In an effort to process wet algal biomass directly, eliminate organic solvent use during lipid extraction, and recover nutrients (e.g., N, P, and glycerol) for reuse, we developed a catalyst-free, two-step technique for algal biodiesel production. In the first step, wet algal biomass (ca. 80% moisture) reacts in subcritical water to hydrolyze intracellular lipids, conglomerate cells into an easily filterable solid that retains the lipids, and produce a sterile, nutrient-rich aqueous phase. In the second step, the wet fatty acid-rich solids undergo supercritical in situ transesterification (SC-IST/E) with ethanol to produce biodiesel in the form of fatty acid ethyl esters (FAEEs). Chlorella vulgaris grown sequentially under photo-and heterotrophic conditions served as the lipid-rich feedstock (53.3% lipids as FAEE). The feedstock and process solids were characterized for lipid components using highly automated microscale extraction and derivatization procedures and high-temperature gas chromatography. Hydrolysis was examined at 250 °C for 15 to 60 min; solids recovered by filtering contained 77-90% of the lipid originally present in the algal biomass, mainly in the form of fatty acids. The effects of reaction time (60 or 120 min), temperature (275 or 325 °C), and ethanol loading (approximately 2-8 w/w EtOH/solids) were examined on the yield and composition of biodiesel produced from the SC-IST/E of the hydrolysis solids. Longer time, higher temperature, and greater ethanol loading tended to increase crude biodiesel and FAEE yields, which ranged from about 56-100% and 34-66%, respectively, on the basis of lipid in the hydrolysis solids. Isomerization and decomposition of unsaturated FAEEs was quantified, and its effect on fuel yield is discussed.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)
- Fuel Technology
- Energy Engineering and Power Technology