Key parameters for the radiative emittance of ashes of solid fuels

The knowledge of the emittance of solid fuel ashes is important for the radiative energy balance in boilers and, hence, crucial for their design. This paper summarizes the most important effects on emittance based on own experiments and makes references to literature. Experimental results will be presented on the spectral emittance of typical minerals (SiO2, CaCO3, MgCO3, SrCO3, CaSO4, MgSO4, Fe2O3) contained in solid fuels ashes, extended by exemplary measurements on natural ashes (coal). The normal emittance is measured in a temperature range between 500 and 1000 °C in the wavelength range from 1.6 to 12 µm in a radiation test rig. The influence of physical surface structure and chemical-mineralogical composition on emittance is discussed. The results show that sizes of ash particles influences emittance. Emittance is increasing with particle size. Surface sintering as well as Fe in the ash also increases emittance. Surface fusion can either increase or decrease emittance based on ash composition. Sulfates and carbonates, typical for ashes under oxyfuel conditions, show characteristic spectral emittance bands. These bands vanish when the sulfates and carbonates being converted to the corresponding oxides at elevated temperatures. These characteristic bands can also be detected in natural ashes which consist of a variety of mineral components.