Thermal ( Steam ) Power plants mainly consists of 4 circuits

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Thermal ( Steam ) Power plants mainly consists of
4 circuits

• Coal and ash Circuit
• Coal produced in the mining site is transported
  to power plant site
• Coal handling equipment for generation of steam
• The combustion of coal produces ash which is
  collected and removed to ash storage yard through
  ash handling equipments

2
Air and gas circuit

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

3
Feed water and steam 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
  boiler
• The condensate leaving the condenser is heated in
  feed heaters through extracted steam from lowest
  pressure extraction point of the turbine
• The 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 sections
• A super heater is used to super heat the wet
  steam from boiler drum and is then supplied to
  prime movers

4
Cooling water circuit

• In the condenser, quantity of cooling water
  required to condense the steam is large and is
  taken either from lake, river or sea
• The 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 river
• The system is known as open system
• Where 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

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Main Components of Thermal Power Plants

• Boiler
• Super heater
• Economizer
• Air preheater
• Reheater
• Steam turbine
• Generator
• Condensers
• Cooling towers
• Pumps

• Coal mills
• FD and ID Fans
• ASH Precipitators
• Water treatment plant

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Fuels used in thermal power plant

• Combustion of fuel is accomplished by mixing with
  air at elevated temperatures
• Oxygen in the air chemically unites with Carbon,
  Hydrogen of fuels and produce heat
• In 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 plants
• Fossil fuels ( Coal, Oil Gas)
• Industrial waste gases
• Synthetic fuels or SYNFUELS

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Coal

• Coal is the principal energy source for India
  because of its large deposits and availability
• Coal originated from vegetable matter, which grew
  millions of years ago
• Trees and plants falling into water decayed and
  later produced peat bogs
• Huge geological upheavals buried these bogs under
  layers of silt
• Subterranean heat, soil pressure and movement of
  earth's crust distilled off some of the bog's
  moisture and hardened it to form coal
• Basically classification of coal is based on
  Physical and chemical composition
• Peat
• Lignite and brown coal
• Bituminous coal
• Anthracite

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Coal analysis

• Two types of coal analysis
• Proximate Analysis
• Gives Behavior of coal when heated
• FC VM M A 100 by mass
• Ultimate Analysis
• Gives chemical elements along with ash and
  moisture
• C H 0 N S M A 100 by mass
• based on
• (a) as-received basis (useful for combustion
  calcu)
• (b) dry or moisture free basis
• (c) dry mineral-matter-free or combustible basis

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Coal properties

• There are certain properties of coal, which are
  important in power plant applications
• They are
• sulphur content,
• heating value
• ash softening temperature
• swelling index
• grind ability,
• weather ability,

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• Desirable properties of coal
• High calorific value
• Small sulphur content ( les than 1)
• Good burning characteristics for complete
  combustion
• High grind-ability index
• High weather-ability
• Grading of coal done on the basis
• Heating value
• Size
• Ash content
• Sulphur content
• Ash softening temperature

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Peat

• Low grade coal
• First stage coal formation
• Contains 90 moisture
• Small amount of volatile matter
• Not suitable for power plants
• Used in domestic purposes
• Requires 1-2 months for drying in sunlight
• Peat (20 water, Dried) has CV of 16 MJ / kg

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Lignite (Brown coal)

• Intermediate stage of coal development
• High amount of moisture 30-40
• Brown in color
• High heating value and carbon compared to peat
• Should be stored to avoid spontaneous combustion
• Used in pulverized form
• Can be air dried easily
• Suitable for local use instead of transporting

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Bituminous coal

• Containing 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 /
  Kg
• Available in two forms CAKING and NON Caking
• Sub-Bituminous is having less moisture, ash than
  lignite and no CAKING power, used in briquette or
  pulverized form
• Semi-Bituminous coal high carbon and heating
  value, contain less moisture, ash, sulphur, VM,
  Has tendency to break into pieces

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Anthracite Coals

• Contains more than 86 fixed carbon VM around 8
• Ignites slowly has high CV of 36 MJ / Kg
• Low ash, zero CAKING power
• Difficult to pulverize Anthracite coal

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Liquid fuels

• They are easy to handle, store and to burn
• They have nearly constant heating values
• They are primarily a mixture of hydrocarbon
  compounds, which may also contain nitrogen,
  oxygen and sulphur
• The bulk of the hydrocarbons belong to the
  paraffin series, like methane (CH4), ethane
  (C2H6) propane (C3H8) and butane (C4H10) which
  are gaseous, and pentane (C5H12) hexane (C6H14)
  and octane (C8H18) which are liquid at STP
• In addition, there can be isoparaffins,
  cycloparaffins and aromatic compounds

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• Carbon 83-87, Hydrogen 11-16,
• Oxygen Nitrogen 0-7, Sulphur 0-4
• There can also be some moisture and sediment
• Crude oil distilled into a number of fractions
  gasoline, aviation fuel, kerosene, light diesel
  oil, heavy diesel oil, lubrication oil
• The heavier fractions are used for boiler fuels
  and chemical production
• The required physical properties of fuel oil are
• specific gravity,
• viscosity, pour point,
• flash point and heating value

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Gaseous fuels

• Transportation of natural gas is made through
  pipelines
• Natural gas is the cleanest of all fossil fuels
• It is free from ash and mixes well with air to
  undergo complete combustion producing very little
  smoke
• It consists of a mixture of the most volatile
  paraffins-methane to pentane
• It has high hydrogen content and produces a
  considerable amount of water vapour when burned
• The heat of combustion varies from 33.5 to 40
  MJ/m3
• Since the major constituent of all natural gases
  is methane
• Liquid natural gas (LNG) is transported by
  special tankers and stored in spherical pressure
  vessels to be used when needed, particularly
  during peak load.

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Other fuels

• Industrial Wastes Byproducts
• blast furnace gas, coke oven gas, and refinery
  gas sugar factory refuse (bagasse) saw mill wood
  dust, rice husk
• Synthetic fuels
• Gaseous and liquid fuels from coal economically
  and environment friendly manner
• Liquid fuels using mixtures of fine coal in oil
  have been known as colloidal fuel, coal-in-oil
  and more recently, coal-oil mixtures (COM)

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Coal handling
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Coal Crushers

• 1.Ring Type Coal Crusher
• 2.Hammer Mill Coal Crusher
• 3.Brad Ford Breaker

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Ring Type Coal Crusher
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Hammer Mill Coal Crusher
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OPERATION

• The coal is fed at the top
• Is crushed by the action of rings that pivot off
  centre on a rotor or by swinging hammers attached
  to it
• Adjustable screen bars determine the maximum size
  of the discharged coal

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Brad Ford Breaker
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Brad Ford Breaker

• Bradford breaker which is used for large capacity
  work
• It consists of a large cylinder made up of
  perforated steel plates to which lifting shelves
  are attached on the inside
• The cylinder rotating slowly at about 20 rpm
  receives coal at one end
• The shelves lift the coal up and then the coal
  drops down by gravity

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Combustion Equipments for Lumped coal burning

• Grate-fired furnaces
• Chamber-type or flame furnaces

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Grate
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Chamber type
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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)

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Fuel bed furnaces (coarse particles)

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

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Overfeeding
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Overfeeding

• Receives fresh coal from top surface has
  following distinct zones
• Fresh or green coal
• Coal losing moisture (Drying zone)
• Coking layer (loosing of VM) Distillation zone
• Incandescent coke ( Fixed carbon is consumed)
  Combustion zone
• Ash layer progressively cooled

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Operations

• Primary air gets warmed up as it flows through
  the ash layer
• As 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 temperature water gas reaction also takes
• The stream while passing through distillation
  zone VM is added
• In distillation zone moisture is added
• Know stream contains N,CO2,CO,H2,VMmoisture

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For combustion of this stream

• A 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
  temperature
• Sufficient fresh air
• Secondary air or over-fire air
• Turbulence
• Providing secondary air at right angles to
  up-flowing gas stream emerging out of fuel bed

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Results

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

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Underfeeding
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Underfeeding

• In underfeeding coal is fed from below
• Primary air passing through holes in tuyeres
  diffuses through spaces in the raw green coal
  picking up moisture
• In distillation zone to stream VM is added
• In incandescent zone VM breaks readily burns with
  secondary air present above it where it is fed
  from top
• VM burning is somewhat cooler need longer time to
  ignite and burn

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Mechanical Stokers

• Overfeeding
• Traveling grate stoker
• Chain grate stoker
• Spreader stoker
• Vibrating grate stoker
• Underfeed stoker
• Single retort
• Multiretort

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Traveling grate stoker
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Traveling grate stoker

• Grate surface is made up of Cast Iron bars joined
  together by links to form endless belt
• Belt wound around two sprockets
• A coal gate regulates the depth of fuel bed
• Simultaneous adjustment of Fuel bed thickness,
  primary air flow controls the burning rate so
  that at the end of its rear ash only remains

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Advantages Disadvantages

• Simple and Initial cost is low
• Reliable in service and maintenance is low
• Gives high rate of heat release per volume of the
  furnace
• Limited coal can be carried on grate
• Clinker problems are common
• Ignition arches are required
• There is always some loss in the form of particles

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Spreader stoker
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Spreader stoker

• Coal from hopper is fed by rotating feeder
• Feeder normally will have Blades fitted on the
  drum
• Fine particles burn in suspension
• Speed of the feeder varies with the steam output
  of boiler
• Grate is made up of CI bars, Links underneath the
  grate are connected to a lever
• Fuels used may be Bituminous, lignite, wood
  waste, baggase
• Coal size used in 6-36 cm

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Advantages and Disadvantages

• A wide variety of fuels can be used
• Clinker formation is reduced
• High temperature preheated air can be used
• Quick response to load variation
• Gives equal pressure drop and proper air
  distribution
• Operation cost is low
• Difficult to operate variable sized coal
  particles
• Fly ash and entrapped carbon particles

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Underfeed stokers
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Multiple retort
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Pulverized coal handling system
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Bowl Mill
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Bowl Mill
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Ball and Race mill
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Pulverization

• Advantages
• Low excess air requirement
• Less fan power
• Ability to use highly preheated air reducing
  exhaust losses
• Higher boiler efficiency
• Ability to bum a wide variety of coals
• Fast response to load changes
• Ease of burning alternately with, or in
  combination with gas and oil
• Ability to release large amounts of heat enabling
  it to generate about 2000 t/h of steam or more in
  one boiler
• Ability to use fly ash for making bricks etc.
• Less pressure losses and draught need.

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Disadvantages

• Added investment in coal preparation unit
• Added power needed for pulverizing coal
• Investment needed to remove fly ash before ill
  fan
• Large volume of furnaces needed to permit desired
  heat release and to withstand high gas temperature

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ASH HANDLING SYSTEM

• Mechanical Handling System
• Hydraulic Ash Handling System
• Low Velocity System
• High Velocity System
• Pneumatic Ash Handling System
• Steam Jet System

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ASH HANDLING FLOW DIAGRAM
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Mechanical Handling System
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Hydraulic Ash Handling System Low Velocity
System
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Hydraulic Ash Handling System Low Velocity
System
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Hydraulic Ash Handling System High Velocity
System
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Pneumatic Ash Handling System