A presentation on

Developments in EAF technologyMetallurgical & Materials Engineering Department, MNIT, Jaipur

Submitted to:- From:-

Dr. N. C. Upadhyay Raghav Saxena

Associate Professor M.Tech. 1st yr

2014PMT5403

Developments in the design of furnace

Ultrahigh Power Transformers

The latest development in transformer design is the 3-legged transformer which permits adjustment of the voltage of each phase independently. This provides the ability to control hot spots in the furnace while maintaining maximum power input.An additional item associated with the UHP transformers is static VAR (Volt Ampere Reactive) generators to obtain flicker control.

Furnace Shell Design

One great development is split shell design in which the shell structure is constructed in 2 separable sections-lower section containing the hearth & the upper section containing the balance of the shell side wall. With this design the upper section with used side wall lining can be replaced with a new lined upper section, thus reducing furnace downtime for a sidewall replacement. This also allows improved working conditions for the actual relining work. The split shell feature is used mostly on larger size continuously operated furnaces.

Another innovation in this field is the tapered shell, in which the diameter of hearth & lower side sections of the shell are of larger diameter than the top opening. The advantages attributed to this shell design are: a) longer side wall lining life due to the larger shell diameter at the hot spot areas (as refractory erosion being inversely proportional to square of this distance); b) improved thermal efficiency because of reduced heat loss & increased reverberatory heating of the charge, and c) greater shell volume to accommodate larger charges.

Design of cooling system for side wall & roof

An important development in furnace shell design is the application of water cooling to those portions of the upper side wall lining & roof which suffer maximum wear rate. This avoids mid-campaign patching.The target areas of cooling are the “hot spots” where side wall is closest to the electrodes & which are subject to intense thermal radiation, arc flare & iron oxide particle which tend to penetrate and flux the refractory lining material. Also heat recovery can also be obtained from the cooling water.

New design of electrodes & electrode holder

Water cooled electrodes are the new technology. This technology consists of employing water-cooled electrode sections between the furnace electrode holder and the conventional graphite electrode which ultimately reduces graphite losses caused by oxidation at high temperatures.

Electronic & computer control

The complexities of power supplies & operation require precise measurement & monitoring of many electrical parameters to achieve & maintain optimum furnace performance.

Under certain situations arc furnaces create disturbances in the power network, called flicker. Such conditions require expensive corrective apparatus, limiting furnace performance & productivity. Developments in electronic control systems have enabled better management of these problems.

Computerized control of the melting process & data logging functions provide statistical means by which the entire process can be optimized. Operational control is the process of ensuring that specific tasks are carried out efficiently. Typical operational control includes:• Calculating the minimum cost of charge materials.• Providing online control of melting process, including scrap-preheating, oxyfuel burner control,

oxygen assisted melting control, controlling the electrical energy & melting profile, thermal model, calculating minimum cost of the alloy additions.

• Providing data logging functions, including heat number, steel grade & chemical analysis, date, shift, furnace number, name of operators, the conditions of bottom, walls & roof, scrap weights & types, flux weights & types, alloy weights & types, and so on.

DC electric arc furnace

In this type arc is struck between a top electrode and a bottom, air cooled electrode. In a DC furnace there is only 1 exposed electrode compared to three in an AC arc furnace . Electrode surface wear is greatly reduced as compared to AC EAF. Also the arc is more stable with DC.

Developments in operational features

Oxyfuel burners & oxygen lancing

An oxyfuel burner uses natural gas or oil, together with pure oxygen, to produce an extremely high-flame temperature. They are used for heating, melting unmelted scrap between the electrodes & refining. The burners are located in the sidewalls of the furnace between the electrodes.They may help to increase the rate of scrap melting in cold spots & thereby makes scrap melting more uniform.

Foamy slag practice

A deep foamy slag formed early in the melting operation may envelope the electrode tips & submerge the arc. With the practice, it is possible to operate with a higher power factor & increased electrical efficiency. It is also possible to operate at high power inputs for a longer part of the melting cycle, thus increasing productivity. Damage to the furnace resulting from arc flare is reduced because the arc is buried. Thermal efficiency is improved because the slag provides an improved medium for energy transfer.

Preheating of scrap & waste heat recovery

For scrap preheating hot off-gases from the arc furnace are passed through the scrap in the charging bucket & a portion of the heat energy in the off-gases, is recovered. The scrap may also be preheated in the charging bucket by burning gaseous fuel. The benefits of scrap preheating are reduction in power & electrode consumption, & improvement in productivity.Scrap is heated on an oscillating conveyor in a preheating section by the hot gases from EAF. Carbon monoxide in the exit gas is also burnt in the preheater to extract its calorific value. The preheated scrap is continuously fed into the liquid heel of metal in the EAF, where the scrap melts by immersion. Energy savings of 80-120 kwh/ton of liquid steel produced has been reported.

Use of hot metal & iron carbide as charge materials

EAF may have the flexibility to use hot metal (liquid pig iron) up to the extent of 80%. The oxidation of carbon in the hot metal supplies additional energy. Oxygen may be introduced to the furnace by any means. As a result of these modifications an appreciable saving in electrical energy consumption may be achieved. The sensible heat available in the hot metal charge, less time on power & exothermic oxidation of carbon & other elements, accounts for the reduction in electrical energy consumption. Shortening of the heat time leads to reduction in electrode consumption.

Another alternative charge material for EAF is iron carbide which contains more than 6% C in the form of Fe3C. It is introduced into molten steel by injecting it below the slag layer pneumatically using a lance. Because of its high melting point (1837 ⁰C), iron carbide does not melt directly. Instead it dissolves in molten steel bath. When the carbon introduced into the bath through iron carbide addition is burnt with oxygen, large amount of heat is released. This saves equivalent amount of electric power. In addition foamy slag operation becomes easier, submerging the arc & improving thermal efficiency. It also contributes to an increase in metallic yield.

REFERENCES

• A.K. Chakrabarti, (2012) Steel Making. New Delhi, PHI publishing.• Ultra high transformer available from: https://www.gedigitalenergy.com/HVMV_Equipment.htm• Split shell image available from: http://www.seekpart.com/product/AC-5-100ton-electric-arc-

furnace-EAF-1437448.html• Tapered shell image available from: http://www.google.com/patents/US4887277• Side wall cooling image available from: http://www.pythongroup.ca/mining-news/article/id/79• Rooftop cooling image available from: http://www.ke-kaercher.com/furnace_roof.htm• Electrode holder cooling image is available from : http://www.pacificinternational.co.in/copper-

tuyeres-cooling-plates-for-blast-furnace-and-electric-arc-furnace-in-steel-melting-shop/• Computerization of EAF image is taken from paper “Developments towards an Intelligent

Electric Arc Furnace at CMC Texas using Goodfellow EFSOP® Technology”.• DC EAF is available from: http://ietd.iipnetwork.org/content/direct-current-dc-arc-furnace• Oxy-fuel burner’s image is available from: http://www.chugai.co.jp/eng_e/04_4_oxy/01.html• Foamy slag’s image is available from http://www.ptie.eu/products/pti-swingdoortm-

system/index.html• Scrap preheating image available from: http://www.climatetechwiki.org/technology/jiqweb-spis