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:
[email protected]; phone: 270779-0202; fax: 270-745-2221. † Western Kentucky University. ‡ North China Electric Power University. § Mingchi University. 2812
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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
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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 2814
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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
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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 2816
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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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