Study on particle radiative properties of lignite, hard coal and biomass fly ashes in the infrared wavelength range.

Fly ash, which is a by-product of combustion in furnaces or boilers, is used in certain materials as an additive for waste recycling. The optical properties of fly ash and the effects on the heat transfer phenomena of materials used in engineering applications differ and depend on the kind of solid fuel and the combustion conditions. The morphological and the radiative properties of fly ash samples of Turkish lignite, biomass, and hard coal that are burned in different thermal power plants were studied herein. The samples, which were collected from cyclones, were morphologically examined using scanning electron microscopy (SEM) images, and their chemical compositions were analysed by energy dispersive x-ray spectroscopy (EDX). Absorbance measurements were made in the wavelength range from 2.5 to 25 μm, and the discrete dipole approximation (DDA) was applied for a numerical assessment of the radiative properties of the samples. The measured absorbance values of all samples for particle diameters of 25 and 75 μm displayed significant differences in the related wavelength range. The most prominent change was observed in the biomass sample, and the hard coal fly ash had the lowest absorbance values in the related spectrum range. Although the particle shapes of lignite and biomass fly ashes are not same, the changes in the measured absorbance values were similar. The effects of the k index of the complex refractive index (CRI) on the radiative properties were examined for values of 0.01, 0.1, 0.3, 0.5 and 1.0. According to the measured absorption values and the calculated absorption efficiency results, the k index may be between 0.3 and 1.0 at the related wavelength range. It was concluded that the effects of the particle size and absorption index of fly ash on the heat transfer properties are important in the specified wavelength range.