Urea injection

Operabihty (ie, pellet formation and avoidance of agglomeration and adhesion) during kiln pyrolysis of urea can be improved by low heat rates and peripheral speeds (105), sufficiently high wall temperatures (105,106), radiant heating (107), multiple urea injection ports (106), use of heat transfer fluids (106), recycling 60—90% of the cmde CA to the urea feed to the kilns (105), and prior formation of urea cyanurate (108). 

The development of an SCR system for vehicle applications requires precise calibration of the amount of urea injected as a function of the quantity of NO emitted by the engine, exhaust temperature and catalyst characteristics. Although model simulations can help in the control, it is necessary to use specific NO sensors which, however, still have problems of sensitivity and transient response. Installing a clean-up catalyst for ammonia would provide more latitude and obtain higher NO conversion ratios without re-emission of ammonia into the atmosphere. 

Figure 9.3. Ammonia, isocyanic acid and urea at various temperatures at the catalyst entrance. Residence time urea injection-catalyst entrance 0.09 s at 440°C.

Below 200°C, reliable urea thermohydrolysis is very hard to achieve, therefore urea dosage is usually stopped in real-world urea-SCR systems in this temperature regime. Another serious problem connected with the urea injection at low temperatures is the formation of white to yellowish deposits, which are observed when urea solution is injected at very low exhaust gas temperatures or if the urea spray forms a thick film at the walls of the SCR system. The analysis of these deposits [26] showed that they mainly consist of urea and some biuret at low temperatures and of cyanuric acid and some biuret at higher exhaust gas temperatures around 350°C. From laboratory investigations of the urea decomposition, it is known that biuret is easily formed from 150 to 190°C [27], whereas the formation of cyanuric acid is predominant from 200 to 300°C, according to the following reactions [12] ... 

They focus on the ID simulation of an urea SCR system. The system includes a model for N02 production on a DOC, a model for urea injection, urea decomposition and hydrolysis catalyst, a model for a vanadium-type SCR catalyst and a model for NH3 decomposition on a clean-up catalyst. The catalyst models consist of a ID monolith model with global kinetic reactions on the washcoat surface, kinetic parameters have been taken from literature or adjusted to experimental data from literature. The complete model was implemented in AVL BOOST (2006). AVL BOOST is an engine cycle and gas exchange simulation software tool, which allows for the building of a model of the entire engine. 

Another recent advancement involving chemists and design is the development of diesel emissions fluid (DEF) systems. Diesel emissions fluid is a 35% urea and 65% water mixture injected into the exhaust stream of a diesel vehicle to improve or reduce the NOx emissions. The urea injection works by decomposition of urea into ammonia when injected into the exhaust stream ... 

A lower temperature limit will be set at 300°C. Using urea injection at lower temperatures, polymerisation products can be formed. Furthermore, at these temperatures ammonium sulfate formation will block most SCR activity. The temperature range in which the catalyst will have to work is consequently from about 300°C to 550°C. 

The main techmcal problem for application of SCR in transient operation lies in the control of the amount of ammonia/urea injected as explained before. Consequently, attention must be paid to the anunonia oxidative capacity of the catalysts. As can be seen in Figure 6 and 7 Ce-MOR does not produce any NO diuing SCR operation or ammonia oxidation whereas the vanadium type catalyst (and also the Cu-MOR) start to produce NO at higher temperatures. 

T problems widi the control of the diesel exhaust using low sulfur fuel as a urea injection can be avoided when the function of NH3 concentration, 440°C... 

The efficiency of such a device is linked to the quantity of urea injected, which depends on the quantity of NOx produced. 

Fig. 20. Proposed simplified diesel exhaust after-treatment system (2010). A diesel oxidation catalyst, wall-flow filter, selective catal5dic reduction with urea injection, and an ammonia decomposition catalyst. All catalysts are deposited on monoliths.

The Thermal De-NOx process was developed by Richard Lyon at Exxon in the early 1970 s and patented in 1975 [1]. It is one of three SNCR (selective non-catalytic reduction) schemes for nitrogen oxides (the others are RAPRENOx, or cyanuric acid injection, and urea injection). Such after-treatment processes are commonly used on stationary combustion systems to control NOx emissions. The Thermal De-NOx process uses ammonia as the additive, and the complex reaction by which the ammonia reacts with nitric oxide has a number of fascinating properties that have prompted considerable research over the past 15 years or so. 

Urea injection control can be quite complex, but it is migrating from open-loop control based on engine operating parameters and engine-out NOx predictions, to closed-loop control based on NOx or urea and temperature sensors. Good urea control also needs to consider the ammonia stored on the catalyst. This will be discussed in detail later. 

Urea injection parameters are determined by NOx quantity in the exhaust (concentration and flow rate), temperature, and the amount of ammonia stored in the catalyst. There is normally closed-loop feedback control using an NOx sensor at the SCR exit, and in many applications, an NOx sensor is used upstream to determine inlet NOx levels. 

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...

In order to get the desired NOx conversion and avoid NH3 slip in the exhaust gases leaving the system, an appropriate urea injection rate is needed. Figure 3.7 shows the effect on NOx conversion and NH3 slip as a function of ammonia-NOx... 

Figure 3.7 shows a steady-state case. In real-world transient operation, the NOx flow varies continuously. This means that good control of the urea injection rate is needed in order to achieve high NOx conversions while avoiding NH3 slip. The urea injection rate is calculated by a control software, depicted as (15) in Fig. 3.6. The software/SCR model can make use of different sensors. Sensors (10) to determine the state of the exhaust gases to be treated (e.g., NOx flow and temperature), sensors (11) to quantify the outlet gas conditions, sensors (12) to determine the state and quality of the reductant (urea) and sensors (13) for determining ambient conditions. The sensors may be physical sensors or virtual sensors. Virtual sensors are models that calculate the sensor value based on a theoretical or empirical model, thus eliminating the need for a physical component. An example of result from a transient control taken from the ETC cycle on a Euro4 engine is shown in Fig. 3.8.

Fig. 3.8 Example of control of a Euro4 SCR system, a NOx concentration before SCR catalyst (gray) and resulting NOx concentration after SCR catalyst (black), b urea injection rate...

Gekas I, Gabrielsson P, Johansen K, Nyengaard L, Lund T (2002) Urea-SCR Catalyst System Selection fOTpuel and PM Optimized Engines and a Demonstration of a Novel Urea Injection System. SAE Technical Paper 2002-01-0289... 

In mobile apphcations, the SCR technique involves the atomization of aqueous solution of urea in the hot part of the exhaust system upstream the SCR catalyst. The urea injected undergoes two different chemical processes to produce the ammonia necessary to sustain the SCR reaction thermal decomposition and hydrolysis. 

A commercial application has been reported by Volkswagen for some EU6 vehicle applications of their new 1.6 and 2.0 1 TDI engines [72]. The system comprises a close coupled DOC followed in short distance by an SCR catalyst coated DPF. In addition, a slip catalyst with combined SCR and oxidation functionality is installed downstream of the DPF. The urea injection is located in-between the DOC and DPF components and a static mixer is used to improve distribution and evaporation within the limited space. Due to the high temperatures that can occur in this very close coupled position, a water-cooled urea injector was selected and the supply lines for the aqueous urea solution are made of highly temperature resistant materials. 

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