Microalgae are a promising renewable source of lipids for biofuel production that could reduce dependency on fossil fuel sources. Additionally, algae are renewable energy source for lipids and proteins co-products similar to soybeans. Protein from soybeans has been used in plastics and adhesives for construction materials since the 1930’s. Unlike soybeans, algae can be grown on marginal lands or in brackish water, so competition for farmable lands and valuable fresh water currently used for producing human food and animal feed can be avoided. Large scale production of algae has the potential to produce lipids and co-products at high levels. In addition to their potential as a renewable source of lipids for biofuels and value-added protein co-products, algae has the ability to utilize growth nutrients from a variety of wastewater sources, providing the additional benefit of wastewater remediation. As plants that undergo photosynthesis, algae consume carbon dioxide emitted from fossil fuel-fired sources, reducing emissions of a major greenhouse gas.
Challenges to widespread utilization of algae for biofuels include: large-scale algae culturing, algae harvesting, and extraction of oil and protein co-products for biofuel production. Two main algae culturing techniques include outdoor open-pond systems and closed photobioreactors. Many different designs have appeared for open-pond systems, but three major types succeeded and are still operated at the commercial scale: raceway ponds, circular ponds, and unstirred ponds. Among the three, raceway ponds are the most commonly used open systems for commercial algae culture because of their comparatively low construction and maintenance costs. Closed photobioreactors culture systems are not directly exposed to the atmosphere; rather they are covered with a transparent material or contained within transparent tubing. Of the different designs of closed systems, most consist of tubes of various shapes, sizes, and lengths constructed of various transparent materials such as glass and plastic.
Several harvesting technologies have been investigated for microalgae. Each presented significant challenges depending on the surface physical and chemical properties of cells. Large equipment and machines are also available for algae harvesting, such as centrifuges and vacuum filters. These equipment, however, are very algal species dependent. Different sizes or cell densities will give different harvesting efficiency. Conveyor belts with textured steel surfaces have recently shown encouraging results as a new method of algae harvesting.
Several algae cell disruption methods have been investigated including: direct extraction, sonication, French press, bead-beater and wet milling . These methods are effective at the laboratory scale but have not been viable at the commercial level for biofuel production. Large equipment and machines, such as bead mills, wet grinding mills, and expellers are commercially available, but are not developed to handle microalgae cells.
Currently, algae lipid production is still too costly, and algal protein extraction methods are not feasible on the industrial scale. Even with highly productive algae producing biomass at near theoretical levels, the current technology is significantly more expensive than fuels generated from petroleum sources. Research interest in algae biofuels has increased around the world and promises advancements to meet the culturing, harvesting and oil extraction challenges in ways that make algae biofuels economically feasible. ExxonMobil has invested $600 million on algae-based fuel research and development in 2009, and algae production facilities are currently operating in the United States and New Zealand.
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