Energy & Fuels 2008, 22, 3557–3558
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Communication Comment on the “Role of SO2 for Elemental Mercury Removal from Coal Combustion Flue Gas by Activated Carbon” Evan J. Granite*,† and Albert A. Presto‡ National Energy Technology Laboratory, United States Department of Energy, Post Office Box 10940 M/S 58-106, Pittsburgh, PennsylVania 15236-0940, and Air Quality Laboratory, Center for Atmospheric Particle Studies, Carnegie Mellon UniVersity, Pittsburgh, PennsylVania 15213 ReceiVed July 3, 2008 We are writing in response to the excellent and timely paper entitled “Role of SO2 for Elemental Mercury Removal from Coal Combustion Flue Gas by Activated Carbon”.1 We offer the following comments:
power plants in the United States are typically conducted upstream of the particulate collection device and at temperatures between 130 and 175 °C. Lower temperatures will significantly enhance the adsorption of mercury by activated carbons.3
Type of Activated Carbon The study by Uddin1 was conducted on activated carbon produced from a coconut shell carbon precursor. Most of the work in the United States on mercury removal from coal-fired stack emissions has been with lignite-derived activated carbons. The coconut shell carbon sorbent may adsorb mercury through a different mechanism than a coal-derived activated carbon. The high Brunauer-Emmett-Teller (BET) surface area of the coconut shell carbon (1250 m2/g) relative to typical lignite carbons (600 m2/g) used in mercury removal trials at U.S. power plants suggest that it may be a better sorbent. Impact of SO2 We agree with Uddin1 that sulfur dioxide will not poison the activated carbon for mercury capture and, in fact, will promote the carbon for mercury capture from simple gas mixtures. This effect has been previously observed by Miller2 and in our laboratories. Impacts of NO, NO2, and HCl Miller has noted that nitrogen oxides can have a profound effect upon the capture of mercury by carbons from flue gases. Miller noted that NO2 will cause desorption of Hg2+ from the surface of activated carbons. These nitrogen oxide species were absent in the study presented by Uddin.1 Hydrogen chloride is also known to have a significant impact upon the adsorption of mercury on activated carbons and was absent from the simulated flue gas.1 Impact of Temperature The studies conducted by Uddin1 were in the temperature range of 60-100 °C. Carbon injection studies at coal-burning * To whom correspondence should be addressed. Fax: 412-386-6004. E-mail:
[email protected]. † United States Department of Energy. ‡ Carnegie Mellon University. (1) Uddin, M. A.; Yamada, T.; Ochiai, R.; Sasaoka, E.; Wu, S. Role of SO2 for Elemental Mercury Removal from Coal Combustion Flue Gas by Activated Carbon. Energy Fuels 2008, 22, 2284–2289. (2) Miller, S.; Dunham, G.; Olson, E.; Brown, T. Flue Gas Effects on a Carbon-Based Mercury Sorbent. Fuel Proc. Technol. 2000, 65-66, 343.
Nature of Sulfur Poisons Jarvis recently reported that sulfur trioxide negatively impacts the ability of activated carbons to remove mercury from coal utility flue gases.4 In previous studies,5-7 we noted that the concentration of sulfur dioxide had little impact upon the capture of mercury from simulated flue gases, whereas the presence of sulfur trioxide had a significant negative impact. Furthermore, we found an inverse correlation between mercury capacity and the concentration of S6+ upon the surface of the activated carbon. This suggests that the S6+ species of sulfur trioxide in the gas and surface sulfate are the actual sulfur poisons for the capture of mercury by activated carbon in coal-derived flue gas. Elemental sulfur has long been suggested as a promoter for the capture of mercury from flue gas. Examples from IndustrysInteresting Puzzle It is noted that, in other industries, sulfuric acid aids in the removal of mercury from fluid streams.6 Sulfuric-acid-impregnated carbons have been successfully employed to remove mercury from liquid hydrocarbons.8 Concentrated sulfuric acid has also been used to scrub mercury from smelter gases via the formation of mercuric sulfate.9 Previous work from this laboratory has also revealed that sulfuric-acid-impregnated activated (3) Granite, E. J.; Pennline, H. W.; Hargis, R. A. Novel Sorbents for Mercury Removal from Flue Gas. Ind. Eng. Chem. Res. 2000, 39, 1020– 1029. (4) Jarvis, J.; Meserole, F. SO3 Effect on Mercury Control. Power Eng. 2008, 54–60. (5) Presto, A. A.; Granite, E. J. Impact of Sulfur Oxides on Mercury Capture by Activated Carbon. EnViron. Sci. Technol. 2007, 41, 6579–6584. (6) 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. (7) Granite, E. J.; Presto, A. A. Response to Comment on Impact of Sulfur Oxides on Mercury Capture by Activated Carbon. EnViron. Sci. Technol. 2008, 42, 972–973. (8) Ohtsuka, K. Acid-Containing Activated Carbon for Adsorbing Mercury from Liquid Hydrocarbons. U.S. Patent 5,891,324, 1999. (9) Habashi, F. Metallurgical Plants: How Mercury Pollution Is Abated. EnViron. Sci. Technol. 1978, 12, 1372.
10.1021/ef8005355 CCC: $40.75 2008 American Chemical Society Published on Web 08/21/2008
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carbon effectively removes mercury from a stream of nitrogen.10 These conditions are significantly different than the flue gas environment. The reasons for these differences are unclear, require further investigation, and make the sulfur-poisoning issue quite interesting. It can be speculated that a mercury sulfate end-product is viable in certain environments but not in coalderived flue gas.11 Pollack12 has also suggested that sulfuric acid will fill the carbon pores and reduce available surface area for mercury adsorption in coal-derived flue gases. Discussion The thought-provoking study by Uddin1 further establishes that sulfur dioxide is not the actual poison for mercury capture (10) Granite, E. J.; Presto, A. A. Novel Sorbents for Removal of Mercury from Flue Gas, In Proceedings of the 23rd Annual International Pittsburgh Coal Conference, Pittsburgh, PA, 2006. (11) Schofield, K. Mercury Emission Chemistry: The Similarities or Are They Generalities of Mercury and Alkali Combustion Deposition Processes? Proc. Combust. Inst. 2005, 30, 1263. (12) Pollack, N. Private Communication, Dec 2007.
Communications
by carbon sorbents. It is noted, however, that the concentration of sulfur trioxide in coal-derived flue gas typically mirrors that of sulfur dioxide; i.e., the concentration of sulfur trioxide increases as the concentration of sulfur dioxide increases. The use of coconut shell activated carbons, the lower sorbent temperatures, and the absence of nitrogen oxides and hydrogen chloride make direct comparisons to electric utility application of activated carbon for mercury control somewhat difficult. We suggest that SO3 (gas), and S6+(surface) in particular are the poisons for adsorption of mercury from coal-derived flue gases.5-7 The examples8,9 from the smelter and hydrocarbon industries where sulfuric acid appears to aid in the removal of mercury make this an interesting problem, with further mechanistic details to be resolved. More studies along the lines of Uddin1 need to be conducted in realistic flue gas mixtures.
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