Gerhart, C., Krimmer, H., Hammer, B., Schulz, B. et al., "Development of a 3rd Generation SCR NH3-Direct Dosing System for Highly Efficient DeNOx," SAE Int. J. Engines 5(3):938-946, 2012, doi:10.4271/2012-01-1078.
In this project funded by the Bayerische Forschungsstiftung two fundamental investigations had been carried out: first a new N-rich liquid ammonia precursor solution based on guanidine salts had been completely characterized and secondly a new type of side-flow reactor for the controlled catalytic decomposition of aqueous NH₃ precursor to ammonia gas has been designed, applied and tested in a 3-liter passenger car diesel engine.Guanidine salts came into the focus due to the fact of a high nitrogen-content derivate of urea. Specially guanidinium formate has shown extraordinary solubility in water (more than 6 kg per 1 liter water at room temperature) and therefore a possible high ammonia potential per liter solution compared to the classical 32.5% aqueous urea solution (AUS32) standardized in ISO 22241 and known as DEF (diesel emission fluid), ARLA32 or AdBlue® . Additionally a guanidine-based formulation could be realized with high freezing stability down to almost -30°C (-11°C for AUS32). The decomposition of this new precursor to ammonia NH₃ could be realized on a gold-doped TiO₂ catalyst completely without any critical side products at temperatures above 240°C.Due to the fact of temperatures above 240°C required for the complete residue-free decomposition to ammonia gas, a side flow reactor concept has been developed for controlled decomposition of the precursor solution. In addition, this reactor concept could be operated with various liquid ammonia precursors such as AUS32 or aqueous guanidinium formate. In this heated catalytic reactor, the decomposition is realized under controlled conditions independent of the main exhaust flow and operation conditions of the engine. NH₃ gas is produced in real-time and directly dosed to the main exhaust flow without any buffer. With a venturi nozzle setup the homogeneous mixing of the NH₃ gas into the exhaust flow is realized. Measurements in steady state and transient cycles show an up to 60% more efficient NOx reduction (DeNOx) on a standard SCR catalyst in comparison to a classical urea to exhaust dosing system. Specially for exhaust conditions below 200°C much higher efficiencies could be achieved by direct NH₃ gas dosing than with liquid AUS32 dosing.