Gargano, Immacolata (2015) INTENSIVE CULTURE OF AUTOTROPHIC MICROALGAE FOR ENERGY VECTOR PRODUCTION. [Tesi di dottorato]

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Abstract

The supply of renewable feedstocks for the production of convenience goods combined with the carbon capture and storage is considered a promising solution to both fossil resources depletion and global warming control. Photosynthetic microorganisms, e.g. microalgae, are good candidates for this challenging bet. Indeed, autotrophic microalgae fix CO2 and are feedstocks for several industries involved in human nutrition, animal nutrition, cosmetics, high-added value molecules, pharmaceuticals, biofuels and wastewater treatments. A critical issue for the intensive production of microalgae is the design of low cost and high efficiency photobioreactors. Main critical issues for the exploitation of microalgae as energy vectors are the biomass dryer and lipid extraction. This PhD thesis was focused on the development of solutions for the critical issues listed above. The main activities were: selection of microalgal strains representative of the main phylum; the optimization of the microalgal cultures by the selection of photobioreactor design and operating conditions (effects of photobioreactor hydrodynamics, light intensity and trend, CO2 in the gas phase, medium composition and medium pH on microalgal cultures in cylindrical and parallelepiped shape photobioreactors); the characterization of the composition of microalgae during the day/night cycle as an effect of the circadian behaviour of microalgae; the characterization of the microalgal growth rate under controlled operating conditions; the characterization of the photochemical process; the optimization of the direct transesterification on wet and dried microalgae to produce biodiesel. Main results were reported. The CO2 concentration typical of power plant exhaust gas (up to 15-18%) enhances lipid productivity, notwithstanding the inhibition effects on pigment synthesis. The optimal pH has been found to be 7.0, but satisfactory microalgae and lipid productivities have been obtained at low pH. The ability of microalgae to be active still a low pH suggests the real possibility to adopt acid conditions to preserve cultures from contamination. Continuous light for 24 hour did not affect the biomass and lipid productivity. The kinetic characterization of the photosynthetic reaction centres in microalgae by means of fluorescence methodology pointed out that: at irradiance lower than 1000 μE m-2 s-1 the photochemical process is controlled by the photons capture while at higher irradiance the photoinhibition competes with the photochemical quenching. The time-scale of repairing process is larger than the other photochemical process whatever the investigated irradiance. The optimization of the alkaline direct transesterification was carried out on Stichococcus bacillaris and validate on microalgal strains characterized by a different cell wall structure. Triglycerides were not converted without an alkaline catalyst and approached a maximum conversion at a catalyst concentration of 1.5% NaOH (w/w). Under alkaline conditions the pre-mixing time did not affect bio-oil yield; the bio-oil yield increased with temperature and approached a maximum at around 65°C; the bio-oil yield did not change significantly with the methanol to biomass weight ratio when the methanol is not limiting; the bio-oil yield gradually increased within the first minutes of reaction. Biomass drying was observed to play an important role in direct transesterification: the bio-oil yield reduced with an increase in biomass water content. A higher bio-oil yield was obtained increasing the methanol/wet biomass ratio. Under alkaline catalyzed conditions the direct transesterification process was more efficient than the acidic ones. The characterization of the specific growth rate and the biomass composition during the day/night cycle of Nannochloropsis sp. was carried out by means of a turbidosotat photobioreactor. Two irradiation strategy were been investigated :"constant irradiance day/night cycle" 16 h of light at 600 μE m-2 s-1 and 8 h of dark; "circadian cycle" 16 h of irradiation at intensity characterized by time-sinusoidal path and maximum of 1500 μE m-2 s-1 and 8 h of dark. The role of circadian clock and of the cell division of Nannochoropsis sp. has been highlighted during tests. The fraction of carbohydrate, protein and lipid of the biomass changes over the day: the lower value has been measured after the cell division, while the maximal values were obtained before cell division. The harvesting during the day of the biomass should be carried out when the concentration of the selected constituent is high. A model to describe the change of the specific growth rate of microalgae during the day was been proposed and validated. The optimization on new photobioreactor systems needs to take into account the different time scale of the photochemical process and the instantaneous growth rate of microalgae. Analysis of the microalgal circadian clock is necessary to understand the right instant of the day at which to harvest the biomass with the set composition. It is also possible suggesting to develop systems to maximize the production of microalgal fractions (carbohydrates, lipids, proteins) according to the application of the process. However, the exploitation of only one microalgal fraction may not be in agreement with the maximum economic profit of the process.

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