POWER July 2010 - 26

MERCURY REMOVAL
Determining AQCS Mercury
Removal Co-Benefits
The U.S. Environmental Protection Agency is expected to propose an emissions
standard for mercury and other hazardous air pollutants emitted by coaland
oil-fired electric generating units in March of 2011. The anticipated rule
would require emission control to meet the various standards using maximum
achievable control technology, as determined by the prescriptive requirements
of the Clean Air Act. In response to the expected rule-making,
utilities will be required to make technology decisions in order to ensure
compliance. One cost-effective approach to compliance may be the use of
" co-benefits " from air quality control systems (AQCSs) already in service
that are designed to remove other pollutants.
By Dr. Corey A. Tyree, Southern Company and Dr. Jonathan O. Allen, PE, Allen Analytics LLC
A
tlanta-based electric utility Southern Company serves 4.4 million customers in the
southeastern U.S. and has more than 42,000 MW of generating capacity. Southern
Company's large coal units have installed emission control equipment such as selective
catalytic reduction (SCR), electrostatic precipitator (ESP), and wet flue gas desulfurization
(wFGD) systems. More than 50% of Southern Company's coal-fired electrical
generating capacity (21,098 MW) is equipped with an ESP + SCR + wFGD technology
combination. One co-benefit of this existing air quality control system (AQCS) equipment
is efficient mercury (Hg) removal.
Mercury emissions from Southern Company's bituminous coal units have been reduced
substantially as a result of co-benefit control. Currently, these AQCSs are designed
and operated for removal of nitrogen oxides (NOx
sulfur dioxide (SO2
), particulate matter (PM), and
). The result is highly variable mercury emissions from a system not
engineered for mercury control. This variability must be accounted for in evaluating cobenefit
systems and their potential to comply with limits on mercury and other hazardous
air pollutants (HAPs) that are expected to be part of the electric generating units' HAPs
maximum achievable control technology rules.
In order to analyze the variability in mercury emissions from co-benefit
control, we gathered and analyzed operating and emissions data for
selected months in 2008 and 2009. The project goal was to determine the
emissions limit that we could reliably meet over the long term with cobenefit
control. The study data includes more than 40 months of wFGD operations
and mercury emissions data from five plants labeled A through E.
Each plant site is equipped with a single boiler, SCR, air preheater,
cold-side ESP, and wFGD system (Figure 1).
We collected a large data set of archived emissions and operating
data, including mercury emissions, and operating variables
possibly connected with mercury emissions, such as boiler
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POWER July 2010

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