Outdoor cultivation of the green microalga Chlorella vulgaris under stress conditions as a feedstock for biofuel

Abstract

The present work investigated the potential of the green alga Chlorella vulgaris to produce high-quality biofuel under culture stress conditions. The cultivation was carried out in a 1000 l open plate tank system, which provides biomass yields comparable to open pond systems, but with less area needed. Algal biomass and lipid content were measured repeatedly. We compared the two solvent systems n-hexane and hexane/isopropanol (HIP) for extraction efficiency of lipids and applied three different extraction methods Soxhlet, soaking, and soaking followed by Soxhlet (soak-Sox). The combination of the HIP solvent and the soak-Sox provided the highest lipid yield (15.8 ± 0.174). Volumetric biomass and lipid productivity were 0.201 g l^-1 day^-1 and 31.71 mg l^-1 day^-1, respectively, whereas areal biomass and lipid productivity were 25.73 g m^-2 day^-1 and 4.066 g m^-2 day^-1, respectively. The fatty acid profile by means of gas chromatography resulted in seven fatty acids from C₁₂ to C₁₈. The most abundant fatty acid methyl esters (FAMES) were palmitic (C16:0), oleic (C18:1), and stearic (C18:0) acids. Lipid synthesis enhanced by optimizing the Kuhl growth medium with replacing nitrate by urea (50% N compared to the original recipe) increased salt content (10 g/l NaCl), ferrous sulfate (0.5 g/l), and sodium acetate addition (1 g/l). With regard to density, kinematic viscosity, gravity, pour point, flash point, and cetane number, the Chlorella-biodiesel comply with ASTM and EN standards thus pointing at the high potential of lipids synthesized by Chlorella as a feedstock for biodiesel production.

Keywords: Biodiesel; Culture medium; Fuel properties; Green alga; Lipid extraction; Open plate tank system.