A technique used in some industrial processes in
which solid particles suspended in a stream of gas
are treated as if they were in the liquid state.
It is useful for transporting powders such coal dust
Fluidized beds, in which solid particles are suspended
in an upward stream, are extensively used in the chemical
industry, particularly in catalytic reactions where
the powdered catalyst has a high surface area.
They are also used in furnaces, being formed by burning
coal in a hot turbulent bed of sand or ash through
which air is passed.
The bed behaves like a fluid,
enabling the combustion temperature to be reduced
so that the production of polluting oxides of nitrogen
is diminished. By adding limestone to the bed with
the fuel, the emission of sulphur dioxide is reduced
High-pressure fluidized beds are also used in power-stations
furnaces in a combined cycle in which the products
of combustion from the fluidized bed are used to drive
a gas turbine, while a steam-tube boiler in the fluid
bed raises steam to drive a steam turbine.
system both increases the efficiency of the combustion
process and reduces pollution.