Al
l
er
t
Adema
DEM-
CFD
MODELLI
NG
OF
THE
DEM‐CFD Modelling of the
ironmaking blast furnace
PhD Thesis
March 2014
Allert Adema
This research described in this thesis was performed in the Department of
Materials Science and Engineering of Delft University of Technology.
This research was performed under project number MC5.06255 in the frame of the
Research Program of the Materials innovation institute M2i in the Netherlands
(www.m2i.nl)
DEM‐CFD Modelling of the
ironmaking blast furnace
Proefschrift
Proefschrift ter verkrijging van de graad van doctor
aan de Technische Universiteit van Delft;
op gezag van de Rector Magnificus prof. ir. K.Ch.A.M. Luyben;
voorzitter van het College van Promoties
in het openbaar te verdedigen op dinsdag 4 maart 2014 om 12:30 uur
door
Allert Tjipke ADEMA
ingenieur in de Technische Aardwetenschappen
geboren te Surhuizum
Dit proefschrift is goedgekeurd door de promotor:
Prof.dr. R. Boom
Co‐promotor:
Dr. Y. Yang
Samenstelling promotiecomissie:
Rector Magnificus
voorzitter
Prof.dr. R. Boom
Technische Universiteit Delft, promotor
Dr. Y. Yang
Technische Universiteit Delft, co‐promotor
Prof.dr. H. Saxen
Åbo Akademi University, Finland
Prof.dr. ir. J.A.M. Kuipers
Technische Universiteit Eindhoven
Dr.ir. T. Peeters
Tata Steel R&D, IJmuiden
Prof.dr. J.H.W. de Wit
Technische Universiteit Delft
Prof.dr. ir. J. Sietsma
Technische Universiteit Delft
Prof.dr. I.M. Richardson
Technische Universiteit Delft, reservelid
Keywords: blast furnace, solid flow, gas flow, modelling, coupled DEM‐CFD,
cohesive zone
ISBN: 978‐94‐91909‐06‐1
Copyright ©2014, by A.T. Adema
All rights reserved. No part of the material protected by this copyright notice may
be reproduced or utilized in any form or by any means, electronic or mechanical,
including photocopying, recording or by any information storage and retrieval
system, without written permission from the author.
Table of contents
Chapter 1 Introduction
1.1.
Blast furnace ironmaking
1
1.2.
Research question of this thesis
7
1.3.
Structure of this thesis
13
References
14
Chapter 2 Blast furnace ironmaking
2.1.
Introduction
15
2.2.
Overview of integrated steelmaking process
15
2.3.
Blast furnace process and equipment
18
2.4.
Chemical reactions
19
2.5.
Burden materials
21
2.6.
Gas and solid flow
24
2.7.
Cohesive zone
25
2.8.
Process disruptions
26
References
27
Chapter 3 Review of blast furnace modelling
3.1.
Introduction
29
3.2.
Continuum modelling
30
3.3.
DEM Modelling
34
3.3.1. Burden flow
34
3.3.2. Raceway modelling
53
3.3.3. Burden charging modelling
56
3.4.
Conclusion
59
References
61
Chapter 4 Development of a DEM‐CFD blast furnace model
4.1.
Introduction
65
4.2.
Modelling methods
66
4.2.1. CFD Modelling
66
4.2.2. DEM Modelling
71
4.2.3. DEM‐CFD Coupling
73
4.3.
Modelling approach
75
4.3.1. Lab scale slot model
75
4.3.2. Pie‐slice model of scaled‐down blast furnace
83
4.3.3. Blast furnace model with different geometries
87
4.3.4. Experimental blast furnace model
91
4.3.5. Charging and burden descent models
104
4.4.
Concluding remarks
112
References
113
Chapter 5 Conclusions and recommendations
5.1.
Conclusions
115
5.2.
Recommendations
118
Appendix A Parallel development of sub‐models
121
References
128
Summary
129
Samenvatting
131
List of publications
133
Acknowledgements
135
Curriculum Vitae
136
Chapter 1
Introduction
1.1. Blast furnace ironmaking
In 2012 the world production of iron was 1100 million tonnes [1] equivalent to approximately 90 million tons per month; about 75 % of this was created from pig iron produced by blast furnace ironmaking. Every second there is a staggering 26 000 kg of liquid iron (called pig or iron hot metal) produced by this process worldwide. The pig iron is further processed into steel for use in e.g. the construction, packaging or automotive industry.
The blast furnace itself is a very large reactor of approximately 40 m high and 15 m wide, producing over 10 000 t/d of pig iron. The process is counter‐current and takes place at high temperatures. The solids, iron oxides (in the form of pellets, pellets or lump ore) and coke, are charged in layers at the top of the furnace as shown in Figure 1.1. Oxygen enriched hot air is blown into the furnace through so called tuyeres at the bottom. Coke reacts with oxygen to form the reduction gas, carbon monoxide, and part of the heat required in the process. As the reduction gas rises through the furnace it reacts to reduce the oxides creating liquid iron and carbon dioxide. Molten iron is tapped from the bottom of the furnace as well as molten slag which contains the remaining nonferrous materials.
Figure 1.1. Zones in the blast furnace [2]
During the descent through the furnace the solid material charged at the top slowly heats up and at a temperature of 1100 °C the ore starts softening and melting. The zone in the furnace where the softening and melting takes place is called the cohesive zone. Below the cohesive zone only coke is present in solid form with liquid iron and slag trickling down. Where the blast enters the furnace the Raceway is created; a large cavity created by the high gas velocity and consumption of coke, with temperatures of approximately 2250 °C. In the centre of the furnace hearth is the so‐called ”deadman”, a slowly moving pile of coke.
Blast furnace ironmaking has been used for centuries and has evolved into an extremely efficient process. Revolutionary improvements such as the use of coke instead of charcoal in the 18th century are combined with a continuous fine‐tuning of the process. The use of coke per tonne of hot metal, the amount of reductant required, has nearly halved in the last 50 years as can be seen in Figure 1.2.
Figure 1.2. Decrease in reductant rate due to technological improvement [3]
A blast furnace today can operate continuously for more than 15 years before it requires the inner lining to be replaced. In Table 1.1 the evolution in blast furnace size and production at Tata Steel, IJmuiden (former Hoogovens and Corus) is shown. From the small Blast Furnace No. 1 to the much larger No. 6 and 7 currently in use, the production has vastly increased. The final or current productivity of each furnace is more than double of what is was initially.
The rapid development of China in recent years has greatly increased the global iron and steel production. Figure 1.3 shows that blast furnace production in China has more than tripled between 1980 and 2000. Growth then really took off and production grew another 5 times from 2000 to 2012 level, a total of 17 times the 1980 level. During the same period iron production in the rest of the world grew to a maximum in 2007 with 23%, after which the global financial crisis caused a severe drop in production. This increase in production went together with a shift to larger volume blast furnaces. Tables 1.2 and 1.3 show that for the initial growth years between 2001 and 2006 the distribution of furnace capacity over the volume ranges still remained the same, but the total number of blast furnaces increased from 196 to 475. After 2006 the Chinese iron production capacity in blast furnaces with an inner volume of less than 1000 m3 went from 41% to the current level of 19%. This decrease of share in capacity shifted entirely to furnaces with an inner volume
>2000 m3. The number of <1000 m3 furnaces has also significantly decreased from 364 to 206. Compared to the rest of the world China still has a relatively large percentage of the total production capacity in small furnaces, but is actively restructuring production to larger and more efficient furnaces [4].
Table 1.1. Development of the blast furnaces at Tata IJmuiden, The Netherlands [2] 1 2 3 4 5 6 7 Hearth diameter (m) 4.8/5.6 4.8/5.6 5.2/5.9 8.5 8/9 10/11 13/13.8 Working volume (m3) 519 519 598 1413 1492 2328 3790 Built 1924 1926 1930 1958 1961 1967 1972 Initial production (t/d) 280 280 360 1380 1700 3000 5000 Most recent production (t/d) 1000 1000 1100 3500 3700 8000 11000 Most recent production (t/d∙m3) 1.9 1.9 1.8 2.5 2.5 3.4 2.9 Last renovation 2002 2006 Demolished 1974 1974 1991 1997 1997
Table 1.3. Total number and percentages of blast furnaces per inner volume range in China [4, 5]
Inner volume (m3) >2000 1000‐2000 <1000 # % # % # % 2001 21 11 29 15 146 75 2006 51 11 60 13 364 77 2013 109 27 90 22 206 51 Rest of world 2013 178 43 144 35 89 22
Table 1.2. Percentage of total production capacity per inner volume range in China [4, 5] Inner volume (m3) >2000 1000‐2000 <1000 2001 38% 21% 40% 2006 38% 21% 41% 2013 59% 21% 19% Rest of world 2013 72% 23% 4%
The world’s largest blast furnace is currently POSCO’s Gwangyang No. 1 with an inner volume of 6000 m3 after finishing relining in June 2013. By using a thinner lining in the blast furnace the volume of an older furnace can be increased, and in this case the inner volume in 1987 was only 3800 m3. Table 1.4 lists the top 15 largest furnaces in the world, and noticeable is that 13 of these are located in Asia. The shift to larger blast furnaces in China can be seen from the fact that the three furnaces from China are all built after 2009. There are currently 27 blast furnaces under construction or scheduled for blow‐in in 2013, 25 of which are in Asia, 10 in China, 7 in India. Of the 27 furnaces, 17 are big furnaces larger than 4000 m3. These include what will become India’s largest blast furnace: NMDC’s Nagarnar No.1 built by Danieli‐Corus with an inner volume of 4506 m3. Tata Steel is building a greenfield steel plant in Odisha: the Kalinganagar No. 1 with an inner volume of 4300 m3.
A bigger blast furnace has a larger production capacity for that single unit; it is however, a trade‐off for productivity. Table 1.5 shows the top 5 highest productivity for blast furnaces >2000 m3 and the productivity of the top 5 largest furnaces. For productivity the working volume of a blast furnace is used; this is the volume from stock level to the raceway and does not include the hearth. The data in the table is limited to furnaces for which the working volume was available.
The highest current productivity for blast furnaces larger than 2000 m3 is achieved by Tata Steel Europe’s Blast Furnace No. 6 in IJmuiden with 3.35 tonnes per day/m3. All top 5 furnaces have productivity above 3 tonnes per day/m3, and this is considerably more than the 5 large furnaces which have an average of 2.5 tonnes per day/m3.
Table 1.4. Top 15 largest blast furnaces [5] Compa n y Count ry Plant Inner vo lu me (m 3 ) Hearth dia m e ter (m) Built La st re line Nomina l ca pa cit y (M t/y e ar )
POSCO S. Korea Gwangyang No. 1 6000 16.1 1987 2013 5.48 Shagang China Zhangjiagang II No. 4 5800 15.7 2009 5.00 NSSMC Japan Oita No. 1 5775 15.6 1972 2009 4.80 NSSMC Japan Oita No. 2 5775 15.6 1976 2004 4.80 POSCO S. Korea Pohang No. 4 5600 15.6 1981 2010 5.31 Severstal Russia Cherepovets No. 5 5580 15.1 1986 2006 3.90 Shougang China Caofeidian No.1 5576 15.5 2009 4.50 Shougang China Caofeidian No.2 5576 15.5 2010 4.50 NSSMC Japan Kimitsu No. 4 5555 15.2 1975 2003 4.53 ThyssenKrupp Germany Schwelgern No.2 5513 14.9 1993 4.30 POSCO S. Korea Gwangyang No. 4 5500 15.6 1992 2009 5.00 JFE Steel Japan Fukuyama No. 5 5500 15.6 1973 2005 4.18 NSSMC Japan Nagoya No.1 5443 15.2 1979 2007 4.25 Kobe Steel Japan Kakogawa No. 2 5400 15.3 1973 2007 3.89 NSSMC Japan Kashima No.1 5370 15.0 2004 4.00
Table 1.5. Blast furnace productivity [5] Compa n y Count ry Plant Inner vo lu me (m 3 ) Working vo lu me (m) Hearth dia m e ter (m) Nomina l product ion (t onnes/day) Productivity (t pd/m 3 ) Top 5 productivity for blast furnaces >2000 m3 inner volume Tata Steel EU NL IJmuiden No.6 2678 2328 11 7800 3.35 Severstal USA Dearborn No.3 2130 1797 9.2 5890 3.28 Ternium Siderar Argentina Ramallo No. 2 2610 2340 10.4 7200 3.08 Wuhan I&S Co. China Hubei No.5,6,7 3200 2700 12.4 8300 3.07 Tata Steel EU UK Port Talbot No.5 2560 2134 10.8 6500 3.05
Productivity of Top 5 inner volume blast furnaces Severstal Russia Cherepovets
No.5
5580 5200 15.1 11000 2.12 Shougang China Caofeidian
No.1,2
5576 4670 15.5 12650 2.71 ThyssenKrupp Germany Schwelgern No.2 5513 4769 14.9 12000 2.52 Hyundai Steel S. Korea Dangjin No.1,2,3 5250 4425 14.8 11650 2.63 NSSMC Japan Kimitsu No. 3 4822 4043 14.5 10600 2.62
A recent topic which has a large influence on global industry is the reduction of CO2 emissions. This has a large impact on blast furnace ironmaking due to the high amount of CO2 it produces. Therefore several large projects have been started in a number of countries. In Japan the governments Cool Earth 2050 initiative has a goal of a 50% reduction of greenhouse gas emissions by 2050. Approximately 7% of CO2 emissions in Japan are from the iron making process, so it is an important sector to reduce greenhouse gas emissions. Part of the response from the Japanese steel industry has been to set‐up the COURSE 50, standing for: CO2 Ultimate Reduction in Steelmaking process by innovative technology for Cool Earth 50. The idea behind COURSE 50 is to reduce CO2 emissions by utilising hydrogen from the coke plant for iron ore reduction in the blast furnace and by applying CO2 separation to the blast furnace top gas and reusing the CO. [6, 7]
In Europe the ULCOS programme, Ultra Low CO2 Steelmaking, has been initiated, aiming to reduce CO2 emissions with 50%. The programme is a consortium of 10 companies, of which Tata Steel Europe is one. Four technology options are investigated, one of which is based on the blast furnace, ULCOS‐BF. The other three are based on smelting reduction (HISARNA), direct reduction (ULCORED) and electrolysis (ULCOWIN/ULCOLYSIS). The HISARNA plant is located on the Tata Steel site in IJmuiden and has had several successful runs.
Figure 1.4. ULCOS‐BF Flowsheet example [8]
The ULCOS‐BF principle is based on top gas recycling, in which the top gas containing approximately as much CO as CO2 is recycled into the furnace after removing the CO2. The second part of CO2 reduction comes from Carbon Capture and Storage (CCS); therefore the efficient removal of CO2 from the top gas is an important part. Experimental campaigns have been done at the LKAB experimental blast furnace (EBF) in which different process set‐ups where used, one of which is shown in Figure 1.4. The recycled top gas can be injected in the raceway with the normal blast or/and at a higher level in the furnace. The test campaigns have shown that the EBF can be successfully operated with top gas
recycling. CO2 emissions in the top gas were reduced by 24% and with CCS a total reduction of 60% should be possible for an integrated steel plant. [8, 9] The above methods describe novel technologies to reduce emissions; the first step should be to increase process and energy efficiency. A highly productive blast furnace using a low reductant rate will reduce emissions and will also be more cost effective. In Japan NSSMC (Nippon Steel & Sumitomo Metal Corporation) is increasing the productivity of its blast furnaces; the same amount of iron can be produced by fewer blast furnaces. This increases energy efficiency and cost effectiveness per tonne of hot metal produced. Techniques used to achieve this are DEM simulation and 3D real time monitoring systems which assist in process optimisation[10].
For a blast furnace to have a high productivity and low reductant rate it firstly requires high quality input materials. The iron oxide ore, in the form of pellets and sinter, and coke need to be of an optimal quality, strength and size. When these demands are fulfilled the blast furnace has to be operated as efficiently and smoothly as possible. This requires control over the burden distribution and knowledge of the processes inside the blast furnace, which can be realised through process modelling and simulation, including CFD‐DEM based approaches.
1.2. Research question of this thesis
Inside the blast furnace a large volume of gas is blown through a slowly descending packed bed. The balance between those two is critical and has a large influence on the stability and efficiency of the process. In the cohesive zone the layers of ore slowly soften and deform after which the melting starts. This greatly reduces the permeability forcing the gas flow through the untouched coke layers, generating an upward force working against the particle descent. The cohesive zone also distributes the reduction gas to the upper part of the furnace where most of the reduction of the iron oxides takes place. Due to the melting of ore a large solid volume is removed from the furnace, and this is one of the drivers for the solid flow above the cohesive zone. The softening and melting of the ore influences the solid flow in the furnace and could potentially create uneven flow or hanging of the material. Because it is connected to many phenomena critical to the blast furnace process, the cohesive zone determines the process efficiency and stability of operation to a large extent [2, 11].Conditions in the blast furnace make prediction of the behaviour of the cohesive zone extremely complex; temperatures are very high and there is a large number of interacting parameters. Observation of the process can only be done indirectly by measuring the properties of the outgoing material flows e.g. hot metal temperature and composition. What exactly is happening inside the furnace is impossible to measure because besides of its sheer size, it is completely filled with a packed bed of solid material and temperatures go up to 2300C. One method of investigating the inside conditions of the blast furnace is to quench it and dig it out, like an archaeological dig. In most cases [12]
the furnace was quenched by spraying water on the top of the burden; however, during the resulting slow quenching reactions continue and the burden also reacts with the quenching water. An example can be seen in Figure 1.5 which shows the results of the dig‐out of the Sumitomo Metals Corporation Kokura No. 2 Blast Furnace in Japan in 1974. The furnace has an 8.4 m hearth diameter and was quenched with water for nearly 5 hours to “freeze” the internal conditions. Figure 1.5. Dig‐out of the Kokura 2 Blast Furnace [12]
A better and faster way is to quench by injection of liquid nitrogen, which has been done in only a few cases. Mannesmannröhren‐Werke quenched their Blast Furnace No. 5 (hearth diameter 8 m) in December 1981 using liquid nitrogen [13]. After quenching the furnace was excavated and samples were taken in a grid pattern and with vertical probes. Figure 1.6 was constructed using the data from the excavation, both (a) and (b) show the location of the cohesive zone during the excavation. It is the zone between the line where temperatures were high enough for melting to start and the line where the ore is molten. Figure 1.6(b) shows the location of softened and partially molten cohesive layers. By comparing the reduction degree of the ore at the different grid points an idea of the intensity of the gas flow can be obtained, this is illustrated in Figure 1.6(a) by the bars in the furnace.
(a)
(b)
Figure 1.6. Dig‐out of Mannesmannrohren‐Werke BF 5 adapted from [13, 14] Another example of a dig out which also shows the influence of the cohesive zone on the process was from two trials of a British Steel experimental furnace [14]. The furnace is small with a working volume of only 1 m3, but this gives the advantage that after quenching with nitrogen the furnace can be used again and dig‐outs can actually be compared. The difference between the two trials was mainly the use of oxygen enrichment in trial B. Results show a poor cohesive zone permeability in trial A and good in trial B. In trial A some cohesive bridges had formed shown in Figure 1.7, which were not present in trial B where melting took place lower in the furnace. The extent of the reduction of the ore above the cohesive zone was very poor in trial A but good in trial B. This is determined primarily by the permeability of the cohesive zone. The poor cohesive zone permeability and formation of cohesive bridges caused operational difficulties during trial A, which were not present during trial B. There was severe hanging, when the burden does not descend. The cohesive masses force the gas flow predominantly along the walls of the furnace.Figure 1.7. Dig‐out of British Steel Pilot blast furnace [14]
These dig‐outs can give a general idea of what the process conditions were in the blast furnace before shut down. The layer structure of the burden can be seen in Figure 1.5 where alternating layers of coke and of ore are descending from the top. In the centre of the furnace the ore and coke are mixed due to the high gas velocity through the centre of the furnace causing fluidization and local mixing. The high gas velocity can also be seen in Figure 1.6(a) where the arrows indicating gas flow intensity are much larger in the centre of the blast furnace. When the ascending gas has heated the burden sufficiently the descending ore layers starts softening; cohesive masses are formed and the layer permeability starts decreasing. This is the top of the cohesive zone as shown in Figure 1.5 where the softening zone starts and in Figure 1.6 as the softening line. The cohesive zone ends when all the ore is molten and only a coke bed remains. This is indicated in Figure 1.5 by the dashed line underneath which only coke is present and in Figure 1.6 by the melting line. The cohesive zones from both dig‐outs have a similar shape, high in the centre of the furnace and sloping towards the walls. This shape is created by the gas flow which is higher in the centre in the furnace. The cohesive zone in Figure 1.6 curves upward nearer to the wall, this is an indication of increased gas flow along the walls.
In the cohesive zone the ore layers become cohesive masses which are shown in both figure 1.5 and 1.6 as black areas, the same can also be seen in Figure 1.7 from the much smaller furnace. Because the cohesive masses have a low permeability, the ascending gas flow is forced through the cohesive zone via the coke layers. The influence of the tuyeres, through which the gas is blown into the furnace, can be seen in Figure 1.5. The
two cross‐sections show 4 tuyeres: 140 mm, 120 mm and 70 mm in diameter and one unused. By decreasing the diameter of the tuyere with an equal pressure drop a lower gas volume enters the furnace. This results in a lower heat supply and less reductant being formed in the raceway, consuming less coke. The effect is an increasingly thicker and lower cohesive zone, with the cohesive zone layers still present below the tuyere.
In the case of the British Steel pilot blast furnace from Figure 1.7, the cohesive masses block the centre of the furnace. This causes severe operational problems; the burden descent becomes very irregular or can even stop altogether. Because the gas can only flow along the wall, there is no reduction taking place in the centre. Since the furnace is small the effect is rather extreme, but it does show the influence of the cohesive zone on the process. These dig‐outs supply valuable information, but have some significant limitations. They are rare because they can only be done on blast furnaces taken out of production, they are very expensive and labour‐intensive. They can never give an accurate picture of the conditions during operation and also do not allow the study of the effect on internal conditions when changing process parameters. Another method which can be used to study the internal conditions is by horizontal core drilling from the walls. It has been applied by Tata Steel on IJmuiden, but this method is limited by the length of samples which can be taken and it can only be done when the blast furnace is out of operation.
The previous discussion clearly shows the need for a tool which can be used to study the inner workings of the blast furnace. Mathematical modelling can deliver such a tool and several models have been successfully used. However, such tools generally require a certain degree of simplification. One major simplification applied is the simulation of solid particles by a continuum method. In such models all the individual, discrete particles are replaced by a single continuous phase with generalized flow characteristics. Simulations are not based on the theoretical flow characteristics of individual particles but on empirical volume averaged parameters. Discrete particle flow is rather complicated and cannot be accurately simulated by these models. What the present research aims to improve over previous models is the use of a Discrete Element Model to simulate the flow of the individual particles.
Discrete Element Modelling (DEM) was developed in the late 70’s by Cundall and Strack [15] but was never extensively applied because of its very heavy computational demands. Due to the increase in computing power DEM came into practical use in recent years and is rapidly gaining popularity. In the DEM method all the individual particles are tracked and for each time‐step the location, velocity and acceleration are calculated. For the present research the DEM method is coupled with Computational Fluid Dynamics (CFD) which can calculate the gas flow in the furnace. A coupling module is used to combine both methods so that e.g. the particles will experience drag force from the gas flow. Two additional models are required to calculate the chemical reactions and to determine the softening and
melting of the ore. Figure 1.8 presents the framework of the full project which is divided into a PhD and Post‐Doctoral part. This thesis presents the PhD work on the burden flow model.
Figure 1.8. Project framework
The full project “Prediction of physical and chemical properties of the burden materials in the
cohesive zone”, MC5.06255, was initiated by Materials innovation institute, M2i (formerly
Netherlands Institute for Metals Research, NIMR), and Delft University of Technology (TU Delft) in cooperation with Tata Steel (then Corus) as industrial partner. The research work was conducted in the group of Metals Production, Refining and Recycling (MPRR) in the Department of Materials Science and Engineering of the TU Delft supervised by Prof. dr. Rob Boom and Dr. Yongxiang Yang. Cooperation with the industrial partner Tata Steel was with Jan van der Stel and Mark Hattink from the Ironmaking group of the Research and Development department.
In order to accurately simulate the cohesive zone additional models are required besides the solid and gas flow models:
softening and melting can be simulated using a thermodynamic model based on the chemical composition and changes of the burden
Chemical reactions taking place in the blast furnace require a kinetic model
Using the combination of the four sub‐models illustrated in Figure 1.8 the final model should be able to simulate the burden descent in the furnace, how the iron ore is reduced by the CO in the ascending gas, when the ore starts softening and melting, and what the thermal profile of the blast furnace is. This results in a model with the ability to simulate the location and properties of the cohesive zone, and its interaction with the blast furnace process.
Solid Flow
Model DEM
Thermodynamic
Melt Formation
Model
Kinetic Model
Iron Ore
Reduction
Burden Flow
Model Cohesive
Zone
Softening and Melt Formation Model
Thermodynamic + Kinetic
Blast Furnace Cohesive Zone Model
DEM + CFD + Thermodynamic + Kinetic models
Gas Flow
Model CFD
For the development of the thermodynamic and kinetic models two post‐doc researchers, Yuko Enqvist and Vilas Tatavadkhar have worked on both modelling and experiments. The modelling was focussed on the thermodynamic and kinetic models but also on the challenging integration of the four models. Experimental work focussed on studying the softening and melting behaviour of the ore materials.
For both the DEM and CFD models commercial software is used [16, 17], which has several advantages as well as disadvantages. The software is ready to use and does not require development from scratch, which also allows for a much easier knowledge transfer within the research group and to the industrial partner over time. Technical support is easily available and the software is continuously improved. Because of a larger user base the accuracy and quality of the results are better determined. Besides high costs the major disadvantage, also encountered in this research, is the lack of adaptability of the software. Users are tied to the software as supplied and only a very limited amount of user adaptation can be made.
1.3. Structure of this thesis
This thesis is built up from 5 chapters. The first gives a short overview of the subject matter as well as the background and structure of the research project. It explains the drive for the research and the aims, and where this thesis fits into the larger research project. Chapter two goes into more detail on ironmaking, explaining the process of producing pig iron from the raw materials, the process this research project aims to model. The third chapter gives an overview of the work which has been presented in literature on the subject of the research project. It shows what has been done on Discrete Element Modelling of the blast furnace, and where this work fits in and where it differs from published work.
In the fourth chapter the development of our model and the results are presented and discussed. Every gradually improved model is applied to a case and also the parallel work in the research project is discussed. Chapter five contains the conclusions and recommendations.
References
1. Worldsteel Association, Statistics archive. [Accessed 2013, 17‐8]; Available from: www.worldsteel.org/statistics/statistics‐archive.html.
2. Geerdes, M., Toxopeus, H., and van der Vliet, C., Modern blast furnace ironmaking. (2009), IOS Press.
3. Ameling, D. and Endemann, G., Ressourcenefficienz: gute argumenten für stahl. Stahl Eisen 127 (2007), p. 85‐93.
4. Zhang, S.‐r. and Yin, H. The trends of ironmaking industry and challenges to
Chinese blast furnace ironmaking in the 21st century. in Proceedings of the 5th
International congress on the science and technology of ironmaking. (2009), Shanghai.
5. VDEh, Blast furnaces worldwide. VDEh PLANTFACTS, Date published: 28‐06‐2013 6. Miwa, T. Development of iron‐making technologies. in Proceedings of the 5th
International congress on the science and technology of ironmaking. (2009). 7. Ariyama, T., Ueda, S., Natsui, S., Inoue, R., and Sato, M. Current technology and
future aspect on CO2 mitigation in Japanese steel industry. in Proceedings of the
5th Internatinal congress on the science and technology of ironmaking. (2009). 8. Stel, J.v.d., Louwerse, G., Sert, D., Hirsch, A., Eklund, N., and Pettersson, M., Top
gas recycling blast furnace developments for 'green' and sustainable ironmaking.
Ironmaking and Steelmaking 40 (2013), p. 483‐489.
9. Zou, G. and Hirsch, A., The Trial of the Top Gas Recycling Blast Furnace at LKAB's
EBF and Scale‐up. La Revue de Metallurgie (2009), p. 387‐392.
10. Ueshima, Y., Higuchi, Y., and Saito, K. Recent topics of iron‐ and steelmaking
technology in NSSMC. in 2013 Annual review of NSSMC. (2013), McGill Metals
Processing Centre (MMPC).
11. Biswas, A.K., Principles of ironmaking. (1981), Cootha Publishing House.
12. Omori, Y., ed. Blast furnace phenomena and modelling. (1987), Elsevier Applied Science.
13. Engel, K., Fix, W., Grebe, K., de Haas, H., and Winzer, G., Ergibnisse von
Untersuchungen an dem mit Stickstoff abgekülten Hochofen 5 der Mannesmannröhren‐Werke AG. Stahl und Eisen 106 (1986), p. 18‐23.
14. Gathergood, D., Jones, J., Juckes, L., and Golding, D., Progressive reduction of
burden in the blast furnace, in Technical Steel Research. (1996), British steel,
Teeside Technology centre.
15. Cundall, P.A. and Strack, O.D.L., A discrete numerical model for granular
assemblies. Geotechnique 29 (1979), p. 47‐65.
16. DEM Solutions, EDEM. Available from: www.dem‐solutions.com. 17. ANSYS, ANSYS Fluent. Available from: www.ansys.com.
Chapter 2
Blast furnace ironmaking
2.1. Introduction
The first iron objects were produced from meteoric iron and the oldest known examples are iron beads from Gerzeh (3500 BC) and a dagger from Ur (3000 BC). Smelted iron objects appeared around 2000 BC and from 1000 BC iron began to replace bronze as the most important material for tools and weapons. Early iron smelting was done in a bloomery furnace, a batch process in which the ore does not become liquid but remains solid while being reduced to iron. The end product was a large block of iron called the bloom which was then further processed by hammering out the impurities. The earliest blast furnaces producing liquid iron were built in China as early as 200 BC. In Europe the process of ironmaking in a blast furnace was first used in the middle ages, since then the process has undergone large changes but is essentially the same. It is the first part of a two stage process to produce steel, the first stage produces crude pig iron which is then refined to steel in the second stage. The name pig iron comes from an old casting process in which the crude iron was cast into a sand bed with a centre runner and a number of side runners. When the iron is solidified it somewhat resembled piglets drinking from their mother pig.
In the medieval process the blast furnace was fed with iron ore and charcoal. Water wheels were used to drive the bellows which pumped air into the furnace as well as to drive hammers for processing of the steel. During the Industrial Revolution the demand for steel increased dramatically with the development of steam engines, railways, bridges and machinery. Blast furnace iron production rapidly expanded and innovated to meet this demand. In the 18th century coke replaced charcoal and steam engines replaced water wheels, the 19th century brought hot air blast. Continuous research and development greatly increased the size, efficiency and productivity of the blast furnace. Table 2.1 shows the development of blast furnaces from the industrial revolution to the modern age. The current process of ironmaking is described in this chapter starting with the place of the blast furnace in the steelmaking process. Focus will then shift to the blast furnace itself and its operating parameters.
2.2. Overview of integrated steelmaking process
Blast furnaces are either located on what is called an integrated steel plant or feed different steel plants at some distance from the site. On an integrated steel plant the whole process from iron ore to steel products takes place as shown in Figure 2.1. The process starts with the unloading of the raw materials from ships or trains. These materials are stored, mixed and pre‐processed for use in the blast furnace. The raw materials consist of two types: the iron containing ferrous oxides and the carbon containing reductants. Ferrous oxides can be iron ore or processed pellets or sinter. Production of sinter generally takes place on‐site;pellets are also commonly processed at the mine site. Only two ironmakers produce pellets at the integrated site: Tata Steel IJmuiden (Netherlands) and Kobe Steel Kakogawa (Japan). This has some big advantages: better control over pellet composition, direct quality control on‐site, pellets are warm and dry, and less handling results in less pellet degradation. The disadvantages are stockpiling of pellet ores, good quality control is required and the fact that an extra operation is required on‐site. The large majority of carbon containing material is coal but oil or gas can also be used in small quantities. Coal is processed to coke by removing the volatile matter and carbonizing in coke batteries. Unprocessed coal fines are directly blown into the furnace with the blast as Pulverized Coal Injection (PCI). To this end large coal grinding facilities are needed on the site. Figure 2.1. Steelmaking process [1] Table 2.1. Development of blast furnaces Year and location Hearth diameter, m2 Reductant rate, kg/tonne of hot metal Iron output, tonne per day 1796 (Gleiwitz, Germany) 0.6 3500 1‐2 (later 4) 1801‐1815 (UK) ‐ 2500 5‐7 1856 (Hasslinghausen, Germany) 2.1 1600 20‐23 1880 (USA) 3.4 1510 120 1902 (USA) 4.4 1000 464 1929 (Bruckhausen, Germany) 6.5 740 1100 1993 (Schwelgern 2, Germany) 14.9 480 10600 2013 (Pohang 4, Korea) 15.6 ‐ 14600
The ferrous oxides and coke are then charged into the blast furnace where the ferrous oxides are reduced, thereby producing liquid slag and hot metal. Slag contains the unreduced components from the burden and consists mainly of SiO2, CaO, MgO and Al2O3. It is much more viscous than the hot metal and its composition is very important for the process. Slag and hot metal are almost continuously tapped from the furnace from a number of tapholes at the bottom of the furnace. The slag is solidified and when it meets certain requirements, can be used in e.g. road construction or as raw material for cement making for building purposes. The liquid hot metal is poured into large torpedo‐shaped ladles, elongated firebrick insulated rail cars called torpedo cars, for transport. Because the liquid iron is in constant contact with coke inside the blast furnace it contains a large amount of dissolved carbon, much higher than required for steel. High carbon content makes the iron very hard but also very brittle and it has to be reduced from approximately 4.5 wt% to 2 wt% ‐ 0.05 wt% depending on the type of steel. This is done in the Basic Oxygen Steelmaking (BOS) process, where the hot metal is poured in a converter into which a water‐cooled lance is used for oxygen blowing. The lance blows oxygen with a velocity higher than Mach 1 onto the hot metal burning off the dissolved carbon. Steel scrap is charged into the furnace before the liquid iron to recycle the steel and use the heat created by the burning of the carbon for melting the scrap. Besides reducing the carbon content the BOS process also removes silicon, titanium and phosphorus. Further impurity removal can be done in the steps before or after BOS e.g. desulphurization in the torpedo or charging ladle. Typical batch sizes of the converter process vary between 100 and 350 tonnes.
After finishing the oxygen blow the steel is tapped from the converter into a steel ladle. A second step of refining follows in which alloying elements can be added and impurities removed to fine‐tune the composition to match specifications. Alloying elements can be added by feeding a wire into the melt or by injecting a powder. Excess dissolved oxygen is removed by deoxidants such as aluminium or silicon. Before casting it is ensured that the liquid metal has a homogenous temperature and composition by stirring the metal using argon injection or Electromagnetic Stirring (EMS). The temperature can be adjusted by scrap cooling or heating in a ladle furnace.
Steel is continuously cast into slabs, blooms or billets with different dimensions depending on the final product, e.g. slabs 1600 mm wide, 250 mm thick and 12 m long. The hot metal is cast into a large tundish, a holding reservoir and distributor which can typically contain 30 to 80 tonnes of liquid steel. From the tundish the hot metal is fed into several vertical water‐cooled copper moulds which solidifies the outer layer of metal. The metal exits the mould supported by rollers and still has a molten core; water is sprayed on the surface for secondary cooling. The next step in the process is the hot or cold rolling. The former is sometimes placed directly after the continuous caster to take advantage of the heat remaining in the metal. During hot rolling, the metal passes between rolls reducing the thickness and width. By carefully managing temperature and cooling rates the physical properties of the steel can be controlled. The deformation by successive cold rolling causes hardening and increases the strength of the steel and the roughness of the steel surface.
2.3. Blast furnace process and equipment
The blast furnace is a counter‐current reactor, the ferrous oxides are charged at the top and the reducing gas is blown in at the bottom. Solids are charged in alternating layers of coke and ferrous oxides to ensure good permeability when the oxides start melting. At the bottom of the furnace oxygen enriched and pre‐heated air is blown in at high velocity through the tuyeres. Gas pre‐heat temperatures are approximately 1200 °C and the flame where the oxygen reacts with the coke has temperatures in excess of 2100 °C. Additional reductants such as pulverized coal, gas or oil are also injected with the hot blast, reducing the amount of more expensive coke required. The Tata blast furnaces at IJmuiden can currently achieve coke rates of 270 kg per tonne of hot metal with pulverized coal injection (PCI) rates of 230 kg/tHM. The lowest sustainable rate achieved in IJmuiden for a week was 250 kg/tHM at which more than half of the reductants are injected. Yearly averages of 260 kg/tHM have been reached. The gasification of the coke by the blast causes a large cavity called the raceway. As the solids slowly descend they are heated by the ascending reductant gas. A large part of the oxides are reduced in the upper part of the furnace with the remainder being reduced in the cohesive zone. As the burden reaches temperatures of around 1100 °C the ore starts softening and melting. In the cohesive zone the permeability of the ore layers decreases and they become nearly impermeable. All of the ore melts and only coke remains solid below the cohesive zone. During melting two liquid phases are created, the liquid iron and the slag, trickling down through the coke bed into the hearth. The liquid iron contains dissolved carbon as well as some impurities which are reduced besides the iron such as silicon, manganese, titanium, sulphur or phosphorus. All the unreduced oxides from the ore and fluxes form a silicate slag.
Around the blast furnace there are several pieces of equipment required for the process shown in Figure 2.2. The oxygen enriched air is pre‐heated in the hot blast stoves: these are large cylinders filled with fire bricks. A burner inside the hot blast stoves burns furnace gas and/or natural gas to heat up the bricks. When the required temperature is reached the burner is turned off and air is blown through, whereby the heat stored in the bricks is transferred to the air. Using multiple stoves the gas is heated continuously. The hot blast is blown into the bustle pipe which runs around the furnace and then injected into the blast furnace via the tuyeres. At the top of the furnace the top gas is removed via the uptakes and a down‐comer into a dust catcher for the removal of fine particles.
The solid materials are stored in the stock house where they are weighed and screened before being charged into the blast furnace. Material is charged into the furnace by skips or conveyor belts which bring the material up and by a charging mechanism which deposits the particles into the furnace while keeping the furnace under pressure. There are two main methods: a bell top or a bell‐less top. In the former the particles are charged into a series of bells; the charge can be distributed over the material surface in the blast furnace by
movable armour plates which deflect the material. A bell‐less top uses a rotating chute to distribute the material and is far more accurate in the distribution of the burden. Figure 2.2. Blast furnace and equipment [2]
2.4. Chemical reactions
In the blast furnace the iron oxides present in the ore are reduced to metallic iron by the carbon present in the coke and additional reductants. In the raceway the carbon from the coke and coal reacts with oxygen to form the main reductant gas CO. O2 + 2C → 2CO (2.1) The iron oxide is present in the form of Hematite (Fe2O3) and the reduction takes place with a decreasing ratio of O to Fe, forming Magnetite (Fe3O4) and Wϋstite (FeO). If we follow the descending burden through the furnace the following reactions take place:
Hematite 3Fe2O3 + CO → 2Fe3O4 + CO2 (2.2) Magnetite Fe3O4 + CO → 3FeO + CO2 (2.3) Wϋstite 2FeO + CO → 2FeO0.5 + CO2 (2.4)
These reactions take place in the upper part of the furnace at increasing temperatures. Hematite starts reducing at about 500 °C, magnetite at 600 °C to 900 °C and the reduction of wϋstite at 900 °C to 1100 °C. At the start of the melting temperature of 1100‐1150 °C FeO is generally reduced to FeO .
Two types of reduction reactions take place in the blast furnace: indirect and direct reduction. The reactions above where CO reacts to form CO2 take place by indirect reduction; the coke is not directly involved in the reaction. Final reduction of FeO0.5 to Fe takes place by direct reduction in a solid‐liquid reaction. In direct reduction the iron is reduced directly by the coke and can be written as: 2FeO0.5 + C → 2Fe + CO (2.5) However, the reaction actually takes place in two parts:
2FeO0.5 + CO → 2Fe + CO2 (2.6)
CO2 + C → 2CO (2.7) The iron oxide reacts with CO gas forming CO2 which then immediately reacts with carbon creating CO. This last reaction is called the Boudouard reaction and describes the balance between CO on the one hand and CO2 and C on the other hand. Figure 2.3 shows both the temperature dependent equilibrium relationship of the Boudouard reaction and the Fe‐O‐C equilibrium diagram. Above 1100 °C all CO2 reacts to form CO, at lower temperatures an increasing amount of CO2 is present. If CO is decomposed very fine carbon is formed with CO2, however, below 400 °C the reaction is very slow and only a negligible amount of carbon forms. Figure 2.3 shows the reduction of magnetite to wϋstite requires only 40% to 20% CO/CO+CO2, depending on the temperature, compared to 60% and higher for the further reduction of wϋstite to metallic iron.
Figure 2.3. The Fe‐O‐C equilibrium diagram combined
with the Boudouard curve [3]
The furnace can be divided into three zones: upper, middle and lower. In the lower zone molten material temperatures reach around 1500 °C and the approximately 2100 °C gas cools down to 900 °C – 1000 °C. All the oxygen reacts with coke to form CO and any formed
CO2 reacts back according to the Boudouard reaction. The reaction of oxygen with the coke generates a very large amount of energy and supplies the heat to the furnace. Direct reduction of iron oxide consumes a large amount of energy and thus significantly cools the lower furnace.
In the middle zone the temperature remains relatively constant at about 900‐ 1000 °C and is called the thermal reserve zone. The effects of endothermal and exothermal reactions, burden heating and heat losses cancel each other. In the thermal reserve zone the wϋstite is slowly reduced in equilibrium with CO as shown in Figure 2.3. By probing it has been shown that inside the thermal reserve zone there also is also a chemical reserve zone where no reactions take place and Fe/FeO and CO/CO2 are in equilibrium. The thermal reserve zone will shrink as the furnace is pushed to higher productivity. The upper zone is where the gas temperature drops from 900‐1000 °C to 100‐250 °C and the solids heat up to 800 °C. Hematite and magnetite are reduced to wϋstite and any moisture in the burden is vaporized. Besides the CO, hydrogen is present as a second reductant in the blast furnace. At temperatures above 821 °C it has a higher reduction efficiency than CO. Hydrogen is formed from the moisture in the blast and in the pulverized coal and can then reduce the iron oxides:
H2O + C → CO + H2 (2.8)
H2 + FeO → Fe + H2O (2.9)
Water generated in the furnace can react with CO to form hydrogen:
H2O + CO → CO2 + H2 (2.10)
Reaction 2.8 only takes place at temperatures above 1000 °C. At lower temperatures the reaction 2.10, called the water‐gas shift reaction, shifts strongly to the right. Because it requires iron as a catalyser the water‐gas shift reaction does not take place in the upper part of the furnace.
2.5. Burden materials
A blast furnace has two main components in its top charged burden: the raw materials that contain iron oxide and the materials that contain carbon. Three types of oxide materials, shown in Figure 2.4, are used: lump ore, sinter and pellets. Lump ore is used directly from the mine where it is screened to get the required size fraction of approximately 6‐25 mm. Lump ore is cheaper than pellets but it has poorer properties for the blast furnace. The ore is weaker than pellets during reduction under the high temperature and pressure in the blast furnace. Because it is naturally occurring it has a wider range of compositions and physical properties. Lump ore is generally only used in small percentages of the total oxide feed.
Sinter is a blocky material made by fusing iron ore fines into larger particles. Originally applied to use revert materials from the whole steel making site in the blast furnace, it is currently used much more widely as one of the predominant sources of oxides beside pellets. The main component of the sinter is iron ore fines, either raw material or a return flow from the process. Return sinter from the sintering process is also added. They are mixed with limestone as a flux to ensure a final slag in the blast furnace with the desired properties for the ironmaking process and for the use as raw material for cement production. The final component is coke fines added as a fuel for the sintering process. After mixing in a rotating drum where 5 % water is added for primary binding the mixture is charged on a sinter strand in a 35‐65 cm thick layer. A sinter strand, shown in Figure 2.5 is a moving grate through which air is being sucked from the bottom. At the start of the strand the coke in the bed is ignited using heated air. The heat generated partially melts and sinters the material. As the sinter moves over the strand the flame front moves down through the bed until it reaches the bottom and all of the material is sintered. At the end of the strand the material breaks off and is further reduced in size. Pellets are created from very fine iron ore (much finer than used for sinter), which has been pre‐processed to separate the iron oxides from gangue materials. These fines are too small for sintering because the bed would be impermeable. Similar to the sintering process the iron ore fines are mixed with fluxes and coke fines in a rotating drum. Then water and a binder are added, the rotation causes snowballing and creates spherical particles. The particles, called green pellets, are then charged onto a grate moving through a furnace and heated to 300‐350 °C. This removes the water and the binder creates a chemical bond to hold the particle together. Burners and the coke fines then increase the temperature to 1250‐1350 °C fusing the particle. Sinter 90% < 25 mm Pellets 11 mm (±2 mm) Lump ore 6‐25 mm Figure 2.4. Blast furnace ferrous burden materials [2]
Figure 2.5. Sinter strand [1]
The second component in the burden is the coke which is supplying carbon for the reduction and the heat to melt the burden. Coke is produced by mixing and grinding coal to the required specifications which is then charged into a coke oven. In the oven the coke is heated to over 1200 °C in an air‐free environment. The volatile matter in the coke escapes and a solid carbon matrix is formed by the carbonization of the coal. After 16 to 24 hours all the material is converted and the hot coke is pushed out and immediately quenched with water to ensure it does not react with oxygen.
With a size of 45‐55 mm coke is larger than the other burden materials. Coke quality is very important for the process; the whole burden is supported by the coke and it has to ensure a good permeability. A large amount of coke can be replaced by injecting coal directly in the raceways, by PCI. The quality demands on pulverized coal are much lower and it is therefore cheaper. There is a limit on the amount of coke which can be replaced by PCI due to the permeability requirements of the burden. This increases the quality demand of the burden materials on strength and degradation resistance. Blast Furnace No. 6 in IJmuiden has achieved very low coke rates of 260 kg/t hot metal year average, globally a more usual rate is above 300 kg/tHM. To achieve such low coke rate the burden permeability has to be as high as possible. For this the pellets and sinter need to be of good quality; both need a narrow size distribution to maximise burden porosity and the particles should not break or degrade during descent. The amount of slag generated should be low, as slag in the inter‐particle pores will decrease the permeability. Because the gas flows through the packed bed of ore and coke layers, the structure of the layers is very important to control the permeability. This also influences the cohesive zone shape and location, which has a very large influence on the pressure drop in the blast furnace. Good control of the burden charging and the resulting layer structure is crucial.
