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Biofuel-burning Microturbine Fig. 11: Calorific Value of Liquid Bio- Fuels in MJ/Litre Liquid biofuel properties and microturbine adaptations Today natural gas, bio-gas, diesel and kerosene are used as fuel to operate microturbines. For liquid biofuel based operation, adaptations of the microturbine system are necessary, due to the different physical properties and chemical composition of biofuels with respect to liquid fossil fuels. Important fuel properties for biofuel applications are presented below. Calorific value Most biofuels have lower heating values by volume than fossil fuels. In order to obtain an equal output and run-time between refilling, in some cases the flow rate and the fuel storage volume has to be increased. Some fuels like methanol, ethanol and their derivates have calorific values less than 50% of those of fossil fuels, thus demanding twice the storage capacity. Biodiesel (RME), Fischer Tropsch diesel and most vegetable oils have a similar energy content as fossil liquid fuels (> 30 MJ/l). Fuel viscosity The viscosity of a fluid is a measure of the fluids adhesive/cohesive or frictional properties. The viscosity of vegetable oils can be ten times higher at room temperature than that of fossil d i e s e l . Th i s m ak es i t d i f f i c u l t , uneconomical or even impossible to pipe and pump vegetable oils at ambient temperature. High viscosity may also lead to cold start problems and prevents a fine fuel/air mixture in the combustion chamber, due to its negative effect on fuel injection properties. These problems can be solved in the following two ways: Blending A fuel with a high viscosity is blended with a fuel of low viscosity. Depending on the mixture ratio, the viscosity can be adjusted to the requirements (e.g. using alcohols with a very low viscosity of 1 – 2 mm2/s). Preheating In this case a fuel heating system is installed, that tempers the fuel to 50°C to 80°C. Figure 13 shows the decrease of viscosity for vegetable oil, RME and diesel with increasing temperature. Impurities Fig. 13: Temperature vs. Viscosity for RME and Diesel Fuel In general, substantially higher contents of ashes are found in biofuels. The relatively high ash content in some biofuels can cause microturbine malfunction, through corrosion, erosion and deposits. In case of erosion, the impact of hard and small particles causes damage on the surface of the turbine blades. Ash fractions within the fuel can form deposits under certain physical and chemical conditions. They cause a reduction in the performance of the microturbine by increasing surface friction losses reducing the flow area and clogging of the heat exchanger. The heat transfer in the heat exchanger is obstructed which reduces the efficiency. - 10 - Fig. 12: Kinematic Viscosity of Liquid Bio- Fuels in mm2/sPDF Image | Biofuel-burning Microturbine Opportunities for Biofuel-burning Microturbines in the European Decentralised-generation Market (BIOTURBINE)
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