Particulate Emissions froi
Coal-Fired Boilers


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

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


                               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

                               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 250-400C. the
resistivity  of most flyashes is

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.


-  40

S  20
          i TVA 500 MW (31
          i PEDCO 45O MW (4)
          I EPfll bOO MW (1)

                                                           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
At the  higher temperatures of
hot-side ESP's, flyash  resistivity

                               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
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

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

                               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

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


                                                                                                            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 '

                               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

                               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

                               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

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

                                                                                        INSTALLED CAPITAL COSTS FOR CONTROL DEVICES
                                                                                       ON 500 MW POWERPLANTS. PROJECTED 1980 S kW
                                                                                                             LOW SULFUR COAL
              ON 500 MW POWERPLANTS. BASE YEAR  1978


43 ng/J
$53 kW
58 36
58 36
58 36
                                                                                                             HIGH SULFUR COAL

S27 kW 38
28 43
32 52
52 36
52 36
52 36
                             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

                      HOT SIDE ESP
                      FABRIC FILTERS
                13               43

                HIGH SULFUR
                                                          COLD SIDE ESP
                                                           lAHUli ML H MS
        13               43
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

       Cost Considerations

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

2)  Generally least expensive of
   devices  to operate

Critical Parameter
Pressure drop, power
consumption  per unit
volume of air flow, usually
measured as crn
1) Can be used for  high-temperature,
   explosive gas streams

2) Can remove gases as well as

3) Compact, easier to retrofit than
   baghouses or ESP's
                                                       1) High water usage, high energy

                                                       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

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

3)  Bag replacement cost is major
   maintenance  item


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
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

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

 1.  Data supplied by GCA/
    Technology Division, EPA
    Contract  No. 68-02-2177.
 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
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
                                                                        EPA -  RTP LIBRARY