Sulphate Production Over The CRTTM: What Fuel Sulphur Level Is Required To Enable The EU 4 And EU 5 PM Standards To Be Met?

The proposals to further lower particulate matter (PM) standards for heavy duty diesel powered vehicles throughout the world have increased interest in diesel particulate filter based aftertreatment solutions such as the Continuously Regenerating Trap (CRTTM). This system has been applied to thousands of heavy duty diesel vehicles in Europe as a retrofit technology over the last six years to meet various local and governmental requirements. For example, the Swedish environmental zones require that all heavy duty diesel vehicles must have better than Euro 2 emissions or at least 80% PM and 60 % hydrocarbon conversion to operate within the cities of Stockholm, Gothenburg and Malmo. The legislated EU PM limit will be decreased from 0.1 g kW⁻¹hr⁻¹ to 0.02 g kW⁻¹hr⁻¹ over the European Steadystate Cycle (ESC) in 2005. It is most likely that this PM control will be brought about by using devices such as the Continuously Regenerating Trap, which operate by oxidising a portion of the engine out NO into NO₂; this NO₂ then combusts the PM at much lower temperatures than is possible using oxygen. However, as well as oxidising the NO into NO₂, these devices also oxidise some of the sulphur (S) in the fuel, to form sulphate species. These species are collected during the PM measurement, and can make a significant contribution to the apparent PM emissions of the system. Therefore, while the soot emissions of the system will be very low, the measured PM emissions can be significantly higher due to sulphate make. This paper describes the results of a controlled study to establish the magnitude of these effects. Specifically, experiments were carried out using fuels with sulphur contents of 2, 10, 20, 30, 40 and 50 ppm. The results show that the 50 ppm fuel S level, which is due to come into force in 2005, causes the measured PM emissions to exceed the 0.02 g kW⁻¹hr⁻¹ standard on the engine used in this study. Indeed, in order to meet this standard reliably and reproducibly a fuel S level of approximately 10 ppm is required. It should be noted that sulphate make will be an issue with any system fitted with a high performance Pt-based oxidation catalyst.