Studies of the mechanical and chemical properties of coke at high temperature
86 pages
English

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Studies of the mechanical and chemical properties of coke at high temperature

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86 pages
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Industrial research and development

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Commission of the European Communities
technical coal research
STUDIES OF THE MECHANICAL
AND CHEMICAL PROPERTIES
OF COKE AT HIGH TEMPERATURE Commission of the European Communities
technical coal research
STUDIES OF THE MECHANICAL
AND CHEMICAL PROPERTIES
OF COKE AT HIGH TEMPERATURE
BRITISH CARBONIZATION RESEARCH ASSOCIATION
Wingerworth
Chesterfield
GB-DERBYSHIRE S42 6JS
Contract No. 7220-EB/818
FINAL REPORT
Directorate-General Energy
1984 EUR 9045 EN Published by the
COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General
Information Market and Innovation
Bâtiment Jean Monnet
LUXEMBOURG
LEGAL NOTICE
Neither the Commission of the European Communities norany person acting on behalf
of then is responsible for the use which might be made of the following
information
©ECSC — EEC— EAEC. Brussels · Luxembourg, 1984 III
CONTENTS
Page
SUMMARY
GENERAL INTRODUCTION 1
PART 1 EXAMINATION OF COKES IN THE HIGH TEMPERATURE DRUM 3
1.1 . Experimental work 3
1.1.1 Initial treatment of coke samples
1.1.2 The high temperature drum
1.1.3 Instrumentation
1.1 Λ Operation -+
1.2 Experimental results 5
1.3 Discussion ofs 6
1.3·1 Coke size analysis . 6
1·3·2 Standard micum tests (>20 mm coke)
1·3·3 Micum tests on the graded coke {jiO-kO mm) 7
1.3·^ High temperature drum tests - 100 revolutions
at ambient temperature 8
1.3.3 Extended half-micum test results using >20 mm coke
and ~j>0-kO mm coke
1.3·6 Thermal treatment in the high temperature drum 9
1·3·7 High temperature drum tests after oxidation v/ith
carbon dioxide 10
1.3-8 The degradation of coke by thermal, mechanical
and chemical oxidation effects.2
PART 2 THE EFFECT OF VARIOUS HEAT-TREATMENTS ON THE TENSILE
STRENGTH OF COKE^
2.1 Experimental methods4
2.1.1 Sample preparation 1-+
2.1.2 Influence of high temperature treatment on coke
tensile strength 15
2.1.3 Influence of partial gasification on coke tensile
strength6
2.1.*f Influence of test' temperature on coke tensile
strength '.7
2.2 Results and Discussion8
2.2.1 Influence of.high-temperature treatment on coke
tensile strength . 1
2.2.2e of partial gasification on coke tensile
strength 22
2.2.3 Influence* of test temperature on coke tensile
strength 2k rv
Page
PART 3 GENERAL DISCUSSION AND CONCLUSIONS 26
References 29
TABLES 30
FIGURES SUMMARY
An account is given of studies carried out on the properties of cokes
at high temperatures. The work consisted of two separate, but complimentary
approaches, namely, the study of the resistance to breakage and abrasion of
samples of coke tested in a refractory drum which could be heated to a
temperature of 1V?0°C and a study of the dependence of the tensile strength
of coke upon test temperature, heat treatment and with partial gasification
of the coke, also ats within the range of 1000-1^+00 C. The
tests carried out in the high-temperature drum are described in Part 1 of
the report and the effect of heat-treatment on coke tensile strength in
Part 2.
The samples of cokes investigated were predominantly obtained from
integrated steelworks in The United Kingdom, but blast-furnace cokes originating
in Belgium and in The Federal Republic of Germany were also tested, as well
as two cokes prepared in the BCRA 17 t test oven, one from a normal and one
from a preheated charge of the same medium volatile coal blend. As the
properties of many of the cokes were rather similar, two cokes, one produced
from a high-volatile blend of United Kingdom coals and one produced from a
blend of low-volatile coals and normally used in foundry coke making practice,
were also examined to widen the range of the general strength characteristics
of the series of cokes.
The principal findings from the work may be summarized as follows. Results
from the drum tests indicated that the strength and abrasion properties of
cokes at high temperatures could not be predicted from their behaviour in
strength tests carried out at ambient temperatures. All cokes displayed, to
some extent, a reduction in strength and a decreased resistance to abrasion
when tested at temperatures generally above their maximum carbonizing
temperatures of 1000-1100 C, a conclusion reinforcing the results of earlier
work ~ . In some cases, there was evidence of an annealing process taking
place at temperatures of the order of 1000 C, resulting in a transient increase
in the physical quality of the coke.
Experiments carried out in the presence of an oxidizing gas greatly
enhanced the extent of coke degradation. From all the data obtained an estimate
was possible of the individual contributions made to total coke degradation
by mechanical, thermal and chemical effects respectively.
It was found, in general terms, that mechanical and thermal effects
only accounted for a small proportion of the total loss of coke (by weight), VI
approximately *f per cent and 11 per cent respectively. By far the largest
influence on the loss of mass could be attributed to the chemical reaction
with oxidizing gases, which accounted for the remainder, 83 per cent.
The effects of temperature on the coke tensile strength were studied in
two ways. In the first, coke was heated to a maximum temperature of 1^50 C,
allowed to cool and the tensile strength measured and compared with the value
found at ambient temperature before heating. The second series of tests involved
the actual measurement of tensile strength at temperatures up to 1390 C. Tensile
strength measurements were also made on samples of coke which vere allowed to
react with carbon dioxide under controlled conditions at a temperature of
approximately 1000 C. Tests were made using 10 per cent carbon dioxide in
nitrogen, and also 100 per cent carbon dioxide, so that the degree of gasifi­
cation extended from about 5 per cent up to over 20 per cent weight loss.
The high temperature treatment ϋν?0°θ) of coke resulted in a significant
reduction in the tensile strength of some cokes but generally the results were
variable and inconclusive. Similarly, although some influence attributable
to the rate of heating and specimen size was implied, the effects could not be
quantified with the data obtained. For the most part there was no marked
influence of heat treatment at 1V?0 C, but in those instances where a strength
reduction occurred, the effects could be accounted for in terms of differential
shrinkage and/or the decomposition of mineral matter.
On the other hand, the tensile strength measured at high temperature was
clearly higher than that at ambient temperature. The magnitude of the increase
varied with different cokes but all the cokes showed significantly higher tensile
strength when measured at 1200 C and showed further increases when tested at
139O C. This behaviour is in accord with that of other carbon materials and can
be attributed to thermal annealing by means of which fissure-like cracks, intro­
duced as a result of differential shrinkage effects during production, are closed.
In contrast, partial gasification of coke in carbon dioxide at about 1000 C
invariably leads to a decrease in the tensile strength due to the pore enlarge­
ment brought about by internal burning and broadly speaking, with the test
conditions used, a 20 per cent weight loss leads to almost 50 per cent reduction
in the tensile strength.
The differences between the high-temperature drum-test results and the
corresponding tensile strength test at high temperature would appear to be
reconcilable in terms of the differences in the size of coke tested and the
recognition that the drum tests assess strength in terms of size reduction. 1 -
GENERAL INTRODUCTION
It is widely recognized that the smooth operation of the blast furnace
is significantly affected by the size distribution of the coke which remains
as the predominantly solid material in the lower part of the blast-furnace
shaft, the bosh and the melting zone.
The results of recent work ''in which coke was withdrawn from the
tuyeres of some forty-five blast furnaces when operating under normal conditions
within the European Community and in Canada, have demonstrated that the
survival of lump coke during its descent of the furnace shaft depends on many
factors. These are, principally, the resistance of the coke to mechanical
and physical forces, the influence of thermal effects, the effect of chemical
reaction with oxidizing gases and the presence, in the burden, of other
constituents, principally alkalis.
Full-scale blast-furnace trials are costly and difficult to carry out
and it was considered that studies of the behaviour of coke at high temperatures
may provide information which could lead to a more adequate characterization
of properties of blast-furnace cokes.
(if)
Previous studies had shown the advantages to be gained by considering
coke strength in more fundamental terms than the data provided by conventional
drum-type or drop tests, such as the Micum or Shatter test. Such tests,
whilst of some valu

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