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Hydrothermal Carbonization and Supercritical Ethanol Transesterification

Project Lead: Bobby Levine

Worldwide energy demand, particularly for 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.  Biodiesel is an attractive replacement for petroleum diesel because it is domestically available, biodegradable, compatible with existing diesel engines, and it reduces tailpipe emissions of most criteria air pollutants. Recent interest in using oleaginous microalgae as a non-edible biodiesel feedstock has grown considerably, largely on the promise of high oil yields (5,000 to 100,000 L/ha-y), the opportunity to capture waste CO2, and the ability to cultivate algae on abandoned or unproductive land using brackish, salt or wastewaters instead of freshwater. 

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 (i.e. hydrochar or carbonized solids), and produce a sterile, nutrient-rich aqueous phase.  In the second step, the wet fatty-acid-rich solids undergo supercritical in-situ (trans)esterification (SC-IST/E) with ethanol to produce biodiesel in the form of fatty acid ethyl esters (FAEEs).  

We grow Chlorella vulgaris or Chlorella protothecoides sequentially under photo- and heterotrophic conditions to produce a lipid-rich feedstock (>50% lipids as FAEE).  The feedstock and process solids are characterized for lipid components using highly automated microscale extraction and derivatization procedures and high-temperature gas chromatography. 

Here is a typical process we use to produce algal biomass, react it, and make biodiesel:

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