OPTIMUM PROCESS CONDITIONS FOR THE PRODUCTION OF PIG IRON BY COREX PROCESS. Abstract

Transcription

1 OPTIMUM PROCESS CONDITIONS FOR THE PRODUCTION OF PIG IRON BY COREX PROCESS Ahmad Wafiq 1, Ahmed Soliman 1, Tarek M. Moustafa 1, and A.F. Nassar 1 1 Chemical Engineering Department, Faculty of Engineering, Cairo University Abstract COREX process for pig iron production is a commercially proven process, and is currently considered as the main competent to blast furnace for pig iron production. COREX process consists of two reactors; the reduction shaft, and the melter-gasifier. The process involves multi-phase, multi-component, having about five raw materials, and three main products. Varying one process input can lead to operation instability and/or result in off-spec products. Macroscopic modeling is always an attractive approach for processes with such complex features. In the present paper a mathematical macroscopic process model has been developed and the Indian Jindal plant was taken as case study. The macroscopic model was efficient tool to determine the window of possible operation scenarios while keeping all process constraints fulfilled. The main raw material constraints are coke amount, and percentage of iron ore fines. In addition to the pig iron quality, the main products constraints are amount and compositions of reduction gases, export gas composition, and the slag s quality. A parametric study has been performed focusing on the freeboard zone in the meltergasifier. The effect of iron ore fines on the free board s temperature and export gas composition has been analyzed. The most flexible operation scenarios have been determined and summarized in an operation chart. This chart can be used for process control as a guide as it combines the controlling process variables. According to the market status, four main operation modes for COREX process have been defined; maximization of iron ore fines, minimization of fuel amount, minimization of Iron bearing materials amount, and minimization of oxygen feed amount. Including the current raw material prices, and to achieve minimum production cost, the minimum possible oxygen feed rate is economically the optimum mode of operation.

2 Introduction Together with the problem of the high capital cost needed, the blast furnace has other drawbacks including the dependence on scarce coking coal as a reductant, and the low flexibility towards utilizing iron wastes as a substitute to the fresh iron ores [1]. Moreover, the increased environmental pressures nowadays cause a lot of problems to any investor thinking to use the blast furnace route for steel production [2]. Among the newly developed smelting reduction processes, COREX process is nearly the only competent to the blast furnace in the field of pig iron production. In COREX process, all the reactions take place in two reactors; namely, the reduction shaft (RS) and melter-gasifier (MG). Currently, 4 plants are utilizing COREX process for pig iron production. These plants are Saldanha Steel Works in South Africa (0.8 mtpa), Jindal South West Steel in India (2 * 0.8 mtpa), Posco in Korea (0.8 mtpa), and Baosteel in China (1.2 mtpa). COREX produces a high quality pig iron using non-coking coal and pure oxygen in an environmental-friendly process. COREX process eliminates the need to coking and sintering plants conventionally used in the blast furnace route. The process is viable at lower production capacities, and this copes with the gradual world shift from the integrated steel plants to smaller mini-mills. Moreover, COREX is more flexible to variations in compositions of raw materials, and it is insensitive to the alkali contents of the raw materials [1]. Because of having a multi-component multi-phase system, previous researches focus on performing macroscopic analysis to reach better understanding of COREX process, and assess the effect of different process parameters. Kumar et al. [3] have studied the effect of the cold crushing strength (CCS) of the burden on the performance of the reduction shaft. In addition, they studied the effect of coal size on the performance of the melter gasifier, and they developed a regression analysis using multiple variables to get an equation for the fuel rate. The addition of coke to the melter gasifier was also a subject of many researches [4-6]. In another important research [7] Kumar et al. have built a macroscopic model to predict the changes taking place on altering any of the input variables using complex mass and energy balance equations. In the present paper a mathematical macroscopic process model has been developed and the Indian Jindal plant has been taken as a case study. The macroscopic model was built to determine the window of possible operation scenarios while keeping all process constraints fulfilled. The most flexible operation scenarios have been determined and summarized in an operation chart. This chart can be used for process control as a guide as it combines the controlling process variables. COREX Process Description RS Process Description As shown in Figure 1, iron ore, pellets and fluxing materials (limestone and dolomite) are charged into RS from the top of the shaft. The reduction gas is injected from the bottom of the shaft at about 850 o C. The gas moves in the shaft upwards counter currently to the solid

3 phase and exits from the shaft at around 250 o C. The solid product is termed as direct reduced iron (DRI) and it is transported from the RS into the MG [5], [7], [8]. MG Process Description In addition to the hot DRI from the RS, non-coking coal, iron ore fines, flux fines and some coke are continuously charged into the MG from their charging bins. Oxygen plays a vital role in COREX process for generation of heat and reduction gases. It is injected through the tuyeres, where it gasifies the coal char generating CO. The hot gases ascend upward through the char bed [5], [7], [8]. The sensible heat of the gases is transferred to the char bed, which is utilized for melting iron and slag and other metallurgical reactions. The temperature of the freeboard zone (shown in figure 2) is maintained between 1000 o C to 1100 o C, and this assures cracking of all the volatiles released from coal. The gas generated inside the MG contains fine dust particles, which are separated in hot gas cyclones. The dust collected in the cyclones is recycled back to the MG through the dust burners, where the dust is combusted with secondary oxygen. The gas from the MG is cooled to the reduction as temperature (About 850 o C) by adding cooling gas as shown in figure 1. Most of the combined gas is fed to the RS, while the excess gas is used to control the plant pressure [5], [7], [8]. Figure 1 Process Flow Diagram of COREX Process Figure 2 Zones in MG Macroscopic Model Development COREX Process is a complex system having a lot of raw materials and 3 main products which are the hot metal, slag and export gas. Changing one input variable can cause process

4 instability, and can also result in off-spec products. Each of the 2 reactors involved (RS and MG) has one of its feeds emerging from the other reactor. Consequently, a macroscopic model based upon simultaneous solution of the material and energy balances on the 2 reactors has been developed. A degree of freedom analysis has been applied in order to determine the feasibility of solution and the required information. The present work will use the data of Jindal Vijayanagar Steel Limited (JVSL) in India as a case study. Consequently, all the input data will be originated from the published information about this plant. Macroscopic Balance on MG In the MG there are 6 raw materials (DRI, coal and coke, iron ore fines, flux fines, primary and secondary oxygen) and 3 products (hot metal HM, slag, and gases). The flux fines comprise limestone, dolomite, and LD slag. From the operating experiences in Jindal plant, Kumar et al. [9] previously published the proximate analysis and ash compositions of the coal and coke, the compositions and amounts of flux fines, composition of iron ore, and composition of the hot metal product. Solomon et al. [10] published a famous research determining the compositions of the different functional groups for various coal types. The latter paper has been used in this model to determine the compositions of different volatiles in coal and coke. In their microscopic model for the MG, Pal and Lahiri [11] used a ratio of between the primary and secondary oxygen, and this ratio has been used in this work. The data required to perform energy balance on the MG are simply the heats of reactions, temperatures and latent heats. The temperatures of all the fresh inputs are about 25 o C [7], and the tapping temperature of HM and slag is about 1470 o C [11]. For the complex pyrolysis phenomenon, Strezov et al. [12] experimentally determined the heat of pyrolysis at its different stages for more than one coal type. The research finding has been used in this macroscopic model. In addition, the final melting temperature and latent heat of melting of slag has been got from the work of Matousek [13]. The main reactions taking place in the MG are [11], [14]: C O 2 CO (1) C + CO 2 2 CO (2) C + H 2 O CO + H 2 (3) CO + Fe 2 O 3 CO FeO (4) H 2 + Fe 2 O 3 H 2 O + 2 FeO (5) CO + FeO CO 2 + Fe (6) H 2 + FeO H 2 O + Fe (7) CaCO 3 CaO + CO 2 (8) CaCO 3.MgCO 3 CaO + MgO + 2 CO 2 (9) 2 CO C + CO 2 (10) (MnO) + [C] [Mn] + CO (11) (SiO 2 ) + 2 [C] [Si] + 2 CO (12) (P 2 O 5 ) + 5 [C] 2[P] + 5 CO (13) 2 (MnO) + [Si] (SiO 2 ) + 2[Mn] (14) (CaO) + [S] + [C] (CaS) + CO (15)

5 CH 4 C (dust) + 2 H 2 (16) Macroscopic Balance on RS In the RS, there are 3 raw materials (Iron ore, flux and reduction gases) and 2 products which are DRI and gases. The same technique applied in the MG is repeated here. The main reactions taking place in the RS are reactions 4, 5, 6, 7, 8 and 9 shown above. Process Constraints In addition to the well-defined givens stated above, Figure 3 and Figure 4 show another type of input data which are the raw materials constraints and products constraints respectively. Figure 3 Raw material constraints for the process SCANMET IV 4 th International Figure 4 Products Conference constraints Process for the Development process

6 Oxygen Amount in Nm3/Ton Ore Because of the system complexity and the difficulty of assuring constant compositions of the raw materials and products, it is normal to have some data in the form of a range of feasible values. These data could be regarded as constraints that should be fulfilled to assure true solution. From a Degree of Freedom perspective, there will be infinite number of solutions for the combined MG and RS balance; however only one solution will be the optimum. Parametric Study Using the Developed Macroscopic Model The effect of the main operational parameters is very important for better understanding of the process. This is achieved by conducting a parametric study. After building the macroscopic model as shown in the previous steps, it becomes easy to perform the parametric study. Several simulation runs have been conducted, and the results can be shown as curves. In each run, all the process constraints have been fulfilled. In this work, the parametric study has been performed on the freeboard zone in the MG as it is considered the real process innovation. The parameter that has been changed in the different runs was chosen to be the percentage of iron ore fines added to the MG. This parameter was specifically chosen so as to assess its effect as an additive to the freeboard zone (referred to later herein as dome ) in the MG. From Figure 5, it is apparent that at constant fuel amount and flux amount, as percentage ore fines increase at constant oxygen amount, the dome's exit temperature decreases. This can be attributed to the increased heat load needed to preheat the ore fines, beside the endothermic nature of the iron ore reduction (note that the heat supply is constant because of the constant amount of oxygen). Conversely, it is apparent that as percentage ore fines increase at constant exit dome temperature, the amount of needed oxygen increases. This is attributed to the increased heat load needed to preheat and reduce the ore fines, and at the same time maintain the dome's temperature within the needed range % Ore Fines T = 1100 C T= 1075 C T= 1050 C T=1025 C T=1000 C Figure 5 Effect of iron ore fines on the dome s temperature and oxygen consumption As shown in Figure 6, as percentage of ore fines increases, the composition of CO 2 in the export gas increases, at nearly constant degree of metallization in RS. This is attributed to the increased amount of CO converted to CO 2 as a result of the iron fines reduction in the MG's free board. It is to be noted that high composition of CO 2 in the export gas lowers its

7 Iron Bearing Materials in kg/thm % CO2 out of RS calorific value, and consequently affects the performance of the dependent facilities. Moreover, and because of having lower iron percent than the iron ore pellets, as the amount of iron ore fines increase, the total amount of iron bearing materials increase accordingly. Thus, this simplified parametric study has clarified that the ore fines can be used as a temperature control parameter for the free board where excessive temperatures can be prevented. In addition, the addition of ore fines should take into consideration the composition of export gas produced, and its effect on the subsequent applications. Moreover, the addition of ore fines should take into consideration the dome's temperature, and its effect of the cracking of volatiles, and consequently the environmental impacts Iron Bearing Materials in kg/thm % CO2 out of RS % Ore Fines Figure 6 Effect of iron ore fines on the composition of export gas and total amount of iron bearing materials Determination of the Most Flexible Operation Scenarios During operation, and especially for this complex system, the process engineers should have a degree of insight to the process. This can be achieved by having a diagram like that shown in Figure 7. This diagram has been created by performing several runs on the developed model. This diagram combines the most important process variables which are the oxygen and fuel amount, percentage ore fines, and the dome's exit temperature. For certain amount of charged fuel (coal + coke), the amount of ore fines to be added can be determined. Then, its effect on the feed amount of oxygen, and on the energy and environmental efficiency of the process, represented in the dome's exit temperature, can be concluded. From the diagram, it is easy to know that at certain fuel amount, increasing percentage ore fines beyond a certain value will not achieve all the process constraints. For example, the curve corresponding to fuel amount of 935 kg/thm has no operation points at 4% ore fines. The latter means that on using 4% ore fines, one or more of the process constraints will be violated. It is to be also noted that the chosen scenarios in Figure 7 are the most flexible. At lower amounts of fuel, it is difficult to utilize iron ore fines, and it is also difficult to reach high exit dome temperature. At higher amount of fuel, large amount of coal volatiles are produced causing temperature drop in the dome, and excessive amount of reduction gases. In this case, purging of some gases is essential which is of course bad energy utilization.

8 Figure 7 Most flexible operation scenarios developed by the macroscopic model

9 Determination of the Best Operation Modes The iron and steel market is subjected to a lot of changes in terms of its raw material prices. Thus, for every market status, the best operation mode for COREX process should be defined. The following four operation modes have been chosen, and the developed macroscopic model has been used to determine the values of the corresponding main process variables. Table 1 summarizes the results. a) Mode 1: Maximization of iron ore fines utilization b) Mode 2: Minimization of fuel amount c) Mode 3: Minimization of iron bearing materials amount d) Mode 4: Minimization of oxygen feed amount Table 1 Main process variables corresponding to the main modes of operation Main Variables Mode 1 Mode 2 Mode 3 Mode 4 1) % Ore Fines ) Iron Bearing Materials in kg/thm ) Fuel Amount in kg/thm ) O 2 Amount in Nm 3 /THM ) Dome Exit Temperature in o C Determination of the Optimum Mode of Operation Regarding the current raw material prices, and with the objective to achieve the minimum production cost, the optimum mode of operation was found to be the minimization of oxygen feed amount. The high consumption of pure oxygen is a characteristic in COREX process. However, it should be noted that this is the optimized mode with regards to the production cost minimization. Sometimes, the environmental pressures may be stronger. For example, the dome's exit temperature corresponding to oxygen feed minimization is only 1000 o C. This temperature may be not enough for the cracking of all the evolved coal volatiles, and this of course will affect the environmental performance of the facility. Consequently, this mode of operation may not be applied in case of stringent environmental pressures. Conclusions COREX process is a multi-phase, multi-component system where varying one process input can lead to operation instability and/or result in off-spec products. To deal with such complex features, macroscopic modeling approach has been applied using Jindal plant data. The developed macroscopic model has proved that there is a wide range of operation reflecting the process flexibility. The most flexible operation scenarios can be determined and summarized in a chart to be used in the process control as a guide. The direct addition of ore fines inside the melter-gasifier can be used as a temperature control parameter for the free board so as to prevent excessive temperature increase; however, it should take into consideration the composition of export gas produced, and its effect on the subsequent applications, and in addition it should take into consideration the dome's temperature, and its effect on the cracking of volatiles, and achieving the environmental constraints. According to the market status, there are four main operation modes for COREX process, where the minimization of oxygen feed amount was found to be the

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