Thermal Power Plant Lay out :
The above diagram is the lay out of a simplified thermal power plant and the below is also diagram of a thermal power plant.
The above diagram shows the simplest arrangement of Coal fired (Thermal) power plant.
Main parts of the plant are
1. Coal conveyor 2. Stoker 3. Pulverizer 4. Boiler 5. Coal ash 6. Air
preheater 7. Electrostatic precipitator 8. Smoke stack 9. Turbine 10.
Condenser 11. Transformers 12. Cooling towers
13. Generator 14. High - votge power lines
Basic Operation :A thermal power plant basically works on Rankine cycle.
Coal conveyor : This is a belt type of arrangement.With this coal is
transported from coal storage place in power plant to the place near by
boiler.
Stoker : The coal which is brought near by boiler has to put in boiler
furnance for combustion.This stoker is a mechanical device for feeding
coal to a furnace.
Pulverizer : The coal is put in the boiler after pulverization.For this
pulverizer is used.A pulverizer is a device for grinding coal for
combustion in a furnace in a power plant.
Types of Pulverizers
Ball and Tube Mill
Ball mill is a pulverizer that consists of a horizontal rotating
cylinder, up to three diameters in length, containing a charge of
tumbling or cascading steel balls, pebbles, or rods.
Tube mill is a revolving cylinder of up to five diameters in length used
for fine pulverization of ore, rock, and other such materials; the
material, mixed with water, is fed into the chamber from one end, and
passes out the other end as slime.
Ring and Ball
This type consists of two rings separated by a series of large balls.
The lower ring rotates, while the upper ring presses down on the balls
via a set of spring and adjuster assemblies. Coal is introduced into the
center or side of the pulverizer (depending on the design) and is
ground as the lower ring rotates causing the balls to orbit between the
upper and lower rings. The coal is carried out of the mill by the flow
of air moving through it. The size of the coal particals released from
the grinding section of the mill is determined by a classifer separator.
These mills are typically produced by B&W (Babcock and Wilcox).
Boiler : Now that pulverized coal is put in boiler furnance.Boiler is an
enclosed vessel in which water is heated and circulated until the water
is turned in to steam at the required pressure.
Coal is burned inside the combustion chamber of boiler.The products of
combustion are nothing but gases.These gases which are at high
temperature vaporize the water inside the boiler to steam.Some times
this steam is further heated in a superheater as higher the steam
pressure and temperature the greater efficiency the engine will have in
converting the heat in steam in to mechanical work. This steam at high
pressure and tempeture is used directly as a heating medium, or as the
working fluid in a prime mover to convert thermal energy to mechanical
work, which in turn may be converted to electrical energy. Although
other fluids are sometimes used for these purposes, water is by far the
most common because of its economy and suitable thermodynamic
characteristics.
Classification of Boilers
Bolilers are classified as
Fire tube boilers : In fire tube boilers hot gases are passed through
the tubes and water surrounds these tubes. These are simple,compact and
rugged in construction.Depending on whether the tubes are vertical or
horizontal these are further classified as vertical and horizontal tube
boilers.In this since the water volume is more,circulation will be
poor.So they can't meet quickly the changes in steam demand.High
pressures of steam are not possible,maximum pressure that can be
attained is about 17.5kg/sq cm.Due to large quantity of water in the
drain it requires more time for steam raising.The steam attained is
generally wet,economical for low pressures.The outut of the boiler is
also limited.
Water tube boilers : In these boilers water is inside the tubes and hot gases are outside the tubes.They
consists of drums and tubes.They may contain any number of drums (you
can see 2 drums in fig).Feed water enters the boiler to one drum (here
it is drum below the boiler).This water circulates through the tubes
connected external to drums.Hot gases which surrounds these tubes wil
convert the water in tubes in to steam.This steam is passed through
tubes and collected at the top of the drum since it is of light
weight.So the drums store steam and water (upper drum).The entire steam
is collected in one drum and it is taken out from there (see in laout
fig).As the movement of water in the water tubes is high, so rate of
heat transfer also becomes high resulting in greater efficiency.They
produce high pressure , easily accessible and can respond quickly to
changes in steam demand.These are also classified as vertical,horizontal
and inclined tube depending on the arrangement of the tubes.These are
of less weight and less liable to explosion.Large heating surfaces can
be obtained by use of large number of tubes.We can attain pressure as
high as 125 kg/sq cm and temperatures from 315 to 575 centigrade.
Superheater : Most of the modern boliers are having superheater and
reheater arrangement. Superheater is a component of a steam-generating
unit in which steam, after it has left the boiler drum, is heated above
its saturation temperature. The amount of superheat added to the steam
is influenced by the location, arrangement, and amount of superheater
surface installed, as well as the rating of the boiler. The superheater
may consist of one or more stages of tube banks arranged to effectively
transfer heat from the products of combustion.Superheaters are
classified as convection , radiant or combination of these.
Reheater : Some of the heat of superheated steam is used to rotate the
turbine where it loses some of its energy.Reheater is also steam boiler
component in which heat is added to this intermediate-pressure steam,
which has given up some of its energy in expansion through the
high-pressure turbine. The steam after reheating is used to rotate the
second steam turbine (see Layout fig) where the heat is converted to
mechanical energy.This mechanical energy is used to run the alternator,
which is coupled to turbine , there by generating elecrical energy.
Condenser : Steam after rotating staem turbine comes to
condenser.Condenser refers here to the shell and tube heat exchanger (or
surface condenser) installed at the outlet of every steam turbine in
Thermal power stations of utility companies generally. These condensers
are heat exchangers which convert steam from its gaseous to its liquid
state, also known as phase transition. In so doing, the latent heat of
steam is given out inside the condenser. Where water is in short supply
an air cooled condenser is often used. An air cooled condenser is
however significantly more expensive and cannot achieve as low a steam
turbine backpressure (and therefore less efficient) as a surface
condenser.
The purpose is to condense the outlet (or exhaust) steam from steam
turbine to obtain maximum efficiency and also to get the condensed steam
in the form of pure water, otherwise known as condensate, back to steam
generator or (boiler) as boiler feed water.
Why it is required ?
The steam turbine itself is a device to convert the heat in steam to
mechanical power. The difference between the heat of steam per unit
weight at the inlet to turbine and the heat of steam per unit weight at
the outlet to turbine represents the heat given out (or heat drop) in
the steam turbine which is converted to mechanical power. The heat drop
per unit weight of steam is also measured by the word enthalpy drop.
Therefore the more the conversion of heat per pound (or kilogram) of
steam to mechanical power in the turbine, the better is its performance
or otherwise known as efficiency. By condensing the exhaust steam of
turbine, the exhaust pressure is brought down below atmospheric pressure
from above atmospheric pressure, increasing the steam pressure drop
between inlet and exhaust of steam turbine. This further reduction in
exhaust pressure gives out more heat per unit weight of steam input to
the steam turbine, for conversion to mechanical power. Most of the heat
liberated due to condensing, i.e., latent heat of steam, is carried away
by the cooling medium. (water inside tubes in a surface condenser, or
droplets in a spray condenser (Heller system) or air around tubes in an
air-cooled condenser).
Condensers are classified as (i) Jet condensers or contact condensers (ii) Surface condensers.
In jet condensers the steam to be condensed mixes with the cooling water
and the temperature of the condensate and the cooling water is same
when leaving the condenser; and the condensate can't be recovered for
use as feed water to the boiler; heat transfer is by direct conduction.
In surface condensers there is no direct contact between the steam to be
condensed and the circulating cooling water. There is a wall interposed
between them through heat must be convectively transferred.The
temperature of the condensate may be higher than the temperature of the
cooling water at outlet and the condnsate is recovered as feed water to
the boiler.Both the cooling water and the condensate are separetely with
drawn.Because of this advantage surface condensers are used in thermal
power plants.Final output of condenser is water at low temperature is
passed to high pressure feed water heater,it is heated and again passed
as feed water to the boiler.Since we are passing water at high
temperature as feed water the temperature inside the boiler does not
dcrease and boiler efficincy also maintained.
Cooling Towers :The condensate (water) formed in the condeser after
condensation is initially at high temperature.This hot water is passed
to cooling towers.It is a tower- or building-like device in which
atmospheric air (the heat receiver) circulates in direct or indirect
contact with warmer water (the heat source) and the water is thereby
cooled (see illustration). A cooling tower may serve as the heat sink in
a conventional thermodynamic process, such as refrigeration or steam
power generation, and when it is convenient or desirable to make final
heat rejection to atmospheric air. Water, acting as the heat-transfer
fluid, gives up heat to atmospheric air, and thus cooled, is
recirculated through the system, affording economical operation of the
process.
Two basic types of cooling towers are commonly used. One transfers the
heat from warmer water to cooler air mainly by an evaporation
heat-transfer process and is known as the evaporative or wet cooling
tower.
Evaporative cooling towers are classified according to the means
employed for producing air circulation through them: atmospheric,
natural draft, and mechanical draft. The other transfers the heat from
warmer water to cooler air by a sensible heat-transfer process and is
known as the nonevaporative or dry cooling tower.
Nonevaporative cooling towers are classified as air-cooled condensers
and as air-cooled heat exchangers, and are further classified by the
means used for producing air circulation through them. These two basic
types are sometimes combined, with the two cooling processes generally
used in parallel or separately, and are then known as wet-dry cooling
towers.
Evaluation of cooling tower performance is based on cooling of a
specified quantity of water through a given range and to a specified
temperature approach to the wet-bulb or dry-bulb temperature for which
the tower is designed. Because exact design conditions are rarely
experienced in operation, estimated performance curves are frequently
prepared for a specific installation, and provide a means for comparing
the measured performance with design conditions.
Economiser : Flue gases coming out of the boiler carry lot of
heat.Function of economiser is to recover some of the heat from the heat
carried away in the flue gases up the chimney and utilize for heating
the feed water to the boiler.It is placed in the passage of flue gases
in between the exit from the boiler and the entry to the chimney.The use
of economiser results in saving in coal consumption , increase in
steaming rate and high boiler efficiency but needs extra investment and
increase in maintenance costs and floor area required for the plant.This
is used in all modern plants.In this a large number of small diameter
thin walled tubes are placed between two headers.Feed water enters the
tube through one header and leaves through the other.The flue gases flow
out side the tubes usually in counter flow.
Air preheater : The remaining heat of flue gases is utilised by air
preheater.It is a device used in steam boilers to transfer heat from the
flue gases to the combustion air before the air enters the furnace.
Also known as air heater; air-heating system. It is not shown in the lay
out.But it is kept at a place near by where the air enters in to the
boiler.
The purpose of the air preheater is to recover the heat from the flue
gas from the boiler to improve boiler efficiency by burning warm air
which increases combustion efficiency, and reducing useful heat lost
from the flue. As a consequence, the gases are also sent to the chimney
or stack at a lower temperature, allowing simplified design of the
ducting and stack. It also allows control over the temperature of gases
leaving the stack (to meet emissions regulations, for example).After
extracting heat flue gases are passed to elctrostatic precipitator.
Electrostatic precipitator : It is a device which removes dust or other
finely divided particles from flue gases by charging the particles
inductively with an electric field, then attracting them to highly
charged collector plates. Also known as precipitator. The process
depends on two steps. In the first step the suspension passes through an
electric discharge (corona discharge) area where ionization of the gas
occurs. The ions produced collide with the suspended particles and
confer on them an electric charge. The charged particles drift toward an
electrode of opposite sign and are deposited on the electrode where
their electric charge is neutralized. The phenomenon would be more
correctly designated as electrodeposition from the gas phase.
The use of electrostatic precipitators has become common in numerous
industrial applications. Among the advantages of the electrostatic
precipitator are its ability to handle large volumes of gas, at elevated
temperatures if necessary, with a reasonably small pressure drop, and
the removal of particles in the micrometer range. Some of the usual
applications are: (1) removal of dirt from flue gases in steam plants;
(2) cleaning of air to remove fungi and bacteria in establishments
producing antibiotics and other drugs, and in operating rooms; (3)
cleaning of air in ventilation and air conditioning systems; (4) removal
of oil mists in machine shops and acid mists in chemical process
plants; (5) cleaning of blast furnace gases; (6) recovery of valuable
materials such as oxides of copper, lead, and tin; and (7) separation of
rutile from zirconium sand.
Smoke stack :A chimney is a system for venting hot flue gases or smoke
from a boiler, stove, furnace or fireplace to the outside atmosphere.
They are typically almost vertical to ensure that the hot gases flow
smoothly, drawing air into the combustion through the chimney effect
(also known as the stack effect). The space inside a chimney is called a
flue. Chimneys may be found in buildings, steam locomotives and ships.
In the US, the term smokestack (colloquially, stack) is also used when
referring to locomotive chimneys. The term funnel is generally used for
ship chimneys and sometimes used to refer to locomotive
chimneys.Chimneys are tall to increase their draw of air for combustion
and to disperse pollutants in the flue gases over a greater area so as
to reduce the pollutant concentrations in compliance with regulatory or
other limits.
Generator : An alternator is an electromechanical device that converts
mechanical energy to alternating current electrical energy. Most
alternators use a rotating magnetic field. Different geometries - such
as a linear alternator for use with stirling engines - are also
occasionally used. In principle, any AC generator can be called an
alternator, but usually the word refers to small rotating machines
driven by automotive and other internal combustion engines.
Transformers :It is a device that transfers electric energy from one
alternating-current circuit to one or more other circuits, either
increasing (stepping up) or reducing (stepping down) the voltage. Uses
for transformers include reducing the line voltage to operate
low-voltage devices (doorbells or toy electric trains) and raising the
voltage from electric generators so that electric power can be
transmitted over long distances. Transformers act through
electromagnetic induction; current in the primary coil induces current
in the secondary coil. The secondary voltage is calculated by
multiplying the primary voltage by the ratio of the number of turns in
the secondary coil to that in the primary.
