Impact of Sulfur Oxides on Mercury Capture by Activated Carbon

Environ. Sci. Technol. 2008, 42, 972–973

Response to Comment on “Impact of Sulfur Oxides on Mercury Capture by Activated Carbon” We are writing in response to the comment on our article titled “Impact of sulfur oxides on mercury capture by activated carbon” submitted by Liu (1). We appreciate the thoughtful comments and suggestions, and offer the following responses. Question 1: What Kind of Mercury Species is Adsorbed? The Hg source used in our experiments emits Hg0 exclusively. We have confirmed this in previous studies using a mercury CEM. Our previous experience with the laboratory-scale packed bed reactor (2) leads us to believe that the mercury in the simulated flue gas reaching the packed bed is practically 100% elemental, and that the contributions of homogeneous reactions or heterogeneous reactions on the surface of the quartz tube or the glass wool packing are negligible. We have confirmed this assumption during recent experiments using the benchscale reactor and a CEM (3). Previous research also indicates that the homogeneous reactions of mercury are extremely slow at the temperatures tested here (4). Liu is correct in noting that oxidized mercury species are thermodynamically favored at low temperatures (5), however the homogeneous oxidation of mercury is kinetically limited at temperatures observed downstream of the furnace (4). Formation of HgO at 200 °C would also present itself readily in the form of a yellow or red stain on the surface of the quartz tube; this phenomenon was observed previously in this laboratory upon the photochemical oxidation of mercury (2), but was not observed in this study. Question 2: What Is the Mechanism of Mercury Adsorption? Our understanding of mercury adsorption on activated carbon stems from the mechanism proposed by Dunham et al. and Olson et al. (6) Mercury is captured on the carbon surface as oxidized mercury (i.e., the mercury is oxidized at the carbon surface). Huggins et al. showed that adsorbed Hg2+ is bound to a soft element such as carbon, chlorine, or elemental sulfur (7). The work of Huggins et al. also suggested that HgO, HgSO4, or Hg(NO3)2 do not form on the activated carbon surface. The oxidized mercury bound to the carbon surface is acidic and requires a Lewis base (electron donating) binding site. Sulfur oxides also bind to electron-donating sites on the carbon surface. Hence, we assert that mercury and sulfur oxides are in competition for the same binding sites on the carbon surface. This result is not entirely novel; our manuscript provides several references that reach the same conclusion. Notably, Miller et al. observed that adding SO2 to a simulated flue gas, with NO2 present to enhance SO2 oxidation, caused Hg2+ to desorb from activated carbon (8). We assume that HgCl2, which is already oxidized, also binds to Lewis base sites on the carbon surface. Thus, we expect that the capture of HgCl2 by activated carbon in real flue gas will also be inhibited by the presence of SO3. Recent tests of activated carbon injection in high-sulfur flue gas containing large fractions of oxidized mercury showed limited mercury capture (9). The suggestion to confirm this assumption by testing activated carbons in simulated flue gases containing HgCl2 merits further consideration. 972 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 3, 2008

The presence of halogens on the carbon surface serves to stabilize the mercurinium ion formed on the carbon surface during mercury oxidation. These surface halogens can be provided by the flue gas or through the use of a halogenated activated carbon. Recent field tests have shown that brominated activated carbons can overcome low halogen concentrations in flue gas and deliver excellent mercury capture in low-sulfur environments (10). Liu’s (1) suggestion to vary the HCl concentration in the simulated flue gas is well-received, but this was outside of the scope of the current study and, considering the effectiveness of brominated activated carbons, is not considered of critical importance at this time. Discussion. Liu (1) is correct in noting the multiple reactions available for producing oxidized mercury in simulated flue gas. However, we stress that the purely homogeneous pathways are kinetically limited at the experimental conditions used here and are unlikely to occur in real flue gas. We further stress that the direct reaction between Hg and HCl to form HgCl2 is unlikely. Formation of HgCl2 requires mercury oxidation, and HCl cannot oxidize mercury as it is fully reduced. Recent, unpublished results from this laboratory testing mercury oxidation catalysts suggest that while HgCl2 is the oxidized mercury compound formed in flue gas, it is not formed from the direct chlorination of Hg0. Schofield presented a mechanism for HgCl2 formation on metal surfaces that requires initial formation of HgO (11). As noted above, HgO is likely not present on the carbon surface, but a similar intermediate product could be required for the formation of HgCl2 across activated carbon. The suggestion to consider the capture of O2 and HCl by the carbon, and subsequent inhibition by S(VI), is intriguing. HCl is acidic, and the occupation of Lewis base binding sites by S(VI) may inhibit its capture along with the capture of mercury. However, we still assert that our conclusions are consistent with reality and are in agreement with the available data.

Literature Cited (1) Liu, Y. Comment on “Impact of sulfur oxides on mercury capture by activated carbon”. Environ. Sci. Technol. 2007, 42, 970–971. (2) Granite, E. J.; Pennline, H. W. Photochemical removal of mercury from flue gas. Ind. Eng. Chem. Res. 2002, 41, 5470. (3) 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. (4) Senior, C. L.; Sarofim, A. F.; Zeng, T.; Helble, J. J.; MamaniPaci, R. Gas-phase transformations of mercury in coal-fired power plants. Fuel Proc. Technol. 2000, 63, 197. (5) Gerasimov, G. Y. Investigation of the behavior of mercury compounds in coal combustion products. J. Eng. Phys. Thermophys. 2005, 78, 668. (6) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. S.; Laudal, D. L.; Benson, S. A. Status review of mercury control options for coal-fired power plants. Fuel Proc. Technol. 2003, 82, 89. (7) Huggins, F.; Yap, N.; Huffman, G.; Senior, C. XAFS characterization of mercury captured from combustion gases on sorbents at low temperatures. Fuel Proc. Technol. 2003, 82, 167. (8) Miller, S.; Dunham, G.; Olson, E.; Brown, T. Flue gas effects on a carbon-based mercury sorbent. Fuel Proc. Technol. 2000, 65–66, 343. (9) Sjostrom, S.; Wilson, C.; Bustard, J.; Spitznogle, G.; Toole, A.; Chang, R. Full-scale evaluation of carbon injection for mercury control at a unit firing high sulfur coal. In Proceedings of the U.S. Environmental Protection Agency 10.1021/es7023093

Not subject to U.S. Copyright. Publ. 2008 Am. Chem. Soc.

Published on Web 01/08/2008

Department of Energy - EPRI Combined Power Plant Air Pollutant Control Symposium: The MEGA Symposium; U.S. EPA - DOE - EPRI: 2006. (10) Sjostrom, S. Evaluation of Sorbent Injection for Mercury Control. Topical Report for Sunflower Electric’s Holcomb Station;Report to U.S. DOE/NETL, Cooperative Agreement No. DE-FC26–03NT41986, ADA-ES, 2005. (11) Schofield, K. Let them eat fish: Hold the mercury. Chem. Phys. Lett. 2004, 386, 65

. Evan Jacob Granite* and Albert A. Presto

U.S. Department of Energy National Energy Technology Laboratory P.O. Box 10940 M/S 58-106 Pittsburgh, Pennsylvania 15090 ES7023093

VOL. 42, NO. 3, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

973