Laboratory Investigation of Selective Catalytic Reduction Catalysts

Jan 15, 2004 - Alkali Metal Poisoning of a CeO2–WO3 Catalyst Used in the Selective Catalytic Reduction of NOx with NH3: an Experimental and Theoreti...
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Ind. Eng. Chem. Res. 2004, 43, 941-947

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Laboratory Investigation of Selective Catalytic Reduction Catalysts: Deactivation by Potassium Compounds and Catalyst Regeneration Yuanjing Zheng, Anker Degn Jensen,* and Jan Erik Johnsson Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Kgs. Lyngby, Denmark

The deactivation and regeneration of selective catalytic reduction (SCR) catalysts poisoned by potassium by a wet-impregnation method was investigated experimentally. Potassium in the form of both chloride and sulfate is a strong poison for the catalyst. The results indicate that potassium titrates the active sites for NH3 adsorption. Simply increasing the operating temperature or the vanadium content in the catalyst cannot compensate the loss of catalyst activity: Increasing the temperature hardly increases the conversion of NO for the strongly poisoned catalysts, and catalysts with high vanadium content become active for oxidizing NH3 to NO, causing a net NO formation. Deactivated catalysts can be regenerated by different methods. Sulfation by gaseous SO2 is efficient provided the poison is first removed by washing. When regenerating by 0.5 M H2SO4, the catalyst regains a higher activity than that of the fresh catalyst at temperatures higher than 300 °C. Heat treatment of the catalyst at 400 °C for 2 h after poisoning to simulate actual operation has no influence on the regeneration by 0.5 M H2SO4. Deactivated catalysts without the heat treatment step regain higher activities than that of the fresh catalyst at all temperatures when regenerated by 1 M NH4Cl. However, the heat treatment step has a negative effect on the regeneration by NH4Cl. 1. Introduction Combustion of most fuels results in the formation of NOx, and for nitrogen-containing fuels such as coal and biomass flue gas cleaning may be necessary to comply with national regulations. For large-scale coal-, oil-, and gas-fired boilers, the most frequent process for NOx reduction is the selective catalytic reduction (SCR) in which NH3 is injected into the flue gas, which is subsequently passed over a catalyst at 300-400 °C. In some countries such as Sweden and Denmark, the SCR process is also being used on plants burning pure biomass or mixtures of biomass and other fuels such as coal. The commercial SCR catalysts consist of TiO2 as high surface area support and V2O5-WO3 as active catalytic components, and they are shaped into channels to allow the fly ash to pass through.1 Renewable energy sources, such as biomass, have become more important because of the more stringent legislation and environmental benefits mainly related to lower emission of greenhouse gases. However, one of the technical constraints of cofiring related to the flue gas cleanup systems is the accelerated deactivation of the SCR catalysts.2 The lifetime of a SCR catalyst in a coal-fired power plant is normally about 3-5 years.1 However, application of SCR catalysts on biomass fired/ cofired boilers in Denmark and Sweden has shown a much shorter lifetime.3,4 In Sweden, SCR tests have been carried out in a 125 MWe circulating fluidized-bed plant firing forest residues and a 75 MWe plant firing pulverized wood.3 When using a conventional honeycomb catalyst, 80% of the original activity remained in the circulating fluidized-bed boiler after 2100 h but only * To whom correspondence should be addressed. Tel.: +4545 25 28 41. Fax: +45-45882258. E-mail: [email protected].

20% remained in the pulverized wood boiler. SCR catalyst tests were also performed in Denmark on catalysts exposed to respectively high- and low-dust conditions in a 150 MWe pulverized-fuel power plant cofiring straw and straw.4 The deactivation of catalyst elements in the high-dust position was measured in a period of about 2860 h, leading to a deactivation of about 35%, of which the first 15% was seen during the first 500 h. The deactivation of the low-dust elements was measured after 2350 h of exposure and was up to 15%. A major difference between typical fossil fuels and biomass is the much higher content of alkali metals in the biomass. Potassium is the main alkali metal in Danish straw and is present in levels of 0.2-1.9 wt %.5 In Denmark, straw and wood are the most abundant biofuels used for power production. During combustion, part of the alkali metals evaporate from the fuel, and as the flue gas temperature decreases, the alkali metals condense and form submicron solid particles by homogeneous and heterogeneous condensation.6-8 The submicron aerosol particles in the flue gas consist of almost pure potassium chloride and sulfate with minor amounts of sodium, phosphorus, and calcium. Replacing the catalyst is a major part of the maintenance cost of the SCR system; hence, it is desirable to prolong the catalyst life. Regenerating the catalyst, instead of substituting it with a new one, can substantially reduce the cost of operating SCR units. The regenerated catalyst should be able to be used for an extended time to avoid frequent regeneration. Potassium has been proposed as an important element for deactivation of SCR catalysts used in biomass combustion,3,4 but it is not clear whether the deactivation is due to pore condensation/blockage or chemical poisoning. Previous studies mainly focused on the effect of alkali oxides, which are not the actual alkali com-

10.1021/ie030404a CCC: $27.50 © 2004 American Chemical Society Published on Web 01/15/2004

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Figure 1. Schematic overview of the experimental setup.

pounds in the flue gas passing through the SCR catalysts in biomass combustion.9-12 The tested catalysts were synthesized as part of these studies and were mainly binary V2O5-TiO2 and a few ternary V2O5WO3-TiO2. In this work, a systematic laboratory study of the deactivation of commercial V2O5-WO3-TiO2 catalysts caused by KCl and K2SO4 is presented. Possible solutions to the fast deactivation of SCR catalyst under biomass combustion by increasing the operating temperature or the vanadium content in the catalyst were addressed. Finally, experiments were carried out to investigate how the catalysts may be regenerated. 2. Experimental Section 2.1. Catalysts. The catalysts were obtained from Haldor Topsøe A/S as plates with different concentrations of the active component (1.0, 1.78, 2.6, 3.0, and 7.3 wt % V2O5). The catalysts contained 5-13 wt % WO3. The catalysts were doped with KCl and K2SO4 at different levels by a wet-impregnation method at room temperature under a small vacuum at 0.8 bar absolute pressure to allow the KCl or K2SO4 solution to penetrate into the catalyst pores. The catalysts were then dried at 150 °C for 15 h. After the impregnation, part of the samples was heat treated at 400 °C for 2 h. The catalyst plates had a dimension of 1.3 mm × 50 mm × 160 mm and were cut into 5 × 5 mm bits for the activity test. In some tests, a size of 16 × 20 mm was also used in a special arrangement as explained below. 2.2. Activity Test. The activities of the catalysts were measured by means of the setup shown in Figure 1. It consisted of three sections: a gas metering and mixing section, a reactor, and an analyzer section. The gas stream contained (on a volume basis) 500 ppm NO, 600 ppm NH3, 5% O2, and 5% H2O in N2, and the flow rate was about 1.2 NL/min. The flow rates of individual gases were controlled by mass flow controllers that provide a constant flow. The actual flow rates were measured by a bubble flowmeter. A flow panel was used to mix the reactant gases from the individual gas cylinders in two different strings and direct the gas streams to the reactor. Water was added by passing part of the nitrogen gas through an evaporator. The reactor was a fixed-bed reactor made of quartz shown in Figure 2. The inner and bottom tubes of the reactor were removable. The catalyst bits (about 0.5-1 g) were placed randomly on the porous quartz plate. Nitric oxide, O2, H2O, and half of N2 were added through the bottom (1 in Figure 2), which functions as a preheating section. Ammonia and the other half of N2

Figure 2. Sketch of the laboratory reactor. All measures are in millimeters. (1) Bottom gas inlet. (2) Top gas inlet. (3) Thermocouple. (4) Gas outlet.

were added from the top of the reactor inlet (2 in Figure 2). With this arrangement, reactive gases could be kept separated until they were mixed, just above the catalysts placed on the porous quartz plate. This minimizes homogeneous and wall-catalyzed reactions in the preheating section, which, in any case, were negligible at the temperatures used here. The reactor temperature was measured below the porous quartz plate by a thermocouple shielded in a quartz tube. The reactor was placed in a three-zone oven for effective temperature control. The activities of the catalysts were measured at 200, 250, 300, 350, and 400 °C, respectively. Nitric oxide was analyzed by a conventional UV analyzer, while NH3 was measured by wet analysis using continuous titration with 0.1 M HCl using a Metrohm 665 Dosimat injector and a 682 Titroprocessor. To test whether bypassing of reactant gas took place through the loose bed of catalyst bits, several experiments were repeated by reloading the catalyst. It was found that the deviation of the activity was less than 6%. Several tests were also made in a different arrangement where larger catalyst pieces of 16 × 20 mm were placed on top of (perpendicular to) each other to simulate the honeycomb layout. The activity deviation from the catalysts tested by random packing of bits was within 5%. An often-used rate expression for NO reduction in the SCR reaction is13-15

KACNH3 -rNO ) krCNO 1 + KACNH3

(1)

The rate expression is based on an Eley-Rideal mechanism where NH3 adsorbs on the catalyst surface while NO from the gas phase reacts directly with the adsorbed NH3. Both the intrinsic chemical rate constant kr and

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the adsorption equilibrium constant KA are of the Arrhenius type. When NH3 is in excess, as is the case in this work, the rate expression (1) effectively reduces to a first-order expression:

-rNO ) k′CNO ) k′0 exp(-E/RT)CNO

(2)

The conversion of NO is defined by

X ) (CNO,in - CNO,out)/CNO,in

(3)

Assuming plug flow in the packed-bed reactor, the firstorder rate constant can be obtained from the conversion of NO as

k′ )

Fgas 1 ln mcat 1-X

(

)

(4)

2.3. Regeneration Methods. The doped catalysts were regenerated by washing with different aqueous solutions and sulfation by gaseous SO2. The solutions used for regeneration were pure water, 0.5 M sulfuric acid, and 1 M ammonia chloride. Each sample (0.5-2 g) was washed in 100 mL of a washing solution at 0.8 bar for 25 min at 25 °C. After washing, the catalyst samples were dried at 150 °C for 15 h. Sulfation was carried out by adding 1000 ppm SO2 to the inlet gas for 30-155 min to reach a steady state in the catalyst activity. Ammonia and SO2 were not added to the system at the same time because SO2 will react with O2, H2O, and NH3 to produce NH4HSO4, which can condense on the catalyst plates and plug the catalyst pores and the gas lines. 2.4. Ammonia Chemisorption. The dominant reaction in the SCR process is given by the following equation:16

4NH3 + 4NO + O2 f 4N2 + 6H2O

(5)

in which a 1:1 stoichiometry of NH3/NO is observed. To investigate the mechanism of the deactivation, some chemisorption studies were made, similar to those of Kleemann et al.17 A gas mixture containing 650 ppmv NH3, 5% O2, and 5% H2O, but not NO, was first passed over the catalyst to saturate the active sites. The addition of NH3 was continued until the inlet concentration was also reached at the reactor outlet. Then NH3 was shut off, and shortly after, 570 ppmv NO was added to the gas. Reaction between NO and chemisorbed NH3 then takes place, and the amount of NH3 on the catalyst was calculated from the amount of NO reduced. It is assumed that the moles of chemisorbed NH3 correspond to the number of active sites. 2.5. Catalyst Characterization. The distribution of potassium in the impregnated catalyst was investigated by scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) on an Oxford JEOL-JSM 5900, which is a high-performance scanning electron microscope with a high resolution of 3.0 nm. Surface area and nitrogen adsorption data at 77 K were measured on a Micromeritics ASAP 2000 automatic surface area and pore-size distribution apparatus. The total surface area was evaluated by the BrunauerEmmett-Teller (BET) method, while the average pore diameter was calculated from the surface area and the BET pore volume. Fourier transform infrared measurements were collected on pressed KBr disks using a Perkin-Elmer 1710

Figure 3. Catalytic activity of a 3 wt % V2O5-WO3-TiO2 catalyst at 250 °C as a function of K loading using KCl or K2SO4. Table 1. BET Area, Pore Volume, and Pore Diameter of Fresh and Poisoned Samples

catalyst

K/V ratio

surface area (m2/g)

pore volume (cm3/g)

average pore diameter (Å)

1.78% V2O5 1.78% V2O5 1.78% V2O5 1.78% V2O5 2.6% V2O5 7.3% V2O5

0 1.8 3.6 7.2 0 0

79.1 63.2 57.0 57.8 68.1 49.4

0.2195 0.1880 0.1715 0.1745 0.1944 0.1551

103.3 110.3 104.6 113.4 103.8 118.2

spectrometer. The crushed catalyst sample (1 mg) was dispersed homogeneously in 100 mg of potassium bromide and pressed into a disk. The spectra were recorded at room temperature. 3. Results and Discussion 3.1. Poisoning Effect of Potassium. Figure 3 shows the influence of increasing potassium loading in the form of both KCl and K2SO4 on the catalyst activity at 250 °C for a 3 wt % V2O5 catalyst. The activity decreases with increasing potassium loading for both KCl and K2SO4, and most of the activity is lost when the molar K/V ratio exceeds about 1.5. There is a tendency that the chlorine is a stronger poison than the sulfate. The results indicate that catalyst deactivation may be expected for all fuels containing potassium provided it is present as KCl or K2SO4 in the fly ash. In full-scale plants, however, the potassium is present as aerosol particles and the mechanisms by which the potassium in this form penetrates the catalyst to the active sites are unknown. The BET surface area and pore volume of the doped catalysts were lower than those of the fresh samples, while the average pore diameter of the fresh catalyst was lower than that of the doped sample (Table 1). The lower surface area and pore volume does not seem to explain the severe decrease in activity. To further investigate the mechanism of the deactivation, some ammonia chemisorption studies as described above were made to discriminate between chemical and physical poisoning of the catalyst. For chemical poisoning, the number of active sites is expected to be lower than that for a fresh catalyst, while for physical poisoning, the sites are still active but less accessible; i.e. the diffusion resistance is larger. Figure 4 shows the results of three experiments: one using an empty reactor, one with fresh catalyst in the reactor, and one with doped catalyst with a K/V ratio of 0.8. It can be seen that the

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Figure 4. Outlet concentration of NO at 250 °C after NH3 chemisorption in the empty reactor and a 3 wt % V2O5-WO3TiO2 fresh catalyst or a catalyst with a K/V ratio of 0.8, doped by KCl.

Figure 6. IR spectra in the OH region of the 1 wt % V2O5-WO3TiO2 fresh catalyst and doped one with K/V ) 1 after activity measurement.

TiO2.18,19 The band due to the V-OH groups is also confirmed at 3642 cm-1. When the catalyst is doped with a K/V ratio of 1, all of these OH bands disappear. This implies that potassium added to the catalyst coordinates to the OH groups and titrates the active sites for NH3 chemisorption. The following reactions involving KCl and K2SO4 may take place:

Figure 5. Ammonia chemisorbed on the 3 wt % V2O5-WO3-TiO2 catalyst surface at 250 °C as a function of the amount of potassium added.

NO outlet concentration for the experiment with fresh catalyst in the reactor increases much slower than those for the empty reactor and doped catalyst. The area between the curves with and without catalyst is proportional to the amount of NO reduced and, thereby, to the amount of NH3 chemisorbed on the catalyst. Figure 5 shows the relations between the molar K/V ratio and the amount of NH3 adsorbed. The adsorbed NH3 decreases almost linearly when the K/V ratio is below 0.5 and then levels off with a further increase of the K/V ratio. The decrease of the amount of NH3 chemisorbed on the catalysts results in a decrease of the pseudo-firstorder rate constant in eq 2 for NO reduction as shown in Figure 3. The results of NH3 chemisorption indicate that the active sites are chemically altered by the presence of potassium. According to the kinetic mechanism proposed by Topsøe et al.,18,19 the SCR mechanism consists of two catalytic cycles. The important first step is chemisorption of NH3 on the Brønsted acid site V-OH. The results of our study indicate that the Brønsted sites are affected by potassium because the amount of NH3 chemisorbed on the catalyst decreases with an increasing amount of potassium. Figure 6 shows the infrared spectra in the OH region of a fresh 1 wt % V2O5 catalyst and a doped one with K/V ) 1. The fresh catalyst shows bands at 3742 and 3674 cm-1, which are due to hydroxyl groups on

-V-OH + KCl f -V-O-K + HCl

(6)

-V-OH + K2SO4 f -V-O-K + KHSO4

(7)

-V-OH + KHSO4 f -V-O-K + H2SO4

(8)

Once the -V-OH sites are in the potassium salt form, they are no longer active for adsorption of NH3 and thus no reduction of NO takes place. This agrees well with the results of Kamata et al.12 and Pritchard et al.20 Kamata et al.12 made an infrared spectroscopic study, which showed that the addition of K2O to the catalyst modified the structure of the surface vanadium species. K2O added to the catalyst might also partially react with V2O5 to form KVO3.12 It has also been shown that K2O directly coordinates with the surface vanadium oxide phase.21,22 Addition of K2O progressively to V2O5-TiO2 catalyst titrates the surface vanadium oxide sites. 3.2. Effect of Temperature and Vanadium Content on the Poisoned Catalyst. A possible way to circumvent the loss of activity could be to increase the operating temperature of the catalyst. According to the simplified rate expression given by eq 1, an increased temperature will increase the rate through the Arrhenius rate constants; see eq 2. Another way to prolong the lifetime of the catalyst could be to increase the content of vanadium. Thereby, the catalyst would be able to take up more potassium on an absolute level and remain active. The latter approach would only be possible for fuels with low sulfur contents (typical for biomass5) because increasing the vanadium content accelerates the undesired oxidation of SO2 to SO3 over the catalyst.23-25 Parts a-c of Figure 7 show the effect on the conversion of NO when the temperature is raised for a 1.78, 2.6, and 7.3 wt % V2O5 catalyst, respectively, doped with different levels of KCl. It can be seen from Figure 7a that increasing the temperature hardly increases the conversion of NO for the strongly poisoned catalysts with more than 1.37 wt % potassium (K/V )

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Figure 8. Catalytic activity of a 1.78 wt % V2O5-WO3-TiO2 catalyst as a function of temperature after doping with a K/V ratio of 3.6 by KCl, regeneration by water washing only, and water washing followed by sulfation for 90 min. The activity of the fresh catalyst is also shown for reference.

Figure 7. Influence of the reaction temperature on conversion of NO at different KCl loadings: (a) 1.78 wt % V2O5; (b) 2.6 wt % V2O5; (c) 7.3 wt % V2O5. The weight of the sample is about 0.5 g.

1.8). From Figure 7b and more clearly from Figure 7c, it can further be seen that the catalysts with high vanadium contents become active for oxidizing NH3 to NO corresponding to the net reaction

4NH3 + 5O2 f 4NO + 6H2O

(9)

It is interesting to note that the capacity for taking up potassium on an absolute level and remaining active does not always increase with the vanadium content in the catalyst. For slightly deactivated catalysts, the 2.6 wt % V2O5 catalyst with 0.26 wt % potassium shows a much better activity than the 1.78 wt % V2O5 catalyst with 0.34 wt % potassium. However, for higher potassium level, the 1.78 wt % V2O5 catalyst with 1.37 wt % potassium, 2.6 wt % V2O5 catalyst with 1.03 wt % potassium, and 7.3 wt % V2O5 catalyst with 1.55 wt % potassium show almost the same level of activity at temperatures below 350 °C. At higher temperatures, the 7.3 wt % catalyst with 1.55 wt % potassium is no longer

active for reduction of NO but generates NO. The results indicate that neither simply raising the operating temperature nor increasing the vanadium content of the catalyst may be an efficient way to solve the deactivation problem. 3.3. Regeneration by Sulfation and Pure Water Washing followed by Sulfation. A regeneration method proposed in the literature is sulfation of the catalyst by SO2 to increase its acidity.26-28 This method has the advantage of being labor extensive and can be performed while the catalyst is in operation. Several deactivated samples were regenerated by sulfation and tested at 250 °C. It was found that sulfation did increase the activity of the catalysts. The increment of the activity after sulfation is about 25% of the activity of the fresh catalyst. This method can only be used to regenerate the slightly deactivated catalyst. SEM/EDX analysis shows that sulfation alone does not remove the poison, and this may be a main reason for the poor efficiency of the method. Then the samples were washed in pure water before sulfation. Figure 8 shows the activities of the catalyst regenerated by water washing only and by water washing followed by sulfation. About 50-72% of the catalyst activity was regained at 250350 °C. However, the activity did not return to the level of a fresh catalyst, and, overall, this method did not seem effective. The activity test showed that it is necessary to remove potassium more effectively. 3.4. Regeneration by H2SO4 Washing. Figure 9 shows the activities of the 1 wt % V2O5-WO3-TiO2 catalysts doped with a K/V ratio of 1 and subsequently regenerated by 0.5 M H2SO4. The regenerated catalyst showed a higher activity than the fresh one at temperatures higher than 300 °C. This is probably due to the promoting effects of surface sulfates and the removal of potassium by washing in H2SO4, whereby the V-OH groups are restored.29,30 Furthermore, sulfate species on the support have been shown to interact with adsorbed water to form Brønsted acid sites on the catalyst surface under SCR conditions, promoting the adsorption of ammonia. SEM/EDX analyses showed that practically all of the potassium was removed by washing in H2SO4 for 25 min. When doped catalysts were heat treated at 400 °C for 2 h prior to regeneration by 0.5 M H2SO4, similar encouraging results were obtained, as seen in Figure

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a PC boiler firing pulverized wood.3 However, the deactivation rate is still significantly higher than that for coal-fired plants. Our results (see section 3.2) indicate that the acceptable level of potassium in a catalyst does not necessarily increase with increasing vanadium content in the catalyst, and this method works only for low loading of potassium in the catalyst, while at higher load the catalyst may become active for the oxidation of NH3 to NO. After 600 h of exposure to the stream of the wood-fired plant, about 0.57-0.75 wt % potassium was already accumulated in the catalyst3 and as a result the relative activity of the biomodified catalyst was only 30%. This again indicates that increasing the vanadium content does not solve the problem satisfactorily. Figure 9. Catalytic activity of a 1 wt % V2O5-WO3-TiO2 catalyst as a function of temperature after doping with a K/V ratio of 1 by KCl, a catalyst further heat treated at 400 °C for 2 h, a catalyst without heat treatment regenerated by 0.5 M H2SO4, and a heattreated catalyst regenerated by 0.5 M H2SO4. The activity of the fresh catalyst is shown for reference.

Figure 10. Catalytic activity of a 1 wt % V2O5-WO3-TiO2 catalyst as a function of temperature after doping with a K/V ratio of 1 by KCl, a catalyst further heat treated at 400 °C for 2 h, a catalyst without heat treatment regenerated by 1 M NH4Cl, and a heat-treated catalyst regenerated by 1 M NH4Cl. The activity of the fresh catalyst is shown for reference.

9. These experiments may be closer to full-scale conditions, where the SCR plant runs at 300-400 °C while deactivation takes place. 3.5. Regeneration by NH4Cl Washing. Figure 10 shows results for regeneration of the catalyst with 1 M NH4Cl (pH ) 4.63). Doped catalysts without heat treatment regain higher activities than that of the fresh one in the whole temperature range. The activity of the catalyst heat treated for 2 h at 400 °C and regenerated by 1 M NH4Cl is similar to that of the fresh catalyst at temperatures below 300 °C but lower at higher temperatures, as shown in Figure 10. At 400 °C, the regenerated catalyst only regains about 70% activity of the fresh catalyst. 3.6. Discussion. One proposed solution to the fast deactivation of the SCR catalyst under biomass combustion is to increase the vanadium content in the catalyst compared to the conventional catalyst for coal- and oilfired plants. These catalysts are often referred to as biomodified catalysts. A full-scale test in Sweden shows that the deactivation rate of a biomodified catalyst was about 2/3 compared to the conventional catalyst in a circulating fluidized-bed boiler firing forest residues and

4. Conclusions A systematic laboratory investigation of the effects of KCl and K2SO4 compounds on commercial V2O5WO3-TiO2 catalyst plates is presented. Potassium in both the form of chlorides and sulfates was shown to be a strong poison for the catalyst. The results of NH3 chemisorption experiments show that the amount of NH3 chemisorbed on the catalyst decreases with an increasing amount of potassium, indicating that the Brønsted sites are chemically altered by the presence of potassium. Neither simply raising the operating temperature nor increasing the vanadium content of the catalyst may be an efficient way to solve the deactivation problem. Increasing the temperature hardly increases the conversion of NO for the strongly poisoned catalysts, and catalysts with a high vanadium content may become active for oxidizing NH3 to NO. Regeneration by washing in water followed by sulfation was not efficient as the regeneration method because of insufficient removal of the poison. The catalyst, with or without heat treatment, regenerated by 0.5 M H2SO4 showed a higher activity than the fresh catalyst at temperatures above 300 °C. Doped catalysts without heat treatment regained higher activities than that of the fresh one in the whole temperature range of 200-400 °C when regenerated by 1 M NH4Cl. The heat treatment step has a negative effect on the regeneration by NH4Cl. Acknowledgment This work is supported by the EU project CATDEACT (NNE5-2001-00164). Supply of the catalyst samples by Haldor Topsøe A/S is gratefully acknowledged. The authors thank Rasmus Fehrmann at the Department of Chemistry for the IR analysis. Nomenclature CNH3 ) gas-phase concentration of NH3 (kmol/m3) CNO ) gas-phase concentration of NO (kmol/m3) E ) activation energy (kJ/mol) Fgas ) gas flow rate (m3/s) k′ ) pseudo-first-order rate constant (m3/g‚s) k0′ ) preexponential factor in the Arrhenius equation (m3/ g‚s) KA ) adsorption equilibrium constant kr ) intrinsic chemical rate constant (m3/g‚s) mcat ) weight of catalyst (g) R ) universal gas constant (8.314 J/mol‚K) rNO ) NO reduction rate (kmol/g‚s)

Ind. Eng. Chem. Res., Vol. 43, No. 4, 2004 947 T ) temperature (K) X ) NO conversion (%)

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Received for review May 14, 2003 Revised manuscript received November 24, 2003 Accepted December 2, 2003 IE030404A