2005 evolution of coke properties while descending through a bf

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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

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Evolution of Coke Properties while

Descending through a Blast Furnace

Tobias Hilding

Welcome Publications


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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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Evolution of coke properties while descending through a blast furnace

Tobias Hilding, Div. of Process Metallurgy, LTU, 2005

1

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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46

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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47

(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

2005 evolution of coke properties while descending through a bf

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