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Show BOller ash, lncl dlng a~ r hea ter, economlzer, superheater hopper and slftlngs ash, 1S carried to the bottom ash conveyor by enclosed dry mechanlcal conveyors. STEAM ~~TOR DESIGN MODIFICATIONS Although mass burnlng of refuse has a long European operatlng hlstory , RDF does not share a simllar European herltage. RDF is largely a USA-developed technology drlVen by the goals of providlng more efficlent, more load responsive and more economlcal bOller facilities, coupled w1th a capablllty to recycle recovered materials. RDF has been burned in various ways since the late 1960s, both alone and in combination with other fuels. First generatlon plants adapted procesSlng equlpment from other lndustries and many lessons were learned. The learning process, typical of new technology, has been followed and state- of-the-art RDF trash- to-energy facilities are being designed and bUllt. The Red Wing and Wllmarth repowering have benefitted from the earlier experiences, which provided the design approach used for the repowerlng. Most combustion problems (slagging, fouling, erosion/corrosion) have been remedied by improvements in process facllities and increased conservatism in the steam generator design. Boiler Modifications The existing boilers are balanced draft with variable speed 10 fans. New electrostatic precipitators were installed in the early 1980s. The existing furnaces are of tube and tile construction. Neither plant was equipped with steam temperature control. The existing Wilmarth boilers were equipped with bare tube economizers. The existing Red Wing boilers (designed and supplied by Foster Wheeler) had extended surface economizers. None of the boilers were equipped with air heaters. Figure 3 shows the pre-conversion boiler outline that is typical of both plant sites. Meeting the obJective of burning 15 tons/hour set the required furnace size, and B&W experience dictated the needs for selective corrosion protection. The existing furnace had to be enlarged to meet the desired objective throughput, while at the same time insuring adequate tube wall life. Experience indicated that a furnace exit gas temperatur~ (FEGT) of 16000F is a practical maximum, given the economic trade-offs of alloy versus conventional gas-side tube materials for this corrosive environment. The relatively high levels of chlorides in the flue gas streams preclude the use of traditional stainless materials as the optimum tubing material choice. Ash materials in the RDF demonstrate slagging and fouling characteristics, not unlike coal, with the added challenge of removal while preventing the exposure of new tube metal to corrosion. The cyclic removal of protective oxide coatings has not retarded corrosion. 8&W's experience indlcates that, at present, the best FEGr to balance the corroslon/slagging/fouling/burnout concern is attained by llmiting the furnace gas outlet temperature to 1600°F at maximum continuous rating (MCR) on RDF. To Precipitator ..- Fig. 3 Preconversion bOiler outline. The furnace could be enlarged, in thlS case, by extending it 14 ft lower. It is desirable to ~duce release rates for RDF below these rates for coal because RDF is u more difflcult fuel to burn and does generate ~orrosive gases. Longer residence times will aid combustion and reduce FEGT and associated furnace slagging. The 14 ft were obtained by going 1nto the b1sement, remov1ng the traditional stoker coal bottom ash hopper and replacing it with a low-head submerged chaln conveyor. Our evaluatlon indicated the 15 tons/ hour could be combusted and the FEGT of 1600°F not exceeded (Figure 4). From Precipitator To Precipitator ~ Air Heater _L-I--tt-....,..-T"..'\ ~~~n Fig. 4 Post conversion boiler outline. Furnace ExtenSion |