As the air flow increases further, the bed becomes less
uniform, bubbles of air start to form, and the bed becomes
turbulent. This is called a bubbling fluidized-bed (BFB),
shown in Fig. 1d. The volume occupied by the air/solids mixture
increases substantially. There is an obvious bed level
and a distinct transition between the bed and the space above.
A further increase in air flow causes the particles to become
entrained and elutriate the bed into the furnace. If the
solids are caught, separated from the air, and returned to the
bed, they will circulate around a loop, defined as a circulating
fluidized-bed (Fig. 1e). Unlike the bubbling bed, the
CFB has no distinct transition between the dense bed in the
bottom of the furnace and the dilute zone above. The solids
concentration gradually decreases between these two zones.
Bed combustion temperature control is fundamental to
fluid-bed boilers. Bed temperature is controlled to limit
emissions, and to limit bed material agglomeration. Agglomeration
can be caused by ash containing high amounts
of alkali (such as sodium and potassium), other metals and
phosphates, combining with alumina and silica to form
low melting point eutectics that coat the bed particles. If
the alkali concentration is too high, the coating can melt
and cause solid particles to join together. As a result, these
larger agglomerated particles become too heavy and hinder