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

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Call number: LD2668 .T4 1984 N46 / Master of Science

Sasol's Research and Development Division has identified several proprietary gas-liquid reactions where very low superficial gas velocities « 0.8 cmls) are required to obtain desired conversions in a bubble column reactor. A review of existing literature has shown that research in bubble column reactors is typically conducted in the superficial gas velocity range of 1 - 40 cmls. Traditionally bubble column reactors are designed via the application of empirical correlations which are only valid under specific conditions. There is a danger of under or over design if incorrect nonadjustable parameters such as liquid dispersion coefficients, mass transfer coefficients and gas hold-up values are used. To this extent, a hydrodynamic study was undertaken at superficial gas velocities lower than 0.8 cmls, to determine whether existing correlations are valid in this little investigated superficial gas velocity regime. Three bubble column reactors were designed and set up to perform hydrodynamic studies: • 22 cm inner diameter QVF glass column, 190 cm tall • 30 cm inner diameter 304 stainless-steel column, 200 cm tall • 30 cm inner diameter QVF glass column, 80 cm tall All measurements were undertaken in an air/water system. Gas hold-up measurements revealed that at the investigated gas flow rates, the gas hold-up was less than 1 % and as such was not investigated extensively. Partition plates were installed into the bubble columns and residence time distribution measurements were undertaken. The bubble columns were found to behave identically to the well known tanks in series model (Levenspiel, 1962). Liquid dispersion coefficients were measured via two methods. Batch liquid measurements were undertaken via the method of Ohki and Inoue (1970) and continuous liquid residence time distribution measurements were also undertaken. Data reduction was performed for both methods using the axial dispersion model to regress the liquid dispersion coefficient EL_ Both methods yielded equivalent results. The effect of distributor plate geometry on EL was also investigated and proved not to affect EL. It was found that existing literature correlations developed at higher superficial gas velocities failed to accurately predict the measured dispersion coefficients obtained in this study_ Correlation of the EL values with column diameter and superficial gas velocity showed EL to be a weak function of diameter as compared to existing correlations. This will have a significant effect on scale-up to larger column diameters. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2005.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1982. / Includes bibliographical references. / by Mark Thomas Nicastro. / Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1982.

The thermodynamic behaviours in a PFBC combustor were simulated for the ash from all of the six coals with sand and limestone as bed material. Ash components determined the ash thermodynamic behaviour at high temperature, and each component had different effects. For assessment of the potential for bed material agglomeration, the temperature at which 15% of the ash would become liquid (T15) was calculated with the coal ash, the cyclone ash and the cyclone ash mixed with varying amounts of limestone. Both the bed ash and fly ash, collected from an industrial PFBC plant, consisted of limestone/lime particles with different extent of sulphation, and coal ash particles. The calcium aluminosilicate material formed on the coal ash particles but not on the limestone particles. The aluminosilicate materials appeared to be formed from fine ash and lime particles at some local hot zones in the boiler. The melted materials may glue ash and bed material particle into large particles leading to bed agglomeration and defluidization. Four mechanisms were proposed for the formation of bed material agglomeration in PFBC, which may occur under different conditions. One mechanism explains the bed material agglomeration with the high localized high temperature zone due to the improper design or operation, while the bed agglomeration through the other three mechanisms results from the unsuitable coals burnt in the PFBC combustor. The maximum char temperature and the minimum T15 were used simultaneously to predict the tendency towards bed material agglomeration in PFBC burning different coals. Both char properties and ash properties should be considered during coal selection process for PFBC, to ameliorate the potential problem of bed agglomeration.

Bibliography: leaves 210-223.

This thesis examines transport phenomena of cupric chloride (CuCI2) hydrolysis within a fluidized bed. Conversions of CuCi2 and steam as a fluidizing gas are numerically investigated using a new non-catalytic gas-solid reaction model, proposed in the literature but here updated for the purposes of the present study. The results are illustrated considering two cases of kinetics for the consumption of particles: Volumetric Model and Shrinking Core Model. Consistent results in terms of the conversion of reactants versus superficial velocity, bed inventory and bed temperature are obtained by developing new solution algorithms abased on each of the above kinetic models. / UOIT