CFD Modelling of HIsarna Off-gas System

HIsarna is a new and breakthrough smelting reduction technology for producing liquid hot metal for steelmaking directly from iron ores. Compared to the conventional blast furnace route, HIsarna achieves a 20% reduction in CO2 emissions by eliminating coking and the need for iron ore agglomeration processes such as sintering and pelletizing and directly receiving fine coal and iron ore. The process is built on a pilot scale capable of producing 8 tons/hour of hot metal. The technology combines the Cyclone Converter Furnace (CCF) technology (developed by Hoogovens/Corus/Tata Steel) with the Smelting Reduction Vessel (SRV) from the HIsmelt technology (developed by RioTinto). Operational since 2010 at the IJmuiden Works of Tata Steel Nederland, it has been continually developed towards industrial demonstration. Fine iron ore and pure oxygen are injected into the CCF. The oxygen is needed as an oxidizer to partially combust the CO-H2 mixture of the off-gas from the SRV. The combustion process supplies heat to pre-reduced ore particles, melting them during their fly time in the CCF. Eventually, the molten ore particles accumulate on the furnace wall, forming a liquid film that drips along the wall and falls into the molten bath of the SRV. Coal is introduced into the slag layer of the bath via a carrier gas to fully reduce pre-reduced iron oxide (FeOx) droplets falling from the CCF above. CO gas is generated in the form of bubbles, rising to the top space of the SRV, where it undergoes partial combustion with oxygen injected through oxygen lances (OL), providing the necessary heat in the SRV.

Through operational analysis of the pilot plant, it has been determined that replacing half of the primary raw material with galvanized steel scrap as a secondary source in the HIsarna process is feasible. This substitution would result in a significant reduction in the injection of fine iron ore. Another advantage is the continuous evaporation of zinc from the scrap surface, accumulating in the off-gas dust, which can later be separated and recovered. In contrast to the blast furnace route, the zinc element does not form a circulating loop inside the reactor but is converted to the oxidized/ferrite form, ultimately ending up in the dust bag and filters.

However, plant measurements and laboratory analysis of the HIsarna dust reveal that the evaporated zinc primarily reacts with available oxygen and iron oxides to form zinc ferrite. This necessitates additional pre-processing steps before feeding into the zinc smelting unit, incurring extra costs. Consequently, the formation of ferrite is deemed undesirable.

In a nutshell, this thesis focuses on developing a precise computational fluid dynamic (CFD) model to predict the behaviour of the HIsarna off-gas system. This model is crucial for predicting temperature and composition profiles within the off-gas system, particularly in zones where data are not measured at the pilot plant. The possibility of zinc ferrite formation reduction and off-gas system is investigated using plant measurements, CFD data analysis, and thermodynamic calculations. Furthermore, the developed CFD model is utilized to propose modification/optimization of the process, reducing iron ore dust escaping the system, reducing post-combustion oxygen consumption, optimizing post-combustion lance, and off-gas system scale-up.

Chapter 1 of the thesis is dedicated to a brief history of ironmaking and introduces the HIsarna process in detail, as well as the research focus and thesis structure. Chapter 2 focuses on establishing and validating a CFD model and offers a detailed description. Chapter 3 provides an extensive discussion of the model selection and sensitivity analysis. This chapter primarily delves into critical insights regarding the reasons behind the choice of sub-models within the CFD model. Flow analysis of the off-gas system is presented in Chapter 4, and in Chapter 5, the behaviour of the escaped ore entering the off-gas system is investigated, and potential solutions to mitigate injected ore losses from the off-gas system are discussed. The modified geometry introduced in Chapter 5 is subjected to analysis using the same validated CFD model, ensuring its effective operation within the entire off-gas system. These findings are discussed in Chapter 6 of the thesis. In Chapter 7, the formation of zinc oxide and zinc ferrite are investigated in the original and modified geometry of the off-gas system, and possible solutions to reduce the ferrite formation are proposed. In Chapter 8, a modification to the oxygen lance is proposed to enhance the combustion of the CO-H2 mixture. This modification involves using a fluidic oscillator instead of injecting oxygen through a conventional nozzle. The results demonstrate an improvement in CO-H2 combustion in the reflux chamber. The proposed geometry is constructed and implemented in the reflux chamber for further evaluation and is discussed in detail.

In Chapter 9 (Part 3), the CFD model developed for the pilot plant is employed to conduct a CFD-based scale-up of the off-gas system to the industrial scale. Within this chapter, the optimized geometry and recommended operating conditions are presented. Conclusions, remarks, and recommendations are presented in the final chapter of the thesis (Chapter 10).