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Enhancement of Mercury Capture by the Simultaneous Addition
of Hydrogen Bromide (HBr) and Fly Ashes in a Slipstream Facility
Yan Cao, Quan-Hai Wang, Jun Li, Jen-Chieh Cheng,
Chia-Chun Chan, Marten Cohron, and Wei-Ping Pan
Environ. Sci. Technol., 2009, 43 (8), 2812-2817• DOI: 10.1021/es803410z • Publication Date (Web): 12 March 2009
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Environ. Sci. Technol. 2009, 43, 2812–2817
Enhancement of Mercury Capture by
the Simultaneous Addition of
Hydrogen Bromide (HBr) and Fly
Ashes in a Slipstream Facility
Y A N C A O , * ,† Q U A N - H A I W A N G , †
J U N L I , †,‡ J E N - C H I E H C H E N G , †,§
C H I A - C H U N C H A N , †,§ M A R T E N C O H R O N , †
AND WEI-PING PAN†
Institute for Combustion Science and Environmental
Technology, Western Kentucky University, Bowling Green,
Kentucky 42101, North China Electric Power University,
BaoDing 071003, HeBei, P.R. China, and Mingchi University,
Taipei, Taiwan
Received December 2, 2008. Revised manuscript received
February 13, 2009. Accepted February 23, 2009.
Low halogen content in tested Powder River Basin (PRB)
coals and low loss of ignition content (LOI) in PRB-derived fly
ash were likely responsible for higher elemental mercury
content (averaging about 75%) in the flue gas and also lower
mercury capture efficiency by electrostatic precipitator
(ESP) and wet-FGD. To develop a cost-effective approach to
mercury capture in a full-scale coal-fired utility boiler burning
PRB coal, experiments were conducted adding hydrogen
bromide (HBr) or simultaneously adding HBr and selected fly
ashes in a slipstream reactor (0.152 × 0.152 m) under real flue
gas conditions. The residence time of the flue gas inside the
reactor was about 1.4 s. The average temperature of the slipstream
reactor was controlled at about 155 °C. Tests were organized
into two phases. In Phase 1, only HBr was added to the
slipstream reactor, and in Phase 2, HBr and selected fly ash
were added simultaneously. HBr injection was effective (>90%)
for mercury oxidation at a low temperature (155 °C) with an
HBr addition concentration of about 4 ppm in the flue gas.
Additionally, injected HBr enhanced mercury capture by PRB
fly ash in the low-temperature range. The mercury capture
efficiency, at testing conditions of the slipstream reactor, reached
about 50% at an HBr injection concentration of 4 ppm in the
flue gas. Compared to only the addition of HBr, simultaneously
adding bituminous-derived fly ash in a minimum amount (30 lb/
MMacf), together with HBr injection at 4 ppm, could increase
mercury capture efficiency by 30%. Injection of lignitederived fly ash at 30 lb/MMacf could achieve even higher
mercury removal efficiency (an additional 35% mercury capture
efficiency compared to HBr addition alone).
1. Introduction
Mercury is a persistent bioaccumulative toxic element (1).
Coal-fired utilities are the major unregulated mercury
emission source of the total anthropogenic mercury emission
* Corresponding author e-mail: yan.cao@wku.edu; phone: 270779-0202; fax: 270-745-2221.
†
Western Kentucky University.
‡
North China Electric Power University.
§
Mingchi University.
2812
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 8, 2009
inventory (2). Mercury occurs in the flue gas of coal-fired
utility boilers as elemental mercury Hg(0), oxidized mercury
Hg(2+), and particle-bound mercury Hg(P) (3). Mercury
transformation (oxidation and adsorption on fly ash) in the
flue gas is critical for effective Hg control by air pollution
control devices (APCDs) in utility boilers, such as cold-side
electricstatic precipitator (ESP), fabric filter (FF), and wet
flue gas desulfurization (W-FGD) (4). Previous work shows
that cold ESP and FF can capture Hg(P), W-FGD can capture
Hg(2+), and selective catalytic oxidation (SCR) can enhance
Hg(0) oxidation (3).
However, the U.S. Environmental Protection Agency (EPA)
Information Collection Request (ICR) Hg emission database
and other field tests regarding the potential effects of
combining an SCR and a wet-FGD on mercury capture
indicate that, compared to wet-FGD alone, mercury capture
increased when bituminous coals were burned, but not when
Powder River Basin (PRB) coals were burned. The low halogen
content of PRB coals is responsible for their failure to enhance
Hg(0) oxidation by SCR (3). On the other hand, mercury
emissions control using an SCR for enhancing the Hg(0)
oxidation depends upon the availability of a wet-FGD to
subsequently capture the Hg(2+). It was reported that only
about 25% of utility boilers had a wet-FGD installed for SOx
control. In comparison, 80% of utility boilers are only
equipped with a cold-side ESP (3). For these utility boilers,
lower Hg capture efficiency was found, especially for PRBfired utility boilers. Future retrofitting of boilers with SCR
and wet-FGD also may not achieve the cobenefit of Hg
capture by APCDs when PRB coal is burned. Activated carbon
injection (ACI) doped with bromine has been a prevailing
technology for mercury control where only particle control
devices are available in coal-fired power plants (5-13). It
has been proven that ACI technologies for mercury capture
are about 90% efficient (14-17). However, adding activated
carbon to fly ash complicates the subsequent use of fly ash,
especially in PRB coal-fired boilers. This is a dominant issue
in the coal-fired power plants of the United States because
PRB coal is the single largest source of coal mined in the U.S.
and makes up the largest coal deposits in the world. In 2007,
the Powder River Basin area alone produced 436 million short
tons (396 million tons) of coal, which is more than twice the
production of second-place West Virginia (18, 19).
The EPA’s ruling on mercury emissions control (Clean
Air Mercury Rule, CAMR) would have cut mercury emissions
from coal-fired power plants by 69% by 2018 (20), but could
create mercury hot spots around the country. On February
8, 2008 the U.S. District Court overturned EPA’s CAMR, which
would ultimately require a 90% cut in mercury emissions
from coal-fired power plants from their current level, or would
require the application of the best available control technology. This was followed on July 11, 2008 with overturn of the
Clean Air Interstate Rule (CAIR) by the Federal Court. With
these recent changes, there is considerable uncertainty on
the future of mercury regulations and regulated entities.
Currently, available mercury control technologies can achieve
90% mercury emission control. The economics of technologies and subsequent impacts of downstream byproduct
utilization will be critical for coal-fired utilities to accept
mercury control technologies.
Our previous studies indicated that HBr addition to PRBderived flue gas at high temperatures (330 °C) could
significantly enhance mercury oxidation (21, 22), but it did
not enhance mercury adsorption on fly ash at such a high
temperature. Therefore, a new test using HBr injection at a
low temperature (155 °C) was conducted and presented in
10.1021/es803410z CCC: $40.75
 2009 American Chemical Society
Published on Web 03/12/2009
FIGURE 1. Schematic of configuration of the slipstream reactor.
this article. Furthermore, we also propose simultaneous
injection of HBr and fly ash into flue gas to enhance Hg(0)
oxidation and subsequently enhance Hg capture by brominated fly ash. Simultaneous additions of HBr and fly ash in
a slipstream reactor (0.152 × 0.152 m) using actual flue gas
were conducted in full-scale coal-fired utility boilers burning
PRB coal. Here we report a series of on-site slipstream tests
to verify this new strategy for Hg emission control and also
optimize the proposed technology while controlling the total
cost for mercury capture.
2. Experimental Section
2.1. Test Facility. The test facility was designed and manufactured to simulate “full-scale” applications of the ductwork
configuration in a coal-fired utility boiler. The schematic
configuration and setup are shown in Figure 1. In this study,
the addition of HBr or the simultaneous addition of HBr and
the selected fly ash in a slipstream reactor (0.152 × 0.152 m)
under a real flue gas situation was conducted in a full-scale
coal-fired utility boiler burning PRB coal. Flue gas was
introduced into the slipstream reactor from the economizer
outlet port of the utility boiler, passing through the slipstream
reactor and then back into the utility’s ductwork. During the
tests, the residence time of flue gas inside the reactor was
about 1.4 s. The average temperature of the slipstream reactor
was controlled at about 155 °C. Tests were organized into
two phases. In phase 1, only HBr was added to the slipstream;
in phase 2, there were simultaneous additions of HBr and
selected fly ash.
Hydrogen bromide (HBr) gas was injected into the system
either from a pressurized cylinder or a diluted HBr acid liquid
injector, at a predetermined concentration using nitrogen
as the carrying gas. The desired spiking concentration of
HBr inside the slipstream reactor could be adjusted by a
mass flow controller or liquid injector. To ensure the
controlled and even distribution of the HBr, two static mixers
were installed at different locations in this facility. The HBr
injection port was located below the Hg sampling port at the
inlet, which left this sampling port unaffected (Figure 1). An
adsorbent screw feeder was used for delivery of adsorbents
(fly ashes or the commercial Darco-LH mercury adsorbent)
into the slipstream reactor. With the assistance of a pressure
balance line located between the adsorbent hopper and the
AC injection port, the injection rate was unaffected by
pressure fluctuations inside the reactor.
2.2. Mercury Sampling and Analysis. The reaction facility
was equipped with a mercury semicontinuous emissions
monitor (SCEM), which was used for measuring mercury
variations during testing. The Ontario Hydro (OH) method
was used for validation of the Hg-SCEM data. For both
methods, an inertial sampling probe was used to sample
the flue gas at the inlet and the outlet sampling ports of
the slipstream facility. The sampling probe temperatures were
controlled to match the temperatures of flue gases in the
reactor at locations of the sampling probe installation. A
detailed description of quality assurance and quality control
(QA/QC) in both methods was given in previous publications
(23, 24).
2.3. Characterization of Coal and Fly Ash. Under low
temperature operation (155 °C) of the slipstream reactor,
PRB coal and ash samples were collected from coal hoppers
and ESP ash hoppers at the same time the slipstream testing
was conducted. Analysis data on coal and ash samples are
presented in Tables 1 and S1 (see Supporting Information),
respectively. It was found that major constituents in coal
and ash samples during two testing phases were almost
identical. During phase 1, the average sulfur and mercury
content in the tested coal was about 0.63% with a relative
standard variation of 23%, and 0.13 ppm with a relative
variation of 28%, respectively. The detectable halogen
constituents, chlorine and fluorine, in coal samples averaged
164 ppm and 43 ppm, respectively. There was also no major
difference in the loss of ignition (LOI) and mercury content
in collected fly ash during phase 1, as shown in Table S1.
During phase 2, the average sulfur and mercury content in
the tested coal was about 0.59% with a relative standard
variation of 20% and 0.12 ppm with a relative variation of
38%, respectively. The chlorine and fluorine content in coal
samples was also lower, averaging 118 ppm and 80 ppm,
respectively. As shown in Table S1, particle-bound mercury
(Hg(P)) and LOI, which were found in ESP fly ash collected,
were about 0.7 ppm and 0.8% for phase 1, and 0.6 ppm and
0.6% for phase 2. It seemed that there were also no major
differences in the LOI and Hg(P) during these two testing
periods. Based on analysis of collected coal and ash samples
from this full-scale utility boiler, it could be concluded that
operation the boiler unit tested was relatively stable. During
phase 2, additional fly ashes were also collected at the outlet
of the slipstream reactor using a standard EPA flue gas
sampling probe, in the front of which a finger filter was
installed for ash sample collection.
3. Results and Discussion
3.1. Mercury Oxidation and Adsorption on Fly Ash during
HBr Addition at 155 °C. Unlike when HBr was added during
a high temperature range (above 300 °C), Hg(VT) (the total
vapor phase mercury) at the outlet of the slipstream reactor
decreased during HBr addition at a low temperature (about
155 °C). Under testing conditions in this study (temperature
of 155 °C and a residence time of 1.4 s), the overall mercury
removal efficiency (as defined in the eq 1) was increased by
increasing the HBr concentrations in the flue gas.
The overall mercury removal efficiency (%) )
[Hg(VT)inlet - Hg(VT)outlet] /Hg(VT)inlet (1)
And the total mercury oxidation efficiency (%) )
[Hg(VT)inlet - Hg(0)outlet] /Hg(VT)inlet (2)
As indicated in Figure 2, HBr addition with concentrations
of 1.1, 1.8, 2.65, and 3.5 ppm introduced into the flue gas
increased the overall mercury removal efficiency inside the
VOL. 43, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
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TABLE 1. Characterization of Coals and Collected Ash at the Outlet of Testing Slipstream Reactora
phase 1
“As deter”
dry basis
ash
vol.
sulfur
carbon hydrogen nitrogen oxygen chloride mercury fluoride bromide
(%) mat. (%) (BTU/lb) Btu (%) (%)
(%)
(%)
(ppm) (ppm)
(ppm)
(ppm)
(ppm)
coal sample
ADL
moisture
%
coal day 1
coal - day 3
coal - day 5
coal - day 7
coal - day 9
coal - day 11
average
relative
variation
11.67
19.14
26.67
26.64
24.11
18.58
21.13
17.10
11.89
6.32
6.50
8.22
13.59
10.60
coal - day 1
coal - day 2
coal - day 3
coal - day 4
coal - day 5
coal - day 6
average
relative
variation
41.16
42.88
43.67
43.40
43.84
43.72
43.11
0.73
0.61
0.62
0.59
0.61
0.61
0.63
12342
11441
11939
11970
11962
11918
11929
73.43
69.05
70.45
70.33
70.55
70.80
70.77
22.6%
17.06
15.98
15.68
24.48
13.99
12.54
16.62
injected fly ashes
bituminous #1
bituminous #2
bituminous #3
SB (PRB)
lignite
darco-LH AC
a
8.66
9.74
7.63
7.10
7.21
7.52
7.98
13.43
17.21
17.17
13.04
21.79
20.98
17.27
7.65
7.77
7.06
8.61
9.41
7.25
7.96
43.82
44.13
44.29
44.07
43.78
45.15
44.21
0.58
0.57
0.55
0.60
0.69
0.57
0.59
4.67
4.76
4.87
4.86
4.86
4.87
4.81
1.02
0.86
0.93
0.95
0.92
0.90
0.93
11.48
14.98
15.51
16.17
15.84
15.30
14.88
6.2%
phase
11833
11820
11927
11681
11557
11872
11782
2 coal samples
75.24
4.46
76.17
4.54
77.44
4.60
76.41
4.55
75.72
4.49
77.77
4.58
76.46
4.54
20.0%
1.28
1.14
1.09
1.06
1.11
1.11
1.13
10.79
9.81
9.27
8.76
8.58
8.73
9.32
3.3%
165
176
143
187
161
152
164
0.14
0.14
0.16
0.10
0.12
0.12
0.13
43
48
58
37
49
21
43
ND
ND
ND
ND
ND
ND
ND
21.3%
27.9%
86.7%
108
117
106
115
140
123
118
0.10
0.11
0.09
0.14
0.12
0.13
0.12
77
81
76
88
80
80
80
26.7%
37.9%
13.5%
ND
ND
ND
ND
ND
ND
ND
LOI
(%)
SSA (BET)
(M2/g)
Na2O
(%)
MgO
(%)
Al2O3
(%)
SiO2
(%)
CaO
(%)
K2O
(%)
SO3
(%)
P2O6
(%)
BaO
(%)
SrO
(%)
Fe2O3
(%)
MnO
(%)
TiO2
(%)
4.57
3.49
35.2
0.75
0.46
4.41
3.62
0.42
5.42
0.7
305
0.01
0.15
0.01
1.12
0.27
1.20
0.95
0.66
4.65
3.15
25.79
18.63
6.00
18.76
18.76
49.75
43.49
23.26
38.39
58.12
3.35
8.88
40.14
25.20
12.41
2.54
2.53
0.60
0.63
0.81
2.39
1.82
24.99
1.96
0.44
0.33
0.12
0.09
0.97
0.17
0.15
0.01
0.01
0.67
0.26
0.13
0.03
0.05
0.42
0.22
12.54
22.21
3.81
5.49
3.82
0.02
0.01
0.01
0.02
0.05
1.81
1.19
0.39
1.72
1.51
LOI, loss of ignition; SSA, specific surace area; ADL, air-dry loss; ND, not detected.
FIGURE 2. Correlation of HBr injection concentrations and mercury removal efficiency in the slipstream reactor.
slipstream reactor to about 30%, 40%, 47%, and 50%,
respectively. The mercury removal efficiency inside the
slipstream reactor was only 5% on average when HBr was
not added. Hence, a net mercury removal efficiency of about
45% was achieved for the maximum addition of 3.5 ppm HBr
into the slipstream reactor. The HBr addition significantly
increased the mercury capture capability of the PRB-derived
fly ash at 155 °C. The curve of the overall mercury removal
efficiency correlated with the HBr addition concentrations,
but became flat as the HBr injection rate increased (Figure
2). This may be due to the interactions among HBr, fly ash,
and mercury. Shorter residence time of HBr within the
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 8, 2009
slipstream reactor and a less developed pore structure of
PRB-derived fly ash (25) may have made the adsorption of
HBr on the fly ash less effective on mercury capture at the
increased HBr injection rates. If this is the case, the adsorbed
HBr or bromine species on the fly ash should be responsible
for the enhanced mercury capture capability of PRB-derived
fly ash.
Because of the total gaseous mercury decrease at the
slipstream reactor’s outlet during low temperature operation
when HBr was injected, we used the total gaseous mercury
at the inlet of the slipstream reactor to calculate the mercury oxidation efficiency, as indicated in eq 2. The total
FIGURE 3. Correlation of HBr injection concentrations and mercury oxidation efficiency in the slipstream reactor.
mercury oxidation efficiency is presented as a ratio between
the difference of the total vapor-phase mercury at the inlet
of the slipstream reactor and elemental mercury at the outlet
of the slipstream reactor [Hg(VT)inlet - Hg(0) outlet], and the
total gaseous mercury concentration at the inlet of the
slipstream reactor (Hg(VT)inlet). This gave the absolute
mercury oxidation efficiency, which included mercury oxidation processes occurring prior to introducing flue gas inside
the slipstream reactor. Figure 3 presents the variation of the
elemental mercury oxidation efficiency during HBr injection
under low temperature operation. Similar to a higher
temperature range (above 300 °C) (22), the addition of HBr
into the slipstream reactor under the lower temperature range
(155 °C) and the shorter residence time (1.4 s) also results
in significant mercury oxidation. The HBr solution addition
and HBr gas injection functioned identically with respect to
mercury oxidation. However, the effectiveness of the HBr
solution addition on mercury oxidation depended on whether
it was prevaporized prior to its injection. The prevaporization
of HBr solution could enhance the mixing of added HBr with
flue gas and fly ash. As indicated in Figure 3, the total mercury
oxidation efficiencies were about 30%, 55%, 70%, and 90%,
at HBr addition concentrations in the flue gas of 0, 0.9, 1.8,
and 3.5 ppm, respectively. The OHM data matched the SCEM
data and confirmed the effectiveness of HBr injection for
enhancing oxidation of elemental mercury under these lower
temperature operation conditions. By subtracting the total
mercury oxidation efficiency without HBr addition from that
with HBr addition, the net mercury oxidation (caused by the
HBr addition) could be calculated. It was found that this net
mercury oxidation efficiency was about 25%, 40%, and 60%
under HBr addition concentrations of 0.9, 1.8, and 3.5 ppm,
respectively.
3.2. Mercury Adsorption during the Simultaneous Addition of HBr and Selected Fly Ashes at 155 °C. Test results
on simultaneous injection of HBr and fly ashes from different
utility boilers are shown in Figure 4. The average mercury
removal efficiency by the original PRB-coal-derived fly ash
was only 3% in phase 2. With the addition of HBr at 4 ppm,
the total mercury removal efficiency increased to about 44%.
Adding HBr at 4 ppm along with commercial Darco LH
adsorbent increased the mercury removal efficiency to 76%.
This may be because the preoxidized mercury can be easily
captured by fly ash (3). Due to concerns regarding increased
LOI content in the fly ash generated when an activatedcarbon-based adsorbent is injected, a group of fly ash
samples from different utility boilers was tested with the
simultaneous addition of HBr at 4 ppm. It was found that
a minimum amount of injected fly ash resulted in the
additional mercury removal for bituminous-derived or
lignite-derived fly ashes, but not always when PRB-derived
fly ash was added (Figure 4).
For example, the simultaneous addition of HBr at 4 ppm
HBr and 10 lb/MMacf of PRB-derived fly ash (subbituminous
coal (SB) - PRB coal in this study) resulted in no increase in
mercury removal efficiency, whereas with the addition at 10
lb/MMacf of bituminous-derived fly ash #1, the mercury
removal efficiency increased to 61%. Increasing the addition
of fly ash #1 to 30 lb/MMacf increased the mercury removal
efficiency to 73%, whereas at the same addition rate, a second
bituminous-derived fly ash (#2) increased it to 76%. However,
the addition of a third bituminous-derived fly ash (#3) did
not achieve any additional mercury removal efficiency. This
fly ash was found to be a bed slag from a circulating fluidized
bed combustor. The lower Brunauer-Emmett-Teller specific
surface area (BET-SSA) (25) in this CFBC combustor slag was
likely responsible for its lower mercury capture capability
despite its higher LOI content. For comparison, bituminousderived fly ash #1 and #2 both presented good mercury
capture efficiency with minimal addition. This may be the
result of their developed pore structure. Interestingly, the
lignite-derived fly ash had even better performance on
mercury capture than the bituminous-derived fly ash. For
the addition of the lignite-derived fly ash at 10 lb/MMacf
and HBr at 4 ppm, mercury removal efficiency was 65%.
Increasing the addition of lignite-derived fly ash to about 30
lb/MMacf increased mercury removal efficiency to over 80%.
Assuming a mass balance of fly ash, the loading of the original
PRB-derived in the flue gas fly ash should be about 220 lb/
MMacf. Therefore, the maximum addition of fly ash at about
30 lb/MMacf would not be expected to dramatically change
the properties of the PRB fly ash generated.
The addition of fly ash without HBr did not significantly
increase the mercury removal efficiency. For example, the
VOL. 43, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
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FIGURE 4. Mercury removal efficiencies by simultaneous additions of HBr (at 4 ppm) and selected fly ashes.
FIGURE 5. Correlation of particle-bound mercury and fluorine, chlorine, and bromine contents on fly ashes.
mercury removal efficiency was about 15% with the addition
of CFBC slag at 30 lb/MMacf compared to 3% without the
CFBC slag addition, which did not significantly increase the
mercury removal. This was also far less than the mercury
removal efficiency of 58% obtained with the addition of both
HBr and CFBC slag. It was likely that the addition of HBr
enhanced mercury capture by fly ash, based on examples of
enhanced mercury capture using brominated activated
carbons. In this case of fly ash, the bromine content should
increase after HBr addition because of adsorption of HBr
when it is added to flue gas. Further study by characterizing
halogen contents of the fly ash (collected at the outlet of the
slipstream facility during tests) indicates, as shown in Figure
5, that there was indeed an increase of bromine content in
the fly ashes. A significant correlation between particle-bound
mercury (Hg(P)) and bromine content in fly ashes was found.
The correlative factor (R2) was about 0.767. But this was not
the case for other coal-derived halogen species in fly ashes,
such as fluorine and chlorine, as indicated in Figure 5. This
may mean that injected HBr in the ash-laden flue gas causes
bromine to bond with fly ash. This brominated fly ash has
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 8, 2009
a mercury capture capability in a low temperature range,
specifically around 150 °C.
HBr addition resulted in an enhanced mercury oxidization
under both high temperature (22) and low temperature
ranges. Therefore, it is likely that a greater occurrence of
oxidized mercury during HBr addition in the flue gas could
also contribute to the enhancement of mercury capture by
fly ash. It is well-known that oxidized mercury can be more
easily captured by fly ash than elemental mercury at a low
temperature range.
The LOI content, BET-SSA, and minor metal oxides of fly
ashes collected during the simultaneous addition of HBr and
fly ash are shown in Table 1. It was found that the LOI contents
and BET-SSA of bituminous-derived fly ashes were higher
than those of fly ashes derived from low rank coal, such as
PRB-derived coal (SB) and lignite-derived fly ashes. It was
understandable that bituminous-derived fly ash had better
mercury capture performance, compared to PRB-derived fly
ash due to the lower LOI and lower BET-SSA of PRB-derived
fly ash. Higher LOI and BET-SSA could enhance the adsorption of both HBr and mercury on fly ashes. However, lignite-
derived fly ash with both comparable lower LOI and BETSSA had even better mercury capture performance. The
reasons for this will be left for further study.
This study confirmed that at a low temperature range
(around 155 °C) and short residence time (about 1.4 s), the
addition of HBr can enhance mercury oxidization and
promote the capture of gaseous mercury by the available fly
ash in the flue gas. The doped HBr on the fly ash should be
responsible for the additional mercury capture on the fly
ash, the extent of which was dependent on fly ash properties.
With a minimal addition of HBr, small amounts of added
bituminous-derived or lignite-derived fly ashes (but not PRB
coal derived fly ash) can enhance mercury capture by injected
fly ash. Therefore, fly ash is not only an inexpensive mercury
adsorbent, but it also has a minimal impact on fly ash
properties for reutilization. In a full-scale utility boiler, longer
ductwork can achieve a longer contact time for HBr, fly ash,
and mercury, by which even higher mercury capture efficiency by fly ash can be expected. This combination of
technology could maximize mercury capture efficiency with
minimized injection rates of both HBr and Hg adsorbents
(such as commercial mercury adsorbents and selected fly
ashes), which would likely control the costs of Hg capture
using less expensive untreated fly ash. This synergistic,
simultaneous injection of both HBr and fly ash could be an
optimal technology and strategy for Hg capture in PRB-fired
utility boilers with a goal of 90% Hg control efficiency with
better economic prospects.
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Acknowledgments
We greatly appreciated Ed Morris Jr. and Steven Derenne of
We-energies for their testing coordination.
Supporting Information Available
One table. This information is available free of charge via the
Internet at http://pubs.acs.org.
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