ºÚÁϸ£ÀûÍø

At the heart of this research of Nicole Stevens lies a simple but powerful idea: iron-oxide, commonly known as rust, can be turned back into pure iron using hydrogen. This reduced iron then acts as an energy carrier. When it is later ‘burned’, it turns back into iron-oxide, releasing clean energy without producing any CO₂. The cycle can then start again. Experiments in fluidized bed reactors show that this circular process can be repeated many times. The particles keep their structure, their composition remains stable, and their ability to store energy does not fade across cycles. This stability is crucial, as it means the system could operate reliably on a large scale.

The surprising strength of low-grade iron

An important question addressed in this research  is whether only high-purity iron powders can be used, or whether the process can also handle more irregular and less refined feedstocks. The answer is encouraging. While low-grade or varied iron materials behave differently during the first reduction cycle, these differences largely disappear after combustion. Once the particles re-enter the cycle, their performance aligns with that of the standard powders used in metal fuel research. This means that the technology could potentially make use of recycled or lower-value iron sources, reducing waste and material costs.

When water gets in the way

Even though the concept works, the reduction process itself faces a major hurdle: water vapor. As hydrogen reduces iron-oxide, water is produced, and this water slows the reaction down. A detailed kinetic study using thermogravimetric analysis reveals just how strongly water vapor hinders the reduction. The more water accumulates, the slower the reaction becomes, because diffusion of hydrogen through the vapor becomes limited. When water vapor is removed or minimized, the reduction process speeds up significantly. Higher gas flow rates, especially in turbulent fluidization regimes, help remove the vapor and improve heat and mass transfer, accelerating the conversion of rust back into iron.

Heat, speed, and the sticky problem

Pushing the system to higher temperatures and higher flow rates improves reduction efficiency, but it introduces a new challenge: particle sticking. When particles begin to clump together, the fluidized bed stops behaving like a fluid, and the system de-fluidizes. Although particle break-up improves at high flow rates, sticking at elevated temperatures remains a critical obstacle. The exact cause of this sticking is still unclear, highlighting an important avenue for future research. What is clear, however, is that avoiding sticking is essential for stable, continuous operation of the iron power cycle.

A circular energy future in sight

This dissertation brings the iron power cycle another step closer to industrial reality. It proves that circular combustion and reduction of iron can take place without any CO₂ emissions. It shows that diverse iron feedstocks can be reused effectively. And it identifies what still needs to be optimized—most importantly, making the reduction process faster and preventing particle sticking. The path ahead involves refining reactor conditions and deepening our understanding of particle behavior at high temperatures. But the message is clear: rust can become a clean, reliable, and circular energy carrier. With further innovation, the iron power cycle could one day play a key role in a greener industrial future.

Research School: JMBC (Fluid Mechanics)

Research Institute:

Title of PhD thesis: . Supervisors: Dr. Giulia Finotello and Prof. Niels Deen.