Evaluation of alternative bed materials in fluidized bed incineration for ash recycling as supplementary cementitious material

This study evaluates different alternative bed materials, sourced from waste materials, to replace quartz sand in fluidized bed incineration of wood. Using alternative bed materials is not only beneficial in terms of the circular economy, but it also aims at tailoring the chemical and mineralogical composition of the resulting bed ash to enable its use as supplementary cementitious material (SCM) in sustainable cements. Seven alternative bed materials were investigated: Construction and demolition waste fine-fraction (CDW-ff), Electric arc furnace slag (EAF slag), Steel Refinement slag (SR slag), two types of used foundry sands, and Municipal solid waste incineration bottom ash. These materials are likely to present more latent hydraulic or pozzolanic properties than the commonly used quartz sand. The study focuses on (i) the physical and mechanical properties of the alternative bed materials relevant to fluidization, and (ii) their chemical and mineralogical properties relevant for upcycling as SCM. The developed testing scheme involves three stages from the laboratory to the pilot scale. First, materials were characterized using XRF, XRD, and particle analysis, followed by XRD-heating-stage, rotary kiln experiments, and cold-flow fluidization. Finally, the most promising materials (CDW-ff and EAF Slag) were tested in bench scale wood incineration reactors. The produced bed ash contains cement-reactive phases, indicating a high potential for use as SCM. This reuse option will help closing material cycles in the sense of a circular economy. These promising findings should be validated at industrial scale, through cement performance testing and critical evaluation of the composition of the resulting ash regarding heavy metal content.