Hello to all,
Im new to boilers and since Im not a mechanical engineer Im not good with terminology.
Id like to understand what fluidized bed boilers are.
For example, in coal fired power plant, if coal is turned to powder and blown into boiler, that is called pulverized boiler. This type of boilers are widely used in power generation applications.
What fluidized bed represent? Where they are used?

Thanks

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

Wow right up my alley. I currently work at a powerplant where we operate 2 CFB,s.
Like ralph had said. The floor of the boiler is lined with many gird nozzled, which are fed from Primary FD air. the air suspends the "bed". With stoker type coal boilers the anthracite coal is pulverized and blown in to the combustor. Well the CFB is alittle different. We burn bituminous soft coal. The coal is fed through a rotary valve type feeder, down a chute then onto the boilers suspended bed. The soft coal is first crushed to walnut sized pieces so they will have lag time in the furnace. The bed is made up of sand. upon start up beach sand is purchased and injected, then when the coal diplaces it it become the Coal- sand- grit bed. Only enough air is used to the grid to keep the bed suspended. it burns at around 1400* to 1600* F and pulverized limestone is added to capture the sulfur in the coal. sorry to be continued duty calls. Doug

continued from last
With the bed in suspension at about 1550* F the limestone undergoes a chemical reaction, it absorbs the sulfur (So2, & So3) particles in the coal and becomes gypsum. These boilers are great at emissions control, ultra low sulfur.
As the bed burns the lighter dust like particles rise above the bed and head to the exhaust. The material enters a Cyclonic Separator area. The flue gas is drawn up and out to stack but any other unburned particles fall down a loop seal return and returns to the bed for further combustion. I can send images if wanted. There is probably 15 to 20 tons of bed material in the boiler under normal run conditions. In the bed the real heavy material is on the bottom and bottom ash coolers/classifiers are used to remove and maintain bed levels.
Another advantage of our CFB? is that they can be load cycled, 100% load to 25% load with out losing any superheat temperature. As with conventional boilers the more load and heat you push through your super heater the more heat you have, but when you back off the boiler load you lose the backend heat and lose super heat. With CFBs they have a Secondary radiant super heater hung off the top furnace inside wall. As boiler load decreases the radiant section maintains it temperature. Also with the fluid bed the boiler bed temps at the cyclonic outlet are 1550*F all the time (not load dependant), as the flue gas enter the super heater lanes.
Some disadvantages are that the lower furnace needs protection from sandblasting effects of the bed, but we just put the refractory line higher. Also with sand as the bed medium, Temps need to be maintained below 1850* F, if not the sand will turn to glass and solidify, also known as slumping the bed, or baking a cake.
We use our boilers as hog boilers; we feed them with all sorts of fuels. We burn coal, biomass, wood waste, sawdust, Tire chips, chipped railroad ties, some construction debris, waste oils. We can use #6 fuel oil or Natural gas for startups. We are at a paper mill so we burn (dry) effluent sludge. Our limestone delivery trucks unload the pulverized limestone, and then drive around to our ash disposal system where they are loaded with fly ash (gypsum). This product is used in low grade concrete for stabilization.
Well enough with the blah blah blah. I guess I am done now. I have much more info if needed.
Thanks Doug