Three-phase AC Electric Arc Furnace

Three-phase AC Electric Arc Furnace

Three-phase AC electric arc furnace is the main equipment for smelting electric furnace steel.

(1) Electrical and Thermal Characteristics.

Although the law of DC arc formation is also applicable to AC arc formation, the instantaneous voltage of AC power supply varies and phase difference exists between three-phase electrodes, so AC arc has its own special law.

When the electrode is supplied with alternating current, the instantaneous current of the arc changes with time, and the instantaneous power changes with time because the instantaneous value of the supply voltage changes with time, which is a fluctuating arc and may also be interrupted temporarily, i.e. it may be a discontinuous arc. An arc can be considered as a current-carrying gas conductor, which is subjected to electromagnetic forces when in a magnetic field, resulting in arc deflection, the size, and direction of which conform to Ampere's law.

(2) Heat Transfer in Furnace.

Three-phase electric arc furnace belongs to the furnace with large capacity and energy consumption, so energy saving is a central issue in furnace thermomechanics.

The heat exchange objects in the electric arc furnace are the electric arc, the charge, and the lining (including the hearth and the top), which together form a closed system.

The electric arc is the heat source, the charge is the heating body, and the lining is the insulation, which isolates the charge and the arc from the outside atmosphere and reduces the heat loss. Under normal conditions, the amount of gas in the furnace is not large. In order to simplify the problem, its role in heat transfer in the furnace is not considered.

Heat exchange in an electric arc furnace is extremely complex. 

The following heat transfer processes take radiant heat transfer as the main one:

1) The arc radiates directly to the liquid level of the molten pool;

2) Surface radiation from electric arc to furnace lining;

(radiation from the inner surface of furnace lining to liquid level of the molten pool;

4) Radiation from surface to surface of lining;

5) The molten pool liquid radiates towards the inner surface of the furnace lining;

6) Conduction and convection heat transfer from the upper surface of the molten pool to the inside of the molten steel;

7) Heat dissipation through the lining, door, and water-cooled parts.

(3) AC Electric Arc Furnace Structure.

The complete set of AC electric arc furnace equipment for steelmaking consists of four parts: the main circuit equipment, the control equipment, and the dust removal equipment.

The main body of the electric arc furnace for steelmaking consists of several parts, such as a hearth, body, furnace cover, electrodes, elevating device, and tilting mechanism.

Determining the shape and size of the furnace body is an important work in the design of an electric arc furnace. The principle of determining furnace shape is to meet the requirements of the steel-making process first. Secondly, it is beneficial to heat exchange in the furnace with small heat loss and full utilization of energy. In addition, a longer lining life is required.

When working, the lining of the electric arc furnace is easy to be damaged due to the high-temperature radiation of the electric arc, the impact of the solid charge, scouring of liquid metal, and furnace shake. Therefore, the life of the lining becomes a technical and economic index of the electric arc furnace. At present, the life of the semi-water-cooled lining of domestic electric arc furnaces is generally between 150 and 300 furnaces.

The brick cover of the electric arc furnace has a circular water-cooled furnace cover ring welded from steel plates to support the refractory material. The furnace cover is arched with refractory (commonly used aluminum plaque in China). There are three symmetrically arranged electrode holes in the furnace cover. The lifetime of refractories for furnace cover is an important technical and economic index. Generally, in China, the service life of the furnace cover is about 100 stoves, and that of the full water-cooled (spray-cooled) furnace cover of Mario Steel Company can reach 5000 stoves.

Because the current entering the electric arc furnace is very large, taking a 10t furnace as an example, the current of each electrode reaches about 13kA, so good conductivity of the electrode is required. At the same time, it should be able to withstand high temperatures and have certain mechanical strength. Generally, a graphite electrode is used. Graphite electrodes are expensive and their consumption index directly affects the cost of steelmaking. The average consumption index of electrodes in China is about 7kg/t, and the advanced index is 4.5kg/t. South Africa uses an external jet cooling device, which reduces electrode consumption to 3.25kg/t.

Advantages of Three-phase AC Electric Arc Furnace

(1) energy concentration, surface power of molten pool up to 560~1200kW/m2, and arc temperature over 3000T; 

(2) The process has great flexibility, can effectively remove impurities such as sulfur and phosphorus, and is convenient for temperature control; 

(3) Compared with converter furnace, scrap steel can be used as furnace charge; 

(4) High productivity and low power consumption (smelting power consumption is 350~600kW h/t); 

(5) Small area and low investment cost.

Disadvantages of Three-phase AC Electric Arc Furnace

(1) More smoke and dust, 2.5~8kg falling steel dust; 

(2) High noise up to 90~120dB; 

(3) Arc flashing interferes with the power grid.

Hani Metallurgy is a Hot Rolling Mill manufacturing company working mainly in the area of bar and wire rod mills, high-speed wire rod mills, high-speed rebar rolling mills, TMT rolling mills, flying shear, cold strip rolling mill, reheating furnace, induction furnace, intermediate frequency furnace, electric arc furnace, ladle refining furnace, blast furnace, continuous casting machine.

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