Energy Fuels 2010, 24, 10–17 Published on Web 09/01/2009
: DOI:10.1021/ef900501p
Development of a Novel Mercury Cartridge for Mercury Analysis† Yan Liu, Zhenghe Xu,* and Steven M. Kuznicki Contribution from: Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada, T6G 2 V4. †Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. Received May 22, 2009. Revised Manuscript Received August 2, 2009
Mercury cartridges, which are a key component of semicontinuous online mercury monitors, capture lowconcentration mercury from the effluent streams of coal-fired power plants and subsequently release highly concentrated pulses of mercury for spectroscopic analysis. The most common sorbent used in mercury cartridges is gold-coated silica beads (Au/SiO2), which form a reversible amalgam with elemental mercury. Ag/MC is a robust composite mercury sorbent, consisting of silver nanoparticles supported on the surface of natural chabazite, which can efficiently capture and release mercury from a real flue gas environment, making the material a potential alternative to Au/SiO2 in mercury preconcentration cartridges. The performance of Au/SiO2- and Ag/MC-based mercury cartridges in capturing low-level mercury in Ar-, SO2-, and NO-containing gas streams was investigated systematically. Both SO2 and NO were determined to be harmful to the performance of an Au/SiO2 mercury cartridge. NO had limited impact on the performance of Ag/MC, but the presence of SO2 led to reduced mercury recovery from the Ag/MC mercury cartridge. Soda lime was proven to be an effective, NO-tolerant SO2 scrubber. Based on these results, a novel SO2- and NO-tolerant mercury cartridge was designed and fabricated using soda lime as a disposable SO2 scrubber and Ag/MC as the reversible mercury sorbent.
large amounts of fly ash in coal-fired power plant exhaust makes real-time detection of mercury impractical with existing technologies.11,12 Taking the diodle laser-based UV absorption technique as an example,12 the presence of ultrafine particulate fly ash in the flue gas and its buildup on fused silica window attenuate the intensity of the laser beam, leading to less-reliable online mercury measurement. For this reason, an extractive-sampling absorption cell was proposed for off-line measurement, which could not avoid other challenges such as accurate and reliable sampling, mercury adsorption on walls, and catalyzed reactions. Currently, off-line analysis is widely utilized in flue gas mercury analysis, coupled with either wet chemistry methods such as the Ontario Hydro method and EPA methods 19 and 101A, or dry methods, as shown in Scheme 1. The most popular detection methods used are cold vapor atomic absorption spectrophotometry (CVAAS) and cold vapor atomic fluorescence absorption spectrophotometry (CVAFS).3,13,17 CVAAS and CVAFS detect only elemental mercury. Because total gaseous mercury is composed of both Hg0 and Hg2þ, mercury reduction must precede spectroscopic analysis. Scheme 1a outlines mercury speciation by a wet chemistry approach, in which oxidized mercury (Hg2þ) is separated with a KCl solution and reduced to Hg0 in an SnCl2 solution prior to detection. The wet chemistry methods are highly sensitive for detecting and measuring mercury at concentrations over
Introduction Accurate, real-time mercury detection systems are required both to implement environmental regulations for the control of mercury emissions from coal-fired power plants and to study mercury transformation and removal in power plant effluents.1-10 The presence of interfering gas components and *Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Seigneur, C.; Vijayaraghavan, K.; Lohman, K.; Karamchandani, P.; Scott, C. Global source attribution for mercury deposition in the United States. Environ. Sci. Technol. 2004, 38, 555–569. (2) Heebink, L. V.; Hassett, D. J. Mercury release from FGD. Fuel 2005, 84, 1372–1377. (3) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. C.; Laudal, D. L.; Benson, S. A. Status review of mercury control options for coal-fired power plants. Fuel Process. Technol. 2003, 82, 89–165. (4) Toxicological Profile for Mercury, (update) Draft; U.S. Dept. Health and Human Services Agency for Toxic Substances and Disease Registry (ATSDR): Atlanta, GA, 1999. (5) Mercury Study for Congress. Vol. V: Health Effects of Mercury and Mercury Compounds; USEPA Office of Air Quality Planning and Standards and Office of Research and Development: Washington, DC, 1997; Report No. EPA-452/R-97-007. (6) Seigneur, C.; Karamchandani, P.; Lohman, K.; Vijayaraghavan, K.; Shia, R.-L. Multiscale modeling of the atmospheric fate and transport of mercury. J. Geophys. Res. 2001, 106, 27795–27809. (7) Devito, M. S.; Tunati, P. R.; Carlson, R. J.; Bloom, N. Sampling and Analysis of Mercury in Combustion Flue Gas. In EPRI 2nd International Conference on Managing Hazardous Waste Air Pollutants; EPRI: Washington, DC, 1993. (8) Krishnakumar, B.; Helble, J. J. Understanding mercury transformations in coal-fired power plants: evaluation of homogeneous Hg oxidation mechanisms. Environ. Sci. Technol. 2007, 41, 7870–7875. (9) Manolopoulos, H.; Snyder, D. C.; Schauer, J. J.; Hill, J. S.; Yurner, J. R.; Olson, M. L.; Krabbenhoft, D. P. Sources of speciated atmospheric mercury at a residential neighborhood impacted by industrial sources. Environ. Sci. Technol. 2007, 41, 5626–5633. (10) Chang, J. C. S. Simulation and evaluation of elemental mercury concentration increase in flue gas across a wet scrubber. Environ. Sci. Technol. 2003, 37, 5763–5766. r 2009 American Chemical Society
(11) Hall, B.; Lindqvist, O.; Ljungstroem, E. Mercury chemistry in simulated flue gases related to waste incineration conditions. Environ. Sci. Technol. 1990, 24, 108–111. (12) Magnuson, J. K.; Anderson, T. N.; Lucht, R. P.; Vijayasarathy, U. A.; Oh, H.; Annamalai, K; Caton, J. A. Application of a diode-laserbased ultraviolet absorption sensor for in situ measurements of atomic mercury in coal-combustion exhaust. Energy Fuels 2008, 22, 3029–3036. (13) Sondreal, E. A.; Benson, S. A.; Pavlish, J. H.; Ralston, N. V. C. An overview of air quality, III: Mercury, trace elements, and particulate matter. Fuel Process. Technol. 2004, 85, 425–440.
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Energy Fuels 2010, 24, 10–17
: DOI:10.1021/ef900501p
Liu et al.
3 μg/dscm, but require well-trained, experienced personnel and a high level of quality assurance to prevent sample contamination and mercury loss.3,13,17 In addition, these costly detection methods do not provide sufficiently frequent online data analysis as required. In contrast, dry methods can produce real-time or near-real-time results, including mercury speciation information, with minimal operator input, and can be readily incorporated into semicontinuous automated mercury monitors.13,17,18 Scheme 1b illustrates mercury monitoring and speciation by a dry method. Mercury species are measured in two parallel streams of flue gas, one for Hg0 detection and the other for total mercury detection after catalytic reduction. Accurate measurement of elemental mercury is fundamental to the operation of semicontinuous mercury monitors. In both the wet and dry methods (see Scheme 1), a mercury cartridge is applied to preconcentrate mercury for measurement, because the concentration of mercury in flue gases of coal-fired power plants is very low (1-10 μg/m3).2,3 Mercury cartridges are used to capture mercury from flue gases over a predetermined time interval and subsequently release concentrated pulses of mercury into a detection stream of high-purity carrier gas. Although this approach creates a time delay in mercury emission monitoring and control, it significantly increases the accuracy and decreases the detection limit of mercury measurement. From a practical perspective, the time required for accurate measurement is more than satisfactory for realtime monitoring of flue gases from coal-fired power plants. SOx, NOx, and HCl are common components of flue gases and have broadband absorption at the detection wavelength of elemental mercury, interfering with mercury detection by CVAFS and CVAAS.3,18 Mercury release from the mercury cartridge normally proceeds offline in high-purity argon (CVAFS) or dry air/nitrogen (CVAAS), which minimizes the effects of SOx, NOx, and HCl on mercury determination by CVAFS or CVAAS. 14-16
Gold-coated silica beads (Au/SiO2) are widely used in mercury cartridges for mercury preconcentration.19-26 The gold film on the silica beads forms an amalgam with elemental mercury at relatively low temperatures. When the mercuryloaded Au/SiO2 is heated, a highly concentrated pulse of mercury is released and can be measured by a mercury analyzer. Heating concurrently regenerates the Au/SiO2 mercury cartridge.19-22 In addition to the complete capture and release of mercury, accurate mercury measurement also requires that the effect of flue gas components on the mercury release profile be minimized. During mercury capture from a flue gas, the Au/SiO2 mercury cartridge is transiently exposed to the acidic components of the flue gas. Exposure to these components may interfere with cartridge performance by (i) affecting the efficiency of mercury capture through competitive adsorption, (ii) affecting mercury release by forming intermediate species, (iii) destroying the gold film, and/or (iv) releasing detrimental components into the mercury detector.21,25,27,28 Previous studies have demonstrated that silver nanoparticles can be synthesized on the surface of natural chabazite (Ag/MC) and used as a mercury sorbent with both high mercury capture efficiency and capacity.20,29-35 Complete mercury release and sorbent regeneration of the Ag/MC composite has also been demonstrated by mercury mass balance analysis.29 Ag/MC is a robust, reversible mercury sorbent that can efficiently capture and release mercury from a real flue gas environment, making the material a potential candidate for use in mercury preconcentration cartridges. Chabazite is a natural tectosilicate mineral of zeolite, which stabilizes silver as nanoparticles. Simple ion exchange followed by thermal annealing for the fabrication of silver nanoparticles on chabazite makes novel Ag/MC mercury cartridges a less-expensive alternative to the synthetic silica beads that are coated by gold through vapor deposition which requires sophisticated devices.29 (25) Kobiela, T.; Nowakowski, B.; Dus, R. The influence of gas phase composition on the process of Au-Hg amalgam formation. Appl. Surf. Sci. 2003, 206, 78–89. (26) Xu, H.; Zeng, L; Xing, S.; Shi, g.; Xian, Y.; Jin, L. Microwaveradiated synthesis of gold nanoparticles/carbon nanotubes composites and its application to voltammetric detection of trace mercury(II). Electrochem. Commun. 2008, 10, 1839–1843. (27) Nowakowski, R.; Kobiela, T.; Wolfram, Z.; Dus, R. Atomic force microscopy of Au/Hg alloy formation on thin Au films. Appl. Surf. Sci. 1997, 115, 217–231. (28) Nowakowski, R.; Pielaszek, J.; Dus, R. Surface mediated Ag-Hg alloy formation under ambient and vacuum conditions-AFT and XRD investigations. Appl. Surf. Sci. 2002, 199, 40–50. (29) Liu, Y.; Kelly, D. J. A.; Yang, H.; Lin, C. C. H.; Kuznicki, S. M.; Xu, Z. Novel Regenerable Sorbent for Mercury Capture from Flue Gases of Coal-Fired Power Plant. Environ. Sci. Technol. 2008, 42, 6205– 6210. (30) Yan, T. Y. A novel process for Hg removal from gases. Ind. Eng. Chem. Res. 1994, 33, 3010–3014. (31) Kuznicki, S. M.; Kelly, D. J. A.; Bian, J. J.; Lin, C.C. H.; Liu, Y.; Chen, J.; Mitlin, D.; Xu, Z. Metal nanodots formed and supported on chabazite and chabazite-like surfaces. Microporous Mesoporous Mater. 2007, 103, 309–315. (32) Liu, Y.; Chen, F.; Kuznicki, S. M.; Wasylishen, R. E.; Xu, Z. A Novel Method to Control the Size of Silver Nanoparticles Formed on Chabazite. Nanosci. Nanotechnol. 2009, 9, 2768–2771. (33) Lin, C. H. C.; Danaie, M; Liu, Y; Mitlin, D; Kuznicki, S. M.; Edward, M. Thermally stable silver nanoparticles formed on a zeolite surface show multiple crystal twinning. Nanosci. Nanotechnol. 2009, 9, 4985–4987. (34) Dong, J.; Xu, Z; Kuznicki, S. M. Magnetic multi-functional nano composites for environmental applications. Adv. Funct. Mater. 2009, 19, 1268–1275. (35) Dong, J.; Xu, Z; Kuznicki, S. M. Mercury Removal from Flue Gases by Novel Regenerable Magnetic Nanocomposite Sorbents. Environ. Sci. Technol. 2009, 43, 3266–3271.
(14) Laudal, D. L.; Heidt, M. K. Evaluation of Flue Gas Mercury Speciation Methods; Energy and Environmental Research Center: Grand Forks, ND, 1997; EPRI Contract No. TR-108988. (15) Method 29-Determination of Metal Emissions from Stationary Sources; U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards: Research Triangle Park, NC, April 25, 1996; Report No. TM-029. (16) Park, K. S.; Seo, Y.-C.; Lee, S. J.; Lee, J. H. Emission and speciation of mercury from various combustion sources. Powder Technol. 2008, 180, 151–156. (17) Laudal, D. L.; Thompson, J. S.; Pavlish, J. H.; Brickett, L. A.; Chu, P. Use of continuous mercury monitors at coal-fired utilities. Fuel Process. Technol. 2004, 85, 501–511. (18) Zamzow, D. S.; Bajic, S. J.; Eckels, D. E.; Baldwin, D. P.; Winterrowd, C.; Keeney, R. Real-time atomic absorption mercury continuous emission monitor. Rev. Sci. Instrum. 2003, 74, 3774–3783. (19) Horvat, M.; Lupsina, V.; Philar, B. Determination of total mercury in coal fly ash by gold amalgamation cold vapour atomic absorption spectrometry. Anal. Chim. Acta 1991, 243, 71–79. (20) Long, S. J.; Scott, D. R.; Thompson, R. J. Atomic absorption determination of elemental mercury collected from ambient air on silver wool. Anal. Chem. 1973, 45, 2227–2233. (21) Dumarey, R.; Dams, R.; Hoste, J. Comparison of the collection and desorption efficiency of activated charcoal, silver, and gold for the determination of vapor-phase atmospheric mercury. Anal. Chem. 1985, 57, 2638–2643. (22) Baeyens, W.; Leermakers, M. Determination of metallic mercury and some organomercury compounds using atomic absorption spectrometry after amalgamation on a gold column. J. Anal. At. Spectrom. 1989, 4, 635–640. (23) Schaedlich, F. H.; Schneeberger, D. R. Cartridge for collection of a sample by adsorption onto a solid surface, U.S. Patent 5,660,795, 1997. (24) Levlin, M.; Ikavalko, E.; Laitinen, T. Adsorption of mercury on gold and silver surfaces. Fresenius J. Anal. Chem. 1999, 365, 577–586.
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Energy Fuels 2010, 24, 10–17
: DOI:10.1021/ef900501p
Liu et al.
Scheme 1. Principles of Semicontinuous Automated Mercury Monitors Utilizing (a) Wet and (b) Dry Speciation Approaches
In this study, a systematic investigation of mercury cartridge performance was conducted to demonstrate the critical role of mercury cartridges in mercury monitoring and investigate the effects of two major interfering gas components (SO2 and NO) on mercury cartridge performance. Two types of mercury cartridge sorbents were compared: gold-coated silica beads (Au/SiO2) and surface-supported silver nanoparticles formed on chabazite (Ag/MC). To mitigate the effects of SO2 on mercury recovery from the Ag/MC mercury cartridge, a novel SO2- and NO-tolerant mercury cartridge was designed and fabricated with soda lime placed upstream of the Ag/MC as a disposable SO2 scrubber.
Evaluation of Mercury Cartridge Performance in an Argon Stream. The experimental setup consisted (from upstream to downstream) of a mercury port, a mercury cartridge, and a mercury detector connected by Teflon tubing and fittings.29 A pulse of mercury-saturated air of known volume at storage temperature was injected into the argon stream from the mercury port. After 5 min, the mercury cartridge was heated by applying 5.6 V to the wrapped coil to release precaptured mercury. The released mercury was detected by a downstream mercury detector (Tekran-2500 cold vapor atomic fluorescence spectrometry). The mercury-saturated air was generated by equilibrating a high-purity mercury ball with air in a 2500-mL flask at room temperature and ambient pressure, same procedure as that described in ref 20. The mercury concentration in mercury-saturated injection is calculated to be 1 μg/L, which is much higher than that in the real flue gases, normally in the range of tens of micrograms per cubic meter. Evaluation of Mercury Cartridge Performance in a Synthetic Flue Gas. The performance of a mercury cartridge in a synthetic gas stream was studied in a semicontinuous mode: mercury preconcentration in the synthetic flue gas, followed by mercury measurement in a high-purity argon stream. The mercury cartridge was first exposed to the synthetic flue gas for a time period of texposure. Mercury-saturated air of known volume was then injected into the synthetic flue gas at time tHg through a mercury port (tHg< texposure). The carrier gas was switched to argon to flush the system for 2-3 min with the objective to remove physically bonded flue gas components from the mercury cartridge and protect the mercury detector. The mercury cartridge was then heated by applying 5.6 V to the heating coil to release precaptured mercury for measurement. After each test in a NO/SO2-containing stream, one or two tests of the mercury cartridge were performed in a high-purity argon stream to determine the effect of exposure to NO and SO2 on cartridge performance. The SO2-containing synthetic flue gas contains 892 ppm SO2, 6.0% O2, 12.0% CO2, and balanced N2. The NO-containing synthetic flue gas contained 318 ppm NO and the balance was N2. X-ray Photoelectron Spectroscopy (XPS) Analysis. The XPS spectra were collected using an AXIS 165 (Kratos) spectrometer equipped with a monochromatic Al KR (hν= 1486.6 eV) X-ray source. The pressure in the sample analysis chamber was maintained at
