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Oxidation stability is a physical property of liquid fuels used to describe the ageing behavior of fuels during transport and storage. In biodiesel and its blends with diesel, oxidation of fuel can result in acid formation and polymer solids which plug filters and leave problematic deposits throughout the fuel supply system. ASTM Fuel Specification Standards include both 100 % biodiesel and biofuel-blends. Of all these fuel standards only ASTM D6751, which is for 100 % biodiesel “as a blendstock,” mentions the test method EN 14112. Although the following fuel specification standards permit the blending of biodiesel into the fuel, to date these standards have not yet identified a suitable oxidation stability test method for blended fuels: ASTM D975 for diesel fuel oils (<5 % biodiesel); ASTM D396 for burner fuel oils (<5 % biodiesel); and ASTM D7467 for diesel fuel oil (6–20 % biodiesel). ASTM D2274 is the most frequently used method to characterize ageing behavior of diesel by determining the polymer (sludge) content of artificially aged fuel. ASTM D02.14 approved two new test methods for evaluation of oxidation stability. ASTM D7545 is a new method for testing oxidation stability of middle-distillates. ASTM D7525 is the sister method for gasoline. These methods utilize a new rapid small scale oxidation tester (RSSOT) named PetroOXY. Within the RSSOT's test chamber, a 5 mL sample of fuel is combined with oxygen, at 700 kPa, and heated to 140°C. This initiates a fast oxidation whose results are determined by the time needed to achieve a pressure drop of 10 % in the system. Testing confirmed that the time needed to achieve the 10 % drop in pressure is directly related to the sample's oxidation stability. The RSSOT-method is quicker than other actual methods evaluating the oxidation stability biodiesel, petroleum middle-distillates, and blends. Round-robins based upon this have been conducted in Europe and Japan.
oxidation stability, diesel, biodiesel
Managing Director, Petrotest Instruments GmbH & Co. KG, Dahlewitz, Brandenburg,