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Ind. Eng. Chem. Res. 2003, 42, 6872-6879
Investigation of a Mineral Melting Cupola Furnace. Part I. Experimental Work R. Leth-Miller,*,†,‡ A. D. Jensen,‡ P. Glarborg,‡ L. M. Jensen,† P. B. Hansen,† and S. B. Jørgensen‡ Rockwool International A/S, Hovedgaden 584, DK-2640 Hedehusene, Denmark, and Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Lyngby, Denmark
Full-scale measurements on a mineral melting cupola furnace for stone wool production have been made to investigate the behavior and increase the understanding of mineral melting cupolas. The work includes probe measurements of gas concentration, solid/melt phase and gas temperature, and melt composition at several vertical and radial positions in the cupola. Four types of experiments were conducted: quenching of an operating cupola, sample collection from the hot part of the cupola using probes through the walls, sample collection through a probe inserted from the top of the cupola, and collection of input and output data. The experiments showed the position of the melting zone, porosity, and other characteristics. The probe experiments indicate that the oxygen is consumed approximately 0.3 m above the tuyeres. The top probe experiments gave the temperature profile in the cupola. Introduction The production of stone wool products involves melting of rock materials and subsequent spinning of the melt into fibers (wool). The melting of the rocks is usually carried out in a cupola furnace. A few experimental investigations of the conditions inside cupolas have previously been reported in relation to the foundry industry.1-3 The results of these measurements are not directly comparable to stone wool cupola operation, however, because the CO concentration is higher in the foundry cupolas and the properties of the iron charged in foundry cupolas are different from the properties of the rocks used in stone wool cupolas. Nevertheless, the information provided by these investigations does provide indications of the conditions in a stone wool cupola. Experimental data for fixed-bed coal gasifiers, found in ref 4, can also be used for qualitative comparisons with stone wool cupola measurements. In stone wool cupolas, the reduction of iron oxides to metallic iron is unwanted, in contrast to the case in the foundry cupolas. The location and mechanism of the reduction of the iron oxides contained in the raw materials in cupolas are not well-known. Mass transport of FeO in foundry slag has been reported to be ratecontrolling,5-8 although others have found that the reduction of iron oxide with CO at the slag-gas interface is rate-controlling.9 Yet other authors report that the CO formation rate in the Boudouard reaction [C(s) + CO2(g) f 2CO(g)] is rate-controlling.10 In all of the proposed mechanisms, the iron reduction can take place only after the iron has melted, i.e., in the lower first meter of the cupola and in the melt bath. Through experimental investigations, this work aims to obtain new insights about the conditions inside the cupolas used for stone wool production. The thermal efficiency of stone wool cupolas is only approximately * To whom correspondence should be addressed. Tel.: +45 4656 0300. Fax: +45 4655 5990. E-mail: rasmus.leth.miller@ rockwool.com. † Rockwool International A/S. ‡ Technical University of Denmark.
50%, i.e., about 50% of the heat released from combustion of the coke is used for heating and melting the rocks, and the rest is lost to the surroundings, mainly to the cooling water and as CO in the flue gas. Thus, there is a great potential for improvement of the energy efficiency of the cupola. To achieve the goals of this work, an elaborate set of experimental investigations was carried out both on the coke11 and raw material12 properties and on full-scale operating cupolas. The latter experiments are reported here after the mineral melting cupola is described. The experimental data were also used to develop a mathematical model of the cupola (see ref 13). Process Description A cupola furnace for melting rocks is a vertical shaft furnace, 4-6 m tall, with a circular cross section with a diameter of 1-2.5 m (cupolas in the foundry industry are often larger); see Figure 1. Preheated air is blasted through a number of tuyeres (4-20) at the bottom of the furnace, and coke and raw materials (rocks, briquettes, limestone) are fed from the top. Some cupolas are charged with mixed charges and others alternating with coke and raw materials. However, because the coke percentage is low (10-15%), the layers mix as they move down through the cupola, and in practice, there is no difference in the two modes of operation. The coke burns at the bottom of the furnace, and the combustion proceeds until oxygen is consumed approximately 0.5 m above the tuyeres. The hot combustion products flow upward through the cupola, heating the raw materials and coke, which move down during operation. Melting of the rocks takes place in a zone about 0.5-1 m above the tuyeres (the melting zone), and the melt is collected in a melt bath. The level in the melt bath is maintained by a simple siphon. The furnace is made of steel and is cooled with water in a cooling jacket to prevent the steel from melting. Evaporation of the cooling water in the jacket circulates the cooling water from the jacket to a tank above the cupola. Steam leaves at the top of the jacket, and water from the tank enters the jacket at the bottom. The cupola operates at approximately 10 kPa
10.1021/ie020449w CCC: $25.00 © 2003 American Chemical Society Published on Web 10/03/2003
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The limestone is mostly added to adjust the viscosity of the melt to the requirements of the spinning process. The raw material particles typically have an average diameter of 0.05-0.15 m. Approximately 30% of the iron oxides in the minerals is reduced to metallic iron in the cupola. The metallic iron sinks to the bottom of the melt bath, where it accumulates and must be tapped off before the iron level reaches the siphon. The iron reduction is an undesired reaction because metallic iron is a waste material, and the production must be stopped two or three times a day for iron tapping.
Figure 1. Cupola furnace.
overpressure at the level of the tuyeres and a slight underpressure at the top. Coke is a fuel made through the pyrolysis of coal at high temperatures (∼1000 °C) for several days. The remaining char contains only very little volatiles.11 The coke used in stone wool cupolas is foundry coke, i.e., coke with an average diameter of ∼0.2 m. This coke typically contains between 3 and 15% ash. The raw materials fed to a mineral melting cupola can be rocks, briquettes, and limestone. The rocks are typically formed in volcanos and could be diabase, gabbro, or basalt. The briquettes are made of various minerals, such as olivine or basalt, diabase, and gabbro. Waste stone wool can also be recycled in the briquettes.
Experimental Section In this section, four experiments are described that aim at measuring the internal state of the cupola. Measurements of the inputs and outputs of the cupola are also described. Gas concentrations of O2, CO, and CO2; melt composition; and gas and solid/liquid temperatures were measured using several different probes. Two types of probes were used: a set of probes inserted horizontally through the wall (wall probes) and probes inserted vertically from the top of the cupola. The location of the melting zone inside the cupola was explored through quenching of an operating cupola by sudden purging with nitrogen. Melt sample collection through the tuyeres and at the siphon outlet was conducted to determine the location of the iron reduction. The full-scale experiments were performed on Rockwool cupolas. Wall Probes. The gas concentrations, melt composition, and solid/liquid temperature in the hot part of an operating cupola were measured using wall probes located from 0.3 to 0.9 m above the tuyeres. Procedure. To perform a measurement, the probe is placed on a stand and driven horizontally into the cupola with an electrical motor. The probe is then left in the cupola for 1-2 min while the samples are collected. A picture of the three types of probes is shown in Figure 2. The gas probe is a steel tube with a tip welded to the end of the pipe and with a hole on the bottom side close to the tip through which the gas is drawn. The temperature probe is a steel tube connected to an IR pyrometer. The melt sample probe is a solid steel rod with three pockets on the top side distributed along the length of the probe. Five gates were installed on the cupola at 0.3, 0.45, 0.6, 0.75, and 0.9 m. The measurements were made at three radial positions: at the wall, at the half-radial position, and at the center of the cupola.
Figure 2. Probes used to measure gas concentrations, temperature, and melt composition in the cupola.
6874 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003
Figure 3. CO profiles at the center of the cupola measured with the probes.
Figure 4. Average CO profile measured with the probes. (Vertical bars indicate the standard deviations of the measurements.)
Results. Gas Concentrations. Figure 3 shows the results of the CO concentration measurements made at the center of the cupola plotted against vertical position. The measurements were made on several days. The concentration is less than 1% at 0.3 m above the tuyeres and then increases to between 5 and 15%. The measurements are very scattered, even though the cupola operation did not change significantly between the measurements on the individual days. The concentrations of O2 and CO2 are not shown because they reflect the same information as the CO concentrations. For clarity, the average of the CO concentration measurements at each position was calculated. Figure 4 shows these results as a function of the vertical position at the wall, halfway from the center, and at the center. The reaction between CO and oxygen is rapid at high temperatures; hence, CO and oxygen cannot coexist in the hot part of the cupola if perfect mixing is assumed. The CO concentration is therefore most likely very close to zero until the oxygen has been consumed. Reactions in the probe might disturb the measurements, i.e., CO and oxygen that might coexist because of nonideal mixing might react in the probe. Above the position where all oxygen is consumed, the CO concentration increases as a result of gasification. At gate 1, 0.3 m above the tuyeres, the CO concentration is close to 0%, indicating that the oxygen level has just dropped to 0% (however, this point is based on only one measurement and is therefore associated with higher uncertainty). At gates 4 and 5, the results indicate that the gasification rate is significantly reduced, probably caused by the decrease in temperature. Figure 4 shows that the concentration of CO rises faster at the center than toward the wall, indicating that the gasification of coke is fastest at the center. The difference in
Figure 5. Temperature profile measured with the probes. (Vertical bars indicate the standard deviations of the measurements.)
gasification rate between the wall and the center is probably caused by the lower temperature at the watercooled wall. Temperature Profiles. Figure 5 shows the average temperature profiles. The temperature is between 1400 and 1650 °C at 0.45 m above the tuyeres and then decreases upward through the cupola. At 0.9 m above the tuyeres, it is between 1250 and 1400 °C. There are two points in the plot that deviate from the expected behavior. At gates 4 and 5 (0.75 and 0.9 m above the tuyeres, respectively), the temperature is rising at the wall. Also, it is surprising that the temperature is lower at the center than halfway to the center. Two factors can be suggested to explain these observations: (1) The high rate of gasification of coke due to less oxygen at the center requires energy, thereby lowering the temperature. (2) The high melting rate at the center leads to faster movement of new colder raw materials down through the cupola. In addition, the measurements represent a mix of the temperatures of coke and raw materials in both solid and liquid form, which makes the interpretation of the temperature measurements more difficult. For all measurements, there were large variances or disturbances. The difficulty of obtaining reliable temperature measurements using the wall probes might be associated with the fluctuations in the temperature readings, which indicate the presence of high-frequency noise. To account for this noise, a reliable filtering procedure should be used in future measurements rather than visual averages. The plots show only an average of the measurements, which is a rather crude way to treat the data, as the precise conditions of the operation were not identical during all the measurements. The operation changed mostly in terms of the melting rate, i.e., the charge composition was relatively unchanged, but the blast air flow rate changed. Furthermore, a cohesive zone might influence the measurements, even though the extent of the cohesive zone, and thus the importance of the so-called windows, is smaller than that found in foundry cupolas as the raw materials melt over a wide temperature range than iron. Nevertheless, the average values presented here still provide valuable information. Sampling of the melt showed that the melting zone extends as far up as over 0.9 m above the tuyeres, but the probes cannot determine the height at which all of the raw material has melted. The melt composition results are described in the following section. Iron Reduction. Iron oxides are present in most raw materials and are essential for stone wool properties,
Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6875 Table 1. Chemical Composition (in % w/w) of the Melt Samples Collected to Investigate the Iron Reduction in the Cupola Furnace composition excluding the free iron samplea
Fe
SiO2
Al2O3
TiO2
FeO
CaO
MgO
Na2O
K2O
P2O5
MnO
siphon tuyere gate 2 gate 3 gate 4 gate 5
0.21 3.6 0.4 0.7 0.6 7.2
42.6 43.5 40.2 40.5 37.5 42.0
19.2 18.3 18.2 18.0 17.1 20.6
1.6 1.7 1.6 1.6 1.5 1.8
5.9 7.5 10.4 9.4 15.1 3.9
19.7 17.5 18.4 19.3 18.0 20.1
7.8 7.7 7.4 7.4 6.7 7.3
1.8 2.0 1.9 1.9 2.0 2.1
1.0 1.0 1.3 1.4 1.5 1.6
0.3 0.5 0.4 0.5 0.5 0.5
0.1 0.1 0.1 0.1 0.1 0.1
a Probe samples are from a different cupola than the tuyere and siphon samples. The gate positions are as follows: 2, 0.45; 3, 0.6; 4, 0.75; and 5, 0.9 m above the tuyeres.
because they provide fire resistance to the wool. In the cupola, approximately 30% of the iron oxides are reduced to metallic iron. The metallic iron is immiscible with the glass melt and sinks to the bottom of the melt bath at the bottom of the cupola. As a result, the level of metallic iron in the melt bath increases during operation, and the metallic iron must be tapped out of the bottom of the cupola before it reaches the outlet of the siphon. As a first step toward understanding the iron reduction, which is a prerequisite to avoiding or reducing it, melt samples were taken out through the tuyeres and from the siphon. The chemical compositions of the samples were analyzed using a magnet to determine the content of metallic iron and X-ray analysis to determine the remaining composition. Melt samples from the probe measurements described above were also analyzed. The values were averaged over two samples for the siphon and the tuyere samples. The results for the probes are averages of the analyses of six samples, two at each of the three radial positions of the pockets in the probe. Results. The results of the chemical analyses are shown in Table 1. The analyses show that the compositions of the samples from the tuyeres and from the siphon are similar when the metallic iron is removed from the tuyere sample. Slightly higher contents of SiO2 and FeO were found in the tuyere sample, whereas the content of Al2O3 was somewhat lower. The reason for the observed differences might be that the melt was not well-mixed above the melt bath, but iron reduction in the melt bath might also have an effect. However, it appears that most of the iron reduction occurs above the melt bath. The compositions of the melt samples taken with the probes show that some metallic iron is formed at all of the positions investigated. The amount is, however, only slightly above that in the siphon, except for probe 5. The high content of free iron in the sample from gate 5 was actually found in only one of the five samples (not six because one of the pockets was empty after one of the probings), while the remaining values were below 1. It is most likely that the one measurement with the extremely high content was an unsuccessful measurement because it was seen only once. This means that the major part of the iron reduction probably occurs below 0.45 m above the tuyeres, which is the lowest position of the wall probes used for collecting melt samples. The conclusion from these measurements is that the iron reduction occurs from the melt bath and approximately 0.5 m up. Reduction above 0.5 m above the tuyeres can be neglected, and the iron reduction in the melt bath is limited and can probably be neglected. The
Figure 6. Gas temperature profiles measured on two different days with the top probe and the temperature profiles obtained with the wall probes.
results are surprising as the reduction was expected to takes place in a zone with a high CO concentration and low O2 concentration, but the measurements show the opposite. This indicates that the mechanism of iron reduction in stone wool cupolas involves reaction of the molten iron oxide with the coke surface, which is the only reducing reactant in the zone from the tuyeres and 0.5 m up. Top Probes. Procedure. In addition to the measurements at the wall probes, the temperature was also measured using a 6-m-long steel probe that was inserted from the top of the cupola a short distance into the rocks and coke packing. The burden of the charge then pulled a probe with a thermocouple inside it downward. The top probe was inserted near the wall, i.e., less than 0.3 m from the wall (cupola diameter was 1.8 m). The results are, therefore, affected to some extent by the water-cooled wall. The precise location of the probe tip cannot be determined as it is the charge that pulls the probe down through the cupola and the flow pattern of the charge is not known. Results. The temperature profiles are shown in Figure 6 for two experiments on the same cupola. The temperature profiles show that, 1 m above the tuyeres, the temperature is approximately 1300 °C. The temperature then decreases almost linearly to approximately 150 °C at the top of the cupola. The two profiles are very similar, and the difference between them is probably due to slightly different paths that the probe traced through the cupola. The temperature profiles obtained with the wall probes are also plotted in Figure 6. The profiles of the different probes are in qualitative agreement, but the wall and top probes were used on different cupolas, and the temperatures obtained with the wall probes are for solid/liquid phase, so the temperature measurements are not immediately comparable.
6876 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003
Figure 7. Cross section of the open quenched cupola. (Picture height and width are 2.5 and 1.8 m, respectively.)
Quenching of a Cupola. Given the high temperatures and the tight packing of the coke and rocks, it is difficult to obtain an overview of the conditions inside the cupola in terms of parameters such as the bulk density and location of the melting zone. Therefore, an operating cupola was quenched to obtain such an overview. Procedure. The quenching of the cupola was carried out by interrupting the blast air and injecting nitrogen to stop the combustion and other chemical reactions and to cool the furnace. The nitrogen was injected from the top of the cupola to prevent the nitrogen from gaining heat from the hottest region and then continuing to melt the rocks above even after the combustion was stopped. Just before the quenching, the cupola was tapped for all iron and melt in the melt bath, and the hole in the bottom was used as the outlet for the nitrogen. The injection of nitrogen was continued until the temperature in all regions in the cupola was below the selfignition temperature of the coke (600 °C). Subsequently, the cupola was allowed to cool to room temperature over several days. The void in the cupola was then filled with cement, and when the cement had cured and dried, the large lump was cut open to provide a picture of the state inside the operating cupola. The quenched cupola was charged with basalt, briquettes, and coke during the period before in the quenching. Results. A cut through the cupola is shown in Figure 7. It can be seen that the melting zone is located slightly higher near the tuyeres than near the center. The extent of the melting zone is not distinctly related to the
position, i.e., near the wall or the center. The briquettes melt over an interval of 0.35 m, from 0.4 to 0.75 m above the tuyeres, and the basalt melts over an interval of 0.25 m, from 0.45 to 0.70 m above the tuyeres. Figure 8 shows an example of a basalt rock in the melting process. A droplet was frozen just before dripping off and was captured in the concrete. Another indication of melting is when the surface edges are not sharp and bubbles are present. There is evidence of iron accumulation in the melt bath caused by iron that solidified during the week of production. Apparently, the iron never melted again, thus decreasing the volume of the melt bath and thereby increasing the need for more frequent tappings. The accumulation is lowest near the tapping holes and highest furthest away. The melt zone is located a little higher near the wall than near the center. This is surprising because it was expected that the furnace would be hottest at the center furthest away from the water-cooled wall. The extent of the melt zone is thus apparently not dependent on the radial position (i.e., whether at the wall or at the center). An impression of the bulk porosity of an operating cupola can be obtained from the cross section of the quenched cupola. Prior to the quenching, the bulk porosity was estimated to be 0.4-0.5, which the picture in Figure 7 supports. The picture has not been systematically analyzed, e.g. by image analysis, so here, it can only be concluded that there is no apparent contradic-
Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6877
Figure 8. Basalt droplet captured in the quenched cupola. (Total picture width corresponds to 70 mm.)
tion between the perception of the bulk porosity before and after quenching. The quenched cupola was charged in alternate layers with coke and raw materials, but there is no evidence of layers in Figure 7. This is not surprising given that the layers charged were relatively thin, e.g., the coke layer could barely cover the cross section even if it were perfectly distributed. The interpretation of the quenching experiment should be taken with some caution, as the quenching did not happen instantaneously. It took at least 15 min before the nitrogen injection was working properly, and after this initial period, the entire melt was, of course, not frozen instantaneously. In this period, part of the melt might have escaped to the melt bath, in which case the visible cross section would not represent the true picture concerning the melted fraction in the operating cupola. Measured Input/Output Data. Some of the input to and standard output from the cupola were measured at several occasions in connection with development
projects in Rockwool. The input parameters include the amount and type of coke and raw materials charged and the amount and properties of the blast air. The output data are the temperature of the melt, the temperature of the flue gas, the CO content in the flue gas, and the cooling water loss. To determine the CO concentration in the flue gas, the O2 concentration must also be measured to determine the amount of false air that dilutes the flue gas. The gas concentrations were measured with an IR analyzer for CO2 and CO, and the O2 was measured in a chemical cell (O2 and CO2 were used to check the oxygen balance). The flue gas temperature is measured with an ordinary thermocouple. The temperature of the melt was regularly measured manually with a thermocouple that was placed in the melt jet as it left the siphon. The cooling water loss (i.e., the amount of water evaporated from the cooling water tank) was not measured continuously, but could be measured manually in terms of how much water was added into the cooling water tank.
6878 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 Table 2. Measured Melt Temperature (°C), Flue Gas Temperature (°C), CO Content in the Flue Gas (%), and Cooling Water Loss (MW) for Five Different Sets of Cupola Operating Conditions operating coke cal vala Vblast/ conditions (%) (MJ/kg) Vblast,max 1 2 3 4 5 a
13.5 16.1 17 15 14
28.465 31.194 31.194 30.721 30.721
0.802 0.802 0.813 1 0.879
Tm (°C)
Tflue COflue CW (°C) (%) (MW)
1501 1487 1481 1501 1485
165 158 176 195 175
6.4 9.6 10.9 6.5 6.6
1.64 1.34 1.52 1.78 1.61
Calorific valve.
The uncertainties of the measurements are estimated as follows, based on many years of experience: (i) melt temperature, (20 °C of ∼1500 °C; (ii) flue gas temperature, (2 °C of ∼150 °C; (iii) CO in the flue gas, (0.5% of ∼5; and (iv) cooling water loss, (0.1 m3/h of ∼4 m3/h. The melt temperature is difficult to measure, because the surface cools when the melt leaves the siphon. The melt is very black, and thus, there is heat radiation loss almost only from the surface and not from deeper in the melt. This results in significant temperature gradients; hence, the value read from the instrument is very dependent on where in the melt the thermocouple is placed. The flue gas conditions were measured where the flue gas should be well-mixed and fluctuations in the measurement are small. The cooling water loss is constant (at least when averaged over 1 h), and thus the precision is good. The flue gas temperature is between 100 and 200 °C, which is a temperature range in which the temperature can be measured very accurately. However, the temperature is measured in a relatively large duct, so the flow profile and heat loss can affect the precision of the measurement. Also, false air will disturb the temperature measurement. Measurement of the CO concentration in the flue gas is also disturbed by false air. The precision of the cooling water loss measurement is affected by the heat loss from the pipes and tanks, etc., in the cooling water system. The heat loss from the surfaces was estimated using heat-transfer correlations from ref 14, and it was found that approximately 1% was lost this way. The total amount of new water filled into the tank was recorded. The measurements usually proceeded for more than 6 h, with the meter indicating how much water had been added into the cooling water tank being controlled regularly (i.e., once per hour), and the average was then found. It was seldom possible to close the energy balance from the measurements. Typically, the energy measured in the output streams was less than the energy supply in the coke and the hot blast air. The reason for the discrepancy is not clear, but can partly be attributed to the uncertainty in the blast air flow rate. This value affects the calculated energy in the flue gas, both thermal and as CO. Other error sources include the leaks in the system that are known to exist (the O2 measurements can only discover leaks in positions with underpressure). Results. The results of the input/output measurements are shown in Table 2. The results provide valuable information for the development a mathematical model of a mineral melting cupola, as presented in the second part of this work.13 In themselves, the measurements contribute little to the understanding of the cupola.
Discussion of Experimental Results. The experiments performed in this work to map the conditions inside a mineral melting cupola furnace have provided new information. The concentration and temperature profiles inside the cupola have been measured, the location of the melt zone has been determined, and the location of the iron reduction has been indicated. These results provide a better understanding of cupola operation. The wall and top probe measurements show behavior similar to those of foundry cupolas, blast furnaces, and fixed-bed coal gasifiers. The measurements presented in refs 1-4 show that oxygen is rapidly consumed and then the CO concentration increases to a steady level when the temperature has decreased so much that the gasification reactions have virtually stopped. Also, the temperature profiles found in gasifiers, blast furnaces, and foundry cupolas qualitatively compare with the profiles found in this work. The location of the melting zone was indicated by both the wall probes and the quenching experiment. There is qualitative agreement, but quantitative differences, between the two experiments, as the quenching indicates that the melting zone lies below 0.75 m above the tuyeres, but melt was collected in the wall probes 0.9 m above the tuyeres. The two experiments were performed on different cupolas, so different operating conditions could explain the discrepancy. It is also likely that the position of the melting zone in the quenched cupola moved downward somewhat during the tapping and/or during the time when the nitrogen injection was not working properly. The results of measurements on full-scale cupolas should be interpreted with caution as it is difficult to determine what the exact conditions were during the measurements. Also, the cupolas are not designed for measurement purposes but for production, so no effort was made, for example, to prevent air from coming in through the charging system every time new raw materials and coke are charged, and this disturbs the flue gas measurements. Conclusion In situ measurements have been performed to investigate the internal conditions of a full-scale operating cupola. The measurements provide new insight. Four types of experiments were conducted: (1) Gas concentration, solid-/liquid-phase temperature profiles, and melt composition in the zone from the tuyeres and 1 m up were measured by probing the cupola through the wall. (2) Gas temperature from 1 m above the tuyeres and up was measured with a probe inserted from the top of the cupola. (3) Quenching of the cupola provided insight into the state of the cupola charge and the location of the melting zone. (4) Measurements of input/ output streams from the cupola provided an energy balance that could be used for parameter estimation in the model presented in the second part of this work.13 The following conclusions can be drawn from the experiments: (i) The oxygen is consumed 0.3 m above the tuyeres. (ii) The temperature is between 1400 and 1650 °C at 0.45 m above the tuyeres and then decreases upward through the cupola. At 0.9 m above the tuyeres, it is between 1250 and 1400 °C. (iii) Iron oxides are reduced on the coke surface in the zone from the tuyeres and 0.5 m up.
Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6879
(iv) One meter above the tuyeres, the temperature is approximately 1300 °C. The temperature then decreases almost linearly to approximately 150 °C at the top of the cupola. (v) The melting zone is located from 0.35 m above the tuyeres to ∼1 m above the tuyeres. (vi) The melt zone is located a little higher near the wall than near the center. This is surprising because it was expected that the furnace should be hottest at the center, i.e., the position furthest away from the watercooled wall. (vii) The bulk porosity is 0.4-0.5. (viii) Charging the cupola with alternating layers of coke and raw materials is no different than supplying it with mixed charges. The experiments with the probes and the quenching provide new qualitative and quantitative knowledge about operating mineral melting cupolas. The measurements provide a good basis for calibrating and validating a cupola model in the second part of this work.13 Note Added after ASAP Posting This article was released ASAP on 10/03/03 with the incorrect title. The correct version was posted on 11/21/ 03. Acknowledgment This work is part of the research program of Rockwool International A/S carried out in cooperation with CAPEC (Computer Aided Process Engineering Centre) and CHEC (Combustion and Harmful Emission Control) at the Department of Chemical Engineering, Technical University of Denmark. The project is funded by Rockwool and Erhvervsfremmestyrelsen (Danish Ministry of
Business and Industry), and the industrial Ph.D. program is administrated by the Academy of Technical Sciences. Literature Cited (1) Piwowarsky, E. Giessereiforschung 1973, 2, 1. (2) Saxe´n, H. Steel Res. 1996, 3, 73. (3) Viswanathan, N. N.; Srinivasanand, M. N.; Lahiri, A. K. Ironmaking Steelmaking 1997, 24, 476. (4) Hobbs, M. L.; Radulovic, P. T.; Smoot, L. D. Prog. Energy Combust. Sci. 1994, 19, 505. (5) Sarma, B.; Cramb, A. W.; Fruehan, R. J. Metall. Mater. Trans. B 1996, 27, 717. (6) Bafghi, M. S.; Fukuda, M.; Ito, Y.; Yamada, S.; Sano, M. ISIJ Int. 1993, 33 (11), 1125. (7) Paramguru, R. K.; Galgali, R. K.; Ray, H. S. Metall. Mater. Trans. B 1997, 28, 805. (8) Siddiqi, N.; Bhoi, B.; Paramgura, R. K.; Sahajwalla, V.; Ostrovski, O. Ironmaking Steelmaking 2000, 27 (5), 367. (9) Min, D.-J.; Fruehan, R. J. Metall. Mater. Trans. B 1992, 23, 29. (10) Sasaki, Y.; Soma, T. Metall. Mater. Trans. B 1977, 8, 189. (11) Leth-Miller, R.; Jensen, J.; Jensen, A. D.; Glarborg, P.; Jorgensen, S. B. J.; Jensen, L.; Hansen, P. Ironmaking Steelmaking, manuscript submitted. (12) Leth-Miller, R.; Jensen, A. D.; Glarborg, P.; Jorgensen, S. B. J.; Jensen, L.; Hansen, P. Thermochim. Acta, manuscript accepted. (13) Leth-Miller, R.; Jensen, A.; Glarborg, P.; Jorgensen, S. B. J.; Jensen, L.; Hansen, P. Ind. Eng. Chem. Res. 2004, 43, 68806892. (14) McCabe, W. L.; Smith, J. C.; Harriot, P. Unit Operations of Chemical Engineering (International Ed.), 5th ed.; McGrawHill, 1993.
Received for review June 17, 2002 Revised manuscript received April 22, 2003 Accepted June 20, 2003 IE020449W
