POWER August 2010 - 58

MERCURY REMOVAL
cury to its Hg2+
form. The SCR process and
its key reactions are shown in Figure 2.
Key Performance Parameters
A number of factors are known to affect the
degree of oxidation of elemental mercury.
Key factors include halogen content (for
example, chlorine, and bromine), temperature,
reductants (such as ammonia [NH3
SO2), catalyst chemistry, and catalyst age,
] and
as well as the expected flue gas constituents
from coal combustion.
Halogen Content. Halogen content in
the fuel and flue gas is an important driver
for achieving mercury oxidation through the
SCR catalyst. Chlorine levels in bituminous
coals such as Illinois Basin coal exceed 500
ppm, resulting in hydrocholoric acid (HCl)
levels in the flue gas of greater than 35
ppm, thus helping to drive greater formation
of oxidized mercury through the SCR.
For subbituminous coals such as Powder
River Basin coal, chlorine levels are typically
<100 ppm, resulting in HCl levels in
the flue gas of <10 ppm. This low level of
halogen in the flue gas can limit the SCR
performance for oxidizing mercury, and this
presents one of the challenges to catalyst
manufacturers. Methods to enhance performance
include improved catalysts that can
be used exclusively or in combination with
halogen additives.
Temperature. Another factor that can
significantly affect the oxidation of mercury
across the SCR is the temperature of
the flue gas. Thermodynamically, elemental
mercury is more likely to be oxidized at
lower temperatures, thus higher temperature
applications present more of a challenge.
In addition, if SO2
oxidation is a concern,
catalyst chemistry, especially at higher
temperatures, must be modified to help reduce
such activity. This can adversely affect
mercury oxidation performance, posing an
additional consideration in the development
of an advanced catalyst.
Reductants. As noted in Figure 2, oxidized
mercury can be reduced back to its
elemental form if there is an excess amount
of reductants such as ammonia and SO2
. Ammonia
is the most significant reductant to be
considered.
Catalyst Chemistry and Age. Catalyst
chemistry is critical to driving mercury
oxidation performance; however, it
must be balanced with the other critical
performance needs from the SCR, including
NOx
activity and SO2
oxidation. Understanding
the interactive effects of the
flue gas conditions and catalyst chemistry
is critical to the optimization process. The
age of the catalyst also affects the degree
of mercury oxidation, as the catalyst po58
tential
decreases with time. The effects of
catalyst aging on mercury oxidation can
be minimized through a combination of
proper chemical formulation and catalyst
management procedures.
Other Glue Gas Constituents. The
relative rate of mercury oxidation is higher
in cases where the inlet concentration of Hg0
is higher. In a study conducted by the DOE,
EPA, and Electric Power Research Institute,
high levels of mercury oxidation over the
SCR were observed at plants firing eastern
bituminous coal where the level of Hg0
high. However, oxidation was shown to be
is
insignificant when most of the inlet mercury
was already in the oxidized form.
Performance Modeling
Information gathered through a combination
of significant laboratory testing and field data
analysis over the past decade has resulted in
the development of highly predictive performance
models. These models provide the opportunity
to prepare a parametric performance
assessment to determine the best methods to
achieve desired performance levels from the
SCR. In order to set the desired performance
target from the SCR, a full system evaluation
2. SCR process schematic and key reactions. Source: Cormetech Inc.
Desired reactions
NO + NH3 + ¼ O2 ➝ N2 + 3/2 H2O
Hg + 2 HCI + ½ O2 ➝ HgCI2 + H2O
NH3 for DeNOx
NOx
Hg + HCI
Undesired reactions
HgCI2 + NH3 + ¼ O2
NOx + NH3
Hg + HCI
(NOx reduction)
(Hg oxidation)
SCR catalyst
N2 + H2O
HgCI2
➝ Hg + 2HCI + ½ N2 + ½ H2O
HgCI2 + SO2 + H2O ➝ Hg + 2HCI + SO3
SO2 + ½ O2
➝ SO3
(HgCI2 reduction by NH3)
(HgCI2 reduction by SO2)
(SO2 oxidation)
Status of the Clean Air Mercury Rule
In 2005, the U.S. Environmental Protection
Agency (EPA) issued the Clean Air
Mercury Rule (CAMR) establishing performance
standards capping mercury emissions
from coal-fired power plants and
creating a market-based cap-and-trade
program. CAMR was set up to reduce
power plant mercury emissions from the
then-current rate of 48 tons per year to
15 tons per year by 2018, a reduction of
nearly 70%.
Not content with the regulatory approach
adopted by the EPA, petitioners
from 15 states, other governmental entities,
and environmental groups filed suit
maintaining that the EPA did not have
the authority to delist power plants from
the federal Clean Air Act list of sources of
hazardous air pollutants. The petitioners
challenged that mercury and other hazardous
air pollutants should be subject to
www.powermag.com
the National Emission Standards for Hazardous
Air Pollutants and thus be subject
to maximum achievable control technology
rules. In 2008, the U.S. District Court
of Appeals in Washington, D.C., vacated
CAMR, leaving regulation to the 20 states
that have implemented their own regulations
covering mercury emissions. Most of
these states have requirements for an 80%
to 90% reduction in mercury emissions.
To address a subsequent lawsuit by multiple
plaintiffs, the EPA developed a draft
consent decree in October 2009 calling
for the EPA to develop air toxin emissions
standards for coal- and oil-fired power
plants under Section 112(d) of the Clean
Air Act. The decree calls for a notice of
proposed rulemaking to be published in
the Federal Register no later than March
16, 2011, followed by enactment of a final
rule by November 16, 2011.
POWER | August 2010
http://www.powermag.com
POWER August 2010

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