V.T REPORT SAIL

VOCATIONAL TRAINING REPORT BOKARO STEEL PLANT, STEEL AUTHORITY OF INDIA LTD. [6-JUNE-2013 TO 29-JUNE-2013]

ASHISH KUMAR JHA B-TECH, S-6 CHEMICAL ENGINEERING. INSTIUTE OF TECHNOLOGY, GURU GHASIDAS VISHWAVIDAYALAYA

VOCATIONAL TRAINING REPORT

JUNE 2013

ACKNOWLEDGEMENT

Before proceeding with the detail of the report, I thank the almighty God for making my vocational training a successful one. I would like to thank Mr. Priyaranjan Kumar and Miss Swati Chatterjee, for letting me enjoy this experience of getting trained at Bokaro Steel Plant, Steel Authority of India Ltd. It has helped me fully and has grown up my knowledge. Besides, I would like to thank the officials of Bokaro Steel Plant, Steel Authority of India Ltd for providing me a good environment and facilities to complete my training. I am also thankful to technicians and field operators and other staff of training department who spared their valuable time and took effort explaining the working of various units of the plant. I am greatly thankful to for their co-operation. I was thoroughly guided by them throughout my training .The information provided to me by them have helped me a lot and would also help me in my long run too .The tremendous effort put by them have motivated me and made me gain confidence in completing this report.

Ashish Kumar Jha B-TECH S6, CHEM. ENGG. I.T. GGV

Page 1 of 31

Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

CONTENTS TITLE 1) Introduction a) Steel Authority Of India Ltd. b) Bokaro Steel Plant 2) Gas Utilities a) Air Separation Unit b) Propane Gas Plant c) Protective Gas Plant 3) By Product Plant a) Composition of coke oven gas b) Process diagram c) Primary cooler d) Tar distillation plant e) Exhauster f) Final cooler g) Ammonia removal h) Benzol recovery plant 4) Energy Management Department a) Blast furnace gas b) Coke oven gas c) Converter gas d) Mix gas e) Gas analysis f) Gas mixing and boosting station 5) Research And Control Laboratory a) Spectrometer 6) Conclusion 7) Reference

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PAGE NO. 03 03 05 06 06 08 10 11 12 13 13 14 14 14 15 17 20 20 20 20 21 21 24 26 27 30 31

Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

Chapter 1 INTRODUCTION STEEL AUTHORITY OF INDIA LIMITED Steel Authority of India Limited (SAIL) is the leading steel-making company in India. It is a fully integrated iron and steel maker, producing both basic and special steels for domestic construction, engineering, power, railway, automotive and defense industries and for sale in export markets. SAIL is also among the seven Maharatna of the country's Central Public Sector Enterprises. SAIL manufactures and sells a broad range of steel products, including hot and cold rolled sheets and coils, galvanized sheets, electrical sheets, structural, railway products, plates, bars and rods, stainless steel and other alloy steels. SAIL produces iron and steel at five integrated plants and three special steel plants, located principally in the eastern and central regions of India and situated close to domestic sources of raw materials, including the Company's iron ore, limestone and dolomite mines. The company has the distinction of being India’s second largest producer of iron ore and of having the country’s second largest mines network. This gives SAIL a competitive edge in terms of captive availability of iron ore, limestone, and dolomite which are inputs for steel making. SAIL's International Trade Division ( ITD), in New Delhi- an ISO 9001:2000 accredited unit of CMO, undertakes exports of Mild Steel products and Pig Iron from SAIL’s five integrated steel plants. With technical and managerial expertise and know-how in steel making gained over four decades, SAIL's Consultancy Division (SAILCON) at New Delhi offers services and consultancy to clients world-wide. SAIL has a well-equipped Research and Development Centre for Iron and Steel (RDCIS) at Ranchi which helps to produce quality steel and develop new technologies for the steel industry. Besides, SAIL has its own in-house Centre for Engineering and Technology (CET), Management Training Institute (MTI) and Safety Organization at Ranchi. Our captive mines are under the control of the Raw Materials Division in Kolkata. The Environment Management Division and Growth Division of SAIL operate from their headquarters in Kolkata. Almost all our plants and major units are ISO Certified.

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Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

Major Units Integrated Steel Plants: • • • • •

Bhilai Steel Plant (BSP) in Chhattisgarh Durgapur Steel Plant (DSP) in West Bengal Rourkela Steel Plant (RSP) in Orissa Bokaro Steel Plant (BSL) in Jharkhand IISCO Steel Plant (ISP) in West Bengal

Special Steel Plants: • • •

Alloy Steels Plants (ASP) in West Bengal Salem Steel Plant (SSP) in Tamil Nadu Visvesvaraya Iron and Steel Plant (VISL) in Karnataka

Ferro Alloy Plant: •

Chandrapur Ferro Alloy Plant

Subsidiary: •

SAIL Refractory Company Limited

Joint Ventures: •

NTPC SAIL Power Company Pvt. Limited (NSPCL): A 50:50 joint venture between Steel Authority of India Ltd (SAIL) and National Thermal Power Corporation Ltd (NTPC Ltd); manages SAIL’s captive power plants at Rourkela, Durgapur and Bhilai with a combined capacity of 814 megawatts (MW).

•

Bokaro Power Supply Company Pvt. Limited (BPSCL): This 50:50 joint venture between SAIL and the Damodar Valley Corporation (DVC) is managing the 302-MW power generating station and 660 tonnes per hour steam generation facilities at Bokaro Steel Plant.

Ownership and Management: The Government of India owns about 80% of SAIL's equity and retains voting control of the Company. However, SAIL, by virtue of its ‘Maharatna’ status, enjoys significant operational and financial autonomy.

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Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

BOKARO STEEL PLANT

Bokaro Steel Plant (BSL), the fourth integrated plant in the Public Sector, started taking shape in 1965 in collaboration with the Soviet Union. It was originally incorporated as a limited company on 29th January 1964, and was later merged with SAIL, first as a subsidiary and then as a unit, through the Public Sector Iron & Steel Companies (Restructuring & Miscellaneous Provisions) Act 1978. The construction work started on 6th April 1968. The Plant is hailed as the country’s first Swadeshi steel plant, built with maximum indigenous content in terms of equipment, material and know-how. Its first Blast Furnace started on 2nd October 1972 and the first phase of 1.7 MT ingots steel was completed on 26th February 1978 with the commissioning of the third Blast Furnace. All units of 4 MT stage have already been commissioned and the 90s' modernization has further upgraded this to 4.5 MT of liquid steel. A new hydraulic coiler has been added and two of the existing ones revamped. With the completion of Hot Strip Mill modernization, Bokaro is producing top quality hot rolled products that are well accepted in the global market. Bokaro is designed to produce flat products like Hot Rolled Coils, Hot Rolled Plates, Hot Rolled Sheets, Cold Rolled Coils, Cold Rolled Sheets, Tin Mill Black Plates (TMBP) and Galvanized Plain and Corrugated (GP/GC) Sheets. Bokaro has provided a strong raw material base for a variety of modern engineering industries including automobile, pipe and tube, LPG cylinder, barrel and drum producing industries. Bokaro Steel is working towards becoming a one-stop-shop for world-class flat steel in India. The modernization plans are aimed at increasing the liquid steel production capacity, coupled with fresh rolling and coating facilities. The new facilities will be capable of producing the most premium grades required by the most discerning customer segments.

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Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

Chapter 2 Gas Utilities Gas utilities unit deals with the handling and distribution of industrial gases like oxygen, nitrogen, propane etc. needed by various other unit of the steel plant. Major plants under this unit are discussed as follow.

Air Separation Unit: An air separation unit separates atmospheric air into its primary components, typically nitrogen and oxygen, and sometimes also argon and other rare inert gases. The most common method for air separation in industries is fractional distillation (cryogenic method). Pure gases can be separated from air by first cooling it until it liquefies, then selectively distilling the components at their various boiling temperatures. The process can produce high purity gases but is energy-intensive. To achieve the low distillation temperatures an air separation unit requires a refrigeration cycle that operates by means of the Joule–Thomson effect, and the cold equipment has to be kept within an insulated enclosure (commonly called a "cold box"). The cooling of the gases requires a large amount of energy to make this refrigeration cycle work and is delivered by an air compressor. Modern ASUs use expansion turbines for cooling; the output of the expander helps drive the air compressor, for improved efficiency. The process consists of the following main steps: 1. Before compression the air is pre-filtered of dust. 2. Air is compressed where the final delivery pressure is determined by recoveries and the fluid state (gas or liquid) of the products. During compression water is condensed out in inter-stage coolers. 3. The process air is generally passed through a molecular sieve bed, which removes any remaining water vapor, as well as carbon dioxide, which would freeze and plug the cryogenic equipment. Molecular sieves are often designed to remove any gaseous hydrocarbons from the air, since these can be a problem in the subsequent air distillation that could lead to explosions. The molecular sieves bed must be regenerated. This is done by installing multiple units operating in alternating mode and using the waste nitrogen gas to desorb the water. 4. Process air is passed through an integrated plate fin heat exchanger and cooled against product and waste streams. Part of the air liquefies to form a liquid that is enriched in oxygen. The remaining gas is richer in nitrogen and is distilled to almost pure nitrogen (typically < 1ppm) in a high pressure distillation column. The condenser of this column

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Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

requires refrigeration which is obtained from expanding the more oxygen rich stream further across a valve or through an Expander. 5. Alternatively the condenser may be cooled by interchanging heat with a re-boiler in a low pressure distillation column (operating at 1.2-1.3 bar abs.) when the ASU is producing pure oxygen. Typical oxygen purities range in from 97.5% to 99.5% and influence the maximum recovery of oxygen. The refrigeration required for producing liquid products is obtained using the Joule-Thompson effect in an expander which feeds compressed air directly to the low pressure column. Hence, a certain part of the air is not to be separated and must leave the low pressure column as a waste stream from its upper section. 6. Finally the products produced in gas form are warmed against the incoming air to ambient temperatures which requires a carefully crafted heat integration.

GAS

BOILING POINT

OXYGEN ARGON NITROGEN

-183.5oC -185.6oC -195.6oC

Process Flow Sheet:

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Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

There are 5 units at Bokaro Steel Plant in which ASU 1 & 2 are shut down and other three are working. ASU 3 & 4 is similar but ASU 5 is different. The difference being as follow:

ASU 3&4 •

•

•

Moisture and carbon dioxide is removed in heat exchanger by cooling process air to -130oC to -140oC at which CO2 liquefies. Heat exchanger is used in pairs having interchangeable waste nitrogen and process air streams. Cryogenic temperature is achieved by cooling process air through heat exchanger, waste nitrogen being cooling media.

ASU 5 •

•

•

Moisture and carbon dioxide is removed by passing process air through drier consisting of alternate beds of molecular sieve and activated alumina. Simple arrangement of heat exchanger is used with fix waste nitrogen and process air streams Cryogenic temperature is achieved by passing process air through TurboExpanders working on principal of Joule-Thompson effect.

The main consumer of ASU is Steel Melting Shop (SMS). Gaseous oxygen is used in SMS to remove impurities present in melted iron by oxidation as various oxides. It is also used to burn fuel gases to provide heat for melting of steel at 1300oC. Gaseous nitrogen is used as substrate for addition of different material to melted steel as it is inert while cooling it as it is very cold.

Propane Gas Plant: Liquid propane is brought and stored at propane gas plant in an underground tank having capacity of 5 ton. It is kept in an underground tank to prevent direct contact with atmosphere due to leakage and spillages. Then the liquid propane is converted to gaseous form in a vaporizer at about 75oC to 80oC. Then the propane gas is send to other units as per requirement. Its main consumers are Slabbing Mill, Continuous Casting Shop, and Hot Strip Mill. Propane gas is generally used for cutting of slabs, slab scarfing, reheating. Sometime propane gas is mixed with coke oven gas to increase its calorific value. Special care should be taken to avoid exposure to this gas. Inhalation of propane causes shortness of breath, headache, drowsiness, and unconsciousness. Direct contact with skin causes frostbite, pain, redness, and blisters. Eye contamination causes impaired vision. The fire extinguishing media used to stop flow of gas are foam, carbon dioxide, dry chemical powder. Following steps should be taken in case of leakage and spills: •

Shut off leaks without risks.

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Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

•

JUNE 2013

Warn everybody that air mixture is explosive.

In the following table a comparison between propane and acetylene shows the use of propane instead of other industrial gases: Properties State Appearance Odor Specific Gravity Flash Point Auto Ignition Temperature Calorific Value (Kcal/Kg) Calorific Value (Kcal/M3) Expansion Rate Maximum Allowable Pressure Burn Velocity (M/Sec) Back Fire Tendency Toxicity Explosion Sensitivity To Impact Explosion Sensitivity To Electricity Explosive Limit (%Air Volume) Explosive Limit(% Oxygen) Effect On Environment Ozone Depletion And Global Warming Cost Of Energy (Rs/Gcal) Tangible Save Annually Intangible Benefits

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Acetylene(C2H2) Gaseous Colorless Garlic like 0.906 -32oC 305oC 11573 13508 ---15 PSIG

Propane(C3H8) Liquid and gas Colorless Odorless 1.573 -156oC 470oC 10985 22250 250 Cylinder rating

6.9 High High High

3.7 Low Low Stable

High

Explodes

2.4-52.3

2.4-9.5

3.0-93 Generates solid waste Yes

2.4-57 Environment friendly Less as compared to acetylene

18500 -------

5500 < 60 million Faster preheat, higher penetration, faster & smoother metal works.

Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

Protective Gas Plant Protective gas is a mixture containing 95% nitrogen gas and 25% hydrogen gas. It is used to provide an inert atmosphere by forming a layer over steel surface during the annealing of steel sheets. This inert layer protect the steel sheets from direct contact with the atmosphere, which may cause corrosion of steel sheets by oxidation. So the only consumer of protective gas plant is Hot Rolling Mill. Protective gas is prepared by cracking of ammonia gas as follow: 2 NH3

800 C

�⎯⎯�

N2

+

3 H2

Liquid ammonia is sent to cracking unit where it is heated to 800oC. Coke oven gas mixed with air is used as fuel for heating. At temperature above 800oC ammonia is decomposed into 3 parts hydrogen and 1 part nitrogen. Now the product stream is sent to a mixer where more nitrogen gas from ASU is mixed to achieve the required composition of 95% nitrogen and 25% hydrogen. Then the gas is dried and filtered and sent to annealing process. A model of protective gas generator is shown in the picture below.

Page 10 of 31

Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

Chapter 3 By Product Plant The coke oven by-product plant is an integral part of the by-product coke making process. In the process of converting coal into coke using the by-product coke oven, the volatile matter in the coal is vaporized and driven off. This volatile matter leaves the coke oven chambers as hot, raw coke oven gas. After leaving the coke oven chambers, the raw coke oven gas is cooled which results in a liquid condensate stream and a gas stream. The functions of the byproduct plant are to take these two streams from the coke ovens, to process them to recover byproduct coal chemicals and to condition the gas so that it can be used as a fuel gas. Historically, the by-product chemicals were of high value in agriculture and in the chemical industry, and the profits made from their sale were often of greater importance than the coke produced. Nowadays however most of these same products can be more economically manufactured using other technologies such as those of the oil industry. Therefore, with some exceptions depending on local economics, the main emphasis of a modern coke by-product plant is to treat the coke oven gas sufficiently so that it can be used as a clean, environmentally friendly fuel. In a by-product coke oven the evolved coke oven gas leaves the coke oven chambers at high temperatures approaching 2000oF. This hot gas is immediately quenched by direct contact with a spray of aqueous liquor (flushing liquor). The resulting cooled gas is water saturated and has a temperature of 176oF. This gas is collected in the coke oven battery gas collecting main. From the gas collecting main the raw coke oven gas flows into the suction main. The amount of flushing liquor sprayed into the hot gas leaving the oven chambers is far more than is required for cooling, and the remaining non-vaporized flushing liquor provides a liquid stream in the gas collecting main that serves to flush away condensed tar and other compounds. This stream of flushing liquor flows under gravity into the suction main along with the raw coke oven gas. The raw coke oven gas and the flushing liquor are separated using a drain pot (the downcomer) in the suction main. The flushing liquor and the raw coke oven gas then flow separately to the by-product plant for treatment. In order to make raw coke oven gas suitable for use as a fuel gas at the coke oven battery and elsewhere in the steelmaking facility the by-product plant must: • • •

Cool the coke oven gas to condense out water vapor and contaminants Remove tar aerosols to prevent gas line/equipment fouling Remove ammonia to prevent gas line corrosion

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Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

• • •

JUNE 2013

Remove naphthalene to prevent gas line fouling by condensation Remove light oil for recovery and sale of benzene, toluene and xylene Remove hydrogen sulfide to meet local emissions regulations governing the combustion of coke oven gas.

In addition to treating the coke oven gas, the by-product plant must also condition the flushing liquor that is returned to the coke oven battery, and treat the waste water that is generated by the coke making process.

Composition of coke oven gas: Raw coke oven gas coming from the coke oven battery has the following typical composition: Dry basis

Actual composition(water saturated at 176°F)

-

47%

Hydrogen

55%

29%

Methane

25%

13%

Nitrogen

10%

5%

Carbon Monoxide

6%

3%

Carbon Dioxide

3%

2%

Hydrocarbons (ethane, propane )

2%

1%

Water vapor

Raw coke oven gas also contains various contaminants, which give coke oven gas its unique characteristics. These consist of: • • • • • •

Tar vapors Light oil vapors (aromatics), consisting mainly of benzene, toluene and xylene (BTX) Naphthalene vapor Ammonia gas Hydrogen sulfide gas Hydrogen cyanide gas

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Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

Process diagram of By Product Plant:

The gas treatment process can be described by describing the different components of the byproduct plant successively as follow:

Primary cooler: The first step in the treatment of raw coke oven gas is to cool it to remove water vapor and so greatly reduce its volume. This is done in the Primary Cooler. There are two basic types, the spray type cooler and the horizontal tube type. In a spray type cooler the coke oven gas is cooled by direct contact with a recirculated water spray, with the contact cooling water being itself cooled externally in heat exchangers. In the tubular type, the coke oven gas is cooled indirectly by flowing across horizontally mounted tubes through which cooling water is pumped. In this case, the cooling water does not come into contact with the coke oven gas and so it can be cooled in a cooling tower for example.

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Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

As the coke oven gas is cooled, water, tar and naphthalene condense out. The condensate collects in the primary cooler system and is discharged to the tar & liquor plant.

Tar Distillation Plant: As the raw coke oven gas is cooled, tar vapor condenses and forms aerosols which are carried along with the gas flow. These tar particles would contaminate and foul downstream processes and would foul gas lines and burner nozzles if allowed to continue in the gas stream. The tar precipitators typically use high voltage electrodes to charge the tar particles and then collect them from the gas by means of electrostatic attraction. The tar precipitators can be installed before, or after the exhauster. By distillation, the coal tar is separated into a number of fractions graded arbitrarily by temperatures in the tar still. These fractions are usually called light oil, carbolic oil, middle oil, creosote oil, heavy oil, anthracene oil, soft pitch and hard pitch. This group of products covers the usual range of results obtained under the first distillation of crude tar. Naphthalene is removed from coke oven gas using wash oil in a gas scrubbing vessel. The vessel may be packed or it may be the "void" type in which the wash oil is sprayed into the gas in several stages. The wash oil is regenerated by stripping out the naphthalene from the wash oil using steam in a still. In many plants, naphthalene removal is integrated with the similar process of light oil removal. The naphthalene is typically recovered as a heavier oil stream that is then often mixed with the tar that is produced in the by-product plant.

Exhauster: The exhauster is a large blower that provides the motive force to induce the coke oven gas to flow from the coke oven battery and through the by-product plant. The exhauster is of prime importance to the operation of the coke oven battery. It allows the close control of the gas pressure in the collecting main, which in turn affects the degree of emissions, for example door emissions, from the battery. A failure of the exhauster will immediately result in venting to atmosphere, through the battery flares, of all of the raw coke oven gas produced.

Final Cooler: The duty of the final cooler is to remove the heat of compression from the coke oven gas that it gained on flowing through the exhauster. This is necessary because the efficiency of many of the by-product plant processes, including the water wash ammonia removal process, is greatly

Page 14 of 31

Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

improved at lower temperature. The final cooler is therefore placed upstream of water wash ammonia scrubbers if these are installed. Final coolers typically cool the coke oven gas by direct contact with a cooling medium, either water or wash oil. An important aspect of final cooler operation is that when the coke oven gas is cooled below the outlet temperature of the primary cooler, naphthalene will condense from the gas. This naphthalene readily crystallizes out from the cooling medium and will foul equipment if not disposed of. In wash oil final coolers the naphthalene dissolves in the wash oil and a side stream of oil is steam stripped to remove the naphthalene. If water is used to cool the coke oven gas, the condensed naphthalene must be absorbed using tar. The tar is either entrained in the cooling water, with a portion of the flow being continuously blown down for treatment, or it takes the form of a tar layer through which the cooling water flows. The tar is continuously exchanged with fresh tar from the tar and liquor plant to dispose of the absorbed naphthalene.

Ammonia removal: Because of the corrosive nature of ammonia, its removal is a priority in coke oven byproduct plants. Historically the removal of ammonia from coke oven gas has yielded one of the more profitable by-products that of ammonium sulfate. The ammonium sulfate process can take various forms but all basically involve contacting the coke oven gas with a solution of sulfuric acid. Variations include the use of an absorber, in which the sulfuric acid solution is sprayed into the gas, or the use of a saturator in which the gas is bubbled through a bath of sulfuric acid solution. The sulfuric acid reacts readily with the ammonia in the coke oven gas to form ammonium sulfate. This is then crystallized out, removed from the solution and dried for sale typically as a fertilizer. Nowadays the cost to produce ammonium sulfate often outweighs the revenue from the product however there are still very many coke plants around the world producing ammonium sulfate. The sulfuric acid used in the ammonia removal process is produced at sulfuric acid plant by Contact Process. The sulfuric acid is also used for washing of steel sheets. The process of manufacturing sulfuric acid can be described as three basic reaction steps. Step 1 - Production of sulfur dioxide. This reaction is described by the equation: S

+

O2

→

SO2

Elemental sulfur is purchased on the international market having been recovered as a byproduct of the oil refining process. This sulfur is melted by steam coils at l40°C in brick lined tanks. The molten sulfur is filtered to remove any impurities (usually iron or organic

Page 15 of 31

Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

compounds). Lime is added to reduce the acidity of the molten sulfur therefore reducing its corrosivity. The molten sulfur is pumped to the burner where it is burnt in an excess of dry air. The gas exiting the burner is maintained at 8 - 9%v/v sulfur dioxide and approximately 830°C due to the heat produced by the exothermic reaction. The sulfur dioxide/air gas mixture is then passed through the hot gas filter, where any ash contamination is removed.

Step 2 - Conversion to sulfur trioxide: The sulfur dioxide is converted to sulfur trioxide by reacting with oxygen over a catalyst. This reaction is described by the equation: SO2

+

1� O 2 2

→

SO3

This reaction occurs in the converter, a four-stage reaction vessel with each stage consisting of a solid catalyst bed through which the gas is passed. The catalyst used is vanadium pentaoxide (V205) and potassium sulphate dispersed on a silica base which forms a porous support, giving a large surface area for reaction. It is believed that the V2O5 increases the rate of the overall chemical reaction by oxidizing the S02 to S03 and being re-oxidized itself by the oxygen in the gas stream. This reaction is exothermic and its equilibrium constant decreases with increasing temperature (Le Chatelier's Principle). The reaction rate is temperature dependent, so that if the temperature becomes too low the equilibrium point will not be reached. In practice, the gas temperature must be maintained between 400 - 500°C to maintain a high reaction rate and also high conversion equilibrium. As the reaction is exothermic, heat is generated across each of the catalyst beds. This heat must be removed between each stage to maintain the optimum reaction temperature into the following stage. The greatest degree of cooling is required between the first and second stages. Cooling after the second and third stages is by injection of dried air. The gas exiting the converter is used to pre-heat the boiler feed water. Step 3 - Absorption of SO3 to form sulfuric acid: The gas is passed to the absorption tower, a packed tower where S03 is absorbed into a counter-current flow of 98 - 99% sulfuric acid. The overall reaction can be described by the following equation, where sulfur trioxide reacts with the free water to produce sulfuric acid: SO3

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+

H2O

→

H2SO4

Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

The circulating sulfuric acid must be maintained at about 98% concentration and 70°C to maximize the absorption efficiency. The acid strength is important because the vapor pressure of sulfur trioxide above sulfuric acid is at a minimum at an acid strength of 98%. At higher concentrations the increased vapor pressure is caused by S03 and at lower concentrations the water vapor pressure increases sharply and the resultant acid mist is not readily re-absorbed and escapes to the atmosphere. The sulfuric acid is circulated at such a rate that there is only a very small increase in concentration through the absorber tower. Dilution water is added to the circulating acid tank and also as atmospheric water absorbed in the drying tower. A stream of sulfuric acid is continuously bled off and cooled through a plate heat exchanger before being passed into the storage tanks. The overall conversion from sulfur to sulfuric acid is greater than 98.5%. The plant operates under an air discharge permit which controls emissions of sulfur dioxide and total acidity. Traditionally mild steel has been used as the primary material of construction for process equipment containing 98% sulfuric acid. The corrosion rate is reasonably low, except at the air/liquid interface where atmospheric moisture encourages corrosion. The trend is now towards more sophisticated materials including Teflon lined steel pipework to reduce iron contamination of the sulfuric acid.

Benzol Recovery Plant: The raw gases issuing from the coke oven contain benzol and its homo­logues as vapors and the recovery of these valuable substances consti­tutes another important source of profit in by-product coke making. Benzols are of supreme importance as raw materials for the manufacture of explosives, and the demand for them was such that they commanded very high prices. It must be remembered, however, that the recovery of benzol from coke oven gas is a profitable undertaking even under normal conditions. Besides their use in the manufacture of explosives which are by no means confined to military purposes, the vast development of the Indian chemical industry insures a market for large quantities of benzol products. Benzene, itself, is an indispensable source of nitro-benzene and aniline with its long series of valuable derivatives used for dyes, drugs, photographic chemicals, etc. The by-product apparatus, which cools the gas and extracts from it tar and ammonia, does not extract, in any important degree, the light oil vapors or so-called benzols. Owing to their vapor tension and the relatively large volume of the coke oven gas these substances pass on as vapors in the gas, of which they make up about one per cent by volume, and are extracted only when the gas is washed with “wash oil" having absorbent or solvent proper­ ties with respect to

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Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

these vapors. Of the total light oil recovered from the carbonization of the coal, 95 per cent results from washing the gas, and only 5 per cent is found in the tar. Previous to treatment with the absorbing oil, the gas should be cooled to a suitable temperature which may vary from plant to plant and depends principally upon such factors as its moisture and naphthalene content, temperature of the wash oil, and percent enrichment desired. This cooling is accomplished by means of the cooler which is preferably of the direct contact type. The water not only acts as a cooling medium, but mechanically washes a large portion of the naphthalene out of the gas and carries it into the separating sump. The cool gas then passes into the benzol washers. These are tall steel towers of the hurdle type, affecting a very intimate and prolonged contact between gas and oil. The debenzolized gas passes out of the last washer through the return main to its point of consumption. The fresh wash oil is pumped from the circulating tank over the hurdles in an opposite direction to the flow of the gas, maintaining the counter current principle that is essential in nearly all washing operations and bringing the fresh washing medium into the system at a point where the gas contains the least light oil vapors. The distribution, of the wash oil over the tops of the washers is a very important matter, and should be made as uniformly as possible. The enriched wash oil accumulates in tanks usually located underneath the washers, and is pumped from them to the benzol recovery plant to be heated for the purpose of releasing its benzol constituents. The light oil produced by the distillation of the wash oil is a mixture of benzol, toluol, xylol and solvent naphtha with a small amount of other substances. Common commercial practice is to include the xylol with the solvent naphtha. The light oil is accumulated in a drain tank and portions are taken for distillation in still. This still is usually known as the crude still, the first distillation of the wash oil being made for the purpose of effecting an approximate separation of several fractions of different boiling points preliminary to washing and final rectification. This and subsequent distillations are made intermittently in stills of large capacity which give better fractionations than are possible in smaller apparatus. The heating is accomplished by means of internal steam coils and a direct steam spray. The benzol and toluol are principally distilled off by indirect heat, using the steam coils, and the higher boiling constituents, xylol, solvent naphtha, etc., are then distilled over by introducing steam directly into the still. After the benzol, toluol, xylol and solvent naphtha have distilled off, a certain amount of wash oil containing naphthalene remains in the still tank. The presence of wash oil in the light oil is due not only to mechanical trapping of the heavy oil during the distillation, but also to the actual distillation of some of its original constituents by agency of the direct steam used. The method of distilling the light oil depends upon the final products that are desired, and the number of distilled fractions may be from one to three or four. In distilling to make motor fuel, one distilled fraction is frequently all that is necessary.

Page 18 of 31

Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

The wash oil remaining in the still is drained into cooling pan, where it is cooled in the air to crystallize out the naphthalene. The wash oil is drained away from the latter into tank and then is returned to the main circulating tank. In large plants where the amount of naphthalene is great, a centrifugal dryer is employed for the purpose of separating the small amount of oil remaining in the naphthalene, and also for reclaiming the naphthalene from the separating sump at the foot of the gas cooler. The crude naphthalene so obtained can be sold as such, or m a y be put into the tar in the coke plant. The products obtained from the crude still will satisfy many commercial purposes in normal times. However, the present demands of chemical manufacturers for benzol and toluol of a high degree of purity have made it advisable to accomplish the complete process of purification at the coke plant to serve this important part of the trade. For this purification the crude benzols are first washed with sulfuric acid and then with caustic soda and water. This operation is accomplished in agitator, which is a large lead-lined vessel with an efficient mechanical mixing device for bringing the acid and benzol into intimate contact. The washed benzol is delivered from the agitator to the still which is provided with a very efficient dephlegmator. The operation of this "pure still" depends upon whether it is desired to make pure benzol and toluol or motor fuel. The distillation of motor fuel is very simple and rapid while the production of pure products must be conducted with more care, laboratory tests being made frequently to check the quality of distillates. In conclusion the By Product Plant produce is summarized in the following table, with the use of products or waste so produced in different part of plant.

Stream

Destination

Coke Oven Gas

Used as fuel gas at the coke oven battery and steel works

Flushing Liquor

Recirculated back to the coke oven battery

Waste Water

Discharged to treatment plant

Tar

Sold as product

Ammonia/Ammonium Sulfate

Sold as product

Light Oil (if recovered)

Sold as product

Sulfur/Sulfuric Acid (if gas is Sold as product desulfurized)

Page 19 of 31

Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

Chapter 4 Energy Management Department High levels of power requirement and rising energy costs represent a major challenge for the steel industry. Gases created as a ‘free’ by-product during steel production processes serve as an attractive energy source option for efficient power generation. In addition to the economic benefit, using these gases as engine fuel reduced industrial CO2 emissions and saves natural energy .The Energy Management Department deals with all the energy need of the plant. The major source of energy in a steel plant is burning of different gases. The main gases used as fuel are as follow:

Blast Furnace Gas: Blast furnace gas is a by-product of blast furnaces where iron ore is reduced with coke into metallic (pig) iron. The gas has a very low heating value of around 900Kcal/Nm3, which on its own is typically not high enough for combustion in a gas engine. The calorific value is low due to lower percentage of combustible components (hydrogen and carbon monoxide). There is the possibility to blend this gas with other off gases to achieve a given calorific value. The main consumers of this gas are blast furnace, coke oven, and power plant.

Coke Oven Gas: Coke gas is a by-product of industrial coke production from pit coal; coke gas is created by high-temperature dry distillation of coking coals in the absence of oxygen. The gas mainly consists of hydrogen (50-60%), methane (15-50%) and a small percentage of carbon monoxide, carbon and nitrogen. With a calorific value of 4000Kcal/Nm3, coke gas constitutes a high-value fuel for effective power generation. Its calorific value is high due to higher percentage of combustible components (hydrogen, carbon monoxide, methane, and unsaturated hydrocarbons). Due to its high calorific value, maximum units of the plant are its consumer.

Converter Gas: Converter gas is created from pig iron during the steel production process. Steel-making technology can be categorized into two different processes: blow molding or open hearth. Within the blow molding process, the pig iron is refined with oxygen or air, lowering the carbon proportion and providing enough process heat to maintain the steel liquid. On the other hand, the open hearth process extracts the oxygen of the added scrap and ore, requiring additional heat supply for the steel-making process. One of the most common open hearth processes is the electrical melting process. Converter gas from the molding and electrical melting processes can

Page 20 of 31

Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

be used as fuel. The gas consists of about 65% carbon monoxide, 15% carbon dioxide, 15% nitrogen and small amounts of hydrogen and methane. Due to high carbon monoxide content, it is very poisonous gas. There is no proper storage and handling facility for converter gas at Bokaro Steel Plant. So the converter gas is burnt in situ and then the combustion product which is not poisonous is discharged into atmosphere. This type of burning is called Bleeding of gas.

Mix Gas: Most of the time the calorific value of the gas required as fuel in different units of plant falls is such that neither pure coke oven gas nor pure blast gas can be used. Then a mixture of these two gases in a varying proportion called Mix gas is used as a fuel. The major consumers of mix gas are Coke Oven Battery, Sintering Plant, Hot Strip Mill, Slabbing Mill etc. Following is a tabulated comparison between coke oven gas and blast furnace gas, showing percentage composition of different components present: Components [calorific value ] Carbon dioxide(CO2) [noncombustible] Oxygen(O2) [noncombustible] Hydrogen(H2) [2590Kcal/Nm3] Carbon monoxide(CO) [3040 Kcal/Nm3] Nitrogen(N2) [noncombustible] Unsaturated hydrocarbons(CmHn) [17000 Kcal/Nm3] Methane(CH4) [8560 Kcal/Nm3]

Blast Furnace Gas

Coke Oven Gas

17.0%

3.00%

0.20%

1.00%

4.20%

56.6%

24.5%

5.20%

54.1%

9.00%

----

2.40%

----

22.8%

The above detail is dated 17-JUNE-2013 and obtained after analysis of a sample of above two gases.

Gas Analysis: These fuel gases should be analyzed before distribution to other units of plant. The gas analysis provides information regarding gases percentage composition and their respective calorific value. Following are the three types of gas analysis based on purpose or point of operation:

Page 21 of 31

Ashish Kumar Jha, B.Tech, CHEM, IT.GGV

VOCATIONAL TRAINING REPORT

JUNE 2013

1. Surrounding air analysisThe first and must analysis is of atmospheric air in the area surrounding the working site. This analysis is done to know the percentage of oxygen present in working atmosphere. If the analyzed percentage is less than that of normal atmospheric air composition (20.6%), then the working atmosphere is not suitable for humans. 2. Fuel gas analysisThis is the analysis of gas mixture to be used as fuel. This analysis is done to know the percentage of oxygen present in the fuel gas. Higher percentage of oxygen means lower percentage of other combustible gases. So this analysis puts a check on the oxygen content of fuel gas. 3. Flue gas analysisThis is the analysis of gases leaving after combustion. This analysis is also done to know the percentage composition of oxygen in the flue gases. Higher percentage (>5%) indicates excess air supply while a lower percentage (