DC Electric Arc Furnace
DC Electric Arc Furnace
DC (direct current) electric arc furnace (EAF) is a furnace for primary steelmaking which represents a different concept in the designs of arc furnaces. DC-EAF has only a single electrode which acts as a cathode and the current flows down from this graphite electrode to an anode which is mounted in the bottom of the furnace. Single graphite electrode exploits the highly efficient heat transfer of the arc generated between the graphite top electrode and the anode provided by the charge of the furnace. A typical view of a DC-EAF is shown in Fig 1.
Fig 1 Typical view of DC-EAF
DC-EAF arc furnace typically comprises a refractory lined cylindrical steel shell, with a central graphite electrode vertically positioned through an opening in the centre of the roof. The anode connection in the hearth of the furnace is in direct contact with the layer of liquid steel which is covered by a layer of the liquid slag. The energy is supplied by means of an open plasma arc (Fig 2) which is generated between the bottom tip of the cathode and the upper surface of the molten slag. At least a central portion of the slag surface is open. Because the furnace is electrically powered, very high temperatures (higher than 1500 deg C) can be attained.
DC-EAF is an alternative to the AC (alternating current) based EAF. The output of the UHP (ultra high power) transformer is converted to DC using a power rectifier usually bridge connected thyristors. DC-EAF equipped with controllable high power rectifier systems ensures a stable arc under all conditions, at the maximum possible power ratings. The arc in a DC-EAF is a sustained high-velocity high-temperature jet, driven by electromagnetic acceleration (the Maecker effect) in the constricted region near the arc’s root on the electrode surface. The arc is generated by the interaction between the fluid flow, the thermal field, and the electromagnetic fields. The self-constricting electromagnetic forces keep this supersonic super-heated plasma jet (Fig 2) reasonably coherent. A DC reactor is used to stabilize the arc further. Furthermore, the surface of the liquid bath (or at least a portion of the surface in the arc attachment zone) is open, i.e. essentially uncovered by unreacted feed material. Schematic diagram of a DC-EAF is shown in Fig 2.
Fig 2 Schematic diagram of a DC EAF
DC-EAF has several unique requirements over AC furnaces in addition to the obvious differences in electrical power supply. The important features of DC-EAF are (i) robust and reliable design, (ii) high flexibility with respect to charge materials (iii) high current density and power usage, (iv) high arc stability, (v) good quality of power even under weak grid conditions, and (vi) independent voltage and current control. Other characteristics of a DC-EAF as compared with AC-EAF are given below.
- The operation is more stable. The melting is uniform.
- It has reduced electrode consumption because of the system regulations. Electrode consumption reduces to around one third. However there is need to improve the spalling and cracking of the electrodes. Lower consumption of electrodes makes it possible to make low carbon heats.
- It has lower lining wear. Refractory consumption is less on the side walls but more on the bottom. Overall saving in refractory consumption is in the range of 15 % to 25 %.
- There is convection stirring of the bath.
- Temperature distribution of the bath is better. Better temperature distribution results into improved heat distribution. The hot spots on the furnace wall in the case of AC-EAF are not there.
- Noise levels are much lower (reduces from 105 dB to 85 dB).
- Installation costs are higher (around 10 % to 35 % more).
- Operating costs are lower (around 15 % to 20 % less).
- There is lesser network disturbance. There is sharp reduction in flicker effect. Flicker level and flicker frequency reduces by half. The need for Var compensation equipment is much lower.
- There is lower energy consumption. There is 5 % to 10 % savings in power.
- Bottom electrodes make the furnace bottom complicated.
- It has lower levels of dust and gas emissions.
The development of DC arc furnace technology
It is generally believed that the DC arc furnace is a recent development. This is only broadly correct. Already in 1881 in Germany and 1885 in Sweden, melting furnaces were described that used DC technology. These furnaces had two approaches to melting namely (i) the concept of a burning arc between a graphite electrode and a metallic charge, as used today, and (ii) the proposal of an arc burning between two horizontally arranged electrodes, in this case melting the charge by radiation only. Since neither high current rapid and controllable DC rectifiers nor a solution to the problem of high thermal losses were available during that time, the DC arc furnace could not be developed into a feasible unit for operation under prevailing industrial conditions.
In the early 1970s, DC technology was developed to such level that it became reasonable to use this technology for arc furnaces. Application of this technology created a new generation of arc furnaces for use in the steelmaking industry, as well as in the ferro-alloy and non-ferrous metal sectors. The single graphite electrode DC furnace exploited the highly efficient heat transfer of the arc generated between the graphite top electrode and the anode, provided by the charge in the furnace. Successful applications in the 1980s and 1990s established melting and smelting advantages of the DC furnace.
Basic design features
In DC-EAF the electrical energy is converted into thermal energy mainly by the arc, which is established between the top electrode tip and the slag bath. The top of the electrode is connected as the cathode, and the conductive bottom system is connected as the anode.
DC furnaces have only one electrode mast arm and a single graphite electrode. This electrode acts as the cathode. Thus the top of the furnace is less complicated and there are fewer component to be maintained in the case of the DC-EAF and in general has fewer components to maintain as compared to AC designs. The electrode is consumed during the steelmaking operation and is to be extended by new pieces. Electrode arm is used for regulation of the electrode. Since there is single electrode the graphite loss due to oxidation is lower when compared with AC arc furnace.
For top electrode there are special requirements of graphite and it needs special grade of graphite. The special requirements when compared with the graphite for electrode for AC-EAF are (i) to have lower thermal expansion coefficient, (ii) to have lower specific electrode resistivity, (iii) to have enhanced transverse thermal conductivity, (iv) to have lower coefficient of thermal expansion, and (v) to have improved homogeneity and coarseness. DC-EAF requires an effective cooling arrangement in the roof and side wall area to counteract the effect of hot liquid slag in direct contact with the refractory material and the increased thermal radiation.
The DC-EAF however, needs a return electrode, the anode, to complete the electrical circuit. This anode is normally referred to as the bottom electrode since it is located in the bottom of the furnace shell. The furnace operates with a hot heel in order to ensure an electrical path to the return anode. Several different designs are available for the bottom return electrode including metal pin return electrodes with non-conductive refractories, billet electrode, metal fin electrodes, and conductive bottom refractory. These are shown in Fig 3.
Fig 3 Different types of bottom anode configurations
In the case of current conducting refractory contact, the refractory lining at the centre of the furnace bottom acts as the anode. The bottom has a circular flange which rests inside a circular channel which is welded to the furnace shell. Inside the channel, the flange is supported by fiber reinforced ceramic blocks. The space between the channel, support blocks, and flange is filled with a refractory ramming compound. This isolates the bottom electrically from the rest of the furnace shell as shown in Fig 4.
The spherical furnace bottom is made of high temperature steel. A circular copper plate is bolted directly to the furnace bottom. Four copper terminals extend down through the furnace bottom from the copper plate and connect to flexible cables which in turn are connected to the bus tubes. The conductive refractory bricks are installed over top of the copper plate. Heat flow from the bottom of the furnace (normally around 15 kW/sq m) is removed by forced air cooling. Due to the large surface area of the bottom electrode, the current density tends to be quite low, normally around 5 kA/ sq m. However, in some furnaces, non-conductive patching material is used in the centre of the furnace in order to force the current to distribute more evenly over the whole bottom. In case proper distribution of the current is not achieved, then it results into hot spots in the centre of the furnace.
Fig 4 Conductive refractory bottom electrode
The billet return electrode configuration uses from 1 to 4 large steel billets around 100 mm to 150 mm diameter but can be as large as 250 mm diameter depending on the size of the EAF. Normally, the design aims for a current of 40 kA to 45 kA per bottom electrode. The billets are in contact with the bath at the top surface and hence, melt back. The degree to which the billet melts back is controlled by water cooling. The billet is inserted into a copper housing through which cooling water is circulated. By providing sufficient cooling, it can be ensured that the billet does not melt back completely. Thermocouples monitor the bottom billet temperature and the cooling water temperature.
An insulating sheath isolates the copper housing from the billet. The billet is connected to a copper base. The copper base provides the connection to a power cable. Typical arrangement of a billet anode is given is Fig 5.
Fig 5 Typical arrangement of a billet anode
The pin type of return electrode uses multiple metal pins of 25 mm to 50 mm in diameter to provide the return path for the electrical flow. These pins are configured vertically and actually penetrate the refractory. The pins extend down to the bottom of the furnace where they are fixed in position by two metal plates. The bottom ends of the pins are anchored to the lower power conductor plate. The bottom contact plate is air cooled and is located in the centre of the furnace bottom. The top portions of the pins are flush with the working lining in the furnace. The pins are in direct contact with the bath, and melt back as the working lining wears away. A return power cable is attached to the bottom conductor plate.
An extensive temperature monitoring system is provided to track lining wear and bottom electrode life. This enables scheduled change out of the bottom electrode. The integral cartridge design which has evolved allows for quick change out of the bottom electrode over a scheduled 8-hour maintenance outage.
The steel fin return electrode uses steel fins arranged in a ring in the furnace bottom to form several sectors. Each sector consists of a horizontal ground plate and several welded on steel fins which protrude upwards through the refractory. The fins are around 1.6 mm thick and are around 90 mm apart. The sectors are bolted onto an air-cooled bottom shell which is electrically insulated from ground and is connected to 4 numbers copper conductors.
Most of the DC-EAFs are operated with long arcs, typically 2 to 3 times those encountered in the conventional UHP furnace operations. As a result, DC-EAF has a higher water flow rates for water-cooled panels.
Refractory lining for DC-EAF
The refractory concept of a DC furnace is to be designed considering the harsher condition in which they operate. DC arc furnaces have special refractories at the bottom because of the anode is installed in the bottom of the furnace. Besides the bottom refractories, other important points which are to be considered for design of the refractory lining are refractory zoning pattern, hearth contour, slag line location, tap-hole size, angle, and location, roof orientation, expansion allowances, burner port location, slag door construction and bottom stirring element.
DC-EAF has special refractory requirements since the return electrode is usually installed at the bottom of the furnace (some DC-EAFs use an alternative arrangement with two graphite electrodes). In the case of a current conducting bottom, the refractory lining at the centre of the furnace bottom acts as the anode. A copper plate is usually connected below the conductive refractory and the return copper bus bar is connected to the plate. In this case special requirements for the refractory are low electrical resistance (preferably less than 0.5 milli ohms per meter), low thermal conductivity, and high wear resistance.
A typical configuration uses a 150 mm thick working lining consisting of carbon bonded magnesia mixes containing 5 % to 10 % carbon. These materials can be installed either hot or cold. Below the working lining a three layer magnesia carbon brick is installed. The residual carbon content of the bricks ranges from 10 % to 14 %. With regular maintenance, this bottom electrode configuration has achieved a bottom life of upto 4,000 heats.
The billet return-electrode configuration uses from 1 to 4 large steel billets (around 250 mm in diameter) depending on the size of the furnace. The billets are embedded in the bottom refractory. The billets are surrounded with a basic refractory brick. The remainder of the hearth is rammed with a special magnesia ramming mix. Magnesia ramming mix is used to maintain the brick area around the electrode. This return electrode configuration has achieved in excess of 1,500 heats on the furnace bottom.
The pin type of return electrode uses multiple metal pins of 25 mm to 50 mm in diameter to provide the return path for the electrical flow. These pins actually penetrate the refractory down to the bottom of the furnace where they are attached to a metal plate. Dry magnesia ramming mix is used for the entire hearth lining. This mix is rammed between the metallic pins. Alternatively magnesia carbon brick can be used in the area around the anode. This helps to improve the furnace bottom life but is more costly. Typical bottom life ranges from 2,000 heats to 4,000 heats depending on the refractory materials used.
The steel fin return electrode uses steel fins arranged in a ring in the furnace bottom to form several sectors. Each sector consists of a horizontal ground plate and several welded steel fins which protrude upwards through the refractory. Dry magnesia ramming mix is used between the fins. The hearth is also lined with this material.
Electrical considerations for DC- EAF
To achieve maximum furnace throughput, furnace power control is to be optimized to ensure maximum power input at all stages of the melting process and during variations in the charge material. At the same time, minimum network disturbance always is to be guaranteed. Power quality is to be at ease with the utility.
DC supply – The required high power again is supplied from a high voltage 3-phase AC network. This is converted to DC by rectification of the output of the furnace transformer. Rectification is achieved by bridge-connected thyristors. Normally 12, 18, or 24-pulse supplies are used in arc furnaces, obtained by multiple, parallel transformers electrically displaced one from another so that their individual pulses overlap uniformly. This electrical displacement, of 15 degrees, 10 degrees or 7.5 degrees, corresponding to the 12, 18 or 24-pulse systems, is made by various coil connections within the transformer. For this reason the transformers used for DC-EAFs are quite different to those for AC-EAF and are generally unsuitable for AC furnace operation.
The volt/amp characteristic of a DC supply consists of a weakly declining drop of DC voltage as the DC current increases. The slope of this line is on the order of 1 volt per kA and is determined by the commutating reactance of the transformer / rectifier combination, not by the arc furnace. In order therefore to limit wide current excursions due to widely different arc voltages thyristors are used in preference to diodes. The conducting instant after current zero (firing angle delay) is under the control of the gate terminal. Each thyristors can, in principle, be turned off within half a cycle. Even so, within the several milli-second delays between an arc-voltage change (e.g. a short circuit) and the control of the thyristors, currents can increase significantly. To reduce the rate of rise of the current it is normal to add a reactor within the DC current loop, the natural reactance of the high current DC loop being inadequate.
These reactors are sized to have an inductance in the 100 micro-Henry to 400 micro-Henry range. Since they take the full DC current, ohmic losses are significant and can only be maintained within acceptable bounds by employing an adequate section of the copper or aluminum making up the coils. Thyristors are each capable of handling a few kA and a few kV of reverse polarity. An arrangement of series and parallel connected thyristors makes up each leg. Fuses and voltage balancing resistors are used as protective measures. Cooling is affected by de-ionized water.
Electrical characteristics of DC-EAF – The thyristors control is normally chosen to hold current constant. Thus the AC current before the rectifier is also constant, as is the primary current. Considering powers on the AC primary, it is seen that constant current means MVA is constant. The characteristic of MW as a function of MVAR is hence a quadrant of a circle for which (square of MW) + (square of MVAR) = (square of MVA) = constant. Normally the slope of the volt / amp line is linear and drops typically 100 V in 100 kA. Thus at 100 kA, for example, the thyristors control can hold constant current over an arc voltage range from about 900 V down to short circuit by varying the firing angle.
Bottom connections – In order to operate with a single DC arc it is necessary to make an electrical connection (the positive anode) to the steel charge. Various solutions have been developed for this issue. A few types of the bottom connections for DC-EAF are shown in Fig 6.
Fig 6 Types of bottom connections
In one type the anode current is shared amongst many steel rods embedded in a rammed refractory block. The rods, with a diameter of around 25 mm, can be one meter long and are linked by a copper plate below the furnace shell. The whole anode block can measure 1 m to 2 m in diameter. A variation on the pin type is to use thin steel sheets, again embedded in refractory. Another variation is to employ a steel billet of diameter 250 mm passing through an insulated sleeve, leading to a cooled copper connection below the furnace shell. In all three of these designs (pin, sheet or billet) the top of the steel conductor melts through the course of the heat. It re-solidifies during power-off and after scrap charging.
An alternative to the steel-to-steel current designs is one where the current is taken through conductive refractories to a large diameter, copper bottom plate. In all bottom connection types there is to be insulation between the anode connection and the furnace shell. This is to reduce the likelihood of current passing through the shell directly to the anode bus bars
The power quality is the main concern of the power supply system in an EAF unit. It is necessary to have compliance with flicker, power factor, and harmonic limits. An effective electrode control algorithm reduced transformer switching and correct sizing of the DC reactor and harmonic filters are necessary to meet the utility requirements. In addition to that, optional active flicker reduction and continuous power factor correction through a Static Var Compensator (SVC) are required to guarantee compliance with the most stringent utility demands and/or permit operation in remote areas with weak grid conditions.
Flicker is mainly caused by reactive power fluctuations. Hence, a fast-forward link which sends information from the rectifier control to the SVC control, allowing the calculation of the actual reactive power consumption of the furnace is required. This information is used to improve the flicker mitigation performance. The result of using a SVC system is (i) higher average power input compared to conventional Var compensation, (ii) continuous power factor correction close to unity, (iii) control of furnace bus voltage, (iv) no unbalanced load in the network, and (v) low flicker and harmonic levels even in weak grids.
Arc stabilizer is necessary for higher productivity. A stable process is essential for productive operation. The use of an optimized DC reactor design (low losses) to smoothen the electrode current helps a stable arc is to be maintained at all times. The result is (i) lower stress on the electrode hydraulic system, (ii) less vibration, (iii) lower electrode consumption, (iv) higher productivity, (v) even greater flicker reduction, and (vi) lower radiation losses.
In the case of the DC-EAF, the thyristors have two copper terminations, one of which is attached to the EAF power cable, and other is attached to the bottom furnace electrode. The bottom furnace electrode is normally rigid, as no movement is required during operation of the furnace. In principle, the termination on the thyristors is analogous to the delta closure, though physically, it differs considerably. With respect to the maintenance issues for the delta closure, however, the same concepts can be applied to the DC operation.
The progress in high power semiconductor switching technology brought into existence low cost efficient DC power supplies. Due to these advances, the high power DC furnace operation became feasible. The DC-EAF is characterized by rectification of three phase furnace transformer voltages by thyristors controlled rectifiers. These devices are capable of continuously modulating and controlling the magnitude of the DC arc current in order to achieve steady operation. DC furnaces use only one graphite electrode with the return electrode integrated into the furnace bottom. There are several types of bottom electrodes conductive hearth bottom, conductive pin bottom, single, or multiple billet, and conductive fins in a monolithic magnesite hearth.
All of these bottom return electrode designs have been proven. The ones which appear to be used most often are the conductive pin bottom where a number of pins are attached to a plate and form the return path and the bottom billet design. The bottom electrode is air cooled in the case of the pin type and water-cooled in the case of the billet design. The area between pins is filled with ramming mass and the tip of the pins is at the same level as the inner furnace lining. As the refractory wears, the pins also melt back.
DC-EAFs operate with a hot heel in order to ensure an electrical path to the return electrode. During startup from cold conditions, a mixture of scrap and slag is used to provide an initial electrical path. Once this is melted in, the furnace can be charged with scrap.
Some of the early benefits achieved with DC operation included reduced electrode consumption (20 % lower than high voltage AC, 50 % lower than conventional AC), reduced voltage flicker (50 % to 60 % of conventional AC operation) and reduced power consumption (5 % to 10 % lower than for AC). The above results have been mainly achieved on smaller furnaces which were retrofitted from AC to DC operation. However, some larger DC furnace installations did not immediately achieve the claimed benefits. Especially, two areas of concern emerged namely (i) electrode consumption, and (ii) refractory consumption.
Several DC furnace operations found that the decrease in electrode consumption expected under DC operation did not occur. Much analysis by the electrode producers indicated that physical conditions within the electrodes are different for AC and DC operations. As a result, for large DC electrodes carrying very large current, an increased amount of cracking and spalling has been observed as compared to AC operations. Hence, it has become necessary to develop electrodes with physical properties better suited to DC operation.
The economical maximum size for DC furnaces tends to be a function of limitations due to electrode size and current carrying capacity. At the present time the maximum economical size for a single graphite electrode DC furnace appears to be around 165 tons. Larger furnace sizes can be accommodated by using more than one graphite electrode.
Several of the early DC operations have experienced problems with refractory wear and bottom electrode life. These problems are directly related to arc flare within the furnace. The anode design has the greatest influence on the arc flare. In all DC furnaces, the electric arc is deflected in the direction opposite to the power supply due to asymmetries in magnetic fields which are generated by the DC circuit. Thus the arc tends to concentrate on one area within the furnace creating a hot spot and resulting in excessive refractory wear. Several solutions have been developed to control or eliminate arc flare. All the bottom electrode designs are presently configured to force the arc to the centre of the furnace.
In the case of bottom conductive refractory and the pin type bottom, it is necessary to provide split feed lines to the bottom anode or a bottom coil which helps to modify the net magnetic field generated. In the billet bottom design, the amount of current to each billet is controlled along with the direction of anode supply in order to control the arc. The bottom fin design utilizes the fact that electrical feed occurs at several points in order control arc deflection. Quadrants located further from the rectifier are supplied with higher current than those located closer to the rectifier.
Some feel that the possibility for increased automation of EAF activities is greater for the DC furnace. This is because with only one electrode, there is increased space both on top and within the furnace. DC furnace is expected to cost from 10 % to 35% more than a comparable AC furnace. However, calculations on payback indicate that this additional cost can be recovered in one to two years due to lower operating costs.
A study has been conducted which has compared AC and DC furnace operations and it has been found that the electrical losses amount to around 4 % in AC operations and 5.5 % in DC operations with the difference in absolute terms is relatively insignificant. The difference in total energy consumption between AC and DC furnaces is likely to be less than 9 kWh/ton in favour of the DC furnace. However many other variables influence the power consumption and it is difficult to develop accurate figures.
DC furnaces experience roughly 25 % less electrode consumption than AC furnaces, this correlating to typically 0.4 kg/ton. This difference appears to be greater for smaller AC furnaces. Flicker is around 60 % lower for DC operations, however, advances in AC power system configurations (additional reactance) has reduced this difference to 40 %.
Some typical results which have been presented for large DC EAF operations are electrode consumption of 1 kg /ton to 2 kg/ton liquid steel, power consumption in the range of 350 kWh/ton of liquid steel to 500 kWh/ton of liquid steel, tap-to tap time ranging from 45 minutes to 120 minutes, and bottom life of 1,500 heats to 4,000 heats. It is important to remember however, that power consumption is highly dependent on operating practices, tap temperature, use of auxiliary fuels, scrap type etc.