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"Energy" Report Provides Performance Benchmarks
02-06-03
While substantial energy efficiency gains have been made in aluminum industry processes, there remains room for improvement. Quantifying both the current state and theoretical minimum energy requirements is the focus of a just-released report entitled "U.S. Energy Requirements for Aluminum Production," which will serve well as a resource for ongoing development efforts.
This report, subtitled "Historical Perspective, Theoretical Limits, and New Opportunities", was prepared by Bill Choate, Senior Technical Staff member of BCS, Inc., and Dr. John Green, retired VP of Technology for The Aluminum Association and now a consultant, under contract to the U.S. Department of Energy. It complements and extends previous reports in this area. Specifically, it complements the "Energy and Environmental Profile of the Aluminum Industry" published in July 1997, although the focus here is much more energy than environment. It also utilizes data developed in the Aluminum Association's "Life Cycle Inventory Report for the North American Aluminum Industry" published in 1998. This new report addresses just the production aspects and not maintenance and use energy. As with the previous reports, it is a U.S.-focused look at the energy issues.
Statistical data covering industry growth as well as energy use and selected environmental impacts is presented. The report is actually quite a bit more than simply a tabulation of numbers, although the Appendices containing this data are extensive and quite useful.
One instructive aspect is the historical context that is presented when describing the changes in processes over the years. It is noted that energy reduction in the U.S. aluminum industry in the past forty years has been substantial, resulting from two primary sources. The first, broadly termed "technical progress", relates to technical process improvements and has accounted for a 21% energy reduction. The growth of recycling is the second area, which has accounted for a 37% reduction in energy usage. Recycling was a small portion of the metal supply (only 18%) in 1960, but has grown quite significantly to the point that it represented 48% of the supply in 2000. Since production of aluminum from secondary sources requires only about 6% of the energy needed for primary production, the impact on energy saving has been significant.
While energy aspects are the primary focus of the report, a subset of the environmental issues involved are addressed as well. The report addresses greenhouse gas (GHG) emissions, and reports overall carbon emission data but does not address other fuel-related or process emissions.
Background information is provided that benchmarks the current industry energy performance and usage in a number of areas. The approach addresses not only onsite energy use but also tacit and feedstock energy values. Onsite energy is that energy required in the actual process, while tacit and feedstock energy accounts for generation and transmission energy losses associated with electricity production, the feedstock energy of fuels used as materials, and the process energy used to produce fuels.
This report does an excellent job of providing both a concise description of the fundamental chemistry as well as practical aspects of aluminum production processes. The aluminum manufacturing process is broken down and analyzed in eight areas, specifically:
- Mining
- Refining
- Anode production
- Smelting
- Casting
- Rolling
- Extrusion
- Shape casting
Perhaps one of the most effective charts in the report is a bar graph showing the process energy used in each of these areas both in terms of actual energy and theoretical minimum energy. The difference between the heights of the bars in each category provides a visual perspective on the size of the energy saving opportunity in each. Even with the significant energy reductions realized from past developments, the aluminum industry still consumes nearly three times the theoretical energy required, and hence there remains room for improvement.
The focus here is in smelting and process heating, since these represent the two largest areas of energy use, comprising 49% and 25%, respectively of the total energy consumed in U.S. manufacturing of aluminum.
Smelting, as usual, comprises the lion's share of the discussion. In this area, an interesting diagram shows the substantial reductions that have occurred in electric energy consumption in the Hall-Héroult process since 1900 due to process improvements. Current efficiency has increased to about 95%, and hence lowering the voltage requirements of cells presents "the largest challenge and best opportunities for improving Hall-Héroult efficiencies". The approaches for achieving voltage reductions are then reviewed in terms of technology developments. These include the areas of inert anode and wetted cathode, as well as applying them in combination. Also reviewed are alternative reduction technologies such as the carbothermic technology and kaolinite reduction. Comparison of both the onsite energy and tacit energy demands of all of these alternatives are summarized conveniently in tabular form, and the potential energy savings that could result support the strong research emphasis placed on this area.
In the area of process heating, melting of aluminum is the largest energy user. The report notes "programs to improve the efficiency of heating and melting while minimizing the formation of aluminum oxide and/or dross provide a much larger impact on decreasing industry energy usage than their energy consumption indicates." Such programs addressing improved process furnace efficiencies and reduced melt loss are underway and appear to be attacking the main problem.
The main body of the report concludes with sections on energy requirements for so-called downstream processing operations such as ingot casting, rolling, extrusion, and shape casting. The energy use in these processes is small by comparison to the two main categories discussed above, and the coverage is briefer as well. It is noted that one of the best ways to reduce process energy requirements is to minimize in-process scrap. In the context of these downstream processing operations this would include cracked or off-specification ingots, edge and end scrap in rolling operations, billet end losses and out-of-specification extruded products, and gates and risers from shape casting. Thus, improvements in metal recovery in these processes will improve overall aluminum production energy efficiency.
In conclusion, "U.S. Energy Requirements for Aluminum Production: Historical Perspective, Theoretical Limits, and New Opportunities" makes a valuable contribution to our knowledge of the aluminum production processes and their energy impact. It should be very effective in meeting its stated goals of providing understanding, setting common benchmarks, identifying process areas where significant improvements can be made, and increasing awareness.
The report is available in .pdf format through the DOE-EERE web site at www.eere.energy.gov/industry/aluminum/pdfs/al_theoretical.pdf
Article courtesy of Secat, Inc. - Research Resource for the Aluminum Industry
www.secat.net
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