Electric Arc Furnace Energy Enhancement
Intensification of the energy supply of electric arc furnaces means increasing the energy input and energy efficiency. According to reports, the energy efficiency of electric arc furnaces in different smelting periods varies greatly. In addition, modern electric arc furnaces attach importance to chemical energy input. With enhanced smelting, the proportion of chemical energy in total energy input continues to increase, reaching up to 60%. In contrast, electrical energy accounts for 40% to 65% of the total energy input.
When total energy input is limited by equipment and cost, improving energy efficiency becomes the core means to enhance energy supply. However, different types of energy have different properties and their energy efficiency needs to be improved through different process means.
Power Enhancement
Electric arc furnaces convert electrical energy into heat energy through an arc with nonlinear resistance characteristics.
When the electric arc furnace is mainly filled with solid charge, the arc is unstable. In order to avoid arc breaking and burning of the water-cooled plate, “short arc” operation is required. The energy is absorbed by the charge in the form of radiation, and the energy efficiency is low.
When a molten pool is formed in the electric arc furnace, “long arc” operation is adopted under the condition of foamy slag submerged arc operation. At this time, the arc burns stably in the foamy slag, and the energy is directly input into the molten pool, with high energy efficiency.
The means of strengthening power supply are: increasing power supply intensity, quickly forming a molten pool, “long arc” operation and using foam slag to cover the arc.
The efforts to increase power supply intensity have benefited from the progress in the manufacturing level of power supply equipment such as transformers, electrodes and short nets. Ultra-high power supply of electric arc furnaces (≥700kW/t) has been achieved, which not only increases power supply intensity, but also helps to improve energy efficiency.
Steel retention operation and continuous charging are conducive to the rapid formation of a molten pool. The established dynamic model of the electric arc furnace well demonstrates the positive significance of steel retention operation for charge melting and molten pool heating. It is found that the smelting cycle of steel retention operation will be shortened by 5 min;
The continuous charging electric arc furnace has the characteristics of “flat molten pool” and charge preheating, which is conducive to the rapid melting of charge and stable combustion of arc;
It is reported that the smelting cycle of the Quantum electric arc furnace with a large steel retention rate and continuous charging in the shaft is only 33 minutes, while the smelting cycle of the widely promoted Consteel electric arc furnace can be reduced to less than 40 minutes.
On the other hand, the “long arc” operation does not rely solely on the “flat molten pool” to be realized. Its basis is to use foamy slag to cover the arc. Only when the foamy slag is covered can the “long arc” operation be used to achieve the circuit characteristics of high voltage, high power factor and high thermal efficiency.
Chemical energy input
The chemical energy input in the electric arc furnace includes the chemical energy released by the reaction of elements in the molten pool, and the chemical energy released by the reaction in the gas phase, that is, the chemical energy released by burner input or secondary combustion.
According to research, in the early stage of electric furnace smelting, the chemical energy released by the burner combustion can be absorbed by the charge through radiation or heat transfer, and its energy utilization efficiency can reach up to 84.02%, and will decrease to 77. 96% within 10 minutes. However, after the molten pool is formed, only a small part of the energy is transferred to the slag surface through radiation, and its energy utilization efficiency will decrease rapidly, which has little impact on the energy status in the furnace.
Carbon-oxygen reaction is a basic link in the steelmaking process. Electric arc furnaces use scrap steel with low carbon content as the main raw material. It is necessary to add carbon to the charge or use a spray gun to transport carbon powder, but a large number of production examples have confirmed that it is difficult to achieve effective carbon increase in the molten pool.
An effective task is to optimize the spray gun to improve its utilization rate. On the other hand, attention should be paid to the matching of carbon injection and oxygen supply. The typical furnace of GMH Steel Plant shows the imbalance of O/C control and the carbon increase and oxidation phenomenon caused by it (Figure 10)
It is worth noting that the energy efficiency of chemical energy can be maintained at a relatively high level, reaching 70%~80%, and can solve problems such as low heating efficiency of electrode point heat sources, limited electrical energy input, and uneven molten pool temperature.
The main problem faced by chemical energy input is the low efficiency of fuel input. In the future, we should focus on how to optimize process equipment in electric arc furnaces and improve input efficiency.
According to Robert’s measurement, the chemical energy contained in the furnace gas of the electric arc furnace (in the form of combustible gases such as CO and H2) can reach 15.0% of the total energy input. The use of secondary combustion technology to release the chemical energy in the furnace gas is conducive to improving production efficiency.
However, it is necessary to ensure the secondary combustion of the furnace gas in the foam slag to obtain an energy utilization efficiency of more than 40%, and try to avoid secondary combustion in free space (energy utilization efficiency is about 10%, and it damages the life of the water-cooled plate). On the other hand, the continuous charging electric arc furnace obtains high-temperature flue gas and preheats the scrap steel by inhaling air to burn the furnace gas.
Physical heat input
Through the main raw materials of electric arc furnace smelting, inputting physical heat into the molten pool is a very effective means of energy supply, with an energy efficiency close to 100%. It can ensure the rapid melting of the charge and the rapid formation of the molten pool, thereby realizing efficient smelting in the electric arc furnace.
Hot charging of hot metal, hot charging of direct reduced iron and preheating of scrap steel are typical processes for increasing physical heat input. It is reported that every increase of 100°C in preheating temperature of scrap steel will reduce power consumption by 10.0~12.5 kW·h/t.
For an electric arc furnace with a charge structure of 100% DRI, the use of hot-charged DRI at 600 °C can reduce power consumption by about 125 kW·h/t; on the other hand, the effect of hot-charging molten iron is quite obvious, and every 1% increase in molten iron ratio will reduce power consumption by 3.5~4.5 kW·h/t.
For molten iron and direct reduced iron, their hot raw materials can be directly obtained through the front-end process, and hot charging can be achieved by only slightly modifying the electric arc furnace equipment; while scrap steel is mainly heated by heat transfer from furnace gas, which means that scrap steel preheating equipment is required. Currently, there are mainly two types: vertical shaft type and horizontal type. The scrap steel preheating effect of the former is significantly better than that of the latter, but there is a “sticky finger” problem;
On the other hand, the electric arc furnace will enhance the preheating effect of scrap steel by configuring high-power burners in the scrap steel preheating section.
Intensification of arc furnace bath reaction
Modern electric arc furnaces inevitably use scrap steel from society with complex sources, resulting in a large amount of residual elements and harmful elements in the molten steel of the electric arc furnace. At present, steel companies still lack effective control methods for residual elements (such as Cu, Sn, etc.) during the production process. The common means are to control the raw materials entering the furnace, such as the classification of scrap steel, and adding clean raw materials such as pig iron, molten iron, DRI, etc. into the furnace for dilution.
On the other hand, controlling harmful elements such as P, S, N, and H requires a series of molten pool reactions, which will reduce the smelting rhythm of the electric arc furnace process and affect product quality. Among them, dephosphorization and desulfurization reactions depend on the slag-metal reaction in the molten pool, which requires improving the reactivity and foaming of the slag, promoting the positive reaction of the slag-metal reaction, and increasing the contact area between the slag and the metal.
Efficient decarburization of the molten pool
In the arc furnace molten pool, the carbon-oxygen reaction is the most important and basic metallurgical reaction, which releases a large amount of chemical energy and produces diffuse CO bubbles, which helps to strengthen energy supply and molten pool stirring.
Using raw materials such as pig iron, molten iron, and DRI in the arc furnace can more effectively input carbon elements into the molten pool (compared with the carbon addition or carbon injection methods mentioned above). According to the report of the Anmi Mexico plant, its efficiency is as high as 95%, but there are two sides. Figure 11 shows the impact of molten iron addition on the production indicators of the arc furnace calculated by Cotchen. It can be found that when the carbon content in the molten pool is too high, the decarbonization reaction will lead to an extension of the smelting cycle. This conclusion also applies to the arc furnace of hot-charged direct reduced iron.
On the other hand, the progress of the decarburization reaction is closely related to the oxygen content in the molten pool. The carbon-oxygen product of the final molten steel in the electric arc furnace is about 30. In contrast, the carbon-oxygen product in the converter can be as low as 13.4. This is the result of the relatively insufficient stirring effect of the electric arc furnace.
An interesting technical solution is to introduce CO2 into the electric arc furnace to enhance the decarburization reaction by utilizing its physical and chemical properties and bubble proliferation characteristics.
The CO2 bubble nitrogen absorption reaction rate constant is 9.6 times that of Ar. Using CO2 instead of Ar as the bottom blowing gas of the electric arc furnace can reduce the nitrogen content of the steel liquid by 16.9%, and has the characteristics of reducing the carbon and oxygen accumulation of the terminal steel liquid, strengthening the stirring of the molten pool, and protecting the bottom blowing refractory materials.
This work is regarded as an important link in the resource utilization of CO2 in the steel industry, and is also conducive to achieving efficient steelmaking.
Efficient dephosphorization in the molten pool
In the steel manufacturing process, the secondary refining process lacks effective control means for phosphorus, so it is necessary to achieve efficient dephosphorization reaction in the electric arc furnace.
Related research focuses on the use of thermodynamic analysis of dephosphorization reaction. The empirical model based on temperature and slag composition has been widely developed and applied. Some work pays attention to the kinetic analysis of dephosphorization reaction. However, it should be pointed out that since the mass transfer coefficient of phosphorus in molten steel (4.7×10-9 m2/s, 1600℃) is much higher than that of carbon (2.0×10-9 m2/s, 1600℃), it is generally believed that the dephosphorization reaction is more limited by thermodynamics.
The arc furnace molten pool heats up slowly, and slag flow is performed during the smelting process, which can effectively avoid the “rephosphorization” phenomenon caused by the molten pool heating.
The current research on strengthening the dephosphorization reaction in the electric arc furnace particularly emphasizes the influence of temperature, basicity and slag composition.
After studying the influence of the solid phase content of (Fe, Mg)O and C2S on the dephosphorization effect of the slag, it was found that an appropriate increase in the solid phase content in the slag will help increase the dephosphorization efficiency; at the same time, the high phosphorus content molten pool and low basicity slag in the electric arc furnace using DRI as raw material will reduce the dephosphorization reaction efficiency [Figure 12, in which lg CPO3 4 – is the phosphorus capacity and the horizontal axis lg (XBO/XAO) is the basicity] [62]. The study proposed that the slag reactivity can be improved by introducing aluminum industry solid waste (red mud and white mud).
The embedded powder spraying technology is a new technology developed to achieve efficient dephosphorization in electric arc furnaces (Figure 13). This technology uses a CO2-O2 mixed gas to input CaO powder into the molten pool through a embedded spray gun set at the slag-steel interface, directly generating composite calcium ferrite molten slag particles. Under high temperature and high oxygen conditions, the “bubble-powder-molten steel-slag” multi-phase multi-phase coupling reaction is used to directly and rapidly perform deep dephosphorization. This technology can also be used to achieve efficient transportation of carbon powder.
Efficient desulfurization in the molten pool
Unlike decarburization and dephosphorization, the steel manufacturing process generally desulfurizes in a reducing environment, including: hot metal pretreatment process and secondary refining process (mainly LF furnace).
However, in the electric arc furnace process, especially the process using DRI as the main raw material, since there is no hot metal pretreatment process and some high-grade steel production does not go through the LF furnace with good desulfurization ability, this requires the electric arc furnace to have a certain sulfur control ability.
Related research points out that in an oxidizing atmosphere, the sulfur in the molten pool can be oxidized by oxygen in the gas phase, causing the sulfur to escape in the form of gas SO2 [Formula (1)] or generate sulfate [Formula (2)].
［S］+2［O］=SO2（1）
［S］+3（O）+（CaO）=（CaSO4）（2）
BASU S conducted laboratory tests on oxidative desulfurization slag and BOF industrial tests and found that oxidative desulfurization has a certain desulfurization capacity. For high-basicity and high-oxidizing slag, its sulfur distribution ratio can reach more than 2.
Due to the complex raw materials entering the furnace, unstable endpoint control, variable dynamic conditions and slag flow operation in the electric arc furnace, the desulfurization rate of the electric arc furnace in previous studies varies greatly within 10%~50%.
Unfortunately, there is a lack of research on arc furnace desulfurization that comprehensively considers the above factors. At the same time, due to the low mass transfer rate of sulfur, the influence of kinetic conditions on it must be considered. Through research, kinetic parameters such as the mass transfer coefficient of sulfur in steel, slag and droplets at different smelting stages are given, and it is pointed out that a good three-phase stirring effect of slag, metal and gas must be maintained to achieve the best desulfurization effect.
Intensive stirring of the melt pool
The electric arc furnace has a shallow molten pool, poor stirring effect, and insufficient dynamic conditions, which leads to slow material energy transfer and metal melt flow in the furnace, inhibiting the rapid progress of metallurgical reactions, resulting in serious overoxidation of the final molten steel, high content of harmful elements, and uneven composition temperature.
In addition to strengthening the carbon-oxygen reaction in the molten pool, the stirring effect of the molten pool can also be enhanced by introducing an external driving force.
Figure 14 shows the source of the driving force for stirring the arc furnace molten pool. The modern arc furnace molten pool is stirred by oxygen supply, decarburization reaction, electromagnetic force (EMforces), bottom blowing stirring (BGI) and electromagnetic stirring (EMS). The effects are shown in Table 3 (Umax and Uˉ in the table are the maximum and average speeds of molten steel flowing in the molten pool, respectively).
Metallurgical workers have formed a composite blowing technology by combining enhanced oxygen supply, controlled O/C input and bottom blowing stirring, achieving good metallurgical effects.
On the other hand, due to equipment limitations, EMS and BGI are difficult to apply simultaneously. It is reported that the former has a stronger stirring effect than the latter, and can eliminate slag eyes and prevent the molten steel from being exposed to the atmosphere, resulting in nitrogen increase in the molten pool.
However, it should be noted that the former has the problem of higher cost and lacks extensive practical application data, while the latter can introduce gases such as CO2, H2, and CH4 to achieve functions such as enhanced stirring, reduced carbon emissions, and reduced molten pool peroxidation.
