Globally, the steel industry is the largest single emitter of CO2 emissions, accounting for seven percent of global greenhouse gas emissions. According to the International Energy Agency, the amount of steel produced is expected to rise from just under two billion metric tons to as much as three billion metric tons in 2050, which would further increase the steel industry's carbon footprint unless it moves away from coal as a reducing agent.
Steel companies are already taking different approaches to achieve this goal. For example, it is possible to reduce iron ore directly with hydrogen, but hydrogen is currently not produced in nearly sufficient quantities to put steel production on a more or less climate-neutral course on its own. Apart from that, green hydrogen is also expected to replace fossil fuels in other areas of the economy.
One possible solution for production would be to generate hydrogen in sparsely populated sun- and wind-rich areas of the world using electricity from solar or wind power plants. So far, however, it is unclear how the gas would then be transported. Liquefying hydrogen and transporting it in tankers is very costly. This process alone would use 30 percent of the energy that hydrogen contains.
Using ammonia would simplify this process considerably, making the additional step of producing the substance itself with hydrogen worthwhile.
Ammonia in direct reduction
"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. Avoiding the splitting would reduce costs by about 18 percent," said Dr. Yan Ma, group leader at the Max Planck Institute for Iron Research (MPIE).
Yan Ma was instrumental in the study, which confirmed that such a process does indeed work. Ammonia was used to convert around 98 percent of the iron ore into metallic iron - the same amount as in direct reduction with hydrogen. The actual reducing agent is still hydrogen, which is catalytically split off from the ammonia in the reactor at around 350 degrees Celsius without any additional effort, thus reducing the iron ore heated to at least 700 degrees Celsius.
The researchers also found that the process is just as fast with ammonia as with hydrogen.
"For industry, speed is a critical factor. If the process is too slow, it is not economically worthwhile," explains Prof. Dierk Raabe, director at MPIE.
Companies can use ammonia in the same plants that run on natural gas or hydrogen, which makes the case for its use on an economic level. Some companies are testing iron production in such direct reduction plants. If sustainably produced hydrogen is not yet available in sufficient quantities, iron ore is reduced in them with natural gas, synthesis gas - a mixture of carbon monoxide and hydrogen usually obtained from fossil raw materials - or other gas mixtures.
"In the future, however, it will be possible to replace the natural gas with variable proportions of hydrogen or ammonia, depending on availability," says Raabe.
Safe transport thanks to protective nitrite layer
The gas is as suitable for iron production as hydrogen but is easier and cheaper to transport. In addition to the better energy balance compared to hydrogen, ammonia offers another advantage, as the experiments of the Max Planck team in Düsseldorf showed. As soon as the freshly produced iron cooled in the ammonia-flowed reactor, a layer of iron nitrite formed on its surface, protecting the iron from rust. When the iron coated with iron nitrite is heated again to produce steel with other components such as manganese or chromium, for example, the protective nitrogen disappears again.
"This is useful when you have to transport the pig iron for further processing. For example, if it is produced right where the sun and wind are tapped as energy sources," Raabe says.
However, ammonia has one disadvantage compared to hydrogen. It is toxic, which requires special precautions in industrial plants. But these are also necessary with hydrogen, which is extremely difficult to capture and explosive.
It will also be several years before the steel industry converts on a large scale from the established blast furnace process with carbon-based reduction to direct reduction.
"Most steel companies are married to their plants because the capital cost is so high. But with ammonia as a hydrogen carrier, the barrier to entry into climate-friendly steel production will hopefully become smaller, especially since our next projects are even aimed at significantly accelerating direct reduction," Raabe said.
