Vegetable oils, recycled greases, animal fats, and future options from algae, bacteria, and fungi, cannot be used as such in conventional high-speed diesel engines. However, these can be converted into diesel compatible fuels with esterification (Fatty Acid Esters) or hydrotreatment processes (Paraffins). Oils and fats as such can be used in modified high-speed engines or in medium-duty diesel engines. In agriculture, there are engines manufactured for operation with "natural pure plant oil". In addition, existing diesel engines can be converted for vegetable oils by modifications into fuel lines, injectors, combustion chambers etc. However, in many regions clean fuels are required also for agricultural engines to meet stringent exhaust emission limits. (Agriforenergy 2011, Barnwal and Sharma 2005). Wood -based tall oil fatty acids are different from oils and fats, since they contain free acids that are aggressive towards materials. Therefore, unlike oils and fats, they cannot be used as such in internal combustion engines.