High-Intensity Resistance Heaters Offer Power and Efficiency for Industrial Heating Processes
While the concentration of implementation efforts continues to focus on aluminum, nonferrous metals such as lead, zinc, magnesium and copper-base alloys – as well as chemicals, petrochemicals, agricultural and forest products – could also benefit. The replacement of less powerful and less efficient resistance-heating systems and heating processes that rely predominantly on lower-efficiency heat-transfer mechanisms can significantly increase the energy-savings potential of the new technology. Current activities already include the melting and processing of lead.
BackgroundIn the spring of 2006, the U.S. Department of Energy sponsored a Kickoff Event that recognized the commercial demonstration of the Isothermal Melting Process (ITM) developed by Apogee Technology, Inc. in partnership with Aleris International, Argonne National Laboratory, the University of Pittsburgh and Drexel University. ITM later received R&D Magazine’s prestigious R&D 100 Award for 2006. D.O.E. support was also obtained for a four-year program to develop an integrated system for melting, over-the-road transport and in-plant dispensation of molten aluminum with unprecedented energy savings over current practices. Project partners for this program are Aleris International and General Motors.
The development of ITM from concept to commercial demonstration required five years of intensive research and development, much devoted to developing a heating system capable of overcoming the severe limitations of existing immersion-heating systems used in aluminum. These heaters in use at the Aleris Urichsville facility operate at 840 kW and provide 2.84 million BTU/hr to melt aluminum at a net 97% energy efficiency. The overall thermal efficiency, including holding and pumping losses, is 84%. An array of 15 such heaters, illustrated in Figure 1, operates at a total power output in excess of 300 kW.
Present State-of-the-ArtNatural-gas air-fired reverberatory furnaces are the mainstay of primary- and secondary-aluminum cast shops and large engineered-casting foundries. They range in capacity from 10,000 to more than 250,000 pounds. Charge components may include solid and molten aluminum, internal and purchased scrap, metallurgical metals and master alloys. These are introduced to the hearth, after which the furnace doors or lids are closed, and it is fired for melting. Energy from combustion is absorbed by exposed refractories, from which radiant heat is reflected to the charge. After melting, the bath must be stirred to assure complete solution and homogeneous composition.
Adjustments in metal chemistry are made when necessary, and additional alloying, stirring and sampling take place. Finally, the on-composition melt may be held for varying periods at controlled temperatures before transferring to other furnaces or casting stations. Additional stirring and/or skimming may be required before transfer. With each iteration, more energy is required, additional oxide losses are experienced and dissolved-hydrogen levels increase.
Reverberatory furnaces impose significant disadvantages that increase in importance with rising energy costs. Among these are:
- Aluminum in the molten state oxidizes when exposed to the atmosphere and products of combustion to form an insulating barrier to the absorption of radiant energy by the melt.
- Heat concentrates at the melt surface, accelerating oxidation, increasing oxide-barrier thickness and forming more harmful oxide species.
- Surface-down heating results in extensive thermal segregation.
- The melt must be stirred to accelerate alloying, assure thermal and chemical homogeneity and remove the oxide-metal skim layer. Stirring is usually accomplished by industrial truck or rail-mounted tools. This requires open-door access, resulting in substantial heat losses.
- Over-firing to increase radiation intensity reduces the life of refractories above the metal line.
- The batch nature of the process results in alternate high-firing and cooling sequences that consume energy and contribute to refractory failure by thermal fatigue.
An approximation of the best energy efficiency obtained by reverberatory furnaces in good condition with the most advanced features and operated competently is 45%. More typically, energy efficiencies across the aluminum industry approximate 20%.
The Isothermal Melting ProcessThe current ITM direct-immersion heater can operate at a heat flux of 150 watts/in2, or three to five times the power of competing commercial electric-resistance immersion units that are typically contained in thermal and mechanical shock-sensitive ceramics. The innovations of the Apogee immersion heater include:
- The material and design of the resistance element
- The use of a dense, more thermally conductive alternative to conventional dielectrics
- A complex multi-component flame-sprayed titanium sheath, which is resistant to mechanical and thermal shock, spalling, erosion and chemical attack in molten aluminum
Continuing intensive research is expected to result in the successful use of these heaters in highly aggressive aluminum-magnesium alloys. It also includes consideration of less sophisticated shrouding for less aggressive operating environments.
Isothermal melting is a continuous process. Molten metal is circulated from a fully enclosed holding section through four in-line bays. This is schematically illustrated in Figure 2.
A radial-flow centrifugal pump is located in the first bay. Solid metal is charged in the second bay. The third bay consists of a high shear-phase reactor in which the melt is homogenized, degassed and oxides and other nonmetallics are removed. Heat corresponding to that required for melting is provided through direct-immersion heating in the fourth bay, after which the stream is returned to the holding section. Molten metal is recovered and directed to crucibles or process furnaces by overflow.
Energy is applied through two independent, self-regulating heating systems. The energy required for melting is provided by direct-immersion (DI) heaters with sufficient heat flux for melting at rates consistent with most industry requirements. Current-design DI heaters operate at a heat flux of 100-150 w/in2 and result in an effective spatial firing rate equivalent to 600,000 BTU/ft2 of metal area. By contrast, typical reverberatory furnaces operate at 115,000 BTU/ft2 of metal area.
Radiant heat transfer dominates in reverberatory-furnace operation, while in ITM, heat transfer occurs through a combination of conduction and forced convection. Through extensive modeling and novel element design, the typical temperature differential between the electric-resistance heat source and molten-metal temperature is less than 425°F. That differential exceeds 2200°F for reverberatory furnaces.
Much lower-output electrical-resistance elements embedded in baffled side-pocket refractory panels (BSPP) of the holding section compensate for system heat losses. The BSPP system is electrically independent from the direct-immersion heaters used for melting and is operated for heat-loss compensation at a flux of 20–35 w/in2. But it also has the capacity to remelt in the unlikely event of freeze-up.
Under idle conditions, DI heaters are essentially off, and heat losses are compensated by the BSPP system. When charging of solid metal begins, temperature depression is detected by the DI-heater control logic, and power is applied to maintain constant bath temperature at a level commensurate with the charge rate. The DI and BSPP control systems are independent and use PID controls with various signal-processing algorithms that provide the basis for optimum fail-safe operation. Monitoring and control can be remotely accomplished electronically through the Internet.
Because ITM melting is based on conduction and forced convection rather than radiation, molten-metal surface area is reduced by almost 70% and footprint by 75%. While conventional radiant-heat transfer is area intensive, ITM-melting heat transfer can be viewed as volumetric because it occurs through depth. Convective heat transfer requires metal circulation within the ITM vessel, and this also assures chemical and thermal homogeneity while offering significant advantages in refractory life. It overcomes substantial safety risks associated with furnace charging and provides the plant environment with significant contrasts in pollution, noise level and physical comfort.
ITM DevelopmentsThe performance and success of the Apogee resistance heating element in immersion heating for melting and in BSPP configuration for holding has led to additional, highly significant concepts and developments.
- The use of highly efficient resistance heating in holding furnaces and the replacement of gas-fired crucible melting/holding furnaces
- Conventional furnaces can be retrofit with side-bay-mounted immersion heaters.
- The BSPP heating system is already used commercially in launders (trough sections through which molten metal is conducted between melting and holding furnaces and connecting furnaces and casting stations) for efficient and precise temperature management and control.
- Internally heated metal transfer ladles avoid the necessity of superheating ladle linings and metal for over-the-road transit and effectively eliminate time and distance limitations for molten-metal shipment. The Turbo Electric Ladle (TeL) developed by Apogee employs BSPP technology to prevent heat losses using trailer-mounted, turbine-generated electrical power.
Aleris and General MotorsIn the most recent D.O.E.-supported four-year advanced-melting, transport and dispensation extension project, Aleris International will employ ITM to melt and supply alloyed molten metal to General Motors. Apogee TeL transfer ladles will accurately maintain conventional holding temperatures and melt quality during transit and will be off-loaded to electrically powered stations at GM, thereby eliminating the need for separate holding furnaces. A technology has also been developed for the controlled dispensation of metal that preserves melt quality from the TeL when transfer to the casting station is performed.
Preliminary estimates show that the current energy requirement of almost 40 BTU/lb-hr of these furnaces can be reduced to less than 9 BTU/lb-hr. On-line molten-metal inventory will be reduced from 172,000 pounds to less than 10,000 pounds. A comprehensive study of comparative energy consumption and costs is under way. It is expected to confirm or exceed preliminary justifications for this program that include annual savings of 12.7 trillion BTUs per year.
ImpactDirect immersion and panel heating by high watt density, high-efficiency electrical-resistance elements with the characteristics required for the temperature and operating environment of the application represent obvious and highly significant advantages to the aluminum industry. The technology’s most apparent and important role is in revolutionizing the melting process. This innovative and significantly more cost-effective and energy-efficient process is perfectly timed to the U.S. aluminum industry’s expanding reliance on melting and recycling to satisfy metal requirements.
Of emerging importance since ITM relies on electricity, the primary energy source can be nuclear, coal, natural gas, biomass, low-energy-content waste gas, oil or hydroelectric turbine, including yet-to-be-developed sources. This versatility is important because of the uncertainty of future energy supply. An example of the value of this versatility is the intriguing possibility of installing ITM units at waste-landfill locations where an availability of aluminum separated from the waste stream coincides with the generation of low-quality, methane-containing waste gases that could be used for power generation.
The ITM process for candidate applications in the aluminum industry alone can result in potential energy savings conservatively estimated at 50 trillion BTUs per year with an annual savings estimated at $750 million. Melt loss is reduced by at least 75% with energy savings of more than 6 kWh/lb.
AcknowledgementsIt’s important to pay tribute to the Office of Industrial Technologies and more recently the Industrial Technology Program of the U.S. Department of Energy, whose assistance was instrumental in accelerating Apogee Technology’s research and development efforts. The D.O.E. has played a unique and vital role in inspiring, motivating, encouraging, facilitating, supporting and coordinating efforts by our most important technological resources – National Laboratories, colleges and universities and U.S. industry – in programs that identify and exploit the most promising new technologies for improved energy efficiency. These are programs in which the government allies itself with industry to achieve goals identified with national priorities: improved industrial process efficiencies, reduced energy consumption, conservation of natural resources, lowered environmental impact and global competitiveness. IH
For more information: Contact Elwin Rooy at Rooy and Associates, 461 Ravine Drive, Aurora, OH 44202; tel: 330-995-0095; e-mail: email@example.com; or Apogee Technology, Inc., P.O. Box 101, Verona, PA 15147; tel: 412-795-8782; e-mail: Info@apogeetechinc.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: isothermal melting, reverberatory furnace, resistance heating element, heated transfer ladle