Biosludge incineration in FBCs: Behavior of ash particles

Abstract

The evolution of ash morphology and metals behavior during incineration of a biosludge and silica sand in a 300-kW fluidized bed facility have been studied. The reactor was operated in the bubbling mode. Analyses of ash particles were performed using a computer-controlled electron probe microanalyzer equipped with four wavelength-dispersive spectrometers. The paper presents data on ash particle structure formation, size/numbers density distribution and migration/distribution of metals inside a supermicron fly ash particle. A mechanistic model of the fly ash evolution process is proposed. The major trends in the suggested mechanism are (1) the massive formation of porous particles (45–110 μm) in the splash zone, (2) their extensive fragmentatio/disintegration along the incineration pathway resulting in the particle size reduction and number density increase, (3) the presence of a phase transition in locally high-temperature regions (1650 K), and (4) the formation of smooth-surfaced compact-structured glassy fly ash submicron (< 0.7 μm) and supermicron (3–30 μm) spheres. A physical model of a compact/glassy supermicron fly ash particle is also developed. Light metal elements (Si, Al, Ca, K, Na) create a multilayer external shell (4–6 μm in thickness) encapsulating heavy metals (Cd, Cu, Ni, Pb) distributed in discrete pockets toward the core of the particle. The distance 4–6 μm does not constitute any definite boundary between these two characteristic regions since no dependence is found between particle size and shell thickness. These data illustrate that heavy trace metals are partitioned inside a biosludge-originated supermicron fly ash particle rather than on the surface, an assumption previously accepted on the basis of fly ash data obtained during coal combustion.