Natural Resources Engineering - Doctoral theses

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Browsing Natural Resources Engineering - Doctoral theses by Subject "biodiesel"

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    Design and optimization of a novel top-lit gas-lift bioreactor for industrial CO2 mitigation and microalgae-sourced biodiesel production
    (2017-03-24) Seyed Hosseini, Nekoo
    Mitigation of CO2 in industrial off-gasses by sparging the gas through photosynthetic microalgae bioreactors is an attractive concept. The goal is for the CO2 to be consumed by the microalgae as a nutrient, which in turn produces lipids suitable for conversion into biodiesel, as well as other value-added bioproducts such as Omega-3 fatty acids and antioxidants. Open systems are considered the most economic outdoor, large-scale cultivation option but have large land space requirements due to their shallow depths (15-35 cm). Consequently, finding sufficient space to locate them close to off-gas sources on industrial sites can be a significant challenge. Shallow depths are also likely to result in low uptake of CO2 and consequently reduced biomass productivity due to short gas residence times in the culture medium. In order to obtain longer gas/liquid transfer times, as well as greater per area productivity, the tanks through which the off-gas is sparged must be as deep as possible. However, to make the tanks deeper and avoid the costs associated with sub-surface artificial lighting, the issue is how to ensure the microalgae receive adequate light exposure. We have, therefore, looked for a novel method for increasing the depth of the tanks through which the off-gas is sparged. To achieve this, we have investigated the use of a gas-lift circulating system in a deep top-lit open bioreactor. In addition to providing CO2, the sparged gas also provides continual vertical circulation of the microalgae to ensure good mixing and an adequate light/dark cycle. Compared to existing shallow open systems, the results obtained showed comparable biomass productivity per unit volume, but importantly around three-times higher biomass productivity per unit area occupied by the bioreactor. The lipid productivity was also increased due to light and hydrodynamic stresses. In order to enhance further light utilization efficiency in the deep cultivation bioreactor, the use of a novel non-energy-consuming light column was also evaluated. The results of using the light column showed a 33% increase in areal biomass productivity and a 16% increase in areal lipid production. The proposed design and developed models can be easily translated into larger scale, onsite production facilities in industrial sectors emitting off-gas. The carbon capturing properties of microalgae can, therefore, help reduce industrial carbon dioxide emissions, whilst at the same time producing biodiesel from the resulting lipids.
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    Utilization of industrial waste heat for the cultivation and harvesting of microalgae
    (2017-03-17) Laamanen, Corey Alfred
    Microalgae sourced lipids that can be transesterified into biodiesel are a promising source of biofuels that can be produced while mitigating industrial carbon dioxide (CO2) in offgasses. There are many advantages to microalgae compared to other bio-feedstocks, including their rapid growth rate, their ability to accumulate significant amounts of lipid, and the possibility of year-round production. However, there are significant limitations to achieving wide spread and economic microalgae mass cultivation and two of these are addressed in this research program. Microalgae cultivation is currently generally limited to climatic zones where temperatures remain above 15°C, which effectively restricts mass cultivation to tropical or sub-tropical regions thereby eliminating the use of a number of worldwide industrial CO2 sources. However, many of these sources also produce significant amounts of waste heat. The capture and repurposing of waste heat to maintain culture temperature and provide an alterative method for harvesting was explored. A dynamic model was developed to determine the potential of waste streams from a nickel smelter to maintain year-round growth in a cold climate. From this model, it was determined that there is more than enough heat to maintain cultivation temperatures even when the ambient temperature drops well below freezing. Harvesting of microalgae prior to lipid extraction is, with current approaches, often cited as an area where costs need to be significantly reduced. As a wholly novel approach, the capture of this waste heat was also explored for the use as a pretreatment for harvesting by flotation. It was determined to be highly effective and crucially avoids the addition and costs of chemical coagulants, which contaminate and restrict the use of the remaining biomass after lipid extraction.