Abstract: The invention relates to a method of combusting tallow, and particularly tallow that may be contaminated with the prion protein considered to be responsible for spreading the Bovine Spongiform Encephalopathy (BSE) infection in cattle, in a standard high-speed compression ignition engine. The invention also extends to combustion of other animal/vegetable-based fats and oils, particularly waste cooking oils which are at risk of contamination by toxic chemicals which may potentially enter the food chain. The heat and power produced by the engine may then be used to generate electricity. In accordance with the invention, the tallow, etc. is combusted in an enriched oxygen atmosphere in the combustion chamber of the engine.