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LICENTIATE T H E S I S
Luleå University of Technology
Department of Chemical Engineering and Geosciences, Division of Process Metallurgy
:|: -|: - -- ⁄ --
:
Evolution of Coke Properties while
Descending through a Blast Furnace
Tobias Hilding
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Evolution of Coke Properties while Descending Through aBlast Furnace
by
Tobias Hilding
Licentiate Thesis
Luleå University of Technology
Department of Chemical Engineering and Geosciences
Division of Process Metallurgy
SE-971 87 Luleå
Sweden
2005
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ACKNOWLEDGEMENTS
I would like to express my gratitude to Professor Björkman and Professor Jan-Olov
Wikström for their supervision and for giving me the opportunity to perform my
research.
Special thanks to Professor Veena Sahajwalla for her supervision and support and Dr
Sushil Gupta for all help and discussion.
Also special thanks to Dr Lars Bentell for fruitful discussions and help.
Thanks to the members of committees JK21057, JK21060 and RFCS 7210-PR-324.
Further thanks to all the employees at Luleå University of Technology, in particular
my colleague Ryan Robinson for the good laughs and business lunches. Also, thanks
to my colleagues at MEFOS and employees at University of New South Wales who
have helped me throughout my studies.
A great amount of thanks to my parents, brother, relatives and my mates. Deep thanks
to my Luleå-love JK, you are the best! We did it!
I would like to especially acknowledge the Swedish Energy Agency, STEM, and
JERNKONTORET for financial support and LKAB, SSAB and Ruukki for supplying
research input.
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ACKNOWLEDGEMENTS..............................................................................1
LIST OF PAPERS.............................................................................................4
SUMMARY .......................................................................................................6
1. INTRODUCTION .......................................................................................8
1.1 BACKGROUND............................................................................................................................................. 8
1.2 STATE OF THE ART ................................................................................................................................... 111.3 OBJECTIVES .............................................................................................................................................. 17
1.4 R ESEARCH QUESTIONS............................................................................................................................. 18
2. METHODS.................................................................................................18
2.1 THE EXPERIMENTAL BLAST FURNACE................................................................................................... 18
2.2 THE STUDIED EBF CAMPAIGNS ............................................................................................................... 20
2.2.1 EBF campaigns followed by excavation ................................................................................................ 202.2.2 EBF trial with high CRI coke................................................................................................................. 242.3 METHODS USED FOR CHARACTERIZATION OF COKE SAMPLES............................................................. 25
2.3.1 TGA/DTA–MS....................................................................................................................................... 262.3.2 CRI/CSR EQUIPMENT ............................................................................................................................. 272.3.3 SIEVING ................................................................................................................................................... 292.3.4 X-RAY DIFFRACTION............................................................................................................................... 292.3.5 CHEMICAL ANALYSES............................................................................................................................. 292.3.6 SCANNING ELECTRON MICROSCOPE ...................................................................................................... 302.3.7 LIGHT OPTICAL MICROSCOPE................................................................................................................. 302.3.8 BET......................................................................................................................................................... 312.3.9 MICRO TEXTURE MEASUREMENT............................................................................................................ 31
3. RESULTS AND DISCUSSION ................................................................32
3.1 VARIATION IN PHYSICAL PROPERTIES ................................................................................................... 32
3.2 EVOLUTION OF CARBON STRUCTURE ..................................................................................................... 35
3.3 ALKALI UPTAKE AND DISPERSION IN COKE ............................................................................................ 37
3.5 EVOLUTION OF COKE REACTION WITH CO2 .......................................................................................... 45
3.6 ISOTROPIC / ANISOTROPIC CHANGES IN THE COKE CARBON MICRO STRUCTURE ............................... 50
3.7 TRIAL WITH HIGH CRI COKE .................................................................................................................. 50
3.7.1 PROCESS ANALYSIS................................................................................................................................. 513.7.2 EVOLUTION OF CARBON STRUCTURE...................................................................................................... 523.7.3 EVOLUTION OF COKE ASH CHEMISTRY ................................................................................................... 533.7.4 EVOLUTION OF COKE REACTION WITH CO2 ............................................................................................ 553.7.5 ISOTROPIC / ANISOTROPIC CHANGES IN THE COKE CARBON MICRO STRUCTURE.................................... 573.7.6 POROSITY DIFFERENCES ......................................................................................................................... 58
4. CONCLUSIONS ......................................................................................... 59
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4.1 EVOLUTION OF COKE CARBON STRUCTURE ........................................................................................... 59
4.2 ALKALI IMPLICATIONS ............................................................................................................................ 59
4.3 COKE R EACTIVITY ................................................................................................................................... 59
4.4 PHYSICAL PROPERTIES INCLUDING COKE STRENGTH AND ABRASION ............................................... 60
5. FUTURE RESEARCH..............................................................................62
5.1 COKE DEGRADATION .................................................................................................................... 62
5.2 OPTIMUM COKE PROPERTIES ...................................................................................................... 62
6. LIST OF ABBREVIATIONS ...................................................................63
7. REFERENCES ..........................................................................................63
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LIST OF PAPERS
The outcome of project JK21057 “Coke Strength at High Temperatures”, that this
thesis is based on is a literature review, two conference proceedings and two journal
papers.
I Hilding, T., Sahajwalla, V., Gupta, S.K., Björkman, Bo, Sakurovs, R.,
Grigore, M., Saha-Chaudhury, N. Study of Gasification Reaction of
Cokes Excavated From Pilot Blast Furnace. Scanmet II, 2004, Luleå,
Sweden.
T. Hilding’s contribution to this publication were as a participant in
excavation of the EBF and investigation of changes of coke from the EBF
utilizing TGA, LECO, and XRD.
II Tobias Hilding, Nouredine Menad, Bo Björkman and Jan-Olov
Wikström. Thermal Analysis of Coke From Different Layers in an
Experimental Blast Furnace. Submitted to Thermochimica acta, 2005
T. Hilding’s contribution to this publication was as a participant in
excavation of the EBF and conduction of all experimental work.
III Tobias Hilding, Sushil Kumar Gupta, Veena Sahajwalla. Effect of Carbon
Structure and Coke-Alkali Reactions on the Coke Behaviour in an
Experimental Blast Furnace. Submitted to ISIJ, 2005
T. Hilding’s contribution to this publication was as a participant in
excavation of the EBF and conduction of all experimental work.
IV Tobias Hilding, Jan-Olov Wikström, Urban Janhsen, Olavi Kerkkonen.
Investigation of coke properties while descending through an
experimental blast furnace. Submitted to ECIC 2005
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T. Hilding’s contribution to this publication was as a participant in probe
material and tuyere core sampling from the EBF and X-ray diffraction
and TGA measurements.
Apart from the supplements above, the following papers have been published during
the thesis work:
Veena Sahajwalla, Tobias Hilding, Anne von Oelreich, Sushil Kumar
Gupta, Bo Björkman, Jan-Olov Wikström, Patrick Fredriksson and
Seshadri Seetharaman. Structure and Alkali Content of Coke in an
Experimental Blast Furnace and Their Gasification Reaction. AIST 2004
T. Hilding’s contributions, see I.
Tobias Hilding, Kelli Kazuberns, Sushil Gupta, Veena Sahajwalla,
Richard Sakurovs, B. Björkman and Jan-Olov Wikström. Effect of
Temperature on Coke Properties and CO2 Reactivity under
Laboratory conditions and in an Experimental Blast Furnace. AIST 2005T. Hilding was responsible for generating most of the data except
laboratory annealing measurements.
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SUMMARY
Due to increasing price and economic pressures, there is a need to minimise coke
consumption. The lesser amount of coke used has indirectly set higher standards for
coke quality and lead to a wish for even more knowledge about its function in the
blast furnace.
Over the last 20 years, coke quality has been strongly dictated by the so-called CSR
value because it was believed that a higher CSR leads to improvement in productivity
and more stable operation. Due to lack of suitable coals, often cokes are made from
coals with relatively inferior quality leading to coke with lower values of the so-
called CRI indicia. Because of this, there was an indirect focus on cokes with lower
CRI values. Therefore, this thesis will address some of the important issues of coke
strength and focus on changes occurring with coke when it passes through a blast
furnace. The main aim of this study is to understand the degradation mechanisms and
reactivity changes of coke in order to investigate the factors that affect coke quality.
Cokes excavated from LKAB’s Experimental Blast Furnace (EBF) are used as a basis
for the research. Two campaigns with similar coke (low CRI/high CSR) but different
blast furnace injection material have been studied. The coke is supplied from SSAB
Tunnplåt Luleå AB. Physical and chemical properties of cokes samples from the EBF
were measured. Evolution of coke properties particularly carbon structure and alkali
uptake were related to CO2 reactivity as well as coke behaviour (e.g. CSR/abrasion).
In addition to this, a trial with very high CRI coke was studied. On the basis of this
study, following conclusions were made.
1. The order of carbon structure and concentration of alkali species were
increased and these were the most notable changes in the coke properties as it passed
through the shaft to the cohesive zone of the EBF.
2. The degree of graphitisation was increased while amorphous carbon
content was decreased in the hotter zones of the EBF. A linear correlation between
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the height of the carbon crystallite (Lc) values and the coke bed temperature was
established to demonstrate the strong effect of temperature on the carbon crystallite
value (Lc) of coke in the EBF.3. The alkali concentration of coke increased with increasing temperature of
the coke bed such that most of the alkali content was evenly distributed in the bulk of
the coke rather than in the periphery of the coke matrix.
4. The CO2 reactivity of coke was found to increase during progressive
movement of the coke from shaft to cohesive zone of the EBF, and was related to the
catalytic effect of increased alkali concentration in coke.
5. The deterioration of coke quality in the EBF, particularly coke strength
(CSR) and abrasion propensity (I drum test), was related to coke graphitisation,
alkalization and reactivity to demonstrate the strong effect of the coke graphitisation
on the propensity of coke degradation.
6. Differential Thermal Analysis indicated that reactions with CO2 are
enhanced as coke descends through the EBF.
In addition, a trial period with poor coke quality was studied by extensive sampling.
The results from this study gave the following additional conclusions:
7. Comparison between high and poor quality coke indicate structure to be
connected with alkali uptake, reaction with CO2 and degradation.
8. Isotropic coke carbon components are more resistant than anisotropic
components when passing through the EBF.
9. Both cokes develop a more ordered structure as they descend through the
EBF.
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1. INTRODUCTION
1.1 Background
Coke has ancient origins and carbonisation of coal is mentioned in text as early as
371 BC. However, coke use as a sole source of fuel in a blast furnace began from
somewhere between the early to mid 1800th century. This coke was made in piles [1].
The knowledge of coke and its properties was lacking in the beginning of the coke
era. The higher demands incurred for better pig iron led to higher demands on the
coke.
The last decade, three consistent themes have appeared pertaining to coke properties
and blast furnace performance. They are related to the viability of the blast furnace,
improvement in blast furnace productivity and efficiency, and blast furnace
operations at lower coke rates.
The most consistent theme of recent literature is that the blast furnace will remain a
dominant method for production of hot metal worldwide [2-9]. Another theme shared
throughout the world relates to significant improvements in blast furnace productivity
[10].
A third common theme relates to coke replacement at the furnace with reductant
injection such as pulverized coal, natural gas and oil. However, coke is essential for
the blast furnace iron making process in order to support the burden and provide gas
permeability, thus a minimum coke burden limit exists.
Coke production has, since the last two decades, gone through some major changes.
The number of aging coke plants steadily increases while very few new plants are
being built, except in China. The coke export from China has however decreased due
to domestic usage. Prices of external coke have since the beginning of 2003 to mid2004 increased by more than 400 %.
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The most dominant hot metal making process in the world today is still the Blast
Furnace (BF) process, and the most important raw material fed into the BF, in terms
of operation efficiency and hot metal quality, is coke. Due to a decrease in the cokesupply and a desire to lower the energy consumption and to reduce CO2 emissions,
developments in the BF sector have long focused on replacing the coke by coal. One
of the major developments in the blast furnace operation is the introduction of
pulverized coal technology in which coke is substituted by Pulverized Coal Injection
(PCI) through the tuyeres. This technique was introduced in the early 1980’s.
Economic and environmental pressures are the primary driving force behind the
promotion of PCI technology. The old coking plants are gradually closing while few
new plants are being built to replace the coke supply, particularly in developed
countries, including Europe. New coke plants are extremely expensive due to
stringent environmental regulations. Therefore, in the future, blast furnace operations
will rely on less coke addition per unit hot metal production. During BF operation at
low coke rates, the coke experiences prolonged residence time. Regardless of
residence time, the coke must maintain satisfactory bed permeability for reducinggases to flow upwards in the furnace and for liquids to flow downwards. Therefore,
high quality coke is essential for future blast furnace operations.
Coke is produced by heating a coal blend in the absence of oxygen. The most
common type of production technique is the so-called conventional or by-product
coke plant, see Figure 1. They are comprised of horizontal chamber ovens, measuring
12 to 18 m long, 3 to 8 m tall, and 0.4 to 0.6 m wide. Several chambers are grouped
to form one battery (Multi-Chamber-Systems). A single battery may consist of up to
85 ovens. The coal mix is charged through charging holes in the oven top. Following
15 to 25 hours coking time, the doors are opened and coke is pushed by the coke
pusher machine out of the oven into a coke quenching car. The coke is then cooled.
The oven chamber is again sealed, initiating a new carbonisation cycle. The gas
evolving on coal carbonisation enters gas treatment facilities and the by-product
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recovery plant. The ovens are run with a slight over-pressure. The coke reaches a
temperature of approximately 1100°C to 1250°C.
Other types of coke
production techniques are
heat recovery coke plants and
non-recovery coke plants.
The heat recovery plants
utilize all the excess gas to
produce heat. The furnaces
are typically called Beehive
furnaces and work with
negative pressure and require
coking times of up to 48
hours.
Coke performs three functions in a blast furnace namely: a thermal function, as fuel
providing the energy required for endothermic chemical reactions and for melting of
iron and slag; a chemical function, as reductant by providing reducing gases for iron
oxide reduction; a mechanical function, as a permeable grid providing for passage of
liquids and gases in the furnace, particularly in the lower part of the furnace. When
coke passes through a blast furnace, the coke degrades and generates fines which
affect bed permeability and affects the process efficiency. The rate at which coke
degrades is mainly controlled by the solution loss reaction, thermal stress, mechanical
stress and alkali accumulation.
Coke quality is often characterized by measuring cold and hot strength, ash
composition and chemistry, which are largely dictated by coal properties. A range of
laboratory tests and procedures have been developed to characterize physical andchemical properties of coke and their potential impacts in the blast furnaces. The
Figure 1. Illustration of a typical coke plant of the
conventional kind.
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most often used and well-known tests are the Coke Reactivity Index (CRI) and the
Coke Strength after Reaction (CSR) developed by Nippon Steel Corporation (NSC)
in Japan in the early seventies, in order to assess the effect of CO2 reactions on coke.There is no universally accepted standard procedure, however NSC/CRI test is
widely recognized around the world and was adopted by ASTM while being
considered for ISO standard [11, 12]. Generally high CSR coke is believed to prevent
the coke from breaking down, improve the permeability of gas and liquid and
increase the productivity as well as decrease the specific coke consumption of the BF
[13].
1.2 State of the art
No international agreement of an ideal way to determine the quality exists as each
industry relies on their empirical experience for the interpretation. These laboratory
tests are designed to test the coke properties under specific set of conditions which
might not be universally suitable. The reproducibility of CRI/CSR values among
different laboratories also varies considerably [14]. Whether the reactivity constitutes
an important factor in determining blast furnace performance has been a subject of
some controversy during the past decades. Some investigators suggest that most of
the reactions involving coke tend to take place in the high temperature zone of the
blast furnace, where diffusion or mass transfer are rate limiting and the mechanical
strength or integrity of the coke was thought to be the significant factor. Others say
coke reactivity is one of the most important factors which control the permeability
and that the lower the coke reactivity the higher is the permeability of the burden.
Coke reactivity in itself might possibly not play a very important role, but the manner
in which the coke reacts could markedly influence its degradation characteristics and
hence the performance of the furnace as a whole [15].
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Consumption of coal matter in coke has an impact on particle porosity. Therefore a
strong interaction exists between the chemical reactivity of coke and its remaining
mechanical strength. The test conditions for CRI and CSR do not truly simulate the blast furnace and are too severe (time, temperatures and exposure of coke to CO2),
although actual field trials have indicated some correlation between the test and the
blast furnace process [16]. However, the CRI/CSR test has the limitations of a single
point test on coke, and includes poor reproducibility and also variable starting
material, varying porosity and particle surface area, and variability in shape and size
[10]. Also important to point out, is that coke is a very inhomogeneous material thus
making it difficult to characterise. Despite some results which counter a general
linear correlation between CSR and CRI, normally low CRI-values lead to high CSR
values. Coke reactivity is mainly influenced by the aging and the maceral
composition of the coal leading to isotropic or anisotropic coke structures (the
isotropic components are more reactive towards CO2), by the ash composition as well
as the carbonisation conditions. From the view of product quality and corresponding
behaviour in the blast furnace, an optimum has to be found between coke CSR, CRIvalues and the carburization of the hot metal [17].
Coke reactivity is influenced by physical properties, including porosity as well as
chemical properties including coke minerals and carbon structure. Reactions with
oxidising gases affect the porous carbon matrix during combustion/gasification. As
coke descends in a BF, its chemical structure is expected to change. The evolution of
pore structure by growth and coalescence leads to increasing or decreasing available
surface areas, changes in pore structure/distribution, gas diffusion and reactivity.
Porous structure of coke is governed by the coking properties of coals, particularly by
maximum fluidity and swelling number [18].
Transformations of inorganic matter upon heat treatment include changes in chemical
bonding, sintering, melting and vaporization as well as mutual interactions withorganic matter. In addition to the catalytic affect on reactivity of carbonaceous
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materials, high temperatures affect particle size of mineral matter and hence the
fragmentation and mechanical stability of the carbonaceous material. Hermann [17]
has evaluated the effect of chemical composition of coal ash on coke reactivity suchthat CaO and SO3 are gasification stimulating components, Fe2O3 an Al2O3 have an
intermediate effect, and P2O5, TiO2, MgO are gasification-inhibiting. Feng et al [19]
have observed that iron is a major catalyst during gasification of bituminous coal as
well as resulting in organised crystalline structures of carbon in the vicinity of the
carbon/iron interface. With increasing burnout, mineral matter could have inhibiting
effect by forming a barrier for oxidizing gases that could influence carbon reactivity
[20].
During its descent through a blast furnace, coke is exposed to extreme reacting
conditions. The prevailing high temperatures in the cohesive zone areas lead to coke
graphitisation i.e. increased ordering of carbon structure. Synthetic graphite has a
highly ordered structure, high fixed carbon content with low levels of ash and volatile
matter. Graphite structure can be described by a regular, vertical stacking ofhexagonal aromatic layers with the degree of ordering characterised by the vertical
dimension of the crystallite Lc, see Figure 2. Each C atom within the aromatic layer
(basal plane) is linked through covalent bonds to three C atoms. However, bonding
between the layers is very weak and can easily be broken by external forces. Natural
graphite has highly ordered structure like synthetic graphite but contains high level of
impurities. The Lc for coal/char/coke can be measured by using X-ray diffraction
profiles [21]. The carbon structure is often believed to influence the carbon reactivity
[22].
As the coke descends through the
blast furnace it is initially dried by
the ascending hot gases. At
temperatures in the 800 – 850 °C
regions, alkali carbonatecompounds are deposited on the
Figure 2. A schematic of crystal structure of
graphite.
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coke surface, causing an increase in reactivity, but do not affect coke size or strength
[23].
Helleisen et al reported that potassium decreases the gasification threshold
temperature from the classical Figure of 950 °C down to 750 – 850 °C, depending on
the amount of potassium and the nature of coke.
When the temperature increases further to 900 – 950 °C, the carbon solution loss
reaction commences and any carbon dioxide produced by the gaseous reduction of
the iron oxides is immediately converted back to carbon monoxide.
The chemical reaction considered as most important is the solution loss reaction,
2CO(g)(g)2
COC(s) + , which normally starts at temperatures around 900 – 1000
°C. Alkalis, in particular potassium, enhance the solution loss reaction significantly
and the reaction starts at considerably lower temperatures by a catalytic effect of the
alkalis [24, 25].
Already in the early 1980’s, Japan raised interest for coke quality at high
temperatures. In order to clarify the degradation of coke in the blast furnace, a series
of fundamental studies on the degradation due to chemical, mechanical and thermal
effects were carried out. The work was based on probe samples and dissections. The
conclusions were as follows;
• When post-reaction strength decreased, the permeability became lower due to a
large amount of fine coke depositing in the lower part of the furnace.
• The tuyere flame temperature and blast velocity have a great influence on the
degradation of coke. Under the higher flame temperature, the cracking of coke
caused by the thermal stress occurs easier. An optimal blast velocity exists to
prevent an inactive dead man and the degradation of coke.
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• The fines originating from coke in the lower part of blast furnace accumulate in
the dead man or travel upward in the furnace. The generation of coke fines is
dependent of the coke strength [26].
Below the cohesive zone the temperature of the coke increases to above 1500 °C.
Coke in the mobile bosh zone (between the cohesive zone and the stagnant deadman
coke) feeds the raceway. This coke is subjected to extremely rapid heating (up to
approx. 2200 °C), combustion and mechanical action in the hot blast. The decrease of
coke rate at high levels of coal injection would lead to higher degradation resulting
from thermal action. The catalytic graphitisation of the coke lump surface by iron and
slag derived from injected coal might also lead to reductions in coke abrasion
resistance [23].
Dissections and probing have indicated a rather complete vaporisation of potassium
in the raceway area, and a sharp rise of potassium towards the centre of the furnace.
The K 2O content in coke ash may reach values as high as 30 % in the centre of thedead man. Alkali distribution in coke is clearly a consequence of the thermal
conditions prevailing along the radius. In the raceway, temperature is the highest, and
alkalis are completely vaporized. In the centre of the furnace, at tuyere level, lower
temperatures exist, promoting the deposition of alkalis on the condensed phases, coke
and slag [25, 27], [28].
Although investigations of cokes from dissected furnaces have provided relationships
between alkali pick-up and coke properties, the actual mechanisms of alkali attack,
and in particular the effect of time of exposure to alkalis, are uncertain [23]. The zone
of maximum alkali pick-up and coke strength reduction is situated near the cohesive
zone.
The fact that coke reactivity in the blast furnace is strongly connected to the alkali
content of coke has been revealed by dissections. Besides a weakening of the pore
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walls in the coke by the solution loss reaction, which is influenced catalytically by
alkalis, there are also observations indicating that alkalis by other mechanisms are
able to decrease the coke stability [24].
Studies of the effect of depositing potassium carbonate (K 2CO3) and potassium
phosphate (K 3PO4) up to 4 % K concentration on BF coke showed that the potassium
clearly increased the reactivity. Porosity measurements and microscopic studies
indicated the reaction to be progressively shifted towards the periphery [28].
The chemical composition of the coke strongly depends on the mineral matter. The
basic elements (Fe, Ca, Mg and alkalis) are included in minerals, which are active at
the CRI test temperature, destroying carbon textures. An increase of the ash basicity
catalyses the coke reactivity.
On the other hand, silicates (Si, Al and alkalis) in coke are inactive during the coal
coking and coke CRI test. Australian and Canadian coals give an increase in ash byfine quartz or kaolinite dissemination. However, the amount of carbonates and
sulphides decreases. Non-reactive silicate dissemination reduces the micro pore
surface of the coke and delays gas penetration into the coke core. This favours a low
CRI and high CSR [29].
Van der Velden [16] wrote “both iron and alkali matter are good catalysts for coke
gasification. Deposition or condensation of these components on coke particles in the
shaft may therefore enhance coke gasification. However, carbon dioxide
concentrations are still very limited and deposition is mainly on the particle periphery
thus no extra pressure is developed on the bulk strength of the coke.”
Coke gasification in the BF preferentially occurs on the coke’s surface. This suggests
that the specifications for reactivity and post-reaction strength of BF feed coke are
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somewhat questionable if no account is taken of the presence of alkalis in the furnace
[28].
However, Gudenau reported the contrary. A damage of coke structure by alkalis is
doubted by investigations that did not find a decrease of coke strength even at alkali
contents of 5 % in coke. Although the blast furnace coke consumption undoubtedly
depends on the alkali-input, this phenomenon cannot be explained with changing
CSR and CRI values and that these values are independent of the alkali content of
coke [30].
Helleisen et al wrote, ”potassium may induce dramatic effects on coke strength at
high temperature, even in the case of good quality coke” based on reference and K
enriched coke studies [25].
According to Beppler et al, alkali contents in coke were found to be lower during
PCI. This was explained by a longer residence time of coke in the BF during PCI andthe heavier stress incurred, thus leading to a higher degree of disintegration. At an
injection rate of 200 kg coal/THM, the coke has to perform about 75 % more direct
reduction work. Further, Beppler et al assumed that an alkali content gradient exists
in the coke lump, and that the alkali-rich layer is abraded to a greater extent as a
result of higher stress [31].
1.3 Objectives
The objectives of this thesis are to;
a) Develop understanding of coke properties and its behaviour in blast furnace,
b) Build-up knowledge regarding the changes of coke properties,
c) Attempt to understand the mechanism of changes,
d) Investigate and attempt to assess the significance of CRI&CSR tests to represent
how coke degrades in operating BF,
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e) Investigate the mode of dispersion behaviour of alkalis, particularly if alkali exists
to a greater extent in the periphery of the coke.
1.4 Research questions
As the coke descends through a blast furnace it experiences fundamental changes in
temperature and atmosphere.
How does the coke change?
Sub-questions to be answered:
In which way is coke degraded?
What factors are affecting the solution loss reaction?
What role does the reactivity play?
What is important, high temperature strength or reactivity?
What affects the strength?
What is the influence of alkali and ash?
2. METHODS
In addition to bench-scale testing, a more comprehensive approach is the pilot-scale
testing of materials under a more realistic industrial environment. Even though these
tests are time consuming and very expensive, data generated in these tests are critical
to provide a comprehensive testing of raw materials such as coke. Coke excavated
from two campaigns was studied in LKAB’s Experimental Blast Furnace (EBF).
Both these campaigns utilised a relatively good quality coke i.e. low CRI (around 20)
and high CSR (around 70). A large number of samples and data were collected during
this campaign. In addition to this, a test with very high CRI and very low CSR was
conducted.
2.1 The Experimental Blast Furnace
A simplified layout of the EBF is shown in Figure 3.
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The working volume is 8.2 m3,
the hearth diameter is 1.2 m,
and the working height is 5.9m. It is equipped with three
tuyeres placed at 120-degree
intervals, and both oil and coal
injection can be used, as well
as other injection materials.
Insulating refractories are
installed to minimize heat
losses, and only the bosh area and the tuyeres are water-cooled. The blast is normally
preheated to 1200 °C in a new type of pebble heaters. The EBF can be equipped with
either a bell-type top with moveable armour, or a bell-less top, for burden distribution
control. Two mechanical stock rods monitor
the burden descent and control the charging
of the furnace. The EBF has one tap hole,which is opened with a drill and closed with a
mud gun. The hot metal and slag are tapped
into a ladle. Probes for temperature
measurements, gas analysis and solid
sampling over the blast furnace diameter are
installed at three different positions, see
Figure 4. To facilitate excavation and repair,
the hearth is detachable and can be separated
from the furnace.
The EBF is run campaign wise and two 6-10 week campaigns normally take place
each year. It has a production rate of about 35-40 thm/day. The normal tap-to-tap
time is 60 minutes and normal tapping duration is 5-15 minutes. Process data arelogged continuously and stored in a database. The data are transferred at regular
Figure 3. The EBF plant and its design.
Figure 4. Illustration of the EBF
and the included probe system.
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intervals to another database from which reports and trend charts are generated and
process calculations are carried out. The coke used has been crushed and sieved to a
fraction of 15-30 mm. After each campaign an excavation is normally performed.Prior to the excavation, the furnace is quenched with nitrogen with the objective to
stop chemical reactions. The EBF-operation together with the excavation gives an
opportunity to map and understand the changes to coke that occur at different levels
in the furnace.
2.2 The studied EBF campaigns
In the present thesis, high quality cokes from two excavations were studied. In
addition a trial with high CRI and low CSR coke was tested and compared with coke
from a reference period. The evaluation in this test is based on solid sampling of coke
through probing.
2.2.1 EBF campaigns followed by excavationThe first campaign took place during
the fall of 2002. This campaign lasted
for almost two months and the furnace
was thereafter quenched with nitrogen
to stop prevailing reactions. A three
week long excavation occurred when
the furnace reached acceptable
temperature.
Two core-drilling events occurred with
success. This was done by removing a tuyere during furnace stoppage and thereafter
inserting a metal cylinder into the furnace to collect burden material. The metal
cylinder is then removed and quenched for later testing. The core was divided intosections and photographed and then the coke was sampled.
Figure 5. Photo of an upside-down piece
from ferrous burden layer 08.
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Material probes have been used frequently during the campaign and the material was
sampled.
Feed coke was sampled every week during the campaign. The feed coke has been
analysed for the following parameters;
• Moisture
• Volatile matter
• Ash
• Sulfur, Nitrogen, Carbon and Hydrogen
• CRI & CSR
• Sieve analysis
During the excavation, samples were taken from each coke layer. This was done at
three different locations for each layer i.e. close to the wall of the furnace, at the
centre, and in the intermediate part (between wall and centre). The volume for each
(a) (b)
Figure 6 a) Photo of layer 3 and b) photo of layer 25 from inside the EBF,
campaign 11.
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sample was around 4 litres. Each layer was photographed in four directions (west,
east, north, south), with digital and analogue camera, see Figure 5 and 6.
The depth was measured at five points (west, east, north, south, centre) for each layer.
A mapping of the locations of the coke layers of interest for campaign 10 has been
made, see Figure 7.
Figure 7. EBFC10. Illustration of how a few selected coke layers
were found when the EBF was excavated. The left hand side
displays cross-section from South to North, and the right hand
side, from West to East. Only the top of the layers are
displayed.
The second campaign with a followed excavation occurred during spring of 2003.
Process differences for the two campaigns can be seen in table I. This campaign
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lasted for almost two months as well and was thereafter directly followed by
quenching with nitrogen and excavation.
Table I. Differences and similarities for EBFC 10 and 11.
Prior to quenching EBFC 10 EBFC 11
Injectant Oil. ~100kg/thm Coal. ~105kg/thm
Ferrous burden LKAB Pellets LKAB Pellets
Coke SSAB coke spring 2002 SSAB coke spring 2003
CRI & CSR 23.2 & 68.8 respectively 19.4 & 71.6 respectively.
Material probes have been used frequently during the campaign. Feed coke was
sampled every week during the campaign, and has been analysed in the same way as
coke was analysed during the EBFC10. During the excavation, samples were taken
from each coke layer.
Figure 8. EBFC11. Illustration of how a few selected coke layers
were found when the EBF was excavated. The left hand side
displays cross-section from South to North, and the right hand side,
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from West to East. Only the top of the layers are displayed.
This was done at six different locations for each layer i.e. in the same radial positions
as used earlier but in two different directions. Apart from more extensive sampling insome areas, the same procedure was used here as for campaign 10. A mapping of the
location of the coke layers of interest for campaign 11 has been made, see Figure 8.
2.2.2 EBF trial with high CRI coke
The trial took place in the spring of 2004 and consisted of two parts i.e. a two day
reference period with a low CRI coke followed by two days of operation with high
CRI coke. During this trial solid sampling occurred at three positions, i.e. in the upper
shaft, lower shaft and through the cohesive zone, see Figure 1. In addition, a tuyere
core drilling was done.
The sampled coke material was separated from slag, fluxes and pellets. Prior to x-ray
diffraction, XRF and TGA reactivity measurements, small coke lumps
(approximately 6-8 cm3) were selected from each probe and crushed to powder (< 75
micron).
The tuyere drill core was divided in four equally large segments and labelled Centre,
Mid 1, Mid 2, and Wall. Thereafter the samples were sieved to fractions of -19 mm,
19-22.4 mm and +22.4 mm. The samples labelled Centre thus represent coke from
the centre of the furnace at tuyere level.
The process parameters were altered as little as possible as the coke type was
changed. The same amount of coal injection was used. The cokes types that were
tested are very different in quality, as can be seen in Table II.
Table II: Properties of the feed coke used in the current study
Parameters Low CRI coke High CRI coke
CRI 19 48CSR 72 35
Fe 0.35 1.05
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SiO2 6.14 4.72
P2O5 0.022 0.053
Al2O3 2.82 2.26
MgO 0.04 0.2
Na2O 0.04 0.11K 2O 0.14 0.22
TiO2 0.18 0.1
2.3 Methods used for characterization of coke samples
To study the cokes, various instruments and methods have been used, i.e.
TGA (Thermal Gravimetric Analysis) and DTA (Differential Thermal
Analysis) with MS (Mass Spectrometry)
CRI (Coke Reactivity Index) and CSR (Coke Strength after Reaction)
Sieving
XRD (X-Ray Diffraction)
SEM (Scanning Electron Microscope) with EDS (Energy Dispersive
Spectroscopy)
LOM (Light Optical Microscope) BET nitrogen adsorption
Each method is described below.
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2.3.1 TGA/DTA–MS
Figure 9 shows the
schematic of Netzsch STA
409 instrument at Luleå
University of Technology,
which can be used for
simultaneous Thermal
Gravimetric and
Differential Thermal
Analysis. Non-isothermal
reactivity was measured by
using a small amount of
coke powder (60 ~ 80 mg)
in an Al2O
3crucible in TGA/DTA equipped with a Quadropole mass spectrometer
with the setting to detecting ions with mass of 1 to 65. The loss in sample weight is
recorded by a very accurate balance ±1 µg. All samples of interest have been reacted
under dynamic heating up to 1300 °C with a heating rate of 10K/minute. Various
gases can be used, but in this study 100 % CO2 or 100 % Ar gas was used.
N2
Gas outlet
Furnace
Sample carrier
protective tube
vacuum
reactive gas
protective gas
inductive displacement
transducer
electromagnetic
compensation system
vacuum tight casing
DSC and TG
carrier
thermostatic
control
evacuation
system
Computer
QMS
radiation shield
Figure 9. Schematic of TGA/DTA furnace used for
non-isothermal reactivity measurement of coke
samples.
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A custom built TGA, see Figure
10, at the University of New
South Wales was used tomeasure the weight loss in coke
samples during isothermal
heating at 900°C for 2 hours
under 100% CO2 and at various
flow rates ranging from 1.5 to
2.0 l/min. The TGA furnace
consists of a recrystallised
vertical alumina (60 mm ID)
tube. Sample temperature is
controlled by an internal
thermocouple located close to
the sample holder.
Approximately 0.2 g sample was placed on a square alumina crucible (30X 30 mm)holder at room temperature. Alumina sample assembly is suspended by a high
temperature stainless wire which is connected to a balance that can measure weight
changes of the order of 1 micro gram (Precisa® 1212 M SCS). The assembly was
kept at low temperature zone in the furnace followed by heating up to 900°C at the
rate of 2°C/minute while 5 l/min of N2 was continuously purged through the furnace
which was regulated by Brooks 5850E mass flow controller. As the furnace reaches
the required reaction temperature, the furnace chamber is raised to move the sample
in the reaction zone followed by reducing the N2 flow to 4 l/min and adding 1 l/min
of CO2. The weight loss of coke sample was continuously recorded by data logger
and used to calculate carbon conversion.
2.3.2 CRI/CSR equipment
In the present work a CRI and CSR equipment was constructed and installed at LTU
see Figure 11. It is based upon the ISO draft for CRI and CSR. It is used for
Figure 10. TGA reactor at UNSW used for
isothermal reactivity measurements.
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determining lump coke reactivity in carbon dioxide gas at elevated temperatures and
its strength after reaction in carbon dioxide gas by tumbling in a cylindrical chamber,
called I-drum.The coke tested should
consist of pre-dried coke
with sizes from 19.0 mm
to 22.4 mm. This sample
is then heated in a
reaction vessel to
1100°C in a nitrogen
atmosphere. For the test
the atmosphere is
changed to carbon
dioxide for exactly two
hours. After the test, the
reaction vessel isallowed to cool down to
about 50°C in a nitrogen
atmosphere.
The comparison of the sample weight before and after the reaction determines the
coke reactivity index and is given as a percentage of the weight loss. The reacted
coke is rotated in the I-drum at 600 revolutions for 30 minutes. The CSR value is
determined by sieving and weighing the amount of the coke passing a 10.0 mm sieve.
The abrasion value is defined as the lack of resistance to abrasion of the coke after
reaction with carbon dioxide in the CRI test, measured as the percentage passing
through a 0.5 mm sieve after tumbling in an I-drum. CSR BF consist of the same step
as for the CSR part of the NSC test but with coke excavated from a blast furnace.
Figure 11. Image of the CRI and CSR equipment.
Inserted is a picture of the reactor when in uplifted
position.
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The furnace consists of three Kanthal Fibrothal 200/200 heating elements and is
controlled in a PC-environment using the software LabVIEW. The gas system
consists of two digital BRONKHORST flow meters. The ISO-draft states that a CO2
flow of 5 dm3/min and a N2 flow of 10 dm
3/min in STP must be used.
2.3.3 Sieving
Samples excavated from the EBF have been sieved by hand, using sieves with a mesh
of 22.4 mm, 19.0 mm, 14.0 mm and 10 mm. Samples for CSR BF were also sieved.
2.3.4 X-ray diffraction
Siemens 5000 X-ray diffractometer at the University of New South Wales (UNSW),
Australia was used to record scattering intensities of samples by using Copper K
radiation (30 kV, 30 mA) as the X-ray source. Samples were packed into an
aluminium holder and scanned over an angular range from 5-105° by using a step
size of 0.05° and collecting the scattering intensity for 5 seconds at each step. The
XRD data was processed to obtain crystallite dimension Lc in carbonaceous
materials. The average stacking height of 002 carbon peak can be calculated using
Scherrer’s equation by using K = 0.9 for Lc. A sharper 002 peak will indicate a larger
crystallite size and a greater degree of ordering in the carbon structure [32]. In most
cases Lc was calculated when Xa was determined by half-width criteria. When the
half-width criteria failed, Xa was determined using centre symmetry method.
2.3.5 Chemical analyses
Samples have been sent to laboratory for XRF chemical analysis. The laboratory at
SSAB Tunnplåt Luleå AB and the laboratory at UNSW have done the XRF analyses
while carbon and sulphur content was measured using LECO analyser at the UNSW.
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2.3.6 Scanning Electron Microscope
Coke pieces were mounted in an epoxy slow-setting resin in plastic moulds (40 mm
diameter). Surfaces were ground on four different grades of silicon carbide paper
(120, 500, 800, and 1200 grit) with distilled water and polished with three different
grades of polishing paper with diamond paste of particle sizes of 15 µm, 9 µm, 3 µm,
and 1 µm. Lubrication fluid was used during the polishing.
Polished samples are fixed on aluminium mounts and coated with a thin layer of
gold-palladium alloy using a Bal-tec MCS 010 sputter coater. The coated specimens
were then examined with a Philips XL 30 scanning electron microscope equipped
with Energy Dispersive X-ray Analysis (EDS) for chemical mapping.
2.3.7 Light Optical Microscope
Coke pieces were mounted in an epoxy slow-setting resin in plastic moulds and
treated the same way as for preparation for SEM.
The coke porosity was measured;
1) Using a Leco 3001 image analysis program. Six polished samples are mounted in a
special holder and placed under the microscope. The microscope measures one coke
piece at the time and the motorized table shifts the samples. The image analysis
software calculates the number of pores seen on the polished surface and also report
pore size and pore size distribution. It measures at magnifications of 520x and 130x
which gives information of macro and micro pores respectively.
2) Using the analySIS 3.2 program and Olympus microscope with 520x
magnification. Coke porosity is calculated as the average value of the samples
measured for each of the four tuyere segments from the tuyere core drillings. Both
procedures were developed by Ruukki in Raahe, Finland.
Light optical microscope has also been used to manually study samples.
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2.3.8 BET
The BET surface area was measured using a FlowSorb 2300 by determining the
quantity of N2 that adsorbs as a single layer of molecules, a so-called monomolecular
layer, on a sample. This adsorption is done at or near the boiling point of the
adsorbate gas. Under specific conditions, the area covered by each gas molecule is
known within relatively narrow limits. The area of the sample is thus directly
calculable from the number of adsorbed molecules, which is derived from the gas
quantity at the prescribed conditions, and the area occupied by each.
2.3.9 Micro texture measurement
The change in the coke microstructure passing the EBF was measured by an
automated microscopic measuring procedure developed at TKS to quantify the
ordering of the coke carbon microstructure. This measuring procedure is based on the
optical physics of the bi-reflectance. The dimension of the bi-reflectance is recorded
using a linear polarising filter in the reflected light of the sample at various
polarisation degrees.
The microscope employed is equipped with a scanning stage, an auto focus system
and a power-driven polarizer in the reflected microscopic light. An adapted image
analysing system enables quantification of the degree of anisotropic and isotropic
components calculated from the optical bi-reflectance.
Using this method at both coke operations (high and low level CRI) the feed cokes
were investigated in comparison to the tuyere coke material sampled by the tuyere
probe. The material of each tuyere core drilling was split radially into four segments.
The material of each segment was screened into three fractions ( 22.4 mm) and than separated into coke, metal and slag components. From
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the crushed feed coke and each tuyere coke material a polished section was prepared
for determination by this microscopic measurement technique.
3. RESULTS AND DISCUSSION
Coke degradation and gasification is influenced by coke porosity, carbon structure
and its minerals. The variation in physical properties, the evolution of carbon
structure and the variation in chemical properties are discussed here.
3.1 Variation in physical Properties
Cokes from the
EBF were tested in
the I-drum to
determine a “Coke
Strength after BF
reactions” value.
Figure 12 illustrates
the result from the
CSR part of the ISO
draft for CRI &
CSR. The Y-axis
corresponds to the
“Coke Strength
after BF reactions”
values and the X-
axis displays the distance below the top of the furnace. As can be seen, a negative
trend line can be easily fitted. The sodium content in the abraded material of the coke
is also plotted as a function of distance.
Figure 13 illustrates the result from the abrasion index part of the ISO draft for CRI
& CSR. The Y-axis corresponds to the “Abrasion Index” values and the X-axis
R2 = 0,93
R2 = 0,91
80,0
81,0
82,0
83,0
84,0
85,0
86,0
87,0
88,0
3,5 4 4,5 5 5,5 6 6,5 7 7,5
Distance (m)
C S R B F
0
0,2
0,4
0,6
0,8
1
1,2
% N a 2 O
i n r e s i d u e f r o m a
b r a s i o n
CSR BF, EBFC10
Na2O %
Figure 12. Coke treated according to the CSR part of the
CRI & CSR ISO-draft. Left Y-axis represents the mass of
coke larger or equal to 10.0 mm after treatment and rightY-axis is sodium content in the coke.
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displays the distance below the top of the furnace. As can be seen, a polynomial trend
line can be fitted. The potassium content in the abraded material of the coke is also
plotted as a function of distance.The increase of
alkali in the
abraded material
corresponds well
with the chemical
analyses of the bulk
coke from
corresponding
layers. There is no
significant
difference, thus
indicating that
alkali not onlyincreases on the
surface of the coke
but that it actually penetrates the whole coke matrix.
Porosity measurements of the coke samples used in this study indicated no significant
variation in the porosity of coke samples from different locations in the EBF.
Examination of EBF coke samples under light optical microscopy indicate that open
pores could have increased marginally as coke descends towards the tuyeres. BET N2
surface area of the EBF coke samples suggest that surface area of cokes did not
change significantly in samples and hence might not have a significant influence on
possible differences of reactivity measurements. It may be noted that there could be
differences in the surface area of coke layers not included in this study. Further
discussion is mainly limited to changes occurring in carbon structure and cokeminerals.
R2 = 0,93
R2 = 0,84
0,0
1,0
2,0
3,0
4,0
5,0
6,0
3,5 4 4,5 5 5,5 6 6,5 7 7,5
Distance (m)
A b r a s i o n I n d e x
0
0,5
1
1,5
2
2,5
3
3,5
% K
2 O
i n r e s i d u e f r o m a
b r a s i o n
Abrasion Index
K2O %
Figure 12. Coke treated according to the CSR part of the
CRI & CSR ISO-draft. Left Y-axis represents the mass of
coke larger or equal to 10.0 mm after treatment and right
Y-axis is sodium content in the coke.
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The measurements of the BET surface measure open and accessible micro pores. The
coke was grinded in a ring mill for 15 seconds to produce a powder. This was donefor coke from both campaigns 10 and 11. In Figure 14 the BET surface is seen for the
specific layers.
(a) (b)
Figure 14. To the left, BET measurements of cokes from EBFC 10 and to
the right from EBFC 11. The layers in campaign 10 and 11 do not have the
same positions.
The excavated coke was also hand sieved using sieves with mesh 22.4, 19.0, 14.0 and
10.0 mm and for each of the three horizontal positions. The peripheral coke becomes
smaller in size (the fraction equal to or above 22.4 mm is reduced) as it travels down
the furnace, in accordance to what one would expect. However, centre coke increases
in size, according to sieving results, as it travels down the furnace. This result is
contrary to what would be expected. This phenomenon can be due to small coke
being predominantly consumed in the centre of the furnace. However, variations were
small and the sampling is difficult for this purpose and could hence result in errors.
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3.2 Evolution of Carbon structure
As the coke descends through the blast furnace, it reacts with upcoming CO2 gases
and loses carbon content. Figure
15 shows that carbon content of
coke samples is decreasing such
that around the bosh region in the
furnace (sample 35) coke
contained approximately 3% less
carbon content. Increased ash
content can be attributed to carbon
loss as well as increased alkali
uptake by coke in the EBF.
Figure 16 shows XRD patterns of
cokes samples from three different
locations. The chemical structure
of coke carbon is increased as
indicated by a sharpening of the
002 carbon peak in cokes taken
from locations 5 to 35. Further,
Figure 16 indicates less
background intensity in the XRD
patterns of coke samples from
lower levels in the EBF (location
35 is less than location 5). Lower
background intensity is often
indicative of decreasing
amorphous carbon content in
coke. Even though amorphous
Figure 15. Variation in carbon content of
EBF centreline coke samples plotted against
distance from top of EBF, tentative
associated temperatures in EBF are also
indicated. From campaign 10.
Figure 16. Variation in background intensity
of XRD patterns of coke samples from three
locations.
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carbon content is not distinctively different in the coke samples shown in Figure 16,
the amorphous carbon content is believed to decrease as hearth coke samples
indicated significantly lower amorphous carbon content than coke samples fromupper parts of the EBF. The results suggest that amorphous carbon is increasingly
depleted as coke descends towards the hearth.
Carbon atoms become more ordered as coke passes from shaft to bosh region as
indicated by the increasingly higher Lc values as shown in Figure 17. This means that
coke structure becomes more ordered during its movement towards lower parts of
furnace. Samples from the hearth were strongly graphitised. Generally, highly
ordered carbons are expected to be
more reactive towards oxidising
gases, including CO2.
The linear correlation suggests
that increase in Lc value isstrongly influenced by
temperature in the EBF even
though other factors such as alkali
and iron species present in coke
could also influence the chemical
structure. The Lc values were
calculated from x-ray diffraction
spectrum after applying
corrections to raw XRD data.
15
30
45
60
75
90
105
120
4 5 6 7 8 9 10
Distance from top of furnace (meters)
L c v a l u e s o f c o k e ( A n g s t r o m )
600
800
1000
1200
1400
1600
1800
2000
T em p er a t ur e of c ok
e b e d l a y er ( o C )
Central layer temperature
Lc values of central layer cokes
Figure 17. Increase in Lc values of coke
during its journey towards cohesive zone in
the EBF and associated temperatures based
on assumptions generated from vertical,
horizontal and inclined temperature probe
measurements. Campaign 10.
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3.3 Alkali uptake and dispersion in coke
The XRF analyses of coke
samples from the centreline
position in the furnace are
indicated in Table III and
IV. Figure 18 plots the
alkali content in coke ash
against the furnace depth
and shows that alkali
content (K 2O and Na2O) in
coke increases as the coke
moves through the shaft to
the cohesive zone. It is
obvious that the alkali
present in recirculation
gases inside the blast
furnace have condensed on
coke surface or penetrated inside the coke matrix followed by reactions with other
minerals. In order to understand the alkali distribution, each coke sample was
analysed for three regions namely outer, middle and core region of sample.
Table III. Chemical composition of EBFC10 coke samples.
XRF(SSAB) Sum ox. SiO2 Al2O3 Fe2O3 CaO MgO K 2O Na2O TiO2 P2O5 SO3
Coke samples from EBF Campaign 10
KL01C 11.4 6.50 3.15 0.58 0.15 0.10 0.37 0.15 0.17 0.04 0.18
KL05C 11.59 5.76 2.63 1.20 0.01 0.06 0.17 0.10 0.16 0.03 1.47
KL10C 12.53 6.39 2.77 1.32 0.04 0.06 0.35 0.16 0.17 0.03 1.25
KL15C 13.26 6.30 2.65 1.06 0.00 0.06 1.24 0.43 0.15 0.02 1.35
KL20C 12.98 5.58 2.57 0.92 0.02 0.07 1.78 0.61 0.14 0.02 1.27
KL25C 13.77 5.83 2.61 0.92 0.02 0.08 2.31 0.67 0.14 0.02 1.17
KL30C 13.89 5.97 2.66 0.97 0.04 0.08 2.07 0.68 0.14 0.02 1.25
KL35C 14.80 5.81 2.64 0.77 0.00 0.08 3.21 0.85 0.12 0.22 1.10
Figure 18. Alkali concentration in EBF coke ash
plotted against distance from top of furnace.
Approximate temperature profile of EBF is also
indicated.
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Table IV. Chemical composition of EBFC11 coke samples.
XRF(SSAB) Sum ox. SiO2 Al2O3 Fe2O3 CaO MgO K 2O Na2O TiO2 P2O5 SO3
Coke samples from EBF Campaign 11
KL01C 10.10 6.13 2.63 1.12 0 0.06 0.18 0.05 0.17 0.026 1.45KL05C 16.59 6.27 4.07 5.63 0.39 0.11 2.35 0.22 0.11 0.025 1.10
KL10NC 10.28 6.23 2.74 1.09 0 0.04 0.16 0.05 0.16 0.032 1.52
KL15NC 10.59 6.31 2.74 1.32 0 0.05 0.26 0.06 0.17 0.03 1.50
KL20NC 11.44 6.66 2.76 1.54 0.14 0.11 0.43 0.07 0.17 0.029 1.45
KL25NC 10.04 5.61 2.44 2.06 0.07 0.05 0.29 0.07 0.16 0.029 1.42
KL30NC 13.93 6.21 2.74 0.83 0 0.06 3.59 0.42 0.12 0.026 1.20
KL35C 18.98 4.63 1.86 17.84 1.94 0.19 1.01 0.19 0.08 0.025 0.65
KL40C 12.44 6.03 2.53 0.97 0 0.05 2.58 0.34 0.13 0.023 1.27
Figure 19-24 provides the
SEM analysis of coke
samples from two widely
different locations in the
EBF (from coke layer 10
and 35 from campaign 10)
at various magnifications.
Figure 19 & 20 illustrates
the inhomogeneity of
mineral distribution in
coke sample 10C. The
EDS analysis (see Table V
to IX) suggested the alkali
content of the
aluminosilicate phases of
coke sample 35C (35th
layer) was higher than in
sample 10C. Alkali also
appear to be incresaingly associated with the carbon matrix when comparing sample
35C to sample 10C. EDS analysis of mineral grains in Figures 19 and 20 indicated
that alkali contents of minerals was in the normal range of often observed
a) Outer coke matrix 10C
b) Central coke matrix 10CFigure 19. a) SEM images illustrating mineral
distribution in upper/outer coke layer in sample
(10C) from upper part of the EBF, b) central core
region of the same coke at various magnifications.
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aluminosilicate phases throughout the coke. The middle sample position can be seen
in Figure 20.
Table V. EDS analysis of coke layer 10. See Figure 19.Values reported in Wt %.
Ima e Point Na K C Al Si Oa 1 6.1 1.8 21.7 12.2 25.9 24.9a 2 17.5 0.8 30.2 6.2 11.4 15.1a 3 0.3 0.4 39.7 0.9 29.5 26.6a 4 0.7 0.3 19.6 3.8 40.3 33.0a 5 3.7 2.3 33.8 14.3 18.0 21.3a 6 1.9 0.8 26.3 5.9 33.8 27.9a 7 1.1 0.3 83.4 1.5 3.0 2.3a 8 0.4 0.2 37.2 0.8 30.2 27.8
a 9 1.5 0.6 16.5 6.9 38.2 32.7 b 1 0.2 0.2 32.8 0.9 65.9 - b 2 0.1 0.3 41.5 0.8 57.3 - b 3 0.4 0.1 91.5 0.5 7.5 -
a) b)
c) d)
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Figure 20. Middle coke matrix 10C
Table VI. EDS analysis of coke layer 10. See Figure 20.Values reported in Wt %.Ima e Point Na K C Al Si O
b 1 1.1 0.6 97.7 0.3 0.3 - b 2 1.5 0.5 97.2 0.4 0.5 - b 3 1.2 1.5 24.4 14.8 58.1 - b 4 1.8 0.3 94.8 1.5 1.6 - b 5 1.5 0.3 90.5 2.8 5.0 - b 6 1.4 0.4 97.6 0.4 0.3 -c 1 1.3 0.8 23.4 3.8 70.7 -c 2 1.3 3.5 13.1 34.6 47.6 -c 3 1.1 0.5 32.8 1.9 63.7 -c 4 1.0 1.2 18.6 2.8 76.4 -
c 5 1.4 2.1 55.4 15.6 25.5 -c 6 1.2 1.4 63.9 11.6 22.0 -c 7 0.9 0.5 96.6 0.9 1.2 -c 8 1.5 1.9 44.3 20.5 31.8 -c 9 0.6 0.4 98.0 0.2 0.9 -d 1 2.4 0.9 - 7.5 11.2 8.5d 2 3.8 0.7 - 14.7 19.7 31.1d 3 1.0 0.9 - 5.8 8.1 13.4d 4 1.1 0.4 - 4.3 6.3 6.9d 5 1.3 1.8 - 22.6 23.5 29.3d 6 1.2 2.3 - 29.3 31.4 32.2d 7 1.0 2.2 - 26.3 32.6 32.8
EDS has been used to create mappings of coke samples as well as for point chemical
analysis. Alkali is found together with Aluminium, Silica and Oxygen, e.g. see Figure
21. The probable compounds are the more common (K,Na)AlSi2O4 and the less
common (K,Na)AlSi2O6. Alkali is found all over the coke matrix. The average alkali
content in coke from layers just above the cohesive zone reached levels to above 4
wt.%.
Table VII. EDS analysis of coke layer 35. See Figure 22. Values reported in Wt %
Point Na K C O Al Si1 2.9 10.1 2.6 31.4 24.7 25.12 0.1 0.1 3.3 39.6 0.6 55.23 3.5 10.8 2.5 30.1 24.8 25.04 1.0 5.6 86.8 3.0 0.5 0.45 3.1 10.4 2.1 30.0 24.5 26.6
6 0.9 5.8 84.8 3.3 0.6 0.6
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Figure 21. EDS mapping of coke from the periphery of layer 35, EBFC10.
Figure 22-24 compare the physical appearance of three regions within the coke
matrix of sample 35C from a lower part of the EBF. In general, the alkali content of
the aluminisilicates analysed in sample 35C were found to be higher when compared
to alkali content of similar phases from the coke sample 10C, see Table V to IX. No
apparent cracks or significant changes in macro pores were visible in coke sample
35C. Visual examination of SEM images of coke 10C and 35C did not indicate any
significant changes in their physical structure. Alkali could influence the surface area,
chemical structure and could also display catalytic effect, see paper IV.
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Figure 22. Periphery of sample from coke layer 35.
a)
b)
d)
c)
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In Figure 23 d), the upper half of the coke matrix is shown. This coke is from the
Figure 23. SEM images of centre of sample 35.
a)
b)
d)
c)
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periphery of layer 35. Images 23 a), b) and c) display an area from the peripheral part
of the coke. Displayed in table VIII are a few points and their estimated content of
some elements. In Figure 24 d), the lower half of the coke matrix is shown. Images24 a), b) and c) display an area from the central part of the coke. Displayed in table
IX are a few points and their estimated content of some elements.
Table VIII. EDS analysis of coke layer 35. See Figure 23. Values reported in Wt %
Point Na K C O Al Si1 4.3 8.5 4.5 29.9 22.8 26.72 3.6 6.1 9.7 29.0 23.2 24.6
3 1.2 2.8 85.7 2.1 1.3 1.74 1.0 2.6 86.8 3.1 0.2 0.55 0.9 2.4 89.1 0 0.8 0.96 3.9 5.2 4.9 29.9 30.7 21.37 1.5 2.1 87.4 1.7 0.3 0.68 3.6 6.3 4.9 30.0 28.2 23.99 3.7 8.0 5.2 29.8 23.9 25.6
Middle coke matrix C35
Figure 24. SEM images illustrating mineral distribution in middle part of coke
layer in sample (35C) from from lower part of EBF at various magnifications.
Table IX. EDS analysis of coke layer 35. See Figure 24. Values reported in Wt %
Point Na K C O Al Si1 3.7 8.6 12.4 27.7 20.3 25.12 3.6 8.8 10.5 29.5 22.1 23.83 1.3 14.9 45.6 16.8 5.0 4.74 3.4 9.7 24.2 20.1 17.4 20.55 2.9 9.5 27.5 25.5 14.7 16.26 3.5 8.4 10.2 28.5 22.6 24.9
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3.5 Evolution of Coke reaction with CO2
Figure 25
compares the non-
isothermal
reactivity of coke
samples from
different locations
in the EBF. This
figure
demonstrates that
as coke descends
in the blast
furnace, its
reactivity
increases. A similar trend was also observed during isothermal reactivity
measurements as shown in Figure 26.
Comparison of isothermal reactivity
of coke samples from upper (5C) and
lower zones (35C) of EBF suggests
that CO2 reaction of cokes from
lower parts of the EBF was faster, see
paper I, II and III. Porosity did not
significantly change for these
samples; therefore the increase in
coke reactivity is most likely related
to the presence of enhanced alkali
concentration. This means that in the
EBF, the coke reactivity can increasedue to catalytic influence of alkali
Figure 25. Non-isothermal reactivity illustrated as loss in wt.
of EBFC10 coke samples with increasing temperature in a
TGA/DTA furnace.
Figure 26. Isothermal reactivity of
EBFC10 coke samples illustrated as loss in
weight of two coke samples (5C & 35C) at
900°C in a TGA furnace.
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even when the carbon structure becomes more ordered. Relatively strong influence of
coke minerals on coke reactivity was also observed in a recent laboratory study of
reactivity based on Australian cokes [33].
Non-isothermal TGA/DTA analyses were made on cokes from EBFC11 as well, see
Figure 27. The results are similar to the results from tests on coke samples from
EBFC10.
In order to further study the differences, derivate TGA plots were generated for both
EBFC10 and EBFC11 cokes. As can be seen in Figure 28 a), samples from 15C and
20C experience a more severe weight loss rate than samples from 01C, 05C, and 10C.
It is even greater for samples from 25C, 30C, and 35C. In Figure 28 b), sample 30C,
and definitely sample 40C, experience a greater weight loss rate. The gasification
threshold temperature was lowered from approximately 1000°C down to 800°C when
comparing top and bottom layers.
This is the samefor cokes
excavated from
campaigns 10
and 11. The
increase of
alkali content
from top layer to
bottom layer is
about 10-fold.
This alkali
uptake
phenomenon
corresponds well with some previous studies.
Figure 27. Non-isothermal reactivity illustrated as loss in wt.
of EBFC11 coke samples with increasing temperature in a
TGA/DTA furnace.
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(a) (b)
Figure 28 a). Derivative of TGA results for selected cokes from EBFC10 and b)
Derivative of TGA results for selected cokes from EBFC11.
Figures 29 a) and b)
give the DTA curves
of coke layers taken
from EBFC10 and
EBFC11 respectively.
To understand
different reaction
transitions of these
coke layers, the
derivative of the DTA
curves are calculated
and shown in Figures
30 a) and b). Both types of figures show the exothermicity or the endothermicity of
reactions involved. Two categories of coke layers appear in the DTA curves.
The first category includes the coke layers KL30C, KL35C and KL43C containing a
high amount of alkaline (K, Na), see Tables III & IV, which are distributed in
Alumina silicate matrices as observed by SEM. The second category of coke layers
includes KL01C and KL05C that contain a low amount of alkaline. DTA results from
900 950 1000 1050 1100 1150 1200 1250 1300Temperature /°C
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0
0.2
DTA /(uV/mg)
30C
35C
05C
01C