POWER April 2013 - 36

EMISSIONS CONTROL
Electrostatic Precipitator Upgrade
Opportunities
The results of stack emissions testing conducted at several coal-fired power
plants during the past three years have provided useful guidance for plant
operators who are required to meet new federal guidelines regarding the
release of particulate matter. The data and guidelines presented here will
assist those who operate plants with electrostatic precipitators to develop
a strategy for filterable particulate emissions control.
By K.S. Kumar and J.A. Knapik, Babcock & Wilcox Power Generation Group Inc.
N
ew Environmental Protection Agency
(EPA) rules for the control of
mercury and air toxics, also known
as the Mercury and Air Toxics Standards
(MATS), were established in 2012. The air
toxics portion of the rules includes control
of hydrochloric acid (HCl) and several
trace metals. After several years of research,
proposals, industry comments, and
stakeholder reviews, the EPA has accepted
the premise that filterable particulate matter
(PM) is a surrogate for virtually all
trace metals. In the final MATS rule, the
EPA determined that limiting stack filterable
particulate emissions below 0.03
pound per million Btu (lb/MMBtu) based
on EPA Test Method 5 should adequately
safeguard public health.
The specification of filterable PM limits,
instead of total particulate matter that
included the highly variable condensable
fractions, has provided additional flexibility
for achieving compliance with existing
electrostatic precipitators (ESPs).
Fortunately, during the past 50 years, a
considerable body of literature has been
generated describing research and fullscale
operating experience with ESPs. Our
approach in this article is to draw from this
literature and provide plant operators with
empirical means to assess their ESP performance.
We review important research
on the various parameters affecting ESP
outlet emissions. This research summary
guides the discussion that follows on the
interpretation of results from recent testing
at several coal-fired power plants. Areas
for further research on reducing stack
filterable emissions are also identified.
Fundamentals of Ash and ESP
Corona Power
Sulfuric acid is an excellent fly ash-conditioning
agent. Proper amounts of this acid
36
can control the ash resistivity to manageable
levels, in the optimal range of 108
to 1,010 ohm-cm. It is known that some
eastern high-sulfur coals with high iron
content in the ash can generate elevated
levels of sulfur trioxide (SO3
) in the flue
gas. Depending on the air heater operation,
it is quite possible for these plants to operate
the ESPs at a flue gas temperature that
is below the sulfuric acid dew point. The
fly ash can then become too conductive for
proper control in the ESP, as the particles
are subjected to the " pith ball " effect, an
electrostatic phenomenon through which
the particles lose their charge at collector
plates and are repulsed back into the
gas stream. The resultant electrical reentrainment
can greatly diminish the ESP
collection efficiency even if there are no
limitations to the introduction of proper
corona current density in the ESP.
On the other hand, it is also possible for
the fly ash in flue gas to be insufficiently
conductive. This has been known to lead
to premature flue gas breakdown inside the
ESP caused by the incipient " back corona "
effect. Back corona severely limits the
amount of corona current density that can
be received by a given electrically energized
section of the ESP. Low ash conductivity
(or high ash resistivity) is believed to
be caused by a combination of factors:
■ Inadequate amounts of free SO3
in flue
gas due to the highly alkaline nature of
the fly ash.
■ A combination of insufficient amounts
of SO3
in flue gas and alternate conductive
species, such as sodium sulfate, in
the fly ash.
■ A combination of high acidity in the fly
ash, high flue gas temperature, and low
flue gas moisture. That can result in much
of the SO3
present in the fly ash to be inwww.powermag.com
effective
to adsorb onto fly ash particles,
rendering it ineffective for providing the
needed conductivity.
Corona Current and Specific
Corona Power
Corona current density is defined as the
maximum corona current that can be introduced
by a power supply per square foot of
collecting surface of a given ESP electrical
section. The inlet fields of the ESP experience
the maximum dust concentrations
and are subjected to flue gas breakdown at
lower current density levels as compared to
cleaner outlet fields. Even in the absence
of ash resistivity-induced limitations, it is
not unusual for the inlet field corona current
density levels to be lower by a factor of four
as compared to the outlet fields.
The relationship among corona current
density (mA/1,000 ft2
), specific corona power
(watts/acfm), a nd collection efficiency
across an ESP has been well researched. It
has been found that when operating an ESP
at ash resistivity levels higher than 5 x 1010
ohm-cm, high ash resistivity-related
to
5 x 1011
premature gas breakdown impacts begin to
be noticed. ESP corona current density starts
dropping rapidly and, depending on the size
of the ESP, filterable emissions may start to
increase.
In most cold-side ESP applications, corona
current density levels could vary from values
as low as 5 mA/1,000 ft2
ashes to levels as high as 75 mA/1,000 ft2
for highly resistive
for
highly conductive ashes. Operating voltages
will vary, depending on plate spacing and
discharge electrode geometry, between 40
and 75 kV, on average.
As an example, for an ESP operating
at 40 kV average, and at a corona current
density level of 50 mA/1,000 ft2
, the average
corona power density would be 2.0
watts/ft2
. If this level of energization were
POWER | April 2013
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POWER April 2013

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