After more than 20 years from its first commercial installation and with 41 applications worldwide, Tenova’s Consteel® has become a proven steelmaking technology, appreciated for its efficient use of energy and raw materials, operational and maintenance ease and environmental friendliness.
|Fig. 1. Conceptual configuration of the Consteel® Evolution™ EAF|
|Fig. 2. Experimental test rig set up at CSM’s combustion station|
The experience gained throughout the years on Consteel® electric-arc furnaces (EAFs) running in a variety of scenarios has given Tenova’s team several ideas of how to improve the technology. Improvements were investigated with a deep look at the complex scrap heating phenomenon taking place inside the Consteel system by means of lab tests and CFD analysis.
The changes introduced with the new Consteel® Evolution™ cover the entire system from the EAF design up to the heating-tunnel section, where the most noteworthy change in the system has taken place. The main driver for the whole development has been the reduction of electrical energy consumption by means of improved scrap heating inside the tunnel by reshaping the tunnel geometry, using EFSOP™ off-gas analysis measurements and using air/gas burners to help the initial melting of the charge.
Replacing electric energy with chemical energy provided by burners reduces the operating costs, particularly for all those areas where natural gas has become largely available and inexpensive in the U.S. with the development of the shale-gas industry.
|Fig. 3. Temperature measurement for a bucket charged with shredded scrap (T8)|
Concept of Consteel® Evolution™
In a standard Consteel system, scrap is conveyed to the EAF through a heating tunnel, where it encounters the off gases coming out of the furnace at about 1500°C (2732°F) and exchanges heat with the gases, which increases the temperature of the scrap before entering the molten bath of liquid steel.
The Evolution concept starts with splitting this heating tunnel in two sections: tunnel A, which remains the same as the standard system, and tunnel B, where scrap is heated by dedicated tunnel burners. Basically a reheat furnace, tunnel B is placed on top of the Consteel conveyor before the scrap enters the standard Consteel, tunnel A.
Tunnel B is refractory-lined and provided with air/natural gas burners located on the roof at a relatively short distance from the scrap surface. In this configuration, the burner flames impinge, relatively undisturbed, on the scrap layer with sufficient momentum to penetrate its cavities, heating it more uniformly.
Tunnel B configuration was studied by a series of laboratory trials to investigate the effects of an impinging flame on the heating process of scrap located inside a conveyor and CFD simulations based on an original scrap model validated by experimental data.
|Fig. 4. Scrap representation developed for the case|
Heat-Exchange Modeling in the Scrap Due to Flame Impingement
Flame impingement heating of solids has been used for many years to enhance the convective heat transfer from combustion products to the charge. Some typical applications include melting of scrap metal, shaping glass, heating metal bars, and metal fabrication and assembly (e.g., soldering, brazing, cutting and welding). In EAF steelmaking, it is common practice to use oxygen/fuel burners in order to achieve a faster and more uniform meltdown of the charge, avoiding cold spots. In such cases, the main goal is to achieve a fast scrap meltdown in a specific zone. On the contrary, in the envisioned application of burners to a continuous charging process, the burners will heat the scrap bed, which has a speed ranging from 1.5 m/min to 5 m/min. Any significant melting must be avoided to prevent meltdowns on the bottom of the steel conveyor. The uniformity of the heat flux is, therefore, an important feature for this new type of scrap heating process.
Very limited literature is available on air/gas burners for scrap heating. Therefore, physical and mathematical models have been set up to evaluate the effects of the various parameters affecting the heating phenomenon. Parameters to consider are:
1. The position of the burner in respect to the scrap bed
2. The burner operating conditions
3. The different shape and layering of the scrap pieces
|Fig. 5. Scrap model validation: 200-kW burner power on shredded scrap at steady-state conditions – experimental vs. CFD results|
An experimental test rig includes a ceramic-fiber-lined furnace characterized by internal dimensions of 2,020 x 1,740 x 1,470 mm and a scrap bucket (800 x 780 x 750 mm) that is water cooled on two sides.
The tests were performed using a commercially available 600-kW Tenova THS burner with combustion air at ambient temperature. The bucket, charged with about 500 kg of scrap (600 mm deep), was instrumented with 75 thermocouples to monitor the temperature evolution during the heating process at different scrap heights from the top (100/200/300/400/500 mm). Four other thermocouples have been placed very close to the scrap surface (5 mm) to monitor the initial heating.
Figure 3 reports the temperature evolution with time for the thermocouple 8 (T8) placed at the center of the scrap bulk. This test was performed using shredded scrap, with three different burner power settings – 200, 400 and 600 kW. In order to prevent the test setup from localized scrap meltdowns, it was chosen to stop the test at a temperature of about 1250°C (2282°F) on the top layer.
These first results provided useful information regarding:
- The required distance between the burner and the scrap surface
- The correct power density to be achieved inside the heating tunnel equipped with multiple burners
- Maximum amount of heat to the charge before the occurrence of a significant superficial meltdown
|Fig. 6. 600-kW burner on a moving shredded scrap – experimental vs. CFD results|
The main effort has been the achievement of a suitable scrap representation because the other sub-model was already set-up and validated at Centro Sviluppo Materiali (CSM) for the modeling of burners and reheating furnaces. Multiphase transport phenomena in porous media is an issue. Literature on the matter approaches this problem assuming local thermal equilibrium between the solid and the fluid phase. That assumption is not realistic in this specific case. Attempts were made by Dr. Gordon Irons of McMaster University to model the scrap melting process in the EAF by oxygen/fuel burners and electric arcs without this assumption.
However, a validated model that also includes the effects of radiation within the scrap pieces is not yet available in the literature. Due to the difficulty of developing a comprehensive model for the complex heating phenomena taking place in such heterogeneous material as the scrap, a simplified model has been set up.
First, the penetration of the flame inside the scrap cavities was obtained considering the scrap as a porous media using the Brinkman-Forschheimer extended Darcy Model. The porosity and permeability of the various scrap types have been defined by using the previously noted McMaster University formulation and parameters (void/filled volume ratio, scrap characteristic length) for scrap characterization. To take into account the convective heat transfer due to penetration of the flame and, at the same time, the heat transfer by radiation, the scrap has been represented with a “groove geometry.” The depth of the grooves has been assumed equal to the flame-penetration length calculated in the previous step, while the groove’s width has been selected in order to maintain the same ratio between area and volume (A/V) of the scrap being considered.
Area (A) and volume (V) have been calculated according to the porosity and characteristic dimension of voids considered in the previous step. Then the scrap has been represented as a solid material with equivalent density (ρequiv) and equivalent conductivity (kequiv) calculated as follows:
where mcharge is the mass flow of scrap inside the conveyor, Scharge is the cross section of the scrap layer, Vcharge is the transport velocity of scrap inside the conveyor, kFe is the conductivity of pure iron, kair is the conductivity of the air and % is the ratio between equivalent density of the charge (ρequiv) and density of iron (ρFe).
|Fig. 7. Scrap heating curve for different scrap types: solid and porous (shredded)|
The accuracy of the model has been verified by comparing the results of the experimental shredded-scrap trials. The test rig – including the furnace, the burner and the bucket – was modeled first for the heating test, with the burner set at 200 kW of power in order to reach a steady-state condition.
Due to the good results for the steady-state case, the test with 600 kW was also considered. In this condition, since the steady condition could not be reached experimentally, a translation velocity was imposed on the scrap layer to simulate the proper residence time under the burner. The quality of the comparison between measured and calculated temperature is very similar to the previous case. Figure 6 reports the temperature map for the vertical plane, just under the burner axis for the physical test and CFD simulation.
Measurement and calculations using different scrap mixes have confirmed the capability of the approach to predict the flame penetration and the effects of flame-impingement heating, including the representation of radiation within the scrap.
|Fig. 8. Example of a heating tunnel equipped with burners: five zones, each made by five burners|
Scrap Heating Modeling in the Consteel Tunnels
The scrap model developed for the representation of scrap heating with burners (tunnel B) has also been applied to the simulation of classic scrap heating by means of the melting process off-gas (tunnel A), considering the effects of draft air intake and the combustion of CO inside the tunnel. Figure 7 reports the comparison of the heating curve in a conventional tunnel (20,000 x 2,000 x 2,000 mm) for the same working condition of the EAF, with scrap modeled as a moving solid (no porosity) and with the new model using shredded scrap. With the new scrap model, a higher average temperature of the scrap is achieved near the connecting car zone (the last part of the conveyor that discharges scrap into the furnace), where the draft air ingress produces a vortex structure that attaches the CO combustion to the scrap surface, generating a velocity field characterized by significant vertical component toward the scrap. These results, coming from CFD simulations, have been confirmed by field observations that have pointed out better heating of the charge inside the last portion of the tunnel when operating a Consteel with a porous scrap charge.
After achieving a satisfactory scrap model, it was possible to continue the CFD study for different configurations of the tunnel implementing the flame-impingement heating concept. The main goal for this work has been the definition of design guidelines for this tunnel in order to achieve the best possible heat-transfer efficiency, with a natural gas consumption similar to those used in a conventional EAF (<9 Nm3/t). It was discovered that a modular approach, such as the one shown in Figure 8, was the way to go.
|Fig. 9. Example of CFD simulation results for tunnel equipped with burners|
Figure 9 reports some CFD simulations for this tunnel configuration. The tunnel length is maintained fixed (12 m), while the burner zones are switched on to maintain a constant specific consumption (5.7 Nm3/t) at a different scrap feeding rate (velocity) – zone 1 and 2 for 1.76 t/min, zone 1 to 3 for 2.64 t/min, zone 1 to 4 for 3.53 t/min and all five zones for 4.4 t/min.
According to these findings, the value of the specific consumption and calculated efficiency are in the range of those typically recognized for wall-mounted oxy/fuel burners used in conventional top-charge EAFs. Considering that combustion is performed with gas and cold air and extrapolating the cold air/fuel burner to an equivalent oxy/fuel burner, an average efficiency higher than 60% is expected, indicating the effectiveness of flame-impingement heating technology in this type of application.
A New Generation of Furnaces
The results shown above refer to the very first stage of the research that has led to the configuration of the new Consteel® Evolution™ system. Thanks to that, great potential has been found for scrap heating by convection. This has helped define the design criteria for both tunnel B and tunnel A, introducing solutions that increase the turbulence in the primary off-gas stream and, hence, the heating of scrap.
These changes in the heating section of the Consteel process led to a revision of how to charge and layer scrap on the Consteel conveyor. The best practice for charge scrap in these new heating conditions is to achieve a porous layer of scrap at the top and place pig iron and other denser materials in the lower layers.
Improvements have also been made in the configuration of the burners. The burners to be used are rated for a power input higher than 700 kW, and their flame has been optimized for the application.
The control system for the burner section and for the entire system will take advantage of Tenova Goodfellow’s EFSOP™ technology to achieve a dynamic optimization of the operational parameters.
Table 1 reports performance level achievable with a Consteel® Evolution™ furnace. IH
For more information: Contact Francesco Memoli, VP Steelmaking, Tenova Core, 100 Corporate Center Drive, Coraopolis, PA; tel: 412-262-2240; e-mail: firstname.lastname@example.org; web: www.tenovacore.com and www.tenovagroup.com.