| Přispěvatelé: |
Strydom, C.A., Bunt, J.R., Matjie, R.H., 20682972 - Strydom, Christiena Adriana (Supervisor), 20164200 - Bunt, John Reginald (Supervisor), 21166625 - Matjie, Ratale Henry (Supervisor) |
| Popis: |
PhD (Chemistry), North-West University, Potchefstroom Campus Mineral matter in coal is the primary cause of ash-related problems (fouling and slagging) during the combustion of coal. It is crucial to incorporate the behaviour of mineral matter under combustion conditions into ash-deposition prediction methods. The novelty of this approach is that the current ash-deposition prediction methods disregard the heterogeneous nature of ash properties, which are the results of the complexity of mineral matter transformations at elevated temperatures. The first objective of this study was to comprehensively characterise and describe the included and excluded mineral matter transformational behaviour at elevated temperatures in order to comprehend the processes and operational problems which could occur during coal utilisation. The combination of density separation through the float-sink method followed by reflux classification eliminated the liberated minerals successfully and produced maceral-rich float fractions (98% maceral content). Three South African feed coal samples for the combustion process were beneficiated to produce carbon-rich and mineral-rich fractions. The main differences between the feed coals were related to the mode of occurrence of mineral matter. The mineralogical, petrographical, and chemical properties of these feed coals and their density separated fractions were investigated using XRD, XRF, QEMSCAN, electron microprobe, and petrography analyses. By integrating these different analytical techniques, more comprehensive determination of the concentrations of mineral matter responsible for industrial ash related problems were possible. Low-temperature ash (LTA) samples of feed coals and the density separated fractions were subjected to high-temperature X-ray diffraction (HT-XRD) to identify the mineral reactions occurred at elevated temperatures under oxidising conditions. Included minerals were predominantly present in the float fractions (1.9 g/cm3). QEMSCAN results indicate that the mineral associations in carbominerites and included minerals in Feeds A, B and C and sinks B and C are responsible for the melt formation during HT-XRD experiments. HT-XRD results indicate the presence of mullite, anorthite, and amorphous aluminosilicate materials formed in the thermally treated LTA samples. The formation of these slagging crystalline and glassy phases could be attributed to either crystallisation during the cooling of the molten solution, or via solid-state reactions at elevated temperatures. Another objective was to relate the coalescence of included minerals and the fragmentation of excluded minerals, during combustion, to the slagging propensities of South African coal samples. The feed, float and sink fractions were subjected to laboratory combustion experiments in order to determine the temperatures where various mineral interactions occur. The mode of occurrence of mineral matter played a crucial role in the formation of high-temperature mineral phases under combustion conditions. Formations of high-temperature minerals, such as mullite and cristobalite, were mainly due to the transformation reactions of kaolinite and quartz at elevated temperatures, respectively. However, the formation of anorthite at elevated temperatures can be attributed to the interaction of fluxing minerals (calcite, dolomite, pyrite and siderite) that are associated with kaolinite in the coal sample. The presence of anorthite, mullite and alumina-silicate glasses at elevated temperatures can, therefore, be used as an indication of the slagging propensity of South African coal. It was proposed that blends of the different density fractions will reduce or minimise clinker and slag formation as well as the abrasive nature of the clinkers or slags. Possible blends to minimise clinker and slag formation include the float and sink fractions of the feed coals in varying proportions based on the specific mineralogical, petrographical and chemical data. A comprehensive knowledge of the included and excluded minerals can be used to prepare a blended feedstock for combustion processes. The operational ash-related problems in the combustion and gasification processes could be minimised by implementing this comprehensive knowledge of the transformation of coal minerals at elevated temperatures. Doctoral |
