- Email: [email protected]
Effects of flue gas constituents on mercury speciation
Fuel Processing Technology 65–66 Ž2000. 157–165 www.elsevier.comrlocaterfuproc
Effects of flue gas constituents on mercury speciation Dennis L. Laudal b
a,)
, Thomas D. Brown b, Babu R. Nott
c
a Energy and EnÕironmental Research Center, 15 North 23rd Street, Grand Forks, ND 58202, USA U.S. Department of Energy Federal Energy Technology Center, 626 Cochrans Mill Road, Pittsburgh, PA 15236, USA c EPRI, 3412 HillÕiew AÕenue, Palo Alto, CA 94303, USA
Received 7 December 1998; accepted 25 August 1999
Abstract Beginning with the 1990 Clean Air Act Amendments, there has been considerable interest in mercury emissions from coal-fired power plants. This past year, the U.S. Environmental Protection Agency ŽEPA. issued both the Mercury Study Report to Congress and the Study of Hazardous Air Pollutant Emissions from Electric Utility Steam-Generating Units, which make clear that EPA views mercury in the environment as a serious issue and that coal-fired utilities are a major source of mercury. For the past 4 years, EPRI and the U.S. Department of Energy ŽDOE. have funded research on mercury measurement, control, and chemistry at the Energy and Environmental Research Center ŽEERC.. The primary goal of bench-scale work was to determine what flue gas constituents affect mercury speciation, specifically how mercury speciation affects measurement methods and the ability of mercury sorbents to absorb mercury. A bench-scale test rig was designed and built to simulate flue gas conditions. The baseline simulated flue gas consisted of O 2 , CO 2 , H 2 O, and N2 . Other flue gas constituents tested include SO 2 , HCl, NO, NO 2 , HF, Cl 2 , and fly ash. The mercury was delivered to system as either elemental mercury ŽHg 0 . or mercuryŽII. chloride ŽHgCl 2 . via temperature-controlled permeation tubes. EERC bench-scale data clearly show that the type of fly ash is important in determining mercury speciation in flue gas streams. Not surprisingly, there appear to be a number of interactions between various flue gas constituents that affect mercury speciation. Depending on concentration, there is clearly an interaction between NO–NO 2 and fly ash, and it is possible that the interaction may be related to the ratio of NO:NO 2 . However, it has been shown that when NO–NO 2 is tested without fly ash, there is no conversion of Hg 0 to Hg 2q. Bench-scale tests clearly show that the chemistry of mercury is very complex and that more research is needed to understand what is
)
Corresponding author. Tel.: q1-701-771-5138; fax: q1-701-777-5181; e-mail: [email protected] 0378-3820r00r$ - see front matter q 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 Ž 9 9 . 0 0 0 8 3 - 1
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D.L. Laudal et al.r Fuel Processing Technology 65–66 (2000) 157–165
occurring. However, it is equally clear that the development of effective mercury sorbents and the ability to accurately model mercury speciation are dependent on understanding mercury chemistry, thermodynamics, and kinetics. q 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Mercury; Measurement; Chemistry
1. Introduction The 1990 Clean Air Act Amendments ŽCAAAs. required the U.S. Environmental Protection Agency ŽEPA. to determine whether the presence of mercury in the stack emissions from fossil fuel-fired electric utility power plants poses an unacceptable public health risk. EPA’s conclusions and recommendations were presented in the Mercury Study Report to Congress w1x and Utility Air Toxics Report to Congress w1x. The first report addressed both the human health and environmental effects of anthropogenic mercury emissions, while the second addressed the risk to public health posed by the emission of mercury and other hazardous air pollutants from steam-electric generating units. Although these reports did not state that mercury controls on coal-fired electric power stations would be required given the current state of the art, they did indicate that EPA views mercury as a potential threat to human health. Therefore, it was concluded that mercury controls at some point may be necessary. EPA, the U.S. Department of Energy ŽDOE., and EPRI have acknowledged that assessing the risk posed by power plant mercury emissions is very complicated w1–3x. Mercury is emitted in such small concentrations Ž1–30 mgrN m3 . that accurately measuring emission rates has been extremely difficult. In addition, researchers discovered that mercury is emitted in various physical and chemical forms, each possessing distinct properties that affect atmospheric transport, control effectiveness, and sampling and analysis methods. Reliable mercury emission and ambient measurement methods are required to achieve the CAAA goal of assessing the potential human health risks from exposure to mercury. Specifically, accurate measurements of mercury emissions from stationary sources and ambient mercury concentrations in the environment are useful for a variety of reasons, including the following: Ø To estimate the anthropogenic flux of mercury to the environment on a local, regional, and global scale; Ø To identify atmospheric transport and transformation processes; Ø To accurately determine background and natural mercury concentrations; Ø To determine deposition and methylation mechanisms in ecosystems; Ø To assess human health risks; Ø To assess mercury bioaccumulation; Ø To ensure compliance of sources with emission regulations, should they become necessary; Ø To determine partitioning among the various effluents of fossil fuel combustion systems; Ø To evaluate the removal efficiency of control technologies; and Ø To evaluate continuous emission monitors ŽCEMs. for mercury;
D.L. Laudal et al.r Fuel Processing Technology 65–66 (2000) 157–165
159
In addition to measuring total mercury accurately, the identification and quantification of individual physicochemical forms Ži.e., species. of mercury are imperative for addressing questions concerning mercury toxicity, bioaccumulation, emission control, and atmospheric fate and transport, because each has distinctive physical, chemical, and biological properties. Mercury emissions from anthropogenic sources occur in three main forms: solid particle-associated mercury; gaseous divalent mercury, Hg 2q; and gaseous elemental mercury, Hg 0 . Estimates of the relative proportions of these species in fossil fuel-fired power plant emissions and in ambient air are scarce because of a lack of reliable sampling and analysis methods for the different mercury species. Although no validated method, wet chemistry, or CEM existed for determining mercury speciation in combustion flue gas, it was speculated that EPA Method 29, which was validated for total mercury, would also provide accurate mercury speciation data. For a mercury speciation measurement method to work, it must be able to separate the Hg 2q and Hg 0 by selective adsorption. In the case of EPA Method 29, this was done by collecting the Hg 2q in impingers of hydrogen peroxide–nitric acid ŽH 2 O 2 – HNO 3 . and the Hg 0 in potassium permanganate–sulfuric acid ŽKMNO4 –H 2 SO4 impingers. Pilot-scale tests at the Energy and Environmental Research Center ŽEERC. and Radian w4,5x showed that under some conditions, a significant percentage of the Hg 0 is captured in H 2 O 2 –HNO 3 impingers and, therefore, reported as Hg 2q. This was particularly true in flue gas streams with high Ž) 1000 ppm. levels of SO 2 . As a result, other mercury speciation methods needed to be evaluated. From 1995 to 1998, EPRI and DOE funded a research program at the EERC to evaluate mercury speciation analysis methods for electric utility power plants. The EERC implemented an extensive testing program using a bench-scale flue gas simulator and a pilot-scale combustion system to evaluate the performance characteristics Žsensitivity, precision, bias, specificity, and interferences. for many of the mercury speciation
Fig. 1. Schematic of Ontario Hydro speciation sampling train.
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D.L. Laudal et al.r Fuel Processing Technology 65–66 (2000) 157–165
methods, with a final goal of validating a mercury speciation method at a power plant. Based on the pilot- and bench-scale tests conducted at the EERC, the Ontario Hydro method developed by Dr. Curtis of Ontario Hydro and the EERC, showed the most promise. The Ontario Hydro method is a modification of EPA Method 29, with the first H 2 O 2 –HNO 3 replaced by three KCl impingers, as shown in Fig. 1. The Ontario Hydro method has now been validated as a mercury speciation measurement method in a full-scale test program at a midwestern power plant w6x. This paper focuses on the bench-scale results for the Ontario Hydro method which eventually led to the method being validated in the field. In addition, these bench-scale tests provided a substantial understanding of mercury flue gas chemistry.
2. Experimental As part of the mercury speciation method testing program, bench-scale tests were conducted using the Ontario Hydro method as well as EPA Method 29, the tris-buffer method, and the mercury speciation adsorption ŽMESA. method on a wide range of flue gas constituents. Some of the flue gas constituents from coal-fired systems, such as SO 2 , NO x , HCl, and fly ash, have been shown to have a substantial impact on determining the mercury species w7x. These same constituents may also react with the impinger solutions, resulting in erroneous mercury speciation measurements. For example, some fly ashes collected on the filter prior to the sampling train may adsorb andror oxidize mercury. The constituents were delivered to the system through a gas manifold using calibrated mass flow controllers. The effect of fly ash on mercury speciation was studied by passing the simulated flue gas through an optional Teflon-coated particulate filter which was contained in a temperature-controlled oven.
Fig. 2. Schematic of bench-scale testing system.
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Table 1 Gas composition for bench-scale tests Žwet basis. Nominal concentration Main gases Oxygen ŽO 2 . Carbon dioxide ŽCO 2 . Water vapor ŽH 2 Ov . Nitrogen ŽN2 .
4% 15% 10% Balance
Variable gases Sulfur dioxide ŽSO 2 . Hydrogen chloride ŽHCl v . Nitrogen oxides ŽNOrNO 2 . Chlorine ŽCl 2 . Elemental mercury ŽHg 0 . HgCl 2
0 or 1500 ppm 0 or 50 ppm 0 or 600r30 ppm 0 or 10 ppm 20 mgrN m3 20 mgrN m3
Known quantities of Hg 0 or HgCl 2 vapor were introduced into the system by flushing nitrogen around calibrated permeation tubes. The quantity of mercury vapor injected into the gas stream was determine by its vapor pressure at specific temperatures. In addition, EPA Method 101A and a mercury CEM were used to routinely check the quantity of mercury being released from the permeation tubes. A straight-tube condenser and water bath were used to maintain and control the temperature of the permeation tubes. All parts of the bench-scale test unit are Teflon, Teflon-lined, or glass, and the entire system, including the gas manifold for mixing the gases, is heat-taped to maintain a constant temperature. A schematic of the bench-scale system is shown in Fig. 2, and the gas compositions used for the testing are presented in Table 1.
3. Results and discussion As shown in Table 2, the Ontario Hydro method was evaluated using a replicated fractional factorial design test matrix. In this way, five variables could be tested using the minimum number of tests yet still provide sufficient data for a statistical analysis of the effects. The statistical results from the test series are shown in Table 3. The q and y signs in the table indicate an increase or decrease in the percentage of Hg 0 collected in the H 2 SO4 –KMnO4 solution relative to the amount of Hg 0 spiked into the simulated flue gas. An effect is significant if its absolute value is greater than that of the t-statistic found in the footnotes at the end of the table. Other than the effects of Cl 2 and fly ash, it is unclear what flue gas constituents affect the mercury speciation ability of the Ontario Hydro method. It is very clear that Cl 2 has a significant impact on the mercury speciation measurement when the Ontario Hydro method is used. This is not entirely unexpected because Cl 2 can react with the Hg 0 to form HgCl 2 in a gas-phase reaction. There is evidence, however, that the effect of Cl 2 may be a result of aqueous phase reactions in the impinger solutions. As stated
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Table 2 Fractional factorial design test matrix for bench-scale test series using the Ontario Hydro mercury speciation methoda Test no.
Fly ash ŽBlacksville.
SO 2 , ppm
HCl, ppm
NOrNO 2 , ppm
Cl 2 , ppm
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
N Y N Y N Y N Y N Y N Y N Y N Y
0 0 1500 1500 0 0 1500 1500 0 0 1500 1500 0 0 1500 1500
0 0 0 0 50 50 50 50 0 0 0 0 50 50 50 50
0 0 0 0 0 0 0 0 600r30 600r30 600r30 600r30 600r30 600r30 600r30 600r30
10 0 0 10 0 10 10 0 0 10 10 0 10 0 0 10
a
Gas and filter temperature was 1758C.
earlier, in addition to the Ontario Hydro method, three other sampling methods were tested in the EERC bench-scale test rig. These included the MESA method, EPA Method 29, and the tris-buffer method. EPA Method 29 and the tris-buffer methods, like
Table 3 Statistical evaluation of EERC bench-scale test results for the Ontario Hydro methoda,b Variable c
Fly ash SO 2 HCl NOrNO 2 Cl 2 Fly ash=SO 2 Fly ash=HCl Fly ash=NOrNO2 Fly ash=Cl 2 SO 2 =HCl SO 2 =NOrNO2 SO 2 =Cl 2 HCl=NOrNO2 HCl=Cl 2 NOrNO 2 =Cl 2 a
Main effect
Two-factor interactions
y12.97"1.35 22.70"1.35 y0.99"1.35 y6.32"1.35 y31.49"1.35 y7.16"1.35 y2.72"1.35 y7.10"1.35 y0.84"1.35 4.53"1.35 y3.73"1.35 23.94"1.35 y5.29"1.35 y2.93"1.35 5.76"1.35
Data in the table are based on the percentage of the measured mercury in the H 2 O 2 –KMnO4 solution while only elemental mercury is injected. b df s19; two-tailed t-test is at 95%; confidence is 2.093 Ž t-statistic is 2.093=1.35s 2.826.. c All fly ash used in this test series was from the Blacksville baseline pilot-scale tests.
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the Ontario Hydro method, are impinger-based mercury-sampling techniques. The MESA method uses solid sorbents to speciate the mercury. The bench-scale results showed that when Cl 2 is added to the system, all the impinger-based methods measured a statistically significant amount of Hg 2q even though only Hg 0 was added. However, the addition of Cl 2 did not significantly change the mercury speciation when measured using the solid-based MESA method w8x. Although this could be problematic when the chloride content of the gas stream is very high, such as is the case in some waste-to-energy systems, it is unknown if Cl 2 exists at flue gas temperatures commonly found in particulate control devices and in stacks from coal-fired systems. Based on pilot-scale tests at the EERC, when a coal is fired with a relatively high chloride content Ž) 50 ppm in the flue gas., the Ontario Hydro method gives very precise and accurate results w2x, indicating that the chloride does not exist as Cl 2 at the temperatures commonly found at the inlet and outlet of particulate control devices Ž250–4008F.. For these tests, EPA Method 26A, which was designed to speciate HCl and Cl 2 by selective adsorption, was used to speciate between HCl and Cl 2 in later pilot-scale tests at the EERC. When a Blacksville eastern bituminous coal was fired in the EERC pilot-scale combustor, the total chloride in the flue gas was 50 ppm of which EPA Method 26A indicated less than 1 ppm was Cl 2 . Very little data are presented in the literature showing the speciation of HCl and Cl 2 using EPA Method 26A on coal-fired systems. However, results at one power plant tested in the DOE Comprehensive Assessment of Toxic Emissions from Coal-Fired Power Plants showed higher levels of Cl 2 w3x than the results at the EERC. Therefore, questions were raised about whether EPA Method 26A correctly speciates chlorine in flue gas with high SO 2 concentrations. Tests conducted by EPA and the EERC have shown that EPA Method 26A does not correctly speciate HCl and Cl 2 in the presence of SO 2 w2,9x. EERC bench-scale data also clearly indicate that fly ash is important in determining mercury speciation in the flue gas streams. There appears to be a growing body of evidence that fly ash is a major factor in determining mercury speciation w2,10,11x. For the bench-scale tests, Blacksville fly ash collected from the pulse-jet baghouse hopper was placed on a filter through which the simulated gas was passed. The bench-scale test results showed that 10–20% of the injected Hg 0 was measured as Hg 2q. Tests were conducted both with the Ontario Hydro method and with a Semtech Hg 2000 continuous mercury analyzer. This analyzer measures only Hg 0 when used without a conversion system. The analyzer showed that there was a decrease in Hg 0 when the simulated flue gas was passed through the bed of ash; however, total mercury measured using the Ontario Hydro method showed a good mass balance. This is very strong evidence that at least some of the Hg 0 is oxidized to Hg 2q as it passes through the ash. In addition to the Cl 2 and fly ash effects, the statistical analyses of the bench-scale results using the Ontario Hydro method show that several two-factor interactions are significant. This makes the interpretation of the statistical data difficult. To more fully evaluate the effects of the variables, it is necessary to compare individual tests. These results are presented in Table 4. As stated earlier, it is clear that Cl 2 has a significant impact on mercury speciation measurement when the Ontario Hydro method is used. This is not altered by having the
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Table 4 Bench-scale data using the Ontario Hydro method Test no.
Variable gas composition, ppm
Oxidized mercury, %
Elemental mercury, %
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Cl 2 Fly ash SO 2 Fly ash, Cl 2 , SO 2 HCl Fly ash, HCl, Cl 2 SO 2 , HCl, Cl 2 Fly ash, HCl, SO 2 NOrNO 2 Fly ash, NOrNO 2 , Cl 2 SO 2 , NOrNO 2 , Cl 2 Fly ash, NOrNO 2 , SO 2 HCl, NOrNO 2 , Cl 2 Fly ash, NOrNO 2 , HCl SO 2 , NOrNO 2 , HCl Fly ash, SO 2 , HCl, NOrNO 2 , Cl 2
84.8 1.0 0.7 28.5 0.3 88.5 1.9 1.3 2.1 78.5 0.5 37.1 78.7 29.0 0.1 46.7
15.2 99.0 99.3 71.5 99.7 11.5 98.1 98.7 97.9 21.5 99.5 62.9 21.3 71.0 99.9 53.7
simulated gas pass through a fixed bed of Blacksville coal fly ash Žcompare Test 1 with Test 6.. However, when SO 2 is added to the simulated gas stream, there is another major effect on mercury speciation. If fly ash is not present, the addition of SO 2 completely eliminates the effect of the Cl 2 Žcompare Test 1 with Test 7.. In Test 1, an average of 84.8% of the injected Hg 0 is captured in the KCl impingers and, therefore, measured as ‘‘oxidized’’ mercury. However, in Test 2, when SO 2 and HCl are added, only 1.9% of the added Hg 0 is captured in the KCl solution. Even if fly ash is present ŽTest 4., the effect of the Cl 2 decreases from 84.8% measured as Hg 2q to 28.5%. It is this relationship between SO 2 and Cl 2 that explains the q22.7 factor ŽSO 2 main effect. shown in the statistical data in Table 3. There is also a significant interaction between NO x and fly ash. The concentration of the added Hg 0 measured as Hg 2q is ) 25% when the NO x is part of simulated flue gas and the gas is passed through the fly ash. However, if fly ash is not present, the effect of the NO x disappears, as shown in Tests 9, 11, and 15. The exception is when Cl 2 is present without SO 2 ŽTest 13.; then the large impact of the Cl 2 becomes dominant. The interaction between NO x and fly ash does not appear to be affected by the addition of either SO 2 or HCl Žcomparing Test 7 with Test 14.. Both NO x and fly ash must be present as is shown in Test 8 Žno NO x . and Test 15 Žno fly ash.. The relationship between NO x , SO 2 , and fly ash is being confirmed by the work done by the EERC and Radian w7,12x. The effects of Cl 2 appear to be so profound that interpretation of the data becomes difficult; therefore, it would be very beneficial in understanding mercury chemistry in flue gas to redo this test series as a full-factorial design without Cl 2 as a variable. Because the concentration of Cl 2 at the temperatures where mercury sampling will be done is very low, based on the bench- and pilot-scale tests, it was decided that the Ontario Hydro method had the potential to accurately speciate mercury.
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165
4. Conclusions and observations Ø Based on the bench-scale tests, there appear to be significant interactions between the fly ash generated firing an eastern bituminous coal and NO x that greatly impact mercury speciation. In the presence of these gases, it appears that this fly ash can catalyze the oxidation of Hg 0 , thereby having a substantial effect on mercury speciation. These results were later confirmed in pilot-scale tests. Ø Cl 2 has a significant effect on mercury speciation as measured by the Ontario Hydro method. In addition, there appear to be significant interactions with fly ash, NO x , and SO 2 . It is not known whether Cl 2 exists or in what concentrations at temperatures typically found in particulate control devices. HCl, however, appears to have little significance in mercury speciation at concentrations and temperatures typically found in ESPs and baghouses.
References w1x U.S. Environmental Protection Agency, Airlink Web Site at: http:rrwww.epa.govrairlinks, 1998. w2x Evaluation of Flue Gas Mercury Speciation Methods EPRI TR-108988, November 1997. w3x S.J. Miller, S.R. Ness, G.F. Weber, T.A. Erickson, D.J. Hassett, S.B. Hawthorne, K.A. Katrinak, P.K. Louie, A comprehensive assessment of toxic emissions from coal-fired power plants: Phase I results from the U.S. Department of Energy study, Final Report to the Pittsburgh Energy Technology Center and the Morgantown Energy Technology Center of the U.S. Department of Energy on Contract No. DE-FC2193MC30097; Energy and Environmental Research Center, University of North Dakota, Grand Forks, ND, October 1996. w4x M.K. Heidt, D.L. Laudal, B.R. Nott, Presentation on EPRIrDOE International Conference on managing hazardous and particulate air pollutants, August 1995, Toronto, Ontario, Canada, 1995. w5x P. Chu, G. Behrens, R. Roberson, Presentation on EPRIrDOE International Conference on managing hazardous and particulate air pollutants, August 1995, Toronto, Ontario, Canada, 1995. w6x Power Plant Validation of the Mercury Speciation Sampling Method, EPRI TR-112588, 1999. w7x S.J. Miller, G.E. Dunham, E.S. Olson, T.D. Brown, Mercury sorbent development in coal-fired boilers, in: Proceedings of the Conference on Air Quality: Mercury, Trace Elements, and Particulate Matter, December 1998, McLean, VA, in press. w8x D.L. Laudal, K.C. Galbreath, M.K. Heidt, T.D. Brown, B.R. Nott, S.K. Jones, A state-of-the-art review of flue gas mercury speciation methods, Final Report for the U.S. Department of Energy and EPRI, EPRI TR-107080, November 1996. w9x B. Ghorishi, B.K. Gullett, Fixed-bed control of mercury: Role of acid gases and a comparison between carbon-based, calcium-based, and coal fly ash sorbents, in: Proceedings of the EPRIrDOErEPA Combined Utility Air Pollutant Control Symposium, August 25–29, 1997, Washington, DC, EPRI TR-108683-V3, 1997. w10x O.W. Hargrove, T.R. Carey, C.F. Richardson, R.C. Skarupa, F.B. Meserole, R.G. Rhudy, T.D. Brown, Presentation on the EPRI-DOE-EPA Combined Utility Air Pollutant Control Symposium, August 1997, Washington, DC, TR-108683-V3, 1997. w11x R.C. Brown et al., Effects of fly ash on mercury oxidation during post-combustion conditions, Abstract from a proposal submitted to the U.S. Department of EnergyrFederal Energy Technology Center, Contract awarded in response to Program Solicitation No. DE-PS26-98FT98200, November 26, 1997. w12x T.R. Carey, O.W. Hargrove Jr., C.R. Richardson, R. Chang, F.B. Meserole, Performance of activated carbon for mercury control in utility flue gas using sorbent injection, Presentation on the Fourth EPRI International Conference on Managing Hazardous Air Pollutants, November 14, 1997, Washington, DC, 1997.
Effects of flue gas constituents on mercury speciation Dennis L. Laudal b
a,)
, Thomas D. Brown b, Babu R. Nott
c
a Energy and EnÕironmental Research Center, 15 North 23rd Street, Grand Forks, ND 58202, USA U.S. Department of Energy Federal Energy Technology Center, 626 Cochrans Mill Road, Pittsburgh, PA 15236, USA c EPRI, 3412 HillÕiew AÕenue, Palo Alto, CA 94303, USA
Received 7 December 1998; accepted 25 August 1999
Abstract Beginning with the 1990 Clean Air Act Amendments, there has been considerable interest in mercury emissions from coal-fired power plants. This past year, the U.S. Environmental Protection Agency ŽEPA. issued both the Mercury Study Report to Congress and the Study of Hazardous Air Pollutant Emissions from Electric Utility Steam-Generating Units, which make clear that EPA views mercury in the environment as a serious issue and that coal-fired utilities are a major source of mercury. For the past 4 years, EPRI and the U.S. Department of Energy ŽDOE. have funded research on mercury measurement, control, and chemistry at the Energy and Environmental Research Center ŽEERC.. The primary goal of bench-scale work was to determine what flue gas constituents affect mercury speciation, specifically how mercury speciation affects measurement methods and the ability of mercury sorbents to absorb mercury. A bench-scale test rig was designed and built to simulate flue gas conditions. The baseline simulated flue gas consisted of O 2 , CO 2 , H 2 O, and N2 . Other flue gas constituents tested include SO 2 , HCl, NO, NO 2 , HF, Cl 2 , and fly ash. The mercury was delivered to system as either elemental mercury ŽHg 0 . or mercuryŽII. chloride ŽHgCl 2 . via temperature-controlled permeation tubes. EERC bench-scale data clearly show that the type of fly ash is important in determining mercury speciation in flue gas streams. Not surprisingly, there appear to be a number of interactions between various flue gas constituents that affect mercury speciation. Depending on concentration, there is clearly an interaction between NO–NO 2 and fly ash, and it is possible that the interaction may be related to the ratio of NO:NO 2 . However, it has been shown that when NO–NO 2 is tested without fly ash, there is no conversion of Hg 0 to Hg 2q. Bench-scale tests clearly show that the chemistry of mercury is very complex and that more research is needed to understand what is
)
Corresponding author. Tel.: q1-701-771-5138; fax: q1-701-777-5181; e-mail: [email protected] 0378-3820r00r$ - see front matter q 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 Ž 9 9 . 0 0 0 8 3 - 1
158
D.L. Laudal et al.r Fuel Processing Technology 65–66 (2000) 157–165
occurring. However, it is equally clear that the development of effective mercury sorbents and the ability to accurately model mercury speciation are dependent on understanding mercury chemistry, thermodynamics, and kinetics. q 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Mercury; Measurement; Chemistry
1. Introduction The 1990 Clean Air Act Amendments ŽCAAAs. required the U.S. Environmental Protection Agency ŽEPA. to determine whether the presence of mercury in the stack emissions from fossil fuel-fired electric utility power plants poses an unacceptable public health risk. EPA’s conclusions and recommendations were presented in the Mercury Study Report to Congress w1x and Utility Air Toxics Report to Congress w1x. The first report addressed both the human health and environmental effects of anthropogenic mercury emissions, while the second addressed the risk to public health posed by the emission of mercury and other hazardous air pollutants from steam-electric generating units. Although these reports did not state that mercury controls on coal-fired electric power stations would be required given the current state of the art, they did indicate that EPA views mercury as a potential threat to human health. Therefore, it was concluded that mercury controls at some point may be necessary. EPA, the U.S. Department of Energy ŽDOE., and EPRI have acknowledged that assessing the risk posed by power plant mercury emissions is very complicated w1–3x. Mercury is emitted in such small concentrations Ž1–30 mgrN m3 . that accurately measuring emission rates has been extremely difficult. In addition, researchers discovered that mercury is emitted in various physical and chemical forms, each possessing distinct properties that affect atmospheric transport, control effectiveness, and sampling and analysis methods. Reliable mercury emission and ambient measurement methods are required to achieve the CAAA goal of assessing the potential human health risks from exposure to mercury. Specifically, accurate measurements of mercury emissions from stationary sources and ambient mercury concentrations in the environment are useful for a variety of reasons, including the following: Ø To estimate the anthropogenic flux of mercury to the environment on a local, regional, and global scale; Ø To identify atmospheric transport and transformation processes; Ø To accurately determine background and natural mercury concentrations; Ø To determine deposition and methylation mechanisms in ecosystems; Ø To assess human health risks; Ø To assess mercury bioaccumulation; Ø To ensure compliance of sources with emission regulations, should they become necessary; Ø To determine partitioning among the various effluents of fossil fuel combustion systems; Ø To evaluate the removal efficiency of control technologies; and Ø To evaluate continuous emission monitors ŽCEMs. for mercury;
D.L. Laudal et al.r Fuel Processing Technology 65–66 (2000) 157–165
159
In addition to measuring total mercury accurately, the identification and quantification of individual physicochemical forms Ži.e., species. of mercury are imperative for addressing questions concerning mercury toxicity, bioaccumulation, emission control, and atmospheric fate and transport, because each has distinctive physical, chemical, and biological properties. Mercury emissions from anthropogenic sources occur in three main forms: solid particle-associated mercury; gaseous divalent mercury, Hg 2q; and gaseous elemental mercury, Hg 0 . Estimates of the relative proportions of these species in fossil fuel-fired power plant emissions and in ambient air are scarce because of a lack of reliable sampling and analysis methods for the different mercury species. Although no validated method, wet chemistry, or CEM existed for determining mercury speciation in combustion flue gas, it was speculated that EPA Method 29, which was validated for total mercury, would also provide accurate mercury speciation data. For a mercury speciation measurement method to work, it must be able to separate the Hg 2q and Hg 0 by selective adsorption. In the case of EPA Method 29, this was done by collecting the Hg 2q in impingers of hydrogen peroxide–nitric acid ŽH 2 O 2 – HNO 3 . and the Hg 0 in potassium permanganate–sulfuric acid ŽKMNO4 –H 2 SO4 impingers. Pilot-scale tests at the Energy and Environmental Research Center ŽEERC. and Radian w4,5x showed that under some conditions, a significant percentage of the Hg 0 is captured in H 2 O 2 –HNO 3 impingers and, therefore, reported as Hg 2q. This was particularly true in flue gas streams with high Ž) 1000 ppm. levels of SO 2 . As a result, other mercury speciation methods needed to be evaluated. From 1995 to 1998, EPRI and DOE funded a research program at the EERC to evaluate mercury speciation analysis methods for electric utility power plants. The EERC implemented an extensive testing program using a bench-scale flue gas simulator and a pilot-scale combustion system to evaluate the performance characteristics Žsensitivity, precision, bias, specificity, and interferences. for many of the mercury speciation
Fig. 1. Schematic of Ontario Hydro speciation sampling train.
160
D.L. Laudal et al.r Fuel Processing Technology 65–66 (2000) 157–165
methods, with a final goal of validating a mercury speciation method at a power plant. Based on the pilot- and bench-scale tests conducted at the EERC, the Ontario Hydro method developed by Dr. Curtis of Ontario Hydro and the EERC, showed the most promise. The Ontario Hydro method is a modification of EPA Method 29, with the first H 2 O 2 –HNO 3 replaced by three KCl impingers, as shown in Fig. 1. The Ontario Hydro method has now been validated as a mercury speciation measurement method in a full-scale test program at a midwestern power plant w6x. This paper focuses on the bench-scale results for the Ontario Hydro method which eventually led to the method being validated in the field. In addition, these bench-scale tests provided a substantial understanding of mercury flue gas chemistry.
2. Experimental As part of the mercury speciation method testing program, bench-scale tests were conducted using the Ontario Hydro method as well as EPA Method 29, the tris-buffer method, and the mercury speciation adsorption ŽMESA. method on a wide range of flue gas constituents. Some of the flue gas constituents from coal-fired systems, such as SO 2 , NO x , HCl, and fly ash, have been shown to have a substantial impact on determining the mercury species w7x. These same constituents may also react with the impinger solutions, resulting in erroneous mercury speciation measurements. For example, some fly ashes collected on the filter prior to the sampling train may adsorb andror oxidize mercury. The constituents were delivered to the system through a gas manifold using calibrated mass flow controllers. The effect of fly ash on mercury speciation was studied by passing the simulated flue gas through an optional Teflon-coated particulate filter which was contained in a temperature-controlled oven.
Fig. 2. Schematic of bench-scale testing system.
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Table 1 Gas composition for bench-scale tests Žwet basis. Nominal concentration Main gases Oxygen ŽO 2 . Carbon dioxide ŽCO 2 . Water vapor ŽH 2 Ov . Nitrogen ŽN2 .
4% 15% 10% Balance
Variable gases Sulfur dioxide ŽSO 2 . Hydrogen chloride ŽHCl v . Nitrogen oxides ŽNOrNO 2 . Chlorine ŽCl 2 . Elemental mercury ŽHg 0 . HgCl 2
0 or 1500 ppm 0 or 50 ppm 0 or 600r30 ppm 0 or 10 ppm 20 mgrN m3 20 mgrN m3
Known quantities of Hg 0 or HgCl 2 vapor were introduced into the system by flushing nitrogen around calibrated permeation tubes. The quantity of mercury vapor injected into the gas stream was determine by its vapor pressure at specific temperatures. In addition, EPA Method 101A and a mercury CEM were used to routinely check the quantity of mercury being released from the permeation tubes. A straight-tube condenser and water bath were used to maintain and control the temperature of the permeation tubes. All parts of the bench-scale test unit are Teflon, Teflon-lined, or glass, and the entire system, including the gas manifold for mixing the gases, is heat-taped to maintain a constant temperature. A schematic of the bench-scale system is shown in Fig. 2, and the gas compositions used for the testing are presented in Table 1.
3. Results and discussion As shown in Table 2, the Ontario Hydro method was evaluated using a replicated fractional factorial design test matrix. In this way, five variables could be tested using the minimum number of tests yet still provide sufficient data for a statistical analysis of the effects. The statistical results from the test series are shown in Table 3. The q and y signs in the table indicate an increase or decrease in the percentage of Hg 0 collected in the H 2 SO4 –KMnO4 solution relative to the amount of Hg 0 spiked into the simulated flue gas. An effect is significant if its absolute value is greater than that of the t-statistic found in the footnotes at the end of the table. Other than the effects of Cl 2 and fly ash, it is unclear what flue gas constituents affect the mercury speciation ability of the Ontario Hydro method. It is very clear that Cl 2 has a significant impact on the mercury speciation measurement when the Ontario Hydro method is used. This is not entirely unexpected because Cl 2 can react with the Hg 0 to form HgCl 2 in a gas-phase reaction. There is evidence, however, that the effect of Cl 2 may be a result of aqueous phase reactions in the impinger solutions. As stated
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Table 2 Fractional factorial design test matrix for bench-scale test series using the Ontario Hydro mercury speciation methoda Test no.
Fly ash ŽBlacksville.
SO 2 , ppm
HCl, ppm
NOrNO 2 , ppm
Cl 2 , ppm
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
N Y N Y N Y N Y N Y N Y N Y N Y
0 0 1500 1500 0 0 1500 1500 0 0 1500 1500 0 0 1500 1500
0 0 0 0 50 50 50 50 0 0 0 0 50 50 50 50
0 0 0 0 0 0 0 0 600r30 600r30 600r30 600r30 600r30 600r30 600r30 600r30
10 0 0 10 0 10 10 0 0 10 10 0 10 0 0 10
a
Gas and filter temperature was 1758C.
earlier, in addition to the Ontario Hydro method, three other sampling methods were tested in the EERC bench-scale test rig. These included the MESA method, EPA Method 29, and the tris-buffer method. EPA Method 29 and the tris-buffer methods, like
Table 3 Statistical evaluation of EERC bench-scale test results for the Ontario Hydro methoda,b Variable c
Fly ash SO 2 HCl NOrNO 2 Cl 2 Fly ash=SO 2 Fly ash=HCl Fly ash=NOrNO2 Fly ash=Cl 2 SO 2 =HCl SO 2 =NOrNO2 SO 2 =Cl 2 HCl=NOrNO2 HCl=Cl 2 NOrNO 2 =Cl 2 a
Main effect
Two-factor interactions
y12.97"1.35 22.70"1.35 y0.99"1.35 y6.32"1.35 y31.49"1.35 y7.16"1.35 y2.72"1.35 y7.10"1.35 y0.84"1.35 4.53"1.35 y3.73"1.35 23.94"1.35 y5.29"1.35 y2.93"1.35 5.76"1.35
Data in the table are based on the percentage of the measured mercury in the H 2 O 2 –KMnO4 solution while only elemental mercury is injected. b df s19; two-tailed t-test is at 95%; confidence is 2.093 Ž t-statistic is 2.093=1.35s 2.826.. c All fly ash used in this test series was from the Blacksville baseline pilot-scale tests.
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the Ontario Hydro method, are impinger-based mercury-sampling techniques. The MESA method uses solid sorbents to speciate the mercury. The bench-scale results showed that when Cl 2 is added to the system, all the impinger-based methods measured a statistically significant amount of Hg 2q even though only Hg 0 was added. However, the addition of Cl 2 did not significantly change the mercury speciation when measured using the solid-based MESA method w8x. Although this could be problematic when the chloride content of the gas stream is very high, such as is the case in some waste-to-energy systems, it is unknown if Cl 2 exists at flue gas temperatures commonly found in particulate control devices and in stacks from coal-fired systems. Based on pilot-scale tests at the EERC, when a coal is fired with a relatively high chloride content Ž) 50 ppm in the flue gas., the Ontario Hydro method gives very precise and accurate results w2x, indicating that the chloride does not exist as Cl 2 at the temperatures commonly found at the inlet and outlet of particulate control devices Ž250–4008F.. For these tests, EPA Method 26A, which was designed to speciate HCl and Cl 2 by selective adsorption, was used to speciate between HCl and Cl 2 in later pilot-scale tests at the EERC. When a Blacksville eastern bituminous coal was fired in the EERC pilot-scale combustor, the total chloride in the flue gas was 50 ppm of which EPA Method 26A indicated less than 1 ppm was Cl 2 . Very little data are presented in the literature showing the speciation of HCl and Cl 2 using EPA Method 26A on coal-fired systems. However, results at one power plant tested in the DOE Comprehensive Assessment of Toxic Emissions from Coal-Fired Power Plants showed higher levels of Cl 2 w3x than the results at the EERC. Therefore, questions were raised about whether EPA Method 26A correctly speciates chlorine in flue gas with high SO 2 concentrations. Tests conducted by EPA and the EERC have shown that EPA Method 26A does not correctly speciate HCl and Cl 2 in the presence of SO 2 w2,9x. EERC bench-scale data also clearly indicate that fly ash is important in determining mercury speciation in the flue gas streams. There appears to be a growing body of evidence that fly ash is a major factor in determining mercury speciation w2,10,11x. For the bench-scale tests, Blacksville fly ash collected from the pulse-jet baghouse hopper was placed on a filter through which the simulated gas was passed. The bench-scale test results showed that 10–20% of the injected Hg 0 was measured as Hg 2q. Tests were conducted both with the Ontario Hydro method and with a Semtech Hg 2000 continuous mercury analyzer. This analyzer measures only Hg 0 when used without a conversion system. The analyzer showed that there was a decrease in Hg 0 when the simulated flue gas was passed through the bed of ash; however, total mercury measured using the Ontario Hydro method showed a good mass balance. This is very strong evidence that at least some of the Hg 0 is oxidized to Hg 2q as it passes through the ash. In addition to the Cl 2 and fly ash effects, the statistical analyses of the bench-scale results using the Ontario Hydro method show that several two-factor interactions are significant. This makes the interpretation of the statistical data difficult. To more fully evaluate the effects of the variables, it is necessary to compare individual tests. These results are presented in Table 4. As stated earlier, it is clear that Cl 2 has a significant impact on mercury speciation measurement when the Ontario Hydro method is used. This is not altered by having the
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Table 4 Bench-scale data using the Ontario Hydro method Test no.
Variable gas composition, ppm
Oxidized mercury, %
Elemental mercury, %
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Cl 2 Fly ash SO 2 Fly ash, Cl 2 , SO 2 HCl Fly ash, HCl, Cl 2 SO 2 , HCl, Cl 2 Fly ash, HCl, SO 2 NOrNO 2 Fly ash, NOrNO 2 , Cl 2 SO 2 , NOrNO 2 , Cl 2 Fly ash, NOrNO 2 , SO 2 HCl, NOrNO 2 , Cl 2 Fly ash, NOrNO 2 , HCl SO 2 , NOrNO 2 , HCl Fly ash, SO 2 , HCl, NOrNO 2 , Cl 2
84.8 1.0 0.7 28.5 0.3 88.5 1.9 1.3 2.1 78.5 0.5 37.1 78.7 29.0 0.1 46.7
15.2 99.0 99.3 71.5 99.7 11.5 98.1 98.7 97.9 21.5 99.5 62.9 21.3 71.0 99.9 53.7
simulated gas pass through a fixed bed of Blacksville coal fly ash Žcompare Test 1 with Test 6.. However, when SO 2 is added to the simulated gas stream, there is another major effect on mercury speciation. If fly ash is not present, the addition of SO 2 completely eliminates the effect of the Cl 2 Žcompare Test 1 with Test 7.. In Test 1, an average of 84.8% of the injected Hg 0 is captured in the KCl impingers and, therefore, measured as ‘‘oxidized’’ mercury. However, in Test 2, when SO 2 and HCl are added, only 1.9% of the added Hg 0 is captured in the KCl solution. Even if fly ash is present ŽTest 4., the effect of the Cl 2 decreases from 84.8% measured as Hg 2q to 28.5%. It is this relationship between SO 2 and Cl 2 that explains the q22.7 factor ŽSO 2 main effect. shown in the statistical data in Table 3. There is also a significant interaction between NO x and fly ash. The concentration of the added Hg 0 measured as Hg 2q is ) 25% when the NO x is part of simulated flue gas and the gas is passed through the fly ash. However, if fly ash is not present, the effect of the NO x disappears, as shown in Tests 9, 11, and 15. The exception is when Cl 2 is present without SO 2 ŽTest 13.; then the large impact of the Cl 2 becomes dominant. The interaction between NO x and fly ash does not appear to be affected by the addition of either SO 2 or HCl Žcomparing Test 7 with Test 14.. Both NO x and fly ash must be present as is shown in Test 8 Žno NO x . and Test 15 Žno fly ash.. The relationship between NO x , SO 2 , and fly ash is being confirmed by the work done by the EERC and Radian w7,12x. The effects of Cl 2 appear to be so profound that interpretation of the data becomes difficult; therefore, it would be very beneficial in understanding mercury chemistry in flue gas to redo this test series as a full-factorial design without Cl 2 as a variable. Because the concentration of Cl 2 at the temperatures where mercury sampling will be done is very low, based on the bench- and pilot-scale tests, it was decided that the Ontario Hydro method had the potential to accurately speciate mercury.
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4. Conclusions and observations Ø Based on the bench-scale tests, there appear to be significant interactions between the fly ash generated firing an eastern bituminous coal and NO x that greatly impact mercury speciation. In the presence of these gases, it appears that this fly ash can catalyze the oxidation of Hg 0 , thereby having a substantial effect on mercury speciation. These results were later confirmed in pilot-scale tests. Ø Cl 2 has a significant effect on mercury speciation as measured by the Ontario Hydro method. In addition, there appear to be significant interactions with fly ash, NO x , and SO 2 . It is not known whether Cl 2 exists or in what concentrations at temperatures typically found in particulate control devices. HCl, however, appears to have little significance in mercury speciation at concentrations and temperatures typically found in ESPs and baghouses.
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