The iron and steel industry is a major energy consumer and carbon emitte. Developing electric arc furnace（EAF） steelmaking process is an important path to achieve carbon peaking and carbon neutrality goals.
Improving the EAF production efficiency is important for the developing of iron and steel industry.
At present，production efficiency of EAF is increased by improving energy input，promoting chemical reactions，reducing energy and material consumption，carrying out solid waste recycling，and improving intelligence level. However，there are also a series of problems such as steel peroxide，ineffective removal of residual elements，low energy efficiency，lack of online detection and control methods，and unclear metallurgical reaction mechanisms.
In the future，with the diversification of raw materials and the scaling up of EAF，the importance of improving energy efficiency，enhancing the stirring of the molten pool，improving the reactivity and foaming of slag and other technologies should be further emphasized. The optimization of related supporting equipment and the classification of scrap need to be further promoted. Replace manual operation and empirical judgment by advanced detection technologies and control models，can also help to improve production efficiency of EAF.
Promoting the progress of related work and achieving efficient and low consumption production of high-quality steel in EAF is the only way for the iron and steel industry to transform and develop towards the future.
Moving low -carbon smelting technology progress in arc furnace
Green Energy and Raw Materials
With the advancement of carbon reduction work in the steel industry, green energy, such as: green electricity and hydrogen energy; green raw materials such as: directly returned the original iron (DRI), scrap steel and biomass, and used in the production of electric arc furnaces. Research.
Green Electricity and Hydrogen Energy
The green electricity generated through the power generation is greatly affected by the weather, and it cannot be stable and effective to power the user. At the same time, arc furnace users have the characteristics of large power demand, sharp fluctuations in electricity, and serious power grid. Therefore, it is necessary to use energy storage systems with large capacity, high output efficiency, and long circular life requirements for energy storage and adjustment. Use the current mature lithium -ion batteries or newly developed air compressed energy storage technologies. Power supply, as shown in Figure 1.
The use of hydrogen gas is used as a beam oxygen gun fuel gas, and replaced the natural gas, gas, etc., forming a hydroxide beam firing oxygen gun, which can achieve the effective use of hydrogen energy in the arc oven, which is in line with the policy requirements of energy conservation and emission reduction of CO2 emissions in the steel industry. Studies have shown that compared with traditional fuel gas, hydrogen has low ignition energy, good flame stability, and high combustible flow speed, which can meet the requirements of steelmaking production to speed up and cold -regional heat supplement. However, the special nature of hydrogen also puts forward special requirements for the nature of the shooting and the design of the burning mouth. The basic characteristics of the hydroxide bundle carrier oxygen gun still needs to be explored.
On the other hand, hydrogen has a restore potential energy, which can react with the N and O elements in the steel solution to achieve deoxidation and nitrogen loss. The hydrogen plasma melting reduction technology makes full use of the restore potential energy of the hydrogen. The hydrogen -containing gas is input through hollow graphite electrodes, and the DC electric arc is ignited between the melting tank to obtain metal iron, and only water vapor is generated as by -products. Among them, the stability of DC arc is considered an important indicator, and a series of tests were conducted to explore the influencing factors (including hydrogen ratio, graphite quality, electrode gap, cutting -edge form, etc.). The concept of hydrogen plasma arc furnace is proposed (Figure 2) to solve environmental problems such as large carbon emissions and severe pollution such as the current arc furnace steelmaking process, and preparation problems such as low cleanliness.
Direct Reduced Iron
Direct reduced iron (DRI) is a product of iron oxide reduction under low-temperature solid conditions. The production capacity is mainly gas-based DRI, which is regarded as an important alternative raw material for scrap steel. It has the advantages of uniform and stable physical and chemical properties and low residual element content, and can meet the needs of electric arc furnace smelting high-quality steel.
There are some process difficulties in using DRI in electric arc furnaces, including: large slag volume, DRI bonding to form “icebergs”, harmful elements such as sulfur and phosphorus input, more power consumption and power-on time;
To this end, it is necessary to optimize the energy input mode of the electric arc furnace and improve the feeding system and physical and chemical characteristics of DRI.
Nucor Steel Arkansas Plant uses the Smart Furnace system to detect arc stability and calculate energy status to control the DRI addition rate and energy supply intensity, thereby avoiding the appearance of “icebergs”.
Production enterprises can effectively reduce power consumption and power-on time by developing high-carbon hot DRI and adding it directly to the electric arc furnace through a nitrogen-sealed chute.
Hydrogen-based DRI is a green raw material provided by using hydrogen-rich gas or hydrogen as a reducing gas based on gas-based DRI.
Since the carbon content in DRI produced using pure hydrogen is low, it is difficult to meet the current needs of arc furnace molten pool stirring. A transitional solution is to use hydrogen-rich gas (mainly coke oven gas) to produce DRI, but it will increase the sulfur content in DRI;
The million-ton hydrogen-based vertical furnace newly built by Baosteel Zhanjiang Steel in 2023 is the first million-ton direct reduction production line that integrates hydrogen and coke oven gas for industrial production. It has been put into trial production.
On April 11, 2024, the construction of Baosteel Zhanjiang Steel’s zero-carbon high-grade thin steel plate plant project officially started. This project is the first “hydrogen-based vertical furnace + electric arc furnace” short-process smelting production line for high-grade thin steel plates. It includes a 220 t high-efficiency green electric furnace, a 220 t electrode arm rotating double-station ladle refining furnace, a 220 t cross-track type double-station RH device, a 2150 mm single-strand slab continuous casting machine and related supporting public and auxiliary facilities. It has an annual production capacity of about 1.8 million tons of zero-carbon high-grade plates, which can achieve seamless connection and efficient utilization of hydrogen-based vertical furnace projects and production capacity.
After the completion of the zero-carbon high-grade thin steel plate plant project, it will use “hydrogen-based vertical furnace direct raw iron + scrap steel” as the main raw material to produce near-zero-carbon slabs, use the existing rolling facilities in the plant to produce high-grade thin steel products, and make full use of green electricity energy to form a complete construction of Baosteel Zhanjiang Steel Zero Carbon Demonstration Plant.
According to estimates, after the project is put into production, compared with the traditional “blast furnace + converter” long process of the same production scale, the whole process will reduce carbon dioxide emissions by more than 3.14 million tons per year, and the whole process will be zero-carbon factory through carbon capture and forest carbon sink.
High-grade steel has strict requirements on the nitrogen content of liquid steel, but the various nitrogen-increasing methods and lack of stirring effect in the electric arc furnace make it difficult to effectively control the nitrogen content of liquid steel. By adding high-carbon gas-based DRI to the electric arc furnace, the nitrogen content of the terminal steel liquid can be reduced by enhancing the carbon-oxygen reaction and the slag foaming effect; but a study has confirmed that this effect is closely related to the physical and chemical characteristics of DRI, and the use of coal-based DRI will increase the nitrogen content of the terminal steel liquid.
Baosteel Co., Ltd. Central Research Institute conducted a series of experiments on a 10 t electric arc furnace to explore the smelting characteristics of DRI in an electric arc furnace. Figure 3 shows the relationship between the DRI charge ratio and the mass fraction of liquid nitrogen in molten steel. It can be found that increasing the DRI charge ratio can effectively reduce the mass fraction of liquid nitrogen in molten steel. Under the full scrap steel charge structure, the mass fraction of liquid nitrogen in molten steel is 0.0075%, and after adding DRI with a mass fraction of 43%, the mass fraction of liquid nitrogen in molten steel will be less than 0.0040%.
Figure 4 shows the change of nitrogen mass fraction of steel liquid with the time after melting under different charge structures. With the continuous blowing of oxygen into the molten pool, the nitrogen mass fraction of steel liquid decreases. Under the full scrap steel charge structure, the nitrogen mass fraction of steel liquid decreases from 0.0075% to 0.0045%. After adding DRI with a mass fraction of 23.78%, the nitrogen mass fraction of steel liquid decreases from 0.0062% to 0.0037%.
It is reported that the nitrogen mass fraction of steel tapped from the electric arc furnace of the Ampere Lázaro Cárdenas plant using a 100% DRI charge structure is less than 0.0025%, which can meet the composition requirements of high-grade steel.
Scrap Steel
Scrap steel is the main raw material for electric arc furnaces. The all-scrap steel electric arc furnace is considered to be the steel production process with the lowest carbon emissions. According to forecasts, the amount of scrap steel resources will increase year by year and will exceed 500 million tons by 2060. At this time, the carbon emissions of the steel industry will drop to about 100 million tons, a decrease of about 95% compared with the peak.
Many works focus on exploring the melting law of scrap steel, so as to achieve rapid melting of scrap steel, promote its resource utilization, and reduce smelting energy consumption and carbon emissions.
Based on the multivariate regression analysis model, it is found that the factors affecting the melting rate of scrap steel are, from large to small, the stirring intensity of the molten pool, the temperature of molten steel/scrap steel, the specific surface area of scrap steel, and the carbon content of the molten pool.
In the electric arc furnace, there are characteristics of large steel retention and low carbon content in the molten pool, so it is necessary to pay special attention to improving the stirring effect of the molten pool;
Related work confirms that there is a large variation space for the convective heat transfer coefficient of molten steel, ranging from 17000 to 55000 W/(m2·K).
The melting law of multiple scrap steel coupling was studied, and it was pointed out that higher porosity and initial temperature will effectively avoid the adhesion between scrap steel, thereby avoiding the appearance of difficult-to-melt steel “iceberg”.
The sources of social scrap steel are complex, with large differences in composition, size, shape, and unstable quality, which requires integrated work of scrap steel recycling, processing, and classification.
Most steel companies adopt manual sorting methods, including the use of handheld XRF analyzers to classify scrap steel according to its composition to make full use of the alloy elements in it;
Laser induced breakdown spectroscopy (LIBS) is used instead of manual non-contact online classification of scrap steel. The analysis speed can reach 25 samples per second, and the accuracy of the processing results is greater than 88%, showing good application prospects.
Emerging work is to classify scrap steel based on image recognition technology. By extracting features such as color and shape of scrap steel, scrap steel is classified, and further algorithm development is carried out to achieve the distinction between rusted scrap steel and rust-free scrap steel.
The technological progress of deep learning and AI big data models will help to further improve the accuracy of scrap steel classification, among which algorithm development is the core of this process.
For a long time, due to the limitations of scrap steel raw materials and the arc furnace steelmaking process itself, it is difficult for all-scrap steel electric arc furnaces to produce high-quality steel products, which is not conducive to the low-carbon development of the steel industry.
Baosteel Co., Ltd. Central Research Institute has conducted a series of studies on this. Through scrap steel management, clean smelting, composition adaptability adjustment, and dynamic manufacturing process implementation, harmful elements such as N, P, and S are efficiently controlled. The arc furnace is used for all-scrap steel smelting, and the production of automotive sheet products such as CR420, DP980 and QP980 is successfully achieved (Figure 5). The product composition, performance and surface quality fully meet the product design requirements (the composition of the arc furnace smelting DP980 (JX9213A6) product is shown in Table 1, and the performance comparison with conventional products is shown in Table 2), and the carbon emission ratio is reduced by 60% compared with the conventional process.
At the same time, Baosteel Steel Pipe and Bar Division vigorously explored the “green electricity + all scrap steel” smelting production process and successfully developed ultra-low carbon emission (BeyondECO®-60%) gas cylinder steel products with carbon emissions reduced by more than 60%. The product has excellent performance indicators such as steel purity, inclusions, yield strength, low temperature toughness, fatigue life, etc., and has passed strict type tests, fully meeting the use requirements of high-quality gas cylinder steel. In the future, Baosteel will accelerate the comprehensive evaluation of related products and carry out product evaluation and application demonstration work with key users.
Biomass
Biomass, as a renewable carbon source, has the characteristics of carbon neutrality and is regarded as an effective alternative raw material for fossil carbon.
It is reported that using biomass carbon source to replace fossil carbon source in electric arc furnace will reduce carbon emissions by up to about 29%. Studies have confirmed the feasibility of waste tires as carbon source and energy in electric arc furnaces. However, it should be noted that biomass other than waste rubber often has the disadvantages of high water content, low energy density and poor grindability. The more common practice is to convert biomass into biochar through pyrolysis process and then use it in electric arc furnace.
At present, biochar is mainly used as carburizing agent and foaming agent in electric arc furnace, with a series of functions such as molten pool carbonization, scrap steel fluxing, and slag foaming. Although laboratory-scale work has proved that it is feasible to use biochar to partially or completely replace fossil carbon used in electric arc furnace steelmaking.
However, experiments on the industrial scale have found that biomass and biochar will burn directly after entering the furnace, and it is difficult to effectively penetrate into the slag layer, thus affecting its metallurgical effect. The researchers proposed two solutions: injecting directly into the slag and using an oxygen lance and a powder spray gun to inject into the molten pool (Figure 6), and successfully applied them at the Deutsche Edels‐tahlwerke GmbH (DEW) and Marienhütte Gmbh (MH) plants, proving that the use of biochar powder instead of coal has no negative impact on the composition of slag and molten steel. Some studies have pointed out that the use of biochar will produce more furnace gas and increase the calorific value of furnace gas.
In addition, a circulation system for the co-production of biomass in electric arc furnaces was proposed. By adding biomass to the electric arc furnace and generating a large amount of waste heat to pyrolyze the biomass or produce green electricity, it will effectively promote the low-carbonization process of the steel industry.
Reduce energy and material consumption
Reducing the energy and material consumption of smelting is an important part of achieving low-carbon smelting in electric arc furnaces. In the carbon emission calculation method provided by the World Steel Association, the carbon emission factors of raw materials and energy in the upstream production process are displayed, which confirms the practical significance of reducing energy and material consumption for achieving low-carbon production in electric arc furnaces.
Energy consumption
The energy input for electric arc furnace production includes primary energy (input in the form of fuel) and secondary energy (mainly electricity). At the same time, some of the energy lost in by-products will be recovered through residual energy recovery technology. Summarizing the effect of electric arc furnace technology progress on reducing energy consumption in the steel industry (Figure 7), optimizing operation management is considered the most favorable energy-saving means.
On the other hand, although modern electric arc furnaces widely use primary energy (oxygen burners in Figure 7) to strengthen smelting, inputting fuel of equal calorific value into the electric arc furnace will be more conducive to reducing energy consumption than using electricity generated by thermal power generation. However, because the fuel is burned in the furnace during actual production, the utilization efficiency is poor, resulting in its energy efficiency often being lower than the ideal value. The addition of carbon powder and natural gas has a very weak effect on reducing the power consumption of electric arc furnaces.
According to reports, the heat dissipation of furnace gas, slag and cooling water in a typical electric arc furnace reaches 20.7%, 7.6% and 5.5% of the total energy intensity respectively. Therefore, the use of waste energy recovery technology is considered an important means to reduce energy consumption. The most widely used technology at present is the waste heat recovery technology of furnace gas.
Studies have shown that recovering furnace gas waste heat by preheating scrap steel is a very effective method.
The waste heat recovery efficiency of this method can reach 53.57%. At the same time, this method is also conducive to shortening the smelting cycle, but it will bring governance difficulties in horizontal continuous charging electric arc furnaces.
In the study, the efficiency of the efficient furnace gas waste heat steam recovery scheme based on stoichiometric combustion is 43%, while under the current general process conditions, the efficiency is only 26%~28%.
Electric arc furnace slag also contains rich sensible heat. The remaining energy recovery technology is mostly achieved by atomizing liquid slag droplets to increase the surface area and reduce the particle size, and the efficiency can reach 40%. However, the relevant technology has not yet been promoted in industry. In comparison, the temperature of arc furnace cooling water is only 45~65℃, which is low-grade waste heat. Existing reports have used it for indoor heating.
Electrode consumption
The production cost of electric arc furnace graphite electrodes is high, and the carbon emission and energy consumption are concentrated. When working, they are subjected to high temperature, oxidation and mechanical effects, which cause chemical and physical losses.
It is reported that the physical loss caused by electromagnetic force, thermal stress and mechanical impact can reach 53% of the electrode loss, and the chemical loss caused by high-temperature flue gas, slag and molten steel oxidation can reach 47% of the electrode loss.
The work of reducing electrode consumption needs to focus on the optimization of the production process of electric arc furnaces, by realizing long arc operation, reducing smelting time, reducing power consumption, uniform molten pool temperature field, optimizing charging system and electrode adjustment system; it also needs to start from the characteristics of graphite electrodes themselves and promote the application of water-cooled composite electrodes, spray electrodes, surface anti-oxidation coatings, impregnated electrodes and other technologies.
Water-cooled composite electrodes and spray electrode technologies reduce the surface temperature of electrodes and their oxidation losses through physical means, which can reduce electrode losses by 10%~40%. The surface anti-oxidation coating (Figure 8) and impregnation electrode technology can improve the thermal vibration resistance, high-temperature oxidation resistance, tip wear resistance and fracture resistance of graphite materials by improving the physical and chemical properties of the electrode itself, thereby reducing electrode loss by 10%~20%.
Consumption of steel materials
During the arc furnace smelting process, iron is lost in the form of dust and slag. The high temperature zone generated by the arc or oxygen blowing will cause the iron element to evaporate into the gas phase, forming dust that escapes with the high-temperature flue gas; the arc furnace end point molten pool is severely overoxidized, and the TFe mass fraction in the slag is often above 20%.
Methods to reduce dust generation include using foam slag to cover the arc and lower the temperature of the fire point zone. It has been reported that by doping CO2 in the O2 jet, the latter reacts with the molten pool elements to absorb heat and lower the temperature of the fire point zone.
On the other hand, the recovery of arc furnace dust has received attention, especially the extraction of valuable elements such as Zn and Cr. The serious overoxidation of the arc furnace end point molten pool is caused by poor molten pool stirring effect. By optimizing the molten pool stirring effect, the TFe mass fraction in the slag is reduced by 3.63%.
In addition, the valuable elements in the slag can also be recovered by using a reducing agent at the end of the smelting (such as adding ferrosilicon at the end of stainless steel smelting, which can reduce the TFe mass fraction in the slag to less than 3%).
Refractory Consumption
In modern electric arc furnaces, high-basicity slag steelmaking uses mostly alkaline refractories. Magnesium-carbon bricks with excellent thermal shock resistance, oxidation resistance and erosion resistance are generally used for construction in parts such as furnace walls that are in direct contact with the molten pool.
Reducing the consumption level of refractory materials depends on the optimization of the performance of the refractory materials themselves. A series of studies investigated the role of different types of antioxidants in extending the life of refractory materials. The oxidation products of antioxidants were found to effectively block the pores produced by carbon oxidation to reduce the mass transfer rate of oxygen in refractory materials.
On the other hand, additional additives or treatment processes, such as nano-MgAl2O4 and high-temperature nitriding processes, have been proven to effectively optimize the performance of refractory materials.
In the production process of electric arc furnace, by adding excessive lime and dolomite into the furnace, high basicity slag with a certain MgO content is formed to alleviate the chemical erosion of slag on refractory materials. However, for electric arc furnaces using DRI as raw materials, this operation directly leads to a sharp increase in slag volume, increasing energy and material consumption, and excessive basicity (the ratio of the mass fraction of CaO to SiO2) is not conducive to slag foaming and produces more adverse effects. A series of works have demonstrated the practical significance of controlling slag basicity at 1.2~2.0 (Figure 9).
The refractory materials in the bottom blowing area are eroded by the bottom blowing gas and highly oxidizing molten steel. It is reported that by optimizing the preparation and process of bottom blowing elements, the life of the bottom blowing elements can be synchronized with the furnace age, achieving a furnace age of more than 700 furnaces.
