The gas is just as suitable for iron production as hydrogen, but is easier and cheaper to transport.hydrogen is the beacon of hope for a climate-neutral economy - also for the steel industry. However, the industry should possibly also rely on ammonia to produce green steel. This is suggested by a study conducted by a team from the Max Planck Institute for Iron Research in Düsseldorf. In it, the researchers show that ammonia is just as suitable as hydrogen for converting iron ore into iron. Ammonia can be produced using hydrogen, which is generated using renewable electricity in sunny countries, for example. However, it is much easier to transport.
The steel industry is the world's largest single source ofCO2 emissions. It accounts for seven percent of global greenhouse gas emissions. And according to the International Energy Agency, the amount of steel produced is likely to increase from just under two billion tons today to up to three billion tons in 2050. The steel industry'scarbon footprint would therefore continue to grow if it does not move away from coal as a reducing agent, which it uses to convert iron ore into iron.
In fact, steel companies are already pursuing different approaches to achieve this goal. For example, it is possible to reduce iron ore directly using hydrogen. However, hydrogen is currently not produced in anywhere near sufficient quantities to bring steel production alone onto a more or less climate-neutral course, not to mention the fact that green hydrogen is also intended to replace fossil raw materials in other areas of the economy. One scenario therefore envisages producing hydrogen in sparsely populated, sunny and windy areas of the world using electricity from solar or wind power plants. However, it is still unclear how the gas will then get to where it is needed. Liquefying hydrogen and transporting it in tankers is not only very expensive, but 30% of the energy contained in the hydrogen would be lost in the process. This would be much easier with ammonia, so much easier in fact that the additional step of producing it from nitrogen in the air and hydrogen would be worthwhile.
Ammonia produces as much iron as hydrogen, and just as quickly
"So we asked ourselves whether ammonia could be used instead of hydrogen for the direct reduction of iron ore without first splitting ammonia back into hydrogen and nitrogen," says Yan Ma, group leader at the Max Planck Institute for Iron Research. "Avoiding the splitting process would reduce costs by around 18%." Yan Ma played a key role in the study published in the journal Advanced Science, which has now shown that this actually works: Ammonia was used to convert around 98% of the iron ore into metallic iron - the same amount as with direct reduction using hydrogen. The actual reducing agent is still hydrogen, which is catalytically separated from the ammonia in the reactor at around 350 °C without any additional effort, thus reducing the iron ore heated to at least 700 °C. Future scenario for steel production: Where electricity from renewable sources such as wind and sun is abundantly available, hydrogen is to be produced in the future using water electrolysis. Together with nitrogen separated from the air, this can be used to synthesize ammonia, which is easier to transport. Like iron ore, this can then be shipped to steelworks, which first produce pure iron (DRI) using direct reduction and then use this to produce various almost climate-neutral steels.
Coils made from sheet steel could be produced using ammonia in the future
The researchers also found that the process runs just as quickly with ammonia as with hydrogen. "Speed is a decisive factor for large-scale implementation," says Dierk Raabe, Director at the Max Planck Institute for Iron Research in Düsseldorf. "If the process is too slow, it is not economically viable, as around two billion tons of steel are already needed every year." Another economic argument in favor of ammonia is that companies could use it in the same plants that can also be operated with natural gas or hydrogen. Some companies are already testing iron production in such direct reduction plants. As long as sustainably produced hydrogen is not yet available in sufficient quantities, iron ore is reduced with natural gas, synthesis gas - a mixture of carbon monoxide and hydrogen usually obtained from fossil raw materials - or other gas mixtures. "In future, however, it will be possible to replace the natural gas with variable proportions of hydrogen or ammonia, depending on availability," says Dierk Raabe.
Protective nitride layer for transportation
In addition to the better energy balance compared to hydrogen, ammonia offers another advantage, as the experiments by the Max Planck team in Düsseldorf showed: as soon as the freshly produced iron cooled down in the reactor through which the ammonia flowed, an iron nitride layer formed on its surface, which protects the iron from rust. "This is useful when you have to transport the pig iron for further processing," explains Dierk Raabe. "For example, if it is produced directly where the sun and wind can be tapped into in sufficient quantities as energy sources." When the iron coated with iron nitride is heated again to produce steel with other components such as manganese or chromium, the protective nitrogen disappears again. However, ammonia has one disadvantage compared to hydrogen: it is toxic, which requires special precautions in industrial plants. However, these are also necessary for hydrogen, which is extremely difficult to capture and explosive.
But regardless of whether hydrogen or ammonia is used: despite the worsening climate crisis, it will probably be several years before the steel industry switches from the established blast furnace process with carbon-based reduction to direct reduction on a large scale. "Most steel companies are married to their plants because the investment costs are so high," says Raabe. "However, with ammonia as a hydrogen carrier, the barrier to entry into climate-friendly steel production will hopefully become smaller, especially as our next projects are even aimed at significantly accelerating direct reduction."
