Produce hydrogen-based directly reduced iron (HDRI) in an EAF
Against the background of “carbon peak” and “carbon neutrality” in China, the traditional and long process of using a blast furnace converter in the steel industry is limited by many problems (such as limitations related to resources, energy, and the environment), and the carbon reduction potential of this process is also limited. The electric arc furnace (EAF) steelmaking process has the advantages of a shorter duration, recyclable resources, and low energy consumption, and the amount of carbon emissions of per ton of steel is far lower than that of the aforementioned longer process. At present, short-process EAF steelmaking involves scrap steel as its main raw material; this is an important aspect of manufacturing in the steel cycle. However, in the recycling process of scrap steel, there are problems such as uncontrollable mass composition and enrichment of impure elements, which will inevitably affect the quality of steel materials. The direct reduction of iron based on hydrogen reduction has the benefits of low carbon emissions, stable composition, and low contents of impurities; it produces high-quality raw materials for the subsequent production of high-quality steel. The new process of producing hydrogen-based directly reduced iron (HDRI) in an EAF will be important for the future production of high-quality steel.
Hydrogen-based directly reduced iron (HDRI) usually refers to iron formed as a consequence of the nonblast furnace ironmaking process of reducing iron ore via direct reduction within a gas-based shaft furnace that reacts with pure hydrogen by more than a 55% proportion. By 17 January 2024, the world’s first project to demonstrate this process had been independently developed and built by CISRI; it operated in Linyi City, Shandong Province, for 300 h, and the HDRI metalization rate reached more than 93%. HDRI can be used as a raw material for high-purity iron or high-end special steel; HDRI is melted or smelted in an electric furnace to form the final product. At present, many electric furnaces use directly reduced iron (DRI) and some scrap steel smelting. In the smelting process that occurs within an electric furnace, the time taken to melt solid furnace material accounts for 60% of the total smelting cycle in the electric furnace; this aspect is what most affects the smelting cycle and energy consumption of the electric furnace. When carrying out DRI or HDRI smelting, the formed pieces of material can easily bond with each other to become larger pieces and may even form an “iceberg”, thereby seriously adversely affecting the smelting conditions.
In the melting of solid materials in an electric furnace solid, researchers have studied the melting mechanisms of scrap steel by changing the characteristics of the melting process (i.e., the temperature, shape, and composition of the melting pool); they did so by mixing in and adding scrap steel. Based on the method of finite difference, they established heat transfer and mass transfer equations, and described and verified both the melting mechanism and melting process. On this basis, a series of rapid melting technologies were developed for scrap steel. Most of these studies focused on studying the melting phenomenon of scrap steel within the iron and carbon melting pool, with the belief that the carburization stage is the key link to scrap steel melting. It is generally believed that the carbon content of molten iron is about 3~8%, and the carbon content of scrap steel is between 0.2% and 0.3%. But iron melt or carburization is often added to the scrap smelting process, so scrap-EAF process has high carbon content in this situation. Although both HDRI and scrap steel have similar carbon contents, the properties of HDRI and scrap steel are significantly different. Therefore, in this study, the use of scrap steel was not considered, and the melting phenomenon of HDRI is instead considered.
In the process of producing HDRI in an electric furnace, melting HDRI is the key to optimization. HDRI has different characteristics from scrap steel: (1) HDRI has a low carbon content, derived from the reducing gas; (2) HDRI is mostly formed into 10~16 mm solid-state balls, whose shapes and compositions are more uniform than those of scrap steel; (3) HDRI has a dense pore structure, with a large number of fine and uniform pores; (4) the density of the HDRI is relatively low; and (5) the melting point of HDRI is far lower than that of scrap steel and generally lower than the melting tank temperature; this means the melting process is mainly controlled by heat transfer. In the future, the carbon additions and emissions of the whole process will be greatly reduced. With this in mind, the influence of the carburizing process on the HDRI melting process is not critical.
At present, the production of HDRI reduced by pure hydrogen gas is minimal; however, DRI obtained via direct reduction of CO and hydrogen gas has already been utilized, and its basic characteristics are similar to those of HDRI. The melting of DRI in an EAF mainly relies on heating the arc of the liquid steel melting tank. The liquid steel melting tank is heated alongside stirring in the melting tank. When DRI is poorly melted, it is more likely to exhibit the “iceberg” phenomenon, which affects the smooth progress of electric furnace smelting; thus, the study of its melting phenomenon is very important. Many researchers have analyzed the characteristics of DRI smelting in industrial production. Cárdenas and others analyzed the influence of DRI on EAF smelting based on material balance and energy balance. When the carbon content of DRI and the metallization rate are high, energy consumption decreases. However, with an increase in DRI gangue content, power consumption, and lime consumption, the amount of slag produced also increases. Kirschen et al.analyzed the influence of different DRI ratios on electric furnace smelting and found that with an increase in the ratio of DRI added, the energy consumption of electric furnace increased. To further study the mechanism of change in DRI within the smelting process, Li and Sharifi studied the reaction between DRI with high carbon content and slag, as well as the process of decarburization. When a DRI ball was immersed in liquid slag, the reaction between the high-carbon-content DRI and the slag caused an increase in the furnace pressure. Pfeiffer et al. designed a thermal simulation device with which to study the interaction between DRI with different carbon contents and HBI (featuring a hot block) and high- and low-carbon contents. However, because of the short melting time of a single piece of DRI in the melting pool, changes in the melting process were not observed. Zhang et al. established a numerical model for the melting of a single DRI particle in an Fe-C melting cell, believing that the DRI melting process is mainly controlled by heat transfer. The melting time was very short, and the authors analyzed the influence of the DRI radius and porosity on the melting time. At present, most studies in the literature focus on the macroscopic influence of the nature and amount of DRI added on electric furnace smelting. Compared with the scrap melting process, there are few studies on low-carbon HDRI obtained via direct reduction of hydrogen or hydrogen-rich gas in the melting process of an EAF melting pool. That said, the process of melting HDRI is critical for direct reduction in an iron arc furnace. Therefore, reasonable methods are urgently needed so that we might study the process of melting HDRI and the bonding phenomenon involved in this process.
Lin, X., Ni, B., & Shangguan, F. Numerical Simulation of the Hydrogen-Based Directly Reduced Iron Melting Process. Processes, 12(3), 537. https://doi.org/10.3390/pr12030537
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