EPA/600/D-85/252
October 1985
NEW TECHNOLOGY FOR THE CONTROL OF AEROSOLS FROM STATIONARY SOURCES
BY: Norman Plaks
Particulate Technology Branch
A1r and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, N.C. 27711
AIR AND ENERGY ENGINEERING RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NC 27711

-------
NOTICE
This document has been reviewed in accordance with
U.S. Environmental Protection Agency policy and
approved for publication. Mention of trade names
or commercial products does not constitute endorse-
ment or recommendation for use.
11

-------
NEW TECHNOLOGY FOR THE CONTROL OF AEROSOLS FROM STATIONARY SOURCES
by: Norman Plaks
Particulate Technology Branch
Air and Energy Engineering Research Laboratory
U. S. Environmental Protection Agency
Research Triangle Park, N.C. 27711
ABSTRACT
The Environmental Protection Agency has underway a program to develop
new technologies for controlling particulate matter from stationary sources,
including both electrostatically augmented fabric filtration (ESFF) and
electrostatic precipitators (ESPs). The first generation ESFF system,
which uses an electrostatic field parallel to the fabric surface, provides
a reduction in pressure drop to about SOS of the pressure drop in conven-
tional fabric filtration for both reverse-air and pulse-jet operation.
Second generation (or Super)ESFF, which utilizes an electrostatic field
perpendicular to the fabric surface, provides reductions in pressure drop
to 10 to 30%. Large diameter corona electrodes in ESPs have decreased
the penetration up to a factor of 4 as compared with conventional small
diameter corona electrodes. Work on the multistage ESP has shown that,
for high resistivity particulate matter, it is possible to construct an
ESP that provides equivalent performance to a conventional ESP 4 to 5
times larger. The E-SOX technology utilizes the multistage technology to
free up space within an existing ESP for sulfur dioxide removal of 60 to
90%, by injection of alkaline reagents. Improved computer modeling
techniques are allowing more rapid and economical ESP designs.
1

-------
NEW TECHNOLOGY FOR THE CONTROL OF AEROSOLS FROM STATIONARY SOURCES
INTRODUCTION
As part of its mission the Environmental Protection Agency undertakes
research to develop pollution control technology for aiding industry in
meeting the requirements of the various air pollution control standards and
regulations. Part of this research and development is in the area of par-
ticulate matter control technology. Out of this program has come a number
of significant advances. This paper describes several in both fabric
filtration and electrostatic precipitation.
FABRIC FILTRATION
Historically, fabric filtration has been used for a considerable time
in industry. Fabric filtration is being applied more frequently to utility
and industrial boiler particulate matter control with increasing use of low
sulfur coals having high resistivity fly ashes.
In the EPA particulate program it has been concluded that, of all the
parameters involved in fabric filtration design, electrostatics, when
properly applied, is the dominant.
Electrostatically augmented fabric filtration (ESFF) is a technique
in which an electrostatic field is established across the fabric to cause
preferential deposition of the particulate matter on some areas while
leaving other areas open to gas flow. The electrostatic field acting on
charges upon the particles causes the selective deposition; the stronger
the charge and electric field the greater is the ESFF effect. The open area
causes a lowered pressure drop for the baghouse. Much of the work to
date has been with the electric field parallel to the surface of the
fabric. Typically the field was generated between electrodes, placed at the
surface of the fabric, running the length of the bag, and placed about 2
cm apart. The electric field strength is from about 2 to 8 kV/cm. In
pulse-jet operation {in which the particle matter is collected on the
outside of the bags) the wire support cage is adapted for developing the
electric field as shown in Figure 1 (1). Adjacent wires are insulated
from each other thereby allowing the cage itself to be electrified. For
reverse-air, which does not use a cage, and in which the particle matter
is collected on the bag interior, stainless steel yarns have been woven
into the fabric warp as shown in Figure 2. The fabric is fiberglass with
a Teflon finish; the yarn is 316L stainless steel having 90 fibers each
22 m in diameter (2). The alternate yarns have the electric field devel-
oped between them, the same as in pulse-jet.
The ESFF effect has been shown for fly ashes on small EPA in-house
baghouses at Research Triangle Park, NC. It has also been shown for both
reverse-air and pulse-jet with the EPA 28 actual m3/min pilot unit having
2.4 m bags on an industrial boiler at the DuPont Waynesboro, VA, facilities.
An ESFF system has been operated for about 14 months, using reverse-air,
2

-------
at the EPA 140 actual m3/min pilot unit at the Southwest Public Service
Harrington Power Station in Amarillo, TX. At the Harrington power plant
the bags were full utility size, each being 9.14 m long and 0.3 m in
diameter as shown in Figure 2.
Typically, for fly ash the pressure drop across an ESFF baghouse is
about 50% of the pressure drop across a conventional baghouse (1,3). This
means that one can minimize the pressure drop and consequently the fan power
or else decrease the number of bags and consequently the capital cost.
A series of experiments were performed on the EPA in-house baghouses
for both reverse-air and pulse-jet using redispersed particulate matter
from a spray drying process for sulfur dioxide capture. In spray drying,
a lime slurry is injected into a chamber to react with the sulfur dioxide.
The resulting mixture consists of fly ash, and reacted and unreacted
lime. For spray drying by-product particulate matter, the pressure drop
was reduced by about a factor of 4 to 6 as compared to conventional
filtration. Figure 3 is a pressure vs time trace which shows dramatically
the effects of turning the field on. At the start, with the power off,
the pressure drop was continuing to rise and would have eventually gone
off the chart if continued. The pressure drop started to respond immedi-
ately to turning on the field.
The ESFF effect that has been discussed is based upon the action of
an electric field upon particulate matter containing only natural charge.
No corona for charging the particles was generated. The measured natural
charge on typical fly ash is about 0.1 to 0.2 t£/g; for spray drying
by-product some of the measured charges were as high as 1.68 yC/g. The
difference in the pressure drop reduction for spray drying material as
compared to fly ash accounts for some of the improvement. The remainder
is due to the ability for developing a stronger electric field with this
material as a result of lower operating temperatures and higher moisture
levels. Varying the electic field has also been seen to affect the
pressure drop reduction. The ESFF effect improves with increasing electric
field and decreases with lowered electric field.
Work is underway on techniques in ESFF for simultaneously increasing
the field strength and increasing the charge on the particles. There is
good indication that by doing this it would be possible to achieve pressure
drop reduction for most particulate matter that will equal that which was
achieved with spray drying material.
Charging of particles occurs when a corona current is caused to flow
across the gas stream containing the particles. Conventional ESFF, in
which the electrodes and resultant electric field are parallel to the
surface of the fabric, is not conducive to generation of a corona discharge
or increasing the field strength; the filtration fabric would rapidly
break down from the c orona formed on its surface. The need to charge the
particles and to increase the field strength has led to a second generation
ESFF which has been termed "Super ESFF. " In Super ESFF a strong electric
3

-------
field, with corona discharge, is developed perpendicular to the fabric
rather than parallel. Super ESFF has been applied to both reverse-air
and pulse-jet filtration.
In reverse-air Super ESFF the electrical field is developed between
an axially located wire, extending the length of the bag, and the interior
surface of the fabric filtration bag which is made electrically conductive
by one of several means (See Figure 4). The electrical field between the
axial wire and the grounded conductive fabric is sufficently strong so
that the wire goes into corona, and the particles entering the bottom of
the bag, containing some natural charge, are given additional charge by
the corona current. In normal fabric filtration the particles follow the
gas streamlines and consequently tend to deposit themselves evenly on the
bag's interior. Under the influence of the electric field the charged
particles leave the gas streamlines and tend to be selectively deposited
on the lower portion of the bag. This leaves the upper portion of the
bag with little or no particle matter upon the surface. Consequently the
majority of the gas flows out through the upper portion of the bag which
provides a significantly decreased pressure drop.
Pulse-jet Super ESFF requires that the electric field be perpendicu-
lar to the external surface of the fabric filtration bag, which is the
surface upon which the particle matter collects. This is accomplished by
developing the electric field between corona discharge wires, placed
parallel between the fabric filtration bags, and the wire anti-collapse
cages inside of the bags; the cages in turn are grounded (see Figure 5).
The strongest electric field is established in the area directly between
the wire and the grounded support cage. It is in this area that the
maximum amount of particle matter is selectively deposited. The fabric
filtration bag surfaces between these areas of maximum deposit provide
the low flow resistance for the majority of the gas which results in the
lowered pressure drop across the bags.
The reverse-air Super ESFF has been operated in a small single-bag
baghouse with bags 1.07 m in length and 0.13 m and 0.2 m in diameter.
The pressure drop ranged from about 10 to 30% of the pressure drop of
conventional reverse-air filtration. The range is a function of particle
matter resistivity and gas temperature. Lower temperatures allow higher
field strengths. Lower resistivities allow higher corona currents. With
high field strength at a temperature of about 70*C and maximum charge
(such as experienced with spray dryer conditions) the pressure drop is
lowest. At higher temperatures of about 150*C, as experienced under the
gas cleaning conditions of steam-electric utilities, especially with high
resistivity particle matter, the pressure drop was at the higher end of
the range.
In a larger reverse-air Super ESFF pilot unit having 0.2 m diameter
bags,7.47 m in length,operating on a stoker-fired boiler at 150*0, the
average pressure drop was about 25% of the pressure drop being achieved
with conventional fabric fi1tration. The fly ash from this stoker-fired
boiler had a very low electrical resistivity due to about 30% unburned
carbon contained within it.
4

-------
Performance from reverse-air Super ESFF appears to offer a significant
improvement over the original ESFF concept described previously, and offers
an even greater improvement over conventional reverse-air filtration. This
again will allow either a more significant reduction in operating costs due
to the need for less fan power, or less capital costs due to the need for
even fewer filter bags as compared to conventional ESFF.
A small amount of work has been done on pulse-jet Super ESFF. A nine
bag ambient temperature baghouse having bags 1.22 m in length and 0.13 m
1n diameter was electrified. Difficulty in stabilizing the resistivity
of the particle matter made a quantitative determination of the pressure
drop reduction unfeasible. However, the data qualitatively indicates that
the pressure drop reduction is significantly improved over the conventional
pulse-jet ESFF previously described. Pulse-jet Super ESFF shows consider-
able potential, and additional work is scheduled.
ELECTROSTATIC PRECIPITATORS
EPA's electrostatic precipitator (ESP) research has concentrated on
improving performance and operation, decreasing size and cost, upgrading
performance for operation with desulfurization systems, integrating
sulfur dioxide control into the ESP, and developing performance and cost
models.
Historically, for operation with low resistivity particle matter,
ESPs have been designed for high corona currents. Even with these high
corona currents the absence of a back corona problem allows achievement
of high electric field strength. As a consequence, the ESPs operated well,
and the particles were easily charged and collected. High corona currents
were achieved by use of small diameter discharge wires {approximately 0.3
cm) or use of electrodes with a multiplicity of discrete discharge points.
Where the allowable useful current is limited by high resistivity, the low
corona onset voltage and fairly steep voltage/current relationship result
in both a decreased electric field strength and low corona current for
charging. The combination of low current density and electric field
produced by conventional discharge electrodes requires that a high specific
collector area (SCA) be provided to meet emission standards. The SCA is
defined as the ratio of the volumetric flow rate of gas to the collector
plate area.
Experimental studies at the EPA 840 actual m^/min field pilot unit
at TVA's Bull Run Power Plant and the in-house 28 actual m^/min pilot ESP
have shown that the use of larger diameter, smooth-surface electrodes
(approximately 1 cm in diameter) can provide a significant improvement in
ESP performance—especially when high resistivity particle matter is
collected. The large diameter electrodes allow operation at high field
strengths and useful current densities for particle charging. As can be
seen from Table 1, the large diameter wires reduce the penetration for
high resistivity particle matter (2 x lO1^ ohm-cm) by a factor of 4, and
the penetration for moderate resistivity (8 x 101" ohm-cm) particle
matter resistivity by a factor of 1.25. Economic analysis has shown that
5

-------
the large diameter wire technology is applicable in both new and retrofit
situations when the particle matter resistivity is greater than about 8 x
1010 ohm-cm. The application of this technology is being further pursued.
In conventional ESPs particle charging and collection take place in
the same electrical section. It has been found that, for the higher resist-
ivity particle matter, separation and optimization of the charging and
collection steps can lead to significant improvements in performance.
A number of precharger techniques have been worked upon including the
trielectrode (4), cold-pipe (5), and charged droplet. One of the most prom-
ising is the cold-pipe precharger shown in Figure 6. It consists of dis-
charge wires interspersed with grounded pipes through which cooling water
flows. The purpose of the cooling water is not to cool the gas stream but
to cool the dust layer that forms on the pipes. Cooling the dust layer
decreases the resistivity, thereby allowing high field strength and current
which puts high charge levels on the particles. The direction of gas flow
for the front view of the cold-pipe precharger, which is on the left side
of the figure, is perpendicular into the paper; for the side view the gas
flow is from left to right.
Combining the cold-pipe precharger with wire-plate collectors using
large diameter wires provides an even further improvement in performance.
Large diameter wires in the collector sections allow development of high
field strengths and also provide sufficient corona current to clamp the
particle layer to the plates, thereby minimizing non-rapping reentrainment.
Figure 7 shows the multistage ESP which consists of collector sections
each of which is preceded by a cold-pipe precharger. With high resist-
ivity particulate matter a multistage ESP will give the same collection
efficiency as a conventional ESP operating with relatively low currents
and fields and having 4 or 5 times the collecting plate area. Operation
of the multistage ESP with the cold-pipe precharger and large diameter
wires in the collector sections has been done on the EPA in-house 28
actual m3/min ESP and on the EPA 425 actual m3/min ESP at Colorado Public
Service's Valmont Power Station.
Multistaging, when retrofitted to an ESP that is working well and
is in compliance with the particulate matter regulations, will have more
capacity than is actually needed. This suggests that some portion of the
ESP internals can be removed. This freed up space, fitted with suitable
nozzles, can be used for removal of sulfur dioxide from a flue gas by
injecting droplets of an alkaline reagent.
This system, which has been named E-SOX, for the removal of sulfur
dioxide along with the particulate matter in an ESP, is shown schematically
in Figure 8. The alkali reagent shown being used here is a high calcium
lime that has been slaked and slurried by techniques that have been
developed for conventional spray drying flue gas desulfurization. E-SOX
is discussed in considerably greater detail elsewhere (6), (7).
6

-------
The E-SOX system has been evaluated on the in-house 28 actual m3/min
ESP using two-fluid nozzles for the alkali reagent injections. The sulfur
dioxide removal results are shown in Figure 9. Two reagents were used ~
sodium carbonate solution and lime slurry. With the sodium carbonate inlet
concentrations of 15 and 30% by weight and at flow rates ranging from 26
to 75 liters per hour, the sulfur dioxide removal ranged, for an inlet con-
centration of 1200 to 1500 ppm, from about 50 to almost 90S, with 2 seconds
of residence time. With a slurried lime injection, whose concentration
was 7 and 15% by weight, SO2 removal ranged from about 40 to almost 602.
Maximum sulfur dioxide was removed at the higher injection rates. The
removal was not found to be highly sensitive to reagent concentration.
For the lime experiments the maximum stoichiometry of calcium to sulfur
was about 1.8 based upon inlet sulfur dioxide. In both cases the injected
droplets were completely dried in considerably less than 2 seconds, and
the deposits on the ESP were removed by normal rapping. In this series
of experiments no attempt was made to optimize the sulfur dioxide removal.
In a typical ESP retrofit, enough of the internals could be removed to
obtain 2 seconds of residence time. The remainder of the ESP can be
multistaged not only to regain original particulate collection performance
but also to capture the injected reagent.
The costs of E-SOX have not been fully worked out. However, it is
expected that the process should be among the lowest cost of the various
flue gas desulfurization techniques. The use of an existing ESP makes it
unnecessary to add equipment, such as a spray drying chamber, for sulfur
dioxide capture; the lime system is the same as is required for some of the
other processes. One advantage of E-SOX is that the injection of a large
quantity of droplets lowers the gas temperature and volume which causes a
reduction in particle resistivity and an increase in SCA. The E-SOX project
is expected to be moved onto a larger pi lot unit using actual flue gas as
the next stage in 1ts development.
E-SOX, if successfully developed, would be a low-cost retrofit for
reducing emissions of sulfur dioxide from coal-fired power plants. This is
significant for several reasons: first, coal-fired power plants with ESP
particulate control technology represent the largest source category for
sulfur dioxide emissions in the U.S.; and second, the high cost of controlling
sulfur dioxide with existing technology is a major issue to be considered
in air pollution policy analysis. It would be especially useful for those
situations in which there is not room for retrofit of a spray drying flue
gas desulfurization chamber.
OTHER ACTIVITIES
In addition to the work just described, EPA has a strong program in
modeling and fundamentals. The purpose of the modeling/fundamentals program
is to provide design and performance models, for the particulate control
devices, which are based on a sound understanding of the underlying physics
of the particulate collection process. The ESP work has resulted in a corn-
computer model that is in widespread application by vendors, users, and
researchers (8), especially for utility applications. Work is underway
to improve the ESP model in the areas of effects of electrode geometry,
7

-------
electrical conditions, and space charge. There is also interest 1n apply-
ing the ESP model to Industrial sources In addition to utilities. A new
modelling technique computes the voltage and current for several common
electrode geometries. The results are presented both numerically and as
a video display of the electric field and the current distribution. The
ability to display the electrode geometry, electric fields, and current
distribution allows the designer to make changes and immediately determine
the effect of the.changes on ESP performance. Work is currently underway
to extend and verify the model capabilities to all electrode shapes and
to also allow changes in collector geometry.
The program has produced a useful model for fabric filters. The current
version of the model does not include electrostatic effects. However, work
is underway to introduce the effects of application of ESFF into the fabric
filtration models. A major effort is underway to extend the usefulness of
these models to the control of particulate matter from various industrial
sources in addition to utilities.
There is additional work being done in fugitive particle control
involving the development of engineering design information for hoods
used for collection of particles and fume. Operation and maintenance
manuals are being developed for ESPs and baghouses. Finally, research is
being done to develop technology to decrease the emissions of condensible
aerosols.
CONCLUSION
This paper has presented an overview of the key projects of the EPA
particulate technology R&D program, which may be of interest to industry.
Work will be continuing in the areas that include ESPs, electrostatically
augmented fabric filtration, and control of fugitive emissions. Discussion
of specific problems in particulate matter control that may make use of
these technology areas, would be welcomed.
8

-------
REFERENCES
1.	VanOsdell, D.W., Ranade, M.B., Greiner, G.P., and Furlong, D.A. "Elec-
rostatic Augmentation of Fabric Filtration; Pulse-Jet Pilot Unit
Experience." EPA-600/7-82-062 (NTIS PB 83-168 625), November 1982.
2.	VanOsdell, D.W., and Furlong, D.A. "Electrostatic Augmentation of Fabric
Filtration: Reverse-Air Pilot Unit Experience." EPA-600/7-84-085
(NTIS PB 84-230 002), August 1984.
3.	Furlong, D.A., Greiner, G.P., VanOsdell, D.W., and Hovis, L.S. "Electro-
static Stimulation of Reverse-Air-Cleaned Fabric Filters." In: Fourth
Symposium on the Transfer and Utilization of Particulate Control
Technology, Volume I, Fabric Filtration, EPA-600/9-84-025a (NTIS PB85-
161 891), pp. 287-302, November 1984.
4.	Pontius, D.H., Bush, P.V., and Sparks, L.E. "A New Precharger for Two-
Stage Electrostatic Precipitation of High Resistivity Dust." In:
Symposium on the Transfer and Utilization of Particulate Control
Technology, Vol. I. Electrostatic Precipitators, EPA-600/7-79-044a
(NTIS PB 295 226), pp. 275-285, February 1979.
5.	Rinard, G., Rugg, D., and Durham, M. "Evaluation of Prechargers for
Two-Stage Electrostatic Precipitators." In:Fourth Symposium on the Trans-
fer and Utilization of Particulate Control Technology, Volume II,
Electrostatic Precipitation, EPA-600/9-84-025b (NTIS PB85-161 909)
pp. 84-95, November 1984.
6.	Sparks, L.E., Plaks, N., and Ramsey, G.H. "Investigation of Combined
Particulate and SOg Using E-S0X. " Ninth Symposium on Flue Gas Desulfur-
ization, Cincinnati, Ohio, June 4-7, 1985.
7.	Martin, G.B., Abbott, J.H., and Sparks, L.E. "Advances In Control Tech-
nology for Acid Deposition," Second U.S.-Dutch International Symposium:
Aerosols, Williamsburg, Virginia, May 19-24, 1985.
8.	Faulkner, M.G., and DuBard, J.L. "A Mathematical Model of Electro-
static Precipitation (Revision 3)" Vols. I and II, EPA-600/7-84-069a
and b (NTIS PB 84-212 679 and 84-212 687), June 1984.
9

-------
Table 1. COMPARISON OF LARGE AND SMALL DIAMETER DISCHARGE ELECTRODES
Resistivity, ohm-cm
Electric field ratio
(EL/ES)
Current density ratio
(j'l/js)
Power ratio
(PL/PS>
Overall migration velocity ratio
(wlM)
Penetration ratio
(Pt|_/Pt$)
8 x 10m
1.60
0.41
0.55
1.12
0.80
2 x 10i?
1.19
0.69
0.81
1.33
0.25
NOTE: Subscript L refers to large diameter electrode, and subscript S refers
to small diameter electrode.
10

-------
Filter Bag
£ (tetrodes
Ceramic
Insulators
Welded Joint
Alternate Electrodes
Connected Electrically
Figure 1. Pulse-Jet ISFF Electrode/Bag Assembly
11

-------
T
0.15 m
iiliiiiii! ill:!1
8.84 m
Total of 44 Electrodes
'!
0.15 m
Standard Top Cap
J,P. Stevens 648 Fibergtas
Fabric Without Electrodes
0.32 cm 316 SS Braid
J.P, Stevens 648 Woven
Electrode Fabric
0.32 cm 316 SS Braid
Top of Thimble
Clamp
Figure 2. Reverse-Air Woven-In Electrode ESFF Bags
12

-------
77°C; 18 g/m3; 30 min cycle; 6% H20
Air/Cloth: 2 cm/s
ESFF Power Off
30 min
¦P"*
Time
Figure 3. Effect of Turning ESFF Field On

-------
Spring
To High Voltage
Power Supply
Outlet
Insulators
High Voltage
Power Supply
Corona Discharge
Electrode
Conductive Filterbag
a
E=3
Cell Plate
Inlet
Weight
Figure 4. Super ESFF Reverse-Air Configuration
14

-------
Insulator
Outlet
Cell Plate
Grounded
Wire Cage
Filterbag
Corona Discharge
Electrode v
Cell Plate
ooo
© ©
ooo
Grounded
Wire Cage
Inlet
High Voltage
Power Supply
Top View
Figure 5. Super ESFF Pulse-Jet Configuration
15

-------
u
0)
60
u
ts
X
u
0)
A.
m
o.
i
¦u
ft
o
o

W
3
«C

-------
Figure 7. Multistage Electrostatic Precipitator

-------
f
Original ESP
Spray	Multistage
Chamber	ESP
Conventional
PC Boiler Plant
Flue Gm
Wires and Plates
removed from
first section
t—*
00
Slurry
Lime Storage,
Handling, and
Slurry Preparation
To Landfill
Recycle
(optional)
Figure 8. Schematic of E-SOX Process

-------
kO
#
a>
&
uj
ff
100
80-
60
I 40
20
Inlet S021200 to 1500 ppm


Range of lime slurry experiments
Range of sodium carbonate experiments
1	2
Time .seconds
4
Figure 9. E-SOX Data for Sodium Carbonate and Lime Slurry Experiments

-------