EPA 600/A-98/075
Mercury Control Research: Effects of Fly Ash
and Flue Gas Parameters on Mercury Speciation
Chun Wai Lee and James D. Kilgroe
U.S. Environmental Protection Agency
National Risk Management Research Laboratory
Research Triangle Park, NC
S. Behrooz Ghorishi
ARCADIS Geraghty & Miller, Inc.
Research Triangle Park, NC
ABSTRACT
In flue gas from combustion systems, mercury (Hg) is typically in the vapor phase at flue
gas cleaning temperatures, and the control of Hg emissions is dependent on the specific Hg
compounds that are present (speciation) and the type of air pollution devices employed. In dry and
semi-dry scrubbing systems, the control of Hg emissions is dependent on the sorption of Hg by
particulate matter (PM) which can be subsequently collected in a PM control device. In wet
scrubbing systems, the principal mechanisms of control are the removal of soluble forms of Hg
and the collection of particle-bound Hg. At combustion temperatures, Hg is believed to be
predominantly in the form of elemental mercury (Hg°). As the flue gas is cooled, thermochemical
equilibrium calculations indicate that Hg°is converted primarily to ionic mercury (Kg"1"1) in the form
of mercuric oxide (HgO) or mercuric chloride (HgCl2). Hg°is insoluble in water, but HgO has a
low solubility while HgCl2 is highly soluble. The oxidation of Hg° to an ionic form depends on
the temperature, the time-temperature profile, the flue gas composition, the reaction kinetics, and
the presence of solids that may catalyze reactions. . .
Bench-scale experiments were conducted to study the effects of flue gas and fly ash
parameters on the oxidation of Hg° in simulated flue gases containing hydrogen chloride (HC1).
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°. On the other hand, a
preliminary investigation indicated that the gas-phase reaction of Hg° with chlorine (C12) is fast. At
40 °C and in the presence of 50 ppm C12, 100% of the input Hg° was oxidized to HgCl2 in less than
2 seconds, indicating that C12 is a much more active chlorinating agent than HC1.
The effects of fly ash composition were investigated using a fixed-bed reactor containing
different model fly ashes (simulated fly ash) consisting of mixtures of some major components
found in coal and municipal waste combustor (MWC) fly ashes. Work to date has focused on the
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potential catalytic oxidation of Hg° by two- and three-component model fly ashes composed of
mixtures of: alumina (A12O3), silica (SiO2), ferric oxide (Fe2O3), cupric oxide (CuO), and calcium
oxide (CaO). Copper and iron oxides were the only two components that exhibited significant
catalytic activity in the surface-mediated oxidation of Hg°. The reactivity of these two metals is
hypothesized to be affected 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 much
more effective in the oxidation of Hg° than iron, and its catalytic activity was less sensitive to the
presence of oxidation inhibitors (SO2 and water vapor). The presence of a small quantity of CuO
(0.1% wt) in the model fly ash caused a 95% oxidation of Hg° in the temperature range of 150 to
250 °C. The same extent of Hg oxidation was obtained by adding 14% (wt) Fe2O3 to the model fly
ash.
INTRODUCTION
Mercury (Hg) emissions from combustion sources are important environmentally because
of their adverse health and ecological effects. Mercury, a trace constituent in a wide variety of
solid fuels including municipal solid waste (MSW) and coal, is readily volatilized during
combustion, and it most likely passes through existing air pollution control systems (APCS) unless
it is removed in a scrubber or adsorbed onto particulate matter (PM) and removed by a PM control
device. The chemical form of Hg in combustion flue gases is an important factor influencing its
control. Previous EPA studies (1) have shown that controlling oxidized Hg (Kg**) emissions is
much easier than controlling elemental Hg (Hg°) emissions. Mercury emissions from municipal
waste combustion (MWC) are believed to be dominated by mercuric chloride (HgQ2), because the
high Hg and chlorine contents in MWC favor HgCl2 formation (2). Hg° is suggested to be the
dominant Hg species in coal combustion flue gases due to lower hydrogen chloride (HC1)
concentrations (3). However, recent pilot-scale coal combustion test results showed that Hg
emissions from combustion of certain types of coal are dominated by the ionic species (4).
Mercury contained in fuels vaporizes completely in the combustion zone of a boiler and
leaves this zone as gas-phase Hg°. Oxidation of Hg° may occur as the flue gas cools to lower
temperatures downstream of the combustion zone. Predicting the chemical form of Hg in flue gas
emissions is difficult, since the transformations of Hg° in the post-combustion region are
unknown, and the major reaction pathways for Hg oxidation in combustion flue gases remain to
be determined (5). The pathways may include heterogeneous fly-ash-mediated surface reactions,
as well as homogeneous gas-phase reactions.
The objective of this study was to conduct bench-scale experiments to identify and
characterize the role of flue gas parameters, such as temperature and composition, and fly ash
properties on the speciation of Hg. Oxidation of Hg° was studied under two different test regimes:
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(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.
The homogeneous tests were designed to study potential gas-phase oxidation of Hg° in the post-
combustion region. A preliminary test was also conducted to study the gas-phase oxidation of Hg°
in the presence of C12. The tests for heterogeneous gas-solid Hgft oxidation were conducted under
conditions simulating the baghouse portion of APCS. The effects of fly ash composition on the
oxidation of Hg° in the presence of HC1 were studied using model (simulated) fly ashes which
were formulated using oxides of silicon, aluminum, calcium, iron, and copper. These oxides are
significant components in a variety of fly ashes ranging from coal to MSW, and they may be
instrumental in promoting catalytic surface reactions involving Hg°, HC1, and other flue gas
components.
EXPERIMENTAL
A schematic of the experimental setup used to study oxidation of Hg° is shown in Figure 1.
A low concentration (40 ppbv) of Hg° vapor was generated by using a Hg permeation tube
surrounded by a temperature-controlled water bath. The Hg° vapor was carried by a nitrogen (N2)
stream and mixed with carbon dioxide (CO2), oxygen (O2), HC1, sulfur dioxide (SO2), and water
vapor (H2O) at a constant total system flowrate of 300 cmVmin (at a standard temperature of 25 °C
and pressure of 101.4 kPa). For the gas-phase tests, the Hg°-laden simulated flue gas stream
passed through the empty oxidation reactor, surrounded by a temperature-controlled furnace, with
2 seconds of residence time at 850 °C. For the heterogeneous tests, 0.25 g of the model fly ash to
be studied was placed in the oxidation reactor which was maintained at the desired bed temperature
by a temperature-controlled heating tape. The heterogeneous tests were performed at much lower
temperatures (e.g., 250 °C) than the gas-phase tests.
At the beginning of each test, the concentration of Hg° generated by the permeation tube
(inlet concentration, HgJ was measured by an on-line ultraviolet (UV) Hg° analyzer, which does
not respond to oxidized forms of Hg. During each test, the post-reaction or outlet Hg°
concentration (Hgoul) was measured continuously using the UV analyzer. Since Hg in the flue gas
exists as either elemental or oxidized forms, the difference between the Hgir and the Kg™,
concentrations 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 x (Hgin- Hgout)/Hgin
Because H2O creates interferences in the UV analyzer, it was selectively removed from the
simulated flue gases by a gas sample dryer prior to entering the analyzer. The entire reaction
system showed no affinity toward Hg° and acids present in the flue gas. The UV analyzer used in
this study responds to SO2 as well as Hg°. Contributions from SO2 were corrected by placing a
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SO2 analyzer (UV, model 721AT2, Bovar Engineering, Inc.) on-line, downstream of the Hg°
analyzer. The SO2 analyzer is incapable of responding to Hg in the concentration range used in
this study. By subtracting the SO2 concentration measured by the SO2 analyzer from the
concentration measured by the Hg° analyzer, the outlet Hg° concentration was obtained.
Experiments were conducted over two temperature ranges: a high range (515 to 754 °C)
and a low range (150 to 250 °C). The high range represents temperatures downstream of
combustion chambers where equilibrium calculations indicate that the speciation of Hg° is
beginning to shift to oxidized forms of Hg. The low range represents temperatures where fly ash
is removed from flue gases by PM control devices (electrostatic precipitators and fabric filters).
The range of experimental temperatures and simulated flue gas compositions explored during the
first phase of the Hg° oxidization tests are summarized in Table 1. The compositions of model fly
ashes used during the experiments are given in Table 2.
RESULTS AND DISCUSSION
Homogeneous Gas-Phase Oxidation
The gas-phase experiments were designed to study the gas-phase oxidation of Hg° in the
post-combustion region of a boiler, including the duct region (in-flight oxidation) upstream of the
APCS, or dry particulate collectors (prior to scrubbers), for residence times of several seconds.
The "base case" simulated flue gas (dry) used in 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 HC1
concentrations: 50,100, and 200 ppmv, typical of U.S. coal combustion processes. The effects of
SO2 and H2O were studied at concentrations of 500 ppmv and 1.7% (mole), respectively, to
simulate the combustion of a low sulfur-containing fuel such as coal.
No gas-phase oxidation of Hg° was observed at temperatures below 500 °C with residence
times of 3 to 4 seconds, which is in agreement with a previous study (5). This indicates that the
Hg species in simulated combustion flue gases did not achieve chemical equilibrium; equilibrium'
calculations predict complete oxidation of Hg° to HgCl2 at temperatures below 600 °C with 50 to
200 ppmv HC1 (5,6). As indicated by the results shown in Figure 2, significant oxidation of Hg°
(about 27%) was observed at the highest test temperature (754 °C) and highest HC1 concentration
(200 ppmv). The results also show that increasing the HC1 concentration caused an increase in the
Hg° oxidation at each of the high test temperatures. These observations are in agreement with the
MWC field test results which indicated that significant oxidation of Hg° to HgCl2 may occur in the
post-combustion zone of a waste incinerator which has relatively high HC1 concentrations (7). The
gas-phase oxidation of Hg° in coal combustion flue gas is not expected to be as high due to the
relatively low HC1 concentrations.
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The effects of adding either 500 ppmv SO2, or 500 ppmv SO2 plus 1.7 % (mole) of H2O to
the base case flue gas on the gas-phase oxidation of Hg° were studied at a temperature of 754 °C.
The results are shown in Figure 3. It was found that the presence of SO2 and H2O inhibit gas-
phase Hg° oxidation. The mechanisms of this inhibition are not known. The gas-phase oxidation
reactions of Hg° are not well understood, and they may include many elementary reactions between
Hg° and compounds of O2, H2, C12, and S (8). A chlorinating agent, such as the Cl-free radical, is
most probably needed in the gas-phase Hg° oxidation reactions. The observed inhibition effects
may be caused by the scavenging of Cl-free radicals by compounds associated with SO2 and HjO.
The results of a preliminary investigation to study the effectiveness of Cl^ as an oxidization
agent show that, at temperatures as low as 40 °C and with 50 ppmv of C12, complete gas-phase
oxidation of Hg° was observed at reaction times of less than 2 seconds. These results are in
agreement with those from studies conducted by other researchers (8). They indicate that C^ is a
much more reactive chlorinating agent than HC1. These results support the postulation that C12 is
an intermediate species in the oxidation of Hg° in flue gases containing HC1. Thus, an important
parameter that influences the oxidation of Hg° is the ratio of HCl/C^ in coal or waste combustion
flue gases.
Heterogeneous Catalytic Oxidation
Experiments were conducted to study Hg° oxidation in the presence of fly ash at
temperatures typically found in PM control devices. The experiments most closely simulate the
conditions in baghouses, where the flue gas penetrates a filter cake of fly ash at temperatures below
250 °C. The potential for fly-ash-mediated reactions was investigated by using a fixed-bed
oxidation reactor (Figure 1). The effects of flue gas composition and fly ash composition were
studied using model fly ashes composed primarly of metal oxide particles (particle diameter smaller
than 50 ^im). The fixed-bed reactor was packed with the model fly ash, and Hg° oxidation across
the bed was effected by passing simulated flue gases through the bed. The base case flue gas
composition during the heterogeneous tests was: 40 ppbv Hg°, 50 ppmv HC1, 5% (mole) CO2, 2%
(mole) O2, and balance N2. H2O (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 composition of simulated fly ashes used in these studies is shown in Table 2. The
effect of potentially reactive fly ash components was studied by adding a number of metallic oxides
to a base-case two-component mixture consisting of alumina and silica. The primary focus was on
metallic oxides commonly found in fly ash: ferric oxide (Fe2O3), calcium oxide (CaO), and cupric
oxide (CuO). Initial tests with the two-component base-case fly ash over a temperature range of
150 to 250 °C did not display oxidation of Hg° in these simulated flue gases either in the presence
or absence of SO2 and H2O. Thus, neither alumina nor silica appears to be active in promoting the
oxidation of Hg° at flue gas cleaning temperatures.
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Experiments with the three-component model fly ashes, conducted using the high iron
content fly ash, showed significant oxidation of Hg°. The gas residence time in the empty reactor
was approximately 2 seconds. As shown in Figure 4, steady state oxidation conditions across the
model fly ash bed were achieved after 15 to 20 minutes of exposure to the simulated flue gases.
These steady state oxidation conditions were maintained for up to'2 hours, the time limit of the
experiments. The heterogeneous mechanisms of Hg° oxidation are not well understood. It is
probable that Fe2O3 produces an active chlorinating agent from gas-phase HC1, which
subsequently attacks Hg° molecules and results in the formation of HgCl2. Hg° oxidation levels as
high as 95% were observed when 14% Fe2O3 was present in the model fly ash. The high Fe
content found in a Pittsburgh coal fly ash (9) suggests that the high Kg** concentrations measured
when burning this coal resulted from the heterogeneous catalytic oxidation of Hg°(4).
The oxidation of Hg° over high-iron-containing fly ash was studied with 500 ppmv SO2
and 1.7% (mole) H2O added to the base-case flue gas. The 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, but it appeared to cause a 15 or 20% increase at 150 °C. However, the
presence of both SO2 and H2O caused a drastic decrease in the catalytic oxidation of Hg°,
especially at the lower temperatures. The inhibition effect of SO2 and H2O may be attributed to the
scavenging of a catalytically generated chlorinating agent such as C12 through a possible reaction
shown below:
C12 + SO2 + 2H2O <=> 2HC1 + H2SO4
The inhibition effect of SO2 and H2O on catalytic Hg° oxidation needs to be considered when
burning sulfur-containing fuels such as coal.
The potential catalytic activities of CaO and CuO and the effects of the Fe and Cu contents
of the three-component model fly ashes on the oxidation of Hg° were also investigated. The
results are shown in Figure 6. The CaO-containing model fly ash did not cause Hg° oxidation in
absence of SO2 and H2O, suggesting that CaO is an inert fly ash component relative to Hg°. CuO
is a much more active catalyst than Fe2O3. A model fly ash containing 0.1% CuO exhibited
catalytic activity for Hg° oxidation similar to that for a model fly ash containing 14% Fe2O3. The
high catalytic reactivity of CuO on clilorinating agent formation from HC1 is also indicated by the
formation of dioxins in MSW combustors (10). The mechanism for the chlorinating agent
formation is presumed to be the Deacon process reaction shown below:
2HC1 + 1/2O2 <=> C12 + H2O
The chlorinating agent (C12) was possibly produced at a much higher rate in the presence of Cu
compared to that in the presence of Fe and led to higher oxidation of Hg° and formation of HgCl2.
As illustrated in Figure 6, lowering the amounts of Fe and Cu in the three-component model fly
ashes reduced the heterogeneous oxidation of Hg°. A model fly ash containing 1% CuO was the
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most active catalyst in these tests; oxidation of Hg° reached approximately 95% over the
temperature range of 150 to 250 °C. While changes in temperature did not significantly affect the
rate of Hg° oxidation for the 0.1% Fe2O3 fly ash (little oxidation) or the 1.0% CuO model fly ash
(>95% oxidation), it significantly affected Hg° oxidation rates for the three other model fly ashes.
Increasing reactor temperatures resulted in increasing oxidization rates.
The effect of SO2 and HjO on the oxidation of Hg° across the fixed beds of the three-
component model fly ashes was studied at a bed temperature of 250 °C. Results are shown in
Figure 7. The inhibition effect of SO2/H2O was observed to be very strong for the Fe-containing
model fly ashes, while a less significant inhibition effect was observed for the Cu-containing
model fly ashes. The three-component model fly ash containing 7% CaO exhibited very interesting
behavior in the presence of SO2 and H2O. It did not show any Hg° oxidation in the presence of
HC1 without SO2 and H2O. However, after an induction period of approximately 20 minutes, the
presence of SO2 and H^O resulted in a steady state Hg° oxidation of approximately 15% across this
model fly ash. Adsorption of SO2 by CaO may have created active sulfur sites on the surface of
the CaO-containing model fly ash, which reacted with the Hg° molecules in the gas phase to
produce mercuric sulfide (HgS) molecules on the ash surface. Sulfite or sulfate may be the active
sites, created by reaction between SO2 and CaO.
CONCLUSIONS
The results of the gas-phase experiments suggest that Hg° oxidation occurring in simple
simulated combustion flue gas mixtures in the presence of HC1 is very slow and proceeds at
measurable rates only at high temperatures (>700 °C) and relatively high HC1 concentrations (>200
ppm). A concentration of 500 ppm SO2 in flue gases inhibits oxidation by HC1. However, the
gas-phase reaction of Hg° and Clj is very fast even at low temperatures (40 °C). The observed SO
effect suggests that homogeneous gas-phase Hg° oxidation is unlikely to occur in coal combustion
processes, as U.S. coals typically have relatively high sulfur and low chlorine contents.
The effects of fly ash parameters on the oxidation of Hg° were studied using model fly
ashes which contained major fly ash inorganic constituents. CuO and Fe2O3 showed significant
catalytic activity in oxidizing Hg° with 50 ppmv of HC1 in the simulated flue gas over the
experimental temperature range of 150 to 250 °C. It is speculated that these two transition metal
oxides produce an active chlorinating agent, probably C^, from HC1 through a multi-step
heterogeneous Deacon process reaction. Subsequent rapid reaction of the chlorinating agent with
the gaseous Hg° produces large quantities of HgCl2. CuO is a much more active catalyst than
Fe2O3, and the production of the chlorinating agent from HC1 in the presence of this Cu catalyst
appears to proceed much faster than with Fe. The increases in catalytic activity of the model fly
ashes with increasing reaction temperature suggest that the catalytic Hg° oxidation process is
2
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kinetically controlled. The presence of SO2 and H2O in the simulated flue gas inhibited the catalytic
activity of the Fe-containing fly ashes more strongly than did the Cu-containing model fly ashes.
The catalytic Hg° oxidation may not be significant in combustion of sulfur-containing fuels such as
coal. CaO exhibited a different behavior than did the Fe and Cu oxide catalysts. Low oxidation of
Hg° (15%) was observed for the Ca-containing model fly ashes when SO2 was added to the
simulated flue gas. Oxidation of Hg° may be caused by the formation of Hg-sulfur bonds on the
surfaces of CaO.
REFERENCES
1. Krishnan, S.V.; Gullett, B.K.; and Jozewicz, W. "Sorption of Elemental Mercury by
Activated Carbons," Environ. Sci. Technol., 28:8, 1506,1994.
2. Hall, B.; Lindqvist, O.; and Ljungstrom, E. "Mercury Chemistry in Simulated Flue Gases
Related to Waste Incineration Conditions," Environ. Sci. Technol., 24:108, 1990.
3. Devito, M.S.; Tunati, P.R.; Carlson, R.J.; and Bloom, N. "Sampling and Analysis of
Mercury in Combustion Flue Gas," Proceedings of the EPRI's Second International Conference
on Managing Hazardous Waste Air Pollutants, Washington, DC, 1993.
4. 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.
5. 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, Toronto, Ontario, Canada, 1997.
6. Krishnan, S.V.; Gullett, B.K.; and Jozewicz, W. "Mercury Control by Injection of Activated
Carbon and Calcium-Based Sorbents," Proceedings of Solid Waste Management Conference,
Thermal Treatment & Waste-To-Energy Technologies, Washington, DC, 1995.
7. Kilgroe, J. D. "Control of Dioxin, Furan, and Mercury Emissions from Municipal Waste
Combustors," Journal of Hazardous Materials, 47, 163-194, 1996.
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8. Hall, B.; Schager, P.; and Lindqvist, O. "Chemical Reactions of Mercury in Combustion Flue
Gases," Water, Air, and Soil Pollution, 56, 3-14, 1991.
9. Bool, L.E.; Helble, 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," DOE Report No.
PSIT-1178/TR-1407, Department of Energy, Pittsburgh Energy Technology Center, September
1995.
10. 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.
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Table 1. Summary of Experimental Temperatures and Simulated Flue Gas
Compositions
Temperature* f°Q
Low Temperature Tests 150 200 250
High Temperature Tests 515 634 754
Flue Gas Composition**
Concentration of HC1 (ppmv)
Concentration of SO2 (ppmv)
Concentration of H2O (mole%)
Concentration of CO2 (mole%)
Concentration of O2 (mole%)
Concentration of N2 (mole%)
50 100 150
0 500
0 1.7
5
2
Balance
* with + 5.0 °C measurement uncertainty
** with + 2.8% measurement uncertainty
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Table 2. Model Fly Ash Compositions (wt%*)
Model Fly Ash
Base Composition
(Two-Component)
Three-Component,
High Fe
Three-Component,
Medium Fe
Three-Component,
LowFe
Three-Component,
High Cu
Three-Component,
Medium Cu
Three-Component,
HighCa
Content
AIA
22
19
22
22
22
22
21
Si02
78
67
77
78
77
78
72
Fe203
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
* with ±0.0017% measurement uncertainty
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Mercury Generation
System
N
Carbon Trap
Manifold
Water Bath
N
,t
3-Way Valve
On-off Valves
TTt
Humidifier
Carbon Trap
Rotameter
Water
Analyzer
'HCI
SO,
Air/CO,
Data Acquisition
Elemental
Mercury
Analyzer
3-Way Valve
CO
CL
CD
o
o
(0
0)
CC
c
o
"^
(0
2
g
3-Way Valve
Water
Removal
System
Figure 1. Schematic of the elemental mercury oxidation reactor system.
12
-------
c
o
'x
O
30
25
20 ^
15 <-
515 °C •634°CH754°C
5 ^
0
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 and H2O.
13
-------
30 f-
25 7-
c20
o
I 15
'x
O
10
O)
X
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 H2O on the gas-phase oxidation of Hg° at 754 °C and three
different HCI concentrations.
14
-------
100
***********
80
60
.g
*j
(0
o° 40
0)
20
o -
o
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 SO2 and H2O; effects of time of
exposure and bed temperature.
15
-------
500 ppmv SO2
1.7%H20
500 ppmv SO2
noH2O
no SO2/H2O
c
g
'•+2
CO
•g
"x
O
100 t
80 -
60 -
o
D)
I
40
20
0
150 200
Bed Temperature (C)
250
Figure 5. Effects of SO2, H2O, 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.
ifi
-------
150°C •200°C •250°C
100 t
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 SO2 and H2O.
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100
500 ppmv SO2
1.7% H20
no SO2/H2O
500 ppmv SO2
no H2O
Figure 7. Effects of SO2 and H20 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.
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NRMRL-RTP-P-313
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA 600/A-98/075
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Mercury Control Research: Effects of Fly Ash and
Flue Gas Parameters on Mercury Speciation
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7.AUTHORS c w_ Lee and j. D> Kilgroe (EPA), and
S. B. Ghorishi (ARCAD1S)
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
ARCADIS Geraghty and Miller, Inc.
P. O. Box 13109
Research Triangle Park, North Carolina 27709
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-D4-0005
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
Published paper; 4-9/97
14. SPONSORING AGENCY CODE
EPA/600/13
i».SUPPLEMENTARY NOTES APPCD projectofficer is Chun Wai Lee, Mail Drop 65, 919/541-
7663. For presentation at 6th Annual Waste-to-Energy Conference, 5/11-13/98,
Miami, FL.
16. ABSTRACT The paper discusses results ofDencn-scale experiments to study the effects
of flue gas and fly ash parameters on the oxidation of elemental mercury (Hgo) in
simulated flue gases containing hydrogen chloride (HCl). Gas-phase studies indicate
that the in-flight post-combustion oxidation of Hgo in the presence of HCl is very
slow and proceeds at measurable rates only at high temperatures (> 700 C) and high
HCl concentrations (>200 ppm). The presence of sulfur dioxide (SO2) and water va-
por (H2O) in the simulated flue gas. significantly inhibits the gas-phase oxidation of
Hgo. On the other hand, a preliminary investigation indicates that the gas-phase re-
action of Hgo with chlorine (C12) is fast. At 40 C and in the presence of 50 ppm C12,
100% of the input Hgo was oxidized to mercuric chloride in less than 2 seconds, indi-
cating that C12 is a much more active chlorinating agent than HCl. The effects of fly
ash composition were investigated using a fixed-bed reactor containing different mo-
del fly ashes (simulated fly ash) consisting of mixtures of some major components
found in coal and municipal waste combustor fly ashes. Work to date has focused on
the potential catalytic oxidation of Hgo by two- and three-component model fly ashes
composed of mixtures of alumina, silica, ferric oxide, cupric oxide, and calcium
oxide.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Pollution
Mercury (Metal)
Fly Ash
Flue Gases
Oxidation
Hydrogen Chloride
Sulfur Dioxide
Water
Chlorine
Pollution Control
Stationary Sources
13B
07B
21B
07D
07 C
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
18
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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