Abstract

Hydrogen-based direct reduction is one of the most promising routes to decarbonize iron and steel production, a sector responsible for roughly 7–9% of global anthropogenic CO₂ emissions. Conventional direct reduction already offers lower emissions than blast furnace ironmaking, but it still relies on fossil fuels such as natural gas, which limits its decarbonization potential. Replacing carbon-containing reductants with hydrogen can dramatically reduce CO₂ emissions, since water vapor becomes the main product gas. However, in general, during gas-based direct reduction, iron ore pellets can undergo volume changes, swelling, cracking, and even disintegration, regardless of the specific reducing gas used. Despite decades of investigations, there is still limited contradictory data and a lack of detailed three-dimensional characterization of how pellets deform, crack, and rearrange internally during reduction, particularly in multi-pellet beds and hydrogen-rich atmospheres. This knowledge gap motivates a closer study of pellet behavior under hydrogen-based direct reduction conditions.

This work investigates the volume change and structural evolution of industrial iron ore pellets during reduction by hydrogen in a multi-pellet bed configuration. Experiments are carried out in a custom-designed furnace at 800°C under 100% hydrogen. In contrast to previous studies that rely primarily on thermogravimetric analysis to monitor mass loss, this study uses mass spectrometry to directly follow hydrogen consumption and water vapour formation throughout the reduction process, which provides a more accurate measure of the reduction degree of the pellets.

The evolution of pellet volume, porosity, and crack networks is examined with a combination of X-ray Computed Tomography (XCT), Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD), and Compressive Strength Testing (CCS). XCT provides three-dimensional, non-destructive images of the pellets at different reduction stages, allowing quantitative analysis of overall swelling, internal pore connectivity, crack formation, and structural rearrangements inside individual pellets. SEM observations and EBSD phase mapping complement these data by revealing local microstructural features, such as pore coalescence, iron growth morphology, interfaces between remaining oxides and newly formed metallic iron, and the evolution of phases at different reduction stages, which relate to volume changes. Additionally, compressive strength is measured on pellets before reduction and at intermediate stages to quantify mechanical degradation and better correlate pore/crack formation with loss of structural integrity.