Process Path for Reducing Carbon Emissions from Steel Industry—Combined Electrification and Hydrogen Reduction

Abstract

This review focuses on the energy structure of iron and steel production and a feasible development path for carbon reduction. The process path and feasible development direction of carbon emission reduction in the iron and steel industry have been analyzed from the perspective of the carbon–electricity–hydrogen ternary relationship. Frontier technologies such as “hydrogen replacing carbon” are being developed worldwide. Combining the high efficiency of microwave electric-thermal conversion with the high efficiency and pollution-free advantages of hydrogen-reducing agents may drive future developments. In this review, a process path for “microwave + hydrogen” synergistic metallurgy is proposed. The reduction of magnetite powder by H₂ (CO) in a microwave field versus in a conventional field is compared. The driving effect of the microwave field is found to be significant, and the synergistic reduction effect of microwaves with H₂ is far greater than that of CO.

1. Introduction

According to the statistics of the World Iron and Steel Association [1], in 2021, global pig iron production was 19.51 million tons, direct reduction iron production was 1.13 million tons, and the global production of 1 ton of steel emitted approximately 1.8 tons of carbon dioxide. The steel manufacturing industry is a typical high-carbon emission industry. In the past 50 years, the global steel industry’s energy consumption per ton of steel has decreased by 61% [2]. However, according to the estimation of the International Iron and Steel Industry Association, there still is a 15–20% reduction potential based on the effective capture and reuse of complementary energy and carbon resources [3,4]. Maximizing carbon emission reduction in steel manufacturing is the most important measure for the metallurgical industry to cope with climate change and the biggest challenge for sustainable development.

Over recent decades, theoretical research of iron metallurgy has been expanded. New methods and technologies are constantly emerging. Under the guidance of metallurgical theory, scientific and technological innovation and progress are continuously transformed into practical productivity in the ironmaking process. However, technological optimization with efficient use of carbon as the core approaches the limit of carbon reduction and zero- or low-carbon breakthrough technology is still in its infancy [5]. Hydrogen energy is currently recognized as the best “green” energy. It is expected to solve the resource and environmental crisis, achieve low-carbon green transformation, and upgrade the steel industry [6]. The development of carbon peaks and carbon neutralization is expected to accelerate the application of hydrogen energy in traditional metallurgical processes [7].

This paper discusses a future, feasible path for hydrogen metallurgy. The research topics of hydrogen and microwave metallurgy are explained from the perspective of the carbon–electric–hydrogen ternary relationship. A feasible new method of iron and steelmaking based on electro-hydrogen synergistic metallurgy to replace carbon metallurgy is discussed. To solve the issue related to the efficient supply of non-carbon heat energy in the “hydrogen metallurgy” process and obtain higher production capacity at the technical level, carbon emissions must be eliminated.

2. Energy Structure Change in Steel Production

Blast furnace ironmaking has a large capacity and high efficiency. However, the contradiction between high energy consumption and environmentally friendly development has become increasingly prominent. Metallurgical practitioners have been exploring the application of hydrogen energy in the metallurgical industry [8,9,10,11], including a series of technologies such as replacing coke with natural gas or biofuel and replacing coal injection with hydrogen-rich gas injection. However, coke reduction is still widely applied. Carbon emission reduction targets are expected to be achieved in the short term [12]. However, eliminating carbon emissions and realizing carbon neutralization smelting with ultra-low carbon emissions or near-zero carbon emissions is challenging.

In recent years, gas-based direct reduction technologies have made great progress [13,14,15,16,17,18]. However, technical optimization with efficient use of carbon as the core approaches the potential limit of carbon reduction [19]. To significantly reduce carbon emissions from steel production and achieve more stringent emission reduction targets, changing the energy supply structure of the production process is required. Hence, strengthening the research and application of energy-saving and low-carbon breakthrough technologies to realize the low-carbon transformation of iron and steel enterprises is imperative.

Existing steel smelting technology is based on fossil fuels as energy sources. This mainly includes coal, natural gas, hydrocarbon mixed fuel, and electric (electric arc furnace). Two main solutions exist for developing low-carbon or zero-carbon processes [20,21]. One solution is to replace coal with hydrogen and electricity in hydrogen-reduced or electrolytic iron ore, followed by introducing carbon capture and storage (CCS) or carbon capture utilization and storage (CCUS) technologies.

The former uses hydrogen and electricity instead of coal to reduce carbon emissions as “prevention”, while the latter uses CCS or CCUS technologies as “treatment”.

Given the characteristics of the existing energy structure and the bottleneck of hydrogen metallurgy technology, eliminating fossil energy and its hydrogen-rich gas without developing hydrogen smelting technologies to reduce gas emissions is impossible. In the short and medium terms, efforts should focus on energy efficiency and energy saving/recycling technologies [4,22] (Figure 1). For example, natural gas and hydrogen-rich direct reduction technologies should be used to further increase the proportion of hydrogen and strengthen the practical design of continuous processes to reduce heat loss. Applying renewable or pure green energy in metallurgical processes [23,24,25,26], such as the development of full hydrogen metallurgy to achieve zero carbon emission technology, should be promoted long-term (Figure 2).

Non-blast furnace smelting is more suited to advancing hydrogen metallurgy technology than blast furnace smelting. At present, stable large-scale hydrogen production is mainly based on coal gas hydrogen or coke oven gas hydrogen production [27]. Hydrogen metallurgy relies on the hydrogen content of the reducing gas being greater than 50%. Hence, hydrogen metallurgy cannot achieve zero carbon emissions, and pure hydrogen metallurgy only exists in the laboratory stage. Hydrogen smelting is limited by the current hydrogen production cost and the immature hydrogen storage equipment and technology. Furthermore, hydrogen reduction absorbs heat and reduces furnace temperature. Consequently, the efficient supply of “non-carbon thermal energy” becomes the most important limiting link.

One major source of carbon-di-oxide emissions in the steel industry is using carbon-based reducers to reduce iron ore to iron. Considering the three-way relationship between carbon–electricity–hydrogen, the author believes that the energy structure of ironmaking and even steelmaking will be modified from the current aspect of “carbon metallurgy” to the process of “carbon-hydrogen” combined with metallurgy.

A gradual transition to a cleaner production of “electric hydrogen” combined with collaborative smelting is expected. Figure 3 shows the carbon-di-oxide emission reduction path drawn from the perspective of the carbon–electric–hydrogen ternary relationship.

As seen from the triangular matrix in Figure 3, a transition to using carbon, hydrogen, and electricity as reducing agents and fuels is taking place.

The carbon–hydrogen line shows that reducing agents undergo a transition from “carbon metallurgy” using coke and coal to “hydrogen-rich metallurgy” using natural gas and coal to syngas, gradually realizing the transition from “gray hydrogen metallurgy” to “green hydrogen metallurgy”.

The left side of Figure 3 shows a gradual transition from carbon to pure hydrogen (green hydrogen) as a reducing agent. The traditional smelting process, represented by a blast furnace and converter, is gradually transitioning to the electrothermal smelting process, represented by an electric furnace. Furthermore, the problems of intermittent power generation and grid connection with renewable energy are solved, revolutionizing the heat source structure. The strong endothermic effect of the hydrogen metallurgy process will eventually be replaced by existing “carbon heat source” protection methodologies, such as arc heat, ion beam, laser, and other “non-carbon heat source” protection strategies.

Currently, only approximately 40% of global steel originates from recycled steel [28,29], leaving room for improvement in scrap steel recycling as an effective measure to reduce carbon emissions in the short term. A rapid increase in the scrap utilization rate is expected to further accelerate the replacement of the “carbon heat source” with an “electric heat source”.

3. Development Path for Carbon Reduction in the Iron and Steel Industry

Figure 4 shows the existing iron and steel metallurgical processes. Its upstream process consumes a large amount of fossil fuels, such as coal. As a result, carbon enters the material flow of iron and steel metallurgy, and the reaction process is discharged into the atmosphere in the form of different compounds. Different process routes require different raw materials and energy structures. Existing processes and technologies can roughly be divided into the following categories [30,31,32,33,34]:

• (1). If the ironmaking process is centered on a blast furnace, a cooking process of coal and a sintering process of iron ore are required. In the blast furnace, coke, pulverized coal, and various hydrocarbons reduce the iron ore to obtain pig iron. During the downward movement of iron-bearing minerals, a reduction reaction occurs with the furnace gas moving upward. The metal liquid and slag are gathered at the bottom of the furnace, and the flue gas produced by the reaction is discharged from the top of the furnace. The blast furnace’s high carbon metal liquid is blown into steel by an alkaline oxygen converter. This process produces vast carbon-di-oxide emissions, dioxins, furans, sulfur-di-oxide, and carbonitrides and has significant energy costs.

• (2). The electric arc furnace smelting process with scrap as the main raw material uses arc heat to melt the selected scrap and other furnace materials. To produce the necessary product components, the chemical composition of the alloy and its element content is adjusted by adding ferroalloy. Energy consumption and emissions are lower because of the absence of coking and sintering processes.

• (3). The direct reduction process uses natural gas or coal to reduce iron ore. The smelting reduction process uses coal to reduce and melt iron ore, and the energy demand of this process is slightly lower than that of the blast furnace process.

Figure 4

Current ironmaking and steelmaking processes.

[Figure omitted. See PDF]

Regarding energy efficiency, the existing steel smelting process technology has been optimized to the maximum extent [35,36,37,38,39]. However, the author suggests a new method of collaborative “electro-hydrogen” steelmaking for the future. Ideas for developing new technologies for disruptive low-carbon metallurgy are shown in Figure 5.

Figure 5 shows several potential collaborative “electricity-hydrogen” low-carbon short-process steelmaking process paths. The processes can be roughly divided according to different combinations of process flow and energy structure:

• (1). Direct reduction non-blast furnace smelting process under microwave field or electromagnetic induction heating. A microwave field or electromagnetic induction is used to heat the furnace burden around the reduction furnace’s tuyere. The hydrogen-reduced direct reduction iron is melted in an electric furnace to produce the required steel composition.

• (2). Hydrogen reduction of iron ore powder under a microwave field or electromagnetic induction heating. Iron ore powder is heated by a microwave or electromagnetic field to promote hydrogen-rich or pure hydrogen ironmaking. The process uses iron-containing concentrate as a raw material. Hydrogen-rich or pure hydrogen smelting of iron-bearing minerals is realized by microwave radiation or an electromagnetic induction device to obtain a direct reduction of iron powder or high-purity iron powder. This method is a low-carbon or carbon-free ironmaking method combining “electricity-hydrogen”. It provides a new strategy for solving the environmental problem of carbon emissions.

• (3). Hydrogen-rich reduction blast furnace/hydrogen metallurgy shaft furnace ironmaking + electric furnace + secondary refining + solid-state steelmaking technology. Hydrogen-rich blast furnaces or molten reduced iron and scrap into the electric furnace smelting. After secondary refining, it is directly solidified by thin-strip continuous casting equipment such as twin-roll continuous casting. After on-line heating by electromagnetic induction, the carbon is removed from the iron sheet as a gas–solid reaction in an H₂/H₂O mixed atmosphere. This process eliminates the high-strength oxygen, reduces the consumption of alloy deoxidation, effectively avoids the formation of inclusions and bubbles in the steel, reduces the steel smelting process, and greatly reduces energy consumption.

• (4). Hydrogen-rich reduction blast furnace/hydrogen metallurgy shaft furnace ironmaking + converter furnace + solid-state steelmaking technology. Hydrogen-rich blast furnace or smelting reduction molten iron and scrap steel into the converter. A semi-steel tapping of the converter is adopted, and after secondary refining, it is directly solidified using thin-strip continuous casting equipment such as twin-roll continuous casting. After on-line heating by electromagnetic induction, the carbon is removed from the iron sheets to the required level as a gas–solid reaction in an H₂/H₂O mixed atmosphere. This process can effectively reduce the burden of refining, reduce the consumption of deoxidizing alloy, greatly improve the cleanliness of the steel, save energy consumption, and reduce emissions. This is conducive to the sustainable development of the metallurgical industry.

4. Electro-Hydrogen Synergistic Metallurgical Advantages

Figure 6 shows the thermodynamic equilibrium diagram of the Fe-O-H and Fe-O-C reduction reactions, reflecting the relationship between temperature and gas reducibility. Gas reducibility (H₂/(H₂ + H₂O) or CO/(CO + CO₂)) is the ratio of the reducing component to the non-inert component of the gas. Thermodynamic data were calculated by FactsageTM 7.2. Hydrogen reduction of iron ore is a strong endothermic process. With an increase in temperature, the stable area of Fe and FeO in the Fe-O-H system expands, while the stable area of Fe in the Fe-O-C system shrinks. At temperatures below 820 K, the order of Fe₂O₃ reduction by H₂ and CO is Fe₂O₃-Fe₃O₄-Fe. The critical reducibility required for CO reduction is less than 0.49, and the critical reducibility required for H₂ reduction is greater than 0.79. At temperatures above 820 K, the order of Fe₂O₃ reduction by H₂ and CO is Fe₂O₃-Fe₃O₄-FexO-Fe. At temperatures above 1100K, H₂ reduction is more advantageous than CO reduction.

In the past 70 years, many basic laboratory studies have confirmed that the reduction rate of iron oxides by H₂ as a reducing agent is much higher than that of CO [40,41,42]. The reduction rate of iron oxides by H₂ is 5~10 times that of CO [43], and the higher the temperature, the more significant the H₂ advantage. The interdiffusion coefficient of H₂/H₂O gases in iron ore sintered particles at 723 K is three times that of CO/CO₂ [44]. Wen [45] studied the reaction rate and iron crystal growth of iron ore powder reduced by CO and H₂. The adsorption of CO and H₂ molecules on Fe and O atoms on the FeO surface and the associated chemical reactions were explored using density functional theory. The simulation results show that the reaction energy barrier of H₂-reducing iron ore powder is much smaller than that of CO. This is because breaking the sp1 hybridization of the C-O molecule requires more energy, which is confirmed at the microscopic scale.

The reduction process of iron ore can be divided into three categories according to its internal layered structure (Figure 7).

• (1). If the diffusion rate is faster than the chemical reaction (such as smaller particles, lower temperature, slower reduction rate, higher porosity), a uniform body (volume) reduction occurs.

• (2). If the pore diffusion is slower than the chemical reaction (large particles, high temperature, low porosity), a topological chemical reduction with a clear interface is obtained.

• (3). If the reaction rate is equal to the pore diffusion rate, a topological chemical reduction with a diffusion interface is obtained.

The complexity of the kinetic process of iron ore reduction is mainly attributed to the following three aspects [46]:

• (1). The dependence of the reduction rate on the structure of the solid iron ore, especially the coupling of porosity, pore structure, and layer-by-layer reduction of iron oxides.

• (2). Changes in the solid structure during high-temperature reduction (expansion/contraction, adhesion, recrystallization, and grain growth).

• (3). The equivalent relationship between the interfacial chemical reaction rate, the pore diffusion rate, and the mass transfer rate of the boundary layer and its time-dependent variation in the reduction process.

Microwave processing is fundamentally different from traditional conduction and thermal radiation heating sources. In traditional heating, heat is applied from the outside to the material’s surface and enters the colder internal region through conduction or radiation, generating a gradient field. In addition to heating a sample evenly across a volume range, microwave radiation has the ability to pass through samples. As a result, the material has an internal reverse temperature gradient and heats up quickly without overheating on the surface. Conventional heating causes the surface reaction to complete before the internal reaction. Consequently, the surface pores may close prematurely, preventing gas reactants from being transported to the internal center. In contrast, under microwave heating, the reaction gas can enter the sample and diffuse into the heat source [47].

Studies have shown that microwave radiation can change the thermodynamic equilibrium [48,49] and significantly improve the chemical reaction and diffusion rates [50,51].

Ferrari [48] studied the gasification reaction of carbon under microwave irradiation and found that microwave irradiation can significantly change the thermodynamic equilibrium of the reaction. Janney [52] used a microwave to heat sapphire at 1773~2073 K and studied the diffusion behavior of oxygen using an O18 indicator. Microwave heating significantly increased the diffusion of O18 and reduced the apparent activation energy of O18 bulk diffusion.

Microwaves distinctly affect diffusion, sintering, and grain growth. In single-crystal diffusion experiments, bulk diffusion is the driving mechanism of the process; for grain growth, ion hopping across grain boundaries is crucial; during the sintering process, a combination of surface, grain boundaries, volume diffusion, and changes of the boundary conditions are essential. Amini [53] studied the ironmaking kinetics of the hydrogen reduction of a FeS-CaO mixture under microwave/conventional heating conditions. The results showed that microwaves accelerated the reduction reaction.

According to the analysis above, applying microwave radiation lowers the reaction energy barrier. It increases atomic diffusion ability and serves as a constant heat source for the reduction process. Hence, combining hydrogen and microwave energy can increase the rate at which iron ore is reduced.

5. Electro-Hydrogen Synergistic Ultra-Short Process

Considering the ternary relationship of “carbon-electricity-hydrogen,” ironmaking will gradually develop toward using “electricity-hydrogen” synergy. Based on the above analysis, this paper proposes an electro-hydrogen synergistic ultra-short metallurgical process and conducts a preliminary exploration of basic research work [54,55,56] (Figure 8).

The suggested ultra-short process of electro-hydrogen synergy can be divided into two paths. The core points are summarized as follows:

• (1). Iron ore is directly reduced to a hydrogen reduction product (DRI) by a hydrogen-rich base under microwave irradiation. The amount of metal phase carburized in the gas-based reduction process is controlled by the amount of C/H in the reducing gas. The hydrogen reduction products obtained by direct reduction are mixed with a certain amount of scrap steel and melted into medium- and low-carbon molten steel in an electric furnace. After secondary refining, the molten steel is directly solidified into thin strips by thin-strip continuous casting equipment such as twin-roll continuous casting. Then, it is heated on-line by electromagnetic induction to prepare low-carbon or ultra-low-carbon steel strips in the form of gas–solid reaction decarburization. Alternatively, traditional medium-thick steel can be prepared by the continuous casting process [57,58].

• (2). Pure iron ore powder is directly reduced to metallic iron powder by pure hydrogen under microwave irradiation. The metal iron powder obtained by direct reduction enters the electric furnace as a high-purity raw material for smelting. However, high-purity iron powder can also be obtained by step-by-step magnetic separation.

The schematic diagram of the oxygen potential change in the process of modern steel smelting is shown in Figure 9. In contrast to the traditional steel smelting process, the new ultra-short electro-hydrogen synergy process has fewer process nodes. Furthermore, it does not involve high-strength oxygen blowing without consuming a large amount of deoxidizing alloy, conducive to controlling inclusions and gases in steel and improving product performance and service life [59,60].

6. Experimental Explorations of Gas-Based Reduction under Microwave Field

The feasibility of hydrogen reduction magnetite powder smelting iron under the microwave effect was explored. Magnetic powder was used as a raw material. The comparative experiments were carried out using a microwave tube furnace/electric heating tube furnace and H₂/CO reducing gases. A mass flow meter was used to regulate the gas flow during the reduction process. The total flow rate of the reducing gas was set to 1160 mL/min. The microwave heating furnace power was 4 kW, and the frequency was 2.45 GHz.

6.1. Comparative Study on the Effect of H₂ Reduction of Magnetite under Microwave and Conventional Fields

Magnetite was reduced by H₂ at different temperatures for 40 min in microwave and conventional heating fields, respectively. Figure 10 shows that the metallization rate of magnetite reduced by H₂ in the microwave and conventional fields increases with increasing heating temperature. However, at a constant temperature, the metallization rate of H₂-reduced magnetite is higher under microwave heating, indicating that microwaves have a strengthening effect on the H₂-reduction of magnetite, originating from the non-thermal effect of the microwave field. Nevertheless, the strengthening mechanism has yet to be clarified. A previous study [61] suggested that microwaves significantly increase the collision frequency between the reaction atoms. The microscopic results of magnetite after reduction at 1000 °C show that microwave and conventional heating fields have different effects on sintering, diffusion, and grain growth. After H₂ reduction under a microwave field, obvious micropores appear in the particles. This is beneficial to the diffusion of gas during the reduction process and significantly affects the kinetic conditions of the reduction reaction. However, the mechanism of microwave-enhanced reduction requires further experimental study. The author believes that using phenomenological statistics and comparative methods to analyze the influence of microwave effects on the pre-exponential factor (representing particle collision entropy) and activation energy (energy barrier) to determine the relationship between reaction rate constant k or diffusion coefficient D and temperature T is necessary. The applicability of the existing hypothesis of microwave-enhanced chemical reaction processes is discussed below.

6.2. Comparison of H₂ and CO Reduction of Magnetite under Microwave Field

Magnetite was reduced by H₂ and CO at different temperatures for 40 min in a microwave field. The metallization rate of magnetite after reduction is shown in Figure 11. Under microwave field heating, the reduction effect of H₂ is better than that of CO. The microstructure of magnetite is reduced layer by layer by H₂, and CO is carried to the center of the particles in a pointed manner. Fine iron ore is reduced by pure H₂, and the angle between the surface and the Fe/Fe_1−xO interface is smaller compared to pure CO [62]. Therefore, the migration paths of V^//_Fe and 2e⁻ with pure H₂ are shorter than with pure CO, leading to a faster movement of the Fe/Fe_1−xO interface. According to crystallographic theory [63], the growth of iron grains is based on the movement of the reaction interface. Therefore, the growth rate of iron grains in pure H₂ is faster than in pure CO.

7. Conclusions

The process path and feasible development direction of carbon emission reduction in the iron and steel industry have been analyzed from the perspective of the carbon–electricity–hydrogen ternary relationship. New ideas for the green, low-carbon transformation of the iron and steel industry and the innovation of ultra-low carbon or near-zero carbon emission technology have been suggested as follows:

• (1). The production of iron and steel materials in China is still dominated by the long process of blast furnaces, and the energy consumption and carbon emission of blast furnaces account for more than 70% of the whole process. At present, the CO₂ emission per ton of steel products in China is about 1.8 tons. The CO₂ emission per ton of steel in the short process of EAF is only about 0.4 tons, which is 1/4 of that in the long process. The EU will gradually reduce the proportion of free quotas included in the CBAM industry from 2026. It is conservatively estimated that the EU quota price will rise to EUR 100/tCO2e (CNY 758/ton) in 2026. At present, the average profit of long-process steel in China is only CNY 338 /ton. If the layout of the low-carbon steel smelting industry in China is not done well in advance, the competitiveness of enterprises related to the industrial chain will be greatly weakened. Planning the ultra-short process of hydrogen-rich, or even full hydrogen metallurgy, in advance will not only help reduce the implied carbon emissions of China’s iron and steel materials but also help China’s downstream industries of iron and steel materials to improve the competitiveness of enterprises. For long-term energy conservation and emission reduction in the iron and steel industry, the iron and steel industry must eliminate carbon dependence by combining electricity and hydrogen as an efficient supply of non-carbon thermal energy in metallurgical processes.

• (2). The metallurgical process of converting electric energy into heat energy by applying microwave radiation in combination with H₂ provides a feasible “electro-hydrogen” synergistic steelmaking method to achieve near-zero carbon emissions.

• (3). The microwave-heated electro-hydrogen synergistic metallurgical technique decreases the reaction energy barrier and enhances the atomic diffusion ability in addition to providing a constant heat source for the reduction process. The microwave-hydrogen combination can produce a superimposed promotion effect on iron ore reduction.

• (4). Compared with the long process of modern steel smelting, the “electro-hydrogen” synergistic metallurgical process does not require high-strength oxygen blowing for decarburization, avoiding the repeated reduction–oxidation–reduction procedure in the smelting process. It is beneficial for controlling inclusions in steel and improving product performance.

• (5). In the exploration experiment, the reduction of magnetite powder by H₂ (CO) in a microwave field/conventional field was compared. The driving effect of the microwave field is significant, and the synergistic reduction effect of microwaves and H₂ is far greater than that of CO. Under microwave field heating, the reduction effect of H₂ is significantly better than that of CO. The microstructure of magnetite is reduced layer by layer by H₂, and CO is carried to the center of the particles in a pointed manner.

Author Contributions

Methodology, L.A. and L.H.; validation, C.S., L.W. and L.H.; formal analysis, L.H. and C.S.; data curation, M.Z., J.W. and L.W.; writing—original draft preparation, C.S.; and writing—review and editing, L.A. and L.H. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data can be obtained from the corresponding author upon reasonable request.

Conflicts of Interest

Author Jie Wang was employed by the company Central Iron and Steel Research Institute. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

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Figures

Figure 1. Development of the steelmaking process from the mid- and short-term viewpoints.

Figure 2. Development of the steelmaking process from the long-term viewpoints.

Figure 3. Pathways of technologies for greenhouse gas emission abatement based on the ternary relationship of carbon–electricity–hydrogen.

Figure 5. Blueprint of a collaborative “electricity-hydrogen” low-carbon short-process steelmaking process path.

Figure 6. Thermodynamic equilibrium of Fe-O-H and Fe-O-C system reduction reaction.

Figure 7. Schematic diagram of three types of reduction processes for iron ore.

Figure 8. Blueprint of the electro-hydrogen synergistic ultra-short metallurgical process.

Figure 9. Oxygen potential variations in the steelmaking processes.

Figure 10. Comparison experiments of H2 reduction of magnetite in microwave and conventional heating fields.

Figure 11. Comparison experiment on the reduction effect of magnetite by H2 and CO under microwave field.

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