Doctor of Philosophy
Chemical and Biochemical Engineering
de Lasa, Hugo
Microalgae has the potential to contribute to carbon dioxide capture, resulting in the production of alternative fuels and valuable chemical products. To accomplish this, high-efficiency photobioreactors must be conceptualized, designed, and established, in order to achieve high inorganic carbon conversion, superior light utilization, and unique fluid dynamics.
In this PhD Dissertation, experiments with Chlorella vulgaris were carried out, in a 0.175L especially designed PhotoBioCREC unit, under controlled radiation and high mixing conditions. This unique design involves 1 mm-2 mm alumina particles, which keep photoreactor walls always clean, without compromising photon transmittance. Sodium bicarbonate (NaHCO3) was supplied as the inorganic carbon containing culture media. The NaHCO3 concentrations studied were in the 18 mM to 60 mM range. The NaHCO3 concentrations, the total organic carbon concentrations and absorbed radiation were measured every 24 hours. The pH was readjusted every day to the required 7.00 level, with the temperature being maintained at 24.3°C ± 0.5°C.
Results showed 29.6% as the best carbon conversion achieved, with a total organic carbon (TOC) selectivity up to 33% ±2.0, by Chlorella vulgaris. It was found that quantum yield efficiencies, for Chlorella vulgaris culture, in a NaHCO3 solution media, were in the 1.9%-2.3% range. It was also proven that maximum reaction rates for organic carbon formation were achieved with a 28 mM NaHCO3 concentration, displaying a 1.18 ± 0.05 value. Based on the experimental data obtained, a kinetic model for inorganic carbon consumption and organic carbon formation was successfully developed and validated for concentrations of NaHCO3 in the 18 mM to 60 mM range.
Thus, the findings of the present PhD Dissertation allowed one to establish best operational conditions, in the PhotoBioCREC unit, for Chlorella vulgaris growth, in sodium bicarbonate solutions, with high inorganic carbon and photon energy utilization.
Furthermore, the rotating flow design, in the near transmission wall region of the PhotoBioCREC prototype, was also demonstrated in a 10.3 L PhotoBioCREC Swirl Reactor prototype. It was proven in this PhD Dissertation, that this scaled-up unit could also benefit from the flow rotational principles of the PhotoBioCREC. It is anticipated that future studies, which will include the developed microalgae growth kinetics, will allow one to demonstrate via numerical simulation and experimentation, the value of scaled PhotoBioCREC Swirl Reactor units, for CO2 derived carbon capture using Chlorella vulgaris culture.
Summary for Lay Audience
The combustion of fossil fuels leads to greenhouse emissions that play a significant role in climate change. Carbon dioxide (CO2) is one of the main components of these emissions. Plants consume CO2 in the process of photosynthesis. However, CO2 fixation in plants is not significant enough to prevent the increase of the CO2 concentration in the atmosphere. For this reason, action must be taken to enhance CO2 fixation and to reduce these emissions.
Microalgae, like plants, offer a unique method for CO2 fixation, through photosynthesis. They can be grown at controlled conditions, in photobioreactors. This approach can allow power plants to reduce carbon emissions, by capturing CO2 in bicarbonate solutions, feeding them later, to photobioreactors, for microalgae growth. The organic matter produced can be used for energy production in the same power station, or alternatively, as a precursor of other products such as biofuels, pharmaceuticals, and food.
Photobioreactors for algae production are however, still under development. Light supply at a constant rate during microalgae growth, is a challenge since microalgae, tend to grow on the photobioreactor walls. Moreover, light absorption efficiency has not usually yet been reported in the technical literature even though light is the photosynthesis driving force.
The objective of this study was to design a new photobioreactor for microalgae culture powered by visible light. This objective was successfully accomplished by using sodium bicarbonate solutions in a 0.175 L vortex flow PhotoBioCREC unit, with a Chlorella vulgaris culture. Inorganic carbon depletion and organic carbon formation were monitored. The promising efficiency of the reactor was demonstrated in terms of its ability to convert inorganic carbon into organic carbon and to transform visible photon energy to produce microalgae. The study was completed, with fluid dynamic and photo absorption studies in a 10.3 L volume PhotoBioCREC Swirl Reactor. Experiments in this larger unit, provided valuable reactor engineering information, required to implement in the near future microalgae growth in scaled-up PhotoBioCREC units, with an induced vortex flow.
Cordoba Perez, Maureen D., "CO2 Derived Carbon Capture Using Microalgae in a PhotoBioCREC Unit" (2021). Electronic Thesis and Dissertation Repository. 8093.
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