Measurements of the Near Infrared Optical Properties of Coal Slags

The spectral complex refractive index has been measured over the wavelength range of 1 μm to 12 μm for three coal ash samples. The measurements were conducted by making spectral transmittance and near-normal reflectance measurements on thin, homogeneous slag wafers prepared from the ash samples. The imaginary part of the refractive index k exhibits a large peak near 9 μm, attributable to Si-O vibrational absorption. Three smaller peaks in the k spectra were observed at 1·2μm, 1·8 μm, and 2·8 μm, which have been identified as due to absorption by the ferrous iron ion Fe2+ and the hydroxyl group OH. The dependence of the ash optical properties on ferrous iron content in the wavelength range of 1 μm to 5 μm suggests that both the total iron content of the ash and the ash oxidation state may affect the contribution of ash to radiative heat transfer in coal combustion systems.