Field study on ash behavior during circulating fluidized-bed combustion ofbiomass. 2. Ash deposition and alkali vapor condensation

Fly ash deposition on heat-exchanger surfaces during fluidized-bed combusti on of biomass causes operational problems such as reduced heat transfer and corrosion of superheater tubes. Ash deposition and alkali vapor condensati on were studied during circulating fluidized-bed combustion of forest resid ue in a 35 MW co-generation plant. A 70 +/- 10% amount of fly ash was depos ited on the heat-exchanger surfaces in the convective back pass between soo t-blowing periods. Practically all the largest ash particles (d > 10 mu m) but only a small fraction of particles d < 3 mu m were deposited. The depos ition efficiency of particles with a given size was not correlated with the ir elemental composition. About 50% of alkali chloride vapors (KCl and NaCl ) were condensed in the convective back pass on fine-mode particles (d < 0. 6 mu m) and the other 50% on the coarse-mode particles. Alkali chlorides we re not effectively deposited since they were depleted in the largest (d > 1 0 mu m) ash particles.