SCR Control

The objective of urea-SCR control is to simultaneously minimize the tailpipe NOx ammonia emissions. Due to the nature of SCR dynamics, intuitively, increasing AdBlue injection rate can generally decrease SCR-out NOx emissions but also result in ammonia slip increases, and vice versa. To achieve low NOx and ammonia emissions simultaneously, specific control approaches need to be designed to handle the SCR nonlinear dynamics. 

With an adequate amount of ammonia being adsorbed on the SCR substrate, high NOx reduction rate can be realized. It is thus believed that consistent SCR NOx reduction can be ensured by ammonia storage control. However, besides the NOx reduction, tailpipe ammonia slip constraint is another objective needs to be taken into account. From the SCR model in Eq. (14.29) and the ammonia adsorption/desorption reactions in Eq. (14.4), it can be seen that high ammonia 

To verify this ammonia storage distribution control approach in practice, a two-catalyst SCR system is developed. A controller is designed to control the AdBlue injection such that the ammonia coverage ratio of the upstream catalyst is kept at a higher value and the ammonia coverage ratio of the downstream catalyst is limited under a lower level as presented in Fig. 14.13. 

---

The laboratory layout is sketched in Fig. 15.4 The power supply is a 3-phase silicon controlled rectifier, SCR, controlled supply capable of delivering 200 A at 50 V. 

Reheat coils are electric (multistage or SCR controlled) or hot water. 

The flue gas monitoring system consisted of analyzers and recorders for continuous monitoring of S02, C02, CO, NO, and O2 from either the combustor or EHE. A digital readout indicator provided the weight of MSW in the hopper. SCR controls were provided for the MSW screw feeder and the DSS feed pump. 

Preparation of V20s/Ti02 catalyst for NH3-SCR Controlling factor of Ti02 support for highly active catalyst... 

Ammonia slip t)T)ically occurs when overstoichiometric amoimts of urea/ammonia are injected. Due to the ammonia storage capacity of the catalyst this is not emitted directly so if an appropriate control action is taken, ammonia slip can be avoided. However, up to today an inexpensive and fast NOx and/or ammonia sensor is not available to accommodate such a control action. Solutions have been proposed in the use of engine maps for predicting the NOx output [9] and/or an additional oxidation catalyst for avoiding ammonia slip [3]. The latter is detrimental to the sulfate emissions and as most of the oxidized ammonia will leave the system as NO, the overall NOx removal efficiency will be lowered. Therefore, it would be convenient if a possible overdose of ammonia will leave the system in the form of nitrogen (N2)- This would solve the SCR control problem. 

Another way to do this, as also mentioned on page 102, is by fast on-off switching. This is done with SCR controllers, as will be described Chapter 21. 

Willems F, Cloudt R, van den Eijnden E, van Genderen M, Verbeek R, de Jaeger B, Boomsma W, van den Heuvel 1 (2007) Is closed-loop SCR control required to meet future emission targets SAE Technical Paper 2007-01-1574... 

Fig. 3.6 Principle layout of installation of an SCR system in a mobile application. (1) Engine (2) Exhaust pipe carrying the exhaust from the engine (5) Urea injection point in exhaust pipe (4) Urea mixing zone (5) Silencer containing the (6) SCR catalyst elements (7) Exhaust outlet (8) AdBlue (urea) tank (9) Urea pump/injector (70) Sensors for measuring exhaust conditions upstream the SCR catalyst (11) Sensors for measuring exhaust conditions downstream the SCR catalyst (12) Sensors for measuring conditions inside the urea tank (13) Ambient sensors (14) Engine model/Engine ECU (15) SCR model/SCR control unit...

According to the SCR control-oriented model shown in Eq. (14.29), the key states of an SCR catalyst are SCR inlet NO(x) concentration, SCR inlet NH3 concentration, exhaust flow rate, SCR catalyst temperature, SCR-outlet NO concentration, SCR-outlet NH3 concentration, and SCR catalyst ammonia coverage ratio. Current vehicle onboard sensors are capable of measuring gas flow rate, temperature, NOx concentration, and NH3 concentration. However, the current production NOx sensors are cross-sensitive to NH3, which make the accurate measurement of SCR-outlet NOx concentration difficult. Without the information of SCR-outlet NOx concentration, closed-loop SCR control is difficult, and so is the diagnostics of SCR NOx reduction capability. Moreover, the catalyst ammonia coverage ratio ( NHa) is also hard to be directly measured. Ammonia coverage ratio is an inherent state in the SCR catalyst which directly affects the catalytic reactions. This state is... 

The complete SCR control architecture for practical applications is summarized in Fig. 14.15, which includes the SCR ammonia coverage ratio observer proposed in Eq. (14.40) (observer 2) and the EKF NO sensor correction approach discussed previously. 

A schematic presentation and a picture of the experimental setup are shown in Figs. 14.16 and 14.17, respectively, which include a medium-duty Diesel engine, Diesel oxidation catalyst (DOC)/Diesel particulate filter (DPF), and two SCR catalysts in series at downstream of the DOC/DPF. For the emission measurement system, a Horiba MEXA 7500 gas analyzer was used to measure the tailpipe NO emissions. Three Siemens VDO (NGK) NO sensors and two Delphi ammonia sensors were used to provide feedback information to the SCR controller and to monitor the emission levels at different locations as shown in Figs. 14.16 and 14.17. 

The NOx sensors were calibrated with a Horiba gas analyzer up to 1500 PPM and the ammonia sensors were calibrated with a FTIR up to 500 PPM. Notice that this experimental setup is to validate the concept of controlling ammonia storage distribution, and multiple NOx and NH3 sensors are used. The number of sensors used for SCR control can be limited in real applications due to cost consideration. 

Hsieh M-F, Wang J (2010) An extended Kalman filter for ammonia coverage ratio and capacity estimations in the application of Diesel engine SCR control and onboard diagnostics. Proceedings of the 2010 American Control Corference, 5875—5879... 

Wang DY et al (2009) Ammonia sensor for close-loop SCR control, SAE 2008 World Congress, SAE International Journal of Passenger Cars- Electronic and Electrical Systems, l(l) 323-333... 

Microscopy. Thin films (1-2 mils) were pressed at 150 C using a Pasedena press fitted with West SCR controllers. Typically, a small sample was placed between Teflon coated aluminum foil sheets, preheated for 30 sec, and held at 25, 000 lb. gauge load for 5 min. Samples were then rapidly transferred to a cooling press and held at 25, 000 lb. gauge load for 5 min. 

Thyristor (SCR) control (see Fig. H-35) of three-phase power can be achieved with... 

---