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United States
Environmental Protection .
Agency
Environmental Sciences Research - ,
Laboratory ", ' '
Research Triangle ParkNC 27711 '' >
Research and Development
EPA-600/S3-81-041 Oct. 1981
Project Summary
Characterization of
Scrubbed and Unscrubbed
Power Plant Plumes
H. M. Barnes
Airborne measurements of scrubbed
and unscrubbed plumes from the
Widows Creek Steam Plant were
made during August 17 to 25,1978,
under the SCRUB program. Data from
the flight program (except size distri-
bution data) and preliminary data
analysis results have been previously
published in a Data Volume.
This report briefly describes the
flight program and methodology of
SCRUB and gives an analysis of the
data gathered. The results cover
plume chemistry, primary aerosol,
aerosol size distributions, and aerosol
formation rates among the scrubbed
and mixed plumes.
Little difference was seen in photo-
chemical aerosol and sulfate formation
in the scrubbed and unscrubbed
plumes. However, measurement noise
and plume mixing may have obscured
moderate differences. The submicron
primary emissions from the scrubbed
unit were only about 14 percent of
those from the unscrubbed unit.
Sulfur dioxide to sulfate conversion
rates in the plumes were between 0
and 3.2 percent per hour. Aerosol
formation rates varied between 0 and
0.30 microns3/(ppb SO2 - hr - cm3).
More sulfate was measured than
could be accounted for by the aerosol
measurements.
Ozone bulges of 40 ppb were typical
in the afternoon 50 kilometers down-
wind of the plant.
Photochemical reactions were most
rapid when the plume was dilute and
the sunlight strong.
Submicron primary emissions from
all units appeared to be mostly sulf uric
acid.
This Project Summary was devel-
oped by EPA's Environmental Sci-
ences 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
The production of primary and sec-
ondary pollutants has been studied
extensively in the plumes of unscrubbed
power plants, but little work has been
done to study the effects of wet
scrubbers on downwind plume chemis-
try. The environmental laws coupled
with the cost of low sulfur coal are such
that the installation of SO2 wet scrubbers
are certain to become more common in
the future. Therefore, it is necessary to
obtain information on any possible
adverse effects from using wet scrubbers
to control SOz emissions from utility
boilers.
A number of studies have been
conducted to establish gas to particle
conversion rates in urban and in
conventional power plant plumes.
These studies indicate conversion rates
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for SC>2 to sulfate aerosols of between 0
and 10 percent per hour.
One of the principal pathways leading
to sulfate formation in the atmosphere
is the S02 + OH radical reaction. Since
the hydroxyl radical is formed photo-
chemically, the conversion rate should
depend on the amount of solar radiation.
SC>2 oxidation may also take place on
the surface of existing particles or in
water droplets. This liquid phase
oxidation may play an important role
under high tiumidity conditions or in
moisture laden power plant plumes.
Thus, the S02 conversion rate may be
affected by the droplets produced by a
wet scrubber on a power plant.
The purpose of the study was to
investigate the S02 conversion rate in
the plume and to determine if it differed
significantly from that found in un-
scrubbed power plant plumes.
Experimental Approach
The Tennessee Valley Authority
Widows Creek Steam Station near
Stevenson, Alabama, was studied in
this project. The plant's six 135 MW
units (Units 1-6) are connected to a
single 1000 foot stack. Two additional
557 MW units have individual 500 foot
stacks and electrostatic precipitators of
90% efficiency. Unit 8 is equipped with a
combined venturi and limestone wet
scrubber having a design efficiency of
80% for SOz removal. The effective
paniculate removal efficiency for Unit 8
is 99.5% by weight. Ammonia (about 10
ppm) is added to the Unit 7 flue gas to
increase the particulate collection
efficiency. Units 1 -6 burn 1 percent
sulfur coal; Units 7 and 8 burn 4 percent
sulfur coal.
The field study involved sampling in
the power plant plume using a Beech-
craft Queen Air fixed wing airplane to
carry the sampling equipment. The
sampling and monitoring equipment is
listed in Table 1.
Three types of flight maneuvers were
used in the program: traverses, spirals,
and orbits. Traverses gave horizontal
distributions of measured parameters.
Spirals gave vertical distributions of the
important species. Orbits were per-
formed to obtain filter or impactor
samples at a specific location.
To produce plume cross sectional
plots, the data from the traverses were
plotted in cross sections with the
endpoints of each traverse fixing the
traverse line. Contours were then
drawn. Since completing a traverse
Table 1. Queen Air Instrumentation
Parameter Manufacturer/Model
Analysis Technique
SOZ
03
Sulfate
Light Scattering
Condensation
Nuclei
Turbulence
Temperature
Elemental
Analysis
Dew Point
Altitude
Meloy 285
Monitor Labs 8440
CSI
MRI Two-Mass
ERT RSP sampler
MRI 1550
MRI 1569
Environment One
Rich 100
MRI 1120
YSI/MRI
Lundgren 4-stage
Impactor
Cambridge Systems 137
Validyne
Flame Photometric
Chemiluminescence
Chemiluminescence
Flash vaporization/flame
photometric
Ion Chromatography
Integrating nephelometer
Light Attenuation
Pressure Fluctuations
Bead Thermistor
Analysis by PIXE
Cooled Mirror
Absolute Pressure Transducer
Airspeed
Position
Aerosol Charge
Acceptance
Data Logger
Stripchart
Recorder
Particulate
Sulfur
Aerosol Size
Distribution
Aerosol Size
Distribution
Aerosol Size
Distribution
Validyne
King KX1 70B/HTI DVOR
Washington U.
MR! Data System
Linear Instruments
Meloy 285
TSI 3030
Roy co 218
Knollenberg ASSP
Differential Pressure Transducer
Aircraft DME/VOR
Aerosol Charge Acceptance
9 -Track Tape - 6 hr. Capacity
Dual channel
Upstream SOz Scrubber/
Measurement of Total Sulfur
Charger/Mobility Analysis i
Optical Particle Counter
Axial Scattering Spectrometry
requires 15 to 60 minutes, these cross
sections give true plume concentrations
only for steady state situations.
A total of ten flights were flown
during August 17-25, 1978. All data
collected are available in a Data Volume
and on magnetic tape. Several flights
provided studies of the scrubbed and
unscrubbed plumes. On August 17 and
23, Unit 7, the largest unscrubbed unit,
was not operating; flights on those days
gave data on the scrubbed and mixed
plumes. On August 19, morning and
afternoon flights were made while all
units were operating.
The weather varied little during the
study period. The days were clear and
the plume well mixed by 10 or 11 a.m.
each day. A moderate to strong inversion
formed each night and broke up by 9
a.m. the following morning. Winds were
moderate at plume elevation, typically
10-15km/hr.
Results and Discussion
Photochemical aerosol formation in
the plume was calculated in two ways:
(1) using sulfate concentration deter-
mined from filter samples and (2) using
aerosol size distributions measured
with the electrical aerosol analyzer
(EAA) and the bag sampling system. The
sulfate results are discussed, then the
EAA, and finally a comparison is made
between the two techniques.
Sulfate Measurements
Three methods were used to deter-
mine sulfate concentrations. Sulfate in
the Lundgren impactor samples was
measured as total sulfur by proton
induced X-Ray Emission. The sulfur
values on the impactor filter and last
stage were added together to give the
sulfur in particles less than one micron
in size. Sulfate on the Two Mass filter^
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was measured by flash vaporization
followed by flame photometric analysis.
Sulfate on the Teflon™ coated glass
fiber filters was extracted with water
and measured by ion chromatography.
Of the three methods the 1C analysis
of the Teflon™ coated filters was judged
to be the most accurate because of its
simplicity and because the technique
was unlikely to introduce errors. Pair-
wise comparisons among the three data
sets showed good correlation (rz=0.74)
between the flash vaporization/FPD
and the 1C values. Other correlations
were not very good. In all data analysis
the FPD values were used since they
correlated well with the 1C results and
since many more FPD samples were
available than 1C ones.
Table 2 shows the sulfate formation
rates obtained using the FPD data.
The data set exhibits significant
'scatter, i.e., some excessively large
sulfate numbers and several negative
values, which are physically unrealistic.
As an entity, the data appear to be
reasonable and generally consistent
with results from other studies showing
0-10% hr"1 conversion rates of SO2 to
sulfate aerosol.
Aerosol Formation
Aerosol formation rates were calcu-
lated using the EAA size distribution
data. The data are shown inTable 3. The
plume excess aerosol volume has been
divided by the S02 concentration to
reduce the impact of plume dilution in
the comparison. Plume excess volumes.
Vpe were calculated using_the following
equation: Vp, = VE_- VB - S02(VVS02*)
where Vp and S02 are the average
aerosol volume and SOz concentration
in the plume at a given distance, VB is
the average aerosol concentration in
the background air, and V*/S~02* is the
ratio of primary source aerosol concen-
tration to S02 concentration obtained
from measurements close to the source.
Table 3 shows that aerosol formation
did not occur significantly near the
source or early in the morning even on
sunny days. Maximum rates were
observed near noon or in the early
afternoon.
The SOz conversion to aerosol was of
interest in these calculations. Using the
EAA data, the SOz conversion rate is the
normalized volume formation rate times
a constant, Cs, which depends on the
aerosol composition. The following
assumptions were made: (1 ) the sulf uric
acid formed by S02 oxidation was
Table 2.
Date
8/17
8/17
8/17
8/19
8/19
8/19
8/21
8/21
8/23
8/23
8/25
8/25
8/25
8/25
In Plume Sulfate
Sample Times
Morning
0853-0933
1024-1116
1129-1159
Afternoon
1551-1635
1732-1805
1821-1852
A verage
0853-1852
Early Morning
071 7-0736
0756-0826
0845-0944
0917-0932
0955-1007
1011-1026
1038-1053
Average
071 7-0826
0955-1053
Afternoon
1407-1424
1516-1529
1553-1618
Morning
0734-0844
0857-0925
0936-1031
0949-0959
A verage
0734-1031
Morning
0944-0950
1134-1231
1252-1259
Afternoon
1441-1531
1710-1719
1822-1828
Morning
0543-0645
0652-0736
0830-0845
O857-0912
A verage
0543-0912
Late Morning
1053-1108
1240-1255
1301-1316
Average
1240-1316
11 May be left over plume.
n. a. - not
applJcable
Formation Based on Filter Measurements
Distance
(km)
0.5
3
9
0.5
3.01
7.0
<9
0.5
9
24
25
45
47
75
<9
45-75
58
0.5
9-13
15-20
30
15
20
15-30
11-14
50
50
70
50
13
0.5
0.5
20
8
45
50
45
47.5
travel time
Sulfur Conversion Rates
(%/hr)
From Start From Previous Distance
n.a
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
0.6 0.4
0.4 <0
1.4 2.7
1.1 1.6
1.3 1.4
1.3±0. 1 1.9±0.4
2.1
6.82
0.2
.8
0.4
0.5±0.2
0.7
n.a.
n.a.
0.6
2.9
3.2
3.0±0.2
very uncertain. 2I Number unreliable.
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Table 3. Aerosol Growth Rate Calculations
Aerosol Growth
Excess
Plume
Distance Vol./
Date Time (km) S02
8/17 Morning
1024-1115 3 <0
1128-1158 9 <0
8/17 Late Afternoon
1732-1751 3 0.055*
8/19 Early Morning
O7 59 -08 25 9 <0
\J / *J& W ^tJ *^ ^»V
0847-0907 24
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Table 4. Comparison
Sample
Time
8/17 Morning
8/17 Late Afternoon
8/19 Early Morning
8/19 Late Afternoon
8/21 Morning
8/23 Morning
8/23 Afternoon
8/25 Early Morning
8/25 Late Morning
of Sulfate and Aerosol Measurements
Sulfate As*
Aerosol Volume
Distance
(km)
0.5
3
9
0.5
3
7
0.5
9
24
25
45
47
75
9-13
58
15-20
15
20
30
11-14
50
13
50
70
0.5
0.5
8
20
45
45
50
Measured Aerosol*
Volume by EAA
3.8
3.1
5.4
0
2
30
0.58
2.3
4.6
3.3
16
9.1
8.8
58
4.3
4.1
0.9
<0
1.7
16
-0.2
1.1
24
6.7
0.58
8.4
<0
<0
3.9
0.50
1.2
3. The EAA data should be better
corrected for altitude and channel
cross sensitivity. This adjustment
might improve the estimates of the
aerosol formation rates. The in-
verted data may then be analyzed
to establish the relative contribu-
tions of gas and liquid phase
chemical reactions to aerosol
formation.
*Plume excess including primary emissions.
indicated by ozone formation in the
plume, was observed about 50 km
downwind on occasions when Unit 7
was and was not operating. The ozone
formation in the plume was more a
factor of solar insolation and plume
mixing with background air than of the
presence or absence of the scrubbed or
unscrubbed plume. The maximum
aerosol formation was measured for
downwind (40 km or more) when the
plume was well mixed and sunlight was
strong. When expressed as pseudo first
order sulfur conversion rates, average
aerosol formation rates of 2.5 percent
per hour were typical between 10 am
and 5 pm. The aerosol concentrations
predicted from the plume excess sulfate
measurements were usually about 3
times higher than the measured con-
centrations; this large disagreement
rprooably has more than one cause. No
useful nitrate formation data were
obtained because the amount of nitrate
collected on the filters was near the
blank values. Observations of N0«
removal from the plume indicate an
afternoon removal rate of 25 percent
per hour, a value much higher than for
S02.
The results of the study indicate
several linesof future research. Specific
recommendations include:
1. The large difference between the
sulfate and aerosol measurements
should be resolved, possibly by
comparing bscat measurements
with the sulfate and aerosol data.
2. The near source aerosol formation
rates should be investigated,
possibly by computing changes in
nuclei mode particle concentra-
tions.
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The EPA author H. M. Barnes (also the EPA Project Officer, see below) is with
the Environmental Sciences Research Laboratory, Research Triangle Park, NC
27711.
The complete report, entitled "Characterization of Scrubbed and Unscrubbed
Power Plant Plumes," was authored by G. R. Markowski, J. L Stith, and L.
Richards of Meteorology Research. Inc., Santa Rosa, CA 95401 (Order No.
PB 82-101 346; Cost: $8.00, 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:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
*U.S. GOVERNMENT PRINTING OFFICE:1981--559-092/3351
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