Elsevier

Chemical Engineering Science

Volume 204, 31 August 2019, Pages 9-26
Chemical Engineering Science

A numerical investigation into the effect of angular particle shape on blast furnace burden topography and percolation using a GPU solved discrete element model

https://doi.org/10.1016/j.ces.2019.03.077Get rights and content

Highlights

  • Investigation of size and shape dispersity on burden topography.

  • Shape polydispersity influenced burden topography.

  • Size polydiserpsity influenced the interlayer percolation.

  • Shape mainly affects macroscopic particle transport and size mainly affects interlayer percolation.

Abstract

In blast furnaces, burden topography and packing density affect the stability of the burden, permeability of gas flow as well as the heat transfer efficiency. A fundamental understanding of the influence and interaction of coke and ore particles on the burden topography and packing density is therefore essential, in particular, the influence of particle shape polydispersity and particle size polydispersity. In this paper we analyze the effect of particle shape and size polydispersity on the coke and ore charge distribution inside a bell-less blast furnace using the discrete element method (DEM). We first validate experimentally the polyhedral particle model with a simplified lab-scale charging experiment. A comparative study between spheres, with rolling friction to account for shape, and polyhedra is conducted for shape and size polydisperse particle systems. It was found that shape polydispersity mainly influenced the topography of the burden, whereas the size polydispersity mainly influenced the inter-layer percolation, i.e. localized particle diffusion, hence the local spatial packing density. The differences between the spherical particle models and polyhedral particle models on the burden topography are also quantitatively and qualitatively presented, especially on the role of particle shape on the push-up of coke in the centre. This study demonstrates that modelling particle shape effects using spheres with rolling friction is insufficient to fully describe the complex behaviour of shaped particles in a blast furnace, as the particle shape has a noteworthy influence on the burden characteristics.

Introduction

Blast furnaces (BFs) are widely used as a continuous smelting process in the steel industry (Iron and Steel Institute of Japan, 2009, Ghosk, 2008). In this process, successive layers of coke and iron ore are continuously charged from the top of a blast furnace, with additional coke, ore and limestone, while air is continuously injected into the lower section of the furnace through the tuyere, so that carbon (C) from the coke reacts with oxygen (O) in the haematite (Fe2O3) and magnetite (Fe3O4) in the iron ore (Biswas, 1981). This produces off-gas in the form of CO (incomplete combustion) and CO2 (complete combustion), and the main product of carbon contaminated iron ore or as pig iron which is tapped from the bottom to be converted into steel. Around 60% of the entire steel industry energy is consumed by BFs that are also responsible for 90% CO2 emissions (Fujita, 2010). Energy consumption in a blast furnace operation depends on (i) percolation uniformity of interstitial gas flow through the packed-bed to enhance efficient heat and mass transfer, and (ii) greater combustion surface area to facilitate complete combustion. Hence, the gas flow distribution and the furnace performance is directly influenced by the burden topography inside a BF (Kuang et al., 2017).

As the burden topography is largely dictated by the particle properties (e.g. shape and size) and the charging process (Dahlstedt et al., 1999, Dudinski and Gammel, 2006), various innovative charging systems were developed, including the bell-less top (BLT) charging system (see Fig. 1(b)) and the Gimbal charging system (SIMETALCIS Gimbal Top) depicted in Fig. 1(c), to offer more charging control than the one and two bell (with a lower and upper bell as depicted in Fig. 1(a)) systems prevailed until 1972. In order to explore the charging performance of these charging systems, many experimental studies on charging behaviour and burden topography were performed (Takahashi and Komatsu, 1993, Guo and Chen, 2004, Baath et al., 2008, Dahlstedt et al., 1999), so were numerous discrete element method (DEM) simulations, especially on BLT charging and burden distribution using mono-sized (Ariyama et al., 2014, Hou et al., 2017, Qiu et al., 2017, Xu et al., 2018, Peters et al., 2018), polydispersed (Mio et al., 2007, Mio et al., 2009, Zhang et al., 2014, Ariyama et al., 2014, Terui et al., 2017), and clumped spherical (up to a dozen spherical particles) particle systems (Yu and Saxen, 2014, Mitra, 2016, Ariyama et al., 2014, Natsui et al., 2009, Terui et al., 2017), as well as coupled DEM with computational fluid dynamics (DEM-CFD) (Hou et al., 2017, Baniasadi et al., 2018).

From numerical simulations incorporating of fluid and thermofluid interactions with the particles, the following five domains of particle flow inside a BF were identified: (i) quasi-stagnant zone, (ii) wall shear zone, (iii) deflecting flow zone, (iv) converging flow zone and (v) transitional flow zone. Distinct mechanical interactions, i.e. force chain networks (Nouchi et al., 2009), and spatial velocity profiles were observed in these flow regimes (Qiu et al., 2017). Previous studies found that the particle size distribution is an important factor in optimising the uniformity of charge layers and in suppressing segregation for spherical particle systems (Terui et al., 2017). The furnace should be charged in such a way that the coarser and finer particles are distributed appropriately on the burden surface to control the burden distribution, so that the uniform percolation of gas through the bed can be achieved as well as the lining of the BF protected. As a consequence, smaller sized sinter should be discharged in the periphery area for protecting the lining and preventing excessive heat losses, and particles of large sizes in the central area to form a strong central gas flow (Ueda and Natsui, 2010, Shen, 2014). In addition, the mean size of the sinter should gradually increase from the periphery to the centre, in order to attain an increasing gas distribution from the periphery area to the central area (a weak gas flow at the wall area with lower porosity and a strong gas flow in the central line of furnace with higher porosity). Creating a thin iron-bearing material layer at the periphery with a small quantity of small size sinters leads to a thin cohesive zone at the lower part of furnace and a reduced pressure drop of whole furnace. Although the benefit of the controlled burden distribution (i.e. burden of a controlled microstructure achieved with an optimal distribution of feed materials) has been anticipated, it is a challenging task to realise it in practice. Hence, only the height of the burden layer is currently controlled in the blast furnace through the control of the mass flow rate in a rotating chute discharging system.

In addition, the importance of particle shape on the burden topography and the gas dynamics through the particle bed was also highlighted (Hilton and Cleary, 2012, International, 2012, Xu, 2006, Mio et al., 2017). Nevertheless, in previous simulations the particle shape was generally modelled using spheres or clumped spheres, and particle angularity was normally ignored. Moreover, numerical studies with shape polydispersity remains scarce (Yu and Saxen, 2014, Mitra, 2016, Ariyama et al., 2014), so the influence of particle angularity on the charging behaviour and burden formation is still poorly understood. Therefore, this study aims to investigate the influence of angularity for polyhedral particles in the layering and stability of particle bed during charging of a BF. In particular, the impact of shape polydispersity and size polydispersity of the coke and iron ore on the burden formation will be explored. Our hypothesis is that only through appropriate models can a thorough understanding be obtained for the charging behaviour as well as burden formation and descent.

Section snippets

Discrete element method

Granular media (GM) is only second to water as the most manipulated substances on the planet (de Gennes, 1999), and is encountered in almost all industries. The understanding of the physical behaviour of GM is critical in design and operation of process equipment as it exhibits various complex phenomena such as percolation, elutriation, agglomeration and flow-induced mechanisms (Scott and Bridgwater, 1975, Pathak et al., 2017). Thus a number of computational approaches were developed (Khakhar

Experimental validation

BlazeDEM-GPU (Govender et al., 2016) has been validated for a number of applications (Govender et al., 2014, Govender et al., 2015, Govender et al., 2015, Govender et al., 2018, Wilke et al., 2016, Govender et al., 2018, Govender et al., 2018) and also used by a increasing number of users in the Americas, Europe, UK, Africa, Asia and Australia. In this study we further verify that the code does indeed capture the macroscopic behaviour of charging for the choice of parameters used in this study.

Modelling assumptions

Fig. 12 depicts the geometrical setup which consists of a filling silo (Part 1) where particles are generated, a hopper (Part 2) through which the particles are discharged onto a chute, rotating at 10 rpm, with a fixed inclination of 60 degrees similar to Xu et al. (2011), denoted as Part 4, filling a cylindrical throat (Part 3). A typical blast furnace has a diameter of around 10 m with a typical maximum particle size of 100 mm, giving a ratio of 100:1 (ignoring fines (Zhang et al., 2014))

Conclusions

In this study we first validated the simulated polyhedral charge layering against experimental charge layering results. This was followed by a study on the effect of shape polydispersity and size polydispersity of polyhedral particle systems on the burden topography. It was found that shape polydispersity mainly influenced the burden topography of the burden, whereas the size polydispersity influenced the inter-layer percolation due to the better flowability of the size polydispersed particle

Conflict of interest

The authors declared that there is no conflict of interest.

Acknowledgments

This work was supported by the MARIE Sklodowska-CURIE Individual Fellowships with acronym DECRON, funded through the People Programme (MARIE Sklodowska-CURIE Actions) of the European Union’s H2020 under REA grant agreement No. 747963. We gratefully acknowledge the support of the NVIDIA Corporation with the donation of the Titan X Pascal GPU used for this research. The financial support of the National Research Foundation (NRF) of South Africa is acknowledged.

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      The repose angle is generally used as a calibration target since it is known to be affected by friction coefficients [143,172–174]. In some studies, the rolling friction coefficient is omitted when complex shapes are used to model irregular particles (e.g., the work of Schott et al. [122], Govender et al. [124] and Bester et al. [175]), so that the calibration procedure is directed towards only the sliding friction coefficient. Others included the rolling friction coefficient since they modelled particles as spheres (e.g., Liu et al. [116], Li et al. [169] and Chibwe et al. [176]), or even though non-spherical particles were used (e.g., Yu & Saxén [43], Chakrabarty et al. [145] and Chen et al. [92]).

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