Methods of Shutting Down a Blast Furnace
Methods of Shutting Down a Blast Furnace
A blast furnace, after it is blown in and the production of hot metal starts, runs for a large number of years before it is put down for relining. This continuous run of the blast furnace is known as campaign life. In modern blast furnaces a campaign life of 15 years to 20 years is expected. When a furnace has reached the end of its campaign, it was usually blown out/blown down. Between the blown in to blown out/blown down, the blast furnace may have to be shut down for short periods due to various reasons. The various types of shut downs of a blast furnace are described below. (Fig 1)
Fig 1 Important methods of shutting down a blast furnace
Very often while running of the blast furnace, a situation can arise when the full productive capacity of the furnace is not required for a period of time. This can be due to some problems which may arise in downstream or upstream facilities. When this occurs, the problem can be solved either by the shutting the blast furnace down or curtailing the operation of the blast furnace by reducing the quantity of hot blast. The hot blast rate is usually reduced until the hot blast pressure at the tuyeres is very low. However, it is a must that a positive pressure is kept in the hot blast system to assure that there is no danger of blast furnace gas coming back from the blast furnace into the blower system. The technique of reducing the hot blast volume to less than 20 % to 25 % of normal is known as fanning.
Fanning has the advantages of keeping the blast furnace gas system pressurized and furnishing a small quantity of blast furnace gas for use as fuel, and enables a resumption of near full operation on relatively short notice. This technique is used for emergency situations or short periods only. Prolonged use, such as 8 hours out of every 24 hours, or on weekends, often results in a hearth buildup and frequently promotes in wall scab formation.
During the campaign life of a blast furnace , there are occasions when the blast furnace is taken off blast for short periods to perform various maintenance functions such as the replacement of tuyeres, tuyere coolers or maintenance of the peripheral equipment. In such instances, the blast furnace is back drafted. In this operation, as soon as the hot blast is stopped, the bustle pipe is put under negative pressure. This is done normally by opening the chimney valve and the hot blast valve to a stove that has already been prepared by heating it to temperature and then shutting off the gas valve. As the blast furnace gas is drawn back into the hot blast stove, air is admitted through the peep sights and stove burner, and the operator makes certain that the gas burns in the stove. During the operation, the bleeders at the top of the blast furnace are also opened to pull some of the blast furnace gas out through the top.
In some of the blast furnaces, a special back draft stack is installed so that it is not necessary to draw the blast furnace gas back through the blast furnace stove. This stack is connected to the bustle pipe or to the hot blast main. In some places, it is closed by a water cooled gate valve at the level of the bustle pipe while in some other places it is closed by a cap valve at the top of the stack which is not cooled. Opening of the valve allows the blast furnace gas to draft to the atmosphere where it burns without difficulty.
In the present day running of the blast furnace, banking of the blast furnace is seldom being practiced. However banking is considered as a standard technique for blast furnace shut down unless the outage is of short duration. Blast furnaces are normally blown down these days which means that they are run without being charged until the burden level reaches the tuyeres. Plans for an extended shutdown or interruption to furnace operation either for a breakdown, scheduled repair or because market conditions indicate a pause in production is desirable, may influence management to blow down a blast furnace.
Banking process in the blast furnace is adapted, since the procedure of banking is useful for short outages. The word banking is used because of a similarity to the operation of banking a fire. The origin is lost in antiquity, however, generally it means covering a fire with ashes or fresh fuel to restrict air, reducing the combustion rate, and thus preserving the fuel for future use.
Banking is also resorted to as an emergency measure when some unforeseen event requires a shutdown of the blast furnace. In case of banking of the blast furnace, the blast is taken off, the blow pipes are dropped and the tuyere openings are plugged with clay to prevent air from drafting through. Thus, the heat of the hearth is preserved and the blast furnace can be returned to operation with a minimum effort. If the downtime exceeds four or five days in duration, some difficulty can be expected in resuming operation, although examples are available that no trouble has been experienced even after a seven day bank.
Normally the banking operation of the blast furnace is carried out as a planned event. Preparations are made depending upon the length of banking time anticipated. If the furnace is to be banked for only a few days, an extra blank or two of coke may be charged without flux and the furnace taken off when the coke descends to the bosh zone. If it is to be banked for a slightly longer time, the ore and lime stone burden is to be reduced by 5 % to 10 % following the coke blank, possibly for ten or fifteen charges before normal charge weight is resumed. This technique is still used very short outages.
A banking burden for a shutdown for an undetermined length of time is very similar to a blow in burden. Prior to the start of a banking burden, miscellaneous iron bearing materials are removed from the charge and a large reduction is made in the amount of limestone charged. Extra coke is also charged before the banking burden. The purpose is to develop a hot, siliceous slag which has a tendency to clean off the lime accumulation on the bosh walls and prevent an excessively high lime slag during blow in. High lime slag has a higher melting point and is apt to cause some operating problems early in the blow-in period. Often during the initial warm up period, temperatures are to be very high in the bosh which results into increase in the reduction of silica to silicon with the result that slag contains a higher proportion of lime. For this reason, effort is made to have a hot, siliceous slag at the time when the blast furnace is banked because a similar condition upon the resumption of operation is expected.
After the initial preparatory charges, a heavy coke blank is charged and subsequent charging is similar to a characteristic blow in burden. Charging continues until the coke blank reaches the upper bosh area of the furnace. At this time, the final casting of the blast furnace is carried out. Effort is made to drain the hearth until a dry blow of the tap hole is observed to ensure a clean hearth for the future startup and eliminate as much as possible the need for melting cold slag early in the blow in period. Prior to the last casting, the blast furnace dust catcher is emptied. Accumulated dust has a tendency to consolidate into a rock like mass if undisturbed for a time and can present a difficult problem after operation begins again.
About the end of the casting, before the blast furnace is taken off, a heavy blanket of ore may be dumped in the blast furnace to cover the upper burden surface, thus reducing the natural drafting tendency of the blast furnace.
At the end of the casting, the tap hole is plugged, hot blast is taken off the blast furnace, bleeders are opened, steam is turned into the dust catcher, the blast furnace is isolated from the common gas system and stove valves are manipulated to draft gas back through the bustle pipe, hot blast main and out through the stove chimney. Blast furnace operators quickly drop the blow pipes and plug the tuyeres with clay. In many places, It is preferred to remove the tuyeres to avoid any chance of a stray water leak permitting water to accumulate in the blast furnace and also to provide an opportunity to observe the coolers for possible leaks. Clay is solidly packed into the tuyere openings and backed up with sand to eliminate any chance of air filtering in. Sometimes this is followed by bricking up the openings as further insurance against air infiltration.
As soon as the blowpipes are down after the final casting, blowers are stopped, and stove burner valves, chimney and hot blast valves are closed to preserve heat as long as possible. As a precaution, blow off valves are opened slightly to prevent a pressure buildup from developing in the hot blast stoves resulting from an undetected water leak or from some unsuspected source.
Within a day or two, the manhole at the blast furnace top is opened and steam is shut off in the dust catcher. Daily inspection of the stock line is important. A slow stock movement is an indication that air is infiltrating and coke is being consumed. A small movement can be expected but a continual drop is undesirable and may force the operators to spray the bosh with a sealing material. A thin mixture of water, clay and water glass is sometimes used because the material is inexpensive and does an effective job.
If the furnace is banked for an extended period, after a lapse of a month to six weeks, water flow is to be reduced on the cooling members and finally, after two to three months, turned off entirely except for the hearth staves.
Very often after a bank of six or more weeks, when tuyeres are opened, all signs of fire in the tuyere area has disappeared. In this case the operators of the blast furnace are happy since full benefit of the coke blank is then be available to supply heat when operation begins.
The blowing out is also called sometimes raking out. Blast furnace is normally blown out when the production from the blast furnace is no longer required. A blown out furnace can be restarted faster and with less effort than starting from a banked furnace, since the conditions approaches those of starting a new furnace. However, the costs in connection with blowing out, raking out and cleaning preparatory to starting is likely to exceed the cost of banking the blast furnace.
When a furnace has reached the end of its campaign (that is, the lining has worn out), it is usually blown out. However these days the practice followed is to blow down the blast furnace.
For the blow out, the operation is discontinued for a short period of time roughly 12 hours to 16 hours before the last casting of the blast furnace is done to permit the installation of water sprays in the top of the furnace and thermocouples in the uptakes. The burden composition is then normally changed to produce a very siliceous slag. This helps in removing as much lime as possible from the bosh and hearth walls. The purpose of this is to prevent the formation of calcium hydroxide which would occur if lime were to come in contact with the cooling water during the later stages of the blow out. The formation of calcium hydroxide from lime imbedded in the lining can generate sufficient force to crack the steel hearth shell or to lift the furnace off its columns.
After installation of the blow out equipment, the blast is put on and charging is continued. The activity of blow out starts with charging of a heavy coke blank in the blast furnace. The volume is to be equivalent to approximately the volume of the bosh. After the coke blank has been charged, washed and screened quartzite lumps of size 25 mm to 50 mm is charged. The blast furnace is kept full early in the blow out and then the stock line is permitted to drift downwards roughly 6 m to 9 m towards the end (when all the iron bearing burden has been reduced). During the blow out, water from water sprays is judiciously used to control the temperature of the blast furnace top. However, the additional charges of quartzite lumps are very effective in keeping top temperature low. A decrease in the hot blast rate is needed as the height of the column of burden material in the blast furnace decreases. From the time that the heavy coke blank is charged in the blast furnace until the blow out is completed requires only around 6 hours to 8 hours.
The coke blow out method is similar as above except that coke is used instead of quartzite lump. Sometimes 20 mm to 25 mm coke screenings are used.
Following the last casting , the stock is watered down as described above. When cooling has progressed far enough, sluice ways frequently are built from a few cooler openings to an open top railroad car and the contents of the furnace washed out with high pressure water jets.
Around 1970s, It was realized that the procedures of banking and blowing out of blast furnace are expensive procedures. Also very often banking results into difficult startups because of water problems, or many unforeseen reasons. Also the blast furnace operators have now enough technical understanding of the process to allow operators to blow the furnace down. Blowing down of a blast furnace means running the blast furnace without charging until the burden level in the blast furnace is reduced to approximately the tuyere level. There are several reasons for the preference of the blowing down technique over the banking or blowing out a blast furnace. Some are when a furnace is blown-down it can be thoroughly inspected for leaking coolers or staves, and no effort is required to stop air infiltration because the furnace is empty. And, if the furnace is to be relined it is faster and less expensive to tear apart an empty furnace that one containing burden. The blow down technique varies from blast furnace to blast furnace but basically the following procedure is followed.
Atomizing water sprays are installed at the top of the blast furnace on a shutdown prior to the blow down, somewhere in the vicinity of the 3 m to 5 m stock line level. Normally four to six spray nozzles are installed equally spaced around the furnace. The purpose of the water sprays is to control the top temperature of the blast furnace. The water flow rate usually needed is around 150 cum per hour at a pressure of 8 atm. Emergency backup water supplies are often provided for safety purpose. Depending on the type of blast furnace top and its condition, emergency water sprays are sometimes installed to fight in case there is a grease fire during the blow down.
Steam sprays are also installed on one to three levels to provide some cooling and to maintain furnace pressure. If one level is used it is generally placed low in the stack at roughly the 12 m to 15 m stock line level. If additional levels are used they are placed roughly equidistant between the top water sprays and the bottom steam sprays. The steam injectors are activated once the burden descends below them. In some places there is also a provision of supplying nitrogen to the steam sprays as backup in case there is a loss of the steam pressure.
Nitrogen injection is provided for purging the blast furnace at the end of the blow down. Usually the nitrogen is introduced through the bustle pipe. If nitrogen is used as backup to the steam injection system, that system can also be used to purge the blast furnace when the blow down is complete. Desired nitrogen flow and pressure levels required are around 150 cum per minute at 3.5 kg/sq cm.
Provision is to be made for the analysis of process off gas. Normally this simply needs recalibration of the top gas analyzer. Analysis of hydrogen and oxygen is needed. Level of hydrogen is to be kept low (below 15 %) and there must not be any oxygen. There is also a requirement of the measurement of the stock line and the probe must be capable of extending well into the furnace.
The rate of hot blast and its temperature is reduced as the burden descends to control the top temperature of the blast furnace. Also hydrogen and oxygen content of the top gas is to be controlled. Typically the maximum top temperature is to be in the range of 300 deg C to 450 deg C.
After the blow down, especially when the blast furnace is to be completely relined (including the replacement of the hearth lining), the salamander is usually tapped. This operation saves days and possibly weeks in relining time which otherwise might be lost in blasting out the heavy chunk of solid iron that get formed if the liquid metal (that accumulates in the hearth as bottom block eroded during the campaign) is permitted to solidify. It is preferable to tap as much liquid iron as possible, because the removal of a solidified salamander costs many days, with additional risks of damaging the blast furnace, due to the use of explosives.
Salamander tapping of a blast furnace is the final tap for draining out the last liquid iron from the blast furnace hearth. Because of its rare occurrence a salamander tapping represents a special job which needs a lot of preparation. Salamander tapping is normally considered , to a large extent, as an art. Heat transfer calculations based on thermocouples located in the under hearth allow the depth of penetration of the iron pool to be estimated. However this point is at best an estimate. Consequently, the location of the drilled hole is somewhat arbitrary and some, usually small, portion of the salamander often remains in the blast furnace after the tapping.
Earlier it used to be difficult to find the best possible location of the salamander tap hole due to a lack of information on the blast furnace hearth interior and thus on the position of the wear line. Without any or insufficient data from thermocouples it was difficult to determine the optimal position to drill or lance the salamander tap hole. Professional experience was normally used in order to determine the drill location and angle to hit the salamander. More than once a number of holes were to be drilled and lanced before the salamander was hit for starting the tapping.
With modern blast furnace hearths being more and more equipped with dense thermocouple grids, thermal calculation of the position of the wear line, and hence of the salamander position, has become possible. Densifying the thermocouple grid improves the calculation accuracy, so that guessing where the salamander may be hit, is now replaced by knowing where the drill hits the wear line and, hence, from where the hot metal can be expected. An additional advantage of a more precise location is the possibility to improve the engineering of the setting around the salamander tap hole.
The salamander tapping is made at preferably the lowest level where liquid iron can be expected in the blast furnace hearth. Normally the salamander tap hole is positioned somewhere close under the cast house floor and usually it is in a difficult to reach area, full with piping, cables, etc. This difficult to access area also has insufficient or poorly accessible escapes routes and present a dangerous area for the operator who is drilling or lancing the salamander tap hole. The important issues during the salamander tapping include (i) location of the salamander tap hole, (ii) environmental aspects (big brown clouds are normally there) and (iii) tapping of maximum of liquid salamander iron.
The salamander tap is always organized to drain as much as possible liquid iron from the hearth and for using it as hot metal charge in the steel melting shop.
In the past, salamanders were tapped after the blow down and after the blast furnace were completely off blast. As a result the salamander had only its own ferro static pressure as the driving force to come out of the blast furnace.
Initial preparations for salamander tapping which are made include (i) drilling a predetermined distance into the furnace bottom below the hearth staves, and (ii) installing a trough or runner for the iron. When all is ready, a long oxygen lance is inserted in the drilled hole and the remaining brickwork is burned through into the pool of iron. Usually the flow of iron is slow and several hours are needed for emptying out the accumulation which may be up to 400 tons to 600 tons.
Not drilling the salamander tap hole completely through into the liquid and lancing the last part, results in an undefined tap hole diameter and sometimes in slowly running casts. These slowly running casts may also be retarded by a decreased hot metal temperature of the salamander, caused by the effect of the hearth cooling system during the waiting time between the end of the blow down and of the start salamander tap.
The salamander hot metal is usually led to the hot metal ladles (normally torpedo) with runners shelled by dam plates. This allowed for a controlled filling of up to three hot metal ladles, as there is no possibility to switch back to a ladle position upstream.
A tilting runner can also be used to exchange an unlimited amount of hot metal ladles, but its disadvantage is the extra height required, lowering the salamander drill angle. The use of the tilting runner requires a bended long runner to get a cross flow in the centre of the tilting runner and the tilting runner has to be actuated.
An important aspect to improve the salamander tapping is to eliminate the safety risks since in most cases the salamander tap hole is situated in a difficult to reach and confined area, with difficult escape routes, inherent to their position directly under the cast house floor.
To check whether or not the furnace was complete drained from liquid iron, a secondary safety salamander tap hole is engineered at some places with the salamander liquid iron to be collected in an open pit. This tap hole is opened when the flow from the main salamander tap hole is reduced. Very small amount is expected of salamander iron is expected from the second tap hole but it aids in the complete drainage of the hearth.