Maximize furnace efficiency to temper aluminum melt costs: Controlling energy, environmental and equipment performance of aluminum melting furnaces can improve melt quality and reduce melting costs.
That was the focus of a workshop called "Furnace Technology: Maximizing Performance of Aluminum Melting Furnaces for Improved Melt Quality, Energy Conservation and Reduced Melting Costs." The workshop was part of the AFS 6th International Conference on Molten Aluminum Processing on Nov. 11-13 in Lake Buena Vista, Florida.
Participants in the workshop were: Steve Robison, AFS technical director, who focused on energy costs and metal loss; Russell Kemp, RMT, Inc., Atlanta, who discussed pollution control concepts; Joseph Danega, Thermtronix Corp., Adelanto, California, who spoke on getting the most out of melting equipment; and Don Whipple, North American Manufacturing Co., Cleveland, who described the different types of burners used to melt aluminum in gasfired furnaces.
Heat, Melt Loss Sources
Using a heat loss diagram (Fig. 1), Robison showed how the efficiency of aluminum melting furnaces could vary tremendously. Heat loss is a primary factor in energy costs.
Only a portion of the heat that a furnace produces is actually available to melt aluminum. Heat is lost through the flue, walls, open metal surfaces, burner inefficiency, furnace design and even inconsistent temperature controls. A conservative estimate of the radiant heat lost through an open metal surface (such as an uncovered crucible or dip well) while melting at 1300F (704C) is 6633 Btu/sq ft/hr. A radiant heat loss while melting at 1400F (760C) can reach 9335 Btu/sq ft/hr. Among Robison's advice to save energy was:
* recycle the heat from the flue to heat the melt or pre-heat charge material;
* check and repair any cracks or holes in the furnace refractory walls;
* properly select and install the refractory material;
* reduce the amount of time a furnace cover remains open.
Pyrometer accuracy indirectly can affect melt costs, Robison warned. If the reading of a pyrometer is off then a foundry will over or under-compensate the temperature of a furnace. Pyrometers need to be kept clean of metal oxides that can inhibit accuracy and regularly calibrated to provide accurate readings. For example, if a pyrometer is not properly calibrated and is reading 50F (10C) lower, the entire furnace of metal will be overheated, resulting in excess energy cost.
Robison also touched on metal loss when the metal is melted but not poured in a casting. Metal loss can be attributed to oxidized charge material that produces excess dross. Meta1 loss adds to melt cost. For example, 1% met loss of 7215 lb of aluminum melted equals 7 lb of lost metal. At a cost of $0.65/lb, $47 can be lost daily and $12,168 annually, he said.
To reduce excessive aluminum in dross, charge materials should be stored indoor to minimize oxidation and pre-heated immediately prior to melting. Dross recovery units are an option to reduce metal loss by reducing the amount of aluminum in dross. Dross should be loose and granular.
"If the dross is shiny and silver, it's not dross. It's metal," said Robison.
If dross contains 70% aluminum, then it may cost a foundry $63 in lost metal for every 100 lb of dross sent to the reclaimer.
NOx Emission Concerns
Kemp offered ideas on how to control nitrogen oxide (NOx) emissions, NOx reacts in the atmosphere along with volatile organic compounds to form ozone. While ozone in the upper atmosphere is great for protecting us from ultra-violet radiation, at ground level it is harmful to respiratory systems. Ozone is a major component of what is commonly referred to as "smog."
In controlling NOx, the NOx is destroyed before the furnace exhaust gases are released to the atmosphere. The control reactions drive the NOx to harmless nitrogen and water vapor.
Two low-cost methods are low NOx burners (staging combustion from a conventional burner that can control 50% of the emissions) and regenerative burners (preheating of combustion air that can control 60%--and achieve 40% in fuel savings).
Basically, the preheating of the combustion air affects the actual flame temperature in a way that reduces the potential for NOx formation. The fuel savings comes from the thermal efficiency of preheating the air using hot exhaust gases (the regenerative part).
Other NOx emission control steps include:
* selective catalytic reduction (SCR) consisting of ammonia and a catalyst (controls 90%);
* selective non-catalytic reduction (SNCR) consisting of ammonia or urea without catalyst (controls 70-85%);
* fuel reburn that has staged natural gas injection (controls 60-75%).
The last method "actually takes natural gas and puts it in the exhaust stream," Kemp said.
The choice to control NOx usually would come directly from tightening regulations in areas that are facing ambient ozone nonattainment or from facilities wanting to do substantial expansions. Those foundries are either subject to the prevention of significant deterioration (PSD) rules or trying to reduce emissions to a point so as to avoid PSD.
Once the choice to control has been made, different control approaches have to weighed. In making the split between SCR and SNCR, the main factor would be the temperature of the offgas stream. SNCR needs gases above about 1700F (926C). SCR, being catalytic, requires either mostly clean offgases or good filtration up front to prevent metals in the gas stream from fouling the catalyst.
With selective reduction principles, these reactions occur readily at temperatures in the 1700-1900F (926-1037C) range, he added. Catalysts must be used to foster the reactions at lower temperatures.
Kemp presented a case study of a metalcaster in a NOx emission trading area that could generate revenue from revamping its environmental equipment.
By installing a $2 million system, the facility could drop its NOx emissions by almost 80%. The changes involved refitting four remelt furnaces with regenerative burners, installing SCR on low-temperature sources and SNCR on high-temperature sources. An option for installing gas reburn and cogeneration on the high temperature sources also was presented.
As a result, the facility could sell 11,000 lb/yr in emission credits to another company for revenue.
Furnace Efficiency in Daily Operations
Danega said that in order to squeak every dollar out of an electric furnace, foundries must consider furnace construction, control and time of use, day-to-day habits, cleanliness and crucible maintenance.
In regard to furnace construction, insulation is the most important aspect for maintaining thermal efficiency, he said. Also, foundries must eliminate gaps in furnace walls, floors and roof. Without a sealed chamber, foundries risk uncontrolled losses of natural convection, radiant heat and combustion heat.
Additional savings can be achieved by: diverting energy to off-periods; limiting controls for on-peak hours; and melting during off-peak hours so holding is on-peak instead of re-melting,
For day-to-day melting habits, Danega suggested foundries keep crucibles covered, pre-heat ingot by capturing inherent heat loss (by placing it on top of the furnace prior to charging, for example), eliminate excessive degassing and keep crucibles as full as possible (when practical).
By ensuring that a crucible is completely covered so that the melting heat doesn't escape, $2000/yr may be saved on energy costs, he estimated. Automatic cover controls can help. For manually operated lids, the operator needs to make sure the lid is covering the opening completely. All covers should be inspected for cracks regularly.
As for excessive de-gassing, Danega said more research and testing needs to be performed to determine the proper amount required. Since de-gassing is used to eliminate porosity defects, each foundry should evaluate the gas level needed for each job rather than using a constant amount, De-gassing often is performed in an open crucible or well (without a cover). Some foundries may de-gas more than necessary and can save by decreasing the energy used in its de-gassing procedure.
Furnace cleanliness provides for maximum melting efficiency as well. Dross and metal build-up may form on uncleaned equipment walls and bottoms, impeding heat transfer and reducing the capacity that a crucible can hold. Foundries should keep crucibles clean by scraping the walls meticulously and dredging sludge from the bottom.
Danega said maintenance of crucibles is important, noting that new crucibles transfer heat efficiently, whereas older crucibles develop cracks and force the level of metal lower. Gaskets, he warned, should be replaced and never reused.
Gas Burner Designs
Turning the focus to gas furnaces, Whipple discussed the different types of burners needed to melt aluminum. He spoke on heat release by combustion and the available heat with air/fuel ratio. Different burners have different heat release patterns. Matching the proper heat release to the style of furnace can be critical to the efficiency of the operation.
"Burner adjustment is structured around natural gas, not oil, butane or propane. The goal is heat. The process is a chemical reaction," he said.
As for air/fuel ratios, Whipple said 1 cu ft of air should equal 100 Btu of natural gas (or 10 parts of air for every 1 part of gas). One cu ft of air has enough oxygen that when combined with the proper amount of natural gas, 100 Btus of energy is released. One cu ft of natural gas will release 1000 Btus of energy when combined with the proper amount of oxygen. Thus establishing the 10:1 ratio.
The air/fuel ratio is used to insure that all the gas put in the furnace is burned (or consumed). If not, excess gas could go out the flue, which means wasted money. If excess air goes out the flue, this too means wasted money. Therefore, a proper ratio is required for an efficient operations
Excess fuel in a furnace also can cause a danger by creating a situation where an explosion can happen. For safety reasons, good air/fuel ratio is "a must."
The three basic types of air/fuel ratio systems are:
* zero-governed systems for premix burners;
* pressure balanced for nozzle-mix burners;
* flow ratio for nozzle mix burners.
The air/fuel ratio set point can be adjusted for the greatest production and efficiency with an aspirator premix burner system or the commonly used nozzle-mix (Fig. 2) burner system.
The two basic types of gas burners are premix and nozzle-mix. Premix burners can be an inspirator (an open burner nozzle) or aspirator (a sealed-in burner). The inspirator needs pressurized gas to induce air. The aspirator requires pressurized air to aspirate gas. The aspirator can be more precisely set up and is not affected by temperature, Whipple added.
With either the premix or nozzle-mix, the proper nozzle is critical, he said.
The two types of nozzles are self-piloting (open or closed burner) and "sealed-in" tunnel burner (closed burner). The self-piloting has piloting holes in the nozzle that causes burning in the tip to stabilize the flame. The sealed-in nozzle has a flame retention step.
With nozzle-mixing burners, the mixing of air and gas takes places within the nozzle. Used with larger installations or for close temperature control, the nozzle-mixing air/fuel ratio typically required is a flow-metered system with either no pressure or temperature compensation, or pressure and temperature compensated (mass flow). Foundries regularly use fixed-port valves with nozzle-mix burners for temperature control. At 12-15 Btu/hr, a hot side, multi-zone flow-metered system can save on energy costs, Whipple said.
Although different styles of flame stabilizers are available, they all are built to burn efficiently and hold that flame on the burner.
For a free copy of this article circle No, 341 on the Reader Action Card.
RELATED ARTICLE: How to Measure Furnace Efficiency
As energy costs continue to escalate, the cost of melting becomes a critical issue affecting foundry profitability. Any increases in melting rate or reductions in energy used for melting are direct bottom-line improvements. The cost of melting and processing can be one of the most overlooked areas of business cost analysis for the aluminum foundry.
Yet, many foundries do not know how much it costs them to melt their metal. Before a foundry can begin to measure improvements and melt cost savings, it must perform a cost study to define its current melting and processing costs.
Although a melt cost analysis may involve considerable time, the basics of a foundry melt cost study are simple:
* utility cost (for example, $/1000 cu ft of natural gas);
* amount of metal melted;
* furnace melt rate lb/hr;
* average melt loss;
* amount of time the furnace is used (and consuming energy);
* furnace utilization--amount of time the furnace is melting and "holding (less energy is used when holding than when melting).
A sample melt cost analysis for both gas fired and electric resistance furnaces are available online at www.moderncasting.com.
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|Date:||Jan 1, 2002|
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