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Date of Award
Doctor of Philosophy (PhD)
In 2016, 37% of total primary energy consumption in the US was supplied by petroleum products. Concerns over availability of fossil fuels and over CO2 emissions have prompted strong interest in finding alternative energy methods that can provide reliable and affordable energy, while also reducing or eliminating their greenhouse gas emissions. Biofuels can provide a reliable energy supply in a similar manner that fossil fuels do while reducing the overall carbon emissions of the process. However, their main disadvantage is the cost and energy intensive processes required to produce the fuels. Wind and solar power are also attractive because they can provide zero-emissions energy at a zero marginal cost. However, these sources are inherently intermittent, which means that they cannot provide a reliable supply of energy. If large scale reliance on intermittent energy sources is ever to become a reality, an enormous amount of energy storage capacity will need to be installed. If batteries, the most common method of energy storage, are to be used for this purpose, higher performance chemistries and lower cost production methods will be necessary.
This research builds on previous studies on wet fuels, which provide an alternative that would make biofuels more cost-competitive and increase their net energy gain. By avoiding energy-intensive dewatering processes, fuels that would otherwise be of limited value can be utilized in carbon neutral or even carbon negative processes. The concept of wet fuels is also used to create a simple, low-cost, environmentally-friendly flame spray pyrolysis method for producing Li-ion battery cathode materials.
In the first part of the dissertation, the spray breakup, structure, and dynamics during wet fuel combustion are studied in detail. Results show that the addition of ethanol to a glycerol / water mixture delays the onset of the liquid-sheet breakup and generates a high concentration of fine droplets in the near-nozzle region. The rapid vaporization of the fine droplets leads to a high fuel vapor concentration near the nozzle, which releases heat upon burning, enhancing ignitability. The combustion of ethanol acts to stabilize the system, while the swirling flow brings heat towards the nozzle, further enhancing stability. Once the system stabilizes, the spray turns into a wide hollow cone, and a strong stable attached flame is observed. These results explain the somewhat unexpected observation that the addition of 10% ethanol can lead to robust flames of glycerol/water mixtures.
The possibility of using dimethyl ether (DME) and DME / methanol / water mixtures as liquid fuels in a swirl-stabilized combustor is also investigated. Results show that the extremely high volatility of DME causes flash breakup to occur, generating a narrow spray comprised of very fine but very high velocity droplets, which inhibits the formation of an inner recirculation zone. This leads to a long, lifted flame with distributed heat released, as opposed to the shorter, more robust flame that is formed when using traditional fuels. When mixed with water at low concentrations, the mixture behaves in a similar way to alcohol / water wet fuels studied previously. As DME concentration increases, the effect of flash vaporization on breakup becomes more apparent, negatively affecting flame stability. As even more DME is added to the mixture, the effect of water on local flame temperature becomes less marked, and the stability of the mixtures is improved, despite the flash breakup.
In the second part of this work, a low temperature flame spray pyrolysis (LT-FSP) process is developed for the synthesis of Li1.2Mn0.54Ni0.13Co0.13O2 utilizing the concept of wet fuel combustion. An ethanol / water mixture was used as a fuel and a swirl-stabilized burner to achieve reactor temperatures lower than what can be attained via traditional flame spray pyrolysis. The effects of reactor temperature, which is controlled via altering ethanol concentration, on the physical properties and the electrochemical performances of the synthesized materials were characterized. Li1.2Mn0.54Ni0.13Co0.13O2 synthesized with 25 wt% ethanol showed the best results and delivered a discharge capacity of 203 mAh g-1 after 100 cycles under C/3. It also achieved good rate capability showing 201 mAh g-1 and 169 mAh g-1 under C/2 and C/1, which is state-of-the-art performance. The proposed LT-FSP method is simple, cost-effective, and is capable of producing 90 g h-1.
In addition, LT-FSP was used to address the hollow sphere issue that has challenged spray pyrolysis synthesis for decades, namely producing particles greater than 2 μm size with a solid (non-hollow) but porous interior morphology by performing slurry flame spray pyrolysis. Tap densities of 1.2 g cc-1 where achieved with half of the seed loading previously demonstrated. Results also show that at low seed loadings the spray dynamics are not affected, but when the loading is above 30 %, droplets become larger, which can negatively affect density. Finally, it was determined that to increase the density further, higher density seed particles are necessary. Li1.2Mn0.54Ni0.13Co0.13O2 produced by slurry spray pyrolysis reproduces the electrochemical performance of the conventional spray pyrolysis, meeting or exceeding the performance of materials produced by co-precipitation.
Richard L. Axelbaum
Ramesh K. Agarwal, Benjamin M. Kumfer, Mark Meacham, David A. Peters,
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