E PA-600/R-98-014
Envirortmerr.al Protection
Agency Februarv 1998
<&EPA Research and
Development
FUNDAMENTALS OF MERCURY
SPECIATION AND CONTROL
IN COAL-FIRED BOILERS
Prepared for
Office of Air Quality Planning and Standards
Prepared by
National Risk Management
Research Laboratory
Research Triangle Park, NC 27711
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FOREWORD
The U.S. Environmental Protection Agency is charged by Congress with pro-
tecting the Nation's land, air, and water resources. Under a mandate of national
environmental laws, the Agency strives to formulate and implement actions lead-
ing 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 pro-
blems today and building a science knowledge base necessary to manage our eco-
logical resources wisely, understand how pollutants affect our health, and pre-
vent or reduce environmental risks in the future.
The National Risk Management Research Laboratory is the Agency's center for
investigation of technological and management approaches for reducing risks
from threats to human health and the environment. The focus of the Laboratory's
research program is on methods for the prevention and control of pollution to air,
land, water, and subsurface resources; protection of water quality in public water
systems; remediation of contaminated sites and groundwater; and prevention and
control of indoor air pollution. The goal of this research effort is to catalyze
development and implementation of innovative, cost-effective environmental
technologies; develop scientific and engineering information needed by EPA to
support regulatory and policy decisions; and provide technical support and infor-
mation transfer to ensure effective implementation of environmental regulations
and strategies.
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 Re-
search and Development to assist the user community and to link researchers
with their clients.
E. Timothy Oppelt, Director
National Risk Management Research Laboratory
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.
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EPA-600/R-98-014
February 1998
Fundamentals of Mercury Speciation and Control in Coal-Fired Boilers
Prepared By:
S. Behrooz Ghorishi
ARCADIS Geraghty & Miller, Inc.
4915 Prospectus Drive
P.O. Box 13109
Research Triangle Park, NC 27709
EPA Contract No. 68-D4-0005
Work Assignment 3-050
EPA Work Assignment Manager:
Chun Wai Lee
Air Pollution Prevention and Control Division
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
Prepared for:
U.S. Environmental Protection Agency
Office of Research and Development
Washington, DC 20460
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ABSTRACT
Understanding the transformation of mercury species in coal combustion flue gases is of
interest to both those responsible for developing control technologies and those responsible for
setting regulations on mercury emissions. Mercury speciation in simulated coal combustion flue
gases was investigated. The effects of coal fly ash and flue gas parameters on the oxidation of
elemental mercury (Hg°) in the presence of hydrogen chloride (HC1), in a simulated post-
combustion region, and in the baghouse portion of air pollution control systems (APCS) were
studied. 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 high temperatures
(>700°C) and high HC1 concentrations (>200 ppm). The presence of sulfur dioxide (SO2) and
water vapor in the simulated flue gas significantly inhibited the gas-phase oxidation of Hg° in the
presence of HC1. On the other hand, a preliminary investigation indicated that gas-phase reaction
of Hg° with chlorine (C12) is fast. At 40 °C and in the presence of 50 ppmv C12, 100% of the input
Hg° was oxidized in the gas-phase within 2 seconds of residence time. These results indicate that
C12 is a much more active chlorinating agent than HC1. The effects of coal fly ash components
and compositions were investigated using a fixed-bed of model or simulated fly ashes. The
primary focus was on evaluating the catalytic Hg° oxidation activity of major mineral
constituents of coal fly ashes: alumina (A12O3), silica (SiO2), iron (III) oxide (Fe2O3), copper (II)
oxide (CuO), and calcium oxide (CaO). Copper and iron oxides were the only two components
that exhibited significant catalytic activity toward a surface-mediated oxidation of Hg°. The
observed catalytic activities were hypothesized to be effected through the formation of a
chlorinating agent (most probably C12) from gas-phase HC1 on the surface of metal oxides (the
Deacon process reaction). Copper was a much more active catalyst than iron, and its catalytic
activity was less influenced by the presence of oxidation inhibitors (SO2 and water vapor) in the
simulated flue gas. The presence of small quantities of CuO (0.1% wt) in the model fly ash
caused a 95% oxidation of Hg° in the temperature range of 150-250°C. The same extent of Hg°
oxidation was obtained by adding 14% (wt) Fe203 to the model fly ash.
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TABLE OF CONTENTS
Page
Abstract ii
List of Tables iv
List of Figures iv
Aknowledgments v
1. Introduction 1
2. Experimental Procedure 4
3. Results and Discussion 5
3.1 Homogeneous Gas-Phase Oxidation of Hg° 5
3.2 Heterogeneous Catalytic Oxidation of Hg° 8
4. Conclusions 12
5. Future Research (Phase II) 13
6. References 15
in
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List of Tables
Page
Table 1 Model Fly Ash Compositions 18
List of Figures
Page
Figure 1 Schematic of the elemental mercury oxidation reactor system 19
Figure 2 Effects of temperature and HC1 concentration on the gas-phase oxidation 20
of Hg° in the absence of SO2 and water vapor
Figure 3 Effects of SO2 and water vapor on the gas-phase oxidation of Hg° at 21
754 °C and three different HC1 concentrations
Figure 4 Heterogeneous oxidation of Hg° across the high iron model fly ash in the 22
presence of 50 ppm HC1 and the absence of SO2/water vapor: effects of
time of exposure and bed temperature
Figure 5 Effects of SO2, water vapor, and bed temperature on the steady-state 23
heterogeneous oxidation of Hg° across the high iron model fly ash in the
presence of 50 ppm HC1
Figure 6 Effects of coal fly ash components/compositions and bed temperature on 24
the steady-state heterogeneous oxidation of Hg° across model fly ashes in
the presence of 50 ppm HC1 and the absence of SO2 and water vapor
Figure 7 Effects of SO2 and water vapor on the steady-state heterogeneous 25
oxidation of Hg° across model fly ashes at 250°C and in the presence of
50 ppm HC1
IV
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Acknowledgments
This research was supported by the U.S. EPA's Office of Air Quality Planning and
Standards, Great Waters Program, with oversight by Dianne M. Byrne and Martha H. Keating.
Chun Wai Lee and James D. Kilgroe of the National Risk Management Research Laboratory
collaborated in the development of the research plan and provided technical guidance on the
project. Jarek Karwowski of ARCADIS Geraghty & Miller, Inc. was responsible for operation of
the experimental apparatus, and Wojciech Jozewicz, also of ARCADIS Geraghty & Miller, Inc.,
provided technical assistance throughout the project.
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1. INTRODUCTION
Title III of the 1990 Clean Air Act Amendments (CAAA) requires the U.S.
Environmental Protection Agency (EPA) to submit a study to Congress on 189 hazardous air
pollutants (HAPs) from industrial sources. This study will include an emission and risk (to public
health) assessment of the HAPs. Among the 189 HAPs, mercury has drawn special attention due
to its increased levels in the environment and its bioaccumulation in the food chain [1,2]. An
EPA report to Congress cites the largest emitters of mercury as coal-fired utilities, medical waste
incinerators (MWIs), municipal waste combustors (MWCs), chloro-alkali plants, copper and lead
smelters, and cement kilns [3]. These sources are estimated to account for over 90% of the
anthropogenic mercury emissions in the U.S. Utility boilers account for nearly 25% of the total
anthropogenic emissions of which more than 90% are attributable to coal-fired utility boilers.
Mercury, a trace constituent of coal [4], is readily volatilized during coal combustion [5].
Mercury is the most volatile species among various trace metals, and major portions of it can
pass through an existing air pollution control system (APCS) unless it is adsorbed onto
particulate matter (PM) and collected or removed by PM control devices [5]. Several methods of
controlling mercury emissions are either in commercial use or development for MWCs and
MWIs [6]. Dry sorbent injection (DSI) of activated carbon followed by fabric filtration (FF) has
shown consistently high (>90%) mercury removal in MWC applications. Spray drying (SD)
followed by FF and electrostatic precipitators (ESPs) followed by 2 stage acid gas wet scrubbers
(WS) have also been found to remove substantial (60-90%) amounts of mercury in MWCs.
However, these technologies have not been as successful in removing mercury from coal-fired
flue gases [7].
The primary reason suspected for the observed differences in mercury capture between
waste and coal combustors is the differences in the mercury species present in waste incineration
and coal combustion flue gases. Thus, an important scientific issue that needs to be addressed is
the chemical form of mercury in combustion system flue gases. A significantly lower mercury
concentration in coal combustion flue gas compared to that in MWCs may also be an important
contributing factor to the observed different mercury capture behavior between the two
combustion systems. Previous EPA studies have shown the relative ease of controlling oxidized
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mercury (specifically mercuric chloride, HgCl2) as opposed to elemental mercury (Hg°) [8,9].
Hall et al. [10] showed that in a simulated MWC mercury is mainly found as ionic mercury
(Hg2+) in flue gas. They postulated that HgCl2 is the most favorable mercuric species due to
thermochemical equilibrium calculations in the presence of relatively high hydrogen chloride
(HC1) concentrations in MWC flue gas. On the other hand, due to the low HC1 concentration,
Hg° is believed to be the prevailing form of mercury in emissions from coal combustion
processes [11]. However, recent pilot-scale coal combustion test results have indicated that
combustion of certain types of coal (Blacksville, a bituminous coal from Pittsburgh No. 8 seam)
can lead to a flue gas mercury species profile dominated by Hg2+ (most probably HgCl2)[12].
Most researchers agree that mercury in coal vaporizes completely in the combustion zone
of a boiler and leaves this zone in the form of Hg° in the gas phase. Some oxidation of Hg° may
occur as the flue gas cools. At the economizer exit, where the flue gas typically enters the APCS,
mercury can be found as Hg° or Hg2+. Predicting emissions of mercury species has been a
problem since the transformation of Hg° in the post-combustion region is not well understood. A
detailed review on the state of knowledge on mercury speciation in coal-fired processes has been
performed by Senior et al. [13]. Their review and another investigation [14] 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 fly-ash-mediated surface reactions as well as
gas-phase reactions. Senior et al. [13] posed two important questions that should be the focus of
the on-going and future research:
1- What is the rate of oxidation of Hg° in the post-combustion flue gas? Answering this
question would lead to prediction of gas-phase mercury speciation at the inlet and outlet
of the APCS.
2- What properties (or which components) of coal fly ash affect the oxidation of Hg°? In
other words, what are the processes through which fly ash components mediate or
catalyze the transformation of gaseous Hg° to its oxidized forms?
Thus, the objective of the Phase I (Phase II is discussed in the Future Research Section) of this
research project was to conduct bench-scale studies to identify and characterize the role of flue
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gas parameters (temperature and composition) and fly ash properties on the speciation of
mercury. The focus of the study was on the oxidation of Hg°. The extent of Hg° oxidation was
determined by comparing the measured inlet and outlet Hg° concentrations across the oxidation
reactor (cf. Experimental Procedure). These measurements provided information on the
conversion of Hg° to its oxidized inorganic compounds. No speculation was made as to the
nature of these compounds, since current analytical methods are not capable of distinguishing
between different oxidized mercury species. The inorganic mercury compounds that are
considered important in combustion processes are HgS(s), HgO(s,g), HgCl2(s,g), Hg2Cl2(s), and
HgSO4(s) [15]. As mentioned earlier, thermochemical equilibrium calculations have shown that,
in the presence of sufficiently high HC1, HgCl2 is the most favorable oxidized species. Since
throughout this study HC1 was present in large excess (mole ratio of HCl/Hg° > 1250), HgCl2
may be the dominant mercury compound formed during oxidation reactions of Hg°.
The chlorine in combustion flue gases is primarily in the form of HC1 at the APCS
operating temperatures. Throughout most of this study, the oxidation of Hg° was investigated
while HC1 was present in the simulated flue gas. Oxidation of Hg° was studied under two
different test programs: (1) homogeneous gas-phase tests and (2) heterogeneous gas-solid tests.
In the gas-phase tests, the effects of flue gas temperature and HC1 concentration on the oxidation
of Hg° were investigated. This test project was designed to simulate and study potential gas-
phase oxidation of Hg° in the post-combustion regions of coal combustion processes. This
oxidation (if any) may occur downstream of the combustion chamber at temperatures favoring
equilibrium concentrations of HC1 and Hg2+. A preliminary investigation was also conducted to
study gas-phase oxidation of Hg° in the presence of C12.
In a second program, heterogeneous, gas-solid phase tests were designed to simulate
conditions in the baghouse portion of the APCS. In baghouses, the flue gas penetrates a fixed-
bed of fly ash and other combustion residues at low temperatures (150-250°C). Fly ash is a
complex mixture of many minerals. The characterization of the structure of coal fly ash is the
subject of on-going research [16]. The amount of information on this important subject is rather
limited at this time. In this project, the effects of coal fly ash composition on the oxidation of Hg°
(in the presence of HC1) were studied using model (simulated) fly ashes. Fly ash composition
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experiments were limited to studies on the effects of the inorganic constituents of coal fly ash.
Model fly ashes were formulated by physically mixing oxides of silicon (Si), aluminum (Al),
calcium (Ca), iron (Fe), and copper (Cu). These oxides are abundant in a variety of coal fly ashes
[16], and they may be instrumental in promoting catalytic surface reactions involving Hg°, HC1,
and other flue gas components.
2. EXPERIMENTAL PROCEDURE
A schematic of the experimental setup used to study the oxidation of Hg° is shown in
Figure 1. Pure Hg°(l) in a permeation tube (VICI Metronics, Inc.) generated the desired Hg°
vapor. The concentration of Hg°(g) was controlled by adjusting the water bath temperature. A
detailed description for the calibration of the Hg° vapor generation system and its quality control
checks have been reported elsewhere [8,9]. The Hg° vapor generated was carried into the
manifold by a nitrogen (N2) stream where it mixed with carbon dioxide (CO2), oxygen (O2), HC1,
S02, and water vapor (H2O) at a constant total system flowrate of 300 cmVmin (at standard
temperature of 25 °C and pressure of 101.4 kPa). A three-way valve placed before the manifold
(Figure 1) diverted the Hg°-laden N2 stream away from the manifold when desired. The first
three-way valve placed ahead of the oxidation reactor was used to direct flow to or away from
the reactor. The reactor inlet Hg° concentration was measured when the valve was turned to the
by-pass mode. It should be noted that the oxidation reactor is made of quartz, and the connecting
lines are made of Teflon. Prior quality control checks [8,9] showed that this system had no
affinity for elemental mercury.
In the gas-phase tests, the oxidation reactor was empty and surrounded by a temperature-
controlled furnace. The gas-phase residence time (in the reactor) was 2 seconds at 850°C. In the
heterogeneous gas-solid tests, the model fly ash to be studied (0.25g, bed length of
approximately 2 cm) was placed in the oxidation reactor which was maintained at the desired bed
temperature by a temperature-controlled heating tape. It should be noted that, in the presence of
HC1, the heterogeneous tests were performed at a much lower temperature (150-250°C)
compared to those for gas-phase tests.
At the beginning of each test, the concentration of Hg° vapor generated by the permeation
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tube (hereafter, inlet concentration, Hgin) was registered by an on-line ultraviolet (UV) Hg°
analyzer (Buck 400A, detection limit of 1 ppb Hg°), while the oxidation reactor was in the by-
pass mode. It is important to note that the UV analyzer does not respond to oxidized forms of
mercury. During each test with the reactor on-line, the post-reaction or outlet Hg° vapor
concentration (Hgout) was measured continuously using the Hg° UV analyzer (experimental
variability of ±5%). Considering the fact that mercury in the flue gas exists in either the
elemental or oxidized form, the difference between the inlet and the outlet Hg° concentration was
used to quantify the extent of oxidation of Hg° in the reactor as a function of experimental
parameters. Percent oxidation was obtained as:
% Oxidation = 100*(Hgin-Hgout)/Hgin
Water vapor creates interferences in the UV Hg° analyzer. Prior to entering the Hg°
analyzer, water vapor was removed from the simulated flue gases. A NAFION® gas sample
dryer (Perma Pure, Inc.) was used to selectively remove water vapor from the effluent of the
oxidation reactor. Repeated quality assurance checks have indicated that this system has no
affinity toward adsorption of Hg° and acid gases present in the flue gas. The UV Hg° analyzer
used in this study responded to SO2 as well as Hg°. For instance, a gas stream consisting of 500
ppm SO2 and 40 ppb Hg° produces a SO2/Hg° signal ratio of 1/12. Contributions from SO2 were
corrected by placing a SO2 analyzer (UV, model 721AT2, Bovar Engineering, Inc.) on-line,
downstream of the Hg° analyzer. The SO2 analyzer is incapable of responding to mercury in the
concentration range used in this study. By subtracting the SO2 signal measured by the SO2
analyzer from the total response of the Hg° analyzer, the outlet Hg° concentration was obtained.
3. RESULTS AND DISCUSSION
3.1 Homogeneous Gas-Phase Oxidation of Hg°
The gas-phase tests were designed to study the potential gas-phase oxidation of Hg° in the
post-combustion region of coal combustion processes. This oxidation (if any) may occur in the
duct region (in-flight oxidation) upstream of the APCS, or dry particulate collectors (prior to
scrubbers), in residence times of the order of seconds. The "base case" simulated flue gas in
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these tests consisted of 40 ppbv Hg°, 5% (mole) CO2, 2% (mole) O2, and a balance of N2. The
effect of HC1 was studied at three concentrations: 50, 100, and 200 ppmv (typical of coal
combustion processes). The effects of SO2 and water vapor were studied at concentrations of 500
ppmv and 1.7% (mole), respectively. No oxidation of Hg° in the empty reactor (gas-phase) was
observed at temperatures lower than 500°C (residence times of 3-4 seconds). This is in
agreement with the results of Senior et al. [13], and once again indicates that the assumption of
gas-phase equilibrium for mercury species in coal combustion flue gases is not valid.
Equilibrium calculations predict complete oxidation of Hg° to HgCl2 at temperatures lower than
600°C and in the presence of 50-200 ppmv HC1 [13, 14].
Gas-phase oxidation of Hg° vapor proceeded at measurable, steady-state rates at
temperatures greater than 500 °C. Figure 2 shows the effects of temperature and HC1
concentration on the gas-phase oxidation of Hg° in the absence of SO2 and water vapor (base case
flue gas). Relatively significant oxidation of Hg° (about 27%) was observed at high temperatures
(>700°C) and high HC1 concentration (200 ppm). These results show that gas-phase Hg°
reactions with HC1 are relatively slow even at temperatures as high as 700°C. Increasing HC1
concentration caused an increase in Hg° oxidation at a high temperature (754 °C). These results
agree with the observations of Hall et al. [10] and MWC field test results, confirming the fact
that, in waste incinerators at high HC1 concentrations (1000-2000 ppmv), significant oxidation of
Hg° to HgCl2 may occur near the combustion zone. On the other hand, the gas-phase oxidation of
Hg° in coal combustion flue gas is not expected to be as significant due to its relatively low HC1
concentration.
Gas-phase reaction of Hg° with HC1 may be affected by the presence of SO2 and water
vapor. Figure 3 shows the effect of adding either 500 ppmv SO2 or 500 ppmv SO2 plus 1.7%
(mole) water vapor to the base case flue gas on the gas-phase oxidation of Hg° at the highest
temperature (754 °C) selected for this study. The presence of SO2 and water vapor inhibited the
gas-phase oxidation of Hg°. To better understand this inhibition effect, one has to obtain a
complete understanding of the detailed Hg° oxidation mechanism. The detailed mechanistic
reactions of Hg° and HC1 are not well understood. There are no obvious reaction pathways for
the gas-phase oxidation of Hg° by HC1. The ones identified as candidates are complicated; they
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probably involve many unknown elementary steps [15]. A chlorinating agent (such as chlorine
free radical, or C12) is most probably needed in the gas-phase Hg° oxidation mechanism. Chlorine
free radicals (or C12 molecules) may be produced in trace quantities from the HC1 in the flue gas;
not much is known on this subject. The inhibition effect of SO2/water vapor may be related to a
chlorine free radical scavenging effect caused by these species.
A preliminary investigation, however, indicated that the gas-phase reaction of Hg° and
C12 is very fast. At temperatures as low as 40°C and in the presence of 50 ppmv C12, complete
oxidation (100%) of Hg° was observed in the gas phase at reaction times of less than 2 seconds.
These results are in agreement with those from studies conducted by Hall et al. [15]. They
showed that at room temperature as little as 2 ppmv C12 was sufficient to oxidized half the
elemental mercury in about 1 second (starting with a mercury concentration of 12 ppbv). The
results obtained in this investigation and those obtained by Hall et al. [15] indicate that C12 is a
much more reactive chlorinating agent than HC1, and it may be an intermediate species in the
oxidation reaction of Hg° and HC1. Thus, the important gas-phase parameter that influences the
oxidation of Hg° is the ratio of HC1/C12 in coal or waste combustion flue gases. No quantitative
data are available on this ratio; therefore, future research on quantifying the effects of
combustion parameters on the HC1/C12 ratio in flue gas could provide important information on
the oxidation of Hg° by HC1.
In summary, the homogeneous gas-phase oxidation of Hg° in the presence of HC1 is very
slow and proceeds at significant rates only at high temperatures (>700°C) and high HC1
concentrations (>200 ppmv). The presence of SO2 and water vapor inhibits the oxidation of Hg°
by HC1. All indications have shown that conditions existing in coal combustion processes are not
favorable for a gas-phase oxidation of Hg° because of low HC1 concentration and high SO2 and
moisture contents in coal combustion flue gases. On the other hand, homogeneous gas-phase
oxidation of Hg° in the presence of C12 is very rapid. The HC1/C12 ratio in coal combustion flue
gases needs to be characterized and quantified in order to obtain a better understanding of
oxidation of Hg° in the gas phase.
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3.2 Heterogeneous Catalytic Oxidation of Hg°
These experiments were designed to simulate conditions in baghouses of the coal-fired
power plants. In baghouses, the flue gas penetrates a filter cake (fixed-bed) of fly ash at
temperatures lower than 250°C. Oxidation of Hg° may occur at these temperatures on the surface
of fly ash through heterogeneous catalytic reactions. If this is indeed the case, one has to identify
components of fly ash that catalytically enhance oxidation of Hg° in the presence of HC1. Of
interest is the effect of the individual components on the extent of Hg° oxidation and the effect of
flue gas species such as SO2 and water vapor. The fixed bed of fly ash build-up in a baghouse
was simulated by the fixed-bed oxidation reactor (Figure 1). The effects of coal fly ash
components and composition were studied using model fly ashes (particle size of less than 300
mesh). The fixed-bed reactor was packed with the model fly ash, and the extent of Hg° oxidation
across the bed was determined (cf. Experimental Procedure). The base case flue gas composition
during the heterogeneous tests was as follows: 40 ppbv Hg°, 50 ppmv HC1, 5% (mole) CO2, 2%
(mole) O2, and balance N2. Water vapor (1.7%, mole) and SO2 (500 ppmv) were added to the
base case flue gas during selected tests to deduce the effects of these species.
The model fly ash studies were started using the base case simulated fly ash. Its
composition (base composition; two-component) is illustrated in Table 1. The effect of fly ash
composition was studied by individually adding a number of metallic oxides to the base case
(three-component fly ashes, Table 1). The primary focus was on the most commonly found
metallic oxides in coal-derived fly ash: Iron(III) oxide (Fe2O3, Puratronic, 99.99%, Alfa Aesar),
calcium oxide (CaO, 99.95%, Alfa Aesar), and cupric oxide (CuO, 99.9%, Alfa Aesar). Different
compositions of these compounds in the three-component model fly ashes were prepared (Table
1). These compositions reflect those ranges typically measured in coal fly ashes [16].
Initial heterogeneous tests with the two-component fly ash (Table 1) yielded no oxidation
of Hg° in the presence or absence of SO2/water vapor in the flue gas over a temperature range of
150-250°C. Thus, alumina (A12O3, fused, 99+%, Aldrich Chemical Company) and silica (SiO2,
fused amorphous powder, 99.9%, Alfa Aesar) could be considered to be inert since they do not
play a role in the oxidation of Hg° in the simulated flue gas and model fly ash tested.
Heterogeneous tests on the three-component model fly ashes were continued using the
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high iron (Fe) content fly ash (14% Fe203, Table 1). Significant oxidation of Hg° was observed
across the fixed bed of this simulated fly ash. The extent of Hg° oxidation as a function of time of
exposure of the fly ash to the base case flue gas (no SO2/water vapor) and bed temperature is
shown in Figure 4. On average, steady state oxidation of Hg° across the bed of fly ash was
achieved after 15-20 minutes of exposure to the simulated flue gas and maintained for at least 2
hours. The initial 20 minutes of the oxidation process can be referred to as the "induction"
period. The reaction mechanisms during the induction period are not understood at this time. In a
separate test, the model fly ash was first exposed to HC1 (no Hg°) and then exposed to Hg° (no
HC1). No oxidation of Hg° was observed. This indicates that HC1 is needed during the oxidation
of Hg°, and the catalysts are not being activated by HC1 during the induction period.
In the steady-state region after the induction period shown in Figure 4, the fly ash
exhibited its best catalytic activity under a particular condition, and continuously catalyzed the
oxidation of Hg°. The heterogeneous mechanisms of Hg° oxidation are not well understood. It is
probable that iron oxide in the fly ash produces an active chlorinating agent from the gas-phase
HC1. This chlorinating agent can be the fly ash on which a surface chlorine radical is attached, a
gas-phase chlorine free radical, or a gas-phase chlorine (C12) molecule. Subsequently, the
chlorinating agent attacks Hg° molecules, resulting in the most probable oxidation state of Hg°;
i.e. HgCl2. The process can be described in terms of a detailed kinetic mechanism consisting of
elementary gas-phase and gas-solid reactions. Future research with focus on constructing such
detailed mechanisms is needed in order to obtain a better understanding of catalytic oxidation of
Hg°.
The effect of bed temperature is also illustrated in Figure 4. Like any kinetically
controlled process, catalytic or non-catalytic, increasing the reaction temperature increases the
rate of reactions, and here leads to higher steady-state oxidation of Hg°. Hg° oxidation as high as
95% was observed when 14% Fe2O3 was present in the model fly ash. The high iron content
found in a Pittsburgh coal fly ash [16] suggests that the high HgCl2 emissions measured when
burning this coal are resulting from heterogeneous catalytic oxidation of Hg° [12].
Coal combustion flue gas contains a significant amount of SO2 and moisture. The
oxidation of Hg° over high iron containing fly ash was studied under more realistic conditions;
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i.e., the presence of 500 ppmv SO2 and 1.7% (mole) H2O. Steady-state Hg° oxidation results are
shown in Figure 5. The presence of SO2 in the simulated flue gas caused a slight decrease in Hg°
oxidation at temperatures of 200 and 250°C. It caused a slight increase at 150°C, which can not
be explained at this time. However, the presence of both SO2 and water vapor caused a drastic
decrease in the catalytic oxidation of Hg°, especially at the lower temperatures. Similar to the
gas-phase tests, the effect of SO2/water vapor can be related to a scavenging of the catalytically
generated chlorinating agent(s) such as C12. One possible global scavenging reaction is shown
below:
C12 + SO2 + 2H2O - 2HC1 + H2SO4
Thus, in practical situations, the inhibition effect of SO2/water vapor is also an important factor
which needs to be considered when the coal combusted contains sufficient iron and other
catalytic metals for catalytic Hg° oxidation to occur.
The potential catalytic activities of CaO and CuO and the effects of the iron and copper
contents of the three-component model fly ashes on the oxidation of Hg° were also investigated.
Table 1 lists all the three-component model fly ashes used for the present study. The steady-state
Hg° oxidation results under the base case flue gas (no SO2/water vapor) are shown in Figure 6.
Since the CaO-containing model fly ash did not exhibit any Hg° oxidation, it may be considered
to be inert fly ash component. As shown in Figure 6, CuO is a much more active catalyst than
Fe2O3. This is apparent for the 200°C and 250°C data shown in Figure 6 when comparing the
same percentages of the two additives. A model fly ash containing 0.1% CuO exhibited similar
catalytic activity for Hg° oxidation as did a model fly ash containing 14% Fe2O3. The high
catalytic reactivity of CuO on chlorinating agent formation from HC1 is also indicated by the
formation of dioxins at low temperatures in the presence of MWC fly ash. Chlorination of
aromatic compounds such as phenol is enhanced by the presence of a copper (I) chloride catalyst
[17] or MWC fly ash [18]. The mechanism for the chlorinating agent formation is presumed to
be the Deacon process reaction:
2HC1 + '/2O2 * C12 + H2O
in which HC1 is converted to C12 in the presence of a metal catalyst. This conversion promoted
by the cuprous chloride catalyst (CuCl) was directly observed by Gullett et al. [17] in a
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laboratory flow reactor. At 400°C, approximately 40% conversion of HC1 to C12 was measured.
It is suggested by this case that copper is very reactive in generating a chlorinating agent through
the Deacon process reaction. The chlorinating agent (C12) was possibly produced at a much
higher rate in the presence of Cu than in the presence of Fe and led to greater oxidation of Hg°
and formation of HgCl2. It should be noted that the homogeneous gas-phase test results also
suggested that C12 may be the required intermediate species in the oxidation of Hg° by HC1.
As illustrated in Figure 6, lowering the amounts of iron and copper in the three-
component model fly ashes reduced the heterogenous oxidation of Hg°. A model fly ash
containing 1% CuO was the most active catalyst in these tests. The oxidation of Hg° reached
approximately 95% in the temperature range of 150-250°C, while temperature had no effect on
oxidation.
Figure 7 shows the effect of SO2/water vapor on the oxidation of Hg° across the fixed
beds of the three-component model fly ashes at a bed temperature of 250°C. The inhibition effect
of SO2/water vapor was observed to be very drastic for the iron-containing model fly ashes, and a
less significant inhibition effect was observed for the copper-containing model fly ashes. The
diminished inhibition effect may be due to the superior catalytic activity of copper.
A three-component model fly ash containing 7% CaO exhibited a very interesting and
unique behavior in the presence of SO2/water vapor; it did not cause any Hg° oxidation in the
presence of HC1. In another words, it did not catalyze the formation of chlorinating agent(s) from
HC1. However, unlike the iron and copper catalysts, the presence of SO2 induced little Hg°
oxidation across this model fly ash. One possible explanation for this observation is that CaO is a
good SO2 sorbent. Adsorption of SO2 may create active sulfur (S) sites on the surface of the
CaO-containing model fly ash. An oxidation reaction between the Hg° molecules in the gas phase
and the active "S" sites produces mercuric sulfide (HgS) molecules which either remain on the
surface or are released back into the simulated flue gas. The outcome of this process is a low
oxidation of Hg° (15%) to HgS mediated by gas-phase SO2 and solid-phase CaO in the model fly
ash.
In summary, it appears that coal fly ashes rich in iron and copper may catalyze the
oxidation of Hg° in baghouses through the chlorinating agents produced by a Deacon type
11
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reaction. Copper has a much more pronounced catalytic activity than iron even in the presence of
oxidation inhibitors, such as SO2 and water vapor.
4. CONCLUSIONS
The formation of oxidized mercury species from Hg° vapor in simulated coal combustion
flue gases was investigated. Although high temperature combustion reactions convert mercury in
coal to gaseous elemental mercury (Hg°), certain coal fly ash and flue gas parameters may be
influential in converting Hg° vapor to gaseous mercuric chloride (HgCl2) in post-combustion
regions, including air pollution control systems (APCSs). Research has indicated the relative
ease of controlling HgCl2 emissions in APCSs as opposed to those of Hg°.
Gas-phase studies revealed that Hg° oxidation in the presence of hydrogen chloride (HC1)
is very slow and proceeds at measurable rates only at high temperatures (>700°C) and high HC1
concentrations (>200 ppm). Furthermore, the presence of high sulfur dioxide (SO2) concentration
in coal combustion flue gases inhibits such oxidation. On the other hand, gas-phase reaction of
Hg° and chlorine (C12) proceeds rapidly even at low temperatures (40 °C). No data are available
on the quantitative ratio of C12/HC1 in coal combustion flue gases. Assuming that this ratio is
small, one may conclude that the conditions in coal combustion processes are not favorable for a
gas-phase oxidation of Hg°.
The effects of coal fly ash parameters on the oxidation of Hg° were studied using model
(simulated) fly ashes. The focus was on the inorganic constituents of coal fly ashes: alumina
(A12O3), silica (SiO2), iron(III) oxide (Fe2O3), calcium oxide (CaO), and copper (II) oxide (CuO).
CuO and Fe2O3 exhibited significant catalytic activity in oxidizing Hg° in the presence of 50 ppm
HC1 and in the temperature range of 150-250°C. Initial indications are that these transition
metals, through a multi-step heterogeneous Deacon process, produce an active chlorinating
agent, most probably C12, from HC1. Subsequent reaction of this agent and gaseous Hg° proceeds
quickly and results in large quantities of HgCl2, the most probable oxidation state of Hg°. CuO is
a much more active catalyst than Fe2O3. A simulated fly ash consisting of a 3.5/1 ratio of alumina
to silica base and 0.1% (wt) CuO exhibited the same catalytic activity (95% Hg° oxidation) as a
model fly ash containing of 14% Fe2O3 with identical alumina and silica contents in the
12
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temperature range of 150-250°C. It appears that the production of the chlorinating agent from
HC1 in the presence of a copper catalyst proceeds much faster compared to that of iron. Overall,
increasing the bed temperature increased the catalytic activity of the model fly ashes, indicating
that the oxidation process is kinetically controlled.
The presence of SO2/water vapor in the simulated flue gas inhibited the catalytic activity
of the iron-containing fly ashes more drastically than those having copper catalysts. The
inhibition effect was more pronounced at the lower temperature studied (150°C). Calcium oxide
(CaO) exhibited a different behavior from those of iron and copper catalysts. It did not show any
catalytic activity in oxidizing Hg° in the presence of HC1 and absence of SO2. However, when
SO2 was added to the simulated flue gas, a low oxidation of Hg° (15%) was observed across the
fixed bed of the CaO model fly ash. This low oxidation may have been caused by the formation
of Hg-S bonds on the surfaces of CaO.
In summary, it appears that coal fly ashes with high iron and copper contents catalyze the
oxidation of Hg° in baghouses. Copper is a much more active catalyst than iron even in the
presence of oxidation inhibitors such as SO2 and water vapor.
5. FUTURE RESEARCH (PHASE II)
There have been indications that oxidation of Hg° in the presence of chlorine (C12)
proceeds faster than in the presence of HC1 [15]. In this study, it was also hypothesized and
shown that C12 may be the critical intermediate reaction species during oxidation of Hg° by HC1.
Additional heterogeneous tests will be performed using C12, instead of HC1, in the simulated flue
gases. Heterogeneous oxidation of Hg° will be studied in the presence of C12. The effects of C12
concentration, flue gas temperature, and coal fly ash components and composition will be
investigated. There have also been indication that nitric oxide (NO), a significant component of
coal combustion flue gases, may affect mercury speciation. Oxidation of Hg° will be investigated
in the presence of NO in the simulated flue gas.
Coal fly ash is a complex mixture of many metal oxides which are formed from minerals
present in coal. The study described here (under Phase I) used simulated fly ashes which
contained three metal oxides. Hg° oxidation studies on more complex fly ashes which contain
13
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more metal oxides need to be performed in order to better understand the synergistic or
antagonistic effects of individual metal oxides. Four-component model fly ashes will be studied.
These model fly ashes will contain A12O3, SiO2, Fe203 (or CuO), and CaO at different
concentrations. Following the four-component fly ash tests, a more complex model fly ash will
be developed to simulate the oxidation behavior of Hg° in coal combustion flue gases. Initial
components of this fly ash are anticipated to be: SiO2, A12O3, Fe2O3, CaO, CuO, Na2O, TiO2,
MgO, and MnO2. Pending the initial results, the composition of the complex model fly ash will
be further improved to represent more realistic conditions.
In the presence of sufficiently high chlorine (HC1 or C12) in the gas phase, the metallic
compounds in the fly ash may be in the form of chlorides. Metal chlorides may also be the
dominant form of minerals in MWC fly ash. Additional heterogeneous tests will be performed in
the presence of simulated fly ashes containing copper and iron chlorides. These results will be
compared with those obtained in the presence of copper and iron oxides.
Coal cleaning prior to its combustion has been considered as a method of pollution
prevention. The cleaning process significantly changes the properties and characteristics of coal
combustion fly ashes. Samples of fly ashes from combustion of raw and cleaned coal will be
obtained. Oxidation of Hg° across these two different types of fly ashes will also be investigated.
Recent research results obtained at the Babcock & Wilcox Clean Environment
Development Facility (B&W CEDF) have indicated that major portions of oxidized mercury can
undergo reduction to Hg° in the specific sections of the APCS [19]. The Hg° oxidation studies
will be followed by a preliminary investigation into the reduction of HgCl2 across the fixed bed
of the complex model fly ash.
Besides baghouses, other air pollution control devices may also be instrumental in
affecting mercury speciation in coal-fired combustion processes. A detailed bench-scale
investigation will be designed to specifically answer the following questions:
• What is the effect of wet scrubber (WS) design and operating conditions on mercury
speciation?
• What WS parameters, such as pH and temperature, affect mercury speciation?
• How does the chemistry in a single stage limestone WS system commonly used for coal-
14
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fired boilers affect mercury speciation and control?
• What are the effects of other likely utility APCS devices such as electrostatic
precipitators (ESPs) on mercury speciation?
• What operating parameters (such as electric field) are most likely to control mercury
speciation in an ESP?
6. REFERENCES
1. Langley, D.G. "Mercury Methylation in an Aquatic Environment," J. Water Pollut.
Contr.Fed,45:44, 1973.
2. Westoo, G. "Methyl Mercury as Percentage of Total Mercury in Flesh and Viscera of
Salmon and Sea Trout of Various Ages," Science, 181: 567, 1973.
3. U.S. Environmental Protection Agency. Mercury Study Report to Congress. Book 1 of 2,
External Review Draft, EPA/600/P-94/002a (NTIS PB95-167334), Environmental
Criteria and Assessment Office, Cincinnati, OH, January 1995.
4. Billings, C.E.; Sacco, A.M.; Matson, W.R.; Griffin, R.M.; Coniglio, W.R.; and Harley,
R.A. "Mercury Balance on a Large Pulverized Coal-fired Furnace," J. Air Pollut. Contr.
Assoc., 23:9, 773, 1973.
5. Klein, D.H.; Andren, A.W.; Carter, J.A.; Emery, J.F.; Feldman, C.; Fulkerson, W.; Lyon,
W.S.; Ogle, J.C.; Talmi, Y.; Van Hook, R.I.; and Bolton, N. "Pathways of 37 Trace
Elements Through Coal-Fired Power Plant," Environ. Sci. & Technol., 9(10): 973, 1975.
6. Brna, T.G.; and Kilgroe, J.D. "The Impact of Particulate Emissions Control on the
Control of Other MWC Air Emissions," J. Air & Waste Mgt. Assoc., 40(9): 1324, 1990.
7. Chang, R.; and Offen, G.R. "Mercury Emission Control Technologies: An EPRI
Synopsis," Power Engineering, November 1995.
8. Krishnan, S.V.; Gullert, B.K.; and Jozewicz, W. "Sorption of Elemental Mercury by
Activated Carbons," Environ. Sci. & Technol., 28:8, 1506, 1994.
9. Krishnan, S.V.; Bakhteyar, H.; and Sedman, C.B. "Mercury Sorption Mechanisms and
Control by Calcium-Based Sorbents," Paper 96-WP64B.05 Presented at the 89th Air &
Waste Management Association Annual Meeting, Nashville, TN, 1996.
15
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10. Hall, B.; Lindqvist, O.; and Ljungstrom, E. "Mercury Chemistry in Simulated Flue Gases
Related to Waste Incineration Conditions," Environ. Sci. & Technol, 24: 108, 1990.
11. Devito, M.S.; Tunati, P.R.; Carlson, R.J.; and Bloom, N. "Sampling and Analysis of
Mercury in Combustion Flue Gas," In Proceedings of the EPRI's Second International
Conference on Managing Hazardous Waste Air Pollutants, Washington, DC, 1993.
12. Laudal, D.L.; Heidt, M.K.; Brown, T.D.; Nott, B.R.; and Prestbo, E.P. "Mercury
Speciation: A Comparison between EPA Method 29 and Other Sampling Methods,"
Paper 96-WP64A.04 Presented at the 89th Air & Waste Management Association Annual
Meeting,, Nashville, TN, 1996.
13. Senior, C.L.; Bool, E.L.; Huffman, G.P.; Huggins, F.E.; Shah, N.; Sarofim, A.; Olmez, I.;
and Zeng, T. "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, June 8-13, Toronto, Ontario, Canada, 1997.
14. Krishnan, S.V.; Gullett, B.K.; and Jozewicz, W. "Mercury Control by Injection of
Activated Carbon and Calcium-Based Sorbents," In Proceedings of Solid Waste
Management Conference, Thermal Treatment & Waste-To-Energy Technologies,
Washington, DC, 1995.
15. Hall, B.; Schager, P.; and Lindqvist, O. "Chemical Reactions of Mercury in Combustion
Flue Gases," Water, Air, and Soil Pollution, 56, 3-14, 1991.
16. Bool, L.E.; Helbe, J.J.; Shah, N.; Shah, A.; Huffman, G.P.; Huggins, F.E.; Rao,
K.R.P.M.; Sarofim, A.F.; Zeng, T.; Reschke, R.; Galline, D.; and Peterson, T.W.
"Fundamental Study of Ash Formation and Deposition: Effect of Reducing
Stoichiometry," Final Report for Contract # DE-AC22-93PC92190, Department of
Energy, Pittsburgh Energy Technology Center, September 1995.
17. Gullett, B.K.; Bruce, K.R.; and Beach, L.O. "The Effect of Metal Catalysts on the
Formation of Polychlorinated Dibenzo-p-dioxin and Polychlorinated Dibenzofuran
Precursors," Chemosphere, Vol. 20, No. 10-12, pp 1945-1952, 1990.
16
-------
18. Gullett, B.K.; Lemieux, P.M.; and Dunn, J.E. "Role of Combustion and Sorbent
Parameters in Prevention of Polychlorinated Dibenzo-p-dioxin and Polychlorinated
Dibenzofuran Formation During Waste Combustion," Environ. Sci. and Technol., 28,
107-118,1994.
19. Evans, A.P.; and Nevitt, K.D. "Mercury Speciation Measurements on a 10 MWe Coal-
Fired Boiler Simulator," Paper 97-WA72B.07 Presented at the Air & Waste Management
Association's 90th Annual Meeting & Exhibition, June 8-13, Toronto, Ontario, Canada,
1997.
17
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Table 1. Model Fly Ash Compositions (wt%)
Model Fly Ash
Base Composition
(Two-Component)
Three-Component,
High Fe
Three-Component,
Medium Fe
Three-Component,
Low Fe
Three-Component,
High Cu
Three-Component,
Medium Cu
Three-Component,
High Ca
Content
A1A
22
19
22
22
22
22
21
SiO2
78
67
77
78
77
78
72
FeA
0
14
1
0.1
0
0
0
CuO
0
0
0
0
1
0.1
0
CaO
0
0
0
0
0
0
7
18
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Mercury Generation
_ Carbon Trap . . ., . ,
System Manifold
. \ i /"**« s^ff \ A»ll% Jj«l»>
i un-oiT valves
*~ — r+j — • (j tf | o v <*> o
3-Wav Valve A ii Jl ^
A i
Water Bath HCI
Humidifier SO2
Air/PO rn
N. ^ Air/UU2 ro
21 i
m
Water Qata Acquisition
Carbon Trap ^
^^* i
f** /^v
3-Way Valve 1
LL ..•;. tT, i;--
c .
.2 ;.>•:'.';':•'-:
CD i
T3
'x
3-Way Valve
..
SO^ Elemental _ y
Analyser ^ Mercurv
R^tameter J"V" Analyzer Water
Removal
System
Figure 1. Schematic of the elemental mercury oxidation reactor system.
19
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c
o
30
25
20
15
2
'x
O
5
0
515 °C •634°C •754°C
50 100 200
HCI Concentration (ppmv)
Figure 2. Effects of temperature and HCI concentration on the gas-phase oxidation of Hg°
in the absence of SO2/water vapor.
20
-------
30
25
c 20
o
I 15
'x
O
o
10
5
0
500 ppmv SO2
1.7% H2O
no SO2/H2O
500 ppmv SO2
50 100 200
HCI Concentration (ppmv)
Figure 3. Effects of SO2 and water vapor on the gas-phase oxidation of Hg° at 754°C and
three different HCI concentrations.
21
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100
as
.g
'•*-•
CO
***********
80
60
o° 40
D)
20
0
0
Bed Temperature (°C)
150 200 250
• + *
20 40 60
Exposure Time (min)
80
Figure 4. Heterogeneous oxidation of Hg° across the high Fe model fly ash (see Table 1) in
the presence of 50 ppm HC1 and the absence of S02/water vapor; effects of time
of exposure and bed temperature.
22
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500 ppmv SO2
1.7% H2O
500 ppmv SO2
noH2O
no SO2/H20
c
_o
'-!-«
CD
100
80
60
O 40
o
O)
I
20
0
150
200
CL
250
Bed Temperature ( C)
Figure 5. Effects of SO2, water vapor, and bed temperature on the steady-state heterogenous
oxidation of Hg° across the High Fe model fly ash (see Table 1) in the presence of
50 ppm HC1.
23
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150°C
200°C
250°C
100
Figure 6. Effects of coal fly ash components/compositions and temperature on the steady-
state heterogeneous oxidation of Hg° across model fly ashes (see Table 1) in the
presence of 50 ppm HC1 and the absence of S02/water vapor.
24
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100
500 ppmv SO2
1.7% H2O
no SO2/H2O
500 ppmv SO2
noH2O
Figure 7. Effects of SO; and water vapor on the steady-state heterogeneous oxidation of Hg°
across model fly ashes (see Table 1) at 250°C and in the presence of 50 ppm HC1.
25
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TECHNICAL REPORT DATA
(Please read Instructions on the reverie before completing}
. REPORT NO.
EPA-600/R-98-014
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Fundamentals of Mercury Speciation and Control in
Coal-Fired Boilers
5. REPORT DATE
February 1998
. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
S. Behrooz Ghorishi
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Acurex Environmental Corporation
4915 Prospectus Drive
Research Triangle Park, North Carolina 27709
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-D4-0005, WA 3-050
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Air Pollution Prevention and Control Division
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Task Final; 3-9/97
14. SPONSORING AGENCY CODE
EPA/600/13
is. SUPPLEMENTARY NOTES APPCD project officer is Chun Wai Lee, Mail Drop 65, 919/541-
7663.
16. ABSTRACT
The report describes the progress of an experimental investigation of the
speciation of mercury in simulated coal combustion flue gases. The effects of flue
gas parameters and coal fly ash on the oxidation of elemental mercury (Hgo) in the
presence of hydrogen chloride (HC1) in a simulated post-combustion region, inclu-
ding the baghouse portion of air pollution control systems, were studied using a
bench-scale setup. Results of the gas-phase experiments indicate that the in-flight
post-combustion oxidation of Hgo in the presence of HC1 in a simulated flue gas is
slow and proceeds at measurable rates only at high temperatures (>700 C) and high
HC1 concentrations (> 200 ppmv). The presence df sulfur dioxide (SO2) and water
vapor in the simulated flue gas significantly inhibits the gas-phase oxidation of Hgo
in the presence of HC1. However, results of a preliminary investigation indicate that
the gas-phase reaction of Hgo with chlorine (C12) proceeds rapidly, suggesting that
C12 is a much more active chlorinating agent than HC1. The effects of the coal fly
ash component and its composition were investigated using a fixed bed of model
fly ashes. The primary focus was to evaluate the catalytic activity of major mineral
constituents of coal fly ashes. Copper and iron oxides were the only two components
that exhibited significant catalytic activity toward surf ace-mediated oxidation of Hgo.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. cos AT I Field/Group
Pollution
Mercury (Metal)
Oxidation
Coal
ombustion
Flue Gases
Emission
Boilers
Fly Ash
Hydrogen Chloride
Chlorine
Pollution Control
Stationary Sources
13 B
07B
07C
21D
21B
14G
ISA
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
31
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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