Solutions for Greener Stainless Steel Production

The steel industry ranks amongst the top three CO2 (carbon dioxide) emitters from the industrial segment, and many steelmakers have set carbon neutral goals for the 2030-2050 timeframe. In general, stainless steel producers have a smaller carbon footprint than most carbon steel producers— on average about half — yet many have focused on investigating and implementing various available decarbonization approaches.

Naturally, a short-term approach for decarbonization would be focused on improvements and minor alterations in parallel to mapping out longer-term options. There are immediate actions relating to procurement, operation practice, and use of proven technologies, that can be implemented to reduce the carbon footprint. An increased use of renewable power, both directly and indirectly, is key here as well.

By Joachim von Schéele, Global Director Commercialization, Linde plc.

Procure Greener

Using green power – based on renewable energy sources – is an obvious step that can be taken wherever possible. The biggest consumer of electricity is the Electric Arc Furnace (EAF) and Induction furnaces. Employing green power in the EAFs could decrease CO₂ emissions across the Americas by 50- 200 kg per tonne of steel produced, depending on how the electricity is produced at the specific location. Naturally, green power should also be used to produce the industrial gases used (oxygen, argon, and nitrogen) as well hydrogen.

Another action that instantaneously results in a reduced carbon footprint, is charging of more scrap. Scrap is defined as ‘carrying zero CO₂’; however, all alloys do ‘carry CO₂’ from their upstream production. Accordingly, it is very important that attention is paid to the carbon footprint when procuring alloys, because those alloys could be a large contributor to the final carbon footprint of the stainless steel product. Nickel Pig Iron (NPI), for example, carries a large carbon footprint – about five times that of the lowest CO₂ emitting alternative for nickel supply. Accordingly, from this perspective, the NPI material should be avoided as much as possible. Some other observations on the carbon footprint relating to the choice of alloys include the use of nickel from sulfide ores instead of laterites, and, if possible, replacing nickel with manganese.

Oxyfuel Combustion Drives Energy Efficiency

Many unit operations in a steel mill use air for the combustion of fuel, which carries 79% ballast (almost all of it nitrogen). This nitrogen is heated up in the furnace and emitted in the flue gas, resulting in wasted energy, higher fuel consumption, and CO2 emissions. Moreover, it hampers the radiative heat transfer from the products of combustion. The use of oxygen instead of air, called oxyfuel combustion, eliminates this nitrogen ballast, and results in:

• Up to 50% fuel and CO2 savings,
• An 80% reduction in flue-gas volume,
• Up to 90% NOx (nitrogen oxides) reduction,
• On-demand production increase, and
• The ability to use of low calorific gases in heating and reheating operations.

Oxyfuel combustion has been successfully applied to several steel mill operations including blast furnace stoves, pelletizing/sintering furnaces, ladle preheating (40-60% fuel savings), reheat furnaces (batch and continuous), and heat treatment furnaces.

The economics of oxyfuel combustion are typically driven by fuel price, but as steel mills adopt green hydrogen fuel to decarbonize their footprint, oxyfuel combustion will become economically necessary. This is because over time hydrogen prices are expected to drop down only to around USD$2/kg, which is equivalent to USD$15/GJ, i.e., hydrogen will always be a relatively expensive fuel. Accordingly, oxyfuel combustion is required to minimize the use of hydrogen. Therefore, the recommendation for steel mills is to convert to oxyfuel combustion now to achieve 20-50% CO₂ reduction and be prepared to blend green H₂ when available, to achieve full decarbonization in the future. In addition, in stainless steel production, the annealing lines can be converted to a flameless oxyfuel operation.

Flameless Oxyfuel Solutions

Increasingly stricter regulations on Nox emissions, combined with ongoing efforts to decrease the carbon footprint, demand the steel industry to use further enhanced processes. The successful development and use of flameless oxyfuel was originally triggered by the need for reduced NOx emissions. Since its full-scale introduction at Outokumpu in 2003, hundreds of installations have been completed using flameless oxyfuel in all kinds of steel reheating and annealing furnaces, aluminium and copper melting furnaces, and in ladle and other vessel preheating systems.

In flameless oxyfuel, the mixture of fuel and oxidant reacts uniformly through the flame volume, with the rate controlled by partial pressures of reactants and their temperature. Fundamentally here, using burners or lances or a combination of the two, is the matter of utilizing velocity in a beneficial way when at same time separating the injection points of the fuel and the oxidant, leaving the traditional design of a burner. In flameless oxyfuel, the combustion gases are effectively dispersed throughout the furnace or vessel, ensuring a more effective and uniform heating of materials even with a limited number of burners installed. Although the first installations took place in reheating and annealing, flameless oxyfuel was quickly adopted for preheating of ladles and converters. The lower flame temperature substantially reduces the NOx formation.

At stainless steel production companies such as Acerinox, Alleima, Outokumpu, and Walsin, flameless oxyfuel systems are used for the preheating of ladles and converters. Typical results include 50% fuel (and CO₂) savings, shorter heating cycle, and higher final preheating temperature. More than 200 such installations of Linde’s OXYGON® flameless oxyfuel are successfully in operation in the steel industry.

Focus On Reheating and Annealing

Reheat furnaces typically consume 1.2-1.6 GJ/t of steel, and oxyfuel solutions can reduce fuel and CO₂ emissions from reheat furnaces by up to 50%. An example of one such solution is Linde’s REBOX® oxyfuel solutions that have been installed on almost 200 batch and continuous reheat furnaces and annealing lines, with production up to 300 t/h and covering a wide range of steel grades. This has already contributed to decarbonization as the installations decreased the consumption of fossil fuel by 20% to 50%. Partial or full conversion of reheat furnaces from air-fuel to oxyfuel combustion is not only an easy first step to decarbonization, but it also prepares the furnace for subsequent hydrogen fuels. Indeed, oxyfuel combustion will be necessary with hydrogen fuels. Since 2003, flameless oxyfuel has been applied to improve temperature uniformity and secure reduction of NOx emissions – a matter particularly important going forward, including the use of H₂. More than 40 REBOX installations have taken place at stainless steel producers. Some examples of those installations are briefly discussed below.

Alleima uses full flameless oxyfuel converted continuous reheating for its production with excellent results. It is interesting to note that Alleima’s operations include production of very high alloyed materials (e.g., for rock drills). This clearly demonstrates that flameless oxyfuel has no negative impact when heating even very high alloyed steel grades. Alleima also has installations using an add-on system that combines air-fuel with oxyfuel to achieve a semi-flameless oxyfuel regime. A large number of installations are also found at Outokumpu’s sites, applied both for reheating and annealing.

Dongbei Steel uses full flameless oxyfuel for a 70 t/h continuous annealing of stainless steel wire rod, a process called Direct Solution Treatment (DST). The fuel used is gasified coal with a calorific value at 2 kWh/m3. Another such DST installation is found at Yongxing, where the fuel is natural gas. This 60 t/h production is the most energy efficient stainless wire rod annealing line in the world, with a total fuel consumption including idling time, etc., at <70 kWh/t.

Use of Hydrogen

Hydrogen combustion results in a 100% H₂O atmosphere in the furnace. After Linde together with steel producers performed pilot tests to develop hydrogen flameless oxyfuel burners and to assess the impact of hydrogen reheating on steel quality, capacity, and uniformity in the reheating process, NOx emissions, safety issues, scaling, it was determined that all results were encouraging, and no negative impacts could be identified.

The engineering steel producer Ovako and Linde decided to make a full-scale demonstration of hydrogen-oxygen reheating at Ovako’s Hofors mill in Sweden. This was the world’s first heating of steel with 100% hydrogen as fuel, carried out in March 2020 with 25 tonnes of ball-bearing steel ingots heated in a pit furnace fired with hydrogen-oxyfuel using Linde’s REBOX Hyox technology. After heating and soaking, the ingots were successfully rolled to bars in the rolling mill. A thorough inspection and analysis of the rolled bars showed that heating using hydrogen as fuel does not impact the quality. Based on this success, a permanent full-scale installation of REBOX Hyox at Ovako Hofors was commissioned in early 2024 in a total of 48 soaking pit furnaces, where both the oxygen and the hydrogen – using a 20 MW alkaline electrolyzer – are produced using renewable energy.

In parallel, several full-scale tests have been carried out with REBOX Hyox on stainless steel ingots. Also, these tests were successful, and no negative impact on the material was identified. Another example of early use of hydrogen is in the cutting operation at continuous casting. Full-scale tests have successfully confirmed the advantages of using hydrogen-oxyfuel at the cutting. Excellent results were demonstrated, including being able to cut with twice the speed when using only half the power. Moreover, EAF burner and injector systems such as CoJet® have been demonstrated to work very well with hydrogen as a fuel. Accordingly, using hydrogen could not only support decarbonization but also improve the operation.

AOD Intelligent Refining System (IRS)

Modern AOD stainless steelmaking demands a real-time accurate estimation of the steel carbon concentration; this is particularly important to identify the earliest time when the heat has met its target carbon content. In practice, this means determining the optimum time to stop the heat decarburization and take a steel sample that will meet the carbon concentration target. The AOD Intelligent Refining System (IRS) is a platform that models carbon content and temperature during decarburization, and controls reduction temperature and chemistry more accurately than ever before. The system also performs all tasks and calculations, including automatic vessel positioning, ratio and inert gas selection, and alloying requirements. The IRS platform minimizes heat time, so consumption of refractory, alloys, and fluxes is minimized therefore improving stainless steel carbon footprint.

OPTIVIEW® is an AOD carbon endpoint imaging-based solution which is a feedback system in real-time that was developed to help operators to achieve an optimum carbon end point practice that results in a reduction or elimination of the additional costs of fluxes and alloys. OPTIVIEW is an add on to the IRS system where the AOD process can be further optimized. More than 100 AOD IRS systems are successfully in operation worldwide.

Conclusion

At most places, use of hydrogen is not yet viable, however, there are actions that can be taken immediately to reduce the carbon footprint of stainless steel production. Areas to focus on are increased use of renewable power, potential to increase the scrap charge, and alloy selection.

Another area to focus on is increased energy efficiency, which can take place without cost penalty and alteration of the existing processes. Use of oxyfuel solutions in the EAF, ladle preheating, reheating and annealing processes, are well proven solutions. Not only would oxyfuel decrease the fuel consumption today, but also – as these solutions are hydrogen-ready – decrease the need for hydrogen going forward. Optimizing the AOD operation, using IRS with OPTIVIEW for example, is also important as it supports more resource efficient production.

Achieving greener stainless steel production is a journey. However, procurement choices and proven technologies are available to implement immediately, which could provide large reduction of the carbon footprint. Renewable energy plays a vital part here, in the EAF and in the production of oxygen, argon, nitrogen, and hydrogen.