FLY-ASH FORMATION AND DEPOSITION DURING FLUIDIZED-BED COMBUSTION OF WILLOW

Fly ash formation and behaviour during combustion of willow (supplemen ted by 20 wt% of wood pellets) was studied experimentally in a 3-12 MW circulating fluidized bed boiler. Particle size distributions in a si ze range d(ae) < 8 mu m were obtained by a Berner low pressure impacto r in the convection path at a temperature of 650 degrees C and downstr eam the convection path (160 degrees C). For particles above 8 mu m si ze analysis was performed from a cyclone sample collected at 160 degre es C. Size distributions contained two distinctive modes. The major in organic elements in the coarse mode particles (d(ae) > 1 mu m) formed from the non-volatilised ash residue were Ca, K, P, Mg and S. The fine ash particles (d(ae) < 1 mu m) formed from vaporised ash forming cons tituents were composed of alkali (mainly potassium) sulphates, chlorid es and, according to chemical equilibrium calculations, carbonates. K2 SO4 was condensed prior to our sampling station at 650 degrees C. Cond ensed KCl and K2CO3 were observed only at the latter sampling station at 160 degrees C. About 50% oi the coarse ash particles were deposited on the heat exchanger surfaces between 650 degrees C and 160 degrees C, whereas the concentration of the fine mode sulphate particles was n ot observed to decrease from 650 degrees C to 160 degrees C. (C) 1998 Elsevier Science Ltd. All rights reserved.