POWER March 2014 - 44

THE FUTURE OF COAL-FIRED GENERATION
4. Doing nothing is not an option.
Unit B Baseline CEMS data over time, showing
calculated effect of an ESP or baghouse (BH)
retrofit. Baseline (red) data reflect actual measurements.
The ACI-ESP (blue) and ACI-BH data
(green) reflect projected (calculated) levels of
control based on test results. The green line at
1.2 lb/TBtu represents the future MATS mercury
standard for these units. Source: Southern Co.
■ No change (24%) ■ ACI-ESP (0%) ■ ACI-BH (0%)
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
July 2009 Jan 2010 July 2010 Jan 2011 July 2011
for the reduction of Hg re-emission.
Testing occurred on a unit that does not
have any hydrocyclones installed after the
FGD. To determine the fate of the Hg, a pilot
hydrocyclone was used to validate the separation
of the Hg in carbon from gypsum. The
Hg-laden carbon was found to be in the overflow
of the hydrocyclone and was effectively
removed from the gypsum underflow. The
addition of the ADA carbon in the FGD was
effective in reducing Hg re-emission to Southern
Co. compliance levels and is a good, costeffective
option for MATS compliance.
PAC Usage Considerations
Extensive testing conducted by Southern Co. underscores
the high variability of mercury emissions
over time and the importance of active,
engineered control such as ACI, even in systems
with SCRs and wet scrubbers. Looking at various
alternatives provides trade-offs that can be
assessed to determine the overall lowest impact
to plant performance while reliably maintaining
compliance and cost-effectiveness.
Although PAC injection with ESP recovery
of the PAC enables the scrubber and the
gypsum to be unaffected, there are potential
ash consequences to take into consideration.
These can be mitigated with some of the newer
PACs that require lower quantities and have
steeper capture responsiveness in the range
of interest and by optimizing the system over
time. Injecting PAC into the scrubber avoids
potential ash effects, but then it could result
in the scrubber chemistry being affected and
may risk gypsum quality. In tests at Southern
Co., the gypsum and chemistry quality were
maintained with scrubber PAC addition.
PAC can significantly contribute to MATS
compliance on scrubbed units with SCRs in
several ways. Depending on the configurations,
fuel, plant goals, and results of field
demonstrations, it may be more beneficial
to utilize the active control of PAC upstream
of the ESP, where it does provide enhanced
balance-of-plant considerations, does not interfere
with scrubber operations, and keeps
mercury out of the scrubber, thus avoiding
any issues with re-emissions.
On the other hand, advanced PACs can
be effective as scrubber additives, enabling
good scrubber operation and sustainable
control levels. Impacts to gypsum need to be
assessed at individual scrubbers.
In either case, when control using advanced
next-generation PACs is applied to the original
baseline CEMS data from the scrubbed units,
the results show that compliance with margin
is achieved continuously and reliably. ■
-Brandon Looney, retail generation
development project manager; Nick Irvin,
advanced energy systems R&D manager;
and Chethan Acharya, research engineer,
are with Southern Co. Joe Wong, chief
technology officer, and Sheila Glesmann
(sheila.glesmann@ada-cs.com), senior
vice president of environmental, are with
ADA Carbon Solutions.
42
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Hg emissions, 30-day average (lb/TBtu)
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