Cold Flow Properties of Biodiesel and the Improvement Methods: A Review

Biodiesel is a mixture of fatty acid alkyl esters (FAAEs) produced by transesterification, which has been promoted as a substitute for petroleum diesel in some areas of China. However, it shows poor cold flow properties (CFPs), such as high pour point, high cloud point, and high cold filter plugging point, meaning that biodiesel would crystallize under low-temperature conditions, thus resulting in engine problems and emission issues. This review focused on the mechanisms related to the CFPs of biodiesel and outlined the factors that initialize and accelerate the crystallization process. The influence of CFPs on engine performance and pollutant emissions was also discussed. Particular emphasis was given to available methods that can improve the CFPs of biodiesel. Modifying the composition of FAAEs, changing the component ratio, and removing minor constituents are three primary directions. Every derivative method was described in detail about the characteristics, shortcomings, and what can be improved. The type and load of cloud point/pour point/cold filter plugging point cold flow improvers were compared and selected on the basis of the improvement effect of CFPs. Microemulsion is a new method that may be used to improve the CFPs of biodiesel. As a result of the variability of the biodiesel components and the complexity of each improving method on the overall fuel properties and performance, the CFP modification methods should be studied and evaluated carefully and comprehensively before being adopted in industrial applications.