Summary of Recent Results Obtained from Using the Controlled Fluidised Bed Agglomeration Method

Abstract

The most promising energy conversion technologies for solid fuels, and biomass in particular, are based on fluidized bed combustion (FBC) or gasification (FBG). These processes enable higher electrical and total efficiency as well as greater fuel flexibility. Due to the relatively low temperatures in FBC and FBG, sulfur emissions and the extent of deposit formation can also be kept to a minimum. However, bed agglomeration could be a potential problem which can decrease both the heat transfer in the bed and the fluidization quality, resulting in poor conversion efficiencies and loss of control of bed operational parameters. In the most severe cases, bed agglomeration can lead to total defluidization, resulting in unscheduled plant shut down. The state-of-the-art concerning methods to determine fluidized bed agglomeration tendencies were recently compiled by Ohman (1997). 1.1. Review of Previous Work It is generally agreed that agglomeration may proceed through several different sintering mechanisms [Skrifvars 1994, Manzoori 1994; Ohman 1997]; viscous flow sintering of liquid silicates; reactive liquid sintering of molten salt systems; chemical reaction sintering by formation of new compounds; solid state sintering; and vaporization preceded by re-condensation. It can further be assumed that, although they all may contribute to the initial mechanisms of the ash transformations, the melting behavior of the ash or coating material probably is decisive for the final agglomeration in the turbulent fluidized beds. 1.1.1. Prediction of Ash Melting Behavior. Several techniques are currently used in the fuel industry to predict ash fusion temperatures from elemental composition. The most