There are 19 super-large blast furnaces more than 4000 m3 running in China currently, and significant progresses of low carbon operation were obtained in recent years. In this paper, low carbon operation technologies of Chinese super-large blast furnaces were illustrated in the two aspects of raw materials preparation and blast furnace operation. Firstly in terms of raw materials preparation, low carbon operation technologies include producing low SiO2 content and high reducibility sinter to reduce slag volume and improve sinter metallurgical properties; optimizing coal blending structure for coking to guarantee coke quality; implementing blending yard technology to stabilize blast furnace operation; controlling harmful elements load to reduce their damage to coke; adopting screening technology to reduce dust amount; recycling small size sinter, nut coke and CDQ dust to improve ironmaking energy efficiency. Secondly in terms of blast furnace operation, low carbon operation technologies include improving gas utilization efficiency by upper adjustment and lower adjustment, the upper adjustment including charging mode, batch weight and stock line level, lower adjustment including the control of blast volume and blast kinetic energy; implementing comprehensive blast technology of high blast temperature, dehumidified blast and high top pressure, promoting economic coal injection concept; conducting low silicon smelting, but should pay attention to its impact on campaign life of blast furnace; controlling thermal load to reduce heat loss, emphasizing middle-part management of blast furnace; development and application of blast furnace visualization technology to guarantee long-term smooth operation of blast furnace.
Reduction of Fe-bearing FINEX process waste and carbon composite pellets from 1373 K to 1573 K to produce DRI (direct reduced iron) for use in the blast furnace was investigated using a modified thermo-gravimetric analyzer. Reduction from the initial Fe2O3 was not uniform throughout the composite pellet. Oxygen removal from the Fe2O3 rich composite pellets over 84% was only observed at 1573 K. Lower temperatures resulted in significantly un-reduced FeOt due to the premature consumption of the carbon. A peripheral boundary of FeOt·Al2O3 and 2FeO·SiO2 phases surrounding the reducible FeOt was observed in some of the partially reduced cross-sectional SEM (scanning electron microscope) images that could hinder reduction. From the apparent activation energy, interfacial reaction seems to affect the kinetics of the Fe-bearing process waste composite pellets. Bursting of pre-dried composite pellets containing less than 2 mass% moisture was simulated in a RHF hearth simulator. From direct observation of pellets charged between 1173 K to 1573 K, medium-sized pellets between 9.4 to 12.4 mm diameter showed less bursting of the pellet, when charged below 1273 K. In addition, modification of the physical strength of the hard bedrock formed from pellet bursting could be softened with additions of SiO2.
A technique for controlling the mixed coke distribution in the ore layer was investigated in order to achieve low RAR operation of the blast furnace. A reduction test under load was performed with various mixed coke ratio distributions in the height direction. As a result, the ore reduction degree with a high mixed coke ratio in the upper part of the ore layer mixed with coke was higher than ore reduction degree with a uniform mixed coke ratio from the upper part to the lower part of the layer. The effect of the charging pattern on the mixed coke distribution in the ore layer mixed with coke was investigated by a scale model experiment. As a result, it was possible to form an ore layer with a high coke ratio in the upper part of the ore layer mixed with coke at the furnace top by controlling the burden shape of the mixed materials stacked in the top bunker. An operating test was carried out at JFE Steel’s Chiba No. 6 blast furnace on the basis of the laboratory test results. Gas utilization improved after this technique was applied.
Increased gas utilization and improved permeability have been desired in order to achieve low RAR (Reducing Agent Rate) operation of blast furnace. Coke mixed charging in the ore layer is one effective measure for realizing these improvements. In this research, burden distribution control technique for mixing small coke at a parallel type bell-less top and at a center feed type bell-less top were developed and investigated in an experiments with scale models of two actual blast furnaces at JFE Steel. The mixed coke ratio was controlled by the discharge pattern of the mixed small coke and the mixing position. With the center feed bell-less top, the optimum patterns was discharge of the small coke from the coke bin after discharge of the ore, and the parallel type bell-less top, the optimum pattern was discharge of the small coke on the ore at the front of an overlapped quarter part. These patterns were applied to two actual blast furnaces, and improvement of permeability was confirmed as a result of increased mixed small coke ratio.
Reduction of CO2 emissions is recognized as an urgent issue for the iron and steel industry. One of the feasible methods for the reduction of CO2 emissions may be the use of H2 gas as a reducing agent in blast furnaces for ironmaking. In order to keep the conditions of the blast furnace stable under high H2 concentration, it is necessary to understand the effects of H2 and H2O gases on the disintegration of the iron ore sinter in the upper part of the blast furnace. Therefore, the effects of the concentration of the reducing gas and of the reduction time on the reduction and disintegration behavior of actual sinters are examined in the present work. Reduction experiments were carried out under N2–20% CO–20% CO2 (CO gas) and under N2–12% CO–17.7% CO2–8% H2–2.3% H2O (CO–H2 gas) at 773 K. The reduction degree of the sinter reduced under CO–H2 gas increased with time. On the other hand, it was once retarded when reduced under CO gas. For the same value of reduction degree, however, the value of the reduction-disintegration index, RDI, for the sinter reduced under CO gas was higher than that of the sinter processed under CO–H2 gas. When reducing hematite to magnetite for long time, e.g., 3.6 ks under CO–H2 gas, the reduction degree calculated from the weight change was larger than that obtained on the basis of the change in the XRD peak intensity.
In the drive to mitigate global warming, the iron and steel industry has made efforts to reduce carbon dioxide gas emissions. Utilization of a composite material consisting of carbonaceous material and iron ore is expected to be an effective way to solve the issue. Volatile matters in carbonaceous materials have a potential to reduce emissions by lowering the reduction temperature. In this study, the reduction by volatile matter in a coal/hematite ore composite was fundamentally examined. Different kinds of coal were chosen as carbonaceous material samples. The reduction experiment using the prepared composites was conducted by varying the heating rate as 0.08, 0.17, and 0.33 K/s, up to a temperature of 1473 K under Ar–5%N2 gas flow. The outlet gas composition was analyzed and the results were used to calculate the reduction degree. Reduction degree of hematite ore initially increases with increasing content of volatile matter in coal. However, beyond a certain point, further increase in the volatile matter content of the coal did not give a significant change in the reduction degree. For the evaluated coals, the amounts and types of gases generated are not different each other. H2 gas seems to form together with solid carbon via decomposition of hydrocarbon gases. Such gases and solid carbon may contribute to the reduction. Moreover, reduction by H2 was promoted in the early stage of the reactions by decreasing heating rate. The data indicate that control of the heating rate is a possible way to promote reduction at low temperature.
The utilization of H2 gas as a reducing agent in blast furnaces is one potential method to reduce CO2 emissions from the iron and steel industry. In order to maintain stable operation of the blast furnace under a high H2 concentration, it is necessary to understand the effects of H2 and H2O gases on the disintegration of the iron ore sinter by reduction under various conditions. In this study, the effect of the reduction of calcium ferrite on the disintegration of sinter was examined. Reduction experiments were carried out under N2 – 20% CO – 20% CO2 (CO reduction) and N2 – 12% CO – x% CO2–8% H2 – (20-x)% H2O (CO–H2 reduction) at temperatures from 773 to 973 K. The reduction of hematite to magnetite mainly proceeds at 773 K. However, longer reduction time is necessary for acicular calcium ferrite as 5.4 ks by CO reduction and 3.6 ks for CO–H2 reduction, while columnar calcium ferrite is not reduced at such temperature. The reduction of acicular calcium ferrite begins after 1.8 ks at 873 K and accelerates disintegration of sinter. At 973 K, on the other hand, disintegration is inhibited because the volume expansion of skeletal hematite becomes small and the cracks formed in the sinter seem to be mended.
Generally, reactions and forming phases during ironmaking can be thermodynamically predicted using equilibrium phase diagram. However, at low temperature it will likely to be different from predicted phases and deviate from equilibrium. Hence, knowledge of solid state reaction at low temperature is required to control the melting behavior of slag phase in blast furnace. Reactions between iron oxide and gangue minerals in ore under at 1373 K under Ar atmosphere were investigated in present work.
In order to elucidate the slag melting behavior of sinters, the in-situ observation of the melt formation behaviors at the interface between synthesized crystalline olivine and wüstite bulks was carried out using a high temperature microscope over a temperature range between 1080 and 1120°C. The melting instantly happens at the interface in the olivine side at the temperatures higher than 1100°C although the thickness of the melting zone does not vary with time during the heating. The thickness of the melting zone is ca. 20 μm irrespective of heating temperature as well as heating duration. The melt formation is considered to be due to the fact that the olivine composition slightely shifts toward 2FeO·SiO2 by substituting Ca2+ with Fe2+ owing to the diffusion of Fe from wüstite into olivine, lowering the liquidus temperature.
To mitigate CO2 emission from a blast furnace, the use of H2 as a reducing agent is considered to be a prominent method. Reduction of iron ore was reported to be improved by H2 addition. In the present research, reduction in a sinter-packed bed by CO and H2 was carried out under various oxygen partial pressures, and the influence of reduction atmosphere on the reaction behavior was investigated. It was confirmed that the reduction rates were higher when using H2/H2O and CO/CO2/H2/H2O mixtures than when using a CO/CO2 gas mixture. The reaction rate constant for iron ore reduction was determined from the experimental result using a numerical model. The influence of the water-gas shift reaction on the reduction rate was analyzed by comparing the results obtained using the numerical model and those in the experiments. Moreover, the longitudinal distribution of the reaction rate in the packed bed was analyzed to examine the influence of the water-gas shift reaction.
To clarify the primary slag melting behavior of sinters, the microstructure and phase changes during the melt producing process were observed: Sinters were heated under the temperature history and the CO/CO2 ratio simulating a blast furnace condition and quenched from several different temperatures between 1000 and 1250°C. The microstructures of the quenched samples were observed by electron probe microanalyzer. The olivine-like melts have been locally observed as primary melts above 1100°C, the compositions of which have higher FeO content and lower CaO/SiO2 ratio with an increase in the heating temperature. On the other hand, microstructures composed of round shape wüstite and 2CaO · SiO2 have been observed on a sample quenched from 1250°C. In that local area, the olivine-like melts cannot be found. This may imply that although the olivine-like melts originally existed between wüstite and 2CaO · SiO2, the melts have been completely dissolved into 2CaO · SiO2 owing to the large solubility of olivine in 2CaO · SiO2, resulting in the resolidification of the melts. The eutectic structure composed of wüstite, 2CaO · SiO2 and 2CaO · Al2O3 · SiO2 have also been observed on a sample quenched from 1250°C.
Maintaining gas permeability is an important issue to realize low coke rate operation of blast furnace. In present study, the melting behavior of the iron ore and the layer structure in low coke rate operation were introduced in to the DEM-CFD model, and then behaviors of gas and moving bed in the blast furnace were simulated. Influence of shape of cohesive zone in low coke rate operation was investigated, and the effect of coke slit in the cohesive zone on gas flow was demonstrated in the calculation result. Surface area of cohesive zone and intersection angle between the burden layers and cohesive zone will determine the activity of coke slit.
Utilization of small coke in the blast furnace was carried out to improve the permeability in the lower part of the blast furnace. However, at high small coke rates, it was thought that some small coke continues to exist in the lower part of the blast furnace because the small coke charging rate is larger than the gasification reaction rate of the small coke. Therefore, the effect of the small coke rate on permeability in the lower part of the blast furnace was investigated. At high small coke rates, residual small coke with a reduced particle size counted to exist in the lower part of the blast furnace after the coke gasification reaction, and the average particle diameter of the coke and the void fraction of the coke packed bed in the lower part of the blast furnace decreased. It was estimated that the increase in the pressure drop of the coke packed bed in the lower part of the blast furnace was larger than the decrease in pressure drop in the cohesive zone, and as a result, the pressure drop in the lower part of the blast furnace increased.
In the current trend, a low carbon operation of blast furnace is going to make liquid permeability severe condition due to thinning of coke layer around cohesive zone. An iron carburization reaction is one of the most important reactions at the cohesive zone, because an enhancement of the reaction has a positive possibility to improve a metal dripping behavior from cohesive zone. Although it is thought ash of carbonaceous material has a negative effect on the reaction, there is not enough correctly focused knowledge on behavior of the ash in iron carburization reaction. In this study, several kinds of carbonaceous material samples with ash remove treatment by acid solution were prepared. The carbonaceous material samples were applied for “in-situ” observation of molten iron formation behavior due to iron carburization reaction under a constant heating rate condition with inert gas atmosphere. It was found that the acid treatment decreased not only amount of the ash in the carbon samples but also Na concentration of the ash. Decreasing of ash content in carbonaceous material decreased initial Fe–C liquid formation temperature because obstruction on reaction area of iron carburization reaction was decreased. Decreasing of Na content in ash caused changing of molten ash’s properties, increasing of melting temperature and decreasing of wettability to iron and carbon. In case of without the acid treatment, it was thought molten ash could behave as a barrier at a reaction interface of iron carburization due to good wettability from lower temperature than initial Fe–C liquid formation temperature.
A low carbon operation is an unfavorable situation for liquid permeability around cohesive zone, because liquid volume will increase against solid coke in there. In order to keep a healthy operation with this technique, information of wetting behavior between liquid iron and coke should be correctly understood. However, there is not enough information about wetting behavior between them, because of many difficulties about wettability measurement from an active reaction between iron and carbonaceous materials. In this study, a sessile drop method with molten sample injection system was applied to measurement of wetting behavior between liquid iron and carbonaceous material at 1673 K for excluding reaction between samples before starting measurement. Carbonaceous material’s substrates were made from mixture powder of graphite and alumina by hot press at 1873 K. From the results, following knowledge was revealed. Molten iron samples un-saturated with carbon showed bigger values of contact angles, 110°–120°, at initial stage, than apparent constant values of them, 85°–100°, at latter stage. It indicated a reaction between iron and carbonaceous materials had obvious effect on wetting behavior between them due to decrease an interfacial energy during the reaction. Mixed alumina powder in the substrate prevented to wetting behavior of iron sample on carbonaceous materials, and they changed their apparent constant contact angles from 115° to 130°. The alumina powder had effects on not only wetting behavior but also reaction between iron and carbonaceous materials.
A low coke rate operation of a blast furnace tends to cause deterioration of the gas permeability. The liquid iron and molten slag dripping under the melting zone influences the gas flow and permeability in the lower part of the furnace. A computational fluid dynamic model, using a particle-based simulation, is presented for characterization of the melt dripping behavior in a packed bed. In this work, the validity of the liquid passing conditions based on the gravity to surface tension ratio was confirmed. The melt shape produces an “icicle,” “droplet,” and “dome” forms based on the change of the surface tension and density absolute values. Even with an identical liquid volume passage, the form of the liquid flow changed by the pressure from the liquid’s upper portion. Even if it has identical gravity and surface tension ratios, the liquid flow changes with the volume of the liquid phase and its form.
For the reduction of the carbon dioxide emission from the steelmaking industries, various approaches to design the blast furnace operation with the injection of the reducing gas to the stack part have been made. To realize this technology, the flow behavior of the injected gas has to be quantitatively understood. This study focused to clarify the mechanism to determine the flow path of the injected gas, and the flow behavior of the stream which is laterally injected to the main stream in the packed bed was discussed through theoretical consideration, experiments and numerical flow simulation. The injected stream flows along the wall of injection side without penetrating the main stream. The dispersion degree coincides with the flow rate ratio of injected stream to total flow under the condition with uniform packing structure and fluid properties. The dispersion degree in the region enough downstream from the injection is independent of the injection velocity and direction. The dispersion degree can be controlled in some extent by controlling the difference in the fluid properties between the main and the injected streams and the distribution of the packing structure.
Liquid dripping in a packed bed of coke in a blast furnace decreases the gas permeability and production stability. Enhancing the liquid flow is desirable to increase the productivity of the blast furnace process. The wettability between the liquid and coke affects the dripping behavior. In present study, the contact angle of a moving droplet on a non-smooth solid surface was investigated considering dripping slag and pig iron droplets in a packed bed of coke. The advancing and receding contact angles of water and mercury on a substrate were measured at room temperature while controlling the wettability and roughness. The angles between the cut surface of the coke and water or mercury were also measured. The roughness of the solid surface affected the movement of the adhering droplets, but the effect of the roughness was significantly altered by the wettability. It was found that the resistance to movement of the liquid increased and decreased under good and poor wettability conditions, respectively. Because the wettability of the liquid phase in the blast furnace changed depending on the temperature and composition of molten slag or iron, the force on a liquid droplet from the coke surface changed depending on the position and composition of the hot metal and molten slag in the coke bed.
Liquid flow in blast furnaces has a significant influence on gas flow and pressure drop. Therefore, the stability of blast furnace operations and productivity are affected by liquid flow. In a furnace, liquid flows in a packed bed consisting of coke. Holdup is an important phenomenon in packed bed flow. It changes with the variation of the packed bed structure and the physical properties of the liquid. In this study, a numerical simulation for packed bed flow is carried out. The effects of a packed bed structure on holdup phenomena were analyzed by the moving particle semi-implicit (MPS) method.
Stable operation under low reducing agent rate condition of blast furnace is necessary to realize to contribute solution of global warming. Under low reducing agent rate operation, large amount of powder materials like fine coke and unburnt pulverize coal generate in the furnace. It is known that the heavy accumulation of these powders in the packed bed of blast furnace deteriorates the permeability, and cause operation trouble in extreme case. Thus it is important to understand the flow and accumulation behaviors of powder in packed bed. This paper discussed the powder motion in the packed bed through numerical simulation under simplified condition. A simplified packing structure, namely an orifice consisting of three spherical particles that touched each other and were in equilateral triangle arrangement, was picked out from the packed bed. The powder trajectories passing through this orifice were tracked by using the discrete element method. The simulation results showed that only a few powder particles initiated the blockage of the particle orifice. Additionally the effects of mechanical properties of the particles on the passage and the blockage behaviors were also revealed.
The effect of the CaO/SiO2 molar ratio on the surface tension of calcium aluminosilicate melts containing magnesia (CaO–SiO2–Al2O3–MgO) has been explored using a ring method at 1723–1823 K; the Al2O3 and MgO contents were approximately 12 and 8 mol%, respectively. The CaO/SiO2 molar ratio of the samples was varied in the range of 1.1–1.7. The surface tension of the CaO–SiO2–Al2O3–MgO system simultaneously increased upon increasing the CaO/SiO2 molar ratio. The present data were compared with the surface tension of the binary calcium silicate (CaO–SiO2) and the ternary calcium aluminosilicate (CaO–SiO2–Al2O3) melts reported in the literatures. The surface tension of the present CaO–SiO2–Al2O3–MgO melts was higher than those of the binary calcium silicate melts and slightly lower than those of the ternary calcium aluminosilicate melts when the polymerization degrees of the melts were comparable. The change in the surface tension was considered from the viewpoint of the local structure of oxygen atoms at the melt surface. Oxygen atoms, which require higher coordination by cations in the bulk, may tend to lose their neighbors at the surface of the melts, which can result in the formation of unsatisfied bonds at the surface. An increase in the number of unsatisfied bonds can yield an increase in the surface tension.
CO2 emissions from blast furnaces should be reduced to curtail the impact of global warming. A promising solution is the low reducing agent rate (RAR) operation of blast furnaces. Char and ash particles derived from pulverized coal affect permeability in the furnace during low RAR operation. In our study, the combustion behavior and ash particle properties of pulverized coal during combustion were investigated. Char particles formed during combustion were sampled using a drop tube furnace, and then analyzed for their combustion ratio and ash particle properties. As a result, the combustion behavior of pulverized coal and properties of ash particles in raw coal were different by a coal type. Moreover, the combustibility of pulverized coal and the variation in ash particle properties during combustion were affected by the structure of the char particles.
Owing to powder accumulation in the packed bed, the permeability of gas and liquid deteriorates, which decrease throughput of the blast furnace. Powder in the moving bed has a large effect on the productivity and efficiency of the blast furnace process when operating with low coke rate and using high-reactivity coke. Therefore, an investigation on the effect of the powder’s physical properties and the moving bed material on transport phenomena and the accumulation mechanism of powder in the blast furnace is essential. In the present study, the motion of powder particles was simulated using the discrete element method (DEM), and the effect of the powder particles’ shape on the behavior and accumulation of powder was investigated. The study used DEM to reproduce the movement of particles by solving the equations of motion of individual particles. Since the particles were treated as spherical objects in DEM, contact friction and rolling resistance were implemented to represent the irregular shape of actual powder particles. The calculation results showed that contact friction and rotational resistance affected the static holdup of the powder. The amount of clogging of powder at a bottleneck in the moving bed increased with increasing coefficients of friction and resistance. Moreover, the impact of the flow velocity of the gaseous phase on the accumulation behavior of powder was examined. The frictional force on powder particles at the bottleneck increased with an increase in the gas velocity. The clogging of powder occurred easily as a result.
The use of high reactivity coke is a technology that dramatically improves the reaction efficiency in blast furnaces by decreasing the temperature of the thermal reserve zone. In this study, a blast furnace shaft simulator was developed to estimate the temperature of the thermal reserve zone and the distributions of the temperature and gas composition in the blast furnace when using cokes with different reactivity. The shaft simulator combines an experimental reaction furnace and a calculation model. Chemical reaction and mass/heat transfer phenomena in the blast furnace are considered in the calculation model so as to calculate the ore and coke reaction rate and the distribution of temperature and gas composition. Relatively small amounts of packed coke and sinter specimens are reacted with the temperature and gas composition controlled based on the calculation results. The coke gasification rate is fed back to the calculation model, and it is then possible to estimate the temperature of the thermal reserve zone and the distributions of the temperature and gas composition in the blast furnace. Shaft simulator experiments with high reactivity coke, such as CIC (Carbon Iron Composite), showed that the temperature of the thermal reserve zone is 140 K lower with high reactivity coke than with conventional coke.
Realizing stable and highly efficient operation of blast furnace under low reducing agent rate condition is one of the key issues to contribute problems of energy, resources and global warming. Under low reducing agent rate operation, chemical and thermal driving force in the blast furnace is weakened, and it is considered that the capability of recovering the process from the fluctuation is deteriorated. Therefore the probing the state inside the blast furnace by utilizing the information from the various sensors is important. This study focused on the relation between pressure distribution on furnace wall and packing structure inside the furnace. The blast furnace is a packed bed reactor with upward gas flow, thus the pressure generally decreases from the tuyere to the top. Partially inverse pressure distribution is sometimes observed as an instable phenomenon of the furnace. This study proposed a mechanism to produce such pressure distribution with simple structural change in packed bed. When a vacant space is formed in the packed bed, the gas flow concentrates to this space and the gas velocity increases. At the downstream end of the space, the gas velocity decreases and generates dynamic pressure. This increase of the pressure forms the inverse pressure distribution. In this study, this mechanism was confirmed through experimental and numerical approaches.