Key Parameters of Fly Ashes Generated from the Industrial Energy Sector Decisive for Their Pro-ecological Applications

Fly ashes belong to important byproducts, accompanying industrial energy production based on combustion technologies. Dependent upon the nature and characteristics of the used fuels as well as combustion technologies and their conditions, fly ashes may exhibit different physicochemical properties, limiting their potential applications as raw materials. A burned hard coal, lignite, biomass, or heavy fuel oil may generate fly ashes of various chemical, mineral, and phase characteristics. Both process conditions and humidity can, in turn, strongly influence final textural properties of such materials. Exhaust gas cleaning technologies also cannot be neglected as important factors potentially affecting textural and functional properties of the fly ashes. For example, denitrification processes, where NH₃ is used as a reducer, can result in the so-called ammonia slip, when excessive, unreacted ammonia undergoes sorption on ash grains, forming the corresponding ammonia salts. Such undesired effects can be problematic for the consecutive use of fly ashes as raw materials or components. The current work was focused on an in-depth analysis of chemical, mineral, and phase compositions of fly ashes of various origin. Their structural, textural, and functional characterizations have been performed by means of X-ray fluorescence, X-ray diffraction, scanning electron microscopy–energy-dispersive X-ray spectroscopy, Brunauer–Emmett–Teller method, Raman and diffuse reflectance infrared Fourier transform spectroscopies. The concentration of NH₃₍ₐq₎ in aqueous effluents was determined by ultraviolet–visible spectrophotometry. On the basis of the obtained results, a hierarchical algorithm for selecting the most important parameters for industrial-scale pro-ecological applications was proposed. The ratio of Si/Al, the main constitutive elements of fly ashes, both content and speciation of iron and/or other transition metals, occurrence of alkali components and carbonaceous residuals, content of ammonia, mineral and phase compositions, grain diameters and morphology, and sorptive affinity of water to surfaces of ashes were selected as crucial parameters, remaining decisive for the successful exploitation of ashes as raw materials.