General Characterisation and Applications of Plant Oils and Biodiesel

Vegetable oils or their derived product, biodiesel, are potential fuels for diesel engines, representing an alternative to fossil fuel. The suitability of the various oils and the respective biodiesels to be used in diesel engines depends on oil characteristics. The most promising crops, suitable for “short chain fuel oil” or biodiesel production are represented by seed/fruit crops, both herbaceous (annual) and tree (perennial) crops. The most suitable annual ones concerning both yield and oil characteristics are sunflower, oilseed rape and soybean; the most suitable perennial one is the oil palm, which is also the most largely grown. A first decisive factor in the choice of an oil crop is among annual and perennial species suitable for a given area. Another key issue are the physical and chemical properties of the pure plant oil, depending on the intended use: there are also climate-related parameters to be considered, as for example the melting point. The melting point indicates the temperature at which a solid material forms the first liquid droplets; this is especially important for processing or application of plant oils as biofuels. Directly related to the melting point is the iodine value, which is a measure for the amount of carbon double bonds present in the respective oil or fat, indicating the degree of saturation. Another important parameter which is crucial for the application of pure plant oil in engines is the kinetic viscosity, which is a measure of a fluid resistance to shear or tensile stress and the reciprocal value of the fluidity. The heating value or calorific value is the amount of heat released during the combustion of a specified amount of it. The cetane number is a measure of the ignitability or more exactly the ignition delay corresponding to the time period between start of injection and start of combustion. Biodiesel is a renewable fuel derived from vegetable oils which can be added at low ratio to most diesel fuels without substantially changing the fuel properties, as it shows comparable qualities as the crude oil based diesel fuel. In contrast to this, the use of pure plant oil in diesel engines is only suitable to a limited extent due to very high viscosities and poor cold flow properties. Biodiesel is chemically different from fossil diesel fuels, which leads to a number of special physical characteristics compared to the crude oil based fuel. It shows a higher viscosity, density, initial and final boiling point, cold-filter plugging point, and flash point. The lower calorific value compared to fossil diesel results in increased fuel consumption when using plant oil or biodiesel as diesel substitute. Pure plant oil and biodiesel show a significant higher flash point than fossil based diesel which is advantageous for handling the fuel especially in applications requiring high safety standards. The sulphur content of the biofuels is far lower than of standard diesel, which is another advantage as sulphur is supposed to have a negative effect on health and environment.