Depending on the operational conditions inside a direct reduction shaft furnace, e.g., ingoing gas temperature, feeding rate of material, and gas composition, the outgoing material will differ. This study investigates how the heating rate affects the reduction during pure hydrogen reduction of commercial iron ore pellets. As expected, the reduction rate increased with increasing heating rate. The heating rate also significantly affected the microstructure evolution inside the pellet. Inside the hydrogen direct reduced pellets, the iron had two appearances: (1) porous iron containing small and numerous intragranular pores, or (2) dense iron with larger but fewer intragranular pores. The pellet reduced with the slowest heating rate consisted of only porous iron, while the faster heating rates comprised porous and dense iron. The amount of dense iron gradually increased with increasing heating rate and was found to start forming at a temperature of around 668°C. The solid iron aggravated the mass transfer through the product layer and decreased the total reaction rate. This led to an expanded spread of the reaction zone as the heating rate increased. Through this work, it was also shown that insignificant reduction took place below a temperature of 450°C. Lastly, the microstructure that evolved during the non-isothermal reduction vastly differs from the microstructure formed during isothermal reduction. Consequently, an effective diffusivity and thermal conductivity that varies with time and temperature must be considered when optimizing the shaft furnace reactor.

As the iron and steel industry now strives for a carbon neutral industry, hydrogenbased direct reduction shaft furnace technology has become an alternative to theheavily fossil-depending blast furnace route. Research questions related to thefuture full-scale production have, therefore, become more interesting. Dependingon the operational conditions, the H2 concentration and temperature will varyacross the length of the reactor. This work studies the effect of nitrogen in ahydrogen-reducing gas during the reduction of commercial iron ore pellets usingthermogravimetric analysis. The reducing gas consisted of either pure hydrogenor a mixture of 90–70 vol% hydrogen and 10–30 vol% nitrogen at 773, 873, 973,1073, and 1173 K. It is found that the reduction rate decreased with decreasingtemperature and increasing nitrogen content. The effect of nitrogen on thereduction rate is more profound than expected from the decreased hydrogenpartial pressure alone. To aid the discussion, partially reduced pellets are studiedusing optical and scanning electron microscopy. It is found that the microstructure is strongly dependent on the temperature but independent of thenitrogen content.

The contents of hydrogen and nitrogen in the liquid steel after melting hydrogen direct reduced iron (H-DRI) inside an EAF just before tapping were investigated in this work. In addition, the contents of hydrogen and nitrogen inside H-DRI were also studied to understand their effects on hydrogen and nitrogen pickup and removal in EAF practice. The contents of hydrogen and nitrogen in the H-DRI were much greater than the solubility limit of pure iron at relevant process temperatures. Hence, a significant amount of the total contents come from adsorption on the pellet pore surface. Furthermore, the contents depend on the surface area and the type of cooling gas. The highest initial hydrogen (65-87 ppm) and nitrogen (100-120 ppm) contents were found in the laboratory-reduced H-DRI. A sudden and drastic decrease of the contents were noted when subjecting the pellet to temperatures of 1400°C, 1500°C, and 1550°C. The significant decreaseof hydrogen and nitrogen content in the pellet was due to the a) increased desorption rate, b) decreased surface area for adsorption and increased diffusion rate of the gas phase, when the temperature was increased. However, applying a longer heating time did not further decrease the contents and the crude steel sample contained a final hydrogen and nitrogen content of 6-22 ppm and 20-30ppm, respectively. Since the solubility limit of hydrogen and nitrogen significantly increase when the steel melts, the adhered gases can dissolve into the liquid metal. Furthermore, due to the small atom size of the hydrogen and fast diffusion rate, a significant amount of the hydrogen that adhered to the pellet pore surface dissolved into the liquid metal. In the case of nitrogen, the initial content (100-120 ppm) dropped below 20 ppm during heating at a temperature of 1400°C. Due to the slower dissolution rate of nitrogen, most of the adhered nitrogen is desorbed from the surface during heating. This preliminary work sheds light on the need for optimizing the refining process when H-DRI is used as input material.

Inclusions in crude steels are linked to employed feedstock material and secondary steelmaking should be tailored correspondingly for optimized praxis. Thus, when implementing hydrogen direct reduced iron (H-DRI) as feedstock for electric arc furnace (EAF) operation, knowledge of the number, size, type of inclusions, and mechanism of formation are needed to support future process development. Crude steel samples were collected from a pilot-scale EAF, employing 100% H-DRI, to study the inclusions originating from the novel feedstock. A sampler containing aluminum was used, and the effect of deoxidation is discussed. Laboratory experiments were conducted to understand the mechanisms of formation and growth of inclusions. It was clear that the inclusions formed because oxide particles merged and grew when the pores coalesced, and the iron grains sintered during the melting of H-DRI. The composition varied between the different inclusions and a total of three inclusion types were found. The size of the inclusions varied mainly between 5 and 70 µm, but the largest inclusion had a diameter of approximately 300 µm. The smallest inclusions (5-10 µm) were solid and contained mainly magnesiowüstite with a high MgO content (Type I-3). Type I-2 was larger (3-60 µm), and contained liquid, spinel, and no or small amounts of magnesiowüstite phase. Type I-1 inclusion varied greatly in size (13-300 µm) and contained all three phases with relatively high MgO content. H-DRI was heated for different durations and quenched prior to melting to study the formation mechanism. The link between feedstock material and inclusions in crude steel is discussed.