Defossilization of Integrated Plants — Benefits and Challenges of Different EAF Designs
The global steel industry is intensifying its efforts to move away from fossil fuels in order to reduce its specific CO2 footprint. Utilizing electricity to melt and blend metallic ferroalloys such as scrap steel, pig iron, direct reduced iron (DRI), and hot-pressed iron (HBI) is beneficial for reducing the CO2 footprint, but it also presents new challenges. Over the past few decades, various electric arc furnace (EAF) designs and process variations have been validated and have contributed to the development of future EAFs with higher DRI and HBI inputs based on steel quality requirements. This article describes and discusses the technology options and process realities of integrated steel mills.
In response to climate change, many regions around the world have chosen to first limit CO2 emissions into the atmosphere, or even demand zero CO2 emissions. Since the steel industry is a significant part of this, strategies for gradually reducing CO2 emissions have been developed to align with local government frameworks and societal interests.
Figure 1 shows three main strategies available to each steel manufacturer. First and foremost is process optimization (1), which is a process that requires direct action and some capital, but not large investments. It directly reduces CO2 emissions per metric ton of steel by increasing output and/or optimizing material utilization. Therefore, external experts can help determine the fastest path to the optimal production strategy. The impact of this strategy is very effective, but also limited. In fact, scrap-based electric arc furnace (EAF) operations can reduce CO2 emissions to below 100 kg per metric ton of steel by reducing power-on time (PON), power consumption per ton of steel (kWh/t), or optimizing the use of different carbon addition methods (carbon blocks in baskets, carbon injection, etc.). To go beyond the limitations of process optimization, investment is first required to adapt to existing technologies (2). Further reductions in CO2 emissions can be achieved by replacing outdated systems with more efficient machines and by switching to low-carbon (e.g., natural gas), carbon-free (e.g., hydrogen or ammonia), or non-fossil fuels. This approach typically requires more investment and sometimes even research activities. Finally, carbon direct avoidance (CDA) in its final stage means investment in new technologies (3) and results in significant investments of billions of dollars or euros. While the first and second steps are best suited for electric arc furnace-based steelmakers, integrated steel mills cannot avoid the third step in order to achieve zero emissions.
Three main strategies to reduce carbon dioxide emissions from the steel industry
When an integrated steel plant chooses to undertake the third step, numerous challenges arise, requiring a shift from oxygen-based steelmaking to electric arc furnace (BOF) steelmaking while maintaining the same secondary metallurgical refining processes and equipment. Beyond the extensive project management required, the operation and maintenance of an electric arc furnace (EAF) differ significantly from those of a conventional converter (BOF), as detailed in Reference 1. This impact encompasses a wide range of issues, from metallurgical problems with significant implications for secondary metallurgy (e.g., ferrous feedstock, impurities, and residual elements), to new slag design and recycling, flue gas treatment considerations, new and various safety risks, and maintenance issues that severely affect EAF steelmaking productivity. The production and performance of a 300-ton EAF differ from that of a 300-ton BOF converter, and most of the data discussed are theoretical in nature, as actual industrial data are difficult to obtain.
Once the decision is made to use an electric arc furnace (EAF) for steel production, a suitable furnace type must be selected to meet the requirements of high capacity and low consumption, ultimately achieving low cost and low CO2 emissions. The following sections will introduce several EAF designs to provide a broad understanding of the capabilities and potential of EAF technology. For applications such as processing large quantities of metallic iron pellets from iron ore, some of these options are theoretical exercises, with entire sectors and educated groups speculating on how EAFs can replace high-performance, high-quality production equipment like BOFs. To enhance the value of the discussion, this paper presents actual production data for different types of EAFs to theoretically illustrate the less-than-optimistic prospects of EAF design.
Different types of electric furnaces and their emissions
Electric arc furnaces (EAFs), especially when using direct reduced iron (DRI), are expected to play a significant role in future steelmaking. Various EAF technologies have been developed and are available on the market; three main design schemes are shown in Figure 2. Of course, many more design options have been developed in the past; therefore, this list is not exhaustive.
Most of these are traditional scrap-based EAFs, top-loading via baskets without any additional preheating technology. Of course, there are many different EAF designs in this category that support feeding via one or more baskets and can be AC or DC powered. If the two furnace shells are very close to each other (double-shell EAF), a single set of electrodes can be used to melt the scrap in both shells separately. This design allows for alternating power supply to the two independent EAF furnaces; however, the increased productivity benefits may be offset to some extent by energy losses within the furnace.
Several concepts exist in the market that utilize the energy contained in flue gas. The earliest and most direct preheating method is to directly feed the flue gas into the scrap basket (traditional scrap preheating). This method has several difficulties, such as the thermal deformation of the scrap basket, material flow issues, and the formation of unpleasant odors in and around the electric arc furnace (EAF). Therefore, different technical solutions have been developed. Currently, the most common method is horizontal preheating of scrap, using a slow-in, fast-return conveyor to continuously feed scrap into the EAF from an opening opposite the transformer. After the EAF taps, approximately 40% of the scrap remains, while the high-temperature flue gas flows in the opposite direction to the scrap in the conveyor. Tenova pioneered the Consteel scrap preheating system in the United States, and some original equipment manufacturers (OEMs) offer this technology.
One effective preheating method is vertical shaft furnace preheating of scrap, as the flue gas flows upward through the scrap shaft. Over the years, many variations of the vertical shaft furnace have been built; the latest versions use water-cooled finger systems to ensure preheating of the entire scrap column. These furnaces still use basket batching, just like conventional electric arc furnaces. However, because scrap preheating lowers the flue gas temperature, additional flue gas treatment is required.
Additionally, conventional electric arc furnaces can be operated using DRI, HBI, pig iron, or molten iron from a blast furnace if specific quality requirements need to be met. These electric arc furnaces require continuous charging via the furnace lid (DRI/HBI) or the upper furnace shell sidewall, furnace door, or eccentric bottom tapping (EBT).
Based on actual industrial results, three main electric arc furnace designs were compared by selecting the most developed and mature concepts and installation numbers. Regarding decarburization, the following data were reviewed:
- CO2 emission range 1 and 2
- Power consumption
Annual performance data from steel mills with appropriate steel output (80 tons) and high performance levels (low outages/high uptime) were selected from a global best-practice database (benchmark) for evaluation. For simplicity and comparability, it was assumed that the material composition of all carbon carriers at all steel mills was identical to calculate emissions for Scope 1. Similarly, for Scope 2 calculations based on EAF electricity consumption, the same grid factor was used for all steel mills.
Annual performance data from steel mills with appropriate steel output (80 tons) and high performance levels (low outages/high uptime) were selected from a global best-practice database (benchmark) for evaluation. For simplicity and comparability, it was assumed that the material composition of all carbon carriers at all steel mills was identical to calculate emissions for Scope 1. Similarly, for Scope 2 calculations based on EAF electricity consumption, the same grid factor was used for all steel mills.
Figure 3 shows the CO2 emissions for Scope 1 and Scope 2, and the sum of the three different electric arc furnace (EAF) design technologies already explained: conventional EAF, scrap preheating EAF, and DRI-based pelletizing EAF. For the last group of DRI-based EAFs, only gas-based emission reduction operations are considered. Within each group, steel mills are sorted in ascending order of steel output weight (size). It is clear that EAF size has no impact on CO2 emission levels.
The minimum emission level in Scope 1 averages 72 kg CO2/t in conventional electric arc furnace units, and up to 98 kg CO2/t in electric arc furnaces using pig iron as the ferrous metal. However, there are limitations on further reducing CO2 by approximately 45 kg CO2/t due to a minimum carbon requirement in operation.
The eight scrap preheating electric furnaces achieved an average of 106 kg CO2/t in Range 1. The main reasons for this difference are the increased carbon injection during the flat melting process to maintain slag foaming, and the fuel consumption of flue gas treatment (vertical furnace) – the horizontal preheater section uses activated carbon for treatment.
The DRI carbon input (1.4% C in this example) results in a higher CO2 level (average 112 kg CO2/t). A certain amount of carbon is required in the DRI to reduce FeO. Any excess C can be used for FeO slag reduction and combustion via oxygen injection. The carbon in the DRI is a more efficient carbon source compared to carbon in the basket feed or injected carbon.
CO2 emissions for Scope 2 are calculated using a grid factor of 0.376 kg CO2/kWh; logically, emissions are calculated based on electricity consumption for each category.
Figure 4 shows the power consumption of three different EAF technology groups. In the selection of these EAFs, size did not have a significant impact; more important was the metal charge mixing ratio (scrap/pig iron/DRI/HBI/molten iron). The average power consumption of conventional EAFs in this database was 386 kWh/ton. Compared to conventional EAFs, well-managed and efficiently operated EAFs could achieve an optimal power consumption of 338 kWh/ton. For the two steel mills using 22-27% pig iron, the impact of poor pig iron melting performance was evident. The overall effect of using a preheated scrap EAF was a reduction in power consumption to 373 kWh/t, with higher preheating efficiency in the shaft furnace. The power consumption of the EAF using DRI was 506 kWh/t (average). The main influences on the required power consumption were the amount of DRI in the ferrous metal feedstock and the melting behavior of the DRI, which were closely related to the composition of the iron ore used to produce the DRI and the temperature at which the DRI was added to the EAF. Lower consumption values could only be achieved through hot DRI (hDRI).
Figure 5 shows a summary and overall overview of CO2 emissions for three different EAF technologies. The total emissions are also the lowest, ranking lowest among conventional electric arc furnaces (range 1) and with a generally good average power consumption.
Optimized design of conventional AC electric arc furnace
To achieve optimal performance for conventional AC electric arc furnaces (EAFs), it is essential to ensure that the entire EAF design and control system comply with relevant safety regulations. This requires a comprehensive strategy that combines high productivity with safe operation. To provide further information, some key components of modern, state-of-the-art EAFs are briefly introduced. Today, the range of steel grades produced using EAFs extends from long products such as rebar to special bar quality (SBQ) to flat products such as electrical steel for transformers. The quality requirements of the final steel products dictate the ferrometallic feedstock needed for EAF smelting, significantly influencing EAF design. Rebar producers may focus on balancing different scrap weights and corresponding costs with scrap quality. Slab producers need to prioritize minimizing residual elements, using the highest possible proportion of raw materials from ore (e.g., DRI, HBI, pig iron, etc.) and the lowest possible residual element content. Generally, EAFs and their operation should be designed with as few charge baskets as possible – ideally, only one charge basket per heat. Since single-basket operation is extremely difficult and rare (except, for example, a vertical shaft furnace with an additional basket), modern conventional electric arc furnaces (EAFs) strive to achieve dual-basket charging operation. In this case, the basket and furnace volume need to be designed accordingly based on the type and volume of the metal charge.
Under these conditions, the design of the EAF high-current system (HCS), secondary short network, high-current water-cooled cable, and conductive crossarm will effectively input electrical energy and reduce maintenance issues. A crucial design tool is the finite network method (FNM) simulation of the entire system. FNM simulation optimizes the design to achieve the highest possible electrical input symmetry during EAF operation. One simulation parameter is the current density distribution from the transformer to the electrode tip. The current density distribution is then designed.
Furthermore, since chemical energy accounts for at least 30% of the current input energy of electric arc furnaces, the burner system used is another important component. Oxygen burners on the furnace wall can achieve thermal power levels up to 6 MW. For direct carbon-oxygen reactions in the molten pool, similar to oxygen blowing in a converter, oxygen lances with oxygen supply capacities up to 2200 Nm³/h can and should be selected in addition to burner mode. Depending on the scrap mixture and other factors, various operating profiles can be selected in state-of-the-art burner systems for automated control and power input execution. The inclined arrangement of the oxygen lance burners provides an ideal angle, offering greater flexibility for oxygen blowing in the molten pool reaction and for heating and cutting scrap in burner mode. Furthermore, it helps accommodate the different melting behaviors of scrap with varying densities.
The design of the electrode cover drive frame structure of the electric arc furnace is another special and important aspect, requiring robustness, strength, rapid upward and rotational movement, and the ability to allow the electrodes to be pulled out and rotated independently without the furnace cover. Special attention should be paid to the support bearing system for optimal maintenance. Furthermore, due to its streamlined structure, the current frame design can significantly increase the size of the electric arc furnace. Figure 8 shows the main loads on this frame and bearing system in modern electric arc furnace designs.
For any new EAF, a key recommended technology is spray cooling for the furnace cover and fourth orifice. Pressureless cooling reduces maintenance requirements and is highly beneficial for the safe operating conditions of the components. The overall weight is reduced because a bulky cooling water piping system is eliminated, and the hydraulic system experiences less pressure. Overall, this reduces the time required for operation. Furthermore, spray-cooled furnace covers are lighter than other options, allowing the upper furnace shell system to rotate faster, which reduces power outages (POFFs) during basket charging and reduces heat loss during furnace cover opening, directly impacting kWh/t metrics and production efficiency. However, special attention is needed for cooling water return. To ensure safe and reliable return water to the electric arc furnace top plate, a Venturi pump must be used to apply suction to the furnace cover return water collection ring. The advantages of this system in terms of operation and maintenance, along with the extremely long service life of the installed spray pump (completely maintenance-free), mean that the significant installation work required for the spray-cooled furnace cover return water suction can be quickly recouped.
“Is the future going to be unmanned?” This is something that can be seen in most American steel mills. During electric arc furnace (EAF) operation, the area around the EAF becomes an inaccessible red zone, making automation systems more crucial than ever. Therefore, deploying automation systems is now more important than ever, with the latest recommendations focusing on keeping operators away from the furnace while utilizing automated taphole cleaning and replacement equipment, as well as automated, camera-controlled taphole filling and guide sand equipment, such as the eccentric hearth EBT taphole cleaning robot (THM) and the guide sand filling robot. Another technology that may be included is an automated furnace door system, capable of safely cleaning the furnace door platform and passageway during operation and safely opening and closing the furnace door during EAF operation. Figure 10 shows a schematic diagram of the furnace door and includes an actual photograph from an EAF workshop in Turkey. These automation systems do indeed facilitate operational standardization, thereby reducing operation time and leading to increased productivity, which is necessary for narrowing the productivity gap between EAFs and converters, especially when using cold DRI or even HBI feed.
Finally, the electrode conditioning system must be fully considered. Systems like ELARC offer a highly dynamic, adaptive control concept that ensures optimal arc stability, reduced electrode consumption, and superior energy efficiency, even under challenging melting conditions. Recent tests have even shown that simple adjustments to parameters and current setpoints can not only increase power input (MW) but also reduce flicker.
The ratio of slag volume to CO2
Whether DRI (Distilled Residue) produced from blast furnace pellets can be added to electric arc furnaces or used as the sole source of iron for electric arc furnace steelmaking has been a long-standing concern. It is generally believed that excessively high gangue content and low iron content in DRI from blast furnace pellets lead to excessive slag in the furnace, resulting in low steelmaking efficiency and uneconomical processes. A direct comparison should provide some insights when studying this topic. Assuming that the chemical compositions of both types of pellets are representative, Table 1 (column titled “Raw Materials”) shows that DR pellets have a higher iron content and smaller amounts of all other oxides. Here, gangue refers to other unmentioned oxides. The carbon content of both types of pellets remains constant.
When simulating the direct reduction of these particles at temperatures below their melting point, the following chemical composition of the DRI made from the two types of particles is listed in the “DRI” (Reduced to DRI) column. It can be seen that the initial 2.2% gangue in the DR pellets and the 8.0% gangue in the BF pellets become 3.1% and 11.2% in the DRI, respectively. For simplicity, it is assumed that the metallization rate is 100%, and that iron oxide exists only in the form of hematite.
When designing steelmaking slag, metallurgical experts recommend balancing acidic/ampholy oxides (such as SiO2 and Al2O3) to the required basicity. Which basicity is necessary to achieve which quality target is beyond the scope of this discussion. Table 2 shows the slag metallurgical considerations and their impact on yield and slag ratio. For a 100% DRI pellet electric arc furnace, the total SiO2 and Al2O3 per metric tonne of pellets are 28.9 kg/t feed and 110 kg/t feed, respectively. When the B2 target (CaO/(SiO2+Al2O3)) is set at 2.2, the demand for pure CaO is 63.6 kg/t feed and 242 kg/t feed, respectively. Adding these together yields 92.5 kg slag/feed and 352 kg slag/feed, respectively. Since the inputs of SiO2 and Al2O3 have no metal yield and may result in a 2% yield loss during the process, the final iron yields for the two DRIs are 95% and 87%, respectively. The slag ratios were 97.3 and 404.7 kg slag/t tap, respectively. Nowadays, a good electric arc furnace operation yields approximately 150 kg slag/t tap, which makes the DR pelleting situation look very attractive, while the BF pelleting situation is inefficient – not to mention the increased volume required for the electric arc furnace and the increased amount of slag that needs to be considered for transportation, processing and sales.
Integrated steel producers are considering smelting furnace technology, learned from non-ferrous metal production, to significantly reduce CO2 emissions. The reason for choosing smelting furnaces over electric arc furnaces is the flexibility of using lower-grade iron ore, even fine powder instead of pellets, thus allowing for DRI (distilled iron refining) in fine powder form. Liquefaction in a smelting furnace is a continuous process, but tapping is done in batches. It needs to be carburized to 2.5-3.5% to obtain “synthetic hot iron.” In the subsequent converter process, oxygen blowing removes [C], providing chemical energy, which is the only energy source after latent heat. One of the main reasons for the infeasibility of high-slag-volume electric arc furnaces, calculated using Table 2, is the consideration of the large slag volume. Further considering and selecting the smelting furnace technology as the primary and initial reason—CO2 emissions—Table 3 shows the subsequent CO2 emissions for Scope 1 and Scope 2 compared to electric arc furnaces and two types of iron ore pellets. This review indicates that smelting furnace technology leads to higher CO2 emissions in order to reduce uneconomical slag volumes and proportions. The oxygen-blown converter remains necessary to remove dissolved carbon and emits 150 kg of CO2 per metric tonne per tap – similar to a scrap-based electric arc furnace (EAF). Therefore, the actual CO2 emissions of the entire DRI-smelting furnace-BOF process route are considerable, a conscious decision that every steel producer must make. Since the annual production capacity of smelting furnaces has not yet exceeded 1 million tonnes, but EAF is a mature technology with a tap weight exceeding 300 tonnes, this consideration may increase skepticism about smelting furnace technology and certainly reveal uncertainty surrounding this strategic decision.
Furthermore, the slag from the smelting furnace can be designed to resemble, or even be identical to, blast furnace slag. No special metallurgical work is performed here regarding dissolved elements in steel. Therefore, the basicity of “synthetic blast furnace slag” can be defined as around 0.9-1.3, giving it the potential to be used in the cement industry after crushing, just like today’s blast furnace slag. The role of FeO in this slag is not entirely clear. FeO, and possibly even iron oxide at a higher oxidation stage, originates from less than 100% DRI metallization. The reduction of FeO to Fe in the smelting furnace is suspected to be incomplete. Some original equipment manufacturers claim that all FeO, and even some SiO2, is reduced to [Si] from the synthetic hot iron from the smelting furnace. If this is not the case, the impact of FeO in synthetic blast furnace slag on cement utilization needs to be evaluated.
in conclusion
Reducing carbon dioxide emissions during steelmaking is a crucial strategic decision that will profoundly impact the business and profitability of steel producers. Correct technological decisions require in-depth analysis, until certain details cannot be overestimated. This article compares different electric arc furnace designs that may be a consideration for profitable future production in both integrated and small-scale plants, and their respective impacts on Category 1 and Category 2 carbon dioxide emissions.
Different industrial solutions were proposed and compared. Among them, electric arc furnaces (EAFs) employing metal charge preheating devices showed low CO2 reduction efficiency. Conventional EAFs, using scrap steel or pig iron as raw materials and with good operation and maintenance, had the lowest emissions and were the most cost-effective with the help of trained personnel and specialized knowledge. Some key details of conventional EAFs were highlighted and described. A brief comparison with alternative smelting furnace technologies reveals that smelting furnaces do indeed produce less slag, a significant advantage, but the disadvantage is a substantial increase in CO2 emissions. Different solutions can be chosen based on the strategic objectives of steel companies transitioning to steel production. If the primary and overall goal is to minimize CO2 emissions, then conventional EAFs appear to be the most profitable option.
Why are electric arc furnaces important for low-carbon steel?
Electric arc furnaces enable steel production using electricity instead of coke, significantly lowering CO2 emissions, especially when powered by renewable energy.
What are the main EAF design types?
The main EAF designs include conventional scrap-based EAFs, scrap preheating EAFs, and DRI-based EAFs, each with different energy and emission profiles.
Does EAF size affect CO2 emissions?
Industrial data show that EAF size has little impact on CO2 emissions; feedstock composition and operating practice are far more influential.
Why does DRI increase electricity consumption in EAFs?
DRI requires additional electrical energy to melt and reduce FeO, especially when charged cold; hot DRI significantly improves energy efficiency.
Are smelting furnaces a better alternative to EAFs?
Smelting furnaces reduce slag volume but generate higher CO2 emissions and still require BOF refining, limiting their advantage for deep decarbonization.
