The Green Steel Deal
Why is steel important?
Steel is one of the most important materials. More than 1.8 billion tonnes are consumed worldwide every year but coal is needed for conventional production and therefore iron and steel production is the largest single emitter of CO2 worldwide.
The importance of steel is often underestimated. Each of us consumes about 400-500 kilograms of steel every year. If you do the experiment of simply looking around you at everything that is actually made of steel, you can see how important this material is.
It starts with the sewing needle and your tools and the kitchen at home and also with the steel that holds the house we are sitting in together. It's not just cars that are very present, of course, but our entire civilisation has a backbone of steel.
What are the industrial challenges for green steel?
The shift away from carbon is an important step towards a climate-neutral economy but the steel industry cannot simply turn the switch. This is a huge task: we aim at the reconstruction of an entire industry sector where a millennia-old process is to be revolutionised. New systems and plants are needed and the dimensions are gigantic. Also: how can the huge demand for hydrogen be met, and as quickly and completely CO2-free as possible? If we could only build these new plants for green steel when sufficient hydrogen is available, we would lose a lot of time.
New production processes can in principle also change the properties of materials. Research therefore needs to address what effect the use of hydrogen has on steel.
In the restructuring of the entire steel industry, large parts of the previous industrial plants in this sector will be completely eliminated in the future.
For example, blast furnaces, the sintering plant as well as the coking plant and the entire coal transport routes will be eliminated. All this will no longer exist in a few years and large new plants will be built that work with hydrogen and methane gas instead of coal, and which look much more like modern chemical plant process technology.
Role of the blast furnace for green steel making
The blast furnace process is the standard method and backbone of global raw iron (pig iron) production. In the blast furnace process, pig iron, i.e. a eutectic Fe alloy with up to 4.3 weight %
carbon and many impurities inherited from the coke and from the ore is produced from iron oxides by smelting reduction. The reduction is mainly carried out by carbon monoxide obtained through the
Boudouard reaction and to a small extent by hydrogen. The reducing agents are formed from hydrocarbons such as coke, oil, crude tar, natural gas, etc. by gasification with oxygen. The gasification
process also provides the energy needed for the process. Compared to combustion, where only heat and CO2 are formed, in the blast furnace carbon is gasified into carbon monoxide, which does most of
the reduction work. CO2 is formed as a product of the redox reactions that take place.
A thermodynamically and stoichiometrically idealized blast furnace process would require around 370 kg of carbon per metric tonne of pig iron produced, a value that is close to the thermodynamic limit of the redox reaction. Under real process conditions, depending on furnace and feedstock, the carbon requirement is in the best practice cases today around 400-450 kg per metric tonne of pig iron. The global average is about 500 kg per tonne of pig iron.
A further significant reduction in carbon consumption is not possible under the given prerequisites and conditions. This also means that a limit has been reached for a further reduction in process-related CO2 emissions. A further CO2 reduction in pig iron production can only be achieved by substituting conventional iron carriers with pre-reduced materials such as the so called Low Reduced Iron for instance from direct reduction.
How does a blast furnace work?
The blast furnace is a counter-current gas/solid/liquid reactor in which the descending column of the top-charged burden materials, consisting of coke, iron ore and fluxes/additives, reacts with the ascending hot gases. The process is continuous with raw materials being regularly charged to the top of the furnace and molten iron and slag being tapped from the bottom of the furnace at regular intervals. Key steps of the process are as follows: upper part of the furnace - free moisture is driven off from the burden materials and hydrates and carbonates are disassociated. lower part of the blast furnace shaft - indirect reduction of the iron oxides by carbon monoxide and hydrogen occurs at 700-1,000°C. Bosh area of the furnace where the burden starts to soften and melt - direct reduction of the iron [and other] oxides and carbonization by the coke occurs at 1,000-1,600°C. Molten iron and slag start to drip through to the bottom of the furnace [the hearth]. Between the bosh and the hearth are the tuyeres [water cooled copper nozzles] through which the blast - combustion air, preheated to 900-1,300°C, often enriched with oxygen - is blown into the furnace. Immediately in front of the tuyeres is the combustion zone, the hottest part of the furnace, 1,850-2,200°C, where coke reacts with the oxygen and steam in the blast to form carbon monoxide and hydrogen [as well as heat] and the iron and slag melt completely. Molten iron and slag collect in the furnace hearth. Being less dense, the slag floats on top of the iron. Slag and iron are tapped at regular intervals through separate tap holes. For merchant pig iron production, the iron is cast into ingots; in integrated steel mills, the molten iron or hot metal is transferred in torpedo ladle cars to the steel converters. Slag is transferred to slag pits for further processing into usable materials, for example raw material for cement production, road construction, etc.
Green steel: reduction of iron oxide by use of hydrogen
Steel is the most important material class in terms of volume and environmental impact. While it is a sustainability enabler, for instance through lightweight design, magnetic devices, and efficient turbines, its primary production is not. Iron is reduced from ores by carbon, causing 30% of the global CO2 emissions in manufacturing, qualifying it as the largest single industrial greenhouse gas emission source. Hydrogen is thus attractive as alternative reductant. Although this reaction has been studied for decades, its kinetics is not well understood, particularly during the wüstite reduction step which is much slower than hematite reduction.
Some rate-limiting factors of this reaction are determined by the microstructure and local chemistry of the ores. In our research, we conduct multi-scale structure and composition analysis of iron reduced from hematite with pure H2, reaching down to near-atomic scale. During reduction a complex pore- and microstructure evolves, due to oxygen loss and non-volume conserving phase transformations. The microstructure after reduction is an aggregate of nearly pure iron crystals, containing inherited and acquired pores and cracks. We observe several types of lattice defects that accelerate mass transport as well as several chemical impurities (Na, Mg, Ti, V) within the Fe in the form of oxide islands that were not reduced. With our studes, we aim to open the perspective in the field of carbon-neutral iron production from macroscopic processing towards better understanding of the underlying microscopic transport and reduction mechanisms and kinetics.
Role of microstructure in hydrogen-based[...]
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