TiO2 Commercial SCR Catalyst

Sep 27, 2008 - The purpose of this study was to examine how the Hg0 oxidation activity and catalytic nature of a commercial V2O5(WO3)/TiO2 SCR catalys...
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Ind. Eng. Chem. Res. 2008, 47, 8136–8141

Mercury Oxidation over the V2O5(WO3)/TiO2 Commercial SCR Catalyst Hiroyuki Kamata,* Shun-ichiro Ueno, Toshiyuki Naito, and Akinori Yukimura IHI Corporation, 1 Shin-nakahara-cho, Isogo-ku, Yokohama 235-8501, Japan

Mercury oxidation by hydrochloric acid over the V2O5(WO3)/TiO2 commercial SCR catalyst was investigated. Both fresh and aged catalysts with honeycomb structure, which were exposed to a coal combustion flue gas in a coal-fired boiler for over 71 000 h, were examined. The aged catalysts were characterized by X-ray and SEM-EDX analysis to examine the presence of ash deposition on the surface. The mercury oxidation rate was enhanced by increasing HCl concentrations and inhibited strongly by the presence of NH3. This behavior could be explained by a kinetic model assuming that HCl competes for the catalyst active sites against NH3. As the catalyst operation time increased, the mercury oxidation rate was observed to decrease considerably in the presence of NH3 while NO reduction rate was apparently nearly unchanged. By examining aged catalysts, deposits stemming from fly ash and SO2/SO3 were observed to accumulate continuously on the catalyst surface. The ash deposited on the surface may partially block the active catalyst sites and decrease their number. The decrease of the number of active sites on the catalyst surface caused NH3 to remain unreacted in the honeycomb catalyst. The decrease of the Hg0 oxidation rate was caused by the inhibition effect of NH3 remaining in the catalyst. 1. Introduction Selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia (NH3) as a reductant has been found to be very useful for treating exhaust gases.1-3 Since the first large-scale commercial SCR system was installed in a coal-fired power plant in Japan in 1983, the role of the SCR catalyst has greatly increased to prevent air pollution caused by nitrogen oxides. In the reduction of NOx, V2O5/TiO2 based catalysts with WO3 are commonly used because of their high catalytic activity, thermal stability, and high resistance to sulfur dioxides. The reduction of NO with NH3 proceeds according to the following reaction: NO + NH3 + 1/4O2 f N2 + 3/2H2O

(1)

Recently, mercury in flue gas discharged from coal-fired boilers has been recognized as a major concern because of its volatility, persistence, and bioaccumulation.4 According to the regulatory schedule, the U.S.Environmental Protection Agency (EPA) recently issued the Clean Air Mercury Rule for reducing Hg emission from coal-fired boilers.5 Mercury in coal combustion flue gas is classified into three chemical forms: elemental mercury (Hg0), oxidized mercury (Hg2+), and particle-associated mercury (HgP).6 Elemental mercury (Hg0) is released from coal during its combustion at high temperatures. Hg0 can be oxidized in the gas phase and/or heterogeneously over ash or unburned carbon particles derived from the coal combustion process into mercuric chloride (HgCl2) by hydrochloric acid. Oxidized mercury (Hg2+) is water-soluble and therefore is effectively captured in a wet-type desulfurization unit (FGD).5 Most of the particle-associated mercury is removed by an electrostatic precipitator (ESP) along with other dominant particulates. Hg0 is insoluble in water and is difficult to be captured in conventional pollutant control equipments, leaving it to be discharged into the atmosphere. Recently the V2O5/TiO2 based SCR catalyst was found to oxidize Hg0 to Hg2+.7-9 Thus, the SCR catalyst was incorporated into mercury removal strategies in which Hg0 oxidized over the SCR catalyst can be removed by ESP and/or FGD. * To whom correspondence should be addressed. E-mail: [email protected].

Because the Hg0 oxidation activity of the V2O5 catalyst was greatly improved by the presence of hydrochloric acid,7,10 the reaction is considered to proceed by the following equation: Hg0 + HCl + 1/2O2 f HgCl2 + H2O 11

(2)

Niksa and Fujiwara have developed a predictive mechanism for Hg0 oxidation over the SCR catalyst by invoking the Eley-Rideal type mechanism. In this mechanism, HCl competes with NH3 for surface active sites, and Hg0 reacts with these chlorinated sites from the gas phase or as a weakly adsorbed species. The active chlorinated sites were also implicated by Lee et al.10 in vanadium oxychloride complexes such as V2O3(OH)2Cl2 and VO2Cl. On the other hand, Senior12 suggested the Eley-Rideal type mechanism in which Hg0 adsorption was in competition with NH3 adsorption and adsorbed Hg0 reacts with gaseous HCl. This mechanism was based on the observation of Hg0 adsorbing on the SCR catalyst.13,14 Lee et al.13 observed that Hg0 adsorbed on the commercial SCR catalyst in the absence of HCl readily desorbs by adding NH3 to the catalyst inlet. To date, there is no general agreement on the reaction mechanism of the Hg0 oxidation over the V2O5/TiO2 based SCR catalyst. Furthermore, little is known about the long-term stability of the Hg0 oxidation activity of the V2O5/TiO2 based SCR catalyst. The SCR catalysts are normally placed upstream of ESP and are exposed to high concentrations of fly ash derived from coal combustion. Because fly ash is composed of several inorganic materials including alkalis, alkaline earth metals, Si, Al, and sulfur-containing compounds, ash deposition on the catalyst surface may deactivate the Hg0 oxidation activity of the catalyst during long-term operation. The purpose of this study was to examine how the Hg0 oxidation activity and catalytic nature of a commercial V2O5(WO3)/ TiO2 SCR catalyst changed over the long-term for flue gas treatment of a coal-fired boiler. The catalyst, before and after service, was evaluated for NOx conversion and Hg0 oxidation rate and other physicochemical properties. This characterization was carried out to clarify the aging process of the catalyst under industrial conditions.

10.1021/ie800363g CCC: $40.75  2008 American Chemical Society Published on Web 09/27/2008

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Figure 1. Schematic structure of the honeycomb catalyst.

2. Experimental Section 2.1. Catalyst. The commercial V2O5(WO3)/TiO2 SCR catalyst with a honeycomb structure used in this study was manufactured over a decade ago by then-current state-of-theart technology and has been in service for more than 10 years (about 71 000 h) at a coal-fired power plant located in Japan. The catalyst was designed to have a high NOx removal efficiency with depressed SO2 oxidation activity and to be durable both catalytically and mechanically. The length of the cell opening of the honeycomb structure is about 6.0 mm, and the wall thickness is about 1.4 mm. Honeycomb catalysts, which had been exposed to the coal combustion flue gas, were taken from an SCR reactor after specific operation periods to investigate the change in catalytic activity and physicochemical properties over time. In this study, three pieces of the honeycomb samples, which had been installed in the SCR reactor prior to the operation, were taken from the reactor one after another at 10 900, 58 000, and 71 170 h of the operation. 2.2. Activity Measurements. The honeycomb catalyst installed in the SCR reactor was about 1 m in length. Monolithic samples for activity measurements were cut from the middle point of the honeycomb catalyst (at z ) 0.5 in Figure 1). The sample consisted of 3 × 3 channels of honeycomb with a length of either 15.3 or 19.9 cm. Measurements of the Hg0 and NO conversions were carried out under steady-state conditions in a continuous gas flow reactor at a total flow rate (STP) of 4 160 cc/min, atmospheric pressure, and a reaction temperature of 653 K. Slightly different space velocity was employed: 2950 h-1 for the experiment examining the effect of NH3 and HCl concentrations on the activity and 2270 h-1 for the experiment examining the effect of the catalyst operation time on the activity. Reactant concentrations (STP) were 8.92 mmol/m3 NO, 8.92 mmol/m3 SO2, 0.89 mol/m3 O2, 4.46 mol/m3 CO2, and 4.46 mol/m3 H2O, with the balance being N2. Hg0 concentration (STP) in the reactant stream was set at approximately 0.055 µmol/m3 throughout the experiments. The concentrations of HCl and NH3 in the reactant stream were varied to determine their impact on the Hg0 oxidation and NO reduction activities of the catalyst. The NH3 concentration (STP) was varied from 0 to 10.7 mmol/m3 corresponding to a NH3/NO molar ratio, R, between 0 and 1.2. HCl concentration (STP) was varied from 0 to 4.46 mmol/m3, which covers the range of HCl concentration in the combustion flue gas of low rank subbituminous and bituminous coals. The schematic experimental setup is shown in Figure 2. The mercury concentration in the reactant and product streams was carefully monitored by a mercury analyzer (Nippon Instruments, MS-1A and DM-6B), which allowed continuous measurement of both Hg0 and Hg2+ concentrations. Effluent (500 cc/min) containing both Hg0 and Hg2+ from the catalyst bed was washed

with aqueous KCl to separate Hg2+ from the total mercury. The aqueous KCl containing only Hg2+ was then passed through a spiral-type glass tube with flowing air (500 cc/min) and a sulfuric acid solution of SnCl2 to reduce the Hg2+ to Hg0. Both effluents, which contain only Hg0 at this point, were passed through additional scrubbers of aqueous KOH to remove SO2. Finally, they were introduced into measurement cells, and the concentration of Hg0 in each effluent was determined individually by cold vapor atomic absorption spectrometry. Appropriate materials such as Teflon and quartz were chosen for the gas sampling line, and to ensure accurate analysis, the temperature was maintained above 393 K to avoid adsorption of mercury onto the equipment. About 4-10% of the total mercury was observed to be in the oxidized form, Hg2+, at the inlet of the catalyst. This oxidation may occur homogeneously in the precatalyst region where the reactant gas stream was preheated at about 673 K. Hg0 conversion was calculated based on the Hg0 concentrations at the inlet and the outlet of the catalyst: in out in Hg0 conversion ) [CHg 0 - CHg0]/CHg0 × 100

(3)

in 0 out0 where CHg and CHg are the Hg0 concentrations of the inlet and outlet steams, respectively. The mercury recovery ratio defined by the ratio of total mercury concentration in the outlet stream of the catalyst to that in the inlet stream was employed to check the analytical accuracy of mercury. The mercury recovery ratio was calculated based on the following equation:

out out in in Hg recovery ratio ) [CHg 0+CHg2+]/[CHg0+CHg2+] × 100 (4) in 2+ out2+ where CHg and CHg are Hg2+ concentrations in the inlet and outlet steams, respectively. Because of the propensity of mercury toward adsorption, higher recovery ratios indicated accurate analysis of mercury concentrations. The NO concentration in the reactant and product streams was determined by chemiluminescence (Horiba, PG-200). The NO conversion was determined by

in out in - CNO )/CNO NO conversion ) (CNO

(5)

in out where CNO and CNO are the NO concentrations in the inlet and outlet streams, respectively. 2.3. Characterization. Because fly ash deposition may be most severe at the inlet of the honeycomb catalyst, characterization was performed on the monolithic samples that were cut from the different positions of the honeycomb catalyst along the gas flow direction. The position from which the sample was taken was denoted as shown in Figure. 1. BET analysis was performed on a piece of honeycomb wall without being ground by nitrogen adsorption at 77 K (Quantachrome instruments, Autosorb1-MP), after degassing at 373 K for 10 h. Samples were taken from the position at z ) 0.5 in Figure 1. To check the presence of poisonous materials and their distributions inside the honeycomb walls, EDX analysis was performed on both the surface and a cross-section of the wall of the honeycomb using a Jeol JSM-5610 scanning electron microscope attached to a JED-2201 energy dispersion X-ray analyzer. To reveal any changes in the crystal phase and the existence of poisonous materials, X-ray analysis was carried out on the ground sample and the honeycomb wall using a Shimadzu XD610 diffractometer with Co KR radiation.

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Figure 2. Experimental setup of the activity test.

Figure 3. Dependence of Hg0 conversion and NO conversion on the NH3/ NO molar ratio, R, at 653 K and HCl concentration ) 0.45 mmol/m3. O: NO conversion; ∆: Hg0 conversion. Curves were calculated by eqs 6 and 7.

Figure 4. Dependence of Hg0 conversion and NO conversion on HCl concentration at 653 K and R ) 0 and 0.75. NO conversion at R ) 0.75 (O); Hg0 conversion at R ) 0 (∆) and 0.75 (0). Curves were calculated by eqs 6 and 7.

3. Results and Discussion 3.1. Catalytic Behavior. Figure 3 shows the Hg0 and NO conversions over the honeycomb type V2O5(WO3)/TiO2 SCR catalyst (fresh) as a function of the NH3/NO molar ratio, R, at 653 K. The NO conversion was observed to increase linearly with R up to R ) 1.0 with a slight decrease of efficiency at higher R. This indicates that the reduction of NO with NH3 proceeds over the catalyst according to eq 1. In contrast, the Hg0 conversion decreased steadily as R rose. Essentially no Hg0 conversion was observed above R ) 1.0. This suggests that Hg0 oxidation was strongly inhibited by the presence of NH3. Figure 4 shows the Hg0 and NO conversions over the honeycomb type V2O5(WO3)/TiO2 SCR catalyst (fresh) as a function of HCl concentration at 653 K. The results clearly indicate that Hg0 oxidation was strongly promoted by the presence of HCl. The decrease of Hg0 conversion at lower HCl concentrations was more apparent in the presence of NH3 than in its absence. The NO conversion was unchanged over a wide range of HCl concentration. This suggests that NO reduction was not influenced by the presence of HCl. 3.2. Hg0 Oxidation over Aged Catalyst. Figure 5 shows the Hg0 and NO conversions with and without NH3 at 653 K as a function of catalyst operation time. The Hg0 conversion at R ) 0.75 was found to decrease with increasing catalyst operation time while almost constant NO conversion was

Figure 5. Dependence of Hg0 conversion and NO conversion on the catalyst operation time at 653 K and R ) 0 and 0.75. NO conversion at R ) 0.75 (O); Hg0 conversion at R ) 0 (∆) and 0.75 (0). Curves were calculated by eq 11.

observed for the same operation period. The decrease of Hg0 conversion at R ) 0 was less significant compared to that at R ) 0.75. The Hg0 conversion of fresh catalyst, 80%, was slightly reduced to 73% at the longest operation time (71 000 h). The inhibition of the Hg0 oxidation by NH3 was more apparent as the catalyst operation time increased.

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Figure 6. EDX analysis of the surface of the aged catalyst (71 000 h). The samples were taken from the position at z ) 0.5 in Figure 1.

Figure 8. SEM-EDX images of a cross-section of the aged catalyst (71 000 h). Samples were taken from the different positions at z ) 0, 0.5, and 1.0 shown in Figure 1.

Figure 7. Concentrations of the main constituents of fly ash deposition on the catalyst surface as a function of the catalyst operation time. The samples were taken from the position at z ) 0.5 in Figure 1. O: Ti; ∆: Al; 0: Si; ∇: Ca; .: S.

3.3. Surface Analyses. Surface examination was performed on the catalyst, which had been exposed to the coal combustion flue gas for 71 000 h, because of its severe deactivation. Figure 6 shows the results of the EDX analysis on the surface of the catalyst wall, which was taken from the middle position of the honeycomb (at z ) 0.5 in Figure 1). On the surface, Si, Al, Ca, S, and Ti were observed to be the main constituents, and a small amount of Fe was also detected. Since Ti and W were primarily observed on the fresh catalyst as main constituents (not shown), an increase in the concentration of Si, Al, Ca, and S on the surface was apparent. Figure 7 shows the concentration of the main constituents deposited on the catalyst surface determined by EDX analysis as a function of the catalyst operating time. The concentrations of Ti, Si, Al, Ca, and S were calculated as their oxides. The concentrations of Si, Al, Ca, and S increased as the catalyst operation time increased. The decrease in the Ti concentration was the result of the deposition of fly ash constituents over the catalyst surface. Since the penetration depth of the X-ray of EDX is normally a few micrometers, the ash deposits may be accumulated considerably on the outer surface of the honeycomb wall. The distribution of deposits inside the honeycomb wall can be seen in Figure 8. Most of the deposit was found to be distributed within 5-10 µm from the surface, and the intensity profiles for the different elements are almost coincident. By using the thickness of the deposit layer, the apparent rate of growth of the deposited layer at the top of the honeycomb can

Figure 9. X-ray profiles of the (a) fresh and (b-d) aged catalysts (71 000 h). (b) z ) 0; (c) z ) 0.5; (d) z ) 1.0. Spectrum (e) was taken on the surface of the aged catalyst at z ) 0. O: TiO2 (anatase); ∆: CaSO4.

be calculated to be approximately 0.6 µm/year. The concentrations of Si, Al, Ca, and S decreased along the direction of the gas flow. This indicates the fly ash deposition occurs mostly at the inlet of the honeycomb catalyst, where the deactivation must be most severe. Figure 9 shows X-ray profiles of the ground sample, for both fresh and aged catalysts (71 000 h). Only an anatase type of TiO2 was observed for all samples. Although Si, Al, Ca, and S were detected by EDX analysis, their concentrations in the samples were too low for detection by conventional X-ray analysis once the samples were ground. The crystalline size of TiO2 was calculated to be 150 Å for the fresh catalyst, and a similar value was obtained for the aged one. This suggests that the sintering of TiO2 carrier is almost nonexistent at the

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temperatures at which the catalyst had been operated (about 630 K). By increasing the catalyst operation time, on the other hand, the decrease of the BET surface area from 120 m2/g of the fresh catalyst to 90 m2/g of the aged one (71 000 h) was observed. This decrease may be reflected to the deposition of ash on the outer surface of the honeycomb wall. To determine the structure of the deposited material observed by EDX analysis, the surface of the honeycomb wall of the aged catalyst (71 000 h) was subjected to X-ray analysis (Figure 9 e). Besides anatase TiO2, dehydrated CaSO4 was observed to exist on the surface. Because the intensity of TiO2, which is the main constituent of the catalyst, dropped considerably, CaSO4 is considered to accumulate over the honeycomb surface. Compounds containing Si and/or Al could not be observed clearly, which may be due to the strong TiO2 peaks. Most of the deposits exist on the surface of the honeycomb wall; therefore, the deposits may physically block active catalyst sites and may change the pore structure near the wall surface. Because most NO reduction reactions over a honeycomb catalyst are considered to occur near the surface of the wall, specifically within about 50 µm from the surface,15 the deposits on the surface may decrease the number of the active catalyst sites and hinder easy access of the reactants to the pores. The decrease of the Hg0 conversion observed at R ) 0.75 with increasing catalyst operation time can be considered in relation to the decreasing number of active sites. 3.4. Kinetic Analysis. The SCR reaction over the honeycomb catalyst is influenced by gas diffusion of the reactant in the honeycomb structure.15,16 Because of the high molecular weight of mercury, the Hg0 oxidation over the honeycomb type SCR catalyst may be affected by the gas diffusion as well. Previous kinetic studies by Niksa and Fujiwara11 and Senior12 take the gas diffusion phenomenon in the honeycomb catalyst into account for their mercury oxidation model. In this study, however, the gas diffusion limitation in the honeycomb was not taken into account for the kinetic analysis as a first approximation. In practice, NO reduction can be expressed by assuming the reaction occurs between NH3 adsorbed on the active sites and gaseous or weakly adsorbed NO.15-17 The reaction rate of NO reduction can be expressed by the following equation: rNO)kNOCNOKNH3CNH3/(1 + KHClCHCl+KNH3CNH3)

(6)

where kNO and KNH3 are the rate constant of NO reduction and the equilibrium constant for NH3 adsorption, respectively. As shown in Figure 3, Hg0 oxidation was strongly inhibited by the presence of NH3, and thus, the adsorption of NH3 on the active sites may obstruct Hg0 oxidation by HCl. NH3 is often considered to adsorb on the surface acid sites, V5+-OH, and participate in the NO reduction in a form of ammonium ion, NH4+.18 Since chlorine has a strong affinity for metal oxides, vanadium oxychloride complexes are expected to be formed by the reaction between gaseous HCl and V2O5.19 This suggests that HCl participates in Hg0 oxidation in a form of adsorbed species and HCl adsorption may compete against NH3 for the catalyst active sites. However, the details of the adsorption sites for HCl and the structure of active complex is not clarified yet. Further study including a spectroscopic analysis is required. Here, we applied the Eley-Rideal type mechanism in which gaseous Hg0 reacts with chlorine species adsorbed on the catalyst active sites: rHg0 ) kHg0CHg0KHClCHCl/(1 + KHClCHCl+KNH3CNH3)

(7)

Table 1. Kinetic Parameters kNO kHg0 KNH3 KHCl kd

mol s-1 m-3 mol s-1 m-3 m3 mol-1 m3 mol-1 h-1

1.95× 108 3.57× 109 5.10× 105 9.42× 102 6.06×10-6

where kHg0 and KHCl are the rate constant of Hg0 oxidation and the equilibrium constant for HCl adsorption, respectively. In both eqs 6 and 7, the competitive adsorption of NH3 and HCl on the catalyst active sites was taken into account. The numerical integration of eqs 6 and 7 from the inlet to the outlet of the catalyst gives NO and Hg0 conversion, respectively. The following equation is applied for the calculation of both NO conversion and Hg0 conversion: NO or Hg0 conversion )

1 F

∫

V

V)0

rdV

(8)

where F and V are the molar flow rate of NO or Hg0 in the inlet stream and the catalyst volume, respectively. In the calculation, the NH3 concentration in the catalyst was assumed to be held by following equation: in in - (CNO - CNO) CNH3 ) CNH 3

(9)

0

In the calculation of the Hg conversion, the concentration of HCl is assumed to be constant through the catalyst, since a consumption of HCl by Hg0 oxidation is negligible. in CHCl)CHCl

(10)

The data shown in Figures 3 and 4 were fit by eqs 6 and 7 with kNO, KNH3, kHg0, and KHCl as parameters by the least-squares method. The curves in Figures 3 and 4 show the calculated results, and the determined parameters are listed in Table 1. Comparing the determined KNH3 and KHCl values, KNH3 is much higher than that of KHCl. This suggests that NH3 adsorption is predominant on the catalyst surface in the presence of NH3 under SCR condition. HCl tends to adsorb on the surface gradually once NH3 on the surface is consumed by the NO reduction. As for the deactivation of the catalyst, the formation and deposition of CaSO4 and silica-alumina proceed continuously under practical SCR condition in which fly ash and SO2 and/or SO3 exist in the flue gas. As mentioned above, the deposition near the surface of the catalyst may partially block the catalyst active sites for both the NO reduction and the Hg0 oxidation reactions. This results in the decrease of the number of the effective active sites of the fresh catalyst. The rates of both reactions are considered to be proportional to the fraction of the remaining active sites, 1 - θd, which are not blocked by the fly ash deposits. By assuming that fly ash concentration in the flue gas is abundant and constant during the catalyst operation, the decay of the reaction rates can be simply expressed by20 r(t) ) rt)0(1 - θd) ) rt)0exp(-kdt)

(11)

where rt ) 0, θd, and kd are the reaction rate of the fresh catalyst, the fraction of the active sites that was blocked by the deposits, and a constant representing the rate of deposit formation on the catalyst surface, respectively. Eq 11 can be applied to both the NO reduction and the Hg0 oxidation. By using the values of the kinetic parameters determined above (kNO, KNH3, kHg0, and KHCl), eq 11 was fit to the experimental data shown in Figure 5 with kd as a parameter. The calculated results are shown in Figure 5 as solid curves, and the determined kd value is listed in Table 1.

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According to the calculations, the fraction of the remaining active sites, 1 - θd, for the reaction decreases continuously from unity of the fresh catalyst to about 0.65 after 70 000 h of operation. This trend is consistent with the accumulated deposits concentrations on the catalyst surface. The ash deposited on the catalyst surface may physically block the part of the active sites and decrease their number. The decrease in the number of active sites on the catalyst surface by the ash deposition reduces both NO reduction and Hg0 oxidation activities of the catalyst. For the reaction conditions employed in this study, the NO conversion apparently remains unchanged even by the gradual decrease of the number of the active sites. However, the decrease of the catalyst reactivity causes NH3 to remain unreacted in the honeycomb catalyst, namely the residence time required for the NO reduction tends to increase with the catalyst operation time. This results in the inhibition effect of NH3 on the Hg0 oxidation being more apparent. The gradual decrease of the Hg0 oxidation rate is caused by the inhibition effect of NH3 remaining in the catalyst. 4. Conclusion Mercury oxidation by hydrochloric acid over the V2O5(WO3)/ TiO2 commercial SCR catalyst was studied. Mercury oxidation is enhanced by increasing HCl concentration. In contrast, NH3 was observed to inhibit Hg0 oxidation. As the molar ratio of NH3/NO increases, the Hg0 oxidation rate decreases steadily. By assuming that HCl competes for the catalytic active sites against NH3, both a promotion effect by HCl and the opposite negative effect by NH3 on Hg0 oxidation can be explained. Adsorption of NH3 on the catalyst active sites may dominate HCl adsorption. By examining aged catalysts, which were exposed to coal-fired flue gas during specific periods, deposits stemming from fly ash and SO2/SO3 were observed to accumulate continuously on the catalyst surface. The ash deposition, which contains CaSO4 and Si and Al oxides, was observed to form on the near surface of the honeycomb. The ash deposited on the surface may partially block the active catalyst sites and decrease their number. Under the experimental conditions employed in this study, the NO reduction rate remains nearly unchanged during long-term exposure to coal combustion flue gas. The Hg0 oxidation rate was nearly constant in the absence of NH3; however, it gradually decreased in the presence of NH3. The decrease of the number of active sites on the catalyst surface by ash deposition causes NH3 to remain unreacted in the honeycomb catalyst. The gradual decrease of the Hg0 oxidation rate is caused by the inhibition effect of NH3 remaining in the catalyst. Literature Cited (1) Bosch, H.; Janssen, F. J. J. G. Catalytic Reduction of Nitrogen Oxides. Catal. Today 1989, 2, 369.

(2) Forzatti, P.; Lietti, L. Recent Advances in DeNOxing Catalysis. Hetrogen. Chem. ReV. 1996, 3, 33. (3) Ozawa, M.; Seo, Y.; Yukimura, A.; Ueda, Y. Recent Technology for IHI Denitrification (SCR) System. Ishikawajima-Harima Eng. ReV. 1999, 39, 356. (4) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. C.; Laudal, D. L.; Benson, S. A. Status Review of Mercury Control Options for Coal-Fired Power Plants. Fuel Process. Technol. 2003, 82, 89. (5) Senior, C. L.; Helble, J. J.; Sarofim, A. F. Emission of Mercury, Trace Elements, and Fine Particles from Stationary Combustion Sources. Fuel Process. Technol. 2000, 65-66, 263. (6) Fujiwara, N.; Fujita, Y.; Tomura, K.; Moritomi, H.; Tuji, T.; Takasu, S.; Niksa, S. Mercury Transformations in the Exhausts from Lab-Scale Coal Flames. Fuel 2002, 81, 2045. (7) Lee, C. W.; Srivastava, R. K.; Ghorishi, S. B.; Hastings, T. W.; Stevens, F. M. Study of Speciation of Mercury under Simulated SCR NOx Emission Control Conditions, paper no. 41. Proceedings of the MEGA Symposium, Washington, DC, May 19-22, 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, paper no. 64. Proceedings of the MEGA Symposium, Washington, DC, May 19-22, 2003. (9) Bock, J.; Hocquel, M. J. T.; Unterberger, S.; Hein, K. R. G. Mercury Oxidation across SCR Catalysts of Flue Gas with Varying HCl Concentrations, paper no. 233. Proceedings of the MEGA Symposium, Washington, DC, May 19-22, 2003. (10) Lee, C. W.; Srivastava, R. K.; Ghorishi, S. B.; Karwowski, J.; Hastings, T. W.; Hirschi, J. C. Pilot-Scale Study of the Effect of Selective Catalytic Reduction Catalyst on Mercury Speciation in Illinois and Powder River Basin Coal Combustion Flue Gases. J. Air Waste Manage. Assoc. 2006, 56, 643. (11) Niksa, S.; Fujiwara, N. A Predictive Mechanism for Mercury Oxidation on Selective Catalytic Reduction Catalysts under Coal-Derived Flue Gas. J. Air Waste Manage. Assoc. 2006, 55, 1866. (12) Senior, C. L. Oxidation of Mercury across Selective Catalytic Reduction Catalysts in Coal-Fired Power Plants. J. Air Waste Manage. Assoc. 2006, 56, 23. (13) Lee, C. W.; Srivastava, R. K.; Ghorishi, S. B.; Hastings, T. W.; Stevens, F. M. Investigation of Selective Catalytic Reduction Impact on Mercury Speciation under Simulated NOx Emission Control Conditions. J. Air Waste Manage. Assoc. 2004, 54, 1560. (14) Eswaran, S.; Stenger, H. Understanding Mercury Conversion in Selective Catalytic Reduction (SCR) Catalysts. Energy Fuels 2005, 19, 2328. (15) Tronconi, E.; Beretta, A.; Elmi, A. S.; Forzatti, P.; Malloggi, S.; Baldacci, A. A Complete Model of SCR Monolith Reactors for the Analysis of Interacting NOx Reduction and SO2 Oxidation Reactions. Chem. Eng. Sci. 1994, 49, 4277. (16) Tronconi, E. Interaction between Chemical Kinetics and Transport Phenomena in Monolith Catalysts. Catal. Today 1997, 34, 421. (17) Bahamonde, A.; Beretta, A.; Avila, P.; Tronconi, E. An Experimental and Theoretical Investigation of the Behavior of a Monolithic TiV-W-Sepiolite Catalyst in the Reduction of NOx with NH3. Ind. Eng. Chem. Res. 1996, 35, 2516. (18) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Chemical and Mechanistic Aspects of the Selective Catalytic Reduction of NOx by Ammonia over Oxide Catalyst: A Review. Appl. Catal., B 1998, 18, 1. (19) Hisham, M. W. M.; Benson, S. W. Thermochemistry of the Deacon Process. J. Phys. Chem. 1995, 99, 6194. (20) Smisth, J. M. Chemical Engineering Kinetics; McGraw-Hill, New York, 1981.

ReceiVed for reView March 5, 2008 ReVised manuscript receiVed July 28, 2008 Accepted August 11, 2008 IE800363G