United States
Environmental Protection
Agency
Industrial Environmental
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-84-085 Sept. 1984
SERA Project Summary
Electrostatic Augmentation of
Fabric Filtration: Reverse-Air Pilot
Unit Experience
D.W. VanOsdell and D.A. Furlong
This report describes a test of electro-
statically augmented fabric filtration
(ESFF) on a pilot-scale, reverse-air-
cleaned baghouse. The pilot unit con-
sisted of two baghouses in a parallel
flow path arrangement. A slipstream
from an industrial pulverized-coal
boiler house was the ash source for the
pilot unit. The fabric under test was a
17-oz (482-g) woven-fiber-glass fabric
with a Teflon® B finish. The principal
independent variables were baghouse
face velocity and ESFF field strength.
The main parameters monitored were
particle collection, baghouse pressure
drop, and electrical power requirements.
It was recognized that successful
operation of a large-scale ESFF bag-
house required the development of a
reliable and practical electrode system.
During this project, a filter bag with
stainless steel electrodes woven into
the fabric was developed and tested in
the pilot unit. Other candidate electrode
designs were also developed and
tested.
Research results show that reverse-
air ESFF can reduce fabric filter pres-
sure drops and (thus) may allow increased
operating face velocities. It was possible
to operate the pilot unit ESFF baghouse
(but not the conventional baghouse) at
2 cm/s face velocity. However, due to
limits on the experiment, the maximum
operating time at 2 cm/s was only a few
weeks, which is insufficient to ensure
long-term operation at that face velo-
city. The flow resistance of the collec-
ted dust is substantially reduced by the
presence of the electric field. Compari-
son of the average baghouse drag
(pressure drop/face velocity) between
the ESFF and conventional baghouses
showed that the ESFF baghouse drag
would range from 54 to 85 percent of
the conventional baghouse drag if ESFF
were used on the bag for its full lifetime.
Based on this and other pilot unit
results, it was estimated that an ESFF
baghouse operating at 2 cm/s would
have an annual cost 11 percent less
than that of a conventional (not electro-
statically augmented) baghouse opera-
ting at 1 cm/s.
The particulate control capabilities of
the ESFF baghouse were about the
same as for conventional filtration; the
ESFF baghouse averaged 99.7 percent
efficiency, and the conventional bag-
house, 99.8 percent.
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 is a well-
established industrial practice, and its
importance is growing for electrical utility
boilers. The fabric filter is roughly
equivalent in price to other utility control
devices and provides increased particle
removal efficiency. However, fabric filter
operating experience is limited, and
enough questions have been raised
because of unexpectedly high baghouse
pressure drops to cause concern in the
industry.
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A concept to improve fabric filtration
through the use of electric fields (electro-
static stimulation of fabric filtration, or
ESFF) was developed at laboratory scale
by researchers at the Textile Research
Institute (TRI), Princeton, NJ, whose work
was partially supported by EPA. TRI
showed that the concept could signifi-
cantly improve filter performance by
reducing filter drag and increasing filter
efficiency. In work supported by EPA, the
Research Triangle Institute (RTI), Research
Triangle Park, NC, and ETS, Inc., Roanoke,
VA, transferred the ESFF technology to
the field by successfully operating a
pulse-cleaned ESFF baghouse on a slip-
stream from an industrial boiler.
Results of this work have been reported:
in brief, the pulse-jet pilot work was
highly successful. A new ESFF electrode
system that was suitable to pulse-jet
baghouses was developed as a replace-
ment for the inside cage. The pulse-jet
baghouse had a reduced residual pressure
drop and a reduced specific dust cake
resistance when compared to a conven-
tional baghouse operated in parallel. In
addition, the ESFF baghouse operated
stably at higher face velocities than was
possible with the conventional baghouse.
Economic analysis indicated that an ESFF
baghouse operated at 3 cm/s would have
a 30-percent economic advantage when
compared to a conventional baghouse at
2 cm/s, which is the approximate upper
limit for conventional pulse-jet bag-
houses operating on coal-fired boilers.
The ESFF effect can best be understood
as the net effect of fundamental forces on
particles—either charged on uncharged—
in the presence of an electric field.
Although only one primary electric field is
applied to the filter, that field interacts
with the filter fibers and collected
particles to form many localized gradients
in the electric field. A particle approach-
ing the filter is thus exposed to a complex
field before it is collected. The electric
field continues to have an effect on the
particle after it is collected.
Several changes in the pattern of dust
deposition on a filter might account for
the reduced pressure drop effects observed
with ESFF:
1. An increased fraction of the collected
dust being deposited on or near
the upstream filter surface.
2. Changes in the pattern of collection
on a filter fiber; e.g., more dendritic
collection and bridging.
3. Formation of a highly nonuniform
dust deposit—at the scale of the
electrode spacing—on the filter
surface.
An increased fraction of the dust
being deposited near the upstream
surface of the filter has been observed in
laboratory studies of ESFF. Collection in
the less dense surface region, which is
enhanced by the electric field, leads to
reduced pressure drop for a given quantity
of dust and reduces the amount of
dust that penetrates the fabric.
Changes in the pattern of dust deposi-
tion on fibers have been observed by a
number of researchers. In general, the
presence of an electric field leads to
increased dendritic particle collection.
If electrostatic forces cause the forma-
tion of a dust deposit with a nonuniform
areal mass density, a reduction in
pressure drop should result. This will be
true, for instance, if coulombic forces
cause most of the dust to collect in the
immediate region of the electrodes,
leaving reduced deposits on the rest of
the filter. Highly nonuniform deposits
have been observed with redispersed fly
ash in a laboratory ESFF filter.
Stabilization of the deposited dust layer
by the electric field has also been studied
as a possible ESFF mechanism. It is
postulated that the initial dust cake
formation in an ESFF filter is quite porous
due to the electical gradient forces, and
that the electric field prevents the
collapse of this porous cake as the
pressure drop across the filter increases.
In ordinary filtration, this porous struc-
ture is assumed to collapse somewhat as
the cake mass and pressure drop in-
crease.
Purpose of Program
The main purpose of this research
program was to develop reverse-air-
ESFF. The work centered on the ESFF
pilot unit and included electrode and
electrical system development as well as
pilot unit tests. The emphasis was on
realistic field operation to provide pres-
sure drop, particle removal, and system
cost information that ultimately could be
used in an economic analysis of ESFF.
Pilot Unit and Operating
Experience
The primary operating mode during the
ESFF pilot unit test program was parallel
operation of a conventional (control)
baghouse and an electrostatically aug-
mented (ESFF) baghouse. Boiler and coal
variations in an operating boiler were
expected to be too large for successful
comparative testing to be done serially
in time in a single baghouse. Testing was
done serially (field on/field off) during
portions of the test program only to allow
different tests to be undertaken at the
same 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 1 is a diagram of the pilot unit.
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 about 150° C (300° F). The baghouses
were normally operated with three bags,
each 20.3 cm (8 in.) in diameter and 244
cm (8 ft) long. The electrical hardware
consisted of high-volatage DC power
supplies, current and voltage instrumen-
tation, and the ESFF electrodes. Opera-
tion was 24 hours each day while testing
was in progress.
A program goal was to develop ESFF
electrodes, and considerable effort was
expended during the program to improve
the electrode system. Two categories of
electrodes were considered: (1) electrodes
that are separate from the filter fabric and
must be mounted close to the filter to
function, and (2) electrodes that are
integral to the filter fabric.
During the pulse-jet program, a special
pulse-jet cage was developed in which
alternate cage rods were electrically
connected together but electrically
isolated from all nearest neighbor rods.
The electric field was thus formed
between the long vertical rods of the
cage. This electrode design was well
suited to a pulse-jet baghouse, because
a cage was needed to support the bag
even in conventional operation. The
cage/electrode was not very different
form conventional cages and was esti-
mated to be only slightly more expensive.
The conversion of the pilot unit from
pulse-jet to reverse-air cleaning required
a different approach to electrode design.
The reverse-air baghouse had the dirty
gas inside the bag, rather than outside.
Collapse of the bag during cleaning could
not be hindered or cleaning would suffer
drastically. The cage-type electrode was
not a desirable choice, and the conver-
sion from pulse-jet to reverse-air opera-
tion was accompanied by the start of a
program to develop an electrode system
integral to a reverse-air bag. While this
electrode development program was in
progress, cage-type electrode systems
such as that shown in Figure 2 were used
at the pilot plant.
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ESFf Bug/house Fan
Control Bughouse Fan
Top
Arrows Show
Normal
1! Filtering Flow
Figun 1. ESFF pilot unit.
Electrodes integral to the bag fabric
were developed in three directions, more
or less concurrently: (1) printed ESFF
electrodes, (2) sewn-on ESFF electrodes,
and (3) ESFF electrodes woven into the
filter fabric.
The concept of using printed electrodes
in the ESFF application was pursued: they
were found to produce a suitable ESFF
effect when applied to a polyester felt.
However, the formulation used was not
suitable for high-temperature flue gas
service. No suitable compounds were
identified, and active research on the
concept was stopped until a suitable
material becomes available.
Sewn-on electrodes were studied and
actually put to use at the pilot unit. The
major anticipated disadvantages were
the large amount of hand labor required
to make the bags and the number of
pinholes made by the stitching. Bags
were constructed and were found to give
a satisfactory ESFF effect but to have
above-average particle penetration.
Electrodes woven into the fabric were
the object of considerable study. A test
run was first made with three electrode
materials—a stainless steel fiber yarn, a
carbon yarn, and a stainless steel/nylon
blend. Despite being the most difficult to
weave of the test yarns, the stainless
Top
steel yarn became the electrode of choice
because of its durability.
Sufficient fabric was made for pilot
plant use, wrth the multif ilament stainless
steel yarns woven in the warp direction at
2-cm spacing. Bags were constructed
from the woven electrode fabric, and the
performance of the woven electrode bag
at the pilot unit was essentially the same
as that of the other reverse-air electrodes.
Results
The primary measure of the ESFF effect
is the reduction of pressure drop at
various baghouse operating conditions.
Both the residual pressure drop and the
final pressure drop of the ESFF bags were
reduced in comparison with conventional
technology. ESFF and conventional
technology are compared here, on the
basis of average drag.
For much of the ESFF test program,
baghouse parameters were deliberately
varied so that relatively long-term
performance at a fixed face velocity
could not be evaluated. This is not a
departure from normal baghouse opera-
tion: baghouse load swings are common-
place. Both the ESFF and conventional
baghouse average drags show a gradual
increase in average drag over the first 2
months of operation, followed by opera-
tion with no overall changes but large
variation about the apparent mean.
Based on these results, data analysis has
been limited to data collected at about the
same time from bags of about the same
age and history.
The data in Tables 1 and 2 were selected
from the overall data pool to provide the
clearest available constrasts between
ESFF and conventional baghouse opera-
tion. Table 1 compares the average drag
in the ESFF baghouse and that in the
conventional baghouse for the two
baghouses operating in parallel. The bags
in both baghouses were constructed of
the same material and were of the same
age. Each data pair compares bags with
about 1 month of use, and each is an
average over 3 or more days' operation.
Table 1 shows that the average drag for a
baghouse operated as an ESF? baghouse
continuously could be expected to be 65
to 70 percent of the drag in a conventional
baghouse.
The data in Table 2 are much like those
in Table 1. The bags were about 1 -month
old and of the same fabric. In Table 2,
however, the comparisons are between
periods of field-on and -off operation in
the same baghouse; i.e., the bags were
operated with an ESFF field for a period of
time and then the field was turned off for
a period of several days. In some cases
the field was then turned on again. The
ratio of drags averages 0.9, compared to
the ratio of 0.65 to 0.70 found for the data
in Table 1.
This comparison of Tables 1 and 2
indicates that applying the ESFF field
continuously is decidely more advanta-
geous than turning the field on and off.
The effects of electric field strength and
face velocity on drag were investigated
during the three test periods. Data from
these tests are given in Figure 3. Data
include both parallel baghouse operation
and field-on/-off operation. Most data
points are averages of 2 to 4 days of
operation. There is clear dependence of
average drag on field strength at all face
velocities tested, as well as significant
dependence of average drag on face
velocity. Halving the face velocity at any
field strength reduced the average drag
by 25 to 50 percent.
Over the entire pilot unit test program,
the bag currents averaged about 130
luA/bag, or about 0.7 W/m* at an applied
voltage of 5.4 kV. The monthly average
currents ranged from 13 to 420 /uA per
bag, which is about 0.07 to 2.5 W/m'.
ESFF performance was not related to
bag current if the electric field was kept
on the bag. Sooty particles from oil firing
and acid condensation during low tern-
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Top Cap
Welded at Points
of Contact
Ceramic Insulators
High Voltage
Electrodes
Bag
Grounded
Electrodes
Approximate Top
of Thimble
Figure 2. Reverse-air "RIGID" cage.
perature caused high currents and
reduced the power supply voltage. Under
these conditions, there was no ESFF field
on the bag, and no ESFF effect. Generally,
the bags recovered without special
efforts to clean them.
The particle removal capabilities of the
ESFF and conventional baghouses were
very similar: the ESFF baghouse averaged
99.7 percent efficiency, and the conven-
tional baghouse, 99.8 percent. This
difference is not significant at the 90-
percent level for the data collected.
A modified baghouse cost model was
applied to a 500-MW power plant for both
conventional fabric filtration and ESFF.
Costs were estimated for both at face
velocities of 1 and 2 cm/s. Conventional
fabric filters are nomally not capable of
operation at 2 cm/s; whether an ESFF
filter can operate at that gas rate remains
uncertain. The pilot unit was operated at
2 cm/s without difficulty, but the
maximum operating time was only a few
weeks. The design pressure drops used in
the model for the ESFF and conventional
baghouses were based on pilot unit
experience. Based on pilot unit experi-
ence, the ESFF bags are expected to cost
two to four times as much as convention-
al bags; this range is used in the ESFF
cost estimates.
The cost estimates provided by the
model for 1 - and 2-cm/s face velocities
and ESFF bag costs of two, three, and four
times conventional bag costs of $7/m2
are given in Table 3. For example, at 1
cm/s, an ESFF baghouse has increased
turnkey capital cost when compared to
conventional technology, increased
variable operating costs (dominated by
bag replacement cost), and reduced total
electrical cost (because of reduced
pressure drop). Overall, the ESFF bag-
house would be expected to cost between
$400,000 and $1,200,000 more than a
conventional baghouse operating at the
same face velocity.
Operating an ESFF baghouse at an
increased face velocity appears to be an
attractive alternative. An ESFF baghouse
at 2 cm/s, using bags costing three times
conventional bags, has an annual cost of
$3,430,000. Experience at the pilot unit
suggests that this is reasonable. The bag
cost estimate may be excessive for a full-
scale production operation. A conven-
tional unit at a normal and achievable 1
cm/s, controlling the same gas stream,
has an annual cost of $3,840,000, giving
the ESFF baghouse an 11 -percent cost
advantage; at a bag cost of twice conven-
tional bag cost, the advantage is 18
percent.
The comparisons above show that the
ability to operate an ESFF baghouse at
increased face velocities is the single
most important variable in the economic
analysis; ESFF bag cost is also critical.
Conclusions
ESFF, utilizing an electric field parallel
to the fabric surface (with no particle
charging), has been applied successfully
at pilot scale on a reverse-air baghouse.
The integral woven-in electrode devel-
oped during this program is a reliable way
to produce an electric field parallel to the
fabric surface and has potential for
commercial development.
At any given face velocity, the pilot
ESFF baghouse had a reduced residual
pressure drop and a reduced rate of
pressure drop increase, compared to the
pilot conventional baghouse.
Paniculate mass emissions from the
ESFF baghouse were not significantly
different from conventional baghouse
emissions.
Operation of a reverse-air ESFF bag-
house at above conventional face veloci-
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Table 1, Drag Comparisons Over Similar Operating Periods (Comparison of Two Baghouses,
~1-Month-Old Bags. Field-On South Baghouse. Field-Off North Baghouse)
Inclusive
Julian date,
day. year
148.81-151.81
148.81-151.81
191.81-196.81
191.81-196.81
25.82-31.82
25.82-31.82
Overall mean
Field
strength,
kV/cm
3.9
0
3.6
0
4.0
0
Face
velocity,
cm/s
2.0
2.0
1.5
1.5
2.0
2.0
Mean of
average drags,
kPa-s/cm
0.40
0.74
0.56
0.85
0.76
0.89
Ftatio of
field on/
off drags
0.54
0.66
0.85
0.68
Difference
of field on/
off drags,
kPa-s/cm
0.34
0.29
0.13
0.25
Table 2.
Drag Comparisons Over Similar Operating Periods (Same Baghouse, Bags, Elec-
trodes. ~1-Month-Old Bags, Sequential Field On/Off Periods)
Inclusive Field
Julian date, strength,
day. year kV/cm
Standard bags, rigid electrodes
246.82-249.82 2.8-3.8
256.82-260.82 3.8
250.82-254.82 0
Standard bags, rigid electrodes
269.82-273.82 3.8
274.82-278.82 0
Sewn spiral electrode bags
343.82-349.82 2.2
350.82-356.82 0
Sewn spiral electrode bags
362.82-363.82 2.4
364.82-365.82 0
Overall mean
Face
velocity,
cm/s
1.25
1.25
1.25
1.4
1.4
1.5
1.5
1.5
1.5
Mean of
average drags,
kPa-s/cm
1.44
1.49 1.46
1.64
1.30
1.55
1.16
1.26
1.48
1.58
Ratio of
field on/
off drags
0.89
0.84
0.92
0.94
0.90
Difference
of field on/
off drags,
kPa-s/cm
0.18
0.25
0.10
0.10
0.16
ties was not clearly demonstrated during
this program.
The average drag of both the conven-
tional and ESFF baghouses increased as
the face velocity was increased. The
average drag decreased as the field
strength was increased for all face
velocities.
The current requried by the ESFF bag-
house did not depend on field strength or
dust cake thickness within normal
operating ranges.
An ESFF baghouse operating at 2
cm/s, using bags costing three times the
cost of conventional bags, was estimated
to have an annual cost 11 percent below a
conventional baghouse at 1 cm/s. Bag
cost and ESFF face velocity are the most
important parameters in the economic
comparison of ESFF with conventional
operation.
Recommendations
Undertaking reverse-air ESFF with
relatively full-scale bags with sufficient
operating time would allow evaluation
of long-term effects and benefits. Extended
operation (a year or so) under field
conditions should provide conclusive
evidence concerning the design face
velocity allowable for ESFF baghouses.
An improved theoretical understanding
of the electrical effects in filters would be
helpful. Currently, generalizations from
data are not possible. Variations in the
inlet to the pilot unit made detailed study
of many operational parameters difficult.
Results of further laboratory-scale work,
particularly at boiler temperatures, could
lead to further model development.
Modeling and onsite testing in various
environments could extend the applica-
bility of ESFF over a range of dusts and
conditions.
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I
K>
0.6
0.5
0.4
0.3
0.2
0.1
1
figure 3.
2 3
Field Strength, kV/cm
Drag as a function of field strength and face velocity.
4
Table3. ESFF Versus Conventional Baghouse Cost
Cost (10* $)
Face velocity = 1 cm/s
Baghouse description:
Bag cost:
ESFF hardware
Collector and supports
Ducting and supports
Ash removal system
Insulation
Ash pond
ID fan
Miscellaneous
Total field cost
Engineering
Contingency
Turnkey cost
Fixed operating costs
Variable operating costs
Cost of electricity
Annual capital cost
Total annual cost
Conv.
$7/m2
0
5.85
0.87
0.62
1.60
1.14
0.08
4.01
14.2
2.83
2.83
19.9
0.10
0.39
0.18
3.17
3.84
ESFF
$14/m2
0.62
5.86
0.87
0.62
1.60
1.14
O.O4
4.01
14.8
2.95
2.95
20.7
0.10
0.69
0.15
3.30
4.24
ESFF
1.02
5.86
0.87
0.62
1.60
1.14
0.04
4.01
15.2
3.03
3.03
21.2
0.10
1.00
0.15
3.39
4.64
ESFF
$28/m2
1.43
5.86
0.87
0.62
1.60
1.14
O.O4
4.O1
15.6
3.11
3.11
21.8
0.10
1.30
0.15
3.48
5.03
Conv.
$7/m2
0
3.64
0.86
0.61
1.10
1.14
0.22
2.74
10.3
2.06
2.06
14.4
0.10
0.30
0.24
2.31
2.95
Face velocity = 2 cm/s
ESFF
0.31
3.65
0.87
0.61
1.10
1.14
0.13
2.75
10.6
2.11
2.11
14.8
0.10
0.52
0.18
2.36
3.16
ESFF
$21 Vm2
0.51
3.65
0.87
0.61
1.10
1.14
0.13
2.75
10.8
2.15
2.15
15.1
0.10
0.74
0.18
2.41
3.43
ESFF
$28/m*
0.71
3.65
0.87
0.61
1.1O
1.14
O.I 3
2.75
11.0
2.19
2.19
15.3
0.10
0.97
0.18
2.45
3.70
Basis: 500-MW boiler, 720 am3/s; conventional average pressure drop of 1.12 kPa at 2 cm/s, 0.42 kPa at 1 cm/s; ESFF pressure drop ofO. 65 kPa at 2
cm/s, 0.19 kPa at 1 cm/s; interest rate of 15%/yr.
•&U. S. GOVERNMENT PRINTING OFFICE: 1984/759-102/10698
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D. W. VanOsdell is with Research Triangle Institute, Research Triangle Park. NC
27709; and D. A. Furlong is 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:
Reverse-Air Pilot Unit Experience." (Order No. PB 84-230 002; Cost: $14.50,
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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
AGENCY
CHICA60
60*0"
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