Making Biofuel from Microalgae
So much potential coexists with so many scientific, environmental and economic challenges
Dueling Cultivation Models
Two basic cultivation systems have been developed for photosynthetic microalgae agriculture: open ponds and closed photobioreactors. The ponds can be simple and shallow, 20 to 30 centimeters deep, with little or no mixing. Or they can be oval raceways with dividers in the middle and paddlewheels to keep the water moving. Ponds represent the cheapest possible system (especially if they consist of simple trenches in the ground with no liners), but they provide limited protection against pests, predators and pathogens that can drop from the air. Ponds also tend to have a relatively low surface-to-volume ratio. In dense cultures, too many algal cells can get stuck in the shade, especially without adequate mixing.
Closed photobioreactors must be constructed of clear materials such as glass or plastic and can come in a variety of configurations, such as flat panels, tubes or simple plastic bags that either hang from a support or lie on the ground. Photobioreactors can have higher surface-to-volume ratios, so they reduce self-shading. The closed design can also help keep out unwanted organisms and reduce the amount of evaporation (thus reducing the amount of water needed for continuous cultivation). But they can have problems with CO2 transfer and with heat and O2 buildup. And they are also much more expensive than ponds. Even though the cost of production of algal biomass for high-value products such as nutraceuticals and food supplements is not as critical as it will be for fuel production, most commercial algae enterprises use open ponds. They control the quality of the crop by choosing conditions that encourage the growth of the production strain rather than the drop-ins.
Our evaluation of production economics indicates that open ponds will be much more cost effective than photobioreactors, but our laboratory hasn’t yet advocated for one technology over the other. Both systems have advantages and disadvantages, and both, at this stage, are too expensive to produce biofuels that are cost competitive with petroleum-based fuels. We will continue to monitor developments that will either reduce production costs or help defray those costs, with other improvements or by identification of value-added coproducts.
Cultivation is only one of the challenges to scaling up algal-feedstock production. Harvesting the cells and removing water to facilitate TAG extraction pose commercialization hurdles. Even under ideal growth conditions, it is difficult to get more than 1 to 2 grams of biomass per liter of culture. Aerobic fermentation processes with industrial bacterial strains such as E. coli can reach 100 grams per liter. The low algal-cell densities require as much as a hundredfold concentration of algal cells before the extraction process can be performed efficiently. Centrifugation can easily achieve this sort of concentration, but it is considered too capital- and energy-intensive for use in fuel production. Other methods such as flocculation and dissolved-air flotation have been adapted from the wastewater-treatment industry and are much cheaper. Flocculation uses inorganic ions or organic polymers (or in some cases takes advantage of the intrinsic properties of the algal cell wall) to get cells to clump together. The clumps can be collected by gravity settling or can be brought to the surface by bubbling with air or other gases—dissolved-air flotation. Both of these methods are less expensive than centrifugation but may require modifications for each algal strain and each set of growth conditions, and achieve just a fraction of the cell concentration that centrifugation produces.
Once algal cells have been adequately concentrated, it is still not easy to extract lipids. Vegetable oils can be removed by pressing seeds, but algal cells are too small and tough for efficient pressing. Hexane solvent extraction is useful with soybeans and other oil seeds, but it can be difficult to get the hexane to penetrate the algal cell walls. Often additional steps, such as sonication or mechanical homogenization, are needed to disrupt an algal cell wall and make the lipids more accessible. These steps add to the overall cost and energy requirements, so the search goes on for the process that works at scale with wet algae.
Although lipids are the key algal component for biofuel production, algal carbohydrates and proteins could be feedstock for other energy products. Methane can be produced with anaerobic digestion, and other biofuels can be made using fermentative and catalytic processes. Given this potential, NREL is also developing a strategy for accurate and detailed carbohydrate and protein quantification in algal biomass.