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Second International Conference on CFD in the Minerals and Process Industries CSIRO, Melbourne, Australia 6-8 December 1999 NUMERICAL ANALYSIS FOR MOVING BED IRON SCRAP MELTING FURNACE 1 1 1 2 1 X. ZHANG , H. NOGAMI , R. TAKAHASHI , T. AKIYAMA and J. YAGI 1 Institute for Advanced Materials Processing, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, JAPAN 2 Department of Mechanical Engineering, Miyagi National College of Technology, 48 Aza Nodayama, Shiote, Medeshima, Natori, 981-1239, JAPAN R Gas constant [J/mol K] ABSTRACT r Radial distance [m] Re Reynolds number ( = Gi d j / µi ) [-] The reduction and melting of oxidized iron-scrap briquettes containing coke breeze in a moving bed reactor Rk Reaction rate of k-th reaction [kg/s m3-bed] has been proposed from the viewpoints of energy saving, Rm Melting rate [kg/s m3-bed] recycling and environmental protection. The aim of this S Source term [(kg m/s, J, kg)/ s m3-bed] study is to investigate the effect of the briquette on Ssg Mass transfer from solid to gas [kg/s m3-bed] operation of the reduction-melting furnace. For this T Temperature [K] purpose, a total mathematical model of the reduction- u Vertical velocity [m/s] melting furnace has been developed based on the rates of v Radial velocity [m/s] ! briquette reduction and solid iron carburization and wetted v Velocity vector [m/s] area in a trickle bed. 2 We Weber number ( = Gi / a j ρiσ i ) [-] x Vertical distance [m] The numerical simulation of the reduction-melting furnace Superscript described three-phase flow phenomena with chemical * Critical reactions and phase changes; specifically, the distributions Subscript of temperature, gas concentration, reduction degree and b Briquette carburization degree in the furnace were calculated. The bb Binder in briquette simulation results under different operating conditions bc Carbon in briquette showed that stable operation is obtained with blowing of c Coke preheated air at 673K or with 8% oxygen enrichment to g Gas air and with coke ratio fixed at 530 kg/thm. i,j Phase k Reaction number NOMENCLATURE l Liquid a Area [m2/m3-bed] ∆Hc n Gas species (O2, CO2, H2O, CO, H2, N2) CD Drag coefficient [-] Greek Cp Specific heat [J/kg K] φ Dependent variable [(kg m/s, J, kg)/kg] d Mean particle diameter [m] Γ Diffusive transport coefficient [kg/m s] fc Gasification degree of a briquette [-] ε Volumetric fraction [m3/m3-bed] ! F Volumetric momentum flux [N/m3] η Distribution ratio of reaction heat [-] Fr Froude number ( = a j Gi 2 / ρ i 2 g ) [-] λ Thermal conductivity [W/m K] Fr,ij Radial interaction force between phases i and j µ Viscosity [Pa s] [N/m3] ν Stoichiometric coefficient [-] Fx,ij Vertical interaction force between phases i and j θ Contacting angle [degree] [N/m3] ρ Density [kg/m3] g Gravitational force [m/s2] σ Surface tension [N/m] G Mass flow rate [kg/m2s] ∆Hc Enthalpy transfer [W/ m3-bed] INTRODUCTION ∆Hk Enthalpy change [J/ kg] Briquettes of oxidized iron-scrap containing coke breeze h Enthalpy [J/kg] are attracting much attention as a new raw material for hij Heat transfer coefficient between phases i and j ironmaking. It offers two benefits; enhancing the reduction [W/m2K] of iron oxide and decreasing the melting temperature of kck Surface reaction rate constant (k=1-4) [m/s] burden due to carburization of iron. However, the effect of kfn Mass transfer rate of n component [m/s] the briquette, properties and their performance in a kk Reaction rate constant for reaction k [1/s] moving coke-bed reactor is not well understood. This m Fractional mass [-] paper, therefore, deals with a mathematical model of a M Molecular weight [kg/kmol] moving bed reactor for melting scrap, where the effect of Nc Dimensionless surface tension, Nc=(1+cosθ) [-] the briquette containing coke breeze is assessed. The P Pressure [Pa] briquette and coke are charged into the furnace (see Figure Pr Prandtl number [-] 293 1), in which three-phase flow phenomena exists, together Gravitational flow of packed particles was described by with phase changes and several major reactions such as the kinematic model (Nedderman and Tuzun, 1979), in combustion, reduction and carburization. The briquette which the horizontal (radial) velocity is proportional to the moves down slowly, starts melting in a cohesive zone and vertical velocity gradient in the horizontal direction trickles in the form of hot metal and molten slag in the ( vs = − B(∂ us / ∂ r ) ). The value of B, the kinematic constant, lower part of the furnace. In contrast, cold air blasted is an empirical coefficient (is set equal to) 2.5 times the through tuyeres flows up through packed materials. For particle diameter. By substituting this relationship into the the simulation of this reactor, three key parameters must continuity equation for packed bed of particles, an be known; the rates of reduction and carburization, and equation is derived which is similar to the diffusion wetted area. these were experimentally evaluated before equation for the vertical velocity component: the development of the mathematical model. Numerical simulation was finally carried out for analyzing the effect ∂ us 1 ∂ ∂ us (2) of the briquette containing coke breeze on reduction = rB − Rm − Ssg ∂x r ∂r ∂r degree and temperature distributions within the reactor. Here, Rm is melting rate and is given by the rate of heat Briquette transfer to particles at the melting temperature. i φi Γ φi S φi Preheating Coke Reduction g 1 0 Ssg ∂Pg Molten iron g ug µg − εg − Fx , gs − Fx , gl ∂x Melting ∂Pg vg g vg µg − εg − Fr , gs − Fr , gl − ε g µ g 2 ∂r r Tuyere λg/Cpg ∑ a gi hgi (Tg − Ti ) + ∑ η gk ( − ∆H k ) Rk c Dripping + ∆H sg g hg i≠ g k Hot metal and slag g mn ρgDg ∑ν nk Rk Hearth k Tap hole s 1 0 − Rm − S sg Figure 1: Schematic diagram of the reduction-melting ∑a si hsi ( Ts − Ti ) + ∑η sk ( − ∆H k ) Rk furnace. s hs λs/Cps i≠s c c k − ∆H sg − ∆H sl s mFe 0 -Rm l 1 0 Rm MATHEMATICAL MODEL l ul µl − ε l ρ l g + Fx , gl − Fx , ls 1. Governing Equations vl l vl µl Fr , gl − Fr ,ls − ε l µ l 2 In order to estimate the effectiveness of different r briquettes (e.g. carbon content), a mathematical model, l hl λl/Cpl ∑a li li c h (Tl − Ti ) + ∆H sl which simulates all phenomena of heat transfer, fluid flow i ≠l and chemical reactions, is developed. The mathematical model consists of equations for conservation of mass, Table 1: Dependent variable, diffusive transport momentum, thermal energy and rate equations of heat coefficient and source term. exchange, chemical reactions and scrap melting. The methodology for mathematical modeling of multiphase Each conservation equation was discretized over the flows has been described in previous papers (Yagi et. al., control volume (Patankar, 1980) and numerically solved 1992a, 1992b and 1994). Based on these investigations, with a convergence criteria of less 0.1 % of fractional transport phenomena can be described by the following residuals under the boundary conditions. Temperature general equation with several assumptions, i.e. steady state dependence of all properties such as specific heat was continuous flow, axisymmetry and no temperature taken into consideration in the computation. gradient within packed materials. 2. Major Reactions ∂ (ε i ρ iuiφi ) 1 ∂ (rε i ρ i viφi ) The various reactions considered in the mathematical + (1) model are shown in Table 2. For convenience, iron ∂x r ∂r melting is also shown, though a phase change is not a ∂ ∂ φi 1 ∂ ∂φ = (ε i Γφi )+ (rε i Γφi i ) + Sφi chemical reaction. The rate of each reaction was ∂x ∂x r ∂r ∂r incorporated as follows. The dependent variable φi, diffusive transport coefficient Γφi and source term Sφi are described in Table 1. The Coke and Gas Phase Reactions source term includes interaction forces and gravity for In the reactions of coke combustion and coke gasification, momentum transfer, melting rate for mass transfer and the total reaction rates, including chemical reaction and heat exchange, reaction heat and melting heat for thermal gas laminar film diffusion, were applied (Muchi et. al., energy transfer. 1966, Field et. al., 1967, Heynert et. al., 1959). Here, the reaction rate of complete combustion of C and O2 was 294 evaluated using the ratio between CO2 and CO (Arthur, * For B1 briquette: f c = 1 - exp(6.91 - 0.00568Ts ) 1951). ( 89.12 exp − 138 × 10 3 / RTs k8 = ) (f c ≤ f c * ) Rk = ac 12 ρ gε g (3) ( 3.633 exp − 116 × 10 / RTs 3 ) * (f c > f c ) − k −1 + kck1 Mn fn For B2 briquette: f c * = 1 - exp(8.18 - 0.00675Ts ) k =1,3,4 n =1,2,3 k8 = ( 27.94 exp − 123 × 10 3 / RTs ) * (f c ≤ f c ) R2 : C4 / C2 = 2500 exp(− 6240 / Ts ) (4) ( 3 0.214 exp 6.8 × 10 / RTs ) * (f c > f c ) In the combustion reaction of CO, the Howard’s equation Carburization Rate (1973) was employed. The carburization rate of solid iron by CO was obtained 0.5 0.5 ( R5 = 3.64 × 10−10 ε g C4 C1 C3 exp − 15106 / Tg ) (5) by thermo-gravimetrical analysis. The diffusion coefficient of carbon in solid iron was also obtained from the carbon content distribution in solid iron by EPMA analysis We assumed that the water gas shift reaction and (Zhang et. al., 1997). combustion of H2 were equilibrium reactions. The carburization mechanism can be described as a two Reaction No. step process of surface carburization and diffusion of Combustion C + 1/2 O2 = CO 1 carbon into solid iron. The surface carburization reaction and C + O2 = CO2 2 proceeds via two elemental reactions, as described below; gasification C + CO2 = 2CO 3 CO = O(ads)+C(in Fe) of coke C + H2O = CO + H2 4 O(ads)+CO= CO2 CO + 1/2 O2 = CO2 5 Here, dissociation of CO is in equilibrium and elimination Gaseous CO + H2O = CO2 + H2 6 of O adsorbed is rate-determining step. As a result, the H2 + 1/2 O2 = H2O 7 reaction rate can be expressed below. Reduction FenOm + x C= n Fe 8 2 K 9 p CO + (2x-m) CO + (m-x) CO2 ′ a C p CO 2 (7) R9=k 9 − k9 Carburization 2CO = C(in Fe) + CO2 9 K 9 pCO+a C K 9 p CO+a C Melting Fe(s) = Fe(l) 10 K9 = 5.34×10-13 exp( 168.8×103/RT) k9 = 1.61×10-8 exp(-42.1×103/RT) Table 2: Reactions in the reduction-melting furnace. k9′= 3.31×10-6exp(-41.6×103/RT) Reactions of Briquette Iron Melting Rate Two kinds of briquette having different amounts of coke The iron melting rate was calculated using equation (8) breeze were used in the numerical simulation. These are which was based on the assumption of the heat transfer described in Table 3. being rate-limiting. T.Fe M.Fe Fe2+ Fe3+ O C C/O Rm = ags hgs (Tg-Tm)/∆Hm (8) B1 70.0 17.1 43.5 9.4 16.4 7.4 0.60 The melting point, Tm, was regarded as the surface melting B2 67.0 17.7 42.3 7.0 15.1 11.2 0.99 temperature of iron. It was measured using a hot-stage C/O: Molar ratio of carbon and oxygen microscope. Table 3: Chemical composition of briquettes (mass%). 3. Parameter Evaluation Reaction rates for the reduction of iron oxide, the gasification of coke and the thermal decomposition of the Before carrying out a numerical analysis using the binder in the oxidized iron-scrap briquette containing coke mathematical model, unknown parameters, such as contact breeze were utilized. The reaction of a single briquette was area, exchange of momentum and heat transfer between studied experimentally by measuring changes of weight heterogeneous phases, must be formulated. and gas volume at fixed temperature in a nitrogen Contact Area between Heterogeneous Phase atmosphere (Zhang et. al., 1995). The result was that reaction of the briquette is coincident with reactions of Obviously, liquid generation decreases both voidage and gasification of coke by CO2 gas and reduction of iron the contact area between gas and solid in the reduction- oxide by CO gas. The reduction of iron oxide is in an melting furnace. We assumed in this mathematical model equilibrium step and the gasification reaction is a rate- that the dynamic hold-up of the liquid was assumed to be limiting step. It was, therefore, concluded that the volumetric fraction of the liquid. reduction rate of iron oxide is evaluated from the gasification rate shown below. Equation (9) can be derived when occupied ratios of the three-phase (gas, solid and liquid) are expressed by their (6) volume fraction εI, respectively. R8 = k 8 mbc ρ b ε b εg+εs+εl=1 (9) 295 agl is the contact area between gas and liquid obtained by gas composition within the furnace. The good agreement using Mada’s equation (Mada, 1963), and als the contact provides support for the underlying assumptions made in area between liquid and solid obtained by using Niu’s the model. equation (Niu et. al., 1996), which is the equation Temperature (K) 500 1000 1500 2000 modified from Onda’s equation (Onda et. al., 1967). ags is the contact area between gas and solid calculated as the 1.0 Obs. Calc. Temp. difference in surface area of solid and contact area Height from tuyere (m) O2 between liquid and solid. They can be expressed as CO 2 CO following equations. 0.5 Conditions: - / / a gl = 0.34 Frls 1 2Wels 2 3 /d s (10) Blast: air 298K als = 6ε s {0.4(Rels / ε s )0.218Wels 00428 Fr ls -0.0238 Nc − 0.0235} (11) 3 6Nm /min ds Coke: 100mm φ a gs = 6ε s /d s - a ls (12) 298K 0.0 0 10 20 30 Gas composition (%) Figure 2: Comparison between calculated and measured Exchange of the Momentum longitudinal distributions of gas composition The interaction force between gas and solid was evaluated and temperature. using Ergun’s equation developed by considering the liquid-gas interaction. It is described by equation (13). MATHEMATICAL SIMULATION " 150µ g a s a gs 1.75ρ g a gs ! ! ! ! (13) 1. Simulating Conditions Fgs = + |v g − v s|(v g − v s ) 36(1 − ε s ) 6 Size of the Furnace The interaction force between dripping liquid and rising The reduction-melting furnace was a cylindrical vessel of gas through pore space in the packed bed was evaluated 1 m inner diameter and 4.5 m effective height. The using Fanning’s equation. This was developed by distance from the tuyere level to the molten iron surface considering both the flows of gas and liquid in the packed was assumed as 400mm. The tuyere opening was 10 % of bed and the contact area between gas and liquid. It is the cross-sectional area of furnace, with a slit width of 25 represented as equation (14). mm. " a gl 3CD ρ g ε l ! ! ! ! Coke Gas (14) Briquette Fgl = |v g − vl|(v g − vl ) a gl + a sl 4d l 4m The interaction force between liquid and solid in the reduction-melting furnace was evaluated by the Kozeny- Ó ƒ 1m 3 Carman equation developed using the contact area 4.5m between liquid and solid. It is given as equation (15). 2 " 180µl as asl ! ! (15) Fls = (vl − vs ) 36(1 − ε s ) x 1 0.025m Blast 0.4m r Heat Transfer 0 0.5m @ 0 The heat transfer coefficients between gas and solid and Figure 3: Size and grid arrangement for reduction-melting between gas and liquid represented as equation (16, 17) furnace. were evaluated using the Ranz-Marshall equation as modified by Akiyama et. al. (1990). The heat transfer Since the furnace was axisymmetrical, the region for coefficient between liquid and solids was evaluated by the numerical analysis was decided as the half of the furnace. equation (18), which is for forced convection heat transfer The flow domain was represented by a 90 by 15 grid, with proposed by Pohlhausen (1921). finer resolution close to the tuyere. hgs = (2.0+0.39Regs1/2Prg1/3)λg/ds (16) Boundary Conditions 1/2 hgl = (2.0+0.39Regl Prg ) λg/dl 1/3 (17) The boundary conditions are given as follows: no flux condition on center axis, zero velocity on the bottom of hls = (0.664Rels1/2Prl1/3) λl /ds (18) the furnace, slip condition on the wall and free boundary at the top. For the wall and the bottom of the furnace, the 4. Experimental Verification heat loss was also considered. Before numerical simulations of the moving bed reactor Regarding the gas phase, the flow rate, the composition were conducted, the mathematical model, together with and the temperature of the inlet gas were specified as rate parameters summarized above, were experimentally constant values and the pressure of outlet gas was verified using a laboratory-scale combustion furnace of a specified as an atmosphere at the furnace top. Regarding coke bed. Figure 2 shows the comparison between the solid phase, burden materials are continuously observed and calculated distributions of temperature and 296 supplied at the furnace top, and the temperature and the (b) is with 8% oxygen enrichment to air with out coke ratio were fixed as constant values. Regarding the preheating. liquid phase, it is formed in the melting zone and continuously discharged from the furnace bottom. 600 800 1000 4m Operating Conditions 1000 The temperature and physical properties of charged 120 0 burden materials for the numerical simulation of briquette 0 3 120 melting process are given in Table 4. 2 Temp. Diameter Porosity Density Material 1400 (K) (mm) (-) (kg/m3) 1400 Coke 298 50 0.52 1000 1 1600 Briquette 298 35 0.48 3280 1640 1660 1600 18 1680 00 20 180 Table 4: Operating conditions for briquette melting 0 0 00 1700 1720 process. Liquid(K) Solid(K) Gas(K) (a) Hot air blast (673K) 2. Results and Discussion 600 Reduction Degree of Briquette 800 1000 4m Figure 4 shows the distributions of reduction degree of the 1000 briquettes, B1 and B2 under the condition of blowing 120 0 3 preheated air at 673K. Significant differences in the results showed that the final reduction degree for the operation 120 0 with briquette B2 was 30% higher than that with briquette 2 B1. The high reduction degree of briquette B2 was caused by a high coke breeze content in the briquette. This clearly 1400 demonstrated the enhancement in reduction rate of 1400 1 briquette with increasing carbon content. 1640 160 0 1660 160 18 0 00 1680 1700 0 0 % 0 % 4m 1800 2600 Liquid(K) Solid(K) Gas(K) 3 (b) Oxygen-enriched air blast (8%O2) 10 10 Figure 5: Computed isothermal lines of liquid, solid and 20 20 gas for charging briquette B2. 30 30 2 40 40 50 In case (a), gas temperature rose to 2400K around the 50 60 60 1 tuyere due to coke combustion, and then decreased to 90 860K at the outlet of the furnace. The charged briquette was heated up while descending and a melting zone 0 appeared in the lower part of the furnace. The molten iron B2 B1 from the melting zone was heated to 1730K by gas and Figure 4: Computed distributions of reduction degree coke. Temperatures of gas and solid phases showed strong of briquette B1 and briquette B2. radial distribution in the upper part of the melting zone due to heat loss from the wall. Condition in Stable Operation In case (b), the gas temperature rose to 2600K around the The briquette reduction-melting process was analyzed tuyere. This maximum temperature was 200K higher than numerically by changing the air preheating temperature ( that in case (a), however, the high-temperature region was 313 ~ 873 K ) and oxygen enrichment ( 0 ~ 14% ), when narrower. Therefore, the melting zone was no longer charging briquette B2 and keeping coke ratio at 530 horizontal, being higher near the wall and lower at the kg/thm. According to the numerical results, the briquette center of the furnace. The temperature of molten iron was was not well melted due to the shortage of high 30K lower than that in case (a), though the gas temperature heat in the lower part of the furnace when the temperature was higher. blast is not preheated to over 600K, or the oxygen Gas Composition Distribution enrichment is below 6% in the case of an ambient temperature blast. Figure 6 shows distributions of gas composition under the conditions of charging briquette B2 and blowing Figure 5 shows numerical simulations of temperature preheated air at 673K. In the region very close to the distributions of gas, solid and liquid in the reduction- tuyere, oxygen was quickly consumed through coke melting furnace when charging briquette B2 and keeping combustion. The subsequent consumption of carbon coke ratio fixed at 530 kg/thm. Case (a) is with air dioxide by the Boudouard reaction led to a significant preheated at 673K but no oxygen enrichment, and Case decrease in its concentration in the radial direction. 297 Correspondingly, the concentration of carbon monoxide increased in the direction away from the tuyere. The The numerical simulation describes three-phase flow utilization factor of carbon monoxide was low due to the phenomena, with chemical reactions and phase changes. use of small particle coke. The carbon monoxide should The distributions of temperature, gas concentration, be recycled. reduction degree and carburization degree in the reduction-melting furnace are calculated. The reduction degree distribution showed that the charging of briquette 4m B2 was better than the charging of briquette B1. This was due to a higher content of coke breeze in briquette B2 than briquette B1. Simulation results for different operating 3 conditions showed that stable melting of the oxidized iron- scrap briquette was been obtained under preheated air blowing at 673K or with 8% oxygen enrichment to air 2 2 blowing in the case of coke ratio fixed at 530 kg/thm. 36 1 32 4 REFERENCES 2 0 0 AKIYAMA T., TAKAHASHI R. and YAGI J., (1990), 4 16 Tetsu-to-Hagane, 76, 848 - 855. CO(%) CO2(%) O2(%) ARTHUR J.A, TRANS, (1951), Faraday Soc., 47, 164. Figure 6: Computed gas concentration for charging ERGUN S., (1952), Chem. Eng. Progr., 48, 89 - 94. briquette B2 with blowing preheated air at FIELD M.A, GILL D.W., MORGAN B.B. and 673K. HAWKSLEY P.G.W., (1967), Combustion of Pulverized Coal, BCURA, Leatherhead, Cherey and Sons, Banbury, Solid Flow Characteristic England. HEYNERT G. and WILLEMS J., (1959), Stahl u. Eisen, Figure 7 shows the solid flow characteristics and melting 79, 1545 - 1554. ratio of iron under the condition of charging briquette B2 HOWARD J. B., WILLIAMS G. C. and FINE D. H., and blowing preheated air at 673K. The solids charged at (1973), 14th Int. Symposium on Combustion, Pittsburgh, the top of the furnace descended at uniform speed in the 975 - 986. upper part. Near the combustion zone the flow of solid is MADA J., SHINOHARA H. and TSUBAHARA M., directed towards the tuyere. Under the tuyere region, an (1963), Kagaku Kougaku, 27, 978 - 982. stagnant region was formed. The residence time of MUCHI I., YAGI J., TAMURA, K. and MORIYAMA briquettes from the top of the furnace to the melting zone A., (1966), J. Japan Inst. Metals (Japanese), 30, 826 - 831. was about 1 hour, as shown by the time lines. The melting NEDDERMAN R. and TUZUN U., (1979), Powder zone formed was almost horizontal with a thickness of 0.2 Tech., 22, 234 - 238. m. NIU M., AKIYAMA T., TAKAHASHI R. and YAGI J., (1996), AIChE J., 42, 1181 - 1186. 0.4 ks NIU M., AKIYAMA T., TAKAHASHI R. and YAGI J., 4m 0.8 (1996), Tetsu-to-Hagane, 82, 647 - 652. 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CONCLUSION ZHANG X., TAKAHASHI R. and YAGI J., (1997), Oxidized iron-scrap briquettes containing coke breeze Tetsu-to-Hagane, 83, 299 - 304. were investigated for use as a new raw material for hot metal production. A total mathematical model of the reduction-melting furnace has been developed based on the theory of mass and heat transport phenomena, and reaction kinetics. The model was experimentally verified by using a laboratory-scale furnace consisting of a coke bed. 298