Herbert Pfeifer

Particle Distribution and Separation in Continuous Casting Tundish

The tundish as a part of a continuous casting machine combines the discontinuous ladle metallurgy with the continuous solidification of slabs in the mould. The tundish plays a major role in the challenging task of “clean steel” production. That means the smallest number of inclusions and high cleanliness in all steel grades after changing the conditions at the inlet of the tundish. Inclusions hinder the metal forming process and lead often to fatigue. The cleanliness of steels is important to fulfil the customers requirements. In the present study inclusion removal was simulated in a 1:3 scaled water model of a single-strand tundish for the production of stainless steels with a particle counter. The particle counter is capable of counting a large number of particles with a wide range of diameters. The separation rate for particle diameters from dP = 1 - 250 μm was determined with a counter for the water model tundish. With similarity conditions for the particles this deposition rate can be transformed to the melt flow in a steel tundish. The separation rate was measured for different flow rates in the water model tundish. A larger flow rate decreased the separation rate. Additionally, the separation rate for the tundish fitted with an impact pad was measured and showed a significant increase of separation for particles with a smaller diameter. Furthermore, the particle distribution in the tundish for different size groups of particles was investigated with and without an impact pad. Numerical simulations were carried out with the finite-volume commercial code FLUENT using the realizable k-e turbulence model. A special boundary condition for the separation of particles at the surface was implemented.

Nitrogen Oxide Formation in the Electric Arc Furnace—Measurement and Modeling

In this article, the results of pilot electric arc furnace (EAF) trials and model calculations regarding the formation of NOx in an electric arc furnace are presented. The results of these investigations confirm that the conditions within the electric arc furnace are favorable for the formation of NOx. During pilot furnace trials, a notable influence of the electric parameters on the NOx formation could not be established. The importance of the influence of the furnace atmosphere, especially the O2 and CO content, could be ascertained clearly. Thermodynamic equilibrium calculations concerning the furnace atmosphere were conducted during the course of the investigations. The results clearly show that the chemical reaction kinetics have a considerable influence on the NOx concentrations measured in the off-gas of electric arc furnaces. Therefore, in addition to the equilibrium calculations, a reactor network model of the EAF was developed subsequently. Using this model, it was possible to reproduce phenomena such as the NOx peak at arc ignition as well as to calculate NOx concentrations, which match measurement data within an order of magnitude.

Heat recovery from EAF off-gas for steam generation: analytical exergy study of a sample EAF batch

The high amount of latent and sensible enthalpy discharged from the melting process in an electric arc furnace (EAF) through the off-gas offers high potential for waste heat recovery. Evaporative cooling systems (ECSs) installed at dedusting systems of some EAFs are utilising this waste heat for steam generation and subsequent usage of steam for further applications. Within the following paper, further optimisation approaches of this waste heat recovery are examined comparatively by means of exergetic analysis. Thereby, the focus is on the excessive intake of false air into the dedusting system to ensure a complete CO post-combustion. While a sheer energetic examination of the enthalpy hardly shows any differences, the exergetic calculation confirms the significantly higher amount of generated steam with controlled false air ingress. For the calculated optimal reference case with stoichiometric post-combustion, between 50 and 70% more steam could be produced. Even though a stoichiometric post-combustion cannot be realised for safety reasons, the calculation shows the potential of controlled false air intake for CO post-combustion.

Increasing the sustainability of steel production in the electric arc furnace by substituting fossil coal with biochar agglomerates

Biochar fines from a wood gasification plant and from pyrolysis of agricultural residues were investigated as substitutes for fossil coal used in the steel production in the electric arc furnace (EAF). During previous tests biochar fines with high specific surface showed problematic burn-off behaviour. Therefore the agglomeration behaviour of the biochar fines was investigated. Different binary and ternary mixtures of biochar with water and binders were tested in a hydraulic stamp press and evaluated with regard to green strength and fatigue strength of the briquettes after 3 days. One selected mixture was used to produce pillow briquettes in a double roll press. The abrasion behaviour of the produced briquettes was tested and compared to an anthracite reference coal (RC). Melting tests in a pilot EAF showed that the agglomerated biochar reacts similar to the RC. The briquetting leads to reduced reactivity and slower burn-off compared to the biochar fines.

Modeling and Simulation of the Off-gas in an Electric Arc Furnace

The following paper describes an approach to process modeling and simulation of the gas phase in an electric arc furnace (EAF). The work presented represents the continuation of research by Logar, Dovžan, and Škrjanc on modeling the heat and mass transfer and the thermochemistry in an EAF. Due to the lack of off-gas measurements, Logar et al. modeled a simplified gas phase under consideration of five gas components and simplified chemical reactions. The off-gas is one of the main continuously measurable EAF process values and the off-gas flow represents a heat loss up to 30 pct of the entire EAF energy input. Therefore, gas phase modeling offers further development opportunities for future EAF optimization. This paper presents the enhancement of the previous EAF gas phase modeling by the consideration of additional gas components and a more detailed heat and mass transfer modeling. In order to avoid the increase of simulation time due to more complex modeling, the EAF model has been newly implemented to use an efficient numerical solver for ordinary differential equations. Compared to the original model, the chemical components H2, H2O, and CH4 are included in the gas phase and equilibrium reactions are implemented. The results show high levels of similarity between the measured operational data from an industrial scale EAF and the theoretical data from the simulation within a reasonable simulation time. In the future, the dynamic EAF model will be applicable for on- and offline optimizations, e.g., to analyze alternative input materials and mode of operations.

Numerical Investigation for the Design of a Central Recuperator with Hybrid Air Preheating

Abstract Industrial furnaces, especially in steel industry but also in non-ferrous, ceramic or chemical industry, are often fired with fossil fuels like natural gas, oil or coke. Considering the transition to green energy, it is necessary to enable the use of renewable energies like wind and solar energy in fossil-fueled furnaces. Resulting from this motivation, a hybrid recuperator for combustion air preheating with off-gas heat and electric energy is being developed in a public research and development project. The hybrid recuperator consists of a conventional tube bundle module and an electric heating module. The design of the prototype is based on numerical investigations to determine pressure loss, increase in temperature and heat transfer. This article introduces the concept of the new hybrid recuperator and initial results of numerical investigations for a small-scale hybrid recuperator geometry.

Approach for Pyrolysis Gas Release Modelling and Its Potential for Enhanced Energy Efficiency of Aluminium Remelting Furnaces

Within the scope of the Advanced Metals and Processes (AMAP) research cluster in Aachen (Germany) the aluminium recycling process in melting furnaces is investigated with regard to resource and energy efficiency. When organic-contaminated material is charged into the furnace, pyrolysis gases are released as soon as the material temperature exceeds approximately 350 °C (662 °F). Those gases mainly consist of hydrocarbons, hydrogen and small fractions of other species. Thus, they are an energetic contribution to the furnace atmosphere and should be considered as such by the burner control unit in order to reduce the amount of unburnt fuel in the off-gas as well as primary energy consumption. This is achieved by post-processing data from lab-scale pyrolysis experiments in MatLab and bringing it into a format suitable for computational fluid dynamics (CFD) simulations. In this article an insight into the modelling approach and the model application in ANSYS Fluent CFD is given.

A Comparison between Two Cell Designs for Electrochemical Neodymium Reduction Using Numerical Simulation

Nowadays, neodymium is almost solely produced by the electrochemical reduction of Neodymium oxide in fused fluoride salts. Thereby, the fluid flow distribution within the electrolysis cell is important for the productivity and efficiency of the process. In this work, the flow field within a conventional cell with vertical electrodes is compared to that of an innovative cell concept with horizontal electrodes by computational fluid dynamics. The numerical model uses the Eulerian volume of fluid approach to track phase boundaries between the continuous phases, while the Lagrangian discrete phase model is applied to compute the rising trajectories of emitted off-gas bubbles. The calculated results indicate that the new cell type is more suitable for the efficient, large-scale production of neodymium, since there is potential to decrease the cell voltage and enhance the current efficiency. By that, the specific energy consumption can be lowered significantly. However, an advanced level of automation is necessary to operate the new cell.

Development of an Energy-Efficient Burner for Heat Treatment Furnaces with a Reducing Gas Atmosphere*

Abstract One of the main reasons for metal loss of semi-finished metal products during heating in reheating and heat treatment furnaces is scale formation. In the presented project a burner is developed which produces a low oxidizing / reducing atmosphere in the furnace. The concept is realized by a recuperative burner, which generates a reducing furnace atmosphere due to fuel rich combustion of natural gas and air. The complete combustion of the furnace atmosphere is ensured by the injection of additional air and takes places in an open radiant tube resulting in a high energy efficiency. In this paper numerical and experimental results are presented and discussed. The numerical results showed the huge impact of the secondary air swirl on the post-combustion in the annular gap which is formed between the open radiant tube and the burner. Mixing phenomena in the annular gap results in a nearly complete post-combustion at low and high swirl angles of the additional combustion air (ω = 0°, ω = 90°). Instead of that, at a swirl angle of ω = 45° the entire reaction from CO to CO2 was not ensured within the boundaries of the numerical model. The quality of the post-combustion was experimentally evaluated by measuring the CO-emissions in the off-gas channel. These were lower than 50 mg/m3 in a wide range of operation. The NOx-emissions are lower than 121 mg/m3 at all tested cases.

Physical Modelling of the Effect of Slag and Top-Blowing on Mixing in the AOD Process

The argon-oxygen decarburization (AOD) process is the most common process for refining stainless steel. High blowing rates and the resulting efficient mixing of the steel bath are characteristic of the AOD process. In this work, a 1:9-scale physical model was used to study mixing in a 150 t AOD vessel. Water, air and rapeseed oil were used to represent steel, argon and slag, respectively, while the dynamic similarity with the actual converter was maintained using the modified Froude number and the momentum number. Employing sulfuric acid as a tracer, the mixing times were determined on the basis of pH measurements according to the 97.5% criterion. The gas blowing rate and slag-steel volume ratio were varied in order to study their effect on the mixing time. The effect of top-blowing was also investigated. The results suggest that mixing time decreases as the modified Froude number of the tuyeres increases and that the presence of a slag layer increases the mixing time. Furthermore, top-blowing was found to increase the mixing time both with and without the slag layer.