Effect of bed material on agglomeration of bed particles in CFB burning pellets from Arundo Donax

This contribution details a piece of innovative work elucidating the effect of bed material on particle agglomeration during CFB combustion of a high-potassium biomass, Arundo Donax. Experiments were performed in a lab-scale CFB at 900 °C using quartz or Si-lean material as bed material, during which bed particles were discharged every 2-h and subjected to be examined. Severe agglomeration was observed for quartz particles but not for Si-lean bed material. Characterisation using SEM-EDX, XRF, XRD, XMT and FactSage calculation showed that quartz particle agglomeration was initiated by molten ash and strengthened by K/Ca-silicate coating layer of low melting-points. A considerable amount (7.4 %) of K₂O was found in the agglomerates, leading to a liquid proportion as high as 32.6 % in the coating layer. The Si-lean bed material rich in mainly Ca/Al/Fe/Mg showed an insignificant agglomeration propensity because its Al constituents have dissolved into the fuel ash to generate K aluminosilicates owing to its lowest Gibbs free-energy, thereby reducing the liquid proportion by 12.1 %. Meanwhile, K-species were found to be captured and retained inside bed particles, inhibiting consequent formation of K-rich layer. With a negligible K₂O content of 2.0 % in particle agglomerates, both the melt-induced and coating-induced pathways for agglomeration initiation, by using the studied alternative bed material, were therefore mitigated.