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  • 18-Jun-2012 12:29 EDT

Characterization of a New Advanced Diesel Oxidation Catalyst with Low Temperature NOx Storage Capability for LD Diesel


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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. One possible solution to improve the low temperature behavior, is the use of newly developed Advanced Diesel Oxidation Catalysts (A-DOC) which are capable to store NOx at low exhaust temperatures (typical of urban driving conditions) when SCR efficiency is low, and to release the stored NOx at higher temperatures (i.e. during extra-urban driving conditions) where the urea injected is effectively forming ammonia for the subsequent NOx conversion.

Experimental tests were therefore carried out in order to assess the performance of an A-DOC when exposed at the emissions coming from a modern Euro 5, 2.0 L displacement turbocharged Common Rail DI Diesel engine for a typical European passenger car: the engine features a DOC and a DPF in close-coupled position, hosted into an on purpose designed dismountable canning, thus allowing an easy switch between different components.

The characterization of these newer DOC formulations was performed over NEDC cycles. Moreover, the catalyst were tested both in fresh and hydrothermally aged conditions in order to have a better understanding relative to robustness and durability of these newer catalyst.

NOx storage capability, which was found to be impressively high for a fresh A-DOC, significantly decreased after aging, thus leading to a final NOx cumulated emissions figure which equals the engine-out value for the aged A-DOC.

Nevertheless, since most of the NOx release from the A-DOC occurs during the EUDC segment, when a downstream SCR would likely have reached appreciable NOx reduction efficiencies, even an aged A-DOC could provide significant benefits in terms of NOx emissions reduction.

However, the analysis of the NO/NO2 share downstream of the DPF, which is of crucial importance for SCR efficiency at low temperature, revealed that the overall conversion efficiency for NO over NEDC was negative, while on the contrary the conversion efficiency for NO2 was remarkably high. As a result, the NO2/NOx ratio downstream of the DPF (i.e. at the inlet of a downstream SCR) remained significantly low during the whole EUDC segment, thus hindering the achievement of high NOx conversion efficiencies and the full exploitation of a synergetic combination of the A-DOC with a downstream SCR.

Federico Millo

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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.
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 (?

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