1.   Introduction

With the rapid development of industries, the greenhouse effect and environmental issues caused by carbon emission have become increasingly prominent. As an important basic supporting industry, the iron and steel industry is a major carbon emitter. Given the current urgent situation of developing a low-carbon economy, carbon emissions can be greatly reduced by strictly limiting new steel production capacity, controlling steel output, and changing the blast furnace-converter process to the short electric furnace process; the development of clean and green metallurgical technology can ensure the sustainable development of metallurgical industry [1–2].

With the proposal of a carbon emission reduction target, the routes and technologies of carbon reduction in the iron and steel industry have been under heated discussion. Given that about 60%–70% of CO₂ emissions in the iron and steel industry are concentrated in the ironmaking procedure, many scholars have carried out detailed research [3–10]. Direct reduced iron (DRI) production by hydrogen-rich gas is the major process for producing sponge iron. Tens of millions of tons of sponge iron are produced all over the world every year. Hydrogen-enriched reduction processes mainly include shaft furnace reduction processes, such as Midrex, HYL-Ⅲ, and fine-ore fluidized-bed processes such as FINMET [11–12]. These processes can only be realized in countries rich in natural gas. China is lacking in natural gas resources, which is an important reason why gas-based reduction has not been developed in this country.

Since the end of the 20th century, China has successively carried out the development and research of gas-based direct reduction technology, such as the industrial test of direct reduction in Bao steel by coal–gas shaft furnace, the semi-industrial test of DRI production by coal to gas shaft furnace in Shaanxi Hengdi company, the test of carbon-containing pellet reduced by coke oven gas in the shaft furnace to produce DRI in Shanxi, and the industrial test of gas-based shaft furnace reduction by coke oven gas in Zhongjin Mining Company [13–18].

As a new green metallurgical technology, pure-hydrogen metallurgy is an important direction of low-carbon development in the field of metallurgy and realization of low-carbon green development, and it has been a concern among scientific researchers at home and abroad [19–20]. Therefore, this paper focuses on the theoretical analysis of the key technical issues in the application of the pure-hydrogen reduction process, which will provide guidance for the industrial application of direct reduction with pure hydrogen.

2.   Key issues in the process design of hydrogen reduction

The basic theory of iron oxide reduction with hydrogen has been deeply and systematically studied. For the basic issues of industrialization and engineering applications, the following aspects need to be further studied.

2.1   Theoretical basis of the pure-hydrogen reduction process

Compared with carbon reduction, hydrogen reduction has great advantages. From the perspective of basic technical theory, the collision diameter of the H₂ molecule is smaller than that of the CO molecule, and the mutual diffusion coefficient of H₂–H₂O is greater than that of CO–CO₂. Thus, H₂ diffuses more easily into mineral powder particles and pellets, which can accelerate the further reduction of materials. In addition, the adsorption capacity of H₂ on iron oxide is also greater than that of CO, and the intrinsic rate of reduction reaction of iron oxide with H₂ is higher than that with CO (about five times higher) at 818°C. In conclusion, the reaction rate of hydrogen reduction is significantly faster than that of carbon reduction [21–25].

The reduction reactions of iron oxide with H₂ are as follows [26]:

Fig. 1 shows the thermodynamic equilibrium diagram of iron oxide reduced by H₂. The figure reveals that iron oxide is easy to be reduced in a pure hydrogen atmosphere at the temperature range of 973–1273 K. However, given that the reduction of iron oxide by hydrogen is an endothermic reaction, when the reducing gas is pure hydrogen, the heat carried by hydrogen is the heat source of the reduction reaction. Therefore, heating hydrogen to the target temperature is an important factor to ensure the reduction of iron oxide.

Figure  1.  Thermodynamic equilibrium diagram of iron oxide reduction by H₂.

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Many studies focused on the reaction rate control steps of the hydrogen reduction process. The major research content and objective are the mechanism of different reactions in the reaction process. In general, the rate control mechanisms, such as external diffusion, internal diffusion, and chemical reaction, are discussed. However, in the actual industrial design and production, few discussions are focused on the control steps of pellet or particle reduction in the industrial equipment, which involves not only external diffusion, internal diffusion, and chemical reaction but also heat transfer in the process. With the reduction reactions as strong endothermic reactions, the heat source is the physical heat carried by hydrogen. The blowing rate and total amount of hydrogen required for heat balance may affect the reaction rate. Therefore, the discussion on the control step should pay attention not only to the reduction reactions but also to the heat transfer. This basic issue is directly related to the design of hydrogen reduction reactors, thus indicating the need for further studies.

2.2   Analysis of reducing gas demand

2.2.1   Gas demand for direct reduction reactions

The demand for reducing gas during direct reduction reaction in the shaft furnace mainly considers two aspects: (a) the flow of H₂ required for direct participation in the reduction reaction; (b) the heat brought into the reactor by H₂ that meets the needs of heat balance in the reduction process. The volume of reducing gas should meet the above two conditions simultaneously.

The theoretical amount of reducing gas required for producing 1 ton of sponge iron from hematite (Fe₂O₃) at 1173 K is computed as follows:

where V_g1 is the theoretical amount of reducing gas for direct reduction reactions, Nm³/t Fe; R_d is the direct reduction degree of DRI, %; Φ is the utilization degree of hydrogen reduction in the FeO → Fe stage at 1173 K; and α is the proportion of H₂ in the reducing gas, vol%.

According to the stoichiometric calculation of the reduction reaction of iron oxide with hydrogen, the reducing gas consumption of 1 t DRI is 600 Nm³/t Fe. Given the reaction equilibrium of hydrogen with iron oxide in the reduction reactor, the gas composition should also be considered when calculating the gas volume required for the reduction reaction. Finally, the volume of reducing gas to be pumped into the reactor is 952 Nm³/t Fe.

2.2.2   Gas volume required for heat balance

In the process of ore reduction, the reaction heat needs to be transferred by a high-temperature gas. When the heat balance is reached, the following is obtained:

where Q_g–in is the physical heat carried by the reducing gas, kJ; Q_g–t is the heat carried away by the furnace top gas, kJ; Q_Fe is the heat carried away by sponge iron, kJ; Q_g–r is the heat required for reduction reaction, kJ; Q_L is the heat loss, kJ.

The reducing gas volume needed to meet the heat balance demand is computed as follows:

where V_g2 is the reducing gas volume required for heat balance, Nm³/t Fe; C_p is the specific heat capacity of the reducing gas, kJ·kg⁻¹·K⁻¹; η_f is the thermal efficiency of the shaft furnace, 0.9; T_in is the inlet temperature of the reducing gas, K; T_t is the temperature of furnace top gas, K; $\Delta {H}_{{\mathrm{H}}_{2}}$ is the enthalpy when H₂ reduces Fe₂O₃ to Fe, kJ/kg; m_Fe is the mass of iron per ton of DRI, kg; T_Fe is the temperature of DRI, K; C_Fe is the specific heat capacity of DRI (kJ·kg⁻¹·K⁻¹) [26].

According to Eqs. (6)–(8), the hydrogen consumption can be obtained (Fig. 2). On the basis of the heat balance calculation for the reduction process, when the temperature of hydrogen entering the furnace is 1173 K, the gas volume required for the pellet reduction process with 65wt% total iron content (TFe = 65wt%) is 2500 Nm³/t Fe. According to a previous calculation, the theoretical amount of hydrogen required for the reduction reaction is 600 Nm³/t Fe. Considering the chemical balance of the reduction reaction, when the temperature is 1173 K, the amount of hydrogen required to be pumped into the reactor is 952 Nm³/t Fe. A large gap exists between the two calculations, which shows that the limiting step of the reduction process is not a thermodynamic equilibrium but the imbalance between the physical heat carried by the hydrogen and the heat demand of the reduction reaction. More high-temperature hydrogen is required to enter the furnace to transfer heat to meet the physical and chemical heat demands for iron ore reduction. Most of the hydrogen does not participate in the chemical reaction but “circulates” as a heat carrier for the transfer of heat.

Figure  2.  Hydrogen consumption of metal iron and metalized pellets at different temperatures.

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The reduction temperature of industrialized shaft furnace by hydrogen-enriched gas has been increased to higher than 1173 K. With the increase in temperature, the equilibrium hydrogen concentration of reduction reaction decreases, and the reduction reaction rate occurs rapidly. Properly increasing the hydrogen reduction temperature is conducive to accelerating the reduction reaction. Fig. 2 shows that increasing the temperature of hydrogen also greatly reduces the flow of hydrogen required. When the temperature of hydrogen rises, higher requirements are put forward for the reduction process and equipment. Therefore, the hydrogen reduction temperature should not exceed 1273 K.

2.3   Development direction of hydrogen reduction reactor and process design

The main forms of hydrogen reduction reactor include shaft furnace, fluidized bed, and fixed bed. Based on the types and particle sizes of iron-ore raw materials, hydrogen reduction reactor adopts different forms. Shaft furnace hydrogen reduction is suitable for high-grade lump ores and oxidized pellets, and fluidized and fixed beds are suitable for fine ores. If the grade of raw iron ore is low, it needs to be dissociated and separated to obtain iron concentrate powder. Different hydrogen reduction reactors are used depending on their particle size distribution.

The development trend of DRI in the world shows that the gas-based shaft furnace process is an effective way to rapidly expand the production of DRI. In 2020, the output of gas-based DRI accounted for about 75% of the total output of DRI, which shows significant development potential and competitiveness [27]. At present, the major process of gas-based reduction ironmaking is Midrex and HYL processes, which have been proven to be successful by large-scale generation units. Fig. 3 shows the typical shaft-furnace hydrogen-reduction process flow. The following measures can be applied in technological design to reduce hydrogen consumption and improve the reduction efficiency.

Figure  3.  Process flow of pure-hydrogen reduction.

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(1) Proper increase of the hydrogen preheating temperature. The higher the hydrogen preheating temperature, the more heat it carries and the faster the reduction reaction rate. However, increasing the temperature will lead to serious adhesion of metalized pellets. Therefore, the hydrogen preheating temperature should be increased to promote the rapid reduction of iron ore pellets while ensuring a smooth reduction process.

(2) Appropriate increase in pressure. When the gas flow rate is constant, increasing the pressure can increase the diffusion coefficient of gas through the pellet, which is conducive to improving the reduction degree.

(3) Proper increase in the temperature of pellets. When the feed temperature of pellets increases, the heat required to be transferred by hydrogen in the reduction process is relatively reduced. An unchanged hydrogen temperature is conducive to promoting the reduction of pellets.

FINMET process, which has been industrialized, is representative of the direct reduction process based on fluidized beds. Given that the hydrogen reduction reaction of iron ore is an endothermic reaction, the following measures can be applied to the process design to reduce hydrogen consumption and improve the reduction efficiency.

(1) In the reduction process, the ore powder preheating process can be designed to improve the temperature of ore powder into the furnace, which can greatly reduce the demand for heat carried through H₂ and reduce hydrogen consumption.

(2) A multistage reactor can be used for the reduction process. The utilization rate of heat carried by H₂ has a great influence on hydrogen consumption, and the hydrogen utilization rate is inversely proportional to hydrogen consumption. The fluidized bed is characterized by a full gas–solid contact. Therefore, a multistage fluidized bed can be used to improve the temperature drop of H₂ and the utilization rate of heat carried by hydrogen to reduce hydrogen consumption and improve the utilization rate of hydrogen.

(3) Proper increase in the preheating temperature of hydrogen. Increasing the preheating temperature not only increases the heat carried by hydrogen but also speeds up the reduction reaction rate. However, increasing the temperature will aggravate the bonding phenomenon of iron ore powder particles. Therefore, it is necessary to increase the hydrogen preheating temperature and promote the reduction speed of iron ore powder under the condition of ensuring smooth operation.

2.4   Properties of molten steel obtained by hydrogen reduction

The contents of residual and harmful elements in DRI produced by hydrogen reduction are very low, and the pure-hydrogen reduction process does not involve the addition of reducing agent carbon, which can realize carbon-free metallurgy. Most of the sulfur and phosphorus in the iron ore will not enter the molten iron, and a new and clean molten steel with ultra-low phosphorus, ultra-low silicon, and sulfur contents can be obtained, which will significantly improve the steel quality and change the subsequent smelting process. Table 1 shows the typical composition of the hot metal obtained by hydrogen reduction DRI when the total iron content of iron ore is 63wt%–67wt%. The consumption of CaO, O₂, deoxidizer, and other resources in the smelting process and all kinds of slag, dust, NO_x, and SO_x will also be greatly reduced.

Table  1.  Typical composition of hot metal obtained by hydrogen reduction of DRI (in laboratory) wt%

O	P	S	Gangue	N	H
3–6	0.001–0.009	0.0001–0.001	2.5–10	0.0015	0.0001

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Due to the low content of impurities, such as S and P, hot metal obtained by hydrogen reduction DRI is suitable for smelting high-quality special steel. Based on the different compositions, DRI can also be used as high-quality raw materials for the electric-furnace smelting process.

The main problem in hydrogen reduction is the high oxygen content in molten steel. A mechanism for deoxygenation will be a new research direction.

3.   Engineering technical issues

3.1   Large-scale and low-cost green hydrogen preparation

Hydrogen, as secondary energy, must be prepared by a certain method. Numerous methods are used to prepare hydrogen. The traditional hydrogen production methods mainly include fossil-fuel reforming, electrolytic water hydrogen production, and industrial byproduct hydrogen. In addition, some new hydrogen production methods include biomass and photocatalytic hydrogen productions.

Hydrogen production from biomass uses microorganisms to biodegrade organic wastewater and waste to obtain hydrogen. This process can not only obtain hydrogen but also dispose of wastewater and waste. This method has attracted attention in recent years and is still in the stage of laboratory research.

The process parameters, cost, advantages, and disadvantages of various current hydrogen production methods are compared (Table 2).

Table  2.  Comparison of hydrogen production processes [28–29]

Process	Technical stage	Scale / (Nm3·h−1)	H2 purity / vol%	By product	Priority areas
Coke oven gas	Mature, industrial application	10000–20000	≥99	CO2	Medium- and large-scale hydrogen production unit
Natural gas	Mature, industrial application	≥5000	39–59	CO2, CO, CH4	Large-scale
Ammonia decomposition	Relatively mature, industrial application	≤50	39–49	N2	Small- and medium-sized applications
Water electrolysis (0.3 CNY/(kW·h))	Mature, industrial application	2–300	69	O2	High-precision requirements, small scale
Methanol cracking	Relatively mature, industrial application	≤20000	39–79	CO, CO2	Medium and high requirements, small and medium-sized

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At present, the main hydrogen production technologies in the world are hydrogen production from fossil energy and chemical raw materials, primarily due to the low cost. In particular, given the green nature, high efficiency, and low cost, hydrogen production by natural gas reforming has a good development prospect. The method of hydrogen production from electrolytic water is a research hotspot, and the technology is relatively mature. Other new hydrogen production methods have not been applied to large-scale hydrogen production units.

From the perspective of hydrogen production cost, among different hydrogen production technology routes, the cost of hydrogen production from fossil energy and industrial byproducts is low, and that of hydrogen production from electrolytic water is high. The technical conditions required for storage and transportation are the same as those of natural gas. Hydrogen can be transported through pipelines, filled into gas cylinders at high pressure, or stored and transported in the form of liquefied gas (liquid hydrogen).

3.2   Material selection of hydrogen reduction reactor

H₂ is a flammable and explosive gas. When heated in a heating furnace or heat exchanger, the container material shall withstand high temperature, high pressure, and medium corrosion. Therefore, when the heating furnace, heat exchanger, and reactor work in a complex environment with high temperature, pressure, and hydrogen, the selection of high-temperature resistant metal materials is very important.

3.2.1   Hydrogen embrittlement of metallic materials

When heat-resistant metal materials are exposed to hydrogen, hydrogen can be decomposed into hydrogen atoms at high temperature and pressure, and it can be adsorbed and penetrated into the surface of metallic materials. The existence of hydrogen atoms can accelerate the hydrogen damage of heat-resistant metals, mainly including hydrogen bubbling, hydrogen embrittlement, surface decarburization, and hydrogen corrosion (internal decarburization) [30–31].

Given that the amount of hydrogen used as a reducing agent is considerably less than that of the hydrogen charged into the reduction furnace, the furnace top gas contains a large amount of hydrogen, which must be recycled. In the process of recycling, if the H₂S content in hydrogen cannot be reduced to a low level, a cumulative effect will be formed, which will cause a high H₂S content in the reduction furnace. The possible reactions are as follows [32–33]:

According to Eq. (10), the reaction equilibrium diagram of steel and H₂S is shown in Fig. 4. When the reduction reactions are carried out at 1123 K, if the H₂S content in the reducing gas is higher than the equilibrium concentration of the reaction (0.25vol%), then the H₂S in the reducing gas will absorb onto the steel surface and react with it, resulting in steel corrosion. When hydrogen contains H₂S, the corrosion of high-temperature H₂ + H₂S is more severe than that of hydrogen or hydrogen sulfide alone. In addition, if the content of H₂S in the reducing gas is higher than the equilibrium concentration of the reaction, then the reduced metal iron will react with H₂S, which will lead to the increase in S content in the DRI.

Figure  4.  Reaction equilibrium diagram of steel and H₂S in hydrogen.

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Therefore, the sulfur content in the reducing gas must be limited. In general, the hydrogen sulfide concentration in H₂ is required to be less than 0.1vol%, and the optimal concentration is less than 0.01vol%. Hydrogen sulfide in hydrogen mainly comes from the reduction of ore. Thus, the content of sulfur in ores should be controlled below 0.1vol%.

3.2.2   Reaction of hydrogen with carbon in steel

The reaction between hydrogen and carbon in steel is a major reaction of steel corrosion, and it must be considered in the selection of metal material for hydrogen reduction reactor. In the hydrogen-enriched reduction process, hydrogen-rich gas is obtained by natural gas reforming. After reforming, the hydrogen-rich gas also contains unreformed CH₄, which can effectively inhibit the corrosion reaction of the generated hydrogen with the carbon in the steel.

When the reducing gas is pure hydrogen, during the heating and reduction process, hydrogen will react with C on the surface of heat-resistant stainless steel to produce CH₄, which will result in micropores, and hydrogen will further diffuse into the heat-resistant steel to cause corrosion.

Fig. 5 shows that the equilibrium concentration of CH₄ increases with the increase in pressure when the activity of carbon (a_C) is 0.3 at different pressures (P). When the concentration of CH₄ is greater than the equilibrium concentration of the reaction, the reaction between hydrogen and carbon in steel is difficult to achieve. When no CH₄ is present in the reducing gas, or the concentration of CH₄ is less than the equilibrium concentration of the reaction, hydrogen will react with carbon in the steel. Therefore, when using pure-hydrogen reduction, some CH₄ should be mixed into the gas to inhibit the reaction between hydrogen and carbon in the steel and improve the service life of heat-resistant steels.

Figure  5.  Reaction equilibrium diagram of hydrogen with carbon in the steel.

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When selecting materials for a hydrogen reduction reactor, different steels must be selected for various temperatures. Fig. 6 shows the relationship between the equilibrium concentration of CH₄ and carbon content at ${P}_{{\mathrm{H}}_{2}}$ = 0.4 MPa. When selecting steel materials, based on the different carbon contents in steel, the concentration of CH₄ should be higher than the equilibrium concentration of reaction, and the carbon in steel should not react with H_2, which will reduce steel corrosion.

Figure  6.  Equilibrium phase diagram of [C] + 2H₂ (g) = CH₄ (g) under different carbon contents.

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3.2.3   Reaction between hydrogen and sulfur in steel

Sulfur mainly exists in steel in the form of MnS or solid-solution sulfur. Under high temperatures, hydrogen will slowly diffuse into the steel or absorb onto the steel surface and react with sulfur in the steel [34]:

The equilibrium phase diagrams can be obtained from Eqs. (12) and (13) (Fig. 7). Hydrogen can react with sulfur in the steel to generate H₂S. If sulfur is properly distributed on the surface of the material, the above reaction easily occurs. If sulfur is distributed inside the material, H₂ in the reducing gas will diffuse into the steel in the form of an H atom and combine with sulfur in the steel. The H₂S generated will exist in the form of gas. When the content of H₂S reaches a certain value, especially when it is located near inclusions and grain boundaries, it will cause stress concentration, cracking, and corrosion. When sulfur in the material exists in the form of MnS, H₂ in the reducing gas will also diffuse into the steel in the form of an H atom. When the hydrogen atom reaches a certain concentration, it will react with MnS to generate H₂S, causing corrosion and hydrogen embrittlement.

Figure  7.  Equilibrium dominant region diagram of the reaction between hydrogen and sulfur in steel.

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At present, no ideal method can be used to prevent sulfur corrosion at high temperatures. Studies have proven that Ni alloy, Cr-bearing steel, pure nickel, Cr–Ni–Co alloy, and Ni–Cr alloy containing more than 40wt% nickel have strong corrosion resistance and good high-temperature performance [35]. However, if the sulfur content in the reducing gas is high, then Ni and NiS, whose softening point is about 645°C, will form eutectics at high temperatures. The molten eutectics penetrate between metal grains, resulting in intergranular corrosion. The higher H₂S content in hydrogen will accelerate the corrosion.

The existing steel is applied in a high-temperature and high-pressure hydrogen atmosphere under relatively mild conditions. For example, the service conditions are hydrogen-enriched gas, and the temperature is relatively low, generally below 900°C. Therefore, corresponding measures should be implemented for the metal materials of the heating furnace, heat exchanger, or reactor. First, new kinds of materials must be developed to adapt to pure-hydrogen, high-temperature, and high-pressure conditions. Second, new coating materials, such as corrosion-resistant sintered alloy coating that can adapt to pure hydrogen and corrosive environment, must be developed for the long-term use of base metal materials.

3.3   N and S balance

In the process of hydrogen reduction, given the flammable and explosive characteristics of hydrogen, the inlet and outlet of the reactor need to be sealed with N₂. Nitrogen will enter the reactor, which will reduce the concentration of hydrogen. Sulfur in the ore is reduced to H₂S, which will also affect the composition of the reduced gas and the quality of the reduction product and equipment. Fig. 8 shows the gas-flow balance diagram of the reduction process.

Figure  8.  Nitrogen and sulfur balance diagram of the reduction shaft furnace. Q_out is the non-circulating gas flow, Nm³/t; [%N]_out and [%S]_out are the content of nitrogen and sulfur in non-circulating gas, kg/Nm³; [%N]_t and [%S]_t are the content of nitrogen and sulfur in the furnace top reducing gas, kg/Nm³; [%N]_c is the nitrogen content of recycling gas, kg/Nm³; Q_t is the flow of reducing gas at the furnace top, Nm³/t Fe; Q_c is the circulating gas flow after dust removal and dehydration, Nm³/t Fe; Q_in is the total amount of reducing gas entering the shaft furnace, Nm³/t Fe; Q_s is the flow of supplementary reducing gas, Nm³/t Fe.

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3.3.1   N₂ balance

The theoretical hydrogen consumption for the production of 1 ton of iron is 600 Nm³. In the process of hydrogen circulation, a small amount of nitrogen will enter the top gas because nitrogen needs to be used as a system-pressure equalizing gas and sealing gas. After the removal of H₂O and dust, if the top gas circulates completely, the concentrations of nitrogen and sulfur will be enriched because of circulating enrichment effect. Therefore, a part of the furnace top gas must not participate in the circulation. The nitrogen balance can be determined by the following formula:

where [N]_c is the nitrogen content of circulating gas after dust removal and dehydration, vol%; [N]_t is the nitrogen content in the furnace top reducing gas, vol%; Q_c is the circulating gas flow after dust removal and dehydration, Nm³/t Fe; Q_t is the flow of reducing gas at the furnace top, Nm³/t Fe; ${Q}_{{\rm N}_{2}}$ is the nitrogen flow brought in the shaft furnace during the reduction process, Nm³/t Fe.

The relationship between the non-circulating gas flow Q_out after dust removal and dehydration and Q_t and Q_c is as follows:

The relationship between [N]_c and [N]_t is determined by the following equation:

By substituting Eq. (14) into Eq. (16), the following can be obtained:

When Q_t is 2200 Nm³/t Fe, ${Q}_{{\rm N}_{2}}$ is 10 Nm³/t Fe, and Fig. 9 can be obtained. Assuming that the nitrogen content in the top gas is 3vol%, the non-circulating gas flow Q_out is 242 Nm³/t Fe. A part of the circulating hydrogen is used to preheat the supplement hydrogen and circulating gas. If considering the gas consumption of the preheating process, the nitrogen content in hydrogen must be controlled at the level of 3.5vol%, with 800 Nm³ total hydrogen consumption for the production of 1 t of iron.

Figure  9.  Relationship between nitrogen content of furnace top gas and amount of non-circulating dehydrated gas.

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3.3.2   Sulfur balance

In the process of hydrogen reduction, given the certain content of sulfur in the ore, sulfur is inevitably present in the reducing gas, mainly in the form of H₂S. According to a previous analysis, when the sulfur content in hydrogen is high, it will have a great effect on the materials of the heat exchanger and reduction reactor. Therefore, the content, distribution, and possible influence of sulfur in the gas during the reaction must be studied in detail.

In the reduction process, surplus hydrogen is recycled. Given that H₂S is generated during the reduction process, if the top gas is completely recycled, then H₂S will be recycled and enriched. Therefore, a portion of the furnace top gas must not participate in the circulation. The sulfur balance can be determined by the following equation:

where S_pellet is sulfur content in pellets for producing 1 ton of DRI, kg/t Fe; [%S]_c is the sulfur content of circulating gas after dust removal and dehydration, kg/Nm³; [%S]_t is the sulfur content of the furnace top gas, kg/Nm³; ${\eta }_{\mathrm{s}}$ is the desulfurization efficiency during iron-ore reduction.

The relationship between non-circulating gas flow Q_out after dust removal and dehydration and Q_t and Q_c is as follows:

where Q_in is the total amount of reducing gas entering the shaft furnace, Nm³/t Fe.

The relationship between Q_s and [%S]_t is computed with the following:

where Q_s is the flow of supplementary reducing gas, Nm³/t Fe.

The desulfurization efficiency (${\eta }_{\mathrm{s}}$) during iron ore reduction is assumed to be 80%. According to Eqs. (18)–(20), when the sulfur contents in the pellet are 0.005wt%, 0.01wt%, 0.02wt%, and 0.03wt%, the relationship between the flow of supplementary hydrogen (Nm³/t) and sulfur content of the furnace top gas can be obtained (Fig. 10).

Figure  10.  Relationship between sulfur content in furnace top gas and amount of non-circulating gas flow.

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According to Fig. 10 and the sulfur balance equation (Eq. (18)), when the sulfur content in the pellet is 0.005wt%, the newly supplemented reducing gas into the furnace is 200 Nm³/t Fe, and the H₂S content of furnace top gas is 30.3 mg/m³. When the content of H₂S in the tail gas is limited to 50 mg/m³, the volume of reducing gas (Q_in) to be pumped in the furnace is positively correlated with the sulfur content in the oxidized pellet. When the contents of sulfur in the pellet and H₂S in the furnace top gas are limited, the amount of supplemented hydrogen must be equal to the amount of hydrogen required for equilibrium. Otherwise, the sulfur in the system cannot reach equilibrium.

3.4   Continuous, smooth, and safe operation

The important factors affecting the smooth operation of the hydrogen reduction process include the following.

(1) Preparation of high-quality oxidized pellets. Oxidized pellets prepared from ultra-high-grade iron concentrate are prone to malignant expansion when reduced in a gas-based shaft furnace. Thus, they are not conducive to gas-based shaft furnace reduction. The grade of iron concentrate should not be extremely high, and a suitable flux should be added based on the composition. This action will not only reduce the cost of mineral processing but also meet the requirements for the direct reduction in a gas-based shaft furnace. The pellet size should be uniform, and an amount less than 5 mm should not be greater than 5%. The compressive strength of a single pellet shall not be less than 1500 N.

(2) Preheating of hydrogen. Increasing the hydrogen temperature speeds up the reduction reaction. The reduction temperature should not be higher than the softening temperature of iron ore. Otherwise, the pellets or iron ore particles will easily melt and bond, hindering the penetration and diffusion of reducing gas. However, obtaining high-temperature hydrogen puts forward higher requirements for the material selection of hydrogen heater and stable heating operation. Therefore, the reasonable increase in the temperature of hydrogen must be comprehensively considered.

(3) Design of reduction device. The structural design of the reduction device directly affects the flow distribution and flow rate of reducing gas. It also affects the contact, reaction, and heat distribution between iron oxide and the reducing gas, which has a great influence on the reduction process. Therefore, the design of reduction devices must comprehensively consider the influence of various elements and include specific measures for possible expansion, sintering, and other situations.

(4) Composition and pressure of reducing gas. The content of hydrogen in the reducing gas should be as high as possible, and the content of impurities should be low. A high hydrogen content means a fast reduction rate. Increasing the gas pressure can increase the H₂ density per unit volume, reduce the gas flow rate, increase the probability of contact with iron ore, and accelerate the reaction rate. In addition, increasing the pressure of the reducing gas can reduce the pressure drop of the feed column and improve the gas permeability, which is conducive to improving the reduction rate.

(5) Security issues. Hydrogen is a kind of high-energy combustible gas, which is colorless, tasteless, nontoxic, flammable, and explosive in a large range when mixed with air and oxygen. It has the characteristics of low natural temperature, wide explosion range, difficult detection due to the lack of odor, easy diffusion and leakage, and enrichment at high altitudes. Therefore, in the process of using hydrogen, safety must be considered from technical aspects, and measures, including sophisticated standards, operating regulations, and safety facilities, need to be applied.

4.   Conclusions

As a new kind of green metallurgical technology, the pure-hydrogen reduction is an important direction of low-carbon development in the field of metallurgy and in realizing low-carbon green development. After the above analysis and discussion, the conclusions obtained are as follows.

(1) After years of development, the basic theory of hydrogen reduction is relatively mature, but considerable basic work still needs to be strengthened for the application of hydrogen reduction. The methods of appropriately increasing the reduction temperature, pressure, and temperature of the iron ore into the furnace are put forward to promote the rapid promotion of reduction reaction.

(2) Many problems exist in the engineering application of hydrogen reduction. In terms of hydrogen preparation, the existing hydrogen production from natural and coal gases has become increasingly mature and has the conditions for industrial application. However, the cost is still high. The technology of hydrogen production by water electrolysis has developed rapidly and has the conditions for industrial application. However, the cost is also high, and its technology and efficiency still need to be further developed and improved. The preparation of low-cost hydrogen is an urgent problem for the pure-hydrogen reduction process.

(3) In the engineering application of hydrogen reduction, the material selection of hydrogen reduction reactor and hydrogen heater is very important. In the reduction process, hydrogen easily diffuses into the steel at high temperature and pressure, forming hydrogen embrittlement cracking on the surface of the steel, and together with H₂S, reacting with carbon, sulfur, etc. in the steel , resulting in corrosion. New types of steel and coating materials must be developed, and the composition of reducing gas must be controlled to jointly solve the material problems of the hydrogen reduction reactor and heater.

(4) A part of the circulating hydrogen (Nm³/t Fe) is used as the heat source of the hydrogen heating furnace. Given the gas consumption of the heating furnace, the nitrogen content in hydrogen should be controlled at the level of 3.5vol%, and the content of sulfur in iron ore concentrate should be controlled below 0.1wt% to ensure that the total hydrogen consumption is 800 Nm³/t Fe, and the H₂S content in reducing gas is 30 mg/m³.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 52104297) and the National Key R&D Plan (No. 2019YFC1905202).

Conflict of Interest

The authors declare no potential conflict of interest.