Microalgal fatty acid composition: implications for biodiesel quality

The fuel properties of microalgal biodiesel are predicted using published microalgal fatty acid (FA) compositions and predictive fuel models. Biodiesels produced from the microalgae investigated are predicted to have extremely poor oxidative stabilities and the majority also have poor cold-flow properties. The cetane number in most cases is out of specification, but less so than the oxidative stability and cold flow. These findings support the idea that feedstocks rich in monounsaturated fatty acids (MUFAs) are desirable for biodiesel but the composition of the saturated fatty acids (SFAs) is also shown to be of great importance. There is an apparent relationship between algal class and the percentage of FAs represented by MUFA. This potentially allows for the identification of high-MUFA algal classes, or at least provides some basis for researchers to make initial selections of target classes for bioprospecting. Comparisons of FA groups between algal classes also show that the SFAs of Mediophyceae contain significantly higher proportions of C14:0, which is in contrast to the normally abundant C16:0 and the Mediophyceae therefore have better cold-flow characteristics than other classes with similar total SFA contents. Certain particularly promising cases for biodiesel production are presented as species level examples of feedstocks that are close to satisfying the biodiesel standards and to further illustrate the challenges that remain. Variation in FA composition as a response to changes in certain environmental variables forms another important facet to feedstock selection and is briefly considered, with suggestions for further research.