(384d) Development of a Computational Fluid Dynamic Modelling Concept, for the Recodust-Process

The RecoDust-Process offers new possibilities for minimum waste strategies in the iron and steel industry. The idea was realized in the Flash-Reactor pilot plant which is meant to recycle steel mill dusts and residues. The process is a new pyrometallurgical treatment for steel mill dusts and residues with low zinc contents. Due to their high iron contents, these dusts are supposed to be reused within the steel production site. For the dust with higher zinc content a recycling is limited or impossible without treatment. A treatment with the state of the art Waelz-Kiln is only economical with zinc contents around 20 %. Many dusts from the blast furnace route are between these boundaries, so they have to be recycled externally or landfilled. Aim of the RecoDust process is the production of two fractions: A zinc rich oxide fraction and an iron rich fraction. The purity of the products has to be high enough to ensure that both fractions can be used as secondary resources in the zinc or iron industry. The process uses a reducing atmosphere provided by an oxygen/natural gas burner with temperatures around 1700 °C. Within these conditions, the fine dust melts and the zinc oxide is reduced and vaporized. The metallic zinc leaves the reaction chamber with the exhaust gas and is transferred into zinc oxide by means of a post combustion. In a final step the exhaust gas is cooled and subsequently in a bag filter system zinc oxide is separated from the flue gas. The non-volatile components gathered on the bottom of the reaction vessel are discontinuously tapped as a slag. The benefit of the RecoDust-Process is the absence of foregoing treatment steps for the dusts. However, a free floating and dry dust with a grain size less than 1 mm represent limits to ensure a fast melting procedure. Unlike most other established processes, no additives are charged into the process, the reducing conditions are adjusted only by control of the oxygen flow in the mixing cyclone and the burner lance. The Flash-Reactor pilot plant installed in the technical center, at the chair of the Thermal Processing Technology at Montanuniversitaet Leoben has a dust mass throughput up to 300 kg/h. In the framework of this project the matter is further development of new burner geometry, dust loading systems and reactor design based on simulation model and furthermore implementation of verified detailed model of the heterogenic system. For a system scale-up it is necessary to build a mathematical model for process description in CFD to support the research. The main process parameters are particle size and composition, gas composition, particle and gas temperature, and burner and reactor geometry. With the “atmosphere particle kinetic model for particle reactions” it is possible to check the reactions in range of the calculation settings [1]. The first case is calculated by the computational fluid dynamic program fluent v15. The fluid transport is solved in Eulerian framework and the dispersed phase is represented as Lagrangian approach. Turbulence was modeled by a realizable-k-ε model, which produces the most stable results. The flue gas composition is calculated with a reduced 17 species mechanism [2] designed to model natural gas combustion. User defined function has been used for particle reactions and heat and mass transfer between fluid and disperse phases. The particle contemplation is based on a non-porous dust particle in different composition and size distribution. For further studies are used the computational fluid dynamic program OpenFOAM v2.4 [3] is used in combination with a Flamelet solver based on LibOpenSMOKE [4]. An advanced Flamelet Equilibrium Hybrid model with discrete ordinate model (DOM) for the radiation and integrated transient thermo parcels, with higher stability of the calculation system is developed. The flue gas composition is calculated with the same reduced species mechanism [2] as by the fluent case. The fluid transport is solved in an Eulerian framework and the dispersed phase with Lagrangian approach. For turbulence modeling the realizable-k-ε model was used. With the Flamelet Equilibrium Hybrid model with transient thermo parcel it is possible to check new burner designs and reactor designs in case of gas flow and particle flow through the Flash-Reactor. The velocity distribution, temperature distribution and the particle carry over out of the reactor are important system parameters to evaluate the geometry design, so if that process parameters meet the expectations of the specification, the geometry is ready for detail simulation, which includes the calculation of the particle kinetic and the fluid phase by mutual dependency. The solution from the Flamelet case is transferred to an OpenFOAM Partially Stirred Reactor model with transient parcel tracking. After the detailed chemical calculation it is possible to evaluate the particle and the slag distribution from the wall film, the bottom slag or the particle carry over into the exhaust pipe, respectively. So the Simulation data can be compared to the experimental results.

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