Thermal conductivity of dry fly ashes with various carbon and biomass contents

Recently, sustainable energy portfolios have added biomass combustion and coal/biomass co-combustion as alternative fuel sources for generation of electricity. Fly ashes that result from combustion of biomass or its co-combustion with coal contain relatively high contents of unburned carbon, while increasingly stringent air quality regulations have also increased the residual carbon content in fly ash produced by coal combustion alone. While previous studies documented the mechanical and chemical behavior of fly ash relatively well, the thermal characteristics of those fly ashes have not been well studied. Therefore, this study evaluated the thermal conductivity of fly ashes with varied carbon and initial biomass contents to quantify the impact of unburned carbon particles and biomass-fired fly ash on thermal conductivity. Observed results demonstrated that the thermal conductivity of fly ashes almost linearly decreased as biomass content increased while the variation of thermal conductivity of fly ashes caused by unburned carbon content was relatively low. In addition, the thermal conductivity of fly ashes was lower than that of natural soils mainly because of the microporous structures of fly ash particles. The trend of thermal conductivity of fly ashes as a function of dry density was consistent with that of natural soils, due to the similar mineralogy of fly ash with that of natural soils. The developed stepwise regression model indicated that the porosity and the specific gravity was the most critical factor in predicting the thermal conductivity of fly ash.