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