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Urea Decomposition at Low Temperature in SCR Systems for Diesel Engines
ISSN: 0148-7191, e-ISSN: 2688-3627
Published October 13, 2014 by SAE International in United States
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Selective catalytic reduction (SCR) has been demonstrated as one of the most promising technologies to reduce NOx emissions from heavy-duty diesel engines. To meet the Euro VI regulations, the SCR system should achieve high NOx reduction efficiency even at low temperature. In the SCR system, NH3 is usually supplied by the injection of urea water solution (UWS), therefore it is important to improve the evaporation and decomposition efficiency of UWS at low temperature and minimize urea deposits.
In this study, the UWS spray, urea decomposition, and the UWS impingement on pipe wall at low temperature were investigated based on an engine test bench and computational fluid dynamics (CFD) code. The decomposition of urea and deposits was analyzed using thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and fourier transform infrared spectroscopy (FTIR).
The TGA experiment shows that urea decomposition started at about 150°C, and exhibited two stages of rapid mass loss. The off-gas in each stage was mainly NH3 and HNCO respectively.
In tailpipes the decomposition rate of the injected urea was significantly influenced by the exhaust temperature. At 185°C about 20% urea decomposed and produced NH3. The UWS wall film had formed at low temperature and resulted in urea deposits. The TGA and DSC experiments showed that most of the deposits formed and sampled below 180°C were urea crystal, and the urea deposits in the tailpipes could decompose gradually at high exhaust temperatures. The deposits formed after several WHTC tests at the inlet of the SCR catalyst was also sampled and analyzed by TGA and DSC, which proposed to be cyanuric (CYA) acid, and started to decompose at about 300°C.
CitationTang, T., Zhang, J., Shuai, S., and Cao, D., "Urea Decomposition at Low Temperature in SCR Systems for Diesel Engines," SAE Technical Paper 2014-01-2808, 2014, https://doi.org/10.4271/2014-01-2808.
- Wang , J. X. , Fu , L. X. , Li , W. B. Automotive Emission Control and Catalytic Converter Chemical Engineering Press Beijing 2000
- Held , W. , König , A. , Richter , T. , and Puppe , L. Catalytic NOx Reduction in Net Oxidizing Exhaust Gas SAE Technical Paper 900496 1990 10.4271/900496
- Koebel , M. , Elsener , M. , Kleemann , M. Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines Catalysis Today 59 3-4 335 345 2000 10.1016/S0920-5861(00)00299-6
- Fang , Howard L. , and Herbert , F. M. DaCosta Urea Thermolysis and NOx Reduction with and without SCR Catalysts Applied Catalysis B: Environmental 46 1 17 34 2003 10.1016/S0926-3373(03)00177-2
- Yim , S.D. , Kim , S.J. , Baik , J.H. , Nam , I.-S. et al. Decomposition of Urea into NH3 for the SCR Process Industrial & Engineering Chemistry Research 43 4856 4863 2004 10.1021/ie034052j
- Kleemann , M. , Elsener M. , Koebel M. , and Wokaun A. Hydrolysis of Isocyanic Acid On SCR Catalysts Industrial & Engineering Chemistry Research 39 11 4120 26 2000 10.1021/ie9906161
- Dong , H. , Shuai , S. , and Wang , J. Effect of Urea Thermal Decomposition on Diesel NOx-SCR Aftertreatment Systems SAE Technical Paper 2008-01-1544 2008 10.4271/2008-01-1544
- Schaber , P.M. , Colson , J. , Higgins , S. , Thielen , D. et al. Thermal decomposition (pyrolysis) of urea in an open reaction vessel Thermochimica Acta 424 131 142 2004 10.1016/j.tca.2004.05.018
- Xu , L. , Watkins , W. , Snow , R. , Graham , G. et al. Laboratory and Engine Study of Urea-Related Deposits in Diesel Urea-SCR After-Treatment Systems SAE Technical Paper 2007-01-1582 2007 10.4271/2007-01-1582
- Ebrahimian , V. , Nicolle , A. , and Habchi , C. Detailed Modeling of the Evaporation and Thermal Decomposition of Urea-Water Solution in SCR Systems AIChE Journal 58 7 1998 2009 2012 10.1002/aic.12736
- Smith , H. , Lauer , T. , Mayer , M. , and Pierson , S. Optical and Numerical Investigations on the Mechanisms of Deposit Formation in SCR Systems SAE Int. J. Fuels Lubr. 7 2 525 542 2014 10.4271/2014-01-1563
- Birkhold , F. Selektive katalytische Reduktion von Stickoxiden in Kraftfahrzeugen: Untersuchung der Einspritzung von Harnstoffwasserlösung Dissertation Universität Karlsruhe 2007
- Birkhold , F. , Meingast , U. , Wassermann , P. , and Deutschmann , O. Analysis of the Injection of Urea- Water-Solution for Automotive SCR DeNOx-Systems: Modeling of Two-Phase Flow and Spray/Wall-Interaction SAE Technical Paper 2006-01-0643 2006 10.4271/2006-01-0643
- Anders , K. , Roth , N. , Frohn , A. The velocity change of ethanol droplets during collision with a wall analyzed by image processing Experiments in Fluids 15 91 96 1993 10.1007/BF00190948
- Hua , L. , Zhao , Y. , Hu , J. , Tao , T. et al. Comparison Between Air-Assisted and Airless Urea Spray for Diesel SCR System by PDA and CFD SAE Technical Paper 2012-01-1081 2012 10.4271/2012-01-1081
- Zhao , Y. Experimental Study of Urea Solution Spray and Decomposition and Ammonia Storage in Selective Catalytic Reduction System for Diesel Engines Doctoral Thesis of Tsinghua University Beijing 2012