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  • 18-Jun-2012 12:19 EDT

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


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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 (figure 1). 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 precursor 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.

Chrstian Gerhart

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Nitrous Oxide (N2O) is a greenhouse gas with a Global Warming Potential (GWP) of 298-310 [1,2] (298-310 times more potent than carbon dioxide (CO2)). As a result, any aftertreatment system that generates N2O must be well understood to be used effectively. Under low temperature conditions, N2O can be produced by Selective Catalytic Reduction (SCR) catalysts. The chemistry is reasonably well understood with N2O formed by the thermal decomposition of ammonium nitrate [3]. Ammonium nitrate and N2O form in oxides of nitrogen (NOx) gas mixtures that are high in nitrogen dioxide (NO2)[4]. This mechanism occurs at a relatively low temperature of about 200°C, and can be controlled by maintaining the nitric oxide (NO)/NO2 ratio above 1. However, N2O has also been observed at relatively high temperatures, in the region of 500°C.
Currently, two consolidated aftertreatment technologies are available for the reduction of NOx emissions from diesel engines: Urea SCR (Selective Catalytic Reduction) systems and LNT (Lean NOx Trap) systems. Urea SCR technology, which has been widely used for many years at stationary sources, is becoming nowadays an attractive alternative also for light-duty diesel applications. However, SCR systems are much more effective in NOx reduction efficiency at high load operating conditions than light load condition, characterized by lower exhaust gas temperatures.

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