Fluidised Bed
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 (mm)
Bed particle size (mm)
Fluidising velocity (m/s)
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 combustion gases.
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 development issue.