The spectral and total emissivity of particulate ash deposits of different chemical composition which originate from lignite and hard coal fired boilers from Alstom Power and RWE Power were investigated with a spectral radiometer. Furthermore the emittance of all major single phases present in ashes and slag have been determined. The single phases under consideration were SiO_2, Al_2O_3, FeO, Fe_2O_3, CaO and MgO. The results on SiO_2, Al_2O_3 and MgO have already been presented by Linka in 2003 and therefore the focus is on FeO, Fe_2O_3 and CaO in the current paper. A spectral radiometer measures the emittance as a function of temperature and wavelength. The temperature varies from 600 to 1200°C in steps of 50K. The wavelength range from 1 to 15 μm has been examined. The measurements confirm the dependency of the spectral emittance on the chemical composition. Especially the content of either FeO or Fe_2O_3 or CaO influences the emittance of ashes and slag. The single phase samples of FeO and Fe_2O_3 show a temperature dependence of the emittance in the wavelength range between 1 and 6 μm. The same character of emittance appears in samples with high fractions of FeO or Fe_2O_3. CaO shows no temperature dependency in the complete range of wavelengths.
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