POWER July 2013 - 41

WATER TREATMENT
For example, a variety of general predictions
about the various phases of mercury,
selenium, and manganese based upon ORP
and pH can be made (see the table).
Dynamic Chemical Processes
By monitoring the ORP of WFGD absorber
process slurry, changes within the system
process and chemistry can be determined.
Estimations of various parameters of WFGD
absorber chemistry can also be determined,
including the dominant oxidation states and
phases of metals within the slurry, the potential
reemission of mercury, and the risk
of accelerated corrosion of the alloy vessel.
Measurement of the system ORP also alerts
operators to the likelihood of problems in
downstream wastewater treatment (WWT)
systems, especially the ratio and concentration
of selenate ions to selenite ions within
an effluent stream.
The chemistry inside a WFGD absorber
is a complex system of hundreds of potentially
changing ionic species and distinct
compounds existing simultaneously throughout
the slurry. The modeling of WFGD absorber
slurries has been based on equilibrium
thermodynamics to date. However, we have
determined that in operating units, WFGD
absorber slurry chemistry is more likely to
be kinetically controlled. Most absorbers are
operating at an unsteady state. Due to kinetic
interaction, mercury could become reduced
to the elemental state, thereby becoming vaporous
and exiting the system boundaries in
what is called mercury re-emission.
Another important aspect of ORP with
relation to WFGD process chemistry is the
reaction of any of the one or more strong oxidizers
present with one or more halide ions
present in solution, thereby resulting in increased
demand for reagent in the absorber
and a possible lowering of pH in the purge
stream after the solids have been removed.
Because persulfate is a powerful oxidant, it
has the ability to convert some halide anions
to their respective elemental state. Specifically,
some chloride may convert to chlorine
under high-ORP conditions:
Similar reactions occur between the
oxidizer(s) and other halide species present
(for example, bromide, iodide, and the
like). Thus, the concentration of chlorine in
absorber slurry is present as three species
in equilibrium within the aqueous phase:
dissolved gas (Cl2
ionic hypochlorite:
), hypochlorous acid, and
Dominant forms of mercury, selenium, and manganese for approximate
ORP levels. Source: Babcock & Wilcox Power Generation Group Inc.
ORP Level
<0 (mV)
0-300 mV
300-500 mV
>500 mV
Mercury
Vapor
Aqueous ion, with the solids
Aqueous ion, with the solids
Aqueous ion, with the solids
Within the typical WFGD operating range,
equilibrium favors HOCl. The formation of
H+
ions associated with this chemical reaction
will cause a decrease in pH as halogencontaining
species are liberated from the
scrubber. In WFGD systems that are operating
in pH control mode, the reagent feed
controls will respond to the lower pH by adding
more reagent. In systems with high ORP
levels, gypsum formation may occur without
the addition of oxidation air when sulfite reacts
with strong oxygen containing oxidizing
agents. Higher gypsum purity can result as
excess limestone, normally an impurity within
the gypsum product, is reacted to buffer
the system from dropping pH.
Due to the electro-reactive nature of mercury,
ORP levels are a main driver in controlling
reemission, dissolution into the slurry,
and solid phase retention. Higher ORP values
in the ART are favored in order to maintain
dissolved mercury. ORP levels above 500 mV
often favor an increase in the dissolved mercury,
with constant total mercury content in
the slurry. A decrease in the ORP in an ART
is an indication of a less-oxidizing environment,
leading to elemental mercury formation
and potentially release (or reemission).
A possible chemical pathway for mercury
reemission to occur is shown in Equation 3:
Swings in ORP value may also cause mercury
to enter the elemental state and be reemitted
from a WFGD tower.
Staying in Compliance
WWT systems are tuned to control metals
present in influents and to produce effluents
within a certain concentration range. Changes
in the influx of these metals to WWT may
disrupt performance if controls and operational
parameters cannot respond quickly. As
swings in WFGD and/or WWT process occur,
ORP may ultimately produce changes in the
dominant state of regulated metals, thus permitting
the WFGD effluent flow rate to affect
the mass flux of each species. In this situation,
detrimental fluctuations in ORP may result in
minimal removal of some metals and potentially
result in out-of-compliance operation.
July 2013 | POWER
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Selenium
Selenite
Selenite
Selenate
Selenate
Manganese
Aqueous ion
Aqueous ion
Aqueous ion
MnO2 solid
Controlling process ORP can lead to improved
efficiency of WWT systems related
to selenium removal. WFGD effluent ORP
controls the precipitation of many regulated
metals, particularly selenium. At low ORP, selenium
exists mainly as selenite (SeO3
-2) and
can be removed by many WWT methods including
chemical precipitation. At higher ORP
levels (greater than about 300 mV,) selenium
will predominantly occur as selenate (SeO4
-2),
which passes through many WWT systems.
The combined effects of over 1,000 ppm of
total oxidizers and low pH potentially associated
with high-ORP WFGD effluent streams
can result in damage to bioreactor stock and/
or increased reagent costs in WWT systems.
Strong oxidizers present within WFGD effluent
have the potential to upset biological processes.
During high-ORP conditions, the WFGD would
essentially be feeding bleach (hypochlorite),
peroxide, and stronger oxidizers to downstream
systems; such oxidizers can damage microbial
health when fed to biological treatment units.
Furthermore, high ORP in a WFGD effluent
can result in low pH of WWT influent. As described
earlier, oxidizers will continue to react
with halide ions in solution, thereby liberating
a hydronium ion and thus lowering pH. When
excess carbonate is available, it may buffer such
impacts. Once unreacted limestone is removed
from the slurry filtrate during dewatering, the
pH buffering capacity of the system rapidly decreases
while oxidizer and halide ion concentrations
remain and are fed to WWT.
Materials Must Resist Corrosion
Materials coming into contact with WFGD
slurry and effluent should be selected with careful
consideration to the corrosive potential of
high-ORP slurries containing ionic manganese.
Strong oxidative content is present in high-ORP
slurry leaving the scrubber, and very low pH
levels may be seen downstream in conjunction
with high ORP. While industry focus has been
given to WFGD ART corrosion, the potential
exists for similar corrosion to occur in process
pumps, dewatering operations, vacuum systems
for gypsum production, as well as in process
piping and WWT equipment.
Alloy 2205 duplex stainless steel (UNS
S32205) is proving susceptible to accelerated
corrosion from slurries that contain precipi41
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