Investigation of Urea Deposits in Urea SCR Systems for Medium and Heavy Duty Trucks 2010-01-1941

With increasing applications of urea SCR for NOx emission reduction, improving the system performance and durability has become a high priority. A typical urea SCR system includes a urea injector, injector housing, mixer, and appropriate pipe configurations to allow continuous urea injection into the exhaust stream and evaporation of urea solution into gaseous products. Continuous operation at various conditions with high NOx reduction is possible, but one problem that threatens the life and performance of these systems is urea deposit. When urea or its byproducts become deposited on the inner surfaces of the system including walls, mixers, injector housings and substrates it can create concerns of backpressure and material deteriorations. In addition, deposits as a waste of reagents can negatively affect engine operation, emissions performance and DEF economy.

Urea deposit behavior is explored in terms of heat transfer, pipe geometry, injector layout and mixing mechanisms. Two geometrical configurations of urea SCR systems are studied to evaluate heat transfer between urea and system and cooling of pipe wall where deposits can form. Multiple influencing factors such as wall temperature, urea injection rate, mixer and etc. are evaluated. Considerations on general urea test and mapping techniques are given. Necessary analytical predictions such as CFD spray modeling and theoretical formulas are performed and evaluated.