DieselNet.com. Copyright © Ecopoint Inc. Revision 2015.10
This is a preview of the paper, limited to some initial content. Full access requires DieselNet subscription.
Please log in to view the complete version of this paper.
Commercial deployment of urea-SCR systems depended on the development of not only the catalyst, but also the urea dosing and injection system. The increase in NOx conversion efficiency of SCR systems that has been seen since the launch of SCR technology on diesel engines around 2005 is largely owed to advances in SCR control and urea injection. The main functions of the urea dosing and injection system include:
The amount of injected urea must match the ammonia demand corresponding to the amount of NOx entering the catalyst and the NOx conversion efficiency at given operating conditions (catalyst temperature and space velocity). If the amount of ammonia is insufficient, a fraction of NOx that otherwise could be reduced will remain unconverted, resulting in a NOx conversion penalty. If the amount of injected ammonia is more than can be consumed in the SCR reactions, it will cause an unacceptably high ammonia slip. In systems that include an ammonia slip catalyst, some of the ammonia may be oxidized back to NO, thus decreasing the effective NOx conversion.
In order to maximize NOx conversion, the most sophisticated urea injection algorithms need to consider not only engine-out NOx emissions, but also the ammonia storage in the catalyst washcoat. NH3 stored in the washcoat—the amount of which changes dynamically, depending on the history of operating conditions—plays an important role in the overall reductant balance. As it is not easy to accurately model ammonia storage and release under transient conditions, SCR control is not a trivial task.
Even if the amount of injected urea matches the actual ammonia demand, uniform flow distribution and thorough mixing of urea/ammonia with exhaust gases must be ensured to achieve high NOx conversion. If the NH3/NOx ratio is not uniform, catalyst channels with insufficient ammonia will show poor NOx conversion while those with an excess of ammonia will be a source of ammonia slip. Flow maldistribution may be minimized by avoiding elbows or other flow disturbances and by providing a sufficient pipe length upstream of the SCR catalyst . In addition, static mixing devices  may be provided downstream, as well as upstream, of the urea injection point. In many SCR systems, especially those for heavy-duty engines, urea is mixed with compressed air before it enters the injection nozzle, in order to improve atomization . Regardless of the injection method, optimization of urea dosing and injection—in terms of accuracy and reproducibility, as well as mixing—is one of the most important areas where SCR performance improvement can be achieved.
The exhaust gas and the SCR catalyst temperature is another important parameter that affects the operation of the SCR system. Since the catalytic conversion of NOx decreases at decreasing temperatures, urea dosing must be reduced accordingly to prevent emissions of ammonia and other urea decomposition products. Another low temperature limit for urea injection—in addition to catalyst activity—is linked to fouling of the system by crystallization of solid urea, as well as by products of incomplete urea decomposition. In systems that use catalysts of high low temperature activity, the low temperature limit for urea dosing of urea is often set by the fouling considerations, not by catalyst activity. The low temperature cut-off point for urea injection is typically somewhere in the 200-250°C range. While this is sufficient to meet NOx emission limits over most regulatory test cycles, off-cycle emissions (such as during low speed urban driving) create pressures to inject urea at lower temperatures. In well designed systems, urea injection may be possible at temperatures as low as about 180°C. However, even if complete decomposition of urea is ensured, prolonged SCR operation at low temperatures may result in the accumulation of other solid deposits, such as ammonium nitrate and/or ammonium sulfate.
Cold temperature properties of urea solutions also need to be addressed in commercial SCR systems. Since the 32.5% urea solution used in SCR systems has a freezing temperature of -11°C—which is not acceptable for colder climates—the storage tanks, tubing, pumps, etc., need to be heated. When the vehicle is shut down, the dosing system components must be protected from damage due to ice expansion, either by purging from urea solution or through freeze proof design.
Future Trends. Urea-SCR technology is increasingly used in diesel engine applications, driven by the engine efficiency and fuel economy demands. The increasing NOx conversion efficiency of SCR systems may eventually enable the elimination of engine-based NOx control strategies—for example EGR—providing more flexibility in optimizing the engine for performance and fuel economy. The development trends in urea dosing systems include: