Making Biofuel from Microalgae
So much potential coexists with so many scientific, environmental and economic challenges
Macro Versus Micro
First, it’s important to get more specific about what type of algae has landed in the alternative-fuel spotlight. Macroalgae, the seaweeds, grow in open waters, both fresh and marine. These aquatic plants are made up mainly of carbohydrates and have been harvested for centuries as food, including the nori used to wrap sushi, and thickening agents such as agar. The Aquatic Species Program explored the potential of macroalgae as fuel but dropped that project due to the significant challenges related to harvesting costs and fuel conversion. Microalgae, on the other hand, are unicellular photosynthetic microorganisms. They are ubiquitous in nature, found in freshwater, seawater, hypersaline lakes and even in deserts and arctic ecosystems. They can be further subdivided into two main categories: eukaryotic algae, possessing defined organelles such as nuclei, chloroplasts, mitochondria and so on, and prokaryotic algae (cyanobacteria or blue-green algae), possessing the simpler cellular structure of bacteria. Although the relatedness of cyanobacteria to nonphotosynthetic bacteria allows for exploitation of genetic-engineering technologies and makes them an attractive starting point for biofuels research, they lack one very important thing that eukaryotic microalgae can possess in abundance—neutral lipids, which are rich in triacylglycerols (TAGs).
Of the eukaryotic microalgae, green algae are the taxonomic group most often referred to as oleaginous, or oil-rich, microalgae. They are ubiquitous in a variety of habitats and grow faster than species from other taxa, and as much as 60 percent of their cell dry weight can be oils. However, the composition of the oils is highly dependent on the species and the conditions in which the algae grow. Oils that are rich in neutral lipids are desirable in a biofuel context because of their potential high fuel yield. Because TAGs are made up of three molecules of fatty acids that are esterified—or altered—to one molecule of glycerol, close to 100 percent of their weight can be converted into fuels. With polar lipids, on the other hand, only one or two fatty-acid molecules are esterified to glycerol and the remaining components (e.g. sugars or phosphate groups) cannot be converted to fuel feedstock. As a result, these types of lipids generate lower fuel yields.
Fatty acids, the building blocks for lipids, are synthesized by enzymes in the chloroplast, of which acetyl-CoA carboxylase (ACCase) is key in regulating the synthesis rates. When cells are actively growing, their metabolic focus is on photosynthesis and the production of biomass. The fatty acids produced are mostly found in polar-membrane lipids, such as phospho- and glycolipids, which are invaluable to photosynthesis. Unfortunately only about 30 to 50 percent of polar lipids can be converted into fuel molecules. But when the cells experience metabolic stress, such as a lack of essential nutrients, including nitrogen, cell metabolism is redirected to reduce the growth rate and favor the production of carbon-storage compounds, mainly carbohydrates and TAGs. Little is known about the regulation of TAG formation at the molecular and cellular level, but greater understanding could lead to the engineering of algae with higher ratios of neutral lipids.
Organic solvents can extract oils from actively growing cells. But oils extracted from stressed cells yield more fuel. In Chlorella vulgaris, a strain that our laboratory has studied extensively, the extracted oil content amounts to about 30 to 50 percent of the biomass under both active-growth and nutrient-limited conditions. However, the fatty-acid content, reflecting the potential fuel yield, can vary from 10 to 50 percent of the biomass over the growth cycle. This illustrates the big discrepancies often seen between the extracted algal oils and the actual fuel-yield potential.
Unlike typical terrestrial oil-producing plants, in which specialized cells yield oils, every algal cell can produce oils. Algal oils, just like oils produced by soy, canola, palm and the less-known jatropha plants, can be made good biodiesel feedstocks through transesterification. In that process, a catalyst creates a biodiesel fuel (consisting of fatty acid methyl esters) by hydrolyzing and methylating fatty acids in the oils. Refining the mixture is typically the next step and involves removing the non–fatty-acid components—such as glycerol, polar lipids and residual pigments—from the fuel. Typical refinery processes such as hydrotreating, cracking and isomerization of the algal oils can also be used to produce renewable gasoline, diesel or jet fuels. These so-called drop-in fuels are much more like traditional petroleum-based fuels and can be blended, like for like, in existing fueling infrastructure.
Once algal oils have been extracted with organic solvents or removed in some other way, the remaining biomass will be made up of approximately equal amounts of carbohydrates and proteins. We expect that this residual material can be used as a feedstock for so-called coproducts to help the overall economics of algae farming. Carbohydrates can be used to produce methane by anaerobic digestion or ethanol by fermentation. Proteins can be used for animal feed or even human food. Other higher-value algal products such as omega-3 fatty acids and antioxidants are already available commercially, but the potential market for biofuels dwarfs market for these nutraceuticals. The search for high-value coproducts with large market size remains an elusive goal for an integrated biorefinery based on algal biomass.
Research published this year by Mark Wigmosta and coworkers at the U.S. Department of Energy’s Pacific Northwest National Laboratory evaluated the amount of land available for cultivation of microalgae and the availability of needed inputs such as water, carbon dioxide and inorganic nutrients—basic requirements for algal growth. This report used fairly conservative assumptions for algal growth rates and lipid content, based on current technologies, and arrived at a value of 57 billion gallons of lipid-based algal biofuels per year. Thus algae represent a feedstock source that could be comparable in size to all the terrestrial biomass that could be harvested for biofuel production combined.
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