Researchers explore direct conversion of wet algal biomass

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Researchers at the University of Michigan may have unlocked algae's true potential as a viable feedstock for biodiesel production. The team published a feasibility paper detailing its novel two-step hydrolysis-solvolysis process that can produce biodiesel directly from wet algal biomass, obviating the need for costly biomass drying, organic solvent extraction and catalysts while providing a mechanism for nutrient recycling. The paper on the process was published in the ACS journal Energy & Fuels.

In the first step, a strain of algae called Chlorella vulgaris, grown sequentially under phototrophic and heterotrophic conditions, was comprised of 80 percent moisture and had a 53.3 percent lipid content. The wet algal biomass then underwent a reaction under subcritical water conditions in temperatures of 250 degrees Celsius, which transformed the wet algae into a paste-like state, according to Phillip Savage, lead researcher on the project. "At large scale that probably wouldn't be applicable for an economical process," he noted.

The goal here, according to researcher Robert Levine who authored the paper, was to hydrolyze-separate from water-intracellular lipids and conglomerate them into an easily filterable solid that retained high lipid content.

"We wanted to convert the lipids into mostly fatty acids in the form of triglycerides or other phospholipids that contained fatty acid groups, so we could hydrolyze those into fatty acids," he said. "We also wanted to release a lot of the nonlipid components into a sterile nutrient-rich aqueous phase that we could recycle into our process from which we could grow additional microorganisms."

In the second step, the remaining solids (containing approximately 80 to 90 percent fatty acids) underwent a supercritical transesterification process with ethanol that produced fatty acid ethyl esters (FAEE). "We chose ethanol instead of methanol because we wanted to use entirely biologically-derived ingredients, and also because the solid that remained after the process will probably have some minor alcohol residue, which we want to be used as fertilizer, animal feed and so forth," Levine said.

Levine found that the yields of FAEE in the crude product were significantly lower than anticipated due to nominal amounts of unreacted glycerides and unidentified compounds."We definitely have more work to do to optimize the process," Levine said. "Eventually we want to get 100 percent crude biodiesel and 100 percent ethyl esters."

Though the experiment yielded promising results, Levine said the process warrants further optimization efforts to overcome glaring variables pertaining to temperature, time and alcohol use. "We use a higher ratio of alcohol-to-oil, that's common in a normal, catalyzed commercial biodiesel plant," he said. "The triglyceride-to-alcohol ratio of one-to-three is stoichiometric, so we're really moving towards just using just enough alcohol so we don't have to recover it, which would save on recovery costs."

Once perfected, Levine added that the process could be applied to larger scale biodiesel production processes. "Our process can actually be very efficient if you do heat integration on your inlet and outlet streams, there's a big advantage to using higher temperatures and pressures," he said.