Hydrogen is a crucial part of the world’s plans to greenify its manufacturing and automobile industries as a fuel whose production and use needn’t emit carbon. But in the steelmaking industry, hydrogen can also be used instead of carbon in an important chemical reaction that contributes to 5-7% of the global greenhouse-gas emissions and 11% of carbon dioxide emissions. That is, if scientists can surmount an old roadblock.
In particular, if the results of a new study are borne out, this barrier may finally be giving way. Researchers in Germany have reported that they may have figured out why using hydrogen as a reactant in a reaction with iron oxide proceeds more slowly than expected, a fact that currently renders the element infeasible as a substitute for carbon.
What is the new study’s context?
The researchers’ paper was published in Physical Review Letters on April 19. Xuyang Zhou, deputy group-leader at the Max Planck Institute for Iron Research, Düsseldorf, and one of the paper’s authors, told The Hindu in an email that research like this is “essential” to “reduce energy consumption” in steelmaking.
India is the world’s second-largest steelmaker, having produced 118.2 million tonnes in 2021. Making one tonne of steel releases 1.8 tonnes of carbon dioxide, making the sector’s decarbonisation plans an integral part of the country’s ability to achieve its climate commitments.
Strong steel consists of a tiny amount – less than 1% – of carbon. To achieve this mix, iron oxide is heated with coke (a form of coal with high carbon content) at 1,700 C inside a blast furnace. The carbon reacts with oxygen to form carbon dioxide, leaving iron with around 4% carbon behind. This iron is remelted and oxygen is blown through it, producing more carbon dioxide and reducing the amount of carbon in the iron to a desirable level.
“The blast furnace ironmaking process is the predominant primary metal production process, with the carbon emissions accounting for approximately 90% of the total value of the entire steelmaking route,” a paper published in December 2021 said. “Therefore, it is under severe pressure to reduce carbon emissions.”
What is the barrier?
In the first step, when oxygen leaves the iron oxide, scientists know that it leaves behind minuscule pores in the iron.
The German team used phase-field models – a mathematical technique that uses partial differential equations to simulate reactions at interfaces – and electron microscopy to find that when hydrogen is the reactant, the departing oxygen combines with it to form water that becomes trapped inside these pores. From here, the water reoxidises the iron and considerably slows the oxygen-removal process.
The researchers suggested a solution. Some pores on the iron oxide surface were connected by narrow channels, and they found that the water content in these channels was “almost always” lower than in the pores.
They hypothesised that the trapped water drained through these channels, allowing hydrogen to replace it and continue the oxygen-removal reaction.
How can the barrier be overcome?
To encourage such channels to be created when the iron oxide is processed, they proposed that a “microfracture structure” should be created on the feedstock to “increase reduction kinetics and improve metallisation,” per their paper.
“We have proposed several strategies for controlling the percolation and connectivity of the pore network, such as altering the fracture toughness, grain size, interface cohesion, and agglomerate size of the oxide,” Dr. Zhou said.
“Creating channels can be achieved by adjusting reduction pressure, temperature, gas composition, and chemical composition or by introducing mechanical deformation to the oxides. This aspect of the research is currently being investigated.”
“There are several challenges to the widespread application of hydrogen direct reduction production, such as reduction kinetics and the high cost of hydrogen reactants,” he added. “It is crucial to address these questions through scientific research and targeted goals.”
Currently, multiple hydrogen-based steelmaking technologies are under development. A promising one is shaft furnace hydrogen direct reduction, which uses clean hydrogen as the oxygen-removal agent. With some fine-tuning, it is expected to be able to reduce carbon dioxide emissions by up to 91%.
