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Sep 10, 2013 - The addition of bromine to coal combustion systems, such as electric utility boilers, provides a way for increasing the capture of merc...
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Impinger-Based Mercury Speciation Methods and Gas-Phase Mercury Oxidation by Bromine in Combustion Systems Paula A. Buitrago,† Brydger Van Otten,‡ Constance L. Senior,§ and Geoffrey D. Silcox*,† †

Department of Chemical Engineering, University of Utah, 50 South Central Campus Drive, Room 3290 MEB, Salt Lake City, Utah 84112, United States ‡ Reaction Engineering International, 77 West 200 South, Suite 210, Salt Lake City, Utah 84101, United States § ADA-ES, Incorporated, 9135 South Ridgeline Boulevard, Suite 200, Highlands Ranch, Colorado 80129, United States ABSTRACT: The analysis of gas-phase mercury speciation in combustion gases containing bromine and SO2 was studied using a commercial mercury analyzer coupled with several different wet conditioning systems; the latter allowed the measurement of total or elemental mercury in the combustion gases. The total side of the conditioning system was modified to decrease measurement bias in total mercury by replacing an aqueous tin chloride solution (2 wt % SnCl2 and 3 wt % HCl) with a solution of hydroxylamine hydrochloride and sodium hydroxide (4 wt % NH2OH−HCl and 20 wt % NaOH). This change reduced measurement bias in total mercury in the presence of 50 ppm bromine species (as HBr equivalent) from 60 to 15%. Additional improvements in total mercury recovery were obtained by frequently cleaning the glass walls of the chiller, an essential component of the mercury analysis train. The required frequency increased with increased levels of Br2 injection through the burner. The impact of cleaning suggests that bromine species may be accumulating on the glass and subsequently heterogeneously oxidizing elemental mercury. Difficulties in recovering total mercury are not seen when using chlorine as the oxidant. The determination of elemental mercury in the presence of bromine and SO2 yielded results that further suggest that mercury may be heterogeneously oxidized on the glass walls of the impingers and chiller. The presence of SO2, whether added through the burner or directly to the KCl impinger, decreased the apparent extent of oxidation of elemental mercury. This decrease may be due to the adsorption of SO2 and possibly SO3 on the glass surfaces, thereby preventing the adsorption of elemental mercury and bromine species and subsequent heterogeneous oxidation of elemental mercury.

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INTRODUCTION The adverse impacts of mercury on human health and the environment, along with regulations to decrease mercury emissions, have made mercury and its reactions in combustion systems the focus of many studies.1−7 Proposed methods for reducing emissions from coal-fired power plants, one of the major sources of anthropogenic mercury, are dependent upon mercury speciation. Three forms of mercury are present in coal flue gas: elemental (Hg0), oxidized (Hg2+), and particulate bound (Hgp). Elemental mercury is more difficult to capture in existing pollutant control devices and is largely emitted to the atmosphere, while the oxidized and particulate-bound forms can be efficiently removed by existing pollutant control devices. One approach to improve mercury capture is to find methods that increase the fractions of oxidized and particulate-bound mercury. The vapor-phase concentration of elemental mercury is conveniently measured using cold vapor atomic fluorescence spectrometry (CVAFS). In some mercury continuous emission monitoring systems (CEMS), wet conditioning systems are used to prepare the sampled flue gas for measurement in the CVAFS unit. These conditioning systems frequently have two channels: one side reduces oxidized mercury species to the elemental state, and the other retains oxidized mercury but allows for the elemental fraction to continue to the analyzer. This arrangement allows just elemental mercury from the sampled gas to reach the analyzer, and the extent of oxidation is determined by the difference in the concentration between the © 2013 American Chemical Society

two channels. Wet conditioning systems generally use an aqueous solution of stannous chloride as the reducing agent and a potassium chloride solution to retain oxidized mercury by the formation of HgCl42−. Wet systems provide semicontinuous mercury measurements with the inconvenience of having to prepare aqueous solutions of reagents. They also tend to suffer from interferences. Linak et al.8 studied the effect of chlorine as Cl2 on mercury analysis using two different wet conditioning systems: the Ontario Hydro (OH) method and the alkaline mercury speciation method. They observed a bias of about 10−20% in elemental mercury for chlorine concentrations as low as 1 ppm in simulated flue gas mixtures. The authors suggested that this was caused by liquid-phase oxidation reactions because of hypochlorite ion (OCl−) in the KCl solution, leading to an apparent increase in the extent of oxidation. The oxidation was prevented by the addition of a reducing agent, 0.5 wt % sodium thiosulfate (Na2S2O3), to the solution. Subsequent investigations have supported the use of 0.5 wt % Na2S2O3.9 The addition of bromine to coal combustion systems, such as electric utility boilers, provides a way for increasing the capture of mercury by existing particulate collection devices or flue gas desulfurization scrubbers.5,7,10,11 Measuring the fate of mercury in the presence of bromine is challenging because bromine Received: July 10, 2013 Revised: September 9, 2013 Published: September 10, 2013 6255

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interferes with the performance of wet conditioning systems.10−13 Dombrowski et al.7 studied the effectiveness of different concentrations of calcium bromide on oxidizing mercury at a wall-fired, 700 MW, pulverized Powder River Basin (PRB) coal-fired unit. For mercury speciation, the authors used CEMS and the OH method. They observed that, by not using a stainless-steel probe for the CEMS and using a Teflon probe at the inlet to the flue gas desulfurization (FGD), the oxidation measurements were consistent between the two methods. However, at the inlet and outlet of the selective catalytic reduction (SCR), they were forced by high temperatures to use a stainless-steel probe. They observed apparent bromine-related interference at the exit of the SCR that resulted in low total mercury concentrations and overprediction of mercury oxidation. Babi et al.13 provide a mechanistic explanation for bromine and iodine interferences in wet conditioning systems that use stannous chloride to produce a total mercury stream. They note that the stability of the mercuric ion complexes has a regular order: HgIn > HgBrn > HgCln > HgFn. Iodide is the strongest interfering ion, and bromide is next. Interferences were not seen with chlorides. The stability of the iodide and bromide complexes is high enough that aqueous solutions of SnCl2 (3 or 6 wt % SnCl2 in 1 or 2 wt % HCl) cannot reduce Hg2+ to Hg0. The stability of the mercuric halide complexes is favored by low pH. Babi et al.13 were able to eliminate interferences by bromide and iodide by increasing the pH to 9 or greater, and they noted that, at high pH, the tin remained in solution because of its amphoteric behavior and the formation of [Sn(OH)6]4−. The objective of this study is to determine the effects of various impinger solutions on the effectiveness of wet conditioning systems that use stannous chloride and potassium chloride for application to mercury speciation in the presence of bromine species. For total measurements, the ability of the stannous chloride solution to reduce oxidized mercury was studied. The confusing effects of sulfur dioxide on measured levels of oxidized mercury were also examined. Different compositions of the impinger solutions and configurations on both sides of the conditioning system were tested. The results obtained from this investigation may be useful for the improvement of mercury measurements by wet conditioning systems.

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Figure 1. Temperature profile in a quartz-lined reactor. Two flue gas samples were taken from the bottom of the reactor, 130 cm from the quartz burner (see Figure 1), for mercury analysis and sent to the conditioning system shown in Figure 2. The

Figure 2. Baseline wet chemical conditioning system.14

temperature at the sampling point on the centerline of the reactor was about 300 °C, and the flue gas was drawn through insulated, 1/4 in. Teflon PFA tubing at the rate of 2.0 actual liters per minute. The length of the tubing ranged from 15 to 50 cm, and the length did not noticeably affect measured levels of oxidation. The impingers operated at 35 °C. In the baseline conditioning system, one stream was bubbled through a 2 wt % SnCl2 and 3 wt % HCl solution to attempt to reduce all mercury to it elemental form. This stream provided the total mercury concentration. The second stream was passed through a solution containing 10 wt % KCl and 0.5 wt % Na2S2O3 to capture oxidized mercury. Sodium thiosulfate was added to prevent oxidation of elemental mercury by halogens in solution.9 The second stream provides the elemental mercury concentration in the flue gas. The tin chloride and KCl solutions were followed by impingers containing 5 wt % NaOH solution to remove acid gases. All impingers were continuously supplied with fresh liquid solutions. A glass-lined chiller removed water from the two gas streams. A Tekran 2537A mercury analyzer measured the elemental mercury concentration of each stream. A four-port sampler controlled which stream was sampled. The concentration of oxidized mercury was calculated by difference using the total and elemental mercury concentrations. At the beginning of each test, a mass balance was closed to verify that both total and elemental mercury streams were showing the baseline concentration, 25 μg/m3, added to the furnace through the burner. When mercury

EXPERIMENTAL SECTION

The combustion reactor used in this study14 is a 50 mm outer diameter × 47 mm inner diameter quartz tube (132 cm in length) located along the center of a high-temperature Thermcraft heater. The tube extends 79 cm below the heater, is temperature-controlled, and has a quartz sampling section at the bottom with a capped end. The peak gas temperature in the electrically heated zone was 1080 °C. A methane-fired, premixed burner made of quartz glass supplied realistic combustion gases to the reactor. All reactants were introduced through the burner and passed through the flame to create a radical pool representative of combustion systems. The burner was fired at about 300 W, producing 6 standard liters per minute of combustion gases. The elemental mercury passing through the burner was generated by a PS analytical mercury calibration gas generator or “CavKit”. The temperature profile shown in Figure 1 was used in all experiments. The low-temperature region (around 350 °C) extended over 3 s, and the profile approximates flue-gas temperatures at the end of the convective pass of a boiler. These low temperatures allowed mercury oxidation reactions to occur. 6256

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oxidation occurs, the elemental concentration should decrease and the total concentration should remain at its baseline value. The major flue gas components are shown in Table 1. The gases and vapors entering the reactor passed through a methane-fired

mercury concentrations, and the total concentration was highly erratic. Figure 3 also shows a prolonged, steady decline in the elemental mercury level at 20 ppm Br addition (as HBr equivalent). When both bromine species and SO2 were present, as in Figure 4, the decrease in the elemental mercury

Table 1. Flue Gas Composition in the Reactor

a

species

concentration

O2 H2O CO2 NO SO2 HCla HBrb Hg0

0.8 vol % 16.5 vol % 7.7 vol % 30 ppm 0−500 ppm 0−500 ppm 0−50 ppm 25 μg/m3

Assuming all chlorine as HCl. bAssuming all bromine as HBr.

burner, and exit concentrations were measured using California Analytical Instruments analyzers ZRH infrared gas analyzer for CO and CO2, paramagnetic oxygen analyzer model 100P, and NOx analyzer model 300-CLD. The concentrations of CO2, O2, NOx, and Hg were measured, while H2O, SO2, HCl, and HBr were calculated by mass balance. The NO concentration was constant at 30 ppm and was generated by the methane flame.

Figure 4. Total and elemental mercury concentrations with the addition of 45 ppm bromine (as HBr equivalent) and 100 ppm SO2 using the baseline conditioning system shown in Figure 2.

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concentration was less dramatic and the total mercury concentration was stable. The data in Figures 3 and 4 suggest that, in the absence of SO2, bromine species may be oxidizing mercury in the conditioning system and that bromide ion may be preventing the reduction of Hg2+ by stannous chloride in the conditioning system. It is also possible that SO2 is inhibiting oxidation in the gas phase of the reactor. In an effort to improve the performance of the conditioning system and to understand these various hypotheses, different reducing and complexing agents were tested on the total and elemental sides. The purpose of the total side of the wet conditioning system is to reduce the oxidized mercury species in the flue gas to elemental mercury for analysis by atomic fluorescence. This mercury reading represents the total mercury concentration in the system and is the sum of the elemental and oxidized species present in the combustion gas. Several modifications to the total side of the conditioning system were tried to address the problems seen in Figures 3 and 4. In all experiments, an initial mercury balance was closed, which just involved the mercury source and the mercury analyzer. A second balance included the reactor and the burner. In these balances, no halogens or SO2 were present. Figures 3 and 4 show the initial closure of the balance; the total and elemental mercury concentrations are the same. If the conditioning system behaved as intended, the injection of halogens would decrease the elemental mercury concentration, while the total mercury concentration would remain unchanged. However, Figure 3 shows a decrease in total mercury as soon as bromine is present; this decrease disappears when the bromine addition is stopped. The decrease in total mercury could indicate low efficiency in the reduction of oxidized mercury by the stannous chloride solution and possibly the oxidation of elemental mercury species on the walls of the chiller or impingers. The baseline configuration for the total side of the conditioning system was an impinger containing 2 wt % SnCl2 and 3 wt % HCl in water, followed by an impinger containing 5 wt % NaOH, as shown in Figure 2. Various modifications of the impinger solutions were tried to increase

RESULTS Previous experimental work5,7,10,15 has shown the effectiveness of bromine as a mercury oxidant in bench- and industrial-scale combustion systems. Previous bench-scale combustion work with SO2 and chlorine9 has shown little effect of SO2 on the extent of oxidation. With SO2 and chlorine, the extents of oxidation were calculated from the difference between the total and elemental mercury concentrations. A similar procedure was expected to work for flue gas mixtures containing bromine species with or without SO2. Contrary to expectations, extents of oxidation by bromine were difficult to calculate by difference because the total and elemental mercury concentrations in the combustion gas could not be reproducibly measured. The lack of reproducibility increased with longer experiments, higher bromine concentrations, and the addition of SO2. A typical performance of the standard wet conditioning system (Figure 2) with bromine addition to the furnace is shown in Figure 3; bromine decreased the total and elemental

Figure 3. Total and elemental mercury concentrations with the addition of 20 and 30 ppm bromine (as HBr equivalent) using the baseline conditioning system shown in Figure 2. 6257

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Figure 5 shows the decrease in total mercury with concentrations of SnCl2 ranging from 2 to 10 wt % and with various HCl levels. The changes were calculated using the difference between the total side concentration and the baseline concentration of 25 μg/m3. No SO2 was added to the reactor, and the concentrations of NO and bromine were 30 and 50 ppm (as HBr equivalent). Figure 5 shows that none of the conditions tested caused a significant improvement in the recovery of total mercury. In an ideal conditioning system, there would be no decrease in total mercury in the combustion gases with the addition of bromine to the combustion system. The addition of 5 wt % NaOH to SnCl2 (case E in Figure 5) was not successful because the carbon dioxide in the flue gas rapidly neutralized hydroxide, leading to precipitation of tin hydroxide. The molar flow rate of carbon dioxide was hundreds of time that of NaOH to the impinger. Figure 6 shows the measured decrease in the total mercury concentration with 50 ppm bromine (as HBr equivalent) when different mixtures of NH2OH*HCl and NaOH were used, instead of the SnCl2−HCl solution. Hydroxylamine hydrochloride has been used for mercury concentration measurements with urine, blood, and hair samples,16−18 mostly as a stabilizing agent. It is also used as a reducing agent in synthetic and analytical chemistry. The decrease in the total mercury concentration shown in Figure 6 was calculated using the difference between the total side value and the baseline concentration of 25 μg/m3. No SO2 was added to the reactor, and there was 30 ppm NO. The data show that a solution of 4 wt % NH2OH*HCl and 20 wt % NaOH allowed for the measurement of 85% of the total mercury concentration. The stability of the measurements was observed for periods of 3−4 h and was dependent upon the initial bromine concentration and the frequency of cleaning of the chiller walls. The longer bromine was in the system, the higher the loss of total mercury. This may be due to the accumulation of bromine species in the conditioning system and reactions to form mercury−bromine complexes. The sensitivity of total mercury loss to cleaning of the chiller and to time suggests that heterogeneous oxidation of elemental mercury may be occurring on the glass surfaces of the chiller. Cleaning of the conditioning system prior to bromine injection improved the recovery of total mercury. The presence or absence of a NaOH impinger after the NH2OH*HCl− NaOH impinger did not make any difference in the measured total mercury concentrations. The results in Figures 5 and 6 were obtained with a NaOH impinger downstream of SnCl2 or NH2OH*HCl . The addition of the reducing agent sodium thiosulfate (0.5 wt %) to the final NaOH impinger had no effect on the recovery of total mercury for any of the conditions tested. The nature of the second impinger or its absence downstream of the first impinger did not seem to affect or give rise to liquid-phase reactions and interferences. The purpose of the elemental side of the wet conditioning system is to retain oxidized species in solution, allowing just the elemental mercury to reach the analyzer. This mercury reading represents the elemental concentration in the flue gas. The elemental side of the conditioning system showed unusual behavior in the presence of bromine species and SO2 that suggested that oxidation of mercury might be occurring in the KCl impinger, the downstream NaOH impinger, or the chiller. Figure 3 shows that an initial mass balance was closed, with the total and elemental mercury concentrations at similar values. Figure 3 also shows that, when bromine was added

the recovery of total mercury: (1) different concentrations of the stannous chloride/HCl solution, including the use of NaOH instead of HCl (Figure 5), (2) pretreatment of the flue

Figure 5. Losses in total mercury when using different concentrations of the SnCl2−HCl solution on the total mercury side of the sample conditioning system with 50 ppm bromine (as HBr equivalent) and 30 ppm NO. All solutions were followed by a second impinger containing 5 wt % NaOH. (A) Baseline conditioning system (2 wt % SnCl2 and 3 wt % HCl), (B) SnCl2 diluted (3 wt % SnCl2 and 2 wt % HCl), (C) SnCl2 concentrated (10 wt % SnCl2 and 6 wt % HCl), (D) 2 wt % SnCl2 and 3 wt % HCl in batch mode instead of flowing, and (E) 5 wt % SnCl2 mixed with 5 wt % NaOH.

gas stream with a hydroxylamine hydrochloride solution before the stannous chloride (Figure 6), (3) replacement of the stannous chloride solution by an hydroxylamine hydrochloride−sodium hydroxide solution (Figure 6), and (4) the addition of sodium thiosulfate to the hydroxide impinger.

Figure 6. Losses in total mercury using a solution of NH2OH*HCl− NaOH, instead of SnCl2−HCl, on the total mercury side of the sample conditioning system with 50 ppm bromine (as HBr equivalent) and 30 ppm NO. A NaOH impinger was installed downstream of all of the configurations. (A) Baseline conditioning system (2 wt % SnCl2 and 3 wt % HCl), (B) NH2OH*HCl (30 wt %) impinger followed by the baseline SnCl2 impinger, (C) NH2OH*HCl (15 wt %) and NaOH (20 wt %) impinger, (D) NH2OH*HCl (12 wt %) impinger followed by the baseline SnCl2 impinger, (E) NH2OH*HCl (10 wt %) and NaOH (20 wt %) impinger, and (F) NH2OH*HCl (4 wt %) and NaOH (20 wt %) impinger. 6258

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without any SO2, a decrease in the elemental concentration was observed that may indicate mercury oxidation by bromine species, in either the gas phase of the reactor or the conditioning system. When bromine and SO2 were added to the furnace, as in Figure 4, an increase in the elemental mercury concentration in the sampled flue gas was observed. Evidence for reactions in the conditioning system rather than gas-phase inhibition in the reactor is given by Figure 7, in which the extent of oxidation in

Figure 8. Percent decrease in the elemental mercury concentration as a function of the bromine species and SO2 concentrations in the system. Four different configurations of the elemental side of the conditioning system were considered. On the total side of the conditioning system, the baseline configuration was used (Figure 2).

that SO2 is preventing the heterogeneous oxidation of elemental mercury in the conditioning system.

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DISCUSSION On the basis of gas-phase kinetic calculations, below 400 °C, a mixture of Br2 and HBr might be expected in the flue gas, whereas for chlorine, HCl is dominant.19 At the experimental conditions of this study, both Br2 and HBr might be present in the flue gas entering the wet condition system. Because chlorine is present predominantly as HCl, it is not expected to have the same potential for oxidation as the bromine species. Although bromine compounds are effective at oxidizing mercury in combustion systems, they appear to interfere with impingerbased conditioning systems. This makes it difficult to determine extents of oxidation. Industrial-scale tests by Berry et al.10 suggest that replacing metallic and glass surfaces by Teflon and quartz and the use of dry conditioning systems in mercury CEMS help decrease losses in total mercury. Under our experimental conditions, there was no contact of the flue gas with metallic surfaces; however, the impingers as well as the condenser in the chiller were made of glass. No significant accumulation of oxidized mercury species on the reactor or impinger walls was detected in this study, and cleaning the impingers did not affect the extent of oxidation. However, frequent cleaning of the chiller was necessary to decrease the effects of what appears to be heterogeneous oxidation. Babi et al.13 investigated interferences in continuous flow techniques for aqueous mercury analysis in the presence of iodine or bromine. They used a Tekran Series 2600 CVAFS mercury system configured in accordance with United States Environmental Protection Agency (U.S. EPA) Method 1631. This method included a SnCl2−HCl solution (3 or 6 wt % SnCl2 in 1 or 2 wt % HCl) as a reducing agent, followed by a NaOH solution. At aqueous-phase concentrations of 5 ng/L mercury, 500 mg/L iodide (from KI), and 4% BrCl, the mercury recovery was zero. They tested several methods to recover mercury in the presence of bromine and iodine: (1) increasing the reaction time between the sample and the SnCl2 solution, (2) pH adjustments of the SnCl2 solution to destroy possible Hg−Br and Hg−I complexes, (3) increasing the SnCl2 solution concentration, and (4) replacing the SnCl2 solution by a potassium borohydride (KBH4) solution. The authors concluded that pH adjustments to the SnCl2 solution to levels greater than 9 gave full recovery of mercury in the presence of

Figure 7. Extent of mercury oxidation in the presence of 40 ppm bromine (as HBr equivalent) and SO2. Baseline configuration was used on the elemental side of the conditioning system (see Figure 2). SO2 was injected at two different points: the burner and the KCl impinger. The total side of the conditioning system used for these measurements is shown in Figure 2.

the presence of bromine species and SO2 is similar when SO2 is injected through the burner or directly into the KCl impinger on the elemental side of the conditioning system. The KCl impinger always contains 0.5 wt % Na2S2O3. This reduces the likelihood that elemental mercury is being oxidized by bromine species in the liquid phase. However, it is possible that heterogeneous oxidation reactions are occurring on the glass surfaces of the impingers and downstream chiller. The decrease in mercury oxidation seen in Figure 7 could then be occurring because SO2 and SO3 are competing with elemental mercury, HBr, and Br2 for surface adsorption sites, thus preventing heterogeneous oxidation of elemental mercury by bromine species. The elemental side of the baseline conditioning system is an aqueous solution of 10 wt % KCl and 0.5 wt % Na2S2O3, followed by 5 wt % NaOH, as shown in Figure 2. To further explore the possibility that surface rather than liquid-phase reactions are responsible for the effects of SO 2, the compositions of the liquid solutions on the elemental side were varied as follows: (1) the baseline solution was used; (2) the KCl solution was prepared without thiosulfate; (3) KCl was replaced with tris(hydroxymethyl)aminomethane (THAM)/ ethylenediamimetetraacetic acid (EDTA); and (4) a single NaOH impinger was used on the elemental side of the conditioning system. The decrease in the elemental mercury concentration was measured with 40 ppm bromine (as HBr equilvalent) with and without 500 ppm SO2. Figure 8 shows that, without SO2, the decrease in the elemental mercury concentration was about 75% and was similar in all four cases. At 500 ppm SO2, the decrease was about 30% and was also comparable at the four conditions. These results are consistent with the hypothesis 6259

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high aqueous-phase levels of bromine (20% BrCl) and iodine (500 mg/L). Our tests at pH levels greater than 9 were not successful because CO2 in the flue gas almost immediately lowered the pH to levels causing the precipitation of tin hydroxide. The molar flow rate of CO2 in the gas phase was hundreds of times the molar flow rate of NaOH to the impingers such that a compensating increase in the flow of NaOH was not practical. Babi et al.13 report that using 1.0 wt % KBH4 as a reductant gives full recoveries of mercury in the presence of iodine but go on to note that KBH4 can interfere with the amalgamation process in the mercury analyzer. Our tests with KBH4 were unsuccessful and gave erratic results, possibly for the reasons noted by Babi et al. With regard to the elemental side of the conditioning system, Figures 7 and 8 show that SO2 appears to limit the oxidation of mercury by bromine. Parallel experiments conducted with chlorine show no impact of SO2 provided that sodium thiosulphate (0.5 wt %) is added to the KCl solution.9 Gasphase kinetic calculations19 show that, at temperatures ranging from 300 to 500 °C, Br2 and HBr are the dominant bromine species expected in the flue gas. Similar calculations performed for chlorine show that HCl is the dominant species. A possible explanation for the effects of SO2 is that it competes with elemental mercury and Br2 for surface sites on the glass walls of the impingers and chiller and, thus, inhibits the heterogeneous oxidation of elemental mercury by Br2. This is consistent with the results of Figure 7, in which the same effect of SO2 was seen whether it was injected through the burner or directly into the KCl impinger. Similar effects may not occur with chlorine because the dominant predicted species, HCl, does not appear to have the same adsorptive characteristics as Br2. In addition, HCl is not a strong oxidant like Br2. It is worth noting that Figure 8 shows no effect of removing sodium thiosulphate from the KCl impinger when injecting Br2 through the burner. This result is not consistent with measurements made when injecting Cl2 through the burner and is not understood, particularly given the results of the gasphase kinetic calculations noted above, showing high levels of Br2 in flue gas below 400 °C.9,19 The addition of 0.5 wt % sodium thiosulfate to the hydroxide impingers on both sides of the conditioning system did not affect the apparent oxidation of elemental mercury on glass surfaces.

chiller. This problem is not seen when using chlorine as the oxidant. The determination of elemental mercury in the presence of bromine species and SO2 yielded results that further suggest that mercury may be heterogeneously oxidized on the glass walls of the impingers and chiller. The presence of SO2, whether added through the burner or directly to the KCl impinger, decreased the extent of oxidation of elemental mercury. This decrease may be due to the adsorption of SO2 and possibly SO3 on glass surfaces, thereby preventing the adsorption of elemental mercury and Br2 and subsequent heterogeneous oxidation of elemental mercury.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: geoff[email protected]. Notes

Disclaimer: Any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the U.S. Department of Energy (DOE). The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper was prepared with the support of the U.S. Department of Energy (DOE), under Awards DE-FG2603NT41797 and DE-FG26-06NT42713. Additional support was provided by the Electric Power Research Institute (EPRI) under Agreement EP-P29435/C13889 between EPRI and the University of Utah.

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REFERENCES

(1) Niksa, S.; Helble, J. J.; Fujiwara, N. Kinetic modeling of homogeneous mercury oxidation: The importance of NO and H2O in predicting oxidation in coal-derived systems. Environ. Sci. Technol. 2001, 35, 3701−3706. (2) Naruse, I.; Yoshiie, R.; Kameshima, T.; Takuwa, T. Gaseous mercury oxidation behavior in homogeneous reaction with chlorine compounds. J. Mater. Cycles Waste Manage. 2010, 12, 154−160. (3) Xu, M.; Qiao, Y.; Zheng, C.; Li, L.; Liu, J. Modeling of homogeneous mercury speciation using detailed chemical kinetics. Combust. Flame 2003, 132, 208−218. (4) Sliger, R.; Kramlich, J. C.; Marinov, N. M. Towards the development of a chemical kinetic model for the homogeneous oxidation of mercury by chlorine species. Fuel Process. Technol. 2000, 65−66, 423−438. (5) Cao, Y.; Wang, Q.; Li, J.; Cheng, J.; Chan, C.; Cohron, M.; Pan, W. Enhancement of mercury capture by the simultaneous addition of hydrogen bromide (HBr) and fly ashes in a slipstream facility. Environ. Sci. Technol. 2009, 43, 2812−2817. (6) Wu, C.; Cao, Y.; Dong, Z.; Cheng, C.; Li, H.; Pan, W. Mercury speciation and removal across full-scale wet FGD systems at coal-fired power plants. J. Coal Sci. Eng. 2010, 16, 82−87. (7) Dombrowski, K.; McDowell, S.; Berry, M.; Sibley, A. F.; Chang, R.; Vosteen, B. The balance-of-plant impacts of calcium bromide injection as a mercury oxidation technology in power plants. Proceedings of the AWMA 7th Power Plant Air Pollutant Control Mega Symposium; Baltimore, MD, Aug 25−28, 2008. (8) Linak, W. P.; Ryan, J. V.; Ghorishi, B. S.; Wendt, J. O. L. Issues related to solution chemistry in mercury sampling impingers. J. Air Waste Manage. Assoc. 2001, 51, 688−698. (9) Cauch, B.; Silcox, G. D.; Lighty, J. S.; Wendt, J. O. L.; Fry, A.; Senior, C. L. Confounding effects of aqueous-phase impinger chemistry on apparent oxidation of mercury in flue gas. Environ. Sci. Technol. 2008, 42, 2594−2599.

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CONCLUSION The analysis of gas-phase mercury speciation in the presence of bromine species and SO2 was studied using several different configurations of wet conditioning systems. The total side was modified to decrease bias in total mercury by replacing an aqueous tin chloride solution (2 wt % SnCl2 and 3 wt % HCl) with a solution of hydroxylamine hydrochloride and sodium hydroxide (4 wt % NH2OH−HCl and 20 wt % NaOH). This change reduced apparent losses in total mercury in the presence of 50 ppm bromine (as HBr equivalent) from 60 to 15%. Additional improvements in total mercury recovery were obtained by frequently cleaning the glass walls of the chiller located between the wet conditioning system and the CVAFS detector. The required frequency of cleaning increased with increased concentrations of bromine species. The impact of cleaning suggests that bromine species may be accumulating on the glass and heterogeneously oxidizing elemental mercury. Oxidized mercury may be accumulating on the glass walls of the 6260

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(10) Berry, M.; Dombrowski, K.; Richardson, C.; Chang, R.; Borders, E.; Vosteen, B. Mercury control evaluation of calcium bromide injection into a PRB-fired furnace with a SCR. Proceedings of the AWMA Air Quality VI Conference; Arlington, VA, Sept 24−27, 2007. (11) Cao, Y.; Wang, Q.; Chen, C.; Chen, B.; Cohron, M.; Tseng, Y.; Chiu, C.; Chu, P.; Pan, W. Investigation of mercury transformation by HBr addition in a slipstream facility with real flue gas atmospheres of bituminous coal and Powder River Basin coal. Energy Fuels 2007, 21, 2719−2730. (12) Wang, Z.; Pehkonen, S. Oxidation of elemental mercury by aqueous bromine: Atmospheric implications. Atmos. Environ. 2004, 38, 3675−3688. (13) Babi, D.; Schaedlich, F. H.; Schneeberger, D. R. Correction techniques for iodine and bromine interferences in continuous flow aqueous mercury analysis. Anal. Bioanal. Chem. 2002, 374, 1022− 1027. (14) Fry, A.; Cauch, B.; Lighty, J. S.; Silcox, G. D.; Senior, C. L. Experimental evaluation of the effects of quench rate and quartz surface area on homogeneous mercury oxidation. Proc. Combust. Inst. 2007, 31, 2855−2861. (15) Van Otten, B.; Buitrago, P. A.; Senior, C. L.; Silcox, G. D. Gasphase oxidation of mercury by bromine and chlorine in flue gas. Energy Fuels 2011, 25, 3530−3536. (16) Bailey, B. W.; Lo, F. C. Automated method for determination of mercury. Anal. Chem. 1971, 43, 1525−1526. (17) Rio-Segade, S.; Bendicho, C. Determination of total and inorganic mercury in biological and environmental samples with online oxidation coupled to flow injection-cold vapor atomic absorption spectrometry. Spectrochim. Acta, Part B 1999, 54, 1129−1139. (18) Peter, F.; Strunc, G. Semiautomated analysis for mercury in whole blood, urine and hair by on-stream generation of cold vapor. Clin. Chem. 1984, 30, 893−895. (19) Silcox, G.; Buitrago, P.; Senior, C.; Van Otten, B. Gas-phase mercury oxidation by halogens: Effects of bromine and chlorine. Proceedings of the 103rd Air and Waste Management Association Annual Conference and Exhibition; Calgary, Alberta, Canada, June 22−25, 2010.

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dx.doi.org/10.1021/ef401314q | Energy Fuels 2013, 27, 6255−6261