Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines
Introduction
Forthcoming European legislation pertaining to heavy duty diesel engines aims at the simultaneous reduction of the emissions of particles and NOx. It is generally assumed that the EURO 4 emission standards proposed for the year 2005 will be no longer feasible by primary measures but will require additional exhaust gas after treatment techniques.
Urea-SCR, the selective catalytic reduction using urea as reducing agent, has been investigated in detail for about 10 years and today is a well established technique for DeNOx of stationary diesel engines [1], [2]. It is presently considered as the most promising way to diminish NOx emissions originating from heavy duty vehicles, especially trucks [3]. In the following we will discuss some challenges and selected problems that arise if the SCR process will be adopted to mobile diesel engines.
Section snippets
Main SCR reactions with ammonia
NOx in diesel exhaust is usually composed of >90% NO. Therefore, the main reaction of SCR with ammonia will beThis reaction implies a 1:1 stoichiometry for NH3 and NO and the consumption of some oxygen. The reaction consuming no oxygen is much slower and is therefore not relevant in lean combustion gases:On the other hand, the reaction rate with equimolar amounts of NO and NO2 is much faster than that of the main reaction (1):It should
Basic requirements for a mobile DeNOx system
A stationary diesel engine is operated at constant (usually full) load and there is no restriction concerning the size of the SCR catalyst. Therefore, the SCR catalyst is usually generously sized. This allows to attain very high values of NOx reduction (DeNOx) at very low ammonia slip. It should be emphasized here that a high DeNOx alone is of no practical value if the ammonia slip is high. High values of ammonia slip will result if the catalyst is not generously sized and if the stoichiometric
Reduced catalyst volumes of selective catalysts
Typical space velocities GHSV in three-way catalysts (spark-ignited engines) amount to ≈100 000 h−1. Correspondingly, the necessary catalyst volume will be of the order of the engine cylinder volume. On the other hand, typical space velocities in stationary SCR catalysts are only of the order of 10 000 h−1. The problem of necessary catalyst size is further aggravated by two unfavorable features of the diesel engine: (a) the widely varying exhaust gas temperatures and (b) the diesel process is
Outlook
Urea-SCR has high potential for the application in diesel driven vehicles. The main emphasis is presently put on the development of systems for heavy duty vehicles, especially trucks. In the case of diesel passenger cars the requirement of an additional reagent (urea) will hardly be accepted by the consumers and the average exhaust temperatures in a typical test cycle are so low that only modest reductions of NOx are feasible. In the case of the diesel passenger car the NOx storage-reduction
Acknowledgements
The financial support of the Swiss Federal Office of Energy (BEW) is gratefully acknowledged.
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