A Study on the Factors Affecting the Formation of Urea Crystals and Its Mitigation for SCR After-Treatment Systems

Selective Catalytic Reduction has established itself to significantly reduce NOx emissions from diesel engines. Typically, in this technology, aqueous urea solution is injected into hot exhaust stream which chemically decomposes to form ammonia and then reacts with NOx to form safe byproducts as H₂O and N₂ over the catalyst surface. However, incomplete thermal decomposition of urea not only reduces the NOx conversion efficiency and increases the ammonia slip, but also leads to the formation of solid crystals that adversely affect the performance of the system by increasing the back pressure and lowering the overall fuel economy.

The present study discusses about the main reasons that lead to crystal formation in a vanadium based SCR system on a six cylinder 5.6l diesel engine and also design considerations of decomposition tube that affect the formation of crystals and ways to mitigate them. CFD analysis of decomposition tube with different dozing angles and its effect on urea mixing and crystal formation was studied using both simulations and experiments. A test cycle was formulated in order to simulate on road driving conditions in both city and highway scenario on test bench. The SCR system was validated through this test cycle on test bench to arrive at a crystal deposition free solution.