Detection of Defluidization in Fluidized Beds

A problem of biomass/waste combustion is formation of agglomerates followed by sedimentation of coarse particles, which finally leads to defluidization. The mechanism of the formation of agglomerates is reaction between bed material (silica sand) and alkali (K and Na) which forms glassy material with low melting point. One approach to avoid this problem is to replace a part of bed material to maintain the concentration of alkali low. To optimize the system to feed/withdraw bed material, monitoring of fluidization is necessary. This work proposes a new method to detect sedimentation of agglomerates at the bottom of fluidized bed. Pressure fluctuation signal and gas velocity are measured in nozzle-type or bubble cap-type gas distributor. The fluidization status at the exit of the nozzle/bubble cap affects pressure fluctuation and/or gas velocity. In this work, cold model experiments were carried out to simulate the defluidization caused by agglomeration. Steel balls were fed to the fluidized bed as simulated agglomerates. The change in pressure fluctuation signal as well as gas velocity in the nozzles with the accumulation were measured. The fluctuation of the pressure was analyzed in terms of frequency spectrum calculated by use of FFT. Difference in frequency spectrum was observed between defluidization zone and normal fluidization zone.