Heat Transfer Analysis in a Blast Furnace Tuyere Nose

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Proceedings of HT 2005: 2005 Summer Heat Transfer Conference

July 17 – 22, 2005, San Francisco, CA

HT2005-72599

HEAT TRANSFER ANALYSIS IN A BLAST FURNACE TUYERE NOSE

David Roldan1, Clifford Tetrault1, Yongfu Zhao2, Mark Atkinson2 and Chenn Q. Zhou1

1Department of Engineering, Purdue University Calumet, Hammond IN 46321

2United States Steel Corporation, Research and Technology Center 4000 Tech Center Drive, Monroeville, PA 15146

Proceedings of HT2005 2005 ASME Summer Heat Transfer Conference

July 17-22, 2005, San Francisco, California, USA

HT2005-72599

ABSTRACT The Blast furnace process is a counter current moving bed

chemical reactor to reduce iron oxides to iron for iron/steel making. In the process, tuyeres are used to introduce hot air (blast) and fuel (gas or pulverized coal) into the furnace for combustion. The nose of a tuyere, composed of copper material, that is exposed to a high temperature environment and a cooling water pipe is embedded to prevent melting of the material.

In this work, heat transfer and temperature distributions have been analyzed using the computational fluid dynamics commercial software, FLUENT®. The computations have included the cooling water flow and conjugate heat transfer in the tuyere nose. Both convection and radiation heat transfer on the surfaces are included. Different geometry and operating conditions were considered. The results have indicated that insufficient cooling in a large area between the nose inlet and outlet pipe can cause failures of the tuyere nose INTRODUCTION

Ironmaking is a capital and energy intensive process. The blast furnace (BF) represents the predominant ironmaking process in the U.S. Currently, more than 53 million tons of BF hot metal is produced annually in the U.S [1]. One of the major advances in the BF ironmaking process has been the pulverized coal injection (PCI) through tuyeres to partially replace metallurgical coke as a source of heat and reductant. Thus causing a reduction of overall cost do to the price of pulverized coal being 40% cheaper than coke which was previously used exclusively in steel production [2].

The BF process is a counter current moving bed chemical reactor to reduce iron oxides to iron. In the BF process, iron-bearing materials, fuel (coke), and flux (limestone and/or dolomite) are charged into the top of the furnace. Hot air (blast) and fuel (gas or pulverized coal) are introduced into the BF through tuyeres for combustion. The injected fuel and part

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of the coke are burned in the raceways of the BF to produce heat and reducing gas for reducing the iron ores [3]. The raw materials are charged at the top of the furnace and require 6 to 8 hours to descend to the hearth of the furnace where they become the hot metal and molten slag. Figure 1 shows the lower part of the furnace. The hot blast enters into a bustle pipe that encircles the furnace and is then introduced into the furnace through tuyeres. The hot air having a high velocity creates a void space in front of the tuyere called a raceway, which is surrounded by a bed of loose coke. Natural gas, pulverized coal and oxygen can also be introduced into the furnace through tuyeres to combine with the coke to release additional energy for increasing productivity and saving energy. This region reaches temperatures of 2100°C [4] which cause wear on the linings and the tuyeres.

Figure 1 Schematic of lower part of a Blast Furnace.

The blast furnace tuyere ends called tuyere noses are made of copper and are designed to be united to the fastening steel rings by means of a special welding technology which warrants the leak tightness at a blast furnace working temperature [5]. The standard make of the tuyeres possesses a single circuit of cooling water while some tuyeres may have double circulation of cooling water. The typical design of the tuyere nose is shown in Figure 2.

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Figure 2. A typical blast furnace tuyere

The Ironmaking Group at Great Lakes Works (GLW) noticed that tuyere failures frequently occurred at the tuyere nose. It was initially suspected that irregular flow of cooling water through the pipe bends in the tuyere might be responsible for the failures. Research was requested to conduct physical water modeling.

Two tuyeres were examined by U. S. Steel (USS) Research and Technology Center. A preliminary visual inspection indicated that both tuyeres had been damaged by liquid iron. One tuyere (shown in Figure 3) had damage in two locations on the nose (at the 2 (Figure 3a) and 3 (Figure 3b) o’clock positions, respectively). Also, a large portion of copper of another tuyere was eroded and liquid iron had penetrated into the water loop (on the right of Figure 1). The most severe erosion on that tuyere nose occurred at two locations, around the 3 o’clock and 9 o’clock positions. The tuyere was sectioned along the nose water pipeline (shown in Figure 4). It was observed that the large area (approximately 4-1/2-in. wide) between the inlet and outlet pipe bend was not protected well by cooling water and had been deeply eroded. The large copper rib at the nose of the tuyere between the nose inlet and outlet pipes is very unusual and questionable.

The inlet and outlet pipe cast into the tuyere was carefully inspected. The bends were completely smooth and no sharp bumps or protruding items were observed. Based on this inspection, it was thought unlikely that significant flow separation or stagnant regions exist at normal tuyere water velocities. After examining the tuyeres, it appears that insufficient cooling on the large area of the nose between the inlet and outlet bends more likely caused the failures. It was then determined that the physical water modeling was unnecessary and efforts would therefore focus on computer modeling of heat transfer and water flow in the tuyere. Through the joint efforts of USS and Purdue University - Calumet (PUC), the computer modeling work was completed. This paper gives an overview of the modeling principles and the computer simulation results.

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(a) at 2 o’clock position

(b) at 3 o’clock position

Figure 3. Tuyeres damaged on the nose

Figure 4. Tuyere nose sectioned along the cooling water loop. A large area (approximately 4-1/2 in. wide) between the inlet and

outlet pipe bend is not protected by water.

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CFD MODEL FLUENT® numerically solves the governing equations of the flow properties on a computational grid with specified boundary. In this work, the 3-D computational domain includes the copper material and the cooling water. The governing equations of flow properties are derived from fundamental conservation laws. The equations of mass, momentum, and enthalpy are all elliptic-type partial differential equations. For convenience in numerical formulation, these equations are arranged in a common form, in Equation (1).

ξ=

∂∂ξ

ξΓ−ξρ∂

∂�

=

S)x

u(x i

ii

3

1i

(1)

in which ξ is a general flow property, xi, i=1-3 are coordinates, ui, i=1-3 are velocity components, Γ is effective diffusivity, and Sξ is the sum of source terms. In this simulation, the only source term is gravity.

Boundary conditions include inlet water temperature and velocity, hot blast temperature and convection heat transfer coefficient, outer surface ambient temperature and radiation coefficient, as well as outflow condition.

For the base case, the following conditions are used: 15 m/s for water inlet velocity, 305 K for inlet water temperature, insulated front wall containing the water inlet and outlet, 0.5 emissiveity for outer and end surface radiation heat transfer, 2200 K for ambient temperature, 1400 K and 156 m/s for hot blast which results in the convective heat transfer coefficient of 112 W/m2-K. The outflow boundary is specified to ensure mass balance. For this simulation, convergence criterion for all the residuals, (except energy) is 10-4; the energy criterion is 10-7.

The computer-modeling domain is given in Figure 5. It

considers three-dimensional steady-state heat transfer in the nose and turbulent flow of cooling water. The hot blast is considered as a convection heat transfer boundary condition.

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Radiation heat transfer from the raceway to the nose is included. Commercial CFD software was used for numerical solution of the heat transfer and liquid flow equations with specified boundary conditions. The governing equations, including mass, momentum, and energy equations, are derived from fundamental principles. A standard k-epsilon turbulence model is used. Relaxation factors are needed for the numeric scheme to ensure convergence. The material properties and computer modeling conditions are show in Table I.

Figure 5. Schematic of geometry used in simulation

RESULTS AND DISCUSSION The objective of the computer modeling was to investigate

what gap size between the inlet and outlet pipe in the tuyere nose could provide sufficient cooling and prevent failure. In the base case, the gap is 2 inches. The typical cooling water velocity is 15 m/s and the inlet water temperature is 90°F. The copper absorptivity is uncertain. It varies in a wide rage, depending on the surface condition and temperature. The absorptivity normally is 0.18 for a highly polished surface, and 0.7 for an oxidized surface [Siegel, 2000]. The outer surface of the tuyere nose is actually covered with a thin layer of hard alloy by inertia gas welding. The value of the absorptivity was not available, thus absorptivity of 0.5 was used in the base case. A total of 12 cases were studied. The impact of absorptivity, gap size between the inlet and outlet pipe, cooling water velocity and temperature was examined. All the variables are listed in Table II.

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Base Case The temperature distribution across the water circle is

given in Figure 6. The velocity vectors colored by temperature distribution are shown in Figure 7. A contour of 1986°F (the melting point of copper) is shown in Figure 8. Erosion would take place more likely in the area with temperature greater than the melting point. Obviously, all hot face areas exceed the critical temperature under the base case condition. This is shown in the 3d plot (Figure 9) were the red color indicates a temperature well above the melting temperature of the copper. However, the most dangerous area is the part between the inlet and outlet pipe as shown in Figure 10. The maximum liquid phase penetration (defined as L in Figure 12) could reach as deep as 1-1/2 inches.

To ensure that the mesh used in the simulations is accurate, a grid sensitivity study was conducted. Figure 11, shows the temperature distribution through the gap of the tuyere, at different mesh sizes. Three meshes were compared with the interval size of 0.35 (coarse case), 0.25 (base case), and 0.125 (fine case) respectively. There is a discrepancy between the base and the coarse mesh, especially between the 0.075 to 0.1m. There is hardly any discrepancy between the base and the finer mesh size cases. Since the finer mesh size takes about 5 times longer to converge than the base, the mesh size of 0.25 was chosen which gave both accuracy and efficiency in computational time.

Figure 6. Temperature contours.

Figure 7. Velocity vectors colored by temperature.

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Figure 8. Erosion profile-1359 K (1986°F).

Figure 9. Multiple cross sections.

1000

1200

1400

1600

1800

2000

2200

0.050 0.075 0.100 0.125 0.150Distance from the hot blast (m)

Tem

pera

ture

(K)

Figure 10. Temperature distribution through the gap

1100

1300

1500

1700

1900

2100


Tem

pera

ture

(K) Mesh-0.125

Mesh-0.25 (base)Mesh-0.35

Figure 11. Grid sensitivity study

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Effect of Coating Layer The nose outer surface has been covered with a hard

weldment material. The hard layer provides good protection from abrasion and from hits by liquid iron and slag. The effect of the layer on heat transfer was studied in this work (Case 2). The most common coating material is nickel-based alloy but the chemical and physical properties are not available. Here, it was assumed as a 1/8-inch layer of nickel metal. A contour of 1359K (1986°F), which is the melting point of copper, is shown in Figure 12 (below). As compared with Figure 6, the line shifts towards outside significantly due to the effects of the coating. The copper phase is below the critical temperature, a considerable improvement as compared with the case without the layer. However, the area between the inlet and outlet pipe shows less change. The maximum liquid phase still reaches as deep as about 1-1/8 inches. That area could be damaged easily by liquid iron or slag hit, and coke particle abrasion.

(a) Case 1 (base case) (b) Case 2 (outer surface coated) Figure 12. Contour of temperature at 1359 K (Erosion profile)

Absorptivity Effect

The absorptivity in the base case is 0.5. As discussed above, the actual value is uncertain. The effect of the absorptivity was examined. Refer to Cases 3 and 4 for the modeling conditions. The temperature profiles, shown in Figures 13 and 14, show minimal change, but upon inspecting the erosion line in Figures 15 and 16 the change become more evident. The temperature distribution along the center line of the gap is shown in Figure 17, illustrating what the actual reduction in temperature is. This is also shown in the temperature of the outlet water shown in Figure 18. The maximum liquid phase penetration, L is used for evaluation. The simulation results are shown in Figure 19. One can see that absorptivity plays an important role in the erosion process. If the absorptivity is less than 0.1, the water flow could provide sufficient cooling for the nose. However, if the absorptivity is greater than 0.2, the water flow could not provide sufficient cooling. The liquid phase penetrates deeply (L>1/2 in.) in the case of a 2-inch gap between the inlet and outlet pipe. This also implies that a polished or shiny nose outer surface might help in extending the tuyere life.

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Figure 13. Temperature contours �=0.3.

Figure 14. Temperature contours �=0.1

Figure 15. Erosion profile-1359 K (1986°F) �=0.3.

Figure 16. Erosion profile-1359 K (1986°F) �=0.1.


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8001000120014001600180020002200

0.050 0.075 0.100 0.125 0.150

Distance from the hot blast (m)

Tem

pera

ture

(K) � = 0.75

� = 0.5� = 0.3� = 0.1

Figure 17. Temperature distribution through the gap.

305.0

305.5

306.0

306.5

307.0

0.1 0.3 0.5 0.75Absorptivity '�'

Tem

pera

ture

(K)

Figure 18. Effect of absorptivity on water outlet temperatures

Figure 19. Effect of absorptivity on liquid phase limit.

Gap Size Effect

In the base case, the gap between the inlet and outlet pipe is 2 inches. In Cases 5 to 7, the gap size varies from 2 inches to 1/4 inch. The temperature profiles, shown in Figures 20, 21 and 22, show changes in temperature distributions. Upon inspecting the erosion line in Figures 23, 24 and 25 the changes become clearer in the reduction in melt penetration. The temperature distribution along the center line of the gap, shown in Figure 26, shows what the actual reduction in temperature penetration is. This is also shown in the temperature of the outlet water temperature shown in Figure 27. The maximum liquid phase penetration, L was compared. The simulation results are shown in Figure 28. One can see that gap is critical to the erosion process. If the gap is less than 1/2 inch, the penetration is limited to about 1/2 inch. Any further decreasing will not make

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much difference. However, if the gap is greater than 1/2 inch, the liquid phase line will protrude into the nose dramatically. This clearly indicates that a 2-inch gap is not proper to use in the normal circumstance.

Figure 20. Temperature contours gap size = 1.5 inches.




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Figure 23. Erosion profile-1359 K (1986°F) gap size = 1.5 inches.



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800

1000

1200

1400

1600

1800

2000


Tem

pera

ture

(K) gap-2in

gap-1.5ingap-0.5ingap-0.25in

Figure 26. Temperature distribution through the gap.

306.00

306.25

306.50

0.25 0.5 1.5 2Gap size (inches)

Tem

pera

ture

(K)

Figure 27. Effect of gap size on water outlet temperature

Figure 28. Effect of gap size on the liquid phase limit

Water Velocity Effect

The effect of the water velocity on the liquid phase line is only minimal. Refer to Cases 8 and 9 for the modeling conditions. The velocities vary form 10 m/s to 20 m/s. The temperature profiles, shown in Figures 29 and 30, illustrate changes in temperature distributions to be very small. Upon inspecting the erosion line in Figures 31 and 32 the changes are shown to only be minimal. The temperature of the outlet water is shown in Figure 33. The penetration varies very little as the water velocity increases from 10 to 20 m/s. Thus, changing water velocity is not a solution to this tuyere failure as shown in Figure 34.



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Figure 29. Temperature contours Velocity=10m/s.

Figure 30. Temperature contours Velocity=20m/s.

Figure 31. Erosion profile-1359 K (1986°F) Velocity=10m/s

Figure 32. Erosion profile-1359 K (1986°F) Velocity=20m/s.

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305.0

305.5

306.0

306.5

307.0

10 15 (base) 20Velocity (m/s)

Tem

pera

ture

(K)

Figure 33. Effect of velocity on water outlet temperatures

Figure 34. Effects of Velocity on liquid phase limit.

Water Temperature Effect

The next case examined is the cooling water inlet temperature. The temperature distributions are shown in Figures 35 and 36 and not much change is observed. This is substantiated by the erosion profiles have no change, shown in Figures 37 and 38. The calculation indicated that the water temperature increases by only 1.3°F as passing through the nose water loop shown in Figure 39. No difference is found in the penetration location as the inlet water temperature increases from 72 to 108°F shown in Figure 40. Refer to Cases 10 and 11 for the modeling conditions. Thus, changing cooling water temperature has no significant effect and is not a solution to this tuyere failure problem either.

Figure 35. Temperature contours Twi=295 K.


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Figure 36. Temperature contours Twi=315 K.

Figure 37. Erosion profile-1359 K (1986°F) Twi=295 K

Figure 38. Erosion profile-1359 K (1986°F) Twi=315 K

290

295

300

305

310

315

320

295 305(base) 315Water Inlet Temperature (K)

Tem

pera

ture

(K)

Figure 39. Effect of water inlet temp. on outlet temp.

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Figure 40. Effect of inlet water temperature on the liquid phase

limit.

CONCLUSIONS This work has determined that insufficient cooling in the

large area between the nose inlet and outlet bends caused tuyere failures at this BF. Increasing the cooling water velocity or decreasing the inlet temperature would not solve the problem. Research suggested that the gap between the water inlet and outlet pipe bend should be reduced to <1/2 inch.

Previously, it has been known that a hard layer on the nose provides good protection from abrasion and hit by liquid iron and slag. The present work explored for the first time that a hard layer shifts the liquid phase line towards the outside significantly. This improves our understanding of the protection mechanism from a heat transfer point of view.

ACKNOLWDGEMENT This work is supported by the Indiana 21st Century

Research and Technology Fund.

REFERENCES 1. Siegel, R and J. R. Howell: “Thermal Radiation Heat

Transfer,” 2nd Edition, Hemisphere Publishing Corp. 1981. 2. Birk, Wolfgang and Alexander Medvedev: “Pressure and

Flow Control of a Pulverized Coal Injection Vessel,” proceedings of the 1997 IEEE International, Hartford, CT. 1997.

3. Feng, Y. Q., D. Pinson, A. B. Yu, S. J. Chew, P Zulli: “Numerical Study of Gas-solid Flow in the Raceway of a Blast Furnace,” Australia. 2001.

4. DeCastro, Jose Adilson, Alexander Jose Da Silva, Hiroshi Nogomi, Jun-Ichiro Yagi: “Industrial Process Simulation Based on a Six-fluid Model: Application to the Analysis of the Blast Furnace Process,” Proceeding of the 2nd International Conference on Computational Heat and Mass Transfer , Brazil. Oct. 2001.

5. Ishii, K., JSPS, ISIJ: Advanced Pulverized Coal Injection Technology and Blast Furnace Operation. 1st edition, Pergamon Publishing Corp. 2000.



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Heat Transfer Analysis in a Blast Furnace Tuyere Nose