The most feasible decarbonization path for the steel industry is currently considered to be green electricity-green hydrogen DRI-electric arc furnace (EAF). Many European steel companies are conducting pilot or industrial trials. However, the high cost of hydrogen and green electricity production, coupled with the lack of carbon tax implications, has hindered the rapid implementation of this short-process approach. Although there is widespread advocacy, it has not yet become a mainstream decarbonization route.

In the past decade or so, investment by private enterprises in EAF steelmaking has led to significant advancements in this technology, completely overturning the previous perception of it as noisy and slow, and essentially achieving the transformation of EAF into a converter-based process.

The main technologies and equipment used in modern electric furnaces in China are as follows:

1. Use of ultra-high power transformers

Modern electric furnaces have generally increased the power of their transformers. The transformer capacity for a 75-ton electric furnace is generally above 60MVA, and now the largest selected is 80MVA, which increases the power transmission capacity and shortens the power transmission time. The transformer capacity for a 120-ton electric arc furnace is 120MVA.

2. EBT eccentric bottom tapping method

In traditional electric arc furnace (EAF) steelmaking, the “old three phases” process is divided into melting, oxidation, reduction, and slag-bearing stages, during which the slag is reducing. However, with the functional differentiation of EAFs, ultra-high-power EAFs are now used in conjunction with ladle refining, resulting in oxidizing slag at tapping. Introducing oxidizing slag into the ladle refining process would have extremely adverse effects. Therefore, to prevent oxidizing slag from entering the ladle refining process, the EBT (Extended Electric Blast Furnace) eccentric bottom tapping process was developed.

The EBT EAF structure replaces the tapping trough of the old three-phase EAF with a tapping box, with the tapping spout vertically downwards at the bottom of the box. A tapping spout opening and closing mechanism is designed at the bottom of the box, and an operating port is located at the top center of the box for easy operation and maintenance of the tapping spout.

The main advantages of the EBT EAF are that it achieves slag-free tapping and increases the usable area of the water-cooled furnace walls. The advantages are as follows:

(1) Reducing the tapping tilt angle simplifies the electric arc furnace tilting structure, lowers the resistance to the short-circuit grid, increases the usable area of the water-cooled furnace wall, and improves furnace lifespan.

(2) Retaining steel and slag operation: Slag-free tapping improves steel quality and facilitates refining operations; retaining steel and slag allows for continued smelting in the electric arc furnace and saves energy.

(3) Tapping from the bottom of the furnace lowers the tapping temperature, saves electricity, reduces secondary oxidation, improves steel quality, and increases ladle lifespan.

Due to its numerous advantages, the EBT electric arc furnace has rapidly gained popularity worldwide. Currently, newly constructed electric arc furnaces, especially those combined with ladle refining, all require slag-free steelmaking and adopt the EBT process.

3. Supersonic oxygen lances and carbon lances are widely used; in particular, cluster oxygen lances have been added, and the furnace door oxygen lances have been eliminated.

Modern electric arc furnaces typically consume more than 35 Nm³/t of oxygen per ton of steel (with a certain plant in Tangshan achieving a peak oxygen consumption of 36 Nm³/t). High-flow-rate oxygen is supplied by oxygen lances at the furnace door or on the furnace wall, with a single lance flow rate exceeding 4500 Nm³/h. For example, Danieli’s telescopic oxygen lance and Constance’s first use of four clustered jet oxygen lances are examples of this. This is the fundamental reason for the high efficiency of current electric arc furnace smelting, also known as the “converterization of electric arc furnaces.”

Working principle and function of oxygen blowing through the furnace door and oxygen lance:

(1) Supersonic oxygen blown in through the oxygen lance at the furnace door cuts large pieces of scrap steel;

(2) After a molten pool is formed in the electric arc furnace, oxygen is blown into the molten pool. The oxygen reacts with the elements in the molten steel to produce an oxidation reaction, releasing heat of reaction and promoting the melting of scrap steel;

(3) Through the stirring effect of oxygen, heat transfer between molten steel is accelerated, thus increasing the melting rate of scrap steel in the furnace and reducing the unevenness of steel temperature;

(4) A large amount of oxygen reacts with carbon in the molten steel to achieve rapid decarburization. The carbon-oxygen reaction releases a large amount of heat, which is beneficial for the molten steel to reach the target temperature;

(5) While blowing oxygen into the slag, a certain amount of carbon powder is injected. The reaction in the furnace produces a large amount of gas, making the slag foamy, i.e., producing foamy slag.

The principle of the clustered jet oxygen lance is to add natural gas burners around the Laval nozzle, so that the oxygen jet of the Laval nozzle is surrounded by a high-temperature, low-density medium, reducing the influence of various airflows in the electric furnace on the central oxygen jet, thereby slowing down the decay of the oxygen jet velocity and maintaining the initial diameter and velocity of the oxygen jet over a longer distance, enabling it to provide a supersonic polymerization jet to the molten pool over a longer distance.

4.the production of foamed slag and strict grading of scrap steel greatly improve the efficiency of electric arc furnaces

To create foamy slag, the same number of carbon lances as the clustered oxygen lances are typically installed. The oxygen and carbon lances together form a carbon-oxygen injection module system. Modular technology combined with PLC metering control of the injection powder quantity and multi-point carbon injection within the furnace is utilized. The carbon and oxygen injected into the molten pool react in the slag to generate a large amount of CO, forming a thick foamy slag. This buries the electric arc under the molten slag, reducing arc radiation heat release and harsh noise, while also benefiting the long-term use of the furnace wall refractory materials. Good foamy slag allows for rapid heating of the molten steel, saving energy.

Foamy slag increases the slag-steel contact interface, accelerating oxygen transfer and the physicochemical reactions between slag and steel, significantly shortening the smelting time for a single heat. Currently, the smelting cycle for producing ordinary carbon steel or rebar has been reduced to around 30 minutes. In electric arc furnaces, the foamy slag thickness is generally required to be at least 25 times the arc column length. Long-arc operation can be used to stabilize the arc and shield it, reducing the heat radiation of the arc light to the furnace lining. Long-arc foamed slag operation can increase the input power of the electric furnace, improving the power factor and thermal efficiency. It can reduce power consumption, shorten smelting time, increase productivity, and reduce refractory material consumption.

Good foamed slag (more than 3 times the arc length) is crucial for the reaction FeO + C = Fe + CO. Appropriate viscosity control of the oxide slag is essential; suitable viscosity ensures good fluidity and metallurgical properties. Excessive viscosity slows the transfer of substances between molten steel and slag, hindering rapid steelmaking reactions; insufficient viscosity exacerbates furnace lining erosion.

5.Horizontal feeding scrap preheating technology

In the preheating section, the hot flue gas from the electric furnace heats the scrap layer. The conveyor belt is equipped with a cover plate capable of withstanding high temperatures. Under flat molten pool conditions, the oxygen injection rate is lower, thus generating less iron oxide waste gas (at least 20% less compared to batch feeding).

The space between the scrap surface and the protective cover is used as a secondary combustion chamber to ensure complete secondary combustion of carbon monoxide. The preheating temperature of the scrap before entering the furnace can reach up to 500°C, which is the design temperature. By controlling the preheating effect of the scrap, it avoids the surface melting and over-oxidation of some scrap before reaching the molten pool of the electric furnace due to excessively high preheating temperatures, as is seen in vertical shaft furnaces, thus eliminating the impact of high-temperature scrap oxidation on the recovery rate.

6.Strict and meticulous scrap steel sorting helps electric arc furnace smelting save electricity and improve efficiency

Modern electric arc furnaces emphasize the classified management of scrap steel. In terms of design, the proportion of the building area occupied by the scrap steel area is increasing to allow for the selective use of scrap steel based on density, size, and trace elements, achieving scrap steel batching and a single-basket feeding system.

Various types of scrap steel recycled from the community, including heavy, medium, and small scrap steel, should be stored separately and managed centrally. The bulk density of scrap steel used in electric arc furnace smelting should be greater than 0.7 t/m³, with a maximum length not exceeding 1.2 m, a maximum cross-section less than 500 mm × 500 mm, and a maximum single weight not exceeding 500 kg. For ultra-high power electric arc furnaces with ECS horizontal and continuous feeding, the maximum size of scrap steel is 1.0 x 0.5 x 0.5 m, the maximum cross-sectional thickness is 0.1 m, and the maximum single weight of any scrap steel is 1 ton. Scrap steel exceeding 0.5 tons cannot exceed 5% of the total weight.

7.Flat-bore smelting process

With horizontal charging, a large amount of steel is retained or hot iron is charged, typically 40-50% of the steel can be retained to form a molten pool in advance. This allows for balanced arc heating and sustained maximum power heating. Simultaneously, the physical heat carried by the retained molten steel or iron accelerates the melting process and improves oxygen utilization.

8.Production Practices

Currently, China can domestically produce most of the major equipment for electric furnaces with a capacity of 130 tons or more, except for a very few pieces of equipment (electrode regulators and hydraulic control systems).

9.Summary of Modern Electric Furnace Development and Carbon Reduction Efforts

The rapid development of modern electric arc furnaces can be attributed to the following factors:

Increased production efficiency: Simplified steelmaking processes, with melting and oxidation occurring only within the electric furnace; charging without opening the furnace lid and tapping while powered on shortens the smelting cycle and reduces electricity consumption; the use of high oxygen levels transforms the electric furnace into a converter, utilizing chemical heat to increase heating speed and save electricity.
Cleaner products: The quality of electric arc furnace steel is often the most significant reason for customer complaints. During my time working in Guangdong, some state-owned enterprises even expressed reluctance to use rebar produced by electric furnaces, primarily due to concerns about steel cleanliness. Modern electric arc furnaces have implemented a series of technical measures to ensure steel cleanliness, thus meeting the requirements of different steel grades. These include furnace bottom stirring technology; the use of clean scrap and direct heat treatment (DR); and endpoint control and slag-free tapping technology (EBT).
Environmental protection and waste disposal: Modern electric furnaces use a large amount of steel left to reduce the impact on the power grid and reduce noise; the use of horizontal feeding scrap steel preheating technology can achieve near-zero emissions and ultra-low emissions; and waste flue gas is recycled for steam.
High level of intelligence and automation: By adopting the above technologies and supplementing them with the classified management of scrap steel, the adoption of intelligent and automated electric furnaces can be realized.

10.Problems in the Development of Electric Furnaces

Scrap Steel Prices: In all-scrap steel smelting processes, the cost of scrap steel accounts for approximately 80% of the cost per ton of steel. Therefore, controlling scrap steel costs is crucial for production cost control. Horizontal heating systems allow for the addition of various types of metal materials to the electric furnace, making it possible to produce using the cheapest scrap steel. Relevant departments have proposed increasing the proportion of electric furnace steel production to over 15% of total crude steel production by 2025, striving for 20%. However, due to market factors and, more importantly, the impact of scrap steel prices, this target is difficult to achieve.
Due to the enrichment of trace elements in scrap steel, dilution with DR (diluted scrap) or molten iron is necessary. Employing refined scrap steel processing and sorting, pre-furnace sorting and storage, and precise scrap steel batching can reduce the enrichment of these harmful elements through management. For high-standard specialty steels, DR can be used for further dilution.
The professional qualifications of practitioners need improvement. Many private enterprises’ electric arc furnace steelmaking has evolved from the original medium frequency furnaces. However, the development of electric arc furnaces in China is disproportionate. Therefore, the understanding of the overall process and equipment among practitioners needs to be improved. Automation and intelligence are mostly achieved through modeling led by process engineers and continuous optimization. Therefore, improving the overall quality of all practitioners will be the key to future development.