The Thermal Behavior Analysis in Tap-Hole Area

Transcription

1 Chna Steel Techncal Report, No. 1, pp. 13-0, (008) Y. C. Ko, C. K. Ho and H. T. Kuo 13 The Thermal Behavor Analyss n Tap-Hole Area YUNG-CHANG KO*, CHUNG-KEN HO* and HSU-TANG KUO** *Department of Steel and Alumnum Research and Development **Blast Furnace Plant-I, Department of Irommakng Chna Steel Corporaton Hsao Kang, Kaohsung 8133, Tawan, R.O.C. The No. blast furnace at Chna Steel Corporaton (CSC) had a splashng problem from ts new campagn n January 006. It was nferred that the splashng was somehow related to the performance of the refractory and coolng system near the tap-hole area. Ths study establshes a blast furnace hearth model to nvestgate the thermal behavor n terms of refractory and mud, n the tap-hole area, by usng a CFD code Fluent. The accuracy of the model was verfed by comparng the smulated temperature dstrbuton wth ste practcal data. Based on the numercal smulaton, t s found that the thermal propertes of mud-core, castable and brck and the convecton heat transfer coeffcent of spool have a sgnfcant effect on the tap-hole area temperature dstrbuton. The developed hearth model can predct the trend of thermal behavor by adjustng materal thermal propertes and s helpful n understandng the temperature dstrbuton of the tap-hole area, whch can be a reference to adjust the coolng operaton. 1. INTRODUCTION Many works about the hearth phenomena of blast furnace, such as molten ron flow, mass and heat transfer, have been done. These mathematcal models gve a basc understandng about molten ron flow and the heat transfer n the blast furnace hearth, whle only a few focuses on the tap-hole area. Vats and dash (1) computed the wall shear stress by solvng the flow feld and concluded that there exsts an optmal tap-hole length for mnmal wall shear stress, whch has also been verfed n ther present work. However, ther model dd not consder varous possble coke-bed postons and shapes, nor the model dd not show expermental valdaton. Dash et al (-3) dscussed the optmal tap-hole angle for mnmal flow- - nduced shear stress on the wall of the blast furnace hearth, but ther study only concentrated on a fxed coke-bed sze and shape. Excessve splashng of a hot metal stream from tap-hole, whch s caused by entraned blast ar, can result n premature trough refractory wear. Ths splash occurs when the gas-lqud flow regme of slug flow s present n the tap-hole. Tap-hole stream trajectores and ther effects on trough refractory, reported by He et al, (4) can be predcted by the theory proposed heren that recognzes that the average velocty n the lqud stream s equal to the sum of the gas and lqud superfcal veloctes. After more than 1 years n operaton, No. blast furnace of Chna Steel Corporaton (CSC) was blown down n October 005 for revampng, and ts 3rd campagn begun n January 006. Durng the revampng, the furnace hearth was equpped wth stave coolng nstead of shell water spray coolng. Ths s the frst tme one of one of CSC s blast furnaces has used stave coolng n the hearth. Unfortunately, splashy tap-hole streams occurred very frequently and severely n the begnnng of castng n ths blast furnace. It was nferred that the splashng was somehow related to the performance of the refractory and coolng system near tap-hole area. Ths study establshes a blast furnace hearth model to nvestgate the thermal behavor n terms of refractory and mud n tap-hole area, by usng a CFD code Fluent.. PHYSICAL SYSTEM In order to save computng tme, a -D model was used to nvestgate the effect of the thermal propertes and coolng parameters. Fgure 1 shows a schematc dagram of the No. blast furnace hearth, n X-Y plane. In the hearth sde wall, copper staves were nstalled n the area of the tap-holes, and cast ron staves were used n the rest of the area. Besdes, water-coolng steel ppes were arranged n the hearth bottom. Then, a 3-D smplfed model was also developed to smulate the temperature dstrbuton n the tap-hole area, as s llustrated n Fg.. As we focused on the tap-hole temperature profle, only hot metal n the tap-hole was

2 14 The Thermal Behavor Analyss n Tap-Hole Area Hot metal n Spool Hot metal out Tap-hole.88m Mud core Mushroom 6m 0.6m (RDN) 0.6m (7RDN) Curbural 7RDN A/I 0.6m (RDN) 0.3m (RN) Fg. 1. Curbural A/I -D Geometrc dmensons and refractory components of hearth. (1) Symmetrc B.C. () Convectve B.C. (3) Adabatc B.C. (4) T=1500 Fg.. () (1) (3) T/H C.L. (4) 3-D model of hearth and boundary condtons. consdered. The hearth wall temperature was set as a constant temperature, 1500 C. The model parameters are lsted n Table GOVERNING EQUATIONS AND NUMERICAL METHOD 3.1 Governng Equatons Based on the prevous work, (5) the Naver-Stoke equatons wth conjugate heat transfer were solved to generate the hot metal flow feld n the hearth and the heat flux across the sde wall and the bottom. The governng equatons for ths conjugate heat transfer problem can be descrbed as follows: Table 1 Standard Values of Model Parameters Iron Lamnar vscosty Pa s Thermal conductvty 16.5 W m -1 k -1 Heat capacty 850 J kg -1 k -1 Densty 7000 kg m -3 Refractores Thermal conductvty of.6 W m -1 k -1 Protecton Wall Thermal conductvty of 150 W m -1 k -1 RN Thermal conductvty of 11 W m -1 k -1 RD-N Thermal conductvty of 7 W m -1 k -1 (T=473.15K); Carbural A/l 8 W m -1 k -1 (T=673K); 10 W m -1 k -1 (T=873K); 11 W m -1 k -1 (T=1073K) Thermal conductvty of 16 W m -1 k -1 (T<473K) ; 7 RD-N 18 W m -1 k -1 (T>1373K) Thermal conductvty of W m -1 k -1 Copper Stave Thermal conductvty of W m -1 k -1 Cast-ron Stave Coke bed Partcle dameter 0.03 m Porosty 0.35 Boundary condton Convecton coeffcent 3000 w m - k -1

3 Y. C. Ko, C. K. Ho and H. T. Kuo 15 ρu x ρu U x j = 0..(1) j p = + x U μ x x j U j + + ρg..() x T T ρc pu = k (3) x x S μ = (4) α m U 3 Dpε α =..(5) 150(1 ε) Where U s the velocty of hot metal (m/s) ρ s the densty of hot metal (kg/m 3 ) g s the gravtatonal acceleraton (m/s ) C p s the heat capacty (J/kg-K) T s the temperature (K) k s the thermal conductvty (W/m-K) α s defned as the permeablty (-) D p s the coke mean dameter (m) ε s porosty (-), and S m s the momentum source term caused by the deadman (Pa/m). To obtan S m, the deadman was modelled as a porous meda, and Darcy s Law, smplfed from Ergun equaton and shown n Equaton 6 for the -D model, was adopted to address the pressure drop n terms of knetc energy and vscous energy losses. Δp L 150 μ(1 ε) 1. 75(1 ε) = υ+ ρυ..(6) 3 3 Dp ε Dpε where p s pressure (N/m ) L s the characterstc length (m) υ s the superfcal velocty (m/s). 3. Assumptons and Boundary Condtons Base case assumptons made for ths study ncluded the followng: (1) The dranage process s steady state, so the hot metal flows n and out at a constant rate; after mudpluggng, the process s unsteady state and the sntered mud temperature vares wth tme. () Chemcal reactons are gnored; (3) The hearth lnng s kept ntact n the begnnng of campagn; (4) Only force convecton and heat conducton are consdered for the heat transfer between the hot metal and refractory ; (5) (5) Turbulence nfluence s neglected; The followng boundary condtons were mposed for all cases: (1) The hot metal level s constant n the -D model, and only hot metal n the tap-hole s consdered n the 3-D model; () The free surface of hot metal s defned as an nlet flow boundary, n whch the hot metal of 1,500 C s unformly dstrbuted nto the hearth n -D model and the hearth wall temperature s defned as a constant temperature, 1,500 C; (3) The convectonal heat transfer n the hearth outsde surface s set accordng to the convectve coeffcent between stave and coolng water; (4) The taphole ext s kept at atmospherc pressure, and a mass flow boundary condton s mplemented to ensure mass balance; (5) The refractory n the free surface level s adabatc; (6) A symmetrcal wall boundary s used through the vertcal axs of the hearth center; and (7) At hearth sde and bottom wall surface, no-slp boundary condtons are employed. 4. NUMERICAL PROCEDURE The set of governng equatons (1)-(5) were dscretzed usng the fnte volume technque n a computatonal doman and can be solved numercally. The dscreterzaton of the momentum and contnuty equatons and ther solutons were solved by means of the segregated solver. A standard pressure nterpolaton scheme was requred to compute the face values of pressure from the cell values. SIMPLE algorthm was appled n pressure-velocty couplng n the segregated solver to adjust the velocty felds by correctng the pressure feld. The second upwnd scheme was chosen to dscrete the convecton term of every governng equaton. As dsplayed n Fgs 1 and, the steady state analytcal doman ncluded the hot-metal regon and refractory. For numercal procedure, the temperature dstrbuton durng dranage was calculated frst. Then, the unsteady state model s proposed to calculate the temperature varaton wth tme n the tap-hole. 5. RESULTS AND DISCUSSIONS 5.1 Temperature Dstrbuton Durng Tappng and Mud Pluggng To nvestgate the temperature dstrbuton of the hearth, a temperature profle was computed by a -D blast furnace model. Fgure 3 shows a general case of -D temperature dstrbuton of the hearth durng tappng and mud-pluggng. The effect of mud sntered tme on temperature after pluggng s not remarkable at the hearth bottom. The man dfference n Fg. 3 was

4 16 The Thermal Behavor Analyss n Tap-Hole Area the heat concentraton n the tap-hole area due to adabatc refractory nstalled between the carbon brck and castable. After pluggng, the mud was heated frst to maxmum temperature, then cooled by the refractory around tap-hole and coolng system. Therefore, the temperature dstrbuton near the tap-hole s more remarkable wth tme. 5. Sensblty Analyss of Refractory S Thermal Propertes 5..1 Temperature Dstrbuton n Tap-Hole Dfferent tappng sequences, such as alternate tappng, mother and daughter tappng and one sde tappng, were used n the mll to prevent splashng. The effect of the dfferent tappng sequences was nvestgated by adoptng a tap-hole temperature measurement, wth tap-hole were measurements beng taken at 0 mnute, 60 mnute and 10 mnute after mud pluggng. The measured temperature profle s shown n Fg. 4. Fg. 3. Temperature contours of hearth wth dfferent tme. Fg. 4. Measured temperature n tap-hole of No. blast furnace of CSC.

5 Y. C. Ko, C. K. Ho and H. T. Kuo 17 The temperature profle of one sde tappng was hgher than the one of alternate tappng due to shorter coolng tme. Comparng to the measured temperatures of the last campagn, the tap-hole temperatures of the new campagn were too low to snter the mud. Ths could be the man reason of splashng. Snce the tap-hole temperature can not measured all the tme, a mathematc model was used to smulate the tap-hole temperature profle n ths study n order to fnd the man nfluence of the refractory and coolng system on the tap-hole temperature dstrbuton. 5.. Effect of Surface Convectve Condton Fgure 5 shows the effect of the surface heat transfer propertes of coolng condton on tap-hole temperature dstrbuton. The effect of water coolng on the stave, wth the heat convectve coeffcent (h) assumed to be 300 and 3,000 wm - k -1, are shown n Fg. 5(a). The effect of ar coolng on the spool surface, an adopted wth convectve coeffcent of 30 and 300 wm - k -1, are shown n Fg. 5(b). The major effect n the tap-hole area of the convectve coeffcent was on the temperature of the spool surface. A hgher convectve coeffcent causes a lower surface temperature of the spool. Both of convectve coeffcents have slght nfluence n the tap-hole temperature dstrbuton Effect of Thermal Property of Refractory The dfferent thermal conductvtes of mud, hgh alumna brck and castable are adopted to nvestgate the effect of the thermal conductvty of the refractory, as shown n Fg. 6. The case wth mud-core around the tap-hole was computed. The nfluence of the thermal conductvty of mud was found not to be remarkable due to the small volume. The temperature profle of the tap-hole has a peak value n the spool area due to the lower thermal conductvty of hgh alumna brck and castable. These adabatc materals nduce heat (a) Convectve coeffcent of water n stave Fg. 5. (b) Convectve coeffcent of spool surface The effect of surface heat transfer propertes of coolng condtons on tap-hole temperature dstrbuton.

6 18 The Thermal Behavor Analyss n Tap-Hole Area (a) Thermal conductvty of mud Fg. 6. (b) Thermal conductvty of hgh Al O 3 brck and castable The effect of thermal propertes of refractores on tap-hole temperature dstrbuton. concentraton n spool. In Fg. 7 shows the effect of mud-core wth lower thermal conductvty. In ths study, the thermal conductvty of the mud-core was 5 Wm -1 k -1, whch s between the conductvty of mud and hot metal. The lower conductvty means that the heat flux nto mud s harder and causes a lower temperature profle than the mud-core. Actually, the mud-core s a mxed layer contanng mud, ron, slag and cracked brck. Therefore, the thermal conductvty of ths layer s hard to defne and control. 5.3 Effect of -D and 3-D Model Smulaton Accordng to the sensblty analyss, t can be found that the thermal conductvty of the mud-core plays an mportant role n tap-hole temperature dstrbuton. We tred to smulate tap-hole temperature wth effectve resstance n the spool area. Fgure 8 shows the temperature profles of the tap-hole by -D smulaton and by measurement. The measured temperature profle s hgher than the smulated one, and the slope s steeper than the smulated one. Ths s due to the lmtaton of the -D model. For a hearth model wth a spool area, t s not an axal symmetry problem and s hard to smulate. Therefore, the studes regardng the spool area of the hearth were smplfed n general, even n the 3-D model. Tryng to better smulate the tap-hole temperature, a smplfed 3-D model was adopted and s shown n Fg.. Fgure 9 shows the temperature profles smulated n the 3-D model wth values of the base parameters shown n Table 1. The smulaton result can get good agreement wth measured data. A smplfed 3-D model can smulate the tap-hole temperature profle. Makng a comparson between Fg. 4 and Fg. 9, t s clear that the temperature profle of the tap-hole s too low to snter mud well and could cause splashng. Therefore, the model can provde some nformaton to ste personnel for dealng wth hot metal splashng and the selecton of new mud for the tap-hole.

7 Y. C. Ko, C. K. Ho and H. T. Kuo 19 Fg. 7. The effect of mud-core on tap-hole temperature dstrbuton.. Fg. 8. Measured temperature and -D calculated temperature n tap-hole. Fg. 9. Measured temperature and 3-D calculated temperature n tap-hole. 6. CONCLUSIONS In ths study, the heat transfer behavor of the sntered mud n the tap-hole area of No. blast furnace at CSC has been analyzed and the calculated results can be summarzed as below: (1) A -D modfed model was developed to nvestgate the nfluence of the propertes of vared refractory and coolng systems on hearth temperature dstrbuton. The thermal conductvty of mud-core plays an mportant role n tap-hole temperature dstrbuton, but t s hard to defne.

8 0 The Thermal Behavor Analyss n Tap-Hole Area () To better smulate the tap-hole temperature profle, a 3-D smplfed model was developed to and valdated by measured data. The smulaton result can get good agreement wth measured data. (3) The 3-D model can provde some nformaton to ste personnel for dealng wth hot metal splashng and for the selecton of new mud n the tap-hole. REFERENCES 1. A. K. Vats and S. K. Dash: Ironmakng Steelmakng, 7 (000), p S. Dash, S. K Ajman, A. Kumar and H. Sandhu: Ironmakng Steelmakng, 8 (001), p S. K. Dash, D. N. Jha, S. K. Ajman and A. Upadhyaya: Ironmakng Steelmakng Int. J. Technol. Adv., 31 (004), p Qngln He, P. Zull, F. Tanzl, B. LEE, J. Dunnng and G. Evans: ISIJ Internatonal, 4 (00), p W. T. Cheng, C. N. Huang and S. W. Du: Chem. Eng. Sc., 60 (005), p