Sub- and Supercritical Water Based Processes for Conversion of Algae Biomass to Fuels and Green Chemicals – A Review

Philip A. Marrone and Bryan V. Yeh

Science Applications International Corporation (SAIC)

60 Wells Ave, Suite 103

Newton, MA 02459

In recent years, there has been increasing motivation to develop alternative liquid fuel and petroleum feedstock options due to concerns over increasing instability in the price and supply of traditional petroleum fuels. Biomass is an ideal feedstock for the production of liquid fuels and green chemicals because it is carbon neutral, is available or can be produced domestically in large quantities, and can spur economic benefits to local industries that produce and/or process it. One type of biomass in particular that has attracted attention is algae. Because of algae’s rapid growth and ability to produce substantial amounts of lipids under the right conditions, there has been significant on-going research and development work on understanding and improving algae growth and processing. On the commercial side, numerous companies have been established to commercialize the process of converting algae to fungible liquid fuels.

Most attempts to process algae into fuel have focused on one of two broad approaches: 1) extract the oil (lipid) content of algae cells after harvesting and convert the oil chemically to biodiesel, or 2) convert the entire algal biomass to fuel via a thermochemical approach and biocrude intermediate. Most processes to date have proven to be economically nonviable. One of the key costs in most processes is drying of the algae after harvesting. A way to get around this bottleneck is to use a hydrothermal medium for algae processing. This not only avoids the high cost of drying but allows one to take advantage of the significant benefits afforded by supercritical water with regards to solubility, density, and mass transport. Although much early work associated with supercritical water centered upon its use as a medium for oxidative waste destruction, more recent interest has been on using it as a medium for non-oxidative thermochemical processing of biomass such as algae. Depending on the conditions utilized, one can generate solid, liquid, or gaseous products which can then undergo further refinement to the desired final product.

This presentation will review and highlight some of the important work performed to date on the use of sub- and supercritical water to process algae into liquid fuels. The focus will include developments both at the research level and commercial scale.