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

Tetsu-to-Hagané Vol. 82 (1996), No. 5

  • Evaluation of Coal Chemical Structure

    pp. 347-352

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    Recent trends and future outlook of researches on chemical structure of coal have been reviewed, especially from aviewpoint of practical use of coal.
    There are three major trends of researches : first, to show the average coal chemical structure ; second, to illustrate the coal structure ; and third, to consider coal as three-dimensional structure.
    The future outlook of the researches has been discussed as follows : first of all, coal should be considered as organic materials that have molecular weight distribution secondly, it should be noted that coal has three-dimensional structure and that weak bridges which are not covalent bonds are important for thermal behavior of coals.
  • Toward the 21st Century :Viewpoints of the Japanese Cokemaking Technology

    pp. 353-360

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    A serious problem is predicted to occur to the domestic cokemaking industry in the early years of the 21st century. With the majority of existing coke ovens worn out or obsolete, a significant shortage of coke production would take place. Presently, considering the environmental pollution and improving the work conditions, the development of an innovative process is strongly required for future.
    The cokemaking industries in Japan have two candidates as the potential process for the future, i.e., the previously developed Formed coke process and the new Conventional cokemaking process. The project for the latter process has been promoted to develop the innovative process by member companies in The Japan Iron and Steel Federation during eight years since 1994. This project is named after SCOPE 21 (The Super Coke Oven for Productivity and Environment enhancement toward the 21st century). A bench scale test will follow in three years since 1996 and the project is expected to proceed to the 100t/d pilot plant test in next three years until the year 2001.
    This process should develop at least the following technical items to realize more economical and clean coke plants.
    1) Pre-coking treatment for higher utilization of poor coking coals
    2) Coking system for higher productibity
    3) Perfect prevention of environmental pollution
    In addition, some basic researches are pointed out for the process development toward the 21st century.
  • Quantitative Evaluation of Hydrogen Transfer Related to the Appearance of Coal Plasticity

    pp. 361-365

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    In order to investigate a relationship between chemical structural changes of coal and their thermoplasticity at the initial stage of coal carbonization process, we conducted the evaluation of donatable hydrogen in coal by the reaction of coal with hydrogen acceptable compounds. We also carried out the measurement of the 13C-NMR of coal followed by the estimation of carbon distributions of coal. To obtain more quantitative information, we applied the magic angle spinning method (this is now believed to be the most quantitative) for the measurement. Gieseler fluidity is known to be one of the representative parameters for coking coal. The tendency of the amount of donatable hydrogen of coal vs. coal rank was similar to that of Gieseler maximum fluidity vs. coal rank. In addition to this, it is interested to note that the behaviour of transferable hydrogen estimated based on 13C-NMR measurement also showed a close relationship to that of the above parameter. These results strongly indicate that the donatable hydrogen seems to be important to the appearance of coal plasticity.
  • Mixed Solvent Extraction Yield and Structural Changes of Heat-Treated Coals and Their Relation to Coal Fluidity

    pp. 366-371

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    Coals were heat-treated at the heating rate of 3°C/min or 100°C/min in an autoclave at 200550°C under nitrogen, and then quenched rapidly to room temperature. The heat-treated coals were extracted with a carbon disulfide-N-methyl -2-pyrrolidinone mixed solvent at room temperature. At the heating rate of 3°C/min, the maximum extraction yields were obtained at around the initial stage of softening, and the maximum yields showed a good correlation with the maximum fluidity of the coals. While, ultimate analysis and FT-IR measurement for the heat-treated coals showed that significant structural changes did not occur before the initial stage of softening. In the stages of maximum fluidity and resolidification, the extraction yield rapidly decreased, especially for caking coals. An increase in heating rate to 100°C/min shifted the temperature which gives the maximum extraction yield to higher temperature and the yield was high compared to the low heating rate (3°C/min), suggesting that the fluidity is increased by increasing the heating rate. Softening mechanism of coal was discussed from various coal structure models including associate model.
  • Thermoplastic Behavior of Coal Particles during Rapid Heating prior to Carbonization

    pp. 372-377

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    During heating up to agglomeration temperature (<470°C) with 5-500 K/min heating rate, thermoplastic behavior of coals with different caking properties has been studied with a quartz-made fluidized bed reactor. When coal particles are heated near agglomeration temperature, the swelling occurs to a larger extent irrespective of coal type, and consequently both particle size and bed height increase without particle adhesion. The degree of the increase becomes larger at higher heating rates. Independently of caking property, the textures of coal particles after heating show that higher heating rates lead to not only the higher degree of swelling and softening but also the larger portion of the particles with vesicles and hollow spheres. Thus, thermoplastic properties of a non-caking coal as well as a caking coal can be improved greatly by heating at 400-500 K/min prior to carbonization.
  • Effects of Preheating Condition of Coal on Produced Coke Properties

    pp. 378-382

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    Rapid preheating of coal was conducted in entrained and fluidized beds. Preheated coal particles were directly introduced to a carbonizer to produce coke under loading pressure of 0.97 MPa. Effects of preheating conditions of coal, i.e., preheating temperature and soaking time, on the properties of produced coke, i.e. micro and tensile strengths, true density and porosity were discussed.
    In the case of entrained bed, bubble formation in coke produced from caking coal was slightly restrained by the rapid preheating. As to the slightly caking coals, preheating at high temperature with long soaking time led to the reduction in the strength of coke. In the case of slightly caking coal, the rapid preheating at the temperature lower than 400°C increased the strength of produced coke and the effects of soaking time was hardly observed.
    Lastly, it was demonstrated that the rapid preheating of slightly caking coal at 350°C in the fluidized bed could increase the strength of coke drastically.
  • Characteristics of Gas Evolution during Coal Carbonization

    pp. 383-387

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    Changes in mass with time due to gas evolution during carbonization were observed for particles and a cylindrical pellet of various coals heated respectively in a thermogravimetric balance and a microautoclave. The experiments were carried out for a range of the heating rate from 1 to 50K/min, the holding temperature from 673 to 823K and the nitrogen gas pressure of 0.1 and 1.0MPa. Effects of these operating variables on the gas evolution characteristics were experimentally examined. It was demonstrated that the gas evolution characteristics depend on the heating rate and the holding temperature as well as coal nature. Moreover, the total amount of gas evolved at a holding temperature was shown to be independent of the heating rate and to increase with the holding temperature. A simple reaction model is proposed for gas evolution which assumes coal to consist of two components having different reactivities for pyrolysis; one producing gas along with formation of semicoke via plastic intermediate and the other converting directly to gas. The model parameters were analyzed by fitting the model to the observed results and successfully utilized to explain observed results at different operating conditions.
  • Effects of Heating Rate and Coal Type on Gas Evolution during Coal Pyrolysis

    pp. 388-392

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    In order to get a decisive conclusion on the effect of the heating rate on gas evolution behaviour during coal pyrolysis, the pyrolysis of coal was systematically investigated in thermo-balance using six coals which have different carbon contents ranging from 81 to 88 (wt%, daf). The heating rate was ranged from 3°C/min to 300°C/min. The evolution rates of C4, C2H4, C2H2, CO and CO2 were measured. As the heating rate increased, the maximum weight decrease rate and the maximum gas evolution rate were shifted to higher temperatures. The gas and coke yields were decreased and the liquid yield was increased with an increase in the heating rate.
    Six coals were heated at constant temperatures ranging from 360°C to 500°C for 180min. The ultimate volatile matter content at each holding temperature was decided. It was found that the ultimate volatile matter content depended on the coal rank at lower temperatures and a large amount of gas was evolved from low rank coals at a temperature as low as 400°C.
  • Mechanism of Moisture Transfer across the Oven Width at Early Stage of Carbonization

    pp. 393-398

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    In order to clarify the moisture transfer mechanism, moisture distribution across the oven width at the early stage of carbonization has been investigated using the test coke oven with the upper heating wall. There is a region having a maximum moisture content below 100°C because of moisture condensation. Subsequently, as temperature is elevated close to 100°C, remarkable moisture vaporization has taken place so that the moisture in coal decreases rapidly. However, there remains a little moisture in coal beyond 100°C.
    Two dimensional coke oven mathematical model was hence updated by introducing this moisture transfer mechanism.
  • Effect of the Load of Mechanical Pressure during the Pyrolysis on the Enhancement of Caking Properties of Slightly Coking Coal

    pp. 399-403

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    We have previously found that the carbonization of coal under the load of mechanical pressure was effective for producing metallurgical coke from slightly coking coals. In this paper, the effect of pressure load on the carbonization behavior of coal was investigated by measuring gas formation rate and several solid properties such as yield, amount of tetrahydrofuran (THF) soluble and pore volume distribution to clarify the mechanism by which the coking properties increase. By increasing the load up to 40MPa, the formations of H2 and CO were significantly suppressed, whereas the solid yield and the amount of THF soluble increased. Furthermore, the solid yield was found to decrease significantly for a slightly coking coal when the load was released at 350400°C. This shows that a fairly large amount of volatile matters are retained in the coal under the pressure load. Judging from these results, thermal plasticity of coal are supposed to be enhanced because relatively low molecule compounds are retained in the coal due to effective hydrogen transfer under the pressure load. Analyses of the THF soluble by GPC and by 1H-NMR clarified that organic compounds with 23 aromatic rings contribute to the enhancement of thermal plasticity of coal.
  • Modeling of Dilatation Behavior of Packed Bed of Coal during Carbonization

    pp. 404-408

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    The new mathematical model is proposed to estimate the dilatation/contraction behavior and coking pressure of packed bed of coal during carbonization. Volume fractions of gas, liquid and solid phase are determined from coal pyrolysis rate equations and the tar devolatilization is also estimated. We assume that the dilatation of packed bed of coal is caused by impermeability of plastic layer where the tar released from coal mainly exists in the void. The gas released from coal pushes the impermeable plastic layer and then the dilatation pressure is detected. The calculated result using this model is compared with one calculated using the ordinary method where the dilatation of the bed is estimated by true dilatation as a function of heating rate. The model proposed in this study can simulate the decrease of permeability caused by tar release and formation of coking pressure in the plastic layer. The coking pressure becomes large with an increase in the heating rate. On the other hand, the ordinary model predicts very large coking pressure in any heating rate. The proposed model may be a useful tool to estimate the dilatation/contraction behavior of packed bed of coal.
  • Estimation of Dilatation of Coal Blend

    pp. 409-413

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    With the aim of developing a model for estimating coking pressure of coal blend, the effect of coal type and carbonization conditions on gas permeability in softened coal layer was investigated. Then the reason why gas permeability becomes low when coal softens was discussed.
    It turned out that it is necessary for estimating the gas permeability to estimate the specific volume of coal blend and the estimation method of the specific volume of coal blend was developed. It was found that the specific volume of coal blend at a temperature can be estimated by calculating a weighted mean value of the specific volume of component coal at the temperature and multiplying the value by an inert factor corresponding to the ratio of coal exceeding its maximum dilatation temperature.
  • Thermal Stress and Deformation of Coke Layer during Carbonization

    pp. 414-418

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    The non-uniform shrinkage and matrix strength of coke during coal carbonization have a great influence on the size of metallurgical coke. The influence of coal properties and carbonization conditions on the thermal stress and deformation of coke layer were evaluated by the carbonization test and a mathematical simulation model. The deformation was defined by the curvature of coke layer. The coke-fissure formation behavior in coke layer was investigated by the carbonization test using thin coal layer. The fissure distributions and curvature of coke layer were measured by X-ray computerized tomography. The thermal tensile stress leads to cause the curvature and coke fissure of coke layer. There is a good relationship between the curvature, fissures and size distribution of coke and the curvature and thermal stress distributions calculated by the mathematical simulation model. The contraction coefficient and Young's modulus of coal have a great influence on the thermal stress and deformation of coke layer. As the thermal stress in coke layer increases along with increase of the heating rate and transverse temperature gradient of coal layer, the size of coke is decreased. It was clarified that the thermal stress and deformation of coke layer are controlled by coal properties and carbonization conditions.
  • Microtexture Change of Semi-Cokes and Its Mechanical Strength

    pp. 419-424

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    Microtexture changes of semi-cokes produced from coking and slightly-coking coals were compared during the carbonization process in the temperature range of 600-1000°C, in order to correlate them with the mechanical strength development mechanism of cokes. The coke produced from Goonyella coking coal at a low temperature of 600°C exhibited a highly-oriented microtexture observed under the high resolution SEM, while the coke produced from Witbank slightly-coking coal at a higher temperature of 1000°C showed many plate-like edges on the surface. It is suggested that the microtexture of the cokes should reflect the fusibility and the caking properties of the original coals. The blending of the two coals was very influential on the carbonization behavior, the coking coal modifying the slightly-coking coal to produce the coke of high mechanical strength. The influences of cold moulding, coal blend ratios and heating rate during the carbonization on the properties of the cokes were also investigated in terms of the apparent density, the porosity, the tensile strength, and the microtexture of the cokes. The combination of the higher heating rate of 10°C/min with the cold moulding was very effective to produce the cokes of the high tensile strength with relatively low density from the coal blends rich in the slightly-coking coal. The tensile strength of the cokes is essentially related to the anisotropic region development and the microtexture orientation extents of the cokes after the resolidification around 600°C, being influenced by the formation of cracks through the calcination around 800°C with the shrinkage and the thermal expansion. The endothermic reactions such as dehydrogenative condensation and the enlargement of aromatic carbon planar stacking proceeded during the calcination around 900°C observed by the high temperature DSC, although such evidence was not definitely observed by Raman spectroscopy. X-ray diffraction showed the evidence for the enlargement of aromatic planar stacking of the coke produced from the coking coal at 1000 °C.
  • Direct Heating Conditions of Coke Pushed out at Lower Temperature

    pp. 425-430

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    In order to improve the heat efficiency and productivity in conventional coke oven, the cokemaking process combining carbonization in a coking chamber and direct heating was investigated. The concept is that the coke pushed out with lower temperature rather than conventional method is post-heated directly by combusting the residual volatile matter in the coke to complete the carbonization. Coke properties and coke strength were investigated with the decrease in final coke temperature, and the effect of direct heating on the improvement of coke qualities and the heating conditions were studied in a test coke oven based on the mathematical estimation for the temperature change in a coke lump. Coke quality deteriorates as final coke temperature decreases and especially when final coke temperature drops below 800°C, the deviation of coke properties across the oven width enlarges which results in marked deterioratin of coke quality. From the analysis of mathematical estimation, the temperature in a coke lump, which is pushed out with the temperature of 800°C at the center of the oven width, would be uniform and increase over 1000°C in a few minutes by direct heating if the ambient temperature could be kept at 1100°C. The result was obtained from the test oven that the coke pushed out at 800°C and post-heated had a strength equal or superior to that of ordinary coke.
  • Estimation of the Pore Partition Strength of Metallurgical Coke

    pp. 431-435

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    Quantitative estimation method for pore partition strength of metallurgical coke was studied with mercury porosimetry. Two peaks were observed in the relationship between intrusion pressure and differential intrusion volume that was measured for a piece of coke sample. One peak occurred because of intrusion to open pores in coke and the other peak occurred because of intrusion to the pores generated by breakage of pore partitions of closed pores. This phenomenon was clarified based on the difference between the result of a piece of coke sample and crushed sample, and based on the observation of coke sample surface with SEM. Therefore, it was ascertained that the pore partition strength could be estimated by quantification of pressure distribution of intrusion resulted from breakage of closed pore partition. The pore partition strength was measured according to this method for a couple of coke sample. The values of pore partition strength were in the range of 0.743.08 MPa from these measurements, and the pore partition strength became larger with the thickness of pore partition and with the coal rank.
  • Quality and Productivity of Coke Carbonized by Experimental Oven Constructed Chamber Wall Made of High Thermal Conductivity Material

    pp. 436-441

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    In order to clear the effect of thermal conductivity of chamber wall on coke quality and productivity, coke was carbonized at the range from 800°C to 1000°C by a test coke oven. The chamber width was from 200mm to 450mm and was equal to that of a conventional coke oven. Using the chamber wall made of stainless steel as a high thermal conductivity material, the wall temperature of combustion chamber side was same as that of coke oven chamber side. Then, the following results were obtained.
    ( 1 ) When the heating rate in coal bed at the range from 400°C to 500°C was equal to that of conventional coke, the size, drum index and reactivity of coke carbonized about 850°C were equal to that of conventional coke.
    ( 2 ) Increasing the heating rate, the drum index of coke carbonized at medium temperature using low fluidity blend coal became high.
    ( 3 ) When coke was carbonized under the condition of wall temperature from 800°C to 850°C, the coking time and productivity were same as that of conventional coke.
    Therefore, increasing thermal conductivity of chamber wall, quality and productivity of coke carbonized at medium temperature were equal to that of conventional coke oven.
  • Effects of Formcoke Shape on Gas Permeability and Internal Thermal Stress

    pp. 442-446

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    In order to improve the gas permeability of formcoke as with a chamber oven coke, an examination was made about the betterment of the shape of formcoke.
    The result has revealed that by employing a concave shape having a U-shaped permeation groove on the surface and making the shape larger, the gas permeabilith and voidity in case of filling can be improved as with the chamber oven coke.
    In addition, it has been made clear that the internal thermal stress responsible for the fissuring of the new shape formcoke can be reduced below that of the conventional formcoke.
  • The Application of Biological Denitrifcation Method for Cokeoven Wastewater Treatment

    pp. 447-452

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    An experimental study was carried out to apply the biological denitrification method for cokeoven wastewater (ammonia liquor) treatment.
    An ammonia liquor contains not only high concentrated phenol, ammonia but also cyanide, thiocyanate and hydrogen sulfide. Therefore the biological denitrification for the liquor has been considered to be very difficult.
    But the nitrificated liquor return system makes it possible to remove almost ammonia in the liquor. The nitrification-denitrification process of ammonia liquor was proceeded on the course of nitrites (NO2) form.
    When the nitrites concentration was kept under 50mg/1, in this system the activated-sludge was very stable. Consequently nitrification and denitrification rate became very fast under the condition of the high total nitrogen (T-N) load (kg-N/kgMLSS·day).
    (nitrification rate:8.9mg-N/gMLSS·Hr, denitrification rate:5.8mg-N/gMLSS·Hr)
    The nitrification liquor return system required that almost all ammonia in the liquor must be removed through process in order to stabilize the process.
    At the same time, chemicals (an organic carbon ex. Methyl alcohol and a alkali salt ex. Na2CO3) were required.
  • Analysis of Oxidation Reforming of Formed Coke Tar and Its Application to Briquetting Binder Production for Formed Coke Process

    pp. 453-457

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    Theoretical and experimental studies were carried out to quantitatively understand the effect of oxidation reforming conditions on the properties of the reformed tar produced in the formed coke process (FCP tar). The oxidation reaction of the FCP tar was a consecutive first order reaction comprising two steps. First, reactive fragments were generated as free radicals by breakage of the alkyl chain of aromatic compounds. The FCP tar was then polymerized by recombination of these fragments. The activation energies of first and second steps were 43.5KJ/mol and 18.8KJ/mol, respectively. At 473K the compressive strength of briquettes of the reformed tar, which had a 1820% toluene insoluble content and viscosity of 18 Pas, was almost equal to that of briquettes of the conventional coke oven pitch. On the basis of this theoretical and experimental analysis, a procedure was established for determining the air blowing conditions for preparing FCP tar for use as a formed coke binder.

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