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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