I. Industrial technology application background
In the field of transportation, lightweight is of great significance to energy saving, consumption reduction, and transportation capacity improvement. According to research, the use of aluminum alloy material can reduce the equipment quality by more than 60%. With the increase of mechanical strength of aluminum profile components, especially aluminum alloy materials can effectively improve corrosion resistance and static torsional rigidity of components, and facilitate recycling after scrap and other important values, the application of aluminum alloy materials is expanding. In the high-end equipment manufacturing industry in the national “13th Five-Year” development plan, high-quality industrial aluminum products are becoming the key basic materials to achieve the goal of technology upgrading and localization of advanced industrial equipment such as large aircraft, automobiles, rail transit trains, aerospace, military industry, and ships.
But in the field of high-end applications such as automotive, aerospace, military industry, the geometrical structure of aluminum to use mechanical properties and surface quality requirements also higher and higher, at present most of the aluminum enterprises to adopt ac induction heating and gas heating, its precision limited processing capacity, in addition to the inadequate precision of extrusion die, is largely former aluminum ingot heating in the heating process of aluminum profile extrusion to axial gradient distribution uniformity and cannot meet the requirements. At present, many high-end aluminum profiles in China still rely on imports. Superconducting DC induction heating technology is of great help to improve the mechanical properties and surface finish of extruded profile products. It is an effective technology path for enterprises to upgrade their products.
In addition, from the aspect of energy saving and consumption reduction, the high temperature superconducting DC induction heating technology is of great significance. According to the author’s recent field investigation of aluminum enterprises, the annual energy consumption of the enterprise exceeds 600 million yuan, and the heating process accounts for more than 60% of the energy consumption of the whole plant. IF heating furnace of 1M W adopts superconducting DC induction technology, it can save electricity up to 2 million k W h per year, directly reduce electricity costs by 1 million yuan, and at the same time save 0.8 million T standard coal, reduce carbon dioxide emissions by 20,000 T, and reduce nitrogen oxide emissions by 300T. China’s aluminum production capacity accounts for half of the world’s total, the country’s aluminum factory heating furnace more than ten thousand, if the use of superconducting DC induction technology for energy saving transformation, its energy saving space is very large. In China’s huge aluminum profile processing industry environment, the high temperature superconducting DC induction heating technology with the advantages of energy conservation and emission reduction and high precision heating has a very great application value. If the new superconducting DC induction technology is used for energy saving renovation, the space for energy saving and consumption reduction is very large.
Superconducting and high temperature superconducting induction heating technology
As early as the end of the 19th century and the beginning of the 20th century, by cooling mercury with liquid helium, it was accidentally discovered that when the temperature dropped to -268.95 ℃ (4.2K), the resistance of mercury completely disappeared, which was also known as superconductivity. After that, scientists of various countries carried out research on superconductivity technology and application.
Superconducting materials, at present there are low temperature superconducting materials and high temperature superconducting materials. Cryogenic superconductivity refers to the properties of superconductivity in a -269℃ (4K) liquid helium environment.High-temperature superconductivity, however, is only the ultra-low temperature and much higher temperature required by low-temperature superconductivity, which usually has a maximum temperature of -194℃ (20 ~ 77K).
In 1999, Sumitomo Chemical Corporation of Japan developed a bismuth (Bi) 2223 strip around a conductively cooled magnet, and verified the fast excitation and long-term operation of the magnet in the 20K temperature region. In 2001, the Japanese SMES research and development center verified the feasibility of a 15kWh high temperature superconducting ring magnet with conduction cooling, the external field of 10T, and stored energy of 72MJ, and the experiment obtained satisfactory results.
The conductive cooled high-temperature superconducting high-gradient magnetic separation system developed by the Los Alamos national laboratory in 1997 generates a magnetic field of 1.6T at 100A.In 2005, the experiment of applying HTS magnet in 95G H Z vibratory gyroscope was carried out in the United States, and satisfactory results were obtained.
In 2005, the Institute of Electrical Engineering, the Chinese Academy of Sciences completed the production of a single-threaded high-temperature superconducting magnet with an inner diameter of 120mm, an outer diameter of 211.2m and a height of 202.8m. The magnet is made of (Bi) 2223, which is cooled by conduction. When the ambient temperature is 20K, the central field strength is 3.2t. The critical current of the magnet is 49.8A under 77K self-field.
In high temperature superconducting technology got great development in recent years, the second generation of the superconducting strip has got the commercial production, but the high temperature superconducting technology and the application of practical progress is not soon, the main global market is still a low temperature superconducting, especially for superconducting magnetic resonance imaging (MRI), according to the European Conectus survey data, the superconducting markets around the world in 2012, the low temperature superconducting occupy the share of 5.2 billion euros, MRI which occupies a 4.1 billion euro market share, and high temperature superconducting market size for 3 0 million euros.
Although the current superconductor market is still the absolute mainstream, with the continuous development of HTS technology, as well as the gradual development of the new HTS business, HTS technology is becoming the research focus of superconductivity. In the long run, its market share will be greatly increased, so the research on HTS application technology is of great significance for expanding the HTS market. Under this development background, it is of great significance to carry out research on superconducting DC induction heating technology and promote its practical application. Now is usually used as YBCO (YBCO), and other high temperature superconducting strip winding superconducting magnet background magnetic field in the core, driven by the mechanical transmission system, such as aluminum ingots and other metal artifacts in magnetic field rotation, workpiece cutting lines to form a vortex and produce joule heat, realize the workpiece heat treatment.
Iii. Application prospect of high temperature superconducting induction heating technology
1. Introduction to heat treatment of aluminum profiles
Pre-heating aluminum ingots is a key process in the production of aluminum profiles before the extrusion process of aluminum profiles processing. Common heating methods of aluminum ingots include electromagnetic induction heating, resistance furnace heating, natural gas heating, and so on.
The natural gas heating method is widely used in civil aluminum profile manufacturing, especially suitable for small and medium-sized extruder production lines less than 30MN. The disadvantage of the natural gas heating method is that it is difficult to control the temperature gradient and the temperature uniformity of aluminum extrusion cannot be controlled. Electromagnetic induction heating is mostly used in the extrusion production line with more than 36MN, high quality industrial profiles, aerospace and military materials, and structural materials. Superconducting DC induction heating is a new induction heating technology developed for aluminum extrusion in recent years.
The traditional induction heating method USES alternating magnetic field to generate eddy currents in the static blank to realize heating of the blank. However, when aluminum, copper, and other metals are not ferromagnetic materials, the heating efficiency is less than 50% when traditional AC induction heating is adopted. Superconducting DC induction heating technology is that the rotation of the aluminum ingots generates relative motion to the static magnetic field, and the cutting magnetic induction lines form eddy currents and generate Joule heat, so as to realize the heating of the aluminum ingots. The heating efficiency is improved to 80% ~ 85%, giving it an obvious competitive advantage.
2. Advantages of HTS induction heating technology
Superconducting induction heating technology takes advantage of the zero resistance characteristic of superconducting materials at critical low temperature to establish a DC magnetic field of about 0.5 ~ 1T. In the DC magnetic field, the aluminum ingot is driven by a motor to rotate, cut magnetic field lines, generate induced current, and heat the aluminum ingot. The basic principle of heating is the same as traditional induction heating, which is Faraday’s law of electromagnetic induction, eddy current effect, and Joule’s law. Comparison of working principle between traditional induction heating and superconducting induction heating; comparison of superconducting DC induction heating technology with traditional AC induction heating technology and gas heating technology. Compared with conventional AC induction heating, HTS induction heating has four advantages.
(1) High efficiency and energy saving
In the superconducting dc induction heating technology, creating a 0.5 T target dc power loss in the superconducting coil of the magnetic field can be ignored, the efficiency of the whole system mainly depends on the drive spindle rotation of the motor, and the mature technology of motor can easily achieve efficiency more than 90%, compared with the traditional induction heating efficiency of about 50%, energy-saving effect is very obvious.
(2) High heating quality
The ingot is heated uniformly and the axial temperature gradient is controlled accurately. The traditional AC induction heating furnace usually adopts the alternating current greater than the power frequency (50Hz), because of the skin effect, the eddy current generated is mainly distributed on the surface of the ingot, and the uniformity of the amplitude heating effect of the ingot is not good. Superconducting DC induction heating can achieve a more uniform amplitude temperature by adjusting the spindle speed, increasing the strength of the magnetic field, and increasing the penetration depth of the eddy current effect. At present, the spindle speed is controlled at 240 ~ 720rpm (equivalent to 4 ~ 12Hz). Compared with the traditional heating furnace, it can obtain a deeper and more uniform axial temperature distribution.
(3) Can heat a variety of non-ferrous metal materials
Traditional AC induction heating is mainly used for heating aluminum and copper due to its low heating quality and uneven heating. High temperature superconducting DC induction heating is more uniform due to the improved heating quality and is also suitable for the heating of magnesium alloy, titanium alloy, nickel-chromium iron alloy, and other special alloys. Foreign researchers have even turned their attention to high-temperature superconducting DC induction heating to non-ferrous metal pre-heating extrusion smelting and other fields.
(4) Simple and convenient installation and maintenance
During the operation of the superconducting coil excitation winding in the superconducting DC induction heating device, the superconducting magnet is static, does not rotate, does not vibrate, and does not have abrasion. The superconducting magnet cooling system adopts the refrigerating machine to conduct cooling, which is simple in structure, easy to operate, and can run for a long time without low-temperature liquid transportation and supplementary operation. In addition, when working, the resistance of the superconducting magnet is very small, even 0, so the insulation requirement of the superconducting coil is reduced. In addition, compared with the conventional AC induction heater, the superconducting induction heating device does not require a high-power AC frequency conversion power supply and does not require the design of a reactive compensation device.
Research on high temperature superconducting DC induction technology at home and abroad and development of heater (equipment)
At the beginning of the 21st century, Norway, Germany, Italy, Russia, and other countries of schools and scientific research institutes for superconducting induction heating technology research, become ac induction heating and electric heating in recent years the mainstream of international academic research, such as the Norwegian university of science and technology and Norway SINTEF energy research institute, Hanover, Germany, Italy’s Leibniz university Havana university, university of bologna, Rome, st. Petersburg, Russia’s national electrical university, etc., the scientific research work for the Italian institutions of higher education and the Ministry of Science and Technology for financing.
In 2002, M.Unde and N.Mnusso from Norway improved the heating efficiency of traditional AC induction heating devices by using superconducting coils. A 10kW AC superconducting induction heating device was developed.
In 2003, they proposed the idea of using DC heating because of the non-negligible AC loss of superconducting coils under AC conditions. Since the theoretical loss of the superconducting DC system is zero, the theoretical efficiency of this method can be as high as 90%.In 2005, the university of bologna in Italy scholar m. Fabbri and Arjun orandi and the team for the aluminum ingot heating temperature distribution in the dc static magnetic field made the simulation calculation in 2007 m. abbrihe and arjun orandi and have been put forward for the magnetic field distribution in induction heating devices, saddle coil magnet structure, 2009, in order to verify the correctness of the simulation model, they developed a model of the dc heating. The model USES 6 XGS26 samarium – cobalt permanent magnets to generate a DC magnetic field, which verifies the rationality of the simulation model.
In 2008, Nikanorov from St. Petersburg Electrotechnical University in Russia and Zlobina from Leibnitz University in Hanover, Germany, etc. developed a THREE-DIMENSIONAL model for aluminum ingots and superconducting coils by using the finite element modeling method, and analyzed the influence of different parameters on the surface temperature distribution of aluminum ingots after heating.
In 2008, Thierry Lubin and Denis Detter et al. in France proposed the method of heating aluminum ingots by applying a rotating magnetic field. Although this method can achieve high power efficiency, it is difficult to make a rotary superconducting electrode because of the high requirement of manufacturing technology.
In 2008, Italy’s m. abbri and a. Morandi, on the basis of the existing research on industrial heating should be the uniform temperature of aluminum processing requirements, using the simulation model of induction heating process was calculated, and the coil design is optimized, the 3 d coil structure, the structure optimization of the aluminum ingot end magnetic field distribution, can reduce the influence of end effect, better high temperature uniformity of induction heating.
In 2008, Germany’s Zenergy Power developed the world’s first high temperature superconducting induction heating equipment and put it into operation in the Vislalu aluminum plant, which is an important step for superconducting DC induction heating technology from the laboratory to the market. The equipment consists of four main parts: high temperature superconducting magnet, refrigeration system, heating and insulation, and the driving system. The cryogenic environment required for the normal operation of the superconducting magnet is mainly provided by the refrigeration system mounted on the top of the superconducting magnet.
In 2014, Jong-Ho Choi of Changwon University in South Korea manufactured a 10kW high temperature superconducting DC induction heating device, wherein the superconducting magnet was wound by YBCO high temperature superconducting strip. The prototype magnet design of the experimental device is a c-type single air gap iron core geometric structure, non-insulated superconducting runway type coil. This heating prototype was used to test the aluminum ingot with a diameter of 8cm and a length of 30C m, and the heating efficiency was 87.5% respectively.
In 2015, on the basis of the previous 10kW high-temperature superconducting DC induction heating device, Jong-Ho Choi et al of Changwon University in South Korea started the feasibility analysis of the 300-kW high-temperature superconducting DC induction heating device. The prototype of the HIGH-TEMPERATURE superconducting DC induction heating device supports the heating of aluminum ingots with a length of 70 cm and a diameter of 23.6mm.SuNam YBCO strip is selected with a width of 12mm and a thickness of 0.15mm. The magnet is designed with a double-cake racetrack coil with an iron core. The length and diameter of the magnet are 62.5cm and 22cm, the number of coils is 300 turns, and the total length of the strip is 3 407m. When the inductance of the magnet with excitation current 440 A is 1.73 H, the central magnetic field strength of the aluminum ingot is 1.1t.
There are a few kinds of research on superconducting DC induction heating in China. In Germany in 2008 developed into a global, 1 dc superconducting after induction heating device has been in the domestic some introductory reported that the Beijing, superconducting technology co., LTD. Small prototype completed the superconductor heating technology of preliminary trials of the small-scale prototype of aluminum ingot size is 30 mm * 80 mm, and experiment of aluminum ingot temperature is the radial surface to the aluminum ingots origin punch, and insert K type electric coupling, and then measured with a multimeter, electric accidentally contact the inner surface of aluminum ingots hole. This experiment verifies that the principle of induction heating technology is completely valid, but it cannot demonstrate the efficiency improvement effect. Meanwhile, the mechanical transmission and motor components of the small prototype also need to be optimized. In recent years, Shanghai Superconductor Technology Co., Ltd. and Jiangxi Lianchuang Optoelectronics Technology Co., Ltd. have carried out technical cooperation in the research of MW high temperature superconducting induction heating technology and the development of heater equipment.