Identifying new ways to clean up the production of steel

According to McKinsey’s quarterly insight on steel industry decarbonisation, published in August 2022, steel occupies a place as one of the most important materials in modern life. However, making steel is also highly carbon intensive, with the production of one metric ton of steel resulting in 1.8 metric tons of CO2 emissions, on average. With steel production accounting for about eight per cent of global CO2 emissions, researchers are investigating new methods of production to provide a faster route for steel industry decarbonisation.

A new paper, released in the Physics journal in April 2023 – ‘Effect of Pore Formation on Redox-Driven Phase Transformation‘ – shows how using hydrogen as a reactant to produce steel could potentially be more environmentally friendly than using carbon, as in the conventional process. Steel is made through a redox (electron-trading) reaction in which iron oxide reacts with another material to produce steel and an oxide by-product – carbon dioxide if the reactant is carbon, water if it is hydrogen.

With more testing, it is hoped that this could provide an alternative method of production seen as essential to reducing greenhouse gas emissions from steel manufacturing. But a number of challenges must be overcome before this switch can be made on an industrial scale.

One of the biggest problems thus far is that researchers have been unable to understand why one of the reactions needed to make steel using hydrogen is mysteriously sluggish. The paper’s authors – Professor Xuyang Zhou and his colleagues at the Max Planck Institute for Iron Research in Germany – knew that the removal of oxygen atoms during this process leaves behind nanometre-to-micrometre-scale pores in the iron oxide, and identified this phenomenon as an opportunity for experimentation. Through simulations and electron-microscopy observations, they discovered that, when hydrogen is the reactant, water trapped in these pores could reverse the reduction (electron-adding) process required to produce steel and actually reoxidise (remove electrons from) the iron, slowing the overall reaction.

In the paper, the team offers a solution to mitigate this slowdown effect. If the pores were sufficiently interconnected to form channels, then the water would have a chance to percolate out of the material before reoxidising it. Zhou says that the team should be able to achieve the required pore morphology by controlling the temperature, pressure, and other parameters during the reaction.

“Steel production currently accounts for seven to nine per cent of global carbon dioxide emissions,” the team write in the introduction to the paper. “Finding an alternative production method is essential to reducing greenhouse gases and addressing climate change, and the use of sustainably produced hydrogen as reductant is reemerging as an alternative and carbon-neutral synthesis pathway.

“We chose the direct reduction of iron oxide with hydrogen as a model system to study such a redox-driven solid state phase transformation because we believe it is the precondition and key process behind a future sustainable global steel industry. Hydrogen-based direct reduction is a fossil-free approach to iron production in which solid iron oxides are exposed to gaseous hydrogen.

“Because of the gigantic annual steel production, hydrogen utilisation strategies that yield high metallisation at fast reduction kinetics are urgently required in order to make such processes commercially viable and efficient. A key to understanding the rate-limiting solid-state transport and reaction mechanisms behind this approach is the study of the role of the microstructure and specifically of the pores that form during the reduction process due to the mass loss of oxygen.”

Rizwan A. Janjua, head of technology at the World Steel Association, is also excited by the research. He says that, assuming the researchers are referring to iron-ore pellets, iron-ore reduction by hydrogen has the potential to be much faster when compared to carbon-monoxide.

“However, as outlined in the report, there are challenges at the forefront of this cutting-edge research,” notes Janjua. “Control of the reduction reaction is key, since the reaction occurs very quickly in the outer layers and pores can get filled quite quickly, hampering the penetration of hydrogen towards the core. Sometimes this can lead to ‘swelling’ and the disintegration of pellets, which is likely to be caused by the expansion of water vapour.

“While the mechanism of connecting the pores is not described in detail, if it is managed successfully, it could certainly advance the research significantly.”

It will not be long before this theory can be put into practice. H2 Green Steel, a spin off from battery manufacturer Northvolt, has started production at Europe’s first green steel plant in Boden, northern Sweden. Using hydrogen to replace coal, the company hopes to roll out the first batches of green steel by 2025.

The company said that it will be producing its own green hydrogen using water from a nearby river. Electricity required for electrolysis and the running of the plant will be provided by nearby renewable resources including hydropower from the Lule river and wind parks in the region, with estimates suggesting the process could cut emissions by 95 per cent when compared with traditional steelmaking.

Innovation and implementation over the next decade will be key to ensuring the steel industry hits its climate reduction targets. This research could go some way to making that a reality.

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