Process Simulation and Control Optimization of a Blast Furnace Using Classical Thermodynamics Combined to a Direct Search Algorithm
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Caltech Authors Process Simulation and Control Optimization of a Blast Furnace Using Classical Thermodynamics Combined to a Direct Search Algorithm JEAN-PHILIPPE HARVEY and AI¨MEN E. GHERIBI Several numerical approaches have been proposed in the literature to simulate the behavior of modern blast furnaces: finite volume methods, data-mining models, heat and mass balance models, and classical thermodynamic simulations. Despite this, there is actually no efficient method for evaluating quickly optimal operating parameters of a blast furnace as a function of the iron ore composition, which takes into account all potential chemical reactions that could occur in the system. In the current study, we propose a global simulation strategy of a blast furnace, the 5-unit process simulation. It is based on classical thermodynamic calculations coupled to a direct search algorithm to optimize process parameters. These parameters include the minimum required metallurgical coke consumption as well as the optimal blast chemical composition and the total charge that simultaneously satisfy the overall heat and mass balances of the system. Moreover, a Gibbs free energy function for metallurgical coke is parameterized in the current study and used to fine-tune the simulation of the blast furnace. Optimal operating conditions and predicted output stream properties calculated by the proposed thermodynamic simulation strategy are compared with reference data found in the literature and have proven the validity and high precision of this simulation. DOI: 10.1007/s11663-013-0004-9 Ó The Minerals, Metals & Materials Society and ASM International 2013 I. INTRODUCTION chemical nature of emissions as well as the productivity of the BF. IRON making is the pyrometallurgical process of From a classical thermodynamic standpoint, a BF can turning solid iron ore materials into liquid hot metal be viewed as a complex system in which several local called pig iron typically saturated with carbon equilibrium states occur, as will be shown in the current (5 wt pct). The process uses injection of air and study. There are a no. of major phases experimentally metallurgical coke in a counter-current flow reactor observed and considered in the fundamental study of called a blast furnace (BF). This is probably one of the this reactor. The first is the gas phase, which originates metallurgical processes that has captivated the most from the combustion of metallurgical coke by air at the attention of scientists and engineers in the past. The bottom of the reactor. This results in N2- and CO-rich major chemical reactions occurring in the BF are (1) the compositions (reducing gas) producing a final exhaust combustion of metallurgical coke by hot air (potentially gas consisting mainly of N2, CO, CO2,H2, and H2O. enriched in O2) at the tuyere level, resulting in the This is a consequence of the reduction of the iron ore. production of a CO-rich reducing gas; and (2) the direct The second set of phases are present in the iron ore and indirect reductions of the iron-rich minerals present which contains iron-rich minerals such as hematite and in the ore by metallurgical coke and the reducing gas, magnetite, as well as impurities (SiO ,AlO , CaO, respectively. The composition of the available iron ore 2 2 3 MgO, Na2O, K2O, P2O5, and S) in various solid forms. may include impurities such as SiO2,Al2O3,P2O5, MgO, The third is the metallurgical coke injected at the top of CaO, Na2O, K2O, and S. Minor chemical reactions such the BF which contains impurities such as ashes, sulfur, as the reduction of these impurities can occur and affect and water. The fourth is the flux [CaMg(CO ) , CaO, drastically the quality of the final pig iron product, the 3 2 MgO, and CaF2] also introduced at the top of the BF and used ultimately to remove different impurities. The JEAN-PHILIPPE HARVEY, Ph.D. Postdoctoral Fellow, formerly fifth is the slag which is formed intentionally at the with the Center for Research in Computational Thermochemistry, bottom of the BF by introducing fluxes to help in ´ Department of Chemical Engineering, Ecole Polytechnique de Mon- removing undesirable impurities. The sixth is the metal- tre´al, C.P. 6079, Station Downtown, Montre´al, QC H3C 3A7, Canada, is now with the Geological and Earth Planetary Sciences Division, lic liquid solution (pig iron) which is the final valuable California Institute of Technology, Pasadena, CA. Contact e-mail: product of the BF. In considering the engineering design [email protected]. AI¨MEN E. GHERIBI, Ph.D. Research Fel- of such reactors, refractory materials used to protect the low, is with Center for Research in Computational Thermochemistry, inside of the BF from chemical attack of the different Department of Chemical Engineering, E´cole Polytechnique de Mon- tre´al. phases can also be considered as important phases. For Manuscript submitted July 18, 2013. a given iron ore chemical composition, the control of the Article published online December 12, 2013. quality of the pig iron is assumed to be dictated mainly METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 45B, FEBRUARY 2014—307 by the local equilibrium established at the bottom of the of the iron ore, input amount of metallurgical coke, BF between the pig iron and the slag phase. Partitioning input amount of O2-enrich air, etc.) with output of the impurities between the liquid pig iron and the slag variables of the process (CO/CO2 ratio in the exhaust will depend on the chemical composition of the slag gas, and Si content in the liquid pig iron) that cannot be phase as well as the temperature and pressure. directly adjusted in actual operations. For example, the Despite accumulated knowledge and experience, the CO/CO2 ratio in the exhaust gas is directly related to the continuous control optimization of the performance of a amount of metallurgical coke used in the BF. Its value BF is still a difficult task. This is because iron ore depends on the amount of hot blast air used, the local chemical compositions are not known exactly and are equilibrium conditions of temperature and pressure to not invariant over operating periods. Iron ore from which the exhaust gas is exposed as well as the other different sources can be used at subsequent periods of phases in equilibrium with this gas phase before it leaves the year depending on availability. Moreover, other the BF, the exhaust gas speed, the chemical composition variables not directly related to the chemical composi- of the iron ore, the global heat balance of the reactor, tion of the materials used in the BF will also modify the etc. Taken as a variable of the black-box model, the CO/ physicochemical behavior and hence the productivity of CO2 ratio will be directly correlated to the variables the BF. These include the sinter percentage of the iron mentioned previously which will create a physically pellets used in the BF, the blast pressure, the heat losses, meaningless model. the burden charging rate, the burden permeability, the Many of the problems quoted previously can be burden distribution practices, the no. of casting, etc. overcome using a thermodynamic classical model. For these reasons, intensive research has been under- Boundary conditions can be obtained for finite volume taken by scientists and engineers to model the BF methods if a complete overall heat and mass balance is process, in total or in part, using various numerical performed using classical thermodynamics calculations methods and tools. The current authors are aware of the along with adequate input/output streams. Key and abundant knowledge of the modeling of the BF. complex chemical reactions basics to several problems Comprehensive reviews on this subject may be found, can be identified from classical thermodynamic calcula- for example, in the study of Ghosh and Majumdar.[1] A tions. If defined correctly, a minimum no. of input, nonexhaustive list of the different approaches that can output and recycling streams, as well as equilibrium be used to model a BF may include the following: reactors, will be used to simulate the BF in the current study. The success of this simulation strategy lies in four (1) Mathematical models such as discrete element ingredients: (1) a good thermodynamic description of all method[2] or finite volume methods[3,4] used to sim- the potentially stable phases observed in the complex ulate chemical kinetics and transport phenomena. chemical system of interest; (2) a robust Gibbs free (2) Binary coding support vector machines algo- energy minimization algorithm to determine the equi- rithm,[1,5,6] neural network models,[7–10] and genetic librium state of complex multicomponent and multi- algorithms[11,12] all based on data-mining methods phase systems; (3) a tool to manage the thermodynamic and used to help optimizing control parameters of properties of each stream and reactor during the the BF. simulation, and (4) a numerical method for exploring (3) Heat and mass balance models.[13] and optimizing the various input streams properties to (4) Thermodynamic (or equilibrium) models.[14,15] solve simultaneously mass and heat balances and to Finite volume methods are attractive techniques if meet all the requirements of the BF. In the current precise temperature, pressure, chemical composition, study, the thermodynamic description of each poten- and flow profiles are to be evaluated throughout the BF; tially stable phase derives from FactSage software this is at the expense of a precise knowledge of several databases,[16] with the exception of the Gibbs free boundary conditions (chemical composition of the slag energy of metallurgical coke (also defined as amorphous and of the pig iron) and physical properties (viscosity, carbon) which is formulated in Section II–A.