Microalga Nannochloropsis sp. Biomass for Biodiesel Production: Conventional (Cell Disruption) and in situ Transesterification

The potential of Nannochloropsis sp. microalga to biodiesel production was assessed. The work deals with the optimization of the cell disruption, a required step when the lipid extraction followed by the transesterification reaction is the option to produce biodiesel, as well as the optimization of the reaction parameters (catalyst, alcohol, temperature and time) of the in situ transesterification (direct conversion of the saponifiable lipids into FAME - fatty acid metyl esters, usually called as biodiesel, in one step). The cell rupture of wet and dried biomass (by oven and freezedryer) using different methods (physical - coffee grinder and ball-mill; and thermodynamic - microwaves and autoclave) was evaluated. The highest lipid recovery (42.4 ± 0.2% g of lipids /g of oven dried biomass) was attained from the oven dried biomass pretreated in a coffee grinder followed by a ball milling. The microalgal oil quality (fatty acid profile, acid value and iodine value) was determined and was not influenced by the biomass state (oven or freeze-dried) nor by the cell rupture method, being mainly composed of palmitic (C16:0), palmitoleic (C16:1) and oleic (C18:1) acids. The linoleic acid (C18:3) content is quite low (< 0.35% w/w) which results in a high quality biodiesel in terms of this parameter. The viability of the in situ transesterification, in order to reduce technological challenges, energy, time, environmental negative impacts and costs of the traditional processes, was studied and optimized The most favourable conditions were found to be 3.16 g of methanol / 100 g of dry biomass, 1.2% v/v of catalyst (H2SO4), for 6 h at 50°C, which allows of 95% esters formation. In terms of the cell rupture, the coffee grinder followed by the ball mill, proved again, to be the most efficient process, promoting an increase in the transesterification reaction rate.