Modelling Urea Distributions in SCR-Exhaust Aftertreatment Systems. A Simple Approach

摘要

The upcoming emission standards EURO 4 and especially EURO 5 for heavy duty trucks cannot be fulfilled by enhancements of motor management and combustion alone. One possible solution to achieve these limits is to have a means of exhaust aftertreatment like selective catalytic reduction (SCR) systems, NO{sub}x storage reduction catalysts or diesel particulate filters. This article focuses on SCR systems. Here a reducing agent (usually an aqueous urea solution) is injected into the exhaust system and it decomposes to ammonia and carbon dioxide. Ammonia selectively reacts with NO{sub}x to produce nitrogen and water, despite the oxidizing atmosphere. In order to safeguard a high NO{sub}x conversion a highly homogeneous urea/ammonia distribution across the entrance of the catalytic bricks is necessary. A simple approach to determine the ammonia distribution is to perform virtual tracing experiments neglecting the physical properties of the air supported urea dosing system. A more complex model would consider the additional mass flow of air and urea and a more detailed modelling approach would take the properties of the urea spray and droplet tracking into account.