POWER July 2013 - 40

WATER TREATMENT
ORP as a Predictor of WFGD Chemistry
and Wastewater Treatment
Recent studies have shown that system oxidation-reduction potential (ORP)
is not only an important factor for predicting wet flue gas desulfurization
(WFGD) absorber chemistry but also may be a predictor of process equipment
corrosion and wastewater treatment requirements.
By S.R. Brown, R.F. DeVault, and D.B. Johnson, Babcock & Wilcox Power Generation Group Inc.
P
urge streams of wet flue gas desulfurization
(WFGD) units, which are one
byproduct of controlling SO2
emissions
from coal combustion, are being increasingly
subjected to stricter wastewater
regulations. Consequently, coal-fired power
generators need a method for controlling the
operational chemistry of these WFGD units.
Upon implementation of a suitable control
method, WFGD bleed stream chemistry and
flow rate may be optimized, thereby resulting
in improved performance of one or more
downstream unit operations. A further benefit
is reduced reagent and additive costs in
various devicies, including the WFGD unit.
One control parameter of interest is the
oxidation-reduction potential (ORP) of the
bleed stream. Much like pH, the measurement
of ORP can be taken in real time and
integrated with other plant-monitoring
data. By incorporating ORP measurements
into a process control scheme for limestone
forced-oxidized WFGD absorbers-along
with various other control variables such
as SO2
removal, absorber pH, reagent flow
rate and/or one or more reaction stoichiometries,
and/or gypsum purity-generators
are able to manage the oxidation states of
various dissolved metals in the slurry and
the potential reemission of mercury. (Also
see " How to Measure Corrosion Processes
Faster and More Accurately, " May 2009 in
the POWER archives and " Mercury Control:
Capturing Mercury in Wet Scrubbers, Parts
I and II, " July and September 2007, respectively,
in the COAL POWER archives-both
available at powermag.com.)
A further benefit is control of the corrosion
rate of the absorber recirculation tank
(ART) and other alloy parts within the system.
Many utilities have had ORP excursion
events in WFGD wastewater discharge
where the ORP readings changed from 150
millivolts (mV) to 300 mV to a reading
above 500 mV. Previously, these fluctua40
tions
have gone largely unexplained. We
have determined that this magnitude of
change in ORP, in an ART, due to coal
composition and upstream air quality control
system (AQCS) effects on WFGD absorber
chemistry, can accelerate corrosion.
One potential solution to fluctuating ORP
readings is to use integrated process controls
designed to tune the upstream operation
of the AQCS train to produce consistent
inlet flow parameters to the WFGD tower,
rather than operating each as an independent
process. The control of the ORP level
in a WFGD system may produce improved
plant operations by reducing the amount of
wastewater treatment necessary and helping
mitigate mercury reemission.
Fundamentals of ORP
ORP is a measure of the potential for a
chemical species either to acquire or release
electrons. The potential is commonly
measured by an ORP probe in units of
millivolts, which can be measured in real
time under online plant process conditions.
Positive readings are indicative of a system
operating in oxidizing conditions; negative
readings indicate a system operating in reducing
conditions. If a material comes into
contact with a solution that has a higher
oxidative potential, then a chemical reaction
may occur in which the solution is reduced
and the material is oxidized.
The ORP of WFGD slurry and effluent is
driven by the presence or absence of strong
oxidizers. Many WFGD units operate at a
moderate ORP range of about 100 mV to
300 mV, thereby achieving, or yielding, a
rather stable voltage reading over time. The
range of 100 mV to 300 mV is referred to
as " low " ORP in this article. Such WFGD
units often have oxidizer concentrations
within the slurry below 200 ppm. Other
WFGD units operate at higher ORP values,
often above 500 mV. Slurries with high
www.powermag.com
ORP almost always contain a high concentration
of at least one strong oxidizer such
as persulfate (S2
O8
(HSO5 ), or hypochlorite (OCl-
-2),
peroxymonosulfate
).
Persulfate has been identified and quantified
using ion chromatography on absorber
slurry samples collected at several sites. This
anion is the most powerful oxidant of the
peroxygen family of compounds, and it becomes
a more effective oxidizer at scrubber
process temperatures above about 120F due
to free radical formation. In several instances,
strong oxidizers were measured at total residual
concentrations over 1,000 ppm in WFGD
absorber slurry samples exhibiting high ORP
after the samples were removed from the system
and analyzed in the laboratory. Operating
WFGD units are observed to swing from one
process condition to the other (high to low
ORP), but few, if any, hold at an intermediate
value for an extended period of time. The
rate and magnitude of these changes in slurry
chemistry are indicative of upstream process
changes affecting absorber chemistry.
Once you determine the WFGD slurry
ORP, you can predict the dominant oxidation
state for the various constituents that
may be present in the absorber slurry. For
many metals, solubility is a function of the
oxidation state. Therefore, once the ORP of
a solution has been determined, a prediction
can be made of the preferred oxidation
state for a given chemical species in a solution.
The predominant species of various
metals, and other compounds or ions, can
thus be determined. Using this knowledge,
the ORP in WFGD slurry can then be controlled
in order to control the speciation of
various metal ions, as well as other compounds
and ions.
The range of potential electrochemical
states of a given material can be found within
a Pourbaix diagram for a given chemical
species and presented as a function of pH
and electrochemical potential versus pH.
POWER | July 2013

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POWER July 2013

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