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Show Other Furnace Mod~fications The 14-ft extension was fabricated from membrane panels and overlaid with 50 mills of high nickel alloy in selective areas to lnhibit corrosion. The connectiun to the existing tube and tile "upper" furnac~ was accomplished, using a full periphery ring header, located outside of the heated area. The header p~rmitted the mismatch of tube side spacings (old 6-inch to new 3-inch) to be matched. The water supply to the furnace extension 1S :rom new lower four-wall headers. The extension is supported by spring hangers from eX1sting support steel (Figure 5) . Plan View Refractory New Membrane 3" Tubes on 4" Centers 3" Tubes on 6·1/ 2" Centers Side Section - _-f...-"'-~ r- _~ 140--- EXIsting Loose Tubes TranSItIon Header .... --- New Membrane Fig. 5 Transition new furnace to old. Natural Gas Firing Capability was maintained to provide heat for start-up and for load carrying as a standby in the event RDF was not available. There were no plans to co fire these fuels. The boiler outlet screen was redesigned to increase the side spacing (now l8-inch total), and thereby improve ~leanability and increase the shielding of the superheaters from direct radiant heat transfer. The increased shielding was attained by rerouting two rows of generating tubes, turning them into new screen tubes. Combustion System The air, firing, and fuel feed systems all had to be modified to process the new fuel. The RDF design heating value of 5750 Btu/lb would vary due to the changes in moisture and constituents as a function of the time of the year. Stoker Ash S H2 C CL H20 N2 O2 Btu / lb Table 1 Fuel Analyses 12.0 0.12 4.55 32.0 0.45 25.0 0.40 25.5 5,500.00 The furna ce enlargement forced the lower1ng o f the existing coal t=aveling grate stokers by 14 ft. That relocation permitted opening the ash discharge height to 18 inches, dictated by B&W experience, to 1nsure free ash discharge. That increase in turn required a new flat arch (to provide radiation shielding ) , new stoker front extension, and a new upper front support for the fuel feeders. Existing grate bars were retained and, as they failed, were to be replaced with ductile iron. Detroit Stoker recommended new rear tuyeres, removal of the rear cok1ng section, new lower front seal, and new grate wear str1ps for the new fuel; all of which were installed. Fuel Feed The distribution of the RDF is critical to combustion efficiency and to minim1ze the potential for lower furnace corrosion. The fuel feed has to be integrated with the air system to insure complete mixing, combustion, and minimized corrosion due to reducing conditions. A sophisticated overfired air system (OFA) has been developed to insure that the best possible mixing is available to allow all the advantages of the increased fuel preparation costs to be realized . The OFA 1S intended to aid in keeping the RDF particle in suspension - enhancing the dehydration - and by creating turbulence increasing its residence time. Our experience indicates that Detroit Stoker's air swept spouts are the best fuel distributors available. We add as many as can be fitted - in this case, four per unit across the front face. Spouts measuring l8-in x 30-in had been used previously, and are the size applied to this project. The air swept spouts must distribute the fuel with some of the fines going into suspension, all the heavies on the grate, and the intermediates mixed. Grate speed has been modified to a 20 ft/hr rate. Feed to the spouts is by two/boiler live bottom bins. Air System The existing unit had no air heaters and because RDF combustion requires hot air, North American Technologies plate-type air heaters were added to provide 355°F combustion air for both the undergrate and overfire air systems. The undergrate system is sized for 50% of the total air required and is not zone controlled. The OFA system is sized for 50% of the total air required. The objective of the combustion system, air plus fuel |