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-
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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.
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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
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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
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Table 4. Correlation Coefficients for Kt with Various Coal and Fly Ash Properties
Variable
Correlation Coefficient
(r)
Specific surface parameter, SJ
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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
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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
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CHICAGO IL
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