Agglomeration Characteristics of Sludge Combustion in a Bench-Scale Fluidized Bed Combustor

In this study, sludge combustion was carried out in a bench-scale fluidized bed combustor to investigate the formation of agglomerates, followed by a thorough identification of bed materials and agglomerates using scanning electron microscopy/energy-dispersive X-ray spectrometry (SEM/EDX), X-ray diffraction (XRD), and X-ray fluorescence (XRF). Agglomeration invariably became more severe at higher temperatures with degraded sand as the bed material compared to fresh sand. Two major crystalline components, hematite (Fe2O3) and quartz (SiO2), were detected in agglomerates, showing that the eutectics of iron and silicate with the viscous phase might be one of the promoters of bed agglomeration. SEM/EDX analysis revealed the agglomeration progress, suggesting the role of Si (from sand particles) and P, Mg, and Ca (from sludge) in initializing the formation of sticky surface of sand particles to act as a glue matter, followed by the formation of an agglomerate bridge contributed most likely by Fe, then by Al, K, and Na. The bed agglomeration observed was possibly attributed to both melt-induced and coating-induced mechanisms. Furthermore, the behavior of mineral matters in sludge combustion was simulated by means of thermodynamic multiphase multicomponent equilibrium (TPCE) calculations. The thermodynamically major low melting point species formed in sludge combustion were predicted. In combining the experimental and computational results, it is believed that alkali phosphates (KPO3 and NaPO3) and the eutectics of Fe2O3 and SiO2 might play the most important role in bed agglomeration, by forming low melting point compounds, in the course of sludge combustion.