United States
Environmental Protection
Agency
Industrial Environmental
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-84-049 May 1984
<>EFV\ Project Summary
Fundamental Study of Sulfate
Aerosol Formation, Condensation,
and Growth
Shui-Chow Yung, Cumbum N. Rangaraj, Benjamin L. Hancock,
Daniel Ugale, and Seymour Calvert
A theoretical and experimental pro-
gram was performed to study the for-
mation and growth of sulfate particles.
Existing theoretical models on acid par-
ticle formation and growth were re-
viewed and evaluated. The formation
and growth of sulfate particles during
slow cooling, rapid cooling, and dilution
cooling of flue gas were experimentally
determined and compared with theo-
ries.
The experimental results show that
the temperature at which the self-
nucleation of sulfuric acid occurs is
lower than the acid dew point tempera-
ture. Thus, if the flue gas is slowly
cooled to between dew point and nu-
cleation temperature, it is possible to
force the sulfuric acid to condense out
on surfaces, rather than forming fine
particles.
The theories, experimental methods,
and results are described in this report.
This Project Summary was developed
by SPA's Industrial Environmental Re-
search Laboratory, Research Triangle
Park, NC, 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).
Introduction
Primary sulfates are significant pollutants,
contributing to the formation of acid rain,
reduced atmospheric visibility, and human
respirable diseases. To illustrate the mag-
nitude of the sulfate problem, more than
one-third of the airborne respirable particles
in the Eastern U.S. are in the form of sulfate.
A major source of atmospheric sulfate par-
ticles is the chemical reaction of SO2 and par-
ticles in the ambient air. However, control
of secondary sulfates through direct control
of S02 emissions has not generally resulted
in a decrease in atmospheric sulfate. There-
fore, it is worthwhile to consider control
techniques for primary sulfates.
Existing particle control systems do not ef-
fectively remove condensible aerosols be-
cause the aerosol precursors are often in the
vapor state when they pass through the con-
trol device. Although the vapors usually will
condense in a wet scrubber, they often form
ultrafine particles which are very difficult to
capture.
The first step toward developing accep-
table technology for reducing condensible
aerosol emissions is to obtain an adequate
data base and understand the mechanisms
involved: the condensation, formation, and
growth of sulfate aerosols in a simulated flue
gas environment.
Under contract to the U.S. Environmental
Protection Agency, A.P.T., Inc., performed
a detailed theoretical and experimental study
of sutfate particle formation and growth. The
theories, experimental methods,and results
are described in this report.
Objectives
This research was a theoretical and ex-
perimental study of sulfate aerosol formation
and growth under conditions that exist in in-
dustrial smoke stacks and the near-stack
plume. The objectives were to develop the
fundamental data and mathematical models
necessary to design emission control stra-
tegies and control devices for sulfate con-
densation aerosols.
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Approach
The general approach was first to define
the mechanisms by which S02 and S03 are
converted to sulfate particles and then use
this knowledge to develop optimum control.
S02 and S03 can convert to sulfate particles
by:
1. Condensation of sulfuric acid vapor (or
water-vapor-associated S03) to form
sulfuric acid drops.
2. Condensation of sulfuric acid vapor on
pre-existing particles such as fly ash,
liquid drops, and acid drops.
3. Sorption of S02 by liquid or solid par-
ticles, followed by oxidation of S02 to
sulfate.
4. Gas-phase oxidation of S0a to S03
which is subsequently condensed by
mechanisms 1 and 2.
5. Chemical reaction of acid vapor with
solid or liquid particles.
This study emphasizes sulfate formation
by mechanisms 1 and 2. Mechanism 4 is not
likely to happen in industrial smoke stacks,
and mechanisms 3 and 5 are system depen-
dent and difficult to generalize.
Sulfate formation due to condensation is
a physical process in which the gas tem-
perature must be below the acid dew point.
Gas can be cooled by heat transfer to the
surroundings, quenching (such as by in-
troducing water sprays), and mixing with
cold gas. All three cooling processes could
occur in industrial smoke stacks and in near-
stack plumes. Therefore, sulfate formation
in typical flue gas mixtures during slow cool-
ing, rapid quenching, and dilution cooling
was experimentally studied. The results ob-
tained in this study plus published data were
then used to verify sulfate formation models
and develop emission control technologies.
Experiments
Apparatus
Figure 1 shows the experimental system
design, which was basically the same for all
experiments. It consisted of a flue gas
simulator for supplying acid-laden gas for
various cooling apparatus arrangements.
Major components of the flue gas simulator
included an acid vapor generator, a steam
generator, a fly ash particle generator, and
a S02 gas cylinder.
Sulfuric acid and water vapors were gen-
erated by evaporating dilute sulfuric acid and
water at controlled rates. Fly ash particles
were produced by re-dispersion. S02 was
metered into the flue gas simulator from the
gas cylinder. Room air was used as the car-
rier gas because the amount of oxygen and
the presence of CO2 and nitrogen oxides in
the flue gas have no effect on sulfate parti-
cle formation in the stack.
Air
Double Pipe Heat Exchanger
-Water
Packed Bed
I
Air
Gas
Slow Cooling
Experiments
Dilution Chamber
-Air
Heater or Cooler
Rapid Cooling
Experiments
"Gas
Dilution Cooling
Experiments
Figure 1. Experimental system design.
The following cooling apparatus was used
in the experiments:
1. Slow cooling — double-pipe heat ex-
changer.
2. Rapid quenching — packed-bed
column.
3. Dilution cooling — parallel-stream dilu-
tion chamber.
Measurement Methods
Gas samples at the cooling apparatus inlet
and outlet were obtained simultaneously to
determine:
1. Inlet acid vapor, water vapor, SO2, and
fly ash particle concentration.
2. The amount of acid vapor condensed
on existing particles.
3. Concentration of newly formed acid
particles.
4. The amount of acid vapor condensed
on walls.
5. Particle size distribution.
Figure 2 shows the sulfate sampling sys-
tem. Train "A" (used at the inlet) consisted
of a cascade impactor (or quartz filter)
followed by a condensation coil, a quartz
filter, three impingers, and gas metering and
moving instruments. Train "B" (used at the
outlet) consisted of an impactor (with final
filter removed) connected in series with a
screen diffusion battery, a quartz filter, a
condensation coil, another quartz filter, three
impingers, and gas metering and moving in-
struments. The instruments upstream of the
condensation coil were heated to and main-
tained at gas temperature. The water-cooled
condensation coil was maintained at be-
tween 60 and 90 °C when SO2 was injected.
The acid collected by the impactor, diffu-
sion battery, and filter was analyzed by ex-
traction and titration. Acid collected by the
condensation coil and impinger was deter-
mined by washing followed by titration.
Results
Rapid Quenching
Rapid quenching experiments simulated
the formation of sulfate particles in a scrub-
ber. The quencher used in this study was a
randomly packed bed of Berl saddles. The
effects of acid vapor concentration (10 to 100
ppmV), water vapor concentration (5 to 15%
by volume), SO2 concentration (0 to 1.3
g/m3), and quench water temperature (20 to
60°C) on nucleated acid particle size distribu-
tion were determined experimentally. The
results are:
1. Higher water temperature and lower
acid and water vapor concentrations
resulted in higher concentration of fine
acid particles. Low water temperature
decreased the gas temperature and
caused additional water vapor conden-
sation on the nucleated particles.
Decreasing the acid and water vapor
concentration reduced the particle
growth.
2. The presence of SO2 had no effect on
acid-bearing particle nucleation. ^
3. In the presence of fly ash particles, •
acid-bearing particles were larger
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Nozzle \
Impactor or
Filter
Heated
i Condenser
i 1 [UL
i i lilr
_ V 1
T
SV-*
&
To Impingers and
-EPA Method5
Sampling Train
Constant Temperature
Water Bath
Nozzle \
Train "A"
I
Casacade
Impactor
uiiiuxionouiieiy , To Impingers
4 IYI IVI IVl PVl L*J ! Ur^ i M-i_an
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Table 1. Predicted and Measured Nucleation Temperatures
Inlet
Run
No.
116/01
116/02
116/04
116/05
117/01
117/02
117/03
118/01
118/02
119/01
119/02
Acid Vapor
Cone., ppm
83.4
49.9
52.1
61.9
29.0
40.2
35.5
22.1
27.6
22.0
31.4
Water Vapor
Cone., Vol. %
10
5.5
4.7
14.5
10.1
5.0
13.8
10.1
14.0
10.1
4.8
Acid Vapor
Cone., ppm
65.2
39.6
46
60.4
11.6
19.9
26.1
19.3
17.3
4.6
17.7
At Nucleation Point
Water Vapor
Cone., Vol. %
10
2.8
2.9
14.5
4.9
2.5
12.2
9.6
11.6
10.6
3.1
Gas
Temperature
°C
106
96
99
109
104
86
103
96
103
91
101
Predicted
Nucleation
Temperature
°C
105
97
98
113
95
91
105
98
102
99
90
120
110
o
3
!
I 100
I
90
Percent by Volume
Water Vapor
80
Figure 3. Predicted nucleation temperature.
and they agree with our experimental
results. Thus, the sulfuric acid nuclea-
tion temperature for flue gas can be
predicted, if the gas pressure, water
vapor concentration, and S03 concen-
tration are known.
2. Avoiding rapid cooling of flue gas
minimizes the formation of fine acid
particles.
3. While the presence of S02 has no ef-
fect on acid nucleation, fly ash particles
160
150
140
130
3 120
5
QJ
100
90
80
Verhog and Banchero
Lisle and
Sensenbaugh
Dew Point Temperature
Predicted Self-Nucleation
Temperature
i
0 10 20 30 40 50 60 7O 80 90 100
Concentration, ppmv
have great effect. Most of the acid
vapor condenses on ash particles if the
gas is cooled slowly.
4. Increasing the water vapor concentra-
tion in the gas stream increases the
acid particle diameter by additional
condensation or solution-induced par-
ticle growth.
0 10 20 30 40 50 100
Sulfuric Acid Vapor Concentration, ppm
Figure 4. Dew point and acid nucleation
temperatures.
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S-C Yung. C. N. Rangaraj. B. L Hancock. D. Ugale, andS. Calvertare withA.P.T.,
Inc., San Diego, GA 92109.
Leslie E. Sparks is the EPA Project Officer (see below).
The complete report, entitled "Fundamental Study of Sulfate Aerosol Formation,
Condensation, and Growth," (Order No. PB 84-179 886; Cost: $ 17.50, 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
Research Triangle Park, NC 27711
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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U.S. GOVERNMENT PRINTING OFFICE: 1984-759-102/957
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