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that will reduce payback periods to 2–3 years. Besides the value of heat, other values are considered, such as process benefits and avoidance of investments in downstream equipment. Anode Baking Anode baking processes generate hot exhaust gases from fuel combustion. These gases represent substantial heat loss. The exhaust gases contain a fair amount of volatiles and tar vapors. Dealing with fouling from tars is also an issue. Exhaust temperatures are 390°–750°F (or 200°–400°C) and contain large amounts of excess air. “Pitch Burn technology” is getting cleaner and could provide further opportunities. Researchers should revisit work that was done in the 1970s and 1980s. The Norwegian Research Council and Norwegian Technology University are working on fundamental areas of research. There may be an opportunity to work with the European institutes to develop applied technology to reduce and recover waste heat. Melting On the primary cast side, holding furnaces use a cold charge to cool down the superheated metal or increase production. The furnaces operate in semi-continuous or batch mode and result in variable exhaust gas flows and temperatures. Most furnaces are gas fired; however, there are a few plants with electric furnaces. Exhaust gases may have some hydrogen fluoride—usually less than 1 ppm—but in some cases it could be 5–10 ppm. This limits the use of conventional heat exchangers to recover heat. Wall losses and opening losses could be a good percentage of the total heat input; however, these losses are reduced by using best practices in furnace design and operations. Heat loss from these furnaces can be considered a low-impact, high-complexity category. On the secondary side, melting furnaces discharge a large amount of heat as exhaust gases. They are at a high temperature—usually greater than 1,600°F (870°C) —and can contain particulates, organic vapors, and flux material vapors and can be classified in the same category as flue gases from gas melting furnaces. Separating these contaminants at higher temperatures can be considered a cross-industry need. Another possibility that should be considered is treating the gases with absorbents to neutralize the “bad actors.” This will eliminate the need to cool the gases and creates cleaner gases that can be used for heat recovery in currently available heat recovery systems. The possibility of pre-cleaning or pre- treating the hot flue gases to remove the contaminants should be considered an R&D need. Use of submerged combustion devices to promote better heat transfer has been considered and tried. Direct immersion heaters are of interest; however, the challenge is in selecting materials that can survive the highly corrosive nature of molten aluminum with acceptable lifetimes and minimum risk of failure. This is a materials issue. Induction heating is a possibility; however, the limitations are in its efficiency and size, as it is difficult to get units in the energy supply size range required for aluminum melters. Other Operations Other operations under the “semi-fabrication” category use a relatively small percentage of the total energy used in aluminum plants. In most cases the waste heat is discharged in the form of clean exhaust gases, which can be recovered using conventional heat recovery systems such as recuperators, regenerators, or regenerative burners. This can be considered an established best practice. Industrial Waste Heat Recovery Page 10PDF Image | Industrial Waste Heat Recovery: Potential
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