Accumulation of Ash-Forming Elements in Blast-Furnace Slag Bed Material during Fluidized Bed Combustion of Woody Biomass, Sludge, and Plastic Waste

Abstract There is growing interest in utilizing waste fractions in energy boilers. These fuels typically increase bed agglomeration in fluidized bed combustion due to their high concentrations of melt-forming compounds. Alternative bed materials, such as blast-furnace slag (BFS), have been proposed as a solution to prevent bed agglomeration. Understanding the interactions of BFS with ash-forming elements is important for understanding its applicability with various waste fuels. Herein, the fate of ash-forming elements was investigated in fluidized bed combustion when using waste-derived fuels together with BFS bed material, and natural sand as a reference. The experiments were conducted using a pilot-scale bubbling fluidized bed reactor. Wastewater sludge, containerboard mill sludge, and containerboard mill sludge with plastics were cofired with wood. Bed particle samples were collected and analyzed for their chemical composition and morphology. Coating of alkali metals onto BFS was reduced by 69–94% in comparison to natural sand, while Ca, Fe, P, and Mn were enriched onto layers of both bed materials. The BFS inner layers were predicted to contain only high-melting-point compounds based on the layer chemical composition and equilibrium calculations, which is favorable to reducing the severity of coating-induced agglomeration. The inner layer was predicted to contain a mixture of silicates containing Ca, Fe, and K that was interpreted to form through diffusion and substitution reactions in the silica structure. The used waste fuel clearly influenced the accumulation of specific elements in bed particles: several heavy metals accumulated with wastewater sludge, while titanium and antimony were found in increased concentrations when using plastic-containing fuels. Overall, the results of this study provide valuable insights into the accumulation of fuel-derived impurities into BFS bed particles, which is important both for assessing strategies to reduce bed agglomeration in waste-fired boilers, as well as potential extraction of valuable elements from bed materials.