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4.4 Aluminum Production The United States has over 300 aluminum production plants in 35 States103 and consumes about 770 TBtu of energy per year.104 Aluminum manufacturing is divided between primary refining of aluminum from bauxite (about 2.5 million tons of aluminum per year) and secondary production of recycled scrap (about 3 million tons of aluminum per year). Primary aluminum production relies on energyintensive electrolytic cells that account for about 15.6 kWh/kg or 60% of the energy associated with primary aluminum production. A small quantity of heat is lost via offgases, while the majority of heat is lost through the cell sidewalls. Secondary aluminum production requires only about onesixth of the energy required for primary production, which has contributed to the increased demand of aluminum recycling. A key step in secondary production is scrap melting in hightemperature furnaces, where waste heat recovery is employed in only about onethird of highcapacity furnaces. Total exhaust gas losses from primary refining and secondary melting total about 9 TBtu/yr (Table 16). Table 16 Unrecovered Waste Heat and Work Potential from Exhaust Gases in Aluminum Refining and Melting Source Energy Consumption Assumed Average Exhaust Temperature Waste Heat 77°F [25°C] Ref Waste Heat 300°F [150°C] Ref Carnot Efficiency Work Potential TBtu/yr °F °C TBtu/yr TBtu/yr TBtu/yr Hall Hèroult Cells Secondary Melting no Recovery with Recovery Note: Sources and assumptions in Appendix A: Documentation of Waste Heat Estimates 4.4.1 Primary Aluminum Production 134.6 1,292 700 9.3 2,100 1,150 2.6 2.2 6.1 4.2 69% 1.8 79% 4.8 63% 0.5 2.2 1,000 538 0.8 0.4 Total 146.1 9.5 6.7 7.1 Primary aluminum production is carried out in HallHèroult cells (Figure 24) where alumina is electrolyzed in a molten bath of fluoride compounds known as cryolite. Furnace operating temperatures are typically around 1,290°F [960°C].105 Waste heat losses in aluminum cells include offgases as well as unusually high sidewall losses. Offgas losses account for a small percentage of waste losses in aluminum cells, accounting for only about 1% of electricity inputs to the cell. Offgases are primarily due to anode reactions and air burning, which cause the production of about 1.5 tons of CO2 per ton of aluminum. Dilution air is usually used to lower the temperature of the heat before the gases are ducted away from the furnace. Losses total about 2.6 TBtu/yr of waste heat. At this time, no plants have developed economical means for recovering off Figure 24 – HallHèroult Cell (Choate, 2003) 43 PDF Image | Waste Heat to Energy Tech Opportunities in US Industry
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