United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati OH 45268
Research and Development
EPA/60Q/SR-92/142 Sept. 1992
Project Summary
On-Site Engineering
Report for the Low-Temperature
Thermal Desorption Pilot-Scale
Test on Contaminated Soil
Michael L. Smith, Arend Groen, Judy Hessling, and Edward Alperin
Performance of the thermal desorp-
tion process for removal of organic con-
taminants, mostly polynuclear aromatic;
hydrocarbons (PAHs), from soils was;
evaluated. The Superfund Site soil
tested was a fine sandy soil contami-
nated with creosote. An optimum oper-
ating temperature of 550°C and an op-
timum operating residence time of 1C)
min, determined from bench studies,
were used in the pilot-scale desorber.
Contaminants removed from the soil
were captured or destroyed In the as-
sociated air pollution control equip-
ment. Test results showed that greater
than 99% of the PAHs were removed
from the soil. The concentration of to-
tal PAHs averaged 4629 mg/Kg in thai
pretreated soils and were below detec-
tion in the post-treated soils.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the treatment evalua-
tion study that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
This study was performed for the U.S.
Environmental Protection Agency (EPA)
to supply information as part of the datsi
base on best demonstrated available tech-
nologies (BOAT) for soil remediation. The
data base will be used to develop soil
standards for land disposal restrictions.
Thermal desorption has been success-
fully tested at both the bench and pilot-
scale on a wide range of organic contami-
nants. The process drives volatile and
semivolatile organics from the soil by heat-
ing the soil to temperatures greater than
the boiling point temperature of the con-
taminants. The contaminants of concern
in the test soil were PAHs, semivolatile
contaminants that boil at temperatures
ranging from approximately 215°C to
greater than 525°C. Vapor pressures of
these compounds vary depending on
whether the contaminant is one compound
or a mixture of compounds. Because boil-
ing points of various mixes of contami-
nants are not known, bench-scale thermal
desorption tests were done to determine
the optimum temperature and residence
time required for their removal. This ther-
mal desorption study was performed in
two phases: bench-scale and pilot-scale.
Based on the bench test results, the pilot-
scale tests were done at an operating
temperature of 550°C and a residence
time of 10 min.
Treatment Systems and
Procedures
For the bench test, a 5-in. diameter by
12-in. long rotating tube unit was used to
evaluate the effects of time, temperature,
and atmosphere. To eliminate any possi-
bility of explosions from gases in the sys-
tem, the rotating tube was purged by ni-
trogen. Treated residues from the tests
provided adequate sample for thorough
analytical characterization.
A schematic diagram of the thermal de-
sorption pilot plant is shown in Figure 1.
The unit consisted of a continuously rotat-
ing desorber tube partially enclosed within
a gas-fired furnace shell. Small baffles
Printed on Recycled Paper
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Hopper
Furnace
Exhaust
Cooling Air
Exhaust
Thermocouple
Assembly
Nitrogen
Treated
Solids
Screw
Feeder
Rotary
Airlock
Ftgun 1. Diagram of thermal separator.
Solids Collection
were located at Intervals within the tube to
provide soil mixing. A stationary thermoweil
was extended from the discharge end into
the tube with six thermocouples to moni-
tor the soil temperature and three to moni-
tor the gas temperature along the tube
length. The furnace was a refractory-lined
chamber. The 14 equally spaced burners
were controlled by a standard burner con-
trol system with appropriate safety fea-
tures. Temperature measurements for fur-
nace burner control or monitoring were
taken by four thermocouples that contact
at various locations on the outer metal
walJ of the rotating tube beneath the fur-
nace refractor. The furnace flue gas was
discharged directly to the atmosphere
through a remotely positioned exhaust
duct. The desorber was rated at 320,000
Btu maximum heat duty. A nitrogen purge
was introduced continuously at a low rate
of 2 ftVmln to the desorber to help flush
contaminants and to maintain an atmo-
sphere that does not support combustion
(I.e., <6% oxygen). To measure residence
time, before the study colored aquarium
gravel was placed Into the feed hopper
and its discharge was visually observed
from the desorber. The average retention
or residence time In the tube was calcu-
lated as the difference between the time
that the colored gravel was placed in the
screw feeder and the time it was dis-
charged. Solids discharged from the
desorber while it was operating at steady-
state were weighed on a digital electronic
scale to determine the soil feed rate.
The Superfund soil was a fine, sandy
soil with 75% of its particles having a
grain diameter between 0.1 and 0.4 mm.
The soil had a relatively low moisture con-
tent of 10% and a Btu value below 500
Btu/lb.
Various temperatures and soil residence
times were evaluated throughout the
bench-scale testing program. The follow-
ing information summarizes the removal
of semivolatile organics during the bench-
scale tests:
Run #1 (300°C at 10 min) removed 96.4%
Run #2 (425°C at 10 min) removed 99.97%
Run #3 (550°C at 10 min) removed 99.995%
Run #4 (300°C at 20 min) removed 97.4%
Run #5 (550°C at 5 min) removed >99.9999%
Based on the bench-scale study results,
temperature and residenceifime~6peratirig""
conditions of 550°C and 10 min were se-
lected for the pilot-scale testing program.
Though the above results indicate the run
at 550°C and a 5-min residence time pro-
vided the highest removal efficiencies for
the semivolatile contaminants, it was an-
ticipated that larger particles would be in-
troduced into the pilot-scale unit and the
feed streams might not be totally uniform
and could contain "hot spots." Therefore,
a temperature of 550°C and a residence
time of 10 min were chosen to allow for
better treatment of the contaminated soil.
Six sets of temporally related soil
samples (waste feed and treated residual)
were collected during the thermal desorp-
tion pilot test to evaluate the performance
of the technology for treating creosote-
contaminated soil. Additional samples of
the off-gases were collected to character-
ize the emissions from the unit. Off-gases
were sampled before the air pollution con-
trol equipment to determine if any degra-
dation products were being formed. The
air samples were analyzed for volatile and
semivolatile organics. Detailed analytical
procedures are included in the On-Site
Engineering Report (OER) for these Ther-
mal Desorption Studies.
Results
Tables 1 and 2 give the concentrations
of the organic contaminants in the soil
before and after treatment on a dry weight
basis. Individual contaminants in the pre-
treatment soil ranged as high as 1200
rng/kg for phenanthrene tojess thanjhe
detection limit for some of the otheTcon-
taminants. All contaminants analyzed in
the post-treatment samples were below
the detection limits. On the average, total
semivolatile organic contaminants were re-
duced from 4629 mg/kg to less than the
detection limit. Hence average removal
for total semivolatile organics was greater
than 99.9%.
No appreciable reduction in lead or ar-
senic was observed during the study be-
cause of the low operating temperature in
relation to the boiling points of lead and
arsenic. Mercury, however, with a boiling
point of 356°C, showed greater than a
90% reduction in soil levels after treat-
ment.
Air sampling was done to characterize
the gases coming off the treatment sys-
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Table 1. Concentrations of Critical Semivolatile Organic Contaminants in Pretreatment Soil Samples
on a Dry Weight Basis (mg/kg)
Test number
Contaminant
Phenol
2-Methylphenol
4-Methylphenol
2,4-Dimethylphenol
Napthalene
2-Methylnapthalene
Acenapthylene
Acenapthene
Dibenzofuran
Flourene
Phenanthrene
Anthracene
Flouranthene
Pyrene
Benzo(a)anthracene
Chrysene ' '
Benzo(b)flouranthene
Benzo(k)flouranthene
Benzo(a)pyrene
lndeno(1,2,3-cd)pyrerie
Dibenzo(a,h)anthracene
Benzo(g,h,i)perylene
Total concentration
critical contaminants
MDLs'
2.5
1.4
2.0
7.0
0.96
11.0
0.41
12.0
4.8
1.2
2.0
4.3
0.61
3.1
1.4
7.0
1.4
1.4
6.6
2.1
0.97
19.0
PQLs
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
,„, M
20
20
20
20
20
20
' 1
<2.5
<1.4
<2.0
<7.0
130
190
38
330
240
360
900
360
610
460
160
"150
1801
—
73
26
<0.97
45
4252
2
<2.5
<1.4
<2.0
<7.0
130
170
32
310
210
330
980
440
640
540
150
140
130
—
62
33
19
<19
4316
3
<2.5
<1.4
<2.0
<7.0
170
180
46
390
290
500
990
400
690
640
210
200
220
—
120
31
<0.97
32
5109
4
<2.5
<1.4
<2.0
<7.0
130
170
34
310
210
340
1100
460
670
620
160
140
130
—
64
<2.1
5
<2.5
<1.4
<2.0
<7.0
150
170
35
320
220
340
1000
430
590
580
160
150
130
—
65
28
<0.97 <0.97
<19
4538
<19
4368
6
<2.5
<1.4
<2.0
<7.0
150
200
42
390
250
460
1200
400
810
640
190
170
160
—
76
30
<0.97
20
5188
* Benzo(b) - and benzo(k)flouranthene were found to co-elute; therefore a total amount is given.
* MDL = method detection limit; POL = practical quantisation limit.
Table 2. Concentrations of Critical Semivolatile Organic Contaminants in Post-treatment Soil Samples
(mg/kg)
Test Number
Contaminant
Phenol
2-Methylphenol
4-Methylphenol
2,4-Dimethylphenol
Napthalene
2-Methylnapthalene
Acenapthylene
Acenapthene
Dibenzofuran
Flourene
Phenanthrene
Anthracene
Flouranthene
Pyrene
Benzo(a)anthracene
Chrysene
Benzo(b)flouranthsne
Benzo(k)flouranthene
Benzo(a)pyrene
lndeno(1,2,3-cd)pyrene
Dibenzo(a,h)anthracene
Benzo(g,h,i)perylene
Total concentration
critical contaminants
MDLs'
0.043
0.023
0.033
0.120
0.016
0.190
0.007
0.210
0.081
0.020
0.034
0.073
0.010
0.052
0.023
0.120
0.024
0.023
0.110
0.035
0.016
0.320
PQLs
0.34
0.34
0.34
0.34
0.34
0.34
0.34
0.34
0.34
0.34
0.34
0.34
0.34
0.34
0.34
0.34
0.34
0.34
0.34
0.34
0.34
0.34
1
<0.043
<0.023
<0.033
<0.120
<0.016
<0.190
<0.007
—
<0.110
<0.035
<0.016
<0.320
<1.58
2
<0.043
<0.023
<0.033
<0.120
<0.016
<0.190
<0.007
<0.210
<0.081
<0.020
<0.034
<0.073
<0.010
<0.052
<0.023
<0.120
<0.047
—
<0.110
<0.035
<0.016
<0.320
<1.58
3
<0.043
<0.023
<0.033
<0.120
<0.016
<0.190
<0.007
<0.210
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Michael L Smith and Judy Hessling are with IT Environmental Programs, Inc.,
Cincinnati, OH 45246; Arend Groen and Edward Alperin are with IT Corpo-
ration, Knoxville, TN 37923.
Richard P. Lauch is the EPA Project Officer (see betow).
The complete report, entitled "On-S'tte Engineering Report for the Low-Tempera-
ture Thermal Desorption Pilot-Scale Teston Contaminated Soil," (Order No.
PB92- 216938/AS; Cost: $26.00, 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:
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
Official Business
Penalty for Private Use
$300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/SR-92/142
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