Transcript No Slide Title

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
Flowchart of steel production
Sintering process concept
region of interest
A schematic representation of sintering process
Preheated air
Sintered part
Heat wave
Initial materials:
1.Blend ore
2.Coke
3.Limestone
Exhaust gas: N2,O2,CO2
Principle of big pellet aging
Induction bed
for combustion/sintering
Large pellets
for aging
Why simulation ?
Why Phoenics ?
There are many parameters involved,
which determine the system behavior.
An experimental investigation would
be too hard and costly.
Many thanks to friendly and
highly skilled support team in
Tokyo
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
Computational domain and its physical prototype
Packed bed
Air inlet
O
Preheated air
A
- 0= 0.25
-R = 2.5 cm
-dp=0.5 mm
-Fe2O3
8.0 cm
Axis
Wall
Spherical
pellet:
Induction
bed :
z
B
r
Exhaust gas outlet
C
-0=0.4~0.9
-dp=2 mm
-Fe2O3,C
CaCO3
4.0 cm
The sintering process chemistry
Hematite (Fe2O3) – 1.0
Preheated air
1. CaCO3=CaO+CO2 Q2 = –1.61106 J/kg
2. C+O2=CO2 Q1= 3.28107 J/kg
3. CaO+Fe2O3=(CaO·Fe2O3) Q3= –1.37106 J/kg
4. (CaO·Fe2O3)=CaFe2O4 Q4=5.07105 J/kg
Hematite (Fe2O3) – 0.82
Carbon(C)
– 0.03
Limestone (CaCO3) – 0.15
The process related physical phenomena
Preheated air
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 (CaCO3 decomposition,
CaO•Fe2O3 melting)
3. Mass transfer (only gas phase)
- convection (O2,N2,CO2)
- diffusion (O2,N2,CO2)
- gas sourcing (CO2) and sinking (O2)
Kinetics of graphite combustion
C+O2= CO2
YO2
Kinetic control
Diffusional control
T
dc
r
k0=6.532105 (m/s•K0.5)
Ea= 1.839105 (J/mol•K)
kr  k T
0.5
0 s
rc   gYO2 Ap,c kov combustion rate
Ap ,c 
k ov 
6(1   )
Yc specific area
dc
1
1
1

kr km
overall rate coefficient
 Ea 
 chemical reaction rate coefficient
exp 
 RTs 
DO 2
k m  Sh
dc
mass transfer rate coefficient
Sh
Sherwood and Nusselt numbers for sphere
2
Re Sc
 U  dc 
km d c

Sh 
 2.0  0.6


DO 2
 g 
 U  dc 

Nu 
 2.0  0.6



 g 
hdc
1
1
2
2
3
 g 


 DO 2 
 g





1
3
1
3


2 0.5
U   VC1  WC1
2
Kinetics of the other reactions
Assumptions
1.
2.
The reaction rates are controlled by heat supply (1,2) or removal (3)
The reactions proceed within a temperature interval T around the
corresponding thermodynamic temperature Td
1. CaCO3=CaO+CO2
2. CaO+Fe2O3=(CaO·Fe2O3)
3. (CaO·Fe2O3)=CaFe2O4
f1 – function of kinetic factor
rl – reaction rate
Ql – reaction heat
Qc – graphite combustion heat
rc- graphite combustion rate
Example for reaction (1)
T=10
1
f1 (T )  1 
Td=1123 K
T Td
1  e T
rl  f1 (T )
1  dHs


(
1


)
Y

Q
r
s
CaCO3
c c
Ql  dt

Heat supply rate
Initial porosity
“Wall” effect
RB
rB

Zone B :    0, B  0.25
Zone A :    0, A  0.4
B
Transition zone:
A
Transition zone
B
Mathematical formulation
A
  0.9   0, A 
RB    rB

  0, A
Equation of motion


 gg 
t
xk
g 

  gU g   , g
  S , g
xk 

where g  U ;  , g  g g ; U  VC1,WC1
S , g
150 gU (1   ) 2 1.75 gU 2 (1   )


2
2
dp

dp

dp - particle diameter
 - void fraction (porosity)
g - gas viscosity
g - gas density
Ergun
equation
Equations of continuity and mass conservation
(  gU )
x
 S1, g ;
C+O2= CO2

 gYi  
t
xk
CaCO3=CaO+CO2
SYCO 2 , g
rc M CO 2 rl M CO 2


 Mc
 M CaCO 3
SYO 2 , g
rc M O 2

; SYN 2 , g  0
 Mc
S1, g  SYCO 2 , g  SYO 2 , g

Yi 
  gU Yi   , g
  SYi , g
xk 

(i = CO2,O2,N2)
rc is the carbon combustion rate
rl is the lime decomposition rate
Mi is the molecular weight
Equation of energy conservation (gas phase)

 g C p,gTg  
t
xk
(C p , gTg ) 

  gU C p , gTg  T , g
  Sex  (1   ) SC
xk


Concept of C combustion
O2
C
Gas-particle heat exchange rate
Sex 
C+O2=CO2
CO2+C=2CO
CO+O2=2CO2
Reaction front
- part of C combustion heat
going directly to solid phase
( =0.5)
Nug
Ap Ts  Tg 
dh
6(1   )
Ap 
dh
SC  Qc rc (1   ) (fixed flux)
Equation of energy conservation (solid phase)

 s H s   
t
xk
St ,s

H s 
 H , s
  Sex  St , s   Sb  SO
xk 

(1   )  s
H s ,old  H s  ;

t
H ,s 
eff (Ts , Yi )
C p ,s (Ts , Yi )
eff   iYi  Rad ; i  Fe2O3 , CaO, CaCO3 , C
i
Rad - radiative conductivity according to Rosseland diffusion model
Hs
3 (1   )
16
3
 ; Ts 
Rad   Ts  s ;  s 
C p,s
2 dh
3
- Stephan-Boltzmann constant (=5.6710-8), s - scattering coefficient
- the reflectivity coefficient (=0.5) , Ts – solid temperature
Equation of energy conservation (solid phase)


xk
S H ,ex 

H s 
 H , s
  Sex  St , s   Sb  SO
xk 

Nug
dh
Ap Tg  Ts 
So  Ql rl  Qmrm  Qf rs
Qi and ri are heat effect and rate of appropriate reactions
l - CaCO3=CaO+CO2
m - CaO+Fe2O3=(CaO·Fe2O3)
f,s - (CaO·Fe2O3)=CaFe2O4
Boundary and initial conditions
Air (Ta)
A
Initial chemical composition and porosity
Zone Fe2O3 C
CaCO3 
A
0.82 0.03 0.15 0.40
B
1.0 0.0 0.0
0.25
Air velocity at inlet
B
W1 is defined from condition gW1=const (1.2)
V1 = 0
Initial temperature
Tg=Ts=25OC
Air temperature at inlet
Setting of solver options
Grid type
: BFC 2048
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
Example of calculated results.Velocity vector
t = 90 s
180 s
330 s
Carbon mass fraction and heat generation
Carbon mass fraction
Heat generation
Solid temperature and limestone fraction
Temperature of solid phase
Limestone fraction
Solid temperature and melted phase fraction
Temperature of solid phase
Melted phase fraction
Solid temperature and solid phase fraction
Temperature of solid phase
Solidified phase fraction
Carbon mass fraction and void fraction
Carbon mass fraction
Porosity
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