United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81 -127 July 1982
Project Summary
Environmental Assessment of
Combustion Modification
Controls for Stationary
Internal Combustion Engines
H. I. Lips, J. AX-jiotterba, K. J. Lim, and L R. Waterland
\
°-P \ oj **»
This report give^presult^i
evaluation of^cornbust^ttmoc*
techniques ^r'-stationary ..
combustion (Id^engjnes, with resp*ect
to NOx control 'fedaqtion effective-
ness, operational impact, thermal
efficiency impact, capitj&and annual-
ized operating costs, and effects on
emissions of pollutants other than
NOX. Currently available operational
adjustments for NOx control can
reduce emissions by about 40 percent,
but significantly increase operating
costs. The total annualized cost to
control can increase the cost of power
by 3 to 14 percent, due to additional
fuel and maintenance requirements.
Combustion modifications can reduce
NOX emissions without significantly
increasing CO and hydrocarbon emis-
sions for most engines. However, the
kinds and distribution of organic
compounds emitted from stationary
diesel engines are not well character-
ized, and therefore are of concern.
This Project Summary was devel-
oped by EPA's Industrial Environ-
mental Research Laboratory, Research
Triangle Park. NC, 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
With the increasing extent of NO*
control applications in the field, and
expanded NO* control development
anticipated for the future, there is
currently a need to ensure that: (1) the
current and emerging control tech-
niques are technically and environ-
mentally sound and compatible with
efficient and economical operation of
systems to which they are applied, and
(2) the scope and timing of new control
development programs are adequate to
allow stationary sources of NOX to
comply with potential air quality stan-
dards. With these needs as background,
EPA's Industrial Environmental Research
Laboratory, Research Triangle Park
(IERL-RTP) initiated the "/Environmental
Assessment of Stationary Source NO«
Combustion Modification Technologies
Program" (NO* EA) in 1976. This
program has two main objectives: {1) to
identify the multimedia environmental
impact of stationary combustion sources
and NOx combustion modification
controls applied to these sources, and
(2) to identify the most cost-effective,
environmentally sound NOX combustion
modification controls for attaining and
maintaining current and projected NO?
air quality standards to the year 2000.
The NOX EA's assessment activities
have placed primary emphasis on.
major stationary fuel combustion NO,
sources (utility and industrial boilers,
gas turbines, 1C engines, and commer-
cial and residential-warm air furnaces);
conventional gaseous, liquid, and solid
fuels burned in these sources; and
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combustion modification controls appli-
cable to these sources with potential for
implementation to the year 2000.
This summary outlines the environ-
mental, economic, and operational
impacts of applying combustion modifi-
cation controls to this source category.
Conclusions
Source Characterization
Stationary reciprocating 1C engines
are the second largest contributor of
NOX emissions from stationary sources in
the U.S. Figure 1 shows that this source
constituted about 19 percent in 1977.
Because of this high level of NOX
emissions and their potential for
control, stationary 1C engines represent
a priority source category for control
evaluation in the NOX EA.
Stationary 1C engines can be classified
into three characteristic size ranges:
large bore, high power, low to medium
speed; medium bore, high speed; and
small.
Large bore engines (>75 kW/cyl)
operate at lower speeds (usually less
than 1000rpm) and burn three major
types of fuel: diesel, natural gas, and
dual fuel (mixture of diesel and gas).
Natural gas engines are spark ignited,
and diesel and dual fuel engines are
compression ignited. Both two- and
four-stroke models are in this size
range, and the engine may be turbo-
charged, which usually increases
efficiency. Typical heat rates are 9 to 11
MJ/kWh (8500 to 10,500 Btu/kWh).
Typical industries using these large bore
engines are municipal electric power
generation, oil and gas pipeline trans-
mission, and oil and gas production. In
these industries, the engine is run
continuously. Based on 1976 data, only
about 1000 to 2000 of these engines
are sold per year, with a total production
value of $80 to $150 million (1976
dollars). Sales have generally been
declining, although sales of diesel
engines for electric power generation
are up.
Medium power engines (7.5 to 75
kW/cyl) exhibit the greatest variety;
some large units equal the power of
large bore engines. However, where
large bore engines produce high power
output at low speeds due to their large
displacement and consequently high
power per cylinder, medium bore
engines have lower power per cylinder
and, therefore, more cylinders for the
same engine horsepower. Fuels burned
in medium power engines are typical
Noncombustion 1.9%
Warm air furnaces 2.0%
Gas turbines 2.0%
Others 4.1%
Incineration 0.4%
Industrial process
Heaters 4.1%
Reciprocating
1C engines
18.9%
Total: 10.5 Tg/yr (11.6 x 10* tons/yr)
Figure 1. Distribution of stationary anthropogenic NO* emissions for the year 1977
(controlled NO* levels).
mobile fuels, either diesel oil or gasoline,
although there are a few (usually
modified) natural gas engines of this
size. These engines are used in miscel-
laneous industrial, commercial, nonpro-
pulsive marine, and agricultural appli-
cations where shaft power is needed
and electric motors cannot be used.
Small engines are mostly one- and
two-cylinder engines of less than 40 hp.
These engines are mostly diesel and
gasoline, one- and two-cylinder models,
with some four-cylinder models. Almost
all have four stroke cycles and are usual-
ly air cooled. Small engines are used
typically in generator sets, small pumps
and blowers, off-the-road vehicles, and
refrigeration compressors for trucks and
railroad cars.
This report focuses on large bore en-
gines since these represent the largest
NOx emitters in the category, and they
are most amenable to combustion
modification control.
Source Emissions
Air emissions in the form of exhaust
gases are essentially the only effluent
stream from stationary 1C engines.
Hydrocarbons (HC) can be emitted from
the fuel before combustion, especially
from natural-gas-fired engines, but
these emissions ar.e considered minor.
There may also be some emissions from
the crankcase caused by blowby, but
this is also a minor source. The cooling
system may release minor water pollu-
tant emissions, and liquid wastes in the
form of used crankcase oil may be
another pollutant. Neither of these is a
major release.
NOX, CO, and HC are the major
pollutants of concern in the exhaust
gases from stationary 1C engines. SOX
emissions are possible if thefuel burned
has appreciable sulfur content, but this
is rarely the case with the clean fuels
burned in these engines. Paniculate
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emissions are low from stationary
engines. Diesel engines may also emit
polycyclic organic matter (POM) at low
levels, but even low level emissions of
these compounds would be of concern
because of their mutagenicity and
potential carcinogenicity.
NOx in 1C engines, as in all combustion
sources, is formed primarily by two
mechanisms thermal fixation and
fuel NOx formation. Thermal NOxresults
from the termal fixation of molecular
nitrogen and oxygen in the combustion
air, and the rate of formation increases
exponentially with local flame tempera-
ture. Fuel NOx results from the oxidation
of organically bound nitrogen found in
certain fuels and primarily depends on
the nitrogen content of the fuel. Since 1C
engines generally burn clean fuels, with
correspondingly low nitrogen contents,
thermal NOx predominates.
Of the other pollutants, HC and CO
are mainly the result of incomplete
combustion. HC emissions are believed
to be caused by three general mecha-
nisms: wall quenching (fuel impinge-
ment on the walls causing the fuel to be
cooled below the combustion tempera-
ture), variations in engine operation
(mixing inside the cylinder, wrong air-
to-fuel ratio, defective ignition, etc.),
and, in two-cycle engines, cooling the
exhaust gases by scavenging air before
combustion is completed. CO emissions
are also formed by the same general
mechanisms.
Typical uncontrolled emission factors
for 1C engines are listed in Table 1. The
HC emissions listed are total HCs; for
natural gas engines, these are mainly
Table 1. Emissions Factors for 1C
Engines, g/kWha
Fuel /VO COJlC~
Gasoline > 15kW 11.9 137 11.2
< 15 kW 7.5 395 27.5
Diesel >375 kW* 17.3 2.4 0.6
<375 kW° 16.6 6.0 2.8
Natural gas 15.4 3.8 6.5
Dual Fuel 11.0 2.7 4.1
* Emission factors for gasoline and
diesel engines are modal averages;
those for natural gas and dual fuel are
for rated conditions. Modal averages
mean that some of the /VOX numbers
are taken from the constant power out
portion of mobile tests.
* Based on an average of rated condition
levels from engines considered.
c Weighted average of two- and four-
stroke engines. Weighting factors =
2/3 for four-stroke and 1/3 for two-
stroke.
methane. Although Table 1 lists factors
for all engine sizes, this report focuses
on the larger engines. Note that NO, is the
major pollutant for large engines.
Control Alternatives
Since NOx is the major pollutant
emitted by stationary large bore 1C
engines, control development has
focused on limiting NOx emissions.
There are three major approaches to
controlling NOx from 1C engines: opera-
tional adjustment, combustion chamber
redesign, and catalytic exhaust gas
treatment. Operational adjustment
techniques can be considered demon-
strated and are finding current appli-
cation. Combustion system redesigns
are currently being developed and have
seen, at best, laboratory scale testing.
The use of catalysts to reduce NOx
emissions from lean-running engines
(selective catalytic reduction) has seen
only laboratory scale testing. Similarly,
early limited testing of NOx reduction
catalysts for rich-running engines (non-
selective catalytic reduction) has been
performed.
The operational adjustment techniques
are derate, ignition retard, air-to-fuel
ratio change, reduced manifold air
temperature, exhaust gas recirculation
(EGR) (both internal-restricting the exit
of exhaust gases from the cylinder, and
external-reintroducmg exhaust gases
into the intake manifold), and water
injection. All these techniques essen-
tially act to lower the peak combustion
temperatures, thereby limiting thermal
NOx formation. These techniques can be
seen used in combustion, although NOX
reductions are not always additive.
Combustion system redesigns have
been aimed at improving cylinder
mixing, enhancing combustion, or
establishing some form of staged
combustion. The first twoallowefficient
combustion to occur under leaner
lower-temperature conditions. The
third, in addition to lowering peak
temperature, lowers oxygen availability
at peak temperature.
For diesel engines, mixing can be
improved by circumferential injection,
chamber shape, or a variable area
prechamber. Combustion in gas engines
can be improved by torch ignition,
multiple spark plugs, high energy spark,
increased turbulence through swirl or
"squish," or diesel fuel injection.
Staged combustion techniques include
divided chambers, open chambers, or
degraded mixing for gas engines, and a
prechamber or pilot injection for diesel
engines.
Catalytic reduction is a flue gas
treatment technique in which exhaust
gas is passed over a reduction catalyst
which reduces N0xto NOz. Nonselective
reduction catalysts can be used with
rich-running engines since very little
oxygen exists in their exhaust. However,
lean-running engines require selective
reduction catalysts which further
require injecting a reducing agent,
ammonia, into the exhaust stream.
Table 2 lists the various combustion
modifications that have been investi-
gated for 1C engines and shows the NOx
reduction and fuel penalties associated
with these controls as a function of
engine type.
Currently, the best demonstrated
controls, the only ones sufficiently
demonstrated to allow meeting the
proposed 1C new source performance
standards (NSPS), are: (1) retarded
ignition or retarded fuel injection, (2)
air-to-fuel ratio changes, (3) increased
manifold air, or (4) in combinations with
the others. The best combination will be
very engine dependent. But in general,
retard is best for diesel-fueled engines,
air-to-fuel ratio changes for natural gas,
and either control for dual fuel. A 40
percent reduction m N0xcan usually be
achieved without causing any major
operational problems, but there are fuel
consumption penalties.
For the future, combustion system
redesigns have the potential for obtain-
ing the same level of NOX reduction (40
percent) but with lower costs and fuel
penalty. For very low NOX emissions,
only catalytic reduction techniques
show promise.
Table 3 compares the estimated
annualized incremental costs of retard,
air-to-fuel increase, and exhaust gas
recirculation applied to various engines
to those of the corresponding uncontrolled
engines Costs in Table 3 represent
annualized costs in mills/kWh (assum-
ing 8000 hours of operation per year)
and are in 1978 dollars. Table 3 shows
that ignition retard increases the total
cost of power 6 to 7 percent, air-to-fuel
increase increases power costs 3 to 7
percent, and EGR increases power costs
5 to 14 percent. Though not shown in
Table 3, manifold air temperature
reduction should only have a small cost
impact, about a 1.5 percent increase in
initial engine cost and an increase in the
cost of power of about 1 percent. Derate
is a viable technique only if spare power
is available elsewhere. Though derating
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Table 2. /VOX Reduction and Fuel Consumption Penalties for Diesel, Dual-Fuel, and Gas Engines
Engine Fuel Type
Control Approach
Derate 3%
6%
1O%
2O%
25%
Retard 2°
4°
8°
Air-to-Fuel 2%
3%
5%
±10%
Manifold 31 1k(100°F)
Air 315k(107°F)
Temperature 318k(11 3°F)
Internal EGR
External EGR 1O%
Retard and Manifold
Air Temperature
Retard & Air-to-Fuel
Retard and Manifold
Air Temperature and
Air-to-Fuel
Air-to-Fuel and
Manifold Air Temperature
Water Injection 5O%
(HzO/fuel ratio) 100%
Catalytic Reduction
(Projected
Combustion Increased
Chamber Mixing
Modifications
(Projected) Staged
Combustion
%/VOx
Reduction
<20
5-23
<20
<40
28-45
7-8
7-15
5
<20
33
<20
10-24
<20
<40
35-65
20
<20
20-30
25-35
50-80
10-30
10-30
Diesel
&BSFC, %a
4
1-5
4
4
2-8
3
0-2
2
;
/
1
0-1
8
16
5-26
0
2
3
2-4
0
<5
0
Dual Fuel
%/VOx
Reduction
<20
7-33
<20
<40
50-73
<20
25-40
18-37
<20
20
<20
25
<20
<40
56
<20
40
50-50
20-40
70-30
&BSFC, %a
4
7-7
3
7
3-5
0
7-3
0-7
7
7
7
2
7
2
2
2
3
0
<5
0-7
Natural
%/VOx
Reduction
<20
<40
5-90
<20
8-40
<20
<40
20-80
28
<20
<20
5-35
<20
33
<20
30-40
<20
<40
77-52
<20
<40
40-65
25-35
60-75
50-50
20-40
70-30
Gas
AfiSFC, %a
2
3
2-72
3
2-7
2
7
5-72
0
0
5
0-5
0
0
3
5-6
4
5
4-77
2
4
6-7
7-2
2-5
0
<5
0-2
Brake specific fuel consumption penalty.
would increase fuel consumption and
raise operating costs, specific figures
are not given because of the difficulty in
specifying highly site dependent costs.
In general, as shown in Table 3, the
incremental initial capital costs of the
available controls range from 0 to 5
percent of an uncontrolled engine's
cost. However, the total annualized cost
to control can increase the cost of power
from an engine by 3 to 14 percent, the
significant impact due to additional fuel
and maintenance requirements.
By combining control techniques, it
may be possible to achieve the same
NOx reductions with a smaller fuel
penalty, or reduce NOx levels more than
could be achieved by each technique
alone. Table 4 compares three different
methods for reducing NOX by 40 percent
from a large bore diesel engine. For this
case, there is a definite advantage to
using combined controls since the two
combined techniques, air-to-fuel ratio
change and manifold air cooling (or air-
to-fuel ratio change and retard), had a
lower brake specific fuel consumption
penalty (BSFC) than retard alone.
Cost estimates for combustion system
redesign controls vary significantly due
to the developmental state of these
techniques. Estimates indicate that
these redesigns will fall between 0.5
and 20 percent of the capital cost of a
large engine, with 3 percent being
typical. Operational and maintenance
costs should increase very little because
the goal of development is to keep
BSFC changes negligible.
Operational Impacts
of Controls
Since engines are currently optimized
for minimum maintenance requirements
and fuel use, any control technique
which varies engine parameters from
standard conditions will impact opera-
tion and maintenance. Some of these
impacts have been well characterized,
especially from control techniques
which involve engine operational changes
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Table 3. Annual/zed Control Costs for 1C Engines3
Control Techniques
Air-
Retard Ratio
Percent Incremental Percent
Uncontrolled Engine /V0X Cost, /V0«
Typical Engine Cost, Mills/ kWh Reduction milts/ kWh Reduction
3000kW Diesel Capital 6 0
(Electrical Maintenance 5 20-30 1.6 20
Generation) Fuel 32 1.2
Total 43 2.8
3000 kW Dual Fuel Capital 6 0
(Electrical Maintenance 5 20-30 1.6 40
Generation) Fuel 20 0.7
Total 31 2.3
3000 kW Natural Gas Capital 6 0
(Gas Transport) Maintenance 5 20-30 1.6 40
Fuel 22 0.8
Total 33 2.4
75O kW Natural Gas Capital 2 0
(Gas Production) Maintenance 5 20-30 1.6 40
Fuel 25 0.9
Total 32 2.5
a Assumes 8000 hours of operation per year, 1978 dollars.
Table 4. Estimated Incremental Cost of Combined Controls for a Large Bore Diesel
Engine at 40 percent NOX Reduction
Incremental Contr°! Technique
Annualized
Control Cost,3 Air-to-Fuel Changes and
mills/ kWh Retard Manifold Air Cooling Air-to-Fuel and Retard
Capital 0 0.1 0
Maintenance 1.6 1.0 1.8
Fuel 2.4 1.5 1.5
Total 4.0 2.6 3.3
a Assumes 8000 hours in operating year, 1978 dollars.
Other control techniques will require generally increase fuel consumption.
various degrees of evaluation before Finally, operation at air-to-fuel ratios
impacts are clearly understood. where misfiring or detonation occur can
Derate, air-to-fuel ratio changes, cause severe engine damage
manifold air cooling, and ignition retard Increased manifold air cooling has
present the fewest problems in operation little operational or maintenance impact
and maintenance. Derate has no impact for a unit that is already intercooled, but
mechanically and can improve durability will increase the size of the heat
because of lower operating temperatures exchanger, water or air pump, control
and pressures However, additional system, and other system components.
engines may be required to replace the Of course, backfilling mlercooling on an
losi power. Fuel penallies are usually engine will add mainlenance attendant
low. to additional temperature reductions.
Air-to-fuel ratio changes in the lean Changes in fuel consumption are small.
direction cause a power loss if larger When properly applied, ignition retard
blowers orturbochargers must be used. has no serious mechanical drawbacks.
If the engine must be operated richer to Some increase in operational and
reduce NOX emissions, other emissions maintenance time would be needed to
(e.g., smoke, CO, and HC) can increase. ensure that the degree of retard is
This could cause an attendant increase always within safe limits. Increases in
in engine maintenance. Changes in air- fuel consumption are moderate. Exces-
lo-fuel ralio in eilher direclion will also sive amounls of relard, however, can
to-Fuel
Change External EGR
Incremental Percent Incremental
Cost, /VOx Cost.
mills/kWh Reduction mills/kWh
0 0.3
0.2 20 2
3.1 0
3.3 2.3
0 0.3
0.2 20 4
0.4 0
0.6 4.3
0 0.3
0.2 20 4
0.4 0
0.6 4.3
0 0 1
0.2 20 4
0.4 0
0.6 4.1
create severe engine problems. Fuel
consumption increases rapidly, power
drops, misfiring can occur, and smoke
levels increase. In addition, mechanical
maintenance will increase if the exhaust
temperature exceeds the safe limits for
valves or the lurobcharger (usually 920
K-1200°F). More frequenl engine
teardown will be required, and higher
initial costs will result for higher
temperature materials.
Exhaust gas recirculation requires
new hardware components which may
require added maintenance, Problems
of fouling the flow passages of the
cooling heat exchanger, the engine
turbocharger, and the aftercooler with
participate must be solved, or frequent
engine teardown will be required.
Under varying load conditions a so-
phisticated control system is required
or the engine may stall or emit unac-
ceptable smoke levels Fuel consump-
tion penalties with EGR are small.
Water injection can cause severe
maintenance problems. Deposits from
untreated water can build up on
internal engine surfaces, and also foul
the lubricating oil. The problem can
lead to major engine maintenance.
Water injection also adds another
system to the engine which must be
maintained and controlled.
Although not demonstrated, com-
bustion system modifications are
expected to present the least impact to
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operation and maintenance. Mainten-
ance requirements can be expected
to increase slightly if additional injectors,
spark plugs, and valves are added to
the chamber. However, because this
control technique involves new design,
many of the additional maintenance
requirements can be designed out.
Fuel penalties are expected to be small.
Catalytic reduction will require no
additional engine maintenance, since it
is a flue gas treatment technique,
rather than an engine modification.
However, operating the catalyst system
may be expensive. Fouling the catalytic
surfaces with particulate may require
frequent regeneration. The catalyst
may also have a relatively short life and
need to be replaced. Another system,
ammonia injection, must be included in
engine operation. Finally, harmful
products of the reaction may be
produced if the catalyst temperature
varies from the proper level, or if
excessive ammonia is injected. The
catalyst must be installed and operated
on an engine before all these effects
can be quantified.
Currently available operational ad-
justment NOx controls can only reduce
emissions by approximatley40 percent,
while significantly increasing operating
cost and maintenance. Advanced com-
bustion chamber redesigns have the
potential of achieving similar NOx
reductions but at lower cost and smaller
fuel penalty. If very low NOX emissions
are required, catalytic exhaustgastreat-
ment is the only developing technique
with that potential.
Table 5 lists achievable control levels
and associated control techniques and
costs for typical diesel, natural gas, and
dual fuel engines, all assumed to be
turbocharged. In the case of natural gas
and dual fuel engines, the obvious
preferred approach from a cost-effec-
tiveness view would be to go directly to
the more stringent control level with air-
to-fuel adjustment. Note that all values
discussed are typical, and may vary from
engine to engine.
Combustion modification controls
can reduce NOX emissions without
significantly increasing CO and HC
emissions from most engines. However,
the kinds and distribution of organic
compounds emitted from diesel engines
are not well characterized and, there-
fore, are of potential concern.
Recommendations
There are two major weaknesses in
the data base for combustion modifica-
Table 5. Projected Control Requirements and Costs for Alternate NO* Emission
Levels
Type
Diesel
Natural Gas
Dual Fuel
A/Ox Emission.
g NOt/kWh output
17
14
12
10
15
12
11
9
11
9
8
7
Control Techniques
Baseline
Exhaust gas
recirculation
Retard
A/F increase + retard
Baseline
Exhaust gas
recirculation
Retard
A/F increase
Baseline
Exhaust gas
recirculation
Retard
A/F increase
Control Cost,
mil/s/kWh
output
2.3
2.8
3.3
4.3
2.4
1.0
4.3
2.3
1.0
tion controls on 1C engines. The infor-
mation on operational effects and long-
term durability of these control tech-
niques is incomplete, especially con-
cerning combustion system redesign
and catalytic exhaust gas treatment.
I nf ormation on combining these controls
to achieve an optimum of low emissions
other than NOX, CO, and total HC is very
limited. The amounts and types of
organics emitted from these large bore
engines are not very well characterized.
The potential mutagenicity of organic
emissions in diesel exhaust is of major
concern.
Research is needed on designing i
high efficiency Iow-N0x emitting engine
Even with the best available contrc
applied, the large bore stationar
reciprocating 1C engine is the highes
NOx emitter on a heat input basis of al
major combustion sources.
EPA is currently sponsoring severe
programs in the health effects area a
well as new engine designs for low-NC
and high efficiency. These program
should help resolve many of the majo
data gaps in the operational ani
environmental impacts of NOX controls
H. I. Lips, J. A. Gotterba. K. J. Lim, and L. R. Waterland are with Acurex
Corporation, Energy and Environmental Division, Mountain View, CA 94042.
J. S. Bowen is the EPA Project Officer (see below).
The complete report, entitled "Environmental Assessment of Combustion
Modification Controls for Stationary Internal Combustion Engines," (Order
No. PB 82-224 973; Cost: $13.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
US.QOVERNMENT PRINTING OFFICE:1M2-S5»-M2-423
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Environmental Protection Information
Agency Cincinnati OH 45268 Protet
Agency
EPA 335
Official Business
Penalty for Private Use $300
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