ALL ABOUT BLAST FURNACE

CONTENTS

1.)    BLAST FURNACE

       

                     1.1  Introduction

              1.2  History Of  Blast Furnace

              1.3  Construction Of Blast Furnace

              1.4  Refractory Used In Blast Furnace

              1.5  Charging System In Blast Furnace

                      1.5.1  Two Bell Charging System

                      1.5.2  Bell Less Charging System

              1.6  Working Principle Of Blast Furnace

              1.7  Different Reactions In Blast Furnace

    2.)  RAW MATERIAL USED IN BLAST FURNACE

            2.1  Iron Ore

                      2.1.1  Different Types  Of Iron Ore

                      2.1.2  Occurrence Of Iron Ore In India

            2.2  Metallurgical coke

            2.3  Flux

                      2.3.1  Different Types Of Flux

                      2.3.2  Occurrence Of Flux

            2.4  Sinter  

3.)  MODERN TRENDS IN BLAST FURNACE                             

             3.1  Paul Worth Bell Less Top

            3.2  Fuel Injection

            3.3  Oxygen Enrichment Of Blast

            3.4  Humidification

            3.5  High Top Pressure

4.)  IRREGULARITIES IN BLAST FURNACE

    

          4.1  Hanging

          4.2  Scaffolding

          4.3  Slip

          4.4  Chilled Hearth

          4.5  Breakout

          4.6  Pillaring

          4.7  Flooding through tap hole

          4.8  choking of gas off take

          4.9  Leaking of tuyers and coolers

5.)  GAS CLEANING PLANT

            5.1  Dust Catcher

                        5.1.1  History

                        5.1.2  Construction

                        5.1.3  Working Principle

            5.2  Water Scrubber

                        5.2.1  History

                        5.2.2  Construction

                        5.2.3  Working Principle

            5.3  Electro Static Precipitator

                        5.3.1  History

                        5.3.2  construction

                        5.3.3  Working Principle

6.)  BLAST FURNACE STOVE OR COWPER STOVE

         

          6.1  Introduction

            6.2  History

            6.3  Construction

            6.4  Refractory Lining

            6.5  Working Principle

7.)  BLAST FURNACE GAS

1.) BLAST FURNACE

1.1  INTRODUCTION:-

                                      The purpose of blast furnace is to chemically reduce and physically convert iron oxide into liquid iron called “hot metal” The blast furnace is a huge, steel stack lined with refractory brick where iron ore, coke and limestone are charged into the top and preheated air is blown into the bottom. The raw materials require 6 to 8 hours to descend to the bottom of the furnace where they become the final product of liquid slag and liquid iron. These liquid products are drained from the furnace at regular intervals. The hot air that was blown into the bottom of the surface ascends to the top in 6 to 8 seconds after going through numerous chemical reactions. Once the blast furnace is started it continuously runs for four to ten years with only short stops to perform planned maintenance.

1.2  HISTORY OF BLAST FURNACE :-

Blast furnaces existed in China from about 1st century AD and in the West from the High Middle Ages. They spread from the region around Namur in    Wallonia (Belgium) in the late 15th century, being introduced to England in 1491. The fuel used in these was invariably charcoal. The successful substitution of coke for charcoal is widely attributed to Abraham Darby in 1709. The efficiency of the process was further enhanced by the practice of preheating the combustion air (hot blast), patented by James Beaumont Neilson in 1828.

The blast-furnace technique started being used in Sweden as early as the 12th century. At the beginning of the 1980's a well-preserved ruin of a blast furnace was excavated in Norberg called Lapphyttan which was abandoned in the 14th century. The investigations showed that even older furnaces had been operating at the same site at the end of the 12th century.

The first blast furnace of Germany as depicted in a miniature in the Deutsches Museum

1.3  CONSTRUCTION OF BLAST FURACE :-

The blast furnaces nearly 30 meter toll (about 100ft) ,welded plate construction with circular cross section lined inside with refractory materials. The cross section area increases from top downwards maximum being at the bosh level and it decreases thereafter. Top or cylindrical part is known as stack. Stack gradually expands and the lowest widest part of the furnace is belly. After belly widest part is bosh, which is connected with hearth. The lower part of furnace, the iron tap holes are located near the bottom of the hearth. Above iron tap hole there is a slag notch for the removed for the slag. The vertical distance between the iron tap hole and the furnace top is the useful height of the blast furnace .

1.4  REFRACTORY USED IN BLAST FURNACE :-

Ø  Attack mechanisms in different regions of blast furnace
Region	Attack mechanism	Resulting damage
Upper stack	Abrasion	Abrasive wear
	Medium temperatures fluctuations	Spalling
	Impact	Loss of bricks
Middle stack	Medium to heavy temperatures fluctuations	Spalling
	Gas erosion	Wear
	Oxidation and alkali attack	Deterioration
Lower stack	Heavy temperatures fluctuations	Severe spalling
	Erosion by gas jets and abrasion	Wear
	Oxidation and alkali attack	Deterioration
	Thermal fatigue	Shell damage and cracks
Belly	Medium temperatures fluctuations	Spalling
	Oxidation and alkali attack	Deterioration
	Abrasion, gas erosion and high temperature	Wear
Bosh	High temperature	Stress attack
	Slag and alkali attack	Deterioration and wear
	Medium temperatures fluctuations	Spalling
	Abrasion	Wear
Raceway and	Very high temperature	Stress cracking and wear
Tuyere region	Temperatures fluctuations	Spalling
	Oxidation (water and oxygen)	Deterioration
	Slag attack and erosion	Wear
	Damage from scabs	Loss of cooling elements and tuyeres
Hearth	Oxidation (water)	Wear
	Zinc, slag and alkali attack	Deterioration
	High temperature	Stress build up and cracking
	Erosion from hot liquids	Break out risk
Iron notch	Heavy temperatures fluctuations	Spalling
(tap hole)	Erosion (slag and iron)	Tap hole wear
	Zinc and alkali attack	Deterioration
	Gas attack and oxidation (water)	Wear and deterioration

 Selection of appropriate refractory combination depending on the wear mechanism is very important. An improper selection of the refractories often leads to a refractory failure which, subsequently, becomes a complex problem to solve.

Ø Blast furnace refractories
Area	Present	Trend
Stack	39 % – 42 %% Al2O3	Super duty fireclay
Belly	39 % – 42 % Al2O3	Corundum, SiC-Si3N4
Bosh	62 % Al2O3, Mullite	SiC-Si3N4
Tuyere	62 % Al2O3, Mullite	SiC self bonded, Alumina-chrome (Corundum)
Lower hearth	42 %-62 % Al2O3, Mullite, Conventional carbon block	Carbon/Graphite block with super micro pores
Tap hole	Fireclay tar bonded, High alumina / SiC tar bonded	Fireclay tar bonded, High alumina / SiC tar bonded
Main trough	Pitch / water bonded clay / Grog / Tar bonded ramming masses, Castables	Ultra low cement castables (ULCC), SiC / Alumina mixes, Gunning repairing technique
Tilting spout	High alumina / SiC ramming masses / Low cement castables	High alumina / SiC / Carbon / ULCC

1.5 CHARGING SYSTEM IN BLAST FURNACE:-

 There are two types of charging system is used in blast furnace to charge the material in the blast furnace.

1.5.1 Two bell charging system :-

        The two bell charging system consists of a revolving material distribution, a small bell and a large bell. The diameter of large bell is usually smaller than the stockline diameter. The lower edge of the upper face of the bell forms a seal against the bottom edge of the large bell hopper. The bells are connected by a rod and move in the vertical direction by means of air cylinders.

The furnace charging is done in four steps

·         Step 1 – The charge material is taken to the furnace top either by a skip car and hoist or by a conveyor belt and is delivered to a receiving hopper. Small bell and large bells both are in closed condition. The charge materials from skip or conveyor are dumped in hopper above the small bell. Gas flowing from top of furnace through uptakes located in the dome (top cone).

·         Step 2 – With the large bell closed, the small bell is lowered and the charge material is dropped on the large bell. This is repeated several times.

·         Step 3 – The small bell is closed to prevent escape of gas to atmosphere. The large bell is lowered and the charge material is discharged into the blast furnace.

·         Step 4 – Both the bells are closed and the system is ready for repeat charging.

With each charge of the material from skip or conveyor, the small bell and hopper rotates to a selected position before the material is discharged. This provides an improved distribution of materials on the large bell. The bells, seating surface of the bells and hopper are hard surfaced. The rod supporting the large bell passes through the hollow rod supporting the small bell, thus permitting independent operation of the bells.  In this system of charging, the small bell, large bell and hopper are subjected to heavy impact and require replacement 2-3 times during a campaign of the BF lining. In this charging system, it is extremely difficult to maintain a gas tight seal for a top pressure higher than 1 Kg/Sq cm. Further two bell charging system has limitations towards burden distribution in the blast furnace. Burden distribution plays a big part in achievement of high productivity in the blast furnace.

1.5.2  Bell less charging system :-

                                 

                                                          The development of Bell less top charging system by S.A. Paul Wurth was a big quantum jump in technology and hence this system rapidly gained in popularity.

The BLT charging system has the following advantages.

·         It allows nearly continuous charging of the BF. While the rotating chute is distributing the contents of one lock hopper bin, the other can be filled.

·         It solves the problem of gas sealing under a high pressure operation

·         It provides flexibility in the distribution of BF burden. It can carry out one ring charging, multi ring charging, spiral charging, sector charging and point charging both in manual and automatic mode. Charge regulating valve provides accurate and constant distribution of burden materials.

·         It provides improved BF operational stability and efficiency leading to better hot metal chemistry control.

·         It contributes to increase in the BF productivity.

·         It reduces BF coke rate and helps in achieving higher injection rates of pulverized coal.

·         It contributes to higher campaign life due to reduced BF wall heat loads.

·         It greatly reduces the maintenance time and frequency of maintenance of top equipment. The chute can be replaced within a short period of time.

·         The top equipment is of light and compact construction compared to other high pressure top charging system.

However the height of the BLT top equipment is more than the two bell type charging system. BLT charging system can be integrated with skip hoist or conveyor belt charging system. BLT charging system has the following main component parts.

·         A movable receiving hopper.

·         One or two material lock hoppers equipped with upper and lower seal valves and a material flow control gate.

·         A central vertical feeding spout

·         A rotating adjustable angle distribution chute

·         A rotational and tilting drive mechanism

·         Hydraulic, lubrication and cooling systems

·         Monitoring and control systems

During the operation of the blast furnaces equipped with BLT charging equipment, the skip or conveyor brings the charge material to the receiving hopper. The material is then filled in the lock hopper which is then sealed and pressurized to the furnace top operating pressure. The lock hoppers are used alternately, that is one is being filled while other is being emptied. By design, the seal valves are always out of the path of material flow to prevent material abrasion. This reduces the probability of sealing problem. The flow control gate open to predetermined positions for the various types of charge materials to control the rate of discharge. Lock hoppers are lined with replaceable wear plates. The lower seal valves and material flow gates are in a common gas tight housing with the material flow chute, which directs the material through a central discharge spout located in the main gear housing.

1.6  Working principle of blast furnace :-

Blast furnaces operate on the principle of chemical reduction whereby carbon monoxide, having a stronger affinity for the oxygen in iron ore than iron does, reduces the iron to its elemental form. Blast furnaces differ from bloomeries and reverberatory furnaces in that in a blast furnace, flue gas is in direct contact with the ore and iron, allowing carbon monoxide to diffuse into the ore and reduce the iron oxide to elemental iron mixed with carbon. The blast furnaces operates as a counter current exchange process whereas a bloomery does not. Another difference is that bloomeries operate as a batch process while blast furnaces operate continuously for long periods because they are difficult to start up and shut down. Also, the carbon in pig iron lowers the melting point below that of steel or pure iron; in contrast, iron does not melt in a bloomery.

Carbon monoxide also reduces silica which has to be removed from the pig iron. The silica is reacted with calcium oxide (burned limestone) and forms a slag which floats to the surface of the molten pig iron. The direct contact of flue gas with the iron causes contamination with sulphur if it is present in the fuel. Historically, to prevent contamination from sulphur, the best quality iron was produced with charcoal.

The downward moving column of ore, flux, coke or charcoal and reaction products must be porous enough for the flue gas to pass through. This requires the coke or charcoal to be in large enough particles to be permeable, meaning there cannot be an excess of fine particles. Therefore, the coke must be strong enough so it will not be crushed by the weight of the material above it. Besides physical strength of the coke, it must also be low in sulphur, phosphorus, and ash. This necessitates the use of metallurgical coal, which is a premium grade due to its relative scarcity.

The main chemical reaction producing the molten iron is:

Fe₂O₃ + 3CO → 2Fe + 3CO₂

This reaction might be divided into multiple steps, with the first being that preheated blast air blown into the furnace reacts with the carbon in the form of coke to produce carbon monoxide and heat:

2 C_(s) + O_2(g) → 2 CO_(g)

The hot carbon monoxide is the reducing agent for the iron ore and reacts with the iron oxide to produce molten iron and carbon dioxide. Depending on the temperature in the different parts of the furnace (warmest at the bottom) the iron is reduced in several steps. At the top, where the temperature usually is in the range between 200 °C and 700 °C, the iron oxide is partially reduced to iron(II,III) oxide, Fe₃O₄.

3 Fe₂O_3(s) + CO_(g) → 2 Fe₃O_4(s) + CO_2(g)

At temperatures around 850 °C, further down in the furnace, the iron(II,III) is reduced further to iron(II) oxide:

Fe₃O_4(s) + CO_(g) → 3 FeO_(s) + CO_2(g

Hot carbon dioxide, unreacted carbon monoxide, and nitrogen from the air pass up through the furnace as fresh feed material travels down into the reaction zone. As the material travels downward, the counter-current gases both preheat the feed charge and decompose the limestone to calcium oxide and carbon dioxide:

CaCO_3(s) → CaO_(s) + CO_2(g)

The calcium oxide formed by decomposition reacts with various acidic impurities in the iron (notably silica), to form a fayalitic slag which is essentially calcium silicate, CaSiO₃

SiO₂ + CaO → CaSiO₃

As the iron(II) oxide moves down to the area with higher temperatures, ranging up to 1200 °C degrees, it is reduced further to iron metal:

FeO_(s) + CO_(g) → Fe_(s) + CO_2(g)

The carbon dioxide formed in this process is re-reduced to carbon monoxide by the coke:

C_(s) + CO_2(g) → 2 CO_(g)

The temperature-dependent equilibrium controlling the gas atmosphere in the furnace is called the Boudouard reaction:

2CO ⇌ CO₂ + C

The "pig iron" produced by the blast furnace has a relatively high carbon content of around 4–5%, making it very brittle, and of limited immediate commercial use. Some pig iron is used to make cast iron. The majority of pig iron produced by blast furnaces undergoes further processing to reduce the carbon content and produce various grades of steel used for construction materials, automobiles, ships and machinery.

1.7  Different chemical reactions in blast furnace :-

All of the raw materials are stored in an ore field and transferred to the stockhouse before charging. Once these materials are charged into the furnace top, they go through numerous chemical and physical reactions while descending to the bottom of the furnace.

The iron ore, pellets and sinter are reduced which simply means the oxygen in the iron oxides is removed by a series of chemical reactions. These reactions occur as follows:

1) 3 Fe2O3 + CO = CO2 + 2 Fe3O4	Begins at 850° F
2) Fe3O4 + CO = CO2 + 3 FeO	Begins at 1100° F
3) FeO + CO = CO2 + Fe     or     FeO + C = CO + Fe	Begins at 1300° F

At the same time the iron oxides are going through these purifying reactions, they are also beginning to soften then melt and finally trickle as liquid iron through the coke to the bottom of the furnace.

The coke descends to the bottom of the furnace to the level where the preheated air or hot blast enters the blast furnace. The coke is ignited by this hot blast and immediately reacts to generate heat as follows:

C + O₂ = CO₂ + Heat

Since the reaction takes place in the presence of excess carbon at a high temperature the carbon dioxide is reduced to carbon monoxide as follows:

CO₂+ C = 2CO

The product of this reaction, carbon monoxide, is necessary to reduce the iron ore as seen in the previous iron oxide reactions.

The limestone descends in the blast furnace and remains a solid while going through its first reaction as follows:

CaCO₃ = CaO + CO₂

This reaction requires energy and starts at about 1600°F. The CaO formed from this reaction is used to remove sulfur from the iron which is necessary before the hot metal becomes steel. This sulfur removing reaction is:

FeS + CaO + C = CaS + FeO + CO

The CaS becomes part of the slag. The slag is also formed from any remaining Silica (SiO₂), Alumina (Al₂O₃), Magnesia (MgO) or Calcia (CaO) that entered with the iron ore, pellets, sinter or coke. The liquid slag then trickles through the coke bed to the bottom of the furnace where it floats on top of the liquid iron since it is less dense.

      

2.)  RAW MATERIAL USED IN BLAST FURNACE

2.1  Iron ore :-

      2.1.1  Different Types Of Iron Ore :-

Major iron compounds

Name	Formula	% Fe
Hematite	Fe2O3	69.9
Magnetite	Fe3O4	74.2
Geothite / Limonite	HFeO2	~ 63
Siderite	FeCO3	48.2
Chamosite	(Mg,Fe,Al)6(Si,Al)414(OH)8	29.61
Pyrite	FeS	46.6
Ilmenite	FeTiO3	36.81

Haematite

·         Reddish; best quality; 70 per cent metallic content.

·         Found in Dharwad and Cuddapah rock systems of the peninsular India.

·         80 per cent of haematite reserves are in Odisha, Jharkhand, Chhattisgarh and Andhra Pradesh.

·         In the western section, Karnataka, Maharashtra and Goa has this kind of ore.

Magnetite

·         Black ore; 60 to 70 percent metallic content.

·         Dharward and Cuddapah systems.

·         Magnetic quality.

·         Karnataka, Andhra Pradesh, Rajasthan, Tamil Nadu and Kerala.

Limonite

·         Inferior ores; yellowish in colour; 40 to 60 per cent iron metal.

·         Damuda series in Raniganj coal field, Garhwal in Uttarakhand, Mirzapur in Uttar Pradesh and Kangra valley of Himachal Pradesh.

·         Advantage == open cast mines == easy and cheap mining.

Siderite

·         ‘Iron carbonate’; inferior quality; less than 40 per cent iron.

·         Contains many impurities {previous post}; mining is not economically variable.

·         However, it is self-fluxing due to presence of lime.

2.1.2  Occurrence of iron ore in india :-

Iron Ore in Orissa

·         The ores are rich in haematites.

·         India’s richest haematite deposits are located in Barabil-Koira valley.

·         Others: Sundargarh, Mayurbhanj, Cuttack, Sambalpur, Keonjhar and Koraput districts.

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Iron Ore in Chhattisgarh

·         Bailadila mine is the largest mechanised mine in Asia [Ore benefication only done here]

·         A 270 km long slurry (a semi-liquid mixture) pipeline from the Bailadila to Vizag plant transports the ore slurry.

·         Smelting is done in Vizag [Vishakhapatnam] iron and steel factory.

·         Bailadila’s high grade ore is exported through Vishakhapatnam to Japan [No iron ore in Japan. But market is huge due to automobile industry] and other countries.

·         The Dalli-Rajhara range is 32 km long [ferrous content 68-69 per cent] range with significant reserves.

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Iron Ore in Jharkhand

·         25 per cent of reserves.

·         First mine in Singhbhum district in 1904.

·         Iron ore of here is of highest quality and will last for hundreds of years.

·         Noamandi mines in Singhbhum are the richest.

·         Magnetite ores occur near Daltenganj in Palamu district.

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Iron Ore in Karnataka

·         Iron ores are widely distributed.

·         High grade ore deposits are those of Kemmangundi in Bababudan hills of Chikmagalur district and Sandur and Hospet in Bellary [Lot of Mining Mafia].

·         Most of the ores are high grade haematite and magnetite.

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Iron Ore in other states

·         Andhra Pradesh (1.02%): Kurnool, Guntur, Cuddapah, Ananthapur, Nellore.

·         Maharashtra (0.88%): Chandrapur, Ratnagiri and Sindhudurg.

·         Madhya Pradesh (0.66%).

·         Tamilnadu: Salem, Tiruchirapalli, Coimbatore, Madurai etc.

·         Rajasthan: Jaipur, Alwar, Sikar, Bundi, Bhilwara.

·         Uttar Pradesh: Mirzapur.

·         Uttaranchal: Garhwal, Almora, Nainital.

·         Himachal Pradesh: Kangra and Mandi.

·         Haryana: Mahendragarh.

·         West Bengal: Burdwan, Birbhum, Darjeeling.

·         Jammu and Kashmir: Udhampur and Jammu.

·         Gujarat: Bhavnagar, Junagadh, Vadodara.

·         Kerala: Kozhikode.

2.2  Metallurgical Coke :-

Coke is made by destructive distillation of a blend of selected Bituminous coals (called Coking coal or Metallurgical coal) in special high temperature ovens in the absence of oxygen until a greater part of the volatile matter is driven off. The resulting product, COKE, consists principally of Carbon. 

Traditionally, chemistry, size & strength (both cold as well as hot) have been considered the most important properties for use in the blast furnace. The quality of the constituent coals determines the characteristics of the resultant coke.

Coke is primarily used to smelt iron ore and other bearing materials in blast furnaces, acting both as a source of heat and as a chemical reducing agent to produce pig iron or hot metal. Coke, iron-ore and limestone are fed into blast furnace, which runs continuously. Hot air blown into the furnace burns the coke, which serves as source of heat and as an oxygen reducing agent to produce metallic iron. Limestone acts as a flux and also combines with impurities to form slag. Coke also serves as a structural material to support the deep bed of coke/iron oxide/limestone that makes up much of the furnace volume. It is in this last role that its properties are crucial. It is important that it does not degrade (e.g. break up into small particles) during its descent through the oxidizing hot gases passing through the stock region of the furnace.

To produce high quality blast furnace coke,

 high quality coal must be used. High quality

 coals are those coals that when coked together

 produce the highest stability and CSR (Coke

Strength after Reactivity) to support the blast

 furnace burden and allow maximum production.

Low Ash Metallurgical Coke is required for metallurgical and chemical industries and is used as the primary fuel where high temperature and uniform heating is required. The industrial consumers of LAMC include integrated steel plants, industry/foundries producing Ferro Alloys, Pig Iron, Engineering Goods, Chemicals, Soda Ash and Zinc units etc.

2.3  Flux :- Mainly two types of fluxes are used in blast furnace for production of pig iron.

  1.) Limestone.    2.) Dolomite.

2.3.1  Types of flux :-

                               There are three types of flux is available in nature.

 1.) Acidic         2.) Basic       3.)Neutral

Acidic flux:

Acidic flux  are those flux which comes from substances like Lewis acid such as SiO_2, H₃PO₄, HCl etc.

Acidic flux is used in metal welding or extractive metallurgy or Smelting.

Silica (SiO₂) is used to remove basic earthly impurities (gangue) such as lime (CaO) or MgO.

Basic flux:

It is similar to Acidic flux but only difference is that we take basic substances like CaO for welding process or extractive metallurgy or Smelting. 

Basic fluxes like lime (CaO), magnesium oxide (MgO) are used to remove acidic gangue such as SiO₂.

Neutral flux:

This type of flux are only used to increase the fluidity of the slag in molten metal.

 

2.4  Sinter:-    Sinter is agglomerated

 mixture of iron ore fine, coke fines and

flux fines.

  SINTERING:-

                        Sintering has been referred to as the art of burning a fuel mixed with ore under controlled conditions. It involves the heating of fine iron ore with flux and coke fines or coal to produce a semi-molten mass that solidifies into porous pieces of sinter with the size and strength characteristics necessary for feeding into the blast furnace.

There are basically the following three types of sinters.

·         Non flux or acid sinters – In these sinters no flux is added to the iron ore in preparing the sinter mix. Non flux sinters are very rarely being produced these days.

·         Self fluxing or basic sinters – These are the sinters where sufficient flux is added in the sinter mix for producing slags of desired basicity (CaO/SiO2) in blast furnace taking into account the acidic oxides in the blast furnace burden.

·         Super flux sinters – These are the sinters where sufficient flux is added in the sinter mix for producing slags of desired basicity in blast furnace taking also into account the acidic oxides in the coke ash in addition to the other acidic oxides in the blast furnace burden.

3.) MODERN TRENDS IN BLAST FURNACE

3.1  Bell less top charging system in blast furnace

The development of Bell less top charging system by S.A. Paul Wurth was a big quantum jump in technology and hence this system rapidly gained in popularity. The BLT charging system has the following advantages.

·         It allows nearly continuous charging of the BF. While the rotating chute is distributing the contents of one lock hopper bin, the other can be filled.

·         It solves the problem of gas sealing under a high pressure operation

·         It provides flexibility in the distribution of BF burden. It can carry out one ring charging, multi ring charging, spiral charging, sector charging and point charging both in manual and automatic mode. Charge regulating valve provides accurate and constant distribution of burden materials.

·         It provides improved BF operational stability and efficiency leading to better hot metal chemistry control.

·         It contributes to increase in the BF productivity.

·         It reduces BF coke rate and helps in achieving higher injection rates of pulverized coal.

·         It contributes to higher campaign life due to reduced BF wall heat loads.

·         It greatly reduces the maintenance time and frequency of maintenance of top equipment. The chute can be replaced within a short period of time.

·         The top equipment is of light and compact construction compared to other high pressure top charging system.

3.2  FUEL INJECTION IN BLAST FURNACE :-

Pulverized Coal Injection (PCI) is a process that involves injecting large volumes of fine coal particles into the raceway of a blast furnace (BF). This provides not only a supplemental carbon source but also speeds up the production of liquid iron besides reducing the need for metallurgical coke for reactions in the blast furnace. The desire to move away from the production of the metallurgical coke with its inherent environmental problems has motivated the use of pulverized coal injection in blast furnace.

Concept and the process of coal injection

The PCI technology is based on the simple concept of carrying the finely ground (pulverized) dried coal by a conveying gas (normally nitrogen) to the blast furnace where it is distributed to different tuyeres and injected through a lance in the blow pipe. In the blow pipe it is mixed with hot blast and then supplied to the blast furnace in the raceway. The raceway propagates coal and coke combustion and melts the solid iron ore, releasing molten iron.

Advantages of PCI

·         PCI uses non coking coal. This coal is cheaper than the metallurgical coke. In most of the blast furnaces PCI replaces 30 to 50 % of the metallurgical coke charge. This results in lower fuel cost

·         The productivity of blast furnace improves since coal injection is accompanied with oxygen injection.

·         A wide range of coals can be injected

·         Injection rates are higher than the injection rates of other fuels such as oil and natural gas.

·         Coal grinding and injection systems are non polluting systems. Hence the overall pollution from coke production for iron making gets reduced by injecting pulverized coal in the blast furnace.

·         Coal supplies are relatively stable when compared with the petroleum fuels supplies.

·         Coal injection system is less costly than the cost of an additional coke oven battery. Lower capital cost means lower depreciation and interest to be charged on hot metal.

3.3  Oxygen enrichment of blast in blast furnace :-

§ OXYGEN ENRICHMENT UPTO 25 % IN THE BLAST IS FOUND TO BE ADVANTAGEOUS .

§ The presence of 79 % nitrogen by volume in blast restricted the temperature generated in the combustion zone . this temp can be increased by decreasing the nitrogen percentage in hot blast .

§ it has been show that only 2 % increase in oxygen reduces the nitrogen burden by about 4 unit per unit weight of coke and higher temperature would be possible.

§ For every percentage increases in oxygen content , increases in production rate of about 3-4 % with marginal shaving of coke. Main reason of shaving of coke is due to cracking of moisture giving rise to hydrogen which acts as a reducing gas up in stack.

 § Disadvantage of – sticking of stocks and increasing Si content in hot metal

§ Generally oxygen enrichment and humidification(endothermic reaction) is done simultaneously .

§ Disadvantages :

                                there is a limitation of excess temp in tuyer region but if temp exceed the 1- bridging occurs 2-sticking of stocks 3- higher silicon content in pig iron .

3.4  Humidification :-

By incorporating moisture through tuyers we can generate double the volume of reducing gas per mole of carbon burnt.

For every carbon burnt one mole of CO and an addition mole of hydrogen will be available as product of burning of coke for reduction in bosh and stack.

 The more the moisture more will be the additional hydrogen available.

Kinetically H2 reduction of iron oxide is faster that By Co because of its smaller size.

 Fe2O3 + H2---- exothermic reaction

 H2O+ C------ Co+ H2

 H2O+ c = co + H2

h⁰ (1200 ⁰c) =+ 2700kcal/kgc

                       = + 1800 kcal/kg h20

this reaction is exothermic in nature . so furnace temperature decrease so to compensate this temperature the blast temperature is increased or adding oxygen inrichment is done.

moisture is introduced in form of water because if steam is introduced the cost increases as it required fule to heat the water into steam.

3.5  High top pressure :-

Aim to increase the gaseous reduction .

Here by throttling back discharge gas the static pressure inside the furnace is increased. Throttling device is located after the second stage of cleaning after scrubbers. here a septum valve is provided just after the wet scrubber and part of the semi cleaned gas is by passed to the big bell hopper to obtain zero pressure differential across the big bell. Major problem face in case of high pressure is noise level emitted at the septum valve control area.

 Result of high top pressure of 0.7-0.85 kg/cm2 gauge top pressure

 A- increase the production rate by 11%, and

b- shaving of coke is 35kg/t of pig iron and

 c- marked decreases in flue dust loss .

 Limit of the effective top pressure is about 3kg/cm2 gauge for obtaining increased production rate and decreased coke rate.

 Benefits of high top pressure:

1.)- increased production rate : due to increase in the contact time between ore and reducing gases  due to increase in the residence time of the gas in the stack portion , and due to high pressure the rate of reduction of ore increases.

 2.)- reduction in fuel consumption that is coke consumption rate .

 3.)- more uniform operation with lower and more consistent hot metal silicon content . It is due to the flexibility of operation .

4.)- furnace campaign life increases sue to increase in the lining life sue to smoother operation.

 5.)- decreases in the dust loss(leads to reduction in load on gas cleaning system) and channeling.

6.)-boudouard equilibrium moves to left side due to which coke consumption decreases.

3.6  BURDEN PREPARATION :-

 1.)-Iron ore fines agglomerated by sintering and pelletisation.

2.)-incorportion of flux in the sinter (fluxed and super fluxed sinter)

 3.)-incorportion of mgo in the form of dolomite in the sinter to offset the ill effect of high alumina under Indian condition.

 4.)-Increased in the strength of pellets with lower fuel consumption. By using low sulphur portland cement , slaked lime etc. as binders to develop the strength. The carbonate bonding (C-B) process, the hydrothermal and the (COBO) process are the typical example of this category.

 In C-B process slaked lime is used as binder during balling and the pellets are hardened in carbon dioxide atmosphere . The calcium carbonate thus formed developed the strength. In COBO process hardening is carried out at 200 degree centigrade. In an autoclave under 15kg/cm2 pressure of co2 to expedite the process.

4.)  IRREGULARITIES IN BLAST FURNACE

=>  The irregularities in blast furnace operation may due to faulty mechanical devices like cooler, valve etc. or due to faulty mechanical operations like tapping, charging etc.

Main three reasons of irregularity in b/f:

§ Due to faulty mechanical devise like any fault in cooler , valves.

§ Due to faulty mechanical operation : ;like charging , taping etc

§ Abnormal physio chemical changes inside the furnace.

Some of the common irregularities are:-

4.1  Hangging:-

                         There is an ununiform descent of burden material due to bridging, scaffolding , wedging.

Main reasons are :

 • Solidification of previously fused slag that is sintering of this into large mass.

• Bridging of ore particles in the vicinity of fine coke parricles which instead of sepereating the ore particles flow in the interstitial position of the ore particles.

• The deposition of excess carbon due to naumaan reversion reaction in the voids and consequent decrease in the permeability of the burden.

• Condensation of alkali vapous in the upper part of stack there by cementing the charge into impervious mass.

• Excess blass pressure resulting in counteracting the downward moment of the stock.

Classification of hanging :Remedies

 • General hanging and bottom hanging :

                                                                          if hanging occurs anywhaere above                the tuyere due to low voidage in the stack.

 • Top hanging :

                            hanging ocuurs in stack due to alkali condensation and carbon deposition Cold blast, reduction in blast pressure, addition of excess lime (helps in solution loss reaction).

4.2  Scaffolding :-

 In this case formation of scaffold (which is a larger mass of material stuck to the furnace wall at top portion.

Due to scaffolding many problems arises:

`          

 • Reduction in the cross section , uneven moment of stock, rise in blast pressure, increase in dust loss, decrease input put, increase in fuel consumption, decrease in amount of gas produced.

• Main reason of scaffold formation is presence of alkali oxides in burden . the alkali vapour condensed on the bricks lining and form low m.p alkali alumino silicates and to this sticking of ore particls progressively resulting in formation of massive block of charge material stuck to the wall. Main reasons of formation of scaffold : Remedies

• Pore furnace design, improper fluxing, heavy burden .

 Remedies to scaffolding:

• Minimises the alkali content of the burden

 • Decreases the fuel rate

• Scaffolding has been successfully removed by jumping the furnace that is the blast is put off suddenly to relieve the pressure under scaffold.

4.3  Slip :-

Slip is defined as sudden falling of burden due to collapse of scaffold, hanging etc.

• Sudden Slippage of burden material generally the scaffold , wedging hanging .

• Results of slip is explosion , chilled hearth

• Remedy :

 allow the f/c to slip on its own by adjusting the blast temp and pressure.

• Main reason may be bad bosh design.

4.4  Chilled hearth :-

Hearth get chilled which affect the tapping . main causes are water leakage from tuyers, excessive moisture in the blast, slip.

 Remedy :-

                   Proper maintenance of tuyers and cooling system of blast furnace.

4.5  Breakout :-

• the slag or metal or both flow out in an uncontrolled manner due to failure of hearth or bosh wall .

• it causes explosion when the metal comes in contacts with coolers.

• when its occurs the tapping is done in an faster way.

Remedy :-

                    Time to time maintenance of bosh and hearth wall.

4.6  Pillaring :-

if the blast is unable to penetrate right upto the center of the furnace it leads to the formation of cold central column of stcok with hot zone around it . it can be found out by inserting a rod.

Remedy :-

                  Pillaring can be eliminated by increasing the blast pressure or increasing coke size.

4.7  Flooding through tap hole :-

In the bosh liquid metal and slag trickle through the permeable coke bed.

An increase in the gas flow can prevent the liquid metal and

slag from flowing downwards, causing it to accumulate in the coke grid and descents suddenly into the hearth, this is known as flooding.

Remedy:-

                    This problem can minimised by using big size of coke

4.8  Choking of gas off takes :-

Furnace operation is effected if dust gets accumulated in the uptakes can choke the uptake.

This happens because of faulty gas off take design and improper joint.

Remedy :-

                     It can be eliminated by proper design of gas uptakes.

4.9  Leaking of tuyers & coolers :-

Leaking tuyers or coolers in the lower paet of the furnace cone be changes if the are no rectified in time.

The leaking of tap hole coolers generates steam which erodes the hearth lining.   

 

Remedy :-

                   It can be reduced by proper maintenance of tuyers and coolers.

          

              5.)  GAS CLEANING PLANT

The process of liquid iron production in the blast furnace (BF) generates gas at the furnace top which is an important by-product of the BF process. This top gas of the blast furnace is at the temperature and pressure existing at the BF top and usually contaminated with dust and water particles. This top gas is having substantial calorific value and is known as raw BF gas or contaminated BF gas. The composition and quantity of this top gas depend on the nature of the technological process in the blast furnace and the type and the quality of the raw materials used for the iron production in the blast furnace. In order to further use the raw BF gas, it is necessary to clean it by using certain process systems which reduces its content of the solid particles.

Typical analysis of BF gas with PCI
Constituent	Unit	Value
CO	% Vol.	20 – 24
CO2	% Vol.	18-23
H2	% Vol.	1.5-4.5
N2	% Vol.	52-57
SO2	mg/cum	10-30
NH3	mg/cum	5-21
Chloride	mg/cum	50-200
Oxides of N2	mg/cum	3-12

Effective removal of a mixture of coarse and fine dust from a very dusty gas necessitates the use of a dust catcher and a multi venturi scrubbing system. Effective cooling requires the use of a gas cooling tower prior to BF gas discharge into the BF gas network in the steel plant.

5.1  Dust catcher :-

he dust catcher is a large cylindrical structure normally with a large diameter and with the required height. It is usually lined to insulate it and prevent the condensation of moisture in BF gas so that the dust remains dry and does not ball up and flow freely into the conical portion of the dust catcher at its bottom for its periodical removal.

The gas is sent to the dust catcher by a single down comer and enters through the top by a vertical pipe that carries the gas downward inside the dust catcher. This pipe flares at its lower extremity like an inverted funnel, so that as the gas passes downward its velocity (and thus its dust carrying potential) decreases, and most of the coarser dust drops out of the gas stream and is deposited in the cone at the bottom of the dust catcher. Since the bottom of the dust catcher is closed, and the gas outlet is near the top, the direction of the travel of the gas must reverse 180 degrees. This sudden reversal in the direction of flow causes more of the dust to get settle down.

             

BF gas after primary cleaning in the dust catcher, where the majority of heavy particles are removed, moves towards the secondary gas cleaning stage (scrubbers) which is the wet cleaning system. 

5.2  Water scrubber :-

The scrubbers for the blast furnace gas

cleaning operate on the basic aerodynamic

principle. A simple analogy of the aerodynamic

 principle is that if water droplets of very

large size are projected to collide with

gas-stream particles of much smaller size

then the statistical chances of collision are

very small. As the size of the water droplets

 is reduced to more nearly the size of the

gas stream particles, the chances of

 collision improve. Studies have shown

 that a surface film surrounding a water

droplet has an approximate thickness of

1/ 200 of its diameter. A gas stream particle

 in flight flows through the streamline film

 around the droplet without collision if it is

having a diameter less than 1/ 200 the

 diameter of the water droplet. But if the

water droplet diameter is much smaller, then collision would occur. A 10 microns aim particle requires water droplets smaller than 2000 microns (200×10) for adequate collection. Efficient scrubbing, therefore, requires atomizing the water to a fineness related to particle size to afford maximum contact with the particles to be captured. Further the probability of a water droplet hitting the dust particles is proportional to the dust concentration. A single dust particle is less likely to hit a single droplet than a swarm of them. To equalize these factors, scrubbers are regulated as to the volume of gas to be scrubbed (measured by pressure drop of the gas stream), and water to be sprayed (measured by hydraulic pressure at the spray nozzles). The scrubbing chamber’s height and diameter are also tailored to the known characteristics of the gas.

After the cleaning of gas in water scrubber the blast furnace gas is send to electro static precipitator.

5.3  Electro static precipitator :-

An electrostatic precipitator (ESP) is a filtration device that removes fine particles, like dust and smoke, from a flowing gas using the force of an induced electrostatic charge minimally impeding the flow of gases through the unit.

Construction:-
of thin wires called discharge or emitting electrodes and other set called collecting electrodes in the form of pipes or plates. The emitting electrodes are placed in the centre of pipes or midway between two plates and are connected usually to negative polarity of high voltage D.C. source of the order of 25-100kV. The collecting electrodes are connected to the positive of the source and grounded.

Working of ESP:-

The high voltage direct current is connected between the framework and the ground, thereby creating a strong electrical field between the wires in the framework and the steel curtains. The electrical field becomes strongest near the surfaced of the wires, so strong that an electrical discharge –“the corona discharge”-develops along the wires. The particles of dust are removed by an intermittent blow usually referred as rapping. This causes the dust particles to drop into dust hoppers situated below the collecting electrodes.
steps in the process of precipitation:-
•    Ionization of gases and charging of dust particles.
•    ii) Migration of the particle to the collector.
•    iii) Deposition of charged particles on the collecting surface
•    iv) Dislodging of particles from the collecting surface.

Parts of ESP:-

1.Precipitator Casing- 

                                    It is an all-welded steel construction, assembled from prefabricated wall and roof panels using panel construction. The precipitator casing is designed for horizontal gas flow. The casing is thermally insulated by insulation.

2. Hoppers –

                     The hoppers are of pyramidal construction type with dry or wet conveying system. The valley angle of the hoppers (angle between hopper corner and horizontal) is never less then 55° and offer more to ensure easy dust flow down to the feed out flange. To ensure free flow of ash into the disposal system lower portions of the hoppers are provided with electrical heaters with thermostatic control

3. Gas Distribution System-

                                                The gas velocity in the precipitator is approximately 1/10th of the velocity in the ducting before the precipitator. It is therefore essential that the precipitator have arrangements to give an even distribution over its entire cross sectional area. Special gas distribution screens are
therefore located at the inlet of the precipitator. The screens are of modular design and hang within a framework in the precipitator-casing inlet.

4. Collecting Electrode System-

                                                    The collecting plates are made of 1.6-mm steel plate and shaped in one piece by roll forming. The 'G' profiled collecting electrode is provided. The upper edge of the connecting plates are provided with hooks, which are hung from support angles welded to the roof structure. The lower edge of each plate has a shock receiving plate, which is securely guided by the shock bar arrangement. This results in a stable collecting
system similar to the emitting system.

5. Emitting Electrode System-

                                                 Wire type/pipe type electrodes are used for emitting electrode. Wire type electrodes give the best current distribution. Therefore they are the ones best suited for difficult dusts with high electric resistivity.

6. Rapping System –

                                   Each collecting plate has a shock receiving plate at its lower end. The plates in one row of each field are interfaced to one another by these shock receiving irons resting in slots in the shock bar thus maintaining the required spacing. The shock bars are kept in alignment with guides located at the front and rear of each shock bar. Each collecting plate is hung on an eccentric positioned hook to ensure that the shock-receiving of the collecting plate is constantly resting against the shock bar. In this manner the highest possible energy is transferred to the collecting plate when the 'tumbling hammer or MIGI(magnetic impulse gravitational impact) coil hits the corresponding shock bar. The system employs 'tumbling hammers which are mounted on a horizontal shaft in a staggered fashion, with one hammer for each shock bar. As the shaft rotates slowly each of the hammers in turn over balances and tumbles, hitting its associated shock bar. The shock bar transmits the blow simultaneously to all of the collecting plate in one row because of their direct contact with the shock bar. A uniform rapping effect is provided for all collecting plates in one row. The rapping frequency should be as low as possible in order to minimize dust losses from rapping. The frequency of each rapping system is adjustable within a wide range.

7. Rectifier Handling- 

                                    The rectifier-control provides all the modern controls and has a spark rate controller unit which controls spark rate of 5 to 10 sparks per minute to maintain optimum dust collection efficiency. The rectifier system provides a smoother control of output current from 10% to 100%
of the rated value and also maintains the constant current output.

After passing through the gas cleaning plant the gas is cleaned 99.99%. Then it is send to the blast furnace stove.

6.)  BLAST FURNACE STOVE

Hot-blast stove, apparatus for preheating air blown into a blast furnace, an important step in raising the efficiency of iron processing. Preheated air was first used by James Beaumont Neilson in 1828 in Glasgow, but not until 1860 did the Englishman Edward Alfred Cowper invent the first successful hot-blast stove. Essentially, the stove is a vertical cylindrical steel shell lined with firebrick and with the interior separated into two chambers: a combustion chamber, in which gases from the blast furnace and from other fuel sources such as the coking plant are burned, and a regenerative chamber filled with a checkerwork of refractory brick heated by the burned gas. Many blast furnaces are served by three stoves; while two are being heated, the air blast passes through the regenerative chamber of the third stove on its way to the blast furnace. Blast furnaces fed with air that has been preheated to temperatures from 900 to 1,250 °C (1,650 to 2,300 °F) can generate smelting temperatures of about 1,650 °C (3,000 °F), significantly reducing the consumption of coke per ton of iron produced.

A hot blast stove is a facility to supply continuously the hot air blast to a blast furnace. Before the blast air is delivered to the blast furnace tuyeres, it is preheated by passing it through regenerative hot blast stoves that are heated primarily by combustion of the blast furnace top gas (BF gas). In this way, some of the energy of the top gas is returned to the blast furnace in the form of sensible heat. This additional thermal energy returned to the blast furnace as heat reduces the requirement of blast furnace coke substantially and facilitates the injection of auxiliary fuels such as pulverized coal as a replacement for expensive metallurgical coke. This improves the efficiency of the process. An additional benefit resulting from the lower fuel requirement is an increase in the hot metal production rate. All of these have a significant effect in terms of reducing the hot metal cost.

History of hot blast stoves

The use of blast furnaces dates back as far as early as fifth century B.C. in China. However, it was not until 1828 that the efficiency of blast furnaces was revolutionized by preheating them using hot stoves in conjunction with the process, an innovation created by James Beaumont Nielson, previously foreman at Glasgow gas works. He invented the system of preheating the blast for a furnace. He found that by increasing the temperature to 300 deg F (149 deg C), he could reduce the fuel consumption from 8.06 tons to 5.16 tons with further reductions with higher temperatures. In 1860, the cooperative use of hot stoves with blast furnaces was further transformed by Edward Alfred Cowper by recycling the top gas of the blast furnace rather than receiving solid fuel as did the earlier designs.

Early designs of hot stoves used with blast furnaces were originally placed on top of the furnace rather than next to it, the current layout used today. They used waste heat from the blast furnace delivered via cast iron pipes to the hot stove to preheat the cold air blast. One major problem with using cast iron pipes was the generation of cracks throughout them. This was remedied by eliminating the pipes and using refractory instead. This also furthered the design of the layout of the hot stove with the blast furnace to the use of two to four hot stoves placed in series beside the blast furnace. This allowed for the heating of one blast stove by blast furnace top gas as the other one was being drained of its heat to preheat the air blast into the blast furnace. As the air blast entered the stove, it was preheated by hot bricks and exited the stove as a hot blast. Cambria Iron Works was the first company in the U.S. to use regenerative stoves in 1854. These stoves were constructed of iron shells lined with refractory and contained multiple passageways of refractory for the blast throughout. A typical stove of this design had about 186-232 sq m of heating surface. In 1870, Whitwell Stoves designed and produced larger stoves with heating surfaces of about 8546 sq m, which could deliver 454-566 deg C hot blast to the blast furnace. These were also the first stoves to use hexagonal refractory checkers, cast iron checker supports, and semi-elliptical combustion chambers to enhance the distribution of gas throughout the checkers.

Construction :-

                      A blast furnace stove is cylindrical tower more then 40 m. Height and 10 m. diameter. It is lined with refractory bricks the tower consist of a checker brick and combustion chamber.

Hot blast stoves (or hot stoves, hereinafter “HS”) are regenerative heat exchangers. Each of them represents a cylindrical structure filled with multiple course grid (checker-work) which is made of refractory bricks. Checker-work is the basic constructive element of hot stove, which defines the process of heat transfer from combustion products to cold blast.

 Full working period of regenerative hot stoves consists of two operating cycles:

·         Cycle of checker-work heating (gas period) when products of combustion of fuel gas (blast furnace gas or mixed gas) enter from the top and, passing through the checker-work, they heat it up;

·         Blast cycle (blast period) when air (cold blast) enters the previously heated checker-work from the bottom and, passing through it upwards, is heated up. The checker-work is thus cooled down.

Characteristics of a modern hot blast stove :-

The calorific value of blast furnace top gas is not high enough in value to achieve the high flame temperature required for the higher hot blast temperatures of 1000 deg C to 1200 deg C.  Hence the blast furnace gas for the stoves is normally enriched by the addition of a fuel of much higher calorific value, such as coke oven gas for obtaining the high flame temperature. However many of the modern blast furnaces are having hot blast stoves, which have burners designed for the use of only the blast furnace gas.

Hot blast stoves of a modern blast furnace have the following characteristics.

·         Achievement of high efficiency combustion – Achievement of high efficiency combustion even in the operation with only blast furnace gas.

·         Smaller heat radiation from the stove body.

·         Low construction costs.

·         High stove service life -Expected service life of a modern stove is around 40 years

·         Complete elimination of stress corrosion cracking.

·         Low concentration of uncombusted CO above the upper surface of checker bricks.

Most blast furnaces are equipped with three hot blast stoves, although in a few instances there are four. The stoves are tall, cylindrical steel structures lined with insulation and almost completely filled with checker bricks where heat is stored and then transferred to the blast air. Each stove is about as large in diameter as the blast furnace, and the height of the column of checkers is about 1.5 times as tall as the working height of the blast furnace. At the modern blast furnaces, the relation of the stove size to the furnace size is even larger. As an example, one typical new blast furnace has a hearth diameter of 9.75 m and a working height of 25.9 m, and it is equipped with three stoves with each stove having an inside diameter of 10.36 m and a checker height of 40 m.

Fig. 1 shows the typical cross sectional views of a conventional two pass hot blast stove. As seen in the figure, the oval shaped combustion chamber occupies around 10 % of the total cross sectional area of the stove. It extends from the bottom of the stove to within around  4 m of the top of the stove dome. A sturdy brick breast wall separates the combustion chamber from the balance of the stove, which is filled with checker bricks resting on a steel grid supported by steel columns.

There is an insulating lining just inside the steel shell. This is normally very thick on the side near the combustion chamber. The combustion chamber is completely surrounded by a brick well wall, which is lined with super duty refractory bricks containing 50 % to 60 % of alumina. For very high hot blast temperatures in excess of 1200 deg C, the entire combustion chamber and the dome are lined with this type of brick. Also, the top 8 m to 10 m of checkers are normally of super duty bricks.

However, for in newer furnaces for the stoves. silica refractories are the ,material of choice for improved stability owing to the elimination of expansion movements in the upper structure during operation. Silica refractories have an additional advantage over alumina refractories, since they are resistant to dust accumulation. For this reason, seven layers of silica checkers are normally installed at the top of the checker shaft, in alumina based stoves.

In erecting the dome lining, arch bricks are used and a space is provided between the brick and the dome to allow for expansion of the ring wall from which it is supported. In some stoves, there is an offset in the steelwork at the top of the ring wall so that the dome brick can be supported independently.

Traditional hemisphere domes, although simple in shape, have a natural instability with a tendency for the upper part of the dome to collapse first. Hence some blast furnaces have an inverted catenary shaped dome. This dome has a statically balanced shape and can be built with a minimum of special shaped bricks. Since the mushroom dome refractories also expand and contract, a hinged support construction allows for these movements, without exerting any force on the structure.

With better gas cleaning facilities available these days,  it is possible to use checkers with smaller flue openings without any danger of plugging of the flues with dirt. With smaller flues, heat transfer rates are better because the ratio of heating surface to checker weight is large and more checker weight are installed in the available space. However, with the smaller flue openings, it became very important to lay up the checkers properly so that the flues match perfectly. Misaligned flues increase the pressure drop through the stoves significantly and prevent effective use of all the heat storage capacity.

The burner for the blast furnace stove is located near the bottom of the combustion chamber. On the majority of hot blast stoves, the burners are external to the combustion chamber. There is a burner shutoff valve between the burner and the stove that is closed to isolate the burner when the stove is on blast, but open when the stove is being fired. The gas and combustion air are partially mixed in the metallic burner but, because of their high velocity through the burner, actual ignition probably does not occur until inside the stove. The mixture of gas and air impinges on the target wall directly opposite the burner port and then makes a 90 degree turn.

Combustion continues while the gas ascends up the combustion chamber. When a stove is to be heated from the cold condition, an igniter is normally used to start combustion but, during normal operation, the residual heat in the target wall is sufficient to cause ignition.

In several modern hot blast stoves, ceramic burners are used. These burners, with their mixing chamber, are installed inside the combustion chamber and the firing is upward in a vertical direction instead of a horizontal direction as with the conventional metallic burner. With this type of burner, shutoff valves are required in both the gas main and the combustion air duct. These valves are capable of withstanding the force of the blast pressure. The ceramic burners have certain benefits because of their special design features.

The port through which the hot blast air exits from the stove is located on the side of the combustion chamber usually  4 m to 7 m above the burner. Between the stove and the hot blast main there is located a water cooled hot blast valve that prevents the high pressure air in the main from entering the stove during the heating process. The hot blast valve is usually located a short distance away from the stove to reduce the amount of radiation it receives from the combustion gases. In several blast furnace shops, the cold mixing air that is used for controlling the temperature of the hot blast is mixed with the hot air from the stove on the stove side of the valve. This is to prevent the valve from being exposed to air at the maximum temperature obtained in the stove dome. Some blast furnaces have a central single cold blast mixer opening that is located in the hot blast main between the closest stove and the furnace itself.

The central system has the advantage of fewer thermal cyclings of the hot blast main with the higher temperature systems. Most of the hot blast valves are of the gate type or of the mushroom type and are 1.2 m to 2.0 m in diameter.

The reheating of the stove requires instrumentation in the dome area, the checker refractory and the waste gas exit area at a minimum. There is an opening in the dome of the hot blast stove through which a thermocouple or a radiation type temperature detector is usually inserted. This instrument is to control the amount of gas and air during the firing process. The temperature monitoring instruments in the dome, checkers and waste gas area are also used to protect the refractories from an overheating condition.

In the plenum chamber below the grid that supports the checkers, there are openings to the chimney and to the cold blast main. Generally, there are two chimney valves, ranging in size from 1.5 m to 2.0 m in diameter, which are opened when the stove is being heated so that the products of combustion are drawn out to the stove stack. When the stove is on blast (heating the blast air), the chimney valves are closed. The seats of the valve are arranged so that when the stove is on blast, the pressure in the stove holds the seats together to prevent leakage. When the stove is to be taken off blast and put on heating, there is a blow off valve that is opened to relieve the pressure. Because of the need to depressurize the stove rapidly, the air is to exit at a very high velocity. Consequently, the blow off valves are equipped with silencers to keep the noise level within tolerable limits.

The cold blast valve is the type that is held closed by the pressure in the cold blast main. Before this valve can be opened, the small ports in the valve disc are opened to pressurize the stove and equalize the pressure on each side of the valve.

At several modern blast furnaces, the stoves are equipped with combustion chambers completely external to the stove shell. These stoves are having external metallic burners situated near the top of the stove.

The advantage of this design is that the entire stove shell can be filled with checkers. Furthermore, the thermal pattern in the stove is much more symmetrical and there are far fewer stresses that tend to distort and rupture the brickwork. However, there have been many stress induced problems that have caused rupturing in the steelwork of the junction section between the combustion chamber and the stove. As a result, frequent repairs to the steelwork are required in this location.

Stove operation :-

These days, with the use of well prepared burdens and good control of burden distribution, the operation of blast furnaces  is much more uniform. Hence blast furnaces are normally operated very near to the maximum hot blast temperature that the stoves can maintain or that the particular burden materials can accept without causing premature melting and poor burden movement.  With higher hot-blast temperature, the blast furnace operation is more efficient because a larger percentage of the heat consumed is furnished by the sensible heat of the hot blast and less fuel is needed in the blast furnace. In the operation of the hot blast system, the ceramic checker work of the stoves is heated by the combustion of blast furnace gas sometimes supplemented by coke oven gas, and then the air from the blowers is passed through the stoves and is heated by the hot checker work. In the heating cycle, the stoves are fired until the temperature of the exit gases at the stack valves has reached an established maximum temperature of around 400 deg C to 450 deg C, while simultaneously being careful not to overheat the stove domes. During the heating cycle the temperature at the dome of the stove is controlled so that it does not exceed a maximum, which is determined primarily by the type of refractory material used for the lining of the dome. If the dome temperature reaches this maximum before the stack temperature reaches its maximum, excess air is added through the burner to hold down the flame temperature and prevent the dome from being overheated while the firing is continued until the stack gas temperature reaches its limit. However, if the dome temperature does not increase rapidly enough to reach its maximum allowable temperature by the time the stack gas temperature reaches its maximum, the blast furnace gas is usually enriched with a fuel of higher calorific value to obtain a faster heating rate.

After the stove has been heated, it is ready to be put on blast. This is done by first shutting off the gas and the air supply to the burner and then closing the burner shutoff valve and the chimney valves. The cold blast valve is then opened in such a way that the air entering the stove brings it to a pressure equal to the blast pressure without reducing the blast pressure excessively. In some of the modern blast furnace installations, the blower controls are switched from constant volume control to constant pressure control during a stove change. In such a system, the blower speeds up so that the stove can be filled and pressurized rapidly without causing a detectable decrease in the blast pressure.

After the stove is filled, the mixer valve (which controls the amount of cold air which is bypassed around the stove to be mixed with the very hot air from the stove to produce the desired hot blast temperature) is set at approximately the correct opening. The hot blast valve is then opened to put the stove on blast and, once the stove is on blast, the hot blast temperature controller automatically adjusts the mixer valve opening to maintain the desired hot blast temperature.

The hot blast stove after its use, is then taken off blast by closing first the cold blast valve and then the hot blast valve. The blow off valve is then opened to depressurize the stove and, after depressurizing,  the chimney valves are opened and the blow off valve is closed. Next, the burner shutoff valve is opened, and the air supply to the burner is turned on. Finally, the gas shutoff valve is opened to obtain the desired gas flow rate.

At modern blast furnace installations, the stove valves are motorized and the valve changing is automated so that only about three minutes are needed for a stove change. With the shorter changing time, the heating time can be increased so that higher hot blast temperatures can be used and greater efficiency can be obtained. The automatic stove changing cycle can be initiated either by having the stove tender push a button when the change is required or by a completely automatic electronic signal. This signal can be based on the extent of the mixer valve opening (as, for example, when the mixer valve is 85 % closed), on the dome temperature, or strictly on a time cycle.

Typically, blast furnaces are equipped with three hot blast stoves, and each stove is kept on blast for around one hour. Thus, the amount of heat that is extracted from the stove while it is on blast must be put back into the stove in the heating period which is simply twice the on blast time minus twice the stove changing time. At some furnaces, there are four stoves. With the extra stove, the firing rate does not have to be as great because the heating cycle is three times the on blast cycle minus twice the stove changing time. Another advantage of the extra stove is that if there is a problem with the stove equipment, the stoves can be repaired one at a time without significantly affecting the operation of the furnace. Fig 2 give a typical layout with three hot blast stoves.

             7.)  BLAST FURNACE PRODUCTS

7.1  Pig iron :-

                           Pig iron is an intermediate product of the iron industry. Pig iron has a very high carbon content, typically 3.5–4.5%. along with silica and other constituents of dross, which makes it very brittle, and not useful directly as a material except for limited applications. Pig iron is made by smelting iron ore into a transportable ingot of impure high carbon-content iron in a blast furnace as an ingredient for further processing steps. The traditional shape of the molds used for pig iron ingots was a branching structure formed in sand, with many individual ingots at right angles^[3] to a central channel or runner, resembling a litter of piglets being suckled by a sow. When the metal had cooled and hardened, the smaller ingots (the pigs) were simply broken from the runner (the sow), hence the name pig iron. As pig iron is intended for remelting, the uneven size of the ingots and the inclusion of small amounts of sand caused only insignificant problems considering the ease of casting and handling them.

Composition of pig iron :-

·         BASIC PIG IRON 3.5-4.5% carbon, <1.5% silicon, 0.5-1.0% manganese, <0.05% sulphur, <0.12% phosphorus

·         HAEMATITE PIG IRON 3.5-4.5% carbon, 1.5-3.5% silicon, 0.5-1.0% manganese, <0.05% sulphur, <0.12% phosphorus

·         NODULAR PIG IRON 3.5-4.5% carbon, <0.05% manganese, <0.02% sulphur, <0.04% phosphorus

7.2  Blast furnace gas :-

                                       

 Blast furnace gas (BFG) is a by-product of blast furnaces that is generated when the iron ore is reduced with coke to metallic iron. It has a very low heating value, about 93 BTU/cubic foot (3.5 MJ/m³, because it consists of about 60 percent nitrogen and 18-20% carbon dioxide, which are not flammable. The rest is mostly carbon monoxide, which has a fairly low heating value already and some (2-4%) hydrogen. It is commonly used as a fuel within the steel works, but it can be used in boilers and power plants equipped to burn it. It may be combined with natural gas or coke oven gas before combustion or a flame support with richer gas or oil is provided to sustain combustion. Particulate matter is removed so that it can be burned more cleanly. Blast furnace gas is sometimes flared without generating heat or electricity.

Blast furnace gas is generated at higher pressure and at about 100–150 °C (212–302 °F) in a modern blast furnace. This pressure is utilized to operate a generator (Top-gas-pressure Recovery Turbine - i.e. TRT in short), which can generate electrical energy up to 35 kWh/t of pig iron without burning any fuel. Dry type TRTs can generate more power than wet type TRT.

Auto ignition point of blast furnace gas is approximate 630–650 °C (1,166–1,202 °F) and it has LEL (Lower Explosive Limit) of 27% & UEL (Upper Explosive Limit) of 75% in an air-gas mixture at normal temperature and pressure.

The high concentration of carbon monoxide makes the gas hazardous.

Composition of blast furnace gas:-

   CO -  21-23%

    CO2 -  18-20%

    N2 -  53-55%

    H2 -  4-5%

    O2 -  0.2-0.5%

             

7.3  SLAG :-

                      Blast Furnace Slag is formed when iron ore or iron pellets, coke and a flux (either limestone or dolomite) are melted together in a blast furnace. When the metallurgical smelting process is complete, the lime in the flux has been chemically combined with the aluminates and silicates of the ore and coke ash to form a non-metallic product called blast furnace slag. During the period of cooling and hardening from its molten state, BF slag can be cooled in several ways to form any of several types of BF slag products.

COMPOSITION

Table -Typical composition of blast furnace slag.

Constituent	Percent
	1949a.		1957a.		1968a.		1985a.
	Mean	Range	Mean	Range	Mean	Range	Mean	Range
Calcium Oxide (CaO)	41	34-48	41	31-47	39	32-44	39	34-43
Silicon Dioxide (SiO2)	36	31-45	36	31-44	36	32-40	36	27-38
Aluminum Oxide (Al2O3)	13	10-17	13	8-18	12	8-20	10	7-12
Magnesium Oxide (MgO)	7	1-15	7	2-16	11	2-19	12	7-15
Iron (FeO or Fe2O3)	0.5	0.1-1.0	0.5	0.2-0.9	0.4	0.2-0.9	0.5	0.2-1.6
Manganese Oxide (MnO)	0.8	0.1-1.4	0.8	0.2-2.3	0.5	0.2-2.0	0.44	0.15-0.76
Sulfur (S)	1.5	0.9-2.3	1.6	0.7-2.3	1.4	0.6-2.3	1.4	1.0-1.9
a. Data source is the National Slag Association data: 1949 (22 sources); 1957 (29 sources); 1968 (30 sources) and 1985 (18 sources).

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