Research progress on the principle and industrial application of hydrogen metallurgy

Hydrogen energy is regarded as the most promising clean energy in the 21st century and will play an important role in the future energy structure reform. Around the goal of achieving zero greenhouse gas emissions, many countries have promoted the utilization of hydrogen energy as a national strategy, and hydrogen technology research and development has become a hot spot. Hydrogen metallurgy is in the field of metallurgy with hydrogen instead of carbon reduction, hydrogen metallurgical reduction process and carbon reduction ratio has different characteristics, in the application of hydrogen metallurgy, iron ore direct reduction and blast furnace coal injection technology in the application of hydrogen energy has made progress. Although there have been many studies on hydrogen reduction of iron oxides, it is still unable to give an exact and reasonable explanation for some reaction behaviors, and it needs in-depth systematic analysis, research and summary to provide theoretical support for the application of hydrogen metallurgy. Combining with the technical accumulation of direct reduction and hydrogen reduction utilization in blast furnace process, the rational application of hydrogen metallurgy principles is emphasized from the aspects of hydrogen metallurgy thermodynamics, kinetics and engineering, hoping to explore the methods and paths to further improve the reduction capacity, efficiency, rate and industrial application of hydrogen metallurgy.

1 Basic research of hydrogen metallurgy

According to the basic theory of metallurgical reaction process, the development of hydrogen metallurgy technology must be designed according to meet the thermodynamic, kinetic and engineering principles of hydrogen metallurgy. Thermodynamics determines the direction, equilibrium conditions and limits of metallurgical reaction processes, kinetics discusses the rate, mechanism and limiting links of metallurgical processes, and engineering studies the macroscopic transfer law, unit operation and reactor characteristics of metallurgical processes. The three are organically combined to develop the maximum output conditions and parameters that can be achieved by hydrogen metallurgy process, find out the method to control and improve the reaction rate efficiency, improve the system problems existing in the operation process, and achieve the purpose of engineering popularization and application.

1.1 Put forward the concept of hydrogen metallurgy

The definition of hydrogen metallurgy is based on the concept of carbon metallurgy. Carbon metallurgy is the representative development mode of iron and steel industry, and the basic smelting formula is Fe2O3+3CO=2Fe+3CO2; The reducing agent is carbon, and the product is carbon dioxide. The basic reaction formula of hydrogen metallurgy: Fe2O3+3H2=2Fe+3H2O; The reducing agent is hydrogen, the end product is water, and the carbon dioxide emissions are zero. Carbon has always been the most important reducing agent in the steel industry, and it also causes a large amount of carbon dioxide emissions [1]. Non-carbon metallurgy is a metallurgical process that does not use carbon-containing substances as fuels and does not use carbon-containing media as reducing agents. Hydrogen is an excellent reducing agent and clean fuel. Research on hydrogen metallurgy technology, which replaces carbon with hydrogen as reducing agent and energy source, can change the environmental status of the iron and steel industry and is the most favorable choice for the development of low-carbon economy, which will bring hope for the sustainable development of the metallurgical industry [2].

1.2 Hydrogen metallurgy thermodynamics

According to the Fe-O-H system equilibrium diagram, below the critical temperature (about 570℃), the order of Fe2O3 reduction by H2 is Fe2O3-Fe3O4-Fe. Above the critical temperature, the order of reduction of Fe2O3 by H2 is Fe2O3-Fe3O4-Feo-Fe. The thermodynamics of hydrogen reduction in the reaction process includes two process routes: low temperature reduction and high temperature melting reduction [3]. In the process of hydrogen direct reduction of iron ore at low temperature, the raw material needs to be preheated due to heat absorption, and the multi-stage fluidized bed is often used for reduction to make up for the shortcomings of low temperature drop and low gas utilization rate. The high temperature molten hydrogen reduction process of iron ore is to inject hydrogen or hydrogen-rich gas into the lower part of the molten reduction furnace, by controlling the carbon combustion rate, and using hydrogen to replace part of carbon as a reducing agent, reduce the heat load required for carbon reduction, and achieve the purpose of accelerating the reduction speed and reducing carbon consumption.

(1) Low temperature reduction reaction includes:

FeO(s)+H2(g)=Fe(s)+H2O(g)

ΔG0=23430-16.16T (1)

FeO(s)+CO(g)=Fe(s)+CO2(g)

ΔG0=-17883+21.08T (2)

FeO(s)+C=Fe(s)+CO(g)

ΔG0=147904-150.22T (3)

(2) high-temperature reduction reaction includes:

(FeO)+CO(s)=[Fe]+CO2(g)

ΔG0=-35421+32.47T (4)

(FeO)+H2(g)=[Fe]+H2O(g)

ΔG0=5892-4.77T (5)

(FeO)+C=[Fe]+CO(g)

ΔG0=130336-138-83T (6)

The content of H2 and CO in the equilibrium state of iron ore reduction reaction at high temperature is higher than that of the equilibrium state gas in the solid state reduction reaction at low temperature. The content of H2 and CO in the equilibrium gas phase is 45.6% and 81.8%, respectively. In summary, the calculation of the thermodynamic equilibrium components of the C-H2-O2-H2O-CO-CO2 system can be obtained [3] :

(1) When the temperature is 1500℃, carbon and oxygen are in excess, and there is no water and carbon dioxide in the equilibrium system. The proportion of oxygen in the gas phase increases, the content of carbon monoxide increases, and the content of hydrogen decreases. The amount of solid carbon is supersaturated, the content of water and carbon dioxide is reduced, and the content of hydrogen and carbon monoxide is increased; The influence of system pressure on equilibrium component content is not significant.

(2) As the temperature increases, the contents of CO and H2O in the equilibrium system increase, while the contents of H2 and CO2 decrease, and the increase of temperature is conducive to improving the hydrogen utilization rate.

(3) When carbon is in excess, the heat load of reactive carbon cannot be reduced only by spraying H2; At high temperatures, although hydrogen can react with iron oxide, carbon can also react with H2O, thus transforming H2O into H2.

According to the traditional direct reduction process practice, literature [4] proposed the carbon-hydrogen melting reduction technical route, which provides heat by oxidizing carbon to CO and uses hydrogen as reducing agent to reduce iron ore. Carbon is used as heat source and partial reducing agent, and H2 is used as main reducing agent, which solves the contradiction of high heat and strong reducing atmosphere for direct reduction of carbon in melting reduction process. The C in the raw material has participated in the reaction and produced a penjue, which provides hydrogen source for hydrogen metallurgy. The reduction of iron oxide mainly relies on hydrogen, and CO also reacts with iron oxide. At the same time, the reaction of CO2 and H2 can generate CO+H2O, so that the emission of C in the reaction process is reduced, which is conducive to the environmental protection effect of direct reduction. Therefore, ensuring a reasonable supply of raw material hydrogen can keep the reduction process normal.

1.3 Hydrogen metallurgy kinetics

The kinetic condition of hydrogen reduction of iron oxide is better than that of CO, and the mass transfer rate of hydrogen is significantly higher than that of CO [5]. Compared with CO, the reduction kinetics conditions of hydrogen-rich gas or pure hydrogen are improved. CO reduction of iron oxide is exothermic reaction, H2 reduction of iron oxide is endothermic reaction, so how to continue to supply heat to the reaction zone is the technical difficulty of hydrogen rich or pure hydrogen reduction.

1.3.1 Low temperature hydrogen reduction

The key technology of low temperature hydrogen reduction is how to strengthen the reaction rate of hydrogen and iron ore and improve the process efficiency. From the kinetic point of view, the reaction rate of hydrogen reduction iron ore at low temperature is slow, and the concentration of hydrogen in the equilibrium gas phase is high. In order to improve the rate of direct reduction reaction at low temperature, there are two technical measures that can be taken [6] : one is to reduce the activation energy of the reaction and activate H2 into H or H+ through the action of physical field; Iron ore can be reduced to metallic iron by activated hydrogen at low temperature. The second is to increase the surface area of the reactant, that is, to reduce the particle size of the iron ore; The particle size is reduced from 45μm to 5μm, and the reaction area can be increased by 9 times.

The low temperature hydrogen reduction reaction of iron oxide initially occurs at the local active point of the interface, and expands inward to form small pores. The generated active iron forms protrusions through surface diffusion and gradually develops. The porous product structure enables the gas reduction reactants and gas products to diffuse smoothly, and the interfacial reaction can proceed smoothly. With the progress of hydrogen reduction reaction, the product layer thickens, the reduction product begins to sintering and densification, and the diffusion of the reduction gas and product is affected, and gradually becomes a limiting link in the reaction process. Its macroscopic performance is that abnormal temperature effect occurs in the reduction process of iron ore, that is, at a certain reduction temperature, the reduction rate does not increase with the increase of temperature, but decreases with the increase of temperature.

At low temperature, the sintering process is slow, and the product structure does not affect the diffusion of gas, so the low temperature hydrogen reduction process is the interface reaction speed control. With the increase of temperature (higher than 700℃), the sintering process accelerates, the influence of product sintering on the reaction rate gradually increases, and the reaction rate control link gradually changes from interface reaction rate control to diffusion rate control. The problem to be solved in low temperature hydrogen reduction (below 1000℃) is to control the rapid increase of the initial chemical reaction rate before the formation of dense structures that affect gas diffusion, and to end the reduction process before the formation of dense product structures.

1.3.2 High temperature hydrogen reduction

The key technology of high temperature hydrogen reduction is to inject hydrogen or hydrogen-rich gas into the lower part of the iron bath furnace, and use hydrogen instead of carbon as reducing agent by controlling the combustion rate of carbon. When the iron ore reduction reaction temperature is greater than 1000℃, the thermodynamic utilization rate of hydrogen-rich gas increases with the increase of hydrogen content, so increasing H2/CO is conducive to improving the comprehensive utilization rate of hydrogen reduction. At the same time, increasing the heat required for H2/CO iron ore reduction increases, and increasing the heat supply in the furnace requires increasing the total amount of reducing gas, which will lead to the reduction of gas utilization. This makes it difficult to achieve the optimal coordination and unity of gas composition and gas utilization in the high-temperature hydrogen reduction furnace, that is, the contradiction between the heat transfer in the reactor and the chemical equilibrium determines the existence of the primary utilization limit of hydrogen-rich gas.

1.4 Hydrogen metallurgical engineering

The research of hydrogen metallurgy engineering started from the development of direct reduction and melt reduction technology, including hydrogen rich reduction and full hydrogen reduction. Due to the limitation of large-scale hydrogen production technology and cost, hydrogen-rich high-temperature melting reduction has been preferentially developed, and controlling the hydrogen-rich content in the reduction gas is the key technology. The production process of hydrogen-rich coal gas reduction iron ore has been gradually industrialized since the middle of the last century, such as Midrex process and HLY-Ⅲ process using natural gas, which both use the principle of high temperature hydrogen reduction, and mainly need to solve the problem of sponge iron bonding [7]. With the progress of modern powder preparation and separation technology, micron grade powder can be produced by iron ore - ultrafine pulverization - magnetic separation purification - refining process. Micron grade mineral powder has good reduction kinetic conditions, and can be reduced at less than 600℃, low energy consumption and can effectively avoid powder bonding in the reactor

The practice of direct reduction and melt reduction engineering has overcome a series of technical difficulties, and the engineering examples of hydrogen energy utilization are summarized in Table 1[7]. The new direct reduction capacity mainly adopts gas-based reduction process to produce high grade direct reduced iron or HBI powered arc furnace. At present, the smelting reduction technology is mainly developed by using iron bath method or Corex, Finex process, and can be applied to the recovery of ferrous solid waste and comprehensive utilization of resources.

2 Progress of hydrogen metallurgy process

2.1 Hydrogen utilization in traditional metallurgical processes

Traditional steel production processes produce large amounts of hydrogen resources, such as coke oven gas. Based on the principle of hydrogen metallurgy, the injection of coal, coke oven gas, natural gas and plastics into blast furnaces is the test and practice of the development of traditional blast furnace hydrogen metallurgy technology [9].

(1) Blast furnace coal injection. Coal injection is a typical case of hydrogen rich reduction applied to traditional blast furnace. The blast furnace bituminous coal is first vaporized at high temperature, and the resulting hydrocarbons are pyrolyzed into hydrogen with iron oxide as catalyst, which reacts with iron ore. The reduction efficiency and technical index of the blast furnace are improved. In order to overcome the negative effects of coal injection, some new BF coal injection technologies are adopted, such as hydrogen rich gas instead of coal powder injection into the blast furnace through the tuyere, which makes the injection process more efficient and energy saving.

(2) Coal gasification technology. Coal gasification technology is a thermochemical process, using oxygen and water vapor as gasification agents, through chemical reactions under high temperature and pressure to convert coal or coal coke into combustible gas. Coal gasification technology has been widely used in chemical industry, and the reductive hydrogen-rich gas obtained by different gas production methods has reference significance for low-carbon metallurgy.

(3) Blast furnace injection waste plastic (waste rubber) technology. The blast furnace sprayed 1kg of waste plastic, equivalent to 1.2kg of pulverized coal. The waste plastic composition is simple, the hydrogen content is 3 times that of pulverized coal, and the blast furnace can reduce the emission of 0.28t of carbon dioxide per 1t of waste plastic. Waste plastics and rubber can be recycled because of their excellent processing properties and durability, but they need the support of plastic classification and processing policies.

2.2 Foreign hydrogen metallurgy process progress

Gas based direct reduction iron making is a classic application of hydrogen metallurgy in iron making technology. Europe attaches importance to and supports the development of hydrogen metallurgy, and regards hydrogen energy as an important energy option to reduce carbon emissions in the future, and is expected to achieve large-scale replacement of fossil fuels. According to the research on the current development status and future potential of hydrogen energy in Europe [9], hydrogen production from fossil fuels plus carbon capture and storage is the current realistic way of low-carbon hydrogen production, and hydrogen production from electrolytic water will gradually become a low-carbon and low-cost method of hydrogen production in the future. In the past decade, the steel industry under the constraints of strict global resource and environmental policies, the world's major steel producing countries began to work on the development of breakthrough low-carbon metallurgical technologies that can significantly reduce CO2 emissions. Recent typical hydrogen metallurgy projects are shown in Table 2[10].

2.3 Domestic hydrogen metallurgy technology development

China's hydrogen metallurgy process research started late, iron and steel enterprises in recent years began to layout the field of hydrogen metallurgy, its typical hydrogen metallurgy projects are shown in Table 3[11]. In the steel industry facing the situation of capacity reduction, structural adjustment and transformation, the cooperation between the hydrogen energy industry and steel enterprises can form a complementary win-win effect. The utilization of hydrogen energy can help iron and steel enterprises to achieve energy saving and emission reduction, industrial extension and transformation, and iron and steel enterprises can provide more and more large-scale industrialization demonstrations for the hydrogen energy industry.

2.4 Hydrogen preparation technology

The development of hydrogen energy is based on the large-scale production of hydrogen by hydrogen-containing compounds. Hydrogen production methods mainly include electrolytic water hydrogen production, fossil fuel hydrogen production and biomass hydrogen production. Hydrogen must be compressed, transported, stored and transferred before reaching the end user. Large-scale production, storage and transportation of hydrogen depend on technological progress and infrastructure construction, which is a difficulty in the development of hydrogen energy industry [12].

The traditional hydrogen production methods are fossil energy reforming and water electrolysis. Hydrogen production from fossil energy reforming is to mix fossil fuel with water vapor, generate hydrogen and carbon dioxide under catalytic action, and produce high purity hydrogen through pressure swing adsorption and membrane separation evaporation. Hydrogen production by electrolysis is a pair of electrodes with an intermediate diaphragm immersed in an electrolyte, and the electricity is applied to decompose the water into hydrogen and oxygen. The production of hydrogen from fossil fuels and electrolytic water emits a lot of carbon dioxide, and this high-carbon hydrogen is called "gray hydrogen" or "black hydrogen". The actual process of hydrogen production is low carbonization, and the acquisition of low-carbon "blue hydrogen" and zero-carbon "green hydrogen" in the sense of the whole life cycle needs to increase carbon capture and storage in the fossil fuel hydrogen production system, or directly use the electricity produced by non-fossil fuels for electrolytic hydrogen production. "Fossil fuel hydrogen production + carbon capture and storage" is a short and medium term low-carbon hydrogen production transition mode, long-term non-fossil fuel power generation electrolytic hydrogen production will gradually become the main low-carbon hydrogen production mode.

Biomass is an abundant renewable resource on earth, and the technology of rapid pyrolysis of biomass to produce bio-oil has been developed rapidly in recent years. Bio-oil can be reformed with water vapor to produce hydrogen, which provides a new way to produce hydrogen from biomass. Due to the low energy density of biomass, the industrial technology of direct hydrogen production needs to be further developed.

3 Direction of industrialization and promotion of hydrogen metallurgy

3.1 Expansion of hydrogen energy in traditional metallurgical processes

(1) Recycling of gas from the top of the blast furnace. The core of the blast furnace top gas recycling process is to remove dust, purify and decarbonize the blast furnace top gas, inject the reducing components (CO and H2) into the tuyre or furnace body, return to the furnace to participate in iron oxide reduction, and use CO and H2 to further improve the blast furnace index, reduce energy consumption and reduce CO2 emissions.

(2) Blast furnace injection of hydrogen-containing substances. The hydrogen rich medium of blast furnace injection mainly includes natural gas, coke oven gas, waste plastics, and used tires [13]. After blast furnace injection of hydrogen-containing substances, hydrogen participates in iron ore reduction, strengthens the adaptability of blast furnace to raw fuel, and realizes the diversification of blast furnace functions, which has practical significance for energy saving and emission reduction of iron and steel industry. The main component of natural gas is CH4, which is sprayed into blast furnace tuyere together with oxygen-rich hot air to reduce the coke ratio of blast furnace. Some blast furnaces in North America and Russia inject natural gas, and the injection volume is 40~110kg/t. Coke oven gas is the product of waste gas after chemical production recovery and purification. There are cases that coke oven gas injected into blast furnace can reduce the coke ratio of blast furnace to less than 200kg/t.

Plastic is a petrochemical product, and blowing old plastic can not only control "white pollution", but also realize the comprehensive utilization of resources. Waste plastics are used in blast furnaces, including sorting, crushing, granulation and other aspects, replacing some pulverized coal from the tuyere into the blast furnace, the maximum injection volume has reached 60kg/t, and the theoretical maximum injection volume of waste plastics is 200kg/t; The processes that need to be improved include plastic granulation and dechlorination treatment.

3.2 Innovation path of hydrogen metallurgy industrialization

According to the "Global Hydrogen Metallurgy Special Report" issued by the China Hydrogen Energy Alliance, the main manifestations of insufficient hydrogen metallurgy technology research reserves in China are: first, the research and development basis of hydrogen high temperature and high safety materials; Second, hydrogen explosion-proof and leak-proof technology reserve; Third, the structure design and process control technology of hydrogen metallurgy reactor; The fourth is the theoretical study of the reaction mechanism of hydrogen metallurgy and the variation of charge characteristics. The proposed layout direction of hydrogen metallurgy technology research and development: exploring hydrogen rich smelting in blast furnaces to achieve low carbon metallurgy; The waste gas in the metallurgical process is converted into chemical raw materials by adding hydrogen to realize zero-carbon metallurgy; Carbon-free metallurgy is realized by pure hydrogen reduction process.

For China's traditional iron and steel complex, the development of hydrogen metallurgy does not have the regional advantages of natural gas resources, restricted by large-scale storage and transportation facilities, the cost of replacing coal with hydrogen is higher, and the lack of hydrogen metallurgy technical foundation accumulation, so it is necessary to seek new development ideas suitable for the characteristics of enterprises.

(1) China's hydrogen production mainly relies on fossil energy, and hydrogen consumption is mainly reflected in the field of transportation and industrial raw materials. The steel industry itself is accompanied by a large number of hydrogen-rich by-products, but at this stage these hydrogen-rich by-products have not been fully separated, purified and efficiently used. Blast furnace is still the main process of ironmaking, increasing the recycling ratio of hydrogen resources around blast furnace should be the preferred way to improve the process technology at this stage.

(2) The iron and steel industry has three important functions of producing iron and steel products, absorbing and disposing of social waste and realizing energy conversion. The research and development and application of functional technologies for absorbing solid waste and energy conversion should receive attention and attention. Such as: Dr. Niu Qiang of the Chinese Academy of Sciences published a case study on the utilization of hydrogen energy by his team through the Internet media. The high-speed injection of solid waste particles containing hydrogen and oxygen/water vapor into a metal molten pool at 1500℃ can produce a rapid reaction of dissolved oxygen and carbon, and stable generation of clean CO and H2 mixed combustible gas at high temperature, which can be reused. Suitable for waste plastics, old rubber tires, organic solid waste, biomass and other treatment and conversion.

(3) A large amount of coke oven gas, blast furnace gas, converter gas and steam are produced in the traditional coal-iron manufacturing process of enterprises, and gas surplus is a common phenomenon. At present, it is mainly used as fuel for power generation of various boiler systems. Further consideration of converting hydrogen-rich by-products into reducing agents for the whole process of metallurgy or chemical products, to get rid of the situation of relying only on carbon as reducing agents, will strongly promote the application of hydrogen metallurgy technology. At this stage, the most reasonable approach for iron and steel enterprises is to tap the deep potential of the traditional process of hydrogen-rich energy conversion and utilization.