OCR Text |
Show baghouses by an induced draft fan. The baghouses are secondary particulate collectors to meet emission standards. The handling of waste solids in the AFB boiler system is almost as elaborate as in the pressurized fluidized bed. The bed solids will be drained from the lower back wall of the bed through four ports. The solids will spill out of the bed by gravity into parallel solid coolers. These coolers discharge into a common pneumatic line that goes to a large holding tank. This tank serves the same purpose as its PFBC counterpart. Discarded solids can be introduced into a pneumatic line that also sweeps out ash from the preheater and baghouses. The AFB boiler employs recycle of elutriated solids. Ash and char particles knocked out in the convection passes and the cyclones are collected in separate locations but are introduced into common pneumatic reinjection lines. There is a pair of these lines, one for each side of the bed where they re-enter the bed below the surface. OVERALL COMPARISON OF AFB AND PFB Some direct comparisons can be made now between the AFB and PFB designs with regard to packaging and space requirements. Elevation views of four boiler types are shown in Fig. 4, revealing the advantageous packaging arrangement possible with an oil-fired boiler. The AFB, although it would appear to be rail shippable, is not because the height is too great. The 15-ft freeboard in the AFB is considered a minimum by most authorities of FBC and to reduce it might jeopardize the combustion efficiency and S0? capture. Two boiler companies were contacted regarding this problem in packaging an AFB boiler. One company agreed that the freeboard height had thwarted their attempts to design a rail-shippable AFB boiler. However, Combustion Engineering has proceeded with the design of a packaged AFB that utilizes a 7 to 8-ft freeboard above a 3-ft expanded bed. This design, illustrated in Fig. 5 is the same as used in the demonstration AFB now being tested at the Great Lakes Naval Training Center. With the short freeboard, this AFB can be rail shipped when the windbox is shipped separately and a depress flatcar is utilized. The boiler module, windbox, and solids feed lines are then assembled in the field. 21-12 |