| For the last few decades, rapidly developing human society has been highlighting mankind’s continued existence on viable energy supplies. Fossil fuels, the dominant form of energy at present, are not renewable and are the major cause of many environmental problems. As a result, clean and renewable forms of energy are gaining attention among people who want to pursue sustainability and eco-friendliness. Biodiesel from microalgae is a great alternative to fossil fuel. Microalgae, with advantages of rapid growth, high lipid content, high photosynthesis rate and suitability of its lipid for biodiesel for production, is getting more and more attentions.Although the culture and harvest of microalgae biomass and the extraction and refinement of microalgae lipid are not of difficulty from a technical standpoint, problems such as low efficiency and high costs in these processes are preventing the realization of commercial establishments for the purpose of microalgae biodiesel production. Therefore, this thesis selects a novel method of attached microalgae culture as its focus point, and studies the effects of nitrogen sources and initial nitrogen concentration on attached microalgae, a kinetic model for attached microalgae concerning growth and storage molecule accumulation, and a novel design of photobioreactor featuring attached microalgae culture, according to which a prototype was built and experimented with. Detailed informations are as follows:In this thesis, to understand the culture conditions concerning nitrogen in attached culture, freshwater microalgae Scenedesmus dimorphus and marine microalgae Nannochloropsis oculata were attach-cultured with nitrogen sources of urea, potassium nitrate and glycine at different initial concentrations, respectively. Different lipid enhancing strategies (e.g. nitrogen-starvation, high light stimulation and combination of both) were applied to investigate the effects on lipid content. The results and conclusions were 1) Optimal nitrogen conditions:for S. dimorphus, the highest adhesive biomass productivity (11.02±0.05 g m-2 d-1) and adhesion rate (98.50±0.10%) were achieved with 12 mM potassium nitrate; for N. oculata, the highest biomass productivity (6.89±0.24 g m"2 d-1) and adhesion rate (77.09±0.42%) were achieved with 18 mM glycine.2) Lipid enhancing strategies:for S. dimorphus, the highest lipid content (32.97±0.22%) was achieved with nitrogen starvation; for N. oculata, the highest lipid content (20.24±0.06%) was achieved with the combination strategy.3) lipid content of attached microalgae could be effectively enhanced by nitrogen starvation; 4) The combination strategy of nitrogen starvation and high light stimulation was considered optimal in regards of lipid content and adhesive biomass productivity.In this thesis, a kinetic model describing biomass X, storage molecule S, chlorophyll B and nitrogen N was proposed. The model was simulated with MATLAB SIMULINK, with Parameter Estimation module being used in parameter regression. Initial values and ranges of pre-estimated parameters, as well as experiment data used, were obtained in literature. A set of estimated parameter was used in model simulation, which resulted in good fit with another experimental data, indicating the accuracy of estimated parameters. The proposed kinetic model was operational in growth prediction for attached microalgae and valuable in photobioreactor design and culture condition selecting.In this thesis, a photobioreactor design featuring vertical adhesion substrates for microalgae was proposed, in which microalgae was attached to adhesion substrates and provided nutrients via culture medium circulation system. Multiple adhesion substrates were installed 1) vertical to the ground for less foot area; 2) parallel to incident light for "light dilution" to effectively use the light energy as well as to avoid photoinhibition. A laboratory-scale prototype was built with working volume of 71 and foot area of 0.06 m2. Tests were conducted under indoor and outdoor conditions, in which the highest adhesive biomass productivity (22.35 g m-2 d-1) and adhesion rate (16.60%) were achieved by S. dimorphus. Harvesting biomass from adhesion substrates were as easy as a few proper squeezes, and microalgae broth with low water content was obtained, in which case could lower the costs of harvesting and dewatering compared to those of suspended culture. A component analysis of extracted lipids carried out by using GC-MS showed that the major components were FAMEs with carbon chain length of C16-C18, indicating the suitability of which for biodiesel production. The simulation run by the above mentioned kinetic model was in agreement with experimental results. In summary, the proposed photobioreactor was able to perform microalgae culture of high density and high efficiency, as well as reducing the costs and complexity of harvest. In addition, it possesses great potential in regards to commercial application. |
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