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Energy Optimizing Furnace

• satyendra
• February 19, 2013
• 15 Comments
• energy optimization furnace, EOF furnace, Hot metal, O2 blowing, scrap preheating, steel making,

Energy Optimizing Furnace

Energy optimizing furnace (EOF) is a furnace for the primary steelmaking. The steelmaking process in the EOF was developed by the mini steelworks pioneer Willy Korf along with his colleagues. The process is operating at the GERDAU Divinopolis plant in Brazil and at JSW SISCOL plant and at Hospet Steel plant of Mukand in India. The first unit went into production in1982.

EOFs of standard capacities 30 t/40 t, 60 t/80 t and 100 t/120 t are available. The basic features and dimensions of the EOFs are (i) hearth surface in the range of 6.6 sq m to 22 sq m, (ii) shell diameter is in the range of 5.3 m to 7.5 m, (iii) total height from working platform to top level is around 17 m to 25 m, (iv) there are one or two numbers of scrap pre-heater stages, and (v) tilting angle for tapping and de-slagging is upto 8 degrees. Various views of the EOF are shown in Fig 1 and a view of the EOF from the working platform is shown in Fig 2.

Fig 1 Views of the energy optimizing furnace

Fig 2 View of the EOF from the working platform

The process principle

EOF is a melting/refining furnace for the production of liquid steel. It is having a scrap pre-heater.  The basic principle consists of working with combined submerged and atmosphere oxygen (O2) blown in an initial charge containing hot metal (HM), preheated solid scrap and fluxes for slag formation. Scrap is preheated to around 850 deg C to 900 deg C by the sensible heat in the off gas in one or two chambers located above the furnace roof. Blown submerged O2 reacts with the carbon (C) from hot metal and generates carbon mono-oxide (CO) bubbles which travel through the liquid bath to the furnace atmosphere. Here CO is burnt to carbon di-oxide (CO2) by the O2 blown through atmospheric injectors and supersonic lances. The bubbling of CO generates a very strong stirring action and increases significantly the bath surface. This allows transfer of a good amount of heat to the bath. The process also constitutes de-slagging and formation of the secondary slag.

EOF has been conceived to utilize the sensible heat of small and medium sized steel converters in an effective way. It is a combined blowing basic O2 steelmaking process where a mix of HM, scrap and direct reduced iron (DRI) forms the charge. O2 is blown through two numbers of submerged tuyeres and one or two numbers of supersonic lances. Post combustion of the emerging gases above the steel bath is done using four numbers of atmospheric injectors and by air leaking in through the door, thus supplying a part of the heat to the metallic bath and rest for scrap preheating for the subsequent heat. The tap-hole and tilting mechanism is designed for efficient slag-free tapping.

The submerged injected O2 reacts with the C of the bath and generates CO bubbles which promote intense bath agitation, beneficial for reaction kinetics and homogenization of the temperature. Once the bubbles leave the bath, CO is burnt with the O2 from the atmospheric injectors. The projections of liquid metal caused by the eruption of the CO bubbles promote an extraordinary increase in bath surface, increasing the exposure to O2 from the supersonic lances and capturing part of the heat generated by after-burning, which is drawn to the bath. The combination of these factors explains the extremely fast decarburizing and temperature rise of the bath, resulting in blowing times similar to those of the BOF (basic oxygen furnace). The possibility of tilting of the furnace, allowing continuous extraction of slag through the slag door as well as tapping at the very moment of finishing decarburizing, as well as the instantaneous release of scrap from the scrap pre-heater, allow tap-to-tap times of even less than 30 minutes.

The process has possibility of using high percentage of solid charge (even greater than 40 %). Combined with C injection devices, the proportion of DRI in the charge can rise to 25 % and even more. The process owes its thermal efficiency due to the factors such as (i) chemical energy released due to the exothermic reactions between the injected O2 and the various elements in the bath including added C, (ii) chemical energy released from the gaseous oxidation reactions in the furnace atmosphere involving CO and H2 (hydrogen) released from the bath, and (iii) sensible heat transferred by the hot gases from the furnace to the cold scrap charged into the pre-heater.

Characteristics of produced steel quality

In EOF, all types and qualities of steels can be produced. The tapped steel is having similar chemistry as been obtained from combined blowing steel making process. Due to the continuous de-slagging during the process, good amount of de-phosphorization with phosphorus (P) content upto 0.008 %,and de-sulphurization with sulphur (S) content upto 0.025 %) can be achieved.

Since EOF process has a high percentage (more than 60 %) of HM in the charge, the tapped liquid steel has a very low content of tramp elements. This is of advantage while producing special grades of steel such as die forging steels, special clean steels, and steels for seamless pipes etc.

The high partial pressure of CO during the whole blowing period results to very low H2 and nitrogen (N2) levels in the tapped steel. In case of high quality and special steels, the tapped liquid steel is treated in secondary steel making units as per need.

The process and main process equipment

EOF is equipped with bottom having refractory lining, split water cooled shell, water cooled roof, sealing between the furnace and scrap pre-heater, HM launder, steel tapping launder, submerged tuyeres, atmosphere injectors and supersonic lance for O2 blowing, oxy-fuel burners for heating-up the new bottom. The major equipments/components of the process are described below.

EOF hearth is of dish shape made from boiler quality plate and is lined with refractory bricks. It holds the liquid steel while processing. Since the process is a basic oxygen steelmaking process, the refractory in the working lining is primarily made of magnesia-carbon (MgO-C) bricks. The back-up lining is made up of magnesite bricks. The MgO-C lining near the tuyere area consists of higher density blocks since there is more erosion of the refractory in this area. During the furnace campaign, the refractory bricks get eroded which is repaired by gunning using magnesite base gunning materials. Special refractory gunning machine is used for hot repair of the EOF bottom.

Submerged tap-hole along with quick back tilting ensures slag free tapping of liquid steel suitable for subsequent secondary refining. The tap-hole is a very important part of the EOF hearth since the entire steel is to be tapped into the steel ladle through the tap-hole. Whenever the diameter of the tap-hole becomes large, the same is brought back to 200 mm using a steel pipe and filling the balance area with gunning material. Before charging the HM into the EOF, the tap-hole has to be blocked properly since otherwise, it can lead to pre-mature opening of the tap hole.

The EOF shell and roof is made with water cooled panels which reduces furnace refractory consumption. The circular shaped EOF shell and compact design keeps heat losses to a minimum. The EOF shell has a HM launder for pouring liquid HM from the HM ladle in to the EOF hearth by a HM charging crane. The HM launder is also refractory lined and it often needs repairs which can be carried out while the EOF is in operation without the loss of any operational time.

EOF shell has slag door on the opposite side for the continuous removal of slag during the process. The slag door can be operated up and down using the pneumatic cylinder. The slag door is also utilized for drawing of the samples for the heat from the liquid steel and also for taking of the temperature of the liquid steel during the heat. It is through the slag door of the EOF, the hot gunning of the refractory is carried out. The cleaning of the tap-hole and blocking the same prior to charging is also carried out through the slag door. The slag door is also known as the working door.

The EOF water cooled roof consists of roof upper piece and sliding skirt. There is no refractory in the EOF roof. Through the top opening in the EOF roof, the hot gases from the EOF travel to the scrap pre-heating area. When the furnace shell tilts backward and forward, it does so along with the EOF roof. Between the sliding skirt and scrap pre-heater lower piece, cast iron chillers are placed to minimize the ingress of atmospheric air into the scrap pre-heater.

The furnace shell and roof are very important to contain the off gases from the steel bath and carry out post-combustion of the gases before the gases travels to the scrap pre-heating area. A negative pressure of 200 mm water column is maintained inside the EOF.

Scrap pre-heater placed immediately above the furnace is provided with either one or two tilting water cooled fingers to support the solid metallic charge which is heated by the furnace off gas. Water cooled inclined chute is also provided below the fingers for additions into the furnace. Scrap pre-heater is an essential part of the EOF. The scrap pre-heater in the earlier EOFs used to be two-stage or three-stage since the EOF was designed for higher percentage of solid charge. Presently most of the EOFs are having single stage scrap pre-heating system.

Scrap preheating is made with water cooled fingers and water cooled panels where scrap for the subsequent heat is pre-heated to 850 deg C to 900 deg C by the off-gases from the EOF. The fingers are divided into two halves and they can be in open or close position using hydraulic actuated cylinders. The fingers are in close position when scrap is charged on to the fingers. The scrap is kept on the top of the fingers during the entire processing of heat where it gets pre-heated by the sensible heat of the off gases. Once the previous heat has been tapped and the tap hole is blocked, the fingers are opened and the scrap is allowed to fall inside the EOF bottom. This is the unique feature of the EOF whereby the scrap pre-heater is placed directly on the top of the EOF roof such that the off gases are collected at the highest temperature possible for pre-heating the scrap. A by-pass line can also be provided for the scrap pre-heater where the gases can be sent through the by-pass channel to avoid melting of the scrap on top of the scrap pre-heater. However, normally in the recent EOFs, the off gases temperature control is carried out through dilution air by the forced draft blower which gets automatically switched on once the off gases temperature goes high.

The lime and ferro-alloys are also discharged by the automatic feeding system below the fingers and are thus discharged into the EOF. CO-CO2 analyzer is also installed in the scrap pre-heater area. Whenever the CO gas percentage increases beyond a particular percentage, the dilution air is automatically switched on using forced draft blower in order to oxidize the same and avoid the explosion.

Prepared scrap, in specially designed bottom discharge scrap bucket, is charged into the EOF top by the charging crane. After scrap charging, the sliding door is closed. The number of scrap charging buckets in circulation is to be sufficient in order to avoid any delay of the EOF operation for want of scrap charging. Whenever the scrap is ready, the scrap charging bucket is placed on the stand over the top sliding door of the EOF. Whenever the scrap is to be charged into the pre-heating area, the scrap bucket is lifted with a hydraulic cylinder so that its bottom discharge flaps open and discharge the scrap on the pre-heater fingers. The scrap needs to be well prepared and no piece is to be more than 400 mm in size so as to cause any damage of the pre-heater fingers, water cooled fingers and roof upper piece when the scrap is discharged into the EOF.

The O2 blowing system is an important part of the EOF for the steelmaking process. The O2 blowing is done through submerged tuyeres, atmospheric injectors, and supersonic lances. The O2 is to be conveyed into the steel bath as well as for post combustion in a precise manner at a particular pressure with flow rate controlled by the instrumentation. The O2 profile specified for the heat processing is to be followed from the beginning to the end.

The O2 injection into the steel bath through submerged tuyeres is one of the unique features of the EOF.  Four numbers of submerged tuyeres in the hearth are placed at 45 degree, 135 degree, 225 degree, and 315 degree position assuming the tap-hole is at 0 degree position. The tuyeres are placed 300 mm above the furnace bottom. The tuyere outer pipe is made of stainless steel in which the copper (Cu) tuyere is placed concentrically. The outer diameter of the Cu tube has helical grooving and it is cooled with the help of de-mineralized (DM) water and N2 gas, which promotes the formation of nugget on the tuyere tip inside the furnace. The nugget formation helps to minimize the tuyere tip consumption to the extent of 2 mm to 3 mm per heat. The entire tuyere assembly has good safety system for trouble free working. The tuyeres are sliding type through the refractory block and when required the tuyere can be pushed inside the furnace to avoid refractory erosion around the tuyeres.

O2 blowing through the tuyeres helps in de-carburization and stirring of the bath. The cooling of the tuyere pipe using DM water is an important and critical feature. As long as the tuyere is cooled and even if the O2 pressure drops, liquid steel from the EOF does not come out. But, if the cooling water fails and O2 is on, there is very fast erosion of the submerged tuyere resulting in EOF hearth break out which can be very dangerous. For this reason, there is a standby DM water tank which can be immediately brought into operation in case of fall in DM water pressure or flow rate.

The post combustion of the off gases within the furnace vessel is also one of the unique features of the EOF. Four numbers atmospheric injectors are fixed on the furnace shell for post-combustion of the gases emerging above the steel bath. The atmospheric injectors are located exactly above the four tuyere positions. The bodies of the atmospheric injectors are also water cooled and all the four injectors point downwards towards the centre of the steel bath. The basic activity in the post combustion is the oxidation of CO to CO2 and the energy thus generated is partly transmitted back to the steel bath and bulk of it flows along with the flue gases in order to pre-heat the scrap for the subsequent heat.

Two supersonic lances force a jet of O2 close to the slag level inside the EOF, which helps in high speed de-carburization as well as thorough stirring of the bath. The supersonic lance is having a Cu tip and steel body which is water cooled. The supersonic lance is re-tractable on the inclined frame work supporting the supersonic lance. Each supersonic lance is placed on either side of the slag door. The O2 supplied by the supersonic lance is primarily used for de-carburization of the steel bath and also partly for post-combustion. EOF being combined blowing process, the O2 injection on the top is primarily through supersonic lance and the side blowing of O2 from the bottom is done through submerged tuyeres. The supersonic lance is normally provided with instrumentation and control systems for safe working.

Oxygen lancing is also done through manual lancing in the slag/metal interface for the quick fluidization of the slag. Generally two numbers manual lances are used through the slag door. Measured quantity of O2 is released through the lancing pipes into the steel bath. The manual lances are also used to clear the tap-hole at the end of tapping from the EOF.

The flow of O2 through submerged tuyeres, atmospheric injectors, and supersonic lance is controlled through a valve stand having the required instrumentation controlled by a computer. In the valve stand, for safety of operation of the submerged tuyeres, there is a facility to change over from O2 to N2 or to Argon (Ar) gas in case a heat is required to be held inside the EOF for a longer period. The valve stand is the heart of EOF operation which controls the precise blow profile of O2 for achieving the desired O2 blow period in the EOF.

The EOF normally contains of two numbers of trolleys, two numbers of bottoms and one number of EOF shell and roof. A spare shell and bottom can be an alternative to spare bottom. This is shown at Fig 3. The entire EOF bottom, shell and roof are mounted on a trolley moving on rails. Two numbers bottom cars of shuttle type are used for quick bottom (or shell and bottom) change during a new campaign. One bottom car carries the EOF in operation while the second car carries the other bottom and is parked for relining at one or other side of the bottom under use. Both cars are equipped with roll collar tracks to tilt the furnace for tapping or de-slagging. The spare EOF bottom is lined with refractory and is kept ready to go into operation. At the end of the refractory campaign of the hearth, the EOF shell is raised using hydraulic cylinders and the bottom in-use is pulled out and the refractory lined spare EOF bottom is brought in place within twelve hours and the EOF is put back into operation. Tilting is performed by high speed hydraulic cylinders. This allows slag free tapping.

Fig 3 EOF with spare bottom and with spare bottom and shell

The lime and alloy feeding system consists of storage hoppers, weighing systems and conveyor belts. Lime and alloy feeding system is provided to feed the exact quantities of any additive in a regulated manner into the EOF bath. This system is used basically to feed lime in the EOF. This system sometimes is used to feed ferro manganese for manganese boil before tapping of steel. This system is also sometimes used to feed the DRI to control the bath temperature.

There is a second automatic feeding system for charging the lime and alloys into the ladle during tapping the steel from EOF. This is mainly for the primary de-oxidation of steel and making reducing slag for subsequent secondary refining processes. Since EOF is high productivity process with short cycle time, lime and alloy feeding system into the EOF as well as in the ladle during tapping is an important part of EOF process for ensuring the requisite additions matching the furnace productivity.

The gas cleaning plant (GCP) is normally wet type. It consists of down-comer, quenching chamber, venturi, cyclone separator, ID (induced draft) fan, and chimney. The down-comer is for conveying of the off gases after the scrap pre-heater system into the quench chamber. The down-comer is refractory lined and it has water spray nozzles to cool the gases as well as to separate the dust from the gases to the extent possible. In the combustion chamber, there is a direction change of the off gases along with a big shower of water which not only lowers the temperature of the gases but also separates some quantity of dust. The venturi is the heart of the GCP system. It not only helps in separating the dust from the off gases due to sudden pressure release but also controls the furnace negative pressure very precisely through electrical actuated pair of flaps. The cyclone separator is the final device for separating the dust from the off gases.

The dirty water collected from the quench chamber and the cyclone separator is conveyed to the thickener after chemical dozing. In the thickener, the dust is allowed to settle down and the clear water is circulated back into the GCP through the pumping system. The clean air (less than 50 milligrams/cum) is pulled by a series of two ID fans (one standby) and let out in the atmosphere through a tall chimney. The clean gas thus generated is primarily pure steam which appears like white cloud when it emerges out of the chimney. The wet sludge, filtered in the above process is collected in the thickener where the solid particles having 68 % to 70 % Fe (iron) is recycled back in the sinter plant.  Dry type GCP is also possible in the EOF.

Air-oil burner, using low S liquid fuel, is used for pre-heating the newly lined EOF hearth and shell. This ensures proper thermal balance while processing the first heat. Thereafter, between the heats, burner is not required to be used. In case of any long stoppage, it is desirable to pre-heat the furnace before charging subsequent heats.

These days EOFs are equipped with very elaborate instrumentation system where total control is through an on-line PLC/computer system. Safety net provided is exceptionally good in order to ensure safe operation and repeatability of the process. The computer has the facility for the automatic data logging and also for raising the alarm system when any sub-system is malfunctioning.

Operating parameters

The typical operating parameters of the EOF are (i) 340 days per annum of furnace availability, (ii) charge composition consisting of 50 % – 90 % of HM with balance of solid charge, (iii) tap to tap time in the range of 30 minutes to 50 minutes, (iv) a tapping temperature of 1700 deg C without a ladle furnace and 1650 deg C with a ladle furnace, and (v) time needed for bottom exchange (between campaigns) is in the range of 12 hours to 24 hours.

The typical specific consumption per ton of liquid steel are (i) HM-778 kg/t (70 %), (ii) Solid charge consisting of pig iron and steel scrap-333 kg/t (30 %), (iii) lime-45 kg/t (depending on P content of the HM, (iv) O2 consumption in the range of 50 N cum/t to 70 N cum/t, (v) N2 consumption in the range of 3 N cum/t to 5 N cum/t, (vi) fuel consumption in the range of 5 million calories per ton (Mcal/t) to 10 Mcal/t, (vii) refractory consumption around 6  kg/t, (viii) gunning material consumption around 4 kg/t, and (vii) metallic yield in the range of 87 % to 89 %.

Process flow of the EOF process and a typical cross section of the shop are shown in Fig 4 and Fig 5.

Fig 4 Process flow of the EOF process

Fig 5 Typical cross section of the EOF shop

Advantages of EOF

Various advantages of the EOF process are given below.

• • There is a wide flexibility with regards to the metallic charge mix. The advantages are more when HM availability is less say 50 % from the ironmaking process. There is flexibility also with respect to solid charge (scrap, pig iron).
  • The process is advantageous in case of low availability of electrical energy.
  • The process has high productivity and good furnace availability.
  • The produced liquid steel has good metallurgical properties, especially with regards to de-phosphorization and de-sulphurization.
  • The liquid steel has low content of tramp elements.
  • The inclusion level in the steel is reduced greatly due to continuous flushing of slag during the blow and also due to slag free tapping.
  • The tapped steel can be directly transferred to the continuous casting machine or can be sent to secondary metallurgy units.
  • EOF has simple process control and can be fully automated.
  • The process is energy efficient.
  • The process works under slightly negative pressure hence there is hardly any dust emissions.
  • There is low noise level.
  • Flexibility with respect to solid charge materials (scrap, pig iron) is another attraction.