Exhaust aftertreatment

J. Dinesen, S.S. Nissen, and H. Christensen, Electrochemical Diesel Particulate Filter, SAE paper 980547, Diesel Exhaust Aftertreatment (SP-1313) 197-201 (1998). 

H. Christensen, J. Dinesen, H.H. Engell, and K.K. Hansen, Electrochemical Reactor for Exhaust Gas Purification, SAE paper 1999-01-0472, Diesel Exhaust Aftertreatment (SP-1414)225-229 (1999). 

Gulati, S.T., Makkee, M., and Setiabudi, A. (2006) Ceramic catalysts, supports and filters for diesel exhaust aftertreatment, in Structured Catalysts and Reactors, 2nd edn. Chapter 19 (eds A. Cybulski and J.A. Moulijn), CRC Taylor Francis, Boca Raton, p. 663. 

Konig A., Herding G., Hupfeld, B., et al. (2001) Current Tasks and Challenges for Exhaust Aftertreatment Research. A Viewpoint from the Automotive Industry, Topics Catal., 16/17, 23. 

This chapter discusses both the development of models and their application. One way of organising this chapter would be to discuss model development first and then go on to consider the applications. However, as the entire reason for developing these models is to have a practical tool for system design, it was decided to start with the application of the models. The next section discusses the physical model for a monolith reactor, which is common to all technologies (except diesel particulate filters) discussed later. Our approach to model development will then be covered in detail, using TWCs as an example. The final section will outline work done on the various technologies used for diesel exhaust aftertreatment. 

Different simulation tools are employed in the development of exhaust aftertreatment systems. Spatially 2D or 3D CFD simulation is commonly used to assess the effect of non-uniform flow conditions in pipes and housing and of non-uniform catalyst inlet flow on temperature distribution and reactions inside the monolith, thus enabling geometry optimization. 

The simulation of combined exhaust aftertreatment systems has also been undertaken by Wurzenberger and Peters (2003) and Wurzenberger and Wanker... 

Another engine cycle and gas exchange simulation software tool which has been extended for exhaust aftertreatment simulation is GT-POWER (2006). This software includes models for engine components as well as templates for DOC, SCR catalyst, NSRC and TWC. Reaction kinetics can be provided by the user, based on templates. Kinetic parameters adaptation is supported with a built-in optimizer tool. 

The hydraulic simulation tool AMESim (2006) has also been extended for exhaust aftertreatment simulation, by including routines developed together with IFP (2006). The software includes models for TWC, hydrocarbon (EIC) trap, NSRC, oxygen storage, DOC and DPF as well as pipes, etc. Catalysts are modeled via 0D approach, hence all transport effects are lumped into reaction kinetic parameters. These kinetic parameters can be adapted by the user. 

The aim of the present section is to illustrate the procedures employed for the derivation of dynamic kinetic models appropriate for simulation of exhaust aftertreatment devices according to the converter models illustrated in the previous section. In particular, it will be shown how to derive global reaction kinetics which are based on a fundamental study aimed at the elucidation of the reaction mechanism. In principle, this approach enables a greater model adherence to the real behavior of the reacting system, which should eventually afford better results when validating the model in a wide range of operating conditions, as typically required for automotive applications. 

Thus, the main function of the DOC is to oxidize CO and unburned HCs. The secondary function, utilized in combined exhaust aftertreatment systems, is the oxidation of NO to N02, which then enables optimum operation of the NOx aftertreatment catalysts placed down the exhaust line (NSRC and/or SCR, cf. Sections VI, VII and VIII, and also DPF). 

However, there are several major drawbacks that hinder practical application of this NOx reduction method in automobile exhaust aftertreatment (i) The NO reduction activity is typically limited to a certain temperature window, for NM-based catalysts it is around the light-off—cf. Fig. 14 and Ansell et al. (1996), Jirat et al. (1999b), Burch et al. (2002) and Joubert et al. (2006). (ii) With low HC concentrations and the exhaust composition met in modern diesel engines, the achieved NOx conversions in real driving cycles are quite low (typically around 5-10%, cf., e.g., Kryl et al, 2005). (iii) The selectivity of NOx reduction is problematic, N20 may form up to 50% of the product (Burch et al., 2002 Joubert et al., 2006). Alternative (Cu-, Co-, Ag-, etc., based) catalysts may provide a wider temperature window or better selectivity for... 

As discussed, the low temperature deNOx efficiency of SCR converters for automotive exhaust aftertreatment can be significantly enhanced by converting part of the nitric oxide to N02, e.g. by means of a DOC located upstream of the SCR. In fact, the so-called fast SCR reaction, involving the reaction between NH3 and equimolar amounts of NO and N02, can be faster by one order of magnitude than the standard SCR in the low-T region (Ciardelli et al., 2007a Koebel et al., 2001). Effective exploitation of fast SCR reactivity is certainly important... 

Exhaust emission legislation has become more and more stringent over the last years, demanding for lower engine raw emissions and more efficient exhaust converters. Simultaneous low emission limits for different species, e.g. PM and NOx, lead to the development of combined aftertreatment systems, consisting of different catalyst technologies and particulate filter. Simulation can make a considerable contribution to shorten the time and lower the cost of the system development. In this publication, the current status of exhaust aftertreatment simulation tools used in automotive industry is reviewed. The developed models for DOC with HC adsorption, NSRC and catalyst for SCR of NOx by NH3 (urea) were included into the common simulation environment ExACT, which enables simulation of complete combined exhaust aftertreatment systems. 

Amberntsson, A., Skoglundh, M., Jonsson, M., and Fridell, E. Catal. Today 73, 279 (2002). AMESim. http //www.amesim.com/applications/automotive/exhaust-aftertreat.aspx (2006). 

N., D Anna, A., D Alessio, A., Zahoransky, R., Laile, E., Schmidt, S., and Ranalli, M. The diesel exhaust aftertreatment (DEXA) cluster A systematic approach to diesel particulate emission control in Europe. SAE Technical Paper No. 2004-01-0694 (SP-1861) (2004). 

Peters, B. Integrated Id to 3d simulation workflow of exhaust aftertreatment devices. SAE Technical Paper No. 2004-01-1132 (2004). 

Koltsakis, G.C. and Stamatelos, A.M. (1997) Catalytic automotive exhaust aftertreatment. Prog. Ener. Combust. Sci., 23, 1. 

Ceramic Catalyst Supports and Filters for Diesel Exhaust Aftertreatment... 

Enabling of exhaust aftertreatment systems to be used on gasoline and diesel engines that will significantly reduce emissions of nitrogen oxides, carbon monoxide, hydrocarbons, and particulates. 

Figure 94. Emission of CO, HC and NO v at the outlet of a gasoline spark ignition engine as a function of the engine lamlida value, and range of the lambda value in which three different catalytic exhaust aftertreatment concepts for lean burn gasoline engines operate. Adapted from [60].

Exhaust gas catalysts has been widely used since the laimching of the 1970 Clean Air Act in the USA and especially after the introduction of stricter regulations in 1981. At present, one of the fastest growing areas of catalyst-based technology is automotive pollution control. All gasoline-fuelled vehicles sold in the USA, Japan and in the European Community must be equipped with exhaust aftertreatment in order to meet the emission standards. Oxidation catalysts for heavy-duty vehicles have only been used for a short period, but following the tightening emission standards there will be an increased demand for such systems. 

Exhaust aftertreatment generally consists of a filter or trap to capture the particulate and a regeneration system to convert it to less harmful materials Trap oxidizer prototype systems have shown themselves capable of 70 to 90 percent reductions from engine out particulate emissions rates and with proper regeneration the ability to achieve these rates for high mileage. Systems have now started to be introduced commercially. 

F Klingsredt, A Kalantar Neyestanaki, R Byggningsbacka, L-E Lindfors, M Lunden, M Petersson, P Tengstrom, T Ollonqvist, J Vayrynen, Palladium based catalysts for exhaust aftertreatment of natural gas powered vehicles and biofuels combustion, Appl. Catal. A General, 209 301 - 316,2001. 

This reduction for the most part can not be achieved via engine modifications alone making exhaust aftertreatment required. Generally, in the U.S. medium and heavy duty engines meet the standards for gas phase hydrocarbon, carbon monoxide and nitrogen oxides for MY 1994. 

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