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