the hot strength of industrial cokes – evaluation of coke properties steel research 12 2014

The Hot Strength of Industrial Cokes Evaluationof Coke Properties that AffeStrength

Juho A. Haapakangas, Juha A. Uusitalo, Olli J. Mattila, StaniDavid A. Porter, and Timo M. J. Fabritius

The strength of coke at high temperatures is of major importatheauanot

efd

esdin

he reliability of the results was

manufacturers will have to

ne option could be the pre-

als.[1] Also, the high price of

ompanies to find alternative

ommon way to replace coke

injectant fuels, commonly

coke is charged, greater

y individual lumps of coke.

oad due to thicker iron ore

rden weight. Another issue

issions. One possibility to

ddition of bio-material or

mix, which can negatively

coke. All of these factors

oke quality and its analysis

methods.

Stresses inside a blast furnace include mechanical

nd abrasion, solution loss

h temperature, and impact

shock by the high-speed hot blast. In industry, coke

strength is characterized by drum indices (Micum, Irsid)

J. A. Uusitalo, Prof. D. PorterMaterials Engineering Laboratory, University of Oulu, P. O. Box 4200,

www.steel-research.de

FULL

PAPERor drum strength after reaction (CSR). Although these

methods are useful in assessing coke quality, they do not

measure coke strength at operational temperatures. Blast

furnace excavations made in Japan have shown that coke

FI-90014 Oulu, FinlandO. J. MattilaRuukki Metals Oy, Rautaruukintie 155, FI-92100 Raahe, Finland

DOI: 10.1002/srin.201300450stresses such as shattering a

reactions, alkaline attack, hig

[ ] J. A. Haapakangas, S. S. Gornostayev, Prof. T. M. FabritiusLaboratory of Process Metallurgy, University of Oulu, P. O. Box 4300,FI-90014 Oulu, FinlandEmail: [email protected] with statistical analyses.

1. Introduction

Coke as a rawmaterial in the blast furnace provides fuel for

ironmaking, serves as a source of reductant gas, provides

structural support for the material bed and serves to

carburize hot metal. Its role as structural support is vital

and for this reason strength is one of the most important

properties of coke. Poor coke strength can cause many

operational problems, such as reduced permeability in the

shaft and hearth areas, undesirable gas and temperature

distribution, and the possibility of hanging of the burden.

In the near future, steel manufacturers worldwide are

faced with challenges regarding the quality of coke. The

availability of prime coking coals is decreasing and their

price is increasing, so coke

utilize coals of lower quality. O

pelletizing of weakly coking co

metallurgical coke is forcing c

fuels for the blast furnace. A c

is to increase the use of

pulverized coal. When less

chemical stresses are faced b

Coke also undergoes higher l

layers and rise of average bu

is the rising price of CO2 em

reduce CO2 could be the a

waste plastics into the coking

affect the strength of the

increase the importance of cwere compared to industrial strength tests and discussed. T

obtained strength results. Both hot and room temperature compressive strength valuesoperation. Despite this, there is little information regardingresearch, the hot strength of three industrial European cokGleeble thermomechanical simulator. The hot strength wastemperatures: room temperature, 1600 and 1750 8C by measof roughly 50 coke samples at each temperature. A significobserved for all three coke grades at high temperatures. Ncoke grades in compressive strength were observed at roomtemperatures, differences were observed in strength and dthe order of magnitude of strength remained the same. Thetemperatures was also studied and discussed based on strstructural properties of coke such as total porosity, pore sizeamount of inerts, and degree of graphitization were determ1608 steel research int. 85 (2014) No. 12ct Its HighTemperature

slav S. Gornostayev,

nce for efficient blast furnacee hot strength of coke. In thisgrades was studied using analyzed at three differentring the compressive strengtht decrease in strength wasable differences between thetemperature. At highormation behavior; however,eformation behavior at highsstrain curves. Severalistribution, pore shape factor,ed in order to explain the 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

size remains mostly unchanged in the upper parts, but graphitization. Only visibly crack-free samples were

selected for strength testing. The method of sample

preparation is shown in Figure 1.

During strength testing, the Gleeble samples were held

for 30 s at their respective test temperatures to allow the

temperature to stabilize throughout the samples. After

heating, compressive forces were measured while the

samples were mechanically compressed 4mm at the rate

of 1mms1. The use of a Gleeble device for testing the hotstrength of coke has been described in detail earlier.[7,8]

Industrial analyses provided by the coke manufacturers

are displayed in Table 1 and 2.

The XRD tests were performed with a Siemens D5000

X-ray diffractometer. The point counting of structural

properties was performed with a Pelcon Automatic Point

Counter.

3. Results and Discussion

3.1. Hot Strength

During previous work[7,8] coke hot strength was measured

for an individual coke grade and a significant decrease in

strength was observed at 1600 8C in comparison to 1000 8C.Afterwards an even further decrease in strength was


FULL

PAPER2. Experimental

All of the coke samples used in the Gleeble experiments

were prepared beforehand and mixed in order to avoid

systematic errors caused by sample location. Coke lumps

were drilled with a hollow drill, cut to appropriate height

and polished to form cylindrical samples of 16mm in

diameter and roughly 12mm in height. This size and shape

has been found appropriate for use in the Gleeble device

using a standard sample holder. The largest coke lumps

were selected for sample preparation as small ones are

inadequate for producing a sufficient number of samples.

After machining, the coke samples were pre-graphitized

in a chamber furnace in order to speed up testing in the

Gleeble by reducing the required sample holding time. The

pre-graphitization was made at the sample temperature

that was subsequently used for testing in the Gleeble. It

was done in order to simulate the thermal stresses and

degree of coke graphitization brought about by the descent

of the coke in an actual blast furnace. The coke samples

were sealed in a graphite container and covered in graphite

powder to avoid gasification. In addition, the furnace was

operated in inert gas. The graphite container containing

the coke samples was placed in the furnace at 1200 8C afterwhich the temperature was raised up to the subsequent

test temperature and held for 60min to ensure completesignificant degradation is observed in the lower parts when

the temperature exceeds 1400 8C.[2]

There are few published studies on the hot strength of

coke, possibly due to difficulties in experimental setup.

The reported hot strength results are somewhat contra-

dictory: increases in compressive coke strength have been

reported at 1200,[3] 1300,[4] 1400,[5] and 1650 8C[6] while adecrease in coke strength was reported at 2000 8C.[4] Itshould be noted that most of the earlier results have not

been statistically reliable due to small sample sizes. The

properties of coke that define its hot strength are largely

unknown.

In this study, the study of compressive strength of

coke was further expanded to include three industrially

produced European coke grades at room temperature and

at elevated temperatures of 1600 and 1750 8C. Also inthis study several coke properties, such as total porosity,

inerts, and degree of graphitization were analyzed in

order to study how they relate to the strength of coke

both and room temperature and at elevated temperatures.

Both the room temperature and hot strength measure-

ments were conducted with a Gleeble 3800 thermo-

mechanical testing device. The obtained results were

intended to give information about the strength of

industrial coke in the lower part of a blast furnace, where

coke is the only solid material and its strength is vital for

good permeability. 2014 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimFigure 1. A) A coke cylinder after drilling, B) a prepared cokesample, C) the graphitization container, and D) coke sampleduring a Gleeble test.observed at 1750 8C. Deformation of coke was alsoreported to be partially plastic at 1600 8C and plasticityfurther increased at 1750 8C. In this study, two new cokegrades were included in the testing.steel research int. 85 (2014) No. 12 1609

The strength of three coke grades wasmeasured at room

temperature and at elevated temperatures of 1600 and

1750 8C. Originally 50 coke samples were tested for eachgrade at each temperature in order to obtain statistically

4mm (33.3%) compression. In some cases, this was

obtained in the beginning of compression and in some

at the very end.

The mean strength and strain values at room tempera-

Irsid

>20mm

Irsid

40mm

Micum

coke 3 the weakest. The difference in the reported drum

strength between cokes 2 and 3 in the Micum test, given in

Table 1, was much less pronounced. Unfortunately, only

Irsid values were available for coke 1.

At 1600 8C, the ultimate strength was significantlydecreased for all coke grades compared to room tempera-

ture. The decrease in strength was similar for all three

coke grades. An increase in test temperature had a varying

effect depending on the coke grade; the strength of

coke 1 suffered a further significant drop when heated

to 1750 8C. The ultimate strength of cokes 2 and 3, on theother hand, seemed to slightly recover as a result of

increasing the temperature, however, as will be shown

in Table 4 this result was without statistical significance.

The possible recovery in strength could be explained by

the increasing plasticity of the coke at higher temper-

atures. As a result, the density and surface area of coke

resisting the compressive force are increased during

deformation resulting in a higher force required for

further deformation. Plastic deformation behavior of

coke at high temperatures has also been reported in a

previous study.[4] The results indicate that the develop-

ment of coke hot strength as a function of temperature

may be material dependent. Despite the varying effects

of temperature, the relative strengths of the various coke

be the softening of bonding between carbon layers as a

result of increased heat energy, which would allow re-

organization of the carbon matrix during compression.

It is noteworthy that none of the three coke grades

tested during this study suffered a catastrophic fall in

strength as a result of high temperatures. This result was

expected since all three cokes are used industrially and

known to function in actual blast furnace operation. The

significant fall in the strength of coke at high temperatures

is contradictory to some of the previous reports by other

researchers.[3,5,6] A possible explanation for this could be

the differences in the coke grades that were analyzed,

different methods of sample preparation (pre-graphitiza-

tion) or the use of different testing equipment. Some of the

studies were also conducted at temperatures below the

melting point of cokes ash components, therefore not

taking into account their effect. The reported significant

fall in strength at 2000 8C and above, and the observedplastic deformation behavior,[4] however, are congruent

with results of this study.

Statistical reliability of the hot strength results was

analyzed using a two-sample t-test assuming unequal

variances (Welchs t-test). A two-tailed test with p 0.05was chosen as the level of significance. The following

hypotheses were set and evaluated:


FULL

PAPERgrades remained constant independent of temperature

(2> 1> 3).

The reason behind plastic deformation behavior of

coke at high temperatures is unknown. It is the belief of

the authors that melting of cokes ash components is not

responsible, since it has been shown that in tuyere coke the

melted ash has migrated into pores, therefore it should

not affect deformation behavior.[10] One explanation could

Temperature Degrees of

freedom

t

Coke 1

Room vs. 1600 8C 70 3.790

Room vs. 1750 8C 67 5.043

1600 vs. 1750 8C 97 2.019

Coke 2

Room vs. 1600 8C 84 5.648

Room vs. 1750 8C 91 4.328

1600 vs. 1750 8C 87 1.443

Coke 3

Room vs. 1600 8C 68 3.955

Room vs. 1750 8C 70 3.394

1600 vs. 1750 8C 97 0.872

Table 4. Statistical reliability analysis of the hot strength values. 2014 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimCritical t p-Value Statistically

significant

1.994 0.000 Yes

1.996 0.000 Yes

1.985 0.046 Yes

1.989 0.000 Yes

1.986 0.000 Yes

1.988 0.153 No

1.995 0.000 Yes

1.994 0.001 Yes

1.985 0.386 NoofWhen the calculated value of t exceeds the critical value

t and p is below the chosen significance level of 0.05, theH0: There is no statistical difference in coke hot strength

between the studied temperatures.

H1: There is a significant difference in the hot strength of

coke at the studied temperatures.steel research int. 85 (2014) No. 12 1611

difference in strength is concluded to be significant. Based

on the results presented in Table 4, the hypothesis H0was accepted in two cases: both for cokes 2 and 3 when

comparing the hot strength between temperatures 1600

and 1750 8C. Therefore, the differences in strength betweenthese temperatures are statistically insignificant. In all of

the other cases, hypothesis H0 was rejected and the

alternative hypothesis H1 was accepted; the difference in

the other studied temperatures is statistically significant.

Digital stressstrain curves were obtained during the

strength tests both at room temperature and at high

temperatures. The curves can be used for analyzing the

deformation behavior of the cokes. Six general types of

stressstrain curves were identified during the tests and

they are presented in Figure 2. For clearer representation,

the actual stressstrain curves obtained from the

Gleeble experiments were re-calculated based on moving

average of 30 measurement points (one measurement

every 0.01 s). It should be noted that the strain values

in Figure 2 do not always precisely reflect the true strain

values since the graphs were re-calculated based on

moving average. The difference can be up to roughly 2%

during the linear elastic phase. These stressstrain type

categories were found to represent the vast majority of all

strength experiments both at room temperature and at

high temperatures.

th


FULL

PAPERFigure 2. General stressstrain curve types (AF) observed during1612 steel research int. 85 (2014) No. 12e compressive strength tests for coke (see text for further details). 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The distribution of all stressstrain curves to the six

general types is presented in Table 5. As observed from the

table, each coke grade was unique in terms of the shape of

the stressstrain curves at high temperatures. At room

temperature, only curve types A and C were observed.

Meanwhile, types B, D, E, and F, which indicated plastic

deformation, were only found at high temperatures.

At room temperature, the ultimate strength values were

obtained at the very beginning of deformation for all three

grades. On a stressstrain curve this is reflected by an

initial almost linear rise in compressive force until fragile

fracture occurs at 04% strain (mean 2.53.0%) and inmost

cases followed by a significant decrease in force required

for further deformation. This type of a stressstrain curve is

type A in Figure 2. Therefore, it can be stated that

deformation behavior was brittle in all room temperature

cases and the obtained stressstrain curves resembled

the deformation of a ceramic material, a result, which has

been reported in previous studies.

The type B curvewas observed at high temperatures and

can be considered to indicate plastic deformation during

compression: as the coke sample is compressed, its density

and surface area are increased and force required for

deformation is increased. For type B curves, the ultimate

strength value was obtained at the very end of the 4mm

was also regularly observed at room temperature, which

could be a sign of layered crushing as reported in a

previous study.[11]

The type D curve, like type C, had a steep linear rise in

the stress curve followed by fracture and a decrease in the

stress curve, however, at high temperatures toward the end

of compression the stress started to strongly increase.

Types E and F both had a very gentle rise in stress curve

during the linear elastic phase. Based on the shape of

the curve, this was followed by fracture half way through

the compression, i.e., about roughly 15% strain and a

decrease in stress required for further deformation. Type F

showed a second increase in the stress curve toward the

end of indicating plastic deformation during compression

and often the highest force value was obtained at the very

end. Plastic deformation behavior (types B, D, and F) could

be considered beneficial for blast furnace performance,

since the strength of coke would increase under compres-

sion at high temperatures.

As was reported in Table 3, the standard deviation of

strength showed a strong decrease at high temperatures.

This can also be observed from the distribution of all

results to specific strength regions, which are depicted in

Figure 3.

The distribution of coke strength becomes increasingly

p

]

.0

.0

.0

.0

.0

.0

.0

.0

l t


FULL

PAPERcompression. This type was observed for cokes 1 and 2 at

high temperatures.

Type C curve is similar to type A with a steep linear rise

in strength, but unlike for type A, there is no dramatic fall

in the force required for further deformation, instead

significant force is still required all the way up to 33%

compression. This type of a curve was common at

higher temperatures for coke 1 and especially coke 3. It

Type A

[%]

Type B

[%]

Ty

[%

Room temperature

Coke 1 76.0 24

Coke 2 88.0 12

Coke 3 72.0 28

1600 8C

Coke 1 2.0 40.0 22

Coke 2 15.0

Coke 3 18.0 46

1750 8C

Coke 1 42.0 18

Coke 2 4.0 10

Coke 3 4.0 56

Table 5. Distribution of stressstrain curves among the six genera 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimnarrower as the temperature is increased. This result

was especially pronounced for coke grades 1 and 3. For

coke 2, the standard deviation decreased at 1600 8C butwas slightly higher at 1750 8C compared to 1600 8C.This result could be explained by the grades possibly

stronger tendency toward plastic deformation as indicated

by the mean strength values. It can be considered that

a narrow strength distribution would be desirable, since

e C Type D

[%]

Type E

[%]

Type F

[%]

22.0 14.0

67.5 17.5

16.0 16.0 4.0

4.0 30.0 6.0

4.0 62.0 20.0

12.0 22.0 6.0

ypes.steel research int. 85 (2014) No. 12 1613

blast furnace operators value consistent quality and

predictability. Strength of coke 3 is, however, clearly the

highest at all temperatures with a low number of weak

samples. Therefore, out of the tested cokes, the distribu-

tion of coke 3 can be considered the most desirable.

The industrial CSR (coke strength after reaction) test is

often called a hot strength test despite the fact that the

actual drum strength is measured at room temperature

after 2-h gasification at 1100 8C in 100% CO2 gas. Based onthe industrial tests for coke depicted in Table 1, coke 1 had

by far the highest CSR value, likely a direct result of its

lower chemical reactivity (CRI). The high CSR value is

usually interpreted as indicating good performance in a

addition to the coke testing protocol.

Figure 3. Distribution of coke strengths.


1614 steel research int. 85 (2014) No. 12

FULL

PAPERThe most authentic strength test could be testing pre-

gasified coke in a thermomechanical simulator, such as

the Gleeble. The pre-gasification could be performed in a

blast furnace gas phase simulator, such as presented in

literature.[13] The tested coke could be also taken from a

dissected blast furnace or an experimental blast furnace,

in which case it would also include the presence of

circulating elements. Including coke gasification to hot

strength testing, however, could not be fit to this article

and deserves a separate investigation.

3.2. The Effect of Heat Treatment on the RoomTemperature Strength

Heat treatment above the temperature of the coking

furnace is expected to have a twofold influence on the

strength of coke: (i) it promotes further graphitization,

which changes the atomic structure of coke into a more

highly ordered one and (ii) it causes significant weight loss

as displayed in Table 6.

Grade 1600 8C[%]

1750 8C[%]

Coke 1 7.2 10.1

Coke 2 4.9 7.6

Coke 3 6.2 8.7

Table 6. Weight loss of coke as a result of 1 h pre-graphitization ina chamber furnace.blast furnace. However, it has been discussed that the

conditions during the CSR test are intensive and the

reaction mechanism differs from an actual blast furnace:

during the CSR test coke is gasified throughout the coke

whereas in a blast furnace reactions occur mainly on the

surface.[12] Despite the usefulness of CRI and CSR tests,

they do not include testing coke at blast furnace temper-

atures. Since no single analysis method is capable of

simulating blast furnace conditions, multiple different

tests are required. The use of a Gleeble could be a useful 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

As was shown in Table 2, coke 1 contained the highest

amount of SiO2, therefore its ash had the highest potential

to react with the carbon matrix (forming SiC and CO) and

to induce weight loss. The weight loss is a result of

chemical reactions, vaporization, and re-organization of

mineral matter in the coke (i.e., decarbonation and

desulphurization[14]). Another issue is the plastic defor-

mation at elevated temperatures. For this reason, a test

was made to study how much merely the preceding

heat treatment affects the room temperature strength

of coke in comparison to it being in a hot state. This

study was done for coke 2 by testing the strength at

aromatic layer is linked through covalent bonds to three C

atoms. However, bonding between aromatic layers is

weak, easily broken by external forces. Non-graphitic

carbon, however, contains cross-linking between the

aromatic layers and much higher force is required to

dissociate them.[15,17] During heat treatment, non-orga-

nized carbon is presumably attached to the edge atoms

of the graphite like layers, which enables the growth of

organized layers but decreases the cross-linking between

dard

tion

]

Strain

[%]

Youngs

modulus

[MPa]

Change in

strength

[%]

3.0 1337.1

3.1 724.3 35.5%

eat treatment at 1750 8C.


FULL

PAPERroom temperature of 50 non-graphitized samples and

50 samples pre-graphitized at 1750 8C. The obtainedstrength values are displayed in Table 7. Youngs modulus

was determined from the angular coefficient of linear

elastic part of the stressstrain curves.

As can be seen from Table 7, the mean room

temperature strength of coke is significantly decreased

by pre-graphitization. As with strength tests at elevated

temperatures, the standard deviation of strength is

much lower. From Figure 4, it can be concluded that

the structure of coke undergoes a process of homo-

genization during heat treatment. In particular, samples

showing high strengths are absent after graphitization.

The result of a decreased strength is likely a combina-

tion of several factors including weight loss and a

resultant increase in porosity, as well as changes in the

crystalline structure of the coke. It has been stated

that as the crystalline structure approaches that of

graphitic carbon, the cross-linking between carbon

layers is decreased and the coke matrix weakened.[15]

The above results support this theory, however the

weight of each factor is unknown and deserves separate

investigations.

For both graphitized and non-graphitized samples

tested at room temperature, the breakage mechanism

was the following: a short elastic phase followed by fragile

fracture. Heat treatment at 1750 8C did not have anynoticeable effect on the shape of stressstrain curves,

except for the angular coefficient of the elastic phase

(Youngs modulus). There was little to no change in the

strain value following heat treatment. Therefore, it is

expected that the plasticity of coke observed at high

temperatures is purely a result of the temperature during

the compression, not the preceding heat treatment.

Mean

strength

[MPa]

Stan

devia

[MPa

Non-graphitized 23.75 8.09

Pre-graphitized (1750 8C) 15.32 5.42

Table 7. Compressive strength values of coke 2 before and after h 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim3.3. Graphitization Degree of Coke

Graphite is a crystalline structure of carbon that consists of

regular, vertical stacking of hexagonal aromatic layers.

When coke is heated to a sufficiently high temperature,

it begins to become more ordered and approach the

structure of graphite. Further graphitization of coke begins

to occur when its temperature exceeds the original coking

temperature; generally above 1100 8C. Graphitization ismade possible by the plastic phase of coking, during which

carbon layers are organized near parallel. As the tempera-

ture of coke is increased, it enables the continuous

rearrangement of the layer-planes to take place by small

stages.[16]

In the structure of graphite, each C atom within an

Figure 4. Distribution of strength of coke 2 samples before andafter heat treatment at 1750 8C.steel research int. 85 (2014) No. 12 1615

the layers. After heat treatment between temperatures of

Figure 5. Shapes of the 002 peaks from the XRD spectra of thestudied coke grades. The background intensities of each curve arescaled for clearer recognition of the shapes.


FULL

PAPER1000 and 2000 8C non-graphitizing carbons become harderand graphitizing carbons become softer.[16] Therefore, it is

important to know the degree of coke ordering, which is

unique for different coke grades.[18]

Some coals are easily graphitized, others are non-

graphitizing and some, such as anthracite, begin to

graphitize at extremely high temperatures (above

2000 8C). Graphitization is inhibited by randomorientationof the graphite like layers as well as strong cross-linking

between the layers.[16] Coke grades have also been

observed to have varying degrees of graphitization even

in similar annealing temperatures.[18]

The size of graphite crystallites in coke can be

represented by the height of carbon net layer (Lc) and

the spread of carbon net plane (La). The growth of Lc during

graphitization is linear as a function of temperature and

is independent of the gas atmosphere and gasification

reaction. La also increases during heating, but is decreased

during gasification.[19] The graphitization degree of coke

can be evaluated by X-ray diffraction method. In this

work, graphitization degree was evaluated by calculating

the Lc values based on the 002 carbon peaks from the

XRD patterns. Calculating the La values is also possible,

but more difficult due to its smaller intensity in the

XRD spectra. The Lc value can be calculated from X-ray

diffraction profiles using Scherrers equation:[17,19]

LcA 0:9l

B cos uB1

where l is the wavelength of X-ray (A), B is the angular

width in radians at half-maximum intensity of [002] peak,

and uB: is the reflection angle of [002] peak.

The calculation is based on the assumption that

crystallites are ideal with no strain or distortion. The

results can be considered precise only for ordered

materials. Images of the 002 peak in the XRD spectra

are displayed in Figure 5.

The Lc value illustrates the average height of graphite

crystals in coke, hence a high value indicates a high degree

of graphitization. The Lc values of the studied coke grades

are presented in Table 8.

The calculated Lc values indicate further graphitization

as a result of heat treatment for all three coke grades.

Similar observations have been made in previous studies.

The coke grades also showed similar Lc values at a given

temperature, therefore they are expected to be similar in

terms of graphitization behavior brought about by high

temperatures.

Strength and degree of graphitization of coke may

indeed be strongly linked: for all three coke grades strength

was significantly decreased at high temperatures as the

degree of graphitization was increased. This observation is

also supported by the fact that the room temperature

strength was significantly lower for coke heat-treated at

1750 8C. However, the significant weight loss and following1616 steel research int. 85 (2014) No. 12 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

more easily through connected pores.[9,21]

Coke 1 Coke 2 Coke 3 Porosity Inerts Reactive

Table 9. Total porosity, contents of inert, and reactive texture.


FULL

PAPERAhigh amount of inerts in coke is another factor that has

been linked with good strength.[20] High amounts of inerts

have also been reported to decrease the mean pore size,

which may partially explain their positive effect on

strength.[20] A decrease of inert size can improve coke

drum strength up to a certain limit.[22] Inert macerals also

have higher hardness than reactivemacerals. The hardnessincrease of porosity during heat treatment should also be

kept in mind as a weakening factor. In this study, all of the

coke grades proved to be graphitizing and very close in

terms of degree of graphitization at each temperature.

Therefore, their differences in strength at a given

temperature are probably decided by other factors.

3.4. Effect of Coke Properties on Their Physical Strength

The strength of coke can be divided into two factors: the

adhesive strength of the solid matrix and the pore

structure. Lower total porosity is beneficial for coke

strength since high porosity indicates a reduction in the

amount of solid material. This result has been observed in

previous publications.[5] According to results of 3D

modeling, total porosity is dominant compared to carbon

matrix bonding strength regarding the mechanical

strength of coke.[9] Pore size distribution is also considered

important, since it affects the distribution of stresses. It has

been concluded that small pore size and high density of

pores produce higher strength compared to a small

number of large pores.[5,20] Thick coke cell walls in theory

should be beneficial for strength, however in practice a

high density of small pores tends to produce a thin cell

wall. Regular distribution of pores has also been suggested

to be beneficial for good strength as cracks can propagate

[A] [A] [A]

Untreated 21.8 20.0 16.9

1600 8C 59.8 62.5 62.8

1750 8C 93.2 81.9 88.9

Table 8. Lc values of coke after heat treatment.of reactive macerals decreases with increasing mosaic

size.[20] Differences exist in the hardness of various coke

textures.[20] However, despite some attempts to link coke

texturewithmechanical strength, correlations between the

two have been poor and it is possible that the effects of

texture distribution are overshadowed by other factors.[23]

In order to explain the differences in strength between

coke grades, total porosity and the contents of inert and

reactive texture were determined. The distribution of these

structural parameters was done by a standard point

2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimcounting method by counting at least 1000 points from

five polished sections for each coke grade. Structural data

obtained by these analyses is displayed in Table 9.

Differences in the content of inert material between the

coke grades proved to be small. The high strength of coke 2

reported in Table 3 could be at least partially explained by

its slightly lower total porosity. However, all of the studied

cokes were fairly close in measured structural character-

istics and due to the heterogeneous nature of coke

substantial amounts of polished sections would have to

be analyzed in order to draw certain conclusions.

Pore size distribution and pore shape factors were

determined with a MATLAB-based image analysis soft-

ware[24] using the same polished sections used for point

counting. Five images of size 3.5mm 2.6mm were takenfrom each five polished sections per grade under 4magnification. The pore shape factor was determined by

identifying pore edges from LOM images by subsequent

thresholding and filtering methods causing the smallest

pores to be excluded from the analysis as they could not be

separated reliably from the darker areas of the coke matrix

itself. After the pore edge detection, the measured pore

surface area was used to determine round-equivalent edge

length, which was divided bymeasured real edge length on

each pore. The software was programmed to ignore any

pore connected to the edge of an image in the calculations.

According to the image analysis results shown in

Table 10, there was practically no difference in the pore

shape factors in any size range. It can be concluded that in

this study no direct correlation was obtained between coke

strength and pore shape factors of pores below 1mm in

diameter. It is thought by the authors that the effects of

pore shapes on coke strength are caused by flaws,

connected pores, and pores of larger size than the ones

measured during this study.[%] [%] texture

[%]

Coke 1 49.0 17.4 33.6

Coke 2 46.0 17.6 36.5

Coke 3 48.1 16.8 35.24. Conclusions

The compressive hot strength of three industrially

produced coke grades was analyzed by testing 50 coke

samples at room temperature, 1600 and 1750 8C. Statisticalreliability of the obtained strength results was analyzed.

The strength testing was performed with a Gleeble 3800

steel research int. 85 (2014) No. 12 1617

the

tot

we

to

we

Re

Pu

Pore edge

l

Number of pores Pore shape factor

Co

72

0 32

0 7

4

1 5

Tab y


FULL

PAPERzation of coke strength as a result of heating.

3. Differences in contents of inert material and reactive

macerals between the studied coke grades proved to be

small. Likewise, the graphitization behavior of all three

grades as a result of heat treatment proved to be similar

at the test temperatures. Therefore, the difference in

strength was likely governed by other factors such as

pore structure, flaws, or the adhesive strength of the2.4.

5.

16sive strength at room temperature. A large statistically

significant drop in ultimate strength (roughly 30%) was

observed for all three grades when tested at 1600 8C.Coke 1 suffered a further statistically significant fall

in strength at 1750 8C. Despite differences in the hotstrength properties between the coke grades, the order

of strength (2> 1> 3) remained the same at high

temperatures.

The standard deviation of strength was significantly

decreased at high temperatures indicating homogeni-1.The following conclusions can be drawn:

Significant differences were observed in the compres-anrmomechanical simulator. Properties of coke, such as

al porosity, content of inerts, and reactive macerals as

ll as degree of graphitization were determined in order

explain the observed strengths. The measured strengths

re also compared to industrially obtained strength

alyses.

.63 5.64 6.12

le 10. Pore size distribution and pore shape factors obtained b1.200.41 51.52 65.12

.410.82 22.44 30.12

.821.22 6.20 8.12

.221.63 3.39 3.720ength [mm]Coke 1 Coke 2carbon matrix. No direct correlation was obtained

between coke strength and pore shape factors of pores

below 1mm in diameter. It is thought by the authors

that the effects of pore shapes on coke strength are

caused by flaws, connected pores, and pores of larger

size than the ones measured during this study.

Preceding heat treatment of coke at 1750 8C caused alarge decrease in the mean strength of coke 2 at room

temperature. This may be explained by weight loss and

increase of porosity during heat treatment or by the

increased graphitization degree of coke.

The stressstrain curves were similar in shape at room

temperature for all three cokes; early linear elastic

phase followed by fragile fracture. In most cases, little

Ke

str

Re

[1

[2

[3

[4

[5

[6

18 steel research int. 85 (2014) No. 12ceived: December 4, 2013;

blished online: April 28, 2014samforce was required after the initial fracture, although on

some occasions significant force had to be applied all

the way up to the 33% deformation, whichmay indicate

layered crushing.

6. At temperatures of 1600 and 1750 8C, all three cokegrades showed unique distributions of stressstrain

curve types. Although the stressstrain curves at high

temperatures were complex, certain general types were

found for each grade. The shape of the stressstrain

curves at high temperatures also indicated plastic

deformation.

Acknowledgments

This research is a part of the Energy Efficiency & Lifecycle

Efficient Metal Processes (ELEMET) research program

coordinated by the Finnish Metals and Engineering

Competence Cluster (FIMECC). The Finnish Funding

Agency for Technology and Innovation (TEKES) is

acknowledged for funding this work. The authors would

also like to thank Mr. Tommi Kokkonen for assisting with

ple preparation and experimental work.

ke 3 Coke 1 Coke 2 Coke 3

.08 0.97 0.97 0.98

.44 0.85 0.84 0.86

.24 0.72 0.73 0.73

.32 0.67 0.66 0.66

.56 0.55 0.54 0.55

image analysis.ywords: blast furnace; coke; high temperature;

ength; Gleeble

ferences

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