro-
matographic (TDU-GC) system designed
and built with funding provided by the
EPA (Cooperative Agreement No. 807815-
01 -0). Samples of the twenty compounds
were prepared and introduced into the
system by several procedures depending
upon their physical state and vapor
pressure.
As seen in Figure 1, the TDU-GC
system is a closed, in-line system. In-
stream instrumentation measures pres-
sure and flow accurately. Most of the
instrumentation controls are located in a
console from which test functions can be
continuously monitored.
To initiate a test, the sample is intro-
duced into the system and gradually
vaporized in a flowing gas stream (i.e.,
nitrogen, air, or nitrogen/oxygen mix-
tures). The vaporized sample passes
through a controlled, high-temperature
tubular reactor where it undergoes ther-
mal decomposition. The products of the
thermal decomposition of the compound
and the remaining parent compound are
swept into a Varian VISTA 4600 high-
resolution gas chromatograph for anal-
ysis. The sample insertion chamber, the
reactor, and the entire transport system
are fabricated of fused quartz to minimize
interaction with the sample.
The rate at which molecules are admit-
ted into the high-temperature reactor is
important in thermal decomposition stud-
ies. In the TDU-GC system, the sample is
deposited into a sample insertion cham-
ber packed with quartz wool. The chamber
is initially kept at or below room temper-
ature. The chamber is slowly heated to
250-300°C by applying a linear temper-
ature program (10-20 °C/min). The sam-
ple molecules are thermally desorbed and
gradually swept into the thermal reactor.
One of the most important components
of the TDU-GC system is the high-tem-
perature reactor. The major portions of
the reactor consists of a narrow-bore
nominal 1 mm ID quartz tube flowpath in
a race-track configuration (3.5 cycles, 1
meter in length). The all quartz construc-
tion and racetrack configuration mini-
mizes the possibility of wall reactions
while simultaneously providing a narrow
residence time distribution and square
wave exposure temperature profile. In
addition, the reactor design has fine bore
entrance and exit tubes to transport the
sample rapidly into and out of the central
portion of the reactor. This design also
minimizes non-ideal temperature pro-
files. The quartz tube reactor assembly
fits into a high-temperature three-zone
Lindberg furnace designed for continuous
operation at temperatures up to 1,200°C
with control to ± 1 °C. The effluent from
the high-temperature reactor zone is
swept through a heated transfer line
toward a Varian 4600 GC with a Vista
CDS 401 dedicated data terminal. A 30:1
splitter between the furnace and the GC
directs only a small portion (—3%) of the
effluent sample to the capillary column in
order to prevent column overloading.
The sample emerging from the reactor
is trapped at the head of the chromato-
graphic column because the GC oven is
maintained at a cryogenictemperature(~
minus 30°C). A fused silica capillary
column was used in the majority of the
investigation. It was 15 meters in length
and contained a dimethylsilicone chemi-
cally-bonded stationary phase. A flame-
ionization detector was used in this
investigation. Throughout the investiga-
tion the signal/noise ratio for peak de-
tection was four (4).
For the ethane analyses, Tedlar bags
were used to capture the reactor effluent
at the splitter. The captured gas was
analyzed using a Varian Aerograph Series
1800 gas chromatograph equipped with
one meter x 4.0 mm ID packed column (5
X molecular sieve, 45/60 mesh) operated
isothermally at 150°C. This system was
utilized to insure resolution Of the three
Cz hydrocarbons.
Sample handling and preparation was
performed in the TDU-GC system's glove-
box compartment. Samples of each test
compound were prepared according to
individual characteristics. Use of solvents,
were avoided whenever possible to facil-
itate observation of products of incom-
plete combustion (PICs). However, for
solid samples requiring preparation in
solution, Eastman Spectra ACS Grade
cyclohexane was selected as the solvent.
Cyclohexane was chosen for its low
thermal stability and high volatility rela-
tive to the specific solutes. In the case of
1,2,3,4-tetrachlorobenzene, the sample
was prepared in a methylene chloride
solution. For some samples, the probe
was removed from the insertion chamber
and solutions were injected into the
quartz wool portion of the probe. After the
solvent was evaporated, the probe with the
deposited sample was returned to the
insertion chamberwhichwasthen heated
at a programmed rate. High vapor pres-
sure liquid-phase samples were prepared
at concentrations of 1Oppm(v/v) in air for
direct gas-phase injection into the inser-
tion chamber.
Results
For each of the twenty test compounds,
the fraction of the feed material unde-
stroyed at a given set of temperatures and
mean reactor residence times (tr) was
determined. This resulted in the genera-
tion of what may be termed thermal
decomposition profiles, i.e., a plot of
logarithms of the fraction remaining (fr)
vs. the reactor temperature (°C) at con-
stant residence time. Thermal decomposi-
tion profiles were generated in flowing air
at four mean residence times, tr=1.0,2.0,
4.0, and 6.0 seconds. An example of the
determination of this family of profiles is
given in Figure 2 for chloroform. The
thermal decomposition profile for chloro-
form is representative of the great major-
ity of the compounds tested and serves to
illustrate several features.
Below400°C, no measurable decompo-
sition occurs for chloroform (the flat
portion of the curve). Between 400°C and
525°C the rate of decomposition begins
to increase. Above 550°C, it increases
rapidly, resulting in an apparently linear
region of steep slope. The chloroform has
-------
Thermal Decomposition Unit
Capture of
Effluent Products
\
Controlled High
Temperature Exposure
Sample Insertion
and Vaporization
Pressure and
Flow Regulation
Compressed Gas
and Purification
High Temperature Transfer
Multifunctional Gas Chromatographic
Instrumentation
Containment or Destruction of
Effluent Products
Figure 1. Block diagram of the thermal decomposition unit-gas Chromatographic system.
100
t
.£
(0
CD
10
0.1
0.01
Figure 2.
CHLOROFORM
O tr • 1.0
D i, -Z.O
A tr -4.0
O V -6.0
-I
100 400 500 600
Exposure Temperature, °C *
700
Thermal decomposition profiles for chloroform in flowing air at mean residence
times of 1.0. 2.0. 4.0, and 6.0 seconds.
been destroyed with 99.99% efficiency by
620°C at a residence time of 2.0 seconds.
with 99.99% efficiency by 620°C at a
residence time of 2.0 seconds.
The data for the test compounds have
been summarized in Table 1 with entries
for the temperature for the onset of
decomposition, Tonset (2) (°C), the inter-
polated temperature for 99% destruction,
T99 (2) (°C), and the extrapolated temper-
ature for 99.99% destruction, T9999 (2)
(°C). All these values are for tr = 2.0
seconds in flowing air. With only the data
presented in this table, the thermal
decomposition profile for the compounds
may be approximately reconstructed. The
table lists the compounds in order of
decreasing temperature required for 99%
destruction efficiency. A slight reordering
occurs if T9999(2) is used for the ranking.
However, the numerical differences for
the reordered compounds are small.
For the conditions possibly encountered
during gas-phase thermal decomposition
in an incinerator (600°C to 1,400°C and
oxygen levels of 0.1 to 21 percent), two
possibly global decomposition pathways
predominate. The first is pyrolysis, for
which the rate of decomposition of the
parent species is independent of the
oxygen concentration. The second is
oxidation, for which the decomposition of
the parent is dependent both on the
oxygen concentration and susceptibility
of the parent species to attack by oxygen
or other oxidizing species.
The global expressions for these two
reaction schemes are
where:
ki and k2 are the global rate
constants for pyrolysis and
oxidation, respectively, and
a, a, and b, are the reaction
order for the decomposition of
species A with respect to A
and 02.
The time dependence is included in this
expression. The temperature dependence
is included in the rate constants for the
two processes. This may be expressed by
the Arrhenius equation:
k = A exp (-Ea/RT)
where: Ea is the activation energy for
the process, cal mole"1
A is the Arrhenius coefficient,
s-1
R is the universal gas constant,
1.99 cal mole"1 °K"1.
For each of the test compounds, when
the thermal decomposition reaction oc-
curred in an atmosphere with a large
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Table 1 . Summary of Thermal Decomposition Data
Compound
Acetonitrile
Tetrachloroethylene
Acrylonitrile
Methane
Hexachlorobenzene
1, 2,3,4- Tetrachlorobenzene
Pyridine
Dichloromethane
Carbon Tetrachlonde
Hexachlorobutadiene
1,2,4- Trichlorobenzene
1 ,2-Dichlorobenzene
Ethane
Benzene
Aniline
Monochlorobenzene
Nitrobenzene
Hexachloroethane
Chloroform
1, 1 , 1 -Trichloroethane
Empirical
Formula
0^3/V
C2C/4
CM
CH<
CaC/s
CeHiCU
CM
CH2C/2
ecu
C4C/6
CeH3C/3
CeHtC/2
C^6
Ce/Ye
CeHjN
Cer/sCl
CeHs/vOz
C2C/6
CHC/3
C^C/3
excess of molecular oxygen relative to the
concentration of the waste material, the
decomposition equation could be simpli-
fied to an expression that is first order in
tKA f*nnr*antratir\n rtf tho camnln %Afhiis*h
'onset (2)
760
660
650
660
650
660
620
650
600
620
640
630
500
630
620
540
570
470
410
390
Taa(2)
900
850
830
830
820
800
770
770
750
750
750
740
735
730
730
710
670
600
590
570
Taa 99 (2)
~ 950
920
860
870
880
850
840
780
820
780
790
780
785
760
750
780
700
640
620
600
pression for the required temperature for
a given level of destruction in an atmos-
phere of flowing air:
T - nm p ir, / ~l'A \
«.-,
4.7x10?
2.6x1 0s
1.3x106
3.5x10s
2.5x1 0s
1.9x10e
I.JxW5
3.0x10'3
2.8 x JO6
6.3xW'2
2.2x1 0s
3.0x10s
1.3x10*
2.8x10»
9.3xW'5
8.0x10"
1.4xW'5
1.9x10?
2.9x1012
1.9x1 0s
fa
(kcal/mole)
40
33
31
48
41
30
24
64
26
59
39
39
24
38
71
23
64
29
49
32
Calculated
Taa(2}(°C)
908
900
910
874
845
834
767
796
824
763
789
766
830
757
726
810
672
64 J
606
60 f
line with the slope equal to -Ea/R and an
intercept of 1 n A.
Regression analyses of this type have
been performed on each of the twenty
tact r+f\mr\nt mrlc In oil /"«aeao t Ka f i rot
resulted in the integrated pseudo-first
order rate expression:
f, = exp(-k2t,)
where: f, is the fraction of the parent
species remaining, and kz =
ka[02] is the pseudo-first order
rate constant
This expression may be combined with
the Arrhenius Equation to yield an ex-
where: TDE is the temperature required
for a given DE, °K, Ea has units
of kcal mole"1 and the other
variables are as previously
defined.
A plot of In fr vs. t, for the four residence
times will yield the rate constant for the
reaction at a given temperature. A plot In
k vs. 1/T for the four experimental
temperatures should then yield a straight
order kinetic plots yielded far better fits
than zeroth or second order plots. The
measured kinetic parameters, A and Ea,
along with the calculated temperatures
for 99% destruction are included in Table
1.
Discussion
Several methods have been proposed
for ranking the relative incinerability of
hazardous organic compounds. Research-
ers at the Mitre Corporation, in conjunc-
tion with EPA, have proposed a scale
-------
based on the heat of combustion per gram
molecular weight (Hc/gram of the pure
compound). Researchers at IT Enviro-
science have proposed a method based
on the laboratory determination of auto-
ignition temperatures (AIT) of the pure
compounds. Researchers at the National
Bureau of Standards(NBS) have proposed
a purely theoretical approach based on
the kinetics of flame-mode thermal de-
composition. Researchers at the Univer-
sity of Dayton and Union Carbide have
proposed scales based on laboratory-
determined rates of gas-phase thermal
decomposition of pure organic com-
pounds in flowing air. Other parameters
might be appropriate as a basis for such a
scale. The correlation of one scale of
incinerability with another would be
additional evidence of its validity, al-
though correlation with a broad range of
incineration data will eventually be re-
quired before a proposed ranking will
receive universal acceptability. Thus, an
attempt was made to correlate laboratory-
generated gas-phase thermal decomposi-
tion data reported in the previous section
with the previously proposed ranking
scales. This comparison was complicated
by lack of overlap of the compounds
investigated.
For the heat-of-combustion scale, the
only readily discerned agreement was the
general increase in thermal stability with
decreasing heat of combustion for the
chlorinated benzenes. Other than for this
group, there was no discernable trend of
agreement, either within a class or be-
tween classes of compounds.
There appeared to be a general positive
correlation for both T99 (2) and T99.99 (2)
with AIT for those compounds with an AIT
below 550°C. Above this temperature,
the gas-phase thermal stability appeared
to vary little with AIT. In essence there
appears to be a positive correlation for the
compounds which have a significant fuel
value as manifested through their low
autoignition temperature.
Direct comparison of thermal decompo-
sition data with the NBS scale was
hampered because only four of the twenty
compounds were ranked by the group at
NBS. However, the thermal stability of
the chlorobenzenes ranked by NBS fell in
the same order as suggested by the T99 99
(2) based ranking. Chloroform was pre-
dicted to be rather unstable due to the
relatively low-energy carbon-chlorine
bond. This thermal instability is evident
from the gas-phase thermal decompo-
sition results, and it is probably even
more fragile than predicted by the NBS
ranking.
Only five of the compounds were
studied at Union Carbide. The most
significant disagreement was for acrylo-
nitrile, the least stable of the five com-
pounds based on the Union Carbide
calculated T99.99 (2), although the data
from this study indicate that its thermal
stability rivals that of methane. Ethane is
slightly less stable than monochloroben-
zene, as measured by Union Carbide; this
trend is reversed in the present data,
although the difference is small. With the
exception of acrylonitrile, the two rank-
ings are similar, although the data from
this study predict the compounds to be
typically more stable by 40°C than that
predicted by Union Carbide.
No single proposed ranking scheme or
molecular parameter was identified that
correlated with all our thermal decompo-
sition data, although trends were observ-
able in homologous subgroups. Using the
principles of chemical reactions, it is
possible to explain the behavior of each of
the twenty compounds on a relative basis
and identify mechanisms that might be
used to extrapolate this limited data to
other compounds not studied in the
laboratory. The twenty test compounds
may be divided into five subclasses which
are discussed in the following paragraphs.
Methane, Dichloromethane,
Chloroform, and Carbon
Tetrachloride
The observed trend in this group is
decreasing thermal stability with increas-
ing chlorine substitution, except for car-
bon tetrachloride, which is intermediate
in thermal stability between methane and
dichloromethane. The carbon-hydrogen
bond in methane has a bond dissociation
energy (BDE) of 104 kcal/mole. The
carbon-chlorine bonds in the other three
compounds are 79,77, and 70 kcal/mole,
respectively, and the carbon-chlorine
bonds weaken with increased chlorine
substitution. This trend disagrees with
the experimental observations.
The data agree with a mechanism
based on abstraction of a hydrogen,
probably by OH. Since the carbon-hydro-
gen BDE decreases with increasing chlo-
rine substitution up to chloroform, one
would predict decreasing thermal stabil-
ity, which is in fact observed. However,
carbon tetrachloride contains no hydro-
gens and thus would not be susceptible to
this mode of attack. It would instead be
expected to decompose by bond rupture.
The implication of this data set is that H
abstraction reaction rates may be faster
at these temperatures than previously
expected.
Benzene, Monochlorobenzene,
1,2-Dichlorobenzene,
1,2,4-Trichlorobenzene,
1,2,3,4-Tetrachlorobenzene,
Hexachlorobenzene, Pyridine,
An/line, Nitrobenzene
The observed trend is toward increasing
thermal stability with increasing chlorine
substitution. Pyridine is more stable,
nitrobenzene less stable, and aniline is
about as stable as benzene. All of the
bonds in benzene, the chlorinated ben-
zenes, and pyridine are probably in excess
of 90 kcal/mole, and one would expect
electrophilic addition to be the predomi-
nant reaction path. The chlorines and the
nitrogen in pyridine are more electro-
negative than hydrogen or carbon, which
leads to a destabilization of the electron-
deficient intermediate resulting from OH
addition; thus, the rate of decomposition
is reduced and a greater thermal stability
results than for benzene.
The stabiliy of aniline and nitrobenzene
relative to benzene can also be explained
by electrophilic attack by a radical such as
OH. However, the effect of resonance
interaction is somewhat different than in
normal electrophilic attack by a cation.
The radical intermediate formed by nitro-
benzene is actually stabilized by reso-
nance. This is opposed to the norm for
cationic electrophilic addition, where
nitro substitution results in destabiliza-
tion because a positive charge is placed
on the electronegative oxygen. This sta-
bilization of the intermediate again leads
to a less stable molecule relative to
benzene. Also in nitrobenzene, the nitro-
gen carbon BDE is 70 kcal/mole which
may be easily broken and does represent
an altenative mode of decomposition.
The radical intermediate formed by OH
attack on aniline would not receive signif-
icant resonance stabilization due to lack
of an octet on the normally very stable
resonance structure involving the lone
pair on the nitrogen. On this basis, one
would expect aniline to be about as stable
as benzene, which is observed. The
nitrogen-hydrogen BDE is only 80 kcal/
mole and may have some role in the
decomposition; however, the similarity in
stability, as opposed to that of benzene,
indicates that electrophilic addition is the
predominant mode of destruction.
Ethane, 1,1,1- Trichloroethane,
Hexachloroethane
One might expect ethane to be de-
stroyed by unimolecular decomposition
through rupture at the weakest bond. The
-------
carbon-carbon BDE in ethane is 88
kcal/mole compared to the carbon-hydro-
gen BDE of 104 kcal/mole in methane.
Thus, one would predict significantly
lower stability than for methane but still
moderate thermal stability for ethane.
Although these compounds are similar
in structure, one would expect the path-
way of decomposition for 1,1,1 -trichloro-
ethane to be by concerted elimination of
HCI, which is a very low-energy process.
Therefore, 1,1,1 -trichloroethaneisoneof
the least stable compounds studied.
Hexachloroethane would have to elimi-
nate CU to proceed by a concerted path-
way. This process is more endothermic
than the elimination of HCI and the
decomposition of hexachloroethane would
instead be expected to proceed through
carbon-chlorine or carbon-hydrogen bond
rupture, both BDEs being approximately
73 kcal/mole. Based on these considera-
tions, hexachloroethane is observed to be
intermediate in stability between ethane
and 1,1,1-trichloroethane.
Tetrachloroethylene and
Hexachlorobutadiene
Both of these compounds are quite
stable. This is probably due to the large
BDEs caused by sp2 hybridization of all
carbon atoms and the lack of hydrogen
atoms available for abstraction by OH or
formation of OH. Carbon-carbon BDEs
would be expected to decrease in the
order ethylene > hexachlorobutadiene >
ethane. This may be used to explain the
relative stability of the series.
Electrophilic addition of OH, however,
may be the predominant mode of attack in
an incinerator. The prediction is the
same, since butadiene can form an allyl
radical intermediate known to be quite
stable.
Acetonitrile and Acrylonitrile
These compounds are very stable, and
all BDEs are 93 kcal/mole or greater.
Based on the mechanism of bond rupture,
one might predict acetonitrile to be less
stable than acrylonitrile for the carbon-
hydrogen bonds would certainly be
stronger in acrylonitrile. This reasoning
would also apply to hydrogen abstraction
reactions.
Carbon-nitrogen triple bonds are ex-
pected to be much less reactive toward
electrophilic addition than double bonds.
Apparently, the only mode of decomposi-
tion for acetonitrile is loss of a hydrogen
through bond rupture or abstraction.
These are both high-energy processes
which account for the stability of aceto-
nitrile. Acrylonitrile may, on the other
hand, be susceptible to addition at the
carbon-carbon double bond. The stability
of the resulting intermediate may be
expected to be somewhat greater than in
tetrachloroethylene through resonance
stabilization. This would account for the
fact that acrylonitrile is less stable than
either tetrachloroethylene or acetonitrile.
Conclusions
When this program was initiated in
April 1982, very limited information ex-
isted on thermal decomposition properties
of organic compounds commonly subject-
ed to incineration. The data generated in
this program provide a consistent initial
data base for the development of a
concept of incinerability.
Preliminary calculations indicate that
"fault" or "failure" modes of incinerator
operation or, equivalently, the "extremes"
of operational parameter distribution
functions, may well control measured
incineration efficiency for full-scale units.
Following this line of reasoning, one
would conclude that the incinerator ef-
fluent would only contain undecomposed
feed material and products of incomplete
combustion which were formed under
these conditions of failure. Thus, "fault"
modes are essentially worst-case condi-
tions and appear to have a dominating
effect on the composition of the inciner-
ator effluent. This suggests that the
majority of future studies should identify
and address these failure conditions.
Although these conditions are not pres-
ently well defined, an atmosphere con-
taining 1% oxygen and a residence time
of 0.25 seconds might be considered
representative.
Furthermore, the temperature at which
this study was conducted is probably
representative of "fault" modes. The
experimental laboratory temperature
range covered 0 to 99.9% destruction of
the feed material, which is typically
several hundred degrees below mean
temperatures quoted for hazardous waste
incineration. If a given incinerator does
not meet the 99.99% destruction effi-
ciency requirement, yet has a high mean
operating temperature, then a likely pos-
sibility for its failure is that a fraction of
the waste feed experiences temperatures
somewhat lower than the mean (where
destruction efficiency is low) i.e., the
destruction efficiency and temperature
range measured in laboratory studies.
Thus, one might expect the actual PICs
emitted from the incinerator to be the
same as those formed under the condi-
tions studied in the laboratory. Further-
more, this reasoning suggests that the
relative thermal stability of hazardous
wastes should be compared at "fault"
mode temperatures since only this frac-
tion of the waste is escaping incineration.
Because selecting a suitable temperature
for comparison of every compound is
difficult and still somewhat arbitrary, a
ranking based on the temperature re-
quired for 99% destruction at 2 seconds
mean residence time is proposed. One
could just as well select 90% or 99.9%,
but examination of the data shows that
the rankings over this range are essen-
tially identical.
Products of incomplete combustion have
not been considered in the full report. PIC
determination would remove much of the
speculation in the discussion of reaction
mechanisms. In previous research, the
formation of numerous PICs from a wide
variety of organic compounds has been
observed. These PICs have, in some
cases, been produced in as much as 50%
yields and have been as hazardous as or
more hazardous than the parent com-
pound. The determination of PICs should
be an integral part of future research.
The reported research has addressed
non-flame, high-temperature, gas-phase
reaction chemistry. Extension of this
work to include so-called flame mode
studies represents a special challenge
due to the difficulties in scaling results.
Of all incineration processes which must
be modeled to full scale, gas-phase
chemical kinetics is the easiest and most
successfully performed. The temporal
and spatial distributions present in small
laboratory- or bench-scale flames are not
easily scaled to the turbulent, poorly-
defined flames present in full-scale sys-
tems. Consequently, an elementary chem-
ical kinetic approach to determining flame
mode destruction efficiencies might prove
most effective.
The importance of the hydroxyl radical
in flames is well documented. Thus, an
experimental program to determine the
rate of attack of hydroxyl radicals on
hazardous wastes would produce easily
scaled kinetic results. Kinetic data of this
type, in combination with measurements
or estimates of hydroxyl radical concen-
trations in full-scale systems would allow
simple scaling of laboratory results to
full-scale. This data over different tem-
perature ranges would be applicable to
both flame and non-flame modes of
destruction.
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Recommendations
• Studies including more thorough ki-
netic investigations and identification
of products of incomplete combustion
should be conducted on a limited
number of selected compounds to
identify the dominant mechanisms of
destruction of hazardous organic com-
pounds as well as the formation of
toxic products of incomplete combus-
tion.
• Further surveys of thermal stability of
hazardous organic wastes should be
conducted under "fault" modes of an
incinerator, as calculations indicate
that these modes control the incinera-
tion efficiency. A representative "fault"
mode might be 1 % oxygen (or less for
destruction systems other than incin-
erators) and a residence time of 0.25
seconds.
• Determining the relative importance
of various "fault" modes such as
reduced gas-phase residence time,
low levels of oxygen, and low exposure
temperature should be emphasized.
• Studies should be performed to ad-
dress the effect of changing the compo-
sition of the organic fraction of the
reaction atmosphere, since this has
the potential of modifying the reaction
pathway, thereby affecting thermal
stability and product formation.
• A laboratory study to determine the
rate of OH attack on hazardous organic
compounds at incineration tempera-
tures should be undertaken in light of
the importance of this reaction inferred
from this study.
• A round-robin test program should be
conducted using a well-defined waste
sample. The waste should be evaluated
by each proposed incinerability ranking
scheme and predictions made concern-
ing the organic composition (both
POHCs and PICs) of the stack effluent.
The results of a trial burn of this waste
at the EPA's Combustion Research
Facility could be used as the basis for
evaluating the test program results.
Barry Dellinger, Juan L. Torres, Wayne A. Rubey, Douglas L Hall, and John L.
Graham are with University of Dayton Research Institute, 300 College Park,
Dayton, OH 45469.
Richard A. Carnes is the EPA Project Officer (see below).
The complete report, entitled "Determination of the Thermal Decomposition
Properties of 20 Selected Hazardous Organic Compounds," (Order No. PB
84-232 487; Cost: $19.00, 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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
•&U. S. GOVERNMENT PRINTING OFFICE: 1984/559-111/10718
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Environmental Protection
Agency
Center for Environmental Research
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