Thesis (Ph. D.)--University of Adelaide, Dept. of Chemical Engineering, 1993? / Includes bibliographical references (leaves 290-303).

Thesis (Ph. D.)--West Virginia University, 2004. / Title from document title page. Document formatted into pages; contains xi, 120 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 103-108).

Thesis (doctoral)--Helsinki University of Technology, 2000. / Includes bibliographical references. Also available on the World Wide Web.

Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2010. / Full text also avaiable online. Scroll down for electronic link.

Heat transfer in circulating fluidized beds was studied in both a 7.3 m high, 152 x 152 mm square, pilot-scale combustor and a 9.3 m high, 152 mm ID transparent cold model unit. Results were obtained for particles of mean size 171-299 µm at superficial gas velocities from 4 to 9.5 m/s and for solids circulation rates up to 70 kg/m².s. For the combustor, results obtained by using membrane walls and a vertical tube as heat transfer surfaces show a strong influence of the cross-sectional area-averaged suspension density on time-averaged, length-averaged suspension-to-surface heat transfer coefficient. The influence of superficial gas velocity is found to be small. Radiation becomes significant at suspension temperatures higher than 400 C and at low suspension densities. Heat transfer coefficients were also found to vary with the lateral position of the tube. The vertical length of heat transfer surface is shown to be an important parameter, allowing seemingly discrepant published results to be reconciled. For the cold model unit, sudden and dramatic peaks in instantaneous heat transfer coefficients were measured using an instantaneous heat transfer probe. Simultaneous heat transfer and capacitance measurements suggest that these peaks are caused by the arrivals of particle strands at the heat transfer surface. Two-probe heat transfer measurements suggest the existence of a characteristic residence length for the strands at the wall in this column. A proposed heat transfer model, based on an overall core-annulus flow structure in the riser, and periodic formation, movement along the wall, and disintegration of strands in the annulus, gives reasonable agreement with a wide range of published data. It accounts successfully for the effects of heat transfer surface length and particle sizes. However, the effect of the heat transfer surface configuration on the flow pattern of particles must also be taken into account to give improved agreement with experimental data. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

A two part study was undertaken to explain the performance of cyclones operated in circulating fluidized bed combustion (CFBC) systems. In the first part, collection efficiency tests were performed on a one-ninth scale polyacrylic cyclone model of the industrial scale cyclone at the 22 MWe CFBC facility at Chatham, New Brunswick. Emphasis was placed on scale-up considerations, loading effects, inlet geometry effects, and flow visualization trials. Experiments were performed at room temperature with inlet velocities between 3.7 and 5.5 m/s, solids loading between 0.05 and 7.5 mass solids/mass air with two different solids systems. There was disappointing agreement between the results from the Chatham unit, scaled according to Stokes Number scaling, and the findings obtained from the cold model unit. There was a minimum in the particle collection efficiency for particles of diameter 2.5 to 3.0 µm, apparently associated with agglomeration effects in the cyclone. Particle collection efficiency was found to increase with increased particle loading for the conditions studied. Changes in the inlet geometry gave inconclusive results. The experimental results were limited by problems associated with feeding and recycling the fines solids system used. In the second part radial gas concentration profiles of a secondary cyclone serving the UBC pilot scale Circulating Fluidized Bed Combustor were performed at temperatures of about 870 ℃. Concentrations of O₂ , CO₂ , NO[formula omitted] , CH₄ , CO and SO₂ were measured. An increase in [CO], and to a lesser extent [CO₂], was measured near the cyclone wall. There appeared to be little radial variation in the concentration of other species. Further work is required to allow the cold model to operate continuously, with particles which can be fed more freely, and to obtain radial gas concentration profiles within the primary cyclone of the UBC CFBC system. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

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

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.

Hydrodynamic effects play important roles in fluidized bed combustion processes. Since the motion of "bubbles" is an important influence on fluidized bed heat transfer, a better understanding of their behavior is necessary for improving the design of fluidized bed boilers. Using a two-dimensional bed, silica sand particles were fluidized with air at room conditions. The bubbling bed was videotaped, and both qualitative and quantitative information were gathered. Bubble characteristics such as size, rise velocity and frequency were studied while particle size and superficial gas velocity were varied. Results were compared with some existing theories and other similar research. The effect of internal surfaces at several heights in the bed was also studied. General bubble behavior agreed well with descriptions from previous research, and the expected spherical-cap bubble shape was observed. Both bubble size and rise velocity increased with particle size and with fluid velocity. Bubble frequency increased with fluid velocity, but decreased with increasing particle size and height in the bed. These results agree with previous work done using optical probes to measure bubble characteristics. Comparisons of data with empirical models showed general agreement. The presence of internal surfaces had the effect of reducing the bubble size, rise velocity, and frequency, and also of reducing the influence of changing particle size and superficial velocity on the bed behavior. / Graduation date: 1992

Thesis (Ph. D.)--Lehigh University, 2003. / Includes vita. Includes bibliographical references (leaves 104-107).