A thesis submitted to the Faculty of Engineering and Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy, Johannesburg 2017 / South Africa (SA) has an energy-intensive coal mining industry, where coal accounts for approximately 72% of total primary energy consumption in the country, particularly in the electricity sector, where 95% of total electricity generated is derived from coal. Pulverised coal combustion has been the preferred technology adopted for power generation in South Africa for many decades. These coal-fired power plants have no flue gas desulphurisation (FGD) equipment fitted at present. Therefore, these plants account for the majority of annual SO2, CO2, and NOx emissions, making them environmentally unsustainable for power generation. Such environmental issues add to the challenges for the power producer, who is required to meet not only energy demand, but also to compete with the export market for quality coals, and to ensure that electricity generation complies with ever-changing air quality standards. Circulating fluidised-bed combustion (CFBC), a technology for the combustion of coal, biomass, waste, has not been adequately explored or tested in South Africa previously. CFB combustion is currently under intense scrutiny amongst researchers evaluating its potential as an economic and environmentally acceptable technology, in particular for the burning of lowgrade coals. The main objective of this study is to undertake a case study using CFBC technology and to establish its potential for use in South Africa as a clean and cost-effective method in power generating for high-ash, low-grade coals. Experimental tests were conducted in a CFBC pilot plant in Finland, using two high ash coals, discarded coal from South Africa (SA) and a better quality coal from Russia for comparative purposes. A review was conducted of discard coals in South Africa in order to establish an inventory in support of their potential utilisation for power generation in circulating fluidised bed boilers. A further study established a comparison between pulverised coal (PC), and fluidised bed (FBC) technologies as a future benefit analysis. All four coals proved to have very high combustion efficiencies, despite significant quality differences in terms of petrographic composition and ash content. More specifically, the SA coals achieved combustion efficiencies of 99.6 %, 99.7 % and 99.8 %, where the Russian coal achieved 98.7 percent. The Russian coal was characterised as being low in ash and high in the reactive maceral vitrinite, the two South African coals possessed high ash content (35 to 45%), one with relatively high vitrinite, and the other very low vitrinite, whilst the South African discard possessed an ash content of 65-70% and extremely low reactive vitrinite content. All these factors lean towards the suitability of SA coals to the CFB technology. In terms of NOx emissions, all coals tested showed that their NOx and N2O emission meet the minimum requirements for small plants as set out by the European and SA standards, i.e. <300 ppm for a plant with generating capacity below 100 MW. This result is in agreement with data from the literature. The emission of SO2 depends on the sulphur content in the initial coal, which also has an impact on the Ca/S Ratio. SO2 emitted from the South African coals was higher than the national permitted standard, due to the low Ca/S ratio used. This was especially the case for South African discard. Vast reserves of discard coal containing from 2MJ/kg to 14 MJ/kg in calorific value have accumulated in South Africa since the last inventory of 2001, i.e. close to 1.5 billion tonnes are in existence. It is apparent that one of the looming challenges regarding discard coal is putting this ever-accumulating material to use. From the combustion results obtained in this research, it is proposed that such materials can be combusted in a CFBC boiler, and that it produces the same efficiency as other coals from South Africa and a clean coal from Europe. Ash distribution within the boiler was found to change in proportion of bed ash to fly ash, subject to the quality of the coal used. This is also likely to change the proportions of sulphur-absorbing sorbents in future. CO2 emissions from the coals under review were found to be very close, in the region of 12.8 to 13.8 percent. / XL2018

Air pollution regulations have driven modern power generation systems to move from diffusion to premixed combustion. However, these premixed combustion systems are prone to combustion instability, causing high fluctuations in pressure and temperature. This results in shortening of component life, system failure, or even catastrophic disasters. A large number of studies have been performed to understand and quantify the onset of combustion instability and the limit cycle amplitude. However, much work remains due to the complexity of the process associated with flow dynamics and chemistry. This thesis focuses on identifying, quantifying and predicting mechanisms of flame response subject to disturbances. A promising tool for predicting combustion instability is a flame transfer function. The flame transfer function is obtained by integrating unsteady heat release over the combustor domain. Thus, the better understanding of spatio-temporal characteristics of flame is required to better predict the flame transfer function. The spatio-temporal flame response is analyzed by the flame kinematic equation, so called G-equation. The flame is assumed to be a thin interface separating products and reactant, and the interface is governed by the local flow and the flame propagation. Much of the efforts were done to the flame response subject to the harmonic velocity disturbance. A key assumption allowing for analytic solutions is that the velocity is prescribed. For the mathematical tools, small perturbation theory, Hopf-Lax formula and numerical simulation were used. Solutions indicated that the flame response can be divided into three regions, referred to here as the near-field, mid-field, and farfield. In each regime, analytical expressions were derived, and those results were compared with numerical and experimental data. In the near field, it was shown that the flame response grows linearly with the normal component of the velocity disturbance. In the mid field, the flame response shows peaks in gain, and the axial location of these peaks can be predicted by the interference pattern by two characteristic waves. Lastly, in the far field where the flame response decreases, three mechanisms are studied; they are kinematic restoration, flame stretch, and turbulent flow effects. For each mechanism, key parameters are identified and their relative significances are compared.

Call number: LD2668 .T4 1979 P63 / Master of Science

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Stephen Gregory Poulos. / M.S.

No description available.

Typewritten / Includes bibliographical references / 1 v. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--Faculty of Engineering, University of Adelaide, 1956

Thesis (Ph. D.)--University of Wisconsin, 1966. / eContent provider-neutral record in process. Description based on print version record.

No description available.

The combustion of a low heating value gas in a swirl burner is investigated. The investigation covers the development of a finite difference analysis of the flow and combustion processes in such a burner. In conjunction with the analytical work, an experimental program was conducted to provide detailed measurements of the three-dimensional velocity distributions within the flow field. The dissertation emphasizes the development and solution of the mathematical model. The finite difference analysis uses the primitive variables of velocity and pressure to describe the flow field. Features of the solution algorithms of several previous authors are incorporated into the analysis. A unique feature of the current approach is the use of a non-staggered grid system. An additional feature is a very straightforward technique for handling boundary conditions which eliminates the need for special treatment of the finite difference equations at boundary points. The solution algorithm is given the acronym CENSIS, derived from CENtered-Cell-Implicitly-Staggered. To illustrate the incorporation of the algorithm into a computer code, a sample program is developed to solve a simplified problem which has a closed form solution. This program, CENCIS-T, is included. Calculations are presented for the swirl burner, and the predicted results are compared with experimental data. The program used for the calculation of swirl burner performance is a more general code called PRIMCO. The PRIMCO code includes variable density and viscosity effects and incorporates a two-equation turbulence model for the Reynolds stress terms. The PRIMCO code also uses a simplified, infinite reaction-rate combustion model. Because of the use of the non-staggered grid system, the CENCIS solution algorithm is less complicated than previous algorithms. As compared to a staggered grid system approach, the current algorithm requires approximately one- third the computations of the former approach. These advantages make this approach considerably easier to code and relatively easy to apply. / Ph. D.

South 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.