POWER December 2012 - 80

MERCURY EMISSIONS
Table 1. Emissions limitations for coal and solid oil-derived fuels.
Source: EPA
Subcategory
Existing: Unit not
low-rank virgin coal
Existing: Unit designed
for low-rank virgin coal
Existing: IGCC
Existing: Solid oil-derived
New: Unit not low-rank
virgin coal
New: Unit designed for
low-rank virgin coal
New: IGCC
Filterable particulate
matter
3.0 E-2 lb/MMBtu
(3.0 E-1 lb/MWh)
3.0 E-2 lb/MMBtu
(3.0 E-1 lb/MWh)
4.0 E-2 lb/MMBtu
(4.0 E-1 lb/MWh)
8.0 E-3 lb/MMBtu
(9.0 E-2 lb/MWh)
7.0 E-3 lb/MWh
7.0 E-3 lb/MWh
7.0 E-3 lb/MWh
9.0 E-2 lb/MWh
New: Solid oil-derived 2.0 E-2 lb/MWh
Note: E = exponent
Hydrogen chloride
2.0 E-3 lb/MMBtu
(2.0 E-2 lb/MWh)
2.0 E-3 lb/MMBtu
(2.0 E-2 lb/MWh)
5.0 E-4 lb/MMBtu
(5.0 E-3 lb/MWh)
5.0 E-3 lb/MMBtu
(8.0 E-2 lb/MWh)
4.0 E-4 lb/MWh
4.0 E-4 lb/MWh
2.0 E-3 lb/MWh
4.0 E-4 lb/MWh
Mercury
1.2 E0 lb/TBtu
(1.3 E-2 lb/GWh)
1.1 E+1 lb/TBtu
(1.2 E-1 lb/GWh)
4.0 E+0 lb/TBtu
(4.0 E-2 lb/GWh)
2.0 E-1 lb/TBtu
(2.0 E-3 lb/GWh)
2.0 E-4 lb/GWh
4.0 E-2 lb/GWh
3.0 E-3 lb/GWh
2.0 E-3 lb/GWh
Table 2. Emissions limitations for liquid oil fuels. Source: EPA
Filterable
Subcategory
Existing: Liquid oil,
continental
Existing: Liquid oil,
non-continental
New: Liquid oil,
continental
New: Liquid oil,
non-continental
Note: E = exponent
establish technology-based emission standards,
known as maximum achievable control technology
(MACT). These standards are based on
technology currently available. Due to the variability
in performance between older units and
new units, the MACT analysis may set different
standards for existing and new sources.
When determining what is considered to
be MACT, the EPA must calculate something
called the " MACT Floor. " For existing units, the
MACT Floor is determined by analyzing a set a
data containing the average emissions for a particular
pollutant. Using this set of data, the EPA
then determines what level of emissions can be
met by the best performing 12% of the data set.
This emission target is the MACT Floor,
and that target becomes the minimum value
that all other existing units must conform to. In
the event that there are fewer than 30 sources
for which data is available, the EPA will isolate
the best five performing units and set that as the
MACT Floor.
When determining the MACT Floor for a
new source, the approach is different. In that
case, the MACT Floor would be set by the
" emission control achieved in practice by the
best controlled similar source. "
December 2012 | POWER
Unfortunately, determining what the MACT
Floor is doesn't necessarily set what the final
MACT limits will be. Upon defining the
MACT Floor, the EPA must then consider a
more stringent " beyond-the-floor " option that
pushes control technology to perform at levels
below the MACT Floor emission level. As a
result of previous litigation (Cement Kiln Recycling
Coal v. EPA, 255 F.3d 855, 857-58, D.C.
Cir. 2001), this analysis must also take into account
costs, energy, non-air quality health, and
environmental impacts.
In the case of the MATS regulation, the EPA
chose to set mercury emission limits for most
units at the MACT Floor limit. However, some
units will be required to reach below-the-floor
limits. In the case of existing units that utilize a
low-rank virgin coal for power generation, the
EPA determined that the MACT Floor emission
limitation was 11 pounds per trillion Btu.
The agency also determined that, based on the
availability of activated carbon injection (ACI)
systems, the incremental cost of further reduction
is " reasonable. "
From this conclusion, the EPA decided that
a limit 64% below the MACT Floor was appropriate,
resulting in a limitation of 4 pounds per
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particulate matter
3.0 E-2 lb/MMBtu
(3.0 E-1 lb/MWh)
3.0 E-2 lb/MMBtu
(3.0 E-1 lb/MWh)
7.0 E-2 lb/MWh
2.0 E-1 lb/MWh
Hydrogen chloride
2.0 E-3 lb/MMBtu
(1.0 E-2 lb/MWh)
2.0 E-4 lb/MMBtu
(2.0 E-3 lb/MWh)
4.0 E-4 lb/MWh
2.0 E-3 lb/MWh
Mercury
4.0 E-4 lb/MMBtu
(4.0 E-3 lb/MWh)
6.0 E-5 lb/MMBtu
(5.0 E-4 lb/MWh)
4.0 E-4 lb/MWh
5.0 E-4 lb/MWh
trillion Btu for existing units using low-rank virgin
coal. This means that those units will now be
required to perform at the same level as a new
unit. And although new units have been granted
a stay of effective dates while the EPA reevaluates
technical issues associated with mercury,
existing units are required to maintain the original
compliance dates (Tables 1 and 2).
Measurement Techniques
The promulgation of MATS created more than
just the issue of how to control these pollutants.
It also addressed the issue of how to measure
the pollutants being emitted. As mentioned
previously, the EPA has already acknowledged
potential problems with measuring low emission
levels for pollutants such as mercury, and
therefore granted a stay for new units. Multiple
options exist for measuring mercury, and each
unit will have to be individually evaluated to
determine which option will obtain the desired
level of accuracy.
Typically, a continuous emission monitoring
system (CEMS) for mercury or a sorbent
trap system meeting the EPA's Performance
Standard 12B is required. Maintenance of a
mercury CEMS system is very labor-intensive,
with high operating costs, particularly for calibration
gases for maintaining system accuracy.
Also, each new mercury measurement system
requires initial certifications, initial linearity
checks, daily calculations, weekly integrity
checks, and quarterly linearity checks. An annual
Relative Accuracy Test Audit (RATA) is
necessary to maintain certification.
A sorbent trap is an extraction system
that directly measures vapor phase mercury
emissions. With this method, a grab sample
is taken from the stack and pulled onto the
sorbent trap. The sorbent captures ( " traps " )
the mercury in the flue gas while an analyzer
will measure the amount of mercury in the
sorbent. Calibration gases are not required
for operation. However, the traps are typically
set up for a seven-day continuous run and
therefore must be replaced weekly. Initial and
annual RATA verification are also required
for certification of this type of system.
For both systems, additional flue gas measurements
must be made to ensure accuracy.
Oxygen, carbon dioxide, moisture, and gas flow
measurements of the unit are needed to support
these systems. Although most large combustion
sources currently measure these parameters for
combustion control and other boiler functions,
some smaller units do not measure some of
these parameters on a continuous basis.
A special exception is allowed for EGUs that
have been determined to qualify as low-emission
mercury sources. In these cases, the EGU
will only be required to perform periodic, oncea-quarter
monitoring of mercury utilizing EPA
Test Method 30B. To qualify as a low-emission
75
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POWER December 2012

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