United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-86/040 Aug. 1986
&EPA Project Summary
Investigation of the Thermal
Destructibility of Hazardous
Wastes Using the Thermal
Decomposition Analytical
System
Douglas L Hall, Wayne A. Rubey, and Barry Dellinger
The laboratory determined gas-phase
thermal decomposition characteristics
of a variety of hazardous organic com-
pounds and mixtures of organic com-
pounds are reported. The thermal sta-
bilities of the compounds are assessed
and compared, and the effects on ther-
mal stability from the variation of sev-
eral parameters are demonstrated.
Possible chemical mechanisms for
the decomposition of the gas-phase
species are reported, including mecha-
nisms for the formation of stable prod-
ucts. The role of oxygen in the thermal
destruction of chlorinated aromatics is
addressed qualitatively, through pro-
posed reaction mechanisms, and quan-
titatively, through the application of
kinetic methods. The thermal decom-
position analytical system used for gen-
eration of these data is discussed in
limited detail, and modifications per-
formed to improve its utility and sensi-
tivity are discussed.
This Project Summary was devel-
oped by EPA's Hazardous Waste Engi-
neering Research Laboratory, Cincin-
nati, OH, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Background and Introduction
Based on a recent report from the
U.S. EPA, 150 million metric tons of fed-
erally regulated hazardous waste were
generated in the United States during
1981. This figure illustrates the pressing
need for proper treatment methods for
detoxification and disposal of haz-
ardous wastes.
Prior to the enactment of the Re-
source Conservation and Recovery Act
(RCRA) in 1976, approximately 80 per-
cent of the hazardous waste generated
was disposed of in or on the land. How-
ever, regulations promulgated under
the authority of RCRA increased the li-
ability of the generator utilizing land dis-
posal methods. This has resulted in
greater interest in and utilization of
other methods of hazardous waste dis-
posal.
Although more immediately costly
than land disposal, incineration is seen
as an attractive alternative for perma-
nent disposal of hazardous organic
wastes. In addition, in many instances,
organic wastes with sufficient heating
value and appropriate physical charac-
teristics are useful as supplemental
fuels in industrial boilers. Both of these
thermal treatment methods result in the
virtual destruction of a large percentage
of the waste and are therefore consid-
ered preferable to land disposal.
This increased utilization of thermal
treatment methods for the destruction
of hazardous organic wastes has given
rise to an increased need to understand
the gas-phase thermal decomposition
behavior of organic compounds. Early
research at the University of Dayton Re-
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search Institute identified several vari-
ables which play an important role in
gas-phase processes. Three of these
variables are exposure temperature,
mean residence time, and composition
of reaction atmosphere. In addition, the
following two major observations were
reported:
• Under appropriate conditions, haz-
ardous organic compounds are
amenable to thermal destruction.
• Under certain conditions, haz-
ardous thermal reaction products
may form that are more difficult to
destroy and may be more toxic
than the parent material.
Information related to the conse-
quences of manipulating the variables
listed above as well as insights into the
formation of thermal reaction products
were obtained in this investigation.
Instrumentation
All of the data described in the report
were obtained using the thermal de-
composition analytical system (TDAS).
This system was designed and built
with funding provided by the U.S. EPA
(Grant No. R805117-01-0). An artist's
rendering of the TDAS is shown in Fig-
ure 1. This is a closed continuous ana-
lytical system that allows controlled
thermal exposure of gas-phase
molecules followed by separation, de-
tection, and identification of the result-
ing thermal reaction products.
Samples are inserted into the TDAS
via a thermally programmable insertion
chamber. Following vaporization from
an insertion probe into a flowing gas
stream, a controlled thermal exposure
of the sample occurs in a narrow bore
quartz tubular reactor located within a
three-zone Lindberg furnace. The sam-
ple is carried to and from the reactor in
quartz transfer lines maintained at non-
degradative transport temperatures. At
the reactor entry and exits points, the
transfer lines constrict to a very fine
bore, providing increased gas velocity
in the temperature transition regions.
The overall result of this reactor assem-
bly design is to enable a nearly square
wave thermal exposure of sample
molecules. Additional detail of the reac-
tor design is included in the complete
report.
The reactor effluent is analyzed using
an LKB model 2091 gas chromatograph
coupled mass spectrometer. The vari-
ous components of the effluent are sep-
arated in temperature-programmed
open tubular columns before entering
the mass spectrometer. Both the total
ion concentration and the electron mul-
tiplier detection devices in the mass
spectrometer were used in the course of
this work.
During this project, a number of ma-
jor modifications were made to the
TDAS which considerably increased its
versatility and sensitivity. These modifi-
cations included:
• Reconfiguration of the insertion re-
gion and addition of a glove box at
the inlet;
• Replacement of the existing gas
chromatographic oven by a modi-
fied oven containing a flame ioniza-
tion detector, on-column injector,
and cryogenic capabilities;
• Total reconstruction of the inter-
face between the gas chro- *
matograph and the mass spec-
trometer;
• Acquisition and installation of soft-
ware and hardware providing mul-
tiple ion detection capability to the
mass spectrometer and dedicated
computer system.
A thorough account of the rationale
for these modifications as well as a de-
tailed description of the modifications
can be found in the full report.
Experimental Procedures,
Results, and Discussion
The High Temperature Destruc-
tion and Reaction of Various
Organic Compounds and Mix-
tures of Organic Compounds
Five Component Aromatic Mix-
ture
A test sample was prepared contain-
ing 2,6-dichlorobiphenyl, 2,2'-dibromo-
biphenyl, 2,7-dichlorodibenzo-p-dioxin,
2,8-dichlorodibenzofuran, and
triphenylene in a toluene solvent. The
solution was injected onto an insertion
probe and the solvent evaporated prior
to analysis. Approximately one (ig of
each component was present for the
analyses.
The results of thermal decomposition
analyses indicate that all of the com-
pounds possess high thermal stabili-
External
Data Base
Interface
Mass Gas
Spectrometer Chromatograph Destruction
Unit
Minicomputer
for
Data Reduction
Figure 1. Artist's rendering of assembled TDAS.
2
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ties, with the temperatures required for
99.99 percent destruction (in air with a
2.0 sec. mean residence time [tr ]) in the
800°C to 850°C range. Various products
of incomplete combustion (PICs) were
detected but not identified during these
analyses.
The relative thermal stabilities for the
five compounds were determined.
Triphenylene was the least stable while
2,6-dichlorobiphenyl was the most sta-
ble. The relative stabilities are explained
in terms of basic chemical principles in
the full report.
The effect of variation of the resi-
dence time at temperature for four com-
ponents of this mixture was also deter-
mined. The data are presented in
Table 1.
Biphenyl Mixture
It was suspected that the biphenyl
compounds in the previously discussed
five component mixture were the pri-
mary precursors of the PICs formed.
Therefore, a mixture of 2,6-dichloro-
biphenyl and 2,2'-dibromobiphenyl was
prepared for analyses using a toluene
solvent. The solution was injected onto
an insertion probe and the solvent al-
lowed to evaporate. Approximately
5 (jig of each component were present
for the analyses.
As anticipated, the results showed
that the predominant reaction products
in both the five component mixture and
this biphenyl mixture were qualitatively
the same. The biphenyls, therefore,
were indeed the primary precursors of
reaction products in the five component
mixture and behaved relatively inde-
pendently of the other three compo-
nents. Dibenzofuran, fluorenone, and
xanthenone were identified as the
major PICs.
No. 2 Waste Stream
The thermal decomposition charac-
teristics of an industrial waste sample
were determined by directly injecting
0.04 (j.l of the liquid fraction of the waste
into the TDAS. It was determined that
this sample could be virtually destroyed
at 800°C (tr = 2.0 sec. in air), except for
low levels of some volatile PICs.
Chloroform
The propensity of single-carbon chlo-
rinated species to form PICs was investi-
gated. A sample of chloroform was di-
rectly injected into the TDAS and
thermally exposed in air at 600°C for
tr = 2.0 seconds.
Significant amounts of pen-
tachloroethane and tetrachloroethylene
were observed as PICs. Given the large
yield of products observed and the po-
tential autocondensation of ethylenes
to form aromatics, it is hypothesized
that even single carbon chlorinated spe-
cies may yield extremely toxic chlori-
nated aromatic compounds when sub-
jected to less than ideal conditions in an
incinerator. Possible pathways for such
reactions are included in the full report.
Pentachloronitrobenzene
(PCNB)
A PCNB sample was prepared for
thermal decomposition studies by dis-
solving the PCNB in toluene. This solu-
tion was injected onto a probe and the
solvent evaporated leaving a 3.6 |j.g
PCNB sample deposited for each analy-
sis.
A thermal profile exhibiting the de-
struction of PCNB and subsequent for-
mation of the PIC hexachlorobenzene
was obtained and is included here as
Figure 2. The decomposition character-
istics of this compound demonstrate
thermal processes not addressed by in-
cinerability ranking scales based on a
single molecular parameter (e.g., the
heat of combustion). At the same time,
the value of laboratory systems for pro-
viding useful data pertaining to product
formation is clearly evidenced. These
points are further expounded in the full
report.
Polychlorinated Napthalenes
(PCNs)
Because of their wide-spread use as
plasticizers, protective coatings, electri-
cal insulating materials, etc., the ther-
mal destructability of this family of com-
pounds is of interest. A commercial
mixture of PCNs (Halowax 1001) con-
taining about 50 percent by weight chlo-
Table 1. Weight Percent Remaining after 750°C Exposure for Respective Mean Residence
Time*
Mean Residence Time, sec.
Compound
0.5
1.0
2.0
4.0
2,6-Dichlorobiphenyl
2,2'-Dibromobiphenyl
2,8-Dichlorodibenzofuran
2, 7-Dichlorodibenzo- P-Dioxin
68.6
82.4
85.3
77.8
36.9
53.0
51.6
40.6
26.4
27.6
26.5
13.6
4.3
6.9
3.9
1.1
*ln flowing air.
Residence Time -2.0 sec. in Air
Figure 2.
400 500 600 700
Exposure Temperature f°C)
Quantitative thermal profiles for PCNB and HCBz.
3
800
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rine was obtained, spiked with 1,2,3,4-
tetrachloronaphthalene and subjected
to a series of tests using the TDAS.
In an atmosphere of flowing air at
tr = 2.0 sec., the original constituents of
the PCN sample were destroyed to lev-
els greater than 99.99% at 800°C. In ad-
dition, unlike the previously discussed
chlorinated biphenyls, there was virtu-
ally no product formation.
Thermal decomposition studies of the
PCN sample conducted in nitrogen
rather than air demonstrated the de-
pendence of thermal decomposition on
the presence of oxygen. In nitrogen, the
temperature required for levels of de-
struction comparable to those in air are
shown to be approximately 200°C
higher. Additional data illustrating the
oxygen dependence of the thermal de-
composition of PCNs was obtained in a
pure oxygen atmosphere. Based on
these data, it is hypothesized that bi-
molecular decomposition pathways in-
volving oxygen are an important mode
of decomposition for these compounds.
Kinetic Data for Selected Com-
pounds
Thermal Degradation of 2,2'A,
5,5'-Pentachlorobiphenyl
(2,2',4,5,5'-PCB)
The gas-phase thermal decomposi-
tion kinetics of 2,2',4,5,5'-PCB in flowing
air were obtained using the TDAS by
measuring the percent of the sample re-
maining as a function of tr at different
temperatures. The sample was pre-
pared by dissolving the PCB in benzene
and then depositing the solution on an
insertion probe. Following evaporation
of benzene, the deposited PCB sample
(approximately 1.2 ng) was inserted
into the TDAS.
The data obtained were fit to an inte-
grated first order rate expression for de-
termination of the pseudo first order
rate constant at various temperatures.
The activation energy (Ea) and Arrhe-
nius coefficient (A) for the thermal de-
composition process were then ob-
tained from the rate constant data by
application of the Arrhenius equation.
The calculated values for Ea and A are
70 kcal mole"1 and 1.3 x 1015 s~1, respec-
tively. These values are presented in
comparison to 20 other compounds in
the report.
Thermal decomposition data for
2,2',4,5,5'-PCB were also obtained in
flowing gas streams with a variety of
oxygen concentrations. The decompo-
sition of this compound was found to be
highly dependent on oxygen concentra-
tion and, in fact, is shown (by kinetic
treatment) to be first order with respect
to the oxygen concentration.
Oxygen Dependence of
Chlorobenzenes
The thermal decomposition-of 1,2,3,4-
tetrachlorobenzene (1,2,3,4-TCBz), pen-
tachlorobenzene (PCBz), and hex-
achlorobenzene (HCBz), including the
oxygen dependence of their decompo-
sition, was investigated. Individual sam-
ples of each compound were prepared
using solvent which was allowed to
evaporate following deposition of the
solutions on the insertion probes. Ther-
mal decomposition studies were con-
ducted over a wide range of tempera-
tures and oxygen concentrations while
the residence time was held constant at
2.0 seconds.
The reaction order for the decomposi-
tion of each compound at three different
temperatures was calculated from the
data obtained at oxygen concentrations
less than or equal to 21 percent. A plot
of these reaction orders as a function of
temperature for the three compounds is
included as Figure 3. The relatively high
oxygen dependence of 1,2,3,4-TCBz and
PCBz at 725°C and 750°C is related to the
ability of these compounds to follow
low energy bimolecular decomposition
pathways involving reactive oxygen
containing species while HCBz is less
amenable to such pathways. At 775°C,
however, sufficient thermal energy is
available for the higher frequency uni-
molecular pathways to predominate
and the oxygen dependence is conse-
quently reduced.
Conclusions
• The laboratory approach, using
thermal instrumentation systems,
has a demonstrated ability to inves-
tigate the gas-phase thermal de-
composition behavior of organic
substances. Information obtained
from such investigations is useful
to insure the safe and economical
operation of incinerator systems
and can provide guidance for the
regulation of such systems.
• Products of incomplete combus-
tion (PICs) are frequently observed
when mixtures of hazardous or-
ganic compounds and pure com-
pounds are exposed to conditions
insufficient for complete oxidation
to occur.
• Somewhat limited data indicate
that the formation of PICs during
the thermal exposure of a mixture
may depend more on the propen- (
sity of the individual constituents to
form PICs than the overall chem-
istry of the mixture.
• PICs form readily from chlorinated
compounds considered thermally
stable (e.g., the temperature re-
quired for 99.99 percent destruc-
tion (T99gg) of the compound is
greater than 800°C) as well as those
of lesser thermal stability (e.g.,
T99.99 <650°C).
• Simple, single carbon chlorinated
compounds may serve as precur-
sors for hazardous, structurally
complex PICs.
• The oxygen dependence of the
thermal decomposition of chlori-
nated aromatic compounds is im-
portant and very complex. It is a
function of both the exposure tem-
perature and the degree of chlori-
nation of the compound of interest.
• The relative thermal stabilities of
organic compounds as well as the
mechanisms of product formation
can be explained using the funda-
mentals of organic chemistry. As
the data base for this type of infor-
mation expands, a priori prediction
of the thermal stability of a wide
range of compounds should be-
come possible.
• Both complex mixtures of haz-
ardous organic compounds and
pure organic compounds can be ef-
ficiently destroyed by combustion
in the gas phase.
Recommendations
• Based on the data obtained in this
project, alternative permitting proc-
esses should be considered to en-
sure environmentally safe incinera-
tion (i.e., destruction of both
POHCs and PICs) rather than per-
mitting processes which simply
evaluate the destruction and re-
moval of POHCs. For example, the
selection of a POHC for trial burn
monitoring could be based on gas-
phase thermal stability [as mea-
sured by the laboratory determined
temperature required for 99 or
99.99 percent destruction (T99 or
T99.99)l, toxicity, and concentration
in the waste. A PIC for emissions
monitoring should be selected
from a laboratory generated data
base identifying the PICs formed
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from a variety of waste streams.
The permit writer should have the
option of ordering laboratory test-
ing of unusual waste streams to es-
tablish the identity of suitable com-
pounds for monitoring.
In some cases, a single test com-
pound will fulfill the requirement for
both PIC and POHC monitoring. For
instance, based on the current data
base, hexachlorobenzene would be
suitable for assessing the overall
combustion efficiency of a waste
stream containing chlorinated or-
ganics.
Experimental data indicate that
even the simplest chlorinated hy-
drocarbons may serve as precur-
sors to larger and potentially toxic
PICs. Because of this, an intensive
research effort should be under-
taken to identify the fundamental
reactions involved in such path-
ways.
If suitable methods cannot be es-
tablished for preventing PIC forma-
tion, the possibility of reducing the
overall toxicity of the PICs formed
should be investigated. This might
involve the addition of chlorine rad-
ical scavengers to alter the inciner-
ation atmosphere, preventing the
formation of toxic chlorinated PICs.
Laboratory studies should be
undertaken to assess the effects of
various incinerator failure modes
(e.g., reduced oxygen, short resi-
dence time, or low temperature) on
both POHC destruction and PIC for-
mation. These failure modes
should be ranked based on their ef-
fect on incineration efficiency.
1.0 I
o
.c
I
o
0.5
f 2.0 sec
\
\
I
725
775
750
Temp. °C
Figure 3. Reaction order in oxygen vs. temperature for 1,2,3,4-TCBz. PCBz, andHCBz.
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Douglas L Hall. Wayne A. Rubey. and Barry Bellinger are with the University of
Dayton Research Institute. Dayton. OH 45469.
Richard A. Carries was the EPA Project Officer (see below for present contact).
The complete report, entitled "Investigation of the Thermal Destructibility of
Hazardous Wastes Using the Thermal Decomposition Analytical System."
(Order No. PB 86-175 700/AS; Cost: $16.95, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
For information contact E. Timothy Oppelt at:
Hazardous Waste Engineering Research 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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