United States
 Environmental Protection
 Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
 Research and Development
EPA/600/S7-86/042 Apr. 1987
 Project Summary
 Pilot-Scale Evaluation of  Top-Inlet
 and Advanced  Electrostatic
 Filtration
A. S. Viner, G. P. Griener,
D. F. Furlong, and R. G. Hurst
  Advanced Electrostatic Augmenta-
tion of  Fabric Filtration (ESFF) was
evaluated on a slipstream from a stoker-
fired boiler. Advanced ESFF, with  its
characteristic high-voltage center-wire
electrode, was compared with conven-
tional filter bags in the same baghouse
using calibrated flow orifices. The ad-
vantage of advanced ESFF was demon-
strated by consistently higher gas flow
rates in  the bags with the corona pro-
ducing electrodes. Analysis of the data
showed that the specific resistance of
an electrostatically enhanced filter was
70% less than that of a conventional
bag. An economic analysis showed a
capital cost savings of 26% with ad-
vanced  ESFF, based  on  doubling the
air-to-cloth ratio for advanced ESFF. In
a second test, the feasibility of using
top-inlet filtration on stoker fly ash was
established. No definitive comparison
with conventional bottom-inlet filtration
could be made  in the  allotted test
period.
  This Project Summary was developed
by EPA's Air and Energy Engineering 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 In-
formation at back).

Introduction
  Under a program funded by the U.S.
Navy,  and with support from the U.S.
Environmental Protection  Agency, per-
sonnel from Research  Triangle Institute
and ETS, Inc., conducted a pilot plant
evaluation of fabric filtration at a bag-
house. The study focused on two different
approaches to fly ash filtration: top-inlet
filtration  and advanced  electrostatic
stimulation of fabric filtration (ESFF). In
designing a baghouse,  several variables
must be considered: the cost of the fan(s);
the rate of the pressure loss  (amount of
energy required to pull air through the
dust-laden fabric); and the ratio of the air
flow  rate  to the  filter area (air-to-cloth
ratio). Generally, it is better to have a low
rate of pressure loss or drop. Attaining a
low pressure drop means ensuring a slow
rate of dust deposition which, in turn, is
related to the amount of fabric area avail-
able for dust deposition. This all points to
the importance of air-to-cloth ratio. The
design of a baghouse poses the problem
of specifying the highest possible air-to-
cloth ratio (to minimize the overall size of
the baghouse and subsequently its cost)
while ensuring that the pressure  loss
limitations (influenced  by fan cost con-
siderations) are not exceeded.
  One form  of ESFF  involves  placing
electrodes of opposite  polarity  on the
surface of the fabric and then generating
an electric field parallel to the fabric. The
most recent version of ESFF (also called
advanced  ESFF)  calls  for placing the
electrode in the center of the filter bag
(Figure 1). Experiments  conducted by the
U.S.  EPA showed that  the electric field
alters the dust deposition pattern and the
structure of the dust cake. The overall
result was a reduction  in the pressure
drop  across the bag. The advanced form
of ESFF was tested during this project.
  The top-inlet design of a baghouse was
the other form of filtration tested at the
pilot  scale. The  standard, bottom-inlet
design allows dirty gas to enter and flow
up the bag. Because of this upward flow,

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            To High-Voltage Power Supply
           •ğ~ Fabric Bag
Gas Flow In
Figure  1.
•fy- High-Voltage Electrode



     Insulator Cable


    Electrode Tension Spring

    Spring-Restraining Bar
  The center-wire configuration
  for advanced ESFF.
large particles  either fall from the gas
into the hopper below or are deposited on
the lower portion of the bag. An elutriation
effect may occur whereby the average
particle size  deposited on the bag de-
creases as the distance from the bottom
inlet  increases. The  resistance of the
dust cake  may increase as  the mean
particle size decreases, producing an in-
crease in flow resistance as the distance
up  the bag  increases.  Based on  this
theory,  it has been  recommended  that
the dust-laden  gas be introduced from
the top, into a bag with openings at both
the top and the bottom (Figure 2). In this
way, the large particles would be retained
in the gas and be distributed more evenly
along the entire length of the bag. This
may both lower the resistance of the dust
cake and make  it more uniform.

Pilot Plant Design
  Located at Cherry Point Marine Corps
Air Station  in  Havelock, NC, the pilot
plant (Figure 3) filtered coal fly ash from a
slipstream from two spreader-stoker-fired
boilers. The boilers are rated at a maxi-
mum of 77,500 Ib steam/hr (9.77 kg/sec).
Only one boiler was  online at any given
time during the test period. The fuel was
an  eastern  coal  of  1%  sulfur and 5%
water and had  a heat content of 31,600
                                          Gas Flow In
                                          Gas Flow Out
                                Figure 2.   Conceptual design of a top-inlet baghouse.
                               kJ/kg (13,600 Btu/lb). The ash's electrical
                               conductivity (relatively high for  a coal
                               ash) resulted  from a high,  unburned
                               carbon content. Identical baghouse com-
                               partments  were located next to each
                               other, with each fed by a separate fan
                               with a capacity of 19.82 mVmin (700
                               ftVmin) of air  drawn from a common
                               duct. This common duct pulled gas from
                               the inlet of the electrostatic precipitators
                               (ESPs) downstream  of  each boiler. A
                               single reverse-air fan provided ambient
                               air for cleaning each compartment.
                                 Because  of the project's focus on the
                               feasibility of advanced  ESFF, the only
                               tests performed  were those  which
                               demonstrated the effect and, to a small
                               degree, the operating range of advanced
                               ESFF. Time constraints dictated comple-
                               tion within 1  year (February 1985 to
                               February 1986). This requirement affected
                               the  experimental  design.  The  central
                               problem in designing the plant consisted
                               of making it possible to test three kinds of
filtration — advanced ESFF, top-inlet, an(
conventional bottom-inlet — in only tw<
compartments as efficiently as possible
The engineers decided to use individua
bag flow monitors (IBFMtm), thereby al
lowing for the simultaneous operation c
advanced ESFF and conventional filtratio
in the same compartment. This  left th
second compartment free for testing th
top-inlet design.
  The IBFM   has a calibrated  orific
plate  at the inlet of the bag on top of th
compartment tubesheet. Pressure tap
on the upstream  and downstream side
of each orifice  plate make it  possible 1
measure the orifice pressure drop. Th
flow into each bag is calculated from dai
on the pressure drop and the temperatui
of the gas. This information,  along wii
the tubesheet  pressure  drop and inl
dust concentration, can then be  used
calculate the dust cake's specific resi
tance and residual drag. A comparison
these parameters produces  a figure

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           Test
          Trailer
           I
           Conventional
            andESFF
           Compartment
              R/A Fan
             Top-Inlet
           Compartment
 Figure 3.    Schematic of the Cherry Point pilot plant.
merit for evaluating the ESFF design. The
data obtained from the top-inlet compart-
ment resulted in  a similar  comparison.
Testing the ESFF and conventional bags
in the same compartment meant that the
bags were exposed to identical conditions,
thereby increasing the confidence in the
results.

Data Acquisition System
  As mentioned earlier, the project had a
time constraint of 1 year. Also, cost con-
siderations precluded  payments for an
on-site  operator.  Thus,  the  engineers
designed an automated system for col-
lecting, retrieving, and analyzing the data.
This system offered a number of advant-
ages, the most important of which were:
elimination of labor costs for data col-
lection and  computer entry;  and long-
distance data  retrii
advantage meant that the data could be
monitored and analyzed daily.
  It was necessary t<
peratures, tubesheet
drops, electrical condi
aval.  The second
 measure the tem-
and orifice pressure
ions, and dust-inlet
loadings. All were automated except for
the dust-inlet loadings (measured  ac-
cording to  standard  EPA  Method 5
procedures). The signals from the various
thermocouples and pressure transducers
fed into a  Molytek  Model  2702  Pro-
grammable Datalogger. The  datalogger,
in turn, interfaced with a Tandy 1200
microcomputer. At 10-minute intervals,
the datalogger transmitted instantaneous
readings to the computer which then
stored the data (with custom-developed
software). The concurrent PC-DOS oper-
ation allowed for simultaneous operation
of the data collection/storage program
and remote communication.  Thus, the
engineers could use a high-speed modem
to call the on-site computer and  read
data files over the phone lines into their
own microcomputers. Once the data files
were retrieved in this fashion, they were
read into a  Lotus 1 -2-3 (C) worksheet for
analysis. The one disadvantage in testing
two kinds of filtration in the same com-
partment was that it precluded on-site
evaluation  of performance based on the
tubesheet  pressure drop. Thus, it was
necessary to analyze the data  before the
results were known.  However,  the ad-
vantages certainly outweighed this one
disadvantage.

Results
  During February and March 1985, the
pilot plant was installed. The  plant was
brought online in  April, with hardware
debugged in April and May. The collection
of data began in June. The plant operated
continuously until the end of September,
except for brief periods due to coal eleva-
tor  repair, a switch from boiler No. 1 to
No. 2, and  electrical failure. At all  other
times, the  plant  required  a  minimum
amount of intervention and collected data
automatically.

Bottom-lnlet/ESFF Compartment
  The feasibility of advanced  ESFF and,
to a lesser degree, of top-inlet filtration is
determined by the ability to operate at
higher face velocities. Therefore, the focus
was on determining if ESFF and top-inlet
filtration  could be used for  prolonged
periods  of  time at face  velocities that
were substantially higher than those of a
conventional  filter. Thus, the test  plan
called for one  long experiment with the
filtration velocity increased gradually. This
experiment was  particularly  important
since it was impossible  to explicitly set
the flow rate through the bottom-inlet
and ESFF bags. Instead, it was possible
only to set  the overall flow rate to the
entire compartment. The engineers deter-
mined the  distribution of flow between
the bottom-inlet and the ESFF bags by
calculating  the drag differential between
the two sets of bags.
  Figure 4  plots the average air-to-cloth
ratio (for each bag) against  the  cycle
number. One cycle represents a 12-hour
period (11 hours and 50 minutes of filtra-
tion, and 10  minutes of  cleaning). The
curves are  separated into two  distinct
groups, with the  exception of the  data
from cycle 50 through 70. The upper set
of curves corresponds to the  advanced
ESFF bags, and the lower set corresponds

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    1.8
    1.5-
 |
 ^  0.9
 3>  0.8
 V  0.7

 .i  0.6
 *  0.5

    0.4-

    0.3-
       10
               30
                         SO
\     i
    70
            r     r     i    i
            ;;o      730
                                                                      150
                                           90
                                  Cycle Number

Figure 4.   Average air-to-cloth ratio for conventional and ESFF bags  in bottom-inlet
           compartment.
to the conventional bags. The overall air-
to-cloth ratio (compartment inlet flow/
total compartment bag area) increases
from an initial value of 0.55 to 0.79
m/min  (1.8 to 2,6 ft/min) at cycle 50.
Then, the reverse air fan failed, resulting
in erratic data. The tubesheet pressure
drop ranged from 0.1  kPa (0.4 in. H20) at
an air-to-cloth ratio  of 0.55 m/min  to
0.87 kPa (3.5 in. H20) at an  air-to-cloth
ratio of 1.26 m/min.
  It was possible to convert the data into
flow rates and then use those rates and
the dust concentration information  to
compute the variation of dust load (kg/m2)
on each bag as a function of time. The
ratio of pressure drop to bag flow velocity
gives the bag drag. Figures 5 and 6 plot
the bag drag as a function of dust load for
the ESFF and conventional bags, respec-
tively. The straight line in each figure
represents the regression fit to the data.
Note the two important features in these
figures: (1) the straight lines show a good
fit to the data, indicating that it is reason-
able to assume  a linear relationship
between drag and dust load;  and (2) the
slopes (i.e., specific resistance) of the two
lines are  quite different. The value  of
drag for the ESFF bag rises much more
slowly than the conventional bag's value.
Thus, by using an  appropriate model of
pressure drop behavior and  measuring
the individual bag  flows, the values for
          the specific resistance and residual drag
          can be determined fairly easily.

          Top-Inlet Compartment
            In  making  these  measurements,  a
          single value for specific resistance was
          assumed as characterizing all the bags in
          the compartment at  a given time. The
          value for residual drag also was assumed
          to be the same for all the bags. Values for
the compartment drag and dust load were
calculated from measurements  of  the
compartment inlet flow and the tubesheet
pressure drop. The test plan for this
compartment was very similar to the plan
for the other compartment, with the face
velocity increasing  from 0.55 to 0.79
m/min. The  results,  however,  were
marred by two problems: the breakdown
of the cleaning fan (mentioned earlier)
and the loss of control over a flow-control
valve. Because of the small amount of
reliable data,  it is doubtful that any
accurate conclusions can  be drawn
about  the  operation  of the  top-inlet
compartment.

Economic Analysis of
Advanced ESFF
  This project  showed  that advanced
ESFF can greatly reduce the rate of pres-
sure rise of a baghouse. This can translate
into  an economic savings in that, for a
fixed average pressure drop, a baghouse
which uses ESFF will be smaller than a
conventional baghouse. Also to be con-
sidered is whether  or not this smaller
size will offset the cost of the additional
hardware. The pressure drop depends on
both the residual drag and the specific
resistance of the dust  cake. Test results
indicated that an ESFF baghouse can be
expected to  have a specific resistance
equal to 30%  of that of a conventional
baghouse. This value was used to esti-
mate pressure drop. Two values were
considered for residual drag: case I made
the ESFF's residual drag equal  to a con-
ventional  baghouse's,  while case II
             0.40
             0.36
              0.32
           *  0.28
           2s

           Q  0.24
              0.20
              0.16
                                          I
                                                I
                                                                     0.5
                                                                                0.4
                                                                                    c:

                             0.05
                              0.10
                               Dust Load,

Figure 5.   Drag correlation for the ESFF bag.
   0.15
 kg/m2
                                                                 0.20
                                                                     0.2
                                                                  0.25

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    040
    036
    032
c
I
£
*   028
Q.
    024
    0 20
    0 16
                   I	i
                                                                      05
                                                                      0.4
                                                                      03
                                                                      0.2
                                                                          o
                                                                          3C
                                                                          C
         0    002   004   006   0.08   0.10
                                Dust Load, kg/m2

Figure 6.    Drag rnrre/ation fnr thf rnnwpn(;pna/ hnn
                                                  0 12
                                                         0.14   0.16
assumed the ESFF value to be half of that
of the conventional baghouse Although
the results from this study present little
evidence  to  support this assumption,
laboratory and pilot-plant studies of other
ESFF geometries have indicated a large
difference  in residual drag   Based on
these assumptions about the air-to-cloth
ratio, bag life,  specific resistance, and
residual drag, the annual operating and
maintenance  costs  were  computed.
Figure 7 plots the annual savings (in the
form of the percent difference in cost
between  an  ESFF and a conventional
baghouse)

Conclusions  and
Recommendations
  On an average, the specific resistance
of the ESFF bags was 70% less than that
of conventional bags.  The IBFMtm system
worked well, proving invaluable to the
success of this  program The automated
data acquisition/analysis system also
worked well and allowed the pilot plant
to operate unattended for days at a time.
Problems with the cleaning hardware in
the top-inlet  compartment significantly
affected the conditions and produced un-
reliable results. Finally, the additional cost
of the ESFF hardware is more than offset
by the savings resulting from the smaller
size of the baghouse. The capital cost of
an ESFF baghouse which was operated
at air-to-cloth ratio of 1.22  m/min would
be 26% less than that of a conventional
baghouse with  an air-to-cloth  ratio  of
0.61 m/min.
                                              40
                                              30
                                           to
                                           Ol
                                           c
                                           I  20
                                           Q.
                                           O

                                           1
                                           C
                                           C
                                           I
                                           Uj
                                              10
                                                      Case II
                                                                                                       Bag Life
                                                 0          03          06         09         12
                                                                 ESFF Gas-to-Cloth Ratio, m/min



                                        Figure 7.   Annual ESFF savings in operating and maintenance costs

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     A  Viner is with Research Triangle Institute, Research Triangle Park, /vc.' 2//UU;
       andG. Greiner, D. Furlong, andR. Hurst are withETS. Inc.. Roanoke. VA 24018.
     Louis S. Wow's is the EPA Project Officer (see below)
     The complete report,  entitled "Pilot-Scale Evaluation of Top-Inlet and Advanced
       Electrostatic Filtration, "(Order No PB87-133 096/AS; Cost: $ 13.95, 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/S7-86/042
              0000329    PS

              U  S ENVIR PROTECTION  AGENCY
              REGION  5  LIBRARY
              230 S DEARBORN  STREET
              CHICAGO                IL    60604

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