&EPA
United States	FPA-R00/R-04/1 ^0
Environmental Protection	bUUm U4/1vjU
Agency	September 2004
Evaluation of SCR
Catalysts for Combined
Control of NOx and
Mercury

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EPA-600/R-04/130
September 2004
Evaluation of SCR Catalysts for
Combined Control of NOx and Mercury
by
Ravi K. Srivastava
Chun Wai Lee
U.S. Environmental Protection Agency
Office of Research and Development
National Risk Management Research Laboratory
Air Pollution Prevention and Control Division
Research Triangle Park, NC 27711
ICCI Project Manager: Joseph Hirschi
Illinois Clean Coal Institute
5776 Coal Drive, Suite 200
Carterville, IL 62918
EPA Project Officer: Ravi K. Srivastava
National Risk Management Research Laboratory
Air Pollution Prevention and Control Division
Research Triangle Park, NC 27711
U.S. Environmental Protection Agency
Office of Research and Development
Washington, DC 20460

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Abstract
In addition to reducing nitrogen oxides (NOx), selective catalytic reduction (SCR) catalysts can
oxidize elemental mercury (Hg°) in flue gases of coal-fired boilers. Therefore, Hg° oxidation
by SCR catalysts upstream of flue gas desulfurization (FGD) systems should create an
opportunity for simultaneous and cost-effective control of mercury, NOx, and sulfur dioxide
(S02) emissions at power plants. This is because, unlike Hg°, oxidized mercury (Hg2+) is highly
soluble in aqueous solutions and, therefore, can be easily removed in FGD systems.
A two-task, bench- and pilot-scale research study was conducted to investigate the effect of
SCR catalysts on mercury speciation in Illinois and Powder River Basin (PRB) coal combustion
flue gases. Task I studied Hg° oxidation by a SCR catalyst using a bench-scale reactor. Illinois
and PRB coal flue gases were simulated in the bench-scale system. Very good oxidation of Hg°
was observed in all the coal flue gases (approximately 90%). It was shown that hydrogen
chloride (HC1) may be the critical flue gas component that causes the conversion of Hg° to Hg2+
under SCR reaction conditions. Since high HC1 concentrations are expected during combustion
of all Illinois coals, firing of these coals should result in significant Hg° oxidation occurring
across the SCR reactors in the field.
Based on bench-scale results, an appropriate SCR catalyst was produced in a pilot-scale size
by Cormetech, Inc. and installed in a pilot-scale combustor for Task II studies. Three different
Illinois coals (from high to low sulfur and chlorine) and one PRB coal were combusted in the
pilot-scale unit. For the high sulfur/low chlorine Illinois coals, about 84% to 68% Hg°, and 15%
to 9% Hg2+ were measured at the inlet of the pilot-scale SCR reactor. The SCR catalyst induced
high oxidation of Hg°, decreasing the percentage of Hg° at the outlet of the SCR to values below
16%. The low sulfur/high chlorine Illinois coal had a relative high amount (21%) of Hg2+ at the
inlet of SCR. The Hg° content of this coal flue gas was decreased from about 73% at the inlet
to about 4% at the outlet of the SCR reactor. The PRB coal tests indicated a low oxidation of
Hg° by the SCR catalyst. The percentage of Hg° decreased from about 96% at the inlet of the
reactor to about 80% at the outlet.
11

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Foreword
The U.S. Environmental Protection Agency (EPA) is charged by Congress with protecting
the Nation's land, air, and water resources. Under a mandate of national environmental laws,
the Agency strives to formulate and implement actions leading to a compatible balance
between human activities and the ability of natural systems to support and nurture life. To meet
this mandate, EPA's research program is providing data and technical support for solving
environmental problems today and building a science knowledge base necessary to manage
our ecological resources wisely, understand how pollutants affect our health, and prevent or
reduce environmental risks in the future.
The National Risk Management Research Laboratory (NRMRL) is the Agency's center for
investigation of technological and management approaches for preventing and reducing risks
from pollution that threaten human health and the environment. The focus of the Laboratory's
research program is on methods and their cost-effectiveness for prevention and control of
pollution to air, land, water, and subsurface resources; protection of water quality in public
water systems; remediation of contaminated sites, sediments and ground water; prevention
and control of indoor air pollution; and restoration of ecosystems. NRMRL collaborates with
both public and private sector partners to foster technologies that reduce the cost of
compliance and to anticipate emerging problems. NRMRL's research provides solutions to
environmental problems by: developing and promoting technologies that protect and improve
the environment; advancing scientific and engineering information to support regulatory and
policy decisions; and providing the technical support and information transfer to ensure
implementation of environmental regulations and strategies at the national, state, and
community levels.
This publication has been produced as part of the Laboratory's strategic long-term research
plan. It is published and made available by EPA's Office of Research and Development to
assist the user community and to link researchers with their clients.
Sally Gutierrez, Acting Director
National Risk Management Research Laboratory
111

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EPA Review Notice
This report has been peer and administratively reviewed by the U.S. Environmental
Protection Agency and approved for publication. Mention of trade names or commercial
products does not constitute endorsement or recommendation for use.
This document is available to the public through the National Technical Information
Service, Springfield, Virginia 22161.
Disclaimer
This report was prepared by Ravi K. Srivastava of U.S.EPA with support, in part, by grants
made possible by the Illinois Department of Commerce and Economic Opportunity through
the Office of Coal Development and the Illinois Clean Coal Institute. Neither Ravi K.
Srivastava of U.S. EPA nor any of its subcontractors nor the Illinois Department of
Commerce and Economic Opportunity, Office of Coal Development, the Illinois Clean Coal
Institute, nor any person acting on behalf of either:
(A)	Makes any warranty of representation, express or implied, with respect to the
accuracy, completeness, or usefulness of the information contained in this report, or that
the use of any information, apparatus, method, or process disclosed in this report may
not infringe privately-owned rights; or
(B)	Assumes any liabilities with respect to the use of, or for damages resulting from the
use of, any information, apparatus, method or process disclosed in this report.
Reference herein to any specific commercial product, process, or service by trade name,
trademark, manufacturer, or otherwise, does not necessarily constitute or imply its
endorsement, recommendation, or favoring; nor do the views and opinions of authors
expressed herein necessarily state or reflect those of the Illinois Department of Commerce
and Economic Opportunity, Office of Coal Development, or the Illinois Clean Coal
Institute.
Notice to Journalists and Publishers: If you borrow information from any part of this
report, you must include a statement about the state of Illinois' support of the project.
iv

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Contents
Section	Page
Abstract	ii
Foreword	iii
List of Figures	 vi
List of Tables 	 vi
Acknowledgments 	vii
Nomenclature	viii
Executive Summary 	1
Objectives	5
Introduction and Background	7
Experimental Procedures 	9
Results and Discussion	13
Task I: Bench-Scale Evaluation of SCR Catalyst for Hg° Oxidation in Simulated
Illinois and PRB Coal Combustion Flue Gases 	13
Task II: Pilot-Scale Evaluation of SCR Catalyst for Hg° Oxidation in the Combustion
of Natural Gas, Illinois, and PRB Coal Combustion Flue Gases	15
Subtask I: SCR Catalyst Evaluation in Natural Gas Combustion 	15
Subtask II: SCR Catalyst Evaluation in Illinois and PRB Coal Combustion	19
Galatia Coal Combustion	19
Turris Coal Combustion	22
Crown II Coal Combustion	23
PRB (Black Thunder) Coal Combustion 	23
Conclusions and Recommendations	25
References	27
v

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List of Figures
Figure	Page
1	Schematic of Bench-Scale SCR/Hg° Oxidation Reactor System 	 9
2	Schematic of the Pilot-Scale Combustor Used to Evaluate the SCR Catalyst 	 10
3	SCR Temperature Profiles for the Doped Natural Gas Test Day 06/17/03;
Hg CEM (PSA) Sampling; Insulation Added Across the SCR System	 17
4	Hg CEM (PSA) Measurement Results for the Doped Natural Gas Test
Day 06/17/03	 18
5	Hg CEM (PSA) Measurement Results for the Doped Natural Gas Test Day
06/18/03; IGS Probe Hg Speciation Bias Check and the Effect of HC1 on Hg°
Oxidation	 19
List of Tables
Table	Page
1	Proximate and Ultimate Analyses of Three Illinois Basin Coals and a PRB Coal ... 13
2	Summary of Simulated Flue Gas Compositions for Task I Studies 	 14
3	Summary of NOx Reduction Results for Task I Studies	 14
4	Summary of Mercury Speciation Results for Task I	 15
5	Hg Speciation Across the SCR Catalyst for the Natural Gas Test Day 06/05/03 .... 16
6	Hg Speciation Across the SCR Catalyst for the Doped Natural Gas Combustion
Test Days; Comparison between OH and PSA	 18
7	Combustion Conditions for the Illinois Turris, Galatia, and Crown II Coals and
the Black Thunder PRB Coal	 20
8	Galatia Coal Combustion Test, OH Sampling Results Measured at the Inlet of
the SCR Reactor	 21
9	Galatia Coal Combustion Test, OH Sampling Results Measured at the Outlet
of the SCR Reactor 	 21
10	Turris Coal Combustion Test, OH Sampling Results Measured at the Inlet
and Outlet of the SCR Reactor 	 22
11	Crown II Coal Combustion Test, OH Sampling Results Measured at the Inlet
and Outlet of the SCR Reactor 	 23
12	First PRB Coal Combustion Test, OH Sampling Results Measured at the Inlet
and Outlet of the SCR Reactor 	 24
13	Second PRB Coal Combustion Test, OH Sampling Results Measured at the Inlet
and Outlet of the SCR Reactor 	 24
vi

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Acknowledgments
The authors would like to acknowledge the support for this work provided by the Illinois
Clean Coal Institute. This work was completed under the ICCI project number 02-1/2.2A-1.
Mr. Joe Hirschi, the ICCI project manager, provided many helpful suggestions. Cormetech,
Inc. provided the SCR reactor and catalyst used in this work. Dr. Thomas W. Hastings, Mr.
Frank M. Stevens, and Mr. Mark A. Conger of Cormetech, Inc. provided SCR
catalyst-related technical guidance and support. Finally, research support provided by Dr. B.
Ghorishi, Dr. W. Jozewicz, and Mr. Jarek Karwowski of ARCADIS under EPA contract
68-C-99-201 is also appreciated.
vii

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Nomenclature
Term	Definition
APPCD
Air Pollution Prevention and Control Division
CAA
Clean Air Act
CEM
continuous emission monitoring
CO
carbon monoxide
co2
carbon dioxide
EPA
U.S. Environmental Protection Agency
FGD
flue gas desul-furization
HC1
hydrogen chloride
Hg
mercury
Hg°
elemental mercury
Hg2+
ionic mercury
Hgp
Hg associated with particulate matter
HgT
total mercury
ICCI
Illinois Clean Coal Institute
IFR
innovative furnace reactor
IGS
isokinetic/inertial gas sampling
nh3
ammonia
NOx
nitrogen oxides
OH
Ontario Hydro
PM
particulate matter
PRB
Powder River Basin
PSA
PS analytical
SCR
selective catalytic reduction
SO,
sulfur dioxide
viii

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Evaluation of SCR Catalysts
Executive Summary
The Air Pollution Prevention and Control Division
(APPCD) of the U.S. Environmental Protection
Agency (EPA), ARCADIS, Inc., and Cormetech, Inc.
conducted an Illinois Clean Coal Institute (ICCI)
co-funded research to evaluate the viability of a
mercury control technology utilizing oxidation of
elemental mercury (Hg°) to ionic mercury (Hg2+) for
subsequent removal by conventional flue gas desul-
furization (FGD) systems. Mercury speciation modi-
fication was implemented via selective catalytic
reduction (SCR) catalysts, currently used for nitrogen
oxides (NOx) control. In December of2001, the EPA
determined that control of mercury emissions from
coal-fired power plants is appropriate and necessary.
For Illinois coal-fired utilities, control of mercury
emissions may be needed to comply with future
regulations. Oxidation of Hg° to Hg2+ prior to an FGD
system is a viable option for controlling mercury
emissions because FGD systems are known to effi-
ciently remove Hg2+. This research generated valu-
able data, which may be used by utilities using
Illinois coals to cost-effectively control mercury,
NOx, and sulfur dioxide (S02) emissions using
SCR/FGD systems.
Currently there are no commercial technologies for
modifying mercury speciation prior to an FGD unit.
This study evaluated the viability of SCR technology
for mercury speciation modification in Illinois and
Powder River Basin (PRB) coal combustion pro-
cesses. Since SCR and FGD are increasingly being
used at power plants to control NOx and S02 emis-
sions, respectively, their combination has the poten-
tial to provide effective mercury control.
The objectives of the research were to conduct bench-
and pilot-scale studies in order to demonstrate Illinois
and PRB coal combustion conditions that favor
conversion of Hg° to Hg2+ by a Cormetech, Inc.
formulated SCR catalyst. These objectives were
accomplished initially through a systematic bench-
scale study of the effect of the simulated flue gas
compositions (three different Illinois and one PRB
coals) on the oxidation of Hg° to Hg2+ in the presence
of the SCR catalyst. Hg° oxidation activity of the SCR
catalyst was established during the bench-scale testing
phase of this study. A pilot-scale version of the
Cormetech SCR catalyst was installed in a pilot- scale
combustor to evaluate and establish Hg° oxidation
capabilities of this catalyst during combustion of three
different Illinois coals (from high to low
sulfur/chlorine) and one PRB coal (very low sulfur
and chlorine).
The research was performed over a 12-month period
(from September 2002 to August 2003) and consisted
of two main tasks. Task I involved bench-scale tests
of a Cormetech SCR catalyst formulation to character-
ize oxidation of Hg° to Hg2+ in the presence of coal
combustion/SCR flue gas species such as NOx,
ammonia (NH3), S02, and hydrogen chloride (HC1).
The ranges of NOx, NH3, HC1, S02, and water (H20)
concentration employed during Task I were those
expected to be found in three representative Illinois
coals and one PRB coal combustion flue gases. The
Illinois coals tested include Galatia [low sulfur (1.13
wt%) and high chlorine (0.29 wt%)], Turris [medium
to high sulfur (3.11 wt%) and low chlorine (0.17
wt%)], and Crown II [high sulfur (3.48 wt%) and low
chlorine (0.13 wt%)]. The PRB coal tested (from
Black Thunder) is a Southern PRB coal with very low
sulfur (0.29 wt%) and chlorine content (about 0.01
wt%). Task I results showed that HC1 is the critical
flue gas component for converting Hg° to Hg2+ under
SCR emission control conditions. Hg2+ was measured
as the predominant species at the outlet of the SCR
1

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Evaluation of SCR Catalysts
catalyst in the four simulated coal tests (PRB,
Galatia, Turns, and Crown II). The HC1 concentra-
tion (8 ppm) used for PRB test was much lower than
those used for the Illinois coal combustion tests
(100-200 ppm). However, it was still much higher
than that of the doped Hg° (19 ppb) in the simulated
flue gas and may be adequate for converting most of
the Hg° to Hg2+. Hg° was found to be the predominant
species at the exit of the SCR catalyst for only one
test, PRB coal that had no HC1 present in the flue gas.
This is believed to be due to the lack of a chlorine
source in this test. The final result of Task I was the
confirmation of the SCR catalyst formulation to be
used in Task II.
Task II was performed in two subtasks. In Subtask I,
a SCR catalyst was installed in a vertical downward
flow configuration in the post-combustion region of
a pilot-scale 34.9 kW (150,000 Btu/h), refractory
lined, down-fired cylindrical furnace. This furnace,
termed the innovative furnace reactor (IFR), has the
capability of firing natural gas or pulverized coal. In
Subtask I, flue gas from natural gas combustion was
doped with appropriate amounts of Hg°, HC1, NOx,
NH3, and S02. It was determined that the catalyst
contribution to the oxidation of Hg° was very low
(only about 8% increase). It was hypothesized that
the absence of particulates (fly ash) might have a
bearing on Hg° oxidation reactions. Indeed, the coal
combustion tests performed later indicated a very
good Hg° oxidation by the SCR catalyst. The natural
gas combustion test indicated that the IFR could be
operated consistently, and most of the injected
mercury could be recovered at the outlet of the SCR
catalyst. Furthermore, it established reliable operation
of the pilot-scale combustion/SCR system.
In Subtask II of Task II, three different Illinois coals
were combusted in the IFR facility. The objective of
Subtask II was to characterize Hg° oxidation for a
spectrum of Illinois coals and to establish any addi-
tional increases of Hg2+ resulting from the SCR
catalyst used in Task I and Subtask I of Task II. Since
combustion of PRB coals will result in the production
of Hg°-dominated flue gases, the effect of the pilot-
scale SCR catalyst on oxidation of Hg° in a typical
PRB coal combustion flue gas was also studied.
Mercury speciation of the flue gas (Hg°, Hg2+, and Hg
associated with particulate matter, Hgp) across the
SCR catalyst was measured using the Ontario Hydro
(OH) method. Attempts were made to measure gas-
phase Hg (Hg° and Hg2+) by using a specially de-
signed dual isokinetic/inertial gas sampling (IGS)
probe. The Hg speciation samplings were performed
at the inlet of the baghouse where high particulate (fly
ash) loading on the filter of the isokinetic OH sam-
pling train may change the speciation of mercury.
Deposition of white powder was observed occurring
on the walls of the first impinger connected to the IGS
probe. Comparison of the data measured by using the
OH method and those using the IGS probe showed the
total Hg in solid and gaseous phases measured by the
OH method was significantly higher than the gas-
phase Hg. The white ammonium salt aerosols were
formed near the inlet of the first impinger connected
to the IGS probe as the ammonia-containing flue gas
cooled down. The aerosols may retain gas-phase Hg
and change in Hg speciation may result. Further work
is needed for the development of the IGS probe for
sampling ammonia-containing flue gas in SCR sys-
tems. NOx conversions by the Cormetech-formulated
SCR catalyst for all these coals were around 90%.
Hg° was found to be the predominant species in the
flue gases measured at the inlet of SCR catalyst for
the three Illinois coals tested. The combustion of the
low sulfur/high chlorine Galatia coal produced a
relative high amount (21%) of Hg2+ at the inlet of
SCR. The Hg° content of this coal flue gas was
decreased from about 73% at the inlet to about 4% at
the outlet of SCR reactor. Turris and Crown II coal
combustion tests exhibited similar mercury speciation
and oxidation behavior. These two coals are very
similar in their properties. During the combustion of
these coals about 84% to 68% Hg°, and 15% to 9%
Hg2+ were measured at the inlet of the pilot-scale SCR
reactor. The SCR catalyst induced high oxidation of
Hg°, dropping the percentage of Hg° at the outlet of
the SCR to below 16%. These results suggest that the
use of SCR systems in boilers firing Illinois coal and
2

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Evaluation of SCR Catalysts
equipped with FGD systems may result in about 85%
to greater than 90% control of mercury.
The PRB coal combustion tests showed that almost
all the Hg measured at the inlet was Hg°. The test
results also indicated a very low oxidation of Hg° by
the SCR catalyst. Based on the measurements con-
ducted in this study, it appears that SCR applications
on PRB coal-fired boilers may not result in significant
increase in Hg2+ content of flue gas. Therefore,
such boilers equipped with wet FGD systems may not
achieve increased mercury removal resulting from
SCR applications.
3

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4

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Evaluation of SCR Catalysts
Objectives
The obj ective of this research program was to investi-
gate Hg° oxidation across selective catalytic reduc-
tion (SCR) systems in Illinois coal combustion
processes. Additional tests were also performed to
characterize SCR catalyst activity in a PRB coal com-
bustion environment. The specific goals of the pro-
posed research were to identify the effect of SCR
catalysts, flue gas compositions, and combustion
conditions (time-temperature history and concentra-
tions of gas-phase species) on the oxidation of Hg°
upstream of the flue gas desulfurization (FGD) units.
This work was synergized with similar collaborative
bench-scale research being conducted by U.S. EPA
and Cormetech, Inc.
This research was performed over a 12-month period
and consisted of two tasks. Task I involved bench-
scale testing of a promising SCR catalyst formulation
to characterize oxidation of Hg° to Hg2+ in the pres-
ence of coal combustion flue gas constituents such as
NOx, NH3, S02, and HC1. Previous EPA investiga-
tions with synthetic and actual fly ashes have indi-
cated that certain transition metals are very active in
catalyzing conversion of Hg° to Hg2+ in the presence
of HC1 and NOx. SCR catalysts contain vanadium (a
transition metal) active sites. In Task I, the role of
this transition metal on Hg° oxidation was identified.
The NOx, NH3, HC1, S02, and H20 concentration
ranges employed during Task I were those expected
to be found in three representative Illinois coals (low
to high sulfur and chlorine) and one PRB coal com-
bustion flue gases. The final result of Task I was the
confirmation of the SCR catalyst formulation to be
used in Task II.
Task II was performed in two subtasks. In Subtask I,
a pilot-scale configuration of the active SCR catalyst
was installed in a vertical downward flow configura-
tion in a pilot-scale 34.9 kW (150,000 Btu/h), refrac-
tory lined, down-fired cylindrical furnace. This
furnace, termed the IFR, has the capability of firing
natural gas or pulverized coal. In Subtask I, natural
gas combustion flue gas was doped with appropriate
amounts of Hg°, HC1, NOx, NH3, and S02. Mercury
speciation of the flue gas (Hg° and HgCl2 content)
across the SCR catalyst was measured using the
Ontario Hydro (OH) method. Once the reliable
operation of the pilot-scale combustion system was
established, three different Illinois coals were com-
busted in Subtask II of Task II. The objective of
Subtask II was to characterize Hg° oxidation for a
spectrum of Illinois coals and to establish any addi-
tional increases of Hg2+ using the SCR catalyst
characterized in Task I and Subtask I of Task II. The
effect of the active pilot-scale SCR catalyst on
oxidation of Hg° in a typical PRB coal combustion
flue gas was also studied.
5

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6

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Evaluation of SCR Catalysts
Introduction and Background
Metallic air toxics, such as mercury, are found in coal
combustion flue gases in elemental or various oxi-
dized forms and exist in the solid, aerosol, or vapor
state. These compounds originate from the raw coal
and are typically enriched in the fine particles
(Markowski and Filby, 1985). Mercury speciation
may be potentially modified by SCR catalysts cur-
rently used for NOx control. In December of 2001,
the EPA determined that control of mercury emis-
sions from coal-fired power plants is appropriate and
necessary. For utility boilers fired with Illinois coal,
control of mercury emissions may be needed to
comply with future regulations. Oxidation of Hg° to
Hg2+ prior to an FGD system is a viable option for
controlling mercury emissions, becauseFGD systems
easily remove Hg2+. Research data is needed to assess
the contribution of SCR catalysts to the oxidation of
Hg° prior to the FGD systems. Preliminary EPA
laboratory results (Ghorishi and Gullett, 1998) have
indicated the effectiveness of alkaline sorbents (dry
FGD units) in removing Hg2+ (but not Hg°) from flue
gases. Previous investigations have also indicated
that many flue gas and fly ash parameters determine
the speciation of mercury (Ghorishi, 1998; Ghorishi
etal., 1999).
The distribution of various forms of mercury in flue
gases is a function of system-specific properties. For
example, various concentrations of chlorine or sulfur
have been shown to significantly affect the expected
equilibrium product distribution (Linak and Wendt,
1993). Ghorishi et al. (1999) have shown that, in
combustion flue gas with the presence of active
simulated or actual fly ashes, Hg° is readily oxidized
by HC1 and NOx at temperatures typical of air pollu-
tion control systems (150-250 °C). The presence of
these constituents in the flue gas can therefore shift
the ratio of Hg2+/Hg° to a higher value and result in
better removal of mercury in F GD systems. However,
quantitative information on speciation of mercury in
the SCR systems in the presence of coal combustion
flue gases at typical SCR operating temperatures
(340-370 °C) is lacking at this time.
The control of mercury emissions has been found to
strongly depend on the form of mercury; these results
were observed at the EPRI High Sulfur Test Center's
4 MWe pilot-scale wet FGD system and at municipal
waste combustor (MWC) tests (Volland, 1991). The
presence of HC1 and NOx in the combustion flue
gases shifts the mercury speciation toward its oxi-
dized form (Hall et al., 1991, Ghorishi, 1998). The
type of coal has also been found to affect different
levels of mercury capture (Felsvang et al., 1992;
1993). The chlorine present in coal is capable of
forming Hg2+, which is less volatile and more wa-
ter-soluble than Hg°, and is, therefore, thought to be
an important factor in the capture of mercury in coal
combustion flue gas (Felsvang et al., 1993; DeVito et
al., 1993). Therefore, an important scientific issue
that is being investigated is the speciation of mercury
in coal combustion flue gases. Knowledge about the
transformation of Hg° to HgCl2 is absolutely crucial
in this regard. Senior et al. (1997) has reviewed in
detail the state of knowledge of mercury speciation in
coal-fired processes. Their review and an investiga-
tion by Krishnan et al. (1994) concluded that the
assumption of gas-phase equilibrium for mercury
species in coal-fired flue gases is not valid, and major
reaction pathways for mercury oxidation in coal
combustion flue gas need to be investigated. These
investigations should include heterogeneous reactions
on active surfaces of particles in the flue gas as well
as gas-phase reactions (Senior et al. 1997).
A preliminary investigation was performed at EPA
7

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Evaluation of SCR Catalysts
laboratories (Ghorishi, 1998) to provide some in-
sights into these questions by studying mercury
speciation in simulated coal combustion flue gases.
Gas-phase studies indicated that the in-flight, post-
combustion oxidation of Hg° in the presence of HC1
is very slow and proceeds at measurable rates only at
temperatures greater then 700 °C and high HC1 con-
centrations (200 ppm). The presence of S02 and H20
in the simulated flue gas significantly inhibited the
gas-phase oxidation of Hg° in the presence of HC1.
These results indicate that conversion of Hg° to Hg2+
needs to be mediated by the presence of an active
surface such as fly ash (or SCR catalyst, as in this
study). The effects of coal fly ash components and
compositions were investigated using a fixed-bed of
model (simulated) fly ashes. The primary focus was
on evaluating the catalytic Hg° oxidation activity of
major mineral constituents of coal fly ashes: alumina
(A1203), silica (Si02), iron (III) oxide (Fe203), copper
(II) oxide (CuO), and calcium oxide (CaO). Copper
and iron oxides exhibited significant catalytic activity
toward a surface-mediated oxidation of Hg°.
SCR has been used extensively to control NOx
emissions from hundreds of utility and industrial
boilers in Japan, Europe, and the United States, and
a large number of SCR systems are under construc-
tion or planned to be installed in the United States. A
recent study (Cichanowicz and Muzio, 2001) esti-
mated that, by the year 2004, in excess of 100 GW of
coal-fired capacity in the United States will be
equipped with SCR to meet the requirements of Title
I of the Clean Air Act (CAA). The study describes
how today's U.S. applications of SCR have evolved
from two prior generations of design in response to
changing application conditions. The first commer-
cial SCR application in Japan occurred in the late
1970s, and a subsequent "wave" of installations took
place in Europe (predominantly Germany) in the
mid-1980s. As a result of this increased SCR market,
present-day SCR catalysts have evolved to feature
thinner walls, improved mass transfer and activity,
and better poison resistance than earlier generations.
Limited field data obtained from European coal-fired
boilers equipped with SCR systems suggest that SCR
catalysts promote the formation of Hg2+ (Gutberlet et
al., 2000). Results of recent pilot-scale screening tests
on the impact of SCR on mercury speciation also
indicate that SCR systems could promote the forma-
tion of ionic Hg (EPRI, 2000). The screening test
results reflected that the impact of SCR on mercury
speciation appears to be highly coal-type dependant.
For the coals tested (three bituminous coals and one
subbituminous PRB coal), only high-sulfur bitumi-
nous coal showed significant increase in oxidized
mercury in the outlet of the SCR. The low-sulfur
(also low chloride) PRB coal showed very little
impact of SCR on mercury speciation. The other two
bituminous coals' results were between the two
extreme cases. Results of the recent field tests con-
ducted in the United States showed there was an
increase in Hg oxidation across the SCR catalysts for
plants firing eastern bituminous coals with sulfur
contents ranging from 1.1% to 4.2% (Laudal et al.,
2003).
The increase in oxidation of Hg° to ionic Hg across
the SCR reactor could result from the changes in flue
gas chemistry caused by the SCR catalysts. The SCR
catalysts (which include oxides of vanadium, tita-
nium, and tungsten) have the potential to catalyze the
formation of S03 (from S02) and Cl2 (from HC1) in
coal combustion flue gases. Both S03 (Galbreath and
Zygarlicke, 2000) and Cl2 (Ghorishi etal., 1999)have
been suggested to react with Hg° to cause its oxida-
tion. However, not much is known on the effect of
the flue gas species, chemical reactions, and reaction
conditions on Hg° oxidation across SCR reactors.
8

-------
Evaluation of SCR Catalysts
Experimental Procedures
Task I studies were performed in a bench-scale
SCR/Hg° oxidation reactor system. A schematic of
the reactor system is shown in Figure 1. The system
consists of preheating and premixing sections, a
mercury generation unit, a SCR reactor, and an
on-line reactor effluent measurement unit. Flue gas
components including carbon dioxide (C02), S02, air,
and nitrogen (N2) were mixed and preheated to 350
°C and then mixed with another preheated stream of
NOx and HC1 at the main heating section. Water, for
simulating the moisture content in the flue gas, was
also pumped into this section at a calibrated rate and
mixed with the other flue gas components. Ammonia
was preheated and added into a section equipped with
static mixing hardware. The gas mixture flowed into
a Pyrex reactor. The alkaline NH3 reacts with the acid
gas components in the gas mixture to form ammo-
nium salts at temperatures lower than the SCR
reaction temperatures, so the simulated flue gas
mixture was preheated by electrical furnaces and
maintained at 350 °C by temperature-controlled
electrical heating tapes. A mercury generation unit
consisting of a mercury permeation tube surrounded
by a temperature-controlled water bath was used to
generate Hg° vapor for the oxidation experiments.
The Hg° vapor was carried by an N2 stream and
mixed with flue gas mixture near the top of the
reactor (4 cm in inner diameter and 35 cm in length).
The flue gas mixture containing Hg° and NH3 passed
through a honeycomb flow straightener to produce a
uniform laminar flow before passing through the
honeycomb SCR catalyst. The effluents exiting the
SCR reactor passed through a drying tube for remov-
ing moisture in the flue gas. The dried gas then
passed to an online ultraviolet S02 analyzer (Bovar
Engineering, Inc., model 721AT2; accuracy of about
±5%) for measuring S02. An online chemilumines-
centNOx analyzer (Advanced Pollution Instrumenta-
tion, Inc., model 200AH; accuracy of about ±5%)
downstream of the S02 analyzer was used for mea-
suring NOx.
Sampling Port
Heating Tape
Water Pump ^
Thermocouple
—2-
Thermocouple
Heating
Tape -
w Electrical Heating .
Furnaces
Static Mixiny Section
Heating
Tape
Mass Flow Controller
-cw-
-mn-
-mK-
¦£>
NO
HCI
Air (Nj, 02)
co2
S02
Nj
nh3
Honeycomb
Flow —
Straightener
SCR Catalyst	

Thermocouple
Mass Flow Controllers
=LJ=LJ=C3=E3=
Exhaust
NOx
Analyzer
so2
Analyzer
Mercury
Analyzer
Water
Removal
System
Mercury
1 Generation
System
Mass Flow
Controller
N2
Electrical
Heating
Furnace
Thermocouple
Figure 1. Schematic of Bench-Scale SCR/Hg° Oxidation Reactor System.
9

-------
Evaluation of SCR Catalysts
A SCR catalyst with a vanadia/titania formulation
and a honeycomb configuration, designated as
Catalyst A, was used in Task I. A small piece (2.2 cm
for both sides and 1.9 cm in length; calculated space
velocity of 2609 hr"1) of the catalyst sample was
placed into the catalyst compartment of the SCR
reactor. The flows of the flue gas components were
maintained at the desired levels by using mass flow
controllers. The variability of the gas concentrations
is about ±2.5%. A constant total flow rate of 400
cm3/min—at a standard temperature and pressure
(STP) of 25 °C and 101.4 kPa, respectively—was
used for all tests. The concentrations of S02 and NOx
at the outlet of the reactor were monitored continu-
ously for 4 hours by using S02 and NOx analyzers to
ensure that NOx reduction reached a steady state.
Then the two gas analyzers were disconnected from
the outlet of the reactor, which was subsequently
connected to a mercury sampling train. The mercury
speciation measurement method known as the OH
method was used for measuring Hg° and Hg2+ in this
study.
In Task II, tests were performed in a pilot-scale 34.9
kW (150,000 Btu/h), refractory lined, down-fired
cylindrical furnace fired with coal or natural gas
(Figure 2). The furnace, termed the IFR, has an inner
diameter of 15.2 cm and overall length of about 4 m.
The IFR is used to simulate and generate a coal
combustion environment and quench rate conditions
similar to those upstream of FGD units in coal-fired
utility boilers. View and injection/probe points tra-
verse the length of the furnace for testing flexibility.
The IFR system was retrofitted with two vertical
sections, an upward and a downward flow. The
two-stage, pilot-scale SCR reactor based on the
bench-scale results was installed in the downward
section. Figure 2 illustrates the dimensions of this
two-stage SCR system. A sampling port was installed
between the two stages. The first stage of the SCR
catalyst is equipped with a flow straightener, a soot
blower, and a honeycomb catalyst. The second stage
has a soot blower and a honeycomb catalyst. The two
honeycomb catalysts are identical and have a pitch of
8.2 mm, wall thickness of 0.83 mm, and length of
Natural Gas
16ft
J Gate Valve
° Va' Temperature Port
O 3" OH/CEM Port
Air
• o»o«O *0
Coal
3ft
5" id Duct
2th Mezzanine
7.3 ft 1 SCR Reactor
includes honeycomb, flow
straightener, and soot blower
SCR
By-pass
Vertical
Furnace
Refractory
Section
8ft
1 ft Sampling Joint
1 Mezzanine
6.8 ft 2"*-'SCR Reactor
includes honeycomb
and soot blower
9ft
23 ft
3ft
Collection
Bin
Floor
Baghouse
Figure 2. Schematic of the Pilot-Scale Combustor Used to Evaluate the
SCR Catalyst.
10

-------
Evaluation of SCR Catalysts
1250 mm; they consist of 8x8 cells (a total of 64).
The calculated space velocity of the two stages when
the IGS probe is operational was 2943 hr"1. This
space velocity is comparable to the 2609 hr"1 used in
bench-scale tests and to SCR space velocities ob-
served in the field. Sampling ports along the horizon-
tal ducts at the inlet and the outlet of the vertical SCR
reactor allow gas and particle monitoring. The IFR is
equipped with a continuous emission monitoring
(CEM) system for 02, C02, CO, NOx, and S02. The
IFR is also equipped with 12 thermocouples in order
to accurately determine the time-temperature history
of the combustor.
Isokinetic OH trains were used for making mercury
speciation measurements at the inlet and the outlet of
the SCR reactor. Two inertial gas sampling (IGS)
probes (Mott Co.) were also used in conjunction with
the isokinetic OH trains at the inlet and the outlet of
the SCR reactor for Hg measurements. The IGS
probe has the capability of separating gas from
particulate matter without the need for a filter, thus
minimizing the potential Hg speciation biases created
by the filter of the isokinetic OH train. The IGS
probes were equipped with venturi flow meters for
accurate axial flow measurement. The focus of the
isokinetic trains for mercury sampling was only on
total mercury data (Hgx), since these samplings were
performed at the inlet of the baghouse with signifi-
cant particulate loading in the flue gas. The IGS
probe data are only for the gas-phase elemental and
ionic mercury. The difference between Hgx results
obtained by the isokinetic OH train and the gas-phase
Hg° and Hg2+ obtained by the IGS probe is the
amount of Hg associated with particulate matter
(Hgp). The IFR is also equipped with a baghouse and
a scrubber to treat the flue gas before releasing it into
the atmosphere.
11

-------
Evaluation of SCR Catalysts
12

-------
Evaluation of SCR Catalysts
Results and Discussion
Task I: Bench-Scale Evaluation of SCR
Catalyst for Hg° Oxidation in Simulated
Illinois and PRB Coal Combustion Flue
Gases
Bench-scale testing of the SCR catalyst was per-
formed in the simulated combustion flue gases of
three different Illinois coals and one PRB coal. These
coals were also tested in the pilot-scale combustor
during Task II studies. Table 1 summarizes the
characteristic of the obtained Illinois coals (Turris,
Crown II, and Galatia) and the PRB coal (Black
Thunder). Turris coal sample contains medium
amounts of sulfur and chlorine. Crown II is a high
sulfur/low chlorine coal sample. Galatia contains
high amounts of chlorine and low amounts of sulfur.
The ultimate analysis results of these coals were used
to determine the composition of the simulated flue
gas in each test case shown in Table 2. The base flue
gas mixture consisted of 350 ppm NOx, 315 ppm of
NH3, 15% C02, 3.5% of 02, 5.3% of H20, 19 ppb of
Hg°, and balance in N2. The nominal substoichio-
metric NH3/NOx ratio of 315/350 was used in the all
tests in order to achieve a nominal 90% NOx reduc-
tion. The PRB/no HC1 test represents a situation
where the CI released from the combustion of PRB
coal reacts with Ca resulting in no HC1 in the flue
gas. This simulation was designed to determine the
effect of HC1 on the oxidation of Hg°.
Results of the NOx reduction for these tests are
summarized in Table 3. NOx reductions of 85% to
88%) were observed for all the tests. These levels of
NOx reduction are similar to those achieved in the
field, suggesting that the reactor system used in the
Table 1. Proximate and Ultimate Analyses of Three Illinois Basin Coals and a PRB Coal.
Content
PRB
Black Thunder
Illinois Turris
(medium S/Cl)
Illinois Crown II
(high S/low CI)
Illinois Galatia
(low S/high CI)
Moisture, %
14.00
16.99
16.07
11.33
Ash, %
5.92
9.26
7.34
6.29
Volatiles, %
37.33
33.89
37.05
34.16
Fixed C, %
42.76
39.85
39.55
48.55
Heating Value (Btu/lb)
9903
10,531
10,877
12,179
C, %
59.71
59.00
60.48
68.31
H, %
3.83
4.32
4.70
4.50
N, %
0.82
1.19
1.07
1.50
S, %
0.29
3.11
3.48
1.13
0, %
15.44
5.96
5.73
6.94
CI, %
NAa
0.17
0.13
0.29
Ha. ppmw
NA
0.07
0.07
0.09
aNA = not available.
13

-------
Evaluation of SCR Catalysts
Table 2. Summary of Simulated Flue Gas Compositions for Task I Studies.
„	PRB PRB/no HC1 Galatia	Turris	Crown II
Concentration
HC1, ppm
8
0
204
134
98
S02, ppm
280
280
934
2891
3116
NOx, ppm
350
350
350
350
350
NH3, ppm
315
315
315
315
315
co2, %
15
15
15
15
15
02, %
3.5
3.5
3.5
3.5
3.5
h20, %
5.3
5.3
5.3
5.3
5.3
H2°. DDb
19
19
19
19
19
Table 3. Summary of NOx Reduction Results for Task I Studies.
NOx Results	PRB PRB/no HC1 Galatia	Turris	Crown II
Outlet NOx Concentration, ppm 44 52 44 43 41
NO, Reduction. %	87	85	87	88	88
present study closely simulated the SCRNOx emis-
sion control conditions in the field.
The mercury speciation results are summarized in
Table 4 (± indicates duplicate test). Two mercury
speciation samples were taken at the inlet of the SCR
catalyst by using the OH method after the PRB and
Galatia tests. Two OH samples were also taken at the
outlet of the mercury generation unit at the beginning
of the test program. The total mercury concentrations
(19.3±1.0 ppb) with very little Hg2+ (0.5 ppb) mea-
sured near the inlet of the catalyst are very close to
those measured at the outlet of the mercury genera-
tion unit (19.2±0.1 ppb Hgx with 0.5 ppb Hg2+). The
two inlet OH speciation results obtained in the
presence of two different simulated flue gas mixtures
(PRB and Galatia tests) showed Hg° was the only
mercury species. The consistent inlet results suggest
that the presences of HC1 and S02 with different
concentrations in the simulated flue gas mixtures do
not cause gas-phase oxidation of Hg° at 350 °C prior
to the SCR catalyst.
The speciation results shown in Table 4 suggest that
HC1 has a significant effect on conversion of Hg° to
Hg2+ under SCR/NOx emission control conditions.
All tests except PRB/no HC1 showed mostly Hg2+ at
the outlet of the SCR catalyst. As shown in Table 2,
the gas mixtures used for PRB and PRB/no HC1 tests
were identical except that 8 ppm of HC1 was added to
the simulated flue gas mixture for the PRB test.
Almost all the mercury measured in the PRB test was
Hg2+, but very little Hg2+ was measured in the
PRB/no HC1 test. The total mercury concentration at
the outlet of the SCR catalyst measured for the
PRB/no HC1 test (13.1 ppb) is about 60% of that
measured at the inlet (19.3 ppb). The results of the
two outlet replicate tests are similar to each other,
indicating good precision. One possible explanation
for the discrepancy in inlet and outlet results is that
the SCR catalyst might have adsorbed Hg° in the
absence of HC1 in the flue gas mixture during the
PRB/no HC1 test. Evidence of Hg° adsorption was
observed during the start-up of the SCR reactor
system. When a gas mixture with 3% 02, 10% C02,
14

-------
Evaluation of SCR Catalysts
Table 4. Summary of Mercury Speciation Results for Task I
„ . ,.	Inlet	PRB PRB/no HC1 Galatia Turris Crown II
Concentration
Hg°, ppb3
18.8±1.0
0.7±0.1
12.6±0.4
0.73
3.3
2.0±0.1
Hg2+, ppb
0.5±0.0
17.8±0
0.46±0.0
16.2
29.4
17.9±0.5
HgT, ppb
19.3±1
18.4±0.2
13.1±0.4
16.9
32.7
20.0±0.3
HS2+. %
2.6
96.7
3.5
95.9
89.9
89.5
a 1 ppb =8.3 |ig/dscm
240 ppm of NO, and 30 ppb of Hg° (balance N2) was
passed through the SCR catalyst at 350 °C, very little
Hg° was measured at the outlet of catalyst by using a
UV mercury analyzer. However, this adsorption of
Hg° by the SCR catalyst requires further research.
Table 4 also shows that Hg2+ was the predominant
mercury species measured at the outlet of the catalyst
for all the Illinois coal simulation tests. It appears that
the relatively high HC1 concentrations present in
these simulated flue gases provide adequate chlorine
for conversion of Hg° to Hg2+. The total Hg concen-
trations (Hgx) measured at the outlet of the catalyst
for the Galatia and Crown II tests (16.9 and 20.0±0.3
ppb, respectively) are comparable to that measured at
the inlet (19.3 ppb). However, much higher outlet
total Hg concentration (32.7 ppb) was measured for
the Turris Test. It is possible that, during this test, a
spike in the generation of Hg° might have occurred.
However, this shows that the SCR catalyst was active
enough to oxidize the relatively high spike of Hg°.
It appears that HC1 is the critical flue gas component
for converting Hg° to Hg2+ under SCR emission
control conditions. Hg2+ was measured as the pre-
dominant specie at the outlet of the SCR catalyst for
the four simulated coal tests even though these coals
had widely different HC1 and S02 concentrations.
The HC1 concentration (8 ppm) used for the PRB test
was much lower than those used for the Illinois coal
combustion tests. However, it is still much higher
than that (19 ppb) of the Hg° in the flue gas and
appears to be adequate for converting most of the Hg°
to Hg2+ in the SCR catalyst. Hg° was found to be the
predominant species for the only test that had no HC1
present in the flue gas. No Hg° oxidation observed for
this test may be due to the lack of a chlorine source.
Task II: Pilot-Scale Evaluation of SCR
Catalyst for Hg° Oxidation in the Com-
bustion of Natural Gas, Illinois, and
PRB Coal Combustion Flue Gases
Task II, Subtaskl: SCR Catalyst Evaluation in
Natural Gas Combustion
Natural gas tests were used to shakedown the system,
to determine IFR temperature profiles, and to assess
the feasibility of operating the pilot-scale SCR
reactor at temperatures around 350 °C. Another
objective of Task II, Subtask I was to assess the
change in Hg speciation across the SCR in a particu-
late-free environment. Natural gas tests were also
used to determine whether mercury speciation in the
IFR duct could be biased by use of the IGS probes.
Initial temperature profile measurements across the
SCR catalyst indicated that, at the nominal IFR firing
rate of 120,000 Btu/hr, atemperature of 350 °C could
be easily achieved in the first section of the SCR
reactor. The axial temperature gradient along the
SCR was about 80 °C, and it was determined that this
axial temperature gradient needed to be reduced.
Therefore, a layer of insulation was added to the SCR
reactor wall, and all the flanges and exposed surfaces
were insulated. The axial temperature gradient for the
insulated system was 50 °C, which was deemed to be

-------
Evaluation of SCR Catalysts
appropriate for the operation of the SCR reactor.
Ontario Hydro sampling for the natural gas tests was
performed on 06/05/03. The new insulation was not
in place during this test, and the axial temperature
profile along the SCR reactor was 80 °C. Natural gas
was combusted at a firing rate of 118,674 Btu/hr. The
excess air was calculated to be 20%. The 02 concen-
tration measured at the outlet of the SCR was 4.8%
(predicted to be 3.8%), and the measured C02 con-
centration was 8.9% (predicted to be 9.6%). Compar-
ison between the measured and the calculated values
indicated a 7% in-leakage based on C02 and a 6%
in-leakage based on 02. The flue gas compositions
measured at the inlet and outlet of the SCR reactor
were identical, indicating no in-leakage between the
inlet and the outlet of the SCR system. The total flow
(including the in-leakage) was calculated to be 29.3
scfm (0.829 scm/min). NOx and S02 were doped into
the flue gas in the vertical section, near the 2nd tem-
perature location, at 600-ppm levels. NH3 and HC1
were doped at 500- and 50-ppm levels (calculated),
respectively. HC1 was injected with S02 and NOx.
NH3 was inj ected at the entrance of the top horizontal
duct before the SCR system. The injected NH3 level
corresponded to an NH3/NOx ratio of 0.83. The SCR
catalyst was conditioned with S02 and HC1 for 4
hours prior to the test. A 90% NOx removal across
the SCR catalyst was observed. Hg° was introduced
in the vertical furnace section of the IFR via a sepa-
rate injection nozzle.
Simultaneous isokinetic OH sampling was performed
at the inlet and the outlet of the SCR catalyst. IGS
probes were not used since there were no particulates
during the natural gas combustion tests. Mercury
speciation results are shown in Table 5.
Table 5. Hg Speciation Across the SCR Catalyst
for the Natural Gas Test Day 06/05/03.
Location Hg° Hg2+ Hgl
(lg/dscm |og/dscm (ig/dscm %
SCR Inlet	722 L86 9*08 20
SCR Outlet 6.35 2.48 8.83 28
Very good total Hg recovery was obtained across the
SCR catalyst. The catalyst showed very little contri-
bution to the oxidation of Hg° (only an 8% increase).
It was hypothesized that the newly installed catalyst
may need more aging in order to achieve a higher Hg°
oxidation rate. The absence of particulates (fly ash)
might also have a bearing on the catalytic Hg° oxida-
tion reactions. A decision was made to continue with
natural gas and Illinois coal combustion tests to
induce aging in this catalyst. The natural gas combus-
tion tests indicated that the IFR can be operated
consistently and most of the injected mercury can be
recovered at the outlet of the SCR catalyst.
The doped natural gas combustion test was repeated
on 06/17/03 after the additional SCR reactor insula-
tion was installed. A mercury CEM known as PS
analytical (PSA) was used to measure Hg speciation
on this test day. Natural gas was combusted at a firing
rate of 127,630 Btu/hr. The excess air was calculated
to be 16%). The 02 concentration measured at the
SCR outlet was 4.8% (calculated to be 3.3%) and the
measured C02 concentration was 9.1% (calculated to
be 9.9%). Comparison between measured and calcu-
lated values indicated an in-leakage of 8% based on
C02 and 7% based on 02 (insignificant). There was
no in-leakage between the inlet and the outlet of the
SCR. The total flow (including the in-leakage) was
calculated to be 31.1 scfm (0.883 scmm). NOx and
S02 were doped at 600- and 700-ppm levels, respec-
tively. NH3 and HC1 were doped at 500- and 50-ppm
levels (calculated), respectively. SCR catalyst was
conditioned for 4 hours prior to the test with S02 and
HC1. A 94%) NOx removal across the SCR catalyst
was observed. Furthermore, the IGS probe was
biased-checked by connecting combustion gas
monitors to the sampling arm of the IGS probe. No
difference in measurement of 02, C02, S02, andNOx
was observed, indicating no leak in the IGS probe.
This is the standard operating procedure for the
bias-check of the IGS probe.
SCR temperatures (top, middle, and bottom) are
shown in Figure 3. As mentioned before, the more
thorough reinsulation of the SCR reactor caused a
16

-------
300 H	1	1	1	1	1	1	t	1	1	1	1	1	¦	1	1	1	1	t	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1—
^ >J CN O M CI O sj CM O Xj CM O Xl CM CD S| CM O s| CM O vj CI CD Sj CM O "-J CM O Nj CM CD vj CM CD sj CM
FncomwiNoj^^^dddcnmaicocdcoN^N^uj'Xiiriifjint^^cQCQfrjcNCNoi^^
c^ir»o,^cNCQ^rir)o^ojcoco^rinoT-(MCQ^rmo^-cMco'^ir:oT-cNcQ^]-ir)o^csJcoxr
¦ • CI> 0"» CD CD CD CD CD CD	*—¦	" t— *—¦ CM CM CM CM CM CM CO CO CO CO CO CO vj Nj vj vj vj vj Iri U"J llrj lO IjTj
-C	_	— —	— —	—	—	—	—
Time
Figure 3. SCR Temperature Profiles for the Doped Natural Gas Test Day 06/17/03; Hg
CEM (PSA) Sampling; Insulation Added Across the SCR System.
decrease in the axial temperature gradient from 80 to
50 °C (377 °C at the top and 327 °C at the bottom).
The axial temperature gradients in the first and the
second sections were 15 and 35 °C, respectively.
The mercury CEM, PSA, was connected to the IFR
duct downstream of the SCR catalyst and just up-
stream of the baghouse. Since no particulate matter
(PM) was present during the natural gas combustion,
the PSA was not connected to the IGS probe. PSA
results are shown in Figure 4. NH3 was added around
the 13:20 time frame. Addition of ammonia caused an
increase in Hg° and, thus, total Hg. This was observed
before during the bench-scale reactor shakedown
tests. Probably, addition of NH3 caused desorption of
previously adsorbed Hg°. The PSA data before the
addition of NH3 (excluding the outlier) can be com-
pared to the OH tests performed on 06/05/03. PSA
results as compared to the simultaneous isokinetic
OH samplings performed at the inlet and the outlet of
SCR catalyst are shown in Table 6.
Considering experimental variability and the fact that
OH and PSA sampling were performed on two
different test days, a relatively good agreement was
obtained. As discussed, this shows that the SCR
catalyst did not contribute to the oxidation of Hg° in
natural gas firing tests. Moreover, the PSA showed a
large variability in the measurement of Hg even when
the outlier result (see Figure 4) was not included. The
reason is unknown and may be related to the sample-
conditioning unit of the PSA.
Before proceeding to the coal tests, the potential
biases of the IGS probe toward changes in Hg specia-
tion needed to be investigated (an obj ective of natural
gas test, Subtask I of Task II). Hg speciation in the
IGS probe must be preserved in order to accurately
quantify the Hg speciation in the duct of the IFR. A
doped natural gas combustion test was performed on
17

-------
Evaluation of SCR Catalysts
E
o
 n
40
30
20
10
PSA results downstream of the SCR on 06/17/03; doped natural gas
combustion; catalyst was conditioned with HCI and S02 for 4 hours.
outlier (total Hg)
Addition ot MH3 at 5UU ppm
<	—	~
.. i
natural gas doped with NO, (S00 ppm), S02 (700 ppm),
HCI (50 ppm), and Hg
<	*¦
total Hg
elemental Hg


~~~ ~~~
+++ ~~~
7:12
8:24
0:36
10:48
12:00
Time
13:12
14:24
15:36
16:48
Figure 4. Hg CEM (PSA) Measurement Results for the Doped Natural Gas Test Day
06/17/03.
Table 6. Hg Speciation Across the SCR Catalyst for the Doped Natural Gas Combustion Test
Days; Comparison between OH and PSA.
Location
Hg°
Hg2+
HgT
Hg2+
|Xg/dscm
(lg/dscm
|ig/dscm
%
7.22
1.86
9.08
20
6.35
2.48
8.83
28
7.8±1.4
	
13.4±3.0
12-60
SCR inlet, OH sampling, 06/05/03
SCR outlet, OH sampling, 06/05/03
SCR outlet. PSA sampling. 06/17/03
test day 06/18/03 to investigate potential Hg specia-
tion biases created by the IGS probe. Combustion
conditions were similar to those obtained on test day
06/17/03 (see above). The mercury CEM, PSA, was
initially connected to the IFR duct downstream of the
SCR catalyst and just upstream of the baghouse.
After two hours of sampling from the duct (from 8:38
to 10:46), the PSA was connected to the IGS probe.
The PSA results are shown in Figure 5. As seen in
this figure, the Hg speciation was preserved through
the IGS probe, indicating that the IGS probe does not
affect the Hg speciation in the duct. It should be
noted that the IGS probe temperature (measured at
the surface) was maintained at 240-250 °F (116-121
°C). Previous unpublished results have shown that, at
higher temperatures [390 °F (200 °C)], the stainless
steel surfaces of the IGS probe causes catalytic
oxidation of Hg° to Hg2+. Thus, it is crucial to main-
tain the temperature of the IGS probe in the low
temperature range mentioned above. During this test,
18

-------
Evaluation of SCR Catalysts
PSA results on 06(18/03; doped natural gas combustion; catalyst was conditioned with HCI and S02
the night before; bias check of the IGS probe and the effect of HCI on Hg oxidation by the SCR catalyst.
40
35
30
E
u
t/J 25
i. 20
I 15
10
5
0
7:12	8:24	0:3G	10:48	12:00	13:12	14:24	15:36	16:48
Time
Figure 5. Hg CEM (PSA) Measurement Results for the Doped Natural Gas Test Day
06/18/03; IGS Probe Hg Speciation Bias Check and the Effect of HCI on Hg°
Oxidation.
GuJier (Hg total)
PSA sampling from IGS probe HCI increased by a factor of 2
	~ from 50 to 100 ppm nominally
•«	~
PSA sarn pi ing from the duct
1	*	¦	total Hg
(*	*		~»»
~~~ ~'
elemental Hg»4	*	
an attempt was also made to assess the effect of
increased HCI concentration on Hg° oxidation by the
SCR reactor. HCI concentration was increased from
50 to 100 ppm. Figure 5 shows that the increase in
HCI concentration in the combustor had no effect on
speciation of Hg, confirming that the SCR catalyst
does not contribute to the oxidation of Hg° in natural
gas firing tests. Note that the catalyst was conditioned
with S02 and HCI for more than 24 hours.
Task II, Subtask II: SCR Catalyst Evaluation
in Illinois and PRB Coal Combustion
Galatia Coal Combustion
Galatia coal was tested under combustion conditions
listed in Table 7 (in the Galatia column). Galatia coal
was combusted at a firing rate of about 140,000
Btu/hr. Comparison between measured and calculated
values of C02 and 02 indicated an insignificant
in-leakage of air into the combustor (about 4%).
There was no in-leakage between the inlet and the
outlet of the SCR. A very good combustion of Galatia
coal was observed as indicated by low CO concentra-
tion (30±7 ppm) measured in the flue gas. The axial
temperature gradient across the SCR catalyst was
around 50 °C (change from 354 to 304 °C). Uncon-
trolled levels ofNOx were around 850 ppm. NH3 was
injected at a nominal NH3/NOx ratio of 0.9. NOx
conversion around the SCR for the combustion of
Galatia coal was about 86%. S02 concentration for
this low sulfur Illinois coal was measured to be
929±63 ppm. The calculated level of S02 under the
combustion conditions was 915 ppm, reflecting very
good agreement with measured values.
Simultaneous isokinetic and IGS probe OH sampling
were performed at the inlet and outlet of the SCR
reactor (i.e., upstream of the baghouse) for the
Galatia test. The isokinetic trains measure all three
forms of mercury, Hg°, Hg2+, and Hgp, and Hgx can
be calculated from the isokinetic train measurement.
When sampling upstream of the baghouse, fly ash
19

-------
Evaluation of SCR Catalysts
Table 7. Combustion Conditions for the Illinois Turris, Galatia, and Crown II Coals and the Black
Thunder PRB Coal.
Parameter
Turris
Galatia
Crown II
PRB
Coal feed rate lb/hr
14.0
11.5
13.4
15.7
IFR firing rate, Btu/hr
147,540
140,424
145,208
155,873
Total air flow, scfm
28.6
28.2
28.4
28.9
Excess air, %
11
18
11
5
Top SCR temperature, °C
373±8
354±1
363±4
380±2
Middle SCR temperature, °C
365±5
342±1
352±3
364±1
Bottom SCR temperature, °C
323±4
304±2
315±2
325±1
Measured CO, ppm (dry)
38±6
30±7
40±4
0±1
Uncontrolled NOx, ppm (dry)
960
850
650
525
NOx conversion across SCR, %
90
86
90
90
Measured S02, ppm (dry)
2921±49
929±63
2739±26
222±2
Calculated S02, ppm (dry)
3064
915
3006
274
Measured 02, % (dry)
2.7
4.0
4.4
4.1
Calculated 02, % (dry)
1.8
3.0
1.9
0.1
In-leakage based on 02, %
5
5
13
19
Measured C02, % (dry)
15.6
14.8
14.3
15.6
Calculated C02, % (dry)
16.0
15.4
15.7
18.0
In-leakage based on C02, %
3
3
9
13
Calculated flow based on 02 in-leakage, scm/min
0.879
0.896
0.944
1.008
Calculated flow based on C02 in-leakage, scm/min
0.858
0.853
0.911
0.856
Measured HC1, ppm (wet)
NMa
246±23
NM
NM
Calculated HC1, ppm (wet)
141
208
96
7.9
SCR inlet PM concentrationb, mg/dscm
5863
3070±555
3946
2418±504
SCR outlet PM concentration13, mg/dscm
3506
1735±194
2560
1606±160
a NM = not measured.
b Measured using the filter weight of the isokinetic OH method at the inlet and outlet of the SCR.
collected in the filter of the OH train may cause
catalytic reaction or retention of mercury, resulting in
a change in mercury speciation. Therefore, the IGS
probe can be used for sampling gas-phase Hg° and
Hg2+, and the difference between Hgx obtained by the
isokinetic train and the gas-phase Hg° and Hg2+
obtained by the IGS probe can be used to estimate the
amount of HgP. During the initial coal combustion
test, the total axial flow for the IGS probe installed at
the inlet and the outlet of the SCR reactor dropped
from between 7 and 8 to below 2.7 scfm. The total
axial flow of the inlet of the IGS probe dropped from
5 to 3 scfm. This drop in the flow might have caused
retention of particulate matter (PM, fly ash) in the
probes. This might have created an unwanted packed
bed in the IGS probe, thus positively biasing the HgP
results, although the results of the 2nd test (see below)
showed that this was not the case. The problem of
drop in sampling flow was corrected by installing an
air pump at the outlet for each of the IGS probes. The
combination of the pump and the eductor (operated
with compressed air) maintained the desired level of
axial flow through the IGS probes. Biases created by
packed beds of fly ash are not expected because the
20

-------
Evaluation of SCR Catalysts
high axial flow prevents formation of such packed
beds in the probe. The total axial flow in the IGS
probes was maintained at around 7-8 scfm.
Results of the simultaneous isokinetic and IGS probe
OH sampling for the Galatia test are summarized in
Tables 8 and 9. The flow through the IGS probes was
maintained at around 8.7 scfm. Relatively good Hgx
recovery was obtained across the SCR catalyst. The
Hgx at the outlet was slightly lower than the inlet (7.2
vs. 9.9 |ig/dscm). This could be due to adsorption of
Hg by the SCR catalysts.
Comparison of the data obtained by using the iso-
kinetic trains and those obtained by using IGS probes
suggests that a considerable portion of the Hg in the
Galatia coal combustion may be associated with the
PM at the inlet of the SCR catalyst. However, the
adsorption of mercury by fly ash is unlikely to be
significant at the SCR inlet temperatures (354 °C),
which is supported by recent field data that showed
very little HgP at the inlet of various SCR systems
firing different types of bituminous coals (Brickett et
al., 2003). Deposition of white powder was observed
on the walls of the first impinger connected to the
IGS probes, indicating formation of fine particulate
aerosols near the inlet of the impinger. Much more
white powder was deposited in the impinger used for
measuring the inlet of the SCR reactor than in the
impinger connected to the outlet train. Ammonium
salts formed near the inlet of the first impinger
connected to the IGS probe as the NH3-containing
flue gas cooled down, which could retain gas-phase
Hg and result in changes in Hg speciation. It is well
known that the reaction of NH3 with sulfuric acid
(H2S04) condensed from S03 as the coal combustion
flue gas cools down produces ammonium sulfate and
bisulfate salts, which could cause severe air pre-
heater plugging problems (Burke and Johnson, 1982).
The capture of Hg° by the aerosols generated by the
reactions of NH3 and S03 under flue gas cooling
conditions has been demonstrated by a recent
bench-scale study (Lee, et al., 2003). The NH3
concentration in flue gas decreased from about 850
Table 8. Galatia Coal Combustion Test, OH Sampling Results Measured at the Inlet of the SCR
Reactor.
Inlet OH Train
Hg°
Hg2+
HgP
HgT
Hg°
Hg2+
HgP
|i,g/dscm
|i,g/dscm
|i,g/dscm
|i,g/dscm
%
%
%
Isokinetic
7.2
2.1
0.61
9.9
73
21
6
IGS probe
1.7
3.9
—
—
—
—
—
Calculated inlet Ha SDCciatioiV
1.7
3.9
4.3
9.9
17
40
43
a Calculated assuming that isokinetic train is used for measuring Hgx and IGS probe is used for measuring Hg° and Hg2+.
Table 9.	Galatia Coal Combustion Test, OH Sampling Results Measured at the
Outlet of the SCR Reactor.
^ ... T	H§0	H§2+ HgP	HgT Hg° Hg2+ HgP
Outlet OH Train	6	,	,	? 0, 0f
[Xg/dscm p,g/dscm p,g/dscm p,g/dscm %	%	%
Isokinetic
0.32
5.1
1.8
7.2
4
71
25
IGS probe
0.1
5.4
—
—
—
—
—
Calculated outlet Hg
SDeciationa
0.1
5.4
1.7
7.2
1
75
24
a Calculated assuming that isokinetic train is used for measuring Hgx and IGS probe is used for measuring Hg° and Hg2+.
21

-------
Evaluation of SCR Catalysts
ppm at the inlet of the SCR reactor to a few ppm at
the reactor outlet as a result of the NOx reduction
reactions that consumed NH3. Thus, the potential for
deposition of ammonium salts in the sampling system
used at the inlet of the SCR reactor is significantly
higher than that used at the reactor outlet. The dis-
crepancies in Hg speciation measured by the iso-
kinetic OH train and the calculated speciation are
much larger for the SCR inlet case (Table 8) than
those for the outlet case (Table 9). The larger discrep-
ancies for the inlet case are consistent with more
white powder deposition found in the first impinger
of the inlet IGS probe. During the initial shakedown
tests of the bench-scale reactor (Task I), white pow-
ders were found deposited near the inlet of an on-line
mercury analyzer connected to the SCR outlet for
continuously measuring Hg°, indicating aerosol for-
mation. The white powder deposits were observed
only when NH3 was added to the simulated flue
gases, and the data measured by using the analyzer
showed continuous drift as the deposit was accumu-
lating during the test. Therefore, the OH sampling
method was used for the Task I tests in order to avoid
the measuring problem created by the aerosol forma-
tion when using the mercury analyzer. It appears that
the formation of ammonium salts in the Hg speciation
sampling/measurement system in the presence of NH3
and under cooling conditions may cause significant
bias of the speciation measurement. Further research
is needed for applying the IGS probe to measure Hg
speciation in SCR systems. Therefore, it was decided
to not use IGS probe in this work.
The OH sampling results show very good oxidation
of Hg° across the SCR catalyst. Hg° was decreased
from 73% to 4% and Hg2+ was increased from 21% to
7P/o across the SCR catalyst. The change in Hg
speciation from predominantly Hg° at the inlet of the
catalyst to predominantly Hg2+ at the outlet was
observed in SCR systems on boilers firing bituminous
coals (Brickett, et al., 2003). The isokinetic OH filter
weights can be used to determine the PM concentra-
tion in the IFR. The average PM concentrations for
the Galatia coal combustion at the inlet and the outlet
of SCR reactor are shown in Table 7. The lower PM
concentration at the outlet may be due to the opera-
tion of the IGS probe at the inlet. This probe draws a
high flow from the duct and, thus, may preferentially
take higher amounts of PM from the flue gas.
Turris Coal Combustion
Turris coal was combusted at a firing rate of about
148,000 Btu/hr. Combustor air in-leakage was about
4%. There was no in-leakage between the inlet and
the outlet of the SCR. A very good combustion of
Turris coal was observed as indicated by the low CO
emissions (38 ppm). The axial temperature gradient
across the SCR catalyst was around 50 °C (change
from 373 to 323 °C). Slightly higher temperatures
were obtained around the SCR catalyst as compared
to the Galatia coal combustion test. Uncontrolled
levels of NOx were around 950 (higher than Galatia).
NH3 was injected at an NH3/NOx ratio of about 0.9.
NOx conversion across the SCR was about 90%>. S02
concentration for this high sulfur Illinois coal was
measured to be 2921±49 ppm. The calculated level of
S02 under the combustion conditions was 3046 ppm,
reflecting good agreement with measured values.
Table 10 summarizes the results of the OH sampling
Table 10. Turris Coal Combustion Test, OH Sampling Results Measured at the Inlet
and Outlet of the SCR Reactor.
Location	Hg°	Hg2+	Hgp	H§T	Hg° Hg2+ Hg"
|Xg/dscm ng/dscm |lg/dscm |j,g/dscm %	%	%
Inlet 4*7 06 L5 6*9 68	9 23
Outlet	03	M	04	3J	9 79 12
22

-------
Evaluation of SCR Catalysts
for the Turns coal test at the inlet and the outlet of the
SCR reactor. Only about half of the inlet Hgx was
recovered at the outlet. This could be due to the
adsorption of mercury by the catalyst. Very little Hg
was associated with the particulate matter. The SCR
catalyst exhibited a strong Hg° oxidation capability,
dropping the amount of Hg° from 68% at the inlet to
9% at the outlet. The Hg2+ was increased from 9% to
79% at the two sampling locations. Such significant
changes in Hg speciation are in agreement with those
observed in the field for bituminous coal-fired SCR
systems (Brickett, et al., 2003).
Crown II Coal Combustion
Crown II coal was combusted at a firing rate of about
145,000 Btu/hr. Combustor air in-leakage was about
11%). It appears that, as the combustor operated for a
longer period of time, air in-leakage increased, but
this level of air in-leakage is considered insignificant.
There was no air in-leakage between the inlet and the
outlet of the SCR. A very good combustion of Crown
II coal was indicated by low measured CO emissions
(40 ppm). The axial temperature gradient across the
SCR catalyst was around 48 °C (change from 363 to
315 °C). SCR temperatures were within the range for
those obtained during the other two Illinois coal tests.
Uncontrolled levels of NOx were about 650 ppm,
lower than those for the Galatia and Turris tests. NH3
was injected at a calculated NH3/NOx ratio of about
0.8. NOx conversion across the SCR was about 90%>.
S02 emissions for this high sulfur Illinois coal tests
measured 2739±26 ppm. The calculated level of S02
emission under these test conditions was 3006 ppm.
Table 11 summarizes the results of the OH sampling
for the Crown II coal combustion test at the inlet and
the outlet of the SCR reactor. Relatively low Hgx
recovery was obtained across the SCR catalyst. The
Hgx concentration measured at the outlet was lower
than that measured at the inlet (4.1 vs. 5.5 |ig/dscm).
The lower HgT measured at the outlet of the SCR
catalyst may be due to the adsorption of Hg by the
catalyst. Very little Hg was associated with the PM.
This was also found during the combustion of Turris
coal. Crown II and Turris coals are very similar in
their properties (see Table 1). Galatia was the only
Illinois coal that generated relatively high HgP during
its combustion. The SCR catalyst exhibited a very
good Hg° oxidation capability. Hg° was reduced from
84%o at the inlet down to 12%> at the outlet, with the
increase in Hg2+ from 15%> to 85%> across these two
sampling locations. The changes in Hg speciation
across the SCR catalyst observed for the Crown II
coal combustion are consistent with those of the two
other Illinois coal tests and also those of the bitumi-
nous coal-fired SCR systems observed in field
(Brickett, et al., 2003).
PRB (Black Thunder) Coal Combustion
Black Thunder PRB coal was combusted in duplicate
runs. The combustion conditions were identical and
are listed in Table 7 under the PRB column. PRB coal
was combusted at a firing rate of about 156,000
Btu/hr. This firing rate was slightly higher than those
used for the Illinois coal tests. In-leakage of air into
the combustor was about 16%>. Similar to the Crown
II test, as the combustor operated for a longer period
of time, in-leakage seemed to increase, but the levels
of air in-leakage are considered to be acceptable.
There was no in-leakage between the inlet and the
Table 11. Crown II Coal Combustion Test, OH Sampling Results Measured at the Inlet
and Outlet of the SCR Reactor.
Location	Hg°	Hg2+	Hgp	H§T	Hg° Hg2+ Hg"
|Xg/dscm ng/dscm |lg/dscm |j,g/dscm %	%	%
Inlet 4*6 08 01 ~ 84 15 1
Outlet	05	3J	01	4J	12 85	3
23

-------
Evaluation of SCR Catalysts
outlet of the SCR. A very good combustion of the
PRB coal was observed although no CO emissions
were measured during the two combustion tests. The
axial temperature gradient across the SCR catalyst
was about 55 °C (change from 380 to 325 °C). SCR
temperatures were slightly higher than those observed
during Illinois coal tests, possibly due to slightly
higher firing rate. Uncontrolled levels of NOx emis-
sions were about 525 ppm, which are lower than
those measured for the Illinois coal tests. NH3 was
injected at a calculated NH3/NOx ratio of about 0.8.
NOx conversion across the SCR was about 90%. S02
emissions for the combustion of this low sulfur coal
was measured to be 222±2 ppm. The calculated level
of S02 under the combustion conditions was 274
ppm, indicating good agreement with the measured
values.
Tables 12 and 13 summarize the results of the OH
sampling for the first and second PRB coal combus-
tion tests, respectively. A good Hgx recovery was
obtained across the SCR catalyst during the second
PRB test (5.1 |ig/dscm at the inlet vs. 4.6 |ig/dscm at
the outlet), which was better than those obtained
during the first test (7.2 |ig/dscm at the inlet vs. 5.1
|ig/dscm at the outlet). The inlet Hgx obtained during
the first test (7.2 |ig/dscm) is also significantly higher
than that measured at the same location for the
second test, which may be an outlier. It appears that
almost all Hg was in elemental form at the inlet with
no appreciable Hg2+ or HgP. The comparison of data
measured between inlet and outlet of the catalyst
shows only a very small drop in Hg°, from 96% to
76%) for the second test and from 97% to 88% for the
first test. This is equivalent to only a 20% oxidation
of Hg° (9%> oxidation of Hg° for the first test). Field
sampling conducted to date indicates the same Hg
speciation profile as shown by this study, mostly Hg°
at the inlet and very low Hg° oxidation. PRB coals in
general have significantly lower chlorine and higher
alkaline contents than those of the bituminous coals.
The low oxidation of Hg° across the SCR catalyst
observed for the two PRB coal tests supports the
hypothesis that HC1, which serves as the chlorine
source, is critical in Hg° oxidation across the SCR
catalyst. The importance of HC1 in Hg oxidation
across the SCR catalyst had been demonstrated in
Task I.
Table 12. First PRB Coal Combustion Test, OH Sampling Results Measured at the
Inlet and Outlet of the SCR Reactor.
Location
Hg°
|Xg/dscm
Hg2+
|Xg/dscm
HgP
|Xg/dscm
HgT
|Xg/dscm
Hg°
%
Hg2+
%
HgP
%
Inlet
7.0
0.2
0.0
7.2
97
3
0
Outlet
4.5
0.5
0.1
5.1
88
10
2
Table 13. First PRB Coal Combustion Test, OH Sampling Results Measured at the
Inlet and Outlet of the SCR Reactor.
Location
Hg°
|Xg/dscm
Hg2+
|Xg/dscm
HgP
|Xg/dscm
HgT
|Xg/dscm
Hg°
%
Hg2+
%
HgP
%
Inlet
4.9
0.2
0.0
5.1
96
4
0
Outlet
3.5
1.0
0.1
4.6
76
22
2
24

-------
Evaluation of SCR Catalysts
Conclusions and Recommendations
A two-task, bench- and pilot-scale research study was
conducted to evaluate the viability of SCR-based
mercury control technology utilizing oxidation of Hg°
to Hg2+ and subsequent removal by conventional
FGD systems. Mercury speciation modification was
implemented through utilization of SCR catalysts,
currently used for NOx control. The effect of SCR
catalysts on mercury speciation was investigated in
Illinois and PRB coal combustion flue gases. This
research was designed to demonstrate the achievable
Hg2+ levels in Illinois and PRB coal combustion
processes utilizing SCR systems.
Task I involved bench-scale testing of a SCR catalyst
formulation to characterize oxidation of Hg° to Hg2+
in the presence of coal combustion/SCR flue gas
species such as NOx, NH3, S02, and HC1. The NOx,
NH3, HC1, and S02 concentration ranges employed
during Task I were based on those expected to be
encountered in flue gases resulting from firing three
representative Illinois coals (low to high sulfur and
chlorine) and one PRB coal (very low sul-
fur/chlorine). Task I results showed that HC1 (with
concentrations as low as 8 ppm) is the critical flue
gas component for converting Hg° to Hg2+ under SCR
emission control conditions. Hg2+ was measured as
the predominant species at the outlet of the SCR
catalyst for all the simulated Illinois and PRB coal
tests. Task I tests established the Hg° oxidation
activity of the formulated SCR catalyst.
Pilot-scale Task II tests were performed using a SCR
catalyst installed in a vertical down-ward flow
configuration in the post-combustion region of a
pilot-scale 34.9 kW (150,000 Btu/hr), refractory
lined, down-fired cylindrical furnace. Three different
Illinois coals and one PRB coal were combusted in
the pilot-scale facility. For the high sulfur/low chlo-
rine Illinois coals, about 84% to 68 % Hg°, and 15%
to 9% Hg2+ were measured at the inlet of the pilot-
scale SCR reactor. The SCR catalyst induced high
oxidation of Hg°, dropping the percentage of Hg° at
the outlet of the SCR to values below 16%. The low
sulfur/high chlorine Illinois coal had a relative high
amount (21%) of Hg2+ at the inlet of SCR. The Hg°
content of this coal flue gas was decreased from
about 73% at the inlet to about 4% at the outlet of
SCR reactor. These results indicate that installation of
SCR systems in Illinois coal-fired boilers that are
equipped with a wet FGD system may achieve about
85%) to greater than 90% control of mercury.
The PRB coal tests indicated a low oxidation of Hg°
by the SCR catalyst. Based on the measurements
conducted in this study, it appears that SCR applica-
tions on PRB (Black Thunder) coal-fired boilers may
not result in significant increase in Hg2+ content of
flue gas. Therefore, such boilers equipped with wet
FGD systems may not achieve increased mercury
removal resulting from SCR applications.
The following recommendation are made based on
the results of this one-year study:
•	For most typical Illinois coals (high sulfur and
medium to low chlorine), installation of SCR
catalysts prior to FGD systems should be very
beneficial. In addition to achieving a 90% reduc-
tion in NOx, about 85% to greater than 90% reduc-
tion in mercury may be possible.
•	A particular low sulfur/high chlorine Illinois coal
(Galatia) showed relatively high Hg2+ content at the
SCR inlet (21%); the rest was predominantly Hg°
(73%>) and very little HgP. However, the Hg con-
tents at the SCR outlet were changed to 71% Hg2+,
25%) HgP, and very little Hg°. Based on these data,
25

-------
Evaluation of SCR Catalysts
installation of an SCR catalyst may results in
greater than 90% reduction of Hg across the PM
control and the wet FGD systems.
•	The benefit of installing a SCR catalyst for the
tested PRB coal (Black Thunder) appears to be
very limited. Installation of an SCR catalyst could
only increase the mercury removal to a maximum
of 20%.
•	Further tests are required to evaluate the long-term
activity of the tested SCR catalysts in both Illinois
and PRB coal combustion flue gases.
• Tests should be conducted to evaluate the effect of
SCR catalyst space velocity on Hg° oxidation in
Illinois and PRB coal combustion flue gas. Higher
space velocities (greater than 3000 hr-1) should be
tested to demonstrate further cost-effectiveness of
this technology.
26

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Evaluation of SCR Catalysts
References
Brickett, L., P. Chu, C.W. Lee, R.K. Srivastava, D.
Laudal, J. Thompson, and C. Wocken, 2003."Impact
of SCR on Mercury Speciation for Coal-fired Boil-
ers." Presented at the Combined Power Plant Air
Pollutant Control Mega Symposium, Washington,
DC, May 19-22.
Burke, J.M., and K.L. Johnson, 1982. "Ammonium
Sulfate and Bisulfate Formation in Air Preheaters."
EPA Report number EPA-600/7-82-025a (NTIS
PB82-237025), April.
Cichanowicz, J.E., and L.J. Muzio, 2001. "Twenty-
Five Years of SCR Evolution: Implications For U.S.
Application And Operation." Presented at the MEGA
Symposium, Chicago, IL, August 20-23.
DeVito, M.S., P.R. Tumati, R.J. Carlson, and N.
Bloom, 1993. "Sampling and Analysis of Mercury in
Combustion Flue Gas." Presented at the Second
International Conference on Managing Hazardous Air
Pollutants, Washington, D.C., July 13-15.
EPRI, 2000. "Pilot-Scale Screening Evaluation of the
Impact of Selective Catalytic Reduction for NOx on
Mercury Speciation." EPRI Report No. 1000755,
December.
Felsvang, K., R. Gleiser, G. Julip, and K. Kragh
Nielson, 1992. "Control of Air Toxics by Dry FGD
Systems." Presented at the Power-Gen '92 Confer-
ence, Orlando, FL, November 17-19.
Felsvang, K., R. Gleiser, G. Julip, K. Kragh Nielsen,
1993. "Air Toxics Control by Spray Dryer." Pre-
sented at the 1993 S02 Control Symposium, Boston,
MA, August 24-27.
Galbreath, K.C., and C.J. Zygarlicke, 2000. "Mercury
Transformation in Coal Combustion Flue Gas." Fuel
Process. Technol., 65-66, pp. 289-310.
Ghorishi, S.B., C.W. Lee, and J.D. Kilgroe. 1999.
"Mercury Speciation in Combustion Systems: Studies
with Simulated Flue Gases and Model Fly Ashes."
Paper # 99-651 Presented at the 92nd Annual Meet-
ing of Air and Waste Management Association, St.
Louis, MO, June 20-24.
Ghorishi, S.B., 1998. "Fundamentals of Mercury
Speciation and Control in Coal-Fired Boilers." EPA
Report number EPA-600/R-98-014 (NTIS PB98-
127095), February.
Ghorishi, S.B., andB.K. Gullett, 1998. "Sorption of
Mercury Species by Activated Carbons and Cal-
cium-Based Sorbents: Effect of Temperature, Mer-
cury Concentration and Acid Gases." Waste Manage-
ment & Research, 16 (6), pp. 582-593.
Gutberlet, H., A. Schliiten, and A. Lieutal, 2000.
"SCR impacts on Mercury Emissions on Coal-Fired
Boilers." Presented at EPRI Workshop, Memphis,
TN, April.
Hall, B., P. Schager, and O. Lindqvist, 1991. "Chem-
ical Reactions of Mercury in Combustion Flue
Gases." Water, Air, and Soil Pollution, 56, 3-14.
Krishnan, S.V., B.K. Gullett, and W. Jozewicz, 1994.
"Sorption of Elemental Mercury by Activated Car-
bons." Env. Sci. & Tech. 28 (8), pp. 1506-1512.
Krishnan, S.V., H. Bakhteyar, and C.B. Sedman,
1996. "Mercury Sorption Mechanisms and Control by
Calcium-Based Sorbents." Paper 96-WP64B.05
27

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Evaluation of SCR Catalysts
presented at the 89th Air & Waste Management
Association Annual Meeting, Nashville, TN, June
23-28.
Laudal, D.L., C.R. Wocker, P. Chu, L.A. Brickett,
and C.W. Lee, 2003. "Evaluation of the Effect of
SCR on Mercury Speciation and Emissions." Pre-
sented at the Air Quality IV Conference, Arlington,
VA, September 22-24.
Lee, Y.P., S.J. Khang, and T.C. Keener, 2003.
"Mercury Removal from Flue Gas with Aerosols
Generated by S03-NH3 Reactions." Paper 69342
presented at the 96th Annual Meeting and Exhibition,
AWMA, San Diego, CA, June 22-26.
Linak, W.P., and J.O.L. Wendt. 1993. "Toxic Metal
Emissions from Incineration: Mechanisms and Con-
trol." Progress in Energy & Combustion Science, 19,
pp. 145-185.
Markowski, G.R., and R. Filby. 1985. "Trace Ele-
ment Concentration as a Function of Particle Size in
Fly Ash from a Pulverized Coal Utility Boiler." Env.
Sci. Tech., 19 (9), 796-804.
Senior, C.L., E.L., Bool, G.P., Huffman, F.E.,
Huggins, N., Shah, A., Sarofim I., Olmez, and T.
Zeng. 1997. "A Fundamental Study of Mercury
Partitioning in Coal-Fired Power Plant Flue Gas."
Paper 97-WP72B.08 presented at the 90th Air &
Waste Management Association Annual Meeting,
Toronto, Ontario, Canada, June 8-13.
Volland, C. 1991. "Mercury Emission from Munici-
pal Solid Waste Combustion." Paper # 91-35.1
presented at the 84th Annual Meeting and Exhibition,
AWMA, Vancouver, BC, June 16-21.
28

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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/R-04/130
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Evaluation of SCR Catalysts for Combined Control of NOx and
5. REPORT DATE
September 2004
Mercury


6. PERFORMING ORGANIZATION CODE
7. AUTHORS
R.K. Srivastava
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
See Block 12.

10. PROGRAM ELEMENT NO.



11. CONTRACT/GRANT NO.
In-house
12. SPONSORING AGENCY NAME AND ADDRESS
U. S. EPA, Office of Research and Development

13. TYPE OF REPORT AND PERIOD COVERED
Final; 09/01/02-08/31/03
Air Pollution Prevention and Control Division
Research Triangle Park, North Carolina 27711

14. SPONSORING AGENCY CODE
EPA/600/13
15. SUPPLEMENTARY NOTES
The EPA Project Officer is Ravi K. Srivastava, mail drop E305-01, phone (919) 541-3444, e-mail
srivastava.ravi@epa.gov.
16. ABSTRACT
The report documents two-task, bench- and pilot-scale research on the effect of selective catalytic reduction
(SCR) catalysts on mercury speciation in Illinois and Powder River Basin (PRB) coal combustion flue
gases. In task I, a bench-scale reactor was used to study the oxidation of elemental mercury (Hg°) in
simulated Illinois and PRB coal flue gases by a vanadium/titanium SCR catalyst. Elemental mercury
oxidation on the order of 90% was observed in all the simulated coal flue gases. It was shown that
hydrogen chloride (HCI) may be the critical flue gas component that causes the conversion of Hg° to
oxidized mercury (Hg2+) under SCR reaction conditions. Since high HCI concentrations are expected during
combustion of all Illinois coals, firing these coals should result in significant Hg° oxidation occurring across
SCR reactors. Based on bench-scale results, an appropriate SCR catalyst was produced in pilot-scale and
installed in a pilot-scale combustor for Task II studies. Three different Illinois coals (from high to low sulfur
and chlorine) and one PRB coal were combusted in the pilot-scale unit. For the high sulfur/low chlorine
Illinois coals, about 68%-84% Hg° was measured at the inlet of the pilot-scale SCR reactor and below 16%
Hg° at the outlet of the SCR. Flue gas from the combustion of low sulfur/high chlorine Illinois coal had 21%
Hg2+ at the inlet of the SCR, and the Hg° content was decreased from about 73% at the inlet of the SCR
reactor to about 4% at the outlet. For the PRB coal tests, the percentage of Hg° decreased from about 96%
at the inlet of the SCR reactor to about 80% at the outlet.
17.
KEYWORDS AND DOCUMENT ANALYSIS


a. DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Coal
Combustion
Exhaust Gases
Mercury (metal)
Oxidation Reduction Reactions
Catalysis
Pollution Control
Stationary Sources
13B
08G, 21D
21B
14G
07 B
07C
07 D
18. DISTRIBUTION STATEMENT

19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
36
Release to Public

20. SECURITY CLASS (This Page)
Unclassified
22. PRICE
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION IS OBSOLETE
forms/admin/techrpt.frm 7/8/99 pad

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