Urea Dosing Control

Hannu Jääskeläinen, W. Addy Majewski

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Abstract: Early applications of urea-SCR technology on mobile diesel engines utilized open loop, or feedforward, control of urea dosing. To meet increasing demands for high NOx conversion efficiency and low NH3 slip, closed loop SCR control strategies were 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. As commercial applications of NH3 sensors become more widely available, model-based, adaptive SCR control strategies have been developed using an ammonia sensor.

Urea Dosing Control Strategies

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. 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.

To be available for the conversion of NOx to N2, ammonia must adsorb onto the SCR catalyst where it can then participate in the in the NOx reduction chemistry. 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 [3250].

[SVG image]
Figure 1. Effect of ammonia stored on the SCR catalyst on NOx conversion

ANR = Ammonia to NOx ratio

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 [2836]. 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. Typical limits for ammonia slip are 10 ppm average and 30 ppm maximum. 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.

[SVG image]
Figure 2. Efect of temperature on NH3 storage capacity for example vanadia and zeolite SCR catalysts

A number of urea dosing strategies are available. 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. Open loop strategies primarily control the rate of urea dosing based on values contained in look-up tables or maps. The look-up table may contain multiple variables including exhaust temperature, engine speed and engine load. 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. Many closed loop dosing systems use the signal from a NOx sensor after the SCR catalyst. Newer approaches are available that use an ammonia sensor located somewhere after the front section of the SCR catalyst. Virtual sensors, where a particular parameter (such as catalyst outlet NOx) is modelled based on the input from other sensors, are also used. 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.

Figure 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 NH3 sensor response as a function of urea dosing rate. SCR inlet NOx can be assumed to be held constant. As urea dosing rate increases, more ammonia is available to convert NOx to N2. This yields the increase in SCR efficiency. Post-SCR catalyst NH3 emissions (NH3 slip) start to increase as urea dosing rate increases and approaches the ideal stoichiometric NH3/NOx ratio (ANR). This leads to the increase in NH3 emissions as urea dosing rate increases. Post-SCR catalyst NOx also decreases yielding the dropping NOx signal. However, NOx sensors are cross-sensitive to NH3 emissions so the NOx sensor signal starts to increase again as NH3 slip increases [2573].

[SVG image]
Figure 3. Effect of urea dosing on SCR efficiency, NH3 slip and NOx sensor output

Superimposed on Figure 3 are some potential SCR efficiencies using open loop control and closed loop control using either a NOx sensor or a NH3 sensor. It should be noted that these potential conversion efficiencies represent only one view of this matter from the mid-2000s. The conditions under which these determinations were made are not clear and the reader should interpret this information qualitatively rather than quantitatively.

While the conversion efficiency of open loop urea dosing control can be well in excess of 90% under some test conditions, in practise it is prudent to limit open loop system conversion efficiency to about 60-70% because of the need to ensure ammonia slip does not become excessive as system performance drifts when the vehicle ages. Closed loop control with a NOx sensor allows in-use efficiencies in excess of 80%. However, challenges related to the cross sensitivity of the NOx sensor to ammonia limits application at higher efficiencies because of the difficulty in maintaining NH3 slip limits over the long term. Closed loop control with a NH3 sensor is able to directly control NH3 slip under some conditions and thus offers the potential for the highest efficiencies [2573][3251].

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