Investigation of urea aqueous solution injection, droplet breakup and urea decomposition of selective catalytic reduction systems

Urea aqueous solution is injected into the hot exhaust flow which acts as the source of ammonia in SCR systems to reduce the NOx emissions of diesel vehicles. However, urea decomposition thus ammonia generation is not always perfect, resulting in solid urea deposit formation accumulated in exhaust pipe wall, on the catalyst surface and mixing element. Decomposition of urea and mitigation of solid deposit formation is a great challenge to implement automotive SCR systems. This study presents a combined experimental and numerical investigation of UWS spray, droplet breakup, urea decomposition and deposit formation of SCR systems. The investigation firstly focused on fundamental spray characteristics of UWS and its effects on urea decomposition. The experiment was conducted with a commercial pressure driven SCR injector into an optically accessible test chamber and z-type shadowgraph imaging was implemented to capture the spray images. Relevant dimensions of spray images were estimated by digital image processing. The injection quantity of the experimental injector was measured to get the injection rate. Temperature distribution in the exhaust pipe and ammonia mass fraction was measured for different exhaust gas temperatures to estimate the urea decomposition. Finally, FTIR was used to analyze the chemical composition of solid deposit formed due to incomplete urea decomposition. The numerical assessment was carried out by using STAR CCM+ CFD code. The Eulerian-Lagrangian approach was implemented for the modeling of multiphase flow and urea water spray droplets. The investigation reveals that higher injection pressure increases the risk of wall impingement and wall wetting that leads to deposit formation. Though injection pressure is dominant on droplet size, exhaust gas temperature has also significant effects on droplet size and evaporation. Spray injection causes local cooling that effects on urea decomposition and relatively higher exhaust gas temperature is required for the complete decomposition of urea. Major chemical composition identified in the deposit sample are unreacted urea, biuret, and cyanuric acid.