Vegetable/edible oils are one of the densest biogenic energy storage systems that are created using photosynthesis. Worldwide, there is a wide variety of plants whose oils can be used as fuel. In Germany rapeseed (canola) oil is mainly used, because its use as a fuel has been researched in greatest detail, rape grows best here for climatic reasons, and so the oil can be produced economically. Sunflowers are also a possible domestic source of fuel, but their oil is significantly more expensive to produce. Another oft-cited domestic alternative is camelina (false flax). Internationally, soy is also used and has significant potential globally.
In contrast to biodiesel, which needs to first be produced by transesterification of the oils and then used as fuel, these oils only have to be extracted from the plant and cleaned and can then be used as fuel.
Vegetable oils have a wide range of application areas. On the one hand they are used materially, for example as an environmentally friendly chain saw oil lubricant in forestry. But they are also used as fuel in stationary power plants (cogeneration plants), for heating, and in vehicle engines. Many common diesel engines (cars, trucks, agricultural machinery or construction equipment) can be modified to use this fuel.
There are two production methods for vegetable oil: In farms or cooperatives, cold pressing is often used as a local technology, while large industrial plants rely on extraction. In cold pressing, the cleaned oil seed is mechanically pressed at maximum temperatures of up to 40°C. Suspended solids are then removed by filtration or sedimentation. However, a residual oil content of more than 10% remains in the presscakes. With oil obtained using downstream solvent extraction, the seeds are first pressed after pretreatment at higher temperatures. The remaining oil is extracted from the remaining oil presscake using solvents (usually hexane), which is then removed from the oil by evaporation. As the oil produced from this process contains more unwanted additives than a cold-pressed oil, further refining processes are then carried out.
The energy content (calorific value) of a liter of rapeseed oil fuel corresponds to that of about 0.97 liters of diesel fuel. Compared to gasoline, the calorific value of rapeseed oil per liter is a good 10% higher.
Since 2012, the quality requirements for rapeseed oil have been specified in DIN 51605. To comply with the limits, the oil mills usually have to post-treat the rapeseed oil to reduce the calcium, magnesium and phosphorus content. To allow other oil plants to be used for fuel production and improve the raw materials base, the German Institute for Standardization (Deutsches Institut für Normung - DIN) has prescribed requirements and test methods in DIN 51623 (Fuels for vegetable oil compatible combustion engines - Fuel from vegetable oil - Requirements and test methods).
Nevertheless, only old engines – usually pre-chamber diesel engines – allow for the use of pure vegetable oil without an engine conversion. Most vehicles today need to be converted to use vegetable oil, because of its higher viscosity and low cetan number. The conversion includes a fuel pipe with a larger diameter, and the installation of a new filter. A heat exchanger, which preheats the vegetable oil fuel, is also important. Even so, at least 10 vol.% diesel must be added to vegetable oil in winter to keep the fuel flowing.
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Status: December 2015
All information subject to change. Errors and omissions excepted.