Gas-Phase Oxidation of Mercury by Bromine and Chlorine in Flue Gas

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Gas-Phase Oxidation of Mercury by Bromine and Chlorine in Flue Gas Brydger Van Otten,† Paula A. Buitrago,‡ Constance L. Senior,§ and Geoffrey D. Silcox*,‡ †

Reaction Engineering International, 77 West 200 South, Suite 210, Salt Lake City, Utah 84101, United States Department of Chemical Engineering, University of Utah, 50 South Central Campus Drive, Room 3290 MEB, Salt Lake City, Utah 84112, United States § ADA Environmental Solutions, 8100 Southpark Way, Unit B, Littleton, Colorado 80120, United States ‡

ABSTRACT: Oxidized mercury species may be formed in combustion systems through gas-phase reactions between elemental mercury and halogens, such as chorine or bromine. This study examines how bromine and chlorine species affect mercury oxidation in the gas phase. Experiments were conducted in a bench-scale, laminar, methane-fired (300 W), quartz-lined reactor, in which gas composition (HCl, HBr, NOx, and SO2) was varied. In the experiments, the postcombustion gases were quenched from the flame temperature to about 350 °C and then speciated mercury was measured using a wet conditioning system and continuous emission monitor (CEM). Bromine was shown to be much more effective in the postflame, homogeneous oxidation of mercury than chlorine, on an equivalent molar basis. The addition of NO to the flame (up to 400 ppmv) had no impact on mercury oxidation by chlorine or bromine. The addition of SO2 had no effect on mercury oxidation by chlorine at SO2 concentrations below about 400 ppmv; some increase in mercury oxidation was observed at SO2 concentrations of 400 ppmv and higher. The addition of chlorine caused minor increases in the extent of oxidation by bromine. The results of this study can be used to understand the relative importance of gasphase mercury oxidation by bromine in combustion systems.

’ INTRODUCTION Mercury emissions from coal-fired power plants are a concern to state and federal governments in the U.S. In March 2011, the United States Environmental Protection Agency (U.S. EPA) proposed rules to regulate mercury emissions from coal-fired power plants, keeping 91% of the mercury in coal from being released.1 This proposed regulation replaces the Clean Air Mercury Rule (CAMR), all aspects of which were vacated by a February 2008 court ruling. As of February 2011, 16 states had passed their own mercury emissions regulations.2 The new federal mercury regulations are expected to be finalized by November 2011. Mercury-specific technology will be required in the U.S. to achieve 70 90% reductions in mercury emissions mandated by the Clean Air Act Amendments and various state regulations. Mercury exists as the elemental form (Hg0) in the hightemperature regions of coal-fired boilers. As the flue gas is cooled, a series of complex reactions begin to convert Hg0 to gaseous oxidized forms (Hg2+) and particulate-bound mercury (Hgp). The extent of conversion of Hg0 to Hg2+ and Hgp depends upon the flue gas composition, the amount and properties of fly ash, and the flue gas temperature and quench rate. Studies in laboratory combustion systems show that increasing the chlorine content of combustion exhaust gases results in an increase in the amount of mercury in the exhaust gas in an oxidized form.3 5 As extensive data on mercury speciation in coalfired boilers became available,6,7 it was noted that the amount of mercury oxidation found in the flue gas at the inlet to gascleaning devices was often significantly less than that predicted from gas-phase equilibrium, leading to the conclusion that the oxidation of mercury by chlorine in practical coal combustion systems must be kinetically limited.8 A number of gas-phase r 2011 American Chemical Society

kinetic models have been developed to predict the homogeneous oxidation of mercury by chlorine compounds in combustion gases.9 However, these models have not be able to model successfully the oxidation of mercury in coal combustion systems. The presence of particles in the combustion gas, consisting of inorganic ash and unburned carbon (collectively known as fly ash), has been suggested to catalyze the oxidation of mercury by chlorine. Inclusion of a heterogeneous oxidation mechanism for mercury based on the interaction of chlorine species and unburned carbon surfaces has been able to model the oxidation of mercury in coal combustion systems.10 According to this homogeneous heterogeneous model, the heterogeneous pathway (using the surface of unburned carbon in fly ash) is dominant for the oxidation of mercury by chlorine species in practical combustion systems. The ability of HCl to promote carbon sites for oxidation of mercury is extensively discussed by others.11 The introduction of bromine compounds into boilers has recently been demonstrated at full scale to be effective at increasing mercury oxidation in flue gas12 14 and enhancing mercury capture by activated carbon.15 On a molar basis, bromine is much more effective than chlorine in increasing the amount of oxidized mercury in the flue gas, when the halogens are added to the coal.12,13 Figure 1 shows data taken from Berry et al.,14 in which bromine was added to a low-halogen, subbituminous coal in a 700 MW coal-fired boiler. The concentration of HBr was measured in the flue gas at the particulate control device, and the concentration of gaseous elemental and oxidized mercury was measured at the inlet to the selective catalytic reduction (SCR) Received: June 7, 2011 Revised: July 22, 2011 Published: July 24, 2011 3530

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Figure 1. Fraction of oxidized mercury at the SCR inlet (ca. 383 °C) as a function of measured HBr content in flue gas at a coal-fired power plant.14

unit, which had a design operating temperature of 383 °C. The data in the figure illustrate that relatively low concentrations of bromine in the flue gas increase the amount of oxidized mercury in the flue gas at temperatures above 350 °C. However, these full-scale results cannot answer the question as to the relative importance of homogeneous and heterogeneous pathways for mercury oxidation by bromine. Recent kinetic modeling16 attempts to explain the full-scale results with bromine addition. The modeling results suggest that bromine speciation is different from chlorine speciation in coal combustion flue gases. As flue gases cool in a practical combustion system, most of chlorine is predicted to be found as HCl, with a very minor amount as Cl, the species thought to be responsible for the homogeneous oxidation of elemental mercury. In contrast, the concentrations of HBr and Br are comparable at flame temperatures, and as the flue gases cool, the concentration of Br is on the same order as that of HBr. For mercury compounds, the kinetic model predicts that homogeneous oxidation of elemental mercury by Br begins as the flue gas cools below 500 °C. The kinetic model predicts mercury speciation observed in full-scale power plants. However, a power plant is a complex system with inputs that are not always well-quantified. More direct laboratory evidence is desirable but difficult to obtain. Homogeneous oxidation of mercury by bromine species has been measured in the laboratory in synthetic flue gas at the relatively low temperature of 23 °C.17 These conditions do not duplicate the chemical transformations that occur in a rapidly cooling flue gas in a coal-fired boiler. It is clear from full-scale boiler data14 that bromine oxidizes mercury in flue gas at temperatures above 380 °C. It has been proposed on the basis of kinetic modeling that this represents homogeneous oxidation.16 However, homogeneous oxidation of mercury by bromine has not been observed directly at these temperatures in combustion gases. Previously, we reported on a study of homogeneous mercury oxidation in a natural-gas-fired furnace, in which mercury and chlorine were added through the burner.18 The time temperature history in the furnace was similar to that in a full-scale boiler, to obtain realistic concentrations of free radicals in the cooling gas. In the previous work, little homogeneous oxidation of mercury was observed with chlorine addition, which is consistent with the idea that the heterogeneous route is the primary

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Figure 2. Temperature profiles in a laboratory furnace.

pathway for mercury oxidation in coal combustion systems. In the present study, we extend the work to compare the homogeneous oxidation of chlorine to bromine in the natural-gas-fired furnace. This is the first study of homogeneous oxidation of mercury by bromine compounds in a combustion furnace.

’ EXPERIMENTAL SECTION The reactor configuration used in this study is described in detail by Fry et al.4 and was developed to study the impacts of the quench rate, HCl, SO2, and NO on homogeneous mercury oxidation. The reaction chamber is made of a 47 mm inner diameter quartz glass tube, 132 cm in length. The top 53 cm of the reaction chamber are enclosed in a Thermcraft high-temperature heater. The remaining quartz tube below the heater is wrapped in four sections of heat tape to provide separately controlled temperature regions. This allows the time temperature profile of the reactor to be manipulated to create various quench rates. Two quench rates were used in these experiments, representative of industrial conditions: 440 K/s (high quench) and 210 K/s (low quench), as shown in Figure 2. The low-temperature region (around 350 °C), at approximately 5 s of residence time, represents flue-gas temperatures at the end of the convective pass of a boiler (economizer outlet). These low temperatures were necessary for the occurrence of mercury oxidation reactions. A 300 W, methane-fired burner made of quartz supplied 6 standard liters per minute (SLPM) of combustion gases to the tubular reactor. All reacting species were introduced through the flame. This helped ensure a radical pool in the reactor representative of real combustion systems and provided appropriate initial conditions for kinetic modeling. Kinetic calculations are not part of this paper. The source of elemental mercury in the reactor was a PS Analytical Mercury Calibration Gas Generator or “CavKit”. Bromine (3000 ppmv Br2 in air) and chlorine (6000 ppmv Cl2 in air) were introduced through the burner. Sulfur dioxide (6000 ppmv in air) or NO was also introduced through the burner for selected experiments. The concentration of NO at the reactor exit was measured, and the flow rate through the burner was adjusted to give the desired concentration. A similar procedure was used to verify the SO2 concentration. For some experiments, the quartz reactor surface area was increased. The reactor normally has 1000 cm2 of surface area. This value was increased to 3000 cm2 by inserting a bundle of thin-walled quartz tubes. The bundle was made of seven tubes arranged in a hexagonal configuration. Each tube was 60 cm in length, 14 mm outer diameter, and 12 mm inner diameter. The tube bundle decreased the residence time in the reactor by 5%. 3531

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Figure 4. Measured concentrations of total and elemental mercury for the injection of 5.5, 8.4, and 10.6 ppmv bromine (as HBr) into the furnace.

Figure 3. Sample conditioning system.

Table 1. Flue Gas Composition for Experimentsa species

A 5 wt % NaOH solution was used to remove acid gases from each of the sample streams. All impingers had a flow-through arrangement on the liquid side, so that their solutions were continually being refreshed. The two sample streams then passed through a chiller to remove any water. A Tekran 2537A mercury analyzer was used to measure the elemental mercury concentration of each stream. A four-port sampler controlled which stream the Tekran analyzer sampled. The concentration of oxidized mercury was calculated as the difference between the total and elemental mercury stream concentrations.

concentration

O2

0.8 vol %

H2O CO2

16.5 vol % 7.7 vol %

NO

30 and 500 ppmv

SO2

0 500 ppmv

HClb

0 500 ppmv

HBrc

0 50 ppmv

Hg0

25 μg N

1

m

3

a

The concentrations of Hg, O2, CO2, and NO were continuously monitored. The concentration of SO2 was checked intermittently. The values for water, HCl, and HBr were based on material balance calculations. b Total chlorine expressed as HCl. c Total bromine expressed as HBr. For mercury analysis, two sample streams were taken from the bottom of the reaction chamber and introduced into a wet-chemical conditioning system, shown in Figure 3. The combustion gases were sampled in the furnace at approximately 350 °C. The gases cooled rapidly after being withdrawn from the combustion furnace because they were in an unheated sample line and because the impingers in the sample conditioning system contained water. Thus, the measured mercury speciation should be representative of the combustion gases at 350 °C. One stream was bubbled through an acidic 2% SnCl2 solution to reduce all mercury to the elemental form. This stream represented the total mercury concentration present in the system. The other stream was passed through a solution containing 10 wt % KCl and 0.5 wt % Na2S2O3 to capture oxidized mercury. This stream represented the elemental mercury concentration present in the system. Sodium thiosulfate (Na2S2O3) has been shown to prevent aqueous oxidation of elemental mercury in the KCl impinge because of the presence of trace amounts of Cl2 in the combustion gas.18,19

’ RESULTS Table 1 gives the flue gas composition exiting the reactor during the experiments. The concentrations of Hg, O2, CO2, and NO were continuously monitored. The concentration of SO2 was intermittently checked after it was shown that its concentration agreed with simple material balance calculations. The chlorine and bromine concentrations were estimated from material balance calculations. The gas composition is not intended to duplicate flue gas compositions in coal-fired power plants; rather, the intent of this work is to study reactions of mercury and common flue gas species in a well-controlled reactor. It should be noted that the minor species, such as SO2, Br2, Cl2, Hg0, and NO, were added through the burner and passed through the flame. Thus, their speciation in the reactor gas depended upon the flame chemistry as well as the reactor temperature profile. The measured concentration of NO produced by the burner was about 30 ppmv, and this concentration was present in all experiments unless it was supplemented by additional NO. A series of experiments was performed in which different species were added through the burner to understand homogeneous mercury oxidation. Before these were added, the baseline mercury concentration was verified by a material balance that was performed in the absence of any added NO, SO2, Cl2, and Br2. Figure 4 shows a typical set of continuous measurements of mercury species as a function of time with bromine addition. The concentrations of total (HgT) and elemental (Hg0) mercury are indicated. The mercury balance considered total and elemental mercury, measured using the continuous emission monitor (CEM) and sample conditioning system, at the beginning and end of each experiment. Experiments were repeated to calculate standard 3532

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Figure 5. Homogeneous mercury oxidation with the addition of chlorine or bromine, 30 ppmv NO.

Figure 6. Oxidation of mercury by chlorine as a function of the quench rate, 30 ppmv NO.

Figure 7. Oxidation of mercury by bromine as a function of the quench rate, 30 ppmv NO.

deviations in the levels of oxidation, and these are shown as error bars in the figures below. Effect of the Halogen. Bromine is a more powerful oxidant for mercury than chlorine, on an equivalent molar basis. This has been observed in full-scale power-plant trials12,13 and can be seen in the laboratory data in Figure 5. No NO or SO2 was added through the burner in these experiments, although the methane

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flame produced approximately 30 ppmv of NO. Bromine radical chemistry is very different from that of chlorine. HCl is the dominant chlorine species for temperatures between 200 and 1000 °C. With bromine, HBr, Br2, and Br are all significant.16 This drastically different halogen chemistry could account for the increased oxidation seen with bromine versus chlorine. It could also account for the different effect of the quench rate seen with bromine versus chlorine. Effect of the Quench Rate. The quench rate in the reactor affects the extent of oxidation to a minor degree, as illustrated in Figures 6 and 7, for chlorine and bromine oxidation. Note the difference in scale for the y axis between the two figures. The quench rate affects the concentrations of free radicals, such as Cl and Br, that are thought to initiate oxidation of Hg0 in the gas phase.3,8,10 For chlorine, the effects of the quench rate are insignificant, while bromine produces markedly more oxidation of mercury at the lower quench rate than at the higher quench rate. Effect of the Reactor Surface Area. Increasing the quartz reactor surface area from 1000 to 3000 cm2 caused a 15% increase in oxidation by bromine. The conditions for this test included bromine concentrations ranging from 5 to 40 ppmv (as HBr equivalent) at the high quench rate. A similar insensitivity to added quartz tube bundles was observed when working with chlorine.4 These results suggest that homogeneous oxidation reactions are responsible for most of the observed levels of mercury oxidation. Effect of the Acid Gas Species. In coal-fired power plants, NOx concentrations can be in the range of 50 1000 ppmv, depending upon the type of NOx control system employed. Typically, NOx (or total nitrogen oxides) consists of 5 10% NO2 with the balance NO. The laboratory reactor produces about 30 ppmv NOx (of which most is presumed to be NO). In some experiments, NO was added through the burner, to produce up to 500 ppmv NO (again, mostly NO). SO2 concentrations in coal-fired power plants vary widely, from 300 to 3000 ppmv. In the laboratory reactor, up to 500 ppmv SO2 was added, by injecting SO2 through the methane burner. Figure 8 shows the effect of NO and SO2 on mercury oxidation by chlorine. Increasing the NO concentration from 30 to 500 ppmv had little or no effect on mercury oxidation, within experimental error. Adding SO2 at a level of 400 ppmv did not affect mercury oxidation by chlorine when the chlorine concentration was 300 ppmv (equivalent HCl) or less. However, at greater than 400 ppmv HCl, there was an increase in mercury oxidation with the addition of SO2. Figure 9 shows that, as with chlorine, the addition of NO did not affect oxidation by bromine. Efforts to resolve the role of SO2 on gas-phase mercury oxidation by bromine in the combustion gas have not been successful to date, because of confounding aqueous chemistry in the impinger system. This portion of the work will be discussed in more detail in a separate paper focused on chemistry in the impinger system. Effects of Mixtures of Chlorine and Bromine. We performed experiments at the high quench rate on the combined effects of bromine and chlorine. Figure 10 shows that, at 25 and 50 ppmv Br (as HBr equivalent), there was a slight increase in oxidation with the addition of chlorine at either 100 or 400 ppmv. The increase was roughly proportional to the concentration of chlorine. The results suggest little, if any, synergy between bromine and chlorine in the homogeneous oxidation of mercury. 3533

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Figure 8. Effect of SO2 and NO on mercury oxidation by chlorine: (a) low quench rate and (b) high quench rate.

Figure 9. Effect of NO on mercury oxidation by bromine at the high quench rate.

’ DISCUSSION The experiments performed in the natural-gas-fired combustor isolated the homogeneous oxidation of mercury by halogens in a gas that was quenched from flame conditions to about 350 °C. Surfaces in the reactor had a small influence on oxidation. In this study, increasing the NO concentration in the flue gas had no effect on Hg oxidation by chlorine or bromine. SO2 did not have a significant effect on mercury oxidation by chlorine, except when the HCl concentration was greater than 400 ppmv. Smith et al.5 observed inhibition or enhancement of mercury oxidation by chlorine when SO2 was added to a laboratory, methane-fired furnace; the effect depended upon the concentrations of SO2 and HCl. There was little effect of SO2 at 400 ppmv SO2 and up to 550 ppmv HCl. However, at a HCl concentration of 200 ppmv and lower concentrations of SO2 (100 200 ppmv), SO2 did affect the level of mercury oxidation observed. To put these results in perspective, the range of concentrations of chlorine and bromine species expected in coal combustion flue gas must be considered. In coal-fired power plants, the concentration of HCl in the flue gas is on the order of 1 10 ppmv when western coals (sub-bituminous and lignite) are burned, but HCl concentrations can vary from 20 to more than 150 ppmv when biturminous coals from the eastern U.S. are burned. These estimates assume coal chlorine concentrations of up to 100 μg/g in sub-bituminous and lignite coals and 300 2000 μg/g in

eastern bituminous coals.6 Bromine concentrations in coals are significantly less than chlorine concentrations. In one analysis of a set of coals from North America, the ratio of Br/Cl in coal was 0.06 ( 0.03.20 Thus, the concentrations of bromine (expressed as HBr) in the flue gas of coal-fired combustion systems would be estimated to be less than 0.5 ppmv in boilers firing subbituminous or lignite coals and 0.5 3.5 ppmv in bituminousfired boilers. In this study, homogeneous oxidation of mercury by chlorine compounds in combustion flue gas was less than 10% for HCl concentrations up to 500 ppmv. Smith et al.5 obtained similar results in a different natural-gas-fired furnace: homogeneous oxidation of mercury was less than 30% up to HCl concentrations of 500 ppmv. In the experiments by Smith et al., the combustion gas was quenched from the flame temperature to about 25 °C. The higher oxidation of mercury as a function of the HCl concentration observed by Smith et al. could be because of a different quench rate or a lower final temperature. In any case, in coalfired boilers, concentrations of HCl are expected to be less than 150 ppmv, which means that homogeneous oxidation of mercury by chlorine should be less than 5% in practical combustion systems. Homogenous oxidation of mercury by bromine was observed to be 10% or less for bromine concentrations in the gas (as HBr) of up to 3.5 ppmv. Thus, for the concentrations of bromine found in North American coals, homogeneous oxidation of mercury by bromine would not be expected to be significant in coal-fired boilers. The addition of bromine to the fuel equivalent to 25 40 ppmv HBr in the flue gas increased homogeneous oxidation in the experiments to 80%. In full-scale boilers, however, the addition of modest concentrations of bromine to the fuel produced substantial increases in the fraction of oxidized mercury. At the Monticello power plant, which fired a lignite sub-bituminous coal blend, the fraction of oxidized mercury at the electrostatic precipitator (ESP) outlet (ca. 150 °C) increased from 30 to 80% when bromine was added to the fuel at a level equivalent to 4.4 ppmv HBr in the flue gas.12 At the Miller power plant, the fraction of oxidized mercury at the SCR inlet (ca. 380 °C) increased from a baseline of 18 42 to 96% when the measured bromine concentration in the flue gas increased to 3.5 ppmv. 3534

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Figure 10. Mercury oxidation at various concentrations of chlorine and bromine at 30 ppmv NO and the high quench temperature profile.

Cao et al.21,22 and Qu et al.23 suggested that the simultaneous presence of bromine and chlorine can affect mercury oxidation through the formation of the reactive interhalogen species, BrCl, and thus increase mercury oxidation relative to each halogen by itself. In this work, no such synergy between bromine and chlorine was observed. On an equivalent molar basis, bromine is far more effective than chlorine in promoting the homogeneous oxidation of mercury in combustion flue gas. A comparison of the results of this study to full-scale trials of bromine addition to coal suggests that heterogeneous oxidation must also be a significant contributor to mercury oxidation via bromine addition in coal-fired flue gas. The performance of bromine additives for mercury control in coalfired power plants, therefore, will be affected by the level of bromine added but also by the presence of fly ash (composition, concentration, etc.) in the flue gas. These factors must be considered when trying to achieve a specific level of mercury control at a given coal-fired boiler.

’ CONCLUSION Bromine was shown to be much more effective in the homogeneous oxidation of mercury postflame than chlorine. The oxidation of mercury by chlorine was unaffected by doubling the quench rate in the furnace, within experimental uncertainty. However, doubling the quench rate resulted in about a 40% decrease in mercury oxidation by bromine. Mercury oxidation is initiated postflame by free radicals, the concentrations of which are sensitive to the cooling rate in the gas. The addition of NO to the flame (up to 400 ppmv) had no impact on mercury oxidation by chlorine or bromine. The addition of SO2 had no effect on mercury oxidation by chlorine at SO2 concentrations below about 400 ppmv; some increase in mercury oxidation was

observed at SO2 concentrations of 400 ppmv and higher. Future work will focus on the effect of SO2 on mercury oxidation by bromine in the laboratory reactor with special attention to impinger chemistry. The addition of 100 and 400 ppmv chlorine (as HCl equivalent) caused minor increases in the extent of oxidation by 25 and 50 ppmv bromine (as HBr equivalent). The results of this study can be used to understand the relative importance of gas-phase mercury oxidation by bromine in combustion systems.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: geoff[email protected].

’ ACKNOWLEDGMENT This paper was prepared with the support of the U.S. Department of Energy (DOE), under Awards DE-FG26-03NT41797 and DE-FG26-06NT42713. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE. Additional support was provided by the Electric Power Research Institute (EPRI) under Agreement EP-P29435/C13889 between EPRI and the University of Utah. ’ REFERENCES (1) United States Environmental Protection Agency (U.S. EPA). Reducing Toxic Air Emissions From Power Plants; U.S. EPA: Washington, D.C., 2011; http://www.epa.gov/airquality/powerplanttoxics/ (accessed on May 17, 2011). 3535

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