Efficient use of power in electric arc furnaces

Abstract

The maximum transfer of electric energy to the metal in an arc furnace depends on the length of arc and the impedance of the electrical supply system from the generators to the arc itself. The use of directly-reduced sponge iron by continuous feeding results in long periods of flat-bath operation, when it is particularly important to keep a short high-current arc to get the heat into the metal rather than to the refractories, which would suffer excessive wear. The paper illustrates, by reference to a 125 tonne furnace, the method of assessing the optimum operating currents and power factors and the effects of differing powersupply systems. The importance of a low-impedance power system is illustrated, and the possibility of being unable to use the maximum furnace power without excessive refractory wear is noted. The particular problems of connecting arc-furnace loads to electrical supply systems are reviewed, and consideration is given to the problem of voltage flicker. The use of compensators is discussed with reference to existing installations, in which strong supplies from the supply-authority system are not economically available. The furnace operating characteristics, which indicate the optimum points of working, have to be checked on commissioning, and the paper outlines the test procedures. The optimum points for each type of charge and steel can be assessed only during their actual production. The importance of proper recording of relevant data is stressed, and reference is made to the use of computers and automatic power-input controllers. IntroductionThe use of the electric furnace is at present the most efficient way of making steel from a cold charge such as scrap or sponge iron. This applies even to the lower grades of carbon steel, because modern practice enables energy losses to be reduced and higher power to be used without increased refractory wear, and therefore allows reduced cycle times which result in daily outputs of about 10 times the furnace capacity. The paper sets out as simply as possible those factors, governing the efficient use of electric power, which must be taken into account during design and operation. There are, of course, many other factors, such as charge control, oxygen lancing, lime and additions handling, computer control, fettling, pit-side practice and operator training. 2The arcThe energy liberated in an electric arc has been estimated to be 96 MJ/h (23 000 kcal/h) per cm 3 of arc volume at temperatures in the centre of the arc varying from 10000°C to 18000°C. The transfer of this very concentrated energy to the charge is made by: (a) Radiation from the arc and the resulting arc flame, whether direct or redirected from walls and roof (b) Radiation from the electrode tip (c) Dissipation of the electron energy at the interface of the arc and the metal. The available information on the efficiency of transfer of this energy is limited, but experiments carried out by the Institut de Recherches dela Siderurgie 1 indicated that heat transfer into the metal during meltdown was...

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“…(1). The importance of short arcs has been demonstrated in a number of published papers [15,16]. RW I(kW V /cm 2 ) = arc power × arc column volts × 1000…”