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Thermal Power Plant Familiarisation (Part- I : Boiler) Presented By-: Mohit Chandra Joshi Engineer National Load Despatch Centre Power Grid Corporation Of India Ltd. Slide 2 Outline Of Presentation Basics of Thermal Power Plant Coal Handling Plant Fuel System Firing System Boiler Fundamentals Water Circulation System Steam Circulation System Air and Draft System Ash Handling System Once Through Boilers Slide 3 Basics Of Thermal Power Plant A thermal power station is a power plant in which the prime mover is steam driven. It is Based upon Rankine Cycle Water is heated, turns into steam and spins a steam turbine which either drives an electrical generator. Then it is condensed in a condenser and then recycled to where it was heated. Efficiency ranges between 33% to 40% depending upon the no. of reheat and regeneration. Performance indicator-: Heat Rate Slide 4 Slide 5 Reheating & Regeneration Reheating is used to control the quality of steam at the turbine exhaust. In the reheat cycle the expansion of steam from the initial state to the condenser pressure is carried out in two or more steps depending upon the number of reheat. Regeneration is a step towards Carnotization of Rankine Cycle. Steam is taken from some specific stages of turbine to heat the feedwater. Slide 6 Daily Burden of a 500MW Unit (At 80% load factor) Coal4800 Tonnes Furnace Oil60 kl Hydro Chloric Acid1200kg Sodium Hydroxide 600kg Condenser Cooling Water2000cu.m/hr/100MW Water for Steam Cycle1500 to 2000cu.m/hr Make up Water60 cu.m/hr Slide 7 Boiler Specification Dry Bottom Furnace-:Ash or slag is removed in the solid or dry state. Wet Bottom Furnace-:40-60% of the ash in coal moves with the flue gases and rest falls through furnace bottom. Tangentially Fired-: Corner fired Pulverised fuel system-: Coal size of the order of 40-60 micron Direct fired -:No Pulverised coal Bunker Balanced Draft - Induced and Forced Draft Fans are utilized in maintaining a vacuum in the furnace Slide 8 Coal Handling Plant In the coal handling system of NTPC stations, three coal paths are normally available Path A - direct conveying of coal from track hopper to boiler bunkers Path B - from track hopper to stockyard Path C - from stockyard to boiler bunkers. Capacity-:1200-2000 t/hr (depending upon calorific value) The 200mm received coal is fed to the crusher house through a series of conveyor belts In line magnetic separators are provided upstream of crushers to prevent damage to the crusher by tramp iron present in the raw coal. Slide 9 Overview of Coal Handling Plant Slide 10 Coal Handling System-:Equipments Conveyor Belt Drive Unit Take Ups Scrapper Vibrating Screen Magnetic Separators Vibrating Feeder Trippers Crusher Slide 11 Fuel System Feeders A raw coal feeder is a device that supplies the pulveriser with an uninterrupted flow of raw coal from the bunker to meet system requirements. Types of Feeders : Volumetric Feeder : R (kg/s)=Q (m 3 /Rotation)*B (m 3 /kg)*N (Rotation/sec) Gravimetric Feeder : R=M (Kg/m)*N (m/s) Slide 12 Raw Coal Feeder Slide 13 Mills Pulverisers or mills are used to grind or comminute the coal. Classification of Mills Speed Category Speed Range Mill Slow Speed Below 50 rpm Ball/Tube/ Drum Mill Medium Speed 50-100 rpm Bowl Mill Ball and race mill High Speed Above 225 rpm Hammer Mill Beater Wheel Mill Slide 14 BOWL MILL BALL MILL Slide 15 Firing System Direct Firing Indirect Firing Methods of Fuel Firing : There are many ways of firing the coal in furnace. They are: Vertical firing Horizontal firing Corner or tangential firing Slide 16 Components of Fuel Firing System Ignition System Ignition Energy is required to initiate the combustion and to keep the flame stable. The auxiliary ignition energy for coal firing is provided by suitably located oil burners. For igniting the oil while starting the oil burners, igniters are used in the firing system. Slide 17 Ignitors Eddy Plate Ignitor Ion Flame Monitoring Ignitor (IFM) High Energy Arc Ignitor (HEA) Scanners Scanners are the flame sensing devices Ultra Violet Scanners Visible Light Scanners Infra red Scanners Slide 18 Oil Guns And Atomisers Fuel oils are used to ignite the coal as well as to supplement the coal firing under various exigencies. Oil guns either concentric tubes or parallel tubes installed through the furnace walls or windbox with atomizer mounted at their tips provide this oil spray to the furnace Atomisation is done for efficient combustion Slide 19 Slide 20 Burner Arrangement Twenty four pulverised coal burners arranged on the corners at a height of 18 to 25 meters Twelve oil burners provided each in between two pulverised fuel burners. Six mills supply the coal to burners at 4 corners. Four wind boxes fixed at 4 corners of the furnace. 13 nozzle in each wind box 6 for coal and 7 for air Slide 21 Slide 22 Boiler Fundamentals The primary function of oil and coal burning systems in the process of steam generation is to provide controlled efficient conversation. Need of Excess Air 3Ts of Combustion 1. Time 2. Temperature 3. Turbulence Slide 23 Arrangement Of Main Boiler Slide 24 Furnace-:A boiler furnace is that space under or adjacent to a boiler in which fuel is burned and from which the combustion products pass into the boiler proper. Boiler Drum-:The function of steam drum is to separate the water from the steam generated in the furnace walls and to reduce the dissolved solid contents of the steam to below the prescribed limit of 1 ppm. Economiser-:The purpose of economiser is to preheat the boiler feed water before it is introduced into the steam drum by recovering heat from the flue gases leaving the boiler. Super Heater-: (1) first stage consists of horizontal superheater of convection mixed flow type with upper and lower banks located above economiser assembly in the rear pass (2) The second stage superheater consists of pendant platen which is of radiant parallel flow type. (3) The third stage supherheater pendant spaced is of convection parallel flow type. Slide 25 Boiler Drum Internals Slide 26 Burners: There are total twenty four pulverised coal burners for corner fired boilers and twelve oil burners provided each in between two pulverised fuel burner. All the nozzles of the burners are inter linked and can be tilted as a single unit from +30 to -30 The oil burners are fed with heavy fuel oil till boiler load reaches to about 25%. Igniters: There are twelve side eddy plate oil/H.E.A igniters per boiler. Ignitors are used for lighting the main oil gun The atomising air for igniters are taken from plant air compressors at 7Kg/cm2 (gauge). There are two ignitor air fans supply air for combustion of ignitor oil. soot blowers are used for efficient on-load cleaning of furnace, superheaters, reheaters and regenerative air heaters Slide 27 Water Circulation System Boiler Circulation System 1. Natural circulation system (P175kg/cm 2 ) 3. Combined circulation system (260kg/cm 2 ) Nucleate Boiling Departure from Nucleate Boiling Circulation Ratio (6 to 9) Slide 28 Economiser The function of an economiser in a steam generating unit is to absorb heat from the flue gases and add this as sensible heat to the feed water before the water enters the evaporative circuit of the boiler. Location-: Economiser is located ahead of airheaters and following the primary superheater or reheater in the gas stream. Economiser coils are designed for horizontal placement which facilitate drainings of the coil and favours the arrangement in the second pass of boiler. Slide 29 Water Walls Water walls completely cover the interior surfaces of the furnace. Water Wall provide the means to heat and evaporate the feed water. These tubes receive water from the boiler drum by means of downcomers connected between drum and water walls lower header. 50 percent of the heat released by the combustion of fuel in the furnace is absorbed by the water walls. The mixture of steam and water is discharged from the top of the water walls tubes into the upper wall header and than passes through riser tubes to the steam drum. Slide 30 Steam Circulation System Riser Tubes Superheater Radiant Superheater Convection Superheater Desuperheater/Attemperator Desuperheating or attemperation is the reduction or removal of superheat from steam to the extent required. Used for control of temperature in load variation. Reheaters This is the part of the boiler which receives steam back from the turbine after it has given up some of its heat energy in the high pressure section of the turbine Slide 31 Air and Draft System Natural Draft-:In natural draft units the pressure differentials are obtained by constructing tall chimneys so that vacuum is created in the furnace Due to the pressure difference, air is admitted into the furnace Induce Draft-:In this system the air is admitted to natural pressure difference and the flue gases are taken out by means of induced Draft fans and the furnace is maintained under vacuum Forced Draft-:A set of forced draft fans are made use of for supplying air to the furnace and so the furnace is pressurized. The flue gases are taken out due to the pressure difference between the furnace and the atmosphere Balanced Draft-:Here a set of Induced and Forced Draft Fans are utilized in maintaining a vacuum in the furnace. Normally all the power stations utilize this draft system Slide 32 Primary and Secondary Air System Slide 33 Different Fans Used In a Power Plant ID Fan PA Fan FD Fan Ignitor air Fan-:These fans, again two per boiler, are used to supply air for cooling ignitors and combustion of ignitor air fuel mixture. Scanner air Fan-:These fans, two per boiler, supply requisite air for scanner cooling. Fan Output Control Inlet Guide Vanes Blade Pitch Control Speed Control Slide 34 Air Preheater It is used to preheat the combustion air (primary and secondary) from the flue gases. Types of Air Preheater -: Recuperative-:Heating medium is on one side and air is on the other side of tube or plate and heat transfer is by conduction through the material which separates the media Regenerative-:the heating medium flows through a closely packed matrix to raise its temperature and then air is passed through the matrix to pick-up the heat Slide 35 Ash Handling System Bottom Ash System Ash slag discharged from the furnace bottom is collected in two water impounded scraper troughs installed below bottom ash hoppers. Ash is continuously, transported by means of the scraper chain conveyor, on to the respective clinker grinders. Crushed ash from the clinker grinders falls into the ash sluice trench and transported to ash slurry sump aided by the ash sluice channel Slide 36 Fly Ash System Electro Static Precipitators - Used to trap fly ash - Reduced ID fan maintenance - Dust laden is charged by absorption of free ions. - Removed from collecting electrode by intermittent blow to hoppers Fly ash collected in the hoppers drop continuously to flushing apparatus where fly ash gets mixed with flushing water and the resulting slurry drops into the ash sluice channel. Ash Slurry System Bottom ash and fly ash slurry of the system is sluiced upto ash slurry pump along the channel with the aid of high-pressure water jets located at suitable intervals along the channel Slide 37 Once Through Boilers When all the water flowing through water wall gets converted into steam in one circulation, it is called once through Boiler. Circulation ratio is one and no Boiler drum for storing water as buffer stock. Water flow through water wall, in a Once through Boiler, is one fifth that of a conventional Boiler. Religious Monitoring of metal temperature of water walls. Orientation of tubes is changed from vertical to spiral. Slide 38 Fluidized Bed Combustion Fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process. FBC plants can be fired with any fuel like coal, biomass etc. Limestone is used to precipitate out sulfate during combustion The technology burns fuel at temperatures of 750-900 C, well below the threshold where nitrogen oxides form. Slide 39 Boiler Startup Metal Temperature above 350 0 C- Hot Start Metal Temperature between 150-350 0 C- Warm Start Metal Temperature below 150 0 C - Hot Start Slide 40 Questions ? Slide 41 Slide 42 Slide 43 Slide 44 Slide 45 Slide 46 Slide 47 Slide 48