Preturbo SCR: Potential and challenges for NOx removal under elevated pressure and temperature.
Deniz Zengel, Karlsruhe Institute of Technology (KIT), Institute of Chemical Technology and Polymer Chemistry (ITCP)
Considering the decrease of the exhaust gas temperature due to a more efficient engine management, a positioning of the exhaust gas aftertreatment system closer to the engine offers great potential to counteract the declining temperature. One possibility is the location upstream of the turbocharger, which apart from higher temperatures up to 180 °C also results in an in¬creased pressure and, thus, a higher residence time of the reactants. This position can significantly improve the NOx removal. Although recent studies report strong benefits for implementing SCR catalysts at such a position, a number of challenges remain. Besides extremely fluctuating conditions, unburnt hydrocarbons or other catalyst poisons may deactivate the catalyst. In addition, formation of deposits and gas phase reactions cannot be excluded at elevated pressure. Hence, this study does not only focus on evaluating the positive effect on catalytic activity but also the influence of possible catalyst poisons, deposit formation and gas phase reactions at elevated pressure.
The positive influence of pressure on the catalytic activity of different SCR catalysts was investigated by measuring coated honeycombs at a dedicated high pressure test bench. The catalysts were investigated between 150-550 °C with typical SCR gas mixtures, at 100.000 1/h gas hourly space velocity and pressures up to 5 bar. The resulting gas mixture was analyzed on-line by an FTIR spectrometer.
To evaluate the influence of hydrocarbons on the catalytic activity, various hydrocarbons with different characteristics have been tested. Besides direct influence on the catalytic activity also permanent ageing effects were studied. Since sulfur-compounds can be present in the exhaust gas, the influence of SO2 under varying pressure was tested. Furthermore, the formation of deposits (e.g. (NH4)2SO4) was investigated during long-term measurements at different temperatures. Finally, the role of gas phase reactions were elucidated at the high pressure test bench to evaluate their impact on the overall performance.
In summary, this study examines the different effects and interactions that can occur at elevated pressure on common SCR catalysts and thus give a solid basis for evaluating NOx-removal under pre-turbo conditions.