Reduction of NOx in Lean Exhaust by Selective NOx-Recirculation (SNR-Technique) Part I: System and Decomposition Process

The SNR-technique is a new NOx aftertreatment system for lean burn gasoline and diesel applications. The objective of SNR is NOx removal from lean exhaust gas by NOx adsorption and subsequent selective external recirculation and decomposition of NOx in the combustion process. The SNR-project is composed of two major parts. Firstly the development of NOx adsorbents which are able to store large quantities of NOx in lean exhaust gas, and secondly the NOx decomposition by the combustion process.

Emphasis of this paper is the investigation of NOx reduction in the combustion process, including experimental investigation and numerical simulation. The NOx decomposition process has been proven in diesel and lean-burn gasoline engines. Depending on the type of engine NOx-conversion rates up to 90 % have been observed. Regarding the complete SNR-system, including the efficiency of the adsorbing material and the NOx decomposition by the combustion, a NOx removal of more than 50% is achievable. By numerical studies the reaction paths of NOx decomposition have been identified As a main result of the reaction analysis it is presumed that the thermal NO production is decreased and additionally the NO reduction via non-thermal NO reactions is increased.

The SNR project was carried out by an international consortium comprising five European countries within the framework of the BRITE EURAM PROGRAMME and with the financial support of the European Community.