NUMERICAL ANALYSIS FOR MOVING BED IRON SCRAP MELTING FURNACE by rma97348

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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.
                                                                                           ONDA K., TAKEUCHI H. and KOYAMA N., (1967),
                                1.2
                                                                                        J. Chem. Eng.,(Japanese), 31,.126 - 133.
                                              0.001 m/s




                                                                             3
                                1.6
                                                                                           PATANKAR S.V., (1980), Numerical Heat Transfer and
                                2.0                                                     Fluid Flow [Hemisphere Pub. Corp.].
                                2.4
                                                                             2             POHLHAUSEN, E., (1921), Z. Angew. Math & Mech.,
                                                                                        1, 115 - 121.
                                2.8
                                                                                           YAGI J., AKIYAMA T. and WANG J., (1992a),
                                3.2                                          1          Chemical Engineering, (Japanese), 37, 207 - 216.
                 0              3.6                                                        YAGI J., AKIYAMA T. and NOGAMI H., (1992b),
               100%                                                          0          Chemical Engineering, (Japanese), 37, 769 - 779.
                                                                                           YAGI J. and NOGAMI H., (1994), Proc.4th Asian Conf.
            Melting ratio   Timelines                         Velocity                  on Fluidized-Bed & Three-Phase Reactors (edited by
Figure 7: Computed melting ratio, time line and velocity                                YOSHIDA K. and MOROOKA S.), 235 - 240, Fukuoka,
          vector of solid for charging briquette B2 with                                Japan.
          blowing preheated air at 673K.                                                   ZHANG X., TAKAHASHI R. and YAGI J., (1995),
                                                                                        Tetsu-to-Hagane, 81, 1043 - 1048.
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.



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