Renewable Energy and Green Metallurgy Technology

Iron and steel are regarded as the foundation for national development, but their processing consumes huge amounts of fossil fuel and produces large amounts of carbon dioxide gas, which is not conducive to the sustainable development of society [1,2]. The traditional metallurgy process consists of coking, ironmaking in a blast furnace, steelmaking in a converter, refining, continuous casting, steel rolling, etc. The utilization of renewable energy to replace fossil fuels, the recovery of waste heat from the metallurgy process, and the development green metallurgy technology could achieve the goal of green and high-quality iron and steel production. The Special Issue titled “Renewable Energy and Green Metallurgy Technology” focused on steel production was established by Dr. Xin Yao and Dr. Huaqing Xie. The Special Issue was published with the support of the North China University of Science and Technology, Tangshan Chuangyuan Fangda Electric Co., Ltd, and Northeastern University. This Special Issue is mainly focused on six selected topics of different aspects of iron and steel production, expanding on biomass energy and solar energy as replacements for fossil fuels, resource utilization of metallurgical slag, low-carbon smelting technology in steel, CO₂ resource utilization, the strengthening mechanisms and smelting processes of non-quenched and tempered steel for automobiles, and high-nitrogen steel smelting technology, aiming to master the mechanism of green metallurgy production and decrease fossil fuel consumption and carbon dioxide emissions from the metallurgy process.

Topics concerning the mechanism of green metallurgy production and the decrease in fossil fuel consumption and carbon dioxide emissions from the metallurgy process are of great interest, and the main relevant areas of research over the last 10 years are ordered as follows.

Biomass energy and solar energy as replacements for fossil fuels. Biomass energy is considered a green energy [3], but the utilization of biomass to obtain valuable high-end products might consume energy. Solar energy, regarded as a renewable energy, could provide energy for chemical reactions [4]. Combining biomass and solar energy to replace fossil fuels could decrease the energy consumed during the metallurgy process. Naveen S summarized the utilization of solar energy to produce biodiesel, bioethanol, biohydrogen, and biomethane [5]. It was concluded that solar energy could promote the energy carrier for biomass technology and obtain high-end products. The high-end products could be utilized during the metallurgy process, thus decreasing fossil fuel consumption and carbon dioxide emissions.

Resource utilization of metallurgical slag. During the metallurgy process, metallurgical slag of high temperature is discharged as a by-product [6], such as blast furnace slag, steel slag, etc. In addition, metallurgical slag could be utilized as a raw material in cement. Sun investigated the thermodynamic characteristics of biomass steam gasification within metallurgical slags [7]. The results implied that blast furnace slag and steel slag are high-quality heat carriers for the process of biomass steam gasification, and the production of syngas could be utilized in the metallurgy process, decreasing the consumption of fossil fuels.

Low-carbon smelting technology in steel. Hydrogen smelting is regarded as an outstanding method for low-carbon ironmaking. Chen investigated low-carbon ironmaking using hydrogen-rich fuel [8]. The results implied that the injection of hydrogen-rich fuel could save coke and strengthen blast furnace smelting, which could provide the groundwork for low-carbon ironmaking.

CO₂ resource utilization. The huge amount of CO₂ discharged from the metallurgy process disturbs the environmental balance. CCUS (carbon capture, utilization, and storage) could achieve the goal of low-carbon steelmaking. McLaughlin reviewed CCUS in the industry [9]. The results implied that CCUS could reduce the negative impacts on the climate and the stored CO₂ could be utilized as fertilizer raw materials, fuel cells, chemicals, etc.

The strengthening mechanisms and smelting processes of non-quenched and tempered steel for automobiles. Non-quenched and tempered steel could be utilized in the automobile field, and the strengthening mechanisms and smelting processes of steel are especially significant. Jiang investigated the microalloying behavior of non-quenched and tempered steel [10]. The results implied that the yield strength of V-Ti-Nb-N steel could achieve 509.7 MPa and the elongation was 19.2%. These results could provide the groundwork for steel use in automobiles.

High-nitrogen steel smelting technology. High-nitrogen steel has the advantages of high strength, good toughness, and corrosion resistance compared with traditional steel [11]. Specifically, high-nitrogen steel with excellent corrosion resistance shows unique advantages in the field of marine equipment. Svyazhin investigated the mechanisms of nitrogen on steel properties and obtained the kinetic characterizations and thermodynamic characterizations of alloying with nitrogen [12]. These results could provide the groundwork for high-nitrogen steel applications.

The Special Issue titled “Renewable Energy and Green Metallurgy Technology” mainly focuses on six selected topics to collect research on the advanced technologies for iron and steel processing, achieving green metallurgy production. Centered around research into the selected topics, thirteen papers were finally accepted and published in the Special Issue. The contributions are listed below:

• Huo, W.; Zhang, C.; Zhang, Y.; Li, X. Numerical Simulation of Mold Slag Entrapment Behavior in Nonoriented Silicon Steel Production Process. Processes 2024, 12, 167. https://doi.org/10.3390/pr12010167.
• Liu, C.; Wang, W.; Qi, X.; Wang, B.; Chen, W.; Zhao, K.; Zhen, J.; Zhang, Q. Numerical Simulation of Heat Transfer of Roller Slag in Centrifugal Preparation of Inorganic Fiber. Processes 2023, 11, 3225. https://doi.org/10.3390/pr11113225.
• Wen, L.; Ai, L.; Hong, L.; Zhou, Y.; Zhu, G.; Sun, C. Diffusion Behavior of Carbon and Silicon in the Process of Preparing Silicon Steel Using Solid-State Decarburization. Processes 2023, 11, 3176. https://doi.org/10.3390/pr11113176.
• Ding, Z.; Xue, Y.; Zhang, L.; Li, C.; Wang, S.; Ni, G. Study on Mushy Zone Coefficient in Solidification Heat Transfer Mathematical Model of Thin Slab with High Casting Speed. Processes 2023, 11, 3108. https://doi.org/10.3390/pr11113108.
• Li, C.; Zhang, Y.; Xue, Y.; Zhang, K.; Wang, S.; Sun, H.; Xie, H. The Kinetic Mechanism of the Thermal Decomposition Reaction of Small Particles of Limestone at Steelmaking Temperatures. Processes 2023, 11, 2712. https://doi.org/10.3390/pr11092712.
• Zhou, M.; Ai, L.; Hong, L.; Sun, C.; Tong, S. Promoting Effect of Microwave Field on Gas Phase Diffusion Limited Magnetite Reduction in Carbon Monoxide. Processes 2023, 11, 2709. https://doi.org/10.3390/pr11092709.
• Li, H.; Xue, H.; Zhang, J.; Zhang, G. Study on Efficient Removal Method of Fine Particulate Dust in Green Metallurgy Process. Processes 2023, 11, 2573. https://doi.org/10.3390/pr11092573.
• Meng, L.; Cui, X.; Liu, R.; Zheng, Z.; Shao, H.; Liu, J.; Peng, Y.; Zheng, L. Research on Metallurgical Saw Blade Surface Defect Detection Algorithm Based on SC-YOLOv5. Processes 2023, 11, 2564. https://doi.org/10.3390/pr11092564.
• Guo, H.; Tan, Z.; Li, H.; Long, Y.; Ji, A.; Liu, L. Dynamic Characteristics Analysis of Metallurgical Waste Heat Radiative Drying of Thin Layers of Sewage Sludge. Processes 2023, 11, 2535. https://doi.org/10.3390/pr11092535.
• Ding, Z.; Liu, Y.; Yao, X.; Xue, Y.; Li, C.; Li, Z.; Wang, S.; Wu, J. Thermodynamic Analysis of Hydrogen Production from Bio-Oil Steam Reforming Utilizing Waste Heat of Steel Slag. Processes 2023, 11, 2342. https://doi.org/10.3390/pr11082342.
• Hui, X.; Qi, M.; Wang, W.; Yang, S.; Zhang, C. A Comprehensive Model for Evaluating Titanium Industry Security in China. Processes 2023, 11, 2286. https://doi.org/10.3390/pr11082286.
• Tong, S.; Li, C.; Ai, L.; Wang, S.; Zhang, S. Behavior of Carbothermal Dephosphorization of Phosphorus-Containing Converter Slag and Its Resource Utilization. Processes 2023, 11, 1943. https://doi.org/10.3390/pr11071943.
• Sun, X.; Ren, J.; Wang, S.; Zhao, D. Effect of Powder Formulation and Energy Density on the Nitrogen Content, Microstructure, and Mechanical Properties of SLMed High-Nitrogen Steel. Processes 2023, 11, 1937. https://doi.org/10.3390/pr11071937.

Focusing on the common surface defects during the continuous casting process of non-oriented silicon steel, contribution 1 proposed a new method for predicting defects in non-oriented silicon steel billets. Based on the actual slab crystallizer parameters, a three-dimensional model of the slab crystallizer was established using Fluent software. The LES+VOF coupling algorithm was combined to conduct in-depth research on the steel flow field and the unique slag rolling behavior in the crystallizer. Combined with its unique production process characteristics, corresponding operating processes were established, and a unique production process suitable for the current stage of non-oriented silicon steel was obtained. These results could be of great benefit to improve the quality of non-oriented silicon steel products further.

Centered around the heat transfer in the process of centrifugal fiber formation, contribution 2 conducted a numerical simulation study and established a heat transfer model using the Fluent module in ANSYS software. The effects of the internal circulating water, slag temperature, the width of the slag film, and the boundary layer thickness were investigated. The results implied that the average temperature of the water outlet was about 6 °C higher than that of the water inlet. When the slag temperature increased by 1 °C, the roll surface temperature in that part increased by 0.9064 °C. The conditions involving a slag width from 11 mm to 17 mm and a slag thickness (the thickness of the boundary layer) less than 1 mm were advantageous for fiber production.

Aimed toward the preparation process of silicon steel, contribution 3 implied that the use of gas–solid reaction decarburization could avoid the inclusion generation and bubble generation and improve steel cleanliness. On this basis, the feasibility of preparing silicon steel via the solid-state decarburization method and its decarburization effect were discussed in an Ar-H₂O-H₂ atmosphere. The results implied that an increase in Si content could promote carbon diffusion to the surface. The self-diffusion coefficient of Si increased with the increasing Si content, and the morphology of the oxide layer was also different. Combined with a molecular dynamics study, temperature had an effect on the crystal structure of atoms.

Researching the solidification processes of iron and steel, contribution 4 took a thin slab mold with high casting speed and discussed the effects of the mushy zone coefficient on the thickness of the solidified shell of molten steel in the mold. Through the numerical simulation of the temperature field using Fluent software, it was found that the size of the mushy zone coefficient had a significant influence on the formation thickness of the solidified shell. In order to make the solidified shell thickness of the thin slab at high casting speed close to the real solidified shell thickness, the mushy zone coefficient should be increased. However, too large a mushy zone coefficient will have adverse effects on the calculation of the solidified thickness, so the mushy zone coefficient should be controlled within a reasonable range.

Focusing on high temperature heat waste during the steelmaking process, contribution 5 proposed to study the decomposition behavior of small limestone particles at steelmaking temperature by using the double extrapolation method. The results implied that with the increasing heating rate, the decomposition temperature increased. With the decreasing limestone particle size, the lower activation energy of decomposition decreased. The thermal decomposition reactions of small limestone particles in different CO₂ partial pressures were modeled by stochastic nucleation. With the CO₂ partial pressure increasing from 25% to 100%, the activation energy and number of reaction stages of decomposition reaction increased.

Aimed toward increasing the reduction rate of the early stage of CO reduction, contribution 6 investigated the effects of increasing temperature and applying microwaves on gas phase diffusion during CO reduction of magnet powder. The results implied that the formation of porous iron productions in the reduction process was dependent on the original gas composition, temperature, and mineral composition. The reduction process (oxygen loss rate) and the local and current partial pressure of CO₂ were also key influencing factors. The reduction process under the control of gas diffusion seriously delayed and restricted the occurrence of bubbling. The reaction diffusion of CO reduction was weak due to the low reduction rate at high temperature, which was conducive to sintering diffusion and led to sintering of the material layer. The microwave had a certain effect on improving sintering. The key to increase the CO reduction rate was to improve the CO diffusion conditions.

Focusing on fine particulate dust discharging during the converter steelmaking process, contribution 7 investigated the agglomeration effect and gravity dust removal efficiency of fine dust in converter flue gas using konjac gum, xanthan gum, and their mixed coagulation solutions. The results implied that the mixed solution of 1 g/L mixed glue and 0.5 g/L SDS had a significant influence on increasing the particle size of fine dust particles. Furthermore, the removal rates of PM2.5 and PM10 were 51.46% and 53.13%, respectively. These results could provide effective technological support for efficient removal of the fine particles during the metallurgy process.

Aimed toward solving poor real-time detection, a high false detection rate, and the difficult deployment of detection models, contribution 8 proposed a surface defect detection algorithm of SC-YOLOv5 based on a metallurgical saw blade. The results implied that the accuracy of the improved YOLOv5 model was 88.5%, and the number of parameters were 31.1 M. Compared to the original model, the calculation amount was reduced by 56.36% and the accuracy was increased by 2.1%. The improved algorithm ensured a high detection rate and greatly reduced the complexity and the extent of parameter calculations. It satisfied the requirements of enterprise mobile terminal deployment and provided an outstanding direction for enterprise applications.

Researching the limited methods and low utilization rate of low-grade metallurgical heat waste, contribution 9 simulated the application of low-grade heat waste from the metallurgy process for sludge drying. The seven drying models were analyzed. The kinetic parameters and effective coefficients of diffusion for the three stages of sludge drying were obtained, containing an increasing rate, a constant rate, and a decreasing rate. The relationship between temperature and the effective diffusion coefficient was established using the Arrhenius equation. The activation energies of the three stages of the sludge drying process were calculated to be 29.772 kJ·mol⁻¹, 37.129 kJ·mol⁻¹, and 39.202 kJ·mol⁻¹, respectively.

Centered around the heat waste of steel slag during the metallurgy process, contribution 10 proposed utilizing steam reforming of bio-oil for the recovery of heat waste from steel slag. The equilibrium productions were obtained through thermodynamic analysis. The results implied that steel slag was an outstanding heat carrier to provide heat for the bio-oil steam reforming reaction. At the optimal temperature of 706 °C and the S/C of 6, the hydrogen yield was 109.13 mol/kg and the hydrogen component could achieve 70.21%. The obtained syngas containing hydrogen, carbon monoxide, and methane from the steam reforming of bio-oil could be utilized for the metallurgy process, reducing energy consumption.

Ti is an important deoxidizer and alloying agent during steelmaking production. Researching the development and structure of the Ti industry, contribution 11 established a comprehensive evaluation system, encompassing aspects of availability, economics, and sustainability. The safety level of the Ti industry chain in China from 2010 to 2020 was assessed through the entropy weight technique for order preference and the gray correlation method was used to determine the negative impact on the safety level of the Ti industry chain. The dimension layers and index system were analyzed in terms of coupling degree and sensitivity.

Aimed toward resolving the problem of phosphorus enrichment in converter slag, contribution 12 proposed gasification dephosphorization of converter slag. Firstly, combined with thermodynamic calculations and experiments, the product of gasification dephosphorization was identified as P₂, and the migration behavior of carbothermal phosphorus reduction was clearly analyzed. Then, the production practice showed that circulating steelmaking with converter slag gasification dephosphorization had multiple purposes of environmental protection, saving energy, and efficient dephosphorization. Finally, the prospect of resource utilization of phosphorus-containing slag was proposed. To ensure the full reduction of P₂O₅ in converter slag, the carbothermal reduction conditions and optimal gasification dephosphorization parameters should be designed appropriately.

Focusing on fabrication techniques of high-nitrogen steel using selective laser melting, contribution 13 investigated the effects of energy density and powder formulations on the microstructural features and nitrogen content of the steel. The results implied that the samples made with elemental mixed powder (EMP) exhibited more non-fusion flaws and poorer density, only reaching a density of 92.36%. Meanwhile, alloy mixed powder (AMP)-prepared samples showed higher structural integrity, reaching a density of 97.21%. With the energy density increasing from 83.3 J/mm³ to 125 J/mm³, the density of EMP samples increased from 88.29% to 92.36%. When energy density, scanning speed, and layer thickness, respectively, reached 104.2 J/mm³, 1000 mm/s, and 30 μm, the optimal mechanical property for AMP was obtained. Under the optimal condition, the ultimate tensile strength, elongation, and yield strength of AMP were 1189.2 MPa, 30.66%, and 958.8 MPa, respectively.

The Special Issue titled “Renewable Energy and Green Metallurgy Technology” summarizes the recent findings on advanced technology used in the traditional metallurgy process, including special steel smelting, waste heat recovery, fine particulate dust removing, the utilization of thin slabs with high casting speed, etc. These results could provide a technological guarantee for the development of renewable energy and green metallurgy. However, other emerging technologies were proposed for green metallurgy production. Hydrogen is regarded as an environmentally friendly energy utilized in iron and steel production. The emerging technology named hydrogen metallurgy technology applies hydrogen as a reducing agent instead of carbon to reduce CO₂ emissions [13]. A large amount of carbon dioxide is discharged from the metallurgy process. The utilization of emerging technology to achieve the goal of CCUS becomes a great challenge for green iron and steel production [9]. In addition, compared with blast converter steelmaking, electric furnace steelmaking has the advantages of lower energy costs, higher production efficiency, higher product quality, and better environmental performance [14]. Mastering the detailed mechanisms of hydrogen metallurgy technology, CCUS, and electric furnace steelmaking will become the promising direction for iron and steel production in the future.