Preece, John Christopher (2006)
Ph.D. thesis, University of Birmingham.
In order to mitigate the effects of climate change and reduce dependence on fossil fuels, carbon-neutral methods of electricity generation are required. Solid oxide fuel cells (SOFCs) have the potential to operate at high efficiencies, while liquid hydrocarbon fuels require little or no new infrastructure and can be manufactured sustainably. Using hydrocarbons in SOFCs introduces the problem of carbon deposition, which can be reduced or eliminated by judicious choice of the SOFC materials, the operating conditions or the fuel itself. The aim of this project was to investigate the relationships between fuel composition and SOFC performance, and thus to formulate fuels which would perform well independent of catalyst or operating conditions. Three principal hypotheses were studied. Any SOFC fuel has to be oxidised, and for hydrocarbons both carbon-oxygen and hydrogen-oxygen bonds have to be formed. Oxygenated fuels contain these bonds already (for example, alcohols and carboxylic acids), and so may react more easily. Higher hydrocarbons are known to deposit carbon readily, which may be due to a tendency to decompose through the breaking of a C-C bond. Removing C-C bonds from a molecule (for example, ethers and amides) may reduce this tendency. Fuels are typically diluted with water, which improves reforming but reduces the energy density. If an oxidising agent could also act as a fuel, then overall efficiency would improve. Various fuels, with carbon content ranging from one to four atoms per molecule, were used in microtubular SOFCs. To investigate the effect of oxygenation level, alcohols and and carboxylic acids were compared. The equivalent ethers, esters and amides were also tested to eliminate carbon-carbon bonding. Some fuels were then mixed with methanoic acid to improve energy density. Exhaust gases were analysed with mass spectrometry, electrical performance with a datalogging potentiostat and carbon deposition rates with temperature-programmed oxidation. It was found that oxygenating a fuel improves reforming and reduces the rate of carbon deposition through a favourable route to CO/CO2. Eliminating carbon-carbon bonds from a molecule also reduces carbon deposition. The principal advantage of blending with methanoic acid was the ability to formulate a single phase fuel with molecules previously immiscible with water.
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