Biofuels from microalgae
Motivation
Microalgae are small (micrometer-scale) unicellular aquatic plants. There are thousands of different species many of which rich in lipids (20-60 % w/w) from which we can make biodiesel. Microalgae can be grown in salt or brackish water as well as freshwater. Their growth is driven by sunlight and the building blocks used are carbon dioxide, water and inorganic nutrients such as nitrate, ammonium and phosphate. As such, microalgae can also be used to remove nutrients from wastewater and carbon dioxide from flue gas (Figure 1).
Under ideal growth conditions 9% of sunlight energy can be stored as chemical energy inside the energy-rich biomass. This fraction is also referred to as the photochemical efficiency.
The photosynthetic pathway of microalgae is almost identical to that of higher plants. Since microalgae are unicellular their cultivations conditions can be tightly controlled in dedicated cultivation systems and, consequently, much higher photosynthetic efficiencies can be achieved: 0.1 - 1 % for higher plants versus 3 - 5% for microalgae (Figure 2). These photosynthetic can be used to estimate arial biomass yields which are in the order of 70 tons of dry matter per hectare per year in state-of-the-art closed photobioreactors. Moreover, it is expected that the photosynthetic efficiency of microalgae cultivation can be further improved with 9% being the absolute maximum.
The high biomass productivity obtained cannot be fully converted into lipids for biodiesel production. At first sight this might seem unfavorable but the biomass remaining after oil extraction contains a lot of value, as well as energy, which can be exploited. Part of the proteins and pigments remaining can be extracted and converted to useful products. Residual biomass (carbohydrates among others) can be used to generate additional fuel or power by biological or thermo-chemical treatment. The remaining inorganic nutrients and water can be recycled.
Challenge
Despite the fact very high productivities can be reached the capital costs as well as the operational costs of existing production systems are high. Especially the energy requirement for mixing and for gas transfer, as well as for microalgae harvesting, are much too high.
Technological and biological solutions must be found to reduce energy requirements for large-scale microalgae production plants. In addition, the productivity must be improved by increasing lipid content and photosynthetic efficiency of microalgae. The integration of oil production with production of value-added compounds and re-use of nutrients and water must yield a sustainable and economic process.
Research agenda
In collaboration with Wageningen University the following research agenda has been identified:
• Reduction of energy input related to CO2 supply and O2 removal
• Increase of lipid productivity of microalgae
• Increase of photosynthetic efficiency in large-scale cultivation systems
• Reduction of energy input related to microalgae harvesting
• Make value from residual biomass, protein
• Analyze scenarios for large-scale algae production sites
Research projects
• Carbon dioxide supply in photobioreactors: a biological approach
• Oxygen removal in photobioreactors: a biological approach
• Biorefinery: make value from protein
• Harvesting of algae for oil extraction
• For efficient, robust and flexible algae plants; an advanced scenario study approach
Participating companies
• Alliander
• Eneco
• Delta
• Essent
• Rosendaal Energy
• Neste Oil
• Hednesford
• Friesland Foods
• Syngenta
• Dow
• Ingrepro
• AF&F
• Hubert Stavoren
