The Chair of Thermal Processing Technology

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1 This is where we focus in regular intervals on the main institutions and organizations active in the field of thermoprocessing technology. This issue spotlights the Chair of Thermal Processing Technology at Montanuniversität Leoben. Chair of Thermal Processing Technology at Montanuniversität Leoben The Chair of Thermal Processing Technology (TPT) was founded in the academic year 1911/12, Professor Harald Raupenstrauch is head of the institute since According to the needs of the industry, the name of the institute and the research areas have frequently changed over the years. Today, the focus of the research at the institute lies on four main areas: High temperature processes, e. g. industrial furnaces in metallurgy, refractory, cement industry, etc. Energy efficiency, especially in the energy-intensive industry mentioned above Process and plant safety and Mathematical modeling and computer simulation. Below, the main projects and processes developed at the institute are briefly introduced. One key element of the research at TPT is a pyro-metallurgical approach to recycle steel mill dusts and residues. Steel mill dusts contain about 50 % iron and up to 40 % zinc depending on the production route. The state of the art process for zinc recovery is the Waelz rotary kiln process. However, dusts from the blast oxygen furnace usually do not have sufficient zinc contents for an economically sensible treatment within the Waelz kiln. For these dusts the RecoDust process is a new technology for the separation of zinc and iron. The pilot plant has a capacity of 300 kg/h. The core of the flash reactor is the reaction chamber in which the dust melts at a temperature of about 1,700 C and at under-stoichiometric conditions. The zinc oxide is reduced and turns into its vaporous state at the reaction temperature and leaves the flash reactor through the flue gas system. In the following post combustion, the reducing agents and the zinc are oxidized and cooled at the same time by the injection of oxygen, water and compressed air. The newly formed zinc oxide precipitates from the exhaust gas flow and can be separated in a baghouse filter. The separated crude zinc oxide has a purity of up to 90 % and can be used as a secondary resource in the production of zinc. The non-volatile elements form a liquid oxide phase at the bottom of the reaction vessel where it is tapped discontinuously (Fig. 1). The oxide phase contains most of the iron and can be used in the steel mill as a return material. The benefits of the RecoDust process are the possibility of treating dusts with a wide zinc content range, the use of only natural gas as a reduction agent and the production of two useful products without getting any waste material. Almost 8 t of residues have been treated successfully and the next work topic is a feasibility study for an industrial sized pilot plant. Another process developed and operated at TPT is RecoPhos. The RecoPhos process aims to recover phosphorus from sewage sludge and sewage sludge ashes. These input materials contain a significant amount of phosphorus in the form of calcium, iron and aluminium phosphates. In the InduCarb reactor, the core of the RecoPhos process, these phosphates react to pure phosphorus, carbon monoxide and silicate slag in the presence of silicon dioxide and carbon. In order to realize this reaction, a very low oxygen partial pressure and a high temperature are needed. Therefore, graphite is heated above 1,500 C inductively and the phosphorus reduction takes place in a thin molten film on the surface of the carbon bed. The reactor is shown in Fig. 2. The overall system starts with a dosing unit. Argon is used to prevent oxygen from reaching the reactor, compromising the reductive atmosphere. Subsequently, the material reaches the reactor. Three coils divide it into three zones. In the upper zone, the sewage sludge ash is molten. The main reaction zone is in the middle of the reactor and at its bottom, reduced metals and slag leave the reactor. Gaseous phosphorus and carbon monoxide are led into a combustion chamber and the emerging gas stream, now consisting of carbon dioxide, water vapour and phosphorus pentoxide, reach a gas scrubber. It is possible to gain pure, elemental phosphorus or phosphoric acid with the RecoPhos process. Other products involve an iron alloy and silicate slag, which can be used as a binder in the cement industry. In addition, the exhaust gas is suitable for heat recovery applications. Momentarily, the RecoPhos process and the InduCarb reactor are further developed to the InduRed process in order to process Fig. 1: Tapping of the TPT flash reactor 105

2 Edition 13 Fig. 2: RecoPhos reactor at TPT steelmaking slags. The overall aim of the current research activities is to investigate the reduction behaviour of phosphorus, manganese and chromium as a function of the oxidation degree of iron. The focus is also laid on the separation of the iron and phosphorus phases. Slags and the treatment of slags in general is one main area of research at TPT. Molten blast furnace slag, e. g., represents one of the largest untapped energy sources in the steel industry. Each year, approx. 400 million t of blast furnace slags are produced worldwide, with a tapping temperature of around 1,500 C. It is normally used as a substitute for cement clinker or as an aggregate material in road construction. Currently, the slag is granulated in wetgranulation plants using large volumes of water. With this commonly used production method it is not possible to utilize the remnant heat energy of the molten slag of about 1.5 GJ/t. Actually, TPT is dealing with two new processes, namely Dry Slag Granulation (DSG) and Boiling Water Granulation (ChemGran). The aim of DSG and ChemGran is to develop a reliable quenching technology for molten blast furnace slag in order to recover the high-temperature waste heat while reaching the process conditions required for the production of glassy slag that is suitable for the cement industry. DSG is based on molten slag atomization, using a variable rotating cup or dish (Fig. 3). The atomized slag is cooled down by air and the hot air is used for heat recovery. In contrast, during ChemGran the particle reacts with boiling water and the slag enthalpy is utilized for process immanent drying of the granulate. Both products are suitable for the cement industry. New industrial production processes, as presented above, are often connected to considerable economical and technical risks. Therefore, the field of process and plant safety defines the performance and design of industrial processes more than ever. The process and plant safety research group at TPT conducts research activities for safe exploitation and production of materials as well as for safe development and manufacturing of industrial products. The main field of work of TPT s process and plant safety research group is industrial fire and explosion prevention as well as emergency response. Problems connected to specific fields of industry (mining, waste treatment, etc.) are investigated as well as a variety of topics from fundamental research. The threat of fires and explosion occurs in nearly every process dealing with flammable liquids, gaseous or solid materials (Fig. 4). Dust explosion hazards e. g. are present in various fields of energy production, materials handling, chemical industries or waste treatment. Theses hazards are still underestimated. Fundamental research at TPT s process and plant safety covers the influence of inert material on explosion behaviour or the effects of non-atmospheric conditions on flame propagation. The focus of the research activities in fire prevention within the research group lies on investigations concerning new and improved extinguishing agents and techniques as well as self-heating and auto-ignition of bulk material. Major accidents and industrial disasters pose huge challenges for internal and external emergency response crews. Together with fire brigades, military forces or public authorities, research activities within TPT process and plant safety deal with the improvement and development of measures and systems for emergency response. Research work covers toxic Fig. 3: Dry slag granulation (DSG) Rotating cup atomizer 106 heat processing

3 Fig. 4: Major industrial fire at a chemical production site releases, major industrial accidents and the improvement of emergency planning and emergency procedures. Since 2015, TPT process and plant safety organizes a postgraduate master course for Process and Plant Safety, Emergency and Disaster Management. The focus of the working group mathematical modeling and computer simulation at TPT are safety relevant processes, particle based reactions and industrial furnaces, kilns and burners. The main objective of modeling safety relevant processes is dust deflagrations. For this task two different models were developed. One model uses a flame speed approach and Eulerian dust tracking. This model is used to calculate dust deflagrations in large geometries with reasonable numerical rescuers. The second model uses Lagrangian dust tracking and a pyrolysis approach for the particles. The pyrolysis gases combust with oxygen in the gas phase. This model is used for detailed analysis of the combustion behaviour of dusts. Besides dust deflagrations, gas deflagrations/explosions are an important research topic. Yet, in this case the main focus lies on the dispersion of flammable gases and especially on combustion behaviour in large geometries. Particle based reactions have a wide field of applications. At the moment, the focus is based on particle reactions with a low volume fraction of particles like the flash reactor, fixed beds and highly resolved individual particles. Two approaches for reacting moving beds are currently in development. The flash reactor is modeled using Lagrangian particles for the dust phase. This model is a two-stage approach. In the first phase, the combustion is calculated using the steady laminar Flamelet approach and thermo parcles with a four-way coupling to the gas phase. In this stage, no heterogeneous reactions are included. The steady results of this first simulation are used as starting conditions for a second model, which calculates the combustion using the Partially Stirred Reactor model and reacting parcles with a full-way coupling. These parcles consider the heterogeneous reactions for the reduction of the zinc oxide, the different iron oxides and the evaporation of the zinc. The InduCarb reactor was modeled for the RecoPhos process using a three-phase Eulerian approach, considering the gas phase, the slag phase and the inductively heated char. The aim of the model is to characterize the residence time distribution, the hold up and the temperature profiles. To determinate the transient behaviour of fixed beds including heterogeneous reactions a model was developed. This model is mostly used to determinate the self-heating potential of large stockpiles of burnable substances, but is also able to describe different adsorption processes in fixed beds. For the investigation of the inner particle effects during different combustion processes of organic substances, a single particle model was created. The single particle model considers a gaseous phase in the pores and a non-mobile phase for the solid and liquid substances by using a Eulerian approach. In addition, the gaseous phase includes heat and mass transport by convection and diffusion/conduction in the pores. The non-mobile phase considers only heat conduction. In order to model the pyrolysis, tar and char heterogeneous reactions are included, as well as a set of homogenous reactions. Due to continuous rinsing requirements on emissions, energy efficiency and product quality burners, kilns and furnaces have to be constantly improved. One set of tools for the improvement are Computational Fluid Dynamic models. These models are used in a three-stage approach. In the first step, the furnace, kiln or burner is modeled in the current state. This model is evaluated by measurements. After the evaluation of the model, the results of the simulation are analyzed in detail, to show the weaknesses and possible solutions. The impact of these solutions is determined by further simulations. Based on the given task, the right models have to be chosen, combined and new models have to be developed. ANNEALING FURNACE The annealing furnace for continuous steel strips is heated by radiation tube burners. Besides the annealing, the reduction of wuestite and a decarbonization of the steel strip take place. To model the movement of the steel strip a pseudo-fluid model was developed. This model is based on an incompressible laminar fluid with fixed velocity, to tweak the Fourier s law to a 107

4 Edition 13 Fig. 5: Convective heat flux [W/m 2 ] (left) and radiation heat flux [W/m 2 ] (right) through the strip surface Fig. 6: Temperature profile [K] of the strip surface moving solid. To model the radiation the Discrete Ordinate Method in collation with the Weighted Sum of Gray Gases model was chosen. The turbulence is modeled by the realizable k-ε model. Fig. 5 shows the heat flux through the strip surface in the simulated annealing furnace. In the figure, the steel strip enters the furnace on the left side, is led meandering between the radiant tubes for heating and exits on the right side. Negative fluxes in the figures mean heat absorption Fig. 7: Results of the NO X Post-Processor for the Sandia Flame D of the strip and positive fluxes mean heat emission. As seen in Fig. 5, the steel strip is mostly heated by radiation in the first part of the furnace. After the first part, the steel strip has nearly the same temperature as the radiation tubes. The temperature profile of the steel strip is shown in Fig. 6. The chemical surface reactions are model by a one-dimensional multi-layer diffusion approach in the steel strip and gas phase concentration on the surface of the steel strip. In order to track the thickness of the different diffusion layers, additional conservation equations are added to the steel strip. FLAMELET NO X POST-PRO- CESSOR The steady laminar Flamelet model is based on fast chemistry. The reaction takes place in asymptotically thin layers, which leads to a one-dimensional combustion normal to these layers. Based on this assumption, the combustion can be described with two conservation equations, one for the mixture fraction and one for the mixture fraction variance. This leads to a numerically efficient combustion model, but slow reactions like the NO X chemistry cannot be modeled by this approach. In order to model the NO X chemistry a detailed chemical reaction model, like the Eddy Dissipation Concept or Partially Stirred Reactor model, in combination with a detailed reaction mechanism are necessary. These models need a conservation equation for every species except one and have to solve all reactions in every cell. This makes such models expensive in terms of computing resources. The developed NO X postprocessor solves specific detailed chemical reactions based on a stationary flow, temperature, turbulence and species field, considering only species with high concentrations like O 2, CO, CO 2, N 2 (Fig. 7). 108 heat processing

5 This post-processor was evaluated on different flames, showing consistent NO concentrations between 51 and 68 %. So it is a numerically effective tool to predict NO X trends and can be used to optimize industrial furnaces. When working on energy-intensive high-temperature processes energy efficiency is always a key aspect alongside product quality. Due to the European Energy Efficiency Directive in late 2012, all countries within the EU are required to use energy more efficiently at all stages of the energy chain. Especially energyintensive industries, as is the foundry industry, are challenged to reduce their energy consumption substantially, to increase their energy efficiency and their environmental performance in general to reach the given target values. In order to do so, a project called Energy Efficiency in the Austrian foundry industry was initiated by the Montanuniversität Leoben in corporation with the Austrian Economic Chamber and the Association of Austrian foundry industry. This project s aim was to create a Microsoft Excel based tool to visualize and quantify the actual energy consumption for a specific product for each company within the Austrian foundries. This was done by evaluating the energy situation within six engaged companies in the industry and then creating all relevant energy and mass flows in an economic and a technical approach. The economic approach is based on the cost allocation of the controlling to create energy figures for one average product. This is called top-down approach. Furthermore, also a thermodynamic approach (named bottom-up) for each plant within the companies based on real data like mass flows, loss of material, machine performance, actual energy consumption for one specific product is implemented in Excel. This technical approach is based on a modular and dynamically changeable surface in VBA (visual basic). By this means, every company within the foundry industry was divided into so-called main modules, which represent a specific part in a foundry company (melting, heat treatment, etc.). These bigger parts are then divided by single units which represent specific plants within those companies (cupola furnace, inductive furnace and so on). This approach enables the derivation of the actual energy consumption of processes and the corresponding manufactured products, and therefore leads to a better understanding of cost generation. Moreover, the methodology identifies energy efficiency potentials and merges them into a model-based approach for the planning, evaluation and optimization of energy consumption in the foundry industry. It is important to mention that this tool may be applied to other energy-intensive industries as well. Summing up, it can be stated that the aim of the scientific work at the Chair of Thermal Processing Technology is the development of new high-temperature processes including the deep understanding of the occurring physical processes and chemical reactions as a basis for the design of single components of lab scale, pilot scale and demonstration scale plants, whereby questions concerning energy efficiency and safety are covered as well. Author: Univ. Prof. Dr. techn. Harald Raupenstrauch Contact: Montanuniversität Leoben Department of Environmental and Energy Process Engineering Chair of Thermal Processing Technology Leoben, Austria Tel.: +43 (0)3842 / tpt@unileoben.ac.at Powered by Organized by The Key Event for Thermo Process Technology InterContinental Hotel Düsseldorf, Germany June