|Process flow diagram for biodiesel production through intracellular lipid hydrolysis and supercritical in situ transesterification (SC-IST/E) using ethanol (EtOH). Credit: ACS, Levine et al. Click to enlarge.|
Researchers at the University of Michigan have developed and demonstrated the feasibility of a two-step hydrolysis-solvolysis process to produce biodiesel directly from wet algal biomass. Their process eliminates the need for biomass drying, organic solvent extraction, and catalysts, and provides a mechanism for nutrient (e.g., N, P, and glycerol) recycling. A paper on the process was published 30 August in the ACS journal Energy & Fuels.
Levine et al. reported that a cursory investigation of the influence of some key process variables resulted in crude biodiesel and FAEE (fatty acid ethyl esters) yields as high as 100 and 66%, respectively, on the basis of lipids within the hydrolysis solids. Considering that about 80-90% of lipids in the original algal biomass were retained in the solids recovered after hydrolysis, the authors noted, the total process yield was somewhat lower.
...dewatering and drying remain energy- and cost-intensive processes. A recent life-cycle assessment (LCA) of algal biodiesel production from Chlorella vulgaris indicated that drying and hexane extraction accounted for up to 90% of the total process energy. These data indicate that drying algal biomass and treating it as a substitute for terrestrial oilseeds in traditional solvent extraction and subsequent transesterification processes is not likely to be a net energy positive route toward sustainable biodiesel production.
A biodiesel production process that obviates biomass drying and organic solvent use for oil extraction could lead to significant energy and cost savings...Herein we propose a two-step, catalyst-free biodiesel production process involving intracellular lipid hydrolysis coupled with supercritical in situ transesterification (SC-IST/E).
—Levine et al.
The team used the alga Chlorella vulgaris as the lipid-rich feedstock (53.3% lipids as FAEE). In the first step of the process, the 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, and produce a sterile, nutrient-rich aqueous phase.
In the second step, the wet fatty acid-rich solids undergo supercritical in situ transesterification (SC-IST/E) with ethanol to produce biodiesel in the form of fatty acid ethyl esters (FAEEs).
Longer time, higher temperature, and greater ethanol loading tended to increase crude biodiesel and FAEE yields, which ranged from about 56-100% and 34-66%, respectively, on the basis of lipid in the hydrolysis solids.
More remains to be understood regarding how whole cells, hydrothermally processed algal biomass, and intracellular constituents influence SC-IST/E and potentially contribute to nonester components in the final fuel product. Additional research and process optimization are likely to improve yields and reduce process inputs (e.g., ethanol), thereby minimizing the overall environmental impact of algal biodiesel production. To be economically viable, biodiesel yields must be above 95% and preferably higher than current norms achieved with alkali-catalyzed processes (~97%).
—Levine et al.
Robert B. Levine, Tanawan Pinnarat and Phillip E. Savage (2010) Biodiesel Production from Wet Algal Biomass through in Situ Lipid Hydrolysis and Supercritical Transesterification. Energy Fuels, Article ASAP doi: 10.1021/ef1008314