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Top-Charged Scrap Steel
C H A P T E R
43 Top-Charged Scrap Steel O U T L I N E 43.1 Adding Fe-Rich Solids to the Blast Furnace
390
43.2 Including Top-Charged Scrap Steel in Our Calculations
390
43.3 No Oxidation of Scrap Steel in the Top Segment
390
43.4 Bottom-Segment Scrap Steel Quantity Specification
391
43.5 Scrap Steel Composition and Bottom-Segment Fe Mass Balance
391
43.6 Amended Bottom-Segment Enthalpy Balance
394
43.7 Nearly Completed BottomSegment Matrix
394
43.8 SiO2 - A Minor but Important Change
394
43.9 Raceway Matrix 43.10 Top-SegmentBottom-Segment Connection
Blast Furnace Ironmaking DOI: https://doi.org/10.1016/B978-0-12-814227-1.00043-9
43.11 Top-Segment Matrix
395
43.12 Top-Segment Equation
400
43.13 Amended Top-Segment Fe Mass Balance
400
43.14 Summary of Top-Segment Calculations With Scrap Steel Added
400
43.15 Calculation of Top Gas Temperature 43.15.1 Top-Segment Input Enthalpy 43.15.2 Top-Segment Output Enthalpy 43.15.3 Top Gas Enthalpy 43.15.4 Top Gas Temperature
402 402 402 402 402
43.16 Calculated Results - Coke Requirement
402
395
43.17 Calculated Results: Top Gas CO2 Emissions
404
395
43.18 Calculated Results: Blast Air Requirement
404
389
© 2020 Elsevier Inc. All rights reserved.
390
43. TOP-CHARGED SCRAP STEEL
43.19 Calculated Results: Raceway Flame Temperature
405
43.20 Calculated Results - CaO Flux Requirements
405
43.1 ADDING Fe-RICH SOLIDS TO THE BLAST FURNACE Many blast furnace operators top charge recycled steel and other Fe-rich solids to their blast furnaces. The advantages are; 1. 2. 3. 4.
accelerated molten iron production; lowered coke consumption; lowered CO2 emission; lowered cost, especially if there is a glut of cheap scrap steel; and 5. in-house consumption of Fe-rich residues avoiding disposal fees. Scrap steel charged to the blast furnace must be granular in nature and well sized to assure that it will pass through the charging system including the bell-less top. Fe-rich residues may be briquetted using a binder to increase their size so that the material is not blown out of the blast furnace. The principle alternatives to top-charged scrap steel are top-charged direct-reduced pellets or hot briquetted iron that have had B95% of their oxygen removed by CO(g) and H2(g) reduction in shaft furnaces. The objectives of this chapter are to; 1. demonstrate how top-charged scrap steel is included in our automated calculation matrices, and 2. show the effect of top-charged scrap steel on coke requirement, CO2 emissions, flame temperature, and top gas temperature.
43.21 Calculated Results - Top Gas Temperature
405
43.22 Summary
406
Exercise
406
For simplicity, we specify that the scrap is dry pure Fe. Chapter 44, Top Charged Direct Reduced Iron, examines a more complex material.
43.2 INCLUDING TOP-CHARGED SCRAP STEEL IN OUR CALCULATIONS Readers have now realized that the sequence of our calculations is always bottom segment then top segment. That is not immediately possible with scrap steel because;
mass top-charged scrap steel
is specified in the top-segment matrix. Our calculations are made possible by means of an out-of-matrix precalculation, as follows.
43.3 NO OXIDATION OF SCRAP STEEL IN THE TOP SEGMENT Conditions near the top of the blast furnace are oxidizing with respect to Fe. It is cool in this region, so we can assume that descending pieces of scrap will be oxidized slowly or not at all (Fig. 43.1).
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43.5 SCRAP STEEL COMPOSITION AND BOTTOM-SEGMENT Fe MASS BALANCE
and combine Eqs. (43.1) and (43.2) to give;
mass scrap steel descending 5 80 into bottom segment
(43.3)
This allows us to begin our calculations in the bottom segment. For matrix purposes, the equation is; 80 5
mass scrap steel descending 1 into bottom segment
where 80 is typed into Cell C33 of Table 43.1, and 1 is typed into Cell AH33 of Table 43.1.
43.5 SCRAP STEEL COMPOSITION AND BOTTOM-SEGMENT Fe MASS BALANCE
FIGURE 43.1 Sketch of 80 kg of scrap steel being charged, descending out of the top segment, descending into the bottom segment, and leaving the blast furnace dissolved in product molten iron. Its Fe is not oxidized during this journey, so each flow is 80 kg of Fe in scrap steel (per 1000 kg of Fe in product molten iron).
In this chapter, we specify that our scrap steel is pure Fe. This is expressed by the following equation;
mass scrap steel descending 5
into bottom segment mass scrap steel descending
5
out of top segment mass top-charged
(43.1)
scrap steel
We now specify that 80 kg of scrap steel is being charged to the top of the furnace as described by the following equation;
mass top-charged scrap steel
5 80 kg=1000 kg of Fe in product molten iron
(43.2)
5
mass scrap
scrap steel steel 100 mass% Fe in scrap steel 100%
43.4 BOTTOM-SEGMENT SCRAP STEEL QUANTITY SPECIFICATION We begin our bottom-segment calculations by specifying that with no oxidation in the top segment:
mass Fe in
(43.4)
which leads to the bottom-segment Fe mass balance;
mass Fe0:947 O descending 76:8 mass% Fe in Fe0:947 O 100% into bottom segment mass scrap steel descending 100 mass% Fe in scrap steel 1 100% into bottom segment mass Fe out 5 1 in molten iron
or
mass Fe0:947 O descending
0:768 into bottom segment mass scrap steel descending 1 1 into bottom segment mass Fe out 5 1 in molten iron
BLAST FURNACE IRONMAKING
TABLE 43.1
Bottom-Segment Matrix With Top-Charged Scrap Steel
The mass scrap descending into the bottom segment variable (Column AH) and quantity equation (Row 33) are notable. The amended Fe mass balance and enthalpy equations are also notable. For clarity, the matrix is presented here in three pieces. The tuyere injectants are 220 kg of pulverized coal (Cell C28), 92 kg of O2 in pure oxygen (Cell C29), and 0 kg of natural gas (Cell C31).
394
43. TOP-CHARGED SCRAP STEEL
mass Fe0:947 O descending 0:768 into bottom segment mass scrap steel descending 1 1 from both sides; into bottom segment
or
subtracting
mass Fe0:947 O descending
0:768 into bottom segment mass scrap steel descending 2 1 into bottom segment mass Fe out 1 1 in molten iron
052
(43.5)
as shown in Row 6 of matrix Table 43.1.
We have; 1. added one new variable,
mass scrap steel descending ; into bottom segment
2. added one new quantity specification mass scrap steel descending equation, 80 5 1; into bottom segment 3. specified that the scrap steel is 100% Fe; and 4. amended the bottom-segment Fe and enthalpy balance equations to include the new variable’s Fe and enthalpy contents.
43.6 AMENDED BOTTOMSEGMENT ENTHALPY BALANCE The descending scrap steel brings enthalpy into the bottom segment. The bottom-segment enthalpy equation must include an additional right-side term; H 930 C mass scrap steel descending scrap steel 2 into bottom segment MWscrap steel
43.8 SiO2 - A MINOR BUT IMPORTANT CHANGE Chapter 32, Bottom-Segment Slag Calculations - Ore, Fluxes, and Slag, calculates the amount of SiO2 in descending ore by following the equation:
which, in the case of pure Fe scrap, is; H 930 C mass scrap steel descending FeðsÞ 2 into bottom segment MWFe H 930 C
FeðsÞ where MW 5 0.6164 MJ/kg of Fe (Table J.1). Together, these give the new enthalpy equation term; Fe
mass scrap steel descending 2 0:6164 into bottom segment
as shown in Cell AH21 of Table 43.1. The enthalpy equation is renumbered to Eq. 43.6 in Row 21 of Table 43.1.
43.7 NEARLY COMPLETED BOTTOM-SEGMENT MATRIX
mass SiO2 in mass Fe in product 5 0:0753 descending ore molten iron
This is not suitable for this chapter because some of the Fe in the molten iron product comes from scrap steel. We now use Eq. (32.2) of Chapter 32, Bottom-Segment Slag Calculations—Ore, Fluxes, and Slag, which is:
mass SiO2 in mass Fe in 5 0:0753 descending ore descending ore
(32.2)
To make this useful, we make the bottomsegment substitution:
mass Fe in
descending ore mass Fe0:947 O into 76:8 mass% Fe in Fe0:947 O 5 100% bottom segment mass Fe0:947 O into 5 0:768 bottom segment
Our bottom-segment matrix is now nearly complete.
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43.11 TOP-SEGMENT MATRIX
or
mass Fe in mass Fe0:947 O into 5 0:768 descending ore bottom segment
(43.7)
Bottom-segment matrix Table 43.1 contains the equation;
Combining Eqs. (32.2) and (43.7) gives;
mass SiO2 in
43.10 TOP-SEGMENTBOTTOMSEGMENT CONNECTION
80 5
descending ore mass Fe0:947 O into 5 0:0753 0:768 bottom segment mass Fe0:947 O into 0:0578 5 bottom segment
mass scrap steel descending 1 into bottom segment
where 80 is typed in Cell C33 and 1 is typed in Cell AH33. We now connect this specification to the top segment by the following equation;
or 05 1
mass SiO2 in
80 5
1 descending ore mass Fe0:947 O into bottom segment
0:0578
(43.9)
(43.8)
as shown in Row 4 of bottom-segment matrix Table 43.1. The matrix is now solved as shown in Tables 43.1 and 43.2.
1. need to be included in the raceway matrix or; 2. our raceway input enthalpy, output enthalpy, or flame temperature calculations. This does not mean that the descending scrap steel does not affect the raceway flame temperature. In fact, it does because it affects the steadystate amounts of O2-in-blast, N2-in-blast, and H2O(g)-in-blast entering the raceway, per 1000 kg of Fe in product molten iron.
(43.10)
by typing 5 C33 in Cell BC34 and 1 in Cell CI34 of top-segment matrix Table 43.3. This is consistent with Fig. 43.1 and Eq. (43.1).
43.11 TOP-SEGMENT MATRIX
43.9 RACEWAY MATRIX Scrap steel does not enter the raceway so it does not;
mass top-charged 1 scrap steel
Fig. 43.1 shows that the top segment has two flows of scrap steel, that is; • top-charged flow, and • descent out of the top-segment flow. They have the same mass but different temperatures, hence different enthalpies. Both flows must be represented in the top-segment matrix - requiring two new variable columns and two additional equations. The variables are;
mass top-charged scrap steel
BLAST FURNACE IRONMAKING
and
mass scrap steel descending out of top segment
TABLE 43.2 Results From Solving Table 43.1 Matrix
They are used in our top-segment calculations.
TABLE 43.3
Top-Segment Matrix Including Top-Charged Scrap Steel and Scrap Steel Descending Out of the Top-Segment
(Continued)
TABLE 43.3
(Continued)
400
43. TOP-CHARGED SCRAP STEEL
43.12 TOP-SEGMENT EQUATION We begin our top-segment calculations with the equation in Fig. 43.1;
mass scrap steel descending 5
ing
mass Fe2 O3 in mass top-charged 0:699 1 1 top-charged ore scrap steel
from both sides, giving;
out of top segment mass top-charged scrap steel
mass top-charged
scrap steel
or
5
1 1
out of top segment
0:699 1
mass Fe0:947 O descending
(43.13)
out of top segment
mass scrap steel descending
mass top-charged 05 2 1 scrap steel mass scrap steel descending 1 1 out of top segment
mass top-charged scrap steel
(43.1)
top-charged ore
from which we now obtain the equation;
mass Fe2 O3 in
05
into bottom segment mass scrap steel descending
5
Eq. (43.12) is put in matrix form by subtract
0:768
mass scrap steel descending
out of top segment
1
as shown in Row 21 of Table 43.3.
(43.11)
as shown in Row 33 of Table 43.3.
43.14 SUMMARY OF TOP-SEGMENT CALCULATIONS WITH SCRAP STEEL ADDED Our top-segment calculations; 1. introduce two new variables;
43.13 AMENDED TOP-SEGMENT Fe MASS BALANCE
Including
the
mass scrap steel descending out of top segment
mass top-charged scrap steel
mass Fe2 O3 in
and
and
variables, the top-
mass scrap steel descending ; out of top segment
2. introduce two new equations, namely, a;
segment Fe mass balance is;
mass top-charged scrap steel
0:699 top-charged ore mass top-charged 1 1 scrap steel mass Fe0:947 O descending 5 0:768 out of top segment mass scrap steel descending 1 1 out of top segment
mass top-charged scrap steel
quantity specification equation, and; (43.12)
where the values “1” are 100 mass% Fe in scrap steel/100% as prescribed in Section 43.5.
mass scrap steel descending mass top-charged 5 out of top segment scrap steel
3. specify that the scrap steel is 100% Fe; and 4. amend the top-segment Fe mass balance equation to include the two new variables. Table 43.3 shows the top-segment matrix with these variables and equations. Table 43.4 shows the solution to the top segment matrix.
BLAST FURNACE IRONMAKING
43.14 SUMMARY OF TOP-SEGMENT CALCULATIONS WITH SCRAP STEEL ADDED
TABLE 43.4
Calculated Values of Matrix 43.3
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402
43. TOP-CHARGED SCRAP STEEL
43.15 CALCULATION OF TOP GAS TEMPERATURE The next few sections describe how to calculate top gas temperature when scrap steel is being charged to the blast furnace. It requires four calculation steps; 1. 2. 3. 4.
top-segment input enthalpy, top-segment output enthalpy, top gas enthalpy, and top gas temperature
43.15.2 Top-Segment Output Enthalpy Top-segment output enthalpy Eq. (40.3) is unchanged by top charging of scrap steel.
43.15.3 Top Gas Enthalpy Calculation of top gas enthalpy with top charging of scrap steel requires subtraction of the term;
as follows.
H 930 C mass scrap steel descending scrap steel out of top segment MWscrap steel
or for 100% Fe scrap;
43.15.1 Top-Segment Input Enthalpy The top charging of scrap steel to the blast furnace requires the addition of the term; H 25 C mass top-charged scrap steel 2 scrap steel MWscrap steel
H 930 C FeðsÞ MWFe out of top segment mass scrap steel descending
mass scrap steel descending
5
out of top segment 0:6164 MJ=kg of Fe
from the right side of top gas enthalpy Eq. (40.5), where H 930 C =MWFe
FeðsÞ
to the right side of top-segment input enthalpy. In our case of pure Fe scrap, this term is;
H 25 C mass top-charged FeðsÞ 2 scrap steel MWFe
5 0:6164 MJ=kg of Fe.
This gives Eq. (43.15) of
Table 43.5. Top gas enthalpy 5 1 BL113 2 BC59 2 15:41 2 BC60 2 15:41 2 BC61 1:359 2 BC62 2 10:5 2 BC63 2 3:152 2 BC64 2 13:84 2 BC65 2 4:77 2 BC66 2 14:13 2 BC67 2 14:13 2 BC80 0:6164
Of course, H 25 C =MWFe 5 0 as Fe is in its most
(43.15)
FeðsÞ
common state at 25 C so the final new term is; 2
mass Fe in top 0 charged scrap steel
43.15.4 Top Gas Temperature There is no Fe in the top gas, so the top gas temperature equation is unchanged by top charging of scrap steel.
as shown by Eq. (43.14) of Table 43.5. Top-segment input enthalpy 5 1 BC50 2 5:163 1 BC51 2 15:16 1 BC52 0 1 BC53 2 16:43 1 BC54 2 15:16 1 BC55 2 16:43 1 BC56 2 11:32 1 BC57 2 14:92 1 BC58 2 5:98 1 BC68 2 2:926 1 BC69 2 7:926 1 BC70 2 13:35 1 BC71 2 11:49 1 BC72 2 1:008 1 BC78 2 15 87 1 BC81 0 (43.14)
43.16 CALCULATED RESULTS COKE REQUIREMENT Fig. 43.2 shows the effect of top-charged scrap on the amount of coke required to steadily produce 1500 C molten iron and slag. An amount of 100 kg of Fe scrap saves B35 kg of coke.
BLAST FURNACE IRONMAKING
TABLE 43.5 Scrap Steel
Equations and Calculations of (1) Top-Segment Input and Output Enthalpies, (2) Top Gas Enthalpy, and (3) Top Gas Temperature With Charging of
404
43. TOP-CHARGED SCRAP STEEL
FIGURE 43.2 Effect of top-charged Fe scrap on the amount of coke needed to steadily produce molten iron and molten slag, 1500 C. One kilogram of Fe scrap saves 0.35 kg of 90% C, 3% Al2O3, and 7% SiO2 coke. The line is straight.
This saving is due to the portion of Fe in the product molten iron that is produced directly from Fe scrap, that is;
FIGURE 43.3 Effect of top-charged scrap steel on the steady-state blast furnace CO2(g) emission from the blast furnace of Fig. 43.1. Scrap steel lowers CO2(g) emission by B0.66 kg/kg of top-charged scrap steel. This decrease is expected because steady-state coke requirement decreases with increasing mass scrap (Fig. 43.2). The line is straight.
• without using any carbon for iron oxide reduction. The saving is smaller than might be expected. This is because the solid scrap must be heated and melted in the bottom segment (Fig. 43.1), that is, by combusting C-in-coke with O2-in-blast.
43.17 CALCULATED RESULTS: TOP GAS CO2 EMISSIONS Fig. 43.3 shows the effect of top-charged scrap steel on the steady-state amount of CO2 that is being emitted in a blast furnace’s top gas. As expected from Fig. 43.2, scrap charging decreases C oxidation, hence CO2(g) emissions. CO2(g) is a greenhouse gas contributing to global warming, so the trend of decreasing CO2(g) emissions seen in Fig. 43.3 is valuable to the environment. In those countries that levy a carbon tax on CO2(g) emissions, the lower CO2(g) emission will also have financial benefits. Such taxes are expected to increase over time.
FIGURE 43.4 Effect of top-charged scrap steel on the amount of dry air required to steadily produce molten iron and molten slag, 1500 C. Blast air requirement decreases by B1 kg/kg of top-charged scrap steel.
43.18 CALCULATED RESULTS: BLAST AIR REQUIREMENT Fig. 43.4 shows the effect of top-charged scrap steel on the blast air requirement. It decreases the dry air requirement by B1 kg of air/kg of scrap steel.
BLAST FURNACE IRONMAKING
43.21 CALCULATED RESULTS - TOP GAS TEMPERATURE
FIGURE 43.5 Effect of top-charged scrap steel on raceway flame temperature. Flame temperature drops about 12 C with 100 kg of top-charged scrap steel. This is a result of all our equations. The line is not quite straight.
This decrease is the result of all our equations. We may postulate that it is mainly due to less coke combustion in front of the tuyeres (Fig. 43.2).
43.19 CALCULATED RESULTS: RACEWAY FLAME TEMPERATURE Fig. 43.5 shows the effect of top-charged scrap steel on tuyere raceway flame temperature. It lowers the flame temperature by about 12 C/100 kg of top-charged scrap steel. Scrap steel does not enter the raceway, so it does not directly affect raceway flame temperature. We may speculate that it has the effect of decreasing coke combustion in front of the tuyeres, Fig. 43.2, thereby lowering flame temperature.
43.20 CALCULATED RESULTS CaO FLUX REQUIREMENTS Fig. 43.6 shows the effect of top-charged scrap steel on CaO flux requirement. The
405
FIGURE 43.6 Effect of top-charged scrap steel on the amount of CaO flux that is required to produce 10 mass% Al2O3, 41 mass% CaO, 10 mass% MgO, and 39 mass% SiO2 molten slag, 1500 C, of Chapter 32, Bottom-Segment Slag Calculations—Ore, Fluxes, and Slag. The CaO requirement decreases with increasing scrap steel. This is because the scrap steel contains no SiO2 while the ore contains 36 mass% SiO2 - which needs fluxing with CaO. Al2O3 and MgO requirements correspondingly decrease.
requirement decreases with increasing mass top-charged scrap steel. This is a consequence of the scrap steel containing no SiO2, decreasing the need for CaO fluxing.
43.21 CALCULATED RESULTS TOP GAS TEMPERATURE Although there is no Fe in the blast furnace’s departing top gas, the amounts of CO, CO2, H2, H2O, and N2 vary with the amount of scrap that is being charged to the furnace— thus changing the top gas temperature (Fig. 43.7). This is the consequence of all our equations. We may speculate that it is the result of less hot nitrogen (Fig. 43.4) rising into the top segment.
BLAST FURNACE IRONMAKING
406
43. TOP-CHARGED SCRAP STEEL
In this chapter, we show that top charging of scrap steel decreases both tuyere raceway flame temperature and top gas temperature. This is unusual because all our previous chapters have shown that if flame temperature is lowered by making a change to blast furnace operation, for example, coal injection—top gas temperature rises—and vice versa.
EXERCISE FIGURE 43.7 Effect of top-charged scrap steel on top gas temperature. The temperature falls about 7 C/100 kg of scrap charge. The line is slightly curved.
43.22 SUMMARY Scrap steel and other Fe-rich solids are often charged to the top of the blast furnace. The procedure; 1. saves coke, 2. increases molten iron production rate, 3. decreases greenhouse gas [CO2(g)] emission, and 4. consumes Fe-rich materials that may otherwise be disposed of. Calculations of this chapter explain and quantify these observations. The calculations are different than our previous calculations because they rely on specifying that the top-charged scrap steel does not oxidize in the top segment of the furnace. This specification is consistent with our Chapter 2, Inside the Blast Furnace, specification that Cin-coke does not oxidize in the top segment of the furnace.
All masses in these calculations are kg/1000 kg of Fe in product molten iron. Throughout this chapter, the reference blast furnace is being injected with 220 kg of pulverized coal and 92 kg of pure oxygen. The 1200 C blast contains 15 g of H2O(g)/ Nm3 of dry air in blast and all the fluxes are oxides. These values are based on an industrial blast furnace. The top charge contains 5 mass% H2O(‘), excluding the scrap, which is dry. 43.1. The blast furnace of Fig. 43.1 is top charging 80 kg of scrap (pure Fe) steel. However, its operators anticipate a shortage of scrap so they start lowering the scrap charge to 40 kg/1000 kg of Fe in product molten iron. They know from Fig. 43.6 that their Al2O3, CaO, and MgO flux requirements will all increase (per 1000 kg of Fe in product molten iron) with this decreased scrap top charge, but not by how much. For the operators, please calculate; 1. how much additional SiO2 must be fluxed, and
BLAST FURNACE IRONMAKING
407
EXERCISE
2. how much additional Al2O3, CaO, and MgO flux will be required when the scrap top charge is decreased from 80 kg of scrap steel to 40 kg of scrap steel. 43.2. Luckily, more cheap scrap steel has become available, and the blast furnace operators of Exercise 43.1 now want to
increase scrap steel charging as much as possible. However, they do not want their top gas temperature to fall below 110 C. What is the upper limit of scrap steel charging that can be used without causing the top gas temperature to fall below this value?
BLAST FURNACE IRONMAKING
43 Top-Charged Scrap Steel O U T L I N E 43.1 Adding Fe-Rich Solids to the Blast Furnace
390
43.2 Including Top-Charged Scrap Steel in Our Calculations
390
43.3 No Oxidation of Scrap Steel in the Top Segment
390
43.4 Bottom-Segment Scrap Steel Quantity Specification
391
43.5 Scrap Steel Composition and Bottom-Segment Fe Mass Balance
391
43.6 Amended Bottom-Segment Enthalpy Balance
394
43.7 Nearly Completed BottomSegment Matrix
394
43.8 SiO2 - A Minor but Important Change
394
43.9 Raceway Matrix 43.10 Top-SegmentBottom-Segment Connection
Blast Furnace Ironmaking DOI: https://doi.org/10.1016/B978-0-12-814227-1.00043-9
43.11 Top-Segment Matrix
395
43.12 Top-Segment Equation
400
43.13 Amended Top-Segment Fe Mass Balance
400
43.14 Summary of Top-Segment Calculations With Scrap Steel Added
400
43.15 Calculation of Top Gas Temperature 43.15.1 Top-Segment Input Enthalpy 43.15.2 Top-Segment Output Enthalpy 43.15.3 Top Gas Enthalpy 43.15.4 Top Gas Temperature
402 402 402 402 402
43.16 Calculated Results - Coke Requirement
402
395
43.17 Calculated Results: Top Gas CO2 Emissions
404
395
43.18 Calculated Results: Blast Air Requirement
404
389
© 2020 Elsevier Inc. All rights reserved.
390
43. TOP-CHARGED SCRAP STEEL
43.19 Calculated Results: Raceway Flame Temperature
405
43.20 Calculated Results - CaO Flux Requirements
405
43.1 ADDING Fe-RICH SOLIDS TO THE BLAST FURNACE Many blast furnace operators top charge recycled steel and other Fe-rich solids to their blast furnaces. The advantages are; 1. 2. 3. 4.
accelerated molten iron production; lowered coke consumption; lowered CO2 emission; lowered cost, especially if there is a glut of cheap scrap steel; and 5. in-house consumption of Fe-rich residues avoiding disposal fees. Scrap steel charged to the blast furnace must be granular in nature and well sized to assure that it will pass through the charging system including the bell-less top. Fe-rich residues may be briquetted using a binder to increase their size so that the material is not blown out of the blast furnace. The principle alternatives to top-charged scrap steel are top-charged direct-reduced pellets or hot briquetted iron that have had B95% of their oxygen removed by CO(g) and H2(g) reduction in shaft furnaces. The objectives of this chapter are to; 1. demonstrate how top-charged scrap steel is included in our automated calculation matrices, and 2. show the effect of top-charged scrap steel on coke requirement, CO2 emissions, flame temperature, and top gas temperature.
43.21 Calculated Results - Top Gas Temperature
405
43.22 Summary
406
Exercise
406
For simplicity, we specify that the scrap is dry pure Fe. Chapter 44, Top Charged Direct Reduced Iron, examines a more complex material.
43.2 INCLUDING TOP-CHARGED SCRAP STEEL IN OUR CALCULATIONS Readers have now realized that the sequence of our calculations is always bottom segment then top segment. That is not immediately possible with scrap steel because;
mass top-charged scrap steel
is specified in the top-segment matrix. Our calculations are made possible by means of an out-of-matrix precalculation, as follows.
43.3 NO OXIDATION OF SCRAP STEEL IN THE TOP SEGMENT Conditions near the top of the blast furnace are oxidizing with respect to Fe. It is cool in this region, so we can assume that descending pieces of scrap will be oxidized slowly or not at all (Fig. 43.1).
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391
43.5 SCRAP STEEL COMPOSITION AND BOTTOM-SEGMENT Fe MASS BALANCE
and combine Eqs. (43.1) and (43.2) to give;
mass scrap steel descending 5 80 into bottom segment
(43.3)
This allows us to begin our calculations in the bottom segment. For matrix purposes, the equation is; 80 5
mass scrap steel descending 1 into bottom segment
where 80 is typed into Cell C33 of Table 43.1, and 1 is typed into Cell AH33 of Table 43.1.
43.5 SCRAP STEEL COMPOSITION AND BOTTOM-SEGMENT Fe MASS BALANCE
FIGURE 43.1 Sketch of 80 kg of scrap steel being charged, descending out of the top segment, descending into the bottom segment, and leaving the blast furnace dissolved in product molten iron. Its Fe is not oxidized during this journey, so each flow is 80 kg of Fe in scrap steel (per 1000 kg of Fe in product molten iron).
In this chapter, we specify that our scrap steel is pure Fe. This is expressed by the following equation;
mass scrap steel descending 5
into bottom segment mass scrap steel descending
5
out of top segment mass top-charged
(43.1)
scrap steel
We now specify that 80 kg of scrap steel is being charged to the top of the furnace as described by the following equation;
mass top-charged scrap steel
5 80 kg=1000 kg of Fe in product molten iron
(43.2)
5
mass scrap
scrap steel steel 100 mass% Fe in scrap steel 100%
43.4 BOTTOM-SEGMENT SCRAP STEEL QUANTITY SPECIFICATION We begin our bottom-segment calculations by specifying that with no oxidation in the top segment:
mass Fe in
(43.4)
which leads to the bottom-segment Fe mass balance;
mass Fe0:947 O descending 76:8 mass% Fe in Fe0:947 O 100% into bottom segment mass scrap steel descending 100 mass% Fe in scrap steel 1 100% into bottom segment mass Fe out 5 1 in molten iron
or
mass Fe0:947 O descending
0:768 into bottom segment mass scrap steel descending 1 1 into bottom segment mass Fe out 5 1 in molten iron
BLAST FURNACE IRONMAKING
TABLE 43.1
Bottom-Segment Matrix With Top-Charged Scrap Steel
The mass scrap descending into the bottom segment variable (Column AH) and quantity equation (Row 33) are notable. The amended Fe mass balance and enthalpy equations are also notable. For clarity, the matrix is presented here in three pieces. The tuyere injectants are 220 kg of pulverized coal (Cell C28), 92 kg of O2 in pure oxygen (Cell C29), and 0 kg of natural gas (Cell C31).
394
43. TOP-CHARGED SCRAP STEEL
mass Fe0:947 O descending 0:768 into bottom segment mass scrap steel descending 1 1 from both sides; into bottom segment
or
subtracting
mass Fe0:947 O descending
0:768 into bottom segment mass scrap steel descending 2 1 into bottom segment mass Fe out 1 1 in molten iron
052
(43.5)
as shown in Row 6 of matrix Table 43.1.
We have; 1. added one new variable,
mass scrap steel descending ; into bottom segment
2. added one new quantity specification mass scrap steel descending equation, 80 5 1; into bottom segment 3. specified that the scrap steel is 100% Fe; and 4. amended the bottom-segment Fe and enthalpy balance equations to include the new variable’s Fe and enthalpy contents.
43.6 AMENDED BOTTOMSEGMENT ENTHALPY BALANCE The descending scrap steel brings enthalpy into the bottom segment. The bottom-segment enthalpy equation must include an additional right-side term; H 930 C mass scrap steel descending scrap steel 2 into bottom segment MWscrap steel
43.8 SiO2 - A MINOR BUT IMPORTANT CHANGE Chapter 32, Bottom-Segment Slag Calculations - Ore, Fluxes, and Slag, calculates the amount of SiO2 in descending ore by following the equation:
which, in the case of pure Fe scrap, is; H 930 C mass scrap steel descending FeðsÞ 2 into bottom segment MWFe H 930 C
FeðsÞ where MW 5 0.6164 MJ/kg of Fe (Table J.1). Together, these give the new enthalpy equation term; Fe
mass scrap steel descending 2 0:6164 into bottom segment
as shown in Cell AH21 of Table 43.1. The enthalpy equation is renumbered to Eq. 43.6 in Row 21 of Table 43.1.
43.7 NEARLY COMPLETED BOTTOM-SEGMENT MATRIX
mass SiO2 in mass Fe in product 5 0:0753 descending ore molten iron
This is not suitable for this chapter because some of the Fe in the molten iron product comes from scrap steel. We now use Eq. (32.2) of Chapter 32, Bottom-Segment Slag Calculations—Ore, Fluxes, and Slag, which is:
mass SiO2 in mass Fe in 5 0:0753 descending ore descending ore
(32.2)
To make this useful, we make the bottomsegment substitution:
mass Fe in
descending ore mass Fe0:947 O into 76:8 mass% Fe in Fe0:947 O 5 100% bottom segment mass Fe0:947 O into 5 0:768 bottom segment
Our bottom-segment matrix is now nearly complete.
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43.11 TOP-SEGMENT MATRIX
or
mass Fe in mass Fe0:947 O into 5 0:768 descending ore bottom segment
(43.7)
Bottom-segment matrix Table 43.1 contains the equation;
Combining Eqs. (32.2) and (43.7) gives;
mass SiO2 in
43.10 TOP-SEGMENTBOTTOMSEGMENT CONNECTION
80 5
descending ore mass Fe0:947 O into 5 0:0753 0:768 bottom segment mass Fe0:947 O into 0:0578 5 bottom segment
mass scrap steel descending 1 into bottom segment
where 80 is typed in Cell C33 and 1 is typed in Cell AH33. We now connect this specification to the top segment by the following equation;
or 05 1
mass SiO2 in
80 5
1 descending ore mass Fe0:947 O into bottom segment
0:0578
(43.9)
(43.8)
as shown in Row 4 of bottom-segment matrix Table 43.1. The matrix is now solved as shown in Tables 43.1 and 43.2.
1. need to be included in the raceway matrix or; 2. our raceway input enthalpy, output enthalpy, or flame temperature calculations. This does not mean that the descending scrap steel does not affect the raceway flame temperature. In fact, it does because it affects the steadystate amounts of O2-in-blast, N2-in-blast, and H2O(g)-in-blast entering the raceway, per 1000 kg of Fe in product molten iron.
(43.10)
by typing 5 C33 in Cell BC34 and 1 in Cell CI34 of top-segment matrix Table 43.3. This is consistent with Fig. 43.1 and Eq. (43.1).
43.11 TOP-SEGMENT MATRIX
43.9 RACEWAY MATRIX Scrap steel does not enter the raceway so it does not;
mass top-charged 1 scrap steel
Fig. 43.1 shows that the top segment has two flows of scrap steel, that is; • top-charged flow, and • descent out of the top-segment flow. They have the same mass but different temperatures, hence different enthalpies. Both flows must be represented in the top-segment matrix - requiring two new variable columns and two additional equations. The variables are;
mass top-charged scrap steel
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and
mass scrap steel descending out of top segment
TABLE 43.2 Results From Solving Table 43.1 Matrix
They are used in our top-segment calculations.
TABLE 43.3
Top-Segment Matrix Including Top-Charged Scrap Steel and Scrap Steel Descending Out of the Top-Segment
(Continued)
TABLE 43.3
(Continued)
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43. TOP-CHARGED SCRAP STEEL
43.12 TOP-SEGMENT EQUATION We begin our top-segment calculations with the equation in Fig. 43.1;
mass scrap steel descending 5
ing
mass Fe2 O3 in mass top-charged 0:699 1 1 top-charged ore scrap steel
from both sides, giving;
out of top segment mass top-charged scrap steel
mass top-charged
scrap steel
or
5
1 1
out of top segment
0:699 1
mass Fe0:947 O descending
(43.13)
out of top segment
mass scrap steel descending
mass top-charged 05 2 1 scrap steel mass scrap steel descending 1 1 out of top segment
mass top-charged scrap steel
(43.1)
top-charged ore
from which we now obtain the equation;
mass Fe2 O3 in
05
into bottom segment mass scrap steel descending
5
Eq. (43.12) is put in matrix form by subtract
0:768
mass scrap steel descending
out of top segment
1
as shown in Row 21 of Table 43.3.
(43.11)
as shown in Row 33 of Table 43.3.
43.14 SUMMARY OF TOP-SEGMENT CALCULATIONS WITH SCRAP STEEL ADDED Our top-segment calculations; 1. introduce two new variables;
43.13 AMENDED TOP-SEGMENT Fe MASS BALANCE
Including
the
mass scrap steel descending out of top segment
mass top-charged scrap steel
mass Fe2 O3 in
and
and
variables, the top-
mass scrap steel descending ; out of top segment
2. introduce two new equations, namely, a;
segment Fe mass balance is;
mass top-charged scrap steel
0:699 top-charged ore mass top-charged 1 1 scrap steel mass Fe0:947 O descending 5 0:768 out of top segment mass scrap steel descending 1 1 out of top segment
mass top-charged scrap steel
quantity specification equation, and; (43.12)
where the values “1” are 100 mass% Fe in scrap steel/100% as prescribed in Section 43.5.
mass scrap steel descending mass top-charged 5 out of top segment scrap steel
3. specify that the scrap steel is 100% Fe; and 4. amend the top-segment Fe mass balance equation to include the two new variables. Table 43.3 shows the top-segment matrix with these variables and equations. Table 43.4 shows the solution to the top segment matrix.
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43.14 SUMMARY OF TOP-SEGMENT CALCULATIONS WITH SCRAP STEEL ADDED
TABLE 43.4
Calculated Values of Matrix 43.3
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402
43. TOP-CHARGED SCRAP STEEL
43.15 CALCULATION OF TOP GAS TEMPERATURE The next few sections describe how to calculate top gas temperature when scrap steel is being charged to the blast furnace. It requires four calculation steps; 1. 2. 3. 4.
top-segment input enthalpy, top-segment output enthalpy, top gas enthalpy, and top gas temperature
43.15.2 Top-Segment Output Enthalpy Top-segment output enthalpy Eq. (40.3) is unchanged by top charging of scrap steel.
43.15.3 Top Gas Enthalpy Calculation of top gas enthalpy with top charging of scrap steel requires subtraction of the term;
as follows.
H 930 C mass scrap steel descending scrap steel out of top segment MWscrap steel
or for 100% Fe scrap;
43.15.1 Top-Segment Input Enthalpy The top charging of scrap steel to the blast furnace requires the addition of the term; H 25 C mass top-charged scrap steel 2 scrap steel MWscrap steel
H 930 C FeðsÞ MWFe out of top segment mass scrap steel descending
mass scrap steel descending
5
out of top segment 0:6164 MJ=kg of Fe
from the right side of top gas enthalpy Eq. (40.5), where H 930 C =MWFe
FeðsÞ
to the right side of top-segment input enthalpy. In our case of pure Fe scrap, this term is;
H 25 C mass top-charged FeðsÞ 2 scrap steel MWFe
5 0:6164 MJ=kg of Fe.
This gives Eq. (43.15) of
Table 43.5. Top gas enthalpy 5 1 BL113 2 BC59 2 15:41 2 BC60 2 15:41 2 BC61 1:359 2 BC62 2 10:5 2 BC63 2 3:152 2 BC64 2 13:84 2 BC65 2 4:77 2 BC66 2 14:13 2 BC67 2 14:13 2 BC80 0:6164
Of course, H 25 C =MWFe 5 0 as Fe is in its most
(43.15)
FeðsÞ
common state at 25 C so the final new term is; 2
mass Fe in top 0 charged scrap steel
43.15.4 Top Gas Temperature There is no Fe in the top gas, so the top gas temperature equation is unchanged by top charging of scrap steel.
as shown by Eq. (43.14) of Table 43.5. Top-segment input enthalpy 5 1 BC50 2 5:163 1 BC51 2 15:16 1 BC52 0 1 BC53 2 16:43 1 BC54 2 15:16 1 BC55 2 16:43 1 BC56 2 11:32 1 BC57 2 14:92 1 BC58 2 5:98 1 BC68 2 2:926 1 BC69 2 7:926 1 BC70 2 13:35 1 BC71 2 11:49 1 BC72 2 1:008 1 BC78 2 15 87 1 BC81 0 (43.14)
43.16 CALCULATED RESULTS COKE REQUIREMENT Fig. 43.2 shows the effect of top-charged scrap on the amount of coke required to steadily produce 1500 C molten iron and slag. An amount of 100 kg of Fe scrap saves B35 kg of coke.
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TABLE 43.5 Scrap Steel
Equations and Calculations of (1) Top-Segment Input and Output Enthalpies, (2) Top Gas Enthalpy, and (3) Top Gas Temperature With Charging of
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43. TOP-CHARGED SCRAP STEEL
FIGURE 43.2 Effect of top-charged Fe scrap on the amount of coke needed to steadily produce molten iron and molten slag, 1500 C. One kilogram of Fe scrap saves 0.35 kg of 90% C, 3% Al2O3, and 7% SiO2 coke. The line is straight.
This saving is due to the portion of Fe in the product molten iron that is produced directly from Fe scrap, that is;
FIGURE 43.3 Effect of top-charged scrap steel on the steady-state blast furnace CO2(g) emission from the blast furnace of Fig. 43.1. Scrap steel lowers CO2(g) emission by B0.66 kg/kg of top-charged scrap steel. This decrease is expected because steady-state coke requirement decreases with increasing mass scrap (Fig. 43.2). The line is straight.
• without using any carbon for iron oxide reduction. The saving is smaller than might be expected. This is because the solid scrap must be heated and melted in the bottom segment (Fig. 43.1), that is, by combusting C-in-coke with O2-in-blast.
43.17 CALCULATED RESULTS: TOP GAS CO2 EMISSIONS Fig. 43.3 shows the effect of top-charged scrap steel on the steady-state amount of CO2 that is being emitted in a blast furnace’s top gas. As expected from Fig. 43.2, scrap charging decreases C oxidation, hence CO2(g) emissions. CO2(g) is a greenhouse gas contributing to global warming, so the trend of decreasing CO2(g) emissions seen in Fig. 43.3 is valuable to the environment. In those countries that levy a carbon tax on CO2(g) emissions, the lower CO2(g) emission will also have financial benefits. Such taxes are expected to increase over time.
FIGURE 43.4 Effect of top-charged scrap steel on the amount of dry air required to steadily produce molten iron and molten slag, 1500 C. Blast air requirement decreases by B1 kg/kg of top-charged scrap steel.
43.18 CALCULATED RESULTS: BLAST AIR REQUIREMENT Fig. 43.4 shows the effect of top-charged scrap steel on the blast air requirement. It decreases the dry air requirement by B1 kg of air/kg of scrap steel.
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43.21 CALCULATED RESULTS - TOP GAS TEMPERATURE
FIGURE 43.5 Effect of top-charged scrap steel on raceway flame temperature. Flame temperature drops about 12 C with 100 kg of top-charged scrap steel. This is a result of all our equations. The line is not quite straight.
This decrease is the result of all our equations. We may postulate that it is mainly due to less coke combustion in front of the tuyeres (Fig. 43.2).
43.19 CALCULATED RESULTS: RACEWAY FLAME TEMPERATURE Fig. 43.5 shows the effect of top-charged scrap steel on tuyere raceway flame temperature. It lowers the flame temperature by about 12 C/100 kg of top-charged scrap steel. Scrap steel does not enter the raceway, so it does not directly affect raceway flame temperature. We may speculate that it has the effect of decreasing coke combustion in front of the tuyeres, Fig. 43.2, thereby lowering flame temperature.
43.20 CALCULATED RESULTS CaO FLUX REQUIREMENTS Fig. 43.6 shows the effect of top-charged scrap steel on CaO flux requirement. The
405
FIGURE 43.6 Effect of top-charged scrap steel on the amount of CaO flux that is required to produce 10 mass% Al2O3, 41 mass% CaO, 10 mass% MgO, and 39 mass% SiO2 molten slag, 1500 C, of Chapter 32, Bottom-Segment Slag Calculations—Ore, Fluxes, and Slag. The CaO requirement decreases with increasing scrap steel. This is because the scrap steel contains no SiO2 while the ore contains 36 mass% SiO2 - which needs fluxing with CaO. Al2O3 and MgO requirements correspondingly decrease.
requirement decreases with increasing mass top-charged scrap steel. This is a consequence of the scrap steel containing no SiO2, decreasing the need for CaO fluxing.
43.21 CALCULATED RESULTS TOP GAS TEMPERATURE Although there is no Fe in the blast furnace’s departing top gas, the amounts of CO, CO2, H2, H2O, and N2 vary with the amount of scrap that is being charged to the furnace— thus changing the top gas temperature (Fig. 43.7). This is the consequence of all our equations. We may speculate that it is the result of less hot nitrogen (Fig. 43.4) rising into the top segment.
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43. TOP-CHARGED SCRAP STEEL
In this chapter, we show that top charging of scrap steel decreases both tuyere raceway flame temperature and top gas temperature. This is unusual because all our previous chapters have shown that if flame temperature is lowered by making a change to blast furnace operation, for example, coal injection—top gas temperature rises—and vice versa.
EXERCISE FIGURE 43.7 Effect of top-charged scrap steel on top gas temperature. The temperature falls about 7 C/100 kg of scrap charge. The line is slightly curved.
43.22 SUMMARY Scrap steel and other Fe-rich solids are often charged to the top of the blast furnace. The procedure; 1. saves coke, 2. increases molten iron production rate, 3. decreases greenhouse gas [CO2(g)] emission, and 4. consumes Fe-rich materials that may otherwise be disposed of. Calculations of this chapter explain and quantify these observations. The calculations are different than our previous calculations because they rely on specifying that the top-charged scrap steel does not oxidize in the top segment of the furnace. This specification is consistent with our Chapter 2, Inside the Blast Furnace, specification that Cin-coke does not oxidize in the top segment of the furnace.
All masses in these calculations are kg/1000 kg of Fe in product molten iron. Throughout this chapter, the reference blast furnace is being injected with 220 kg of pulverized coal and 92 kg of pure oxygen. The 1200 C blast contains 15 g of H2O(g)/ Nm3 of dry air in blast and all the fluxes are oxides. These values are based on an industrial blast furnace. The top charge contains 5 mass% H2O(‘), excluding the scrap, which is dry. 43.1. The blast furnace of Fig. 43.1 is top charging 80 kg of scrap (pure Fe) steel. However, its operators anticipate a shortage of scrap so they start lowering the scrap charge to 40 kg/1000 kg of Fe in product molten iron. They know from Fig. 43.6 that their Al2O3, CaO, and MgO flux requirements will all increase (per 1000 kg of Fe in product molten iron) with this decreased scrap top charge, but not by how much. For the operators, please calculate; 1. how much additional SiO2 must be fluxed, and
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EXERCISE
2. how much additional Al2O3, CaO, and MgO flux will be required when the scrap top charge is decreased from 80 kg of scrap steel to 40 kg of scrap steel. 43.2. Luckily, more cheap scrap steel has become available, and the blast furnace operators of Exercise 43.1 now want to
increase scrap steel charging as much as possible. However, they do not want their top gas temperature to fall below 110 C. What is the upper limit of scrap steel charging that can be used without causing the top gas temperature to fall below this value?
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