United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-85/010 May 1985
Project  Summary
Development and  Evaluation  of
Improved   Fine  Particulate
Filter  Systems

Richard Dennis, John A. Dirgo, and Marc A. Grant
  The filterability of fly ashes emitted
by  coal-burning  power  stations is
described,  including that  of  several
ashes   generated  by  low  sulfur
western  U.S.  coal  combustion  that
are best controlled by fabric filtration.
Chemical and  mineralogical analyses
of the  coals were examined to deter-
mine  possible  relationships between
coal and ash properties and filtration
behavior. Both  fly ash size and  coal
ash content correlated strongly with
the fly ash specific resistance coeffi-
cient,  K2.  Weaker,  but  discernible,
correlations were shown for electrical
charge behavior and method of  coal
firing. Coal sulfur content, ash fusion
properties,  and  chemical structures
originally expected to influence parti-
cle size showed no  clear-cut effects
on filtration characteristics. The rele-
vant  literature  on   pulse  jet  filter
theory  and applications was assessed
to  develop coherent guidelines for
designing predictive filter models. The
effects of jet size  and location, jet air
volume, and the  intensity and dura-
tion of the jet  pulses were related to
pressure loss.  Energy  transfer  from
the jet  pulse to the fabric was ex-
plored  in terms of jet pressure,  sole-
noid valve  action, the  ratio of pulse
volume to bag volume, and the kinet-
ic properties of the  felt bags.  Finally,
predictive equations  were developed
for estimating  pressure loss  over a
broad range of collector design  and
operating parameters.
  This  Project Summary  was devel-
oped by EPA's  Air and Energy Engi-
neering  Research  Laboratory,   Re-
search   Triangle  Park, NC,  to  an-
nounce 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
  The primary objective of the research
summarized  in  this report was  to  in-
vestigate possible relationships between
coal and ash properties and fly ash filtra-
tion characteristics.  It was postulated that
certain  chemical and  physical properties
of coals might have some predictive value
in determining   the  specific  resistance
coefficients,   K2, of  their  resultant  fly
ashes. Since this parameter has a signifi-
cant impact  on filter system performance,
a reliable estimation method would facili-
tate the design and evaluation of reverse-
air  and/or  mechanical-shake-cleaned
fabric filter systems. Additionally,  it was
expected that some coal and ash proper-
ties (e.g., particle size, surface, and adhe-
sion  characteristics)  would  determine
how well a  fabric might be cleaned. In
some modeling equations, dust removal is
defined  by a cleaning  parameter, ac, that
indicates the fraction of  the filter surface
from which the  dust cake is removed  by
the cleaning  action.
  A second study objective was to extend
the capabilities of the existing  EPA/GCA
filtration model to include pulse jet collec-
tors or,  alternatively,  to develop a new
model if the former model could not  be
modified practically. The proposed bases
for developing a modeling  protocol were
the results  of  past  and  present  GCA
studies  as  well  as those  of other  re-
searchers. Additional information sources
were correlations deriving from the pres-

-------
ent investigations of coal and ash proper-
ties  and their  impact on  filter  perform-
ance.

Background

Coal and Fly Ash Properties
  Reliable  prediction of fabric filter  per-
formance depends on accurate estimation
of two major variables: K2,  the  specific
resistance coefficient for the dust, and ac,
a cleaning parameter that indicates the
fraction  of the fabric  superficial  dust
loading removed during the cleaning proc-
ess.  K2, which defines the  gas permeabili-
ty of a deposited dust layer,  is especially
important in determining the pressure loss
for  fabric  filter  systems  cleaned  by
reverse-air  and/or  mechanical  shaking.
Although theoretical relationships exist for
calculating  K2,  the predicted results  may
be very inaccurate  because of difficulties
in measuring the parameters  contributing
to K2  variability. Complications may  also
arise in  field  practice  when non-steady
state conditions, moisture  condensation,
or chemical reactions increase adhesion to
the fabric.
  Many  coal  and/or  fly  ash properties
have been  identified that may exert first,
if not second,  order effects on  K2; e.g.,
particle  size,  shape,  hardness,  surface
roughness, and  the chemical,  hygro-
scopic, and  hydration  characteristics  of
the  ash  constituents.  The  above  factors
provided the guidelines for  selecting  the
coal  types and classes that were eval-
uated  in this study.
  Regional distributions for U.S.  produc-
tion  of bituminous,  subbituminous,  and
lignitic coals. Table  1, show that western
coals account for only  25 percent of the
annual  tonnage.  Recent  indications  of
proportionately greater production of  low-
sulfur western coals (whose fly ash emis-
sions are better controlled  by fabric filtra-
tion  than  by  electrostatic  precipitation)
suggested, however, that  western coals
be given a strong weighting in this study.
Analyses  of  the  physical  and  chemical
properties  of eastern coals also  indicated
that Regions 1, 2,  and 3  coals  could  be
treated as  a single group to facilitate final
sample selections.
Pulse Jet Filtration
  Assessment  of the  relevant  literature
pertaining to the theory and application of
pulse jet filters revealed no general model-
ing procedures for predicting filter system
performance, although models have been
proposed  for  filter  systems  utilizing
combinations of bag collapse and reverse
flow or  mechanical shaking for periodic
fabric cleaning. Extensive EPA sponsored
studies  and  the  findings  of several  in-
dependent groups  have  shown that very
distinct differences  in the overall operation
of  pulse  jet  filters preclude any direct
adoption  of  the  mathematical  models
developed for the other cleaning methods.
It  has also been  established that particle
removal  is  caused  principally  by   the
mechanical  projection  of dust  from  the
pulsed fabric and not by air flushing. Ad-
ditionally, most researchers now recognize
that only a small fraction  (~1  to  5  per-
cent) of the dust  dislodged from  a  bag
ever reaches the  dust hopper, regardless
of  the  nearly 100 percent removal  at-
tainable  with  proper  equipment  design
and operation   (and  under  conditions
where the dust is not subsequently com-
pacted or cemented to fabric surfaces by
adverse  condensation  effects).  The very
brief pulse durations, —0.1  s, explain  the
rapid redeposition  of dislodged dust and
hence  the presence of a semipermanent
surface  dust cake, Wc,  referred to as  a
        cycling  layer.  Preliminary  studies  have
        shown that the total pressure loss across
        a conventional  pulse jet filter bag should
        be represented by three rather  than two
        components;  i.e., the  contribution  from
        the cleaned fabric with its residual dust
        holding  that remains with  the fabric, the
        loss  associated  with  the  cycling  or
        reposited layer, and finally the contribu-
        tion from the fresh layer of dust that is
        captured during the interval between each
        pulse.  The object of  the present study
        was to determine, by whatever combina-
        tion  of  theoretical   and  empirical  ap-
        proaches that appeared feasible,  how the
        available data  and   that   derived  from
        measurements  performed   during   this
        study  might be adapted to design a  prac-
        tical   predictive  model  for  estimating
        pressure loss.

        Technical  Approach

        Selection  of  Coals and
        Fly Ashes
          The  classification of  the fly  ashes in-
        vestigated  in this program  is shown  in
        Table  2. Restriction  of the number of
Table 1.    Estimated 1980 Coal Production by Coal Producing Region "
Region
1 Northern Appalachian
2 Southern Appalachian
3 Alabama
4 Eastern Midwest
5 Western Midwest
6 Western
States
PA, WV(nlb, OH, MD, Ml
WV(s), VA, KY(e), TNfn)
AL, GA, TN(s)
KYM, IN, IL
AR, IA, OK, KS, MO, TX
CO, WY, MT, SD, ND,
UT, NM, AZ, ID, WA, AK
Production
10* tons/yr
189(22.71'
192 (23. 1)
32 (3.8)
171 (20.6)
39 (4. 7)
209 (25. 1)
'Includes bituminous, subbituminous, and lignitic coals.
^Letters in parentheses refer to north, south, east, and west.
'Numbers in parentheses refer to percent of total production.

Table 2.    Classification of Fly Ash Samples by Selection Criteria
Characteristic:
• Coal producing region:'   1
  No. of samples          3
2
1
3
1
4
0
5
1
6
8
• Boiler firing method:
No. of samples
• Sulfur content: b
No. of samples
• Ash content:
No. of samples
• Base/acid ratio:
No. of samples
Pulverized coal
10
Low(<1%)
9
Low(<5%)
3
Low«0.17%)
4

Medium (1-3%)
4
Medium (5- 15%)
9
Medium (0. 17-0.331
6
Stoker-fired
4
High (>3%)
1
High (> 15%)
2
High (>0.33%)
4
"Arabic numerals refer to coal regions.
bWhlen a range of values is used to characterize a specific coal or ash property, the midpoint of thai
 range is used to categorize the sample.

-------
samples  to 14 was necessitated  by the
program scope, and the preponderance of
western  coals reflects best  estimates of
the future gas volumes  to be controlled
by fabric filters.  Because high sulfur con-
tents accentuate  fly ash  hygroscopicity
while low  sulfur contents enhance elec-
trical charge effects, coal sulfur contents
of 0.35  to 3.5  percent were surveyed.
Total ash  contents of 3.3 to 23  percent
were investigated because it  was believed
that higher ash  contents,  in conjunction
with  a fixed heating rate, would reduce
heat transfer to  individual  particles, such
that large, irregularly shaped mineral  par-
ticles would be less likely to melt.  Among
the  many characterizing ratios  for  the
mineral  constituents of  fly  ash  used to
predict ash slagging and fouling  proper-
ties, the  base-to-acid (B/A) ratio appeared
to have some predictive value through its
impact  on  melting  temperatures.  Thus,
several  B/A  levels were included  in the
samplings listed  in Table 2.  The  distribu-
tion  of  fly ash  samples  was  generally
representative with  respect  to the prin-
cipal  coal firing  methods. Pulverized coal
combustion,   far  more   common  than
stoker-firing on the basis of tonnage con-
sumed,  was  the source  of 10 of the fly
ash samples  while only 4 fly ashes were
generated by stoker firing.

Determination of Coal
Properties and Chemical
Constituents  of Fly Ashes
  Fly ash suppliers provided  most  of the
information on coal properties and fly ash
chemical  compositions  summarized  in
Table 3.  In general, coal  data describing
proximate  analyses  and sulfur  contents
were  more complete than those  for the
chemical composition of the resultant fly
ashes.  When  sample  information was
missing,  source  data specified by the fly
ash  suppliers for  their  coals (including
state of origin, region,  seam, and—where
possible—mine)  were used as a  supple-
mental source.

Laboratory Measurements of
Kt  and Fly Ash Size
Properties
  Fly ash K2 values (Table 3) were deter-
mined with a bench scale filtration system
using all glass (twill weave) fabric  panels
and  resuspended fly ashes  at a  nominal
filtering   velocity,  V,  of 0.61  m/min (2
fpm). Increases in uniformly distributed fly
ash loadings, W, (300  to 700 g/m2) cou-
pled with the corresponding increases in
pressure  loss, P,  for filtration at  a con-
stant velocity, V, permitted estimation of
K2 for the resuspended fly ashes; i.e., K2
= P/VW. The same test system was also
used, with minor modifications, to deter-
mine the  relationship between pulse jet
pressure  and  dust  dislodgement  from
Dacron felts.
  Particle  size  parameters  were  deter-
mined by Andersen Mark III cascade im-
pactor wherein samples were extracted by
a short probe from the central section of
the  inlet manifold.  This technique  pro-
vides the best possible description of the
dust  that  actually  deposits on the filter
surface.  Cumulative  size   distributions
were plotted on log-probability paper for
the  two  impactor  sizings  performed for
each fly ash.  The  aerodynamic mass me-
dian diameter, aMMD, and  the geometric
standard deviation,  5g, estimated  for each
pair of  curves  showed excellent  agree-
ment in most cases.

Results

Coal Properties  Versus
Fly Ash Filterability
  Relevant coal  and fly ash properties for
each sample are listed in  Table  3 along
with  boiler type.  Laboratory  derived K2
values,  particle  size   properties,  and
qualitative estimates  of the electrostatic
behavior of the fly  ash  in the test system
are also presented.  In Table 4, correlation
coefficients are listed for the relationships
between K2 and various coal  and fly ash
properties, including  the  particle  specific
surface parameter,  Sg.

K2  and Particle  Size
  As stated earlier, K2 values were deter-
mined  by  experimental  measurements
because  of  limitations  of  the  classical
theory.  One theoretical concept, however,
proved  useful in the present  study: the
relationship between  K2 and the  specific
surface parameter where K2 is predicted
to be proportional  to S|.  The term S0,
which  characterizes  the  surface/volume
ratio  for the polydisperse  particle  size
system   constituting  the dust cake,  is
readily  computed from the size  parame-
ters  determined by cascade impactor
measurements, the  mass  median diam-
eter,  MMD, and the geometric standard
deviation, 5g:  S0 = (6/MMD) (101-151 ^
*s).  The regression line generated from the
data shown in Table 3 supports the K2-S§
relationship with the  r2 value statistically
significant at the p  =  0.003 level. Unfor-
tunately, the data point scatter shown in
Figure 1 precludes use of these data as a
predictive  tool because the  95  percent
confidence interval embraces a range of
0.85 to 5.2 for a predicted mean  K2 value
of 3.0  N«min/g«m.

Effect of Coal  Firing  Method
on K2
  The  method of coal firing usually  in-
fluences  fly   ash  size properties,  with
stoker-fired boilers producing coarser  fly
ashes  than  pulverized-fired  or  cyclone
boilers.  Unfortunately,  only  semiquan-
titative  relationships   could  be  inferred
from   the  present   observations:   (1)
because of limited data, and  (2)  because
size properties can also be affected by ad-
ditional factors not defined in this study
(e.g.,  air/fuel ratio,  boiler  load  level,
system geometry,  gas residence time, and
settlement  losses).  Therefore,  although
the  average   K2  value  determined   for
stoker  fired ashes was  3.6  N«min/g»m
versus 2.2  N-min/g^m for the pulverized
firing  method (the expected  result),  the
difference was not statistically significant
based  upon the limited number of obser-
vations.


Effect of Electrical Charge
on K2
  In the absence of charge leakoff (that is
enhanced by  the electrical conductivity of
ionizable materials in the  dust layer),  the
accumulation  of particles bearing similar
charges is expected to expand the dust
layer  due  to mutual  repulsion.  Conse-
quently,  a  lower  K2 value  is anticipated
because  of increased  dust  cake  porosity
as  suggested by  the  circled points  in
Figure  1. Note, however, that stoker firing
may also have  contributed to lower  K2
values   associated  with  the   charged
deposits.


Effect of Sulfur Content on K2
  The  manner in  which coal sulfur con-
tent affects fly ash filtration properties is
not clearly understood,  although it  has
been  established  that  sulfur in various
forms  can  affect  ash  fluid  properties. If
the viscosity of the molten ash is lowered
sufficiently, it appears reasonable  that gas
stream  turbulence and  shearing action
might  lead  to droplet shatter.  On  the
other hand,  particles  that  have  melted,
because  of  their  viscous  nature,  may
serve   as  irreversible  collision sites   for
small particles undergoing Brownian diffu-
sion. When the coal sulfur fraction is due
mainly to its  iron  pyrite content, signifi-
cant  separation   of  FeS2  during  coal
upgrading will lower  the  basic phase  of

-------
the ash (i.e., the Fe203 contribution) and Effect of Coal Ash Content on K2 on the basis of coal ash content appeared
hence reduce the base/acid ratio. The ex- |t appeared that an increase in coal ash to confirm the presence of coarser par-
pected results, as discussed in the follow- content should result in less heat transfer ticles 
-------
Table 4.    Correlation Coefficients for Kt with Various Coal and Fly Ash Properties
                          Variable
                                                   Correlation Coefficient
                                                           (r)
             Specific surface parameter, SJ 
-------
tion  forces determined by pulse intensity
(pressure) and other  factors.  Therefore,
once the pulse cleaning parameters  are
set,  the recycling  loading is essentially
constant  and  independent  of  filtration
velocity and inlet dust concentration. This
facet of pulse jet  collector  performance
lends itself  to nearly constant gas flow
and  pressure loss as well as low effluent
loadings,  all  distinct advantages  when
coupled  to industrial processes or power
plant operations.

Approach to Modeling
  It  was decided  that the current absence
of reliable methods  for predicting dust
cake adhesion  and  gas permeability prop-
erties would require semi-empirical model-
ing  equations  for  predicting  pressure
losses for  pulse jet  collectors.  For this
reason,  the   terms  representing   the
pressure loss at the cessation of the pulse
for  (a)  the cleaned  fabric and  (b)  the
recycling layer were  combined. The new
descriptor, (PE)AW,  designated as the ef-
fective residual pressure loss,  is uniquely
defined by the dust/fabric combination of
interest,  the  filtration  velocity,  and  the
parameters describing the pulse jet clean-
ing system.  In  turn, (PE)AW can be related
to the rate  of pressure  increase within  a
pulsed bag,  d(Ap)/dt, the latter a function
of the pulse air volume, bag volume, and
the  rate at which  compressed  and  ex-
trained  air is  ejected into a  bag. With
respect to  a   single  bag, the following
equation  allows  computation  of total
pressure loss,  P:
       P = (PE)AW +  k CK2 V2At
(1)
where k is a constant that depends on the
choice of units, C is the inlet dust  con-
centration, K2  is  the specific resistance
coefficient  for the freshly deposited  dust
layer  that arrives  over  the  time interval.
At, and V is  the face velocity or air/cloth
ratio  for a group  of bags being  cleaned
sequentially (one at  a time). The pressure
loss normalized  with respect to  velocity
(otherwise referred  to as the filter drag)
may be calculated by the general relation-
ship:
                rib
              i  = 1
                          -1
                                     (2)
where S is the average system drag, nb is
the total  number of bags,  A is the total
cloth area, a;  = A/nb, and Si ranges from
S =  (PE»AW/V to 
-------
    R. Dennis, J, A. Dirgo,  and M. A. Grant are with CCA/Technology Division,
      Bedford, MA 01730.
    Louis S. Hovis is the EPA Project Officer (see below).
    The complete report,  entitled "Development and Evaluation of Improved Fine
      Particulate Filter Systems," (Order No. PB 85-177 244/AS; Cost: $16.00,
      subject to change) will be available only from:
           National Technical Information Service
           5285 Port Royal Road
           Springfield, MA 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
        QCQ0329   PS

        U S  ENVIR  PROTECTION  «GINCY
        REGION 5  LIERARY
        230  S  DEARBCRN  STREET
        CHICAGO               IL

-------