How Two Companies Determined
Their Mercury Control Strategy to
Stay MATS Compliant Year-Round
The hot summer months, when the load demand is high, higher ambient temperatures and high ammonia flows negatively impact mercury
control and likely affect compliance. Here’s how Southern Company and the City of San Antonio Public Service determined that a co-benefits
strategy would not keep them MATS compliant year-round.
SOUTHERN COMPANY
During 2008-2009, Southern Company monitored mercury emissions from five of its plants firing exclusively Central Appalachian bituminous coals.1 During this period,
the coal mercury ranged from 0.02 to 0.19 ppmw and chlorine ranged from 84 to 2,352 ppmw. The equivalent uncontrolled mercury emissions based on the coal
mercury were calculated to be 1.51 to 14.47 lb/TBtu.
Although the EPA emissions factors predicted 90% control, analysis of the emissions from the five Southern Company plants indicated that 90% control was achieved only 47% of
the operating time. The factors limiting greater controls were primarily attributed to reduced oxidation across the SCR and periods of mercury re-emissions from the wet scrubber.
In the SCR-wet scrubber scenario, a plant may be in compliance until the hot summer months when the load demand is high, or until an operator alters scrubber operation that
result in an increase in mercury re-emissions are increased. In the summer scenario, the temperatures will consistently be elevated due to high ambient temperatures, ammonia
flows will be high to maximize load, and corresponding velocities through the SCR will be high. All of these factors are expected to lower the fraction of oxidized mercury.
Activated carbon is often considered for supplemental mercury control. Unfortunately, mercury control using activated carbon is also more challenging when gas
temperatures are elevated, and on units firing bituminous coals where the coal sulfur results in higher levels of SO3. Thus, relying on SCR-wet scrubber for mercury control,
rather than a low risk mercury control scenario, can represent a very challenging case for compliance during periods of the year when load demand may be the highest.
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CITY OF SAN ANTONIO PUBLIC SERVICE ENERGY
City of San Antonio Public Service (CPS) Energy reported on the compliance strategy for Spruce Unit 2.2 This boiler is an 800 MW pulverized coal unit with SCR, fabric
filter and wet scrubber. Originally, it was assumed that the co-benefit strategy would suffice for mercury emissions control. However, after start-up of the unit in 2010, the
mercury stack emissions were high enough that it was felt that the rolling average mercury emissions would not be in compliance year-round.
Two strategies were tested at CPS: activated carbon injection into the fabric filter, and addition of bromine to the coal. Both ACI and bromine injection demonstrated the
ability to control mercury stack emissions to below the regulated level.
The results of the testing and consideration of balance of plant impacts led the utility to the conclusion that ACI would be the best solution for
the plant for the following reasons:
• The costs for implementing ACI or bromine injection were similar
• The ACI system didn’t have to achieve high mercury removal across the fabric filter, because there was additional capture across the wet scrubber, which means that
moderate rates can be used
• ACI was observed to promote the oxidation of mercury across the fabric filter, which means that it enhances removal across the wet scrubber
• ACI results in the greater part of the mercury removal occurring in the fabric filter and relatively little mercury being removed in the wet scrubber, lowering the
mercury content of the scrubber blowdown stream and the gypsum by-product
Since the plant had concerns about potential impacts of mercury control technology on gypsum by-product and the wastewater discharge stream, activated carbon was
deemed to be the best control strategy.
Many plants firing bituminous coals with SCRs and wet scrubbers are planning to rely on co-benefits from these systems for mercury control. While this strategy may be
appropriate for most of the year, especially under a compliance scenario where the averaging period is 30 or 90 days, some evidence suggests that compliance throughout
the year is not necessarily assured. In such cases, additional options might be halogen addition to the fuel and/or activated carbon injection.
Don’t leave yourself exposed to non-compliance risk. If you’re unsure you will stay compliant throughout the year, the
comprehensive ADA® Health Check will evaluate your systems and provide you actionable recommendations.
SOURCES:
1
Tyree, C.A.; Allen, J.O. Determining AQCS Mercury Removal Co-Benefits. Power, July 2010.
2
Blythe, G.; Bissell, J.S.; Labatt L.S. Optimization of Mercury Control on a new 800 MW PRB Fired Coal Plant. Presented at Power Plant Air Pollution Mega Symposium, Baltimore, MD, August 20-23, 2012.
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