Development of a 3rd Generation SCR NH₃-Direct Dosing System for Highly Efficient DeNOx

In this project funded by the Bayerische Forschungsstiftung twofundamental investigations had been carried out: first a new N-richliquid ammonia precursor solution based on guanidine salts had beencompletely characterized and secondly a new type of side-flowreactor for the controlled catalytic decomposition of aqueous NH₃precursor to ammonia gas has been designed, applied and tested in a3-liter passenger car diesel engine. Guanidine salts came into the focus due to the fact of a highnitrogen-content derivate of urea. Specially guanidinium formatehas shown extraordinary solubility in water (more than 6 kg per 1liter water at room temperature) and therefore a possible highammonia potential per liter solution compared to the classical32.5% aqueous urea solution (AUS32) standardized in ISO 22241 andknown as DEF (diesel emission fluid), ARLA32 or AdBlue® .Additionally a guanidine-based formulation could be realized withhigh freezing stability down to almost -30°C (-11°C for AUS32). Thedecomposition of this new precursor to ammonia NH₃ could berealized on a gold-doped TiO₂ catalyst completely without anycritical side products at temperatures above 240°C. Due to the fact of temperatures above 240°C required for thecomplete residue-free decomposition to ammonia gas, a side flowreactor concept has been developed for controlled decomposition ofthe precursor solution. In addition, this reactor concept could beoperated with various liquid ammonia precursors such as AUS32 oraqueous guanidinium formate. In this heated catalytic reactor, thedecomposition is realized under controlled conditions independentof the main exhaust flow and operation conditions of the engine.NH₃ gas is produced in real-time and directly dosed to the mainexhaust flow without any buffer. With a venturi nozzle setup thehomogeneous mixing of the NH₃ gas into the exhaust flow isrealized. Measurements in steady state and transient cycles show anup to 60% more efficient NOx reduction (DeNOx) on a standard SCRcatalyst in comparison to a classical urea to exhaust dosingsystem. Specially for exhaust conditions below 200°C much higherefficiencies could be achieved by direct NH₃ gas dosing than withliquid AUS32 dosing.