Crystalline-SiO₂-Component-Induced Cracking in Coal Ash Deposits Exposed to Temperature Changes

Frequent load changes or daily shut down and restarting have been imposed on thermal power plants in many areas to fill the gap between the demand for electric power and the variable outputs from renewable energy sources. These operations result in temperature changes in boiler furnaces, and the temperature changes may induce cracks in problematic coal ash deposits, which would contribute to their spontaneous shedding from heat-transfer surfaces. In this work, we microscopically investigated the origin of temperature-change-induced cracks in ash deposits. Analyses of the coal ash clinker generated in a utility boiler indicated that many cracks were generated close to the crystalline SiO₂ components, such as quartz or cristobalite. To elucidate the impact of crystalline SiO₂ components on the generation of cracks, we conducted temperature cycle tests for coal ash clinkers in an electric furnace. The results suggested that cracking was promoted by temperature changes that cover the transition temperature of crystalline SiO₂ components. Moreover, we successfully demonstrated that the addition of quartz additives could promote the generation of cracks significantly in coal ash clinkers. The findings in this work will advance the cracking behavior in ash deposits during temperature changes. Factors for the propagation of cracks or other factors for the shedding should be discussed in further studies, which will contribute to developing a method for promoting the shedding of ash deposits during temperature changes caused by load changes or daily shut down and restarting of thermal power plants.