Steam Power PlantSolar
Asst Prof : Electrical and Electronics
Chitkara University, Punjab
Fluidised Bed Combustion
• A fluidised bed combustion may be defined as the bed of solid particles
behaving as a fluid.
• When a gas is passed through a packed bed of finely divided solid
particles, it experiences a pressure drop across the bed.
• At low gas velocities, this pressure drop is small and does not disturb the
particles. But if the gas velocity is increased further, a stage is reached,
when particles are suspended in the gas stream and packed bed becomes
a ‘Fluidised Bed’
• With the further increase in gas velocity, the bed becomes turbulent and
rapid mixing of particles occurs.
• The behaviour of this mixture of solid particles and gas is like fluid.
Burning of fuel in such a state is known as fluidised bed combustion.
1. On distributor plate are fed fuel and inhert material
dolomite(CaMg(CO 3) and from bottom air is suspended.
2. The high velocity of air keeps the solid feed material in suspending
condition during burning.
3. The generated heat is rapidly transferred to the water passing
through the tubes immersed in the bed and generated steam is taken
4. During burning sulphur di oxide formed is absorbed by the dolomite
and prevents its escape with the exhaust gases. The molten slag is
tapped from the top of the bed.
5. The primary objective of the inhert material is to control the temp
of the bed, it accounts 90% of the bed volume.
It should remain in motion with the fuel and at high temp to tune
• 1. As a result of better heat transfer the unit size and capital cost are
• 2. It can respond rapidly to change in load demand.
• 3. Low combustion temperatures 800-950°C inhibits the formation of
nitric oxide and nitrogen oxide.
• 4. Since combustion temperature is low the corrosion of tube is reduced.
• 5. There is no need to crush the coal to a pulverized form so cost of
crushing is reduced.
• 6. Pollution is controlled as combustion of high sulphur content can be
• 7. FBC can use solid, liquid or gaseous fuel.
• 8. Combustion temp can be controlled accurately.
• 9. 70% ash containing coal can be burned in FBC, conventional combustion
system becomes unstable even with above 48% ash.
• A boiler may be defined as a closed vessel in
which steam is produced from water by
combustion of fuel.
• The performance of a boiler is measured in terms
of its evaporative capacity which is also called as
“Boiler Power”. It is defined as the amount of
steam produced in kg/hour. It may also be
expressed in kg of steam per kg of fuel burnt or
kg/hr/m2 of heating surface.
• Boilers are classified according to the following
1. According to the position of the principle axis
2. According to flow of water and hot gases.
• Water tube (Babcock, Wilcox, Striling, Yarrow Boiler)
• Fire tube(Cochran, Lancashire, Locomotive)
3. According to position of furnace
• Internally fired – furnace/fire is inside the boiler shell – (Cochran,
• Externally fired- (Babcock, Wilcox, Striling, Yarrow Boiler)
4. According to application
• Mobile (Marine, Locomotive…)
5. According to circulating water
• Natural circulation (circulation of water takes place due to natural
convention currents produced by application of heat) Lancashire,
Babcock, Wilcox boiler.
• Forced circulation (Water circulation is done by forced pump) Velox,
Lamont, Benson boilers.
• 6. According to steam pressure
• Low pressure – Produce steam at a pressure below 80bar, Cochran,
Cornish, Lancashire, Locomotive boilers
• High pressure - at a pressure 80 bar and above-Babcock, Wilcox,
Velox, Lamont, Benson boilers.
Water Tube Boilers
In water tube boilers,
water is circulated
through tubes and
hot products of
combustion flow over
Water Tube Boilers
• Water tube boilers can further be classified as follows:
• Horizontal straight tube boilers
• Bent tube boilers
• Cyclone fired boilers
• Water tube boilers have the following advantages:
• High pressures (about 140 kg/cm2) can be obtained
• Large heating surfaces can be obtained by use of large
number of tubes – therefore steam can be generated
• Efficiency is higher because of high velocity of water in
tubes which improves heat transfer
Fire Tube Boilers
• In fire tube boilers, the
hot combustion gases
pass through the tubes,
which are surrounded
Fire Tube Boilers
• Fire tube boilers can further be classified as follows:
• External furnace
• Internal furnace
• Fire tube boilers have the following advantages:
• Low cost
• Fluctuations of steam demand can be easily met
• Compact in size
• Disadvantage of fire tube boilers is that they contain more
water in the drum and if the flue gas circulation is poor,
they can not quickly meet the steam demand. For the same
output, the outer shell of fire tube boiler is much larger
than the shell of a water tube boiler
• Vertical multitubular boiler, has a number of
horizontal fire tubes.
• Shell diameter: 2.75m
• Height: 5.79
• Working Pressure: 6.5bar(1bar = 100,000 Pa)
• Steam capacity: 3500kg/h max 4000
• Heating surface: 120m2
• Efficiency: 70 to 75%
Babcock and Wilcox water-tube boiler
• Horizontal straight tube boiler, stationary or
Diameter of drum: 1.22 to 1.83m
Length: 6.096 to 9.144m
Size of water tubes : 7.62 to 10.16cm
Size of super heater tubes: 3.84 to 5.71cm
Working pressure: 40bar(max)
Steaming capacity: 40000kg/h (max)
Efficiency: 60 to 80%
Angle of inclination of tubes: 15deg.
• It is mainly employed in locomotives although
it may be used as stationary boiler.
• It is compact in size and its capacity for steam
production is quite high for its size and can
raise the amount of heat quickly.
• It consists of cylindrical barrel with rectangular fire box at one end
and smoke box at other end. Coal is introduced through the fire
hole into the gate which is placed at the bottom of fire box.
• The hot gasses generated are deflected by an arch of fire bricks, so
that the walls of the fire box may be heated properly.
• Fire box is entirely surrounded by water.
• The hot gasses pass from fire box to smoke box by series of fire
tubes, then they are discharged to atmosphere.
• Heat of hot gasses is transmitted to water, the steam generated is
collected over the water surface.
• Here natural draught cant be used as it need high chimney height,
the forced draught is created by the exhaust steam.
1. High steam capacity.
2. Low construction cost.
3. Low installation cost.
1. Chances of corrosion.
2. Difficult to clean some water spaces.
3. Overload cause overheating.
4. Practical limitations of pressure and capacity.
Accessories used in steam power plants
• Accessories are the auxiliary plants required for
steam boilers for their proper operation and for
the increase of their efficiency.
1. Feed Pumps:
Feed pump is pump which is used to deliver feed
water to the boiler.
a) Reciprocatory pumps consists of cylinder and
piston, these are continuously run by steam
from the same boiler to which water is to be fed.
b) Rotary Pumps are centrifugal type are either run
by small steam turbine or by an electric motor.
• An economiser is a device in which the waste
heat of the flue gases is utilised for heating the
1. Independent type: It is installed in the chamber
apart from the boiler settings. The camber is
situated ate the passage of the flow of the flue
gasses from boiler to chimney.
2. Integral type: it is the part of the boiler heating
surface and is installed in within the boiler
Advantages of Economiser
• The Temperature range between various parts
of the boiler is reduced which results in
reduction of stresses due to unequal
• If the boiler is fed with cold water, it may
result in chilling the boiler metal.
• Evaporation capacity of boiler is increased.
• Overall efficiency of boiler is increased.
• The function of the air
pre-heater is to increase
the temperature of air
enters the furnace. It is
generally placed after
the economizer; so the
flue gases pass through
the economizer and
then to the air
• The function of a super heater is to increase the
temperature of the steam above its saturation
point. The super heater is very important
accessory of a boiler and can be used both on
fire-tube and water-tube boilers. The small
boilers are not commonly provided with a super
• Super heaters are located in the path of the
furnance gases so that the heat is recovered by
the superhearter from the hot gases,
• Steam consumption of engine or turbine is
• Erosion of turbine blade is eliminated.
• Efficiency of steam plant is increased.
Types of Superheaters
• Convective Superheaters: Makes the use of
heat of flue gases.
• Radiant Superheaters: It is placed in the
furnace and the wall tubes receives heat from
burning fuel through radiant process. It is used
where high amount of superheat temperature
• In a reheat turbine the
steam first enters high
speed turbine so its
temp and pressure
reduces before entering
low speed turbine so a
reheater is used to
reheat the cooled
Feed Water Heaters
• A feed water heater is a power plant component
used to pre-heat water delivered to
a steam generating boiler. Preheating the feed
water reduces the irreversibilities involved in
steam generation and therefore improves
the thermodynamic efficiency of the system. This
reduces plant operating costs and also helps to
avoid thermal shock to the boiler metal when the
feed water is introduced back into the steam
• The main purposes of the condenser are to condense the exhaust
steam from the turbine for reuse in the cycle and to maximize
turbine efficiency by maintaining proper vacuum.
• As the operating pressure of the condenser is lowered (vacuum is
increased), the enthalpy drop of the expanding steam in the turbine
will also increase. This will increase the amount of available work
from the turbine (electrical output). By lowering the condenser
operating pressure, the following will occur :
• (a) Increased turbine output
• (b) Increased plant efficiency
• (c) Reduced steam flow (for a given plant output)
It is therefore very advantageous to operate the condenser at the
lowest possible pressure
• There are two primary types of condensers
that can be used in a power plant :
• (a) Direct Contact
• (b) Surface
• Direct contact condensers condense the
turbine exhaust steam by mixing it directly
with cooling water
• Steam surface condensers are the most commonly used
condensers in modern power plants. The exhaust steam
from the turbine flows on the shell side (under vacuum) of
the condenser, while the plant’s circulating water flows in
the tube side.
• The source of the circulating water can be either a closed-
loop (i.e. cooling tower, spray pond, etc.) or once through
(i.e. from a lake, ocean, or river).
• The condensed steam from the turbine, called condensate,
is collected in the bottom of the condenser, which is called
• The condensate is then pumped back to the steam
generator to repeat the cycle.
• These are used to supply of pure water as make up feed for
the boilers. Raw water is evaporated by using extracted
steam then condensed to give distilled and pure feed water.
• The film type evaporators: In this type water is sprayed on
the surface of tubes through which steam is passed. As the
water falls on the surface of the heated tubes it evaporates.
• The submerged type evaporators: In this type the bundle of
tubes is submerged in water. Vapors formed in the shell
pass out of the shell through a moisture separator and
enter a feed water condenser.
• Cooling Towers have one function :
• Remove heat from the water discharged from the
condenser so that the water can be discharged to the
river or recirculated and reused.
• The importance of the cooling tower is felt when the
cooling water from the condenser has to be cooled.
• The cooling water after condensing the steam,
becomes hot and it has to be cooled as it belongs to a
• The Cooling towers do the job of decreasing the
temperature of the cooling water after condensing the
steam in the condenser.
• When water is reused in the process, it is
pumped to the top of the cooling tower and will
then flow down through plastic or wood shells,
much like a honeycomb found in a bee‟s nest.
• The water will emit heat as it is downward
flowing which mixes with the above air flow,
which in turn cools the water. Part of this water
will also evaporate, causing it to lose even more
Natural Draft cooling tower
• Natural draft towers are typically about 120 m
high, depending on the differential pressure
between the cold outside air and the hot humid
air on the inside of the tower as the driving force.
No fans are used.
• Mechanical draft towers uses fans (one or more)
to move large quantities of air through the tower.
They are two different classes :
• (a) Forced draft cooling towers
• (b) Induced draft cooling towers
• A steam turbine is a mechanical device that
extracts thermal energy from pressurized
steam, and converts it into rotary motion.
• A steam turbine is a prime mover in which the
potential energy of the steam is transferred
into the kinetic energy, and later in its turn is
transformed into mechanical energy of the
rotation of the turbine shaft.
Classification of Steam Turbines
Action of steam
C. Combination of impulse and reaction.
According to number of pressure stages
A. Single stage turbine : with one or more velocity stages usually of
small power capacity, used for driving centrifugal pumps, blowers etc
B. Multi Stage turbine: Wide range of capacities from large to small.
According to direction of steam flow
A. Axial Turbine: Steam flows in a direction parallel to axis of the
B. Radial Turbine: Steam flows in a direction perpendicular to the axis
of the turbine
According to method of governing
A. Turbines with throttle governing: fresh steam enters through one
or more simultaneously operated throttle valves.
B. Turbines with nozzle governing: fresh steam enters through two or
more consecutively operated regulators
According to steam conditions at inlet to the turbine
A. Low pressure turbines, 1.2 to 2 ata
B. Medium pressure turbines, 40 ata
C. High pressure turbines, above 40 ata
D. Very high pressure turbines, 170 ata and higher at temp of 550deg
E. Turbines of supercritical pressure, 225ata and above
According to their usages in Industry
A. Stationary turbines with constant speed of
B. Stationary turbines with variable speed of
rotation : turbo-blowers, pumps
C. Non stationary turbines with variable speed:
steamers, ships, locomotives
• The interior of a turbine comprises several
sets of blades, or “buckets” as they are more
commonly referred to. One set of stationary
blades is connected to the casing and one set
of rotating blades is connected to the shaft.
• The sets intermesh with certain minimum
clearances, with the size and configuration of
sets varying to efficiently exploit the
expansion of steam at each stage.
• The main difference lies in
the way the steam expands
while it moves through
• In Impulse turbine steam
expands in the nozzles and
its pressures does not alter
as it moves over blades.
• In reaction type the steam
expands continuously as it
passes over the blades and
thus there is gradual fall in
pressure during expansion.
• A steam nozzle is
defined as a passage of
through which heat
energy of steam is
converted to kinetic
energy. It produces a
steam jet with high
velocity to drive steam
• An impulse turbine has fixed nozzles that orient the steam flow into high speed
• These jets contain significant kinetic energy, which the rotor blades, shaped like
buckets, convert into shaft rotation as the steam jet changes direction.
• A pressure drop occurs across only the stationary blades, with a net increase in
steam velocity across the stage.
• As the steam flows through the nozzle its pressure falls from inlet pressure to the
• Steam Power Plant pressure (atmospheric pressure, or more usually, the
condenser vacuum). Due to this higher ratio of expansion of steam in the nozzle
the steam leaves the nozzle with a very high velocity.
• The steam leaving the moving blades is a large portion of the maximum velocity of
the steam when leaving the nozzle.
• The loss of energy due to this higher exit velocity is commonly called the “carry
over velocity” or “leaving loss”.
• In the reaction turbine, the rotor blades themselves are arranged to
form convergent nozzles. This type of turbine makes use of the
reaction force produced as the steam accelerates through the
nozzles formed by the rotor.
• Steam is directed onto the rotor by the fixed vanes of the stator. It
leaves the stator as a jet that fills the entire circumference of the
• The steam then changes direction and increases its speed relative
to the speed of the blades.
• A pressure drop occurs across both the stator and the rotor, with
steam accelerating through the stator and decelerating through the
rotor, with no net change in steam velocity across the stage but
with a decrease in both pressure and temperature, reflecting the
work performed in the driving of the rotor.
Main Differences of Impulse and Reaction Turbine
• 1) In impulse turbine the water flows through the nozzles and impinges on the buckets where as in
reaction turbine the water is guided by the guide blades to flow over the moving vanes.
• 2) In impulse turbine the entire water energy is first converted in kinetic energy but there is no
energy conversion in reaction turbine.
• 3) ) In impulse turbine the water impinges on the buckets with kinetic energy where as in reaction
turbine the water glides over the moving vanes with pressure energy.
• 4) In impulse turbine the work is done only by the change in the kinetic energy of the jet but in
reaction turbine the work is done partly by the change in the velocity head, but almost entirely by
the change in pressure head.
• 5) In impulse turbine the pressure of flowing water remains unchanged and is equal to the
atmospheric pressure but in reaction turbine the pressure of flowing water is reduced after gliding
over the vanes.
• 7) In impulse turbine the water may be admitted over a part of the circumference or over the
whole circumference of the wheel but in reaction turbine the water must be admitted over the
whole circumference of the wheel.
• 8) It is possible to regulate the flow of water without loss in impulse turbine but in reaction turbine
it is not possible to regulate the flow without loss.