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Top Segment With Carbonate Fluxes
C H A P T E R
42 Top Segment With Carbonate Fluxes O U T L I N E 42.1 Understanding the Impact of Carbonate Fluxes on the Blast Furnace Process
379
42.2 Amended Top-Segment Variables and Equations for Carbonates 380 42.3 New Variable and Its Associated Equation for Carbonates 42.4 Amended Top-Segment Input Enthalpy Equation With Carbonates Added
381
42.5 Top-Segment Output Enthalpy With Carbonates Added
386
42.6 Top Gas Enthalpy
386
42.7 Top Gas Temperature
386
42.8 Results
386
42.9 Summary
386
Exercises
388
381
42.1 UNDERSTANDING THE IMPACT OF CARBONATE FLUXES ON THE BLAST FURNACE PROCESS
lump iron ore. Small carbonate flux additions are also used for final slag chemistry control. These carbonates decompose in the top segment to form solid oxide and CO2 gas by reactions such as;
Previous top-segment chapters have assumed that our slag components enter the blast furnace as oxides, for example, CaO and MgO. This is the case for many industrial blast furnaces, especially those being charged with self-fluxing sinter and pellets. Many blast furnaces charge Ca and Mg carbonate fluxes, especially those that charge
CaCO3 ðsÞ-CaOðsÞ 1 CO2 g MgCO3 ðsÞ-MgOðsÞ 1 CO2 g
Blast Furnace Ironmaking DOI: https://doi.org/10.1016/B978-0-12-814227-1.00042-7
(42.1) (42.2)
The product solids descend into the bottom segment, where they ultimately form molten slag (Fig. 42.1).
379
© 2020 Elsevier Inc. All rights reserved.
380
42. TOP SEGMENT WITH CARBONATE FLUXES
42.2 AMENDED TOP-SEGMENT VARIABLES AND EQUATIONS FOR CARBONATES Replacement of oxide fluxes with carbonate fluxes changes;
mass top-charged mass top-charged to CaO flux CaCO3 flux
mass top-charged mass top-charged to MgCO3 flux MgO flux
1: the variable
FIGURE 42.1 Conceptual blast furnace top segment with CaCO3 and MgCO3 fluxes (and moisture) in the top charge. We specify that all the CO2(g) from carbonate decomposition and all the H2O(g) from moisture evaporation leave the furnace without reacting. Notice that Ca and Mg enter the furnace in carbonates but descend out of the top segment as oxides.
and 2: the variable
It also changes the top-segment CaO mass balance; The product CO2 joins the gases that are rising through the top segment, ultimately ending up in the furnace top gas (Fig. 42.1). Decomposition reactions (42.1) and (42.2) are endothermic. They absorb heat from the rising bottom-segment gases, ultimately resulting in top gas that is cooler than when the topcharged fluxes are oxides (Chapter 47: Bottom Segment Calculations with CO Injection). In this chapter, we examine the effects of charging carbonate fluxes to the blast furnace, especially their effect on top gas composition, enthalpy, and temperature. Top-segment flows with carbonate fluxes [plus top-charged H2O (‘)] are shown in Fig. 42.1. The objectives of this chapter are to; 1. calculate top gas composition, enthalpy, and temperature with top-charged carbonate fluxes, and 2. compare these calculated quantities obtained with top-charged oxide fluxes.
05
mass top-charged
1
CaO flux
mass CaO descending
1
out of top segment
1
to
mass top-charged
0:56 CaCO3 flux mass CaO descending 1 1 out of top segment
05
(42.3)
where 0.56 is 56 mass% CaO in CaCO3/100%. The top-segment MgO mass balance changes to; 05
mass top-charged
0:478 MgCO3 flux mass MgO descending 1 1 out of top segment
(42.4)
where 0.478 is 47.8 mass% MgO in MgCO3/100%.
BLAST FURNACE IRONMAKING
381
42.4 AMENDED TOP-SEGMENT INPUT ENTHALPY EQUATION WITH CARBONATES ADDED
42.3 NEW VARIABLE AND ITS ASSOCIATED EQUATION FOR CARBONATES
05
mass top-charged
CaCO3 flux mass top-charged
Carbonate decomposition introduces one new variable to our top-segment matrix. It is;
mass top-gas CO2 from carbonates decomposition
This variable is added to the right-most side of matrix Table 41.1 in new Column CH. All this CO2 comes from CaCO3 and MgCO3 decomposition and is best described by the following equation;
mass top-gas CO2 from carbonates decomposition 5
mass top-charged 44:0 mass% CO2 in CaCO3 100% CaCO3 flux
1
mass top-charged
MgCO3 flux 5
mass top-charged
0:440
CaCO3 flux 1
52:2 mass% CO2 in MgCO3 100%
mass top-charged MgCO3 flux
mass top-charged
1
0:440
0:522 MgCO3 flux mass top-gas CO2 from
carbonates decomposition
1
(42.5)
This is added to the bottom of matrix Table 41.1 in a new Row 33. Table 42.1 shows our top-segment matrix with these changes. Table 42.2 gives us its calculated values. The equivalent bottom-segment matrix equations remain unchanged because all these activities take place in the top segment only. We now calculate top-segment input enthalpy, output enthalpy, top gas enthalpy, and top gas temperature from calculated values of Table 42.2.
42.4 AMENDED TOP-SEGMENT INPUT ENTHALPY EQUATION WITH CARBONATES ADDED Replacement of CaO and MgO by CaCO3 and MgCO3 in a furnace’s top charge requires two changes to our top-segment input enthalpy equation. They are;
0:522
or
0:440 CaCO3 flux mass top-charged 1 0:522 MgCO3 flux mass top-gas CO2 from 1 5 carbonates decomposition mass top-charged or subtracting 0:440 CaCO3 flux mass top-charged 1 0:522 from both sides: MgCO3 flux
H 25 C CaO
MWCaO
H is replaced by
CaO
MWCaO
MWCaCO3 H
H 25 C is replaced by
25 C CaCO3
25 C MgCO3
MWMgCO3
5 212:06
5 2 13:20
with units of, MJ/kg of substance. This is shown by the terms;
BLAST FURNACE IRONMAKING
BC78 ð12:06Þ
TABLE 42.1
Top-Segment Matrix With Top-Charged CaCO3 and MgCO3 Flux
(Continued)
TABLE 42.1
(Continued)
Column CH and Row 33 are new. Columns BJ and BK have changed.
42.4 AMENDED TOP-SEGMENT INPUT ENTHALPY EQUATION WITH CARBONATES ADDED
TABLE 42.2 Top-Segment Matrix Table 42.1 Calculated Values
Mass CO2 of Row 102 in top gas from carbonate decomposition is new.
BLAST FURNACE IRONMAKING
385
386
42. TOP SEGMENT WITH CARBONATE FLUXES
42.8 RESULTS
and BC79 ð13:20Þ
in Row 112 of Table 42.3.
42.5 TOP-SEGMENT OUTPUT ENTHALPY WITH CARBONATES ADDED Top-segment output enthalpy Eq. (40.3) is unchanged by switching to CaCO3 and MgCO3 fluxes.
Table 42.3 shows that top gas temperature with CaCO3 and MgCO3 fluxes is 24 C as compared to 168 C with CaO and MgO fluxes. This temperature (24 C) is too low for industrial blast furnace operations. In industrial practice, the operator would either need to enrich the sinter and/or pellets with CaO and MgO or increase the overall blast furnace fuel rate to achieve an acceptable top temperature. This low top gas temperature the result of some heat in the rising gas of Fig. 42.1 being used for the endothermic decomposition reactions; CaCO3 ðsÞ-CaOðsÞ 1 CO2 g
42.6 TOP GAS ENTHALPY Top gas enthalpy Eq. (40.5) is also unchanged by switching to CaCO3 and MgCO3 fluxes.
and MgCO3 ðsÞ-MgOðsÞ 1 CO2 g
lowering the enthalpy and temperature of the top gas.
42.7 TOP GAS TEMPERATURE Top gas temperature is not changed by CaCO3 and MgCO3 themselves, but it is changed by the CO2 from their decomposition. Top gas temperature contains the term:
42.9 SUMMARY Carbonate fluxes are readily represented in our top-segment calculations. The major change is introduction of a new variable;
2
3 mass CO2 4 departing 5 in top gas
In this chapter, we replace that term by: 82 < 4 :
9 3 = mass CO2 mass top-gas CO2 from 5 from reactions 1 carbonates decomposition ; departing in top gas
The new term is represented in two places by; ðBC96 1 BC102Þ
mass top-gas CO2 from carbonates decomposition
and an equivalent quantity equation, (42.5), Row 33. Also; • • • •
CaO mass balance, MgO mass balance, top-segment input enthalpy, and top gas temperature
equations are slightly modified.
in Row 119 of Table 42.3.
BLAST FURNACE IRONMAKING
TABLE 42.3 Equations for Calculating Top-Segment Input Enthalpy, Output Enthalpy, Top Gas Enthalpy, and Top Gas Temperature
The input enthalpy and top gas temperature equations have been modified to represent CaCO3, MgCO3, and mass CO2 in top gas from carbonate decomposition.
388
42. TOP SEGMENT WITH CARBONATE FLUXES
Replacement of CaO and MgO fluxes with CaCO3 and MgCO3 results in cooler top gas. This is because endothermic carbonate decomposition reactions absorb heat from the rising blast furnace gases - lowering their enthalpy and temperature.
EXERCISES 42.1. The blast furnace management of matrix Table 42.1 wants to charge its slag’s CaO flux as CaCO3(s) but its slag’s MgO flux as MgO(s). They want to know how much this will change the furnace’s top gas temperature. Please calculate this for them. Before doing so, can you make a prediction?
BLAST FURNACE IRONMAKING
42 Top Segment With Carbonate Fluxes O U T L I N E 42.1 Understanding the Impact of Carbonate Fluxes on the Blast Furnace Process
379
42.2 Amended Top-Segment Variables and Equations for Carbonates 380 42.3 New Variable and Its Associated Equation for Carbonates 42.4 Amended Top-Segment Input Enthalpy Equation With Carbonates Added
381
42.5 Top-Segment Output Enthalpy With Carbonates Added
386
42.6 Top Gas Enthalpy
386
42.7 Top Gas Temperature
386
42.8 Results
386
42.9 Summary
386
Exercises
388
381
42.1 UNDERSTANDING THE IMPACT OF CARBONATE FLUXES ON THE BLAST FURNACE PROCESS
lump iron ore. Small carbonate flux additions are also used for final slag chemistry control. These carbonates decompose in the top segment to form solid oxide and CO2 gas by reactions such as;
Previous top-segment chapters have assumed that our slag components enter the blast furnace as oxides, for example, CaO and MgO. This is the case for many industrial blast furnaces, especially those being charged with self-fluxing sinter and pellets. Many blast furnaces charge Ca and Mg carbonate fluxes, especially those that charge
CaCO3 ðsÞ-CaOðsÞ 1 CO2 g MgCO3 ðsÞ-MgOðsÞ 1 CO2 g
Blast Furnace Ironmaking DOI: https://doi.org/10.1016/B978-0-12-814227-1.00042-7
(42.1) (42.2)
The product solids descend into the bottom segment, where they ultimately form molten slag (Fig. 42.1).
379
© 2020 Elsevier Inc. All rights reserved.
380
42. TOP SEGMENT WITH CARBONATE FLUXES
42.2 AMENDED TOP-SEGMENT VARIABLES AND EQUATIONS FOR CARBONATES Replacement of oxide fluxes with carbonate fluxes changes;
mass top-charged mass top-charged to CaO flux CaCO3 flux
mass top-charged mass top-charged to MgCO3 flux MgO flux
1: the variable
FIGURE 42.1 Conceptual blast furnace top segment with CaCO3 and MgCO3 fluxes (and moisture) in the top charge. We specify that all the CO2(g) from carbonate decomposition and all the H2O(g) from moisture evaporation leave the furnace without reacting. Notice that Ca and Mg enter the furnace in carbonates but descend out of the top segment as oxides.
and 2: the variable
It also changes the top-segment CaO mass balance; The product CO2 joins the gases that are rising through the top segment, ultimately ending up in the furnace top gas (Fig. 42.1). Decomposition reactions (42.1) and (42.2) are endothermic. They absorb heat from the rising bottom-segment gases, ultimately resulting in top gas that is cooler than when the topcharged fluxes are oxides (Chapter 47: Bottom Segment Calculations with CO Injection). In this chapter, we examine the effects of charging carbonate fluxes to the blast furnace, especially their effect on top gas composition, enthalpy, and temperature. Top-segment flows with carbonate fluxes [plus top-charged H2O (‘)] are shown in Fig. 42.1. The objectives of this chapter are to; 1. calculate top gas composition, enthalpy, and temperature with top-charged carbonate fluxes, and 2. compare these calculated quantities obtained with top-charged oxide fluxes.
05
mass top-charged
1
CaO flux
mass CaO descending
1
out of top segment
1
to
mass top-charged
0:56 CaCO3 flux mass CaO descending 1 1 out of top segment
05
(42.3)
where 0.56 is 56 mass% CaO in CaCO3/100%. The top-segment MgO mass balance changes to; 05
mass top-charged
0:478 MgCO3 flux mass MgO descending 1 1 out of top segment
(42.4)
where 0.478 is 47.8 mass% MgO in MgCO3/100%.
BLAST FURNACE IRONMAKING
381
42.4 AMENDED TOP-SEGMENT INPUT ENTHALPY EQUATION WITH CARBONATES ADDED
42.3 NEW VARIABLE AND ITS ASSOCIATED EQUATION FOR CARBONATES
05
mass top-charged
CaCO3 flux mass top-charged
Carbonate decomposition introduces one new variable to our top-segment matrix. It is;
mass top-gas CO2 from carbonates decomposition
This variable is added to the right-most side of matrix Table 41.1 in new Column CH. All this CO2 comes from CaCO3 and MgCO3 decomposition and is best described by the following equation;
mass top-gas CO2 from carbonates decomposition 5
mass top-charged 44:0 mass% CO2 in CaCO3 100% CaCO3 flux
1
mass top-charged
MgCO3 flux 5
mass top-charged
0:440
CaCO3 flux 1
52:2 mass% CO2 in MgCO3 100%
mass top-charged MgCO3 flux
mass top-charged
1
0:440
0:522 MgCO3 flux mass top-gas CO2 from
carbonates decomposition
1
(42.5)
This is added to the bottom of matrix Table 41.1 in a new Row 33. Table 42.1 shows our top-segment matrix with these changes. Table 42.2 gives us its calculated values. The equivalent bottom-segment matrix equations remain unchanged because all these activities take place in the top segment only. We now calculate top-segment input enthalpy, output enthalpy, top gas enthalpy, and top gas temperature from calculated values of Table 42.2.
42.4 AMENDED TOP-SEGMENT INPUT ENTHALPY EQUATION WITH CARBONATES ADDED Replacement of CaO and MgO by CaCO3 and MgCO3 in a furnace’s top charge requires two changes to our top-segment input enthalpy equation. They are;
0:522
or
0:440 CaCO3 flux mass top-charged 1 0:522 MgCO3 flux mass top-gas CO2 from 1 5 carbonates decomposition mass top-charged or subtracting 0:440 CaCO3 flux mass top-charged 1 0:522 from both sides: MgCO3 flux
H 25 C CaO
MWCaO
H is replaced by
CaO
MWCaO
MWCaCO3 H
H 25 C is replaced by
25 C CaCO3
25 C MgCO3
MWMgCO3
5 212:06
5 2 13:20
with units of, MJ/kg of substance. This is shown by the terms;
BLAST FURNACE IRONMAKING
BC78 ð12:06Þ
TABLE 42.1
Top-Segment Matrix With Top-Charged CaCO3 and MgCO3 Flux
(Continued)
TABLE 42.1
(Continued)
Column CH and Row 33 are new. Columns BJ and BK have changed.
42.4 AMENDED TOP-SEGMENT INPUT ENTHALPY EQUATION WITH CARBONATES ADDED
TABLE 42.2 Top-Segment Matrix Table 42.1 Calculated Values
Mass CO2 of Row 102 in top gas from carbonate decomposition is new.
BLAST FURNACE IRONMAKING
385
386
42. TOP SEGMENT WITH CARBONATE FLUXES
42.8 RESULTS
and BC79 ð13:20Þ
in Row 112 of Table 42.3.
42.5 TOP-SEGMENT OUTPUT ENTHALPY WITH CARBONATES ADDED Top-segment output enthalpy Eq. (40.3) is unchanged by switching to CaCO3 and MgCO3 fluxes.
Table 42.3 shows that top gas temperature with CaCO3 and MgCO3 fluxes is 24 C as compared to 168 C with CaO and MgO fluxes. This temperature (24 C) is too low for industrial blast furnace operations. In industrial practice, the operator would either need to enrich the sinter and/or pellets with CaO and MgO or increase the overall blast furnace fuel rate to achieve an acceptable top temperature. This low top gas temperature the result of some heat in the rising gas of Fig. 42.1 being used for the endothermic decomposition reactions; CaCO3 ðsÞ-CaOðsÞ 1 CO2 g
42.6 TOP GAS ENTHALPY Top gas enthalpy Eq. (40.5) is also unchanged by switching to CaCO3 and MgCO3 fluxes.
and MgCO3 ðsÞ-MgOðsÞ 1 CO2 g
lowering the enthalpy and temperature of the top gas.
42.7 TOP GAS TEMPERATURE Top gas temperature is not changed by CaCO3 and MgCO3 themselves, but it is changed by the CO2 from their decomposition. Top gas temperature contains the term:
42.9 SUMMARY Carbonate fluxes are readily represented in our top-segment calculations. The major change is introduction of a new variable;
2
3 mass CO2 4 departing 5 in top gas
In this chapter, we replace that term by: 82 < 4 :
9 3 = mass CO2 mass top-gas CO2 from 5 from reactions 1 carbonates decomposition ; departing in top gas
The new term is represented in two places by; ðBC96 1 BC102Þ
mass top-gas CO2 from carbonates decomposition
and an equivalent quantity equation, (42.5), Row 33. Also; • • • •
CaO mass balance, MgO mass balance, top-segment input enthalpy, and top gas temperature
equations are slightly modified.
in Row 119 of Table 42.3.
BLAST FURNACE IRONMAKING
TABLE 42.3 Equations for Calculating Top-Segment Input Enthalpy, Output Enthalpy, Top Gas Enthalpy, and Top Gas Temperature
The input enthalpy and top gas temperature equations have been modified to represent CaCO3, MgCO3, and mass CO2 in top gas from carbonate decomposition.
388
42. TOP SEGMENT WITH CARBONATE FLUXES
Replacement of CaO and MgO fluxes with CaCO3 and MgCO3 results in cooler top gas. This is because endothermic carbonate decomposition reactions absorb heat from the rising blast furnace gases - lowering their enthalpy and temperature.
EXERCISES 42.1. The blast furnace management of matrix Table 42.1 wants to charge its slag’s CaO flux as CaCO3(s) but its slag’s MgO flux as MgO(s). They want to know how much this will change the furnace’s top gas temperature. Please calculate this for them. Before doing so, can you make a prediction?
BLAST FURNACE IRONMAKING