Thermal power plant

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THERMAL POWER PLANT

• By
• Ashvani Shukla
• CI
• reliance

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Introduction

• Thermal power generation plant or thermal power
  station is the most conventional source of
  electric power. Thermal power plant is also
  referred as coal thermal power plant and steam
  turbine power plant. Before going into detail of
  this topic, we will try to understand the line
  diagram of electric power generation
  plant.A thermal power station is a power plant in
  which heat energy is converted to electric power.
  In most of the world 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. 

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Theory of Thermal Power Station

• The theory of thermal power station or working of
  thermal power station is very simple. A power
  generation plant mainly consists of alternator
  runs with help of steam turbine. The steam is
  obtained from high pressure boilers. Generally in
  India, bituminous coal, brown coal and peat are
  used as fuel of boiler. The bituminous coal is
  used as boiler fuel has volatile matter from 8 to
  33 and ash content 5 to 16 . To increase the
  thermal efficiency, the coal is used in the
  boiler in powder form.

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• In coal thermal power plant, the steam is
  produced in high pressure in the steam boiler due
  to burning of fuel (pulverized coal) in boiler
  furnaces. This steam is further supper heated in
  a super heater. This supper heated steam then
  enters into the turbine and rotates the turbine
  blades. The turbine is mechanically so coupled
  with alternator that its rotor will rotate with
  the rotation of turbine blades. After entering in
  turbine the steam pressure suddenly falls and
  corresponding volume of the steam increases.
  After imparting energy to the turbine rotor the
  steam passes out of the turbine blades into the
  condenser. In the condenser the cold water is
  circulated with the help of pump which condenses
  the low pressure wet steam. This condensed water
  is further supplied to low pressure water heater
  where the low pressure steam increases the
  temperature of this feed water, it is again
  heated in high pressure. For better understanding
  we furnish every step of function of a thermal
  power station as follows,
• 1) First the pulverized coal is burnt into the
  furnace of steam boiler.
• 2) High pressure steam is produced in the boiler.
  
• 3) This steam is then passed through the super
  heater, where it further heated up.
• 4) This supper heated steam is then entered into
  a turbine at high speed.
• 5) In turbine this steam force rotates the
  turbine blades that means here in the turbine the
  stored potential energy of the high pressured
  steam is converted into mechanical energy.

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6) After rotating the turbine blades, the steam
has lost its high pressure, passes out of turbine
blades and enters into a condenser. 7) In the
condenser the cold water is circulated with help
of pump which condenses the low pressure wet
steam. 8) This condensed water is then further
supplied to low pressure water heater where the
low pressure steam increases the temperature of
this feed water, it is then again heated in a
high pressure heater where the high pressure of
steam is used for heating. 9) The turbine in
thermal power station acts as a prime mover of
the alternator.
Rankin cycle
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Working of Rankin cycle

• A typical Thermal Power Station Operates on a
  Cycle which is shown below.

The working fluid is water and steam. This is
called feed water and steam cycle. The ideal
Thermodynamic Cycle to which the operation of a
Thermal Power Station closely resembles is the
RANKINE CYCLE. In steam boiler the water is
heated up by burning the fuel in air in the
furnace the function of the boiler is to give
dry super heated steam at required temperature.
The steam so produced is used in driving the
steam Turbines. This turbine is coupled to
synchronous generator (usually three phase
synchronous alternator), which generates
electrical energy. The exhaust steam from the
turbine is allowed to condense into water in
steam condenser of turbine, which creates suction
at very low pressure and allows the expansion of
the steam in the turbine to a very low pressure.
The principle advantages of condensing operation
are the increased amount of energy extracted per
kg of steam and thereby increasing efficiency and
the condensate which is fed into the boiler again
reduces the amount of fresh feed water. The
condensate along with some fresh make up feed
water is again fed into the boiler by pump
(called the boiler feed pump). In condenser the
steam is condensed by cooling water. Cooling
water recycles through cooling tower. This
constitutes cooling water circuit. The ambient
air is allowed to enter in the boiler after dust
filtration. Also the flue gas comes out of the
boiler and exhausted into atmosphere through
stacks. These constitute air and flue gas
circuit. The flow of air and also the static
pressure inside the steam boiler (called draught)
is maintained by two fans called Forced Draught
(FD) fan and Induced Draught(ID) fan.
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Rankin cycle
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TYPE OF THERMAL POWER PLANT

• 1. CO-GENERATION POWER PLANT
• 2. CAPTIVE POWER PALNTS
• 3. SUBCRITICAL POWER PLANTS
• 4. SUPER CRITICAL POWER PLANTS
• 5. ULTRA SUPERCRITICAL POWER PLANTS

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Co-generation Power Plant

• Cogeneration is also called as combined heat and
  power or combine heat and power. As it name
  indicates cogeneration works on concept of
  producing two different form of energy by using
  one single source of fuel. Out of these two forms
  one must be heat or thermal energy and other one
  is either electrical or mechanical energy.
• Cogeneration is the most optimum, reliable,
  clean and efficient way of utilizing fuel. The
  fuel used may be natural gas, oil, diesel ,
  propane, wood, bagasse, coal etc. It works on
  very simple principle i.e. the fuel is used to
  generate electricity and this electricity
  produces heat and this heat is used to boil water
  to produce steam , for space heating and even in
  cooling buildings. In conventional power plant ,
  the fuel is burnt in a boiler , which in turn
  produces high pressure steam. This high pressure
  steam is used to drive a tribune, which is in
  turn is connected to an alternator and hence
  drive an alternator to produce electric energy.
  The exhaust steam is then sent to the condenser,
  where it gets cool down and gets converted to
  water and hence return back to boiler for
  producing more electrical energy. The efficiency
  of this conventional power plant is 35 only. In
  cogeneration plant the low pressure steam coming
  from turbine is not condense to form water,
  instead of it its used for heating or cooling in
  building and factories, as this low pressure
  steam from turbine has high thermal energy. The
  cogeneration plant has high efficiency of around
  80 - 90 . In India, the potential of power
  generation from cogeneration plant is more than
  20,000 MW.

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• Need for Cogeneration
• a) Cogeneration helps to improve the efficiency
  of the plant.
• b) Cogeneration reduce air emissions of
  particulate matter, nitrous oxides, sulphur
  dioxide, mercury and carbon dioxide which would
  otherwise leads to greenhouse effect.
• c) It reduces cost of production and improve
  productivity.
• d) Cogeneration system helps to save water
  consumption and water costs.
• e) Cogeneration system is more economical as
  compared to conventional power plant
• Types of Cogeneration Power Plants
• In a typical Combined heat and power plant system
  there is a steam or gas turbine which take steam
  and drives an alternator. A waste heat exchanger
  is also installed in cogeneration plant, which
  recovers the excess heat or exhaust gas from the
  electric generator to in turn generate steam or
  hot water. There are basically two types of
  cogeneration power plants, such as- Topping
  cycle power plant Bottoming cycle power plant
  Topping cycle power plant- In this type of
  Combine Heat and Power plant electricity is
  generated first and then waste or exhaust steam
  is used to heating water or building . There are
  basically four types of topping cycles. a)
  Combined-cycle topping CHP plant - In this type
  of plant the fuel is firstly burnt in a steam
  boiler . The steam so produced in a boiler is
  used to drive turbine and hence synchronous
  generator which in turn produces electrical
  energy . The exhaust from this turbine can be
  either used to provide usable heat, or can be
  send to a heat recovery system to generate steam,
  which maybe further used to drive a secondary
  steam turbine.

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• b) Steam-turbine topping CHP Plant- In this the
  fuel is burned to produce steam, which generates
  power. The exhaust steam is then used as
  low-pressure process steam to heat water for
  various purposes.
• c) Water- turbine topping CHP Plant- In this type
  of CHP plant a jacket of cooling water is run
  through a heat recovery system to generate steam
  or hot water for space heating. d) Gas turbine
  topping CHP plant- In This topping plant a
  natural gas fired turbine is used to drives a
  synchronous generator to produce electricity. The
  exhaust gas is sent to a heat recovery boiler
  where it is used to convert water into steam, or
  to make usable heat for heating purposes.
• Bottoming cycle power plant - As its name
  indicate bottoming cycle is exactly opposite of
  topping cycle. In this type of CHP plant the
  excess heat from a manufacturing process is used
  to generate steam, and this steam is used for
  generating electrical energy. In this type of
  cycle no extra fuel is required to produce
  electricity, as fuel is already burnt in
  production process.
• Configuration of Cogeneration Plant
• Gas turbine Combine heat power plants which
  uses the waste heat in the flue gas emerging out
  of gas turbines. Steam turbine Combine heat
  power plants that use the heating system as the
  jet steam condenser for the steam turbine.
• Molten-carbonate fuel cells have a hot exhaust,
  very suitable for heating.
• Combined cycle power plants adapted for Combine
  Heat and Power.

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2. Captive power plant

• A captive power plant is a facility that is
  dedicated to providing a localised source of
  power to an energy user. These are
  typically industrial facilities or large offices.
  The plants may operate in grid parallel mode with
  the ability to export surplus power to the local
  electricity distribution network. Alternatively
  they may have the ability to operate in island
  mode i.e. independently of the local electricity
  distribution system. Captive power plants are a
  form of distributed generation, generating power
  close to the source of use. Distributed
  generation facilitates the high fuel efficiency
  along with minimising losses associated with the
  transmission of electricity from centralised
  power plants.Captive power plants are used to
  generate the power for ourselfs or out plant load
  or house load.it will be synchronized to grid for
  import and export the power depend upon our
  requirement.

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SUB CRITICAL POWER PLANT
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• In a coal based power plant coal is transported
  from coal mines to the power plant by railway in
  wagons or in a merry-go-round system. Coal is
  unloaded from the wagons to a moving underground
  conveyor belt. This coal from the mines is of no
  uniform size. So it is taken to the Crusher house
  and crushed to a size of 20mm. From the crusher
  house the coal is either stored in dead storage(
  generally 40 days coal supply) which serves as
  coal supply in case of coal supply bottleneck or
  to the live storage(8 hours coal supply) in the
  raw coal bunker in the boiler house. Raw coal
  from the raw coal bunker is supplied to the Coal
  Mills by a Raw Coal Feeder. The Coal Mills or
  pulverizer pulverizes the coal to 200 mesh size.
  The powdered coal from the coal mills is carried
  to the boiler in coal pipes by high pressure hot
  air. The pulverized coal air mixture is burnt in
  the boiler in the combustion zone.Generally in
  modern boilers tangential firing system is used
  i.e. the coal nozzles/ guns form tangent to a
  circle. The temperature in fire ball is of the
  order of 1300 deg.C. The boiler is a water tube
  boiler hanging from the top. Water is converted
  to steam in the boiler and steam is separated
  from water in the boiler Drum. The saturated
  steam from the boiler drum is taken to the Low
  Temperature Superheater, Platen Superheater and
  Final Superheater respectively for superheating.
  The superheated steam from the final superheater
  is taken to the High Pressure Steam Turbine
  (HPT). In the HPT the steam pressure is utilized
  to rotate the turbine and the resultant is
  rotational energy. From the HPT the out coming
  steam is taken to the Reheater in the boiler to
  increase its temperature as the steam becomes wet
  at the HPT outlet. After reheating this steam is
  taken to the Intermediate Pressure Turbine (IPT)
  and then to the Low Pressure Turbine (LPT). The
  outlet of the LPT is sent to the condenser for
  condensing back to water by a cooling water
  system. This condensed water is collected in the
  Hotwell and is again sent to the boiler in a
  closed cycle. The rotational energy imparted to
  the turbine by high pressure steam is converted
  to electrical energy in the Generator.

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Super critical power plant

• WHAT IS CRITICAL ABOUT SUPER CRITICAL POWER
  GENERATION Supercritical " is a thermodynamic
  expression describing the state of a substance
  where there is no clear distinction between the
  liquid and the gaseous phase (i.e. they are a
  homogenous fluid). Water reaches this state at a
  pressure above around 220 Kg Bar ( 225.56 Kg /
  cm2) and Temperature 374.15 C. In addition,
  there is no surface tension in a supercritical
  fluid, as there is no liquid/gas phase boundary.
• WHAT IS CRITICAL ABOUT SUPER CRITICAL POWER
  GENERATION
• By changing the pressure and temperature of the
  fluid, the properties can be tuned to be more
  liquid- or more gas like. Carbon dioxide and
  water are the most commonly used supercritical
  fluids, being used for decaffeination and power
  generation, respectively.

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CHALLENGES FOR ADOTION OF SUPER CRITICAL
TECHNOLOGY

• Up to an operating pressure of around 190Kg Bar
  in the evaporator part of the boiler, the cycle
  is Sub-Critical. In this case a drum-type boiler
  is used because the steam needs to be separated
  from water in the drum of the boiler before it is
  superheated and led into the turbine. Above an
  operating pressure of 220Kg Bar in the evaporator
  part of the Boiler, the cycle is Supercritical.
  The cycle medium is a single phase fluid with
  homogeneous properties and there is no need to
  separate steam from water in a drum. Thus, the
  drum of the drum-type boiler which is very heavy
  and located on the top of the boiler can be
  eliminated Once-through boilers are therefore
  used in supercritical cycles. Advanced Steel
  types must be used for components such as the
  boiler and the live steam and hot reheat steam
  piping that are in direct contact with steam
  under elevated conditions STEAM GENRATION IN NA

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STEAM GENRATION IN NATURAL CIRCULATION ONCE
THROUGH BOILER
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STEAM GENRATION IN NATURAL CIRCULATION ONCE
THROUGH BOILER
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BOILER FOR SUPERCRITICAL ONCE THROUGH POWER PLANT

• O Once through Boiler technology, which
  originated in Europe, has evolved into the most
  effective application for Supper Critical Steam
  condition.
• There are no operational limitations due to once-
  through boilers compared to drum type boilers.
• In fact once-through boilers are better suited
  to frequent load variations than drum type
  boilers, since the drum is a component with a
  high wall thickness, requiring controlled
  heating. This limits the load change rate to 3
  per minute, while once-through boilers can
  step-up the load by 5 per minute.
• This makes once-through boilers more suitable
  for fast startup as well as for transient
  conditions.

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• CHANGE FROM NATURAL CIRCULATION TO ONCE THROUGH
  IS MORE IMPPORTANT THAN THE SWITCH FROM SUB-TO
  SUPER CRITICAL

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BOILER FOR SUPERCRITICAL ONCE THROUGH POWER PLANT

• Once-through boilers have been favored in many
  countries, for more than 30 years.
• They can be used up to a pressure of more than
  300 Kg Bar without any change in the process
  engineering. Wall thicknesses of the tubes and
  headers however need to be designed to match the
  planned pressure level.
• Once-through boilers have been designed in both
  two-pass and tower type design, depending on the
  fuel requirements and the manufacturers general
  practice.
• For the past 30 years, large once-through
  boilers have been built with a spiral shaped
  arrangement of the tubes in the evaporator zone.
• The latest designs of once-through boilers use a
  vertical tube arrangement

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BOILER CONCEPTS SUPRCRITICAL BENSON TYPE
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SUPERCRITICAL ONCE THROUGH POWER PLANT TURBINE
GENERATOR

• The Turbine designs for a Super Critical plant
  are similar to the sub critical with the only
  special materials required for the casings and
  walls for withstanding high Temperatures and
  pressures.
• High Pressure (HP) Turbine In order to cater
  for the higher steam parameters in supercritical
  cycles, materials with an elevated chromium
  content which yield higher material strength are
  selected.
• Intermediate Pressure (IP) Turbine Section In
  supercritical cycles there is a trend to increase
  the temperature of the reheat steam that enters
  the IP turbine section in order to raise the
  cycle efficiency. As long as the reheat
  temperature is kept at 560 DEGC there is not much
  difference in the IP section of Sub critical and
  Super Critical plants.
• Low Pressure (LP) Turbine Section The LP turbine
  sections in supercritical plants are not
  different from those in subcritical plants.

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• CHALLENGES FOR ADOPTION OF SUERCRITICAL
  TECHNOLOGY
• O DNB (DEPARTURE FROM NUCLEATE BOILING) DO (DRY
  OUT)
• O DAMAGING THERMAL STRESSES ARISING OUT OF
  TEMPERATURE DIFFERENCE AT EVAPORAOR OUTLET
• SPIRAL WATER WALL,TUBING HEAT FLUX

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SPIRAL WATER WALL, TUBING HEAT FLUX
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TUBE TEMPERATURE EVAPORATOR OUTLET
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