United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 277t 1
Research and Development
EPA/6QO/S7-90/011 June 1990
&EPA Project Summary
An Evaluation of the E-SO
Process on the EPA Pilot
Electrostatic Precipitator
X
Louis S. Hovis
The E-SOX Process makes use of an
electrostatic precipitator (ESP) for
combined sulfur dioxide (SO2)
removal and particulate collection.
The concept of spray drying is
introduced to the inlet and/or first
section of the ESP in which electrical
components are removed. Because
of the many ESPs at coal-fired power
plants, the process is well suited to
retrofitting. The work described in
this report was a small pilot-scale
evaluation of the process to obtain
the information needed to undertake
a planned 5 MWe field pilot
demonstration. The results from this
evaluation indicate that a 50 - 60%
removal of SO2 at a calcium to sulfur
ratio of 1.2 - 1.4 can be obtained.
Furthermore, this reduction in SO2
can be achieved without degrading
the particulate emissions even
though the process requires a
reduction in the collecting surface of
the ESP. The utilization of a
temperature-controlled electrode
precharger to compensate for loss of
collecting surface is also described.
This Project Summary was
developed by EPA's Air and Energy
Engineering 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
E-SOX is a retrofit process for coal-fired
boilers, which combines in a single unit
electrostatic precipitator (ESP)
technology for collecting particles and
spray dryer technology .'for sulfur dioxide
(SO2) removal. The process uses a
modified existing ESP equipped with an
auxiliary system for preparation and
injection of a lime slurry into the ESP.
The front end of an existing ESP is
converted to a spray ..chamber where
contact is made between gaseous SO2
and lime slurry droplets. Water also
evaporates in this converted section of
the ESP so that the reaction product,
excess lime, and fly ash that enter the
remaining portion of the ESP are
sufficiently dry for efficient ESP
operation. A dry solid waste product
containing reaction products, unreacted
lime, and fly ash is collected. That portion
of the ESP not converted to a spray
chamber is left with electrical
components intact to operate as a
particle collector. However, to provide the
contacting chamber sufficient space for a
finite drying time of 1s or greater, the
ESP will lose 25 to 30% of its collector
plate surface. At the same time, the
particulate load will increase by a factor
of 3 or more. This particulate, a large
fraction of which is calcium based, has an
extremely high electrical resistivity at
normal ESP operating temperatures. The
sorbent reaction with SO2 will eliminate
the effects of sulfur trioxide (SO3) that
would lower this resistivity. On the other
hand, lowering the gas temperature by
the water spray will more than
compensate for resistivity increases due
to ash composition. Advanced ESP
technology to aid in maintaining good
particulate collection performance is
available for E-SOX retrofitting if required.
Cooled pipe precharging, for example,
can be introduced in retrofit ESPs to
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compensate for less collector surface or
changes in resistivity characteristics of
the participate.
The concept of E-SOX appears to offer
an attractive option for acid rain
mitigation. An economic evaluation of E-
SOX based on reasonable rates of SO2
removal and lime utilization has indicated
that it can be cost effective. A large pilot-
plant evaluation is underway at Ohio
Edison's Burger Station sponsored by the
U.S. Environmental Protection Agency
(EPA) and the Ohio Coal Development
Office. Preparation for that pilot plant
evaluation started in 1987 and the actual
testing is being carried out in 1989. This
report covers work that was performed in-
house at EPA to verify the original results
and to define the parameters that control
SOg removal. The work- reported was
completed and the technology
transferred for use in starting up the field
site evaluation.
The E-SOX concept raises two
fundamental questions which can be
answered only by experiment. The first
question concerns the feasibility of
removing substantial SO2 by contacting
the rapidly moving gas with slurry
droplets and drying the droplets within
the space of one ESP section. The
second question has to do with
maintaining an acceptable level of ESP
performance under a reduced collector
area and an increased particulate loading.
The results of experiments to partially
answer these questions are reported
here.
Test Facility
All the experiments were conducted in
the ESP pilot-plant located at EPA's Air
and Energy Engineering Research
Laboratory (AEERL). The pilot-plant
consists of a four-section, single-lane
ESP operating at a flue gas capacity
equivalent to 0.47 m3/s".Outside air is
heated to 149°C by a natural gas heater.
Gaseous SO2 is injected into the heated
air to the desired concentration (usually
1,500 to 2,500 ppm) to simulate burning
of moderately high sulfur coal. The ESP
is operated under about 0.5 kPa negative
pressure so that no SO2 is released to
the room. Fly ash is aspirated counter
currently into the simulated flue gas
stream just before the cocurrent injection
of the lime slurry. Lime slurry containing
10 - 20% solids is pumped through a
spray nozzle designed to provide an oval
"Readers mom familiar with nonmetric units may
uso the conversion factors at the back.
spray pattern in the 1.27 x 0.381 m
contact chamber. The atomized slurry
droplets have a 1.5 - 2 s residence time
in the chamber to evaporate most of the
water. The evaporation causes a flue gas
temperature drop and results in a
relatively dry, powder-like product which
contains the unused lime, the absorbed
and reacted SO2, and fly ash. The spray
chamber consists of the entrance section
and the first of four ESP sections with all
the electrical internals (i.e., discharge
electrodes) removed. The electrical
configuration of the ESP is flexible but,
for most experiments reported, two cold
pipe prechargers were used: one in the
connecting space between sections 1
and 2 and one between sections 3 and 4.
Conventional wire-plate electrodes were
assembled in sections 2 and 4.
Summary of Results
The primary objectives of the E-SOX
experiments carried out at EPA using the
0.47 m3/s modified pilot ESP were to
verify E-SOX as a competitive retrofit
process for S02 removal and to
determine the critical parameters which
influence the degree of SO2 removal.
Once the critical factors were determined,
they could be adjusted within limitations
of the process to give the best conditions
for SO2 removal and sorbent utilization.
To meet these objectives, experiments
were planned to investigate impacts of
indirect variables as well as those directly
influencing the lime slurry/SO2 reaction.
SO2 Removal Dependence on
Critical Factors
A number of tests were performed in
which only concentration and rate of
injection of slaked lime were varied. In
essence, this permitted an examination of
the effect of the two most critical
parameters on SO2-removal; the
temperature of approach to saturation
(ATAS) and the stoichiometric ratio of
calcium to sulfur (Ca/S) in the slurry/gas
mixing. When other parameters are held
constant, including spray chamber
geometry, gas flow rate, S02 concen-
tration in the gas, and the inlet gas
temperature, these injection parameters
can be manipulated to give a ATAS and
Ca/S combination. There is a lower limit
on ATAS for adequate droplet drying and
an upper limit on the solids concentration
for consistent spraying. The fixed
conditions are listed in Table 1.
The removal of SO2 as a function of
approach temperature for various
stoichiometric ratios is shown in Figure 1.
This correlation between approach to
Table 1. E-SOX Fixed Conditions for SO2
Removal Studies
Air Flow
Inlet temperature
SO2
concentration
Fly ash
concentration
Nozzle
configuration
Inlet chamber
28 m3/min
149°C
2000 ppm
1.9 g/l
Single two-fluid CasterJet
oval spray pattern
1.5 m spray chamber
plus 1.2 m ESP section
"saturation"and SO2' removal shows that
good removal is possible at very low
stoichiometric ratios, but only at very
close approach temperatures. At these
close approaches the droplets are not
completely evaporated and excess
moisture will pass into the ESP. The
lower practical ATAS is believed to be
between 16 and 17°C. Stoichiometric
ratios of 1.3 to 1.4 produced a removal of
50% or better at a 17°C approach to
saturation. Results indicate a marginal
improvement in SO2 capture at a ratio of
1.4. The leveling off of removal rate with
increasing Ca/S is also reflected in the
plot of percent removal as a function of
Ca/S in Figure 2.
Particulate Removal
The second fundamental question
about E-SOX as a retrofit concerns
maintenance of an acceptable level of
particulate removal. In conjunction with
the SO2 removal testing, the ESP
electrical configuration was"varied to
determine effects of the increased load
and change in characteristics of the
particulate matter collected by the ESP.
With sections 2, 3, and 4 energized and
containing 0.32 cm wires, the ESP has an
18.8 s/m (96 SCA). With only two of the
sections energized, the ESP was reduced
to 12.5 s/m (64 SCA). Cold pipe
prechargers, between sections 1 and 2
and between 3 and 4, could be activated,
but the SCA would remain the same. The
data in Table 2 "show that high mass
efficiencies were obtained during a 4-fold
increase in particulate loading and a 50%
reduction in the SCA. There appears to
be no significant change in efficiency with
the amount of particulate as long as
some moisture is present. The ESP
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60
50
1"
o
I
DC
w
o
CO
40
30
6
O
S02
2000 ppm
Ca/S
• 1.5
O 1-4
A 1.3
A 1.2
...a.i.1
n
13.9
16.7 19.4 19.4
Approach to saturation, °C
Figure 1. Effect of approach temperature on S02 removal at several stoichiometric ratios.
55
50
as
75
(D
o:
t. * -..40
35
7.0
J.T
7.2 7.3
Ca/S
7.4
7.5
emission rates listed in Table 2 also
indicate that under E-SOX
conditions the ESP can meet or
exceed the NSPS standard of 43
ng/J. For fly ash only, the efficiency
was reduced severely without
moisture addition. In this case, the
38 kV could be maintained on the
cold pipe precharger, but not on the
wires because of back corona.
Evaporation of water during the
drying step lowers the temperature
which accounts for the low
resistivity of the lime sorbent/fly
ash mixture in the E-SOX process.
As a consequence of the low
resistivity, back corona is not a
problem.
Future Work K
The primary immediate E-SOX
follow-up will occur at the Burger
Plant of Ohio Edison where the
EPA, Ohio Coal Development
Office (OCDO), Babcock & Wilcox
Research Division and Southern
Research Institute are evaluating
the process. The evaluation is
being carried out in a 5 MWe pilot
ESP which is connected by a
slipstream off of the main ducts
between the boilers and the plant
ESP. The work plan at this site has
been designed to verify the SO2
removal results and the ESP
efficiencies that have been obtained
in the in-house process and
reported here. The work, having
been done on a larger unit, should
provide experience in design that
will be more meaningful for a full-
scale demonstration. The pilot
evaluation is slated for completion
in late 1989 with results to be
reported in the spring of 1990.
Figure 2. Effect of stoichiometric ratio on SO2 removal at two approaches to saturation.
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Table 2. E-SO, Particulate Removal Efficiencies for Various ESP Electrical Configurations
Sections Energized
38 kV
2,3,4
2,3,4
2,3,4
2,3
2,3
2,3,4
2,3,4
2.3.4
Cold Pipe
38 kV
Yes
No
No
No
No
No
Yes
Yes
Approach
Temperature (°C)
17
17
17
17
19
56
56
89*
Particulate
E-SOX & fly ash
Fly ash
E-SOX & fly ash
E-SOX & fly ash
Fly ash
Fly ash
Fly ash
Fly ash
ESP Efficiency
(%)
99.5
97.9
98.5
98.6
95.6
97.6
97.8
89.0
Emission Rate
(ngtJ)
7.319
13.776
12.915
37.884
24.969
12.915
16.359
55.104
*No moisture injection
NONMETRIC EQUIVALENTS
Readers more familiar with nonmetric units may use the following conversion factors:
Metric
°C
"C (app to sat.)
cm
g/i
kPa
m
MWe
nglJ
Multiplied by
9/5 x"C + 32
9/5 x °C
0.394
0.526
4.00
3.28
2128
3000
0.0023
Yields nonmetric
"F
"F (app to sat.)
in.
in. H2O
ft
cfm
acfm
lb/106 Btu
The EPA author, Louis S. How's, also the EPA Project Officer (see below), is with Air and Energy Engineering Research
Laboratory, Research Triangle Park, NC 27711.
The complete report, entitled "An Evaluation of the E-SOX Process on the EPA Pilot Electrostatic Precipitator," (Order
No. PB90-216 4411 AS; Cost: $17.00, 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:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S7-90/011
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