Computational Fluid Dynamics and Experimental Studies of the Urea-SCR System

Licentiatavhandling, 2008

Volatile emissions produced by the burning of fossil fuels may be converted on a catalyst to less harmful products. During lean combustion an excess of oxygen in the exhaust gases together with a catalyst effectively oxidizes carbon monoxide and hydrocarbons into carbon dioxide and water. This leaves a deficit of reductants to reduce nitrous oxides. In order to manage the conversion of nitrous oxides into nitrogen a commonly used technology is selective catalytic reduction (SCR). In the SCR system an extra reductant such as ammonia or hydrocarbons are added to complete the conversion of nitrous oxides over the after treatment catalyst. During the last few years SCR-systems for mobile applications using urea as an onboard source of ammonia have been developed and commercialized. Commonly a solution of urea and water (commercially known as AdBlue) is injected trough an atomizer upstream of the catalyst. However, several problems still exist with the urea-SCR system. During conversion of the urea into ammonia and isocyanic acid stable high molecular compounds that can accumulate within the exhaust gas system may form. The abundance of these by-products depend on the conditions under which urea is injected. In order to optimize spraying conditions new models and deeper understanding of the urea decompositions processes are needed. In this thesis Computational Fluid Dynamics (CFD) has been used to simulate the injection and decomposition of AdBlue for a urea-SCR application. The results from the simulations were found to be sensitive to which sub models that were used. Especially the amount of AdBlue hitting the wall was found to be strongly dependant on the modeling choices. Further, flow reactor experiments using Differential Scanning Calorimetery and gas phase Fourier Transformed Infrared spectroscopy (FT-IR) have been undertaken to study the pyrolysis of urea. Results show that the conditions under which urea is pyrolyzed governs which by-products are formed.