An alternative to the use of biodiesel is the use of vegetable oils or rendered animal fats as a fuel.
Using relatively unmodified oils or fats eliminates the problems associated with toxic and caustic precursor chemicals and residual biodiesel alkalinity as the oil is used without altering its chemical properties.
One possibility for the disposal of these products is as a fuel for transport or other uses. Conversion of waste oils and fats to biodiesel fuel has many environmental advantages over petroleum based diesel fuel. However it is not commercially available in Australia and the ‘back-yard’ production of biodiesel may present serious risks as the process uses methanol, a toxic and flammable liquid, and sodium or potassium hydroxide, both of which are caustic. By-product disposal may present further difficulties and environmental considerations may preclude production in sensitive areas.
An alternative to the use of biodiesel is the use of vegetable oil or rendered animal fats as fuel.
While the use of vegetable or animal oils and fats as fuels may be somewhat surprising at first, when examined in an historical context we can see that the compression ignition engine, first developed to a usable level of functionality by the French-born Rudolf Diesel near the end of the 19th century, was originally designed to operate on vegetable oil.
In 1900, Rudolf Diesel demonstrated his new compression ignition engine at the World Exhibition in Paris running on peanut oil. In 1911 he wrote "The engine can be fed with vegetable oils and would help considerably in the development of agriculture in the countries that use it."
It was about this time that new drilling technology and exploration techniques were developed and together these ushered in the age of cheap and plentiful fossil fuels. Consequently, the use of vegetable and animal oils and fats as fuels has only been used for a few special purposes such as in racing fuels or in environmentally sensitive areas where petroleum spills tend to cause more serious problems than do spills of animal and/or vegetable derived fuels.
After some one hundred years of using liquid petroleum fuels, we are now finding that there are unforeseen side effects, the foremost perhaps being the so-called Enhanced Greenhouse Effect.
In Australia, transport use contributes some 16% of Australia’s greenhouse gas emissions. Of this, diesel fuel contributed about 17% or 11,705,000 tonnes of CO2 equivalent. An additional 1,622,000 tonnes is released from diesel fuel used for electricity generation. On top of greenhouse gas emissions is the vexing question of how little - or much - is left.
However oils of vegetable and animal origin, unlike fossil fuels, have to potential to be produced not only on a sustainable basis but also could be greenhouse gas neutral, or at the very least, emit substantially less greenhouse gases per unit energy than do any of the fossil fuels.
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. 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. 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 polymerize. However this process also increases the melting point of the oil and turns the oil into margarine.
Conversion of a vehicle to operate on Waste Cooking Oil
An alternative to the use of biodiesel is the use of vegetable oil or animal fats as a fuel. The differences amongst fats and oils, whether of animal or vegetable origin, relate mainly to the level of saturation in the carbon chain. Generally speaking, the lower the number of double bonds, the higher the melting point of the triglyceride and the greater the stability of the triglyceride to polymerisation and spontaneous oxidation. From an engine use point of view, it is preferable to use saturated fats as fuels as they are more stable and less resistant to oxidation, particularly under the elevated temperatures and pressures as found in an engine environment. However due to their higher melting points, difficulty may be encountered in starting the engine without pre-heating of the fat.