Ammonia Preparation in the Tailpipe: Spray/Wall Interaction and Deposit Formation
Marion Börnhorst, Karlsruhe Institute of Technology (KIT), Institute of Chemical Technology and Polymer Chemistry (ITCP)
Selective Catalytic Reduction (SCR) is used in lean-burn engines to remove nitrogen oxides from the emitted products. Automotive diesel applications use AdBlue®, a 32.5 wt.-% urea-water solution (UWS) as a precursor for the reducing agent ammonia, which is released by chemical decomposition above 130°C. Due to highly transient conditions, incomplete evaporation of UWS spray may lead to the formation of liquid films at the tailpipe walls. Depending on temperature and residence time, solid intermediates and by-products tend to deposit from the liquid film.
Film and deposit formation are investigated at a hot gas test bench at different operating conditions. Deposits generated in the test bench are analyzed concerning chemical composition by TG (Thermogravimetry), HPLC (High Performance Liquid Chromatography) and NMR (Nuclear Magnetic Resonance) measurements. Kinetic data is used to develop a model for urea decomposition. Simulation of the decomposition process shows adequate agreement with experimental data. However, the decomposition mechanism is still not understood completely. Particularly the decomposition kinetics of high temperature products remain a challenge.
Spray/wall interaction is investigated in detail by infrared thermography and high-speed imaging techniques resulting in a comprehensive characterization of spray/wall interaction regarding hydrodynamic and thermal effects. These data represent a valuable basis for CFD modeling of the process.
This study investigates the different processes in the tailpipe leading to solid deposits and provides concepts for prediction and reduction of their formation.