Selective Catalytic Reduction of NO x - and briefly about its deactivation Marie Louise Dahl Thomsen April 17, 2006

Power Plants

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

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.

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

 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)

Log Angeles, California Beijing, China Mexico City Smog = Ozone

Daily cycle of pollutant concentration

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)

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)

WO 3 has many advantages  WO 3 makes the catalyst stronger  WO 3 increases the active sites Forzatti et al, 1999

SCR Catalysts Morsing et al, 2003

Heterogenic catalyst  Adsorptions mechanism  Elay-Rideal mechanism  Langmuir-Hinshelwood mechanism Jacobsen et al, 2002

SCR catalyst reactions  6 NO + 4 NH 3 → 5 N H 2 O  6 NO NH 3 → 7 N H 2 O  O 2 makes the reaction faster  4 NO + 4 NH 3 + O 2 → 4 N H 2 O  No ammonia out so only add 80-90% NH 3 Bosch and Janssen, 1988

Me=Vanadium or Tungsten Pritchard et al, 1995

Site Nomenclature

(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

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)

Common for SCR catalysts  Works in temperatures between C  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

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

Placement of SCR catalyst Soud and Fukasawa, 1996

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

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

Regeneration Left: Deactivated SCR catalyst Right: Regenerated SCR catalyst

Proposal: Inert layer may help The layer could be metal-oxides ex: Al 2 O 3, TiO 2 and ZrO 2

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

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, (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, (1996)  Folkedahl, B.C., Zygarlicke, C.J., Gosnold, W.D., “Biomass Impacts on SCR Performance”, EERC Proposal No (2001)  Forzatti, P. Lietti L., “Catalyst Deactivation” Catal. Today 52, (1999)

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 (1995)  Topsøe, Nan-Yu, “Catalysis for NO x abatement – Selective Catalytic Reduction of NO x by Ammonia. Fundament and Industrial aspects” pages , December (1997)

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   &SubmitB=Submit &SubmitB=Submit  