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Modelling of SCR in diesel exhaust systems with injection of urea spray is complex and challenging but many models use only the conversion observed at the brick exit as a test of the model. In this study, the case modelled is simplified by injecting ammonia gas in nitrogen in place of urea, but the spatial conversion profiles along the SCR brick length at steady state are investigated. This is a more rigorous way of assessing the ability of the model to simulate observations made on a test exhaust system. The data have been collected by repeated engine tests on eight different brick lengths, all which were shorter than a standard sized SCR. The tests have been carried out for supplied NH 3 /NOx ratios of α 1.5, excess ammonia, α 1.0, balanced ammonia, and α 0.5, deficient ammonia. Levels of NO, NO 2 and NH 3 have been measured both upstream and downstream of the SCR using a gas analyser fitted with ammonia scrubbers to give reliable NOx measurements. A CFD model based on the porous medium approach has incorporated a kinetic scheme available in the open literature. Comparison of CFD simulations with observed data is presented and the results are discussed. NOx conversion is significant in the first 30 mm of the brick for α 0.5 and in the first 90 mm for α 1.0 and 1.5. The ammonia level influences NOx conversion, which is generally under-predicted by the current model for α 0.5 and over-predicted for α 1.5. Measurements show that NO 2 conversion exceeds NO conversion in the first section of the monolith, which the model fails to predict. INTRODUCTION Selective catalytic reduction, SCR, with aqueous urea has become one of the primary methods of NOx reduction for both heavy and light-duty diesel applications over the past few years. The urea is thermally hydrolysed in the vehicle's exhaust to produce ammonia, which in turn reacts with the NOx over a catalyst. SCR technology is advantageous for light-duty applications as it has a minimal impact on fuel consumption and costs, yet has high NOx reduction efficiencies [1], and [2]. NOx reduction is affected by the exhaust gas temperature. For low gas temperatures the fast SCR reaction is the dominant reaction where approximately equi-molar quantities of NO and NO2 are present in the exhaust. The amount of NH3 injected requires careful control, and can be critical under some conditions. Over-injection of NH3 into the exhaust could potentially provide a high NOx conversion, but is likely to result in NH3 slip [3]. The addition of an NH3 oxidation catalyst downstream of the SCR is an option, but this may cause NO formation, which in turn will result in the vehicle not complying with emissions standards [4]. Underinjection of NH3 will avoid NH3 slip, but will provide an inferior NOx conversion. The control of the NO2: NOx ratio in the exhaust stream and the amount of NH3 injected into the exhaust at any one time are therefore very important. This can be investigated experimentally, but this is a time consuming process and is not cost effective [5]. Simulation modelling can be considered a useful tool in reducing development time and costs. Spatial Conversion Profiles within an SCR in a Test Exhaust System with Injection of Ammonia Gas Modelled in CFD using the Porous Medium Approach 2010-01-2089 Published 10/25/2010 M. P. Sturgess, S. F. Benjamin and C. A. Roberts Coventry University Copyright © 2010 SAE International THIS DOCUMENT IS PROTECTED BY U.S. AND INTERNATIONAL COPYRIGHT It may not be reproduced, stored in a retrieval system, distributed or transmitted, in whole or in part, in any form or by any means. Downloaded from SAE International by Coventry University, Wednesday, May 23, 2012 06:53:14 AM