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Simulation of sintering of iron ore packed bed with variable porosity S. V. Komarov and E. Kasai Institute of Multidisciplinary Research for Advanced Materials Tohoku University Japan Phoenics User Conference Melbourne,2004

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Flowchart of steel production

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Sintering process concept region of interest

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A schematic representation of sintering process Sintered part Heat wave Initial materials: 1.Blend ore 2.Coke 3.Limestone Preheated air Exhaust gas: N 2,O 2,CO 2

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Principle of big pellet aging Induction bed for combustion/sintering Large pellets for aging

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Objective of this study Development of a Phoenics-code based model which could predict influences of such parameter as - void fraction - pellet size -initial temperature and flow rate of gas -coke and limestone content -ignition time on heat propagation over induction bed to large pellets Why simulation ? There are many parameters involved, which determine the system behavior. An experimental investigation would be too hard and costly. Why Phoenics ? Many thanks to friendly and highly skilled support team in Tokyo

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Computational domain and its physical prototype z r OA BC Exhaust gas outlet Air inlet Wall Axis Sphericalpellet: - 0 = R = 2.5 cm -d p =0.5 mm -Fe 2 O cm 8.0 cm Preheated air Induction bed : - 0 =0.4~0.9 -d p =2 mm -Fe 2 O 3,C CaCO 3 Packed bed

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The sintering process chemistry 1.CaCO 3 =CaO+CO 2 Q 2 = – J/kg 2.C+O 2 =CO 2 Q 1 = J/kg 3.CaO+Fe 2 O 3 =(CaO·Fe 2 O 3 ) Q 3 = – J/kg 4.(CaO·Fe 2 O 3 )=CaFe 2 O 4 Q 4 = J/kg Hematite (Fe 2 O 3 ) – 0.82 Carbon(C) – 0.03 Limestone (CaCO 3 ) – 0.15 Preheated air Hematite (Fe 2 O 3 ) – 1.0

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The process related physical phenomena 1.Momentum transfer 2.Two phase heat transfer - convection (gas) - diffusion (gas,solid) - radiation (interparticle space) - heat exchange (gas-solid interface) - heat generation (C combustion) - heat absorption (CaCO 3 decomposition, CaOFe 2 O 3 melting) 3. Mass transfer (only gas phase) - convection (O 2,N 2,CO 2 ) - diffusion (O 2,N 2,CO 2 ) - gas sourcing (CO 2 ) and sinking (O 2 ) Preheated air

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Kinetics of graphite combustion Diffusional control Kinetic control r Y O2 T dcdc C+O 2 = CO 2 combustion rate specific area overall rate coefficient chemical reaction rate coefficient mass transfer rate coefficient k 0 = (m/sK 0.5 ) E a = (J/molK)

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Sherwood and Nusselt numbers for sphere Sh

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Kinetics of the other reactions 1.CaCO 3 =CaO+CO 2 2. CaO+Fe 2 O 3 =(CaO·Fe 2 O 3 ) 3. (CaO·Fe 2 O 3 )=CaFe 2 O 4 Assumptions 1.The reaction rates are controlled by heat supply (1,2) or removal (3) 2.The reactions proceed within a temperature interval T around the corresponding thermodynamic temperature T d Example for reaction (1) T=10 T d =1123 K f 1 – function of kinetic factor r l – reaction rate Q l – reaction heat Q c – graphite combustion heat r c - graphite combustion rate Heat supply rate

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Initial porosity Wall effect Mathematical formulation RBRB B A rBrB Transition zone B A

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Equation of motion where Ergun equation d p - particle diameter - void fraction (porosity) g - gas viscosity g - gas density

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Equations of continuitymass conservation Equations of continuity and mass conservation r c is the carbon combustion rate r l is the lime decomposition rate M i is the molecular weight C+O 2 = CO 2 CaCO 3 =CaO+CO 2 (i = CO 2,O 2,N 2 )

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Equation of energy conservation (gas phase) Concept of C combustion Gas-particle heat exchange rate O2O2 C Reaction front C+O 2 =CO 2 CO 2 +C=2CO CO+O 2 =2CO 2 - part of C combustion heat going directly to solid phase ( =0.5) (fixed flux)

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Equation of energy conservation (solid phase) Rad - radiative conductivity according to Rosseland diffusion model - Stephan-Boltzmann constant (= ), s - scattering coefficient - the reflectivity coefficient (=0.5), T s – solid temperature

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Equation of energy conservation (solid phase) Q i and r i are heat effect and rate of appropriate reactions l - CaCO 3 =CaO+CO 2 m - CaO+Fe 2 O 3 =(CaO·Fe 2 O 3 ) f,s - (CaO·Fe 2 O 3 )=CaFe 2 O 4

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Air (T a ) Boundary and initial conditions B A Air velocity at inlet Initial chemical composition and porosity Zone Fe 2 O 3 C CaCO 3 A B W1 is defined from condition g W1=const (1.2) V1 = 0 Initial temperature T g =T s =25 O C Air temperature at inlet

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Setting of solver options Grid type : BFC Time dependence: unsteady 1s 600 step = 600 s Flow : laminar One-phase mode (ONEPHS=T) Total number of iteration : 100 Global convergence criteria : 0.5% Equation formulation : Elliptic GCV Differencing schemes : Hybrid

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Example of calculated results.Velocity vector t = 90 s180 s330 s

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Carbon mass fraction and heat generation Carbon mass fraction Heat generation

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Solid temperature and limestone fraction Temperature of solid phaseLimestone fraction

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Solid temperature and melted phase fraction Temperature of solid phaseMelted phase fraction

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Solid temperature and solid phase fraction Temperature of solid phaseSolidified phase fraction

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Carbon mass fraction and void fraction Carbon mass fraction Porosity

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Conclusions Phoenics code has been applied to the problem of iron ore sintering process which includes coke ignition and flame front propagation through the sintering bed It is shown that Phoenics can be used to simulate transient two-phase problems under one-phase setting option Ground coding allows to simulate gas flow, heat and mass transfer through bed of variable porosity The predicted results seem to be realistic but the model needs to be validated against experimental data

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