Unconventional alternative biofuels: Quality assessment of biodiesel and its blends from marine diatom Navicula cincta

The use of microalgae as triacylglycerides (TAG) feedstock for biodiesel is a widely researched field in the development of new technologies. In order to underpin the potential of microalgal biodiesel as a possible substitute in the biofuel industry, more detailed data is therefore required on these aspects. This study assesses the growth, TAG accumulation, kinetics of dissolved nutrients and the quality of biodiesel and its blends from the marine diatom Navicula cincta grown in a photobioreactor, using seawater enriched with nutrients and soil extract. Acid-catalyst transesterification was tested for biodiesel production. This species presented during nutrient depletion: 1) a total lipid content of 49.7% of ash-free dry weight (AFDW) with neutral lipid (mainly TAG) as the dominant fraction (ca. 81.5% of total lipids), and 2) favourable fatty acids for biodiesel, such as palmitic acid (28%) and palmitoleic acid (46%). The yield of biodiesel (B100) derived from diatom N. cincta (B-NC) and analysed by Gas Chromatography was 97.6% of FAME, being the FAME composition similar to the fatty acid profile of N. cincta neutral lipids. The fuel properties (including FAME percentage, cetane number, heat of combustion, color and sulfur content) of diatom blends prepared with petrodiesel in a 7% (B7(NC)) and 10% (B10(NC)) vol ratio were evaluated using a PetroSpec (PAC) and compared with commercial soybean blends (B7(S) and B10(S)). B7 and B10 derived from N. cincta met the requirements of ASTM D6751 and presented improved cetane numbers (50.2 and 51.6, respectively) with respect to soybean blends. Furthermore, biodiesel derived from the studied species showed the lowest sulfur content (0.0056% w/w). The rapid accumulation of neutral lipids in response to natural nutrient deficiency in N. cincta and its capacity of autoflocculate are good selling points for the use of this microalga in the biofuel industry.