POWER April 2013 - 38

EMISSIONS CONTROL
powered in an ESP with a specific collecting
area (SCA) of 200 ft2
/1,000 acfm, then
the specific corona power would be 400
watts/1,000 acfm. If the SCA were twice
as large, at 400 ft2
rona current density of 25 mA/1,000 ft2
would produce the same specific corona
power of 400 watts/1,000 acfm.
For a given power plant operating at a
given load and a given ESP configuration
reduced corona current density, changes
in coal chemistry, for example, can lead
to much reduced specific corona power.
This lowering of corona current density
has the potential to cause an increase in
stack emissions. A given ESP operating
with proper mechanical integrity can
shift from days and months of very well
energized operation to sudden episodes
of ESP electrical sections operating with
low corona current. While the worsening
of ESP collection can generally be
seen as increased opacity, as monitored
through a continuous opacity monitor at
the ESP outlet, caution needs to be exercised.
Stack opacity is not always a good
indication of increasing PM rates because
it is greatly dependent on particle size.
Smaller PM tends to have less impact on
the PM rate while it has a large impact on
refracting and attenuating the light, which
is then seen as higher opacity. Conversely,
larger PM (such as re-entrainment from
hopper leakage) may not impact the opac/1,000
acfm, then a coity
monitor, but it can have a significant
impact on the PM rate.
There is wide variability in plate-toplate
spacing in the collector electrode
ESP designs. Historically, ESP size was
reported in terms of SCA and usually in
units of square feet per 1,000 acfm. Today's
ESP evaluation practice is to report
the ESP size in terms of total residence
time. This will avoid the misinterpretation
of a lower SCA precipitator at higher
plate spacing as a smaller size unit, even
though the treatment times for both may
be identical. Engineering evaluations and
research on the application of wider plate
spacing technology have revealed that collection
efficiencies of ESPs with two different
plate spacing and SCAs can be the
same, as long as other parameters such as
specific corona power are equivalent.
Performance Variability at a Given
Specific Corona Power
ESP filterable emissions can vary considerably
for a given specific corona power. This
is a result of several factors.
First, ash cohesivity can vary depending
on the nature of flue gas and ash constituents.
It is well known that ammonia conditioning
has reduced filterable emissions considerably,
principally due to control of sulfuric
acid concentrations and the introduction of a
cohesive agent that is a byproduct of the ammonia-sulfur
compounds. Several approach1.
Capturing ash. Control of ash re-entrainment through ammonia injection is one means
of reducing filterable emissions. Source: J.T. Reese, J. Greco, Experience with Fly-Ash Collection
Equipment Serving Steam-Electric Generating Plants, Journal of the Air Pollution Control
Association, August 1968, Vol. 18, No. 8.
Gas temp 270F (below acid dewpoint with NH3 feed)
Gas temp 310F (above acid dewpoint)
Gas temp 270F (below acid dewpoint)
100
90
80
70
60
50
40
325
38
350
375
400
80
es are reported in the literature, such as:
■ Injecting ammonia for control of particulate
re-entrainment from carbonaceous
particulates.
■ ESP performance enhancement through
ammonia injection on high SO3
gas on eastern coals.
by ammonia injection (Figure 1).
-laden flue
■ Maintaining operation above the sulfuric
acid dew point through sufficient mitigation
of SO3
Considerable research with novel additives,
showing similar results, has also been
conducted for the purposes of reducing ash
resistivity and increasing ash cohesivity.
Second, fine particulate matter may
vary from plant to plant, depending on
fireside conditions. A significant number
of coal-fired power plants have introduced
some form of fireside management to reduce
flame temperature, and thus reduce
nitrogen oxide (NOx
relation between fireside NOx
) emissions. The coremissions
and
submicron particulate content has
been thoroughly researched by the Electric
Power Research Institute (EPRI). Figure 2
shows the relationship between nitric ox2.
Fly ash in the gas. Fly ash fine particulate
content versus NO emissions at several
coal-fired plants is illustrated. Source: M.W.
McElroy, R.C. Carr, and G.R. Markowski, Size
Distribution of Fine Particles from Coal Combustion,
Science, Vol. 215, January 1982.
360 MW, front wall
540 MW, tangential
520 MW, opposed wall
113 MW, roof fired
360 MW, tangential
25 MW, front wall
40
Rated full load flow
425
Gas flow (103 cfm)
www.powermag.com
450
475
500
500
Nitric oxide (ppmv)
POWER | April 2013
1,000
Collector efficiency (%)
Mass in submicrometer mode (mg/m3)

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POWER April 2013

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