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Bottom Segment With Moisture in Blast Air
C H A P T E R
12 Bottom Segment With Moisture in Blast Air O U T L I N E 12.1 The Importance of Steam Injection for Blast Furnace Control
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12.2 H2O(g) Through-Tuyere Quantity Equation
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12.3 H2O(g) Concentration, kg H2O(g) per kg of Dry Air in Blast
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12.4 Through-Tuyere H2O(g) Input Quantity Equation
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12.5 Steady-State Bottom-Segment Hydrogen Balance
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12.6 Amended Bottom-Segment Carbon Balance
119
12.7 Amended Steady-State Oxygen Balance
119
Blast Furnace Ironmaking DOI: https://doi.org/10.1016/B978-0-12-814227-1.00012-9
12.8 Amended Steady-State Enthalpy Balance
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12.9 Matrix and Calculations
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12.10 Effect of H2O(g) Concentration on Steady-State Through-Tuyere H2O(g) Input
121
12.11 Effect of H2O(g) Concentration on Steady-State Carbon Requirement
122
12.12 Explanation
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12.13 Summary
122
Exercises
122
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© 2020 Elsevier Inc. All rights reserved.
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12. BOTTOM SEGMENT WITH MOISTURE IN BLAST AIR
12.1 THE IMPORTANCE OF STEAM INJECTION FOR BLAST FURNACE CONTROL All blast furnace plants inject steam into their blast air, Fig. 12.1. The primary objective of adding steam is to maintain a constant concentration of H2O(g)-inblast while atmospheric H2O(g)-in-air (humidity) is varying due to changes in air temperature and relative humidity that occur between day and night and between seasons. Moisture in the blast air can impact the blast furnace performance; steam injection keeps the blast furnace operating steadily and smoothly. The moisture in blast is kept constant over a 24-hour period and can be modified for seasonal changes. H2O(g) concentration in blast is measured with a cooled mirror dew point system or dew cell at point P in Fig. 12.1. The dew cell’s output is calibrated to give a readout in grams of H2O(g) in blast per Nm3 of dry blast (includes air plus added oxygen enrichment). This chapter examines the effects of H2O(g)-in-blast. As compared to Chapter 11, Methane—CH4(g)—Injection, it requires; 1. an H2O(g) input quantity specification (in place of CH4(g) input quantity specification
of Chapter 11, Methane—CH4(g)—Injection), and 2. modified steady-state C, O, H, and enthalpy balance equations. The objectives of this chapter are to; 1. show how H2O(g) through-tuyere input is included in our matrix calculations, and 2. indicate how H2O(g)-in-blast affects blast furnace C-in-coke and O2-in-blast air requirements for steady production of 1500 C molten iron.
12.2 H2O(g) THROUGH-TUYERE QUANTITY EQUATION The amount of H2O(g) entering a blast furnace through its tuyeres is a function of; 1. its blast’s measured H2O(g) concentration, usefully expressed as grams of H2O(g) in blast per Nm3 of dry blast, and 2. the amount of dry air entering the blast furnace, kg per 1000 kg of Fe in product molten iron.
FIGURE 12.1 Conceptual blast furnace bottom segment with H2O(g) in blast air. H2O(g) is always present in ambient air. Its concentration is topped up to the blast furnace operator’s prescribed level by injecting steam into this ambient air. The mixture is then heated to 1200 C, pressurized and blown into the blast furnace. The concentration of H2O(g) in blast is measured downstream of the stoves after the blast has been heated at 1200 C (point P).
BLAST FURNACE IRONMAKING
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12.5 STEADY-STATE BOTTOM-SEGMENT HYDROGEN BALANCE
12.3 H2O(g) CONCENTRATION, kg H2O(g) PER kg OF DRY AIR IN BLAST Appendix O shows how to calculate H2O(g) in dry air concentration expressed as kg H2O(g)-in-blast per kg of dry-air-in-blast from measured H2O(g) concentrations expressed as grams H2O(g)-in-blast per Nm3 of dry-blast. It also shows that a typical industrial H2O(g) concentration of 15 g H2O(g)-inblast per Nm3 of dry-blast is equivalent to 0.0118kgH2O(g)/kg of blast per kg of dry-airin-blast. We use this concentration throughout this chapter.
12.4 THROUGH-TUYERE H2O(g) INPUT QUANTITY EQUATION The basic blast furnace through-tuyere H2O(g) input quantity equation is;
mass through-tuyere
input H2 OðgÞ mass dry air 5 in blast H2 OðgÞ concentration in blast; kg per kg of dry air
or, both sides;
052
input H2 OðgÞ mass O2 1 0:0118 in blast mass N2 0:0118 1 in blast
1
(12.2)
as shown in matrix Table 12.1.
12.5 STEADY-STATE BOTTOMSEGMENT HYDROGEN BALANCE In its most basic form, the bottom segment hydrogen balance is: mass H in 5 mass H out
(12.1)
mass H in through-
tuyere input H2 OðgÞ mass H in 5 ascending H2 mass H in 1 ascending H2 O
This expands further to:
mass through-tuyere
11:2 mass% H in H2 O 100%
100 mass% H in H2 100%
input H2 OðgÞ
which is usefully expanded to;
mass through-tuyere
from
In terms of our input H2O(g) and the H2(g) and H2O(g) ascending out of the bottom segment (Fig. 12.1), this expands to:
With an H2O(g) concentration of 0.0118 kg of H2O(g)-in-blast per kg of dry-air-in-blast, Eq. (12.1) becomes; mass through-tuyere 1 input H2 OðgÞ mass input 5 0:0118 dry air
1 input H2 OðgÞ mass O2 5 0:0118 in blast mass N2 0:0118 1 in blast mass through-tuyere 1 subtracting input H2 OðgÞ
where the input masses are kg per 1000 kg of product molten iron.
mass through-tuyere
5
mass H2 out
in ascending gas 1
mass H2 O out in ascending gas
BLAST FURNACE IRONMAKING
11:2 mass% H in H2 O 100%
TABLE 12.1
Bottom-Segment H2O(g) Injection Matrix
Eq. (12.2) shows that 0.0118 kg H2O(g)/kg of dry air is being blown through the blast furnace tuyeres. It also shows altered C, H, O, and enthalpy balances. The effects of altering H2O(g)-inblast air are determined by changing the quantity in cells F14 and G14 as shown in Appendix O. Try it!
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12.7 AMENDED STEADY-STATE OXYGEN BALANCE
or
mass through-tuyere 5
mass through-tuyere
input H2 OðgÞ mass H2 out
0:112
1
1
mass Fe0:947 O into
mass O2
23:2 mass% O in Fe0:947 O 100%
100% O in O2 100%
ascending gas
0:112
mass CO out in
tuyere input H2 OðgÞ mass H2 out 1 1 in ascending gas mass H2 O out 1 0:112 in ascending gas
in blast 5
mass H in through-
88:8 mass% O in H2 O 100%
bottom segment
1
or both sides;
input H2 OðgÞ
in ascending gas mass H2 O out 1 0:112 in ascending gas mass through-tuyere 0:112 from subtracting input H2 OðgÞ
05 2
1 (12.3)
57:1 mass% O in CO 100%
mass CO2 out in ascending gas
1
72:7 mass% O in CO2 100%
88:8 mass% O in H2 O 100%
mass H2 O out in ascending gas
(12.4)
or
12.6 AMENDED BOTTOMSEGMENT CARBON BALANCE
mass through-tuyere
input H2 OðgÞ
Without CH4(g) injection, the steady-state bottom-segment carbon balance reverts to: 052
mass C in
descending coke mass CO out
1
mass Fe0:947 O into
mass O2 in blast
5
(7.4)
Including the oxygen in through-tuyere H2O(g), this chapter’s bottom-segment oxygen balance is;
0:232
1
mass CO out in
ascending gas
1
0:571
mass CO2 out
in ascending gas
1
12.7 AMENDED STEADY-STATE OXYGEN BALANCE
bottom segment
1
0:429 in ascending gas mass CO2 out 0:273 1 in ascending gas mass C out 1 1 in molten iron
1
1
0:888
mass H2 O out in ascending gas
0:727 0:888
mass through-tuyere 0:888 input H2 OðgÞ mass Fe0:947 O into mass O2 1 0:232 1 from 1 bottom segment in blast
or
subtracting
both sides:
BLAST FURNACE IRONMAKING
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12. BOTTOM SEGMENT WITH MOISTURE IN BLAST AIR
mass through-tuyere
052
2
input H2 OðgÞ mass Fe0:947 O into
With this new value and subtracting (1) 320 MJ per 1000 kg Fe, and (2) the left side of Eq. (12.6), from both sides of Eq. (12.6), the enthalpy equation becomes:
0:888
0:232 bottom segment mass O2 1 2 in blast mass CO out in 1 0:571 ascending gas mass CO2 out 0:727 1 in ascending gas mass H2 O out 1 0:888 in ascending gas
2 320 5 [mass through-tuyere input H2 O(g)] ð10:81Þ (12.5)
[mass Fe0:947 O into bottom segment] ð 23:152Þ [mass C in descending coke] 1:359 ½mass O2 in blast 1:239 ½mass N2 in blast 1:339 1 ½mass Fe out in molten iron 1:269 1 ½mass C out in molten iron 5 1 [mass CO gas out in ascending gas] ð2:926Þ 1 [mass CO2 gas out in ascending gas] ð7:926Þ
12.8 AMENDED STEADY-STATE ENTHALPY BALANCE With through-tuyere input H2O(g), enthalpy Eq. (11.6) becomes; H
1200 C H2 O g [mass through-tuyere input H2 O(g)] MWH2 O 1 [mass Fe0:947 O into bottom segment] ð 23:152Þ 1 [mass C in descending coke] 1:359 1 ½mass O2 in blast 1:239 1 ½mass N2 in blast 1:339 5 ½mass Fe out in molten iron 1:269 1 ½mass C out in molten iron 5 1 [mass CO out in ascending gas] ð2:926Þ 1 [mass CO2 out in ascending gas] ð7:926Þ 1 [mass N2 out in ascending gas] 1:008 1 [mass H2 out in ascending gas] 13:35 1 [mass H2 O out in ascending gas] ð11:50Þ 9 8 > = < 320 MJ bottom segment conductive; >
1
> :
convective and radiative heat loss per 1000 kg of Fe in product molten iron
> ; (12.6)
where the new term is italicized. It replaces the CH4(g) term in Eq. (11.6). From Table J.1, the new enthalpy value is: H
1200 C H2 O g MWH2 O
5 2 10:81 MJ=kg of H2 OðgÞ
1 [mass N2 out in ascending gas] 1:008 1 [mass H2 gas out in ascending gas] 13:35 1 [mass H2 O gas out in ascending gas] ð 211:50Þ (12.7)
The remainder of the equations of this chapter remain unchanged.
12.9 MATRIX AND CALCULATIONS Table 12.1 is our matrix with through-tuyere H2O(g) input. Notice that the H2O(g) enters the furnace at blast temperature, 1200 C in this case. Eqs. (12.2), (12.3), (12.5), and (12.7) are new. Solving the matrix gives one calculated result. It indicates that steady-state operation of the Fig. 12.1 bottom segment with 0.0118 kg of H2O(g) per kg of dry air at 1200 C requires; • 399 kg of C in descending coke, cell C19, and • 302 kg of O2-in-blast, cell C20 both per 1000 kg of Fe in product molten iron. And by Eq. (7.16), the whole furnace C-in-coke requirement is also 399 kg/1000 kg of Fe in product molten iron. Figs. 12.212.5 plot this and other calculated values.
BLAST FURNACE IRONMAKING
12.10 EFFECT OF H2O(G) CONCENTRATION ON STEADY-STATE THROUGH-TUYERE H2O(G) INPUT
FIGURE 12.2 Steady-state dry air requirement as a function of H2O(g) concentration in blast.
FIGURE 12.3 Effect of H2O(g) concentration in blast air on steady-state through-tuyere H2O(g) input mass. H2O (g) input mass is obtained from Table 12.1, Cell C29.
12.10 EFFECT OF H2O(g) CONCENTRATION ON STEADYSTATE THROUGH-TUYERE H2O(g) INPUT Before discussing the effect of throughtuyere input H2O(g) on C-in-coke and O2-inblast requirements, we examine the effect of H2O(g) in blast concentration on;
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FIGURE 12.4 Effect of H2O(g) concentration in blast air on blast furnace steady-state C-in-coke and O2-in-blast requirement for steady production of 1500 C molten iron. Both increase slightly.
FIGURE 12.5 Steady-state blast furnace C-in-coke and O2-in-blast requirements as a function of mass throughtuyere H2O(g) input. H2O(g) input mass is obtained from Table 12.1, Cell C29.
1. the blast furnace’s input air requirement, and 2. the equivalent mass input H2O(g). Fig. 12.2 plots the dry air requirement for steady production of molten 1500 C iron as a function of H2O(g)-in-blast concentration. It increases slightly.
BLAST FURNACE IRONMAKING
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12. BOTTOM SEGMENT WITH MOISTURE IN BLAST AIR
Fig. 12.3 plots the equivalent amount of H2O(g) that enters the blast furnace. This increases markedly with increasing H2O(g) concentration in blast. This is due to; 1. the increasing H2O(g) in blast concentration, and 2. the slight increase in air requirement, Fig. 12.2.
12.11 EFFECT OF H2O(g) CONCENTRATION ON STEADYSTATE CARBON REQUIREMENT Fig. 12.4 shows the effect of blast H2O(g) concentration on steady-state blast furnace Cin-coke and O2-in-blast requirements. Both increase slightly. Section 12.12 explains these results.
12.12 EXPLANATION Fig. 12.5 replots input C-in-coke and O2-inblast requirements of Fig. 12.4 as a function of through-tuyere H2O(g) input mass. Both increase. The C-in-coke requirement increases by 0.46 kg for every kg of H2O(g) input. This increase is a consequence of all Table 12.1 equations. We may, however, postulate that it is mostly due the large negative enthalpy that injected H2O(g) brings into the furnace, 210.81 MJ/kg, (Appendix J). This negative enthalpy must be offset by burning additional carbon in front of the tuyeres. This also explains the increased O2-in-blast requirement of Fig. 12.4. It is the oxygen that is needed to burn the abovementioned additional carbon in front of the tuyeres. Its requirement increases by 0.24 kg for every additional kg of through-tuyere H2O(g) input.
12.13 SUMMARY All blast furnaces blow H2O(g) through their tuyeres. The H2O(g) is from; 1. natural humidity in the blast’s input air, and 2. steam that is injected into the humid air. The effect of through-tuyere H2O(g) input is examined much like CH4(g) injection, Chapter 11, Methane—CH4(g)—Injection. The input concentration of H2O(g) is specified in equation form and the C, H, O, and enthalpy balances are modified to represent H2O(g) at 1200 C in place of CH4(g) at 25 C. H2O(g)-in-blast increases the blast furnace’s steady-state C-in-coke requirement per 1000 kg of Fe in product molten iron—thereby increasing product iron cost. The benefits of this extra cost are; 1. smooth, steady furnace operation, and burden descent; 2. quick blast furnace start-ups, especially when hanging occurs; 3. rapid flame temperature adjustment (Chapter 19: Raceway Flame Temperature with Moisture in Blast Air) by changing injected steam quantity (Fig. 12.1); and 4. ability to quickly control the hot metal thermal state and silicon content.
EXERCISES 12.1. For its furnace start-up, blast furnace team of Table 12.1 wishes to increase their H2O(g) in blast concentration to 25 g/Nm3 of dry air in blast. Please predict for them the amounts of C-incoke and O2-in-blast that will be needed for steady production of 1500 C molten iron with this H2O(g)-in-blast concentration. You may wish to use Appendix O.
BLAST FURNACE IRONMAKING
EXERCISES
Please give your answers in kg per 1000 kg of Fe in product molten iron. 12.2. Will the requirements of Exercise 12.1 be affected by blast temperature? Determine this by changing blast temperature of Exercise 12.1 to 1300 C. Use Appendix J.3. Examine Fig. 12.1 before completing this exercise. The H2O(g)-in-blast concentration remains at 25 g/Nm3 of dry air in blast. 12.3. Blast furnace plant of Table 12.1 is running out of coke. It can only afford to run the furnace with 395 kg (or less) of C-in-coke per 1000 kg of Fe in product molten iron. What is the maximum concentration of H2O(g)-in-blast that can be used while meeting this 395 kg of Cin-coke limitation. Please express your answer in grams H2O(g) in blast per Nm3
123
of dry air in blast. The blast temperature is 1200 C, as in Table 12.1. 12.4. Blast furnace of Table 12.1 is using 15 g H2O(g)/Nm3 of dry air. Humid air of Fig. 12.1 contains 10 g H2O(g)/Nm3 of dry air. How much steam must be added to this humid air to attain the prescribed 15 g H2O(g)/Nm3 of dry air in blast? Please express your answer in; 1. g per Nm3 of dry air, 2. kg per kg of dry air, and 3. kg per 1000 kg (t) of Fe in product molten iron. 12.5. Blast furnace of Exercise 12.4 is producing molten iron at 400 t/h. At what rate must steam be injected into humid air of Fig. 12.1 to continuously meet the blast’s specified 15 kg of H2O (g)/Nm3 of dry air. Remember that the molten iron is not pure Fe.
BLAST FURNACE IRONMAKING
12 Bottom Segment With Moisture in Blast Air O U T L I N E 12.1 The Importance of Steam Injection for Blast Furnace Control
116
12.2 H2O(g) Through-Tuyere Quantity Equation
116
12.3 H2O(g) Concentration, kg H2O(g) per kg of Dry Air in Blast
117
12.4 Through-Tuyere H2O(g) Input Quantity Equation
117
12.5 Steady-State Bottom-Segment Hydrogen Balance
117
12.6 Amended Bottom-Segment Carbon Balance
119
12.7 Amended Steady-State Oxygen Balance
119
Blast Furnace Ironmaking DOI: https://doi.org/10.1016/B978-0-12-814227-1.00012-9
12.8 Amended Steady-State Enthalpy Balance
120
12.9 Matrix and Calculations
120
12.10 Effect of H2O(g) Concentration on Steady-State Through-Tuyere H2O(g) Input
121
12.11 Effect of H2O(g) Concentration on Steady-State Carbon Requirement
122
12.12 Explanation
122
12.13 Summary
122
Exercises
122
115
© 2020 Elsevier Inc. All rights reserved.
116
12. BOTTOM SEGMENT WITH MOISTURE IN BLAST AIR
12.1 THE IMPORTANCE OF STEAM INJECTION FOR BLAST FURNACE CONTROL All blast furnace plants inject steam into their blast air, Fig. 12.1. The primary objective of adding steam is to maintain a constant concentration of H2O(g)-inblast while atmospheric H2O(g)-in-air (humidity) is varying due to changes in air temperature and relative humidity that occur between day and night and between seasons. Moisture in the blast air can impact the blast furnace performance; steam injection keeps the blast furnace operating steadily and smoothly. The moisture in blast is kept constant over a 24-hour period and can be modified for seasonal changes. H2O(g) concentration in blast is measured with a cooled mirror dew point system or dew cell at point P in Fig. 12.1. The dew cell’s output is calibrated to give a readout in grams of H2O(g) in blast per Nm3 of dry blast (includes air plus added oxygen enrichment). This chapter examines the effects of H2O(g)-in-blast. As compared to Chapter 11, Methane—CH4(g)—Injection, it requires; 1. an H2O(g) input quantity specification (in place of CH4(g) input quantity specification
of Chapter 11, Methane—CH4(g)—Injection), and 2. modified steady-state C, O, H, and enthalpy balance equations. The objectives of this chapter are to; 1. show how H2O(g) through-tuyere input is included in our matrix calculations, and 2. indicate how H2O(g)-in-blast affects blast furnace C-in-coke and O2-in-blast air requirements for steady production of 1500 C molten iron.
12.2 H2O(g) THROUGH-TUYERE QUANTITY EQUATION The amount of H2O(g) entering a blast furnace through its tuyeres is a function of; 1. its blast’s measured H2O(g) concentration, usefully expressed as grams of H2O(g) in blast per Nm3 of dry blast, and 2. the amount of dry air entering the blast furnace, kg per 1000 kg of Fe in product molten iron.
FIGURE 12.1 Conceptual blast furnace bottom segment with H2O(g) in blast air. H2O(g) is always present in ambient air. Its concentration is topped up to the blast furnace operator’s prescribed level by injecting steam into this ambient air. The mixture is then heated to 1200 C, pressurized and blown into the blast furnace. The concentration of H2O(g) in blast is measured downstream of the stoves after the blast has been heated at 1200 C (point P).
BLAST FURNACE IRONMAKING
117
12.5 STEADY-STATE BOTTOM-SEGMENT HYDROGEN BALANCE
12.3 H2O(g) CONCENTRATION, kg H2O(g) PER kg OF DRY AIR IN BLAST Appendix O shows how to calculate H2O(g) in dry air concentration expressed as kg H2O(g)-in-blast per kg of dry-air-in-blast from measured H2O(g) concentrations expressed as grams H2O(g)-in-blast per Nm3 of dry-blast. It also shows that a typical industrial H2O(g) concentration of 15 g H2O(g)-inblast per Nm3 of dry-blast is equivalent to 0.0118kgH2O(g)/kg of blast per kg of dry-airin-blast. We use this concentration throughout this chapter.
12.4 THROUGH-TUYERE H2O(g) INPUT QUANTITY EQUATION The basic blast furnace through-tuyere H2O(g) input quantity equation is;
mass through-tuyere
input H2 OðgÞ mass dry air 5 in blast H2 OðgÞ concentration in blast; kg per kg of dry air
or, both sides;
052
input H2 OðgÞ mass O2 1 0:0118 in blast mass N2 0:0118 1 in blast
1
(12.2)
as shown in matrix Table 12.1.
12.5 STEADY-STATE BOTTOMSEGMENT HYDROGEN BALANCE In its most basic form, the bottom segment hydrogen balance is: mass H in 5 mass H out
(12.1)
mass H in through-
tuyere input H2 OðgÞ mass H in 5 ascending H2 mass H in 1 ascending H2 O
This expands further to:
mass through-tuyere
11:2 mass% H in H2 O 100%
100 mass% H in H2 100%
input H2 OðgÞ
which is usefully expanded to;
mass through-tuyere
from
In terms of our input H2O(g) and the H2(g) and H2O(g) ascending out of the bottom segment (Fig. 12.1), this expands to:
With an H2O(g) concentration of 0.0118 kg of H2O(g)-in-blast per kg of dry-air-in-blast, Eq. (12.1) becomes; mass through-tuyere 1 input H2 OðgÞ mass input 5 0:0118 dry air
1 input H2 OðgÞ mass O2 5 0:0118 in blast mass N2 0:0118 1 in blast mass through-tuyere 1 subtracting input H2 OðgÞ
where the input masses are kg per 1000 kg of product molten iron.
mass through-tuyere
5
mass H2 out
in ascending gas 1
mass H2 O out in ascending gas
BLAST FURNACE IRONMAKING
11:2 mass% H in H2 O 100%
TABLE 12.1
Bottom-Segment H2O(g) Injection Matrix
Eq. (12.2) shows that 0.0118 kg H2O(g)/kg of dry air is being blown through the blast furnace tuyeres. It also shows altered C, H, O, and enthalpy balances. The effects of altering H2O(g)-inblast air are determined by changing the quantity in cells F14 and G14 as shown in Appendix O. Try it!
119
12.7 AMENDED STEADY-STATE OXYGEN BALANCE
or
mass through-tuyere 5
mass through-tuyere
input H2 OðgÞ mass H2 out
0:112
1
1
mass Fe0:947 O into
mass O2
23:2 mass% O in Fe0:947 O 100%
100% O in O2 100%
ascending gas
0:112
mass CO out in
tuyere input H2 OðgÞ mass H2 out 1 1 in ascending gas mass H2 O out 1 0:112 in ascending gas
in blast 5
mass H in through-
88:8 mass% O in H2 O 100%
bottom segment
1
or both sides;
input H2 OðgÞ
in ascending gas mass H2 O out 1 0:112 in ascending gas mass through-tuyere 0:112 from subtracting input H2 OðgÞ
05 2
1 (12.3)
57:1 mass% O in CO 100%
mass CO2 out in ascending gas
1
72:7 mass% O in CO2 100%
88:8 mass% O in H2 O 100%
mass H2 O out in ascending gas
(12.4)
or
12.6 AMENDED BOTTOMSEGMENT CARBON BALANCE
mass through-tuyere
input H2 OðgÞ
Without CH4(g) injection, the steady-state bottom-segment carbon balance reverts to: 052
mass C in
descending coke mass CO out
1
mass Fe0:947 O into
mass O2 in blast
5
(7.4)
Including the oxygen in through-tuyere H2O(g), this chapter’s bottom-segment oxygen balance is;
0:232
1
mass CO out in
ascending gas
1
0:571
mass CO2 out
in ascending gas
1
12.7 AMENDED STEADY-STATE OXYGEN BALANCE
bottom segment
1
0:429 in ascending gas mass CO2 out 0:273 1 in ascending gas mass C out 1 1 in molten iron
1
1
0:888
mass H2 O out in ascending gas
0:727 0:888
mass through-tuyere 0:888 input H2 OðgÞ mass Fe0:947 O into mass O2 1 0:232 1 from 1 bottom segment in blast
or
subtracting
both sides:
BLAST FURNACE IRONMAKING
120
12. BOTTOM SEGMENT WITH MOISTURE IN BLAST AIR
mass through-tuyere
052
2
input H2 OðgÞ mass Fe0:947 O into
With this new value and subtracting (1) 320 MJ per 1000 kg Fe, and (2) the left side of Eq. (12.6), from both sides of Eq. (12.6), the enthalpy equation becomes:
0:888
0:232 bottom segment mass O2 1 2 in blast mass CO out in 1 0:571 ascending gas mass CO2 out 0:727 1 in ascending gas mass H2 O out 1 0:888 in ascending gas
2 320 5 [mass through-tuyere input H2 O(g)] ð10:81Þ (12.5)
[mass Fe0:947 O into bottom segment] ð 23:152Þ [mass C in descending coke] 1:359 ½mass O2 in blast 1:239 ½mass N2 in blast 1:339 1 ½mass Fe out in molten iron 1:269 1 ½mass C out in molten iron 5 1 [mass CO gas out in ascending gas] ð2:926Þ 1 [mass CO2 gas out in ascending gas] ð7:926Þ
12.8 AMENDED STEADY-STATE ENTHALPY BALANCE With through-tuyere input H2O(g), enthalpy Eq. (11.6) becomes; H
1200 C H2 O g [mass through-tuyere input H2 O(g)] MWH2 O 1 [mass Fe0:947 O into bottom segment] ð 23:152Þ 1 [mass C in descending coke] 1:359 1 ½mass O2 in blast 1:239 1 ½mass N2 in blast 1:339 5 ½mass Fe out in molten iron 1:269 1 ½mass C out in molten iron 5 1 [mass CO out in ascending gas] ð2:926Þ 1 [mass CO2 out in ascending gas] ð7:926Þ 1 [mass N2 out in ascending gas] 1:008 1 [mass H2 out in ascending gas] 13:35 1 [mass H2 O out in ascending gas] ð11:50Þ 9 8 > = < 320 MJ bottom segment conductive; >
1
> :
convective and radiative heat loss per 1000 kg of Fe in product molten iron
> ; (12.6)
where the new term is italicized. It replaces the CH4(g) term in Eq. (11.6). From Table J.1, the new enthalpy value is: H
1200 C H2 O g MWH2 O
5 2 10:81 MJ=kg of H2 OðgÞ
1 [mass N2 out in ascending gas] 1:008 1 [mass H2 gas out in ascending gas] 13:35 1 [mass H2 O gas out in ascending gas] ð 211:50Þ (12.7)
The remainder of the equations of this chapter remain unchanged.
12.9 MATRIX AND CALCULATIONS Table 12.1 is our matrix with through-tuyere H2O(g) input. Notice that the H2O(g) enters the furnace at blast temperature, 1200 C in this case. Eqs. (12.2), (12.3), (12.5), and (12.7) are new. Solving the matrix gives one calculated result. It indicates that steady-state operation of the Fig. 12.1 bottom segment with 0.0118 kg of H2O(g) per kg of dry air at 1200 C requires; • 399 kg of C in descending coke, cell C19, and • 302 kg of O2-in-blast, cell C20 both per 1000 kg of Fe in product molten iron. And by Eq. (7.16), the whole furnace C-in-coke requirement is also 399 kg/1000 kg of Fe in product molten iron. Figs. 12.212.5 plot this and other calculated values.
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12.10 EFFECT OF H2O(G) CONCENTRATION ON STEADY-STATE THROUGH-TUYERE H2O(G) INPUT
FIGURE 12.2 Steady-state dry air requirement as a function of H2O(g) concentration in blast.
FIGURE 12.3 Effect of H2O(g) concentration in blast air on steady-state through-tuyere H2O(g) input mass. H2O (g) input mass is obtained from Table 12.1, Cell C29.
12.10 EFFECT OF H2O(g) CONCENTRATION ON STEADYSTATE THROUGH-TUYERE H2O(g) INPUT Before discussing the effect of throughtuyere input H2O(g) on C-in-coke and O2-inblast requirements, we examine the effect of H2O(g) in blast concentration on;
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FIGURE 12.4 Effect of H2O(g) concentration in blast air on blast furnace steady-state C-in-coke and O2-in-blast requirement for steady production of 1500 C molten iron. Both increase slightly.
FIGURE 12.5 Steady-state blast furnace C-in-coke and O2-in-blast requirements as a function of mass throughtuyere H2O(g) input. H2O(g) input mass is obtained from Table 12.1, Cell C29.
1. the blast furnace’s input air requirement, and 2. the equivalent mass input H2O(g). Fig. 12.2 plots the dry air requirement for steady production of molten 1500 C iron as a function of H2O(g)-in-blast concentration. It increases slightly.
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12. BOTTOM SEGMENT WITH MOISTURE IN BLAST AIR
Fig. 12.3 plots the equivalent amount of H2O(g) that enters the blast furnace. This increases markedly with increasing H2O(g) concentration in blast. This is due to; 1. the increasing H2O(g) in blast concentration, and 2. the slight increase in air requirement, Fig. 12.2.
12.11 EFFECT OF H2O(g) CONCENTRATION ON STEADYSTATE CARBON REQUIREMENT Fig. 12.4 shows the effect of blast H2O(g) concentration on steady-state blast furnace Cin-coke and O2-in-blast requirements. Both increase slightly. Section 12.12 explains these results.
12.12 EXPLANATION Fig. 12.5 replots input C-in-coke and O2-inblast requirements of Fig. 12.4 as a function of through-tuyere H2O(g) input mass. Both increase. The C-in-coke requirement increases by 0.46 kg for every kg of H2O(g) input. This increase is a consequence of all Table 12.1 equations. We may, however, postulate that it is mostly due the large negative enthalpy that injected H2O(g) brings into the furnace, 210.81 MJ/kg, (Appendix J). This negative enthalpy must be offset by burning additional carbon in front of the tuyeres. This also explains the increased O2-in-blast requirement of Fig. 12.4. It is the oxygen that is needed to burn the abovementioned additional carbon in front of the tuyeres. Its requirement increases by 0.24 kg for every additional kg of through-tuyere H2O(g) input.
12.13 SUMMARY All blast furnaces blow H2O(g) through their tuyeres. The H2O(g) is from; 1. natural humidity in the blast’s input air, and 2. steam that is injected into the humid air. The effect of through-tuyere H2O(g) input is examined much like CH4(g) injection, Chapter 11, Methane—CH4(g)—Injection. The input concentration of H2O(g) is specified in equation form and the C, H, O, and enthalpy balances are modified to represent H2O(g) at 1200 C in place of CH4(g) at 25 C. H2O(g)-in-blast increases the blast furnace’s steady-state C-in-coke requirement per 1000 kg of Fe in product molten iron—thereby increasing product iron cost. The benefits of this extra cost are; 1. smooth, steady furnace operation, and burden descent; 2. quick blast furnace start-ups, especially when hanging occurs; 3. rapid flame temperature adjustment (Chapter 19: Raceway Flame Temperature with Moisture in Blast Air) by changing injected steam quantity (Fig. 12.1); and 4. ability to quickly control the hot metal thermal state and silicon content.
EXERCISES 12.1. For its furnace start-up, blast furnace team of Table 12.1 wishes to increase their H2O(g) in blast concentration to 25 g/Nm3 of dry air in blast. Please predict for them the amounts of C-incoke and O2-in-blast that will be needed for steady production of 1500 C molten iron with this H2O(g)-in-blast concentration. You may wish to use Appendix O.
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EXERCISES
Please give your answers in kg per 1000 kg of Fe in product molten iron. 12.2. Will the requirements of Exercise 12.1 be affected by blast temperature? Determine this by changing blast temperature of Exercise 12.1 to 1300 C. Use Appendix J.3. Examine Fig. 12.1 before completing this exercise. The H2O(g)-in-blast concentration remains at 25 g/Nm3 of dry air in blast. 12.3. Blast furnace plant of Table 12.1 is running out of coke. It can only afford to run the furnace with 395 kg (or less) of C-in-coke per 1000 kg of Fe in product molten iron. What is the maximum concentration of H2O(g)-in-blast that can be used while meeting this 395 kg of Cin-coke limitation. Please express your answer in grams H2O(g) in blast per Nm3
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of dry air in blast. The blast temperature is 1200 C, as in Table 12.1. 12.4. Blast furnace of Table 12.1 is using 15 g H2O(g)/Nm3 of dry air. Humid air of Fig. 12.1 contains 10 g H2O(g)/Nm3 of dry air. How much steam must be added to this humid air to attain the prescribed 15 g H2O(g)/Nm3 of dry air in blast? Please express your answer in; 1. g per Nm3 of dry air, 2. kg per kg of dry air, and 3. kg per 1000 kg (t) of Fe in product molten iron. 12.5. Blast furnace of Exercise 12.4 is producing molten iron at 400 t/h. At what rate must steam be injected into humid air of Fig. 12.1 to continuously meet the blast’s specified 15 kg of H2O (g)/Nm3 of dry air. Remember that the molten iron is not pure Fe.
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