7.2. Mass and heat transfer between a fluidized bed and a freely moving submerged sphere

Abstract

For fluidized bed combustion and gasification of solid fuels, but also for various other fluidized bed processes such as drying, granulation and evaporation, mass and heat transport to (or from) a particle freely moving in the fluidized bed is of great importance. The combustion rate of a millimeter-sized char particle in a fluidized bed of limestone and ash at 800 °C and 1 bar is, for instance, highly determined by the transport rate of oxygen to the char particle (char is completely devolatilized coal). At the same time it can be influenced considerably by the rate at which the generated heat of combustion is carried off to the bed. As another example, the rate of volatiles release during the first 10 to 20 seconds of the coal particle's residence in the fluidized bed is largely controlled by the transport of heat from the dense phase of the fluidized bed to the devolatiHzing coal particle.