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Measuring temperature of the atmosphere in the steelmaking furnace Ren Pyszko a, Tom Brestovic b, Natlia Jasminskb, Marin Lzrb, Mrio Machu a, Michal Pukr c, Renta Turisovd aFaculty of Metallurgy and Materials Engineering, VB Technical University of Ostrava, 17. listopadu 15, 708 33 Ostrava-Poruba, Czech Republic bFaculty of Mechanical Engineering, TU Koice, Department of Power Engineering, Vysokokolsk 4, 042 00 Koice, Slovak Republic cFaculty of Mechanical Engineering, TU Koice, Department of Engineering for Machine Design, Automotive and Transport, Letn 9, 040 01 Koice, Slovak Republic dFaculty of Mechanical Engineering, TU Koice, Department of Safety and Quality, Letn 9, 040 01 Koice, Slovak Republic a r t i c l ei n f o Article history: Received 26 May 2015 Received in revised form 10 July 2015 Accepted 27 July 2015 Available online 6 August 2015 Keywords: Steelmaking furnace Temperature measurement Thermocouple Protection tube FEM simulation a b s t r a c t The paper presents a measurement of temperature in a steelmaking furnace with an oxy- gen refi ning process, specifi cally in a tandem furnace. Knowledge of the temperature is important for optimizing the manufacturing process and extending of the life cycle of the inner furnace lining. Conditions in the furnace are very demanding; the temperature may temporarily exceed 2000 ?C and exhibits changes of 60 ?C s?1. The atmosphere in the furnace is mainly oxidizing and its chemical composition is very variable. In the atmo- sphere there are also present liquid and solid compounds which act chemically and abra- sively on the lining and on the sensor. These conditions make it impossible to use a suction pyrometer, contactless optical or acoustic measurement methods. It was decided to use a thermocouple in a protective tube. The sensor was inserted into the furnace through a steel bushing with water cooling. For the fi rst measurement the type B thermocouple in Al2O3 protective tube inside an outer shielding tube made from SiC was used. The second sensor used the type C thermocouple in a special thick-walled protective tube made from sintered SiC. The sensor does not measure the temperature of the gas itself, since it is affected by radiation of the surrounding areas. Conditions for heat transport from the furnace to the sensor are variable and diffi cult to determine; therefore, the accurate identifi cation of the atmosphere temperature is not possible. Limit values of the temperature interval of the atmosphere were determined, for hypothetical cases of athermanous and ideally diathermic atmospheres. ? 2015 Elsevier Ltd. All rights reserved. 1. Introduction Steel is produced at high temperatures when the tem- perature of the molten steel itself is about 1600 ?C and temperatures in the working space of the steel manufac- turing aggregates (an oxygen converter, a tandem furnace or an electric arc furnace) are even higher. For this reason, the demands on the internal refractory lining of these aggregates are very high. The lining gradually degrades and wanes, while the cost of the renewal is signifi cant. Another negative effect on the lining, in addition to high temperatures, is the corrosive effect of the furnace atmo- sphere, which is more intense at high temperatures. The adverse impact on the life cycle of ceramic linings is also caused by cyclical temperature changes triggered by the nature of the batch of the steel production technology (in contrast, for example, to the blast furnace). /10.1016/j.measurement.2015.07.052 0263-2241/? 2015 Elsevier Ltd. All rights reserved. Corresponding author. Tel.: +421 55 6022360. E-mail address: michal.puskartuke.sk (M. Pukr). Measurement 75 (2015) 92103 Contents lists available at ScienceDirect Measurement journal homepage: /locate/measurement For these reasons, it is very useful to measure the tem- perature in a working space of the furnace in order to opti- mizetheproductiontechnology.However,generally speaking, this is a very challenging task, whether it comes to a temperature measurement of the furnace atmosphere, or to a measurement of the temperature of the lining. The temperature of the furnace atmosphere could be, especially for production units based on the oxygen pro- cess (a tandem furnace and an oxygen converter), an important indicator of the ongoing technological process regularity. At the same time, knowing the temperature can serve as information characterizing a thermal load” of the lining. The problem is how to measure the temper- ature in practice while protecting the sensor from temper- ature and corrosive effects of the furnace atmosphere. The temperature measurement of gases is a diffi cult task in general, but in the case of the steel furnace the measure- ment is greatly complicated due to an extreme pollution of the furnace atmosphere. For the purpose of the develop- ment and experimental tests of the sensor, the tandem fur- nace was chosen. The objective was to measure the temperature just below the crown of the furnace. 2. Description of measurement conditions in a tandem furnace The tandem furnace consists of two furnace hearths: preheating and refi ning (Fig. 1). Each hearth is equipped with an obliquely retractable refi ning oxygen nozzle and another nozzle inserted from above for post-combustion of carbon monoxide (CO). The furnace is closed by a crown from above, which is composed of panels, allowing for a quick replacement. Lining consists of hanging fi red refrac- tory magnesiachrome bricks. Both hearths are connected by a channel that allows fl owing exhaust gases from the preheating into the refi ning hearth. The fl ue gases with high content of CO are produced in the refi ning hearth, and are subsequently post-combusted in the preheating hearth. Fumes from the preheating hearth are discharged into the exhaust. The functions of both hearths alternate. Refi ning time takes ideally about 70 min, the entire time of the cycle, which includes preheating and refi ning, prac- tically takes about 160 min. According to preliminary approximate calculations, the combustion temperature of the atmosphere in the tandem furnace may temporarily exceed 2000 ?C. Preliminary mea- surement which used a platinum thermocouple, confi rmed that the temperature in the connecting channel and in the exhaust temporarily exceeded 1800 ?C. At the same time, the upper limit of the measuring instrument range has been reached and, moreover, destruction of the thermo- couple occurred. Additionally, it was shown that the fl ue gastemperaturehas signifi cant fl uctuationreaching almost 40 ?C s?1in the connecting channel and almost 60 ?C s?1 in the exhaust. These temperature fl uctuations have an undesirable effect both on the lining and on the sensor. The chemical composition of the furnace atmosphere is highly variable in time. The concentration of CO in the exhaust of the preheating hearth ranged from 0 to 25 vol % (an average of 1.7 vol%) during preliminary measure- ments, the CO2concentration ranged from 0 to 33 vol% (an average of 9 vol%), and the O2concentration ranged in the interval from 0 to 21 vol% (an average of 16 vol%). The correlation analysis showed that CO2and O2are related inversely (the Pearsons correlation coeffi cient was negative ?0.45), similarly, but more weakly related CO and O2 (the correlation coeffi cient was ?0.12), whereas the correlation between CO2and CO was directly propor- tional (the correlation coeffi cient was 0.7). It follows from the above data that there is a predomi- nantly oxidizing atmosphere in the tandem furnace which can, however, temporarily change into reducing. In the atmosphere, aside from the analyzed gases, are present also other gaseous, liquid and solid compounds, in particular FeO, CaO, MgO, possibly also graphite, Al2O3, SiO2, Cr2O3and other which act both chemically and abra- sively on the lining, and hence on the temperature sensor. These components also cause the formation of smoke in the atmosphere, which signifi cantly affects heat transport by radiation. The dynamic effect of the fl owing gas on the sensor should not be neglected either. While designing the sensor, mechanical effects of the furnace vibrations and shocks as well as movement of the lining in case of the furnace tilting during tapping needed to be taken into consideration. CO temperature measurement point refiningpreheating flue gases refining nozzle refining nozzle post-combustion nozzles scrapand pigiron steel Fig. 1. Diagram of the tandem furnace. R. Pyszko et al./Measurement 75 (2015) 9210393 3. Measurement of the temperature in furnaces using contactless methods There are number of ways of measuring temperature of gases and fl ames in boilers and furnaces with relatively stable fl ame and constant composition of fl ue gases 1, which can be divided mainly into contact and non- contactmethods29.Butmeasuringtemperatures exceeding 2000 ?C in a polluted atmosphere with a vari- able composition in steelmaking furnaces, from a technical point of view, is a diffi cult task and literature does not offer practically applicable methods for an on-line long-term process measurement, in contrast to methods of measuring temperature of the charge 10,11. In case we intend to measure the furnace atmosphere temperature by means of an optical pyrometer in a diathermic atmosphere, the device will measure the back- ground temperature, i.e. the temperature of the lining or the charge surface across the pyrometer. In the athermanous or rather partially diathermic smoked atmosphere there is a problem with unknown emissivity of the gas and especially of the smoke particles, whose temperature would be measured in this case. In the furnace, gas mixtures of heteropolar molecules (CO, CO2, H2O, etc.) occur that are not 100% diathermic and radiate. The radiation of gases is selective; gases radiate in narrow wavelength intervals. Integral emissivity of gases is very low and nonlinearly dependent on the gas temperature. Measurements must be conducted spectrally in narrow wavelength intervals. In a tandem furnace, there are red fumes” which cannot be considered as a gray body. Radiation will be diffi cult to be spectrally defi ned. The result of the measurement thus depends on pollution and chemical composition of the atmosphere that change over time. On the market there are spectral pyrometers specially designed for measurements of the combustion processes. Instruments use a selective character of the gases spectral emissivity. Devices operating at the wavelength of about 3.9lm are useful for the measurement through the fl ame, e.g. the temperature monitoring of the lining in the combus- tion chambers. A luminous fl ame is required, which cannot be found in a steel furnace. Devices operating at a wave- length close to 4.26lm measure the temperature of CO2 as a component of the exhaust gas. Devices operating at a wavelength of 4.6lm measure the temperature of CO. The composition of fl ue gases in the steelmaking furnace changes rapidly and signifi cantly. Thus, it makes it diffi cult to measure the temperature either with spectral pyrome- ters or with other contactless methods, such as acoustic measurement. Therefore, in such an environment, it is recommended to insert an enclosed ceramic tube with the optical pyrom- eter into a measured space and read the temperature of the bottom of the tube. By doing this, however, the problem shifts into the area of the contact measurement on an interface atmosphere/tube (optical pyrometer is only a means for sensing the temperature of the tube). This cre- ates a problem, how to fi nd the material of the protection tube that could withstand the aggressive environment (in this case inside the tandem furnace) and thus substantially losing the main advantage of the contactless measurement to measurement using a thermocouple. In this situation, it was decided to focus on the use of a thermocouple. Thermocouple advantages are relatively low cost, high accuracy, little or no maintenance and measuring that is unaffected by a variable gas composition. 4. Temperature measurements in the steelmaking furnace by thermocouple sensors The most common design of the thermocouple for an industrial use is the bar thermocouple, which is used both for plant and laboratory measurements 12,13. The sensor is protected by an enclosed protective tube which is, for lower temperatures, usually made from metallic materials, and for higher temperatures, from ceramic materials. Bar thermocouples of a range exceeding 1600 ?C are not com- mon and are available only by some manufacturers. None of the mass-produced bar thermocouples has met specifi c requirements of the measurement in the tandem furnace. Moreover, when a bare thermocouple is inserted into a fl ame for measuring the temperature of gas, errors arise due to the radiative exchange between the thermocouple and surroundings. Therefore, in order to measure the fl ue gas temperature in combustion installations, the suction pyrometers with shielding against radiation between the sensor and surrounding surfaces are used 14,15. Gases are sucked by an ejector and fl ow around the shielded thermocouple with velocity of the order of 101102m s?1. In the case of the tandem furnace the suction thermo- couple could not be used due to extreme pollution of the furnace atmosphere. This would cause the precipitation of slag and clogging of channels through which the gas stream. For the above mentioned reasons, a custom designed bar thermocouple was used. The thermocouple was placed in a special protective tube while measured values were mathematically corrected to refl ect an infl uence of the ambient radiation and the temperature drop in the protec- tion tube wall. An essential parameter for the choice of the protecting material, besides the temperature, is in particular a compo- sition of the furnace atmosphere. There is a variety of materials suitable for the protection of sensors that could operate at temperatures over 2000 ?C, but generally only in an inert or, in some cases, in reducing atmosphere. The problem is, however, an oxidizing atmosphere. For measuring temperatures up to 1700 ?C could be used, for example, the type B thermocouple (Pt 30% Rh vs. Pt 6% Rh). The maximum temperature, which this type of the thermocouple measures up until it is damaged, is 1800 ?C. The type B thermocouple can be used without protection only in an oxidizing or inert atmosphere. In the steelmaking furnace, the thermocouple must be pro- tected from contamination by slag. A pair of wires usually extends through a double-capillary made from a sintered Al2O3, ensuring their electrical insulation. Thermocouple with a double-capillary is inserted into a protective tube, usually also made from a sintered Al2O3. Two protective tubes with closed ends are often used, one tube inserted 94R. Pyszko et al./Measurement 75 (2015) 92103 into the second, usually of different materials. If it comes to protective tubes, it is not only the temperature that a tube should be able to withstand, but also other necessary prop- erties that the same material may not meet simultane- ously. For example, it is resistance to rapid temperature changes, impermeability to gases, and resistance to a chemical attack of the environment (oxidative, reductive or other chemical infl uences). The main objective of the outer tube thus may be the protection against thermal shocks, while the inner tube can provide the necessary hermeticity. For measuring temperatures above 2000 ?C, the type C thermocouple (W 5% Re vs. W 26% Re) is practically usable. This type of the thermocouple may be used in a vacuum, in an inert or hydrogen atmosphere. In an oxidiz- ing environment tungsten oxidizes very quickly and disin- tegrates.Forthetemperaturemeasurementina steelmaking furnace, the thermocouple shall be protected against any environmental infl uences. This is not just about the protection from the environment in a furnace, but also against an ambient air present in the protective tube. In the case of the type C shell thermocouple, wires are placed in a thin walled metal tube fi lled with MgO or HfO2as an insulator, or are guided by a double-capillary of the same material. The casing is hermetically sealed. For extremely high temperatures of up to 2300 ?C type C shell thermocouples are produced with molybdenum (Mo) or tantalum (Ta) protection shell. The maximum tem- perature is limited by the construction of the thermocou- ple and the insulating material in the shell. Although molybdenum withstands temperatures of up to 2400 ?C in vacuum (its melting point is 2620 ?C), in oxidizing atmo- sphere at temperatures above 200 ?C it degrades. As a result, the shell thermocouple used in an oxidizing environment cannot be used without any additional protection. In addition, if there is graphite in the environment with a temperature above 1600 ?C present in any form, it attacks and distorts the molybdenum shell. For applica- tions where there is such a risk, it is recommended to cover the molybdenum shell with a thick layer of tungsten, thereby minimizing the undesirable carbonizing process. This arrangement, however, does not solve the problem of the oxidation, since tungsten oxidizes when heated in the air above 600 ?C. Tungsten is also disturbed by a super- heated water steam. When using this type of sensor for a measurement in a steel furnace, it is necessary to ensure an additional protection against oxidation, e.g. by spraying supplementary protective layer or by using an additional outer protective tube. For tantalum the limit temperature in an oxidizing environment is 300 ?C; however, tantalum is sensitive to reducing environment and for measuring of the highest temperatures, it is suitable only in a vacuum. They are also metal thermocouple shells intended for very high temper- atures fo