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The stability of biodiesel is dependent on storage conditions such as contact with ambient air and metals,
exposure to sunlight and high temperature conditions which accelerate oxidation reactions. In addition,
biodiesels are more susceptible to degradation when compared to fossil diesel because of the presence of
unsaturated fatty acid chains which are prone to oxidation. The stability of biodiesel is categorised according
to oxidation stability, storage stability and thermal stability. Oxidation instability can led to the formation
of oxidation products such as aldehydes, alcohols, shorter chain carboxylic acids, insolubles, gums and
sediments in the biodiesel. Thermal instability is concerned with the increased rate of oxidation at higher
temperature, which in turn increases the weight of oil and fat due to the formation of insolubles. Storage
stability is the ability of liquid fuel to resist change to its physical and chemical characteristics brought about
by its interaction with its storage environment, such as contamination with metals. These fuel instabilities give
rise to the formation of undesirable substances in biodiesel beyond acceptable limits as per global biodiesel
standards such as those of the American Society for Testing and Materials (ASTM D6751) and European
Standards (EN 14214). When such fuel is used in the engine, it impairs engine performance through fuel filter
plugging, injector fouling, and deposit formation in the engine combustion chamber and various components
of the fuel system. We review the stability of biodiesel made from less common vegetable oils of African
origin and synthetic antioxidants used in improving the stability of produced biodiesels. |
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