Alternative methods for the extraction of hydrocarbons from botryococcus braunii

Lipid extraction is a critical step in the development of biofuels from microalgae. The use of toxic and polluting organic solvents should be reduced and the sustainability of the extraction procedures improved in order to develop an industrial extraction procedure. This could be done by reducing solvent amounts, avoiding use of harmful solvents, or eliminating the solvent at all. Here we describe two new processes to extract hydrocarbons from dried and water-suspended samples of the microalga Botryococcus braunii. The first one is a solvent-based procedure with switchable polarity solvents (SPS), a special class of green solvents easily convertible from a non-ionic form, with a high affinity towards non-polar compounds as B. braunii hydrocarbons, into an ionic salt after the addition of CO2, useful to recover hydrocarbons. The two SPS chosen for the study, based on equimolar mixtures of 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) and an alcohol (DBU/octanol and DBU/ethanol), were tested for the extraction efficiency of lipids from freeze-dried B. braunii samples and compared with volatile organic solvents extraction. The DBU/octanol system was further evaluated for the extraction of hydrocarbons directly from algal culture samples. DBU/octanol exhibited the highest yields of extracted hydrocarbons from both freeze-dried and liquid algal samples (16 and 8.2%, respectively, against 7.8 and 5.6% with traditional organic solvents). The second procedure here proposed is the thermochemical conversion of algal biomass by using pyrolysis; this process allowed to obtain three valuable fractions, exploitable for energy purpose, fuel production, and soil carbon storage: a volatile fraction (37% on dry biomass weight), a solid fraction called biochar (38%) and, above all, a liquid fraction named bio-oil (25%), almost entirely composed by hydrocarbon-like material, thus directly usable as fuel