| OCR Text |
Show The distributor in the second-stage fluidized-bed combustor, which should be water cooled to avoid overheating, could be a part of the in-bed heat transfer surface. Air lances should be provided to prevent plugging. The rest of the design does not differ significantly from the conventional fluidized-bed combustion boiler. This conceptual design would offer the following features beyond the conventional single-stage FBC approach: o Potential to burn whole or chopped tires, either alone or as a component in a coal/tire fuel mix. o High chemical (fuel) to thermal energy conversion efficiency. o Efficient waste heat recovery from spent bed materials. o Multi-stage combustion to insure complete and clean combustion. o Greater design flexibility to meet the basic requirements for tire incineration. UNKNOWNS IN THE FBC OF SCRAP TIRES There are still certain unknowns which should be considered. These unknowns are: o Maximum Tire Size When the size oi the tire must be reduced, energy will be required to accomplish the size reduction work. Two things should be considered: solid handling, and rate of combustion. From the experimental findings, solid handling seems the major element of decision on tire shred size. With two-stage FBC, some flexibility in size tolerance may be possible. o Maximum Bed Depth Residence time in the fluidized bed may be controlled by several variables. Among them, the bed depth and fluidizing air velocity are the major controlling variables. In the first stage, the residence time required for pyrolysis should be under 1 minute, while in the second stage, much less residence time will be required for complete combustion. Sufficient allowance has been made in the design to accomodate the change of bed depth in both stages. o Optimum Oxygen Level The air supply should be optimized to reduce the volume of flue gas generated, in order to reduce particulate cleaning equipment costs. Coal-fired fluidized-bed combustion boilers use 15 percent to 20 percent excess air to insure complete combustion. For tire combustion, a higher level of oxygen will be required. The oxygen level can be monitored and the operating conditions changed accordingly. 11-32 |
