Carbon Composite Agglomerates Research Articles

New Trends in the Application of Carbon-Bearing Materials in Blast Furnace Iron-Making

The iron and steel industry is still dependent on fossil coking coal. About 70% of the total steel production relies directly on fossil coal and coke inputs. Therefore, steel production contributes by ~7% of the global CO2 emission. The reduction of CO2 emission has been given highest priority by the iron- and steel-making sector due to the commitment of governments to mitigate CO2 emission according to Kyoto protocol. Utilization of auxiliary carbonaceous materials in the blast furnace and other iron-making technologies is one of the most efficient options to reduce the coke consumption and, consequently, the CO2 emission. The present review gives an insight of the trends in the applications of auxiliary carbon-bearing material in iron-making processes. Partial substitution of top charged coke by nut coke, lump charcoal, or carbon composite agglomerates were found to not only decrease the dependency on virgin fossil carbon, but also improve the blast furnace performance and increase the productivity. Partial or complete substitution of pulverized coal by waste plastics or renewable carbon-bearing materials like waste plastics or biomass help in mitigating the CO2 emission due to its high H2 content compared to fossil carbon. Injecting such reactive materials results in improved combustion and reduced coke consumption. Moreover, utilization of integrated steel plant fines and gases becomes necessary to achieve profitability to steel mill operation from both economic and environmental aspects. Recycling of such results in recovering the valuable components and thereby decrease the energy consumption and the need of landfills at the steel plants as well as reduce the consumption of virgin materials and reduce CO2 emission. On the other hand, developed technologies for iron-making rather than blast furnace opens a window and provide a good opportunity to utilize auxiliary carbon-bearing materials that are difficult to utilize in conventional blast furnace iron-making.

Effect of Slag Composition on Reaction Kinetics of Carbon Composite Agglomerate in the Temperature Range of 1273 K to 1573 K (1000 °C to 1300 °C)

To effectively use high Al2O3 iron ore in carbon composite agglomerate (CCA) reduction process, the effect of slag composition on the isothermal reaction kinetics of CCA was investigated between 1273 K and 1573 K (1000 °C and 1300 °C). Reduction of the particular oxide by CO and gasification reactions of the carbon by CO2 were separately measured by quadruple mass spectrometry gas analysis. To better understand the reaction kinetics, the conventional unimolecular reaction model was modified to get two rate equations for reduction and gasification controls with the reaction thermodynamic driving force incorporated. Based on the gas composition data and modified kinetic model, the rate-controlling steps of CCA at various temperatures and reaction stages were determined. In case, the gasification reaction controls the overall reaction, CaO-Al2O3 Slag increased the reaction rate by accelerating the gasification reaction while CaO-SiO2 Slag decreased the rate by retarding the gasification. As the reduction reaction became dominant in controlling the overall reaction, the addition of slag component had adverse effect on the rate although such effect diminished at higher temperature. Finally, the activation energy values calculated by conventional unimolecular reaction model were compared with those evaluated by modified model. It was found that the gas composition data during the course of reaction and the modified model are of great value in interpreting the reaction kinetics when the controlling reaction is changed over with temperature and reaction time. This study is helpful to understand the theoretical aspects on the usage of high Al2O3-based iron ores for DRI production.

Composite Pellets – A Potential Raw Material for Iron‐Making

Coke constitutes the major portion of iron‐making cost and its production causes severe environmental concerns. In addition, lower energy consumption, lower CO2 emission and waste recycling are driving the Iron and steel making industry to develop “coke free, zero waste or green processes”. In the present article, an overview of possible ways to recognize a reasonable improvement in iron and steel making industry is summarized. The present discussion is focusing on the following approaches: Replacing expensive coke with relatively less expensive alternate fuels having carbon as well as significant amount of hydrogen such as coal, waste plastic and biomass materials. Producing agglomerates from cheaper raw materials (secondary resources) as well as improving their performance in BF. Making the process towards higher carbon utilization by shifting the wustite equilibrium towards lower CO/CO2 ratio by using high reactive coke or catalytic activated one. Recycling the unused CO in the top gas by removing CO2 from the gas stream. Much attention has been paid to carbon composite agglomerates (CCA) as a promising raw material for future iron making. Production, mechanical and chemical suitability, reduction behavior, etc. are being elaborated. In addition, other possible ways to utilize CCA in alternate iron‐making process has been explored.

A Mathematical Model for Carbothermic Reduction of Dust-Carbon Composite Agglomerates

Recycling iron–bearing dust from steel mills has gained a considerable attention in the past two decades to recover the valuable metals in dust while improving the sustainability of steel production. As a method for extracting the metals from dust, the reduction of dust–carbon composite agglomerates using a rotary hearth furnace (RHF) has been practiced in the steel industry. The use of low–grade carbonaceous reductants, such as low–rank coal and waste plastic, is of steelmakers’ interest to further enhance the waste recycling using the RHF process. However, applying these materials as reductants has proven to be a challenging task since the impact of the released volatile gas from such reductants on reduction reactions is not predictable. In addition, the reduction kinetics of dust pellet in RHF is more complicated than the reaction behaviour of sintered ore in blast furnace due to the higher furnace temperature, faster reduction and rapid gas evolution inside the agglomerate. To predict the reaction behaviour of the dust–carbon composite in RHF, a mathematical model was developed. The model takes into consideration heat and mass transfer as well as the reduction reaction of iron and zinc oxides, gasification of carbon and release of volatiles. The simulated behaviour of a dust pellet by the proposed model provides beneficial information to promote recycling an expanded range of waste materials in the RHF process. The modelling approach and calculation results are discussed in the present paper.

The Role of Molten Slag in Iron Melting Process for the Direct Contact Carburization: Wetting and Separation

Rapid iron melting initiation and molten slag separation which take place in carbon composite iron ores are quite interesting phenomena, but their mechanisms have not been established yet. Since the size of carbonaceous material and iron ore particles is tiny in carbon composite iron ores, molten slag can dynamically play a critical role in the iron melting initiation and the slag separation. The behavior of the molten slag was clarified by direct observation of phenomena occurring in a simplified carbon composite agglomerate sample (iron disk–slag particle–graphite or coke particle) via a confocal laser scanning microscope combined with an image furnace. It was found that the molten slag wetting induced the iron melting initiation by attracting graphite or coke to iron and the molten slag separation took place with iron melting propagation. However, the high content of coke ash could suppress the iron melting initiation. In addition, the mechanism of molten slag separation from Fe–C melt was verified by phase field modeling.

炭材内装鉄鉱石塊成物の還元における高ＶＭ炭の利用

Since high VM coal is inexpensive and deposits are abundant all over the world, the use of high VM coal is highly desirable in the carbon composite iron ore reduction process. However, when high VM coal is used, deterioration of DRI strength will occur. In order to solve this problem, different carbonization conditions were applied to high VM coal and different agglomeration methods were tested. As a result, when the non-carbonized coal was used and iron ore/coal composite briquettes were made under high pressure, high strength DRI was obtained. By this method, the porosity was effectively reduced in the carbon composite agglomerates, internal heat transfer was improved, and sintering of reduced metal was promoted.

Numerical Analysis of Blast Furnace Performance Under Charging Iron-Bearing Burdens With High Reducibility

The reducibility of iron-bearing burdens was emphasized for improving the operation efficiency of blast furnace. The blast furnace operation of charging the burdens with high reducibility has been numerically evaluated using a multi-fluid blast furnace model. The effects of reaction rate constants and diffusion coefficients were investigated separately or simultaneously for clarifying the variations of furnace state. According to the model simulation results, in the upper zone, the indirect reduction of the burdens proceeds at a faster rate and the shaft efficiency is enhanced with the improvement under the conditions of interface reaction and intra-particle diffusion. In the lower zone, direct reduction in molten slag is restrained. As a consequence, CO utilization of top gas is enhanced and the ratio of direct reduction is decreased. It is possible to achieve higher energy efficiency of the blast furnace, and this is represented by the improvement in productivity and the decrease in consumption of reducing agent. The use of high-reducibility burdens contributes to a better performance of blast furnace. More efforts are necessary to develop and apply high-reducibility sinter and carbon composite agglomerates for practical application at a blast furnace.

Numerical Evaluation on Lower Temperature Operation of Blast Furnaces by Charging Carbon Composite Agglomerates

This paper places emphasis on evaluating ironmaking operation at lower temperature. Blast furnace operation with carbon composite agglomerates (CCB) charging and/or lower operation temperature has been numerically examined using a modified multi‐fluid blast furnace model under constant thermal conditions of the raceway. The numerical calculation shows that a lower in‐furnace temperature level is achieved under the operation with the CCB charging. With CCB charging, the location of the cohesive zone shifts downward and the temperature of the thermal reserve zone decreases. The decrease in heat requirements for solution loss, sinter reduction and silicon transfer reactions compensates the increase in heat demands for CCB reduction and direct reduction, and rather improves the efficiency of blast furnaces. Consequently, the productivity improves, the coke rate shows a notable decrease and the total reducing agent rate also tends to decline compared with conventional operation without CCB charging. Therefore, charging carbon composite agglomerates contributes to the enhancement of blast furnace performance. Furthermore, the model predicts that higher operation efficiency is achieved if the melting and/or tapping temperature could be dropped. The innovative technology of lower temperature ironmaking is expected to be applied in industrial blast furnaces after resolving the problems in engineering and economic evaluations.

Numerical Simulation of Innovative Operation of Blast Furnace Based on Multi-Fluid Model

A multi-fluid blast furnace model was simply introduced and was used to simulate several innovative iron-making operations. The simulation results show that injecting hydrogen bearing materials, especially injecting natural gas and plastics, the hydrogen reduction is enhanced, and the furnace performance is improved simultaneously. Total heat input shows obvious decrease due to the decrease of heat consumption in direct reduction, solution loss and silicon transfer reactions. If carbon composite agglomerates are charged into the furnace, the temperature of thermal reserve zone will obviously decrease, and the reduction of iron-bearing burden materials will be retarded. However, the efficiency of blast furnace is improved just due to the decrease in heat requirements for solution loss, sinter reduction, and silicon transfer reactions, and less heat loss through top gas and furnace wall. Finally, the model is used to investigate the performance of blast furnace under the condition of top gas recycling together with plastics injection, cold oxygen blasting and carbon composite agglomerate charging. The lower furnace temperature, extremely accelerated reduction rate, drastically decreased CO2 emission and remarkably enhanced heat efficiency were obtained by using the innovative operations, and the blast furnace operation with superhigh efficiency can be realized.

Numerical analysis on blast furnace performance with novel feed material by multi-dimensional simulator based on multi-fluid theory

Multi-dimensional blast furnace operation simulator based on multi-fluid theory and reaction kinetics is applied to the novel operations of blast furnace. The effective use of carbon composite agglomerates (CCB) in blast furnace is expected to have several advantages to improve furnace efficiency. In this study, mathematical expression of reduction behavior of CCB was introduced into the blast furnace simulator and the effect of charging CCB to blast furnace and accompanying temperature lowering were numerically examined. The calculation results showed the increase in productivity and decrease in reducing agent rate with CCB charging while reduction of iron-bearing materials was retarded due to temperature decrease in stack region. Thermal analysis revealed that this improvement of heat efficiency is caused by the decrease in heat requirements for solution loss, sinter reduction and silicon transfer reactions, heat outflow by top gas and wall heat transfer.

Numerical Analysis on Charging Carbon Composite Agglomerates into Blast Furnace

The carbon composite agglomerates have been numerically examined. The previously presented multi-fluid blast furnace model is modified to simulate blast furnace operation with carbon composite agglomerates charging. The model considers the behavior of carbon composite agglomerates, namely, conservation equations of chemical species contained in this material including chemical reactions and phase transformations are newly added. A series of simulating calculations are performed to examine the effect of charging carbon composite agglomerates. The result reveals that with increase in carbon composite agglomerates charging ratio in-furnace temperature level tends to decrease, the location of cohesive zone shifts downward, and the reductions of carbon composite agglomerates and sinter are delayed. On the other hand, the productivity tends to increase while coke rate and total reducing agent rate show decrease. The energy consumption for unit production of hot metal is lowered. The performance of blast furnace is improved in the tested range mainly due to the decrease in heat requirements for solution loss, sinter reduction and silicon transfer reactions, heat outflow by top gas and wall heat loss.

Numerical Analysis on Blast Furnace Performance under Operation with Top Gas Recycling and Carbon Composite Agglomerates Charging

The blast furnace operations with top gas recycling, carbon composite agglomerates (CCB) charging, cold oxygen blast and waste plastics injection are numerically evaluated by means of multi-fluid blast furnace model on the premise of constant raceway conditions and hot metal temperature. These evaluations are in comparison with conventional operation. Simulation results reveal that CCB charging and/or cold oxygen blast lead to low temperature level in the shaft region, which retards the reduction of CCB. On the other hand, injection of waste plastics enriches hydrogen gas in the furnace. And top gas (after CO2 removal) recycling tremendously enhances the concentrations of H2 and CO in the whole furnace, which greatly promote the indirect reduction of sinter in shaft despite lower temperature. Under the innovative operations, the efficiency of blast furnace shows evident improvement, especially in the case with simultaneous injections of treated top gas through shaft and tuyere. The model computations predicted that total heat input decreases while the productivity increases under the operation with top gas recycling. Additionally, top gas recycling and waste plastics injection also contribute to considerably reduce carbon dioxide emission. It is expectable to develop high efficiency blast furnace with improved productivity and lowered environment load through exploiting the processes presented in this study. If scrubbing and fixing of CO2 and manufacturing of process oxygen are commercially available, these innovative processes will possess broad applying potential.

Non-spherical Carbon Composite Agglomerates: Lab-scale Manufacture and Quality Assessment

For recycling steel work dust and improving blast furnace (BF) performance, a few types of non-spherical carbon-iron oxide agglomerates were manufactured in laboratory scale and their strength and reducibility were evaluated using a shatter and a tumbler tester, a simulator for lumpy zone in BF and a softening/melting tester. The flat or ragged shape was expected to enhance reducibility and to prevent the agglomerate from unwilling rolling down to the center of BF. Agglomerate in the shape of column with concave triangle cross-section (Tetra) showed the highest yield during extruding and almost same strength compared with a commercial spherical cold-bond pellet. In reducibility Tetra excelled sinter but spherical carbon composite pellet, which denied shape effect. Reduction disintegration of Tetra initiated at 600°C and reached to the maximum of 60% at 860°C, the entrance of thermal reserve zone, meaning the issue of cement bonding on disintegration remained unsolved. In case of single use, Tetra did not melt down up to the upper limit temperature of the softening/melting tester, causing S-value enlarged. Using Tetra mixed with sinter was recommended as a trial of admixing 50% Tetra to sinter accelerate reduction rate of sinter, improving BF efficiency by 45 kg/t decreases in RAR; moreover, the mixture remarkably reduced the pressure loss of cohesive zone.

Pellet property requirements for future blast-furnace operations and other new ironmaking processes

The reducibility of iron-bearing burdens was emphasized for improving the operation efficiency of blast furnace. The blast furnace operation of charging the burdens with high reducibility has been numerically evaluated using a multi-fluid blast furnace model. The effects of reaction rate constants and diffusion coefficients were investigated separately or simultaneously for clarifying the variations of furnace state. According to the model simulation results, in the upper zone, the indirect reduction of the burdens proceeds at a faster rate and the shaft efficiency is enhanced with the improvement under the conditions of interface reaction and intra-particle diffusion. In the lower zone, direct reduction in molten slag is restrained. As a consequence, CO utilization of top gas is enhanced and the ratio of direct reduction is decreased. It is possible to achieve higher energy efficiency of the blast furnace, and this is represented by the improvement in productivity and the decrease in consumption of reducing agent. The use of high-reducibility burdens contributes to a better performance of blast furnace. More efforts are necessary to develop and apply high-reducibility sinter and carbon composite agglomerates for practical application at a blast furnace.