Main application knowledge of UHP electric arc furnace

Ultra-high-power electric arc furnaces mainly use arcs to heat and melt scrap steel. The so-called arc refers to a form of discharge between the cathode and the anode, which produces ions with the characteristics of low voltage, high current and high energy density. In the arc column, with the increase of internal energy, some molecules will dissociate into atoms, and then ionize after being excited to form plasma. Plasma is different from solid, liquid and gas, it belongs to the fourth state of matter.

The temperature of each part of the arc is 3500-4000 ℃ at the cathode point; the highest is 15000-20000 ℃ near the cathode; the highest is about 6000 ℃ outside the arc column, and the closer to the inner side, the higher the temperature. The AC arc is different from the DC arc in that its voltage and current change direction twice in each cycle, that is, a 50 Hz power supply is used.

When the current decreases, the charged particles within the arc column recombine and the plasma state tends to disappear. The arc can persist with additional reverse voltage and current until this plasma state disappears. The AC arc is easier to extinguish than the DC arc, and the main factors that can stabilize the arc are: high current, short arc, large reactance percentage, and high power frequency; the less furnace gas flow, the more stable the arc; the more furnace gas pressure The higher the arc, the more stable the arc; the presence of slag can also improve the stability of the arc; the less affected by the external magnetic field, the more stable the arc.

1. Arc of electric arc furnace

The electric arc is the heat source in the electric arc furnace, which determines the thermal conditions in the furnace. Due to the characteristics of the arc itself, it determines the electrical characteristics of the electric arc furnace equipment. This in turn determines all aspects of electric arc furnace steelmaking.

Arc is a gas discharge phenomenon. When a certain voltage is applied between the two electrodes, the gas can discharge itself, and at the same time, the gas between the two electrodes is ionized, and a large number of charged particles - free electrons and positive ions appear, and a conductive channel appears between the electrodes, and the current density reaches several ka/cm2 , the gas temperature reaches several thousand degrees Celsius.

2. Conditions for the stable combustion of the electric arc

According to the nature of the power supply, arcs can be divided into two categories: DC arcs and AC arcs. Most of the arcs used in industrial applications are AC arcs. The voltage and current of an ideal AC arc are sinusoidal waveforms, and the magnitude and polarity of the voltage and current change periodically with time.

In order to ensure the combustion of the arc, in addition to a certain number of charged particles, an electric field of a certain strength is also required between the end faces of the two electrodes, that is, there should be an arc voltage of a certain size. The voltage at which the arc begins to burn is called the starting voltage or arcing voltage. After the external voltage exceeds the maximum value, it begins to decrease. When the arc voltage decreases to a certain value, the arc is extinguished, and the voltage at this time is called the arc extinguishing voltage. Due to the influence of arc temperature, the arc voltage is slightly larger than the arc extinguishing voltage. For a high-power AC arc, at the moment of arc extinction, the temperature of the arc area does not decrease significantly, and it can be approximately considered that the arc voltage is equal to the arc extinction voltage.

If the reactance x=o in the external circuit, the arc current and the arc voltage are in phase. When the external voltage u is less than the arcing voltage, the arc is extinguished and the current is 0. It can be seen from the horizontal axis of time that there will be a period of time before and after the power supply voltage crosses 0:00. At this time, the arc is extinguished, which is the source of the unstable combustion of the AC arc. In order not to extinguish the arc, a certain inductance is required in the circuit, that is, a phase difference should be maintained between the arc current and the power supply voltage.

The polarity of the AC arc changes rapidly during combustion. When the graphite electrode is used as the cathode in the negative half cycle, it has a relatively stable cathode spot, which is easy to form a stable arc; on the contrary, when it is in the positive half cycle, the cathode spot is at a high speed on the molten steel surface due to the steam ejected from the cathode molten pool. Turning, the shape of the arc becomes irregular. In addition, during the melting period, due to the presence of cold steel, the arc continuously jumps from one charge to another, and the arc voltage and current waveforms fluctuate violently and irregularly. All of the above conditions lead to the instability of the arc under operating conditions. The drastic changes of the AC arc during operation make the arc current and arc voltage wave distorted seriously, which will lead to the generation of a series of high-order harmonics and reduce the power supply quality of the power grid.

3. Electric arc in steelmaking arc furnace

If the steelmaking furnace is an AC high-power arc, the reactance in the circuit outside the arc must have a sufficient magnitude accordingly. This is to meet the circuit conditions, so that the arc current has a certain size.

In the three-phase steelmaking electric arc furnace, each phase arc will be affected by the magnetic field established by the other two-phase arcs, so it is moved by the electromagnetic force to the outer side of the electrode end close to the furnace lining, which is the arc blowing. Due to the existence of the blowing effect, the angle between the arc column and the metal surface is reduced to 45°-75°. If the high temperature airflow, that is, the arc filling, rushes towards the furnace wall and throws particles such as metal, slag and graphite at high speed, a hot spot will be formed on the furnace wall above the slag line near the arc. Because of the highest heat load here, it is also the most chemically attacked.

The flow of electric current through the molten steel creates a magnetic field and causes the molten steel to agitate. On a high-power electric furnace, about 20%-30% of the arc heat is transferred to the molten steel by this stirring effect, and the molten steel moved per minute accounts for about 9% of the total weight.