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Energy & Fuels 2006, 20, 1398-1405
Targeting by Comparison with Laboratory Experiments the SCR Catalyst Deactivation Process by Potassium and Zinc Salts in a Large-Scale Biomass Combustion Boiler Ann-Charlotte Larsson,† Jessica Einvall,† Arne Andersson,‡ and Mehri Sanati*,† School of Technology and Design, DiVision of Chemistry, Va¨xjo¨ UniVersity, SE-351 95 Va¨xjo¨, Sweden, and Department of Chemical Engineering, Lund UniVersity, P.O. Box 124, SE-221 00 Lund, Sweden ReceiVed February 21, 2006. ReVised Manuscript ReceiVed April 28, 2006
The deactivation of a commercial selective catalytic reduction (SCR) catalyst of type V2O5-WO3/TiO2 has been studied in this work through comparisons of results from a full-scale biomass combustion plant with those from laboratory experiments. In the latter, the catalyst was exposed to KCl, K2SO4, and ZnCl2 by both wet impregnation with diluted salt solutions and deposition of generated submicrometer aerosol particles by means of an electrostatic field. The reactivity of freshly prepared and deactivated catalyst samples was examined in the SCR reaction, for which the influence of the different salts and the method of exposure were explored. Chemical and physical characterizations of the catalyst samples were carried out focusing on surface area, pore volume, pore size, chemical composition, and the penetration profiles of potassium and zinc. Particledeposition deactivation as well as commercially exposed catalyst samples were shown to impact surface area and catalyst activity similarly and to have penetration profiles with pronounced peaks. Salt impregnation influenced pore sizes and catalyst activity more strongly and showed flat penetration profiles. Deposition of submicrometer-sized particles on the monolithic SCR catalyst has been shown to induce deactivation of the catalyst with characteristics resembling those obtained in a commercial biomass combustion plant; the laboratory process can be used to further assess the deactivation mechanism by biomass combustion.
1. Introduction Selective catalytic reduction (SCR) is commonly used for the reduction of NOx emitted from biomass combustion plants. A V2O5-WO3/TiO2 monolithic type of catalyst is most often used. However, in commercial operation, the catalyst deactivates because of the deposition of compounds that are present in the flue gas. As a consequence of the deactivation, the lifetime of the catalyst is reduced. This is a drawback from both an environmental and economical view. The deactivation of SCR catalysts under biomass combustion conditions has been ascribed to the potassium salts1 that are present in the fly ash in the form of submicrometer particles with a size of around 100 nm2. The particles have been shown to consist of alkali salts, mainly potassium sulfates and chlorides,3,4 which are deactivation agents for not only SCR catalysts but also oxidation catalysts.5 Previous laboratory studies of potassium deactivation of SCR catalysts have been conducted using wet impregnation techniques for the deposition of the salt on the catalyst.6-12 In these
studies, in addition to alkali metals, alkaline earth metals as well as arsenic and zinc chloride were pointed out as deactivating agents. Of the compounds investigated, the alkali metal oxides were identified as the strongest poisons to the catalyst. The degree of the poisoning was directly related to the basicity of the metals in the following order: Cs2O > Rb2O > K2O > Na2O > Li2O.6,7 The different salts were also compared, and the order of the poisoning effect in the absence of SO2 was found to be KCl > NaCl > K2SO4 > Na2SO4.6,7 In this study, a commercial V2O5-WO3/TiO2 monolithic catalyst was deactivated in different ways. The approach followed consisted of laboratory-scale experiments in which the SCR catalyst was either exposed to or impregnated with potassium salts as well as zinc chloride. Finally, the laboratoryscale results were compared with those obtained for a deactivated commercial SCR catalyst, which is used in a biomass combustion boiler. To obtain information about the deactivation modes, we characterized the catalysts both physically and chemically.
* To whom correspondence should be addressed. E-mail: mehri.sanati@ vxu.se. Fax: 46-470-708756. Phone: +46-470-708943. † Va ¨ xjo¨ University. ‡ Lund University. (1) Khodayari, R.; Odenbrand, C. U. I. Appl. Catal., B 2001, 33, 277291. (2) Pagels, J.; Strand, M.; Rissler, J.; Szpila, A.; Gudmundsson, A.; Bohgard, M.; Lillieblad, L.; Sanati, M.; Swietlicki, E. Aerosol Sci. 2003, 34, 1043-1059. (3) Strand, M.; Pagels, J.; Szpila, A.; Gudmundsson, A.; Swietlicki, E.; Bohgard, M.; Sanati, M. Energy Fuels 2002, 16, 1499-1506. (4) Wierzbicka, A.; Lillieblad, L.; Pagels, J.; Strand, M.; Gudmundsson, A.; Gharibi, A.; Swietlicki, E.; Sanati, M.; Bohgard, M. Atmos. EnViron. 2005, 39, 139-150. (5) Moradi, F.; Brandin, J.; Sohrabi, M.; Faghihi, M.; Sanati, M. Appl. Catal., B 2003, 46, 65-76.
(6) Chen, J. P.; Buzanowski, M. A.; Yang, R. T.; Cichanowicz, J. E. J. Air Waste Manage. Assoc. 1990, 40, 1403-1409. (7) Chen, J. P.; Yang, R. T. J. Catal. 1990, 125, 411-420. (8) Lisi, L.; Lasorella, G.; Malloggi, S.; Russo, G. Appl. Catal., B 2004, 50, 251-258. (9) Kamata, H.; Takahashi, K.; Odenbrand, C. U. I. J. Mol. Catal. A: Chem. 1999, 139, 189-198. (10) Khodayari, R. Selective Catalytic Reduction of NOx: Deactivation and Regeneration Studies and Kinetic Modelling of Deactivation. Doctorial Dissertation. Lund Institute of Technology, Lund, Sweden, 2001; ISBN 917874-122-X (11) Zheng, Y.; Jensen, A. D.; Johnsson, J. E. Ind. Eng. Chem. Res. 2004, 43, 941-947. (12) Herrlander, B. Presented at the 83rd Annual Meeting and Exhibition of the Air and Waste Management Association, June 24-29, 1990, Pittsburgh, PA.
10.1021/ef060077u CCC: $33.50 © 2006 American Chemical Society Published on Web 05/28/2006
SCR Catalyst DeactiVation by K and Zn Salts
Energy & Fuels, Vol. 20, No. 4, 2006 1399
Figure 1. Experimental setup of the aerosol particle deposition. Table 1. Operating Conditions of the Commercial Biomass Combustion Planta
2. Experimental Section 2.1. Catalyst. A commercial catalyst of type V2O5-WO3/TiO2 was used in the deactivation experiments. The catalyst, supplied by Cormtech Inc., was of a monolithic honeycomb type with a vanadia content of 1 wt %. All samples used had a square cell opening of 6.4 mm, a wall thickness of 1 mm, and a sample length of 160 mm. 2.2. Deactivation Species. The most abundant compounds found in the submicrometer particles in the flue gas from biomass burning, KCl, K2SO4, and ZnCl2, were used as model compounds for the study of the deactivating process. 2.3. Deactivation by Exposure of the SCR Catalyst to Generated Potassium and Zinc Salt Particles in LaboratoryScale Experiments. The deposition of aerosol particles was carried out in the set up shown in Figure 1. The catalyst samples consisted of 1 monolithic channel with a length of 160 mm. The aerosol salt particles were generated by a pneumatic atomizer (AGK-2000). A liquid solution of deionized water and the respective salt with a concentration of 50 g/L was fed continuously into the atomizer reservoir. The gauge pressure was 1 bar. To decrease the relative humidity and avoid the growth of particles by condensation of water, we mixed the flow from the atomizer (4 L/min) at 20 °C with particle-free, dried, pressurized air (7 L/min). The gas mixture was then passed through an inertial impactor to separate large solution droplets before entering an electrical oven set at 300 °C. Here, the gas flow through the catalyst channel was regulated to 1 L/min. To enhance the particle deposition rate, we applied an electrostatic field to the catalyst channels.14 Because of heat losses, the average temperature in the catalyst channels was 200 °C, as measured with a Microtherma 2 thermometer. The relative humidity, measured with a TH-Calc 8720 from TSI, was less than 1% at 200 °C. According to Hinds,13 the drying time of the generated particles at these conditions is in the range of milliseconds, with the residence time of the gas in the oven being seconds. Therefore, with the experimental conditions used, the generated particles can be considered to be completely dry, resulting in deposition of the salt in the form of solid particles on the catalyst. The generated particle flow was physically characterized using the setup in Figure 1. The sampling was carried out isokinetically using a glass sampling probe. To decrease the gas temperature, particle concentration, and relative humidity, we diluted the gas sample with particle-free, dry, pressurized air (20 °C) to give a (13) Hinds, W. C. Aerosol Technology: Properties, BehaVior, and Measurements of Airborne Particles; John Wiley & Sons: New York, 1999. (14) Larsson, A.-C.; Einvall, J.; Sanati, M. Aerosol Sci. Technol. 2005, submitted.
T (°C) normal gas-flow rate mn3/h O2 concentration (vol %) H2O concentration (vol %) NOx concentration (ppm by volume) SO2 concentration (ppm by volume) a
350 170 000 3 20 150
