The Latest Research on the Sustainable Development of the Iron and Steel Industry
According to the world metal reports, recent steel industry carbon emission reduction technology innovation includes the following aspects:
1. Reducing blast furnace production, using scrap or direct reduction iron EAF steelmaking.
2. Application of hydrogen from green electricity in direct reduction iron production.
3. Electrolytic iron ore based on hydrometallurgy.
4. Molten oxide electrolysis.
5. European ultra-low carbon dioxide steelmaking (ULCOS) project ( blast furnace top gas recycling, HISarna ).
6. Using gas to produce chemical products
7. Application of slag in cement industry
The International Energy Agency proposes a sustainable development plan (SDS) to achieve the goals of the Paris Agreement. In order to achieve this plan, the International Energy Agency has proposed that the steel industry must reduce the carbon dioxide emission intensity of crude steel by 1.9% annually from 2017 to 2030.
The carbon dioxide emission intensity of crude steel has been on a downward trend since 2009 (a year-on-year decrease of 1.8% in 2017). However, to reduce the unit carbon dioxide emissions of steel products, the steel industry still needs to work harder and continue to carry out technological innovation.
Recently, there have been several recent developments in reducing the environmental footprint of the steel industry. Many such projects, technologies or innovations fall into the following categories:
1) Hydrogen reduction of iron ore.
2) Using of green electricity.
3) Utilization of biomass.
4) End treatment technology.
This article discusses the current steel production process routes and their carbon dioxide emissions issues. It also reviews existing and developing low-emission technologies (LETs). These technologies can fundamentally change the steel industry.
1.1 Main Steel Process Routes
There are three main process routes for steel production:
1) The integrated route adopts blast furnace, basic oxygen converter and coke oven (70% of BF-BOF method).
2) Direct reduced iron (DRI) production, used in electric arc furnace steelmaking (DRI-EAF method accounts for 5%).
3) Electric arc furnace steelmaking with scrap steel (scrap-EAF method accounts for 24%).
1.2 Regional Differences in Iron and Steel Production Technology
The predecessor of the modern BF-BOF steelmaking process has existed for hundreds of years.However, the electric arc furnace steelmaking began in the early 20th century. In the 1960s, due to the availability of a large amount of scrap steel, the electric arc furnace steelmaking was promoted and applied. The limitations of the early EAF were the lack of electricity (especially cheap electricity) and scrap supply, and they could only produce low grade steel products. In the past 50 years, electric arc furnaces have been continuously improved, so that nearly 80% of steel products can be produced by electric arc furnaces. Countries with low-cost electricity and scrap or natural gas for DRI production usually produce large amounts of steel by arc furnace. In 2018, 68% of crude steel in the United States was produced by the electric arc furnace process, while only 12% in China. In China, sufficient supply of scrap and cheap electricity are still restrictive factors.
2 Emission Reduction Strategy
2.1 Process Route Selection : Traditional Integrated Steelmaking, EAF Steelmaking, Direct Reduction Iron
According to Japan ' s power emission coefficient, the discharge of EAF steelmaking process, especially using scrap as raw material, is significantly lower than the traditional BF-BOF process. To achieve more environmentally friendly steelmaking, the transition from the global steel industry to EAF processes can significantly reduce carbon dioxide emissions. If the BF-BOF route accounts for 60 % and the arc furnace route accounts for 40 % ( using 100 % NG-DRI raw material ), the carbon dioxide emissions can be reduced by about 20 % to 1.62 tCO2 / tHRC.
Low-carbon electricity and sufficient scrap supply are necessary conditions for EAF steelmaking to replace traditional BF-BOF steelmaking. In addition, only when DRI/HBI is added to the electric arc furnace production, the electric arc furnace may become a process suitable for the production of all steel grades. The availability and cost of natural gas are a major factor restricting the production of DRI/HBI.
In China, the world ' s largest steel producer, EAF steelmaking is currently restricted because of limited power and low-cost DRI and scrap supply. Replacing some converters with arc furnaces may be a trend in the global steel industry.
2.2 Blast Furnace Improvement
As the current BF-BOF route accounts for 70% of steel production, the short-term strategy to reduce the carbon emission intensity of steel is to incrementally modify and improve the existing blast furnace route, thereby reducing carbon dioxide emissions per ton of steel. Improvements to the BF-BOF route include technologies that utilize electricity, such as hot air overheating, top gas recirculation, furnace or coking process improvements, natural gas injection into the blast furnace to partially offset the demand for coke, and the use of alternative fuels such as biomass.
Using electrical technology in blast furnace can reduce carbon dioxide emissions, on the premise that electricity comes from renewable power or green national grid. The reliability and maintainability of plasma torches have been significantly improved since they were introduced into blast furnaces in the 1980s. Electric energy is used to generate high-temperature and high-speed plasma flow.
Dry quenching is an improvement of the existing coking technology, using inert gas to recover the heat of the red hot coke. The recovered heat generates steam in the boiler, which is used for other purposes such as power generation. Dry quenching coke moisture content is low, saving the amount of blast furnace coke.
Many blast furnace improvement projects focus on the use of biochar to replace metallurgical coal / coke. Similar to other petrochemical carbon sources, biochar releases carbon dioxide to the atmosphere, and the released carbon dioxide is considered to be the balance of carbon dioxide absorbed during biomass growth. Therefore, biochar is considered to be greenhouse gas (GHG) neutral.
One of the main problems in using biomass as carbon substitute in blast furnace is that the high activity of biochar leads to a significant reduction in coke quality.
2.3 Hydrogen-based steelmaking project
A long-term solution to reduce carbon dioxide emissions from steel is to promote the technology of replacing carbon with hydrogen as iron reducing agent.Hydrogen was used in the reaction process to generate water and avoid the production of carbon dioxide.
Hydrogen reduction has two ways : 1 ) Hydrogen injection into blast furnace can reduce the amount of coal / coke required. 2 ) In EAF steelmaking, hydrogen can be used as a substitute for NG-DRI to produce H2 - DRI.
2.4 Molten reduction technology
Molten reduction is an alternative coal-based ironmaking process, which depends on coal gasification in molten iron. The melting reduction process includes two regions : pre-reduction region and melting reduction region. Coal enters the melting reduction zone, where coal is gasified to produce heat and hot gas rich in carbon monoxide. Heat melts iron in the molten reduction zone, and hot gas is transported to the pre-reduction zone. Then, the hot gas pre-reduced the iron oxide before the iron oxide entered the molten reduction zone for final reduction. Melt reduction technology eliminates coking process and tends to avoid iron ore agglomeration process, significantly reducing carbon dioxide emissions. One disadvantage of most smelting reduction processes is that they need a large amount of oxygen and the cost is high. At present, less than 1 % of steel is produced by smelting reduction process.
HISARNA and FINEX are the two most common melting reduction technologies.
HISARNA is part of the ULCOS project aimed at reducing CO2 emissions by 50 per cent during steelmaking. Since 2007, Tata Steel, Rio Tinto Group and ULCOS project group have been developing HISARNA technology, which can directly produce iron with iron ore and coal without any pretreatment.
Compared with the traditional ironmaking route, carbon dioxide emissions are reduced by 20 %, and the use of biomass or scrap in HISARNA furnace can further reduce carbon dioxide emissions by 50 %.
3. Direct carbon avoidance options
At present, two projects in the experimental stage are studying new process routes, which have the potential to completely change the steel industry and decarbonize the steel industry : molten oxide electrolysis ( MOE ) and iron ore electrolysis projects. Both are green electrical technologies.
3.1 Molten oxide electrolysis
Company B is developing an iron ore carbon-free steel production process.
This process is in small-scale pilot development level.
The MOE process uses iron ore as raw material to selectively reduce iron by inert anode and more stable molten oxide electrolyte layer.
It regularly removes pure iron from the electrolytic cell, adds alloys, and then process and cast the steel in accordance with typical downstream steelmaking equipment. In order to maintain the target chemical properties and alkalinity, the gangue composition in iron ore combines with the flux to form a molten oxide layer.
3.2 Iron ore electrolysis project
The Siderwin process is a new project in the European steel industry, headed by ArcelorMittal, using electrolytic cells to produce metallic iron.
When iron ore is introduced into the electrolyzer and the current flows through the electrode, iron is attracted to the cathode and oxygen is attracted to the anode. The project is funded by the EU Horizon 2020 plan and is currently in the pilot phase. A 3 m industrial battery is under construction to test various iron sources as feed materials, including scrap iron.
The production of metallic iron involves three main steps :
1 ) Hematite reacts with soluble ferrous iron to form magnetite : Fe2O3 + HFeO2- → Fe3O4 + OH-
2 ) Electric coupling between magnetite and iron : Fe3O4 + Fe + 4OH- → HFeO2- 3 ) Electrocrystallization of iron under cathodic polarization : 3HFeO2- + 3H2O + 6e- → 3Fe + 9OH-
4 Waste gas recycling
Carbon capture, storage and utilization can also play a role in the progress towards low-emission steel production. Carbon capture, storage and utilization technologies capture carbon dioxide from waste gas and reuse it as raw materials for the production of various chemical products to avoid the use of coal or natural gas raw materials. Carbon dioxide captured from steel waste gas can be used to improve oil recovery of oil wells, and can also be converted into higher value products, such as bioethanol, biomethanol or polymer. Carbon dioxide can also be stored in cement or used as feed for algae growth. However, to completely eliminate carbon dioxide emissions from the current iron and steel industry, a large number of cement or algae are needed, which will increase the difficulty of these technologies.
Bioethanol is usually produced from yeast fermentation of sugar in biomass such as corn or sugarcane. It is used as a substitute for gasoline. Bioethanol is remarkable because it is obtained from renewable resources. Compared with fossil fuels, bioethanol is less toxic and produces slightly less carbon dioxide emissions.
Methanol can be produced by blast furnace gas or coke oven gas to reduce carbon dioxide emissions by avoiding the use of fossil fuels. Synthetic gas for methanol synthesis can be a mixture of hydrogen, carbon dioxide and carbon monoxide. However, compounds such as N2 must be removed from the gas for methanol production.
The steel industry is investing a lot of money to develop technologies to reduce its carbon footprint. Short-term solutions such as gradual improvement of blast furnaces or transition to EAF steelmaking can help reduce carbon emissions per unit of steel production. However, if carbon dioxide emissions are to be significantly reduced, new alternative processes must be improved, such as the use of hydrogen as a reducing agent or green smelting reduction technology to decarbonize the iron and steel industry ( Fig. 2 ). In addition, Government support for green energy and the phase-out of the fossil fuel industry is critical to advancing these breakthrough technologies as sustainable and cost-effective alternatives to existing processes.