A major factor promoting development of
atmospheric fluidised bed combustion systems (AFBC) is their
ability to fire high ash coals, retain sulphur within the bed
through the addition of limestone and low NOX emissions. The main
types of fluidised bed combustors (FBC) used for large-scale
plants are known as bubbling beds and circulating beds.
Operating conditions for fluidised beds are shown
in the following table.
Top feed size
Bed particle size
Bed temperature )
In a bubbling fluidised bed the gas velocity is
increased until the whole bed becomes a turbulent mass of solids
and bubbles, but there is no carry-over of bed material with the
When the gas velocity is increased further, some
bed material is entrained in the gas stream and leaves the
combustor. The bed material is separated in an external cyclone and
most of the solids are returned to the combustor. These fluidised
beds are known as circulating or fast fluidising systems.
Pressurised fluidised bed combustors (PFBC) are
based on fluidised bubbling technology. This technology gives the
same fuel flexibility and reduced emissions as AFBC, but with
enhanced thermal efficiency and greater heat input for the same bed
area. The thermal efficiency enhancement results mainly from
expanding hot flue gas, at pressures in the range of 10–16
bar, through a turbine. The greater heat input possible with PFBC
means a smaller unit, in terms of physical size, can be built for a
given generating capacity, reducing construction time and costs.
Hot gas cleaning is needed before the turbine and is a major