Fuels combustion effects on a passive mode silver/alumina HC-SCR catalyst activity in reducing NO x

Abstract

The activity of a Ag/Al 2O 3 catalyst in reducing NO x emissions in a passive mode hydrocarbon selective catalytic reduction (HC-SCR) was investigated using exhaust gas from the diesel engine operation on diesel, biodiesel (RME) and low temperature Fischer–Tropsch synthetic diesel (SD). The HC 1:NO x ratio in the engine exhaust from the combustion of these fuels followed the order: diesel > SD > biodiesel and this order was mirrored in the catalyst activity in reducing NO x in presence of hydrogen (1000 ppm). Compared to diesel fuel, biodiesel combustion produces a higher amount of NO x with reduced concentrations of HCs, while both HC and NO x emissions were reduced in the engine exhaust from the combustion of SD fuel. Although, a higher NO x reduction in the SCR process was seen in the case of diesel fuelling, due to higher HC 1:NO x ratio in the engine exhaust compared to biodiesel (RME) and SD, at low exhaust temperatures (190 °C) there was a gradual loss of the catalyst, NO x reduction activity. The incorporation of EGR within the engine operation, increased significantly HC 1:NO x ratios in the exhaust, mainly by lowering the NO x concentration. Under these conditions, which can assumed to represent typical HC 1:NO x ratios of a modern automotive diesel engine, higher NO x conversion was seen with SD, followed by diesel and biodiesel. For all the cases examined here the SD fuelling provides the lowest tailpipe NO x emissions. Hydrogen addition, i.e. at 500, 1000, 1500 and 3000 ppm in the passive mode Ag/Al 2O 3 SCR catalyst needs to be optimized for the different HC 1:NO x ratios and hydrocarbon species in the exhaust from the combustion of the three fuels.