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Experimental assessment of coal combustion hazards and modeling of coal ash sintering

Nowak-Woźny D., Valdes L., Urbanek B., Rybak W.

Archivum Combustionis

2016

Vol. 36 nr 2

69--82

Assessing properly the characteristics and properties of coal combustion hazards requires a holistic understanding of the ash sintering process. The formation of melts and sinters of coal ash is responsible for the operational problems which are usually found in the combustion technologies. A series of experiments on the sintering behavior of coal ash was performed using a variety of experimental and numerical methods. Fusion behavior of coal ash under rising temperatures was characterized by standard Ash Fusion Test (AFT), which fail to indicate exactly at what temperature the first melt/sinter occurs in comparison with the thermochemical model calculated by FactSage. In addition, slagging indices based on ash chemical composition and ash fusibility were calculated indicating a medium slagging potential. Scanning electron microscope/energy dispersive X-ray spectrometer (SEM-EDS) analyses of ash samples were used for clarifying the ash melting mechanism at 700°C and 1000°C. Calcium, aluminum, sulfur and magnesium play a significant role in the formation of a thin molten layer of slag phase at higher temperatures. Meanwhile, the mineral transformations of sintered ash samples were analyzed by pressure drop test, obtaining the sintering temperature at 847°C. The resistivity of annealed coal ash samples at different temperatures was registered by Keithley 6517B high resistance meter; showing its sensitivity to the structural and microstructural changes, especially in the intergranular boundaries area. Resistivity values increased with increasing temperature from 700°C. Surface electrical transport is facilitated by a coherent and sintered ash residue. Three mechanisms of ash microstructural changes are suggested: ion diffusion from the grain bulk to the surface, melting of the surface layer as a result of chemical and physical processes, and ash grains smoothing and sliding. These mechanisms are important precursors of the sintering processes and seem to be responsible for sintering tendency.