/ Steel (Ferrous): EAFs
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.
utilize Oxy-natural gas burners to increase productivity, decrease electric use
and increase electrode life.
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.
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.
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
- Use of natural
gas to stabilize electrode tip
- Hot metal
- Reducing scrap
use (replace with iron pellets)
- Molten metal
stirring in the EAF (to reduce nitrogen)
- Slagmelt process
(to increase yield)
- EAF dust recycling