Comparison of Marine Microalgae Culture Systems for Fuels Production and Carbon Sequestration

Abstract

The dual problems of global fossil fuels supplies and global warming focus attention on the need to develop technologies that can provide large amounts of renewable fuels without contributing to global warming. The capture of power plant flue gas CO2 using microalgae cultures is one potential technology that could meet this objective. The central R&D issues are the design and operation of low-cost algal mass culture systems and the development of algal strains and cultivation techniques that can achieve very high biomass productivities.The major objective of this project was to develop mass culture techniques that could result in greatly increased biomass productivities, well above the about 50 metric tons per hectare per year (mt/ha/y) currently achievable. In this project, two marine microalgae species, the diatom Cyclotella sp.. and the green alga Tetraselmis sp., were cultivated on seawater in both open ponds and closed photo bioreactors, under a variety of different cultivation conditions. Simultaneous operation of the closed photo bioreactors and open ponds demonstrated similar productivities, under the same operating conditions. Thus the very expensive closed systems do not provide any major or inherent advantages in microalgae production over open ponds.Mutants of Cyclotella sp. were developed that exhibited reduced pigment content, which theoretically would result in greatly increased productivities when grown under full sunlight.However, in open ponds, these mutant strains exhibited similar productivities as the parental strains. The mutant strains all grew relatively slowly, suggesting that additional mutations masked whatever inherent potential for increased productivities may have resulted from the reduced pigment content. Research is still required to develop improved low pigment strains.When open pond cultures were exposed to intermittent sunlight, by partially covering the ponds with slats, solar conversion efficiencies increased dramatically, by over 50%. Although such techniques are not directly applicable to practical processes, the experiments demonstrated the inherent potential of algal mass cultures to achieve very high productivities.Nitrogen limited pond cultures demonstrated that it is possible to produce biomass with a potentially high content of carbohydrates or oils (although these were not directly measured in these experiments), without reducing achievable productivities. This suggested that microalgae biomass suitable for conversion to biofuels (ethanol or biodiesel) could be produced without compromising productivity. Experiments combining both light modulation and nitrogen limitation indicated possibly synergistic effects.The goal of developing practical and economic processes for the sustainable production of renewable fuels with microalgae pond cultures using power plant flue gases as sources of CO2 was advanced by these studies, but requires more work. Most important is the research, development and demonstration in outdoor pond cultures of algal strains with low pigment conte...