Improving Thermal Efficiency in Aluminum Scrap Melting
In today’s business climate, companies that operate thermal-processing systems in the recycling area must focus on higher productivity, increased product quality and reduced environmental impact. The recycling of aluminum scrap involves combustion as the main heat source, so meeting these challenges at the lowest cost depends largely on the quality and integrity of the combustion system and its interaction with the scrap.
Aluminum manufacturing is energy intensive, and approximately one-third of the cost to produce aluminum from ore is associated with the use of energy and environmental compliance. Over the past 40 years, the primary aluminum industry has reduced its overall energy intensity by nearly 60%. The alternative to produce primary aluminum from ore is to recover aluminum from scrap. Secondary aluminum ingot consumes only about 6% of the energy required to produce primary aluminum. Further, to achieve a given output of ingot, recycled aluminum requires only about 10% of the capital-equipment costs compared with those required for the production of primary aluminum. Any process that improves the recovery of scrap aluminum is making about an order of magnitude change in the energy associated with aluminum production.
Secondary aluminum processors have recently faced new competition from the export of aluminum scrap to China. Meeting this challenge requires improving the melting process and the combustion system through the use of techniques for reducing the energy required to melt scrap and maximizing the recovery of ingot from scrap. Some of the best-practice techniques involve:
• Air-fuel ratio control
• Preheated scrap
• Preheated air
• Bath stirring
• Improved furnace insulation and maintenance
• Oxidation-loss reduction
• Heat-loss reduction
The focus of this article is on showing how to measure the energy savings that can accrue by adopting these techniques. The downloadable Excel workbook AlMeltCalc simulates the production rate and energy savings obtainable by combustion and furnace operating changes. You can use this workbook to simulate an existing furnace and calculate how much energy will be saved by making changes. The results provide a basis for calculating the ROI for each change.
Scrap aluminum is processed for return to market in a series of steps: preprocessing, melting, alloying, refining and casting. Two general types of scrap are available: furnace-ready scrap and scrap requiring some preprocessing. A number of methods are used to melt aluminum scrap. Most consist of refractory-lined boxes that evolved from conventional furnace designs using combustion systems based on natural gas for fuel. As the recycling industry matured, pre-existing technologies were adapted to make better use of combustion energy, produce cleaner metal and minimize the amount of aluminum lost to oxidation in the furnace.
The reverb furnace is the industry workhorse. In batch mode, lighter aluminum scrap is placed on the bottom of the furnace and covered by a layer of heavier scrap. Flux is added, and melting and charging continue until the furnace has reached capacity. The bath is then skimmed and tapped. The furnace is completely drained to eliminate any chance that scrap containing moisture will be added to a molten-aluminum heel. The drawback to the batch process is that the presence of flux gives a thicker dross layer and entrains more metallic aluminum.
A more versatile version of this furnace has a side well for charging scrap (Fig. 1). Scrap and flux are added to the well and pushed into the main bath, where much of the melting occurs. The melting scrap flows under a weir, leaving behind the dross and flux. These furnaces produce a clean bath with less dross and are capable of lengthy periods of continuous operation.
A stack melter is another variant of the traditional reverb hearth melter. The furnace flue gases exit the furnace through a scrap charging area, which preheats the scrap. These furnaces have much higher energy efficiency than conventional reverb furnaces.
Melt cleanliness is a big factor in the marketability of the cast product. Most melt shops filter the molten aluminum through porous ceramic blocks before casting to remove the last traces of flux and oxide dross. The melt may be degassed to remove hydrogen, and it can be further refined by a chlorine treatment.
Energy Required to Melt Aluminum
The energy needed to heat and melt aluminum requires bringing solid aluminum to the melting point of 1221°F (661°C), melting the aluminum and heating the molten aluminum to the desired casting temperature. Equation 1 gives the required heat to bring aluminum from 77°F to its casting temperature.
Btu/pound = 0.281(cast T, °F) + 116 
So, 500 Btu/pound are required to reach a casting temperature of 1370°F (743°C). Yet, many melt shops use a lot more energy than that. How can this be?
First, the calculation neglects heat loss, which could amount to 250 Btu/pound, with more for smaller furnaces. Next, furnaces may operate at a slightly negative pressure, thus drawing in appreciable amounts of leak air. This air requires more fuel to be burned to heat it to the exhaust temperature of 2100°F (1150°C). Finally, an operator may increase the fuel rate to get a higher meltdown rate, thereby increasing the temperatures of the exhaust gases and the bath. All of these factors contribute to increasing the specific energy consumption to more than four times that obtained from Equation 1. At this rate, only about 25% of the combustion heat is used for the intended purpose (Fig. 2).
What can be done to decrease the specific energy consumption? Some actions have already been mentioned, but just how effective are they? We can calculate this by simulating an aluminum melting furnace in a way that relates all of the process variables. This simulation is available as a downloadable workbook at
www.industrialheating.com/AlMeltCalc. Calculations are available in SI and AES units.
Simulating the Operation of a Side-Well Aluminum Melting Furnace
A typical side-well reverb melting furnace (Fig. 1) consists of a number of burners firing inside the hearth. A charging well and a pump well (when present) are attached to the furnace hot wall outside the furnace. Both wells are connected to each other and to the furnace hearth by arches. This permits melt circulation between the furnace chambers.
Specifications for a 250,000-pound-capacity side-well reverb melting furnace are:
• 580-square-foot bath area, 30-inch bath depth
• 266-pounds/minute melting rate
• 512-cfm natural gas firing rate; ~1,000 Btu/scf
• 4.66×106 Btu/hour heat loss
• 2140°F (1171°C) products of combustion (POC) stack gas temperature
• 1420°F (771°C) aluminum bath temperature
• Cold-air oxidant at 6% excess over stoichiometric
• 250-cfm leak-air rate
Workbook AlMeltCalc shows that at continuous operation of scrap meltdown, the basis-case operation consumes 1,970 Btu/pound (HHV), which is only 28% of the fuel’s energy. Let’s look at ways to double this efficiency or do even better.
Methods for Improving Energy Efficiency
If you want improved performance from your furnace, you need to know how it performs. Your first step is to measure your energy use along with the critical factors that affect energy consumption. Once you know your furnace’s characteristics, compare them to best-practice benchmarks. Measurements of furnace gas temperature at the point where the POC enters the flue, the temperature gradient in the bath, variations in air-fuel ratio, the furnace pressure and Btu/pound melted are all important factors. You can use this information to improve furnace efficiency and produce better castings, and it doesn’t have to take 10 years to pay off.
The basis-case operation requires 1,970 Btu/pound, whereas the benchmark goal for a large cold-air furnace is 1,900 Btu/pound for steady melting operation. Figure 2 shows that most of the heat from a furnace is lost up the flue as sensible heat in the POC. Let’s look first at how controlling the air-fuel ratio can shave a bit off that loss. There are two main principles of combustion management:
• Provide more oxygen than you theoretically need to ensure that all the fuel burns up.
• Do not use too much oxygen.
The basis-case operation uses 6% excess air to the burner, but leaks bring in an additional 250 cfm of air. If the furnace were to operate at 3% excess air, with half the leak air, the melting energy would drop to 1,860 Btu/pound – a 6% improvement. In a typical day, at 20 hours of melting, the furnace will melt 320,000 pounds. Therefore, controlling the oxygen content to the furnace saves about 35,000 cubic feet of natural gas per day. So, the first recommendation is to control the burner’s fuel-air ratio closer to stoichiometric (perhaps going to mass-flow controllers), making sure the flue damper is positioned at high fire rates to give a slight positive or at best a neutral internal pressure. A side benefit from lowering %O2 in the POC is that less dross forms.
Recovering Heat from the Flue Gas
Even with better control of air-fuel ratio, the stack gas still carries out over 53% of the combustion heat. One way to capture some of this energy is to heat scrap with the flue gas. Some furnaces have this ability built in, while others may require a retrofit. The flue gas passes into a charging stack above the charging well and heats the scrap as it descends. An added advantage is cutting down on radiation heat losses from the well surface. Heating the scrap to 800°F (427°C) while cutting the heat loss by 20% brings the energy consumption to 1,370 Btu/pound while still melting at the basis-case rate of 266 pounds/minute.
This technique may not be feasible when melting finer scrap. As an alternative, the flue gas can be used to heat the combustion air. One way is to install a recuperator in the flue, which could bring the burner air to 1100°F (Fig. 3). Even higher temperatures are possible by using burners with an internal recuperator. Using 1100°F (593°C) air brings the energy consumption to 1,300 Btu/pound, a decrease of 34% in energy consumed. Another option is to use regenerative burners, which store flue gas in a ceramic matrix and subsequently release it to the incoming air. This requires burners operating in pairs, one extracting flue-gas heat while the other is heating the air (Fig. 4). This technique can give air preheat temperatures approaching 2000°F (1093°C). For burner air at 1900°F (1038°C), the melting energy is then only 1,006 Btu/pound. When added to better air-fuel ratio and furnace pressure control, the energy consumption is cut in half with a similar decrease in CO2 and NOx production.
Advancements in burner designs combined with good operating and maintenance practices can lead to substantial cost savings, increased productivity and less material loss. As with any process, no one system or combination will act as a magic wand to obtain the most satisfactory results. The workbook available at www.industrialheating.com/AlMetCalc allows you to explore various avenues for saving energy to help make good choices for meeting your goals. IH
The author gratefully acknowledges the help of Ray Peterson of Aleris International, David White of The Schaefer Group and Don Whipple of Bloom Engineering in the preparation of this article.
For more information: Contact Art Morris, chief scientist, Thermart Software; tel: 858-451-5791; e-mail: firstname.lastname@example.org; web: www.thermart.net.
1. Gershtein, V. Y.; Baukal, C. E.; Hewertson, R. J.; “Oxygen-Enrichment of Side Well Aluminum Furnaces,” Industrial Heating, May 2000, p.41
2. Curry, Dan, “The Basics of Pulse Firing,” Industrial Heating, October 2011, p. 73
3. Kaufman, Jared S.; Marino, Josh; “Regenerative Burners or Oxy-Fuel Burners for Your Furnace Upgrade,” Industrial Heating, June 2011, p. 41
Minimizing Aluminum Melt Loss
The primary form of melt loss in aluminum melting is by oxidation to form dross, which must be skimmed from the aluminum melt before casting. Dross is not a waste – it is a by-product with significant value, containing from 15-80% metal. A special section of this article (available only online at www.industrialheating.com/dross) shows ways to minimize dross formation, save energy and recover aluminum from dross. The main points are:
• Techniques for melt-surface temperature control
• Air-fuel ratio and leak-air control practice
• Proper burner selection
• Post-skimming practice for handling dross
• Safe disposal of waste products