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Waste Vegetable Oil As A Diesel Replacement Fuel

Index of Waste Vegetable Oil As A Diesel Replacement Fuel
Properties of Triglycerides as Fuels
Oils and their melting point and Iodine Values
In Car installation
Problems Costs Exhaust emissions and Cost efficiency
Saturated oils and possible improvements

Properties of Triglycerides as Fuels


A large amount of research has gone into examining Diesel’ s dream of using raw vegetable oils as fuels and when one speaks of growing crops for liquid fuels it is often assumed that the oil will be used after only basic extraction and filtering.
Work has been conducted to examine these oils as fuel replacements or additives. For example in the late 1970’ s and early 1980’ s, research was undertaken at Murdoch University (Perth, Australia) into the use of eucalyptus and other plant oils as a fuel component. [6] In New Zealand, there are considerable problems with the disposal of surplus tallow from the processed meat industry and a large amount of work was conducted in the early 1980’ s on the use of tallow as a fuel.
Experience has shown that the use of unsaturated triglyceride oils as a fuel may cause significant problems that can affect the viability of their fuel use. But this is not always the case and in many circumstances these problems can either be dealt with or are acceptable to the user.
While power output and tailpipe emissions using plant or animal oils are in most cases comparable with those when running on petroleum diesel fuel, the main problem encountered has due to the higher viscosity of the triglyceride oils and their chemical instability. These can cause difficult starting in cold conditions, the gumming up of injectors and the coking-up of valves and exhaust.
The viscosity of plant and animal fats and oils varies from hard crystalline solids to light oils at room temperature. In most cases, these ‘ oils’ or ‘ fats’ are actually a complex mixture of various fatty acids triglycerides, often with the various components having widely varying melting points. This may give the oil or fat a temperature range over which solidification occurs, with the oil gradually thickening from a thin liquid, through to a thick liquid, then a semi-solid and finally to a solid.
High melting points or solidification ranges can cause problems in fuel systems such as partial or complete blockage as the triglyceride thickens and finally solidifies when the ambient temperature falls. [3] While this also occurs with petroleum based diesel, particularly as the temperature falls below about ~ -10 to -5° C for ‘ summer’ formulations and ~ -20 to -10° C for ‘ winter’ diesels, it is relatively easy to control during the refining process and is generally not a major problem.
Many vegetable oils and some animal oils are ‘ drying’ or ‘ semi-drying’ and it is this which makes many oils such as linseed, tung and some fish oils suitable as the base of paints and other coatings. But it is also this property that further restricts their use as fuels.
Drying results from the double bonds (and sometimes triple bonds) in the unsaturated oil molecules being broken by atmospheric oxygen and being converted to peroxides. Cross-linking at this site can then occur and the oil irreversibly polymerises into a plastic-like solid.
In the high temperatures commonly found in internal combustion engines, the process is accelerated and the engine can quickly become gummed-up with the polymerised oil. With some oils, engine failure can occur in as little as 20 hours.
The traditional measure of the degree of bonds available for this process is given by the ‘ Iodine Value’ (IV) and can be determined by adding iodine to the fat or oil. The amount of iodine in grams absorbed per 100 ml of oil is then the IV. The higher the IV, the more unsaturated (the greater the number of double bonds) the oil and the higher is the potential for the oil to polymerise.
While some oils have a low IV and are suitable without any further processing other than extraction and filtering, the majority of vegetable and animal oils have an IV which may cause problems if used as a neat fuel. Generally speaking, an IV of less than about 25 is required if the neat oil is to be used for long term applications in unmodified diesel engines and this limits the types of oil that can be used as fuel. Table 1 lists various oils and some of their properties.
The IV can be easily reduced by hydrogenation of the oil (reacting the oil with hydrogen), the hydrogen breaking the double bond and converting the fat or oil into a more saturated oil which reduces the tendency of the oil to polymerise.
However this process also increases the melting point of the oil and turns the oil into margarine.
As can be seen from Table 1, only coconut oil has an IV low enough to be used without any potential problems in an unmodified diesel engine. However, with a melting point of 25° C, the use of coconut oil in cooler areas would obviously lead to problems. With IVs of 25 – 50, the effects on engine life are also generally unaffected if a slightly more active maintenance schedule is maintained such as more frequent lubricating oil changes and exhaust system decoking.
Triglycerides in the range of IV 50 – 100 may result in decreased engine life, and in particular to decreased fuel pump and injector life. However these must be balanced against greatly decreased fuel costs (if using cheap, surplus oil) and it may be found that even with increased maintenance costs that this is economically viable.

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