- Email: [email protected]
MELTING THE RAW MATERIALS
Mineral Wool
2 MELTING THE RAW MATERIALS The cupola furnace is an aggregate for melting of magma rocks and supplements where coke is used as the energy source [8–12]. The furnace is being filled downwards with preliminary prepared packages of magma rocks and coke. There, several chemical and heat processes occur. The melting process is controlled by the composition and the amount of inserted materials and by the underdraft. The underdraft is essential for proper operation of the cupola furnace and is used as a melting velocity regulator, and it directly influences the melt characteristics. It is enabled by concentrically displaced nozzles across the furnace periphery, through which the air is supplied at an exactly determined amount and temperature. The air can be enriched with oxygen. Because of the underdraft, coke burns out more intensively. As a result, extremely hot gas is created which travels through the cupola furnace upwards passing the heat energy over to the magma rocks and coke. The gas leaves the cupola furnace cooled down to approx. 400°C. Because of heat transfer, the inserted materials in the zone above the nozzles begin to melt and flow towards the bottom of the cupola. The melt level is controlled with a siphon. 2.1
Cupola furnace operation
According to its operation, the cupola furnace can be divided into several zones. This is shown in Fig. 2.1 [8]. The first zone can be characterised as the heating zone. There, the raw materials are heated losing damp and flame-loss. At 300–400°C, the indirect reduction of iron oxide starts. This zone is located in the temperature range 200–800°C. The second zone can be designated as the zone of re-formation of the inserted materials, Fig. 2.1. The temperature in this zone varies from 800°C to 1250°C. In the zone, dolomite (CaCO 3 , 8
Melting the Raw Materials Gas Gas output output
RawInput input materials raw materials
Gas temperature Raw material temperature
Heating zone
Reduction zone Oxidation zone Melting zone 500 1000 1500 2000
°C
Fig. 2.1. Cupola furnace scheme (mineral wool production) [8].
MgCO 3 ) decays, and CO 2 is released. Amphibolite can be crushed because it becomes more fragile at high temperatures. Also, further reduction of iron oxide takes place. Casting coke is inactive until it reaches the temperature of 1000°C. Direct reduction of iron oxide (FeO) is possible. The third zone is thin. It includes only the melting of the inserted materials, Fig. 2.1. One could say that magma rocks and dolomite melt in the temperature range from 1250 to 1350°C. The melting of magma rocks depends on minerals and granularity. With temperatures so high, the solidity of amphibolite decreases. Because of considerable loading at the top of the furnace, the material underneath crushes and becomes easier to melt. Something similar occurs to dolomite which decomposes at ~800°C. CO 2 is released. Grains of CaO and MgO burn at up to ~1200°C and then melt at higher temperatures. When melting the briquettes, more unknown quantities occur. They include thin calcinated bauxite and crushed melt that is not fiberised. All these substance together, bound by cement, build a compact structure. Briquettes consist mostly of waste material that is formed in the production process. The larger part is composed of the already fiberised mineral material and fiberisation products which form in the fiberisation process under 9
Mineral Wool
the furnace. The cement phases are bonded to each other and sublimate at ~600°C. The fourth zone is the zone of coke burning. It is located above the nozzles (shown in Fig. 2.1). It ends in the melting bed. In this zone, the coke reaches the temperature at which it starts to burn out with oxygen, ~600–700°C. The underdraft can be cold, or it can be heated which additionally speeds up the burning of coke. It is known that coke can burn into carbon dioxide CO 2 and carbon monoxide CO. In the first case, much more heat energy is being released. This burning is called full oxidation and is possible in the parts where the base coke receives more underdraft. This means that there is enough oxygen for combustion, Fig. 2.1. At high temperatures above 600°C, the thermodynamic balance favours the formation of CO. Out of CO 2, CO can form on the way through the base coke. The reaction depends on temperature, the CO 2 /coke contact surface and on the time the gases stay in the area of base coke. From the thermic point of view, this reaction is undesired since it consumes fuel (casting coke) and is endothermic. More CO increases the atmosphere reduction and therefore the capability of Fe 2 O 3 to reduce into pig iron. Casters and mineral wool manufacturers use casting coke in their cupola furnaces because it is less reactive. CO forms more easily and the thermic effect of casting coke is greater. In the fourth zone, the temperature is at its highest right above the melting bed. We distinguish between the peak temperature of gases, of coke and of the inflowing melt. Danish researchers [8] show the following peak temperatures of: melt ~1500°C, coke ~1800°C and gas above 2000°C. It has been known that the melt heats up to 1300–1500°C when crossing the zone, and preserves the temperature at the exit from the cupola furnace (Fig. 2.1). The fifth zone is at the bottom of the cupola furnace. It is also called the zone of separation. In this part, because of the physical principles, pig iron is separated from the rest of the silicate melt (slag) and sinks because of its high density. 2.1.1 Processes and chemical reactions in the cupola furnace The cupola furnace works as an oriented heat transformer. The inserted rocks and coke that heat up and melt travel downwards. The melt flows out of the siphon. Occasionally, pig iron is drained at the bottom of the cupola furnace. Combusted gases travel in the 10
Melting the Raw Materials INPUT
OUTPUT
stone dolomite briquette coke
melt cupola furnace
pig iron smoke gas
blow away oxygene
Fig. 2.2. Scheme of cupola furnace operation.
opposite direction. They heat up the inserted materials at the mouth of the cupola furnace. Materials entering and leaving the cupola furnace are schematically presented in Fig. 2.2. Several chemical endothermic and exothermic processes take place in the cupola furnace at the same time. Multiphase systems with phase transitions of aggregation are present. These processes consist of the heating, melting and overheating of inserted materials above the melting point of mineral compounds. In mineral material, Fe 2O 3 appears as the oxide with the highest valency. The reduction of this oxide produces oxides with lower valency and pig iron. The carbon from hard coke and carbon monoxide serves as a reducer. Iron oxides appear as Fe 2 O 3 , Fe 3 O 4 and FeO. In theory, the reduction can occur with hydrogen H 2 which enters the furnace with the aid of damp in cold underdraft. At high temperatures, steam dissociates into H 2 and CO. In terms of quantity, the first two reducers (coke and CO) dominate. Air humidity is negligible. The humidity of the inserted coke evaporates immediately after its insertion and mixes with smoke gases. This is why the reduction with hydrogen normally does not occur or is negligible. In the cupola furnace, the reductions of all three oxides, Fe 2 O 3 , Fe 3 O 4 , FeO, can take place with help of CO and C. But carbon in the form of hard coke at temperatures lower than 1000°C is relatively inactive. Up to this temperature, the reduction happens only with CO. More relevant reactions that influence the reduction are [8]: Warm-up pre-reduction zone ~350 to 900°C 1) 3Fe 2 O 3 + CO = 2Fe 3 O 4 + CO 2 11
Mineral Wool
2)
Fe 3 O 4 + CO = 3FeO + CO 2
Reduction zone (indirect reduction) ~800 to 1100°C 3) FeO + CO = Fe + CO 2 4) CaMg(CO 3 ) 2 = MgO + CaO + 2CO 2 Reduction zone (direct reduction) >1000°C 5) FeO + C(coke) = Fe + CO 6) FeO n + C(coke) = Fe n + CO 7) C(coke) + CO 2 = 2CO Underdraft with air, oxygen (base coke) 10) C(coke) + ½ O 2 = CO 11) C(coke) + O 2 = CO 2 2.2
Influence of underdraft on cupola furnace operation
Gases are the main heat carriers in the cupola furnace. They transfer heat convectively as they flow in the opposite direction through the porous inserted materials. It would be ideal if the gas would flow through the homogenous deposit of materials. However, it turns out that a homogenous deposit cannot be produced. The structure of deposition is influenced by a wide granulation range of coke in mineral rocks. The difference in density between coke and mineral materials causes different pouring (trajectories) and loading as well constructional form of the feed shaft. Gases flow through the part of the deposit where resistance is at its lowest or where respectively the permeability is the highest. If the granulation of the deposit is too small, this can cause a significant drop of pressure in the gas flow and force the gases to stop in the middle of the deposit. To prevent this, the underdraft system has to induce an overpressure at the bottom of the cupola furnace. The gases in the cupola furnace are the main factors of the melting process. The amount of blown-in air enables the inserted coke to combust into the determined ratio CO/CO 2 . This gas ratio shows the amount of heat released by coke in the cupola furnace. Large quantities of CO mean that the heat released by coke has been significantly reduced. The ratio of these two gases depends on the underdraft through the base coke. Here, the most CO/CO 2 smoke gases are formed. The flow through the deposit of base coke is determined by the 12
Melting the Raw Materials
overpressure, underdraft amount and by the temperature and permeability of base coke. The amount of underdraft plays an interesting part in the capacity or efficiency of the furnace. According to the given amount of inserted coke, increased underdraft enhances the melting efficiency. Exaggerated underdraft increase can cause rapid melting. As a result, bigger chunks of rock stay in the melting bed, and the melt leaving the cupola furnace is not hot enough. This has a detrimental effect on the fiberisation process. The underdraft reduction leads to reduced efficiency. Temperatures in the cupola furnace and, above all, the highest temperature can be changed with the heated and oxygen-enriched underdraft. We can simply imagine that the increase of underdraft temperature and higher percentage of oxygen O 2 in underdraft result in a rise of maximum temperature in the cupola furnace. This temperature is higher than the melt temperature. The oxidation zone is located in the vicinity of nozzles in the cupola furnace (Fig. 2.1). Intensive burning takes place in the empty space around the nozzles which formed after the combustion of base coke. This space is also known as the combustion space. From the walls surrounding the combustion space, pieces of coke flake off. Because of the turbulent underdraft, the pieces of coke whirl and combust with oxygen enriched smoke gases, and CO 2 forms. C(casting coke) + O 2 → CO 2 An enormous amount of heat is released. The temperature in the combustion space reaches 2000–2500°C [12]. The combustion space and the vortex flow of gas are schematically shown in Fig. 2.3. Figure 2.3 [12] shows the oxidation zone around the nozzles and towards the centre of the furnace. This zone is similar to the combustion space. In these two zones, the amount of oxygen is sufficient for coke to burn into CO 2 . A rich layer of CO 2 forms and passes through the deposit of base coke. The oxidation zone can expand across the combustion space if the underdraft has a high enough pressure and if the permeability of the base coke deposit allows for it. Figure 2.4 [10] shows the profile of smoke gases in relation to the distance from the nozzles. At the distance of 0.6 m, the oxygen concentration is negligible and that of CO 2 is maximal. The distance to which the oxygen can penetrate into the interior of the cupola furnace depends on the distribution of the nozzles, their form, the pressure and amount of underdraft. 13
Mineral Wool
Fig. 2.3. Combustion space in the vicinity of the nozzles [10].
Concentration % 50 CO CO2
40
O2
30
T °C 2400 CO
2200
T
20
2000 1800
O2
CO2 1600
10 0
1400 0
0.5
1.0 1.5 Distance from the nozzle
2.0 mm
2.5
Fig. 2.4. Smoke gas profile according to the distance from the nozzles [10].
The formed CO 2 passes through the deposit of base coke. The passing CO 2 reacts with the white-hot coke and its surface and CO forms: C(cast coke) + CO 2 → 2CO The oxygen enriched underdraft makes the base coke combust more intensively which causes an enormous rise of temperature in the lower part of the cupola furnace. The level of base coke is therefore connected with the 14
Melting the Raw Materials
sufficiency of the outlet temperature and the efficiency of the cupola furnace. If this level is not high enough, the melting zone can reach the area of highest temperature. For this reason, the melt running through the rest of the base coke deposit can not heat up properly and leaves the cupola furnace ‘cold’. This has a negative influence on the viscosity which is an important factor of melt fiberisation. If, however, the level of base coke is too high, the melting process is slowed down because the melting zone is higher and therefore somewhat cooler. The melt at the furnace exit is well heated. 2.3
Measuring temperature and concentrations in the cupola furnace
Chemical processes and phase transitions from solid rock materials into the melt depend on the temperature. It is most important to know the local temperature distributions in order to understand the processes in the cupola furnace. To determine the vertical temperature distribution of gases in the cupola furnace, we measured: z temperatures of smoke gases; z carbon dioxide concentration in dry smoke gases; z carbon monoxide concentration in dry smoke gases; z oxygen concentration in dry smoke gases; z pressure difference between the exterior and interior of cupola furnace. Figure 2.5 shows the measuring positions and the measuring probe for measuring the concentration of O 2, CO, CO 2, depth, temperature and pressure. In our case, the measurements of the temperature–gas profile have been performed at the wall of the cupola furnace. In the narrowest part of the furnace, the so-called belly, the distance between the probe and the furnace wall was 70 mm. Figure 2.6 shows the gas temperature vertically along the cupola furnace. It also shows how melt temperature depends on the vertical distance from the nozzles. One can see that by closing the inlet nozzles, the temperature rises monotonously. At a distance of 120 cm from the nozzles, the first temperature fluctuations appear (local temperature fluctuations) and the temperature gradient increases. The measured maximum temperature is reached in the area of ~60 cm above the nozzles. The temperature curve then starts to decrease monotonically to the point where the 15
Mineral Wool
temperature
pressure concentration
depth
Fig. 2.5. Measuring probe.
Distance from the nozzle
cm
250
200
150
100
50
0
400
500
600
700
800
900
Temperature
1000 1100 1200 1300 1400 1500 o
C
Fig. 2.6. Temperature profile of the cupola furnace.
measurement was terminated, i.e., 40 cm above the nozzles. The temperature profile helps to determine the zones of the furnace on theoretical grounds. The highest temperature of 1367°C was measured 570 mm above the nozzles. The temperature decreased with depth to 1142°C when the temperature sensor was destroyed 16
Melting the Raw Materials
25 CO2 CO O2
Concentration
%
20 15 10 5 0 0
25
50
75
100
125
Distance
150
175
200
225
250
275
cm
Fig. 2.7. Gas profile of the cupola furnace.
and the measurement terminated. This happened 330 mm above the nozzles. In the case where the probe was going deeper, the temperature reduction is possible only if the probe enters the area of increased air flow which could have caused local cooling. The temperature measurements were carried out simultaneously with the measurements of the chemical concentration of smoke gases. Figure 2.7 shows the distribution of O 2 , CO 2 and CO concentrations in the cupola furnace. These concentrations also depend on the distance from the nozzles. The results of the measurements show that O 2 and CO 2 have the highest concentration. In the area from 0 to 200 cm, the concentration of O 2 increases in direction towards the nozzles and stabilizes somewhere at 18 vol.%. The concentration distribution of CO 2 seems to be the mirror image of oxygen concentration. The CO 2 concentration stabilizes at 1.5 vol.%. The CO content at a distance of 110 cm from the nozzles is hardly noticeable. From 110 cm to 130 cm above the nozzles, CO rises to its maximal level of 1.69 vol.%. This result tells us that, during the measurement, the cupola furnace operated with good efficiency. Figure 2.8 shows the concentration of O 2 , CO 2 and CO in relation to the temperature in the cupola furnace. Structurally, the diagram is similar to the one in Figure 2.7. Besides the chemical concentration of smoke gases, the static 17
Mineral Wool
cm
300
Distance from the nozzle
350
250
measured
200 150 100 50 0 -5
0
5
10
15
Pressure loss
20
25
Pa
Fig. 2.8. Concentrations shown according to the temperature in the cupola furnace. 25
Concentration
%
20
15
CO2 CO O2
10
5
0 400
500
600
700
800
900
Temperature
1000
1100
1200
1300
1400
o
C
Fig. 2.9. Pressure relation to the distance from the nozzles.
pressure in the vicinity of the measuring probe has been simultaneously measured. The pressure conditions in the cupola furnace are shown in Fig. 2.9 as the relation of pressure to the distance from the nozzles. One can see that the pressure created by the underdraft and the negative suction pressure influence the pressure in the cupola furnace. The negative pressure passes over to an overpressure of 248 cm above the nozzles. The dynamics of pressure relation to the position – layer depth lead us to conclude that the course of the curve is typical for a turbulent flow through the porous layer with the stressed square law of pressure relation to the depth.
18
2 MELTING THE RAW MATERIALS The cupola furnace is an aggregate for melting of magma rocks and supplements where coke is used as the energy source [8–12]. The furnace is being filled downwards with preliminary prepared packages of magma rocks and coke. There, several chemical and heat processes occur. The melting process is controlled by the composition and the amount of inserted materials and by the underdraft. The underdraft is essential for proper operation of the cupola furnace and is used as a melting velocity regulator, and it directly influences the melt characteristics. It is enabled by concentrically displaced nozzles across the furnace periphery, through which the air is supplied at an exactly determined amount and temperature. The air can be enriched with oxygen. Because of the underdraft, coke burns out more intensively. As a result, extremely hot gas is created which travels through the cupola furnace upwards passing the heat energy over to the magma rocks and coke. The gas leaves the cupola furnace cooled down to approx. 400°C. Because of heat transfer, the inserted materials in the zone above the nozzles begin to melt and flow towards the bottom of the cupola. The melt level is controlled with a siphon. 2.1
Cupola furnace operation
According to its operation, the cupola furnace can be divided into several zones. This is shown in Fig. 2.1 [8]. The first zone can be characterised as the heating zone. There, the raw materials are heated losing damp and flame-loss. At 300–400°C, the indirect reduction of iron oxide starts. This zone is located in the temperature range 200–800°C. The second zone can be designated as the zone of re-formation of the inserted materials, Fig. 2.1. The temperature in this zone varies from 800°C to 1250°C. In the zone, dolomite (CaCO 3 , 8
Melting the Raw Materials Gas Gas output output
RawInput input materials raw materials
Gas temperature Raw material temperature
Heating zone
Reduction zone Oxidation zone Melting zone 500 1000 1500 2000
°C
Fig. 2.1. Cupola furnace scheme (mineral wool production) [8].
MgCO 3 ) decays, and CO 2 is released. Amphibolite can be crushed because it becomes more fragile at high temperatures. Also, further reduction of iron oxide takes place. Casting coke is inactive until it reaches the temperature of 1000°C. Direct reduction of iron oxide (FeO) is possible. The third zone is thin. It includes only the melting of the inserted materials, Fig. 2.1. One could say that magma rocks and dolomite melt in the temperature range from 1250 to 1350°C. The melting of magma rocks depends on minerals and granularity. With temperatures so high, the solidity of amphibolite decreases. Because of considerable loading at the top of the furnace, the material underneath crushes and becomes easier to melt. Something similar occurs to dolomite which decomposes at ~800°C. CO 2 is released. Grains of CaO and MgO burn at up to ~1200°C and then melt at higher temperatures. When melting the briquettes, more unknown quantities occur. They include thin calcinated bauxite and crushed melt that is not fiberised. All these substance together, bound by cement, build a compact structure. Briquettes consist mostly of waste material that is formed in the production process. The larger part is composed of the already fiberised mineral material and fiberisation products which form in the fiberisation process under 9
Mineral Wool
the furnace. The cement phases are bonded to each other and sublimate at ~600°C. The fourth zone is the zone of coke burning. It is located above the nozzles (shown in Fig. 2.1). It ends in the melting bed. In this zone, the coke reaches the temperature at which it starts to burn out with oxygen, ~600–700°C. The underdraft can be cold, or it can be heated which additionally speeds up the burning of coke. It is known that coke can burn into carbon dioxide CO 2 and carbon monoxide CO. In the first case, much more heat energy is being released. This burning is called full oxidation and is possible in the parts where the base coke receives more underdraft. This means that there is enough oxygen for combustion, Fig. 2.1. At high temperatures above 600°C, the thermodynamic balance favours the formation of CO. Out of CO 2, CO can form on the way through the base coke. The reaction depends on temperature, the CO 2 /coke contact surface and on the time the gases stay in the area of base coke. From the thermic point of view, this reaction is undesired since it consumes fuel (casting coke) and is endothermic. More CO increases the atmosphere reduction and therefore the capability of Fe 2 O 3 to reduce into pig iron. Casters and mineral wool manufacturers use casting coke in their cupola furnaces because it is less reactive. CO forms more easily and the thermic effect of casting coke is greater. In the fourth zone, the temperature is at its highest right above the melting bed. We distinguish between the peak temperature of gases, of coke and of the inflowing melt. Danish researchers [8] show the following peak temperatures of: melt ~1500°C, coke ~1800°C and gas above 2000°C. It has been known that the melt heats up to 1300–1500°C when crossing the zone, and preserves the temperature at the exit from the cupola furnace (Fig. 2.1). The fifth zone is at the bottom of the cupola furnace. It is also called the zone of separation. In this part, because of the physical principles, pig iron is separated from the rest of the silicate melt (slag) and sinks because of its high density. 2.1.1 Processes and chemical reactions in the cupola furnace The cupola furnace works as an oriented heat transformer. The inserted rocks and coke that heat up and melt travel downwards. The melt flows out of the siphon. Occasionally, pig iron is drained at the bottom of the cupola furnace. Combusted gases travel in the 10
Melting the Raw Materials INPUT
OUTPUT
stone dolomite briquette coke
melt cupola furnace
pig iron smoke gas
blow away oxygene
Fig. 2.2. Scheme of cupola furnace operation.
opposite direction. They heat up the inserted materials at the mouth of the cupola furnace. Materials entering and leaving the cupola furnace are schematically presented in Fig. 2.2. Several chemical endothermic and exothermic processes take place in the cupola furnace at the same time. Multiphase systems with phase transitions of aggregation are present. These processes consist of the heating, melting and overheating of inserted materials above the melting point of mineral compounds. In mineral material, Fe 2O 3 appears as the oxide with the highest valency. The reduction of this oxide produces oxides with lower valency and pig iron. The carbon from hard coke and carbon monoxide serves as a reducer. Iron oxides appear as Fe 2 O 3 , Fe 3 O 4 and FeO. In theory, the reduction can occur with hydrogen H 2 which enters the furnace with the aid of damp in cold underdraft. At high temperatures, steam dissociates into H 2 and CO. In terms of quantity, the first two reducers (coke and CO) dominate. Air humidity is negligible. The humidity of the inserted coke evaporates immediately after its insertion and mixes with smoke gases. This is why the reduction with hydrogen normally does not occur or is negligible. In the cupola furnace, the reductions of all three oxides, Fe 2 O 3 , Fe 3 O 4 , FeO, can take place with help of CO and C. But carbon in the form of hard coke at temperatures lower than 1000°C is relatively inactive. Up to this temperature, the reduction happens only with CO. More relevant reactions that influence the reduction are [8]: Warm-up pre-reduction zone ~350 to 900°C 1) 3Fe 2 O 3 + CO = 2Fe 3 O 4 + CO 2 11
Mineral Wool
2)
Fe 3 O 4 + CO = 3FeO + CO 2
Reduction zone (indirect reduction) ~800 to 1100°C 3) FeO + CO = Fe + CO 2 4) CaMg(CO 3 ) 2 = MgO + CaO + 2CO 2 Reduction zone (direct reduction) >1000°C 5) FeO + C(coke) = Fe + CO 6) FeO n + C(coke) = Fe n + CO 7) C(coke) + CO 2 = 2CO Underdraft with air, oxygen (base coke) 10) C(coke) + ½ O 2 = CO 11) C(coke) + O 2 = CO 2 2.2
Influence of underdraft on cupola furnace operation
Gases are the main heat carriers in the cupola furnace. They transfer heat convectively as they flow in the opposite direction through the porous inserted materials. It would be ideal if the gas would flow through the homogenous deposit of materials. However, it turns out that a homogenous deposit cannot be produced. The structure of deposition is influenced by a wide granulation range of coke in mineral rocks. The difference in density between coke and mineral materials causes different pouring (trajectories) and loading as well constructional form of the feed shaft. Gases flow through the part of the deposit where resistance is at its lowest or where respectively the permeability is the highest. If the granulation of the deposit is too small, this can cause a significant drop of pressure in the gas flow and force the gases to stop in the middle of the deposit. To prevent this, the underdraft system has to induce an overpressure at the bottom of the cupola furnace. The gases in the cupola furnace are the main factors of the melting process. The amount of blown-in air enables the inserted coke to combust into the determined ratio CO/CO 2 . This gas ratio shows the amount of heat released by coke in the cupola furnace. Large quantities of CO mean that the heat released by coke has been significantly reduced. The ratio of these two gases depends on the underdraft through the base coke. Here, the most CO/CO 2 smoke gases are formed. The flow through the deposit of base coke is determined by the 12
Melting the Raw Materials
overpressure, underdraft amount and by the temperature and permeability of base coke. The amount of underdraft plays an interesting part in the capacity or efficiency of the furnace. According to the given amount of inserted coke, increased underdraft enhances the melting efficiency. Exaggerated underdraft increase can cause rapid melting. As a result, bigger chunks of rock stay in the melting bed, and the melt leaving the cupola furnace is not hot enough. This has a detrimental effect on the fiberisation process. The underdraft reduction leads to reduced efficiency. Temperatures in the cupola furnace and, above all, the highest temperature can be changed with the heated and oxygen-enriched underdraft. We can simply imagine that the increase of underdraft temperature and higher percentage of oxygen O 2 in underdraft result in a rise of maximum temperature in the cupola furnace. This temperature is higher than the melt temperature. The oxidation zone is located in the vicinity of nozzles in the cupola furnace (Fig. 2.1). Intensive burning takes place in the empty space around the nozzles which formed after the combustion of base coke. This space is also known as the combustion space. From the walls surrounding the combustion space, pieces of coke flake off. Because of the turbulent underdraft, the pieces of coke whirl and combust with oxygen enriched smoke gases, and CO 2 forms. C(casting coke) + O 2 → CO 2 An enormous amount of heat is released. The temperature in the combustion space reaches 2000–2500°C [12]. The combustion space and the vortex flow of gas are schematically shown in Fig. 2.3. Figure 2.3 [12] shows the oxidation zone around the nozzles and towards the centre of the furnace. This zone is similar to the combustion space. In these two zones, the amount of oxygen is sufficient for coke to burn into CO 2 . A rich layer of CO 2 forms and passes through the deposit of base coke. The oxidation zone can expand across the combustion space if the underdraft has a high enough pressure and if the permeability of the base coke deposit allows for it. Figure 2.4 [10] shows the profile of smoke gases in relation to the distance from the nozzles. At the distance of 0.6 m, the oxygen concentration is negligible and that of CO 2 is maximal. The distance to which the oxygen can penetrate into the interior of the cupola furnace depends on the distribution of the nozzles, their form, the pressure and amount of underdraft. 13
Mineral Wool
Fig. 2.3. Combustion space in the vicinity of the nozzles [10].
Concentration % 50 CO CO2
40
O2
30
T °C 2400 CO
2200
T
20
2000 1800
O2
CO2 1600
10 0
1400 0
0.5
1.0 1.5 Distance from the nozzle
2.0 mm
2.5
Fig. 2.4. Smoke gas profile according to the distance from the nozzles [10].
The formed CO 2 passes through the deposit of base coke. The passing CO 2 reacts with the white-hot coke and its surface and CO forms: C(cast coke) + CO 2 → 2CO The oxygen enriched underdraft makes the base coke combust more intensively which causes an enormous rise of temperature in the lower part of the cupola furnace. The level of base coke is therefore connected with the 14
Melting the Raw Materials
sufficiency of the outlet temperature and the efficiency of the cupola furnace. If this level is not high enough, the melting zone can reach the area of highest temperature. For this reason, the melt running through the rest of the base coke deposit can not heat up properly and leaves the cupola furnace ‘cold’. This has a negative influence on the viscosity which is an important factor of melt fiberisation. If, however, the level of base coke is too high, the melting process is slowed down because the melting zone is higher and therefore somewhat cooler. The melt at the furnace exit is well heated. 2.3
Measuring temperature and concentrations in the cupola furnace
Chemical processes and phase transitions from solid rock materials into the melt depend on the temperature. It is most important to know the local temperature distributions in order to understand the processes in the cupola furnace. To determine the vertical temperature distribution of gases in the cupola furnace, we measured: z temperatures of smoke gases; z carbon dioxide concentration in dry smoke gases; z carbon monoxide concentration in dry smoke gases; z oxygen concentration in dry smoke gases; z pressure difference between the exterior and interior of cupola furnace. Figure 2.5 shows the measuring positions and the measuring probe for measuring the concentration of O 2, CO, CO 2, depth, temperature and pressure. In our case, the measurements of the temperature–gas profile have been performed at the wall of the cupola furnace. In the narrowest part of the furnace, the so-called belly, the distance between the probe and the furnace wall was 70 mm. Figure 2.6 shows the gas temperature vertically along the cupola furnace. It also shows how melt temperature depends on the vertical distance from the nozzles. One can see that by closing the inlet nozzles, the temperature rises monotonously. At a distance of 120 cm from the nozzles, the first temperature fluctuations appear (local temperature fluctuations) and the temperature gradient increases. The measured maximum temperature is reached in the area of ~60 cm above the nozzles. The temperature curve then starts to decrease monotonically to the point where the 15
Mineral Wool
temperature
pressure concentration
depth
Fig. 2.5. Measuring probe.
Distance from the nozzle
cm
250
200
150
100
50
0
400
500
600
700
800
900
Temperature
1000 1100 1200 1300 1400 1500 o
C
Fig. 2.6. Temperature profile of the cupola furnace.
measurement was terminated, i.e., 40 cm above the nozzles. The temperature profile helps to determine the zones of the furnace on theoretical grounds. The highest temperature of 1367°C was measured 570 mm above the nozzles. The temperature decreased with depth to 1142°C when the temperature sensor was destroyed 16
Melting the Raw Materials
25 CO2 CO O2
Concentration
%
20 15 10 5 0 0
25
50
75
100
125
Distance
150
175
200
225
250
275
cm
Fig. 2.7. Gas profile of the cupola furnace.
and the measurement terminated. This happened 330 mm above the nozzles. In the case where the probe was going deeper, the temperature reduction is possible only if the probe enters the area of increased air flow which could have caused local cooling. The temperature measurements were carried out simultaneously with the measurements of the chemical concentration of smoke gases. Figure 2.7 shows the distribution of O 2 , CO 2 and CO concentrations in the cupola furnace. These concentrations also depend on the distance from the nozzles. The results of the measurements show that O 2 and CO 2 have the highest concentration. In the area from 0 to 200 cm, the concentration of O 2 increases in direction towards the nozzles and stabilizes somewhere at 18 vol.%. The concentration distribution of CO 2 seems to be the mirror image of oxygen concentration. The CO 2 concentration stabilizes at 1.5 vol.%. The CO content at a distance of 110 cm from the nozzles is hardly noticeable. From 110 cm to 130 cm above the nozzles, CO rises to its maximal level of 1.69 vol.%. This result tells us that, during the measurement, the cupola furnace operated with good efficiency. Figure 2.8 shows the concentration of O 2 , CO 2 and CO in relation to the temperature in the cupola furnace. Structurally, the diagram is similar to the one in Figure 2.7. Besides the chemical concentration of smoke gases, the static 17
Mineral Wool
cm
300
Distance from the nozzle
350
250
measured
200 150 100 50 0 -5
0
5
10
15
Pressure loss
20
25
Pa
Fig. 2.8. Concentrations shown according to the temperature in the cupola furnace. 25
Concentration
%
20
15
CO2 CO O2
10
5
0 400
500
600
700
800
900
Temperature
1000
1100
1200
1300
1400
o
C
Fig. 2.9. Pressure relation to the distance from the nozzles.
pressure in the vicinity of the measuring probe has been simultaneously measured. The pressure conditions in the cupola furnace are shown in Fig. 2.9 as the relation of pressure to the distance from the nozzles. One can see that the pressure created by the underdraft and the negative suction pressure influence the pressure in the cupola furnace. The negative pressure passes over to an overpressure of 248 cm above the nozzles. The dynamics of pressure relation to the position – layer depth lead us to conclude that the course of the curve is typical for a turbulent flow through the porous layer with the stressed square law of pressure relation to the depth.
18