POWER July 2013 - 42

WATER TREATMENT
tated, or non-solubilized, manganese species
and/or high ORP levels. Within the WFGD
absorber, the majority of the corrosive attack
is observed below the slurry level in the ART.
Under high-ORP conditions, the rate of corrosion
can accelerate. When the ORP of operating
WFGD slurries is pushed above about
500 mV, manganese normally soluble as ionic
Mn+2
will oxidize and precipitate out as MnO2
.
When this precipitate contacts metal as part of
a deposit, it serves as a galvanic cathode to exacerbate
the fluoride and chloride driven under-deposit
corrosion mechanism. Corrosion
thus accelerated by manganese precipitation
can be rapid and severe.
In many applications ceramic tile and linings
are considered as alternatives to alloy.
Wall-papering with UNS N10276 (Hastelloy
C-276) has been performed in some installations.
Some plastics and resins may also be
susceptible to attack from high ORP levels.
Strong oxidizers within the slurry effluent
may react with and thereby degrade some
polymer bonds, because high-ORP filtrate
samples were observed to weaken and discolor
HDPE bottles in a laboratory setting.
Within substances such as fiberglass-reinforced
plastic, such a reaction with resins
could potentially cause some dissolution of
the resin and/or lead to fiber delamination.
DON'T WASTE TIME AND MONEY
COLLECTING DUST. CLEAN YOUR AIR.
Plantwide Impact
The WFGD ART ORP and the WWT influent
process ORP both need to be controlled. Currently,
studies are being performed to define
techniques to maintain steady ORP and to allow
for greater fuel flexibility. Such control would
afford utilities the option to obtain the most
cost-effective fuel, maintain a constant effluent
for wastewater, and provide a better treatment
scheme. Coal yard and boiler operators will
need to work in conjunction with the plant's
continuous emission monitoring systems, the
WFGD system, and WWT plant operators to
implement improvements that integrate the entire
process. Learning how a parameter change
upstream affects the WFGD and WWT systems
is quickly becoming crucial. With full control
of the AQCS process train and dewatering systems,
control of WWT influent is expected.
The effects of blending coals, staging comscan
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bustion, and swinging load can create issues
with AQCS equipment, especially in environments
where WFGD units are employed. The
WFGD system serves as the catch basin for
all flue gas byproducts as well as any fine ash
not captured in an electrostatic precipitator or
pulse jet fabric filter. Undesired ORP levels,
or undesired fluctuations therein, can cause
problems ranging from mercury reemission to
increased corrosion and improper treatment of
WFGD effluent. Adjustments to combustion
processes may affect operating parameters of
the WFGD environment as well as the WWT
systems. Combustion systems need tuning to
allow for efficient power generation, compliance
with existing regulations, and flexibility
to comply with anticipated, tighter water and
solid discharge regulations.
Due to the potential for aggressive or acCONGRATS!!
celerated
corrosion of some alloy material in
high-ORP environments, care should be taken
on selection of alloys or materials in the WFGD
slurry and filtrate contact zones. Constituents of
the WFGD slurry will include limestone, gypsum,
halide ions, and metals from the burned
coal as well as silica from the ash and other
up-stream constituents. The ORP levels in the
tank can cause the metals to undergo phase
partitioning or to change their solubility due to
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42
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POWER | July 2013
changes in their oxidation state. ■
-S.R. Brown (srbrown@babcock.com) is
AQCS engineer, R.F. DeVault (rfdevault@
babcock.com) is research chemist, and
D.B. Johnson (dbjohnson@babcock.com)
is field service engineer for Babcock &
Wilcox Power Generation Group Inc.
http://www.bit.ly/ZMg6Ar http://www.martin-eng.com http://www.powermag.com
POWER July 2013

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