BIOSLUDGE INCINERATION IN FBCS - BEHAVIOR OF ASH PARTICLES

The evolution of ash morphology and metals behavior during incineratio n of a biosludge and silica sand in a 300-kW fluidized bed facility ha ve been studied. The reactor was operated in the bubbling mode. Analys es of ash particles were performed using a computer-controlled electro n probe microanalyzer equipped with four wavelength-dispersive spectro meters. The paper presents data on ash particle structure formation, s ize/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 m echanism are (1) the massive formation of porous particles (45-110 mu m) in the splash zone, (2) their extensive fragmentation/disintegratio n along the incineration pathway resulting in the particle size reduct ion and number density increase, (3) the presence of a phase transitio n in locally high-temperature regions (1650 K), and (4) the formation of smooth-surfaced compact-structured glassy fly ash submicron (< 0.7 mu m) and supermicron (3-30 mu m) spheres. A physical model of a compa ct/glassy supermicron fly ash particle is also developed. Light metal elements (Si, Al, Ca, K, Na) create a multilayer external shell (4-6 m u m in thickness) encapsulating heavy metals (Cd, Cu, N, Pb) distribut ed in discrete pockets toward the core of the particle. The distance 4 -6 mm does not constitute any definite boundary between these two char acteristic 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 particl e rather than on the surface, an assumption previously accepted on the basis of fly ash data obtained during coal combustion.