Correspondence pubs.acs.org/IECR
Comment on “Mercury Oxidation by UV Irradiation: Effect of Contact Time, UV Wavelength, and Moisture Content” Evan J. Granite* National Energy Technology Laboratory, U.S. Department of Energy, P.O. Box 10940 M/S 83-312, Pittsburgh, Pennsylvania 15236-0940, United States
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ir: I am writing in response to the excellent and timely article titled “Mercury Oxidation by UV Irradiation: Effect of Contact Time, UV Wavelength, and Moisture Content”.1 Sensitized oxidation of elemental mercury using 253.7 nm ultraviolet radiation can be an excellent strategy for capture of this pollutant; our technique (the GP-254 Process) has been examined at laboratory scale, bench scale, and in slipstreams of real flue gas.2−9 Sorbents and fly ashes have a tendency to exhibit greater capacities for oxidized mercury, in comparison to elemental mercury.10 Oxidized mercury is soluble in water, whereas elemental mercury is highly insoluble. Depending upon where the ultraviolet radiation is applied, sensitized oxidation can enhance the removal of mercury within the particulate collection device or wet scrubber of a coal-fired power plant.2−9
1 H2 2
Hg + NO2 + 253.7 nm light → HgO + NO This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
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OTHER APPLICATIONS FOR IRRADIATION WITH 253.7 NM LIGHT Activated carbons are now employed in many coal-burning power plants in the United States to capture mercury. Note that sulf ur trioxide can poison some of the activated carbons used for the removal of mercury.18,19 This suggests that the application of 253.7 nm radiation could be beneficial in cases of high sulfur trioxide levels in coal flue gases, possibly as a polishing step by Reactions 1−5 downstream of the carbon injection. It has also been recently suggested that NO2 can hinder the performance of activated carbons for the capture of mercury (by oxidizing sulfur dioxide to sulfur trioxide),7,20,21 and this ultraviolet (UV) process could again remove mercury (in this case, through reactions 4 and 5, as well as other reactions). The application of 253.7 nm radiation also has the potential to serve as a low-cost method to remove mercury from various waste incinerator flue gases.2−4,9 The untreated flue gas from incinerators typically contain much higher concentrations of mercury than coal flue gas, which should facilitate photochemical oxidation. New power plants must remove nearly all the mercury present within the flue gas; the application of 253.7 nm light at the back end of a power plant can serve as a simple polishing step, ensuring this high level of mercury capture. The application of 253.7 nm UV light can also mitigate the mercury emissions from chloro-alkali facilities.4
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In the reaction mechanism, elemental mercury serves as a sensitizer for the formation of ozone, and ozone oxidizes mercury to form mercuric oxide. The term sensitizer means that elemental mercury in the 6(3P1) excited state induces chemical reactions to occur. When the mercury collides with another atom or molecule, it can transfer its energy from the excited state to this species, setting the stage for chemical reactions. Activation energy for chemical reactions are typically supplied by heat, but can also be supplied by light. Gunning and Noyes discovered the sensitized oxidations of mercury by moisture, hydrogen chloride, nitrogen dioxide, sulfur trioxide, and carbon dioxide under the influence of 253.7 nm light.12−17 Some of these reactions include the following:
Hg + HCl + 253.7 nm light → HgCl +
Hg + CO2 + 253.7 nm light → HgO + CO
Reactions 1−6 can facilitate the oxidation of mercury in a coal-derived flue gas.2−9
IMPACT OF FLUE GAS CONSTITUENTS: 253.7 NM LIGHT An untreated flue gas from a power plant burning low-sulfur Eastern bituminous coal can contain 5%−7% H2O, 3%−4% O2, 15%−16% CO2, 20 ppm CO, 10 ppm hydrocarbons, 100 ppm HCl, 800 ppm SO2, 10 ppm SO3, 500 ppm NOx, and 1 ppb total Hg, with the balance (∼73%) being N2 (2,8,10). The mercury will exist in elemental, oxidized, and particulate-bound forms. Dickinson and Sherrill demonstrated the photochemical formation of mercuric oxide via the sensitized formation of ozone in 1926:11
Hg + H 2O + 253.7 nm light → HgO + H 2
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Hg + 2O2 + 253.7 nm light → HgO + O3
Hg + SO3 + 253.7 nm light → HgO + SO2
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FUTURE RESEARCH FOR IRRADIATION OF FLUE GAS WITH 253.7 NM LIGHT The thought-provoking study by Gruss1 further establishes the potential of 253.7 nm irradiation as a method for the removal of mercury from gas streams. Previous work suggested using sleeves and wipers around the lamps to allow insertion into various harsh fluids;2−5 the contacting methods for introducing the light into dirty and particulate-containing fluids can be a fruitful area for further studies. An early crude estimate of 0.35% parasitic power requirement for 90% mercury oxidation and removal at a coal-burning power plant was made;4,5 the power consumption by the lamps has been identified as a significant operating cost for the process,2−5 and optimization of the contacting method for introducing the light into the fluid
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DOI: 10.1021/acs.iecr.7b02831 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Correspondence
(15) Moore, H. R.; Noyes, W. A. Photochemical studies: II. The activation of a mercury surface by light. J. Am. Chem. Soc. 1924, 46, 1367. (16) Pierce, W. C.; Noyes, W. A. A further study of the reaction between nitrogen dioxide and liquid mercury. J. Am. Chem. Soc. 1928, 50, 2179. (17) Strausz, O. P.; Gunning, H. E. The decomposition of carbon dioxide by Hg 6(3P1) and Hg 6(3P0) atoms. Can. J. Chem. 1961, 39, 2244. (18) Presto, A. A.; Granite, E. J. Impact of Sulfur Oxides on Mercury Capture by Activated Carbon. Environ. Sci. Technol. 2007, 41, 6579− 6584. (19) Presto, A. A.; Granite, E. J.; Karash, A. Further Investigation of the Impact of Sulfur Oxides on Mercury Capture by Activated Carbon. Ind. Eng. Chem. Res. 2007, 46, 8273−8276. (20) Senior, C. Dry Sorbent Injection for SO3 and MATS Application; CSC Publishing: St. Paul, MN, 2013. Available via the Internet at:http://www.solvair.us/SiteCollectionDocuments/articles/ 20130201_AIR_Pollution_Control.pdf (accessed July 11, 2013). (21) Olson, E. S. Mercury-Carbon Surface Chemistry. In Mercury Control for Coal-Derived Gas Streams; Wiley−VCH: Weinheim, Germany, 2015; Chapter 23, pp 377−387.
can reduce this expense. More-efficient lamps can also reduce the power requirements for obtaining high levels of mercury oxidation and removal. The ultimate disposition and form of the mercury, be it as a compound deposited within a photoreactor, a compound dissolved within the wet scrubber, or a compound adsorbed or deposited upon fly ash or sorbent particles, merits further examination. Mercuric oxide and mercurous sulfate films were formed when simulated flue gas was irradiated by 253.7 nm light in quartz photoreactors.2 Other compounds could be formed in real flue gas through secondary reactions between mercuric oxide or mercurous chloride formed by the sensitized oxidation reactions described by Reactions 1−6 and the many components of flue gas.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Evan J. Granite: 0000-0001-7668-8364 Notes
The author declares no competing financial interest.
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REFERENCES
(1) Gruss, A. F.; Rodriguez, R.; Mazyck, D. W. Mercury Oxidation by UV Irradiation: Effect of Contact Time, UV Wavelength, and Moisture Content. Ind. Eng. Chem. Res. 2017, 56, 6131−6135. (2) Granite, E. J.; Pennline, H. W. Photochemical Removal of Mercury from Flue Gas. Ind. Eng. Chem. Res. 2002, 41, 5470−5476. (3) Granite, E. J.; Pennline, H. W. Method for Removal of Mercury from Various Gas Streams, U.S. Patent 6,576,092; June 10, 2003. (4) Granite, E. J.; Pennline, H. W. The GP-254 Process for Photochemical Removal of Mercury from Flue Gas. Presented at the Federal Laboratory Consortium Technology Transfer Conference, Orlando, FL, May 2005. (5) McLarnon, C.; Granite, E. J.; Pennline, H. W. The PCO Process for Photochemical Removal of Mercury from Flue Gas. Fuel Process. Technol. 2005, 87, 85−89. (6) Jia, L.; Dureau, R.; Ko, V.; Anthony, E. J. Oxidation of Mercury under Ultraviolet (UV) Irradiation. Energy Fuels 2010, 24, 4351−4356. (7) Uffalussy, K. J.; Granite, E. J. Novel Capture Technologies: NonCarbon Sorbents and Photochemical Oxidations. In Mercury Control for Coal-Derived Gas Streams; Wiley−VCH: Weinheim, Germany, 2015; Chapter 21, pp 339−356. (8) Granite, E. J.; Pennline, H. W.; Hoffman, J. S. Effects of Photochemical Formation of Mercuric Oxide. Ind. Eng. Chem. Res. 1999, 38, 5034−5037. (9) Granite, E. J.; Pennline, H. W. Photochemical Removal of Mercury from Flue Gas. In Proceedings of 223rd ACS National Meeting; American Chemical Society: Washington, DC, 2002. (10) Granite, E. J.; Pennline, H. W.; Hargis, R. A. Novel Sorbents for Mercury Removal from Flue Gas. Ind. Eng. Chem. Res. 2000, 39 (4), 1020. (11) Dickinson, R. G.; Sherrill, M. S. Formation of Ozone by Optically Excited Mercury Vapor. Proc. Natl. Acad. Sci. U. S. A. 1926, 12, 175. (12) Gunning, H. E. Primary processes in reactions initiated by photoexcited mercury isotopes. Can. J. Chem. 1958, 36, 89. (13) Pertel, R.; Gunning, H. E. Photochemical separation of mercury isotopes. Can. J. Chem. 1959, 37, 35. (14) McDonald, C. C.; McDowell, J. R.; Gunning, H. E. Photochemical separation of mercury isotopes: the reaction of Hg202 6(3P1) atoms, photoexcited in natural mercury vapor, with hydrogen chloride. Can. J. Chem. 1959, 37, 930. B
DOI: 10.1021/acs.iecr.7b02831 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
