Electrification in primary aluminum
The aluminum industry is in a bind. Global demand shows no sign of slowing, yet there is a growing need to reduce emissions in its carbon-intensive production processes. A switch from gas burners to electrification could be part of the answer.
Aluminum is already one of the world’s most versatile minerals. It is used to manufacture everything from airplanes and cars to food packaging and cooking utensils, and plenty more in between. Population growth and increased prosperity, particularly in China, are driving global demand. So too is growing interest in environmental care, since aluminum is increasingly being used to build lighter vehicles and aircraft, and its recyclability makes it a more sustainable option compared with many other metals.
However, its biggest drawback is that the primary production process requires vast amounts of energy and involves high levels of CO2 emissions. In fact, at the current global average, every ton of aluminum produced results in 11.5 tons of CO2 being emitted. Overall, 0.8% of the world’s greenhouse gas emissions come from aluminum production.
Promising inert anode technology
The vast majority of these emissions – around 90 percent – are caused by the reduction process, where refined alumina is smelted into molten aluminum. For this reason, this part of the production process is the focus of a number of promising initiatives. Elysis, a joint venture between Rio Tinto and Alcoa, is currently working with inert anode technology to develop a smelting process that will emit oxygen instead of carbon. Rusal, one of the world’s leading manufacturers of low-carbon aluminum, is also investigating the potential of inert anode technology and hopes its plant in Krasnoyarsk, Russia, will be in mass-scale production by 2023.
“If inert anode technology can be perfected, it will mean CO2 emissions in the reduction process will be almost zero, and that will be a revolution,” says Ole Stadum, Sales Area Manager, Kanthal. “In the meantime, there is not much we can do in this part of the process today. However, there are other steps aluminum producers can take further downstream to reduce energy consumption and CO2 emissions – namely replace gas burners with electric heating.”
A move for increased efficiency
After the reduction area, the production process involves a number of heating applications as the metal is transported to the holding furnace and later the casting house. For safety reasons, all equipment that comes into contact with molten metal needs to be preheated and completely dry, since any contact with moisture or humidity could cause an explosion. Historically, much of this preheating is done by gas burners. However, by using electric heating equipment instead, aluminum producers can greatly improve energy efficiency and reduce CO2 emissions in these applications to virtually zero.
“This is because electric heating is much more precise and efficient,” Stadum explains. “The overall efficiency of a gas burner system is typically in the range of 15 to 30 percent, while the rest of the heat provided by the burners is lost through the chimney or direct to the surroundings. However, with electrical components, the efficiency is typically 90 to 95 percent. We have quite good figures from customers who we have replaced their existing gas/oil burners with electric solutions and seen huge savings.”
For the siphon/tapping tubes used to take the molten metal out of the potcells, electric air flow heaters can be used for preheating and cleaning. For the transport crucibles, metallic cartridges or silicon carbide (SiC) heating elements can be used.
Reducing casthouse energy consumption
In the casthouse, holding furnaces converted from burners to electric will reduce both energy consumption and dross formation. Once the metal has been mixed and alloyed in the holding furnace, heated by cartridge elements in a radiation-tube system, a launder is typically used to transport the metal to the casting station. If the launder is preheated sufficiently, it can help prevent a temperature drop and allow for lower temperatures in the holding furnace, thus saving energy and reducing dross generated in the holding furnace.
This is a function that flow heaters can perform very effectively. Or alternatively, SiC heating elements or ceramic fiber modules with metallic elements can be used to provide a hinged solution that allows the launder to be opened and closed easily for cleaning.
For higher-quality grades of aluminum that require filtering between the holding furnace and casting house, cartridge or SiC heating elements can be used to heat and clean filters between casting cycles.
Flow heaters add efficiency
For preheating degassing units, flow heaters have proved to be very efficient for reaching high temperatures in much shorter time compared with gas burners. They can also be used for fast preheating of ceramic foam filters (CFFs) after cartridge replacements and between production runs.
Inside the casting station, ceramic fiber modules with molybdenum disilicide (MoSi2) elements are very effective at drying casting molds of all moisture after the application of graphite coating.
In the rodding shop, electric ladle heating systems can be used for drying and preheating ladles and crucibles for cast iron. MoSi2 elements can also be used for drying stub holes in anode blocks and drying out graphite coated stubs. Cathode sealing can be conducted in a safe and efficient way with using MoSi2 electric heating.
Many possibilities to reduce CO2 emissions
In each of these applications, electric solutions will result in fast, efficient heating with lower energy consumption and reduced CO2 emissions compared with traditional gas/oil burners.
“We are seeing many aluminum production plants moving toward greater sustainability, and since there is not much that can be achieved in the reduction area, it is important that they reduce emissions as much as possible in the other production steps downstream,” Stadum says. “Switching from gas to electricity will have a huge saving in energy consumption and emissions. Even if gas and electricity prices are equal, the efficiency of electric heating is so much greater that the results will be positive.”
Products that can help aluminum producers make the transition to electrification
Fibrothal® heating modules: Preheating and maintaining high temperatures in the launder.
Superthal® heating modules: Drying and preheating of casting molds; drying stub holes in anode blocks; drying graphite coating studs.
Heliothal™ ladle heating systems: Drying and preheating of transport ladles and crucibles for cast iron.