United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-82-062 Apr. 1983
&ER& Project Summary
Electrostatic Augmentation of
Fabric Filtration: Pulse-Jet Pilot
Unit Experience
D. W. VanOsdell, M. B. Ranade, G. P. Greiner, and D. F. Furlong
This report summarizes the develop-
ment of the parallel-field electrostatic-
ally augmented fabric filter (ESFF) on a
pilot-scale pulse-cleaned baghouse.
The pilot unit consisted of parallel
conventional and ESFF baghouses
installed on a slipstream from a pulver-
ized coal boiler. Teflon and fiberglass
fabrics were investigated under a wide
variety of operating conditions. The
major parameters studied were
particulate collection (total mass and
size dependent), baghouse pressure
drop, and electrical characteristics.
The results of this research show that
the ESFF baghouse has significant
advantages over conventional
baghouses. The flow resistance of the
collected dust is substantially reduced.
Under the same operating conditions,
an ESFF baghouse has about half the
pressure drop of a conventional bag-
house. Alternatively, the flow through a
given area of fabric (face velocity) can
be increased at constant pressure drop
in the ESFF baghouse. Experience at
the ESFF pilot unit suggests that face
velocity can be doubled. An economic
projection based on these results
indicates that the ESFF would reduce
the annualized cost of the filter by 30
percent. Particulate control capabilities
of the ESFF baghouse were about the
same as for the conventional baghouse:
outlet loadings averaged less than
0.017g/std m3.
This Project Summary was developed
by EPA's Industrial Environmental
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).
Background
The use of fabric filters to remove
particles from gas streams, well
established in industrial practice, is of
growing importance in electrical utility
applications. Fabric filters boast high
particulate collection efficiencies and are
competitive in price with other control
technologies. Their major drawback in
coal-fired applications has been
unexpectedly high pressure drops at
some installations. Reduction of the
pressure drop for fabric filters is, thus,
important and capable of significantly
influencing the choice of equipment for
control of particulate emissions.
The deliberate use of electrical effects
to improve the performance of
conventional fabric filters has only
recently received commercial
consideration, although research has
been underway for some time. Conven-
tional design practice does not consider
the electrical properties of either the dust
or the fabric. Research has shown,
however, that electrostatic forces on
particles in a fabric filter can be
important, and that their importance can
be increased by charging the particles or
by applying an electric field. The
electrostatic forces influence the ways
the particles interact with the fabric and
other particles. If the interaction with the
fabric results in increased particle
collection at the fabric surface (reduced
filtration in depth), cleaning could be
expected to be more efficient and the
residual pressure drop of the filter to be
reduced. If the electrically influenced
particle-to-particle interactions result in
a dust cake that is more porous than
normal, the flow resistance of the
collected dust is reduced. Research
indicates that both reduced residual
-------
pressure drop and reduced dust cake flow
resistance can be achieved in electro-
statically enhanced fabric filtration
(ESFF).
The many ways to take advantage of
electrostatic forces in fabric filters can be
grouped into three broad categories: (1)
allow the natural particle charges to
accumulate on the fabric filter, (2) charge
the incoming particles, then allowcharge
accumulation on the fabric, and (3) apply
an electric field at the fabric surface. The
first category describes many existing
fabric filters; it is the natural result of the
use of nonconductive fabrics. The
commercially available Apitron filter
utilizes the second approach (1). Acorona
precharger is placed at the inlet of a filter
bag. The incoming particles are charged
(and a significant fraction collected)
within the precharger. The remaining
dust is collected by the fabric filter.
Charge accumulation by the fabric and
collected dust causes an electric field to
develop, which further enhances
collection. The work discussed in this
report utilizes the third approach, an
externally applied electric field at the
fabric surface. Electric fields can be
applied either parallel or perpendicular to
the fabric surface. Laboratory and
theoretical work have shown that the use
of an electric field parallel to the fabric
surface results in improved fabric-filter
performance (2). An external harness,
similar to that shown in Figure 1, was
tested in a small laboratory fabric filter
(3). The flow resistance of the dust cake
was significantly reduced at relatively
low power levels. Several fabrics were
tested in a range of electrical conditions:
the results, in general, were encouraging.
Purpose of Program
The main purpose of this research
program was to evaluate the concept of
ESFF, as developed in the laboratory, on
particulate in a slipstream from an
operating coal-fired boiler. It was
recognized that the laboratory work,
although very encouraging, had been
done under carefully controlled
conditions, which were not similar to the
flue-gas environment. The laboratory-
test dust was reentrained fly ash, but in
air at room temperature. Relatively low
fabric dust loadings had been used. There
were many questions concerning the
operability of the ESFF system in the field
other than the straightforward problems
of design and materials selection. The
approach taken was to attempt to use the
laboratory design at field conditions. The
relative merit of the ESFF system
compared to the conventional fabric filter
could then be evaluated.
Secondary objectives of the program
were to: evaluate performance with
respect to particle size, estimate the
effects of different coal types, and
evaluate the economics of the ESFF
system.
Pilot Unit and Operating
Experience
The basis of the ESFF pilot-unit test
program was parallel operation of
To Power Supply
\H
Fiberglass Yarn^.
-Electrodes •
Stainless Steel
wire - 0.58 mm
?-&
High Voltage Harness
loll
irnes
Installed on Bag
Figure 1. Harness for applying electric
field parallel to the surface of a
fabric filter bag.
identical conventional and ESFF bag-
houses. Boiler and coal variations were
expected to be too large for successful
testing to be done consecutively in time.
The pilot unit was operated on a
slipstream from an industrial pulverized-
coal boiler house. The coal fed to the
boilers was highly variable: sulfur
content ranged from 0.6 to 2.9 percent
(average about 1.3 percent), and ash
content from 6 to 27 percent (average
about 13 percent).
Figure 2 is a schematic of the pilot-unit
installation. The pilot-plant capacity was
about 9 mVmin (300 ftVmin) in each
baghouse; average inlet mass loading
was about 0.7 g/m3 (0.3 gr/scf). The inlet
temperature was around 150° C (300° F).
Each baghouse was operated with up to
five bags, 11.5 cm (4.5 in.) in diameter,
and 2.44 m (8 ft) long. The electrical
hardware consisted of high-voltage DC
power supplies, current and voltage
instrumentation, and the ESFF
electrodes. Operation was 24 hours each
day while a test was in progress.
The initial electrode design was a high-
temperature version of the electrode
harness shown in Figure 1. Material
problems and inherent design weak-
nesses led to rapid deterioration of the
outside harness. An improved electrode
design was developed and mounted
Outlet (Top)
Inlet
(Bottom)
Trailer
,,,__,_ -JJWT: — -
Test Ports
Inlet
(Botti
ESFF
House
Figure 2. Schematic of ESFF pulse-jet pilot upit
Outlet
(Top)
-------
inside the bag, replacing the normal
pulse-jet cage. The electrodes remained
vertical but were supported by and
insulated from the horizontal cage wires.
This new design was an important
improvement, greatly increasing the
reliability of the ESFF baghouse.
Results
The most important results obtained at
the ESFF pilot unit concerned
improvements in the pressure drop char-
acteristics of the fabric filter. Three
aspects of this phenomenon are
discussed below.
The effect of ESFF on dust-cake flow
resistance can be readily observed in a
plot of pilot-unit pressure drops as a
function of time, shown in Figure 3. The
pressure drop of the ESFF baghouse does
not increase as rapidly as that of the
conventional baghouse. Because the
same amount of dust is being deposited
on the bags in both baghouses, the
difference must be in the resistance of
the dust cake to flow. This reduction in
dust-cake flow resistance has been
quantified as pressure drop ratio (PDR),
defined as:
PDR =
(APf - AP,) ESFF
(APf - APi) Conventional
where APf = pressure drop just prior to
cleaning,
AP, = pressure drop just after
cleaning,
ESFF refers to the ESFF baghouse,
and
Conventional refers to the conven-
tional baghouse.
Figure 4 shows the relationship between
PDR and electric-field strength at the bag
surface for fiberglass and Teflon fabrics
for a range of face velocities. At 3 kV/cm
the increase in pressure drop over a
cleaning cycle for the ESFF baghouse
was a little less than half that for the con-
ventional baghouse. PDR at the pilot unit
was not a strong function of field strength
for either fiberglass or felted Teflon
fabrics. The Teflon results are in general
agreement with the laboratory work.
Figure 5 shows another advantage of
the ESFF baghouse over conventional
technology—a reduced residual pressure
drop. The ESFF baghouse had achieved
stable operation at a residual pressure
drop of 0.5 kPa (2 in. H20), while the
conventional baghouse under identical
conditions had a residual pressure drop of
more than 1.5 kPa(6 in. H2"0). The ESFF
residual pressure drop averaged about 60
percent of that for the conventional
baghouse. It was possible, through
repeated off-line cleanings, to return both
baghouses to about the same pressure
drop. The difference in residual pressure
drop developed again soon after normal
operations resumed.
From the standpoint of process
economics, the most useful manifesta-
;.00p-
tion of the reduced flow resistance of the
dust cake collected under ESFF
conditions is the ability of an ESFF
baghouse to operate at high face
velocities. Experience at the pilot unit
showed that the conventional baghouse
could not be operated in a stable fashion
above about 2 cm/s (4 ft/min). The ESFF
baghouse was operated for nearly a
month at greater than 2 cm/s, and it was
possible to stabilize the operation at up to
5 cm/s (10 ft/min) by increasing the
cleaning frequency.
Paniculate control by the two
baghouses appeared to be essentially the
Control Baghouse
0.25
ESFF Baghouse
Notes: 1. Cleaning at 15 minute intervals
2. ESFF voltage 5.9 kV
3. Teflon Bags; July 31. 1980
4. Face, velocity 3 cm/s (6 ft/min)
JO
20
30
Time, minutes
40
50
60
Figure 3. Pressure drop plot for pilot baghouse.
Pilot Plant Woven Glass
Inside Electrodes
Pilot Plant Teflon
Inside Electrodes
0.0
234
Electric Field, kV/cm
Figure 4. PDR-field strength relationship for Teflon and glass fabrics.
3
-------
same. The average outlet loading from
the ESFF baghouse was 0.017 g/std m3,
and that from the conventional baghouse
was 0.016 g/std m3. Impactor measure-
ments of the size-dependent paniculate
removal efficiency indicated that the ESFF
baghouse was more efficient than the
conventional baghouse. However, the
conventional baghouse was cleaning
much more frequently than the ESFF
baghouse, and the increased penetration
might well be due to seepage. The
electrical requirements of the ESFF
baghouse were modest. The outside
harness electrodes, imbedded in the dust
cake, averaged about 50 //A of current per
bag. Current was roughly proportional to
voltage for the outside electrodes. The
inside cage/electrode combination drew
only about 10 to 20 x/A per bag, and the
current was not affected by voltage within
the normal operating range. The current
was fairly stable, but some drift did occur
7.50-1
1.25-
1.00-
-------
27,200 kg steam/hr (60,000 Ib/hr)
1130 am*/hr (40,000 acfm)
180
160
140
120
a
I
c
700
80
60
Conventional
face velocity = 2 cm/s
ESFF
2 cm/s
ESFF
4 cm/s
Bag Cost $48.44/m2 (4.50/ft*)
4-Year Bag Life
A/'cofwentlonal = 0.75 + 0. 75 (G/C), kPa
A/>ESFF =0.75 + 0.25(G/C). kPa
G/C in cm/s
the fabric and the dust, particle size,
natural charge of the dust, gas
conditions, and similar parameters.
References
1. Felix, L. G., and J. D. McCain. Apitron
Electrostatically Augmented Fabric
Filter Evaluation. EPA-600/7-79-
070 (NTIS No. PB 294716), February
1979.
2. Lamb, G. E. R., and P. A. Costanza.
Electrical Stimulation of Fabric
Filtration: Part II. Textile Research
Journal, 48: 566-573, October 1978.
3. Lamb, G. E. R., and P. A. Costanza. A
Low-Energy Electrified Filter System.
Filtration and Separation, 17: 319-
322, July/August 1980.
0.05
Cost of Electricity, $/kWh
0.10
Figure 6. Pulse-jet baghouse cost: pulverized-coal industrial boiler.
D. W. VanOsdell and M. B. Ranadeare with Research Triangle Institute, Research
Triangle Park, NC 27709; G. P. Greiner and D. F. Furlong are with ETS, Inc.,
Roanoke, VA 24018.
Louis S. Hovis is the EPA Project Officer (see below).
The complete report, entitled "Electrostatic Augmentation of Fabric Filtration:
Pulse-Jet Pilot Unit Experience," (Order No. PB 83-168 625; Cost: $11.50,
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
U. S. GOVERNMENT PRINTING OFFICE: 1983/659-095/1934
-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
9
PROTECTION AGENCY
PS
U S
REGION b LIBA
EJO 3 DE&KBORfc STREET
CHICAGO TL bftbO<4
------- |