Seasonal Influence on the Yield and Chemical Composition of Biofuels Derived from Raceway-Grown Microalgae
This study evaluates the influence of seasonal harvesting conditions on biomass productivity, oil yield, and biodiesel-related FAME composition of native microalgae—predominantly Chlorella sp., with minor populations of Scenedesmus and Ankistrodesmus—cultivated in an open raceway pond integrated into a pig-slurry biogas plant in Soria, Spain. Processing biomass harvested across four seasons showed that oven drying at 45 °C yields stable seasonal biomass concentrations around 7.80–8.80 g/L, while extended outdoor air-drying during spring and summer reduces yields to 2.86 and 4.33 g/L due to the absence of active growth conditions. Lipids extracted via the Soxhlet method using a dichloromethane: methanol mixture (9:1 v/v) exhibited a wide range of 2.5–18.9 g oil per 100 g dry biomass. Marked seasonal variations were observed: winter samples demonstrated an outstanding lipid accumulation capability, yielding the highest oil production (18.9 g/100 g biomass and 1.47 g/L) and a crude ester content reaching 60.75% FAMEs, which reached a maximum of 93.3 wt.% after purification via column chromatography. Conversely, spring, summer, and autumn samples suffered from limited lipid accumulation, with dry-basis oil yields remaining below 4.2 g/100 g biomass. Extensive characterization confirmed that the residual biomass has limited application as a solid fuel because of its low higher heating values (<10 MJ/kg−1) and exceptionally high ash contents, which climbed from 15.98–60.02% to 41.14–51.91% after extraction.