POWER July 2010 - 30

MERCURY REMOVAL
attempt to understand the co-benefit process
using time series analysis of process data.
Interpretations of observed correlations
should include physical explanations based
on engineering analysis and evaluation of
scientific literature.
One downstream effect of load change is
change in operation of the SCR. Ammonia is
injected into the SCR in order to reduce NOx
and NH3
,
NOx in the flue gas. The amount of NOx
flow is controlled in proportion with
generated
based on heat input (lb/MBtu) is approximately
constant, so the NH3
which is known to inhibit heterogeneous
mercury oxidation.
SCR outlet temperature is also affected
by load, and temperatures are at the high
end of the design range for full load. SCR
temperature is expected to affect the rate of
mercury oxidation, but the optimal value
for flue gas is not known. Thus, unit load
likely affects mercury control indirectly by
altering the fraction of Hg0
oxidized in the
SCR. Supporting this conclusion is our observation
that mercury control was reduced
to an average 71% when SCR units were
bypassed.
The observation that load affects mercury
oxidation is consistent with different
mercury oxidation mechanisms. Heterogeneous
oxidation of mercury has been
flow scales
approximately with unit load. Increased NH3
resulted in increased NH3
per surface area,
observed on SCR catalysts, carbon-based
materials, metals, and metal oxides. One
explanation is that HCl competes for surface
sites with NH3
and that Hg0
reacts with
these chlorinated sites either from the gas
phase or as a weakly adsorbed species; or
perhaps Hg sorption on catalytic sites follows
reaction with HCl. Another explanation
is a noncatalytic surface reaction of
surface-bound HgSO4
with HCl. The proposed
noncatalytic reactions may occur
mainly in SCR units because these operate
at temperatures in the range for mercury
oxidation, have relatively long residence
times, and have high surface areas.
Although the specific reaction mechanism
is unknown, any heterogeneous reaction
scheme will depend on gas velocity, proximity
of surfaces, reactant concentrations,
residence time, and temperature history. In
the case of heterogeneous mercury oxidation,
bench- and pilot-scale experiments have
shown that the reactions are temperature-dependent,
promoted by time proximate to the
surface, promoted by chloride, and inhibited
by ammonia.
Efficient mercury oxidation is critical for
co-benefit control. Evaluation of stack emissions
data suggests mercury oxidation was
highly variable. Bituminous coal chloride
concentrations varied by more than an order
of magnitude and averaged 724 ppmw.
Mercury oxidation is sensitive to fluctuation
3. Mercury control varies with unit load. Month-long time series of fractional load
and mercury emissions for Plant C are illustrated. Source: Southern Company
Fractional load
1.0
Hg emissions
3
in coal chloride concentration for concentrations
below 1,500 ppmw. Though not directly
observed in this analysis, coal composition
probably affected mercury oxidation efficiency
and, thus, co-benefit control.
Mercury oxidation appears to vary with
unit load. The effects of temperature and
NH3
on mercury oxidation cannot be decoupled
in this work, but theoretical predictions
and testing performed at pilot scale and full
scale suggest that both temperature and NH3
affect mercury oxidation. Currently, SCR
units are designed and operated for removal
of NOx
. Both SCR temperature and NH3
flow are key parameters for SCR design and
operation. Our results suggest that operation
of SCR units below the peak design temperature
and NH3
flow would likely improve
mercury oxidation efficiency. However,
optimization of existing SCR units for mercury
oxidation would likely result in lower
NOx
effects such as NH3
bisulfate formation.
removal and potentially unwanted side
slip and ammonium
Mercury Captured in a Wet Scrubber
Wet flue gas desulfurization units designed
to capture SO2
ly capture water-soluble Hg2+
have been shown to effective.
Hg0 is nearly
insoluble in water, so it is not efficiently
captured by wFGD units. Once captured,
Hg2+
may become associated with wFGD liquor
solids-that is, sequestered-and then
separated from the wFGD liquor.
The characteristic time for mercury re0.8
2
0.6
1
moval
from wFGD liquor is much longer
than for gas-phase processes in the flue
gas, which are on the order of seconds or
less. The amount of mercury in wFGD liquor
is the product of wFGD liquor volume
and mercury concentration. Wet FGD units
may contain more than a million gallons of
aqueous suspension, and dissolved and particulate
mercury concentrations for these
plants during the study period averaged 76
and 377 ppb, respectively. Thus, wFGD liquor
may be on the order of 1 lb and 4 lb of
dissolved and particulate mercury. Using a
characteristic heat input rate of 0.17 TBtu/d,
a potential emission rate of 6.56 lb/TBtu,
and assuming that 86.5% of mercury is
collected in wFGD liquor, the wFGD units
collect approximately 1 lb mercury per day.
The characteristic time for mercury in the
wFGD liquor can be calculated as this stock
divided by the rate of mercury addition; for
the assumptions used here, the characteristic
time is 1.1 and 4.4 days for dissolved and
particulate mercury, respectively.
In one operational experiment, a fault
0.4
5
10
15
Day of month
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20
25
30
in the limestone feed system for Plant A's
wFGD system resulted in a short-term increase
in pH to 6.0, above the normal opPOWER
| July 2010
Fraction load
Hg emissions (lb/TBtu)
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POWER July 2010

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