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Selective Catalytic Reduction of NO x - and briefly about its deactivation Marie Louise Dahl Thomsen April 17, 2006
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Power Plants
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Overview Why DeNo x and SCR catalysts Why clean the flue gas SCR catalysts mechanisms Placement of SCR catalyst Deactivation caused by alkali metals, especially potassium Research results of deactivation Proposal of minimum deactivation
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Why DeNOx and SCR Catalysts? NO x is air polluting NO x is the sum of nitrogen oxides which are NO, N 2 O and NO 2 Typically 90-95% NO x -components in the flue gas will be in form of NO. The form N 2 O is hardly not there.
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Why clean the flue gas? Acid rain: 2 NO (g) + O 2 (g) → 2 NO 2 (g) 2 NO 2 (g) + H 2 O(l) ↔ HNO 3 (aq)+HNO 2 (aq) SO 2 + ½ O 2 → SO 3 SO 3 + H 2 O → H 2 SO 4
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Nitrogen oxides can react with the sun and become to ozone: NO 2 + UV-sunlight → NO + O NO 2 + UV-sunlight → NO + O O 2 + O → O 3 (ozone) O 2 + O → O 3 (ozone)
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Log Angeles, California Beijing, China Mexico City Smog = Ozone
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Daily cycle of pollutant concentration
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NO x is bad for our health It is observed that NO x gases weaken our immune defense by especially getting virus. NO x gases are also a reason to other illnesses as pneumonia and allergy. (Topsøe, 1997)
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Catalysts in general The most common definition of a catalyst is that a catalyst makes a reaction go faster without being used. Not always true → deactivation Most used SCR catalyst: V 2 O 5 -WO 3 -TiO 2 V 2 O 5 -WO 3 -TiO 2 Developed of the Japanese in 1977 (Topsøe, 1997, 1998)
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WO 3 has many advantages WO 3 makes the catalyst stronger WO 3 increases the active sites Forzatti et al, 1999
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SCR Catalysts Morsing et al, 2003
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Heterogenic catalyst Adsorptions mechanism Elay-Rideal mechanism Langmuir-Hinshelwood mechanism Jacobsen et al, 2002
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SCR catalyst reactions 6 NO + 4 NH 3 → 5 N 2 + 6 H 2 O 6 NO 2 + 8 NH 3 → 7 N 2 + 12 H 2 O O 2 makes the reaction faster 4 NO + 4 NH 3 + O 2 → 4 N 2 + 6 H 2 O No ammonia out so only add 80-90% NH 3 Bosch and Janssen, 1988
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Me=Vanadium or Tungsten Pritchard et al, 1995
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Site Nomenclature
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(1) NH 3 + V 5+ -OH ↔ V-ONH 4 (2) V-ONH 4 + V=O ↔ V-ONH 3 -V 4+ -OH (3) NO + V-ONH 3 -V 4+ -OH → N 2 + H 2 O + V 5+ -OH + V 4+ -OH (4) 2V 4+ -OH ↔ H 2 O + V 3+ + V=O (5) O 2 + 2V 3+ → 2V=O (6) H 2 O + V 5+ -OH ↔ V 5+ -OH 3 O [Dumesic et al, 1996] Proposed reaction mechanism Topsøe et al, 1997, 1998
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So far we know NH 3 adsorbs on Bronsted acid sites to give NH 4 species, and on Lewis sites to give coordinated NH 3 species NO does not adsorb on V 2 O 5 Each N 2 molecule contains one N from NO and one from NH 3 (Elay-Rideal mechanism)
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Common for SCR catalysts Works in temperatures between 300- 400C Need a high specific surface area (high porosity) (high porosity) Lose activity over time because of ex. poison, fouling or sintring. Need to be changed, because they deactivate
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Site Nomenclature for SCR placement AH = Air preHeater ESP = ElectroStatic Precipitator H-ESP = High temperature ESP FGD = Flue Gas Desulphurization GGH = Gas-Gas Heater SCR = Reactor for SCR
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Placement of SCR catalyst Soud and Fukasawa, 1996
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Chen et al, 1990 Alkali metals are among the strongest poisons. The strength of the poison follows the order of basicity: Cs 2 O > Rb 2 O > K 2 O > Na 2 O > Li 2 O
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Deactivation of SCR catalysts, caused by potassium Studstrupværket Cofiring of coal and straw After 2860 hours – The SCR catalyst deactivate with 35 % deactivate with 35 % Technical University of Denmark Flue gas with KCl After 1100 hours – The SCR catalyst deactivate with about 50% Yuanjing Zeng et al, 2005
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Regeneration Left: Deactivated SCR catalyst Right: Regenerated SCR catalyst http://www.envica.com/de/index.php
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Proposal: Inert layer may help The layer could be metal-oxides ex: Al 2 O 3, TiO 2 and ZrO 2
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Conclusion SCR catalysts remove NO x from flue gas We care about the environment SCR catalysts deactivate over time caused potassium and also alkali metals in general Still need research in SCR catalysts
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References Bosch, H., F. Janssen, “Catalytic Reduction of Nitrogen Oxides. A review on the Fundamentals and Technology”, Catal.Today, 2, 369 (1988) Chen, J.P., Yang, R.T.,”Mechanism of Poisoning the V 2 O 5 /TiO 2 Catalyst for the Reduction of NO by NH 3 ”, J.Catal. 125, 411-420 (1990) Christensen, K.A., M., Livbjerg, H., “The Combustion of Straw – Submicron Aerosol Particles and Gas Pollutants” J. Aerosol Sci., Vol. 26 suppl., pp s173-s174 (1995) Dumesic, J. A., Topsøe, N., Y., Topsøe, H., Chen, Y., Slabiak, T. “Kinetics of Selective Catalytic Reduction of Nitric Oxides by Ammonia over Vanadia/Titania”, J. Catal. 163, 409-417 (1996) Folkedahl, B.C., Zygarlicke, C.J., Gosnold, W.D., “Biomass Impacts on SCR Performance”, EERC Proposal No. 2002-0017 (2001) Forzatti, P. Lietti L., “Catalyst Deactivation” Catal. Today 52, 165-181 (1999)
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References Forzatti, P. Lietti L., “Recent Advances in De-NOxing Catalysis for Stationary Application” Heter. Chem. Rev., 3(1), 33 (1996) Huges, R., “Deactivation of Catalysts” Academic Press, 1984 Jacobsen, Claus J.H., Schmidt, Iver, Boisen, Astrid, Johannsen, Kim, “Katalytisk Kemi – Et Spørgsmål om miljø og Ressourcer” Haldor Topsøe A/S (2002) Morsing, P., Slabiak, T., “SCR DeNO x ”, Haldor Topsøe A/S, Denmark, November (2003) Pritchard, S., Difrancesco, C., Kaneko, S., Kobayashi, N., Suyama, K., Lida, K., “Optimizing SCR Catalyst Design and Performance for Coal- Fired Boilers”, Presented at EPA/ERPI 1995 Joint Symposium on Stationary Combustion Nox Control, May 16-19 (1995) Topsøe, Nan-Yu, “Catalysis for NO x abatement – Selective Catalytic Reduction of NO x by Ammonia. Fundament and Industrial aspects” pages 125-134, December (1997)
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References Topsøe, Nan-Yu, “Infrared Spectroscopic Investigations on Environmental DeNO x and Hydrotreating Catalyst”, The Haldor Topsøe Research Laboratories, Lyngby, Denmark (1998) Sloss, L.L., “NO x Emissions from Coal Combustion”, IEA Coal Research, London, UK (1991) Soud, H.N., Fukasawa, K., “Developments in NO x Abatement and Control”, IEACR/89, IEA Coal Research, London, UK (1996) Yuanjing Zeng, Jensen A.D., Johnsson J.E., “Deactivation of V 2 O 5 -WO 3 - TiO 2 SCR catalyst at a biomass-fired combined heat and power plant”, Technical University of Denmark, 2005 http://www.iea.org/Textbase/stats/oecdcountryresults.asp?oecd=Denmark &SubmitB=Submit http://www.iea.org/Textbase/stats/oecdcountryresults.asp?oecd=Denmark &SubmitB=Submit http://www.envica.com/de/index.php http://www.envica.com/de/index.php
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