Mercury Oxidation over a Vanadia-based Selective Catalytic

(2, 3) Cost-effective methods for removing mercury from coal-fired flue gas have received increased attention because of recent limitations placed on ...
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Energy & Fuels 2009, 23, 253–259

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Mercury Oxidation over a Vanadia-based Selective Catalytic Reduction Catalyst Sheng He, Jinsong Zhou,* Yanqun Zhu, Zhongyang Luo, Mingjiang Ni, and Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang UniVersity, Hangzhou 310027, China ReceiVed August 31, 2008. ReVised Manuscript ReceiVed NoVember 16, 2008

The process of the reaction among elemental mercury (Hg0) and reactive flue gas components across the selective catalytic reduction (SCR) catalyst was studied in a laboratory-scale reactor. Prepared vanadia-based SCR catalysts were characterized and analyzed to understand the potential reaction pathways. Mercury oxidation was observed when pro-exposure of the SCR catalyst to HCl, followed by passing through Hg0/N2 in the absence of gas-phase HCl. At testing conditions, Hg0 was found to desorb from the catalyst surface by adding HCl to the gas steam, which implies that HCl adsorption onto the SCR catalyst is strong relative to the mercury. Surface analysis verified the absorption of HCl onto the SCR catalysts, and the potential reaction pathways were proposed. Indeed, the monomeric vanadyl sites on the catalyst surface were found to be responsible for the adsorption of both Hg0 and HCl, which means they are active for mercury oxidation. Furthermore, the detailed Langmuir-Hinshelwood mechanism was proposed to explain the mercury oxidation on the SCR catalyst, where reactive Cl generated from adsorbed HCl reacts with adjacent Hg0.

Introduction Mercury is a leading concern among the toxic metals in air addressed in the 1990s Clean Air Act Amendments (CAAA) because of its volatility, persistence, bioaccumulation in the environment, and its neurological health impacts.1 Coal-fired power plants are major point sources for mercury discharges into the atmosphere.2,3 Cost-effective methods for removing mercury from coal-fired 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 (EPA).4 Mercury emission reduction by enhancing its removal in the already available air pollution control devices (APCD) has the potential of providing a reliable and cost-effective mercury control approach. The predominant forms of mercury in coal-fired flue gas are elemental (Hg0) and oxidized (Hg2+). The speciation of mercury in the flue gas affects the amount of mercury retained in the APCD because the chemistry of Hg0 in flue gas is different from that of Hg2+. Hg0 is difficult to capture with typical APCD because it is highly volatile and nearly insoluble in water. However, Hg2+ can be removed in the wet flue gas desulfurization (WFGD) facilities of coal combustion processes due to its high solubility in aqueous solutions. So a promising method for mercury removal from coal-fired flue gas is catalytic * Corresponding author. Phone: +86 571 87952041; fax: +86 571 87951616; e-mail address: [email protected]. (1) U. S. Environmental Protection Agency. The Clean Air Act Amendments of 1990, Section 114; Government Printing Office: Washington, DC, 1990. (2) Keating, M. H.; Mahaffey, K. R.; Schoeny, R.; Rice, G. E.; Bullock,O. R.; Ambrose, R. B.; Swartout, J.; Nichols, J. W. Mercury Study Report to Congress, EPA-452/R-97-003; U. S. Environmental Protection Agency: December 1997. (3) U. S. Environmental Protection Agency. A Study of Hazardous Air Pollutant Emissions from Electric Utility Steam Generating Units: Final Report to Congress. U. S. EPA Office of Air Quality Planning and Standards, U. S. Government Printing Office: Washington, DC, 1998. (4) U. S. EPA Clean Air Mercury Rule; U. S. Environmental Protection Agency: Washington, DC, 2005. Available at http://www.epa.gov.

oxidation of Hg0 to Hg2+, subsequently captured by WFGD. For this reason, factors that promote mercury oxidation upstream of the WFGD are essential components of this approach. Selective catalytic reduction (SCR) catalysts may be used for the catalytic oxidation step, which has been wildely employed for controlling nitrous oxides (NOx) emissions as well as oxidizing Hg0 in the flue gas of coal-fired power plants.5 Therefore, mercury oxidation across existing SCR system followed by WFGD removal has been regarded as an economic means for “co-benefit” mercury reduction resulting from NOx and SO2 controls imposed by the CAIR. However, to use these devices in a mercury control strategy, a utility must be able to obtain predictable and reliable mercury oxidation. SCR systems have been observed to convert Hg0 to Hg2+ in coal-fired power plants.6,7 However, the extent of mercury oxidation across SCR heavily depends on coal type. The range of mercury oxidation for plants burning bituminous coal varies widely from 30 to 98%, whereas much lower mercury oxidation of 0-26% across SCR exists in plants burning sub-bituminous coal.8,9 The effect of SCR for oxidizing mercury has also been tested at the laboratory and pilot scales for a variety of different HCl concentrations, NO concentrations, NH3/NO ratio, and (5) EPRI Technical Report No. 1005400; Electric Power Research Institute: Palo Alto, CA, 2002. (6) Laudal, D. L. Effect of selective catalytic reduction on mercury. 2002 Field Studies Update; Product ID 1005558; Electric Power Research Institute: Palo Alto, CA, 2002. (7) La Marca, C.; Cioni, M.; Pintus, N.; Rossi, N.; Malloggi, S.; Barbieri, A. Macro and Micro-Pollutant Emission Reduction in Coal-Fired Power Plant. Presented at the Seventh International Conference on Energy for a Clean Environment (Clean Air 2003), Lisbon, Portugal, July 7-10, 2003. (8) Machalek, T.; Ramavajjala, M.; Richardson M.; Richardson. C.; Dene, C.; Goeckner, B.; Anderson, H.; Morris, E. Pilot eValuation of flue gas mercury reactions across an SCR unit, Proceedings of the Combined Power Plant Air Pollutant Control Symposium s The Mega Symposium, Washington DC, May 19-22, 2003. (9) Laudal, D. L.; Pavlish, J. H. Galbreath, J. S. Thompson, G. F. PilotScale EValuation of the Impact of SelectiVe Catalytic Reduction for NOx on Mercury Speciation; Final Report 2001-EERC-12-03, Energy & Environmental Research Center: Grand Forks, ND, Dec 1, 2001.

10.1021/ef800730f CCC: $40.75  2009 American Chemical Society Published on Web 12/23/2008

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reaction temperatures.10-12 The SCR catalyst, alone, cannot ensure optimized mercury oxidation. It is evident that HCl in the flue gas is critical for mercury oxidation, which increases with increasing HCl content in flue gas. Meanwhile, NH3 injection for NOx reduction inhibited mercury oxidization within SCR. Increasing the gas space velocity results in decreasing the extent of mercury oxidation. The exact mechanisms for mercury oxidation by SCR catalysts and their dependence on flue gas properties are currently unknown, but several possibilities have been proposed. Niksa and Fujiwara13 believed that the oxidation of Hg0 across an SCR catalyst occurs via absorbed HCl on the V2O5 active sites followed by reaction of the absorbed HCl with either gas-phase Hg0 or weakly bound Hg0. This reaction mechanism does not account for the Hg0 absorption to the SCR catalysts and cannot explain the observation that NH3 injection caused Hg0 to desorb from the catalysts surface. Senior14 proposed a model that assumes an Eley-Rideal reaction between adsorbed Hg0 and gas-phase HCl. The Deacon process has been suggested as a means to produce chlorine (Cl2) in the gas via a reaction between V2O5 and HCl.15 The gaseous Cl2 could then react with Hg0 in the gas phase to form Hg2+, but direct evidence for the Deacon reaction in SCR catalysts has not been given. An understanding of the chemical reactions inside the SCR reactor may help to alter the operation of an SCR system, to maximize mercury oxidation and capture. At present, it is impossible, given the sufficient and available experimental evidence, to determine the mercury oxidation mechanism across SCR catalyst. High uncertainty of the mechanism and kinetics for mercury oxidation results in limitations in predicting the extent of oxidation achieved over different catalysts and the development of novel cost-effective catalysts. Further detailed laboratory research focusing on the fundamental steps is required to improve the understanding of this reaction. The purpose of this work was to obtain an understanding of the fundamental mechanisms governing mercury oxidation across the SCR systems. The mercury adsorption and oxidation across the SCR catalyst was studied in detail in a bench-scale SCR system. The role of specific oxidant HCl was examined, and surface characterization of the catalyst was carried out to understand the detailed mercury reaction pathways. Experimental Section Catalyst Preparation. The SCR catalyst used in this study is typically vanadium pentoxide supported on TiO2, which is applied widely in commercial situations. Isovolume impregnation method was used to prepare vanadia-based SCR catalysts. Pure TiO2 power (99.9% anatase, Millennium Inorganic Chemicals Inc.) pretreated at 500 °C for 12 h was impregnated with ammonium metavanatate (NH4VO3) dissolved in oxalic acid ((COOH)2 · 2H2O). The amount of V2O5 supported on TiO2 was determined by the concentration (10) Bock, J.; Hocquel, M.; Unterberger, S.; Hein, K. Mercury oxidation across SCR catalysts of flue gas with Varying HCl concentrations. In Proceedings of the DOE-EPRI-USEPA-AWMA Combined Power Plant Air Pollutant Control Symposium-The MEGA Symposium; 2003. (11) Lee, C. W.; Srivastava, R.; Ghorishi, S.; Hastings, T.; Stevens, F. I J. Air Waste Manage. Assoc. 2004, 54, 1560–1566. (12) Blythe, G. Pilot Testing of Mercury Oxidation Catalysts for Upstream of Wet FGD Systems, Quarterly Technical Progress Report to U. S. DOE/NETL; U. S. Department of Energy Agreement NO. DE-FC2601NT41185; URS Corporation: 2003. (13) Niksa, S.; Fujiwara, N. J. Air Waste Manage. Assoc. 2005, 55, 1866–1875. (14) Senior, C. J. Air Waste Manage. Assoc. 2006, 56, 23–31. (15) Gutberlet, H.; Schlu¨ten, A.; Lienta A. SCR impacts on mercury emissions on coal-fired boilers. Presented at EPRI SCR Workshop, Memphis, TN, April 19-21, 2000.

He et al.

Figure 1. Schematic diagram of the SCR reactor system.

of NH4VO3 in the solution. After standing for 24 h at room temperature, water was removed slowly in a rotary evaporator and solid was thus obtained. The product then was dried at 120 °C overnight and calcined at 600 °C for 6 h in air. The SCR catalyst with the V2O5 content of 1 wt % (by weight) was used in this study and is denoted as 1 wt % V2O5/TiO2. Catalyst Characterization. The SCR catalysts were characterized and analyzed by various techniques. The examination of crystallinity and its dispersivity in the fresh SCR catalysts was obtained by X-ray diffraction (XRD, Thermo ARL X’TRA). XRD patterns were obtained in the 2θ range from 15 to 80 with 2° min-1 scanning rate and 0.04° data interval, using nickel-filtered Cu KR radiation (λ ) 1.54 Å) and typical operating condition of at 45 Kv and 100 mA. The multipoint Brunauer-Emmett-Teller (BET) surface areas and pore analysis were calculated from the nitrogen (N2) adsorption isotherms at 77 K with relative pressures (P/P0) up to 0.99, using a Quantachrome Autosorb-1 automated analyzer. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250) was used to examine the valence states of elements on the catalysts surface, coupled with a spherical capacitor analyzer and Al KR (hν )1486.6 eV) as the radiation source. To compensate for a systematic error in XPS measurement, the energy positions were first adjusted by comparing the surface energy of absorbed C on the surface of the specimen with that of the standard binding energy (284.8 eV, C 1s). The chemical functional groups on the SCR catalyst surface analysis were conducted using Fourier transform infrared spectra (FT-IR, Thermo Nicolet 830). The sample powders were mixed with potassium bromide (KBr), ground, and pressed into self-supporting disks. The weight ratio of the sample to KBr is 1:100. The skeletal spectra in the region 4000-400 cm-1 were recorded with a resolution of 4 cm-1. Apparatus. A schematic drawing of the SCR reactor system used to study the oxidation of Hg0 is shown in Figure 1. An Hg0 permeation tube (VICI Metronic, Inc. USA) was used to generate a constant quantity of Hg0 vapor carried by N2, which was introduced to the inlet of the gas mixer. Other simulated flue gas components including HCl, NOx, O2, and N2 were supplied by gas cylinders and were introduced into the top of the gas mixer at constant flows. Ammonia (NH3) gas was injected into the flue gas separately. All the gas components were mixed and preheated to the desired temperature and then passed through the SCR reactor. The quartz plug-flow type reactor (10 mm in diameter) was heated by temperature-controlled electric furnace to maintain SCR reaction temperature. The SCR catalyst particle (size: 250-350 µm) was used in this study. The W/F ratio (weight of catalyst to flow rate) was 60 mg s/mL. Mass flow controllers (MFC) were used for controlling all the gases that flowed into the reactor system. Teflon lines were chosen for gas sampling in order to avoid corrosion and

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Figure 2. X-ray diffraction pattern of the SCR catalyst and the support.

to ensure accurate analysis. All lines that mercury passed through were heated up to 190 °C to prevent mercury deposition on the inner surface, especially for oxidized mercury. A mercury continuous emission monitor (DM-6A/MS-1A, Nippon Instruments Inc., Japan) was used to measure Hg0 and Hg2+ concentrations in the experiments. Effluent (500 mL/min) containing both Hg0 and Hg2+ from the catalyst bed was extracted and washed with aqueous KCl to separate Hg2+ from Hg0. The aqueous KCl containing only Hg2+ was then passed through a spiral-type glass tube with flowing air (500 mL/min) and a sulfuric-acid solution of SnCl2 to reduce the Hg2+ to Hg0. Both effluents, which contain either Hg0 or Hg2+, were then passed through another scrubber of aqueous KOH to remove SO2. Finally, they were introduced into measurement cells, and the concentration of mercury in each effluent was individually determined by cold vapor atomic absorption spectrometry analyzers with a low detection limit of 0.1 µg/m3. An online NOx analyzer of NGA 2000 MLT (Rosemount Analytical Inc., U.S.) also was used to check the NOx removal performance by the SCR. Calculation. At the beginning of the tests for mercury oxidation activity, the mercury monitor measured the concentration of Hg0 at the mixer inlet. Until having established the stable and consistent mercury feeding, the Hg0-contained flue gas was switched to the SCR reactor, and the mercury monitor measured mercury concentration in outlet flue gas. At the end of each test, the mercury monitor was switched to SCR inlet to verify the mercury feed rate. The level of mercury oxidation was estimated according to eq 1. 2+ 0 Mercury oxidation ) [Hg2+ out - Hgin ] ⁄ Hgin × 100%

Hg2+ in

Hg2+ out

(1)

Hg2+

where and are concentrations in the inlet and outlet 0 is the Hg0 concentrations in the inlet streams respectively, Hgin stream.

Results and Discussion Catalyst Property. As shown in Figure 2, the SCR catalyst used in this study has the crystallinity of TiO2 in its anatase form. It is ascertained that supporting vanadium oxides on TiO2 anatase leads to very active oxidation catalysts, more active than those obtained with other supports. Since no visible crystalline can be observed, it can be concluded that V2O5 is evenly dispersed in TiO2. The BET surface area of the prepared SCR catalyst was 70.46 m2/g, the total pore volume was 0.3139 cm3/ g, and its average pore diameter was 178.2 Å. The VOx surface density can be calculated as approximately 0.9 atoms/nm2, which is much lower than the monolayer density of 7.9 atoms/nm2.16,17 (16) Sanati, M.; Andersson, A. J. Mol. Catal. 1990, 59, 223–255. (17) Wachs, I. E. Catal. Today 1996, 27, 437–455.

Figure 3. Results of the SCR NOx removal performance at 300 °C.

The prepared SCR catalyst was also tested for NOx reduction activity. Figure 3 showed the NOx concentration profile at the outlet of the SCR reactor for activity test. Average NOx reduction of 92% was obtained during the tests with NH3/NOx ) 1. The prepared SCR catalyst exhibited strong NOx reduction catalytic activity at 300 °C. Heterogeneous Mercury Oxidation on a SCR Catalyst. The HCl in flue gas is primary for the mercury oxidation on the SCR catalysts. Lee et al.18 observed adsorption of Hg0, with no oxidation, for a simulated flue gas that contained no HCl (350 °C). Furthermore, the Deacon process for generating Cl2 from HCl is known to be catalyzed by appropriate metal oxide at high temperature.19 Catalysts

4HCl(g) + O2(g) 98 2Cl2(g) + 2H2O(g)

(2)

Gutberlet et al.15 observed the production of Cl2 across SCR catalysts, then the gaseous Cl2 could react with Hg0 in the gas phase. Therefore, it seems likely that the observed increased mercury oxidation by SCR catalysts in the presence of HCl might at least partly be caused by the intermediate products Cl2 from the Deacon process. The first group of data was obtained to examine the roles of a SCR catalysts and HCl in mercury oxidation. Results are illustrated in Figure 4. The first column was achieved in the absence of SCR catalyst, serving as the baseline. In this testing case, 50 ppmv HCl, 5% O2, and Hg0 simultaneously flowed through the SCR reactor. The result showed that only 1.5% of the mercury oxidation efficiency was reached, when the mercury effluent reaching a stable state. The gaseous reactions Hg + HCl and Hg + O2 are hindered by a very high energy barrier and cannot be considered as an important path under test conditions.20,21 The second column, also serving as the baseline, was obtained by Hg0 (balanced with N2, 5% O2) flowed over the SCR catalyst, with no HCl addition to the gas steam. There was almost no mercury oxidation, as expected. For the third (18) Lee. C. W.; Srivastava, R. K.; Ghorishi, S. B.; Hastings, T. W.; Stevens, F. M. Study of mercury under simulated SCR NOx Emission control conditions. Presented at the Department of Energy-Electric Power Research Institute, U.S. Environmental Protection AgencysAir & Waste Association Combined Power Plant Air Pollutant Control SymposiumsThe Mega Symposium, Washington, DC, May 19-22, 2003. (19) Pan, H.; Minet, R.; Benson, S.; Tsotsis, T. Ind. Eng. Chem. Res. 1994, 33, 2996. (20) Hranisavljevic, J.; Fontijn, A. J. Phys, Chem. 1997, 101, 2323– 2326. (21) Galbreath, K. C.; Zygarlicke, C. J. EnViron. Sci. Technol. 1996, 12, 818–822.

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Figure 4. Application of SCR catalysts and HCl at 300 °C.

column, the gas steam containing Hg0, with 50 ppmv HCl and 5% O2, flowed through the SCR catalyst until both Hg2+ and Hg0 concentration achieved a stable state. The result showed that 64% of the influent mercury was oxidized. As can be seen, mercury oxidation can be realized as a heterogeneous reaction in the presence of both SCR catalyst and HCl under testing conditions. The fourth column was the result of mercury oxidation; while HCl + O2 continuously flowing through the SCR catalyst, Hg0 balanced with N2 was brought separately into the gas stream downstream and near the SCR catalyst. There was no appreciable change on the effluent concentration of Hg0 during the testing time (overnight). It was negligible that the Deacon reaction converts the HCl in flue gas to Cl2 and thereby promotes the mercury oxidation. Senior et al.22 found that the equilibrium concentration of Cl2 is as small as about 1% of the HCl concentration. Zhao et al.23 observed no appreciable mercury oxidization with 3 ppmv at 473 K. A modeling proposed by Niska and Fujiware13 also indicated that gas-phase reactions alone are not enough to account for observed extents of mercury oxidation due to the slow reaction rate between Cl2 and Hg0. Therefore, another mechanism is likely responsible for heterogeneous mercury oxidation. To understand the mechanism of the mercury oxidation on the SCR catalyst, the process of mercury oxidation among gaseous Hg0, 50 ppmv HCl, and 5% O2 on SCR catalyst was recorded and analyzed. Throughout the test, N2 was used as the balanced gas, and the total gas flow rate was maintained at a constant of 1 L/min. Fresh V2O5/TiO2 catalyst was applied in the test. In Figure 5, from point “a” to point “b”, the gas steam was bypassed and the inlet gas was sampled to ensure a stable influent mercury concentration. At point b, the gas steam was switched to the SCR reactor, and the sample was taken from the exit of the reactor. The concentration of mercury showed continuous disturbance due to the resistance of the SCR catalyst but no reduction of effluent concentration of mercury in the initial 2 h. However, the Hg0 concentration decreased rapidly after 2 h, and the Hg2+ was simultaneously detected by the mercury monitor. It seemed that HCl strongly adsorbed onto the SCR catalyst surface in the initial 2 h. Once the HCl adsorption reaches saturation, gaseous Hg0 adsorbs onto the (22) Senior, C.; Bool, L., III.; Huffman, G.; Huggins, F.; Shah, N.; Sarofim, A.; Olmez, I.; Zeng, T. A fundamental study of mercury partitioning in coal fired power plant flue gas, Paper 97-WP72B.08. In proceedings of the Air and Waste Management Association’s 90th Annual Meeting and Exhibition; Air and Waste Management Association: Pittsburgh, PA, 1997. (23) Zhao, Y.; Mann, M. D.; Pavlish, J. P.; Mibeck, B. A. F.; Dunham, G. E.; Olson, E. W. EnViron. Sci. Technol. 2006, 40, 1603–1608.

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catalyst surface and subsequently reacts to the adsorbed HCl to form Hg2+. In turn, the release of Hg2+ from the catalyst surface results in further adsorption of HCl and Hg0. The removal of HCl from gas stream at point “c” resulted in a rapid increase of Hg0, which indicates the termination of mercury oxidation. Finally, the effluent Hg0 concentration was almost the same as the inlet concentration of mercury. The observation of almost no Hg0 oxidation for this test may be due to the lack of a reactive Cl source in this test. This indicated that the presence of HCl in flue gas is indispensable to the mercury absorption and oxidation. Adsorption of HCl and Hg0. Although Niska et al.13 asserted that HCl adsorbed onto the SCR catalyst surface, no direct experimental evidence was given. The adsorption of HCl on the SCR catalyst surface was confirmed in this study. Preexposure of fresh SCR catalyst to HCl (balanced with N2) for 2 h according to the former test’s result ensures a saturated adsorption of HCl. Then, a gas steam containing only with Hg0 flowed through the pretreated SCR catalyst. The result of mercury removal and oxidation behavior is shown in Figure 6. As shown in Figure 6, when a gas steam Hg0/N2 without any other components, was passed through the fresh SCR catalyst, there was no evident adsorption and oxidation of Hg0. Although many researchers observed that Hg0 absorbs on the various sorbents, including the SCR catalysts,24-26 it is dubious that Hg0 is chemically or physically adsorbed to the sorbent and catalyst surface. The poor adsorption performance of Hg0 in pure N2 environment indicates that the physically adsorption is weak at testing temperature. The reported mercury absorption may be considered as chemical adsorption in the presence of other flue gas components (HCl, SO2, and NOx,). Further investigation into the nature of the behavior of mercury adsorption onto the SCR catalyst should be carried out. However, the performance of mercury adsorption across the HCl pretreated catalyst was distinctive to the fresh catalyst. Hg0 was vastly captured by the HCl pretreated catalyst. Furthermore, the mercury oxidation was observed simultaneous in the absence of HCl in gas steam. It can be confirmed that HCl assuredly adsorbed onto the surface of the SCR catalyst. During the experiment, the reduction of total mercury (Hg0 and Hg2+) concentration indicated that SCR catalyst retained some Hg0. This affirmatively suggested that the HCl adsorbed onto the SCR catalyst and reacted to adsorbed mercury to form Hg2+. The addition of HCl into the flue gas caused a rapid desorption of mercury from the SCR catalyst, as demonstrated in Figure 7. Fresh SCR catalyst after saturation with Hg0 under a flow of Hg0 balanced with pure N2 at 300 °C was used in this test. The continued release of Hg0 was observed when 50 ppmv HCl was added to the gas steam and Hg0 was removed at the same time. The adsorption of Hg0 onto the SCR catalyst in a pure N2 atmosphere seems to be weak in nature, because Hg0 is readily desorbed from the catalyst surface when HCl was added. This strongly indicates that the both Hg0 and HCl adsorb onto the catalyst surface and compete for the active site. However, HCl is easier and stronger to adsorb onto the active sites. It can be speculated that the presence of HCl in N2 environment inhibits mercury adsorption onto the catalyst. The (24) Hocquel, M. The behaVior and fate of mercury in coal-fired power plants with downstream air pollution control deciVes; VDI Verlag: Du¨sseldorf, Germany, 2004. (25) Eswaran, S.; Stenger, H. Energy Fuels 2005, 19, 2328–2334. (26) Senior, C. Oxidation of mercury across SCR catalysts in coal-fired power plants burning low rank fuel, Final Report to DOE/NETL, U. S. Department of Energy Agreement No. DEFC26-0.NT41728; Reaction Engineering International: 2004.

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Figure 5. Process of mercury oxidation between gaseous Hg0, HCl, and O2 on SCR at 300 °C.

in the adsorption of mercury. Eom et al.28 detected the Ooxygen state that corresponds to the weekly adsorbed mercury to vanadia sites. The equation should be:

Figure 6. Mercury absorption and oxidation across the SCR catalyst, Hg0 balanced with N2 at 300 °C.

Figure 7. Release of mercury from SCR catalyst surface by adding HCl at 300 °C.

experiments carried by Straube et al.27 showed that in the absence of V2O5 almost no mercury adsorption was observed. V2O5 contents from 2.5 to 4.5 wt % caused a further increase (27) Straube, S.; Hahn, T.; Koeser, H. Appl. Catal., B 2008, 79, 286– 295.

Hg(g) + OdV5+fHg · · · OsV4+ (3) The competition of HCl and Hg0 on the catalyst surface suggests that the same vanadia sites are both responsible for the adsorption of HCl and Hg0. Surface Analysis and Reaction Mechanism. To understand the role of vanadia sites in HCl adsorption, the SCR catalysts were analyzed by XPS and FTIR. Before the measurement, the fresh SCR catalysts were subjected to 50 ppmv HCl for two different time periods at 300 °C. After the exposure, the catalyst samples were purged at the same temperature in Ar for 0.5 h to remove the physically adsorbed HCl, and then they were cooled to room temperature. In XPS analysis, the V 2p and O 1s peaks were used in analyzing the chemical state, and the results are shown in Figure 8. The binding energy of V 2p measured for fresh SCR catalyst (516.0 eV) is slightly lower than the value reported in the literature (516.8-517.7 eV for bulk V of V5+).29,30 This result is consistent with the reports that the binding energy of V 2p is lower for the highly dispersed vanadia, which corresponds to the coverage of the support below monolayer than for bulk V (V5+).31,32 By the peak deconvolution, two peaks corresponding to 517.1 and 515.9 eV, which can be assigned to oxidation state of V5+ and V4+ respectively. Although, XPS did not detect Cl existing on the catalyst surface directly due to the detection limit, the chemical environments of V and O were modified by the adsorbed HCl. The increased peaks of V5+ with HCl pretreated catalysts were observed because of forming Cl-V5+. The shift of the valence from V4+ to V5+ was considered to occur due to the strong electronegativity of Cl. The reaction should be: V4+sΟΗ + Cl-fClsV5+sΟΗ

(4)

(28) Eom, Y.; Jeon, S. H.; Ngo, T. A.; Kim, J.; Lee, T. G. Catal. Lett. 2008, 121, 219–225. (29) Nickl, J.; Schild, Ch.; Baiker, A.; Hund, M.; Wokaun, A. J. Anal. Chem. 1993, 346, 79–83. (30) Nogier, J. Ph.; Delamar, M. Catal. Today 1994, 20, 109–123. (31) Reiche, M. A.; Bu¨rgi, T.; Baiker, A.; Scholz, A.; Schnyder, B.; Wokaun, A. Appl. Catal., A 2000, 198, 155–169. (32) Madia, G.; Elswner, M.; Koebel, M.; Raimondi, F.; Wokaun, T. Appl. Catal., B 2002, 39, 181–190.

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Figure 8. XPS spectra of the V 2p and O 1s peaks of (1) fresh SCR catalyst, (2) SCR catalyst pretreated with HCl for 2 h at 300 °C, and (3) SCR catalyst pretreated with HCl for 10 h at 300 °C.

According to the bonding energy scale of O 1s from the study of Danielle et al.,33 the component c of the O 1s peak with the binding energy of 529.9 eV represents the O 1s level of oxygen atoms O2- in the lattice. Component d of the O 1s peak represents the hydroxyl groups (-OH), and component e represents the O 1s of water molecules. The increasing O 1s peak of -OH indicated new -OH formed due to the adsorption of HCl. In fact, the vanadium oxides supported on TiO2 reacts with HCl according to following reactions:8,13 VO2 + 2HClfV(OH)2Cl2

(5)

V(OH)2Cl2fVOCl2 + H2O

(6)

V2O5 + 2HClfV2O3(OH)2Cl2

(7)

V2O3(OH)2Cl2fVO2Cl2 + H2O

(8)

V2O5 + 2HClf2V(OH)2Cl

(9)

To understand the actual form of adsorbed HCl on the catalyst, the catalyst samples were also analyzed by FT-IR, and the results are shown in Figure 9. With regard to the structure of VOx on the surface of the TiO2 support, it has previously been reported that isolated monovanadate species and polymeric polyvanadate species exist under the monolayer capacity.17,34 Furthermore, isolated monovadate is the predominant species below 3 wt % VOx loading.35,36 Tarama et al.37 showed that V2O5/γ-Al2O3 and V2O5/SiO2 exhibit a noticeable absorption at 1023 cm-1. Frederickson et al.38 showed stretching vibration of VdO occurred at 1020 cm-1 for bulk V2O5. Miller et al.39 (33) Dupin, D.; Gonbeau, P.; Vinatier, P.; Levasseur, A. Phys. Chem. Chem. Phys. 2000, 2, 1919–1924. (34) Cristiani, C.; Forzatti, P.; Busca, G. J. Catal. 1989, 116, 586–589. (35) Went, G. T.; Oyama, S. T.; Bell, A. T. J. Phys. Chem. 1990, 94, 4240–4246. (36) Machej, T.; Haber, J.; Turek, A. M.; Wachs, I. E. Appl. Catal. 1991, 70, 115–128. (37) Tarama, K.; Yoshida, S.; Ishida, S.; Kakioka, H. Bull. Chem. Soc. Jpn. 1968, 41, 2840–2845. (38) Frederickson Jr, L. D.; Hausen, D. M. Anal. Chem. 1963, 35, 818– 827. (39) Miller, F. A.; Cousins, L. R. J. Chem. Phys. 1957, 26, 329–331.

Figure 9. FT-IR spectra obtained of (1) fresh SCR catalyst, (2) SCR catalyst pretreated with HCl for 2 h at 300 °C, and (3) SCR catalyst pretreated with HCl for 10 h at 300 °C.

also observed the presence of a strong absorption band located at 1035 cm-1 and identified it as arising from the VdO stretching motion. For the fresh catalyst in this paper, the absorption band at 1052 cm-1 may be attributed to the VdO stretching vibration. However, the band intensity of at 1052 cm-1 decreased with increasing HCl pretreated time and almost vanished completely when catalyst was pretreated with HCl for 10 h. This means the adsorbed HCl can also be located near the VdO and can interacted with the vanadyl groups via the valence bond. The adsorbed HCl onto the catalyst surface may react according to the following reaction: V5+dO + HClfClsV5+sOH (10) The SCR catalysts have significant surface areas as well as active sites, which could retain the reactive Cl generated from adsorbed HCl and serve as a reactive Cl source for reactions with Hg0 to form Hg2+. Once adsorbed HCl layer forms on the SCR catalyst surface, mercury oxidation between reactive Cl

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enon that injection of NH3 decreases the extent of mercury oxidation and releases the retained mercury from the catalyst due to the competition for the active sites.13,24,44 Conclusions

Figure 10. Mechanism of the mercury oxidation on the vanadia-based SCR catalysts.

and adjacent adsorbed Hg0 occurs with covalent bond via the Langmuir-Hinshelwood mechanism. On the basis of the experimental data and surface analysis in this study, one model of the mercury oxidation process on the vanadia-based SCR catalyst is proposed and illustrated in Figure 10. According to the mechanism, first, HCl and Hg0 adsorb onto the vanadia sites to form HgCl2 and VsOH species. Then, the reoxidationof the VsOH species by oxygen follows to form VdO and H2O. Indeed, vanadia sites are responsible for the NOx reduction activity. The popular mechanisms of the DeNOx reaction involve the ammonia species adsorbed on vanadia sites forming intermediate species, which are thought to be critical in the SCR reaction.40-43 Therefore, it can explain the phenom(40) Gasior, M.; Haber1, J.; Machej, T. J. Mol. Catal. 1988, 43, 359– 369. (41) Hiroyuki, K.; Katsumi, T. C. U. J. Catal. 1999, 185, 106–113.

A series of tests for mercury oxidation were performed across a laboratory-scale SCR system using a vanadia-based catalyst. The homogeneous oxidation of mercury was inappreciable, and the Deacon reaction was not observed. The adsorption behavior of Hg0 was distinctive from the fresh catalyst and HCl pretreated catalyst. Passing Hg0/N2 through the HCl preadsorbed SCR catalyst, the mercury oxidation was observed. Addition of HCl caused a rapid desorption of mercury from the catalyst. These results prove that HCl adsorbs onto the catalyst surface in a pure N2 environment and competes with weekly adsorbed Hg0 for the active sites. Furthermore, the adsorption of HCl onto the catalyst was confirmed by the XPS and FT-IR surface analysis, and reaction pathways were deduced. Results show that the vanadia sites on the SCR catalyst are responsible for the adsorption of HCl and that active Cl is formed to react with adjacent Hg0. Therefore, the mercury oxidation on the SCR catalyst should be regarded as a heterogeneous reaction via the Langmuir-Hinshelwood mechanism. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant No. 50476056), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20050335057), and the Talents Program of Natural Science Foundation of Zhejiang Province, China (Grant No. R10753240). EF800730F (42) Tops¢e, N. Y. H.; Tops¢e, H.; Dumesic, J. A. J. Catal. 1995, 151, 226–240. (43) Ozkan, U. S.; Cai, Y.; Kumthekar, M. W. Appl. Catal., A 1993, 96, 365–381. (44) Machalek, T.; Ramavajjala,M.; Richadson, M.; Richardson, C. Pilot evaluation of flue gas mercury reactions across an SCR unit. In Proceedings of the DOE-EPRI-USEPA-AWMA Combined Power Plant Air Pollutant Control Symposium-The MEGA Symposium; 2003.