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Title: Survey of catalysts for oxidation of mercury in flue gas

Abstract

Methods for removing mercury from flue gas have received increased attention because of recent limitations placed on mercury emissions from coal-fired utility boilers by the U. S. Environmental Protection Agency and various states. A promising method for mercury removal is catalytic oxidation of elemental mercury (Hg{sup 0}) to oxidized mercury (Hg{sup 2+}), followed by wet flue gas desulfurization (FGD). FGD cannot remove Hg{sup 0}, but easily removes Hg{sup 2+} because of its solubility in water. To date, research has focused on three broad catalyst areas: selective catalytic reduction catalysts, carbon-based materials, and metals and metal oxides. We review published results for each type of catalyst and also present a discussion on the possible reaction mechanisms in each case. One of the major sources of uncertainty in understanding catalytic mercury oxidation is a lack of knowledge of the reaction mechanisms and kinetics. Thus, we propose that future research in this area should focus on two major aspects: determining the reaction mechanism and kinetics and searching for more cost-effective catalyst and support materials. 91 refs.

Authors:
;  [1]
  1. United States Department of Energy, Pittsburgh, PA (United States). National Energy Technology Laboratory
Publication Date:
OSTI Identifier:
20813179
Resource Type:
Journal Article
Resource Relation:
Journal Name: Environmental Science and Technology; Journal Volume: 40; Journal Issue: 18; Other Information: Evan.Granite@netl.doe.gov
Country of Publication:
United States
Language:
English
Subject:
01 COAL, LIGNITE, AND PEAT; FLUE GAS; MERCURY; OXIDATION; CATALYSTS; COAL; FOSSIL-FUEL POWER PLANTS; SELECTIVE CATALYTIC REDUCTION; DESULFURIZATION; ACTIVATED CARBON; FLY ASH; METALS; OXIDES; REVIEWS

Citation Formats

Albert A. Presto, and Evan J. Granite. Survey of catalysts for oxidation of mercury in flue gas. United States: N. p., 2006. Web. doi:10.1021/es060504i.
Albert A. Presto, & Evan J. Granite. Survey of catalysts for oxidation of mercury in flue gas. United States. doi:10.1021/es060504i.
Albert A. Presto, and Evan J. Granite. 2006. "Survey of catalysts for oxidation of mercury in flue gas". United States. doi:10.1021/es060504i.
@article{osti_20813179,
title = {Survey of catalysts for oxidation of mercury in flue gas},
author = {Albert A. Presto and Evan J. Granite},
abstractNote = {Methods for removing mercury from flue gas have received increased attention because of recent limitations placed on mercury emissions from coal-fired utility boilers by the U. S. Environmental Protection Agency and various states. A promising method for mercury removal is catalytic oxidation of elemental mercury (Hg{sup 0}) to oxidized mercury (Hg{sup 2+}), followed by wet flue gas desulfurization (FGD). FGD cannot remove Hg{sup 0}, but easily removes Hg{sup 2+} because of its solubility in water. To date, research has focused on three broad catalyst areas: selective catalytic reduction catalysts, carbon-based materials, and metals and metal oxides. We review published results for each type of catalyst and also present a discussion on the possible reaction mechanisms in each case. One of the major sources of uncertainty in understanding catalytic mercury oxidation is a lack of knowledge of the reaction mechanisms and kinetics. Thus, we propose that future research in this area should focus on two major aspects: determining the reaction mechanism and kinetics and searching for more cost-effective catalyst and support materials. 91 refs.},
doi = {10.1021/es060504i},
journal = {Environmental Science and Technology},
number = 18,
volume = 40,
place = {United States},
year = 2006,
month = 9
}
  • The use of noble metals as catalysts for mercury oxidation in flue gas remains an area of active study. To date, field studies have focused on gold and palladium catalysts installed at pilot scale. In this article, we introduce bench-scale experimental results for gold, palladium and platinum catalysts tested in realistic simulated flue gas. Our initial results reveal some intriguing characteristics of catalytic mercury oxidation and provide insight for future research into this potentially important process.
  • The use of precious metals and platinum group metals as catalysts for oxidation of mercury in flue gas is an active area of study. To date, field studies have recently focused on gold and palladium catalysts installed at pilot-scale. In this work, we introduce bench-scale results for gold, platinum, and palladium catalysts tested in realistic simulated flue gas. Initial results reveal intriguing characteristics of catalytic mercury oxidation and provide insight for future research.
  • This paper introduces a predictive mechanism for elemental mercury (Hg{sup 0}) oxidation on selective catalytic reduction (SCR) catalysts in coal-fired utility gas cleaning systems, given the ammonia (NH{sub 3})/nitric oxide (NO) ratio and concentrations of Hg{sup 0} and HCl at the monolith inlet, the monolith pitch and channel shape, and the SCR temperature and space velocity. A simple premise connects the established mechanism for catalytic NO reduction to the Hg{sup 0} oxidation behavior on SCRs: that hydrochloric acid (HCl) competes for surface sites with NH{sub 3} and that Hg{sup 0} contacts these chlorinated sites either from the gas phase ormore » as a weakly adsorbed species. This mechanism explicitly accounts for the inhibition of Hg{sup 0} oxidation by NH{sub 3}, so that the monolith sustains two chemically distinct regions. In the inlet region, strong NH{sub 3} adsorption minimizes the coverage of chlorinated surface sites, so NO reduction inhibits Hg{sup 0} oxidation. But once NH{sub 3} has been consumed, the Hg{sup 0} oxidation rate rapidly accelerates, even while the HCl concentration in the gas phase is uniform. Factors that shorten the length of the NO reduction region factors that enhance surface chlorination, promote Hg{sup 0} oxidation. This mechanism accurately interprets the reported tendencies for greater extents of Hg{sup 0} oxidation on honeycomb monoliths with smaller channel pitches and hotter temperatures and the tendency for lower extents of Hg{sup 0} oxidation for hotter temperatures on plate monoliths. The mechanism reproduces the reported extents of Hg{sup 0} oxidation on a single catalyst for four coals that generated HCl concentrations from 8 to 241 ppm, which covers the entire range encountered in the U.S. utility industry. Similar performance is also demonstrated for full-scale SCRs with diverse coal types and operating conditions. 28 refs., 5 figs., 3 tabs.« less
  • The feasibility of oxidizing elemental mercury in coal combustion flue gas using catalytic material impregnated onto fabric filters was explored. TiO{sub 2}, Au/TiO{sub 2}, and Pd/Al{sub 2}O{sub 3} were studied based on promising results in previous research. Several fabric coating methods were investigated to determine the best way to load a filter. A spray coat method was found to have the highest initial loading and had the lowest losses after simulated pulse-jet cleaning. The oxidation performance of the catalyst-coated filters was tested using a simulated flue gas in a bench-scale reactor under conditions similar to those found in a baghouse.more » Au/TiO{sub 2} and Pd/Al{sub 2}O{sub 3} were effective, yielding mercury oxidation ranges of 40-60% and 50-80%, respectively. A 19 kW research combustor equipped with a baghouse was used to fire a range of coals and further test the performance of Pd/Al{sub 2}O{sub 3}. Results obtained warrant further development of this technique as a means of mercury pollution control. 19 refs., 2 figs., 5 tabs.« less
  • SiO{sub 2}/V{sub 2}O{sub 5}/TiO{sub 2} catalysts were synthesized for removing elemental mercury (Hg{sup 0}) from simulated coal-combustion flue gas. Experiments were carried out in fixed-bed reactors using both pellet and powder catalysts. In contrast to the SiO{sub 2}-TiO{sub 2} composites developed in previous studies, the V{sub 2}O{sub 5} based catalysts do not need ultraviolet light activation and have higher Hg{sup 0} oxidation efficiencies. For Hg{sup 0} removal by SiO{sub 2}-V{sub 2}O{sub 5} catalysts, the optimal V{sub 2}O{sub 5} loading was found between 5 and 8%, which may correspond to a maximum coverage of polymeric vanadates on the catalyst surface. Hg{supmore » 0} oxidation follows an Eley-Rideal mechanism where HCl, NO, and NO{sub 2} are first adsorbed on the V{sub 2}O{sub 5} active sites and then react with gas-phase Hg{sup 0}. HCl, NO, and NO{sub 2} promote Hg oxidation, while SO{sub 2} has an insignificant effect and water vapor inhibits Hg{sup 0} oxidation. The SiO{sub 2}-TiO{sub 2}-V{sub 2}O{sub 5} catalysts exhibit greater Hg{sup 0} oxidation efficiencies than SiO{sub 2}-V{sub 2}O{sub 5}, may be because the V-O-Ti bonds are more active than the V-O-Si bonds. This superior oxidation capability is advantageous to power plants equipped with wet-scrubbers where oxidized Hg can be easily captured. The findings in this work revealed the importance of optimizing the composition and microstructures of SCR (selective catalytic reduction) catalysts for Hg{sup 0} oxidation in coal-combustion flue gas. 33 refs., 5 figs., 2 tabs.« less