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Dynamic behaviour of liquid-solid systems: Modelling and experiments applied to the blast furnace hearth
Abstract and Figures
The ironmaking blast furnace is an extremely large and complex piece of equipment, which is responsible for the majority of the global crude steel production. Despite its enormous economic impact, much of its working remains unknown. This is mostly due to the extremely high temperature and harsh conditions inside the furnace, which make any internal measurements impossible. This work focusses on the bottom section of the furnace (also known as the hearth), where the liquid iron is collected and tapped, and a bed of unreacted carbon particles floats in this bath of liquid metal. This section of the blast furnace is especially interesting, as the flow of liquid metal damages the heat-resistant walls, thereby determining the lifetime of the furnace.
Both numerical and experimental methods were used to investigate this complex system. First, an existing computer simulation technique, which was previously mostly used for gas-solid systems, was extended to handle the heavy and viscous liquid metal. This new model was compared with experiments on drinking water softening reactors, and was found to predict the liquid-solid behaviour well.
Next, a novel experimental technique termed Magnetic Particle Tracking (MPT) was used to investigate liquid-solid systems. In this technique, sensors measuring magnetic field strength are used to track the movement of a single magnetic particle within a system of non-magnetic particles. First, a rotating-drum filled with cork particles and either water or air was used to show the working of MPT in liquid-solid system, and show the importance of phase between the particles for their behaviour.
A lab-scale model of the blast furnace hearth was constructed, in which room-temperature water and hollow particles were used to represent the liquid iron and carbon particles. The behaviour of the bed was observed under various conditions and alternating liquid levels, and MPT was used to investigate the movement of a single particle within the dense bed. The newly developed simulation technique was used to recreate this system numerically, in order to further verify the accuracy of the model. Additionally, the computer model allows the visualise the flow of the liquid, which was not possible in experiments.
With the numerical model verified through lab-size simulations, it was scaled-up towards the large size of the industrial blast furnace. A series of simulations of a 5 m diameter blast furnace hearth was conducted, in which the particle and liquid iron movement were observed under various circumstances. The mutual interaction between the solid and liquid phases was found to be of great importance. On one hand, the position of the floating particle bed influences the flow pattern of the liquid iron, increasing or decreasing the eroding flows along the furnace walls. On the other hand, the varying liquid level alters the force acting on the particles, influencing the movement and renewal rate of the particle bed.
Lastly, a 10 m diameter hearth simulation was conducted, in which additional calculations for the temperature distribution and carbon dissolution process were implemented. This offers a promising outlook on future work, in which modern supercomputers and models combining various physical phenomena can be used to gain insight in large industrial processes, and study systems which could not be looked into before.
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The blast furnace campaign length is today usually restricted by the hearth life, which is strongly related to the drainage and behavior of the coke bed in the hearth, usually referred to as the dead man. Because the hearth is inaccessible and the conditions are complex, knowledge and understanding of the state of the dead man are still limited compared to other parts of the blast furnace process. Since a number of publications have studied different aspects of the dead man in the literature, the purpose of the current review is to compile the findings and knowledge in a comprehensive document. We mainly focus on contributions with respect to the dead man state, and those assessing its influence on the hearth performance in terms of liquid flow patterns, lining wear and drainage behavior. A set of common modeling approaches in this specific furnace area is also briefly presented. The aim of the review is also to deepen the understanding and stimulate further research on open questions related to the dead man in the blast furnace hearth.
Liquid-solid systems are frequently encountered in industrial processes and it is broadly recognised that numerical simulations are a useful tool for gaining insight in these processes. In this study, the unresolved CFD-DEM approach is extended with a complete momentum coupling for liquid-solid flows. Established correlations are used for the drag and lift forces, while new implementations are introduced for the unsteady interaction forces. A virtual mass force model based on the work of Felderhof (Felderhof 1991) is introduced, which accounts for the local particle volume fraction and the liquid-solid density ratio. The Basset history force, which is usually neglected due to computational difficulties related to its implementation, is evaluated according to the approach proposed by Parmar et al. (Parmar, 2018). A liquid fluidised bed is used as a demonstration case for the extended model. In this work, it is shown that with appropriate stabilisation measures, the Basset history force is approximated accurately (within 5%), while computational efficiency is maintained ( < 30% increase in computational time). Furthermore, the relevance of the complete momentum coupling is demonstrated by analysis of the solids mixing in the liquid fluidised bed. It is shown that when accounting for the complete interaction force, solids mixing is up to 20% slower compared to simulations with the drag-only approach.
In full-scale drinking water production plants in the Netherlands, central softening is widely used for reasons related to public health, client comfort, and economic and environmental benefits. Almost 500 million cubic meters of water is softened annually through seeded crystallisation in fluidised bed reactors. The societal call for a circular economy has put pressure on this treatment process to become more sustainable. By optimising relevant process conditions, the consumption of chemicals can be reduced, and raw materials reused. Optimal process conditions are feasible if the specific crystallisation surface area in the fluidised bed is large enough to support the performance of the seeded crystallisation process. To determine the specific surface area, crucial variables including voidage and particle size must be known. Numerous models can be found in the literature to estimate the voidage in liquid-solid fluidisation processes. Many of these models are based on semi-empirical porous-media-based drag relations like Ergun or semi-empirical terminal-settling based models such as Richardson-Zaki and fitted for monodisperse, almost perfectly round particles. In this study, we present new voidage prediction models based on accurate data obtained from elaborate pilot plant experiments and non-linear symbolic regression methods. The models were compared with the most popular voidage prediction models using different statistical methods. An explicit model for voidage estimation based on the dimensionless Reynolds and Froude numbers is presented here that can be used for a wide range of particle sizes, fluid velocities and temperatures and that can therefore be directly used in water treatment processes such as drinking water pellet softening. The advantage of this model is that there is no need for applying numerical solutions; therefore, it can be explicitly implemented. The prediction errors for classical models from the literature lie between 2.7 % and 11.4 %. With our new model, the voidage prediction error is reduced to 1.9 %.
This study analyses operational data of two small size blast furnaces (BF) with respect to the occurrence of raceway blockages. These blockages mainly occur due to erratic movements in the burden. In a previous project the authors have investigated the various types of raceway blockages and tested different approaches to implement a reliable way to detect such blockages at a very early stage.[1] –[3] Early detection of severe blockage events is important to avoid tuyere damages and trigger operational reactions like, e.g. the shutdown of pulverized coal injection (PCI) branches. The most promising algorithm of the previous project has been implemented in the process control system of the BFs and enhanced with the additional calculation of a strength factor serving as an indicator for the severity of a blockage event. The system is now collecting data on raceway blockages since the beginning of 2019. Since there is now a good data basis available, this paper presents the latest findings on blockage event statistics and the correlation with other operational data of the BF.
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Non-uniform blast injection often occurs in blast furnace (BFs) ironmaking process, but the consequences in the inner state remain unclear under industrial operating conditions. This paper presents a numerical study of three-dimensional (3D) flow and thermochemical behaviors inside a 5000-m3 commercial BF under non-uniform blast injection. This is based on a recently developed computational fluid dynamics (CFD) process model. The model has been validated under experimental and industrial conditions. It is used here to study the effects of tuyere closure on the asymmetric distributions of 3D inner state and the overall performance of the BF. The numerical results show that the increase in inactive tuyere number substantially enlarges the deadman size and shifts the highest deadman boundary from the BF center to the periphery, consistent with the lab-scale experimental observations. Also, the tuyere closure causes significantly asymmetric distributions of gas, solid and liquid velocities and temperature, as well as reactions. The phenomenon occurs mainly in the low part of the BF and is more pronounced at a larger inactive tuyere number. When the inactive tuyere number is large, a "W" shaped cohesive zone (CZ) is observed over the inactive tuyeres, the same as observed by a low-temperature hot experimental BF model. Corresponding to this "W" shaped CZ, a cavity forms in the CZ, beneath which a strong liquid flow develops, causing the substantial non-uniformity in liquid temperature distribution. Additionally, tuyere closure affects carbon consumption and liquid temperature oppositely in the regions over the active and inactive tuyeres. These effects are canceled out by each other, leading to inconsiderable variations of overall performance indicators like top gas utilization factor and liquid temperature.
The three-dimensional (3D) model of erosion state of blast furnace (BF) hearth was obtained by using 3D laser scanning method. The thickness of refractory lining can be measured anywhere and the erosion curves were extracted both in the circumferential and height directions to analyze the erosion characteristics. The results show that the most eroded positions located below 20# tuyere with an elevation of 7700 mm and below 24#–25# tuyere with an elevation of 8100 mm, the residual thickness here is only 295 mm. In the circumferential directions, the serious eroded areas located between every two tapholes while the taphole areas were protected well by the bonding material. In the height directions, the severe erosion areas located between the elevation of 7600 mm to 8200 mm. According to the calculation, the minimum depth to ensure the deadman floats in the hearth is 2581 mm, corresponding to the elevation of 7619 mm. It can be considered that during the blast furnace production process, the deadman has been sinking to the bottom of BF hearth and the erosion areas gradually formed at the root of deadman.
Size segregation of granular mixtures is one of the common types of segregation in a rotating drum. This phenomenon occurs in process equipment such as mixers, ball-mills and rotary-kilns. Here, we studied the segregation of bidisperse spherical glass-beads with size ratio ranging from 0.1 to 0.9 in a rotating drum by experiment and DEM modelling. In the experimental part, solidified-bed sampling was applied. For size ratios larger than the transition amount (0.26), the smaller component tended to accumulate in the core region with a maximum core-shell segregation around the size ratio 0.5, whereas in the opposite case (smaller size-ratios), they percolated near the drum wall. At ratios around the transition ratio (0.26) and at size ratios approaching unity, the most uniform concentration of beads was attained based on the relative standard deviation (RSD).
Blast furnace (BF) remains the dominant ironmaking process worldwide. Central coke charging (CCC) operation is a promising technology for stabilizing BF operations, but it needs reliable and quantified process design and control. In this work, a multi-fluid BF model is further developed for quantitatively investigating flow-thermal-chemical phenomena of a BF under CCC operation. This model features the respective chemical reactions in the respective coke and ore layers, and a specific sub-model of layer profile for the burden structure for the CCC operation. The simulation results confirm that the gas flow patterns and cohesive zone’s shape and location under the CCC operation are quite different from the non-CCC operation. Under the CCC operation, the heat is overloaded at the furnace center while the reduction load is much heavier at the periphery regions; the profiles of top gas temperature and gas utilization show bell-shape and inverse-bell-shape patterns, respectively. More importantly, these differences are characterized quantitatively. In this given case, when the CCC opening radius at the throat is 0.35 m, the cohesive zone top opening radius is around 0.50 m, and the isotherms of CCC operation become much steeper (~ 80 deg) than those of non-CCC operation (~ 60 deg) near BF central regions. In addition, it is confirmed that carbon solution-loss reaction rate can be decreased significantly at BF central regions under CCC operation. The model helps to understand CCC operation and provides a cost-effective method for optimizing BF practice.
The actual state and packing condition of deadman in blast furnace (BF) hearth have always been a matter of great concern. For it directly affects thermal state of BF hearth and relates to the smooth operation and longevity of BF. In this study, a commercial BF was dissected and then core drilling and image processing methods were used to obtain typical samples and characteristics of deadman. The results show that the deadman, with a radius of 80.09% of hearth radius, floated in hearth and surrounded by molten iron. The sample in horizontal direction has an average coke size of 28.04 mm and a void fraction of 0.52 while the vertical sample has an average coke size of 30.55 mm and a void fraction of 0.50. It should be noted that the average diameter of coke is the reference value which will be different depending on different assumptions. The coke size of vertical sample shows decreasing trendfrom topside to middle part and then keeps slight fluctuations.. And the void fraction in the middle of the vertical sample is the largest and gradually decreases toward both the upper and lower ends. Changes in these features of deadman are caused by the renewal of deadman and the flow of molten iron.