POWER July 2015 - 33

ENVIRONMENTAL ISSUES
1. Horizontal flow. In a horizontal-flow wet electrostatic precipitator (WESP), flue gases
move across collecting plates and particles are negatively charged by the electrodes. These
designs tend to be less expensive than other arrangements. Courtesy: Babcock & Wilcox Power
Generation Group
Roof wash
nozzles
Outlet
plenum
Purge air
system
Operating
floor roof
Insulator
compartments
Transformer
rectifiers
Inlet
plenum
2. Vertical flow. In a vertical-flow WESP,
flue gases can move up or down through
the collecting plates. These designs have a
smaller footprint but can be more costly depending
on arrangement and wetted material
selection. Courtesy: Babcock & Wilcox Power
Generation Group
Flow
distribution
device
Collecting
pan
Spray wash
nozzles
Collecting
plates
Discharge
electrodes
WESP Types
WESPs come in different orientations and
configurations, with horizontal- and vertical-flow
types being typical. The choice of
which to use is usually driven by arrangement
and cost. Plants with limited footprints
usually use a vertical design because it takes
less space. With no space limitations, the
horizontal design is usually preferred as a
lower-cost option.
Figure 1 shows a horizontal-flow WESP.
Flue gas enters the WESP horizontally and
flows through the gas passages, where the
particulate matter and acid mist are negatively
charged from the corona created by
the discharge electrodes. The particulate and
acid are then collected on the grounded curtains.
The curtains or collecting plates can be
10 meters or higher.
In China, labor is inexpensive, while highgrade
alloy is very expensive, especially if it
must be imported. Hence, Chinese designs
focus on reducing the amount of alloy used
in the WESP and tend to use as much locally
available materials as possible. Flakeglass
lining of the WESP casings and some support
beams are common. Alloy is reserved for the
electrodes and critical structural supports.
July 2015 | POWER
Internal
walkways
Figure 2 shows a vertical-flow, plate-type
WESP, which is integrated with a WFGD.
In this example, gas flows up through the
WESP. As with the horizontal-flow type, as
little alloy as possible is used in Chinese designs.
A second version of the vertical WESP
is standalone and can be either an upflow or
downflow design. The WESP casing can be
square or round. Different shapes of collecting
electrodes, such as hextube, can also be
considered besides plate.
Impact of Water Washing on
Construction Materials
Water washing in a WESP is comparable to
rapping in a DESP. In a WESP, the electrodes
are cleaned of particulate matter and acid
mist, as opposed to only dry particulate being
removed from the plate in a DESP.
The two major WESP washing methods
are continuous and intermittent. Continuous
washing is most commonly used for horizontal-flow,
plate-type WESPs. Continuous
washing involves spraying fine atomized water
in the gas passage or on the plate to create
a film of water on the collector plate. This
film will flow down, keeping the plate wetted
to increase the pH and reduce the chloride
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Supporting
structure
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doors
concentration, which allows a lower-grade
alloy, such as 316L or alloy 2205, to be used.
Another advantage of a continuous washing
system is the ability to keep the WESP energized
at all times, which removes the need
for oversizing, because the fields are not deenergized
for washing as with an intermittent
wash design. While a continuous wash can
use more water than an intermittent wash, the
wash water can be recycled. In this case, a
caustic addition system is added as well as a
purge stream that is sent back to the WFGD
for use as make-up water (Figure 3).
Caustic addition is controlled by maintaining
tank pH in the range of 5 to 7. The
blowdown rate is set to control the chloride
concentration and/or the total suspended solids
in the system. It is very common to keep
the blowdown rate to a reasonable level that
does not affect the overall WFGD/WESP water
balance at full-load operation.
Continuous washing is not commonly
used in a multi-field vertical design, because
droplets sprayed to coat the collecting surface
will be re-entrained into the flue gas and
can be suspended in the collecting zone. This
causes excessive sparks, which decreases
WESP performance.
Intermittent washing can be used on both
vertical and horizontal WESP configurations.
The wash system is activated every shift for
the front fields, and every 12 to 24 hours for
the later fields. The wash system is sectionalized
with the electrical fields because the
transformer/rectifier (T/R) set needs to be
33

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POWER July 2015

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