Oxygenated Biofuel Blends: Stability, Combustion and Emissions

Adeniyi, Oladapo M. (2021) Oxygenated Biofuel Blends: Stability, Combustion and Emissions. Doctoral thesis, Northumbria University.

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Abstract

The oxygenation effect of bioethanol and the superior physicochemical properties of HVO on engine combustion and emissions has made both biofuels viable alternatives to reduce the full use of conventional diesel in the existing diesel engine without modification. Their improvement of engine performance and emissions reduction have led to various research works on how most of the possible shortcomings of these renewable biofuels could be managed. A major part of this research is to elaborate on the application of bioethanol as an oxygenated addition to conventional diesel with emphasis on the high concentration of bioethanol. In this study, the stability of diesel bioethanol blends and diesel-bioethanol-HVO blends were investigated and achieved by using biodegradable nonionic surfactants (Span 85 and Span 80) to eliminate the major problem of phase separation. Seven blends such as Diesel Bioethanol(70%-30%), Diesel- Bioethanol(60%-40%), Diesel Bioethanol(50%-50%), Diesel-Bioethanol(40%-60%), Diesel Bioethanol-HVO(40%-10%-50%), Diesel-Bioethanol HVO(50%-25%-25%), and Diesel-Bioethanol-HVO(70%-20%-10%) were investigated. The significance of stability of fuel blends during combustion is essential to the life of the internal combustion engine, so each blend was investigated for phase separation after the blending process. Different phase separations of unstable blends were formed over time, and surfactants were applied to the separated blend to control the phase separation. All blends were stable after the application of surfactants with the required compositions except for the dieselbioethanol blend (40%-60%) which was only stable for 10 weeks.

The oxygenation effect of the fuel blends on the combustion performance was analysed and compared with the reference diesel, which results in an overall improvement. The results obtained from the blends showed reductions in engine brake power, torque and BMEP due to the effects of lower heating value and density of bioethanol in the blend. The maximum cylinder pressures of diesel-bioethanol blends (70%-30%) and (60%-40%) are higher than the reference diesel, with slight variations for the diesel-bioethanol-HVO blends. This is due to the improved air-fuel mixture of the diesel-bioethanol blends and the chemical composition of HVO which influences the fuel atomisation. Longer ignition delay periods and higher heat release rates were observed for all the blends except for the diesel-bioethanol-HVO blend (40%-10%-50%), which shorter ignition delay period and lower heat release rate were due to the dominant effect of the cetane number of HVO. The higher BTE and BSFC for both diesel-bioethanol blends and the diesel-bioethanol- HVO blend (70%-20%-10%) are due to the longer ignition delay period of these blends, while the lower BTE and BSFC for the remaining diesel bioethanol-HVO blends are due to the HVO composition in the blend. Lower exhaust gas temperatures for all the blends except diesel-bioethanol-HVO (40%-10%-50%) are due to the oxygenated effect of bioethanol, while the higher exhaust gas temperature for diesel-bioethanol-HVO (40%- 10%-50%) could be associated with the effect of the heating value of HVO.

Diesel-Bioethanol-HVO blend (40%-10%-50%) showed the overall best emission performance among all the fuel blends, with significant reductions in CO, CO2, NOX and THC emissions which were due to the dominant renewable effect of HVO on the blend composition coupled with the oxygenated effect of bioethanol composition in the blend. The results from the soot particles development, carbon compositions and the smoke opacity show reductions in the blends with a significant reduction in the dieselbioethanol- HVO blend (40%-10%-50%). This is due to the dominant renewable effect of HVO on the blend composition coupled with the oxygenated effect of bioethanol composition in the blend. The general reductions in soot particles, carbon compositions and smoke opacity of all the blends show the effect of the fuel-bound oxygen molecules which influence the combustion quality and reduce the formation of CO2 as the end product of the combustion reaction.

The result obtained from the independent combustion analysis of diesel-bioethanol blend (50%-50%) when compared to the reference diesel showed reductions in engine brake power, torque and BMEP due to the effects of lower heating value and density of bioethanol. The higher cylinder pressure was due to the improved air-fuel mixture, while the longer ignition delay period and higher heat release rate were due to the lower cetane number of bioethanol. The higher BTE and BSFC were due to the longer ignition delay, and the lower exhaust gas temperature was due to the oxygenated effect of bioethanol. The corresponding emission analysis result for the diesel-bioethanol blend (50%-50%) showed significant reductions in all emissions, which was due to the higher composition of bioethanol in the blend.

This investigation shows that the oxygenation effect of bioethanol coupled with the renewable properties of HVO has the potential to improve engine performance and exhaust emissions while maintaining their carbon-neutral benefits to the environment.

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