Commitments to achieve substantial decarbonization across metal-producing industries have put a focus on energy efficiency and the use of hydrogen as a fuel. The global aluminium production's average total carbon footprint (Scope 1, 2, and 3) is approximately 12 tonnes of CO₂e per ton of aluminium. The main part of this carbon footprint originates from primary operations, particularly electricity generation, which might take a long time and a considerable effort to address. However, there are great opportunities for immediate decarbonization in remelting and recycling. We will discuss how to decrease the carbon footprint from the remelting and cast house operations, but obviously, all segments of the industry must play their part to reduce emissions.

CO₂ emissions from the cast house are almost entirely tied to the fossil fuel used to heat the furnaces. To reduce the carbon footprint, there are basically only two routes available: to reduce the energy consumption; to reduce the specific CO₂ emission, i.e., g CO₂ per kWh.

When burning fossil fuel with air – containing only 21% oxygen but 78% nitrogen – up to 50% of the energy is lost through the chimney as the nitrogen is heated up in the furnace and emitted in the flue gas. Accordingly, the presence of nitrogen results in wasted energy, higher fuel consumption and CO2 emissions. Moreover, it hampers the radiative heat transfer from the combustion products, which is the dominant mechanism at elevated temperatures. Using oxygen instead of air, called oxyfuel combustion, eliminates this nitrogen ballast.

Oxyfuel

Using oxyfuel technology, removing nitrogen from the process will effectively reduce energy consumption, thereby reducing CO2 emission by 35%. As the flue-gas volume is substantially decreased, it also reduces the load on the filter system by about 75%.

In addition to fuel saving, avoiding the nitrogen ballast in the furnace atmosphere will increase the heat transfer to the metal in the furnace by about 40%, resulting in a 40% increased melt rate. This will reduce the CO2 emission even more per ton of melted aluminium.

Early attempts to apply oxyfuel in the aluminium industry suffered because these installations operated with burners with very high flame temperatures, which created hot spots and increased dross formation. However, hot spots can be avoided by using Flameless Oxyfuel, which combines the two benefits of fuel savings and melts rate increase, but at a low flame temperature. The features of Flameless Oxyfuel technology are described here, together with results from numerous full-scale installations in aluminium melting furnaces.

Low-Temperature Oxyfuel (LTOF) – technology and results

The development of LTOF is based on Linde’s Flameless Oxyfuel technology platform, and it has successfully opened up the market for oxyfuel in the aluminium industry during the last 15 years. Linde´s LTOF technology uses a volume combustion regime, where the combustion products are recirculated back into the flame, thereby reducing the flame temperature very effectively. This takes place without loss of efficiency concerning fuel savings and melt rate increase.

In Flameless Oxyfuel, the mixture of fuel and oxidant reacts uniformly through the reaction flame volume, with the rate controlled by partial pressures of reactants and their temperature. In Flameless Oxyfuel, the combustion gases are effectively dispersed throughout the furnace, ensuring more effective and uniform heating even with a limited number of burners installed.

The focus of using LTOF is on reverberatory furnaces with the highest demand for thermal homogeneity. The increased melt rate has been the primary goal in most more than 50 LTOF installations worldwide. However, with the increased focus on carbon footprint reduction, fuel-saving has become a more prominent driver.

To summarize, results from installations of LTOF show the aluminium industry now has a tool to:

• Increase the melt rate by about 40%
• Reduce the energy consumption by 30-50% i.e., a 30-50% reduction of the carbon footprint
• Reduce the off-gas volume with 80-85% per ton of aluminium
• Reduce the low frequency noise in the plant, since no combustion air fans are needed

These benefits can be obtained without

• Changing the recovery of aluminium
• Increasing refractory wear

Hydrogen ready

A feature of the LTOF technology, which is becoming increasingly important, is that it is ready for using hydrogen as fuel. An LTOF system designed for a conventional fossil fuel can swiftly be converted into hydrogen, entirely or with a mix of fossil and hydrogen fuels.

LTOF has a peak temperature below the temperature for the formation of thermal NOX, supporting the reduction of NOX emissions. Repeated tests and evaluations have confirmed that this feature is also maintained when using hydrogen as fuel.

For the aluminium industry to be ready to move towards hydrogen combustion, it is important to create a clearer understanding of the consequences of the change in the furnace atmosphere.

H2 and furnace atmospheres

In an industrial furnace, a certain amount of air in-leakage is unavoidable. The resulting atmosphere compositions when combusting natural gas using air-fuel and oxyfuel, respectively, will approximately be:

Switching to hydrogen as a fuel will give these atmospheres:

By switching from fossil to hydrogen fuel in melting furnaces, the CO2 footprint of a cast house can be reduced considerably. However, this will result in a substantial increase of the water vapour concentration in the furnace atmosphere and likely change the oxidation behaviour of molten aluminium alloys. To address this important topic, Linde carried out a series of tests where Al-Mg alloys were melted in different atmosphere compositions due to fuel type and burner set-up variations. The pilot-scale tests were done together with, among others. Hydro, Alcoa, and SINTEF.

The results show that hydrogen combustion in an oxyfuel configuration leads to less oxidation on liquid Al-Mg alloys than hydrogen in an air-fuel configuration. The tests also showed that as little as 5% CO2 in the furnace atmosphere significantly suppresses oxidation.

Hydrogen dissolution into the metal will most likely increase with the increased H2O concentration in the atmosphere. With state-of-the-art degassing technologies, this should not be an issue; however, it must be verified in full-scale operation.

The substantially increased water vapour concentration may also affect the flue-gas treatment system. Additionally, it raises questions concerning methods for emission control and furnace diagnostics at very high water vapour concentrations.

Going carbon neutral

LTOF is the stepping stone to the carbon-free melting of aluminium with the radical reduction of CO2 emission with fossil fuels and the fact that the technology is completely hydrogen ready. Accordingly, LTOF provides short-term fuel savings and the potential for increased melt rates and paves the way for carbon-neutral aluminium melting. Moreover, as LTOF also reduces NOx emissions even when using hydrogen as fuel, this transition can take place without any negative trade-offs. Full-scale production tests with LTOF and hydrogen are planned with multiple aluminium-producing companies.