&EPA
Controlling
Particulate Emissions froi
Coal-Fired Boilers
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Introduction
Every year, millions of tons of
participates are emitted into
the air we breathe Most
come from natural sources
like forest fires, dust storms,
and volcanoes. But a grow-
ing percentage is produced
by manmade sources like
factories and powerplants
Some particulates are non-
toxic. But others, especially
those from fossil fuel
combustion and industrial
processes, can be dangerous
to human health. That's why
the Environmental Protection
Agency (EPA) has identified
particulates as one of the six
air pollutants having "poten-
tial for widespread adverse
effects on human health .ml
welfare." Acting on the
authority of the Clean Air Act
Amendments of 1970, EPA
has set a National Ambieni
Air Quality Standard for total
suspended particulates in our
air of 75 micrograms per
cubic meter (annual average).
Since 1970, the national
average concentration of
particulates in the atmosphere
has dropped by almost
20 percent, and the down-
ward trend is continuing. But
we still have a long way to
go before we can meet the
National Ambient Air Quality
Standard. With an increasing
number of industries and
powerplants scheduled to be
built in the coming decades,
paniculate pollution will get
worse - - unless we take steps
to control it
That's exactly what's happen-
ing at the Industrial Environ
mental Research Laboratory
(IERL). part of EPA's Office of
Research and Development in
Research Triangle Park. North
Carolina. There, the Partic-
ulate Technology Branch
(PATB) is working to find
more effective and econom-
ical ways to reduce the
amount of particulates emitted
into our air.
Because coal combustion is
currently the largest source of
manmade particulate pollu-
tion, a major focus of PATB's
work is on controls for flyash.
Electric utilities now burn
vabout 700 million tons of
Boa I per year : •
more th.m 3 million tons of
fly.ish into the atmosphere.
And with dwindling supplies
of oil and nat is. coal
Consumption i : to
double in i1 ; 1 b years.
As coal consumption
increases, more stringent
particulate emission standards
will be required to keep our
air safe to breathe. Like
other branches of IERL-RTP,
PATB supports EPAs Office of
Air Quality Planning and
Standards by providing
technical information for
setting realistic, attainable
limits for particulate
emissions.
But that's only part of PATB's
job. To help utilities and
industries meet air quality
standards. PATB has pro-
grams underway to improve
the efficiency and cost-
effectiveness of the three
particulate control devices in
use today - - electrostatic
precipitators. wet scrubbers,
and fabric filter baghouses.
EPA - RTP LIBRAP
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Electrostatic
Precipitators
For many years, electrostatic
precipitators (ESP's) have
been the mainstay of the
utility industry's participate
control program. With low
maintenance requirements
and high collection effi-
ciencies. ESP's have gained
widespread acceptance. But
the proposed tightening of
participate emission standards
and the growing dependence
on low-sulfur coal could
greatly increase the costs of
installing and operating ESP's.
As a result, some utilities are
now considering alternative
control devices.
When utilities burn high-
sulfur coal, conventional
ESP's can easily meet the
current New Source Perform-
ance Standards (NSPS) of
43 ng/J (0.1 lb/106 Btu).
With design and operating
modifications, they may be
able to meet the proposed
standards of 13 ng/J (0.03
lb/106 Btu). But costs
increase markedly as collec-
tion efficiency is improved.
To cut emissions in half,
costs go up nearly 20
percent.
ESP collection efficiency
becomes an even more
serious problem with low-
sulfur coal. Most coal burned
today is eastern coal mined in
Pennsylvania. Illinois. West
Virginia, and Kentucky. Over
the next two decades,
however, many powerplants
will be switching to western
coal mined in Montana,
Wyoming, and Colorado
Since western coal has a
lower sulfur content - - less
than 1 percent compared to
2 to 3 percent for eastern
coal - - it emits fewer sulfur
oxides into the air.
But there's a trade off
Flyash from low-sulfur coa
has higher electrical resistivity
than flyash from high-sulfur
coal. This means that low
sulfur flyash is more difficult
for conventional ESP's to
collect. To solve this
problem, three techniques to
increase ESP collection
efficiency are currently being
tested: 1) enlarging the
specific collecting area.
2) raising the operating
temperature, and 3) adding
conditioning agents to the
exhaust gases.
Enlarging the specific collect-
ing area (SCA) significantly
improves ESP efficiency.
But it also increases capital.
operating, and land invest-
ment costs. Utilities switching
from high-sulfur to low-sulfin
coal usually increase ESP
specific collecting area by a
factor of 3
Raising the operating temper-
ature of ESP's is one of the
newest ideas for improving
collection of flyash from
low-sulfur coal Conventional
ESP's operate at 1 SOT or
below Recent tests show
that when ESP's are installed
in the flue gas duct upstream
of the air heater, where they
operate at 250-400°C. the
resistivity of most flyashes is
reduced.
The use of hot-side ESP's is
increasing despite some initial
problems with materials and
temperature Several units
are now in service and
operating satisfactorily. The
choice of a hot-side ESP
depends on fuel character-
istics and the size of the
conventional unit that would
otherwise be required.
Adding conditioning agents to
the exhaust gases may be the
least expensive approach to
increasing ESP collection
efficiency However, this
technique may produce
secondary emissions that
must be carefully evaluated.
Further research is needed to
find other ways to improve
ESP efficiency. Meanwhile
some utilities are turning to
wet scrubbers or fabric filter
baghouses as alternative
p.uticulate control devices.
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1
S 20
i TVA 500 MW (31
i PEDCO 45O MW (4)
I EPfll bOO MW (1)
.
U
y
HIGH-SULfUR COAL
LOW SULFUR COAL
Electrostatic precipitators put an
electric charge on particles in the
gas stream, collect the particles
on a grounded metal plate, and
remove them periodically by
flushing or vibrating the plate
0 300 400 600
SPECIFIC COLLECTING AREA |tt' 1000 aclm |s/mH
Capital cost of cold-side ESP's
increases significantly as specific
collecting area is increased.1
SPECIFIC COLLECriNG AREA CAPITAL COST
•• .
Low-sulfur coal requires greater
collection area for the same
efficiency.2
At the higher temperatures of
hot-side ESP's, flyash resistivity
drops.3
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Scrubbers
Wet scrubbers have been
used as paniculate collection
devices for more than 50
years Although they are less
expensive to install than
baghouses or electrostatic
precipitators, scrubbers cost
more to operate. They
require large amounts of
water and electricity and
usually create a slurry that
must be processed separately.
They also tend to be less
efficient than the other
control devices, particularly
for capturing fine particles.
But efforts are underway to
reduce costs, and some new
scrubber designs are now
being tested. One of these is
the mobile bed scrubber.
Commonly used in power-
plants to control SOx
emissions, mobile bed
scrubbers are also used in a
few powerplants for panic-
ulate control. Performance of
mobile bed scrubbers for
particulates is being studied
at the TVA Shawnee Power
Station m Paducah, Kentucky
Another type of wet scrubber,
a ventun/spray tower, is also
being tested at the Shawm>c
Power Station. In this wet-
scrubbing system, the flue gas
is cleaned with 3 slurry of
lime or limestone. Sulfur
dioxide is absorbed into the
liquor, and flyash is removed
by impact with the slurry
droplets. This scrubber
system removes more than 99
percent of the flyash while
collecting over 80 percent of
the SOx
One way of evaluating
scrubber systems is to
compare cut diameters at
various pressure drops. Cut
diameter is the particle size
for which the scrubber
collection efficiency is a
specified value, usually
50 percent. Pressure drop is
a measure of the unit power
consumption of the scrubber.
High collection efficiency foi
fine particles requires a high
pressure drop But a high
pressure drop means increased
energy consumption and,
therefore, greater cost Power
requirements for reasonably
efficient, high-energy venturi
scrubbers can reach 3
percent of the net pl.mt
output.
Various types of wet
scrubbers have been used for
years to control paniculate
emissions from industrial
sources However, significant
improvements will be nec-
essary before wet scrubbers
can be used effectively and
economically on tomorrow's
coal-fired boilers
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PRESSURE DROP'OPERATING COST
Scrubbers spray small droplets of
water into particle-laden gas
streams. Particles collect on
the water droplets and are
removed with the water.
Higher pressure drop allows
collection of smaller particles.
But as pressure drop increases,
so does operating cost.2
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Baghouses
Since the early 1 900's,
fabric filter baghouses have
been widely used to treat
gases emitted from industrial
processes like metal smelting
and chemical and fertilizer
production. Only recently
have baghouses become an
attractive option to the
electric utility industry
Utilities are now interested in
fabric filter baghouses because
they are highly efficient for
collecting fine particles. Even
more important, the type of
coal burned has little effect
on collection efficiency.
Advances in fabric durability
and versatility have also made
baghouses more widely usable.
Two coal-fired powerplants
have been using fabric filters
successfully since 1973 -
the Nucla Plant of the
Colorado Ute Electric Station
and the Sunbury Station of
the Pennsylvania Power and
Light Company. Both instal-
lations can easily meet
today's particulate emission
standards and the proposed
future standards.
Another fabric filter baghouse
has been installed at a South-
western Public Service power-
plant in Amanllo. Texas.
PATB is now evaluating this
baghouse -- collecting data
on performance, operating
life, and cost effectiveness.
Currently, about 50 baghouses
are operating, under construc-
tion, or on order for utility
boilers.
Baghouse performance and
costs primarily depend on air-
to-cloth (A/C) ratio, the
volume of gas to be cleaned
divided by the area of fabric
used. The optimum A/C ratio
for most utility baghouses is
between 2 and 3 Lower A/C
ratio improves collection effi-
ciency but requires larger
equipment, increasing capital
cost Although higher A/C
ratio requires smaller equip-
ment, the lower capital cost is
offset by an increase in oper-
ating cost This is because
more energy is needed to
move the gas against a higher
pressure drop and bags must
be replaced more frequently.
Although the durability of
fabric filters has been a
problem in the past, improve-
ments have been made to
ensure adequate bag life.
Effective air seals, good flow
distribution, adequate insula-
tion, and other precautionary
measures can now prevent
corrosion and filter plugging.
Utilities burning low-sulfur
coal may find fabric filter
baghouses to be the most
cost-effective control device
for new powerplant
installations.
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Gas streams enter the baghouse
and are passed through porous.
flexible fabric filters. Trapped
particles are removed by shaking
or flexing the fabric.
2 4
Ml In I IOTH RATIO
Air-to-cloth (A/C) ratio affects the
cost of fabric filter baghouses.
The optimum A/C ratio for most
utility baghouses is between 2
and 3 '
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Economics
During the next few years,
more utilities will be consider-
ing wet scrubbers and fabric
filter baghouses as alterna-
tives to ESP's. As this
happens, utilities will need as
much information as possible
to be able to compare cost
and performance of all three
devices.
Electrostatic Precrpitators
ESP performance varies with
the type of coal burned
Efficient control of flyash from
low-sulfur coal requires the
use of oversized conventional
precipitators. hot-side precip-
itators. or flue gas condi-
tioning agents But costs are
high for each of these
alternatives. In most cases,
however, ESP's cost less to
operate than the other control
devices.
Scrubbers
Scrubbers are frequently the
least expensive control device
to purchase. But because
they must operate at high
pressure drops to meet
present particulate emission
limits, they are often the most
expensive to operate. In
addition, scrubbers have
difficulty meeting opacity
standards because of their
low efficiency for collecting
fine particulates (Opacity
standards are presently at 20
percent.) In the future, scrub-
bers may be used most
effectively as particulate con-
trol devices in conjunction
with flue gas desulfunzation
(FGD) systems
Baghouses
Fabric filter baghouses are
highly efficient for collecting
fine particulates -- and their
efficiency does not depend on
the type of coal burned. For
low-sulfur coal, installation
and annual operating costs
appear to be less than for
ESP's Interest in baghouses
nuy increase as particulate
emission standards become
more stringent. This has
already happened in the State
of New Mexico. New Mexico
restricts emissions to no more
than 22 ng/J (0.05 lb/106
Btu) of total particulate and
no more than 9 ng/J (0.02
lb/106 Btu) of particulate less
than 2 micrometers in
duimeter. Fabric filter
baghouses can easily meet
these standards
The following tables and
graphs show general cost and
performance comparisons for
the three control devices.
PATB does not recommend
one over another Each has
certain advantages and dis-
advantages for a given situa-
tion. PATB's goal is to
optimize the technology for
each of the devices so that
utilities can make reliable
choices for specific
applications.
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INSTALLED CAPITAL COSTS FOR CONTROL DEVICES
ON 500 MW POWERPLANTS. PROJECTED 1980 S kW
LOW SULFUR COAL
ANNUAL COST (CAPITAL AND OPERATING) FOR CONTROL DEVICES
ON 500 MW POWERPLANTS. BASE YEAR 1978
EMISSIONS
43 ng/J
22
13
ESP
EPA
$53 kW
68
81
EPRI
40
47
52
FABRIC FILTER
EPA EPRI
58 36
58 36
58 36
VENTURI
SCRUBBER
EPA
59
73
NA
HIGH SULFUR COAL
EMISSIONS
ESP
EPA EPRI
S27 kW 38
28 43
32 52
FABRIC FILTER
EPA EPRI
52 36
52 36
52 36
VENTURI
SCRUBBER
EPA
59
73
NA
Compared to ESP's and venturi scrubbers, fabric filters become more
cost-competitive as participate emission standards become more
stringent. (Based on data from the Electric Power Research Institute
and EPA.)1
LOW-SULFUR
COAL
HOT SIDE ESP
FABRIC FILTERS
13 43
PARTICULATE EMISSION LIMIT nfl J
HIGH SULFUR
COAL
COLD SIDE ESP
lAHUli ML H MS
13 43
PARTICULATE EMISSION LIMII
At the proposed paniculate emission limit of 13 ng/J, fabric filters
appear to cost less than ESP's, especially for low-sulfur coal.
(Current limit is 43 ng/J.}1
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Devices
Advantages
Disadvantages
Cost Considerations
ELECTROSTATIC
PRECIPITATORS
Critical Parameter
Specific collecting area
(SCA), usually measured as
ftVacfm (s/rn)
1) Most cost-effective for moderate
efficiency, high-sulfur coal
2) High efficiency with low
pressure drop
1) Problem collecting flyash from
low-sulfur coal or mixed fuels
2) Collection efficiency varies with
nonuniform inlet loading, gas
flowrate, and temperature
1} More expensive than scrubbers,
comparable to baghouses to
purchase
2) Generally least expensive of
devices to operate
WET SCRUBBERS
Critical Parameter
Pressure drop, power
consumption per unit
volume of air flow, usually
measured as crn
H2O
1) Can be used for high-temperature,
explosive gas streams
2) Can remove gases as well as
particulates
3) Compact, easier to retrofit than
baghouses or ESP's
1) High water usage, high energy
consumption
2) Slurry disposal can be a problem
1) Varies greatly with application
and scrubber type
2) Generally less expensive than
ESP's and baghouses to pur-
chase, more to operate
BAGHOUSES
Critical Parameter
Air-to-cloth ratio, gas
volume to be cleaned
divided by cloth area,
usually measured as
ft acfm (s/m) of cloth
1} Highly efficient for a variety
of coals
2) May be easier to retrofit to boiler
burning low-sulfur coal than to
install hot-side ESP
1) Limited bag life (about 4 years)
2) High pressure drop required for
efficient, fine particle collection
1) More expensive than scrubbers,
comparable to ESP's to purchase
2) Generally less expensive than
scrubbers, more than ESP's to
operate
3) Bag replacement cost is major
maintenance item
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Looking Ahead
In the coming decades, the
most critical problem in the
control of paniculate emis-
sions will be collection of
fly;ish from coal combustion.
PATB has programs underway
to improve the three most
effective control devices
available today These
programs will help utilities
meet clean air standards as
they switch to coal to meet
our Nation's immediate need
for plentiful fuel
Pilot-scale testing to improve
electrostatic precipitators
began in late 1977. The
pnxjram has two goals: 1) to
reduce problems in new
installations caused by flyash
resistivity, and 2} to demon-
strate environmentally accept-
able flue gas conditioning
agents for retrofit situations.
PATB is also working to
develop more effective wet
scrubbers for use with both
low- and high-sulfur coals
The goal is to design an opti-
mum scrubber system -- one
that improves collection effi-
ciency and mist elimination
while reducing energy
requirements
As part of the program to
improve fabric filter systems.
PATB is testing the use of
baghouses for combined SOx
and particulate control Work
is also continuing to deter-
mine the life expectancy.
cleanabihty. and physical and
chemical resistance of differ-
ent fabric media.
PATB will also be focusing
attention on pollution problems
that haven't been addressed
before — like fugitive emissions
from hard-to control sources
such as mining sites, con-
veyors, and storage piles of
coa and other materials.
As these programs progress.
new information will be
generated To make sure this
information reaches everyone
concerned, PATB will be
sponsoring a variety of
technology-transfer symposia,
conferences, and publications.
One of the most important
effects of PATB's work in
coming years will be improved
standards for the quality of
our Nation's air. By law.
emissions limits must be
backed up by technology
that's proven to be efficient
and economical As PATB
develops and tests more
efficient and cost-effective
particulate control devices,
emission standards can be
improved, making our air
cleaner and healthier to
breathe.
PATB's goal for the next 5
years is to make particulate
control devices 10 times more
effective than today's equip-
ment At the same time,
PATB is aiming to cut costs
by a factor of 10
Thjt's an ambitious goal. But
to provide the energy we
need -- without damaging the
air we breai - it's a goal
that must be achieved
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References
1. Data supplied by GCA/
Technology Division, EPA
Contract No. 68-02-2177.
1978.
2 U.S. Environmental
Protection Agency. Indus-
trial Environmental Research
Laboratory. Research
Triangle Park. Annual
Report. 1976.
3 Oglesby, S and G. Nichols.
"A Manual of Electrostatic
Precipitator Technology"
Parts I and II. Southern
Research Institute, EPA
Reports APTD 0610 and
061 1 (NTIS PB 196380
and 196381), August
1970.
This report has been reviewed
by the U.S. Environmental
Protection Agency and
approved for publication
Mention of trade names or
commercial products does not
constitute endorsement or
recommendation for use.
Prepared by Acurex Corpora-
tion under EPA Contract
68-02-2611; Task 14.
Photos courtesy of Acurex
Corporation; U.S. Environ-
mental Protection Agency.
IERL-RTP; U.S. Environmental
Protection Agency. Project
Documenca; Envirotech Cor-
poration; Pacific Power &
Light Company; Salt River
Project
EPA - RTP LIBRARY
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