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Show ~nprepared state has selective removal of bu lky ltems such as refrlgerators, mattresses, etc. , and the remainder is fed into a furnace, where, f~nally, other noncombustibles are discharged for dlsposal. The RDF method further refines the as-recelved refuse by shredding, magneti cally separating the lron, and by employing mult iple stages of screening and air c lassification; after which it is recla imed to yield recyclable products and a fuel. The fuel now sized can be burned in a conventional stoker-fired bOller dedicated to RDF combustion. The RDF method proposed by NSP proved to be the most economical solution for the counties and the utility. NSP contracted with the count ies to provide solid-waste-disposal service for 20 years and constructed a processlng facility for municipal solid wastes. ThlS processing plant is centrally located, relative to the waste generators. The output, RDF, must be hauled in solid waste 20-ton transfer trailers, specially designed with hydraulic rams for unloading the RDF at the plants. The processing facility receives 1000 tons/ day of municipal solid waste and separates it into approximately 700 tons/ day of RDF, 50 tons/day of iron, and 250 tons/ day of residue, such as glass, steel, and aluminum. During the planning, design, and construction of the processing facility, the 35-year-old boiler plants were redesigned and modified. The utility contracted with three boiler vendors to perform engineering studies covering the conversions from coal to RDF. Babcock & Wilcox was selected to redesign and provide the material to permit conversion of the four boilers from coal firing to RDF. The objective of the repowering was to process 15 tons/hour of RDF for each boiler. This presentation discusses the plant design decisions, specifics in the steam generator design modifications, construction and operational problems/ solutions, and the expected versus actual performance attained. FUEL RECEIVING AND CONVEYING SYSTEM The fuel is transported from the Newport facility to the Red Wing and Wilmarth steam plants by a fleet of 20 ton transfer trucks. The trucks unload the fuel into walking-floor receiving pits. The RDF is conveyed mechanically to four live bottom fuel feed bins of 2-1 / 2 ton capacity. Fuel is fed to each boiler by two feed bins. Figure 1 provides a schematic representation of the fuel receiving and conveying system. Fuel is supplied to the boilers by four (per boiler) IS-inch by 30-inch pneumatic air distributors (air swept spouts). Hot air to the air swept spouts comes from the overfire air system. Fuel distribution on the grate is controlled by rotating dampers on the overfire air supply. The pulsating air and OFA system is necessary to achieve uniform distribution of the fuel from the front to back of the furnace. The fuel is fed to the air swept spouts by two (per boiler) live bottom bins. Each live bottom bin is equipped with six screw-type feeders feeding two alr swept spouts. Fuel feed rate is controlled by varying the speed of the screw feeders. Balanced draft dampers are provided in each fuel feed chute to seal the boiler when a feeder is out of SWing Door With SWitch Arch Breaker Roll Fig. 1 RDF receiving and conveying diagram. service. The dampers also prevent backdrafts and possible fires in the fuel feed system in the event of fluctuations in furnace pressure. Bottom ash, fly ash, and boiler ash is conveyed by a series of mechanical flight conveyors (Figure 2). The bottom ash conveying system consists of a three-foot submerged drag chain conveyor. The flooded upper trough quenches the ash as it falls off the grate and also provides an effective boiler seal. The ash is pulled from the submerged trough by the flights and dewatered on the 50-ft inclined (40 0 from horizontal) section of conveyor. The conveyor operates at 8 ft/ min to minimize wear and maximize dewatering of the ash by increasing the time the ash is on the dewatering section. Dewatering of the ash to approximately 25% moisture content is important to minimize the potential for freezing in the trucks. All conveyor wear parts are of abrasion-resistant material (400 BHn). Superheater Hoppers Air Heater Hoppers Boller 2 To Precipitator Inlet Botler Water Tank Precipitators. Units 1 2 :--c::=~=~ Fig. 2 Ash receiving and conveying system. |