Performance of diesel particulate filter catalysts in the presence of biodiesel ash species

The utilization of bio-fuels, such as biodiesel, is expected to contribute significantly towards the planned 10% of renewable energy within the EU transport sector by 2020. Increased biodiesel blend percentages may change engine exit flue gas ash composition and affect the long-term performance of cleaning technologies, such as oxidation catalysts and diesel particulate filters.

In this work the performance of a commercial catalyst has been studied for conversion of diesel particulate matter (SRM 2975) at 10% O2, in the presence of salts simulating ash species derived from engine oil and biodiesel. Simultaneous thermal analysis experiments of the diesel particulate matter showed that it was dominated by soot with approximately 10 wt.% condensed hydrocarbons.

The presence of a commercial catalyst (5:1 wt. ratio of catalyst to soot), in tight contact with the diesel particulate matter, decreased the temperature at which the oxidation rate peaked from 662 ± 1 °C to 526 ± 19 °C. The introduction of biodiesel ash species such as Na2CO3, K2CO3 or K3PO4 decreased the peak conversion temperature further (422 ± 12; 404 ± 4 and 423 ± 7 °C), with a limited dependence on ash concentration.

A deterioration in catalytic conversion, i.e. increased peak conversion temperatures, was seen for two engine oil ash species and one mixed ash species (from engine oil and biodiesel) – CaSO4 (569 ± 6 °C), Ca(H2PO4)2 (699 ± 13 °C) and K2SO4 (581 ± 16 °C). Kinetic parameters (A and Ea), obtained from Arrhenius plots of the data, showed a lower activation energy in the presence of the commercial catalyst (Ea = 91 ± 5 kJ/mol) or CeO2 (Ea = 62 ± 8 kJ/mol) compared to pure SRM 2975 (Ea = 220 ± 3 kJ/mol).

The obtained kinetic data were able to describe the peak conversion temperature and the associated part of the mass loss curve, but an initial low-temperature gradual increase in conversion was not adequately described.