Investigation of urea deposit formation in vehicles with Selective Catalytic Reduction system

Abstract

In order to reduce NOX emissions from a diesel engine a urea-water solution called AdBlue is injected into the exhaust system. Risk of deposit formation is high due to unwanted side reactions that may occur during the decomposition of AdBlue during transient engine operation. The objective of the thesis is to increase the knowledge about the formation and decomposition of urea deposits and to develop an experimental method for studying these phenomena. The knowledge will form a base for a kinetic model for deposit formation and decomposition. A literature review was performed to identify unwanted side-reactions that may occur when urea is injected and what parameters influence the formation of deposits. In order to test how these parameters influence the rate of deposit formation, a test set-up was designed for engine bench tests rig at Volvo Cars. The tests were carried out with consideration to critical parameters and limitations of the test rig. Urea deposits were detected at all operation points, even at temperatures as high as 450˚C. All deposits formed during the tests could be decomposed when dosing was stopped and the temperature was increased above 450˚C. A correlation between theoretical calculations, i.e. a heat balance over the wetted area, and experimental results could be seen. The effect of energy transfer can be seen in the experiments as a temperature drop on the surface where the spray hits. The build-up and break-down have been captured on film and shows a non-linear rate. It is to some extent possible to predict if there will be deposits in the system by a theoretical comparison of the available energy in the exhaust gases to the energy required to decompose the urea-water solution. One parameter that has a great influence on the energy calculations is the area that the liquid spray hits, i.e. the area were the main heat transfer occurs. If this surface can be increased the amount of urea-water solution that can be decomposed into ammonia will increase. A better surface distribution and spray dispersion will provide better control of the amount of liquid that will reach the surface and a better base for predicting deposit formation with energy calculations.