Influence of Base Diesel Fuel upon Biodiesel Sludge Formation Tendency

摘要

With the rise in the introduction of biodiesel fuel throughout the world, there has arisen the technical issue of sludge formation due to oxidation degradation. The main causes of this phenomenon are the methyl ester degradation products such as short-chain organic acids and oligomer compounds such as dimmer, trimer and tetramer acids, caused by the cleavage of the double bond in the fatty acid methyl esters (FAMEs) at high temperatures, and by the occurrence of polymerized matter. The authors focused on this sludge formation and carried out forced thermal oxidation with various combinations of diesel fuel (aromatic content about 20 vol%) with Rapeseed Methyl Ester (RME) and Soybean Methyl Ester (SME). As the results, the settlement of viscous sludge was observed. It was found that this phenomenon was pronounced with the low concentration blend (around 20 to 40 vol%) biodiesel at which the maximum value was indicated. Also, as the results of examination of hydrocarbon blends (cetane and binary aromatic hydrocarbons) as well as commercially available diesel fuels, it was found in laboratory experiments that if the aromatic ratio of the base diesel fuel was lower than approximately 30 vol%, sludge formation became significant. Alternatively, if the aromatic ratio of the base diesel fuel was raised above approximately 30 vol%, the sludge was greatly reduced. On the other hand, upon confirming the solubility in hydrocarbons of sludge created by the thermal degradation of the fuel, it was found that the sludge was more soluble in aromatics or methyl ester than in paraffin. In addition, by adding the correct amount of butylated hydroxytoluene and observing the sludge creation, it was found that it was possible to achieve reductions. That is to say, the implications were that the occurrence of this sludge was from the FAME radical reaction in high-temperature oxidation, and solubility of the formed fuel sludge. From the above, it can be reasoned that effective countermeasures would be the addition of antioxidant to suit the biodiesel concentration, adjustment of the aromatic structure of the base diesel oil, and temperature reduction (load reduction) of the automobile fuel system.