A model of post-combustion and heat transfer at smelting reduction with a foamy slag

A smelting reduction reactor with post-combustion in a foamy stag has two slag layers. In the lower, iron oxide from the slag is reduced to iron by carbon. In the upper, the gas from the reduction is post-combusted to CO₂+H₂O. The post-combustion heat is transferred from the rising post-combustion gas to the upper slag, and, therefrom, by slag circulation to the lower slag layer where the endothermic reduction reaction takes place. Unavoidably, slag circulation also transfers part of the char into the upper slag where it is oxidised. A kinetic post-combustion and heat transfer model describes these phenomena. The main process parameters, namely real post-combustion degree, carbon oxidation, heat transfer efficiency, distribution of temperatures of gas and slag in the single regions of the reactor, coal consumption, and iron production rate are given as functions of the ratio of post-combustion oxygen to gas evolution rate of the system. The main influences on which heat efficiency and off-gas temperature as the most important output parameter depend are discussed. Controlled stirring and a regular gas through-put are of primary importance. The required values are given. A comparison with pilot plant results of the CSM process shows good agreement with the model results.     

Flame formation and post-combustion at blowing of oxygen into a carbon monoxide containing slag foam

A combined post-combustion model (CPM) for smelting reduction was developed in a multi-national research project supported by the European Coal and Steel Community. The project partners were CSM, Rome, Hoogovens, Ijmuiden, MPI, Düsseldorf, and TUB, Berlin. This paper is a report about a flame model developed by TUB, Berlin. An oxygen jet is blown into a carbon monoxide containing slag foam. The jet entrains carbon monoxide and slag droplets. Carbon monoxide is combusted by oxygen to carbon dioxide and the developed heat is transferred by radiation from the gas to the surrounding slag and by radiation and convection to the entrained droplets. The droplets are mixed with the slag at the flame end so that also their heat content is finally transferred to the bulk slag. The model consists of a detailed treatment of the entrainment processes, the combustion reaction taking into account the carbon dioxide dissociation equilibrium, the enthalpy changes, and the heat transfer processes. One obtains two ordinary differential equations describing the temperature and composition of the flame gas as functions of the flame pass-way. They are solved numerically by the Runge-Kutta method. As the results, the main flame properties, namely the flame velocity, the diameter and the upwards angle of the flame, the amount of gas and slag entrained from the surroundings to the flame, the oxygen utilisation, from which the post-combustion degree is calculated, the flame temperature along the flame pass-way, and the total heat transfer from the flame gas to the slag, are described as functions of various internal and external parameters. The presented flame model is part of a general post-combustion and heat transfer model of a smelting reduction process with post-combustion in the slag.     

HIsmelt炼铁工艺

非高炉炼铁HIsmelt工艺特征是:向熔池内直接喷吹铁矿粉和煤粉,形成熔炼炉下部的熔炼和上部鼓入富氧热风形成的二次燃烧。其核心是:通过喷溅完成上部燃烧区(高氧势)到下部熔炼区(低氧势)的热传递。西澳奎纳纳(Kwinana)建成首座80万t/a的HIsmelt工厂。2005年4～10月完成热调试,从此进入长时间的生产摸索期。主体冶炼工艺表明其生产率可达设计产能的75%～80%,主工艺喷吹煤比约为800kg/t。热矿喷吹系统(外围)可进一步优化,如果2010年市场形势好转,将对其进行改进,并会将熔炼炉排出的煤气引入矿粉预热器,以期提高生产率,接近设计产能的100%,且煤比降至约700kg/t。由于一些原因,工厂作业率不是很理想。低的作业率(即频繁开停)导致耐材寿命很短。为了延长耐材寿命,2008年工厂安装了一套铜质水冷渣线冷却器,显著延长了炉役周期,并极大提升了工厂的操作水平。HIsmelt的熔炼炉是关键所在,除此外其他影响作业率的因素在奎纳纳已不是问题。目前就是不断解决限制产能发挥的各种问题。     

Heat transfer in an iron bath of a two-stage smelting reduction process

Mechanisms of the heat transfer by means of iron and slag drops in smelting reduction are discussed. The quantity of slag on the molten iron exists in two forms depending on the different processes: either a thin slag layer or a voluminous slag layer. In the case of a small amount of slag, iron and slag drops are spattered in the free space above the bath furnace. By smelting reduction with a high post-combustion degree, the spattered iron drops are quickly oxidized in the free space. The spattered slag drops transfer heat from the free space to the molten iron. A definition for the heat transfer efficiency in the iron bath of a two-stage smelting reduction process is presented. Experiments on heat transfer were made in an experimental EOF-furnace with 100 kg iron. The post-combustion degree and the heat transfer efficiency were on a high level, because inleaked air was used.     

A model of post-combustion in iron-bath reactors,part 2: results for combustion with oxygen

The paper describes calculated results of post-combustion of CO, of heat transfer and of reoxidation for the conditions of a 170-t bottom-blown oxygen converter obtained on the basis of a model published in a previous paper. Calculations were carried out for heat transfer from gas to melt alternately via slag and via metal droplets suspended in the gas phase and acting as the transfer medium. In the case of slag droplets, heat transfer efficiencies of 80% are attainable. In the case of metal droplets, reoxidation limits the heat transfer efficiencies below 50%. The real degrees of post-combustion remain below 20%. Radiative heat transfer amounts to about 20% of total heat transfer.     

A model of post-combustion in iron-bath reactors,part 3: theoretical basis for post-combustion with pre-heated air

This paper describes the post-combustion of carbon monoxide with pre-heated air, heat transfer from gas to melt via a mixture of metal and slag droplets, the microkinetics of reoxidation of metal droplets in the gas consisting of CO₂, CO and N₂, and the total heat and mass balance in the gas space and in the iron-bath. The computer program presented here Is flexible and can simulate various process modes of iron-bath reactors. The production rate of the reactor and the carbon consumption can be calculated herewith.     

A model of post-combustion in iron-bath reactors,part 4: results for post-combustion with preheated air

This paper describes calculated results of post-combustion with pre-heated air, heat transfer from gas to the melt via a mixture of metal and slag droplets, reoxidation at the metal droplets, CO evolution rate from the melt, production rate of the reactor, and carbon consumption of various process modes of a 15-t reactor with or without oxygen blowing through the vessel bottom. Using pre-heated air, the achievable post-combustion degree is higher than using pure oxygen. Nevertheless, slag droplets as heat carrier are necessary. With 50 % slag droplets the real post-combustion degree can be higher than 45 %. With 90 % slag droplets it can be even higher than 80 %. The operating point of the reactor is discussed.     

Towards coal based continuous steelmaking Part 1 – Iron ore fines and scrap to low carbon steel via melt circulation

Abstract

Integrated iron- and steelmaking is effected by depositing a mixture of powdered coal and iron ore fines onto a moving melt surface without prior agglomeration. The resulting metallised solid raft is propelled out of the ironmaking loop onto the melt surface of the first of two steelmaking loops. By progressively adding oxygen to the gas first produced in ironmaking, decarburisation is conducted not with oxygen directly but rather by CO₂ and H₂O so that subsurface formation of CO is never permitted, via careful manipulation of the rates of gas phase mass transfer, interfacial chemical kinetics and liquid phase mass transfer. During ironmaking, infiltration of the melt via capillary rise greatly enhances the rate of metallisation. All the endothermic heat is supplied from beneath the circulating melt, which picks up its heat when post-combustion of CO and H₂ is completed after gases have first passed through the steelmaking loops.     

Influence of post-combustion heat transfer efficiency on fuel reduction in COREX process

Abstract

The influence of post-combustion, heat transfer efficiency, and the degree of metallisation is studied with regard to coal saving and export gas in the COREX process using a thermochemical model. An increase in post-combustion ratio decreases the coal rate and the amount of reduction gas, which influences the achievable degree of metallisation, when no additional reduction gas is added to the system. Further post-combustion with oxygen generates a higher temperature than that with air at all post-combustion ratios. A higher heat transfer rate is necessary to keep the temperature of the gas within the refractoriness limits of the lining material. It is asserted that the post-combustion ratio, the degree of metallisation, and the medium for post-combustion must be adjusted according to the refractoriness of the lining material of the melter–gasifier, the efficiency of heat transfer mechanisms from the post-combustion zone to the bath, the gas handling efficiency, and the thermal energy requirement of the surplus gas, when considering the use of post-combustion in the COREX process.     

Steady state model for current and temperature distributions in an electric smelting furnace

A two-dimensional mathematical model has been developed for predicting current and temperature distributions in an electric smelting furnace. The current distribution is calculated by solving differential equation for magnetic field intensity H_θ with A.C. input. Temperature distribution in the furnace is evaluated by solving the equation for conservation of energy. The data used for the model is based on the ferro manganese production. The effect of various operating parameters such as electrode depth, electrical conductivity of the charge and the slag on the power consumption, heat loss, slag and top gas temperatures have been studied in the present model.     

Fundamentals of post-combustion in steelmaking vessels

Basic equations for the measurement of post-combustion in oxygen steelmaking are given. A thermodynamic phase diagramm of the system Fe-C-O-H for the post-combustion reactions to CO₂ and H₂O has been set up. According to this diagram both CO₂/(CO + CO₂) and H₂O/(H₂ + H₂O) ratios fall when temperatures exceed the melting point of FeO_n (1377°C). Typical curves for post-combustion as a function of blowing time have been included for both K-OBM and KMS heats. An appropriate index for post-combustion is the specific heat flow in MJ/t · min or m³/t · min of (CO₂ + H₂O).     

Physical modeling of gas/liquid mass transfer in a gas stirred ladle

The absorption of gas through the plume eye and of an injected gas in a steelmaking ladle process was investigated, using a physical model of CO₂ absorption into a NaOH solution. The results show that the inert gas escaping through the plume eye is ineffective in protecting the bath from the atmosphere, and placing an oil layer (simulated slag) decreases the absorption rate significantly. Increasing the flow rate of the inert gas not only exposes more of the liquid surface to the CO₂ atmosphere, but also increases the mass transfer coefficient at the surface. The overall mass transfer between an injected CO₂ gas and NaOH solution includes the mass transfer through the surface of the bath as well as the mass transfer in the bubble dispersion zone. The difference between the mass transfer in the bubble dispersion zone and the overall mass transfer was found to be significant for relatively low gas flow rates. The mass transfer coefficient of CO₂ in the bubble dispersion zone was estimated using available information regarding the bubble size and velocity. Mass transfer coefficient estimated for the constant bubble frequency regime shows a dependence on gas flow rate. However, if a constant characteristic size of bubbles is assumed as an alternative approach, the mass transfer coefficient is independent of the gas flow rate.     

A model of post-combustion in iron-bath reactors,part 1: theoretical basis

The paper describes heat and mass balances, thermodynamic equilibria, and kinetics of the post-combustion reaction in iron-bath reactors and of the subsequent transfer of heat from gas to melt via metal and slag droplets suspended by the melt into the gas space above the melt. Heat and mass balances, equilibria, and kinetics of the reoxidation reaction of droplets, which accompanies the transfer of heat, are also described. Finally, the dynamics of droplet behaviour are presented.     

Steady heat transfer in melt-irrigated blast-furnace zone

The steady heat transfer in the irrigated zone of the blast furnace is considered, taking account of the filtration of hot metal and slag through the coke bed. In this zone, the fluxes of coke, hot metal, and slag are heated simultaneously and exchange heat with one another, by both convection and radiation. Convection ensures heat treatment between the gas and all the batch materials and also between the slag and coke, since the slag running through the coke partially covers its surface. Radiant heat transfer develops among the coke, hot metal, and slag. It is more intense at higher temperatures. The heat transfer among the coke, hot metal, and slag in the irrigated zone of the blast furnace has a considerable influence on the temperature field in this zone. To calculate the heat transfer at the shoulder and hearth, information is required regarding the distribution of the thermal effects of direct reduction of iron, silicon, and manganese over the height of the irrigated zone and the proportion of the coke surface covered by slag.     

Effect of oxyfuel burner ratio changes on energy efficiency in electric arc furnace at Co-Steel Lasco

Abstract

Trials were conducted on Co-Steel Lasco's electric arc furnace (EAF) to evaluate the effect of oxyfuel burner ratio changes on furnace energy efficiency and productivity. Carefully controlled trials with the collection of numerous process and sample data over 331 heats provided statistically significant results. The oxygen available for post-combustion in the furnace was increased by 19%. As the burners were operating at their maximum oxygen constraint, this was achieved by decreasing the natural gas consumption by 43%. Results of this study indicate that decreases in terms of specific electrical energy consumption (4·0%), power on time (5·0%), and tap to tap time (4·5%) were realised. Slag chemistry, electrode consumption, and yield were not affected. Measurement data support the finding that more heat was transferred into the steel with the post-combustion burners: a flat bath was achieved earlier; high bath temperatures were reached more quickly; power on time was reduced, leading to reduced electrical energy consumption. Analysis suggests that furnace thermal energy losses were reduced by less air inleakage, less incomplete methane combustion, and earlier achievement of foamy slag conditions.     

Dynamic Model of Basic Oxygen Steelmaking Process Based on Multi-zone Reaction Kinetics: Model Derivation and Validation

A multi-zone kinetic model coupled with a dynamic slag generation model was developed for the simulation of hot metal and slag composition during the basic oxygen furnace (BOF) operation. The three reaction zones (i) jet impact zone, (ii) slag–bulk metal zone, (iii) slag–metal–gas emulsion zone were considered for the calculation of overall refining kinetics. In the rate equations, the transient rate parameters were mathematically described as a function of process variables. A micro and macroscopic rate calculation methodology (micro-kinetics and macro-kinetics) were developed to estimate the total refining contributed by the recirculating metal droplets through the slag–metal emulsion zone. The micro-kinetics involves developing the rate equation for individual droplets in the emulsion. The mathematical models for the size distribution of initial droplets, kinetics of simultaneous refining of elements, the residence time in the emulsion, and dynamic interfacial area change were established in the micro-kinetic model. In the macro-kinetics calculation, a droplet generation model was employed and the total amount of refining by emulsion was calculated by summing the refining from the entire population of returning droplets. A dynamic Fe_tO generation model based on oxygen mass balance was developed and coupled with the multi-zone kinetic model. The effect of post-combustion on the evolution of slag and metal composition was investigated. The model was applied to a 200-ton top blowing converter and the simulated value of metal and slag was found to be in good agreement with the measured data. The post-combustion ratio was found to be an important factor in controlling Fe_tO content in the slag and the kinetics of Mn and P in a BOF process.     

Transport phenomena in electric smelting of nickel matte: Part II. Mathematical modeling

A three-dimensional mathematical model was developed to simulate the distributions of electrical potential, heat release, temperature, and velocity in the slag and matte in a six-in-line 36 MVA capacity furnace for smelting nickel calcine. From Part I of this series, it was found that there was a substantial electrical potential drop at the electrode surface, likely due to arcing through evolved carbon monoxide. The incorporation of this phenomenon into the model permitted accurate calculation of the current, power, and temperature distributions in the slag and matte. The slag was found to be thermally homogenized due to the evolved gas, and to a lesser extent by natural convection. In contrast, the matte was thermally stratified; this finding was attributed to poor momentum transfer across the slag/matte interface. Ninety percent of the electrical energy was used in smelting reactions in the calcine; to simulate the heat transfer from the slag to the calcine, a heat transfer coefficient was deduced from plant data. The implications of these findings for stable furnace operation are discussed.     

不同矿种H2-C熔融还原过程中CaO-SiO2-Al2O3-MgO系熔渣的起泡行为

 在1400℃下,通过300kg级氢碳熔融还原热模拟试验,用渣线测定装置研究了不同矿种在氢碳熔融还原过程中CaO-SiO2-Al2O3-MgO系熔渣的起泡行为,测定了熔渣的起泡高度、泡沫渣厚度、熔渣起泡率等参数。通过计算得到了气泡大小和熔渣的物理化学性质。结果表明,泡沫渣厚度随表观气体流速(反应速率)的增加而增加,表观气体流速相近时又随熔渣的黏度增大而增加。熔渣起泡率最高的矿种为生矿,为3.4。通过对泡沫渣厚度的因次分析,得到了泡沫渣厚度与熔渣黏度、密度、表面张力、气体表观流速、气泡大小及重力加速度的关系。     

Effect of CaO Addition on Iron Recovery from Copper Smelting Slags by Solid Carbon

We investigated the effect of flux (lime) addition on the reduction behavior of iron oxide in copper slag by solid carbon at 1773 K (1500 °C). In particular, we quantified the recovery of iron by performing typical kinetic analysis and considering slag foaming, which is strongly affected by the thermophysical properties of slags. The iron oxide in the copper slag was consistently reduced by solid carbon over time. In the kinetic analysis, we determined mass transfer coefficients with and without considering slag foaming using a gas holdup factor. The mass transfer of FeO was not significantly changed by CaO addition when slag foaming was ignored, whereas the mass transfer of FeO when slag foaming was considered was at a minimum in the 20 mass pct CaO system. Iron recovery, defined as the ratio of the amount of iron clearly transferred to the base metal ingot to the initial amount of iron in the slag phase before reduction, was maximal (about 90 pct) in the 20 mass pct CaO system. Various types of solid compounds, including Mg₂SiO₄ and Ca₂SiO₄, were precipitated in slags during the FeO reduction process, and these compounds strongly affected the reduction kinetics of FeO as well as iron recovery. Iron recovery was the greatest in the 20 mass pct CaO system because no solid compounds formed in this system, resulting in a highly fluid slag. This fluid slag allowed iron droplets to fall rapidly with high terminal velocity to the bottom of the crucible. A linear relationship between the mass transfer coefficient of FeO considering slag foaming and foam stability was obtained, from which we concluded that the mass transfer of FeO in slag was effectively promoted not only by gas evolution due to reduction reactions but also by foamy slag containing solid compounds. However, the reduced iron droplets were finely dispersed in foamy and viscous slags, making actual iron recovery a challenge.