By Ganti S. Murthy and David Hackleman
First generation biofuels, such as ethanol from corn are an initial effort to produce transportation fuels domestically and mitigate carbon dioxide emissions.
However, capacity limitations associated with corn production mean that it’s not possible to meet the 150 billion gallons a year of transportation fuels used solely through such first-generation methods. Therefore second-generation biofuels such as ethanol production from cellulosic feedstocks have been proposed. Though available in much higher quantities, cellulosic feedstocks such as corn stover, wheat and grass straw, switch grass, forest thinnings and hybrid poplars still require arable land, fresh water, fertilizers and pesticides for their production. Additionally, geographically diverse sources of production necessitate innovative harvesting, transportation and processing technologies. Domestically, the Renewable Fuel Standard mandates production of 35 billion gallons of biofuels by 2020, and is encouraging innovation.
Due to the challenges in production, harvesting and processing technologies, ethanol from cellulosic feedstocks is yet to be produced in significant quantities. The environmental impact that energy crops have on soil quality and concerns about arable land and water use have led researchers to take a look at a third generation of biofuels - biofuels from algae. According to some estimates, if we allocate about two percent of the total land area of the U.S. for algae biofuels production, we can meet all the current transportation fuels needs.
Algae are microscopic plants that can grow in diverse environments such as the oceans to brackish water lakes in Texas. Seaweed (a macro algae) is also used as a major food supply in East Asian countries, and indeed somewhat here in the U.S. Some strains of algae accumulate lipids (oils) up to 60 percent of their body weight, while other strains accumulate starch. Algae have been shown to yield over 800 gallons per acre, compared to oil yields of 48 gal/acre from soybeans and 600 gal/acre from oil palms grown in tropical regions, It is believed that algae has the potential to produce up to 5000 gal/acre of biodiesel, given proper strain selection and nutrient supply. Due to their higher productivities (~30 times compared to soybeans) and higher lipid content, their potential for biodiesel production has been extensively investigated. An aquatic species program of the U.S. Department of Energy was initiated in the late 80’s for investigating the biofuels potential of algae. The program identified the cost of production as one principal obstacle in adoption of the algae technology on a large scale (Sheehan et al, 1998). Some of the important factors in high production costs were low productivities and high nutrient costs.
Algae biofuels have the potential to be a sustainable alternative to producing energy without concerns about food vs. fuel, or the use of valuable agricultural lands.
The simplest approach for utilization of algae biomass is in gasifier processes that combine heat and power production. Another approach is based on existing anaerobic digestion technology, which utilize algae biomass for methane production. This approach has the advantage of employing wet algae without the need for additional drying. However, for bio-fuels production, the processes for algae require that the biomass be dried and then the oil present extracted. Oil in algae can be converted to biodiesel using the existing standard and commercially available technology. The remaining solids that are rich in protein and carbohydrates can be used as protein rich animal feed. Additionally, the carbohydrates in the algae biomass can be converted into sugars and fermented to produce ethanol.
past and present
Micro algae can be produced in two systems: open ponds and photobioreactors. Open pond cultivation systems generally consist of shallow (30-50cm) open trenches with paddle wheels to circulate water. Temperature is not generally controlled and light intensity is dependent on sunlight. Photobioreactors are closed bioreactors that permit exchange of light and energy without material exchange from surroundings. Growing algae in photobioreactors confers many advantages, such as greater productivity, reduced contamination, and better control when compared to open pond systems. Since operating conditions in open systems are not completely controlled and they have greater possibility of contamination, the productivities are lower compared to photobioreactors (Chisti, 2007). However, capital and operating costs are also lower as compared to photobioreactors Benemann and Oswald (1996).
Algae strains have a significant influence on production costs and end products. More than 3000 strains of algae have been identified for their potential use in biofuels production as a part of Aquatic Species Program. Currently many of these strains are part of collections at University of Hawaii and University of Texas for academic and industrial research. Strain selection has to be optimized based on desired end product, location and method of production.
Similar to terrestrial plants, algae requires water, nutrients and CO2 for their growth. Agricultural waste water, partially treated municipal sewage water and industrial waste waters can be used as potential sources of nutrients for algae. For example, Sawayama (et al 1995) investigated production of algae oil produced using a thermochemical process from Botryococcus braunii grown of secondary treated sewage. Algae also have been shown to selectively absorb heavy metals present in waste waters. Selected strains of algae can be grown to remove trace amounts of heavy metals and purify waters. Removal of nutrients from waste waters by algae will reduce the biological oxygen and chemical oxygen demand of the waste waters. This water can be used for irrigation of agricultural crops or industrial use. Researchers have shown that algae can grow faster in a CO2 rich medium. Using algae for biological sequestration of CO2 in flue gases has also been studied. Algae grow optimally in an enriched carbon dioxide (3-5 percent v/v) medium that can be produced by blending air and flue stack gases from coal power plants (Sawayama et al, 1995; Maeda et al, 1995). Maeda (et al in 1995) investigated a high temperature tolerant (35°C) Chlorella strain that could grow well in up to 15 percent CO2. They also studied growth rates at various sulfur oxide (Sox) (80 ppm. max.) and nitros oxide (Nox) (240 ppm. max.) concentrations. Brown (1996) reported that Monoraphidium minutum can tolerate even higher SOx (200 ppm.) and NOx (150 ppm.) levels. Some of the NO dissolved in water and was available as nitrite suggesting increased nitrogen availability for algae. Cultivation of micro algae near power plants can provide a cheap source of CO2, thus reducing production costs. A few commercial companies utilizing CO2 from power plants are in initial stages of development.
In order to achieve a viable commercial production of algae biofuels, three critical aspects of the algae system need to be mastered: production, recovery and processing. Efficient production of algae biomass by leveraging waste water treatment, use of flue gases and waste heat can reduce the production cost of algae as a feedstock for fuels and chemicals. The second aspect of the integrated system is the separation and recovery of algae from the medium. Researchers have investigated centrifugation, chemical and biological flocculation, hydrocyclones and filters, to name a few.
The third aspect of the integrated system is the production of fuels and chemicals from algae. Presently, the solvent extraction process technology is the only mature technology for extraction of algae oil from algae biomass. In this process, the oil present in the algae is dissolved using an organic solvent such as hexane. The oil/ hexane mixture is distilled to separate the two, while the remaining solids are dried and can be used as animal feed. However, the use of algae biomass in animal feed has to be monitored for presence of free nucleic acids which can cause decreased weight gain in animals. Heavy metals contamination is an issue that requires strict monitoring.
Much attention is being focused on development of integrated systems that combine waste water use and CO2 from power plants for algae production to reduce costs. Research is also directed towards strain improvement for producing specific high value products from algae. Harvesting and processing technologies are being developed to reduce the overall process costs, with an aim to bring the algae biomass to about $400 a ton.
Biofuels production from algae is a long-term sustainable alternative that does not divert valuable agricultural land away from food production. Algae can be grown on waste waters and at higher productivities using CO2 from power plants. These strategies–while improving water and air quality–also reduce production costs by supplying nutrients required by algae for their growth.
Algae biomass thus produced can be used for sustainable production of methane, biodiesel, bioethanol and animal feed using one of the many technology options.
Presently, significant technological hurdles exist for large-scale commercial production of algae biofuels. However, the advantages are compelling. By not diverting resources from food production–or requiring fresh water for production–algae biofuels are one of the long-term solutions to challenges humanity faces today.
Benemann, J. R., Oswald, W. J. 1996. Systems and economic analysis of microalgae ponds for conversion of to biomass. The U .S. Department of Energy, Pittsburgh, PA. Report:DOE/PC/93204–T5.
Brown, L. M. 1996. Uptake of carbon dioxide from flue gas by microalgae. Energy Convers. Mgmt. 37:1363–1367.
Chisti, Y. 2007. Biodiesel from microalgae. Biotechnol. Adv. 25:294–306.
Maeda, K., Owada, M., Kimura, N., Omata, K., Karube, I. 1995. CO2 fixation from the flue gas on coal-fired thermal power plant by microalgae. Energy Convers. Mgmt. 36:717–720.
Sawayama, S., Inoue, S., Dote, Y., Yokoyama, S. 1995. CO2 fixation and oil production through microalgae. Energy Convers. Mgmt. 36:729–731.
Sheehan, J., Dunahay, T., Benemann, J., Roessler, P. 1998. A look back at the us deparment of energy’s aquatic species program-biodiesel from algae. National Renewable Energy Laboratory, Golden CO. Report:NREL/Tp–580–24190.
Ganti Murthy is an Assistant Professor in the Oregon State University Biological and Ecological Engineering Department. He is conducting active research in energy development from renewable biological resources including Algae. http://bee.oregonstate.edu/Faculty/Murthy/Murthy.htm.
David Hackleman has retired from the position of Linus Pauling Chair, College of Chemical, Biological, and Environmental Engineering and continues to have research in the area of sustainable energy development.