Thermal ( Steam ) Power plants mainly consists of 4 circuitsCoal and ash CircuitCoal produced in the mining site is transported to power plant siteCoal handling equipment for generation of steamThe combustion of coal produces ash which is collected and removed to ash storage yard through ash handling equipments

FD or ID fans are used for supply the air to combustion chamber of the boiler through air-preheaterThe air preheater is placed in the path of flue gases to preheat the airThe flue gases produced by combustion of fuels in the boiler furnaces after passing around boiler tubes and super heater tubesPass through a dust collector or precipitator where most of dust is removed before venting it of to atmosphere through chimneyAir and gas circuit

Prime mover develops power by utilizing steam generated in the boiler Then condenser is used to condense the steam coming out of prime mover A pump is used to feed the condensate to the boilerThe condensate leaving the condenser is heated in feed heaters through extracted steam from lowest pressure extraction point of the turbineThe feed water may also be supplied from external source to compensate any loss of steam and water.In the boiler shell and tubes water circulation is setup due to density difference of water between low and high temperature sectionsA super heater is used to super heat the wet steam from boiler drum and is then supplied to prime moversFeed water and steam circuit:

In the condenser, quantity of cooling water required to condense the steam is large and is taken either from lake, river or seaThe cooling water is taken from upper side of the river and then passed through the condenser The hot water is then discharged to lower side of the riverThe system is known as open systemWhere water is not available in abundant water from condenser is cooled either in cooling pond or in cooling tower the system is known as closed system

Cooling water circuit

Main Components of Thermal Power Plants

BoilerSuper heaterEconomizerAir preheaterReheaterSteam turbineGeneratorCondensersCooling towersPumpsCoal millsFD and ID FansASH PrecipitatorsWater treatment plant

Essentials of steam power plant

1. A furnace to burn the fuel. 2. Steam generator or boiler containing water. Heat generated in the furnace is utilized to con- vert water in steam. 3. Main power unit such as an engine or turbine to use the heat energy of steam and perform work. 4. Piping system to convey steam and water. In addition to the above equipment the plant requires various auxiliaries and accessories depending upon the availability of water, fuel and the service for which the plant is intended. The flow sheet of a thermal power plant consists of the following four main circuits : Feed water and steam flow circuit (ii) Coal and ash circuit (iii) Air and gas circuit (iv) Cooling water circuit. A steam power plant using steam as working substance works basically on Rankine cycle. Steam is generated in a boiler, expanded in the prime mover and condensed in the condenser and fed into the boiler again. The different types of systems and components used in steam power plant are as follows : (i) High pressure boiler (ii) Prime mover (iii) Condensers and cooling towers (iv) Coal handling system (v) Ash and dust handling system (vi) Draught system (vii) Feed water purification plant (viii) Pumping system (ix) Air preheater, economizer, super heater, feed heaters. Various equipments

Fuels used in thermal power plantCombustion of fuel is accomplished by mixing with air at elevated temperaturesOxygen in the air chemically unites with Carbon, Hydrogen of fuels and produce heatIn thermal power plants normally steam is produced from water by using combustion heat of fuels (Except in Gas turbines)Various fuels were used in thermal power plantsFossil fuels ( Coal, Oil & Gas)Industrial waste gasesSynthetic fuels or SYNFUELS

Coal Coal is the principal energy source for India because of its large deposits and availabilityCoal originated from vegetable matter, which grew millions of years agoTrees and plants falling into water decayed and later produced peat bogsHuge geological upheavals buried these bogs under layers of siltSubterranean heat, soil pressure and movement of earth's crust distilled off some of the bog's moisture and hardened it to form coalBasically classification of coal is based on Physical and chemical compositionPeatLignite and brown coalBituminous coalAnthracite

Coal analysisTwo types of coal analysisProximate Analysis Gives: Behavior of coal when heatedFC + VM + M + A = 100% by mass

Ultimate Analysis Gives: chemical elements along with ash and moistureC + H + 0 + N + S + M + A = 100% by mass

Desirable properties of coal High calorific valueSmall sulphur content ( les than 1%)Good burning characteristics for complete combustionHigh grind-ability indexHigh weather-abilityGrading of coal done on the basisHeating value SizeAsh contentSulphur content

Peat

Low grade coal First stage coal formationContains 90% moistureSmall amount of volatile matterNot suitable for power plantsUsed in domestic purposesRequires 1-2 months for drying in sunlightPeat (20% water, Dried) has CV of 16 MJ / kg

Lignite (Brown coal)Intermediate stage of coal developmentHigh amount of moisture 30-40%Brown in colorHigh heating value and carbon compared to peatShould be stored to avoid spontaneous combustionUsed in pulverized formCan be air dried easilySuitable for local use instead of transporting

Bituminous coalContaining 46-86% of fixed carbon and 20-40% of VM Ash content may vary 6-12 %High percentage of volatile matter CV of 32 MJ / KgSub-Bituminous is having less moisture, ash than lignite , used in briquette or pulverized formSemi-Bituminous coal high carbon and heating value, contain less moisture, ash, sulphur, &VM, Has tendency to break into pieces

Anthracite CoalsContains more than 86% fixed carbon &VM around 8%Ignites slowly has high CV of 36 MJ / KgLow ash, zero CAKING powerDifficult to pulverize Anthracite coal

Coal handling

Coal handling systemIn Case of Rail Delivery Most Commonly Used are :

Wagon Tippler Hopper Hydraulic Gates Opening

Hydraulic Gates Opening Operation The gates bellow the wagons are hydraulic controlled By opening the gates the coal flows downwards due to gravitational force and is moved to the bellow pit

Demerits

Due to the dust the Hydraulic will not work smoothly Require a lot of maintenance

Wagon Tippler Hopper

Crushers and breakers used to crus the big size coal into small size coal

Driers used to dry the coal if it contains lot of moisture it is also further done in the pulverizes

Magnetic sprayers used to separate the iron parts that come along with the coal from the mines these may damage the parts of pulverizes or may have a bad effect on the boiler Preparation

Coal CrushersWhy? Reduce size from 200mm to 20mm

Types of crushers 1.Ring Type Coal Crusher2.Hammer Mill Coal Crusher3.Brad Ford Breaker

Ring Type Coal Crusher

OperationThe coal is fed at the top left corner Is crushed in between the plane loose rings and the adjustable plates.The loose rings with the jaws used to take the crushed coal down words Adjustable plate and the screen determine the maximum size of the discharged coal

Hammer Mill Coal Crusher

OPERATIONThe coal is fed at the top Is crushed by the action of rings that pivot off center on a rotor or by swinging hammers attached to itAdjustable screen bars determine the maximum size of the discharged coal

Brad Ford Breaker

Brad Ford BreakerBradford breaker which is used for large capacity workIt consists of a large cylinder made up of perforated steel plates to which lifting shelves are attached on the insideThe cylinder rotating slowly at about 20 rpm receives coal at one endThe shelves lift the coal up and then the coal drops down by gravity

Transfer Belt Conveyor- The belt is

Belt conveyor

Screw conveyor

Bucket & Grab bucket elevator

Skip Hoist and flight conveyor

Stoker and Re-claimer Stoker terms for storing the coal into the stock yard

Re-claimer means to take the coal back from the stock yard

It moves on the rail track along with adjustable length of belt conveyor

Combustion Equipments for Lumped coal burningGrate-fired furnacesChamber-type or flame furnaces

Grate

Chamber type

Combustion Equipment For Burning Coal Fuel bed furnaces (coarse particles)Pulverized coal furnaces (fine particles)Cyclone furnaces (crushed particles)Fluidized bed furnaces (crushed small particles)

Fuel bed furnaces (coarse particles)

There are two ways of feeding coal on to the grate Overfeeding Underfeeding

Overfeed stokerUnderfeed stoker

Overfeeding

OverfeedingReceives fresh coal from top surface has following distinct zonesFresh or green coalCoal losing moisture (Drying zone)Coking layer (loosing of VM) Distillation zoneIncandescent coke ( Fixed carbon is consumed) Combustion zoneAsh layer progressively cooled

OperationsPrimary air gets warmed up as it flows through the ash layerAs it passes through the incandescent coke layer(1200 C) Carbon converted to Carbon dioxide releasing heat continues till oxygen is consumed, if layer is thick CO2 is converted to CO reducing layer temperatureThe stream while passing through distillation zone VM is addedIn distillation zone moisture is addednow stream contains N,CO2,CO,H2,VM&moisture

For combustion of this streamA hot ignition point ( In the range of 1000-1300oC Done by providing a fire brick lined arch which stores up the heat and remains at high temperatureSufficient fresh airSecondary air or over-fire air TurbulenceProviding secondary air at right angles to up-flowing gas stream emerging out of fuel bed

ResultsCarbon is in CO, CO2 both are color lessCarbon with hydrogen in VM which will be cracked to form free carbon at high temperature suspended in gas streamIf Burner is not designed properly or operated properly leads to unburnt carbon particles this appears as black smoke on chimney top

Underfeeding

UnderfeedingIn underfeeding coal is fed from belowPrimary air passing through holes diffuses through spaces in the raw green coal picking up moistureIn distillation zone to stream VM is addedIn incandescent zone VM breaks readily burns with secondary air present above it where it is fed from topVM burning is somewhat cooler need longer time to ignite and burn

Mechanical StokersOverfeeding Traveling grate stoker Chain grate stoker Spreader stokerVibrating grate stokerUnderfeed stokerSingle retortMultiretort

Traveling grate stoker

Traveling grate stokerGrate surface is made up of Cast Iron bars joined together by links to form endless beltBelt wound around two sprocketsA coal gate regulates the depth of fuel bedSimultaneous adjustment of Fuel bed thickness, primary air flow controls the burning rate so that at the end of its rear ash only remains

Advantages & DisadvantagesSimple and Initial cost is lowReliable in service and maintenance is lowGives high rate of heat release per volume of the furnaceLimited coal can be carried on grateClinker problems are commonIgnition arches are requiredThere is always some loss in the form of particles

Spreader stoker

Spreader stokerCoal from hopper is fed by rotating feederFeeder normally will have Blades fitted on the drumFine particles burn in suspensionSpeed of the feeder varies with the steam output of boilerGrate is made up of CI bars, Links underneath the grate are connected to a leverFuels used may be Bituminous, lignite, wood waste, baggaseCoal size used in 6-36 cm

Advantages and DisadvantagesA wide variety of fuels can be usedClinker formation is reducedHigh temperature preheated air can be usedQuick response to load variationGives equal pressure drop and proper air distribution Operation cost is lowDifficult to operate variable sized coal particlesFly ash and entrapped carbon particles

Underfeed stokers

Multiple retort

Pulverised coal

Why pulverisers?Size reduction (mm to micron)Other things:Drying (primary airapprox 300 degree)Feeding

Types of PulverisersBall and tube mill (low speed < 75 rpm)Ball and tube race mill (medium speed 75-225 rpm)Bowl mill (medium speed 75-225 rpm)Impact or hammer mill (> 225 rpm).depending upon the nature of coal

Pulverised coal firingPulverised coal firing is done by two system :(i) Unit System or Direct System. (ii) Bin or Central System.Unit System. In this system the raw coal from the coal bunker drops on to the feeder. Hot air is passed through coal in the feeder to dry the coal. The coal is then transferred to the pulverising mill where it is pulverised. Primary air is supplied to the mill, by the fan. The mixture of pulverised coal and primary air then flows to burner where secondary air is added. The unit system is so called from the fact that each burner or a burner group and pulveriser constitute a unit.

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Unit systemAdvantages

The system is simple and cheaper than the central system.

(ii) There is direct control of combustion from the pulverising mill.

(iii) Coal transportation system is simple

Bin or Central System. Crushed coal from the raw coal bunker is fed by gravity to a dryer where hot air is passed through the coal to dry it. The dryer may use waste flue gases, preheated air or bleeder steam as drying agent. The dry coal is then transferred to the pulverising mill. The pulverised coal obtained is transferred to the pulverised coal bunker (bin)The transporting air is separated from the coal in the cyclone separator. The primary air is mixed with the coal at the feeder and the mixture is supplied to the burner.Advantagesl. The pulverising mill grinds the coal at a steady rate irrespective of boiler feed.2. There is always some coal in reserve. Thus any occasional breakdown in the coal supply willnot effect the coal feed to the burner.3. For a given boiler capacity pulverising mill of small capacity will be required as compared tounit system.Disadvantages1. The initial cost of the system is high.2. Coal transportation system is quite complicated.3. The system requires more space.

Bin or central system

Pulverised fuel firing system

Pulverized coal handling system

Ball and tube mill

Ball and tube mill

Ball and tube mill

Ball and tube mill

Ball and tube millMills can be designed and manufactured for very large capacity. Also each mill can supply coal to two sets of burner. Boiler can be designed with lesser number of mills.Higher power consumption per tons of coal and high starting torque makes drive motor large.Suitable for base load stations only where load variation is less. Response to load change is very fast. During operation presence of relatively large quantity of pulverized coal in grinding zone acts as a storage reservoir. In case of sudden increase in fuel demand fuel flow can be increased almost instantaneously. Start up and shutdown time is longer.Product fineness is very good.

Bowl Mill

Bowl Mill

Bowl mill

Bowl millReplacement of grinding elements takes lesser time.Capacity and product fineness deteriorate with wearing out of grinding elements. Rate of erosion of bowl and rings are very high.Frequent shutdown of mills are required for adjustment of loading springs which becomes loose for wearing out of roller and bowl.Ingress of non crushable material may damage mill internals.Reject rate is very high. Elaborate mill reject system becomes essential for larger units. Disposal of mill reject is another problem to be tackled by power station.

Impact/Hammer Mill

Impact/Hammer Mill

Impact/Hammer MillA simple, compact and low cost mill.

Mill can be designed for very high temperature. This facility makes this mill suitable for use with high moisture lignite fuel.

Wear of grinding elements reduces mill capacity.

Product fineness is low.

Ball and Race mill

Ball and race millCapacity and product fineness does not deteriorate appreciably with wearing out of grinding elements because of self compensating mechanism.Hydro -pneumatic loading system maintains constant loading pressure. But it is costly and needs maintenance.Response to load change is slow compared to tube mills.Start up and shut down time is less. Ingress of non crushable material may damage mill internals. Chances of grinding zone fire are almost nil.

Ball and race mill

Ball and race mill

PulverizationAdvantages

Low excess air requirementLess fan powerAbility to use highly preheated air reducing exhaust lossesHigher boiler efficiencyAbility to bum a wide variety of coalsFast response to load changesEase of burning alternately with, or in combination with gas and oilAbility to release large amounts of heat enabling it to generate about 2000 t/h of steam or more in one boilerAbility to use fly ash for making bricks etc. Less pressure losses and draught need.

DisadvantagesAdded investment in coal preparation unitAdded power needed for pulverizing coalLarge volume of furnaces needed to permit desired heat release and to withstand high gas temperature

Ash handling systemA large quantity of ash is, produced in steam power plants using coal. Ash produced in about 10to 20% of the total coal burnt in the furnace. Handling of ash is a problem because ash coming out of the furnace is too hot, it is dusty and irritating to handle and is accompanied by some poisonous gases.

It is desirable to quench the ash before handling due to following reasons:1. Quenching reduces the temperature of ash.2. It reduces the corrosive action of ash.3. Ash forms clinkers by fusing in large lumps and by quenching clinkers will disintegrate.4. Quenching reduces the dust accompanying the ash.

Handling of ash includes its removal from the furnace, loading on the conveyors and deliveredto the fill from where it can be disposed off.

ASH HANDLING EQUIPMENT

Mechanical means are required for the disposal of ash. The handling equipment should perform the following functions:

(1) Capital investment, operating and maintenance charges of the equipment should be low.(2) It should be able to handle large quantities of ash.(3) Clinkers, soot, dust etc. create troubles, the equipment should be able to handle them smoothly.(4) The equipment used should remove the ash from the furnace, load it to the conveying systemto deliver the ash to a dumping site or storage and finally it should have means to dispose of the stored ash.(5) The equipment should be corrosion and wear resistant.

ASH HANDLING FLOW DIAGRAM

Ash handling

Hydraulic Ash Handling System Low Velocity System

Hydraulic Ash Handling System High Velocity System

Pneumatic Ash Handling System

Dust collection system

The size of dust particles is designated in microns (1 = 0.001 mm). Dust particles are mainly ash particles called fly ash intermixed with some quantity of carbon ash material called cinders. Gas borne particles larger than 1 in diameter are called dust and when such particles become greater in size than 100 they are called cinders. Smoke is produced due to the incomplete combustion of fuels, smoke particles are less than 10 in size.The disposal smoke to the atmosphere is not desirable due to the following reasons :

A smoky atmosphere is less healthful than smoke free air.

2. Smoke is produced due to incomplete combustion of coal. This will create a big economicloss due to loss of heating value of coal.

3. In a smoky atmosphere lower standards of cleanliness are prevalent. Buildings, clothings,furniture etc. becomes dirty due to smoke. Smoke corrodes the metals and darkens the paints.To avoid smoke, the coal should be completely burnt in the furnace.

Types of dust collectorsThe various types of dust collectors are as follows :1. Mechanical dust collectors.2. Electrical dust collectors.Mechanical dust collectors. Mechanical dust collectors are sub-divided into wet and dry types.In wet type collectors also known as scrubbers water sprays are used to wash dust from the air. The basic principles of mechanical dust collectors are shown in Fig. As shown in Fig. by increasing the cross-sectional area of duct through which dust laden gases are passing, the velocity of gases is reduced and causes heavier dust particles to fall down. Changing the direction of flow of flue gases causes the heavier particles of settle out. Sometime baffles are provided as to separate the heavier particles. Mechanical dust collectors may be wet type or dry type. Wet type dust collectors called scrub scrubbers make use of water sprays to wash the dust from flue gases.Dry type dust collectors include gravitational, cyclone, and baffle dust collectors.

Electrostatic Precipitators. It has two sets of electrodes, insulated from each other that maintainan electrostatic field between them at high voltage. The flue gases are made to pass between thesetwo sets of electrodes. The electric field ionises the dust particle; that pass through it attracting them to the electrode of opposite charge. The other electrode is maintained at a negative potential of 30,000 to 60,000 volts. The dust particles are removed from the collecting electrode by rapping the electrode periodically. The electrostatic precipitator is costly but has low maintenance cost and is frequently employed with pulverised coal fired power stations for its effectiveness on very fine ash particles and is superior to that of any other type.

ESP

4.13.1 FLY ASH SCRUBBERIt is similar to a mechanical ash collector but has a flowing water film on its inner walls. Due to this film, the collected ash is removed more rapidly from the apparatus to the bin. The degree of ash collection in scrubbers varies from 0.82 to 0.90. The dust laden gas enters through the inlet pipe.

Cinder Catcher. Cinder catcher is used to remove dust and cinders from the gas. In this catcherthe dust laden gas is made to strike a series of vertical baffles that change its direction and reduce its velocity. The separated dust and cinders fall to the Hopper for removal. Cinder catchers are ordinarily used with stoker firing.

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