Algae are simple, plant-like organisms but do not have roots, stems, and leaves. There are almost 300,000 alga species distributed throughout the world in seawater, freshwater, and wastewater. Algae are typically photosynthetic, namely fixing CO₂ in the presence of sunlight to manufacture their own food, but some are heterotrophic with no requirement of light, assimilating organic compounds such as glucose and acetic acid as carbon sources. Most of the current research and development efforts have focused on microalgae due to their high growth rate and oil content.
Algae contain oils, sugars, and functional bioactive compounds that can be used for commercial products. Recently, special attention has been given to cultivation of microalgae as an "energy" crop with the aim of replacing traditional oil crops for biodiesel and bio-oil production.
Oil-rich microalgae species are the most productive fuel crops, providing 10–100 times higher biomass and oil yield than land oil crops (Table 1). For example, algae with 50% lipid content and a dry biomass productivity of 50 g/m²/day can potentially produce 10,000 gallons oil/acre/year (Pienkos, 2007). By comparison, soybeans only produce 48 gallons oil/acre/year. Algae can be cultivated under different climatic conditions and harvested year-round, but do not compete with arable land for food and feed production.
Microalgae also have many potential environmental benefits, including greenhouse gas mitigation by fixing CO₂ in the atmosphere, i.e., photosynthesis of CO₂ to fuel by sunlight, and bioremediation of wastewater by efficiently removing nutrients (e.g., nitrogen, phosphorous) and heavy metals.
Algae need light, CO₂, water, and macro- and micronutrients for photosynthesis. Sunlight, air, and seawater or waste water can basically meet the requirement for algae growth. However, optimal cultivation conditions can achieve better algae growth. For example, light wave-lengths between approximately 450 nm (blue) and 650 nm (red) are usually preferred (Marsh, 2008). The light intensity is also important as photo inhibition occurs after exposure to too intensive light. For some algal species, a dark cycle is important for processing photosynthates produced in a light cycle for lipid production. The typical temperature range for algal cultivation is 25–35°C. CO₂ supplied at an optimal concentration 350–1000 ppm and flow rate is also important for maximum algae growth, as is water salinity and macro- and micronutrients in water. The pH of the culture media is preferred to be maintained between 7 and 9. Mixing is required to make sure algae are evenly exposed to light and nutrients.
Algae can be cultivated in low cost production systems such as open ponds. This system is typically operated in a continuous mode with a fixed supply of culture media, water, and nutrients. Contamination is the main risk of open systems. Cultivating algae in an enclosed bioreactor, in which the system is strictly controlled and no contamination occurs, is an alternative to overcome the problems with open systems. However, the high investment and operating costs are the main problem for the enclosed bioreactor. This system is suggested for algae cultivation for production of high value fatty acids (e.g., eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA]).
|Table 1. Comparison of potential oil yields of algae and other oil seeds (Pienkos, 2007).|
|Crop||Oil yield gallon/acre|
|10g/m²/day at 15% Triglycerides||1,200|
|50g/m²/day at 50% Triglycerides||10,000|
Harvesting and Processing Algae
Microalgae cell harvesting generally involves two major solid-liquid separation steps. The first step, flocculation, aggregates the algal cells and improves the effectiveness of the second step. Several methods such as filtration, centrifugation, or gravity sedimentation can be used in the second step for microalgae biomass harvest. Filtration is suitable for the large size microalgae but has low efficiency. Centrifugation can harvest most kinds of microalgae but can damage the cell membranes due to high centrifugation speed and shear stress. Gravity sedimentation is generally used for sewage-cultured algae recovery, but is time consuming and requires space for settling ponds or tanks. Compared with other recovery methods, microalgae biomass from sedimentation generally has a much higher moisture content, which can substantially increase the cost of biomass drying for the further downstream process. Therefore, filtration or centrifugation preceded by flocculation-flotation is generally adopted as the method for microalgae biomass recovery.
Oil extraction methods used for vegetable oil extraction are suitable for microalgae oil extraction. Three well-known oil extraction methods are press extraction, hexane solvent extraction, and supercritical fluid extraction. Some new oil extraction methods are emerging, including enzymatic extraction, osmotic shock, and ultrasonic-assistant extraction.
Converting Algae to Biofuels and Bioenergy
Algal oil can be converted to biodiesel and other fuels via several processes (Figure 2). During transesterfication, algal oil reacts with alcohols in the presence of a base catalyst to produce glycerol and biodiesel (methyl or ethyl ester). Biodiesel yield from transesterfication is more than 90% and the biodiesel quality is comparable to conventional petroleum diesel (Amin, 2009). For some algae species, the whole cell can function as a bioreactor to directly produce fuels such as hydrogen and ethanol. The starch-rich algae can also be used as feedstock for fermenting ethanol and hydrogen. Thermo chemical conversion of oil-rich algae can produce fuel gas and/or bio-oil. A technology for gasification of algal biomass with high moisture contents has been developed to produce methane and hydrogen (Elliot and Sealock, 1999). Thermal reactions by pyrolysis, liquefaction, or hydrogenation mainly convert algae to bio-oils. Algae residues remaining after oil extraction or thermal conversion can be digested under anaerobic conditions to produce biogas for electricity.
|An open pond for growing algae in Israel (Source: Seambiotic).||Bioreactors used to grow algae (Source: PetroAlgae).|
|Figure 1. Typical algae cultivation systems (LaMonica, M. 2008a &b).|
Future of Algae for Biofuel and Bioenergy
Algae has the potential to be a significant source for the biofuels described above. However, the high cost of algae production remains a hurdle. An algal strain with high lipid content and high growth rate, a cost-effective cultivation system (open or closed system), and low cost harvesting and lipid extraction systems are critical to the future of a commercial-scale algae to fuel production system.
With depletion of petroleum-based fuel sources, rising crude oil and gas prices, and global warming related to use of fossil fuels, domestic production of biofuels and bioenergy from renewable resources could become attractive. The advantages of renewable energy and products, such as replacement of fossil fuel, biodegradability, and reduced air pollution and greenhouse gas emissions, can outweigh disadvantages of higher current cost and lower fuel economy. Current biodiesel production uses renewable resources but these sources face a dilemma in that they are either competing with food supplies (i.e., oilseed crop) or cannot meet U.S. diesel demand (i.e., waste oils). By producing algae instead of land oil crops for biodiesel production, farmland can be reserved for food production. Since resources for algae cultivation (e.g., water, land, CO₂, sunlight, nutrients) are easily accessible, algae have a good potential to be commercialized in the future.
|Figure 2. Conversion of algae to biofuels (Revision based on Pienkos, 2007).|
- AlgaeVenture Systems. algaevs.com
- Amin, S. 2009. Review on biofuel oil and gas production processes from microalgae. Energy Conversion and Management. 50: 1834–1840.
- Elliot, D.C., and Sealock, L.J. 1999. Chemical processing in high-pressure aqueous environments: Low temperature catalytic gasification. Trans IChemE 74: 563–6.
- LaMonica, M. 2008. Joint venture to use coal emissions to grow algae for biofuels. news.cnet.com/8301-11128_3-9973649-54.html.
- LaMonica, M. 2008. Race to algae-based biodiesel heats up. cnet.com/news/race-to-algae-based-biodiesel-heats-up/.
- Marsh, A.A. 2008. The use of algae. scribd.com/doc/32918391/A-Study-Into-theCultivation-of-Algae-Presentation-2008
- Pienkos, P. T. Potential for biofuels from algae. NREL/PR-510-42414. Golden, CO: National Renewable Energy Laboratory (NREL), 2007.
Reviewed by Drs. Harold Keener and Peter Ling.