Fluidized bed combustion and fragmentation of wet sewage sludge

Abstract

Thermo-chemical conversion of sewage sludge in fluidized beds is an attractive path to reduce the amount of landfilled waste and to mitigate the environmental impact related to sewage sludge disposal. The present study is directed to the characterization of the devolatilization and combustion patterns of lumps of wet sewage sludge in a fluidized bed combustor, with a focus on particle segregation, morphological changes and comminution phenomena associated with in-bed drying, devolatilization and char burn-out. The experimental investigation was carried out with the aid of different and complementary experimental techniques. The phenomenology of devolatilization and char burn-out has been investigated in an optically accessible quartz bench-scale fluidized bed combustor. Self-segregation of fuel lumps to the top of the bed during devolatilization was recorded. Stratified combustion of the emitted volatiles was observed to take place above the bed surface in a flameless fashion. Morphological changes of wet sewage sludge after in-bed devolatilization and burn-out has been accomplished by SEM/EDX and image analysis. The formation of balloon-like particles consisting of a porous outer layer enclosing a big single cavity is reported. The physical and chemical features of ash and char particles have been characterized. Particle fragmentation phenomena have been characterized by varying the oxidative vs. non-oxidative nature of the fluidizing gas, the initial size of the fuel particle, the bed material size, the fluidization velocity. Particle fragmentation occurs mainly during drying and pyrolysis/devolatilization and is very limited during char burn-out. Fragmentation is reported to take place when the initial size of the lump of wet sludge exceeds a critical value which depends on the bed material size and excess gas superficial velocity. Particle fragmentation is strongly affected by the size of bed material and moderately by the excess gas superficial velocity.