Note for Power Plant Engineering - PPE By UPTU Risers

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1.1 THERMAL POWER PLANT 1.1.1 THERMAL POWER STATION: A thermal power station converts the chemical energy of the fossil fuels into electrical energy. In such plants, this action is achieved by generation of steam in boilers and then fed this high pressurized steam to turbine, which is mechanically coupled with generator and responsible of electrical energy generation.  After which this coal is fed to the stoker and burn it under proper conditions to boil the water in boilers. COAL STORAGE COAL HANDLING PLANT Mechanical energy Generator Steam Water Steam Coupling turbine Flue gases Boiler Electrical energy ASH HADLING PLANT BOILER FUEL GAS ASH STORAGE FIG.1.2 COAL & ASH CIRCUIT Thermal energy FIG.1.1 BASIC LAYOUT OF THERMAL POWER PLANT 1.1.3 MAIN PARTS & THEIR WORKING: Main steam station of a thermal plant works on the Rankine cycle. The plant can be divided into four basic circuits: (a) Coal & Ash Circuit:  Coal is transported from supply points to storage through transportation medium like by road/train or from others.  After proper treatment i.e. passing the coal through curshers, dryers and magnetic seperators (for sizing, removal of magnetic substances or imuprities) this coal is fed to the boiler furnace through a conveyer belt. By: DEEPAK PANDEY BOILER PREHEATER FORCED DRAFT FAN CHIMNEY 1.1.2 SITE SELECTION: Selection of site of any power plant plays an important role in the economy of the station. Site selection is based on various important factors, some of which for thermal power plant is given as: (a) Cost of land: Cost of land should be reasonable and further extensions, if necessary should be possible. (b) Nature of land: The type of the land selected as site should have good bearing capability to withstand the load of the plant. (c) Availability of fuel: Thermal power stations requires huge amount of fuel per day. Therefore it is necessary that the location of the plant should in such that the fuel may available at low cost and it should be easy to deliver fuel from coal fields at a low transportation charges and within time. (d) Availability of water: Thermal (steam) power plants requires large amount of water because water is used as working fluid wich is respectively evaporated and condenced. Abundant quantity of cooling water for condenser should also available and its large amount is required for ash handling. It is therefore, necessary to locate the power plant near water body. (e) Easy transportation facility: It is also a very important consideration. It is always necessary to have easy transportation by which transportation of fuel and heavy machinery becomes an easy task. (f) Waste disposal facility: It is an important factor because proper disposal of wate products is very important, since they affects environment and may create serious problems. (b) Air & Gas Circuit:  For combustion of fuel, air is required and this air is supplied through the forced draught fan and induced draught fan.  The air fed to the boiler is pass through the air pre-heater to exhaust energy of flue gases coming out from the boiler.  Then it passes through the economizer and dust collector and then finally released to the atmosphere through chimney. ECONOMIZER FURNACE INDUCED DRAFT FAN FIG.1.3 AIR & GAS CIRCUIT (c) Feed Water & Steam Circuit:  The exhausting of turbine is condensed by condenser. Is first heated in closed feed water heater through extracted steam.  This water is then passed though deaerator (which plays a role to reduce the dissolved oxygen content), then this feed water is pumped to a high pressure heater to the boiler through economizer.  Some part of steam and water is lost in circulation of different componets of the system, adding a make-up water is necessary in feed water to compensate it.  In boiler, water is converted into high pressure steam, which is fed to super-heaters to absorb its moisture and make it dry.  This high pressurized dry steam is now fed to the turbine through the main valve. (d) Cooling Water Circuit:  Colling water is used to condense the steam in condenser and maintaining it on a low pressure.  In such process a large ammount of cooling water is required, which may be taken from river side or any other reservoir. email: d.pandey2906@gmail.com Page 1

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TURBINE CONDENSER CIRCULATING WATER EVAPORATOR VALVE SUPER HEATER CONDENSER ECONOMIZER WATER BOILER BOILER TO ALTERNATOR MAKE-UP WATER TANK TO ALTERNATOR SUPERHEATER TURBINE STEAM COOLING WATER PUMP RIVER OR POND FEED WATER PUMP FEED WATER HEATER CONDENSATE PUMP FIG.1.5 COOLING WATER CIRCUIT FIG.1.4 FEED WATER & STEAM CIRCUIT 1.1.4 BOILERS:  A device (closed vessel) which is use to produce steam under high pressure.  It is one of the major components of the steam power plant.  Boilers are broadly classified as: fire tube boiler & water tube boiler. Generally water tube boilers are used. (a) Fire Tube Boiler:  In these boilers, the products of the combustion pass through the tubes which are surrounded water.  They have low initial cost.  In such boilers, the water volume is more and circulating being poor so they cannot meet quickly changes in staem demand.  Depending on the tubes, they may be further classified as: vertical and horizontal tube boilers.  They may be internally fed or externally fed.  In and internally fed boiler, grate and combustion chamber enclosed within the boiler shell while in externally fed boilers all these are separate and distinct from boiler shell. FURNACE HOT GAS HOT GASES OUTLET BOILER AIR PREHEATER SUPER HEATER FLUE GAS AIR STEAM STEAM DRUMS STEAM OUTLET FIRE TUBE (b) Water Tube Boiler:  In this boiler, water tubes are surrounded by hot gases which are produced due to the combustion of fuel.  These tubes are interconnected by common water storage and to steam outlet.  Depending on the positions of tubes, water tube boiler may classified as: vertical, horizontal and inclined tube boilers.  The circulation of water takes place naturally or it is a forced process as per requirment and condition. COOL C O C MB H U A S M T BE IO R N ECONOMISER ASH FIG.1.7 WATER TUBE BOILER FIG.1.6 FIRE TUBE BOILER By: DEEPAK PANDEY 1.1.5 SUPERHEATERS & REHEATERS:  These are used to superheat steam to the desired value.  Superheated steam contains more heat than saturated steam at same pressure & it has been heated above the temprature corresponding to its pressure.  Due to superheating of the steam, the heat provides more email: d.pandey2906@gmail.com Page 2

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energy to the turbine and thus power output is more.  Corrosion of the turbine blads reduces and the steam can be transmitted for longer distances with low heat loss.  Superheater removes the last traces of moisture from the saturated steam leaving the boiler tube and increases the temprature of this steam.  Superheaters may be classified as: (a) Convection superheaters, (b) Radiant superheaters or may be combination of both.  A convection superheater is located where gas stream receives most of the heat convection.  A radiant superheater is placed near to the furnace where heat is received by radiation.  In modern boilers, reheaters also used to suoerheat the partly expended steam from the turbine.  This action dries the steam through the last stage of turbine. 1.1.6 STEAM TURBINE:  A steam turbine utilized the heat energy of steam by converting it into mechanical energy which drive the generator.  When steam passes through the small opening attain high velocity and then it if used to rotate the turbine.  Steam primover is of two types: (a) impulse turbine; (b) reaction turbine.  Both types of turbines have various stages in which pressure drop takes place.  Usually the number of stages in a reaction turbine are more thean in an impulse turbine of same rating.  In an impulse turbine stationary nozzels are used to attain a high velocity.  Nozzles converted potential enery of steam due to pressure in kinetic energy.  A reaction turbine has no nozzels.  A potential drop of pressure is used to allow tha steam into the moving blades. 1.1.7 CONDENSER:  Condenser are used to decrease the exaust pressuer of the steam below atmospheric pressure, which improves the efficiency of the power plant.  Condensers also help to recover steam and this provides a source of good pure water to the boiler and this process reduces the capacity of water softening plant.  Condensers are of two types: (a) Jet/contact condensers; (b) Surface Condenser.  Jet/Contact Condenser:  In such condensers, colling water is mixed with exaust steam.  Generally water from river or any other storage is used for this purpose through centrifugal pumps.  After use this warm water is returned to the river.  Surface Condenser:  It consist of a air-tight cylindrical shell having a chamber at each end.  Cooling water circulated through the tubes.  The steam enters from the top and get condensed due to contact with the surface of these cooling water tubes and leaves from the bottom. By: DEEPAK PANDEY AIR INJECTION WATER SPRAY CONDENSER SHELL EXHAUST STEAM TO BOILER PUMP COOLING POND FIG.1.8 JET/CONTACT CONDENSER EXHAUST STEAM PLATE WATER BOX CONDENSATE TO EXTRACTION PUMP COOLING WATER OUTLET COOLING WATER INLET FIG.1.9 SURFACE CONDENSER 1.1.8 ECONOMISER:  An economiser is a heat exchanger wich extract heat from flue gases and this exctracted heat is used to raise the temprature of feed water.  The use of an economiser results in saving in coal consumption and higher boiler efficiency. It also increases the evaporation efficiency of boiler.  Economiser is generally used in all mordern power plants.  Economiser contains thin water tubes of small diameter, placed between two headers.  Flue gases flow outside the tubes and feed water enters from one header and leaves through the other.  The temprature range of the various parts of the boiler is redused which is responsible for reduction of stress due to unequal expansion. email: d.pandey2906@gmail.com Page 3

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 A regenerative air preheater forms a compact and efficient heat exchanger for steam power plant due to its large surface area provided by rotor element. FLUE GASES FEED WATER OUTLET FEED WATER INTLET FLUE GASES FIG.1.10 ECONOMISER 1.1.9 AIR PREHEATER:  Air preheaters are used to extract more heat from the flue gases coming out from economiser.  Cooling of flue gases by 200C raises the plant efficiency by 1%.  Air preheaters may be classified as: plate type or tubular type or regenerative type.  In plate type air heater, alternate narrow lanes are there for gas na dair passages. In which two fluids flow in opposite directions. SHAFT COLD AIR ROTOR ELEMENT COLD FLUE GAS HOT AIR HOT FLUE GAS FIG.1.11 AIR PREHEATER (REGENERATIVE)  In tabular air heaters, the gases flow inside the tubes and the air passes over the tube exteriers.  The regenerative preheaters are the most popular preheatear which uses a cylendrical rotor made of corrugated steel plates, which is fixed on to a shaft and rotates at a speed of 2 to 4 rpm.  The rotation of rotor alternatively passes through flue gases and air zones.  Flue gases heated the rotor element in their zone and this element transfer this heat to air when they are in air zone. By: DEEPAK PANDEY 1.1.10 FUEL/COAL HANDLING SYSTEM: In most of the thermal plants coal is used as fuel and more tha half of the operating cost is on account of coal, therefore its handling is very important for proper operation of plant. Requirments odf a good fuel handling syatem are:  It should be reliable;  Should require minium mantinance and operating cost;  Should fulfill the requirment during peak periods, etc. Various stages of a fuel handling system is as: (a) Delivery of fuel: Fuel(coal) may be delivered by sea or Fuel Delivery river, rail or road. Coal selection Road/River/Rail is depend upon the system capacity in tonnes per hour, location of the plant with Unloading transportation medium and the storage facility. Preparation (b) Unloading: In unloading the choice of equipment will depend on how the coal is Transfer received. (c) Preperation: Proper Outdoor storage preparation of coal is very important before its Indoor Storage combustion. It may contain various other elements so coal preparation plant may include: In-plant Handling (i) Crushers (ii) Sizers (iii) Dryers Measurement (iv) Magnetic separators. (d) Transfer: Tranfer of coal from unloading point to storage site Furnace Firing from where it is discharged to the firing equipment, depends FIG. 1.12 COAL on local conditions. Various HANDLING equipments, which are used are following: (i) Belt conveyors, (ii) Screw conveyors, (iii) Bucket elevators, (iv) Grab bucket conveyers, (v) Skip hoists, (vi) Flight conveyers etc. (e) Outdoor/Dead Storage: Outdoor or dead storage is used to store the coal for a long duration of time span. In most of the plants such storage stores coal for 15 days to 1months or more than that. (f) Indoor/Live storage: Such a storage constitutes coal requirments of the plant for a day. The live storage can be provided with bunkers and coal bins. (g) Inplant Handling: This refers to handling of coal between the final storage to the firing equipment. Equipments used for inplant handling are the same as used for coal transfer. (h) Coal Weighing: A correct measurment of coal enables one to have an idea of total quantity of coal delivered at the site and also whether or not proper quantity has been burned as per load on the plant. (i) Weigh bridge, (ii) Belt scale, (iii) Automatic recording system. email: d.pandey2906@gmail.com Page 4