Steam Power Station (Thermal Station)
• A generating station which converts heat energy into
electrical energy through turning water into heated
steam is known as a steam power station.
• A steam power station basically works on the
RANKINE cycle.
Steam is produced in the boiler by utilizing the heat of
combustion. The steam is then expanded in the prime
mover (steam turbine) and is condensed in a condenser
to be fed into the boiler again. The steam turbine drives
the alternator which converts mechanical energy of the
turbine into electrical energy.
Flue gases from the boiler pass through super-heater,
economizer, air pre-heater and are finally exhausted to
atmosphere through the chimney.
Chimney
Bus-bars
induced
draught fan
Hot air
isolators
Air pre
heater
C.B.
isolators
Forced draught fan
Flue gases
Economizer
Transformer
Turbine
Flue gases
Fuel storage
and
handling
Boiler
R
Y
B
Super
heater
Valve
Exciter
Exciter
Alternator
Flue gases
Feed water
heater
Feed water
pump
Exhaust
steam
∼
Condensate
extraction
pump
∼
∼
Water
treatment
chamber
Condenser
Circulating
water pump
Cooling
Tower
River
Schematic arrangement of Steam Power Station
Equipment of Steam Power Station
Water
treatment
plant
Condenser
Steam
generating
equipment
Prime
mover
Electrical &
control
equipment
Boiler
Alternator
Boiler
furnace
Exciter
Super
heater
Transformers
Switchgear
Economizer
Pre
heater
Control
room
Water treatment plant
Boilers require clean and soft water for longer life and better
efficiency. However, the source of boiler feed water is
generally a river or lake which may contain suspended and
dissolved impurities and dissolved gases. Therefore, it is
very important that water is first purified and softened by
chemical treatment and then delivered to the boiler.
Source water is stored in storage tanks, where suspended
impurities are removed through sedimentation, coagulation
and filtration. Dissolved gases are removed by aeration and
degasification. The water is then ‘softened’ by removing
temporary and permanent hardness through different
chemical processes. The resulting pure and soft water is fed
to the boiler for steam generation.
Boiler
A boiler is a closed vessel where the heat of combustion
is utilized to convert water into steam at high
temperature and pressure.
Types of boilers:
Water tube boilers:
Fire tube boilers:
-Water flows through the tubes and
the hot gases of combustion flow
over these tubes
- Requires less space
- High working pressure
- Less liable to explosion
-The
hot
gases
of
combustion pass though
the tubes surrounded by
water
- Not suitable for large
capacity plants
Boiler furnace
- A chamber in which fuel is burnt to liberate heat energy.
- Provides support and enclosure for the combustion equipment
- Walls made of refractory materials that resist change of shape or
physical properties at high temperatures (fire clay, silica, kaolin).
Types of wall construction
Plain refractory walls:
Suitable for small plants where the furnace temperature is not high.
Hollow refractory walls with arrangement for air cooling:
Suitable for large plants. Air is circulated through hollow space to keep
furnace walls at low temperature.
Water walls:
Recent development suitable for large plants. Plain tubes arranged side by
side on the inner face of refractory walls. Tubes are connected to the upper
and lower headers of the boiler. The boiler’s water is made to circulate
through these tubes. Water walls absorb radiant heat in the furnace.
Super-heater
Wet steam produced in the boiler is passed through a super-heater
where it is dried and superheated by flue gases on their way to
chimney. Superheating provides two principal benefits; the overall
efficiency is increased, and too much condensation in the last
stages of turbine, liable to cause blade corrosion, is avoided. The
superheated steam from the super-heater is fed to steam turbine
through the main valve.
Radiant Super-heater
Convention Super-heater
-Placed in furnace between the water
walls.
-Placed in the boiler’s tube bank
- Receives heat from burning fuel
through radiation process
- Requires careful design accounting
for being superheated
- Its temperature falls with increased
steam output
- Receives heat from flue gasses
through convention process
- Its temperature increases with
increased steam output
Economizer
The economizer is a feed water heater that extracts a part of
heat of flue gases to increase the feed water temperature.
Air pre-heater
The air pre-heater extracts heat from flue gases and
consequently increases the temperature of the air supplied
for fuel burning. Increased thermal efficiency and increased
steam capacity per square meter of boiler surface are attained
accordingly. Types depend on heat transfer method
Recuperative type:
Regenerative type:
-Consists of a group of steel tubes,
through which flue gases are passed.
-Consists of a slowly moving
corrugated metal plates drum
- Heat is transferred from gases to air
flowing externally.
- Flue gases flow continuously on
one drum side, and air on the
other permitting heat transfer .
Condenser
- Condenses steam at turbine’s exhaust creating low pressure.
- Permits steam’s expansion in prime-mover at very low pressure.
- Accordingly helps energy conversion in prime-mover.
- Condensed steam can be reused as feed water in surface type
Jet condenser:
Surface condenser:
-Cooling water and exhaust steam are
mixed together
- Consists of a bank of horizontal
tubes enclosed in cast iron shell.
- Requires low initial cost, less floor
area, less cooling water and low
maintenance charges.
- Cooling water flows through the
tubes, while exhaust steam flows
over its’ surface giving up its heat
and condensing.
- Limited to industrial sizes; ∼ 1000 kW
with 50 – 125 mm.Hg vacuum.
-Condensate is wasted and High
power is required to pump water
- Prevail in Large installations;
(best vacuum: 12 – 50 mm.Hg)
- Condensate can be used as feed
water, Less pumping power is
required and Better vacuum is
created at turbine exhaust.
- Requires high initial cost, large
floor area, high maintenance
Prime-movers
Steam Engines
Steam Turbines:
- High
efficiency,
Simple
construction, less floor area, low
maintenance requirements
Impulse Turbines:
Reaction Turbines:
-Steam attains high velocity as it
completely expands in stationary
nozzles or fixed blades, while
pressure over moving ones remains
constant.
-Steam partially expands in
stationary
nozzles,
while
remaining expansion occurs
during flow over moving
blades.
- Rotation results by impulse force
of steam impinging the moving
blades.
- Rotation results by reaction
force originated from steam’s
momentum.
Generator
Alternator:
- Cylindrical rotor type
- Nitrogen or air cooled
Exciter:
- Separate (old)
- Brushless (most used)
Switchyard
- Contains transformers and switchgear.
- Provided with protective lightning arrestors.
-Transformers:
- Main step-up transformers
- General service transformers
- Auxiliary transformers
Switchgear:
- Circuit breakers
- Switches
- Relays.
Control room
- Contains panels of measuring instruments and communication
arrangements.
- Contains control equipment needed for alternator, feeder,
automatic voltage regulator, synchronizing gear and protective gear.
- Separate battery room and either a motor/generator set or a
rectifier are installed to supply make and break switchgear circuits.
Steam plant Efficiency:
Steam plant Efficiency = Thermal efficiency × Electric efficiency
Thermal efficiency depends on the following factors:
- Pressure of steam entering the turbine (increases noticeably with increased
pressure)
- Temperature of steam entering the turbine (increases with increased temperature)
- Pressure in the condenser (increases with decreased condenser pressure; usually
kept at 0.04 kg/cm2)
To increase thermal efficiency beside high steam pressure and temperature, and low
condenser pressure, reheating of steam between turbine stages, and bleeding
steam for heating feed water may be adopted
ηthermal
Heat equivalent of mech enery transmitted to turbine shaft
=
Heat of feul cobustion
Huge amount of heat is lost in condenser (∼50%).
Unavoidable as energy conversion requires temperature
difference, necessitating low steam temperature in condenser,
while the greater the temperature difference, the greater the
heat lost is.
Low Efficiency
(∼30%).
Heat is losses at other various stages of plant (∼20%).
η overall
Heat equivalent of elctric enery output
=
Heat of feul cobustion
Low Efficiency
(∼29%).
Advantages
(i) Less initial cost as compared to other generating stations.
(ii) It can be installed at any place
(iii) It requires less space as compared to the hydroelectric power station.
(iv) The cost of generation is less than that of the diesel power station.
Disadvantages
(i) Pollution due to the production of large amount of smoke and fumes.
(ii) Higher running cost as compared to hydroelectric plant.