Study of Mercury Oxidation by a Selective Catalytic Reduction Catalyst in a Pilot-Scale Slipstream Reactor at a Utility Boiler Burning Bituminous Coal

Abstract

One of the cost-effective mercury control technologies in coal-fired power plants is the enhanced oxidation of elemental mercury in selective catalytic reduction (SCR) followed by the capture of the oxidized mercury in the wet scrubber. To better understand Hg oxidation chemistry within a SCR, the Institute for Combustion Science and Environmental Technology at Western Kentucky University set up a pilot-scale SCR slipstream facility at a selected utility boiler burning bituminous coal. The greatest benefit of this scaled-down SCR slipstream test is the ability to investigate the effects of Hg oxidation in a SCR using actual flue gas with fly ash included and to isolate and control specific flue-gas compositions with spike gas additions. The average sulfur, chlorine, and mercury contents in the burned coal were 1.67% and 731 and 0.13 ppm, respectively. CaO and Fe 2 O 3 and loss on ignition of the fly ash, which are reported to possibly affect Hg speciation, are approximately 1.65, 14.6, and 2.6% on average, respectively. The maximum concentrations of spike gases were 500, 25, 2000, 50, and 15 ppm for HCl, Cl 2 , SO 2 , SO 3 , and HBr, respectively. Semicontinuous mercury emission monitors were used to monitor the variation of mercury speciation at the inlet and outlet of the SCR slipstream reactor, and the American Society for Testing and Materials certified Ontario hydro method was used for data comparison and validation. This paper is the first in a series of two in which the validation of the SCR slipstream test and Hg speciation variation in runs with or without SCR catalysts inside the SCR slipstream reactor under special gas additions (HCl, Cl 2 , SO 2 , and SO 3 ) are presented. Effects of HBr additions on mercury speciation within the SCR will be presented in the second part of the series. Tests indicate that the use of a catalyst in a SCR slipstream reactor can achieve greater than 90% NO reduction efficiency with a NH 3 /NO ratio of about 1. There is no evidence to show that the reactor material affects mercury speciation. Both SCR catalysts used in this study exhibited a catalytic effect on the elemental mercury oxidation but had no apparent adsorption effect. SCR catalyst 2 seemed more sensitive to the operational temperature. The spike gas tests indicated that HCl can promote Hg 0 oxidation but not Cl 2 . The effect of Cl 2 on mercury oxidation may be inhibited by higher concentrations of SO 2 , NO, or H 2 O in real flue-gas atmospheres within the typical SCR temperature range (300-350 °C). SO 2 seemed to inhibit mercury oxidation; however, SO 3 may have some effect on the promotion of mercury oxidation in runs with or without SCR catalysts.

Oxidation Catalysts for Elemental Mercury in Flue Gases—A Review

“…This behavior was attributed to the competitive adsorption of SO 2 , SO 3 and HCl on the SCR catalyst surface active sites [72]. Also, Cao et al [71] observed that Hg el oxidation decreased once the SO 2 concentration in the flue gas was increased. On the other hand, they observed that Hg el oxidation increased with increasing the SO 3 content to a maximum of 50 ppm [71].…”

“…Also, Cao et al [71] observed that Hg el oxidation decreased once the SO 2 concentration in the flue gas was increased. On the other hand, they observed that Hg el oxidation increased with increasing the SO 3 content to a maximum of 50 ppm [71]. The addition of H 2 SO 4 to a simulated flue gas increased the Hg oxidation activity of a commercial SCR catalyst, especially in the presence of HCl [76].…”

“…Besides NO x reduction, SCR catalysts are also active in the oxidation of volatile organic compounds [69,70]. In an additional side reaction, SCR catalysts also convert gaseous Hg el to Hg ox , particularly in the presence of HCl [21,23,25,71]. Currently, SCR catalysts are being developed as multi-pollutant control devices.…”

“…Modifications of SCR catalysts for achieving high Hg el oxidation performance are of interest as well. In order to determine their efficiency in oxidizing Hg el from flue gases, the SCR catalysts were tested at in the laboratory and in pilot [71][72][73][74] and full scale conditions(e.g., [75]). Some of these Hg el oxidation results are presented in Table 3.…”

“…Laboratory-scale studies proved that the activity of SCR catalysts for Hg el oxidation under simulated flue gas conditions is strongly linked to the halide species present and their concentrations [3,21,23,53,71,78,80]. Since HCl is the dominant halide species in flue gases, Hg el oxidation is considered to proceed according to the following overall equation:…”