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gas waste heat. The waste heat loss is a small fraction of total energy inputs; therefore recovery installations are unlikely to have desired payback periods. Moreover, the physical arrangement of cells would make it difficult to retrofit any heat exchange equipment. Meanwhile, sidewall losses in aluminum cells are unusually high compared to other process furnaces. Molten cryolite is highly corrosive; therefore, cells maintain a “frozen ledge,” where the cryolite adjacent to the cathode lining is kept solid. This requires high rates of heat transfer away from the furnace. Consequently, the furnace is controlled so that as much as 45% of the energy input to the cell is lost via conduction, convection, and radiation from the sidewall. This accounts for about 55 TBtu of waste heat per year. Despite the high level of waste heat loss, no technologies have been developed for recovering this heat. There also do not appear to be any ongoing efforts to develop recovery methods. However, there may be possibilities to explore new methods of waste heat recovery, such as using thermoelectric technologies to control furnace heat losses. By closing or opening the electrical circuit connected to the thermoelectric device, it may be possible to control heat losses in order to control the frozen ledge, while recovering a portion of the heat lost. There may also be opportunities for other technologies — such as thermophotovoltaic generation — to recover waste heat. 4.4.2 Secondary Aluminum Production Secondary aluminum production involves recycling aluminum scraps, both “new” scrap (created in aluminum processing steps including scrap from drilling and machining of castings, scrap from aluminum fabrication, etc.) and “old” scrap or postconsumer scrap. Scrap is first pretreated to remove paints, oils, etc. before it is sent to a melting furnace. In the melting furnace, impurities are further removed via fluxing, in which NaCl and/or KCl is mixed with the molten metal in order to both separate impurities and to prevent the molten aluminum from oxidizing. The most common furnace used in secondary melting is the reverberatory furnace (Figure 25), though other options include roundtop melters, induction furnaces, tower melters, vortex melters, and flotation melters.106 Reverberatory furnaces can have energy intensities ranging anywhere from about 1,200 to over 2,500 Btu/lb107 with typical values around 1,800 Btu/lb without heat recovery.108, 109 In many cases, the actual energy consumption associated with producing a final pound of product is much higher, since yield losses in shape casting can be as high as 45%, essentially requiring that 2 pounds of aluminum must be melted for every pound of final cast product. Exhaust gas temperatures leaving the furnace are as high as 2,0002,200°F [1,090°C1,200°C], which can lead to as much as 60% of the energy input being lost to flue gas waste heat.110 There are about 400 aluminum melting furnaces in operation,111 of which over 300 have capacities greater than 40,000 lbs. Of these, only about onethird employ waste heat recovery technologies, due to the increased complexity and capital costs associated with heat recovery.112 The secondary aluminum industry has historically struggled with heat recovery technologies; several plants have previously attempted recovery techniques such as recuperative air preheating, only to quickly abandon these systems when maintenance costs proved burdensome.113 Challenges originally faced by the industry included corrosion from chlorides and fluorides released during fluxing operations, secondary combustion of volatiles in the recuperator, and overheating.114 Several of these issues have been partially addressed, thanks to improved operations and increased field experience with recuperators. Secondary combustion of volatiles is less common, since many producers now delaquer scrap before the melting process. Meanwhile, wastegas bypasses can be used during the fluxing operation to prevent corrosive gases from coming in contact with the heat exchanger. Overheating 44 Figure 25 GasFired Aluminum Reverberatory Tilting Furnace (Source: Seco/Warwick Corporation)PDF Image | Waste Heat to Energy Tech Opportunities in US Industry
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