Transformation of inherent and extraneous minerals in feed coals of commercial power stations and their density-separated fractions

Abstract

The main objective of this work is to relate the coalescence of inherent minerals and the fragmentation of extraneous minerals to the slagging propensities of South African pulverised feed coals during combustion. By incorporating the behaviour of inherent mineral matter or extraneous mineral matter in these coals under combustion conditions into ash-deposition prediction methods, the heterogeneous nature of the ash properties, which were disregarded in previous conventional ash deposition predictions, is considered in the study. The mode of occurrence of mineral matter in feed coals plays a crucial role in the formation of high-temperature mineral phases under combustion conditions. The float and sink fractions of the three different coals evaluated in this distinctive alternative approach provide different chemical and mineralogical properties of the derived ashes when subjected to elevated temperatures under oxidising conditions. Formation of significant concentrations of high-temperature minerals (such as mullite and cristobalite) is mainly due to the transformation reactions of extraneous kaolinite and quartz which are not associated with the extraneous fluxing minerals at elevated temperatures. However, the formation of anorthite at elevated temperatures can be attributed to the interaction of either inherent or extraneous fluxing minerals (namely calcite, dolomite, pyrite, and siderite) that are associated with either inherent or extraneous kaolinite in the coal samples under the oxidising condition. Furthermore, the anorthite, mullite, and calcium/magnesium/iron/aluminosilicate and silica glasses in ashes are formed either via crystallisation during the cooling of the high-temperature molten solution or via the solid state reactions. These high-temperature minerals and their glasses present in ashes can therefore be used as the indicators of the slagging propensity of coals. The implementation of results from this unique case study, will be of great significance to other industrial combustion processes to minimise or control ash deposition, slagging, and equipment erosion problems by either blending the density-separated fractions of coals or coals from different mines based on the chemical and mineralogical properties to prepare suitable feed coals. Furthermore, this unique alternative approach can be followed to further evaluate other feed coals in the global power stations during combustion.