POWER October 2011 - 66

MERCURY REMOVAL
An SCR Can Provide Mercury
Removal Co-Benefits
Complying with various state (and expected federal) requirements governing
mercury removal from the stack gas of coal-fired power plants has usually
been achieved by adding an expensive activated carbon injection system.
Now there is another alternative: a catalyst that features higher mercury
oxidization activity than conventional catalysts while maintaining the
same SO2 to SO3 conversion activity-and all at a lower operating cost.
Full-scale installations are under way at several Southern Company plants
that burn a variety of coals.
By Anthony Favale, Stephen Guglielmo, and Peter Jin, Hitachi Power Systems America Ltd.; Yoshinori Nagai, Babcock Hitachi,
K.K., Japan; and Corey Tyree, Southern Company Research & Environmental Group
I
n March 2005, the U.S. Environmental
Protection Agency (EPA) announced two
final rules for air pollution that apply to
coal-fired power plants: the Clean Air Interstate
Rule (CAIR) and the Clean Air Mercury
Rule (CAMR). CAIR was intended to
reduce nitrogen oxides (NOx
ides (SOx
) and sulfur oxlevels
of ambient O3
) emissions that contribute to high
and particulate matter
(PM2.5). CAMR separately addressed the reduction
of mercury (Hg) emissions from U.S.
power plants. Although CAMR was eventually
vacated, the release of state rules requiring
mercury reduction continued.
These prior federal rules have been replaced
by new regulations focused on hazardous
pollutants (HAPS), such as the Utility
MACT (maximum achievable control technology)
Rule proposed in March 2011 and
slated to become final in November 2011.
In particular, mercury is a major focus of the
new regulations, creating new mercury emission
control challenges for power generating
station owners. In particular, the draft Utility
MACT Rule requires that mercury emissions
meet a concentration level of 11 lb/TBtu for
coal with a heating value less than 8,300 Btu/
lb and 1 lb/TBtu for coals with a heating value
equal to or greater than 8,300 Btu/lb for
existing units. For new units, the targets are
a function of generation: 0.040 lb/GWh for
coal less than 8,300 Btu/lb and 0.00001 lb/
GWh for coals equal to or greater than 8,300
Btu/lb-about one-thousandth the levels required
for existing units.
The challenge for coal-fired plant owners
is to select the most cost-effective and reliable
method to meet these new, aggressive
mercury reduction standards.
A number of technologies have been
shown to reduce mercury emissions, such as
66
activated carbon injection (ACI) and halogen
injection (which reduces the speciation of
Hg). However, these systems potentially create
other problems, such as adding to the cost
of installing and operating the equipment, as
well as the time and cost to maintain it.
There are also other negative economic
consequences of mercury control. For example,
ACI may increase carbon in the fly ash,
reducing its marketability. In our experience,
sometimes the easiest and most cost-effective
way to control mercury is to leverage the
co-benefit of the air quality control system
(AQCS) equipment already present.
Different Forms of Mercury
It is well known that increasing the proportion
of oxidized mercury (Hg2+
the form of water-soluble mercuric chloride
(HgCl2
duction because HgCl2
) allows for high Hg emission recan
be removed in
downstream equipment such as a particulate
control device (PCD) and the wet flue gas
desulfurization (FGD) system. Therefore, to
increase the proportion of Hg2+
upstream of a
wet FGD system will facilitate higher overall
mercury removal for the plant. Selective
catalytic reduction (SCR) catalyst has demonstrated
the ability to increase Hg2+
verting elemental mercury (Hg0
) to Hg2+
by conin
coal
combustion flue gases.
In general, an SCR catalyst can oxidize
elemental form mercury (Hg0
form (Hg2+
) in gaseous form and particulate
form (Hg(P)); however, the mercury oxidation
rate on the SCR catalyst correlates with the
SO2
oxidation/conversion rate to SO3
that can
cause air heater fouling, flue corrosion, and
visible stack plumes. Several downstream SO3
mitigation technologies have become commercially
available in recent years, but these
www.powermag.com
) to its oxidized
SCR Catalyst Designed to Oxidize Hg
Through extensive research and development,
pilot testing, and field demonstration,
Hitachi has successfully developed a new
type of SCR catalyst, TRAC, which satisfies
the high-Hg0
oxidation and low-SO2
oxidation
requirements for low-chlorine coal-fired
power plants. TRAC has been successfully
tested and demonstrated in several slipstream
pilot facilities in the U.S. It has been commercially
available with full mercury oxidation
guarantees since its first installation in a
640-MW PRB-fired U.S. plant in 2008.
Since 2008, TRAC catalyst with high mercury
oxidation activity has been supplied to
several utilities worldwide, including SouthPOWER
| October 2011
) existing in
systems can have high initial and operating
costs, performance limitations, and maintenance
concerns. Therefore, an SCR catalyst
with high Hg0
oxidation activity and a low SO2
to SO3 conversion rate is economically optimal,
especially for U.S. coal-fired plants using
high-sulfur coal. (See " SO3
's Impacts on Plant
O&M, Part I, " Oct. 2006; " Part II, " Feb. 2007;
and " Part III, " Apr. 2007 in POWER's archives
at www.powermag.com.)
The effect of SCR catalyst on Hg0
oxidation
appears to depend on the coal type. Power
plants burning high-chlorine (Cl) coals,
such as U.S. eastern bituminous coals, tend to
show relatively high Hg0
to Hg2+ conversion
across the SCR, whereas plants burning lowCl
coals, such as Powder River Basin (PRB)
coals, tend to show limited or no Hg0
to Hg2+
conversion across SCR catalyst. In such applications,
an enhanced mercury oxidation
catalyst capable of achieving higher mercury
removal across the plant's AQCS would be
highly desirable. (See " Determining AQCS
Mercury Removal Co-Benefits, " July 2010.)
http://www.powermag.com http://www.powermag.com

POWER October 2011

Table of Contents for the Digital Edition of POWER October 2011

Contents
POWER October 2011 - Cover1
POWER October 2011 - Cover2
POWER October 2011 - Contents
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