OCR Text |
Show vessel. This natural circulation boiler has two steam drums and two mud drums. The flue gas exhausts at 900°F into two stages of cyclones where the suspended particulates are removed. The hot exhaust gas is then expanded in a turbo-expander which provides power to compress the feed air. At this relatively low temperature, it is expected that the expander will have an acceptable life although experimental verification would be necessary. The flue gas is then ducted through an economizer and out the stack. Because of the pressurization throughout the combustion system, the tasks of purging spent solids from the bed and removing flyash from the dust collection are difficult. In order to keep from losing pressure during operation, each disposal system will have to employ a batch mode of removal. For the flyash removal systems, each cyclone will empty into pressurized tanks which, through an arrangement of valves, serve as large lock-hoppers. In turn, these tanks will discharge (at atmospheric pressure) into pneumatic conveyance lines that carry the hot ash to a silo. Spent solids will be withdrawn from the vessel through a single side wall port just above the tuyere plate. Under normal operation, the solids will leave the vessel under their own weight and will fall into a pressurized tank (similar to the cyclone ash tanks). After depressuriza-tion, the solids fall into a water-jacketed, screw-driven solids cooler. Exiting the cooler, the solids are pneumatically conveyed upward into a large holding tank. From this tank, the solids can be reinjected (via another F-K pump) into the vessel for load-following bed depth control or discarded for removal. The tank also serves as a prefilled source of bed material for quick startup. AFB BOILER DESIGN The AFB boiler is designed for a 4-ft static bed depth and 8-ft/sec superficial velocity. Early in the development of the AFB layout it became evident that the AFB height would prohibit rail-shipping. Hence, the rail size limits were not imposed on the AFB, and it was considered not packageable. The bed is rectangular in shape and is latitudinally divided into two equivalent cells. The dimensions of the bed are 27 ft long and 15 ft wide, which yields approximately 400 sq. ft of plan area. 21-10 |