Urea/SCR Technology for Removing NOx from Diesel Engine

In-Sik  Nam, POSTECH

Urea/SCR Technology for Removing NOx from Diesel Engine

In-Sik Nam

Department of Chemical Engineering/School of Environmental Science and Engineering,
Pohang University of Science and Techlogy(POSTECH), Pohang, Korea

ABSTRACT

The diesel engine is one of the most promising vehicle propulsion technologies for improving fuel economy while emitting low CO2 to the atmosphere. However, the engine exhaust emissions including nitrogen oxides (NOx) and particulate matter (PM) still remain a critical environmental issue. Particularly, the effective reduction of NOx from advanced diesel engines has become a primary concern in view of energy saving policy and ever-tightening environmental regulations for the recent automotive industry [1].

Among promising NOx reduction technologies currently available, the LNT and the urea/SCR technologies are the two leading ones despite their inherent shortcomings still remaining as important technical issues to be resolved [2-3]. A practical advantage of the urea/SCR technology over the LNT rests on the fact that it does not require the use of a large amount of noble metals and complex engine controls which is the essential requirement for the LNT technology [4]. The HC/SCR technology lags behind due primarily to its insufficient low-temperature activity and narrow temperature window on top of its poor tolerance toward H2O and SO2 [5].

The urea/SCR technology is undoubtedly one of the commercially proven and reliable technologies to meet the stringent worldwide environmental regulations including EURO V and SULEV for reducing NOx emissions from the diesel engine [1,6-7]. Most widely used catalysts for the urea/SCR are CuZSM5, FeZSM5 and V2O5/TiO2 catalysts [8-9]. However, their low-temperature activity is insufficient for application to the next generation vehicles equipped with advanced diesel engines. Hydrothermal stability along with sulfur tolerance of these catalysts is another issue for their commercial application to the aftertreatment system of the diesel engine [10-13]. When the urea/SCR reactor is installed right after the DOC+DPF system to prevent any negative effect of the particulate matter and hydrocarbons emission from the diesel engine [14], degradation of NH3/SCR activity due to the hydrocarbon (HC) slip from the DOC+DPF system has been reported [13-15].

In this presentation, the urea/SCR technology to remove NOx from the diesel engine will be reviewed, topics ranging from the underlying catalysis and reaction kinetics to the recent research trend and commercial applications. Included in the discussion will be bimetallic catalyst systems aimed at improving the low temperature activity of the current urea/SCR catalysts and their catalytic characteristics [16] as well as commercial urea/SCR systems such as BlueTec? developed by Mercedes-Benz [17]. The critical role of NO2 in the feed gas stream for the SCR reaction will be revisited in the light of its parametric significance for the emission control system [18]. Also presented will be our catalytic reaction kinetics model for the urea/SCR process deduced from the experimental data independently obtained for the NH3/SCR and the urea decomposition reactions over the CuZSM5 catalyst in order to design the urea/SCR system for its application to heavy-duty diesel engines [9, 19]. A monolith reactor model has been developed and numerically simulated using the kinetic parameters obtained from the kinetic study over a packed-bed flow reactor containing catalyst particles in the 20/30 mesh size [9]. The monolith reactor model in combination with the reaction kinetics has yielded reasonably accurate predictions of the reactor performance for the NOx reduction by the urea/SCR process. The model has proven to be capable of describing the effect of different reaction conditions on the NOx reduction efficiency, a critical issue for the commercial application of the urea/SCR process in the automotive diesel engine.

REFERENCES

[1] R.M. Heck and R.J. Farrauto, Catalytic Air Pollution Control: Commercial Technology, John Wiley & Sons, Inc., New York, 2002, Chap. 8.
[2] J.-H. Park, M.S. Han, S.J. Park, D.H. Kim, I.-S. Nam, G.K. Yeo, J.K. Kil, Y.K. Youn, J. Catal., 241 (2006) 470.
[3] S.J. Park, H.A. Ahn, I. Heo, I.-S. Nam, J.H. Lee, Y.K. Youn, H.J. Kim, Top. Catal, 53 (2010) 57.
[4] W.S. Epling, L.E. Campbell, A. Yezerets, N.W. Currier, E.P. James, Catal. Rev., 46 (2004) 163.
[5] E. Jobson, Top.Catal., 28 (2004) 191.
[6] H.J. Kwon, Y.J. Kim, I-S. Nam, H-J. Kim, G.K. Yeo, J.W. Choung, M.S. Cha, 2008 AIChE Annual Meeting, Philadelphia, PA. Nov. 16-21 (2008).
[7] J.H. Baik, S.D. Yim, I-S. Nam, Y.S. Mok, J-H. Lee, B.K. Cho, S.H. Oh, Top. Catal., 30/31 (2004) 37.
[8] R.Q. Long, R.T. Yang, Catal. Lett. 74 (2001) 201.
[9] J.H. Baik, S.D. Yim, I-S. Nam, Y.S. Mok, J-H. Lee, B.K. Cho, S.H. Oh, Ind. Eng. Chem. Res., 45 (2006) 5258.
[10] J.-H. Park, H.J. Park, J.H. Baik, I.-S. Nam, C.-H. Shin, J.-H. Lee, B.K. Cho, S.H. Oh, J. Catal. 240 (2006) 47.
[11] H.J. Kwon, Y. Kim, I.-S. Nam, S.M. Jung, J.-H. Lee, Top. Catal., 53 (2010) 439.
[12] B.-W. Soh, I.-S. Nam, Ind. Eng. Chem. Res., 42 (2003) 2975.
[13] J. Andersson, L. Eurenius, E. Olsson and M. Skoglundh, Appl. Catal. B: Environ., 72 (2007) 71.
[14] D. Chatterjee, T. Burkhardt, T. Rappe, A. G?thenke, M. Weibel, SAE Int. J. Fuels Lubr., 1 (2009) 440.
[15] I. Heo, Y. Lee, I.-S. Nam, J.W. Choung, J.-H Lee, H.-J. Kim, Micropor. Mesopor. Mat., in press. (doi:10.1016/j.micromeso.2010.02.005)
[16] Y.J. Kim, H.J. Kwon, I.-S. Nam, J.W. Choung, J.K. Kil, H.-J. Kim, M.-S. Cha, G.K. Yeo, Catal. Today, 151 (2010) 244.
[17] Green Car Congress, Energy, Tech., Issues and Polishes for Sustainable Mobility, 12 January 2006.
[18] J.H. Baik, J.H. Roh, S.D. Yim, I.-S. Nam, J.-H. Lee, B.K. Cho, S.H. Oh, Stud. Surf. Sci. Catal. 159 (2006) 441.
[19] S.D. Yim, S.J. Kim, J.H. Baik, I-S. Nam, Y.S. Mok, J-H. Lee, B.K. Cho, S.H. Oh, Ind. Eng. Chem. Res., 43 (2004) 4856.

Download Presentation: