Hearth Coke Bed Buoyancy in the Blast Furnace: Experimental Study with A
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2006:235 CIV MASTER’S THESIS Hearth Coke Bed Buoyancy in the Blast Furnace Experimental study with a 3-dimensional cold model PÄR SEMBERG MASTER OF SCIENCE PROGRAMME Luleå University of Technology Department of Chemical Engineering and Geosciences Division of Process Metallurgy 2006:235 CIV • ISSN: 1402 - 1617 • ISRN: LTU - EX - - 06/235 - - SE Abstract ABSTRACT The effect of buoyancy forces acting on the hearth coke bed or the “deadman” of the blast furnace has attracted a great deal of interest during recent years of blast furnace research. This thesis presents an effort to elucidate the particle movement patterns connected to formation of a coke free layer in the hearth. In the work, the impact of different pressure distributions on the behaviour of the particle bed was studied with an experimental 3D-cold blast furnace hearth model. The bases for the work were the results from tests along with two different numerical models from a previous study, Hearth coke bed buoyancy, a preliminary investigation, made in 2004 at Bluescope Steel Research, Port Kembla, Australia. In the model, a bed of plastic particles in water were subjected to different pressure distributions, and float-sink motions induced by accumulation and drainage of the water through a valve in the bottom. The results showed that the bed was quite resistant to internal particle movements, when subjected to different linear pressure distributions along the radius. However, previous studies have suggested the downward pressure under the raceways to be severely reduced, and when going below 15:85 in pressure ratio between the peripheral and central area, it was observed that the particles moved internally. As the central load was descending in the bed, upward particle movements were observed along the walls, as well as from the centre towards the walls on the bottom. Particle movements were strongly dependent on the sink-float motions, and moved relative to one another only during drainage, when particles under the central weight moved down faster than under the peripheral reduced pressure area. This mechanism resulted in formation of a peripheral free space in the bottom of the hearth. Addition of an agglomerate of particles to the bottom of the particle bed, resulted in less particle movements and retarded formation of the peripheral free space. Initially it was intended to carry out these tests also numerically using Bluescope Steel’s particle simulation package DPSim. Because of problems with validating DPSim for the application with float-sink motions induced by buoyancy forces, this section was constrained to a sensitivity analysis on the program. The study showed that the float-sink behaviour of the particle bed was insensitive to friction parameter adjustments in the tested range. It was also discovered that the assigned simulation time was of significant importance. This indicates that the common practise of reducing the runtime may not be valid for this application. i ii Acknowledgements ACKNOWLEDGEMENTS This Thesis was carried out at Bluescope Steel Research, Port Kembla, Australia, as the final step on my way to a master’s degree in Chemical Engineering at Luleå University of Technology, LTU. First, I would like to express my sincere gratitude to Doctor Paul Zulli at Bluescope Steel Research and Professor Bo Björkman at the faculty of process metallurgy, LTU, for making this numerable experience in Australia possible for me. I would also like to thank Paul for supervising, for inspiration and encouragement, and for the freedom I have been given throughout my work. I would like to thank Bryan Wright, David Pinson and Ian Bean. Bryan and David for supervising and support with the numerical work, and Ian for helping with the experimental equipment. Thanks to Bryan also for helping me with everything and anything from the first day of my work until the last. Finally, I would like to thank all the others in the unit for six very rewarding months as a member in the group. I have very much appreciated the open door policy, all valuable discussions and feedback, and your longsuffering patience with my almost never ending questions. Pajala, March 2006 Pär Semberg iii iv Table of contents TABLE OF CONTENTS INTRODUCTION 1 1 LITERATURE REVIEW 3 1.1 Overview of the blast furnace 3 1.1.1 The upper zone 3 1.1.2 The cohesive zone 4 1.1.3 Force modelling in the upper- and cohesive zone 4 1.1.4 The lower zone 4 1.1.5 Force balance in the lower zone 7 1.2 Conclusion 12 2 EXPERIMENTAL 15 2.1 Introduction 15 2.2 Equipment 15 2.3 Experiments conducted 16 2.4 Uniform, central and peripheral pressure distributions 17 2.4.1 Equipment 17 2.4.2 Results 18 2.4.3 Discussion 21 2.5 Laser tracking of bed movement 21 2.5.1 Equipment 22 2.5.2 Results 23 2.5.3 Discussion 25 2.6 Tests with extreme distributions 27 2.6.1 Equipment 27 2.6.2 Results 28 2.6.3 Discussion 32 2.7 Bottom profile 33 2.7.1 Equipment 33 2.7.2 Results 34 2.7.3 Discussion 35 2.8 Faoled bed 36 2.8.1 Equipment 36 2.8.2 Results 37 2.8.3 Discussion 38 3 NUMERICAL WORK 39 3.1 Introduction 39 3.2 Method 40 3.3 DPSim Buoyancy model and modelling parameters 40 v 3.4 Experimental equipment and procedure 42 3.4.1 Box test for estimating the angle of repose 42 3.4.2 Experimental buoyancy model 43 3.5 Results 44 3.5.1 Box tests 44 3.5.2 Buoyancy tests 45 3.6 Discussion 54 CONCLUSIONS AND RECOMMENDATIONS 57 REFERENCES 59 APPENDIX 1 APPENDIX 2 vi Introduction INTRODUCTION The blast furnace has been used for producing iron from ore since at least the 14 th century in the Nordic countries, and the process is still today responsible for the majority of the steel production in the world. In the past, “know how” of the blast furnace process most probably was limited, and obtained by practical experiences. Not until recently, during the 20th century, have efforts in terms of applied research been made. The process is capital intensive, and in order to be cost efficient the furnace must be run at long operational lifetime, high availability and high efficiency(Wright, 2002). Today there is a fairly good understanding of the general principles governing the blast furnace ironmaking process. The blast furnace is basically a high temperature, counter current, multiphase reactor where descending iron oxides are reduced by ascending carbon monoxide gases, produced in the lower level of the vessel. The ore and coke are packed in discrete layers to maintain sufficient permeability in the bed, even after the ore loses its permeability due to softening and melting further down in the shaft. From the softening/melting zone where all the ore has smelted, only coke remains intact with the molten metal on the bottom of the vessel, which is known as the hearth. Based on temperature variations in the refractory lining of the hearth, it has been suggested that the coke bed under some conditions is floating on the molten metal and slag, and sometimes sitting on the bottom of the furnace. During recent years, quite a lot of effort has been made to explain the principles underlying the floating/sitting state of the hearth coke bed. However, the mechanisms proposed are novel, and not yet well understood. At Bluescope steel research, a preliminary study, “Hearth Coke Bed Buoyancy- a preliminary investigation” was carried out experimentally, and numerically in order to shed some light on the fundamental principles governing the hearth behaviour. This work was an approach to model the hearth coke bed by force balances, with results indicating that bouyancy forces, and the casting cycle possibly could explain sitting and floating behavior of the dead man. The aim of this thesis is to use the findings from the preliminary study and take a step further towards a deeper understanding for the physical phenomenas and conditions of the blast furnace hearth. Of particular interest, is the force balance of the hearth and the importance of buoyancy forces on the proposed floating and sitting states of the hearth coke bed or “deadman”. The previous work included preliminary evaluations by both experimental and numerical methods, and both these paths will be considered in this work as well . 1 2 1 Literature review 1 LITERATURE REVIEW 1.1 OVERVIEW OF THE BLAST FURNACE The question of whether the coke in the hearth is floating or sitting during operation can be answered by a complete force balance on the whole furnace. In order to gain some understanding of the parameters possibly affecting the state of the deadman, the different parts of the furnace are first explained. 1.1.1 The upper zone The principal raw materials of iron ore, coke and limestone are fed from above through a chute that distributes the raw materials into discrete layers of coke and iron ore, see fig 1.1. The ore layers are blends of sinter, lump and pellets. The coke is usually fed to make the coke layer slightly thicker in the center in order to enhance productivity and obtain optimum gas utilisation. As coke is combusted in the blast zones known as raceways in the bottom of the shaft, the bed slowly moves down as the iron oxides are reduced by the ascending CO-gases. The ore, mainly hematite, in the reduction process goes through the oxidic states magnetite and wustite before formation of iron and melting occur further down. The residence time in the upper zone is usually 4-6 hours depending on radial location in the furnace.