Process Description :
The electric are furnace (EAF) is the principle furnace type for the electric production of steel. The primary application of the EAF is for the remelting of steel scrap; however, EAFs can be charged with limited amounts of iron scrap, pig iron, and direct reduced iron.


ELECTRODES
PRODUCING ARC AND HEATING THE CHARGE


Equipments :
The most common configuration for the EAF in the steel industry is circular with a dish shaped hearth. A typical direct three phase AC furnace has three movable graphite electrodes mounted in the roof. The roof is typically removable for scrap and flux charging. After charging, the electrodes are lowered just above the scrap and an arc is struck that provides the heat for melting the scrap primarily through radiation but also through the current resistance through the metal. Scrap can be added several times during a cycle.




Fume and Dust Collection System
All EAFs have a dust collection and gas clean-up system attached to the furnace. Various systems are used including baghouse, dust collector, sidedraft, direct evacuation, fourth hole systems, and wet scrubbers.

Oxygen Lancing
Oxygen lancing is used on some furnaces to increase the rate of carbon removal from the melt. This practice is often accompanied by coke or coal injection to first increase and then decrease carbon to achieve a controlled product specification.

Water Cooled Panels
Replacement of sidewall and/or roof refractories with water cooled panels decreases the cost and downtime associated with refractory replacement and allows an increased rate of power usage in the furnace thereby increasing productivity.

Oxy-fuel burners
Oxy-fuel burners are used on the EAFs. They increase the effective capacity of the furnace by increasing the speed of the melt and reducing the consumption of electricity and electrode material.

Electrode Saver Equipment
Typical electrode control moves the electrodes in place based on sensed arc voltage and current flows. This type of control can lead to electrode breakage if there is excessive slag or other nonconductive material in the melt. Electrode saver equipment combines current and voltage control with a mechanical tension sensor that stops the mast motor in cases of abnormal tension.

Power Demand Control
EAF melt shops can control electric power demand and thus reduce costs by controlling furnace operation so as to minimize power demand spikes and power factor corrections.

UHP Transformers
Transformer ratings per ton of steel melted have increased dramatically over time. Some shops have over 700 kva/ton of steel of capacity. Three legged transformers are also used that can individually control voltage for each phase to control furnace hot spots while maintaining maximum power input.

Static VAR Generators
EAFs create power quality problems in terms of power factor adjustment and flicker. Some shops are turning to static VAR generators to control power factor and flicker and thus reduce utility charges.

Bottom Tapping
Several furnaces have the taphole located in the furnace bottom. When the heat is ready to tap, a plug is removed and the heat is tapped in less than a minute. This design feature can reduce tap times by 3-7 minutes and better slag control can be achieved.

Materials Handling
Tap to tap times of 90 minutes have become commonplace for new UHP meltshops. New methods for automatic scrap and flux handling are being developed that keep pace with the faster operating cycles.

Scrap Preheating
The hot furnace gases are used for scrap preheating. Scrap preheating can save 4-50 kWh/ton and reduce tap-to-tap times by 8 to 10 minutes.

DC Arc Furnace
Pioneered in Europe, single electrode furnaces operating on direct current have recently been commercialized. This design reduces noise and electrical flicker, increases efficiency, and reduces electrode consumption.





Combustion Technology :
Oxy-fuel burners mounted in the wall of the EAF provide additional melting energy. The PYRETRON Plus burners shown can also be used for injection of carbonaceous fines (coal, coke). Also shown in the schematic are the PYRELANCE water cooled slag door lance manipulator and an optional afterburner incinerator to reduce CO and hydrocarbon emissions levels from the EAF.





Energy Consumption :
Energy consumption varies from 350-700 kWh/ton of steel produced. The "typical" EAF without oxyfuel burners uses 475 kWh/TON. As shown, the use of oxyfuel burners reduces electric consumption to 425 kWh/ton.