Post-SCR catalyst NOx also decreases yielding the dropping NOx signal. This leads to the increase in NH 3 emissions as urea dosing rate increases. Post-SCR catalyst NH 3 emissions (NH 3 slip) start to increase as urea dosing rate increases and approaches the ideal stoichiometric NH 3/NOx ratio ( ANR). This yields the increase in SCR efficiency. As urea dosing rate increases, more ammonia is available to convert NOx to N 2. SCR inlet NOx can be assumed to be held constant. For urea dosing, system dynamics make pure closed loop control very difficult and most closed loop strategies rely heavily on an open loop strategy to provide feedforward control.įigure 3 shows typical trends in SCR efficiency (note the reversed order of efficiency values on the left y-axis), post-SCR NOx sensor response and post-SCR NH 3 sensor response as a function of urea dosing rate. Virtual sensors, where a particular parameter (such as catalyst outlet NOx) is modeled based on the input from other sensors, are also used.
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Newer approaches are available that use an ammonia sensor located somewhere after the front section of the SCR catalyst. Many closed loop dosing systems use the signal from a NOx sensor after the SCR catalyst. Closed loop control strategies use a sensor to provide feedback and are thus able to adjust urea dosing to more accurately reflect operating conditions and to account for long term drift. The look-up table may contain multiple variables including exhaust temperature, engine speed and engine load. Open loop strategies primarily control the rate of urea dosing based on values contained in look-up tables or maps. The choice of strategy depends on a number of factors including: the SCR catalyst NOx conversion required, the allowable ammonia slip limits, drive cycle effects and the requirements for long term system robustness. Effect of temperature on NH 3 storage capacity for example vanadia and zeolite SCR catalystsĪ number of urea dosing strategies are available. In practical terms then, the urea dosing control problem then becomes one of storing sufficient ammonia on the SCR catalyst to achieve the required emission targets while limiting ammonia slip to the required limits. Typical limits for ammonia slip are 10 ppm average and 30 ppm maximum. The more ammonia that is stored on the catalyst at low catalyst temperatures, the more that would be released when ammonia slip conditions are encountered. However, under conditions such as increasing catalyst temperature (Figure 2), ammonia can be desorbed from the catalyst and result in the release of unreacted ammonia-ammonia slip. Effect of ammonia stored on the SCR catalyst on NOx conversion
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While there are numerous factors that impact SCR conversion efficiency, ammonia storage on the SCR catalyst is one important one that can influenced by the urea dosing control system the more ammonia that is stored on the catalyst, the higher the NOx conversion, Figure 1. To be available for the conversion of NOx to N 2, ammonia must adsorb onto the SCR catalyst where it can then participate in the in the NOx reduction chemistry. The ultimate objective of urea dosing control then is to lower tailpipe NOx emissions sufficiently to meet regulatory limits over the required test cycles and to meet the additional requirements of low ammonia slip and urea consumption. While very high conversion efficiencies with urea-SCR NOx aftertreatment systems are possible using simple dosing approaches, the combination of high conversion efficiency, minimum urea consumption and minimum ammonia slip is much more difficult to achieve. Model-based, adaptive SCR control strategies using an ammonia sensor have also been developed. In a common approach, a number of control strategies have been using two NOx sensors-one positioned upstream, the other downstream of the SCR catalyst.
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To meet increasing demands for high NOx conversion efficiency and low NH 3 slip, closed loop SCR control strategies were developed. Abstract: Early applications of urea-SCR technology on mobile diesel engines utilized open loop, or feedforward, control of urea dosing.