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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 92 (2006), No. 3

  • Control of the Size Degradation Behavior of Coke

    pp. 106-113

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    Studies on the size degradation behavior of coke were reviewed. Coke is brittle material and the defects that cause volume breakage and surface breakage have been discussed. A possibility that large pores in coke act as "Griffith" flaws was suggested.
    The history of coal blending technologies for using low-rank caking coals, which form "weak" coke with fingery structure when carbonized alone, was reviewed. The concept of the degree of void filling during coal softening was discussed.
    In order to control the size degradation behavior of coke, the mechanical qualities indicated by drum tests such as the JIS drum test and the MICUM test should be related to the structures and properties of coke.
  • Desirable Coke Properties for Blast Furnace in Future

    pp. 114-121

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    The operation condition of blast furnace in Japan has been currently changed by the following issues ; green house problems, shortage of coke supply and high production derived from rapid growth of Asian countries. From these backgrounds, high productivity and low reducing agent rate operation in blast furnace will be demanded for Japanese steel industries.
    In this paper, the roles of coke, which strongly affects on the blast furnace operation, are discussed, and in-furnace phenomena at low reducing agent rate and high productivity conditions are clarified. On the basis of these discussions, the desirable coke properties to attain the above purpose are described. As a result, the increase in coke diameter and strength to keep permeability and the control of coke reactivity are evaluated to be important so as to achieve low reducing agent rate and high productivity operation of blast furnace.
  • High Temperature EPR Study on the Thermoplastic and Re-solidification Phenomenon of Coal upon Heating

    pp. 122-126

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    High temperature in-situ EPR studies were carried out to evaluate the thermoplastic and re-solidification characteristics of coal during heat treatment. The change in EPR spectra up on heating demonstrated the differences of coking properties of sample coals. The peak-to-peak height, ΔIpp, of spectra for coking coals showed minimum value at around the maximum fluidity temperature. This decreases in ΔIpp is supposed to result from the relaxation of aggregate structure of coal molecules. At higher temperature range, above the re-solidification temperature of sample coals, the peak-to-peak line width, ΔHpp, decreased with increase in temperature. Since the ΔHpp, is affected with spin-hydrogen interaction and spin exchange interaction, the decreases in the value of ΔHpp indicate the development of polyaromatic structure during heat treatment. In order to evaluate the coking properties of coal quantitatively, curve fitting was attempted for the EPR spectra obtained during heat treatment. The EPR spectra were deconvoluted into three components. The change in spin concentrations calculated from the components clearly exhibited the differences of coking properties of sample coals.
  • Evaluation of Blending Effect Based on the Structural Analysis of Semicoke

    pp. 127-131

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    In order to evaluate effects of low-quality coal on blending with good-quality coal for the purpose of making metallurgical coke, thermal decomposition behavior of coal blends was analyzed and the structural features of semicoke samples obtained by the heat treatment of single coal or coal blends were evaluated by solid-state NMR. Thermal behavior of coal was examined by thermogravimetric analysis, the results showing synergistic effects: blending poor-quality low rank coal with good-quality coal tended to promote thermal decomposition reaction, while blending poor-quality high-rank coal will suppress pyrolysis. NMR measurements both with single pulse excitation mode and with dipolar dephasing mode gave the distribution of various types of carbon in detail so a fraction of bridgehead aromatic carbon to total aromatic carbon could be evaluated, this being an index of the size of polycondensed aromatic hydrocarbon which would be contained in the semicoke samples. The structural parameters and the size of polycondensed aromatic hydrocarbon unit was compared between semicoke sample derived single coal and that from blended coal, the results clearly indicating synergistic blending effect. Although two different behavior was observed for two different coal blends, both cases had positive effects on the development of aromatic ring size during heating.
  • Mechanism of Thermoplasticity for Coal Blends Containing Low-quality Coals

    pp. 132-136

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    Thermoplasticity for coal blends containing a large amount of slightly-caking Enshu coal was evaluated by a dynamic viscoelastic technique using a temperature-variable rheometer. It was found that overlapping of temperature range showing high thermoplasticity for blended original coals during heating is the most important factor to enhance the thermoplasticity of their blends; A Gregory-Enshu coal blend (1:1) showed a high synergistic effect for thermoplasticity, because their temperature ranges of thermoplasticity were quite overlapping. The mechanism of thermoplasticity for blend coals is explained by the "Continuous Self-Dissolution Model" proposed before, which has been used for explaining of thermoplasticity for caking coal alone. Addition of solvent-soluble component in a caking coal to coal blend was significantly effective to enhance the overlapping of thermoplastic temperature range for the coal blend, since the solvent-soluble component has originally a wider temperature range of thermoplasticity during heating.
  • Analysis of Carbon Structure in Cokes at Molecular Level

    pp. 137-144

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    The quality of coke strongly depends on their carbon structure, but so far most of the researches for the microscopic analysis of cokes structure have been conducted at optical-microscopic level. It is essentially important to understand carbon structure in cokes from molecular point of view. In this study, several types of cokes have been analyzed with several means such as X-ray diffraction, transmission electron microscopic observation and temperature programmed desorption, all of which have been utilized to understand the molecular structure of the cokes. From the results thus obtained, it was found that the structural difference at molecular level between the cokes from caking and slightly caking coals is not so remarkable as one expected. However, there is a small but distinct difference between the two types of cokes. This study demonstrates that the use of several analysis techniques gives different views of coke structure and they are quite useful for the understanding of the carbon molecular structures.
  • Examination of the Carbonization Behavior of Coals by Using Raman Sectroscopy and Kinetic Analysis of Hydrogen Formation

    pp. 145-151

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    Caking coals, which are main raw materials of metallurgical coke, are carbonized through softening, melting, and subsequent resolidification by heat treatment. The carbonization process after resolidification is poorly understood because of the difficulty of the analysis of the solid amorphous carbons, although it is recognized that the carbonization process affects significantly the strength of coke. In this study, the carbonization process was examined from two aspects: one is the analysis of the hydrogen formation rate during carbonization and the other is the examination of the carbonized coke by the Laser Raman spectroscopy. Formation rate of H2 was analyzed by using the so-called distributed activation energy model to obtain the distribution of activation energy for the hydrogen formation reactions. It was found that the peak activation energy decreased with the increase of the caking property of coal, indicating that the formation of hydrogen is closely related to the caking property of coal. It was proposed to utilize absolute intensity of Raman spectrum for characterizing amorphous carbons. The absolute intensity of Raman spectra for coals carbonized at 600-1300°C decreased with increasing heat treatment temperature. The decrease was well correlated with the amount of H2 produced up to the final temperature, indicating that the absolute intensity decreased by the condensation reaction of aromatic rings and/or cross-linking reactions. It was also found that the peak around 1600 cm-1 disappeared and the sharp peak around 1580 cm-1 appeared by further heat treatment over 2000°C when Raman spectroscopy was interpreted in terms of absolute intensity. These results show that the Raman spectrum reflects the proceeding of carbonization of coal.
  • Investigation of Micro Carbon Structure of Coke and Semi-coke Using Hand-picking Method

    pp. 152-156

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    Coal is a complicated mixture, which consists of inorganic matters and organic matters. Therefore, coke formed through pyrolysis of coal is also a complicated mixture. On the other hand, there are many reports for the relation between carbon structure of coke and the strength, and the carbon structure of bulk coke is used experientially as one of index for evaluation of coke quality. It is important for an essential understanding to get information based on the each microstructure because of complexity of coke. In this study, the technique of sampling arbitrary tiny parts was developed, and the carbon structure of the tiny coke partial was analyzed. Consequently, it was revealed that high quality coke had uniform carbon structure, but the structures of inferior coke were heterogeneous. It was recognized that crystallinity of coke is influenced by the kind of mineral matter and its distribution in addition to the type of organic matter such as soft carbon and hard carbon.
  • Differences in the Coking and Non-coking Coals from the Standpoint of Carbon Structure

    pp. 157-163

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    The purpose of the present study is to clarify the differences in the carbon structure of cokes derived from coking coals and non-coking coals. The structures of the carbonized coals at 1000°C were estimated from the X-ray diffraction and Raman spectroscopy measurements of the carbons heat-treated at 3000°C. This was based on the conventional recognition in carbon science such that the structures of carbons become obvious after heat-treatments at graphitization temperature, 3000°C. Three polymers and five coals confirmed the above recognition from the analyses of the carbons prepared at different temperatures. Essentially, coking coals and non-coking coals could be classified to graphitizing and non-graphitizing carbons, respectively. By close inspections of carbon (002) X-ray diffractions of 3000°C treated coals, it was found that the degrees of graphitizing and non-graphitizing abilities of coals were not as complete as the typical polymer-derived carbons. The Raman spectra of the carbons from non-coking coals prepared at 3000°C revealed the inhomogeneous structures of the carbons, because they gave position-dependent Raman spectra. Structural models of the cokes from coking and non-coking coals by taking account of continuous self-dissolving model were proposed. The important point of the model is that cokes consist of ordered matrix and disordered 'islands' in the matrix. The differences between the coking coal-derived ones and the non-coking coal-derived ones are found in the size and the distribution of the disordered islands.
  • Failure Strength of Metallurgical Coke-An Approach from Materials Mechanics-

    pp. 164-170

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    The failure strength of metallurgical coke is examined on the basis of materials mechanics and fracture mechanics. A novel concept of the "rule of scaling" is addressed for the macro-, meso-, and the micro/nano-structures of coke that comprises complex pores and cracks embedded in a carbonaceous matrix. It is emphasized that the failure strength of coke grains having the dimension of meso-scale (scale from about 5 to about 30 mm) is essential for mechanically qualifying the grade of metallurgical coke.
    Through instrumented spherical indentation testing, the mechanical characterization in mm-scale is conducted for the cokes made from various types of coals encompassing from caking to non-caking coals (strongly coking to poorly coking coals). The indentation test results confirm that the discrepancy in the mechanical properties (elastic modulus, yielding strength, and work-of-indentation) of these cokes in mm-scale is insignificant, whereas there exists a crucial discrepancy in the values of their drum indices (DI). The Weibull statistic is applied to the compressive failure load of coke grains with mm-scales (6 to 15 mm grains and 15 to 25 mm grains). The results of Weibull statistics for the compressive failure loads are successfully related in a quantitative manner to the DI-values.
  • Evaluation of Mechanical Property in Matrix of Metallurgical Coke by Nano-indentation Method

    pp. 171-176

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    The elastic modulus of coke matrix is measured with a nano-indentation method to investigate the effect of coal brands, blending conditions, coking temperature and holding time at the final coking temperature on the mechanical property.
    The elastic moduli of active component and inert carbonized at 880°C are 26-28 GPa and 45-48 GPa, respectively, regardless of coal brands and blending conditions.
    The rise in the final coking temperature increases the elastic moduli of both active component and inert. In addition, the elastic moduli of active component are smaller than those of inert.
    Until the final coking temperature reaches 850°C, the elastic modulus of active component and inert increases with an increase in the holding time. On the other hand, if the final coking temperature is above 850°C, the elastic modulus is hardly dependent on holding time.
    The elastic modulus of matrix of fusinite is smaller than that of isotropic inert because fusinite has a large amount of small pores. The true elastic modulus of matrix of fusinite is almost equal to that of isotropic inert.
  • Strength Evaluation of Coke Structure by Hardness Test

    pp. 177-183

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    Hardness of coke texture and elemental structure including coke matrix and pore was measured by micro Vickers test and Brinel test respectively. Vickers hardness of coke depends only on coke texture. Isotropic and Mosaic texture, which are derived from reactive of low rank coal, have largest hardness and Leaflet texture, which is derived from reactive of high rank coal, has lowest hardness. Average hardness of coke texture is therefore lower for the high rank coal.
    Brinel hardness was measured using an aluminum ball of 20 mm. The hardness was strongly influenced by porosity. Hardness of coke made from high rank coal tends to be lower than that made from low rank coal. Brinel hardness was well explained by porosity and coke texture hardness by micro Vickers test. Brinel hardness test was accompanied by the destruction of coke structure. The hardness was thought to stand for the resistance against destruction. Therefore strength of coke matrix is confirmed not to be weak even in case of coke made from low rank coal.
  • Evaluation of Coke Strength Considering Pore Shapes by Using a Homogenization Method

    pp. 184-190

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    In this study, the effect of pore on coke strength is investigated using a homogenization method. The stress analysis using a homogenization method is carried out for two cases of pore shapes. Case 1 is the round pores with the same diameter, which is regularly arranged in the unit cell. Case 2 is the irregular shape pores arranged randomly in the unit cell.
    In Case 1, the stress in the coke rises with an increase in the porosity. The stress distribution is determined by the porosity and is independent of the pore diameters and the number of pores. In Case 2, the stress in the coke with the irregular pore shapes increases with porosity and is larger than that with regular pore shapes. This is because that the stress concentration occurs at the sharp edges of pores. In addition, the stress analysis is carried out for the unit cell composed of circular pores which has different diameters and randomly arranged pores. As a result, the existence of circular pore reduces the stress in the coke and hardly lowers the rigidity of the coke while the arrangement of pores has little effect on lowering the rigidity.
  • Visualization and Modeling of Coke Pore Formation Mechanism

    pp. 191-197

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    Formation mechanism of coke pores is investigated based on the visualization and digital treatment of laser microscopic observation in order to estimate porosity of cokes prepared from coking and non-coking coals including their blended coals under various conditions. The estimated porosity of the cokes by multi-layered image analysis was well correlated with the coke properties including mechanical strengths previously evaluated, successfully revealing the influences of the different heating rate during carbonization and the different blending ratios on their changes, although the estimated values of the porosity by the multi-layered image analysis method were a little bit lower compared to their real values measured by JIS method. Modeling of coke pore formation in the relation to the mechanical strength is discussed based on the above results.
  • Analysis of Coal Swelling Behavior during Carbonization

    pp. 198-205

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    The swelling behavior of a softening coal particle during carbonization was numerically analyzed. In this study, since the swelling of a coal particle was caused by the growth of bubbles in the coal particle, the bubble growth due to the inflow of volatile matters to the bubbles was calculated involving nucleation, coalescence in the coal particle and emission of bubbles to particle outside during the softening stage.
    The production of volatile matters was evaluated by the FLASHCHAIN model.
    The pyrolysis experiment of a coal particle was carried out and the expansion behavior was photographed. Then, the swelling ratio was obtained. The calculated results of swelling ratio differed from experimental results because the gas evolution from coal particle was incorrect. In addition, the secondary decomposition of tar and the model of the coal softening behavior were required for the accurate prediction of the swelling behavior.
    Bubble growth rate was assumed to be the function of viscosity in this study. In order to understand the effect of the viscosity on swelling ratio, the variation of swelling ratio of the coal with viscosity change was investigated by varying assumed viscosity. The swelling ratio calculated with low viscosity was low because bubble nucleation was decreased by low viscosity. The swelling ratio with high viscosity was inhibited because high viscosity leads to low bubble growth rate. The property of softening coal, which is difficult to measure, can be estimated by more accurate analysis.
  • Effect of Coke Pore Structure on Coke Tensile Strength before/after CO2 Reaction and Surface-breakage Strength

    pp. 206-212

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    The surface-breakage behavior of coke is examined to make clear the influence of the pore structure on the tumbler strength. This behavior is formulated numerically on first order rate equations consisted of coarse and fine powder generation rates. Each generation rates are estimated from particle size distribution of coke after the tumbler test. The relations between pore volume of coke and the powder generation rate constants are investigated. The rate constant of the coarse powder generation increases with increasing coarse pore volume of coke, and the rate constant of the fine powder generation depends on the fine pore volume. The tumbler strength estimated from the pore volume of coke is in a good agreement with the measured value. Furthermore, the effect of coke pore structure on tensile strength before/after CO2 reaction is studied. The pore volume of coke over 100 μm has a large influence on tensile strength. Tensile strength after CO2 reaction is affected by the pore volume under 1 μm. In the coke with larger pore volume under 1 μm, the coke degradation is suppressed after CO2 reaction because gasification occurred preferentially at the surface of the coke.
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  • Thermoplastic Performance of the Solvent Extracted Ashless Coal and Its Applicability for Coke Material

    pp. 213-222

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    This study concerns with the thermoplastic performance of "Hyper-coal", and its applicability for the coke material, which replaces to the high value coking coal. Hyper-coal (HPC) is an ashless coal, which is made by applying the solvent de-ashing technology. Coal is extracted with the coal derived recycling solvent, which consists mainly with 2-ring aromatics, at 360380°C, and the solid material (RC, insoluble coal including ash) is removed by gravity settling. From the coal extraction examination with a number of coals, we found that HPC has a high heat value (>35 MJ/kg) and an excellent thermal plasticity. HPC revealed much more excellent fluidity from lower temperature to higher temperature range than that of the parent coal. Moreover, not only from the coking coal, but also from the low rank coal, which has no thermal plasticity, the produced HPC revealed an excellent thermal plasticity.
    In addition, the mixture of HPC and RC also revealed the thermal plasticity, even though the parent coal was brown coal.
    The characteristics and strength of the cokes, made by addition of HPCs were investigated. The thermal plasticity was improved when HPC was added to various coals. The value of the I-type coke strength was maximum level when the HPC addition into blended coal was at 0.5 wt%. But excessive addition of the HPC deteriorated I-type coke strength. To get the reason, the coke breeze particle distributions in I-type coke strength tests were examined. We confirmed that amounts of fine breeze (<0.25 mm) correlated with I-type drum index.
  • Improvement in Blast Furnace Reaction Efficiency through the Use of Highly Reactive Calcium Rich Coke

    pp. 223-231

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    A method to produce coke in 'lump' form with high strength and reactivity through the addition of a catalyst was investigated in order to improve blast furnace reaction efficiency. The addition of Ca compounds to coal before carbonization was found to considerably increase the reactivity of the coke at a low temperature range equivalent to the thermal reserve zone temperature of a blast furnace. Furthermore it was proved that strong, highly reactive 'lump' form coke could be produced by adding a Ca-rich non-caking coal and adjusting the coal blend composition. Based on this fundamental study, the Ca-rich coke was successfully produced in coke ovens on a commercial scale, both at Kimitsu and Muroran works. The use of the Ca-rich coke in the Muroran No. 2 blast furnace was found to cause a decrease in the reducing agent rate by 10 kg/t-p. This technology, producing coke of high reactivity and strength through catalyst addition, is promising as a means of improving the reaction efficiency of a blast furnace.
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