3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Application of Oxy-fuel combustion on South African Coals using Thermogravimetric Analyses (TGA)(2017) Molise, DorcasThe quality and grade of South African coal is declining simultaneously with depleting seams. This has a negative impact on power generation and the economics of coal mining and power production. The reason is that good quality coal is more difficult to mine and hence costly, thus affecting coal prices and the ability of mines to supply coal quality of the required specifications. There is a global environmental awareness around the CO2 greenhouse gas and its effect on global warming. Legislations are becoming more stringent in limiting the amount of greenhouse gases and air pollutants we produce. In power generation, the most prominent greenhouse gas is carbon dioxide (CO2) and the most prominent air pollutants are oxides of Nitrogen and Sulphur (NOx and SOx). Oxy-fuel combustion (OFC) is a process change that can reduce the production of CO2 by increasing the concentration of oxygen in combusting air. A study is presented here, that focuses on the application of this process (OFC) to South African coals. Three different coal types were studied and characterized by conventional proximate and ultimate analyses and further characterized and graded by more specialized analyses; petrographic analyses and the quantitative evaluation of minerals b scanning electron microscopy (QEMSCAN). The gasification of the coals was then modeled to determine, qualitatively, its magnitude in comparison to combustion (oxidation) in oxy-fuel combustion. However, when modeling and conducting experiments to determine this, it was found that existing empirical formulae used to quantify char burnout are not suitable for all South African types of coal. The formulae found in literature (for both oxidation and gasification) could only be applied to two of the three samples. For the two samples that were successfully modeled, it was found that reactivity in gasification was probable but not to a significant level. For the third sample that couldn’t be modeled successfully, a recommendation was made that a new model be developed to take into account the nature of low grade, high inertinite South African coal. This is required in order to successfully formulate the char burnout of South African coals and thus depict with certainty, the applicability of Oxy-fuel combustion on South African coals. Such a step would benefit the forthcoming studies on modeling the char burnout of South African coal and therefore contribute to addressing the challenge of declining coal quality in South Africa.Item Prediction of spontaneous combustion in coal by use of thermogravimetry(2016) Mthabela, Zamashinga AmandaThe self-heating of coals is a complex problem which has been occurring for centuries. This problem has been fatal to coal miners, an economical challenge to coal mines and a health risk in a release of greenhouse gases to the public in general. Therefore, everyone is affected by the self-heating of coal, which leads to spontaneous combustion when the ignition temperature is reached. There are many test methods that have been used to test spontaneous combustion in coal, but all have one common factor or disadvantage of requiring long periods of time before a conclusion can be deduced. This then creates a need for a rapid and reliable method to test the liability of coal to self-heat in the coal industry and thus the motivation for this project. The thermogravimetry analysis (TGA) method was selected to test the liability of coal to self-heat due to its short analytical duration. The Smith-Glasser oxidation test was selected to validate the TGA results obtained. The main aim of this project is to investigate the reliability of the TGA method to predict the propensity of coal to self-heat. 29 samples from different regions of South Africa were used, prepared to 250 μm for all the analyses and self-heating tests. All samples were analysed for proximate, calorific value, sulphur and petrographic properties before the spontaneous combustion liability tests began. The TGA method followed two tests: 1) the O2 adsorption and 2) the ignition test. Five different heating rates (3, 5, 7, 10, and 20) °C/min were run in order to obtain five derivative slopes which would be used to obtain the TGspc index. The oxygen adsorption test studies the mass increase at low temperature under exposure of air between the temperature ranges of 100 – 300°C. The Smith-Glasser oxidation test method studies the reaction of coal with O2 and calculates the O2 absorbed per amount of coal tested. The Smith-Glasser test results collaborated with most of the other analytical results, and with the TGA results to a certain extent. The TGA spontaneous combustion liability test requires additional analytical work to back up its results because the results do not appear as accurate as the Smith-Glasser oxidation test. It also requires repeatability tests to ensure the integrity of the results.Item Combustion and physicochemical properties of raw and thermally treated bamboos(2015) Makwarela, OliveSouth Africa is economically vulnerable to climate change because its economy is powered by electricity generated from coal fired power stations. There is a need to reduce the reliance on fossil fuel energy not only because of greenhouse gas emissions but also energy security. Bamboo is touted as a renewable energy source, however, like other woody biomass material, it has poor physicochemical properties and low energy densities. Therefore, the bamboo samples utilized in this study were subjected to thermal pre-treatment methods to improve on their combustion and physicochemical properties. Bamboo samples of 1, 3 and 4+ years old were subjected to torrefaction at 250°C and 280°C as well as low temperature carbonisation at 350°C and 400°C. A standard HGI method was modified during the course of this research for studying the grindability of the raw and treated bamboo material. The fuel properties and combustibility of these raw and thermally treated bamboo materials were then studied using thermogravimetric analysis. The raw bamboo samples exhibited a CV ranging from 17 MJ/kg to 18 MJ/kg, whereas the torrefied samples and the carbonised samples had a CV ranging from 25 MJ/kg to 28 MJ/kg and 28 MJ/kg to 30 MJ/kg, respectively. The 4 year old bamboo carbonised at 400°C had the highest CV of 30.24 MJ/kg. The CV improvement occurred as a result of molecular modification observed through an increase in fixed carbon content from 16 to 74%. The energy yields ranging from 48 to 74% were achieved for the torrefied samples and 44 to 54% for the low temperature carbonised samples, depending on the age of the bamboo sample. At torrefaction temperatures tested, the 4 year old bamboo had the highest mass and energy yield, whereas at carbonisation temperatures, 3 year old bamboo had the highest. The number of differential thermogravimetric peaks was observed to decrease from 2 to 1 as the thermal treatment temperature increased to a carbonisation range (350-400) °C. This can be attributed to the less VM content in the carbonised samples. The raw bamboo and thermally treated bamboo had higher reactivity, lower ignition and burnout temperatures compared to that for coal. Blending of coal with bamboo (raw and thermally treated) appeared to increase the reactivity and lower the ignition temperature during co-firing. The activation energies of the individual fuels ranged from 56 to 289 kJ/mol, using the Ozawa model. Bamboo samples carbonised at 400°C had the highest activation energy, irrespective of age. The activation energy was also the highest when co-firing a blend with the highest proportion of coal. Based on the co-firing tests undertaken in the TG analyser in which a percentage of coal is blended with various proportions of raw and thermally treated bamboo, the results showed that as the percentage of coal in the blend increases there is less interaction or influence of biomass. The role of biomass is to aid with ignition of devolatilization in the coal at lower temperatures. At the carbonisation stage, biomass behave more like coal in principle. It was confirmed in this study that in terms of combustibility, the torrefied bamboo samples had a greater capacity to provide lower ignition and burnout temperatures over the low carbonised bamboo samples utilized, and this might support its application as a source of fuel in an industrial burning combustor. The carbonised 4 year old bamboo appears to be the preferred alternative source fuel to be fired solely in an existing pulverised boiler in South Africa or co-fired with coal due to the carbonised bamboo samples exhibiting the higher CV and more coal-like combustion profile.