Many secondary melting furnaces are of the reverbatory and side-well-charged type and employ natural-gas-fired burners. Throughout the melting and/or holding process, furnace atmospheric oxygen is allowed to contact the surface of the melt, and aluminum oxide is formed. The aluminum oxide combined with fluxing salts forms dross on the surface of the melt, similar to the slag that forms on steel, which acts like a sponge trapping pure aluminum and reducing process yields.
Aluminum-oxide formation is a complex phenomenon that is largely a function of both bath temperature and oxygen level. Increasing bath temperatures and higher levels of oxygen availability will increase dross formation rates. Thus, there are two essential parameters to control dross formation – fuel-air equivalence ratio and melt temperature. Firing natural gas at stoichiometric ratio would reduce dross formation but has other implications including CO and VOC emissions. As a result, most melters operate in the 5–10% excess-air range. With conventional burner technology, however, this typically results in 1–2% dross loss.
Aluminum-Melting ApplicationHauck Manufacturing Co. has developed and patented a new ultra-low-NOx gas-fired burner, named TriOx, which utilizes a three-staged air-injection design for maximum production efficiency while minimizing dross formation and emissions. A four-burner TriOx system was installed on a new 190,000-pound side-well-charged aluminum-melting furnace at Metalico, Inc. in Syracuse, N.Y., in March 2007 (Fig. 1).
The furnace is designed for melting aluminum scrap in the side well while casting from the main furnace chamber in which the burners are fired. Total furnace heat input is approximately 24 MMBtu/hour. A high volumetric-flow circulating pump moves molten aluminum from the main chamber through the side well.
The furnace is additionally equipped with pressure control to minimize tramp-air infiltration and uses cascade temperature control to transfer from roof to bath thermocouple for optimum melting and holding efficiency. At furnace temperatures above 1600°F (870°C), the burners operate in Invisiflame® mode, a patented design feature, which when combined with optimum burner placement on the furnace results in diminished oxygen levels near the molten-bath surface.
Testing the System
A 24-hour test was conducted to fully evaluate metal loss over numerous loading/tapping cycles. Throughout the test, the burners were operated in Invisiflame mode with approximately 5% excess air. During the test, the side well was charged with scrap at the average rate of 10,200 pounds per hour. The scrap consisted of sheet bales of building scrap (approximately 30 x 48 x 62 inches), dried machine turnings and compressed chips called pucks (approximately 3 inches thick by 6 inches in diameter).
The side bay was drossed off each time the type of scrap being charged was changed. The main furnace chamber was drossed off only at the end of the test. Fluxing salt was used in the side bay to help remove impurities. The weight of all scrap and fluxing salts used was carefully measured throughout the test.
Casting was continuous at an average rate of 9,460 pounds per hour. Final cast product, dross from the side bay that included many impurities found particularly in the sheet-bale material and the final dross removed from the main chamber at test completion were carefully measured. During the test, the average bath temperature in the main chamber was 1442°F (783°C) and the average gas temperature in the main chamber was 1862°F (1017°C).
Reduced Dross Formation
The amount of dross that was produced from the main chamber during the test was 970 pounds. This represents a metal loss of 0.4% based on total scrap – 245,000 pounds – charged into the furnace. Compared to the average side-well melting furnace using cold-air burners producing 1–2% dross loss in the main heating chamber, a 0.4% dross loss on the Metalico furnace represents a minimum 250% reduction in metal loss compared to typical furnaces of this design.
Conservatively, comparing the 0.4% to typical 1% dross losses results in metal savings of approximately 1,480 pounds/day at the current production rate. Further, assuming furnace utilization is 90% and it has a scrap charge rate of only 70% of its full rate, the melt-loss savings would amount to 338,000 pounds of aluminum per year. At $1.20/pound of aluminum, savings of over $400,000 per year are possible, resulting in complete combustion-system payback of well under six months.
The reduction in dross formation is primarily attributed to the low oxygen concentration on the bath surface combined with the Invisiflame or distributed flame pattern created in the furnace by the TriOx burners. The burners are installed on the side wall of the furnace and inclined down at an angle of 10 degrees relative to horizontal.
Figure 2 shows a Fluent® Computational Fluid Dynamic (CFD) model of the air and natural gas flow profiles exiting the burner. The fuel flow – shown in red – is substantially deflected downward as it exits the burner with far less momentum than the air jets (blue). As a result, the low oxygen-concentration region of the flame is directed toward the bath surface, a condition that minimizes oxygen availability at the bath surface to minimize dross formation. In addition, the flame is stretched in the longitudinal direction as air and fuel mixing is intentionally delayed for ultra-low nitrogen-oxide (NOx) emissions. The stretched-flame pattern extends the hot com-bustion-gas region throughout the main firing chamber as opposed to creating localized hot spots, as is often the case with high-momentum burners angled down, which additionally suppresses oxide formation. Note that the high circulation rate of the molten-metal pump efficiently moved cold metal into the main chamber, aiding heat transfer and minimization of dross losses. The oxygen distribution on the bath surface is shown in Figure 3.
Even though the burners are firing at approximately 5% excess air, corresponding to 1% O2 in exhaust gas, the O2 profile along the bath surface on the burner axis is only an average of 0.495% due to the burner air/fuel flow and resulting flame profile.
ConclusionA novel four-burner system resulted in melt-loss reductions an order of magnitude less than conventional burner systems on a side-well melter, demonstrating significantly higher product yield while simultaneously providing ultra-low NOx, CO and VOC emissions. For producers striving to strike a balance between environmental regulations and production output, recent technology advancements in burner design can provide an attractive payback.IH
For more information:Contact James Feese, P.E., vice president industrial products for Hauck Manufacturing Company, PO Box 90, Lebanon, PA 17042; tel: 717-272-3021; fax: 717-273-9882; e-mail: firstname.lastname@example.org.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: aluminum oxide, stoichiometric ratio, low NOx, dross, computational fluid dynamics, side-well melter
SIDEBAR: Scrap MetalWhile energy continues to be a major cost of producing secondary aluminum, Metalico Aluminum Recovery, a Syracuse, N.Y.-based scrap-metal recycler producing aluminum deox products has found increasing product yield offers a very attractive payback. Since building a new furnace complete with a four-burner combustion system in March 2007, the recycler has enjoyed exceptionally low metal-dross loss rates. Benefits beyond the low dross rates, attributed to the unique mixing design of the burner, which minimizes oxygen concentration near the bath surface, include ultra-low nitrogen-oxide (NOx) emissions combined with reliable maintenance-free burner operation. The system has operated trouble free for nearly two years in almost continuous operation.
The system is capable of meeting tomorrow’s emissions standards today, and in many applications the return on investment from switching to the new-technology burners and combustion-control system can pay for itself in less than a year. The reduction in metal loss combined with the added value of exceptionally low maintenance provides an even more attractive payback. The latest technology in burners represents a state-of-the-art approach in which not only NOx emissions are reduced dramatically, but the actual air-fuel mixing process is tailored to absolutely minimize available oxygen and peak flame temperatures, the primary drivers for metal loss. The new TriOx burner combined with a thermal-head control system can improve product yield on many retrofit and new applications.