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Thermal power plant

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Title: Thermal power plant


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THERMAL POWER PLANT
  • By
  • Ashvani Shukla
  • CI
  • reliance

2
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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  • THANK YOU
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