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Improving Remelt Furnace Operations
02-06-03
Increasing environmental demands for reduced emissions, especially of NOx, and improved energy efficiency in remelt furnaces used for melting secondary aluminum have focused attention on alternative combustion technologies, especially oxy-fuel burners. Increased oxygen in the furnace environment, however, can produce greater oxidation of the aluminum melt surface, with increased dross formation, leading to reduced thermal transfer to the aluminum metal, and increased melt loss. Balancing these apparently competing effects has been the focus of research work on better understanding the causes of molten aluminum oxidation as well as designing combustion approaches to ideally obtain low NOx emissions and minimize melt loss.
Substituting oxy-fuel mixtures for air-fuel mixtures in reverberatory furnaces used for melting of secondary aluminum alloys has been suggested previously. The purpose was to reduce NOx emissions by eliminating the nitrogen component present in air. However, past efforts to incorporate oxy-fuel combustion methods have been limited by flame emissivity limiting heat transfer and/or excessive heat transfer leading to overheating of reverberatory furnace roof refractories. In addition, the increased cost of supplying oxygen often outweighed the savings and benefits created by its use.
One project to address this situation, sponsored by the Department of Energy's
Office of Industrial Technologies, was entitled "High-Efficiency, High-Capacity,
Low NOx Aluminum Melting Using Oxygen-Enhanced Combustion". With Air Products
and Chemicals as the lead in this project, two results were obtained. A novel
air-oxy-natural gas burner that achieves high productivity and energy efficiency
with low NOx emissions was demonstrated. In this case, the flame was enriched
within a range of 35-50% oxygen, which optimizes burner thermal performance while
consuming roughly half the oxygen of traditional oxy-fuel systems. Measured performance
results on a commercial furnace at Wabash Alloys plant in East Syracuse, NY were
a 30 percent increase in furnace productivity and a 40 percent reduction in specific
fuel consumption relative to air-fuel operation, as well as a substantial reduction
of NOx emissions. The second major accomplishment was the demonstration of a low
cost, onsite oxygen storage technology. A vacuum-swing-adsorption (VSA) system
using a patented high-efficiency molecular sieve to remove nitrogen from the air
coupled with a proprietary sieve-storage technology that reduces oxygen delivery
costs by allowing the sizing for average rather than peak furnace oxygen demand
was installed. The technologies involved can be retrofitted to existing aluminum
melting furnaces and are adaptable to different furnace configurations and melting
of different grades of scrap. Further information is available at http://www.eere.energy.gov/industry/aluminum/pdfs/lownox.pdf.
A second project underway with DOE-OIT support addresses the combined issues of
burner efficiency improvement and reduced dross formation on the aluminum melt.
Entitled "Low-Dross Combustion System", this project, led by the Gas Technology
Institute (GTI), addresses the problem by the development of burners that allow
control of flame shape and oxygen distribution within the flame. Like the technology
described above, the focus is on an air-oxy-fuel mixture, with the optimum oxygen
range expected to be in the 30-35% range. The major innovation being pursued in
this project is more specific control of the flame itself. Through a concept labeled
the Self-Optimized Combustion System, a flame is produced that has a fuel-rich
zone on the bottom to minimize exposure of the aluminum surface to oxygen, and
a fuel-lean zone at the top to ensure complete combustion and reduce emissions.
While conceptually easy to imagine, the realization of this concept in practice
is more complex. However, recent results presented by GTI indicate success in
producing such "stratified" flames in the laboratory. In practice, an optical
sensor would be used to ensure that the flame maintains the desired shape and
composition. Future plans in this project include the testing of burners utilizing
the new technology at Wabash Alloys Wabash, IN plant. Since these burners will
be able to be retrofit into existing furnaces, the ability to evaluate not only
emissions and economics but also dross formation over an extended run time will
be possible. A fact sheet is available on this project at http://www.eere.energy.gov/industry/aluminum/pdfs/lowdross.pdf.
Reductions of melt loss through innovative burner design are based on the current
understanding of dross formation mechanisms. This understanding involves the formation
of oxides and spinels with rates depending on furnace atmosphere, turbulence,
and alloy constituents for example. While some improvements can be anticipated
from technologies such as those described above, further reductions will require
a more quantitative understanding of formation mechanisms and potential alternative
solutions. This is the focus of the program being led by Secat, Inc. entitled
"Reduction of Oxidative Melt Loss." With the support of the excellent capabilities
of three National Laboratories and the University of Kentucky, diverse samples
of industrial drosses have been collected and are being analyzed. Coupled with
specialized analytical experiments, including in situ oxidation studies at Argonne
National Lab's APS unit, the goal is to better understand the common features
of drosses and their formation. The ultimate goal is to decrease melt loss from
4% to 2%. Possible solutions identified in the early stages of this project are
control of furnace atmosphere, use of minor alloying elements that dramatically
affect the kinetics of oxide formation, and decreasing the melt surface/furnace
atmospheric interface through various barrier techniques. A description of the
elements of this project can be found at http://www.eere.energy.gov/industry/aluminum/pdfs/meltlossreduction.pdf.
Article provided courtesy of The Aluminum Association - www.aluminum.org
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