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In November 2002 viewers of the BBC car magazine programme "Top Gear" were shown an interesting demonstration. Around 5 litres of used cooking oil from a local grill were filtered through a piece of cloth, a teaspoon of ethanol was added and the mix poured into the empty tank of an old Volvo diesel car. The car started, went off around the test track and voilà - the future of the energy supply for our vehicle fleet was secured.

Well not quite. What was not discussed in the programme were a range of issues such as durability, cold start ability, environmental performance, etc. which might have spoiled the good story. The aim of this article is to "fill in the blanks" in this debate and position straight vegetable oil in a niche where it could make a significant contribution to both agricultural economy and sustainability of the transport system.

Interest in using vegetable oil as a fuel for vehicles goes back a century to Rudolf Diesel who saw vegetable oil as the long-term source of fuel for diesel engines

The interest in vegetable oil goes back a century to Rudolf Diesel who saw it as the long-term source of fuel for diesel engines. More recently, the agricultural sector has become the stronger driver, as vegetable oil obviously constitutes an outlet for agricultural products. Initially, it was believed to be possible to use these oils directly with only minor processing. Testing by the engine industry has shown that unmodified diesel engines will initially operate satisfactorily, but quickly develop durability problems, mainly in the area of fuel injectors, piston rings and lubrication oil stability. For this reason the engine needs to be modified. Such modifications can at present be made by a number of facilities mainly in Germany, where more than 5,000 vehicles are currently using SVO1. The latest development is that SVO has been formally recognized as a biofuel alongside more well-known cousins such as biodiesel and bioethanol, via the passing of an EU directive on promotion of biofuels, which specifically mentions "pure vegetable oil from oil plants" as a fuel.

Production and consumption pathway for vegetable oil

A wide range of crops can be used for SVO production, but in practice only rapeseed oil is being used today in Europe. The production path is relatively simple as it consists simply in growing and harvesting the seeds, pressing the oil at low temperature and filtering the final product to remove impurities. Thus the production technology can be applied on almost any scale.

A wide range of crops can be used as a source of vegetable oil, but in practice only rapeseed oil is widely used as a fuel in Europe today

Production is constrained by availability of agricultural land in terms of total extent of useable land; in terms of competitions with other crops (food, etc.); in terms of crop rotation needs (rape can only grow every 5-7 years on the same land) and because of the EU-US Blair House Agreement2, which limits the area available for oilseed production in the EU.

Oil pressing on a small scale in a decentralized structure will generally result in a lower overall yield of oil, as cold pressing cannot extract all oil from the seeds. Typical yield values lie around 77%, even though values above 87% can be found. For larger oil mills, using hexane solvent extraction, the typical yield is around 98% of total oil content in the seeds3. Using hexane extraction increases the cost of processing, as the hexane used for extraction must be removed before the protein meal (the leftover from pressing the seeds) can be used as animal feed. It is thus mainly an economic trade off to be considered when the protein meal is intended to be used as a feed stuff.

Germany has established standards for vegetable oil fuels which are also used by a number of other countries

For the German market a quality standard exists (RK-Qualitätsstandard)4, which SVO must meet. Several other countries refer to the same standard as well. Meeting this is not a problem for pure cold-pressed rapeseed oil, but may be a problem for some oils available on the market, especially if pressed at higher temperatures to increase pressing yield. Thus a larger scale adoption would require a market separation of the different oils, to ensure a consistent quality for consumers. Used cooking oils would have similar problems, and are generally not considered a resource for SVO but rather for biodiesel.

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