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Global climate change is one of the biggest challenges facing the world. Global economy is critically dependent on fossil fuels. Environmentally benign and sustainable technologies that mitigate carbon dioxide emissions are some of the options to reduce/reverse the impact on climate. Cultivation of algae (Fig. 1) using flue stack gases from coal power plants and nutrient rich waste waters is one of the sustainable alternatives that can also reduce dependence on foreign oil.
Algae are microscopic plants that can grow in diverse environmental conditions. Many species store up to 60% of their body weight (dry basis) as lipids. Due to their higher productivities (> 30 times compared to soybeans) and higher lipid content their potential for biodiesel production has been extensively investigated. Aquatic species program of the U.S. Department of Energy identified, amongst others, cost of production as a 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, nutrient costs. Cultivation of micro algae near power plants can provide a cheap source of carbon dioxide thus reducing production costs. It has been shown by many researchers that algae grow optimally in an enriched carbon dioxide (3-5% v/v) medium that can be easily produced by blending air and the flue stack gases from power plants. These strategies have been studied by a number of research groups since 1960’s, notably by Prof. Mayers group in Israel (REF). Many advances in technology have made it possible to overcome difficulties faced by earlier researchers and lead to a commercially viable solution for carbon dioxide mitigation.
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 incoming solar insolation. 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 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.
However, capital and operating costs are also lower as compared to photobioreactors. Therefore to achieve economically viable algal biodiesel production it is essential to design efficient systems combining both open pond and photobioreactor technologies with optimized performance for the algae strains. Additionally, development of efficient processes to recover algae is critical for economic viability of algal biodiesel. Most of the proposed strategies to recover algae from the growth media such as centrifuges, screens and bio/chemical flocculation are expensive or unreliable in a large scale operation. Although some of these processes are used in commercial production of Spiriluna, the economics of operations are different economics as it is sold as human food. Therefore to produce biodiesel from algae, it is critical to use simple, reliable and low cost algae recovery processes.
Rapid sand filtration is one of the low cost technologies that could be used to harvest/concentrate algae after cultivation in open pond systems. In some of the literature in the rapid sand filter design, it is indicated that the process is effective for particles >100m assuming no interaction effects between algae and the sand particles. However, most of the algae of interest such as Chlorella and Dunaliella sp. are <10m. Some researchers were able to achieve >90% separation using smaller effective particle size and lower flow rates. Hence there are conflicting reports of the efficacy of this process in literature.
The objectives of this senior design project will be to design, develop and evaluate some elements of the integrated micro algae cultivation and processing system utilizing carbon dioxide as carbon source to produce renewable fuel and fertilizer.
The specific objectives of the work are: