Worldwide energy demand, particularly for high-density liquid transportation fuels, continues to rise as populations grow and become more affluent. Concurrently, concern over climate change, declining petroleum reserves, and national security has encouraged the use of biodiesel, a mixture of fatty acid alkyl esters commonly derived from the transesterification of vegetable oils. Recent interest in using oleaginous microalgae as a non-edible biodiesel feedstock has grown considerably, largely on the promise of high oil yields (1,000 to 100,000 L ha-1 y-1), the opportunity to capture CO2 from flue gases, and the ability to cultivate it on abandoned or unproductive land using brackish, salt or wastewaters instead of freshwater. Despite recent advancements and substantial public and private investment, large-scale algal biofuel production has not reached cost parity with the petroleum products they seek to replace. While energy inputs and costs must be reduced at each stage of algal biofuel production, harvesting, dewatering, and lipid processing remain significant challenges.
Instead of attempting to remove lipids from cells in an aqueous suspension or convert them directly to biodiesel in the presence of water, we propose a novel two-step, catalyst-free biodiesel production process involving intracellular lipid hydrolysis coupled with supercritical in-situ (trans)esterification (SC-IST/E). In the first step, wet algal biomass (ca. 80% moisture) is reacted at subcritical water conditions to hydrolyze intracellular lipids, conglomerate cells into an easily filterable solid which retains the lipids, and produce a sterile, nutrient-rich aqueous phase. In the second step, the wet fatty acid (FA)-rich solids are subjected to SC-IST/E with ethanol to produce biodiesel in the form of fatty acid ethyl esters (FAEEs). In this paper, we demonstrate that it is possible to carry out in-situ hydrolysis of cellular lipids in wet algal biomass, retain those lipids within a filterable solid, and then produce biodiesel using supercritical ethanol. In an effort to maximize lipid productivity and efficient carbon substrate utilization, Chlorella vulgaris was grown phototrophically in a bubble column reactor and then heterotrophically. Wet algal biomass contained about 50% total lipids as FAEEs and was used in hydrolysis reaction as a fresh paste or dried and rehydrated. The effects of hydrolysis processing time and temperature were investigated and lipid conversion to FA and smaller glycerides was quantified. Esterification reaction conditions and alcohol loading were investigated as predictors of biodiesel yield and biodiesel was analyzed for impurities in accordance with standard methods (ASTM 6584/EN14105 and EN14103).