Raceway Formation Research Articles

Mechanical Analysis of the Forces Involved in a Pilot-Scale Blast Furnace Raceway Formation by Means of CFD/DEM Simulations

The CFD/DEM approach was used for investigating the forces playing a role in a furnace raceway formation and stability. The configuration is an actual pilot-scale hot blast furnace filled only with coke particles. In such a system, the raceway was unstable, with successively a growing phase and a collapse. The CFD/DEM numerical simulations were coupled with a core-shrinking model to mimic coke particle combustion. However, the kinetic reactions and heat transfers were not numerically predicted. Instead, the characteristic combustion timescale of one coke particle was imposed, and the combustion zone was adjusted to match the global combustion measured in the pilot-scale experiment. The results showed that the standard contact model was not enough to resist the pressure exerted by the granular weight on the raceway. However, the addition of a cohesive force, through the Johnson-Kendall-Roberts (JKR) model, allowed the qualitative reproduction of the gas pressure fluctuations and the collapse cycles in accordance with the experiment. A sensitivity analysis of the flow rate showed that CFD/DEM is able to reproduce quantitatively the time between two collapses, as observed in the experiment. Predicted raceway size and shapes are also in agreement with the experimental observations in the range of investigated parameters.

Effect of particle shape on raceway size and pressure drop in a blast furnace: Experimental, numerical and theoretical analyses

Experiments and simulations are conducted to explore the changing laws of blast furnace (BF) raceway morphology and pressure drop with cylindrical particles. Experimental data show that there are five typical stages for the pressure drop during the raceway formation. The closer the aspect ratio (Ar) of the particle to 1, the bigger the raceway size and the wider the particle moving band will be. When the raceway is in stable stage, the pressure drop can be ascribed to the cooperative action of the bed height, inlet gas velocity and Ar. Numerical results reveal that the formation of large raceway for sphere-like particles is due to the small drag and contact forces. The contact forces in the prolate particle system are very large and thus result in a small raceway. Finally, the influence of particle shape is employed to improve a raceway size predictive correlation which can increase the average calculational accuracy by 3.4%.

Importance of considering interstitial fluid effects in the kinetic theory of granular flow for raceway formation prediction

Owing to its computational efficiency, the two-fluid model is widely used in computational fluid dynamics (CFD) to describe gas–solid flows in large-scale industrial processes. The motion of the solid phase is commonly modeled by the kinetic theory of granular flow (KTGF). The main conceptual difference between the two commonly used KTGF closure relations, namely the closure models by Lun e al. (1984) and Agrawal et al. (2001), is the treatment of the interstitial fluid. We theoretically compare both models and investigate their effects on simulation results. The interstitial fluid effects are essential during the raceway shape prediction but can be neglected in the fluidized bed regime. The correct prediction of the raceway size and shape is essential for predicting the processes in the raceway zone of ironmaking blast furnaces. Our results show that considering the interstitial fluid effects in the KTGF enables more accurate predictions in the raceway regime.

Numerical Analysis of Raceway Formation in Isothermal and Non-reactive Packed Bed

Numerical Analysis of Raceway Formation in Isothermal and Non-reactive Packed Bed

CFD-DEM study of coke combustion in the raceway cavity of an ironmaking blast furnace

A void zone, the so-called raceway, may be formed when gas is laterally injected into a particle packed bed and the char particles may combust when the gas enters at a high temperature for example coke combustion in the raceway of ironmaking blast furnaces. Experimental and numerical studies of multiphase flow in the raceway have been conducted widely in recent years, however, the study of reacting flows in the raceway is still limited at a particle scale. This work is to study the gas-solid reacting flows in the raceway using a particle scale CFD-DEM approach, featuring heat and mass transfers and chemical reactions between the gas and solid phases. The simulation results are comparable with the measurements in the physical experiments and previously calculated results. The key phenomena of flow and thermochemical behaviour related to the raceway formation at the particle scale are obtained. Then the model is used to study the dependency of raceway formation and thermochemical behaviour on several key raceway variables including gas inlet velocity, particle size, bed height and particle discharge rate. The simulation results indicate that before generating a stable raceway, a larger blast inlet velocity or a larger discharge rate can form a larger raceway cavity, while the effect of packed bed level shows an opposite trend. The model provides fundamental insights into the complex reacting flows in the raceway zone for a better understanding and optimization in operations.

Influence of particle shape on microstructure and heat transfer characteristics in blast furnace raceway with CFD-DEM approach

Coupled computational fluid dynamics and discrete element method (CFD-DEM) simulations are conducted to investigate the influence of particle shape on microstructure and heat transfer characteristics in a blast furnace (BF) raceway. Spherical and non-spherical particles including tetrahedron-like and octahedron-like shapes are involved in this work. The applicability of the coupled approach and the accuracy of combined sphere method (CSM) to investigate the kinetic and thermodynamic behaviors are validated firstly based on the experimental results. Then, the influences of particle shape on raceway formation, microstructure, and particle temperature evolution are comprehensively explored. The numerical results show that the tetrahedron-like particle system presents a relatively stronger interlocking efficiency and thus leads to a smaller raceway size. Additionally, the non-uniform performance of temperature in this system is more striking than the other two cases. However, the bigger raceway size exists in the octahedron-like particle system and the average particle temperature is also higher than other cases; meanwhile, this system possesses more powerful rotational kinetic energy but lower drag force required to form and maintain the raceway than tetrahedron-like system under the same inlet velocity. These findings could be beneficial for the in-depth understanding of microstructure and heat transfer characteristics within a BF raceway.

Numerical study on pulverized coal combustion in a raceway from the viewpoint of coal plume

The combustion efficiency of pulverized coal particles in a blast furnace depends strongly on the particle flow behaviors and the shape of the main combustion zone, i.e., the raceway shape. With a coupling method, a raceway formation model and a multiphase reaction model were integrated to predict the raceway shape. The pulverized coal combustion under different operating conditions were simulated, and the combustion performance was analyzed regarding a low-burnout region inside the coal plume. The results show that the raceway shape affects the expansion degree of the plume and then the combustion performance. Increasing the oxygen enrichment rate in the blast significantly promotes the combustion reactions of particles in the inner region of the plume. With an optimal blast rate, the overall burnout rate can be improved with the centered particles having the highest oxygen accessibility.

Numerical investigation of the effects of size segregation on pulverized coal combustion in a blast furnace

Pulverized coal combustion in a blast furnace raceway can play a critical role in blast furnace iron-making operations. This research study integrated the particle combustion and the raceway formation processes using a refurbished coupling method based on computational fluid dynamics (CFD). The CFD model was validated, and there was good agreement between the simulation results and the two sets of experimental data. The combustion behaviors of pulverized coal particles in an industrial blast furnace were also simulated over a wide size range. The simulation results showed that size segregation occurred along the vertical direction in the raceway. Along the coal plume, the effectiveness of the size segregation process can be significantly reduced by the turbulence generated owing to the existence of the lance, the expansion of the gas jet near the tuyere tip, and the gas recirculation at the front end of the raceway. The burnout rate of particles smaller than 60 μm was also shown to be sensitive to the degree of weakening of the size segregation. The particle distribution in the coke bed indicated that the group of particles with diameters equal to 52.5 μm was associated with the largest proportion of unburnt char. This research extended the applicability of the existing numerical methods and provided a better understanding of the pulverized coal injection (PCI) process from the viewpoint of size segregation, which is beneficial to the further optimization of PCI usage

Raceway Formation in a Moving Bed

Raceway is formed in the blast furnace when high velocity air is blown into a bed of coke particles. The blast furnace is treated as a moving bed as the burden descends to replace the coke particles consumed by the combustion in front of the tuyeres. In this study, an experimental setup is constructed using Perspex sheet to replicate the moving bed and a detailed study of raceway size is carried out with respect to different parameters such as gas velocity, bed height and particles flow rate. Compressed air is used as the gas phase and granules of Linear Low Density Polyethylene (LLDPE) are used as the packing particles. Results confirm the existence of the cavity hysteresis phenomenon in a stationary packed bed. For moving beds, it is found that the cavity hysteresis phenomenon is absent, except at a very low particles flow rate. It is also observed that raceway size in the moving bed is independent of the bed height and solid mass flow rate, except for a very low solid/particles flow rates. Results confirm the previous hypothesis that if there are no frictional forces then there would not be any raceway hysteresis in the system. The study also confirms that the raceway size obtained in a stationary packed bed for decreasing gas velocity condition is similar to that obtained in the moving beds.

CFD-DEM simulation of raceway formation in an ironmaking blast furnace

This paper presents a CFD-DEM study of gas-solid flow in the raceway region in an ironmaking blast furnace (BF). In the simulation, 120,000 spherical particles are packed into a full-scale 2D slot BF geometry. Gas is injected into the geometry via tuyeres, generating raceways in different sizes and shapes under different conditions. It is observed that the raceway characteristics are much more complex under the full-scale BF than those observed under the laboratory scale in the literature. Three kinds of raceways can be identified: anti-clockwise circulating raceway, clockwise circulating raceway, and plumelike raceway. The results are analysed in terms of solid flow patterns, and flow and force structures of particles. The simulation has a good agreement with the observed in physical experiments in terms of two contrary circulating gas vortexes located upon and below tuyeres during raceway formation. Moreover, it is also indicated that operational variables have significant effect on one of the gas vortexes, which becomes the main circulating gas flow stream and determines the gas circulating direction.

A Mechanism Model for Raceway Formation and Variation in a Blast Furnace

In this article, a previous mechanical model is extended to predict raceway penetration in a blast furnace (BF) and to dynamically illustrate how raceway penetration varies over time after the blast velocity varies based on Newton’s second law. The model is validated by industrial measurements, and more precise predictions have been obtained using the present model. Moreover, the effects of combustion reactions on the raceway shape and size are taken into account in the present model. The mechanism for raceway formation and variation revealed by the present model is as follows: Fast movements of packed bed above a raceway roof due to blast blowing rate variation make raceway size vary rapidly and form its prototype; combustion reactions modify raceway size and shape, and they maintain its stability.

Raceway formation in laterally gas-driven particle beds

The formation of a circulating region of particles or ‘raceway’ occurs in many processes when gas is laterally injected into a bed of particles. The granular dynamics occurring during raceway formation is still, however, not fully understood as there are significant technical difficulties to experimentally gathering data within an active particle bed. We use a coupled Discrete Element and Navier–Stokes numerical solver to simulate these systems in order to investigate the dependency of raceway formation and dynamics on gas injection velocity, particle shape and bed pressure. Lateral gas injection into a particle bed is found to give a transition from static to circulating particle flow at a specific gas inflow velocity. A theoretical model is presented, based on a force balance at the gas inlet location, giving an expression for this gas inflow velocity. This model gives good agreement with numerical simulations, showing that raceway formation is governed by the balance between lateral bed loading and the drag forces on particles around the gas inlet. Furthermore, we find that non-spherical particles can form raceways at lower gas inflow velocities than spherical particles. Highly aligned, packed, regions are also observed in simulations of non-spherical particles, closely corresponding to the location of high-density regions found in experimental measurements.

高炉羽口から入射したマイクロ波反射強度によるレースウェイ形成挙動の解析

The blast furnace raceway formation under the intensive coal injection by measurement of micro wave reflection gunned through a tuyere is discussed. As the flow rate of coke as the momentum of coke into raceway decreases by combustion of coal injected into tuyeres, the depth of raceway defined as the maximum position of micro wave reflection is easy to contract. It is expected that this new technology could detect the raceway collapse phenomena in short time and that the stability of raceway in coal injection is different from that in all coke operation. Finally the paper ends by summarizing the effect of raceway formation on unsteady phenomena forcing functional disorder on blast furnace performance of burden distribution for high productivity performance.

Analyses on Blast Furnace Raceway Formation by Micro Wave Reflection Gunned through Tuyere

The blast furnace raceway formation under the intensive coal injection by measurement of micro wave reflection gunned through a tuyere is discussed. As the flow rate of coke as the momentum of coke into raceway decreases by combustion of coal injected into tuyeres, the depth of raceway defined as the maximum position of micro wave reflection is easy to contract. It is expected that this new technology could detect the raceway collapse phenomena in short time and that the stability of raceway in coal injection is different from that in all coke operation. Finally the paper ends by summarizing the effect of raceway formation on unsteady phenomena forcing functional disorder on blast furnace performance of burden distribution for high productivity performance.

Cold model study of raceway under mixed particle conditions

The raceway phenomenon has been studied using mixed particle systems in a two-dimensional cold model. No study is available in the existing literature for the raceway under the mixed particle conditions that prevail in operating blast furnaces. Two types of material, quartz and polypropylene, in different shapes and sizes have been considered. These particles are mixed in various proportions to study the effect of mixed particles on void and raceway formation and raceway breaking. The effects of particle diameter, density, and gas flowrate have been studied. The results obtained from the mixed particles have been compared with the results obtained from particles of uniform size. Experiments have been carried out using models made of glass and Perspex in order to study the effect of fabricating materials on the results. Some interesting results have been obtained in the study and are reported here. Correlations have been proposed to predict void and raceway formation, raceway growth, and raceway breaking under mixed particle conditions.

Particle‐scale modelling of gas–solid flow in fluidisation

AbstractVarious approaches have been proposed to model the gas–solid two‐phase flow at different time and length scales, including the so‐called two‐fluid model (TFM), direct numerical simulation (DNS) and combined continuum and discrete model (CCDM). This paper briefly discusses the key features of these models and their relative merit with special reference to modelling gas fluidisation. Focus is then given to CCDM in which the motion of individual particles is obtained by solving Newton's second law of motion and fluid flow by the Navier–Stokes equation based on the concept of local average. The applicability of CCDM is highlighted by its successful simulation of complicated phenomena associated with the transition between fluid‐like and solid‐like behaviour in raceway formation and bed expansion. At the same time, the usefulness of the resulting particle‐scale information is demonstrated in elucidating the fundamentals governing the gas–solid flow. Finally, areas for future development are discussed.© 2003 Society of Chemical Industry

99/00617 Blast furnace models to analyse raceway zone formation and to predict lining life

99/00617 Blast furnace models to analyse raceway zone formation and to predict lining life

97/04081 Transient heat transfer analysis in vacuum furnaces heated by radiant tube burners

This paper presents a CFD-DEM study of gas-solid flow in the raceway region in an ironmaking blast furnace (BF). In the simulation, 120,000 spherical particles are packed into a full-scale 2D slot BF geometry. Gas is injected into the geometry via tuyeres, generating raceways in different sizes and shapes under different conditions. It is observed that the raceway characteristics are much more complex under the full-scale BF than those observed under the laboratory scale in the literature. Three kinds of raceways can be identified: anti-clockwise circulating raceway, clockwise circulating raceway, and plumelike raceway. The results are analysed in terms of solid flow patterns, and flow and force structures of particles. The simulation has a good agreement with the observed in physical experiments in terms of two contrary circulating gas vortexes located upon and below tuyeres during raceway formation. Moreover, it is also indicated that operational variables have significant effect on one of the gas vortexes, which becomes the main circulating gas flow stream and determines the gas circulating direction.

A fundamental study of raceway size in two dimensions

Blast furnace raceway zones are formed by the force of the air blast injected through the tuyeres evacuating a region of the packed bed directly in front of these tuyeres. Raceway depths in blast furnaces have been historically predicted through the use of empirical correlations based on measurements on cold and hot models. These correlations are not found to be universal in application, however, with many researchers finding only fair agreement between their experi-mental data and the correlations proposed by other researchers. We present here an alternative physical mechanism approach for raceway formation based on examination of the fundamental properties of the system. The study includes two-dimensional experiments where raceway depths and shapes are measured and an accompanying theoretical and numerical analysis of the under-lying mechanisms. Gas flow distributions around the raceway zone are also examined. It is found that the raceway size for given blast conditions and particle properties is such that the total gas drag on the solids vertically above the raceway balances the solid bed weight, with some allowance for solid holdup by walls. The formulation of this theory leads to the further conclusion that the total surface area of the raceway walls as a fraction of the horizontal cross-sectional area of the container or furnace is a unique function of three factors: (a) the gas flow rate as a fraction of the gas flow rate required to fluidize the bed, (b) a particle Reynolds number calculated at the particle's incipient fluidization velocity, and (c) the shape of the horizontal cross section of the bed.