Research progress on the principle and industrial application of hydrogen metallurgy

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].