Reactivity and structure of soot generated at varying biofuel content and engine operating parameters

Since the EU demands the use of renewable energy sources, also in the transport sector, the usage of biodiesel fuels increases continuously. As the physicochemical properties of soot are important for the oxidation behavior during the regeneration of diesel particle filters (DPF) and literature provides contradictory information about the reactivity and the structure of biodiesel soot, the aim of this work was to investigate the reactivity of soot produced by a diesel engine operated with fuels of different biodiesel content at varying engine operating parameters. By temperature-programmed oxidation (TPO) an increasing reactivity with increasing biofuel ratio, except for the conditions with lowest injection pressure was determined. This implies that soot generated with 100% biofuel (consisting of rapeseed oil methyl ester) is more reactive than soot generated with commercial gasoline station fuel containing up to 7% biodiesel, while soot from fossil fuel is even less reactive. In addition, reactivity increases with increasing injection and boost pressure. Raman microspectroscopy (RM) analysis gave very similar spectra for the received soot samples, i.e. all generated soot samples possess a similar graphitic nanostructure. Additionally, a trend in reactivity with the particle size as well as the content of Fe, Zn, and Cu in the soot, which was determined by inductively coupled plasma mass spectrometry (ICPMS), could be revealed. The soot reactivity is therefore not only determined by one parameter but a product of many soot properties like nanostructure, particle size and/or inorganic components as impurities. When following the structural change in the DPF during regeneration, a decreased structural order was observed in the beginning. After a certain time (ca. 40 s) a homogeneous oxidation of the soot takes place, which does not change the structure of the soot inside the DPF. This coincides with findings of a constant oxidation rate after a certain time. Overall, it can be ascertained that no graphitization of the soot takes place inside the DPF during regeneration. (C) 2015 Published by Elsevier Inc. on behalf of The Combustion Institute.