The Production of Algal Biodiesel Using Hydrothermal Carbonization and In Situ Transesterification.

Abstract

Increasing demand for liquid transportation fuels and growing concerns about the impacts of our continued reliance on petroleum have encouraged the use of plant-based, alternative fuels. Recently, interest has grown in using oleaginous microalgae as a biofuel feedstock, largely on the promise of high oil yields and the ability to use abandoned or unproductive land along with brackish, salt, or wastewaters instead of freshwater. In this work, we developed a novel biorefinery concept for the production of algal biodiesel that incorporates nutrient recycling while obviating biomass drying and organic solvent use for lipid extraction.

We first grew algae, both photo- and heterotrophically, to produce biomass containing lipids suitable for conversion into biodiesel. This biomass was then dewatered to a 15–25% solids paste and reacted in and with subcritical liquid water (180–250 °C) in a process known as hydrothermal carbonization (HTC). During HTC, about half of the mass of the algae cell dissolves into the aqueous phase, forming a nutrient-rich co-product, while the remainder conglomerates into a hydrochar that can be recovered by filtration or centrifugation. Our work revealed that nearly all of the lipids present in the algae remain in the hydrochar and that these lipids can be converted into fatty acid ethyl esters without prior extraction by in situ transesterification (IST). We also found that the aqueous phase co-product supported algal growth and could replace roughly 50% of nitrogen and phosphorous fertilizers used in common media.

Based on our experimental work, we modeled the mass and energy flows associated with a hypothetical algal biorefinery capable of producing 5 billion gallons per year of biodiesel and found that HTC and supercritical IST can significantly reduce the energy associated with processing wet algal biomass relative to wet hexane extraction and traditional transesterification, perhaps by as much as 50%. In addition, this work highlighted the importance of effectively converting the non-lipid fraction of the biomass into usable nutrients as well as a source for on-site heat and power.