"In the past, waste edible oils and fats were often used in the production of animal feeds. However due to links between BSE and this practice, the use of waste fats for animal feed is not as common as it once was and this has resulted in surplus quantities becoming available. This has led to significant disposal problems.
Waste oils and fats can be used as renewable fuel resources. Conversion of waste oils and fats to biodiesel fuel is one possibility but poses some difficulties such as in the use of toxic or caustic materials and by-product disposal. Conversion to biodiesel may also decrease the economic attractiveness of using waste oils as fuels.
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.
This paper discusses the use of waste vegetable and animal oils and fats as unmodified fuels in compression ignition engines."