Fluid Dynamics of the Bottom Bed of Circulating Fluidized Bed Boilers
The fluid dynamic conditions in the furnace of a fluidized bed boiler influence the combustion and the production of harmful emissions. A substantial amount of the bed material in a circulating fluidized bed (CFB) boiler is present as a dense bubbling bottom bed. The bottom bed, characterized by a constant pressure gradient, was present in spite of fluidization velocities that were several times greater than the terminal velocity of an average sized bed particle. The gas velocity at which the bottom bed disappears depends not only on the particle properties, but also on the total amount of solids in the CFB loop, and the geometrical features of the CFB unit, such as the dimension of the return duct.
The experimental work was carried out in a 12 MWth CFB boiler and in a cold CFB. Three different distributions of the bubble flow in time and space, termed fluidization regimes, were identified in the cold CFB: the multiple bubble regime with many small bubbles evenly distributed in the bed; the single bubble regime, characterized by the presence of only one bubble at a time in the bed; and the exploding bubble regime with large, single, irregular voids stretching from the air distributor to the bed surface. These bubbling conditions were observed during variations in the gas velocity and the distributor pressure drop. A comparison with the 2 m2 cross-section CFB boiler showed that the boiler always operates in the single or in the exploding bubble regime, which indicates a bubble flow that is not continuous and not well distributed over the cross-section of the bed. The conditions in the boiler are influenced by the relatively large area of gas passage and the low pressure drop of the boiler air-distributor.
A simple model was used in which the bed is regarded as an oscillating mass supported by a gas volume. It was shown that the bubble frequency of the single bubble regime is the natural frequency of the bed and the gas volume in the air supply system.
circulating fluidized bed boilers
circulating fluidized beds