Water-in-diesel emulsion and microemulsion fuels: composition, structure and effect on emissions

Sammanfattning: Water-in-diesel fuels, as emulsions and microemulsions, give reduced emissions of nitrogen oxides (NOx) and particulate matter (PM) in combustion in diesel engines compared to regular diesel. These fuels can contribute to an improved local environment in densely populated areas as they can replace regular diesel in existing vehicles without any engine modification requirements. If the diesel is of Fischer-Tropsch type, it can be produced from biomass, giving an even more sustainable approach. The objective of this thesis is to investigate the difference between diesel and water-in-diesel fuels, as emulsion and microemulsion, regarding the NOx and PM emissions and to assess a correlation between water drop sizes and the emission levels for the two different water-in-diesel fuels. Emulsion and microemulsion fuels have been prepared with ten percent water and two different types of diesel, regular diesel from petroleum (EU-diesel), and Fischer-Tropsch (FT) diesel. A large number of surfactants as emulsifiers and two different types of stabilizers have been evaluated to form emulsions with long-term stability. Also for preparing the microemulsion a large number of surfactants have been tested to achieve a microemulsion existing over a large temperature range. The water drop sizes in the emulsions have been determined by use of the nuclear magnetic resonance (NMR) diffusometry technique, among others. For emulsions stabilized with a new type a material, microfibrillated cellulose, the NMR investigation revealed a considerable amount of micelle sized water aggregates in the emulsion besides the emulsion water drops. These small aggregates are believed to affect the stability of the emulsion negatively. Emulsions stabilized with a polymeric surfactant did not show presence of these small aggregates and the stability of these emulsions was much better. The microstructure investigation by use of NMR diffusometry of microemulsions based on EU-diesel showed that the surfactants used not only reside at the interface between diesel and water but also form aggregates in the continuous domain. Emission analyses were performed after combustion in both a heavy-duty diesel engine and a light-duty diesel engine, i.e. a truck and a car engine, respectively. The soot emission reduction in the heavy-duty engine was large, 81 % and 89 % reduction for an emulsion and for a microemulsion, respectively. The effect on NOx was small, however. The differences in emissions between an emulsion and a microemulsion were 42 % for soot and 7 % for NOx in favour of the microemulsion. In the light-duty engine a comparison was made between emulsions based on EU-diesel and on FT-diesel. The effect of the water addition was more pronounced for the EU-diesel than for the FT-diesel.