Hi,
Fluidized bed boiler use air ( typically from the FD fan) to keep the coal and gravel ( pea gravel used at the unit I worked with in the late 1980s) suspended in the combustion chamber; looks like a bubbling lava bed. Our unit also had sand in the bed and what was called a "external heat exhanger". The sand and coal ash would be lifted out of the chamber by the air flow, cyclone seperators would remove the hot sand and divert it to the external heat exchanger ( a mini fluidized bed with only sand being kept fluidized by the air).... the main combustion chamber had a temperture setpoint of approx1450 F to optimize sulpher capture ( may be alittle off on that value, it was quite afew years ago...) hot recycle (eg, hot sand from external heat exchanger) and
cold recycle (eg, sand from a section of the EHE with steam generating tubes, thus making it "cooler") was sent back to the main chamber to maintain a "across the bed differential pressure" ...and help control to its temperture setpoint.

Coal (for fuel) and crushed limestone ( to capture the sulpher) was fed via chute into the top area of the chamber.

Baghouse on the outlet end collected the ash, fine sand and limestone/sulpher......

The EHE (external heat exchanger) was sort of a new idea at the time, I dont know if units of today used them ( it was a learning curve for users and manufacturers!!)

The most important thing to know is: too much material in the main chamber for the fan to hold suspended in air-> big problem,
called "slumping the bed". turns into a big metorite in the bottom..many hours jackhammering, not fun.

It was a power plant at a state university .... approx 30 MW electric generation, its main purpose in life was steam heat throughout the campus.....

RTK