1 DC graphitization furnace
DC graphitization furnace (DC graphitization furhace)
Use carbon roasted products and resistance materials as the furnace core, and pass in direct current. A resistance furnace for producing artificial graphite products. Due to the resistance of the furnace core (mainly the resistance of the resistance material), when the current flows through, the electrical energy is converted into heat energy, and the carbon roasted product is heated to a high temperature of 2000 to 3000°C to complete the graphitization process and become artificial graphite. It and the AC graphitization furnace both belong to the Acheson furnace.
A brief history In the 1960s, DC graphitization technology began to develop in developed countries in Europe and the United States. Compared with AC graphitization furnaces, it has significant advantages such as large capacity, good product quality, and low energy consumption. Therefore, It has aroused widespread interest and concern from all over the world. China's DC graphitization furnace started a little later. In October 1972, Beijing Carbon Turbine Plant used a 3000kV?A rectifier transformer with a 9m furnace for the first time in production. Compared with AC furnaces, it not only shortened the power transmission time, but also saved more than 25% of electricity. In January 1973, Nantong Carbon Factory used a 13500kV?A rectifier transformer with an 18m furnace and put it into production. It also achieved results of shortening the power-on time by 20h and reducing the power consumption to less than 4000kW?h/t. In September 1975, the 16000kV?A large DC furnace of Jilin Carbon Factory and the 3340kV?A DC furnace of Shijiazhuang Graphite Electrode Factory were put into production at the same time. As of 1986, China's original 136,000 kV?A AC graphitization furnace accounted for only 27% of the total installed capacity of graphitization furnaces that year. The installed capacity of DC graphitization furnace has reached 175,000 kV?A, accounting for 73%. This has brought China's graphitization technology to a new level.
Furnace structure and characteristics DC graphitization furnace and AC graphitization furnace have exactly the same structure except for the power supply equipment. The power supply equipment of the DC graphitization furnace consists of a three-phase AC main regulator and a transformer and corresponding rectifier equipment.
Powering the furnace with direct current has the following advantages: (1) Since the power supply transformer used is three-phase, it will not have an unbalanced impact on the three-phase load on the power grid. It can increase the capacity of the transformer, strengthen the graphitization process, and increase the capacity of the graphitization furnace. (2) The power factor on the entire power supply line is high, reaching more than 0.9, and the effective utilization of electric energy is improved. (3) DC electricity has no alternating magnetic field and inductance losses, and no electrical losses such as surface effect and proximity effect, and its electrical efficiency is high.
Intensification of the graphitization process The improvement of power supply conditions of DC graphitization furnace has created conditions for intensification of the graphitization process. Since the power grid has no limit on the capacity of the transformer, high-power transformers and rectifier units can be used. The loss of DC power is small and the utilization rate is high, so the furnace core can get more electric energy. If matched with a furnace core of appropriate size, the power per unit volume can reach more than 160kW/m3 (60% greater than AC furnace), and the current density can reach more than 2.0A/cm2 (100% greater than AC furnace). Such conditions are met. , rapid power transmission can be achieved, so that the graphitization temperature reaches 2700°C in a short period of time (about 400°C higher than that of the AC furnace). Due to the shortened power transmission time, the furnace productivity can be increased and the power consumption of graphitization can be reduced, which can generally be reduced to less than 4000kW·h/t (about 20% lower than the AC furnace). The increase in graphitization temperature makes graphitization proceed more efficiently. more complete, thus improving product quality. In short, the DC graphitization furnace can achieve high-power, high-density, and fast-curve operations, so that graphitization production can achieve the goals of high yield, high quality, and energy saving. This is the strengthening of the graphitization process. Take the 16000kV?A DC graphitization furnace and the 5000kV?A AC graphitization furnace as examples. Its technical and economic indicators are shown in the table.
To strengthen the graphitization process, in addition to using a large-capacity rectifier transformer unit on the equipment, the length and core area of ??the furnace should be appropriately increased and matched with the transformer, and other measures should be taken in process operations. The following measures: (1) Use resistance materials with low resistivity and insulation materials with low thermal conductivity and low electrical conductivity; (2) Improve the quality of roasted blanks; (3) Use furnace loading methods with large and small specifications and offset 1/2D installation. Furnace method; (4) Realize mechanization of loading and unloading, shorten the cooling time of the furnace, and increase the turnover rate.
2 Series graphitization furnace
Serial graphitization furnace (lengthwise graphitization furnace)
A type of baked product that directly passes current into the series connection , a resistance furnace that uses the resistance of the product itself to convert electrical energy into heat energy and graphitize the product.
Brief history: This type of furnace is also called Kastner furnace, which is HY. Castner first invented and patented it in 1896. Its basic principle is to place the roasting electrodes horizontally in the furnace, connect them in series along their axes, and then fix them between two conductive electrodes. In order to reduce heat loss, the roasting electrodes are The surrounding area is covered with insulation material. After energization, the current flows directly to the electrode, relying on its own resistance to generate heat, and the temperature rises rapidly. It only takes about 10 hours to reach the temperature required for graphitization, which greatly shortens the production cycle.
During the power transmission process of the series-connected furnace, the current is evenly distributed within the electrode, so that when the electrode heats up, the temperature difference between the surface and the inside is very small. Although the temperature rises at a high speed, it will not cause the product to crack, making the It is possible to shorten the production cycle. At the same time, because it does not rely on resistance materials to transfer heat, of course there is no heat consumption. These two items alone form the basis for the series connection furnace to be more energy-saving than the Acheson furnace, and it also has The production operation adopts automated control and improves working conditions and other advantages.
Although the tandem furnace is superior to the Acheson furnace in terms of process methods, due to the technical difficulties in the furnace structure itself, industrial production in various countries around the world has been restricted for a long period of time. , far less widely used and developed than the Acheson furnace. By 1974, the former Federal Republic of Germany's SGL Corporation announced a new patent application for a tandem furnace. In 1980, the American Great Lakes Carbon Company built an in-line graphitization workshop in the United States. In 1978, the former Federal Republic of Germany's KHD Company announced their single-unit furnace. The V-shaped tandem furnace test is successful and the product can be put on the market. Its basic parameters are: graphitization temperature, diameter of electrodes that can be produced, length of rows of electrodes in the furnace, production cycle, input DC current, input DC voltage, voltage control range once Voltage, frequency, current density, power consumption. Judging from the above results, the tandem furnace has the ability to compete with the Acheson furnace.
Structure The basic structure of the furnace is shown in the figure.
It can be seen from the picture that the electrodes are arranged in a V shape in the furnace. The current enters through one end of the furnace and comes out at the other end. In addition to the hearth, there are also movable side walls and end walls with electrodes. , the outer steel frame is lined with refractory material, one end is fixed and sealed, connected with a busbar, the other end is movable, which can compensate for the expansion and contraction of the electrode during graphitization, and the connecting mechanism is installed on the rail car, which connects and fixes the aluminum The electrodes on the busbar and the movable end wall of the furnace have high-current insulating switches and hydraulic systems to provide telescopic contact pressure for the electrodes arranged in rows. The vehicle is equipped with a cooling system to cool the contact plates and cables.
The production operation rail car is positioned first, and the current is sent from the busbar to the furnace head end wall electrode through the contact plate and water-cooled cable. The hydraulic jack presses the rows of roasting electrodes against each other, and can adjust and stabilize the electrodes. The expansion and contraction produced during the graphitization process. For example, when an electrode with a diameter of 350 to 650 mm is graphitized, the current density reaches 25 to 50 A/Cm 2, and the contact between the carbon layers needs to be maintained at 0.4 to 1.0 MPa. Contact pressure.
The tandem furnace can heat up at a rate of up to 600°C/h without cracking, and the larger the electrode diameter, the better the process technical indicators, which is exactly the opposite of the Acheson furnace production. However, when the electrodes are ready to transmit power, the contact resistance between the contact surfaces of the electrodes must be very low, otherwise the heating of the contact surface will exceed the electrode itself, causing the temperature difference between the joint and itself to cause the joint to crack. The solution is to rely on The hydraulic facility installed at the end not only pressurizes the electrodes to maintain close contact, but also must specially process the contact surface, and apply a layer of glue composed of graphite powder and resin on the processed surface to achieve good results.
KHD Company compared the thermal balance between the test furnace and the Acheson furnace, showing that the thermal efficiency of the tandem furnace is as high as 49%, which is twice as high as the Acheson furnace.
Prospects Since the tandem furnace has made a breakthrough in today's industrial production, and it is superior to the Acheson furnace in terms of energy saving, product quality, production cycle, operating environment, etc., Europe, the United States and El Ben It has been used in industrial production in various countries.
China has also conducted a large number of theoretical research and experimental explorations on the process and equipment of internally heated tandem furnaces, and has achieved preliminary research results, laying the foundation for the development of tandem graphitization technology.
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