Iron / Steel (Ferrous): EAFs

An Electric Arc Furnace (EAF) uses a number of electrodes (usually three) to produce an electric arc to melt a steel scrap charge. A typical EAF is about 20 feet in diameter and about 15 feet high with a removable roof that usually swings open for charging the scrap (or other materials). The three electrodes are lowered through openings in the roof, and the electric arc melts the scrap. After melting, the furnace is tapped (or tilted) to remove the molten material for further refining and processing. The major energy consumed is electricity. The arc itself is a plasma of hot, ionic gasses in excess of 6,000°F. Electrodes are susceptible to breakage (if an over current situation arises) and are a major consumable in an EAF shop.

Modern EAFs utilize Oxy-natural gas burners to increase productivity, decrease electric use and increase electrode life.

Natural Gas Use:
Most shops that utilize Oxy-fuel burners currently consume between 100 and 150 SCF of natural gas per ton in the EAF melt shop. This can result in significant volume use. An EAF shop producing 500,000 tons of steel annually would consume between 50,000 and 75,000 MCF of natural gas per year.

Currently, the primary use of natural gas has been to assist melting of scrap after charging cold (or preheated) scrap. Burners usually fire about 5 to 10 minutes out of an overall melt time of about 60 minutes (some of the latest installations are approaching 40 minutes). The Oxy-gas burners are located in "cold spots" to assist the electric arc in melting the charge. Generally, Oxy-fuel burners are not operated without the electric arc being on.

The table below illustrates an energy heat balance at a "typical" EAF shop.

To melt steel scrap, it takes a theoretical minimum of 300 kWh/ton. To provide superheat above the melting point of 2,768°F requires additional energy. For typical tap temperature requirements, the total theoretical energy required usually lies in the range of 350 to 370 kWh/ton. However, as EAF steel making is 55 to 65% efficient, a resulting total equivalent energy input is usually in the range of 560 to 680 kWh/ton for most modern operations.

This heat energy can be supplied from a number of sources as shown in the table below. The energy distribution is highly dependent on local material and consumable costs and is unique to the specific melt shop operation. A typical balance for both older, and more modern EAFs, is given in the table.

EAF facilities could utilize additional natural gas to optimize productivity, lower costs and provide greater control over the final metallurgy of the heat. Several possibilities exist as operators strive to shorten the period of melting to more closely match the performance of a BOF facility. These applications include:

• Scrap preheating (prior to scrap loaded in EAF)
• Foamy slag
• Scrap preheating without electrodes on
• Post-combustion techniques
• Use of natural gas to stabilize electrode tip
• Hot metal feeding
• Reducing scrap use (replace with iron pellets)
• Molten metal stirring in the EAF (to reduce nitrogen)
• Slagmelt process (to increase yield)
• EAF dust recycling