Multidimensional and Multiphase Numerical Analysis of...

Seminar Nasional Besi dan Baja II ( ), Aula Barat, ITB, 20-21 Oktober 2011 Sesi I.2 - hal. 1

Multidimensional and Multiphase Numerical Analysis of Blast Furnace Ironmaking Operation

Sungging Pintowantoro Jurusan Teknik Material dan Metalurgi FTI, Institut Teknologi Sepuluh Nopember (ITS),

Kampus ITS Sukolilo, Surabaya 6011, Indonesia [email protected]

Abstract

The production of pig iron in the blast furnace is the major route among various iron making processes. The improvement in productivity, coke consumption and fuel use within the process has become the major issue in steelworks. For these reason that pulverized coal injection (PCI) has become a standard in most blast furnace operation with injection rates varying from 100 to over 200 kg/thm, in order to decrease coke consumption. With increased in PCI rate into blast furnace, more unburned pulverized coal (UPC) and coke degradation would be generated. The large amount of powder deposited (static powder) would be deteriorate permeability and possible hanging of the burden in the blast furnace. In order to overcome these problems, it is necessary to elucidate the flow characteristics of powder deposited for the achievement of high PCI rate and the stable operation of the blast furnace. Previous blast furnace models have considered up to four phases in the blast furnace (solids, gas, liquids and powders). Recently, although there have been extensive effort to understanding of flow characteristic of powder in blast furnace found in previous literatures, the behavior of powders using separate treatment of dynamic and static powder in blast furnace has yet to be reported. Therefore, they need to be considered as different phases possessing own motions, energy and phase composition. Although various mathematical models of the blast furnace have been reported in previous literatures, most have made many simplifications to the governing equations, and several important blast furnace features like three-dimensional and transient behaviors have been ignored. It is well known that thermo physical properties of powder from unburned PC and coke degradation are quite different each other, such as density, viscosity, specific heat and thermal conductivity. Therefore, they need to be considered as different phases possessing own motions, energy and phase composition. The other influence to control and optimize permeability, especially in the deadman region is the motion of coke particles. In this study, a mathematical model, which does consider the flow characteristic of powders in the blast furnace with eight major phases simultaneously interacting via multiphase chemical kinetics and physical changes, is developed. The model solves rigorous conservation equations for all phases simultaneously; moreover the model was extended to consider eight phases for analyze total powder deposition in blast furnace quantitatively. This model is applied to simulate the actual and several advanced blast furnace operations.

KEY WORDS: blast furnace; mathematical modeling; multidimensional; multi-phase flow; pulverized coal injection; chemical reactions; static powders holdup.

I. Introduction

The blast furnace (BF) represents the predominant ironmaking process in the world. More

than 95% hot metal produced from iron ore comes from blast furnaces. It is apparent that

the lead role of blast furnaces in hot metal production will remain unchanged well into the

future. A blast furnace is capital and energy intensive.[1] To maintain and improve the

competitiveness of the blast furnace process, it is necessary to achieve a considerable

decrease in coke and total energy consumption for primary metal production along with

minimization of environmental impacts. Pulverized coal injection (PCI), which provides


auxiliary fuel for partial coke replacement, has proven both economically and

environmentally favorable. It can result in substantial improvement in the furnace

efficiency and thus contribute to the reductions of energy consumption and environmental

emissions. In order to accelerate reducing the coke consumption, it is necessary to

increase the PCI rate to overcome the productivity barrier due to the reliance of coke. The

BF process is a counter current moving bed chemical reactor to reduce iron oxides to iron.

As shown in Figure 1, iron-bearing materials, fuel (coke), and flux (limestone and/or

dolomite) are charged in alternate layers into the top of the furnace. In the lower part of

the blast furnace, hot blast gas blown from tuyere push coke particles away from the

tuyere and form individual raceway zones. Additionally, this configuration results in

discontinuous regions with high carbon consumption rate, and leads funnel flows of lump

solids toward these regions. The motion of liquid phases (molten metal and slag) is

affected by both flows of gas and solid phases. The liquid flow region is determined by

the interactions among these phases, and steep distributions of temperatures and

compositions are formed in the combustion zone of the furnace.

Figure 1. Blast Furnace ironmaking process

Under high PCI rate condition, powder behavior in the lower part of BF has been attracting

technical and scientific interests[2-6], not only unburned char or unburned pulverized coal

(UPC) but also fine coke (FC) particles increases[7,8]. Accumulation of such fine particles

in the furnace deteriorates permeability of the packed bed[9], which may cause in stability

of furnace operation. Therefore such three-dimensional features and powder behavior in

the lower part are required to be quantitavily understood to comprehend for the prediction

of the main product quality and operation efficiency of the blast furnace. The authors have


developed mathematical models of blast furnace, which are based on multi-fluid theory.[10-

14] Recently theses models included accumulated powder (static hold-up) as a new phase

to analyze powder deposition behavior of powder in blast furnace and granular bad

filter.[15-17] Although these models provided the useful information and expectations,

simultaneous handling of the pulverized coal and the fine coke with dynamic-static

separation has yet to be reported. In this study a multi-dimensional and multiphase based

operation simulator of blast furnace including this feature and heat transfer and chemical

reactions regarding accumulated powders was developed, and this new mathematical

model was applied to the actual blast furnace operation to examine the behavior of

unburned pulverized coal and fine coke in the blast furnace.

II. Mathematical Models

II.1. Governing conservation equations Governing conservation equations for all phases are expressed via a general conservation

equation, represented by Equation 1, which is independent of the coordinate system.

( ) ( ) ( )ii

SUt

iiiiiiiii

φφ φεφρεφρε

+Γ=+∂

∂graddivdiv

r

(1)

In the Equation 1, the subscript i stands for phase being considered (i= gas, solids(coke,

sinter, pellets, ore, scrap, static powders(PC and Fine Coke (FC)), dynamic powders (PC

and FC), hot metal and slag). φΓ is the effective transfer coefficient which depends on the

state variable to be solved. For instance, if the general conservation equation is the

momentum conservation the transfer coefficient assumes the dynamic viscosity, on the

other hand, if the conservation equation is of energy, then the transfer coefficient assumes

the thermal conductivity and so on. The phase composition is calculated using the

conservation Equation 1 with the mass fraction of each component selected as dependent

variable. Two powders considered in this study are pulverized coal (or unburned char) and

fine coke. Although these two powders have similar chemical composition, they show

different physical properties, particle structure, and so on. The fine coke is generated as

the coke fragments detached from the lump coke particles. Thus it fundamentally has

same chemical component and properties as the lump coke particles but the diameter.

Contrarily the pulverized coal undergoes chemical and thermal processes during its

combustion process. As a result, the unburned char usually has a porous structure and

shows various textures such as network, balloon, skeleton, and so on. It is usually

considered that the unburned char has smaller average diameter and apparent density

than the fine coke. Consequently these two powders indicate the different fields of flow,

temperature and reaction each other. Therefore the unburned char and fine coke are

treated as different phases in this model.


Regarding the static powders, their treatment was changed from the previous work.[15,17]

The static powder is treated as a component of the solid phase in the modified model

while the previous model handles is a phase. The solid phase consist of multiple

components such as coke, lump ore, sinter, pellet and so on. These components have

only material balance equations and share temperature and velocity fields among the solid

components. The phases have momentum and heat balance eqeuations and continuity

equations. Table 1 gives the complete set of chemical species solved in this model.

Table 1. Phases and chemical species considered in this model

Phases Chemical species ( iω )

Gas CO, CO2, O2, H2, H2O, N2, SiO, CH4, C2H6, C3H8, C4H10 Ore Fe2O3, Fe3O4, FeO, Fe, CaO, Al2O3, MgO, SiO2, H2O, gangue sinter Fe2O3, Fe3O4, FeO, Fe, CaO, Al2O3, MgO, SiO2, H2O, gangue pellets Fe2O3, Fe3O4, FeO, Fe, CaO, Al2O3, MgO, SiO2, H2O, gangue

Solid

Coke C, SiC, SiO2, Al2O3, CaO, MgO, H2O, gangue Hot metal Fe, C, Si

Slag FeO, SiO2, Al2O3, CaO, MgO,gangue Dynamic powders of UPC (DPC)

C, SiO2, Al2O3, CaO, MgO, volatiles, gangue

Dynamic powders of FC (DFC)

C, SiC, SiO2, Al2O3, CaO, MgO

Static powders of UPC (SPC)

C, SiO2, Al2O3, CaO, MgO

Static powders of FC (SFC)

C, SiC, SiO2, Al2O3, CaO, MgO

0

10

20

30

40

50

60

0 2 4 6

Radial position (m)

Dia

me

ter(

mm

)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1V

olu

me

fra

ctio

n (

-)Coke diameter

Ore diameter

Ore

Coke

Figure 2. Grid arrangement for three-dimensional analysis

Figure 3. Radial distribution pattern of burden materials


II.2 Boundary conditions

The numerical grid used to simulate the blast furnace process in a three-dimensional

frame is shown in Figure 2. The calculation was carried out in whole the circunferential

direction of the blast furnace, simulating the real shape of the blast furnace. The grid was

constructed by linear distribution of the coordinate lines. The model uses as boundary

conditions the blast composition, flow rate and temperature at the tuyere nose. At the top

of the blast furnace the solid composition, volume fraction and diameter distributions are

specified. At blast furnace wall, a heat loss coefficient is specified based on operational

data of wall heat loss, allowing its variation with the blast furnace wall.

III. Results and discussions

The actual blast furnace operation is simulated. The input data used in this simulation was

taken as the monthly average values. The operational parameters used as input data are

sumarized in Table 2. Figure 3 shows the burden distribution pattern used as boundary

conditions for solid phase at the charging level. Coke was mainly charged at the central

region while ore was charged preferentially in the peripheral region. The mass balance for

all species was lower than 1% and good agreement of the measured and calculated

global parameters like prodution, slag rate and so on is verified in Table 2.

Table 2. Comparison of global parameters Calculated Measured Error (%)

Production (kg/s) 103.5 104.3 0.7 Coke rate (kg/thm) 313.6 313 0.2 Slag rate (kg/thm) 252.8 261 3.1

2

2

COCO

CO

+

53.3 50.6 5.3

Top gas temperature (0C)

248 240 3.3

Figure 4 shows the predicted three-dimensional pressure distribution of gas phase and

the cohesive zone location based on the melting of iron bearing materials. The pressure

drop is higher just in front of the tuyere where the gas velocity is very high and in the

cohesive zone where partially melted materials offers high flow resistance. The cohesive

zone is defined by the temperature of stating melting of the charged materials and finishes

when all the materials which compose the iron bearing materials(sinter,ore and pellets)

are completely melted.


3.653.603.473.123.052.922.852.68

Pressure(atm)

a. Gas pressure b. Cohesive zone Figure 4. Three-dimensional pressure drop and cohesive zone location predicted by the

model based on the melting rate

2100190018001600140012001000700300

a. Gas b. Solid c. PC d. Hot metal e. Slag

Figure 5. Three-dimensional temperature distribution for the major phases in the blast furnace

In Figure 5 the predicted temperature distributions of the major phases in the blast

furnace are shown. The gas and solid temperature distributions are quite similar due to

the strong interactions between these phases. Particularly, the temperature distribution in

the blast furnace is closely related with the burden distribution pattern shown in Figure 3.

Start

End

17001600158015701550150014001300


These features of the temperature patterns are due the strong dependence of the gas

flow, chemical reactions and physical properties on the bed porosity. Therefore, as coke

was mainly charged at central region, the main stream of the gas path also is in this region

which in turn, makes the iso-temperature surface higher in this region. The hot metal and

molten slag temperature iso-surface presents also similar pattern with the slag

temperature usualy been higher due to the higher residence time in the dripping zone

compared with hot metal. The temperature of pulverized coal phase is similar to the gas

phase due to the permanent contact of gas and pulverized coal phases through the blast

furnace.

0.03180.02790.02390.01990.01590.01190.00800.0040

0.11840.10360.08880.07400.05920.04440.02960.0148

2.01E-041.78E-041.56E-041.34E-041.11E-048.91E-056.69E-054.46E-052.23E-051.06E-05

Metal Slag PC

( )−mε

( )−pcε

( )−slagε

Figure 6. Predicted phase volume fraction for discontinuous phase in the blast furnace

Figure 6 shows the phase volume fractions predicted by the 3-D model. For the hot metal

and slag phases the two region of high concentration of liquids can be observed: one is

just beneath the cohesive zone and the other in the dead man. The liquid accummulation

in dead man is due to the abrupt decrease in the coke porosity which offers higher

resistence for the liquid flow. Beneath the cohesice zone the liquid formed has difficuty to

drop down due to lower viscosity and the upward gas flow. Pulverized coal volume fraction

presents higher concentration in the raceway region and decreasing rapidaly as the

combustion occurs. In the upper part of the blast furnace the volume fraction of the

pulverized phase become higher due to the higher resistence of the packed bed.

Figure 7 shows the mass fraction for CO and CO2 in the gas phase. CO is generated in

the raceway and the gas mass fraction is kept almost constant until the reduction

reactions start, then the CO is consumed generating CO2. A small quantity of CO2 is

generated in front of the tuyere, where the concentration of oxygen is higher and the


temperature is still low, then sollution loss reaction occurs and almost all CO2 is converted

in CO. Therefore, in the lower part of the blast furnace the mass fraction of CO2 in the gas

phase is very small. On the contrary, in the upper part of the blast furnace, reduction of

iron bearing materials occurs and CO2 is generated. Sollution loss also occurs in the

upper part, however it is significant only in temperature above around 1000 oC. Hence, in

the upper part of the blast furnace CO is consumed while CO2 is generated which justify

the predicted pattern of the CO and CO2 mass fraction within the blast furnace.

0.380.310.240.170.10

0.320.250.190.130.06

a. CO b. CO2

Figure 7. Three-dimensional mass fraction distribution in the gas phase

Figure 8. Dynamic powder velocity distributions for two powders


Calculated dynamic powder velocity distributions are shown in Figure 8. Solid line within

the figure shows the locations of deadman and cohesive zone. The powders carried

upward by the gas phase released from the raceway, passes through the deadman zone

and then flows upward. In around the axial and above the cohesive zone of furnace for all

of calculation shows similar flow pattern. In the deadman region the direction of velocity

dynamic powder of FC shows trend to moves downward. The fine particles of FC heavier

than UPC, hence the dynamic powder of FC get into the deadman is unable to be

entrained by upward gas flow in the deadman, and they head to downward.

Figure 9 shows the static powder distribution for two powders have been considered in

this model. The higher amount of static powder of UPC was found at the center of

deadman region. In this region, the velocities of gas lower than other region because far

away from the raceway, therefore more accumulation of powder from UPC occurs. For the

static powder generated from FC (SFC), the higher amount of static powder was found in

the beneath and front of the raceway zone. The dynamic powder of FC which heavier

particles than UPC undergoes strong friction from solid phase, once it flows into packed

bed region from the raceway. Therefore the velocity of this phase decreases drastically in

front of the raceway zone, and the volume fraction increases in this region.

Figure 9. Static Powder distributions for two powders [m3/m3]


Hence, the amount of SFC larger than SPC in this region. Regarding the cohesive zone,

with increasing the PCI rate from 100 to 200 kg/thm has tended to move up. The addition

of UPC to the existing FC to lower permeability in the deadman zone resulting in reduced

coke bed stability. The combustibility within the tuyere giving a greater volume of gas

being injected to the race way caused an increase blast momentum. The increasing blast

momentum caused the degradation of raceway coke faster and than led the carry over of

FC larger. Therefore, the heat exchange between gas and other phases (solid, liquids and

powder) increases due to more heat arises by exothermic reactions (combustion and

carbon dissolution) of SPC and SFC in the solid component.

Figure 10. Deposition rate of powder distribution for two powders

The processes that lead powder deposition depend on the nature of the chemical and

physical conditions inside the furnace: temperarure, residence time, fluid dynamics and

the locally reactions environment. Deposition rate of powder distribution for two powders

were depicted in Figure 10. The higher deposition rate was found partially in the above

cohesive zone near the wall and also in the middle of the shaft region. The ascending

velocities of the gas or dynamic powder below the root of cohesive zone has tend to

decreases due to against the solid and liquid flow downward around the wall of furnace.

Therefore, residence time all of phases increases resulting the increased deposition rate

of powder. With increased of PCI, the amount of dynamic powders generated from UPC

and FC increased, but not consume by such reactions (solution loss and water gas) in the

shaft region. Hence, the high deposition rate of powder occurs in this region, especially in

the middle shaft due to the denser of solid particle.


IV. Conclusions

A three-dimensional mathematical model of the blast furnace has been developed. The

model comprehends the full circunferential blast furnace and is based on the multi-fluid

theory. A mathematical model, which does consider the flow characteristic of powders in

the blast furnace with eight major phases simultaneously interacting via multiphase

chemical kinetics and physical changes, is developed. The model solves rigorous

conservation equations for all phases simultaneously; moreover the model was extended

to consider eight phases for analyze total powder deposition in blast furnace

quantitatively. This model is applied to simulate the actual and several advanced blast

furnace operations.

Reference:

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2. K. Kadaguchi, T. Goto, R. Ito, T. Yabata and M. Shimizu: Kobe Steel Eng. Rep., 46, 2, 1996.

3. A. Maki, A. Sakai, N. Tagakaki, K. Mori, T. Ariyama, M. Sato and R. Murai: ISIJ Int., 36, 650, 1996.

4. M. Sato, R. Murai, T. Ariyama, A. Maki, A. Shimomura and K. Mori: Tetsu-to-Hagane, 84, 37, 1998.

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7. K. Takeda and N. Ishiwata: CAMP-ISIJ, 10, 123, 1997. 8. T. Akiyama and Y. Kajiwara: Advanced Pulverized Coal Injection Technology and

Blast Furnace Operation, ed. by K. Ishii, Pergamon-Elsevier Science Ltd., Oxford, 2000.

9. M. Ichida, T. Nakayama, K. Tamura, H. Shiota, K. Araki and T. Sugisaki: Tetsu-to-Hagane, 78, 1132, 1992.

10. P.R.Austin, H. Nogami and J. Yagi: ISIJ Int., 37, 458, 1997. 11. P.R.Austin, H. Nogami and J. Yagi: ISIJ Int., 37, 748, 1997. 12. J. A. Castro, H. Nogami and J. Yagi: ISIJ Int., 40, 637, 2000. 13. J. A. Castro, H. Nogami and J. Yagi: ISIJ Int., 42, 44, 2002. 14. H. Nogami, P. R. Austin, J. Yagi and K. Yamaguchi: ISIJ Int., 44, 500, 2004. 15. S. Pintowantoro, H. Nogami and J. Yagi: ISIJ Int., 44, 304, 2004. 16. M.A. Ribas, H. Nogami, J. Yagi: ISIJ Int., 45, 303, 2005. 17. S. Pintowantoro, H. Nogami and J. Yagi: ISIJ Int., 45, 204, 2005.

Riwayat Singkat Penulis

Dr. Sungging Pintowantoro, menyelesaikan pendidikan S1 jurusan di Teknik Kimia ITS, 1994, S2 Teknik Kimia ITS, 1999 dan S3 di Material-Engineering Tohoku University, Japan, 2005. Memulai karir sebagai process-engineer di perusahaan Polimer pada tahun 1994 dan sejak 1999 sebagai Dosen di Jurusan Teknik Material dan Metalurgi FTI – ITS. Posisi saat ini adalah Dosen di Jurusan Teknik Material dan Metalurgi FTI – ITS.

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