POWER July 2017 - 25

ENVIRONMENTAL ISSUES
Retrofitting Electrostatic
Precipitators to Meet
Current Emission Limits
Electrostatic precipitators (ESPs) offer a tried-and-true technology for particulate
matter control. However, existing ESPs can be challenged to meet
the latest regulatory requirements. Retrofitting power supplies and other
equipment offers the potential for improved ESP performance.
Jose Sanchez and Tom Keeler
T
he U.S. Environmental Protection
Agency's Mercury and Air Toxics
Standards (MATS) rule, which became
effective in April 2015, provides emission
limits for filterable particulate matter (PM)
and other hazardous air pollutants, such as
mercury, acid gases, trace metals, and organics,
from coal-fired power plants. Under the
MATS rule, the maximum achievable control
technology total particulate emissions limit is
0.03 lb/MMBtu for existing coal-fired plants.
To meet these emissions limits for control
of PM, one of the most-reliable and
often-utilized control technologies is an electrostatic
precipitator (ESP), which is a wellestablished
and mature technology. However,
as regulatory requirements have become increasingly
more stringent, many existing precipitators
are being challenged to collect PM
beyond their original design specifications.
For example, some plants may now use
dry sorbent injection for acid gas control and
activated carbon injection for mercury control,
with the expectation that the existing
ESP will remove the PM. In some instances,
depending on ESP size and condition, the
ESP may no longer operate as efficiently under
these conditions.
Few, if any, new precipitators are currently
being built and installed in today's U.S.
coal-fired marketplace. As a result, plants
that are seeking to meet the new limits, and
that have sufficiently sized ESPs in good
mechanical and working condition, are aiming
to enhance performance of their existing
ESPs through upgrades or rebuilds of the
ESPs with newer power supplies. Modernday
power supplies that deliver higher average
and peak voltage operation provide a
viable means of boosting the operating performance
of an existing precipitator.
Over the past several years, the Electric
Power Research Institute (EPRI) has conJuly
2017 | POWER
1. More power. This image shows a scope trace and voltage-current curve comparing
a high-frequency power supply/switched-mode power supply (HFPS/SMPS) to a conventional
60-Hz transformer-rectifier (TR) power supply. The high operating frequency of HFPSs/SMPSs
produces a nearly pure direct current output waveform. Source: Electric Power Research Institute
(EPRI)
ESP = electrostatic precipitator
T/R = transformer rectifier
Spark-over level
Time (msec)
ESP current (mA)
ducted a number of studies on the effectiveness
of retrofit options for ESPs to meet new
environmental regulations. The most recent
study surveyed power supply technology advancements,
such as high-frequency power
supplies (HFPSs) and new three-phase lowand
mid-frequency power supplies. The
study also documented case studies of ESP
retrofits at five different plants. The study and
its findings are described in an EPRI report,
ESP Upgrades and Optimization: Operating
Under MATS Conditions (3002006099).
ESP Power Supplies
When a plant is considering re-powering an
existing precipitator to meet an emissions
goal, the plant owners should first undertake
a full evaluation of the precipitator and plant
operating conditions to assess whether the
re-powered precipitator could reasonably be
expected to meet the goal and to determine
what additional steps, if any, might also have
www.powermag.com
to be taken to attain that goal. Additional
steps might include mechanical repairs, gas
and particulate flow modifications, additional
electrical sectionalization, rapper sequence
modifications, and flue gas conditioning.
If the ESP is found to be in good mechanical
order, and operating near the desired
emissions level, then replacement of the
power supply may provide the best means of
ensuring its performance.
Precipitator performance is directly related
to the operating voltages in the system,
and the power supply is the key system component
providing the high-voltage (HV) direct
current (DC) power (voltage and current)
directly to the discharge electrodes needed
for electrostatic precipitation.
Historical Development of ESP
Power Supplies
Throughout the developmental history of
ESPs, their design and performance have
25
ESP voltage (kV)
ESP voltage (kV)

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POWER July 2017

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