introduction to thermal powerplant,type of thermal powerplant,captive powerplant,rankin cycle,co-generation powerplant,subcritical powerplant,supercritical powerplant,theory of operation,working principle,parts of powerplant,boiler,turbine,etc
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.
3. THEORY OF THERMAL POWER
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.
4. 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
5. 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
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.
6. 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
12. 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
13. 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
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
14. 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.
15. 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.
16. 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
20. 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
23. 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
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
24. 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
28. 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
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.
29. CHANGE FROM NATURAL CIRCULATION TO ONCE THROUGH IS MORE
IMPPORTANT THAN THE SWITCH FROM SUB-TO SUPER CRITICAL
30. BOILER FOR SUPERCRITICAL
ONCE THROUGH POWER PLANT
Once-through boilers have been favored in many countries, for more than
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
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
32. SUPERCRITICAL ONCE THROUGH POWER PLANT – TURBINE
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
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
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
33. 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