Improved melters save energy

Reduced emissions also targeted

09/10/2013


Illustration of the method of melting used in a cokeless cupola furnace. Note the use of ceramic refractory balls for heat transfer in lieu of the use of coke. Illustration from DOE-EERE. (see PDF at bottom of page)Few industrial processes consume energy at the scale of metal melting equipment. According to a recent DOE study, the metal castings industry uses about 55% of its energy in metal melting alone. Ferrous and non-ferrous scrap metal reprocessing facilities and specialty steel smelters use similar large proportions. 

Two challenges are finding practical new energy-conserving technologies, and deploying steps already demonstrated on existing equipment. Many different energy sources are used in these processes, and the thermal performance can often be improved. Natural gas is a major energy source for metal melting, and will increase in this role in a new, more efficient future.

Obstacles to Energy Improvements

The path to efficiency improvement is not easy. Much of the use of melted metal is in the various castings industries, and many of these are small businesses that are capital-constrained. Few can afford to deploy cutting-edge technologies or experimental applications. For most, energy improvement must come from practical process steps and improvements to existing equipment.

The range of these metal melting industries is broad, from companies that melt down aluminum waste to those that make cast iron and steel automotive and other parts. It includes crucible melting of brass and bronze, and production of specialty steel alloys. Each industry is different, and no single technology has the potential to make universal savings.

Tough Environment for Materials

Further, because of the high temperatures encountered, especially with iron and steel, there are major limitations on the types of materials that can be used for equipment improvements, as well as limitations on the working life of these parts when installed. Promising improvements include development of sophisticated new ceramic refractory materials that resist abrasion and last far longer than earlier products. In some cases these can replace existing refractory surfaces and keep the melter on line longer. This reduces energy use and total emissions.

With products such as aluminum, owners need to avoid excessive handling of the molten metal and excessive introduction of oxygen in the processes. Still, there are very promising areas for improvement, and often natural gas is a fuel that plays a primary role.

Capturing Process Energy to Improve Efficiency

One major source for energy recovery is in combustion exhausts. Whether coal, coke or natural gas are the fuels, with existing technology much of the process heat goes up the stack. Recuperation or regeneration systems are practical tools for recovery of this heat. The recovered heat in turn can be used for pre-heating combustion air, or in some cases for preheating the furnace charge materials such as scrap metal or ingots.

Oxygen – in Limited Amounts

Oxy-fuel firing, as at this Kentucky aluminum recycler, reduces energy consumption. Photo courtesy Owls Head Alloys.A second important tool for many metal operations is oxy-fuel firing. Current burner technology allows introduction of a controlled amount of oxygen for combustion, which makes a hotter flame and quicker melt. The more quickly metal gets to the molten stage, the less energy is needed. But tight controls on oxygen levels are also important, because for some metals – especially aluminum – excess oxygen causes metal oxidation and a lower economic yield of molten metal. For these applications, increasing the oxygen level from the atmospheric 21% to perhaps 30% or 35% may be the optimum. Beyond this, the waste of metal becomes unacceptable.

Eclipse Inc. is a major manufacturer of burners for industrial processes, including melting furnaces. According to Jim Fisher from Eclipse, the company’s emphasis in this area is on oxy-fuel burners for ferrous and hard non-ferrous metals such as copper. “For non-ferrous metals we use a series of velocity burners that deliver very high stir motions that keeps the melt pool surface very uniformly heated.”

Reducing Coke Volume

Coke is refined from bituminous coal and is a rich source of carbon. Historically, coke served in the charge of certain melters such as cupola furnaces as both a fuel and a source of carbon for steel alloys. However, as a furnace fuel coke is expensive, and contributes significantly to carbon emissions and particulates. Today, cupola designs are available to operate without coke, or in some cases need only a small charge for metallurgical purposes.

ROBOTEC LLC is a Birmingham, Alabama firm which does engineering and provides designs and equipment for improved industrial melting furnaces. According Stefan Graf from ROBOTEC, changing a cupola furnace from coke to natural gas is not a simple step. “It requires a process change for the customer, including charge material, charge composition and furnace process characteristics. However with this changeover and commitment, the customer can expect huge benefits.”

Reduce Use of Expensive Coke

He notes that with an oxygen-enriched natural gas-fired cupola, one ton of liquid metal can be produced for about $8.00 for natural gas and $7.00 for oxygen for a total of $15 per ton. This compares with about $50 to $75 per ton using coke.

According to Graf, a cokeless cupola furnace uses a system of superheated ceramic spheres as heat exchange media to hold the heat in the melting zone. These spheres will need to be added with each furnace charge. Graf indicates that companies such as ROBOTEC can assist with a cost comparison study and will help to design the required furnace charge for maximum molten metal production and minimum emissions.

He points out that with the elimination of coke, the emission reductions are an important benefit, and in fact may be necessary to keep the plant operating. “We see a reduction of the carbon footprint by 70% compared with coke-fired cupolas. Particulates are about 1.0 mg per cubic meter, which will reduce the baghouse size. The exhaust gas temperature is about 200 degrees C with low carbon monoxide levels.”

Energy and Emissions Projects

Regardless of the size of operation, owners are increasingly looking for ways to improve process efficiency, recognizing that a more energy-efficient process inherently has lower emissions. The DOE Energy Efficiency and Renewable Energy division has done extensive work on melting technologies. In addition, there are experienced private consultants that can evaluate individual operations and make practical recommendations. Metal melting will always be energy-intensive. But major improvement is possible. 

DOE Metal Melting Technologies

Eclipse Burners

ROBOTEC LLC

This story appeared in the Summer 2013 Gas & Technology supplement. See additional stories below.



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