Mercury oxidation over selective catalytic reduction ( SCR ) catalysts

The vanadium-based SCR catalyst used for NOx-control promotes the oxidation of elemental mercury Hg to Hg in flue gases from coal-fired power plants. Hg is water soluble and can effectively be captured in a wet scrubber. This means that the combination of an SCR with a wet FGD can offer an effective control option for mercury. Laboratory experiments have been carried out to elucidate and quantify the inhibition by the DeNOx reaction on the Hg oxidation by HCl over commercial SCR catalysts at different operating temperatures. In the presence of NO and NH3, the following three net reactions have been identified as relevant for the mercury chemistry over the SCR: R1: 2 HCl + Hg + 1/2 O2 ↔ HgCl2 + H2O R2: 2 NH3 + 3 HgCl2 ↔ N2 + 3 Hg + 6 HCl R3: 2 NO + 2 NH3 + 1⁄2 O2 ↔ 2 N2 + 3 H2O Reaction R1 is the oxidation of Hg by O2 and HCl, reaction R2 is the reduction of HgCl2 with NH3 and reaction R3 is the DeNOx reaction. The importance of the different reactions on the Hg oxidation depends on the SCR operating temperature. At T>325oC, reduction of HgCl2 with NH3 will take place. The observed Hg oxidation will reflect the relative rate of the Hg oxidation via reaction R1 and the HgCl2 reduction via reaction R2. For T=250-375oC, the DeNOx reaction will inhibit the kinetics of reaction R1 by consuming active vanadia-sites that must be oxidized to regain activity for Hg oxidation. Mercury oxidation over SCR catalysts 2 / 14 www.topsoe.com Information contained herein is confidential; it may not be used for any purpose other than for which it has been issued, and may not be used by or disclosed to third parties without written approval of Haldor Topsøe A/S. Model predictions suggest that the Hg oxidation over high dust SCR reactors is limited by external mass transport, when the HCl concentration in the flue gas exceeds 13 ppmv. This means that existing commercial SCR catalysts are sufficiently active for the Hg oxidation for many applications. For T>350oC and low HCl, the surface reactivity will be limiting for the overall Hg oxidation. A high Hg oxidation can be achieved just the same by installing an additional catalyst layer, so a catalyst volume with negligible concentrations of NO and NH3 is available for the catalytic oxidation. Introduction The worldwide anthropogenic emission of mercury was in 2005 estimated to be 1,930 tons per year. 45% of this comes from the combustion of fossil fuels. The emitted mercury will deposit on land or water, where it can transform into methyl mercury and thereby enter the food chain. Humans are most likely exposed to methyl mercury through the consumption of fish. The primary health effect of methyl mercury is an impaired neurological development for fetuses, infants and children. Mercury is present in coal in the order of 0.1 ppmw, which yields concentrations in combustion gases at power plants in the range 1-20 μg/Nm. The amount of mercury emitted to the atmosphere will depend on the fuel, the operating conditions and the air pollution control devices (APCDs) installed. Existing APCDs for control of other pollutants is found to remove some mercury. The approach of integrating mercury control with other regulatory actions, such as NOx, particulate and/or SO2-removal offers a reduced compliance cost compared to introduction of dedicated mercury control options. Three mercury species are normally considered in flue gases from coal-fired power plant: Elemental (Hg), oxidized (Hg), and particulate bound mercury (Hg). This mercury speciation will determine the capture in existing APCDs:  Hg is very volatile and difficult to capture  Hg is water soluble and can effectively be captured in a wet desulphurization device.  Hg can effectively be removed in a particulate control device. Mercury oxidation over SCR catalysts 3 / 14 www.topsoe.com Information contained herein is confidential; it may not be used for any purpose other than for which it has been issued, and may not be used by or disclosed to third parties without written approval of Haldor Topsøe A/S. Based on measurements at over 80 coal-fired power plants in the US, it was shown that mercury removals for plants burning bituminous coals generally were higher than those burning subbituminous and lignite coals. The lower removals were in turn an effect of a larger fraction of mercury being on the elemental form Hg. Mercury control by the combination SCR + wFGD A wet flue gas desulphurization (FGD) will capture oxidized mercury Hg in the flue gas with efficiency around 90%, whereas almost no Hg will be captured. An increased fraction of Hg will therefore enhance the mercury removals in a wet FGD. The selective catalytic reduction (SCR) catalyst used for NOx-control has been shown to promote the oxidation of Hg to Hg in flue gases from coal-fired power plants. For bituminous coals, the combination of an SCR with a wet FGD have been shown to give superior mercury removals compared to systems without the SCR. Table 1 shows the range of mercury capture for the two configurations. Control configuration Wet FGD SCR + wet FGD Mercury removals 43-87% 56-97% Table 1. Mercury removals according to references The combination with an SCR poses an option for mercury capture by a wet FGD for flue gases that would otherwise primarily contain elemental Hg. Mercury oxidation over SCR reactors The oxidation of Hg over full-scale SCR reactors has been reported in the range of 498% depending on coal rank/type, operating conditions and catalyst type/geometry. Hg is oxidized by halogens (via O2) in the flue gas. Chlorine is primarily responsible for the oxidations, since this halogen is typically present in the highest concentrations in coal. The net oxidation takes the form: Hg + 2 HCl + 1⁄2 O2 ↔ HgCl2 + H2O The concentration of chlorine in the coal appears to be the major determining factor for the observed Hg oxidation across different SCR applications, where an increasing oxidation is seen for increasing HCl. There is a tendency of lower oxidation achieved over Mercury oxidation over SCR catalysts 4 / 14 www.topsoe.com Information contained herein is confidential; it may not be used for any purpose other than for which it has been issued, and may not be used by or disclosed to third parties without written approval of Haldor Topsøe A/S. SCR reactors for subbituminous coal combustion compared to bituminous coals, which in part is due to a typically lower concentration of chlorine in lower rank coals. In the SCR reaction, NOx is reduced by reacting with NH3 over a vanadium catalyst according to the reaction stoichiometry: 4 NO + 4 NH3 + O2 ↔ 4 N2 + 6 H2O This will be referred to as the DeNOx-reaction. The oxidation of Hg is a beneficial side reaction over commercial SCR reactors that are optimized for NOx reduction. An interesting controversy is posed for the now two feasible reactions over the SCR, because it turns out the DeNOx reaction inhibits the Hg oxidation. The inhibition is typically explained by a competitive adsorption between NH3 and Hg, but (as will be shown here) the explanation is not as simple. The fundamental understanding of the catalytic Hg oxidation and the relevant chemistry over the SCR is in its early stage. Furthermore, the available experimental investigations on the effect of the two pivotal parameters, HCl and the DeNOx-reaction, only seem to cover a narrow range of conditions for each individual study. Combined this makes the means to optimize the Hg oxidation over SCR catalysts for different applications unclear. This study serves to elucidate and quantify the effect of the DeNOx reaction on the Hg oxidation by HCl over commercial SCR catalysts at different temperatures. Experimental Catalyst Commercial corrugated-type monoliths obtained from Haldor Topsøe A/S are applied in this study. The catalysts are based on V2O5 and WO3 dispersed on a fiber reinforced TiO2 carrier. Laboratory setup The mercury chemistry over SCR catalysts is studied in a laboratory setup at Haldor Topsøe A/S. Here a simulated flue gas containing Hg is passed through a SCR reactor and the change in mercury speciation is measured for different operating temperatures and gas compositions. A schematic drawing of the setup is given in Figure 1. The setup consists of a mixing module, where all gases are mixed and preheated, a SCR reactor, a reduction unit (to reduce all HgCl2 to Hg) and a mercury (Hg)-analyzer. Mercury oxidation over SCR catalysts 5 / 14 www.topsoe.com Information contained herein is confidential; it may not be used for any purpose other than for which it has been issued, and may not be used by or disclosed to third parties without written approval of Haldor Topsøe A/S. Figure 1: Schematic illustrating of laboratory setup. All tubing in contact with mercury consists of Pyrex glass, which is heated to 140oC to avoid precipitates and adsorption on the surfaces. Hg is introduced into the gas via VICI Metronics Dynacal® Permeation devices. H2O is added by passing part of the gas through a bubble-flask with H2O at 37oC. The remaining gas components HCl, NO, NH3, O2 and N2 are added from gas bottles. The simulated flue gas in this study contains the concentrations given in Table 2. Table 2. Component Concentration Hg 2.5-25 μg/Nm O2 4 %(vol) H2O 5 %(vol) HCl 2.5-55 ppm NH3 0-100 ppm NO 0-100 ppm Table 2 Composition of the simulated flue gas The reactor consists of Pyrex glass and contains a single monolithic SCR channel. Experiments across the channel are run isothermally with a temperature profile of +/2oC. Mercury is analyzed in the Lumex RA-915+ analyzer, which uses cold vapor atomic absorption spectrometry to measure gaseous elemental mercury Hg continuously. The reduction unit serves the purpose of reducing all HgCl2 in the gas stream to Hg, which enables a total mercury measurement (Hg) with the analyzer that only detects Hg. Hg/HgCl2 permeation tubes Bubble flask H2O (T=37oC)