(254c) Hydrothermal Carbonization and Supercritical Ethanol in Situ Transesterification for the Production of Algal Biodiesel

The production of transportation fuels from microalgae is complicated by the fact that algae are typically grown suspended in a dilute aqueous media and have a high demand for nitrogen and phosphorous.  In this work, we explore a new and environmentally preferable strategy for algal biofuel production.   There are three major components of the bio-refinery model we have developed: 1) cultivation of oil-rich algae using recycled water and nutrients; 2) treatment of the harvested algae through hydrothermal carbonization to produce a biochar; and 3) conversion of the carbonized algal biomass into transportation fuels using in situ transesterification with supercritical ethanol. 

Lipid-rich Chlorella was grown in bioreactors until it contained approximately 50% oil, then carbonized for 15 to 90 min at 180 to 235 °C.  Biochar obtained by filtration retained >90% of lipids of the original biomass and was converted into biodiesel by reacting the solids with ethanol at 270-325 °C for 15-180 min.  Fatty acid ethyl yields in excess of 90% were obtained while employing 10-100 times less ethanol than previously reported for comparable transesterification reactions.  The nutrients liberated during hydrothermal carbonization were used to produce additional algal biomass, thereby recycling N and P.  Overall, our work suggests that it is possible to produce net-energy positive algal biofuels with significantly less environmental impact than the petroleum fuels they replace.