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Coal fired electric power plants23Presentation Transcript
NTPC Ltd. ,BARH(PATNA)
IIMT INSTITUTE OF ENGINEERING
ELECTRICITY GENRATION FROM COAL
to:Dr. P. K S
Presented by: MUNNA KUMAR
Electricity Generation From
• Let’s start at the beginning.
• Almost everything in our homes and
businesses today run on electricity.
• That electricity has to come from somewhere.
• Some sources of electricity are nuclear power,
solar power, wind power, and most important
for us Coal Power.
• Coal was used to heat water in boiler room pipes to
• The steam was used in a reciprocating (piston)
steam engine to produce mechanical energy
• The mechanical energy was converted into
electricity by a dynamo (generator)
Inside the dynamo room
Boiler plant can be divided in to three parts.
i) water circuit
ii) steam circuit
iii) The air and fuel gas circuit.
1) Water circuit
In the water circuit, water is fed from the boiler feed pumps into the boiler
through economiser. In the economiser, it receives some heat from the
departing flue gas before it goes to the boiler drum. The drum acts as a
reservoir for the various water walls of the boiler and also acts as a
separation chamber where water is removed from the steam before the
steam goes to the superheaters.
From the boiler drum the water passes down through pipes called down
comers to headers at the bottom of the boiler water walls.
The tubes which makeup the walls contain a mixture of
steam bubbles and water. This mixture being low dense
than water in the down comers, rises rapidly and reaches
back to the drum and its place is taken by the water
flowing through down comers. This produces what we call
is natural circulation.
The steam and water mixture which is returned to the
drum is separated so that water only (with no steam
bubbles) is returned to the down comers, and steam
only(with no water droplets) passed to the super heaters.
2) Steam circuit
Dry steam from the boiler drum goes to the various
superheater sections. Steam from the boiler drum passes
through the superheater connecting tubes to the primary
superheater, which is positioned in the convection pass.
The steam then flows from the primary superheater outlet
header to the secondary superheater located in the
Steam then goes to the final superheater which is located in
the combustion chamber in the outlet section, it then leaves
the final super heater outlet header and passes to the main
stem pipe which has a boiler stop valve.
To burn the fuel in the combustion chamber air is required. After
combustion, the hot gases are to be evacuated from furnace through the
heat absorbing surfaces. This air and gas flow is created by the boiler
draught system, which may be either natural or mechanised.
The air drawn from the atmosphere is first routed through an air heater
where air is heated by the outgoing flue gases. The hot air is then admitted
to the furnace through wind box. In coal fired boilers part of this hot air is
used for drying the coal in the pulvariser and transporting the pulvarised
coal to furnace.
The gases pass through the radiant heat release zone and then through
various superheaters and reheaters (in reheat boilers). Normally there will
be a primary superheater and secondary superheater.
After passing through the air heater the flue gas goes to the chimney. In
between the air heater and chimney it is customary to provide
precipitator to remove the flyash from the flue gas (especially in coal
fired boilers) and induced draught fans to suck out the flue gases from
the furnace (in balanced draught/induced draught boilers).
water is the working medium which transfers the heat energy available in
the fuel to the turbine in the form of steam.
22° C reduction in flue gas temperature increases boiler efficiency by 1%
Water is chosen as the medium because of the
a) its easy availability
b) its low viscous property
c) it has high specific heat
d) Its non-reactivity with surfaces with which it comes in
The following can be termed as boiler
1. Boiler drum
2. Water walls
4. Reheaters and
BOTTOM RNG HDR& Z-PANEL
1 PASS W.W
1 PASS W.W O/L HDRS
ROOF I/L HEADER
2ND PASS UPPER C-HDR
2nd PASS LOWER C-HDRS
LTSH I/L HEADER
LTSH O/L HEADER
PASS ROOF O/L HDR(REAR
Air Pre Heater
210 MW Boiler: Water and Steam Circuit
The drum acts as reservoir for water &
saturated steam and also provides
separation and purification of steam.
The feed water to the drum reaches
the drum from the boiler feed pump via
A stronger material for use in boiler
drums is Ducal W30.
Methods of Steam Separation:
1. By Gravity separation
This is employed for boilers having low generation rates.
2. By use of Baffles
These are in the form of obstacles in the direct path of steam towards outlet.
WATER WALL SYSTEM:
In the boiler the walls of the combustion chamber are
formed by tubular wall sections which not only form the
enclosure for the furnace but also provides the
evaporating surface for the feed water.
The water from the boiler drum is admitted in to the
water wall tubes through the downcomers and bottom
As the water circulates through the waterwall tubes,
which receive heat from the furnace radiation, water
partially evaporates into steam.
Water-steam mixture then return back to the boiler
Super heaters (SH) are meant for raising the steam temperature above the
The superheated and reheated steam temperature around 540°C and pressure 165
i) SH (Reheater also) can classified into convection and radiation type
as per heat transfer process.
The super heaters and reheaters which are placed above the furnace and can view
the flame are called radiant type.
ii) Super heater may be classified also according to the shape of the tube banks
and the position of the headers, such as pendant SH, platen SH, horizontal SH,
Ceiling SH, wall SH etc.
iii) They may be classified according to their stages of superheating they perform,
like primary SH, Secondary SH, Final SH etc.
Reheaters (RH) are provided to raise the temperature of the steam
from which part of energy has already been extracted by HP turbine.
Though super heaters are designed in such a way that heat
absorbed by radiant and convection super heaters always try
to maintain the steam temperature constant in practice the
necessary control is achieved by using de-super heater.
All modern boiler contact type de-super heaters by which feed water are sprayed
directly into the steam for required cooling.
Amount of feed water to be sprayed is controlled by automatic control system
which is designed to maintain a set final steam temperature. Provision of manual
control is also there for emergency.
The economiser absorbs heat from the flue
gas and adds it mainly as sensible heat to the
The material used in the manufacture of
furnace wall tubes for coal fired boiler is
ordinary carbon steel but in the 500 MW oil
fired units the major proportion of the furnace
is constructed from the 1% Cr. ½% Mo Alloy.
In 660 MW units also this material is used for
whole of the furnace.
The Boiler Auxiliaries :
• Draft system
• Air heaters
• The combustion process in a furnace can take place only when it
receives a steady flow of Air and has the combustion gases
• The Boiler draft system includes Air and Flue gas flow.
• All modern large utility boilers are fired under "balanced draft"
condition, i.e. where draft is zero. This condition is created by the
combination of "forced draft" and "Induced draft".
• deposits resulting from the combustion of coal will be deposited on
the boiler tubes at various zones will be cleaned by soot blowing for
effective heat transfer while on-load.
• The air heater is required for efficient combustion in the furnace and
also for drying wet coal in the milling plant. to recover "waste" heat
from the flue gas to increase boiler efficiency
MILLING PLANT :
• raw coal from the bunker is fed at a regulated rate to the mills through
• Air required for drying and transporting the pulverized coal from the
mill is obtained from the FD fan.
• Hot air is drawn through air heaters and cold air directly from FD fan
• The drying and grinding takes place inside the mills. The pulverized
particles are being carried from the mill to the classifier, which is
directly mounted on the mill.
• The medium is directed into the burners through various fuel
Type of Pulverizers:
• The principles upon which an electrostatic precipitator
operates are that the dust laden gases pass into a
chamber where the individual particles of dust are given
an electric charge by absorption of free ions from a high
voltage D.C. ionising field.
• They are removed by an intermittent blow usually
referred to as rapping. This causes the dust particles to
drop into dust hoppers situated below the collecting
The following fans are used in the boiler houses:
1. Forced Draft fan (F.D. Fan):
To take air from atmosphere to supply all the Combustion air. Speeds vary between
600 to 1500 r.p.m.
2. Induced Draft Fan (I.D. Fan):
Used only in balanced draft units to suck the gases out of the furnace and throw them
into the stack.
Handles flue gases at temperatures of 125 to 200oC.
Speed generally does not exceed 1000 rpm.
3. Primary Air Fans (P.A. Fans) or Exhauster Fan:
Used for pulverized system
Primary air has got two functions viz. Drying the coal and transportation into the
Usually 1500 r.p.m.
• As mentioned before,
something has to turn the
rotor in order to generate
• In our case the prime mover
happens to be a steam
• Steam comes out of the tubes
in the boiler and into a
manifold then into the turbine.
• As the steam passes over the
turbine blades, torque is
produced as a result of the
Turbine Generator and Condenser
• The rate of steam flow controls how fast the turbine
rotates and therefore the frequency of the electricity
• As the steam moves through the turbine energy is
extracted which results in a pressure drop.
• Therefore the LP turbine is located at the exit of the
HP turbine to extract the maximum amount of
energy from the steam before it is sent to the
• As electricity is generated it leaves the building
though a very large circuit breaker and a series of
transformers before it enters the power grid.
Path of Electricity
• After the steam leaves the LP turbine it travels to the
condenser, where it is condensed back to liquid
• The condenser is a heat exchanger that cools the
steam while is passes over tubes that have cold
water running through them.
• The cold water removes energy from the heated
steam causing it to condense which is necessary for
the water to be re-used as feed.
• There is another reason why the condenser is
necessary which we will discuss shortly.
• The water that runs through the tubes in the
condenser must be cooled down in order to
condense the steam.
• This is accomplished using very large cooling
towers, in which the water is atomized by
sprayers and cooled down by atmospheric
conditions and fans.
• The substance leaving cooling towers is
sometimes mistaken for smoke, but it is in fact
just water vapor.
• After the condensate is collected in the
hotwell of the condenser it is pumped through
the feed system.
• The feed pump increases the pressure of the
feed water in order for it to flow back into the
boiler to be turned back into steam to start
the cycle over again.
• This stage turns out to be the 4th and final
stage of something called a heat engine.
Basic Heat Engine
Laws of Thermodynamics
• 1st: “The increase in internal energy of a system
is equal to the amount of heat energy added
to the system minus the work done by the
system on the surroundings.”
Laws of Thermodynamics
• 2nd: The temperature differences between systems in
contact with each other tend to even out and that work
can be obtained from these non-equilibrium differences,
but that loss of heat occurs, in the form of entropy, when
work is done.
• 2nd: It is impossible to produce work in the surroundings
using a cyclic process connected to a single heat
reservoir (Kelvin, 1851).
Laws of Thermodynamics
• The second law also states that the maximum
efficiency of a heat engine can be determined by:
η = 1-(Th/Tc)
• Efficiency is also equal to the work output over the
η = Δ W/Δ QH
• An ordinary power plant operates between
the temperatures of 565C and 25C which leads
to maximum efficiency of around 64%.
• However, due to the losses mentioned earlier
the usual observed efficiency is about 35%.
• This shows how much of the energy stored in
the coal is just wasted instead of being
converted to electricity.
• The previous part of the presentation was to
explain the process behind electricity
generation that occurs after the coal portion.
• I will now go through some of the
components dealing with the coal aspect of
the power plant ending at the boiler which is
where the steam cycle began.
• Most of what I will discuss is particular to the
Co-gen plant that I visited.
• After the coal is mined and loaded into trucks
it is delivered into chutes that lead to the
• The breaker is a drum with hammers in it that
rotates and breaks the coal down into pieces
about 4” diameter, which fall through the
screen and onto the conveyor belt which leads
to the storage facility.
• After the coal has been sufficiently reduced in
size, it enters a storage facility, in this case, a
• The coal is then stacked using a machine you
will see in the next slide, which rotates and
places the coal around the perimeter of the
• This same machine also takes coal from the
pile and delivers it to the crusher building.
• The coal is delivered from the storage facility to a
device conveniently called a coal crusher.
• This machine takes the 4” pieces of coal and through
a series of rollers converts the fuel into a fine
• This powder is necessary for proper combustion in
• All newer state of the art power plants are set up to
work with pulverized coal, but older plants may
operate with lumped coal.
Fuel Feed System
• The conveyor belt
delivers the crushed
coal to a series of fuel
feeders, which inject
coal into the boilers
along with a mixture of
high pressure air for
• The final stage for the coal coincides with the
first stage of the steam plant.
• The boiler is usually the largest component of
the coal power plant climbing as high as 200 ft.
• Inside the boilers the pulverized coal is burned
while it more or less floats with the aid of HP
• Lining the entire inside of the boiler are tubes
which carry the feed water to be turned into
Inside the Boiler
After the Boiler
• As the coal is burned a large amount of ash is
• Some falls to the bottom and is collected, then
mixed with water and sent to the ash pile.
• Very light ash particles also escape with the
exhaust gasses which are captured by a bag
• This fly ash is collected in a silo until a certain
level is reached when it is pumped to the ash
pile via HP air.
• This ash has a basic pH and has
some beneficial uses.
• After a truck delivers coal to the
plant it is filled with ash to return to
the reclamation site.
• The high pH helps treat acid mine
• The fly ash is used as a substitute to
make Portland cement.
• Another use of fly ash is structural
fill for highway embankments and
the fill under new highways.
Exhaust Gasses and the Stack
• One of the most controversial aspects of a coal
power plant is what comes out of the stack.
• During the combustion process dangerous
gasses and particulates are released, such as
NOx, SOx, and CO2.
• Controls are in effect for each of these in new
plants, however older plants spew thousands of
pounds of each of these into the atmosphere
• The intermittent blown down is given by manually
operating a valve fitted to discharge pipe at the lowest
point of boiler shell to reduce parameters (TDS or
conductivity, pH, Silica etc) within prescribed limits so
that steam quality is not likely to be affected
• TDS level keeps varying
• fluctuations of the water level in the boiler.
• substantial amount of heat energy is lost with
intermittent blow down.
• A steady and constant dispatch of small
stream of concentrated boiler water, and
replacement by steady and constant inflow
of feed water.
• This ensures constant TDS and steam
• This type of blow down is common in highpressure boilers.
Boiler Water Treatment
• Internal Water Treatment: It is carried out by adding
chemicals to boiler to prevent the formation of scale by
converting the scale-forming compounds to free-flowing
sludges, which can be removed by blowdown.
• Limitation: Applicable to boilers, where feed water is
low in hardness salts, to low pressures- high TDS content
in boiler water is tolerated, and when only small
quantity of water is required to be treated.
• Internal treatment alone is not recommended.
External Water Treatment
• Propose: External treatment is used to remove suspended
solids, dissolved solids (particularly the calcium and
magnesium ions which are a major cause of scale formation)
and dissolved gases (oxygen and carbon dioxide).
• Different treatment Process :
– ion exchange;
– reverse osmosis and
• Demineralization is the complete removal of all salts.
• This is achieved by using a “cation” resin, which exchanges the cations
in the raw water with hydrogen ions, producing hydrochloric, sulphuric
and carbonic acid.
• Carbonic acid is removed in degassing tower in which air is blown
through the acid water.
• Following this, the water passes through an “anion” resin which
exchanges anions with the mineral acid (e.g. sulphuric acid) and forms
• Regeneration of cations and anions is necessary at intervals using,
typically, mineral acid and caustic soda respectively. The complete
removal of silica can be achieved by correct choice of anion resin.
• When heated in boiler systems, carbon dioxide
(CO2) and oxygen (O2) are released as gases and
combine with water (H2O) to form carbonic acid,
dissolved gases, such
as oxygen and carbon
dioxide, are expelled
by preheating the feed
water before it enters
Exhaust Gasses and the Stack
Reduce Stack Temperature
• Stack temperatures greater than 200°C
indicates potential for recovery of waste heat.
• It also indicate the scaling of
transfer/recovery equipment and hence the
urgency of taking an early shut down for
water / flue side cleaning.
22o C reduction in flue gas temperature
increases boiler efficiency by 1%