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Training Report

  1. 1. 2013 Training Report on Mejia Thermal Power Station Pintu Khan Asansol Engineering College 1/22/2013
  2. 2. Copyright Notice Copyright © 2012 by AEC All rights reserved. No part of this publication may be reproduced, distributed, or transmitted in any form or by any means, including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the publisher, except in the case of brief quotations embodied in critical reviews and certain other non-commercial uses permitted by copyright law. Page | 1
  3. 3. Preface This Project Report has been prepared in fulfilment of Industrial Training to be carried out in third year of our four year B.TECH course. For preparing the Project Report, we have visited Mejia Thermal Power Station under Damodar Valley Corporation during the suggested duration for the period of 21 days, to avail the necessary information. The blend of learning and knowledge acquired during our practical studies at the company is presented in this Project Report. The rationale behind visiting the power plant and preparing the Project Report is to study the mechanical overview, electrical overview, various cycles and processes (viz. Steam Generation, Turbo Generation and Balance of Plant) of power generation and details of control and instrumentation required in thermal power plant. We have carried out this training under well experienced and highly qualified engineers of MTPS, DVC of various departments’ viz. Mechanical, Electrical, Chemical and Control & Instrumentation depts. We have taken the opportunity to explore the Electrical Department, its use, necessity in power plant and maintenance of various instruments used for monitoring and controlling the numerous processes of power generation. We have tried our best to cover all the aspects of the power plant and their brief detailing in this project report. All the above mentioned topics will be presented in the following pages of this report. The main aim to carry out this training is to familiarize ourselves with the real industrial scenario, so that we can relate with our engineering studies. Page | 2
  4. 4. Acknowledgement I take this opportunity to express my profound gratitude and deep regards to Mr. P.K. Dubey for his exemplary guidance, monitoring and constant encouragement throughout the course of this thesis. The blessing, help and guidance given by him time to time shall carry me a long way in the journey of life on which I am about to embark. I also take this opportunity to express a deep sense of gratitude to Mejia Thermal Power Station, DVC, for their cordial support, valuable information and guidance, which helped me in completing this task through various stages. I am also thankful to the Director (HRD), the Chief Engineer and Project Head, Mr. G. Nandesu (Asstt. Manager HR) for providing me opportunity to carry out my vocational training in MTPS. I am obliged to staff members of Mejia Thermal Power Station, DVC for the valuable information provided by them in their respective fields. I am grateful for their cooperation during the period of my assignment. Lastly, I thank almighty, my parents, brother, sisters and friends for their constant encouragement without which this assignment would not be possible. Signature of the Trainee Page | 3
  5. 5. Table of Contents Page No.  Introduction 5  Damodar Valley Corporation 5  Basic needs and overview of a power plant 7  Mejia Thermal power station 9  MTPS Unit Overview 11  Coal Handling Plant 12  Coal Mill 15  Furnace and Boiler 17  Steam Turbine 20  Introduction to Water Treatment 23  Pre Treatment of Water 24  DM Plant Treatment 25  Waste Water Treatment 26  Steam/Water Circuit of MTPS 27  Components of Steam/Water Cycle 29  Cooling Towers 32  Air and Flue Gas Path 33  Electrostatic Precipitators 35  Ash Handling Plant 40  Electrical System Overview 43  Generator 43  Excitation System 44  Transformers 45  Control and Instrumentation 50  Automatic Voltage Regulator 53  AC and DC Power Flow in MTPS 55  Switchyard 56  Frequency Control 60  Voltage Control 61  National Grid 62  Central Load Dispatch 63  DVC: Transmission and Distribution Network 65  Conclusion 67  Bibliography 68 Page | 4
  6. 6. INTRODUCTION Electricity generation is the process of generating electric power from sources of energy. Electricity is most often generated at a power station by electromechanical generators, primarily driven by heat engines fuelled by chemical combustion or nuclear fission but also by other means such as the kinetic energy of flowing water and wind. There are many other technologies that can be and are used to generate electricity such as solar photovoltaic and geothermal power. In Indian subcontinent the abundance of coal leads to establishment of thermal power stations and governing bodies namely DVC, NTPC, TATA power acts as pioneers in the generation of electricity. Damodar Valley Corporation Damodar Valley Corporation was established on 7th July 1947.It is the most reputed company in the eastern zone of India. DVC is established on the Damodar River. Vision: To foster integrated development of Damodar Valley Command Area and achieve par excellence in its multifaceted activities of control of floods, provision of irrigation, generation, transmission and distribution of electrical energy and also soil conservation, unified tourism, fisheries, socio-economic & health development of villages within a radius of 10 KM of its projects. To establish DVC as one of the largest power majors of Eastern India while discharging the responsibilities of its other projects adequately. In order to achieve this goal against the backdrop of the competitive market scenario in the power sector, the objective of the Corporation has been redefined. Generation: Entrusted with the responsibilities of providing the vital input power for industrial growth in the resource rich Damodar Valley region, DVC has been practically operating as a pioneer, using latest available technologies to supply bulk power at reasonable rates to the major industries. DVC has maintained its lead role in the eastern region by adopting itself to the challenges of time and technology during the course of last 64 years. DVC has been generating and transmitting power since 1953 and has succeeded not only in meeting the needs of consumers but has also helped to increase the demand of power which itself is an index of development. Therefore, DVC, a legacy to the people of India, emerged as a culmination of attempts made over a whole century to control the wild and erratic Damodar river. The Page | 5
  7. 7. river spans over an area of 25,000 km2 covering the states of Bihar (now Jharkhand) & West Bengal. Infrastructure: With the time DVC developed and expanded its infrastructure, seven thermal power stations with a capacity of 8910MW, three hydroelectric power stations with a capacity of 147 MW. Presently DVC has more than 60 substations and receiving stations more than 5500-circuit km of transmission and distribution lines.DVC has also four dams, a barrage and a network of canals that play effective role in water management. The construction of check dams, development of forests and farms and upland and wasteland treatment developed by DVC play a vital role in eco conservation. Thermal Power Stations: Sr.No. Plant State Installed Capacity in MW 1 Bokaro Thermal Power Station B Jharkhand 630 2 Chandrapura Jharkhand 1140 3 Durgapur Thermal Power Station West Bengal 350 4 Mejia Thermal Power Station West Bengal 2340 5 Koderma Stage-1 Jharkhand 1000 6 Durgapur Steel Thermal Power West Bengal 1000 Station 7 Raghunathpur phase-1 Thermal West Bengal 1200 Power Station Total : 8,910 Hydel Power Station: Sl. No. Plant State Installed Capacity in MW 1 Maithon Dam Jharkhand 63.2 2 Panchet Dam Jharkhand 80 Total : 147.2 Joint Venture Stations: Sl. No. Sl. No. State Installed Capacity in MW 1 Bokaro Power Supply Corporation Jharkhand 302 Limited(BPSCL) 2 Maithon Power Limited Jharkhand 1050 Total: 1352 Page | 6
  8. 8. Basic Needs and Overview of a Thermal Power Pl ant The idea that STEAM has potential energy and can be converted into kinetic energy was given by famous scientist, Sir. James Watt. This idea became the governing principal of many mechanical processes and finally led to the success of Thermal Power Energy. The need of establishing a Thermal Power Plant came to engineers by the realization of the fact that Hydel Power could be utilized only for certain period of time in a year. This section will give the basic requirements for Thermal Power Plant.  SITE REQUIREMENT: - The basic requirements of thermal power plant is determined by the type, size and other specifications of the plant. It is required to know the immediate capacity of the power plant after construction and the extension of capacity in the future, to determine the area required for construction of the plant. The basic things that are taken into consideration are <1>Station Building <2> Coal Store <3>Cooling Towers <4>Switch yard compound <5>Surrounding areas and approaching.  GEOLOGY: - The geology of the site should be cost effective and the subsoil must be able to with stand huge load of foundation.  WATER REQUIREMENT: - Water is required in power plant for two basic needs, first is for steam generation and second is for cooling purpose. Thermal Power Plant requires huge volume of water, nearly of about 3 to 4 Tons/hr/MW only for steam generation. So site of plant must also have reliable and huge water sources located near to it.  COAL: - Coal is the prime requirement of any thermal power plant, it is the main source of fuel as it is most economic and residue of coal after combustion is also used by many industries like cement industries, so the plant must have reliable sources of coal and regular supply in huge amount like 20,000 Tons per week.  TRANSPORT: - It is one of the another vital factor of the plant as huge burden lies on transportation in daily basis because of huge need of coal, furnace oil, hydrochloric acid and other chemical products along with mechanical products.  DISPOSAL OF EFFLUENTS: - Due to heavy rate of coal combustion residual volume is also high. The main residual product is ash. The plant must have facilities like ash pond to dispose them safely without harming the environment.  TRANSMISSION: -The plant area must have route available for transmission over head cables to the nearest grid lines or load points which will be capable of accepting the generated power output of the power station.  CLIMATIC CONDITION : - The tropical climate is best for erection of thermal power plant, because areas having high humidity and fluctuating temperature lead to dew point and condensation which as a result damages the electrical machines and corrodes the insulation and over head cables. Page | 7
  9. 9.  PROXIMITY OF AIRFIELDS:- The airfields must be studied properly to avoid mishaps as the chimney height ranges from 500 to 600 fts and boiler housing is of 200 fts in general.  PERSONNEL REQUIREMENTS: - To run a plant smoothly requirement of skilled and unskilled personnel is very important. So recruitment of workers and skilled personnel should be made carefully and in adequate amount.  AMENITIES: -Some considerations like availability of hospital, educational institutes and other facilities must be taken into account. Page | 8
  10. 10. Mejia Thermal Power Station Mejia Thermal Power Station also known as MTPS is located in the outskirts of Raniganj in Bankura District. It is one of the 5 Thermal Power Stations of Damodar Valley Corporation in the state of West Bengal. The total power plant campus area is surrounded by boundary walls and is basically divided into two major parts, first the Power Plant area itself and the second is the Colony area for the residence and other facilities for MTPS employees. Technical Specification of MTPS long with Specialities Installed capacities: 1) Total number of Units: - 4*210 MW with Static Generators 2*250 MW with Brush less Type Generators 2*500 MW with Brush less Type Generators 2) Total Energy Generation: -2340 MW 3) Source of Water: - Damodar River 4) Sources of Coal: - B.C.C.L and E.C.L, also imported from Indonesia 5) Required Water Consumption: - 6) Approximate coal requirement: - 73, 00,000 Tons/annum at 75% PLF (Plant Load Factor) 7) Ash Deposited per annum: - 1.30 million Tons /annum SPECIALITIES OF MEJIA THERMAL POWER PLANT:  The plant is designed and engineered by both Bharat Heavy Electricals Ltd (BHEL) and Damodar Valley Corporation.  Pipelines of 17km long and 1473mm in diameter spiral welded MS pipes laid to transport river water from upstream of Durgapur barrage by pump sets of 500KV pump motor set.  Rail cum Road Bridge across Damodar River near Raniganj Station.  2KM Merry Go Round Railway System.  20mtr high RCC multiple flue stack.  Direct ignition of pulverized coal introduced for reduction in consumption of fuel oil.  Ball and Tube type Mills for more mill rejects and less maintenance cost.  Boiler of 200ft height and four corner firing system for better combustion. Page | 9
  11. 11.  All major and hazardous systems like Steam Generation and Turbo Generation section are incorporated with FSSS (Furnace Safety Supervisory System) for better safety.  Other logic systems like EAST and ATRS are also incorporated.  Water treatment Plants along with two artificial water reservoirs and Two Demineralization Plants loaded with PLC system.  Chimney height up to 600fts for less pollution.  The plant is loaded with latest technology sensor, transducers and transmitters for more accurate analyzing of various processes.  All the units are loaded with intelligent smart microprocessor based systems known to be DCS systems provided by KELTRON, SIEMENS and MAX-DNA for process control.  Station Service Transformers of 6.6KV step-down type are also available for better distribution of power inside the plant for various requirements.  Switchyard with individual step-up generator transformers of ONAN/ANOF/AFOF cooling Transformers of 220KV for supply to national grid, along with other safety instruments. Details of MTPS Generating Units Gen. Name of Original Present Year of Special Features Unit Manufacturers capacity capacity commissioning Boiler TG (MW) (MW) 1 BHEL BHEL 210 210 March , 1996 DIPC Boilers with zero 2 BHEL BHEL 210 210 March, 1998 reject tube mills. 3 BHEL BHEL 210 210 September, 1999 4 BHEL BHEL 210 210 February, 2005 5 BHEL BHEL 250 250 February, 2008 6 BHEL BHEL 250 250 2009 7 BHEL BHEL 500 500 2010 8 BHEL BHEL 500 500 2010 Page | 10
  12. 12. TPS Unit Overview Page | 11
  13. 13. Coal Handling Plant Coal: The Black Diamond Coal is the basic and the oldest raw material used on large scale throughout the world. Throughout history, coal has been a useful resource. It is primarily burned for the production of electricity and/or heat, and is also used for industrial purposes, such as refining metals. A fossil fuel, coal forms when dead plant matter is converted into peat, which in turn is converted into lignite, then sub-bituminous coal, after that bituminous coal, and lastly anthracite. This involves biological and geological processes that take place over a long period. Coal Handling Plant In a coal based thermal power plant, the initial process in the power generation is “Coal Handling”. Coal is extracted from the ground by coal mining, either underground by shaft mining, or at ground level by open pit mining extraction. The huge amount of coal is usually supplied through railways. A railway siding line is taken into the power station and the coal is delivered in the storage yard. The coal is unloaded from the point of delivery by means of wagon tippler. It is rack and pinion type. The coal is taken from the unloading site to dead storage by belt conveyors. The belt delivers the coal to 0m level to the pent house and further moves to transfer point 8. The transfer points are used to transfer coal to the next belt. The belt elevates the coal to breaker house. It consists of a rotary machine, which rotates the coal and separates the light dust from it through the action of gravity and transfer this dust to reject bin house through belt. The belt further elevates the coal to the transfer point 7 and it reaches the crusher through belt. In the crusher a high-speed 3-phase induction motor is used to crush the coal to a size of 50mm so as to be suitable for milling system. Coal rises from crusher house and reaches the dead storage by passing through transfer point 8. Stages in Coal Handling plant Page | 12
  14. 14. Ultimate Analysis of Coal Carbon : 49.63% Hydrogen : 3.66% Sulphur : 0.47% Nitrogen : 0.91% Oxygen : 6.4% Moisture : 5.0% Ash : 34.0% Total : 100% Operation of a Coal Handling Plant  The purpose of the Coal handling plant in a thermal power plant is to process raw coal & insure against their regular supply of coal which is dependent on many players in the supply chain.  The function of a CHP is to receive process, store, and feed the Coal bunkers consistently over the entire life of the Power plant.  Coal is received from mines in the form of lumps, the sizes varying from 100mm to 350mm, in two types of wagons through Rail; BOBR meaning Bogie Open Bottom Rapid discharge & BOXN meaning Bogie Open High Sided Side discharge Wagon  BOBR wagons are unloaded in Track Hoppers & BOXN Wagons are unloaded by Wagon tipplers.  Coal is then supplied to the crusher house through Roller screens or Vibrating feeders to sieve the coal before feeding to the crusher; 20% of the coal that is received is already <20mm size so this is separated & only larger lumps are fed to the Crusher.  The crusher breaks the lumps to sizes <20mm which is the input size to the coal Pulverisers.  The crushed coal is fed to the conveyors in the crusher house through Belt feeders; Coal is either directly fed to the coal bunkers or to the Stacker/Reclaimers for stocking when the bunkers are full.  The stacking is done to insulate the plant against the erratic supply of coal;  CERC allows stocking of1½months stock of coal for Pithead plants.  In case of non-receipt of wagons the coal from the stockpile is reclaimed through the Stacker/Reclaimers & fed to the coal Bunkers. Page | 13
  15. 15.  To increase redundancy certain Plants also have Emergency reclaim Hoppers near the Crushed coal Stock pile where the dozers are used to feed coal to the bunkers when the Reclaimers breakdown.  Coal is conveyed by means of conveyor Belts in the coal handling plant. Components of a Coal Handling Plant 1. Stockpile: Stockpiles provide surge capacity to various parts of the CHP. Coal is delivered with large variations in production rate of tonnes per hour (tph). A stockpile is used to allow the washplant to be fed coal at lower, constant rate. A simple stockpile is formed by machinery dumping coal into a pile, either from dump trucks, pushed into heaps with bulldozers or from conveyor booms. Taller and wider stockpiles reduce the land area required to store a set tonnage of coal. Larger coal stockpiles have a reduced rate of heat loss, leading to a higher risk of spontaneous combustion. 2. Stack: Travelling, luffing boom stackers that straddle a feed conveyor are commonly used to create coal stockpiles. 3. Reclaimer: High-capacity stockpiles are commonly reclaimed using bucket-wheel reclaimers. These can achieve very high rates. Tunnel conveyors can be fed by a continuous slot hopper or bunker beneath the stockpile to reclaim material. Front-end loaders and bulldozers can be used to push the coal into feeders. Sometimes front-end loaders are the only means of reclaiming coal from the stockpile. This has a low up-front capital cost, but much higher operating costs, measured in dollars per tonne handled. Reclaimer pouring coal into stack 4. Crush House: After hand picking foreign material, coal is transported to the Crush house by conveyor belts where it is crushed to small pieces of about 20 mm diameter. The crushed coal is then transported to the store yard. Coal is transported to bowl mills by coal feeders. 5. Tipplers: Coal from the coal wagons is unloaded in the coal handling plant. This unloading is done by the “Tipplers”. This coal is transported up to the raw coal bunkers with the help of conveyor belts. Crusher Page | 14
  16. 16. 6. Pull chord switch: A series of such switches are arranged in series at a 1m distance on the side of conveyor belt. The power supply to rotor of the conveyor belt is established only if all switches in series are connected. 7. Vibrating feeder: The coal stored in a huge hub is collected on the belt through vibrations created by the vibrating feeder. 8. Flap gates: These are used to channelize the route of coal through another belt in case the former is broken or unhealthy. The flap gates open let the coal pass and if closed stop its movement. 9. Magnetic separator: These are used to separate the ferrous impurities from the coal. 10. Metal detector: This are detect the presence of any ferrous and non-ferrous metal in the coal and sends a signal to a relay which closes to seize the movement of belt until the metal is removed. It basically consists of a transmitter and a receiver. The transmitter consists of a high frequency oscillator, which produces oscillations of 1500 Hz at 15V. The receiver receives this frequency signal. If there is any presence of metal in the coal then this frequency is disturbed and a tripping signal is send to relay to stop the conveyor belt. 11. Belt weightier: It is used to keep an account of the tension on the belt carrying coal and is moves accordingly to release tension on the belt. 12. Reclaim hopper: Reclaimation is a process of taking coal from the dead storage for preparation or further feeding to reclaim hoppers. This is accomplished by belt conveyors. Coal Mill A pulveriser or grinder is a mechanical device for the grinding of many different types of materials. For example, they are used to pulverize coal for combustion in the steam- generating furnaces of thermal power plants. The MILL consists of FEEDER, MILL for pulverization of coal (BALL & TUBE TYPE MILL) and CLASSIFIER. The stacked coal in the bunker is dropped to the feeder automatically; the feeder is housed with a conveyor belt system with motors and pulleys. The feeder actually governs the amount of coal to be transferred to the ball & tube mill for pulverizing. The flow of coal is maintained by the speed/rpm of the conveyor belt of the Page | 15
  17. 17. feeder. The coal from the bunker drops to the feeder s conveyor belt at a constant rate determined by the bunker level, in this condition higher the rpm of the conveyor belt greater will be the rate of volume of the coal transferred to the mill. In the same way if the rpm is lower then lesser will be the volume of coal transferred to the mill. Thus the coal from the feeder is transported to the mill where the pulverization takes place. Here the ball & tube method is utilized for pulverizing of coal to 20micron diameter size. This type of mill consists of arrangement of iron alloy balls inside a MTPS Unit 3: Coal Mill tube like structure that is rotated by its Specification auxiliaries. The coal is fed to the tube at its two ends where it is crushed to the Ball Tube Mill: (3Nos.-CM # 2AB, 3AB, 3EF) Type: BBD4760 above mentioned size, these pulverized Capacity: 77 Tonne/Hour coal is taken back from the mill to the Power Rating: 2.25MW classifier. In case of ball and tube type Primary Air Fan: (3Nos.-PA FAN # 2AB, mills, there are 3 mill units; out of which 2 3AB, 3EF) must be running and 1 for standby while Type: NDV20H 3 the unit is running on load. The classifier Capacity: 65.9 m /sec Total Head Developed: 806 mmWC consists of strainers; the primary air brings Power Rating: 850KW the coal from the mill to the classifier where the pulverized coal is passed through strainers. The strainers allow 80% (approx.) of the coal to pass from 200 mesh and rest is fed back to the mill for further pulverization. Here the primary air is utilized to maintain the temperature of the coal up to 80 C-90 C for better combustion. The classifier has 4 outlets and each ball and tube type mills have 6 such classifier (2for each mill unit). The coal from each outlets of a classifier goes to each of the 4 corners of the furnace; therefore coal from each outlets of all the 6 classifier goes to all the 24 elevations (A-B-C-D-E-F of each corner) of furnace in all. All transport of coal from mill to the furnace is done by the primary air produced by PA fans. Page | 16
  18. 18. Furnace and Boiler  What is Boiler? A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including boiler-based power generation, cooking, and sanitation. Here in MTPS, the boiler is a rectangular furnace about 50 feet (15 m) on a side and 130 feet (40 m) tall. Its walls are made of a web of high pressure steel tubes about 2.3 inches (58 mm) in diameter. Types of Boiler: Fire Tube Boiler: In fire tube boiler, hot gases pass through the tubes and boiler feed water in the shell side is converted into steam. Fire tube boilers are generally used for relatively small steam capacities and low to medium steam pressures. As a guideline, fire tube boilers are competitive for steam rates up to 12,000 kg/hour and pressures up to 18 kg/cm2. Fire tube boilers are Fire tube Boiler available for operation with oil, gas or solid fuels. For economic reasons, most fire tube boilers are nowadays of “packaged” construction (i.e. manufacturers shop erected) for all fuels. Water Tube Boiler: In water tube boiler, boiler feed water flows through the tubes and enters the boiler drum. The circulated water is heated by the combustion gases and converted into steam at the vapour space in the drum. These boilers are selected when the steam demand as well as steam pressure requirements are high as in the case of process cum power boiler / power boilers. Most modern water boiler tube designs are within the capacity range 4,500 – 120,000 kg/hour of steam, at very high pressures. Many water tube boilers nowadays are of “packaged” construction if oil and /or gas are to be used as fuel. Solid fuel fired water tube designs are available but packaged designs are less common. Water tube Boiler The features of water tube boilers are: Page | 17
  19. 19.  Forced, induced and balanced draft provisions help to improve combustion efficiency.  Less tolerance for water quality calls for water treatment plant.  Higher thermal efficiency shifts are possible Note: In MTPS Water tube Boilers are incorporated. Furnace: A furnace is a device used for heating. The name derives from Latin fornax, oven. The boiler furnace auxiliary equipment includes coal feed nozzles and igniter guns, soot blowers, water lancing and observation ports (in the furnace walls) for observation of the furnace interior. Furnace explosions due to any accumulation of combustible gases after a trip-out are avoided by flushing out such gases from the combustion zone before igniting the coal. The coal is ground (pulverized) to a fine powder, so that less than 2% is +300 micro meter (μm) and 70-75% is below 75 microns, for a bituminous coal. It should be noted that too fine a powder is wasteful of grinding mill power. On the other hand, too coarse a powder does not burn completely in the combustion chamber and results in higher un-burnt losses. The pulverized coal is blown with part of the combustion air into the boiler plant through a series of burner nozzles. Secondary and tertiary air may also be added. Combustion takes place at temperatures from 1300-1700°C, depending largely on coal grade. Particle residence time in the boiler is typically 2 to 5 seconds, and the particles must be small enough for complete combustion to have taken place during this time. This system has many advantages such as ability to fire varying quality of coal, quick responses to changes in load, use of high pre-heat air temperatures etc. One of the most popular systems for firing pulverized coal is the tangential firing using four burners corner to corner to create a fireball at the center of the furnace. Boiler Operation: The water enters the boiler through a section in the convection pass called the economizer. From the economizer it passes to the steam drum. Once the water enters the steam drum it goes down to the downside the steam drum. The steam separators and dryers remove water droplets from the steam and the cycle through the water walls is repeated. This process is known as natural circulation. Pulverized coal is air-blown into the furnace from fuel nozzles at the four corners and it rapidly burns, forming a large fireball at the center. The thermal radiation of the fireball heats the water that circulates through the boiler tubes near the boiler perimeter. The Page | 18
  20. 20. water circulation rate in the boiler is three to four times the throughput and is typically driven by pumps. As the water in the boiler circulates it absorbs heat and changes into steam at 700 °F (370 °C) and 3,200 psi (22,000 kPa). It is separated from the water inside a drum at the top of the furnace. The saturated steam is introduced into superheat pendant tubes that hang in the hottest part of the combustion gases as they exit the furnace. Here the steam is superheated to 1,000 °F (540 °C) to prepare it for the turbine. Boiler is the main section where the steam is produced by coal combustion. Boiler consists of boiler drum, water walls, wind box, heaters. The boiler has 13 elevations named as AA-A-AB-B-BC-C-CD-D-DE-E-EF-F-FF. Coal is inserted into the boiler from A-B-C-D-E-F elevations. BC is used for insertion of Heavy Oil and Light Oil after atomization with steam and air respectively. DF is used for insertion of oil i.e. only heavy oil. Both the elevations have Oil Gun mounted for insertion of oil in proper ratio into the boiler. Liquid fuel (viz. Heavy Oil and Light Oil) is used for initial light up process. Other elevations are used to insert secondary air from wind box. The furnace is divided into two sections named as first pass and second pass separated by Goose Neck. The combustion takes place in the first pass and the heating of steam through super heaters takes place in the second pass. Boiler Drum: - Boiler Drum is the part of boiler where the dematerialized water is stored and is inserted into the boiler. It is also houses the steam that is formed in the boiler. Water stored in the drum comes down to the top of the boiler and forms a Water Ring which is then inserted into the boiler through the water walls. Water Walls are basically tubes along the walls of the furnace, it is here where the water is converted into steam at 1300C and then the produced steam is taken back to the boiler drum. The drum has a propeller that rotates at high speed and makes the steam and water separated due to centrifugal force. The pressure of boiler drum is 150kg/sq.cm and must be always maintained. Water in the drum comes from feed control station via economizer. Page | 19
  21. 21. Steam Turbine Mechanical Construction of Turbine Assembly The 200/210 MW turbine installed in MTPS is of condensing-tandem-compound, three cylinder, horizontal, disc and diaphragm, reheat type with nozzle governing and regenerative system of feedwater heating and is directly coupled with the A.C generator. TURBINE CASING: - The turbine assembly comprises of three types of casing. 1) High Pressure Casing 2) Intermediate Pressure Casing 3) Low Pressure Casing OTHER TURBINE COMPONENTS: -  ROTOR: - The rotor is basically the main rotating part of the turbine which is also called the shaft and is attached with the rotor of the A.C generator via coupling. Rotor is basically divided into 3 categories and they are as follows: - Cross section of a turbine a) HIGH PRESSURE ROTOR: - This is basically made of single Cr-Mo-V steel forged with internal disc attached to T-shoot fastening designed especially for stabilizing the HPT and preventing the axial shift. b) INTERMEDIATE PRESSURE ROTOR: - This is made from high creep resisting Cr- Mo-V steel forging and the shrunk fit disc are machined from nickel-steel forging. This basically adjusts the frequency of the blades. Page | 20
  22. 22. c) LOW PRESSURE ROTOR: - This is made from the above mention alloy used in IP Rotors; blades are secured to the respective disc by riveted fork root fastening. Wires are provided in all stages of this to adjust the frequency of the blades.  BLADES: - Blades are single most costly element fitted in the turbine. Blades fitted in the stationary part are called guide blades and those fitted in the rotor are called moving or working blades. Blades are of basically three types, they are as follows: - a) Cylindrical ( constant profile) blade b) Tapered cylindrical blade c) Twisted and varying profile blade.  SEALING GLANDS: - To eliminate the possibility of steam leakage to the atmosphere from the inlet and the exhaust end of the cylinder, labyrinth glands of the radial clearance type are provided which provide a trouble free frictionless sealing .  EMERGENCY STOP VALVES AND CONTROL VALVES: - Turbine is equipped with emergency stop valves to cut off steam supply and with control valve regulate steam supply. Emergency stop valves are provided in main stream line and control valves are provided in the hot reheat line.  COUPLING: - Since the rotor is made in small parts due to forging limitations and other technological and economic reasons, the couplings are required between any two rotors. The coupling permits angular misalignment, transmits axial thrust and ensures axial location.  BEARING: - Journal bearing are manufactured in two halves and usually consist of bearing body faced with anti-friction tin based habiting to decrease coefficient of friction. Bearings are usually force lubricated and have provision for admission of jacking oil. Thrust bearing is normally Mitchell type and is usually combined with a journal bearing, housed in spherically machined steel shell. The bearing between HP and IP rotor is of this type. The rest is of journal type.  BARRING GEAR: - The barring gear is mounted on the L.P rear bearing cover to mesh with spur gear L.P rotor rear coupling. The primary function of the barring gear is to rotate the rotor of the turbo generator slowly and continuously during the start-up and shut sown process when the temperature of the rotor changes.  TURBINE LUBRICATION OIL SYSTEM: - The LUB-OIL system of turbine comprises of following category. a) MAIN OIL PUMP: - It is mounted on the front bearing pedestal and coupled through gear coupling to the rotor. When the turbine is running at its normal Page | 21
  23. 23. speed of 3000rpmthen the oil to the governing system (at 20 kg/sq.cm) and to the lubrication system (at 1 kg/sq.cm) is supplied by this pump. b) STARTING OIL PUMP: - It is a multi-staged centrifugal oil pump driven by A.C powered electric motor. It provides the oil requirement for starting up and stopping of the turbine. It provides oil to the governing system and to the lubrication system until the turbine is running at speed lower than 2800rpm. c) STANDBY OIL PUMP: - This is a centrifugal pump driven by A.C motor. It runs for initial10 minutes at the starting to remove air from the governing system and fill up oil to it. d) EMERGENCY OIL PUMP: - This is a centrifugal pump driven by D.C motor. This pump is foreseen as a backup oil pump to A.C oil pumps. This pump automatically cuts in when the A.C power fails in the power station. e) JACKING OIL PUMP: - This pump enables the complete rotor assembly to be raised upor to be floated in the bearing assembly during the start-up and shut down process of the process. Thus this prevents the damage to the bearings when the shaft is too low for hydrodynamic lubrication to take place. JOP sucks and delivers oil to the journal bearings at 120kg/sq.cm for lifting of the rotor. f) OIL COOLERS: - The oil of governing and lubrication system is cooled in the oil coolers by the circulating water. There are five such coolers, 4 are for continuous operation and 1for standby. Specification of Turbine (LPT) U #1 to U #4 Mega Watt : 210 R.P.M. : 3000 Steam Pressure : 150 Kg/cm2 (Abs) Steam Temperature : 535 C Reheat Steam : 535 C Make : BHEL U #5 & U #6 Mega Watt : 250 R.P.M. : 3000 Steam Pressure : 150 Kg/cm2 (Abs) Steam Temperature : 537 C Reheat Steam : 537 C Make : BHEL Page | 22
  24. 24. Introduction of Water Treatment in Thermal Power Plants In Thermal power plants, plenty of water is needed for generation of electricity. Now question is for what purpose we need water here? There are two purposes: 1. As a Working Fluid 2. As Cooling water Water which is used as a working fluid needs some treatment. Reasons to choose Water as a Working Fluid: • It is only common substance available & exists in 3 states (Ice, water, steam)at normal temperature. • Having high specific heat mean heat carrying capacity is high. • Having low specific volume than air. • Low Cost • High Availability • Non-reactive But water is universal solvent; it dissolves many gases, salts, metals etc. so no source of water is pure. Water contamination depends upon source of water. There are 3 sources of water mainly; 1. Surface Water 2. Ground Water 3. Recycled Water Impurities in Water Impurities present in water are grouped into 4 categories: 1) Suspended Matter – • Mean any matter floating or suspended nature in water • Microorganisms • Grits 2) Dissolved Salts – • Ca, Mg, K, Chlorates, Sulphates, Silicates etc. 3) Dissolved Gases – • Oxygen, Carbon di oxide, Ammonia etc. Page | 23
  25. 25. 4) Silica A 210 MW unit typically requires 30,000 to 33,000 m3/h of water. A large part of this water is used for condenser cooling and a small quantity is used for boiler feed makeup and other uses. Total Water Management in Mejia Thermal Power station consists of: 1. Pre Treatment of Water 2. Treatment of water for boiler feed 3. Treatment of water for condenser cooling 4. Treatment of wastewater for disposal or recovery of water for reuse. 1. Pre Treatment of Water:  Aerator: Aerators are various devices used for aeration, or mixing air with another substance, like water. It also converts turbulent water flow into laminar water flow.  Coagulation & Flocculation Basin: One of the first steps in a conventional water purification process is the addition of chemicals to assist in the removal of particles suspended in water. Particles can be inorganic such as clay and silt or organic such as algae, bacteria, viruses, protozoa and natural organic matter. Inorganic and organic particles contribute to the turbidity and colour of water. The addition of inorganic coagulants such as aluminium sulphate (or alum) or iron (III) salts such as iron(III) chloride cause several simultaneous chemical and physical interactions on and among the particles. Within seconds, negative charges on the particles are neutralized by inorganic coagulants. Also within seconds, metal hydroxide precipitates of the aluminium and iron (III) ions begin to form. These precipitates combine into larger particles under natural processes such as Brownian motion and through induced mixing which is sometimes referred to as flocculation. The term most often used for the amorphous metal hydroxides is “floc.” Large, amorphous aluminium and iron (III) hydroxides adsorb and enmesh particles in suspension and facilitate the removal of particles by subsequent processes of sedimentation and filtration.  Clarifiers: Waters exiting the flocculation basin may enter the sedimentation basin, also called a clarifier or settling basin. It is a large tank with low water velocities, allowing floc to settle to the bottom. The sedimentation basin is best located close to the flocculation basin so the transit between the two processes does not permit settlement or floc break up. Sedimentation basins may be rectangular, where water flows from end to end or circular where flow is from the centre outward. Sedimentation basin outflow is typically over a weir so only a thin top layer of water—that furthest from the sludge—exits.  Gravitation Filter: The most common type of filter is a rapid sand filter(gravity filter). Water moves vertically through sand which often has a layer of activated carbon or anthracite coal above the sand. The top layer removes organic compounds, which contribute to taste and odour. The space between sand particles is larger than the smallest suspended particles, so simple filtration is not Page | 24
  26. 26. enough. Most particles pass through surface layers but are trapped in pore spaces or adhere to sand particles. To clean the filter, water is passed quickly upward through the filter, opposite the normal direction (called back flushing or backwashing) to remove embedded particles. 2. Treatment of water for boiler feed: Boiler feed water treatment for high pressure boilers are almost standard. Raw water is clarified and filtered for removal of un-dissolved impurities and demineralised for removal of dissolved salts. Dissolved oxygen is removed in a thermal de-aerator. Residual dissolved oxygen is removed by hydrazine.  DM Plant: A DM plant generally consists of cation, anion, and mixed bed exchangers. Any ions in the final water from this process consist essentially of hydrogen ions and hydroxide ions, which recombine to form pure water. Very pure DM water becomes highly corrosive once it absorbs oxygen from the atmosphere because of its very high affinity for oxygen. The capacity of the DM plant is dictated by the type and quantity of salts in the raw water input. However, some storage is essential as the DM plant may be down for maintenance. For this purpose, a storage tank is installed from which DM water is continuously withdrawn for boiler make-up. The storage tank for DM water is made from materials not affected by corrosive water, such as PVC. The piping and valves are generally of stainless steel. Sometimes, a steam blanketing arrangement or stainless steel doughnut float is provided on top of the water in the tank to avoid contact with air. DM water make-up is generally added at the steam space of the surface condenser (i.e., the vacuum side). This arrangement not only sprays the water but also DM water gets de-aerated, with the dissolved gases being removed by a de-aerator through an ejector attached to the condenser. Normal Water Treatm ent Page | 25
  27. 27.  Presence of silica in boiler feed water is harmful as silica tends to volatilize along with steam and get deposited as glassy and hard deposits on the turbine blades. It has been established that concentrations of silica in excess of 0.03 mg/l invariably causes problems in turbine operation. Suitable lower silica level should be maintained boiler water to maintain silica less than 0.02 mg/l in steam leaving the drum.  Silica in water is present mostly as reactive or dissolved silica. In surface waters, a small quantity of non-reactive silica (in colloidal dimensions) may also be present during parts of the year especially during the monsoon. A DM plant removes reactive silica almost completely, to less than 0.005 mg/l. However, non-reactive silica is not removed and finds its way into the boiler drum where it gets converted into reactive silica under the operating conditions of high pressure and temperature. The station chemists usually overcome this problem by having increased blow-downs during these periods. 3. Treatment off wastewater and its disposal or recovery and reuse off water: Water is a scarce resource and Thermal Power stations are today being compelled to minimise consumption of water to the extent possible. It is possible to recover and reuse water from most of the waste streams generated in a thermal power station. The main waste streams are: · Gravity filter backwash water · Wastewater generated from the DM plant · Ash pond overflows water · Boiler blow down and turbine drains. · Recovery of water from treated sewage Page | 26
  28. 28. Steam/Water Circuit of Power Plant (MTPS): A thermal power station is a power plant in which the prime mover is steam driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated; this is known as a Rankin cycle. This section deals with supplying of steam generated from the boiler to the turbines and to handle the outgoing steam from the turbine by cooling it to form water in the condenser so that it can be reused in the boiler plus making good any losses due to evaporation etc. WATER PATH: - Water comes from the water reservoir to the demineralization plant (DM Plant) for removal of all minerals present in normal water for making it non-conductive and increasing the efficiency of the overall system. After DM plant water goes to the boiler drum via condenser and the feed control station. STEAM GENERATION PROCESS: - Water from the boiler drum comes down to the top of the boiler and forms a ring head and finally goes to the boiler through the water walls. The boiler/furnace is lit up by four corner firing technique; this produces a ball of fire and reaches a temperature of 1200 C. This as a result converts the water in the water walls into steam at high pressure. This steam is sent back to the boiler drum where it is separated from the water with the help of high speed propeller. The steam is taken to the super heaters via water pipes where it is converted to superheated steam for total moisture removal. After super-heaters the steam divides into two ducts called Main Steam Left (L) and Main Steam Right(R) and finally reaches the turbines. Page | 27
  29. 29. TURBINES are form of engine and hence it requires suitable fluid for working, a source of high grade energy and a sink of low grade energy, the fluid when flows through the turbine the energy content of it is continuously extracted and converted into its useful mechanical work. The turbines used in thermal power plants are of STEAM GAS type which uses the heat energy of the steam for its working. Turbine Cycle is the most vital part of the overall process; this is where the mechanical energy of the steam is converted to electrical energy via turbine assembly. The turbine assembly comprises of three turbines named as High Pressure Turbine (HPT), Intermediate Pressure Turbine (IPT) and the Low Pressure turbine (LPT). The steam that is generated in the SG section comes to the HPT through main steam lines via control valves. The steam when strikes the HPT have 540 C at 150kg/sq.cm pressure. This high pressure superheated steam rotates the turbine, the speed of the turbines is controlled by the controlling the amount of steam through control valves. Generally only 3%-4% steam is enough to rotate the turbine at3000rpm at no load but at full load condition 100% steam is required to rotate the turbine at 3000rpm, because to produce power at 50Hz frequency the rpm required is 3000. The HPT is a single head chamber type of turbine. Page | 28
  30. 30. One part of the exhaust steam from HPT is taken to re-heaters through cold reheat line (CRH line) which are again of mechanical type; for restoring the superheated properties of the steam for further use. The reheated steam is brought back to the IPT via HRH (hot reheat steam) line. And the other part of the exhaust steam is taken to the HP heaters (i.e. to HPH-6) The reheated steam is mechanical energy is utilized by the IPT which is a double head chamber type turbine, where steam enters from the top-mid section of the turbine and leaves the turbine from the front and back section. The exhaust of IPT is divided into 3 parts, one goes for the HP heaters (HPH-5), another goes to the de- aerator and the last part goes to the LPT. The exhaust steam of the LPT Is divided into 4 parts, 3 of them goes for the Low Pressure Heaters (LPH-1, LPH-2, LPH-3) for heating the condensate, and the last part goes to the condenser for the steam condensation process and regeneration of water. The condensation is done to minimize the production of DM water to make the process cost effective. The steam is converted to water and extracted by CEP from the condenser and transported to Gland Sealing Coolers (GSC) via Ejectors (EJE). The GSC cools the sealing of the ducts; the condensate is taken to the LPH from the GSC for heating at lower pressure to increase the enthalpy of the water for better efficiency. Water after LPH reaches the de-aerator where the oxygen is removed from it and is taken to the BFPs, the BFPs increases the pressure of the water up to 160kg/sq.cm and sends it the high pressure heaters (HPH-5 & HPH-6). HPH increases the temperature of the water once more and transfers it to the Economizer, in economizer the temperature of water is again increased by the flue gas and is finally is transported to the steam generation process via the Feed Control Station SOME IMPORTANT COMPONENT OF STEAM/WATER CYCLE: A fossil fuel steam generator includes an economizer, a steam drum, and the furnace with its steam generating tubes and super-heater coils. Necessary safety valves are located at suitable points to avoid excessive boiler pressure.  De-aerator: Typically, the condensate plus the makeup water then flows through a de-aerator that removes dissolved air from the water, further purifying and reducing its corrosiveness. The water may be dosed following this point with hydrazine, a chemical that removes the remaining oxygen in the water to below 5 parts per billion (ppb) Page | 29
  31. 31. CONDENSATE SYSTEM Courtesy SIEMENS OS220EA, C&I, MTPS, DVC  Condenser: The condenser condenses the steam from the exhaust of the turbine into liquid to allow it to be pumped. If the condenser can be made cooler, the pressure of the exhaust steam is reduced and efficiency of the cycle increases. The surface condenser is a shell and tube heat exchanger in which cooling water is circulated through the tubes. The exhaust steam from the low pressure turbine enters the shell where it is cooled and converted to condensate (water) by flowing over the tubes as shown in the adjacent diagram. For best efficiency, the temperature in the condenser must be kept as low as practical in order to achieve the lowest possible pressure in the condensing steam. Page | 30
  32. 32. Typically the cooling water causes the steam to condense at a temperature of about 35 °C (95 °F) and that creates an absolute pressure in the condenser of about 2–7 kPa (0.59–2.1 in Hg), i.e. a vacuum of about −95 kPa (−28.1 in Hg) relative to atmospheric pressure. The large decrease in volume that occurs when water vapour condenses to liquid creates the low vacuum that helps pull steam through and increase the efficiency of the turbines. The condenser generally uses either circulating cooling water from a cooling tower to reject waste heat to the atmosphere, or once-through water from a river, lake or ocean. The cooling water used to condense the steam in the condenser returns to its source without having been changed other than having been warmed. The lower portion of condenser where the condensed water stored known as Hotwell.  Economizers: These are heat exchange devices that heat fluids, usually water, up to but not normally beyond the boiling point of that fluid. Economizers are so named because they can make use of the enthalpy in fluid streams that are hot, but not hot enough to be used in a boiler, thereby recovering more useful enthalpy and improving the boiler's efficiency. They are a device fitted to a boiler which saves energy by using the exhaust gases from the boiler to preheat the feed.  GSC: Gland steam condenser is meant for condensing the steam which was used for sealing the LABYRINTH GLAND and reusing it in cycle.  Low Pressure Heater: A Heater is located between the condensate pomp and either of the boiler feed pump. It normally extracts steam from low pressure turbine.  High Pressure Heater: A heater located downstream of boiler feed pump. Typically, the tube side design pressure is at least 100Kg/cm2, and the steam source is the high pressure turbine. [The heating process by means of extraction of steam is referred to as being regenerative. Page | 31
  33. 33. Cooling Towers : The condensate (water) formed in the condenser after condensation is initially at high temperature. This hot water is passed to cooling towers. It is a tower- or building-like device in which atmospheric air (the heat receiver) circulates in direct or indirect contact with warmer water (the heat source) and the water is thereby cooled (see illustration). A cooling tower may serve as the heat sink in a conventional thermodynamic process, such as refrigeration or steam power generation, and when it is convenient or desirable to make final heat rejection to atmospheric air. Water, acting as the heat-transfer fluid, gives up heat to atmospheric air, and thus cooled, is recirculate through the system, affording economical operation of the process. With respect to drawing air through the tower, there are three types of cooling towers: Natural draft — Utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential compared to the dry, cooler outside air. Warm moist air is less dense than drier air at the same pressure. This moist air buoyancy produces an upwards current of air through the tower. Induced draft — A mechanical draft tower with a fan at the discharge (at the top) which pulls air up through the tower. The fan induces hot moist air out the discharge. This produces low entering and high exiting air velocities, reducing the possibility of recirculation in which discharged air flows back into the air intake. This fan/fin arrangement is also known as draw-through. Forced draft — A mechanical draft tower with a blower type fan at the intake. The fan forces air into the tower, creating high entering and low exiting air velocities. The low exiting velocity is much more susceptible to recirculation. With the fan on the air intake, the fan is more susceptible to complications due to freezing conditions. Another disadvantage is that a forced draft design typically requires more motor horsepower than an equivalent induced draft design. The benefit of the forced draft design is its ability to work with high static pressure. Such setups can be installed in more-confined spaces and even in some indoor situations. This fan/fill geometry is also known as blow- through. Page | 32
  34. 34. Air and Flue Gas Path In fossil-fuelled power plants, water is taken to the boiler or steam generator where coal is burnt. The boiler transfers heat energy to the water in form of latent heat of vaporization or enthalpy by the chemical reaction of burning coal. External fans, such as PA fans and FD fans, are provided to give sufficient air for combustion. The air and flue gas path equipment include: forced draft (FD) fan, air preheater (APH), boiler furnace, induced draft (ID) fan, fly ash collectors (ESP or electrostatic precipitator), flue gas stack, etc. External fans re provided to give sufficient air for combustion. INDUCED DRAFT (ID) FAN: - FD & ID Fan This fan is used to create negative pressure in the 3 Phase Asynchronous Motor furnace, i.e. furnace pressure is lower than the Make : BHEL Connection : Y atmospheric pressure, as a result of which the fire Type : S. Cage ball inside the furnace cannot come out of the Insul. Class : F Frequency : 50Hz furnace. ID fan also drives the flue gas throughout its path and above processes and finally ejects it out of the chimney. It sucks air from inside the furnace and ejects it to the atmosphere. Mechanically ID fan consists of one 3-phase asynchronous type motor, a hydro coupling unit for coupling rotor shaft of the motor and the rotor shaft of the fan, scoop unit, a pair of journal bearings and lubrication oil system. It is the only fan which have hydro coupling because this gives more accurate control to its speed for maintaining the negative pressure more precisely since controlling of negative pressure is the most vital factor in any thermal power unit. The lube-oil system has two motors out of which one remains standby; for maintaining perfect pressure of lubrication throughout the ID fan assembly. The second motor automatically starts up when the oil pressure drops below a certain level; this motor increases the oil pressure in the system. Water cools down the oil flowing in the tubes inside the coolers. There are three ID Fans in each unit of thermal power plant, named as ID-A, ID-B, ID-C. FORCE DRAFT (FD) FAN : Forced Draft (FD) fans purpose is to provide a positive pressure to a system. This basic concept is used in a wide variety of industries but the term FD Fans is most often found in the boiler industry. Fans for boilers force ambient air into the boiler, typically through a preheater to increase overall boiler efficiency. Inlet or outlet dampers are used to control and maintain the system pressure. The outlet of the FD fan divides into 5 ways; 2 goes to the air-preheater, and remaining 3 goes to the PA fan supplying cold air. Mechanically FD fans consist of one 3- Page | 33
  35. 35. phase asynchronous type motor, a pair of journal bearings and lube-oil system. Unlike ID fan these fans have direct coupling of rotor shaft of the motor and rotor shaft of the fan. The lube-oil system is designed same as ID fans. There are 2 FD fans in a single unit. ***NOTE: - THE SYNCHRONIZATION OF ID-FAN AND FD-FAN IS VERY IMPORTANT AS THESE TWO FANS COMBINELY BALANCE THE PLANT. WHEN WORKING TOGETHER IT IS CALLED BALACED DRAFT. PRIMARY AIR FAN/PA FAN:- Primary air fan is used for mixing of cold air of FD fan outlet and hot air of air-preheater outlet. The main function of this is to transport the pulverized coal from the mill to the furnace via classifier. Mixing of hot and cold air is necessary because it is needed to maintain the temperature of the pulverized coal from 80⁰C-90⁰C for better transport of coal and better combustion in the furnace. Mechanically the construction of PA fan is same as FD fans along with the lube-oil system. There are 3 PA fans in a single mill of ball and tube type. SCANNER AIR FAN / SC FAN/SA FAN:- The scanner air fans are relatively smaller in size and consume low power as compared to the above mentioned fans. These are simple motor operated fans that suck air from atmosphere and utilize it to cool the flame scanners (explained in C&I section later) inside the furnace. AIR-PREHEATER: - The flue gas produced as a result of combustion of fossil fuel in the furnace is taken to the air-preheater. The air-preheater is used to heat up the atmospheric air to make hot air used for combustion and transport of coal dust from mill to furnace; which is called secondary air. This heater has a unique process of heating, it has a shaft attached to a rotating wheel type structure (like turbine but arrangement of blades are different). Atmospheric air sucked by FD fans passes through one side of the rotating shaft and the hot flue gas passes through another side. This way heat of the flue gas gets transferred to the atmospheric air and it gets heated. There are two air-preheaters named as AH-A and AH-B. These heaters can be found beside the boiler in the burner floor. CHIMNEY:- A chimney is a structure which provides ventilation for hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. Chimneys are typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the stack, or chimney, effect. Page | 34
  36. 36. Electrostatic Precipitators An electrostatic precipitator (ESP) or electrostatic air cleaner is a particulate collection device that removes particles from a flowing gas (such as air) using the force of an induced electrostatic charge. Electrostatic precipitators are highly efficient filtration devices that minimally impede the flow of gases through the device, and can easily remove fine particulate matter such as dust and smoke from the air stream. In contrast to wet scrubbers which apply energy directly to the flowing fluid medium, an ESP applies energy only to the particulate matter being collected and therefore is very efficient in its consumption of energy (in the form of electricity). PRINCIPLE OF ESP: In the electrostatic precipitator the particles are removed from the gas stream by utilizing electrical force .A charged particle in the electrical field experiences a force proportional to the size of the charge and to the strength. The precipitation process therefore requires.  A method of charging the particles electrically.  A means of establishing an electrical field and  A method of removing the collected particles. An industrial ESP includes a large number of discharge electrodes. Pirated wires and rows of collecting electrodes plates forming passage through which the gas flows with velocity. High voltage is applied to the discharge electrodes resulting in the high electric field near the wire and an associated corona producing gas ions .The ions collide with and held by, the dust particles and this in turn become electrically charged the particles moved towards the grounded collecting electrode plates from which the accumulated dust is dislodged by rapping the dust falls to the bottom of the precipitator casing from which it is removed by different methods. PARTS OF THE PRECIPITATORS: The various parts of the precipitators are divided to two groups. Mechanical system comprising of casing, hoppers, gas distribution system, collecting and emitting system, rapping mechanisms, stair ways and galleries. Page | 35
  37. 37. Electrical system comprising of transformer-rectifier units, electronic controllers auxiliary control panels, safety interlocks and field devices. 1. MECHANICAL SYSTEM: A. Precipitator casing: The precipitator casing is an all welded construction, consisting of prefabricated wall and the roof panels. The casing is provided with inspection doors for entry into the chamber. The doors are of heavy construction with machined surfaces to ensure a gas tight seal. The roof carries the precipitator internals, insulator housing, transformers etc. The casing rests on supports, which allow for free thermal expansion of the casing during operation. Galleries and stairways are provided on the sides of the casing for easy access to rapping motors, inspection doors, transformers. B. Hoppers: The hoppers are adequately sized to hold the ash, Baffle plates are provided in each hopper to avoid gas sneak age. An inspection door is provided on each hopper. Thermostatically controlled heating elements are arranged at the bottom portion to the hopper to ensure free flow of ash. The precipitator casing is an all welded construction, consisting of prefabricated C. Gas distribution systems: The performance of the precipitator depends on even distribution of gas over the entire cross section of the field. Guide vanes, splitters and screens and screens are provided in the inlet funnel to direct the flue gas evenly over the entire cross section of the ESP. D. Collecting Electrode System: The collecting plates are made of 1.5mm cold rolled milled steel plate and shaped in one piece by roll forming .The collecting electrode has unique profile designed to give rigidity and to contain the dust in a quiescent zone free from re-entrainment .The 400mm collecting plates are provided with hooks to their top edge for suspension .The hooks engage the slots of the supporting angles 750mm collecting plates in a row are held in position by a shock bar at the bottom. The shock bars are spaced by guides. Page | 36
  38. 38. E. Emitting Electrode System: The most essential part of the precipitator is emitting electrode system.4 insulators support this. The frames for holding the emitting electrodes are located centrally between collecting electrode curtains. The entire discharge frames are welded to form rigid bars F) Rapping Systems: Rapping systems are provided for collecting and emitting electrodes. Geared motors drive these rappers. The rapping system employs tumbling hammers, which are mounted on the horizontal shaft. As the shaft rotates slowly the hammers tumble on the shaft will clean the entire field. The rapper programmer decides the rapping frequency. The tumbling hammers disposition and the periodicity of rapping are selected in such a way that less than 2% of the collecting area is rapped at any instant. This avoids re-entrainment of dust and puffing at the stack. The rapping shaft from the gear motor drive by a shaft insulator. The space around the shaft insulator is continuously heated to avoid condensation. G) Insulator Housing: The support insulators, supporting the emitting electrodes housed in insulator housings. The HVDC connection is taken through a bushing insulator mounted on the insulator housing wall. In order to avoid the condensation on the support insulators, each insulator is provided with one electrical heating element. Heating elements of one pass are controlled by one thermostat. 2) ELECTRICAL SYSTEM: A) High Voltage Transformer Rectifier (H.V.R) with electronic controller (E.C) The transformer rectifier supplies the power for particulate charging and collection. The basic function of the E.C is to feed precipitator with maximum power input under constant current regulation. So, thereby any flash over between collecting and emitting electrodes, the E.C will sense the flash over and quickly react by bringing the input voltage ton zero and blocking it for a Page | 37
  39. 39. specific period. After the ionized gases are cleared and the dielectric strength restored, the control will quickly bring back the power to the present value and raise it to the original non-sparking level. Thus the E.C ensures adequate power input to the precipitator while reckoning the electrical disturbances within the precipitator. Regulated ac power from E.C is fed to the primary of the transformer, which is stepped up and rectified to give a full wave power output. The transformer rectifier is mounted on the roof of the precipitator while the E.C is located in an air-conditioned control room. B) Auxiliary control panel (A.C.P) The A.C.P controls the power supply to the EP auxiliary i.e. rapping motors and heating element dampers etc. The complete A.C.P. is of modular type with individual modules for each feeder. Each module houses the power and control circuits with meters, push buttons, switches and indicating lamps. Following are the modules for the outgoing feeders  Hopper heaters for each field  Support insulator heaters  Shaft insulator heaters  Collecting electrode rapping motor for each field The program control circuit for the sequence and timing of operation for rapping motors is included in the A.C.P. For continuous operation of the rapping motors, the programmer can be bypassed through a switch. Thermal overload relay is provided for overload protection to the rapping motors. Local push buttons are available for tripping the motors to meet the exigencies and for maintenance purposes. Ammeters with selector switches to indicate line currents of motors and heating element feeders are provided. Indicating lamps are provided “main supply on”, “overload trip”, “local push button activated”, “space meter on”, and “control supply on”. Potential free contacts are provided for remote indication for rapping motor trip due to overload. C) Safety Interlock: A safety interlock system is incorporated to prevent accidental contact with live parts of the precipitator and enable energisation only when the ESP is boxed up. The interlock system covers all the inspection doors of casing, insulator housing and disconnecting switches. Warning: familiarity with this system may felon the operating personnel bypass the interlock. As this would defend the very purpose of the interlocking system, such a temptation should be resisted and the sequence of operation at every stage should be systematically followed. Page | 38
  40. 40. D) Disconnecting switch: Each field is provided with one disconnecting switch for isolation of emitting system from the associated transformer .In the on position the emitting system is connected to the transformer and in the OFF position it is grounded. ESP in MTPS: ESPs continue to be excellent devices for control of many industrial particulate emissions, including smoke from electricity-generating utilities (coal and oil fired), salt cake collection from black liquor boilers in pulp mills, and catalyst collection from fluidized bed catalytic cracker units in oil refineries to name a few. These devices treat gas volumes from several hundred thousand ACFM to 2.5 million ACFM (1,180 m³/s) in the largest coal-fired boiler applications. For a coal-fired boiler the collection is usually performed downstream of the air preheater at about 160 °C (320 deg.F) which provides optimal resistivity of the coal-ash particles. For some difficult applications with low- sulphur fuel hot-end units have been built operating above 371 °C (700 deg.F). The flue gas after passing through the air-preheaters comes down to lower temperature that is feasible for releasing into the atmosphere, but one vital job remains still left out, i.e. to remove the carbon content of the gas so that it does not harm the atmosphere. This job is done by ESP, the flue gas after air-preheater comes to the ESP unit. ESP actually works on the principal of CORONA DISCHARGE EFFECT ; the ESP unit houses two electrode plates called emitting plate and collecting plate. The emitting plate is supplied with a very high DC negative potential (in order of**), this results into ionizing of air molecules surrounding the emitting plate which is called corona effect. The collecting plate is grounded and a positive potential develops on it, as a result when the flue gas pass through between them the carbon particles are attracted to the collecting plates. The collecting Inside ESP, MTPS plates are attached to hopper where the ashes get deposited by hammering action on the collecting plate. For a 210MW unit 24 such hoppers are present in each ESP; these hoppers have mechanical transport system for proper disposal of ash. For better corona effect the emitting plate is made corrugated because this way more air molecules get ionized as corona discharge points are more in number in corrugated plate. Exciter Transformer of ESP, MTPS Page | 39
  41. 41. Ash Handling Plant  What is Ash? Ash is the residue remaining after the coal is incinerated.  What is Ash Handling? Ash handling refers to the method of collection, conveying, interim storage and load out of various types of ash residue left over from solid fuel combustion processes.  Why Ash Handling System is required?  In Thermal Power Plant’s coal is generally used as fuel and hence the ash is produced as the by-product of Combustion. Ash generated in power plant is about 30-40% of total coal consumption and hence the system is required to handle Ash for its proper utilization or disposal.  CHALLENGES OF ASH HANDLING:-  Indian coal presents high ash content generally which tends to be inconsistent. Design of the system has to adequately cover anticipated variations and be capable of handling the worst scenario.  System has to be environmentally friendly.  System has to be reliable with least maintenance problem.  System has to be energy efficient.  Ash terminology in power plants:-  Fly Ash ( Around 80% is the value of fly ash generated)  Bottom ash (Bottom ash is 20% of the ash generated in coal based power stations.)  What is fly ash?  Ash generated in the ESP which got carried out with the flue gas is generally called Fly ash. It also consists of Air pre heater ash & Economiser ash (it is about 2 % of the total ash content).  What is bottom ash?  Ash generated below furnace of the steam generator is called the bottom ash. Page | 40
  42. 42. Volume of ash and properties The ash handling system handles the ash by bottom ash handling system, coarse ash handling system, fly ash handling system, ash disposal system up to the ash disposal area and water recovery system from ash pond and Bottom ash overflow. Description is as follows: A. Bottom Ash Handling System Bottom ash resulting from the combustion of coal in the boiler shall fall into the over ground, refractory lined, water impounded, maintained level, double V-Section type/ W type steel- fabricated bottom ash hopper having a hold up volume to store bottom ash and economizer ash of maximum allowable condition with the rate specified. The slurry formed shall be transported to slurry sump through pipes. C. Air Pre Heater ash handling system Ash generated from APH hoppers shall be evacuated once in a shift by vacuum conveying system connected with the ESP hopper vacuum conveying system. D. Fly Ash Handling System Fly ash is considered to be collected in ESP Hoppers. Fly ashes from ESP hoppers, extracted by Vacuum Pumps, fly up to Intermediate Surge Hopper cum Bag Filter for further Dry Conveying to fly ash silo. Under each surge hopper ash vessels shall be connected with Oil free screw compressor for conveying the fly ash from Intermediate Surge Hopper to silo. Total fly ash generated from each unit will be conveyed through streams operating simultaneously and in parallel. E. Ash Slurry Disposal System Bottom Ash slurry, Fly ash slurry and the Coarse Ash slurry shall be pumped from the common ash slurry sump up to the dyke area which is located at a distance from Slurry pump house.  ADVANTAGES:- i) Commercial utilization of ash in: iii)Energy Efficient –Cement additives. iv)High reliability –Brick plants. v)Long Plant Life –Road making, etc. vi)Least maintenance ii) Saving of water - vii)Environment concern a precious commodity. Page | 41
  43. 43. ELECTRICAL SYSTEM OVERVIEW ELECTRICAL SYSTEM OF A THERMAL POWER PLANT BASICALLY CONSISTS OF THE FOLLOWING PARTS:-  GENERATOR  SWITCHYARD  POWER DISTRIDUTION SYSTEM GENERATOR The transformation of mechanical energy into electrical energy is carried out by generator. The A.C generator or alternator is based on the principal of electromagnetic induction and generally consists of a stationary part called stator and a rotating part called rotor. The stator houses the armature windings and the rotor houses the field windings. A D.C voltage is applied to the field winding in the rotor through slip rings, when the rotor is rotated, the lines of magnetic flux is cut through the stator windings. This as a result produces an induced e.m.f (electromotive force) in the stator winding which is tapped out as output. The magnitude of this output is determined by the following equation:- E = 4.44/O f N volts Where E = e.m.f. induced; O =Strength of magnetic field in Weber; f= Frequency in cycles per second or in hertz; N = Number of turns in the winding of the stator; Again, f = P n/120; Where P = Number of poles; n = revolutions per second of the rotor. From the above expression it is clear that for the same frequency number of poles increases with decrease in speed and vice versa. Therefore low speed hydro turbine drives generators have 14to 20poles where as for high speed steam turbine driven generators have 2 poles. Generator Components Rotor: Rotor is the most difficult part to construct; it revolves at a speed of 3000rpm. The massive non-uniform shaft subjected to a multiplicity of differential stresses must operate in oil lubricated sleeve bearings supported by a structure mounted on foundations all of which poses complex dynamic behaviour peculiar to them. It is also an electromagnet and to give it the necessary magnetic strength the Page | 42
  44. 44. windings must carry a fairly high current. The rotor is a cast steel ingot and it is further forged and machined. Very often a hole bored through the centre of the rotor axially from one end to the other for inspection. Slots are then machined for windings and ventilation. Rotor winding: Silver bearing copper is used for the winding with mica as insulation between conductors. A mechanically strong insulator such as micanite is used for lining the slots. For cooling purpose slots and holes are provided for circulation of cooling gas. The wedges the windings when the centrifugal force developed due to high speed rotation tries to lift the windings. The two ends of the winding are connected to slip rings made of forged steel and mounted on insulated sleeves. Stator: The major part of the stator frame is the stator core, it comprises of inner and outer frame. The stator core is built up of a large number of punching or section of thin steel plates. The use of cold rolled grain-oriented steel can contribute to reduction of stator core. Stator windings: Each stator conductor must be capable of carrying the rated current without overheating. The insulation must be sufficient to prevent leakage current flowing between the phases to earth. Windings for the stator are made up from copper strips wound with insulated tape switch is impregnated with varnish, dried under vacuum and hot pressed to form a solid insulation bar. In 210MW generators the windings are made up of copper tubes through which water is circulated for cooling purpose. Generator Cooling and Sealing System 1) HYDROGEN COOLING SYSTEM: Hydrogen is used as cooling medium in large capacity generators in view of its high heat carrying capacity and low density. But in view of its explosive mixture with oxygen, proper arrangement for filling, purging and maintaining its purity inside the generator have to be made. Also in order to prevent escape of hydrogen from the generator casing, shaft sealing system is used to provide oil sealing. The system is capable of performing the following functions:- a) Filling in and purging of hydrogen safely. b) Maintaining the gas pressure inside the machine at the desired value all the time. c) Provide indication of pressure, temperature and purity of hydrogen. d) Indication of liquid level inside the generator. 2) Generator Sealing System: Seals are employed to prevent leakage of hydrogen from the stator at the point of rotor exit. A continuous film between the Page | 43
  45. 45. rotor collar and the seal liner is maintained by means of oil at the pressure which is about above the casing hydrogen gas pressure. The thrust pad is held against the collar of rotor by means of thrust oil pressure, which is regulated in relation to the hydrogen pressure and provides the positive maintenance of the oil film thickness. The shaft sealing system contains the following components. a) A.C oil pump. b) D.C oil pump. c) Oil injector. d) Differential Pressure Regulator e) Damper tank. Excitation System 1) STATIC EXCITATION:  Alternator terminal voltage is used here.  SCR- based controlled rectifier is supplied is supplied from alternator output through step down transformer.  SCR gate signal are derived from alternator output through CT & PT.  Rectifier output voltage is fed to the alternator field winding.  To generate the alternator output, it is run at rated speed with its field supplied from a separate D.C supply bank.  This scheme is less expensive & requires little maintenance.  Excitation energy depends on alternator speed. 2) BRUSHLESS EXCITATION:  Main shaft of prime movers drives pilot exciter, main exciter & the main alternator.  Pilot exciter is a permanent magnet alternator.  Pilot exciter feeds 3-phase power to main exciter.  Main exciter supplies A.C power to silicon diode bridge rectifier through hollow shaft which feeds the D.C to the field of main alternator.  SCR gate signals are derived from alternator output through CT & PT.  This scheme is mainly employed in turbo alternators. Page | 44
  46. 46. Specification of Generators PARAMETERS UNIT-1 UNIT-2 UNIT-3 UNIT-4 UNIT-5 UNIT-6 Maker BHEL BHEL BHEL BHEL BHEL BHEL Kw 210000 210000 210000 210000 250000 250000 P.F 0.85 Lag 0.85 Lag 0.85 Lag 0.85 Lag 0.85 Lag 0.85 Lag KVA 247000 247000 247000 247000 294100 294100 Stator Volts- Volts- Volts- Volts- Volts- Volts- 15750 15750 15750 15750 15750 16500 Amps- Amps- Amps- Amps- Amps- Amps- 9050 9050 9050 9054 10781 10291 Rotor Volts- 310 Volts- Volts- Volts- Volts- Volts- Amps- 310 310 256 292 292 2600 Amps- Amps- Amps- Amps- Amps- 2600 2600 2088 2395 2395 R.P.M. 3000 3000 3000 3000 3000 3000 Hz 50 50 50 50 50 50 Phase 3 3 3 3 3 3 Connection YY YY YY YY YY YY Coolant Hydrogen Hydrogen Hydrogen Hydrogen Hydrogen Hydrogen & water & water & water Gas pressure 3.5 BAR(G) 3.5 3.5 2 BAR(G) 3 BAR(G) 3 BAR(G) BAR(G) BAR(G) Insulation B B B F F F class Year of 1994 1994 1992- 2004 2006 2006 manufacture 1993 TRANSFORMERS  It is a static device which transfers electric powers from one circuit to the other without any change in frequency, but with a change in voltage and corresponding current levels also.  Here the transformers used are to transfer electric power from 15.75KV to 220KV or 400KVthat are provided to the national grid.  The step-up generator transformers are of ONAN/ANOF/AFOF cooling type. Page | 45
  47. 47. Neutral Grounding Transformer(NGT):  The NGT is used to prevent the generator from earth faults.  It comprises of primary winding and secondary winding, the secondary winding is connected with a high value resistance. Whenever earth fault arises heavy current flows to the primary winding and as a result an e.m.f is induced in the secondary.  The voltage drop across the resistance is sensed by the NGT relay and it actuates to actuate the Generator Circuit Breaker (GCB) and thus the generator is tripped.  Limited Earth-Fault Earthling System: Generators and other apparatus installed at higher voltage levels are exposed to much greater fault energy… in the order of thousands of MVA. Earth-fault currents could damage iron structures in generators, motors, and transformers, so that they can't be repaired, but instead must be replaced… at great cost! Hence, some method of current limiting, like NGT (Neutral Grounding Transformer) or NGR (Neutral Grounding Resistor) is beneficial. Power Transformer:  Power Transformers enhances the productivity as well as maximizes the capacity level of the high power supply equipments.  These are ultimate for the regular power without any cut off. They are used for control high voltage and frequency for the different systems.  Power transformers having the following standards: They can assist three phases. There ratings are up to 2000 KVA. Copper and aluminium winding material is used in this. Applicable Standards are IS, IEC, ANSI, JIS, etc. It is sufficient for primary as well as secondary voltage. Auto Transformer:  High voltage auto-transformers represent an important component of bulk transmission systems and are used to transform voltage from one level to another.  These auto-transformers are critical for regional load supply, inter-regional load transfers and for certain generator/load connections.  Major or catastrophic failures to this equipment can have severe consequences to electric utilities in terms of increased operating costs and customer load losses.  To minimize the impact of this type of failures, utilities may carry some spare units to guard against such events. These spare units are going to cost utilities money (utility cost) to purchase, to store and to maintain and utilities should try to strike the right balance between the utility cost and the risk cost (if spare units are not there). Page | 46
  48. 48. Advantages of Autotransformers: 1. Its efficiency is more when compared with the conventional one. 2. Its size is relatively very smaller. 3. Voltage regulation of autotransformer is much better. 4. Lower cost. 5. Low requirements of excitation current. 6. Less copper is used in its design and construction. 7. In conventional transformer the voltage step up or step down value is fixed while in autotransformer, we can vary the output voltage as per out requirements and can smoothly increase or decrease its value as per our requirement. Applications: 1. Used in both Synchronous Motor and Induction Motor. 2. Used in electrical apparatus testing labs since the voltage can be smoothly and continuously varied. 3. They find application as boosters in AC feeders to increase the voltage levels. Generating Transformer (GT):  This is a type of Power Transformer where the LV winding is connected to the generator through the bus duct and HV winding to the transmission system. In addition to the features of Power Transformer, our Generator Transformer is designed to withstand over voltage caused by sudden load throw off from the generator. It is built as a single or three phase unit and located in power stations.  Normally generating voltage is 15.75KV from generator. If we want to transmit that power to 220 KV busbar. This voltage must be stepped up, otherwise if we transmit at same voltage level as generation voltages that is associated with high transmission loss so the transformer which is used at generator terminal for stepping up the voltage is called Generating transformer. Specification of GT MAKER BHEL MVA HV- 150/200/250 LV- 150/200/250 VOLTS HV- 245 KV LV- 15.75 KV RATED CURRENT HV- 151/482/602 LV- 3505/7340/9175 PHASE 3 FREQUENCY 50 TYPE OF COOLING OFAF/ONAF Page | 47

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