The effect of temperature upon minimum fluidization velocity

Abstract

The fluidization behavior of bubbling fluidized beds of coarse particles was investigated between 700 and 1000 °C for superficial gas velocities ranging from 0.6 to 1.5 m/s. The objective of the study was to highlight modifications in the bed behavior with the operating temperature at conditions under which the role of hydrodynamic forces (HDFs) or interparticle forces (IPFs) was dominant. To this end, the experimental work was divided into two phases. In the first phase, the influence of temperature on the hydrodynamics of a bubbling fluidized bed of coarse particles for which HDFs were dominant was investigated. In the second phase, the surface characteristics of the fluidized particles were primarily modified through the formation of eutectics resulting from a chemical reaction between the bed material and alkali/alkali earth metal based reagents that were introduced into the bed throughout the periods of solid fuel combustion at elevated temperatures. This then triggered changes in their fluidization characteristics with increasing temperature while IPFs were present in the bed. Experimental results revealed that the flow dynamics of a bubbling bed of coarse particles at high temperature was principally influenced by the variation of the gas density with temperature when IPFs did not play a discernible role. Nevertheless, with the presence of different levels of IPFs in the bed, a multiplicity of behaviors was realized at elevated thermal levels. Consequently, the physical and/or physico-chemical changes in the fluidized particles due to an increase in temperature and the variation in the physical properties of the fluidizing gas should be seriously considered when attempting to successfully design and operate gas–solid fluidized beds at high temperature.