Thermal Power Plant (ST):
Steam is an important medium of
producing mechanical energy. Steam has the advantages that it can be raised
from water which is available in abundance it does not react much with the
materials of the equipment of the power plant and is suitable at the
temperature required in the plant. Steam is used to drive steam engines, steam turbine
etc. Thermal electrical power generation is one of the major methods. For a
thermal power plant the range of pressure may vary from 10 kg/cm2
critical pressure and the range of temperature may be from 2500C
to6500C.
Figure: Steam Power Plant
Main parts of a steam power plant:
1. Boiler
(Babcock and Wilcox)
2. Steam
turbines (LPT, IPT and HPT)
3. Super
heater
4. Heaters
(LPH and HPH)
5. Condenser
6. Air
Compressor
7. Pumps
8. Economizer
9. Generator
Figure: Overall water and steam cycle of steam turbine units.
Working principle:
1. At
first water is feed into the boiler and heated by the furnace heat. A boiler
contains 9 furnace where each furnace having about 12000C.
2. In
boiler the separation line separates the steam from water. Water evaporates
through natural circulation with down comer.
3. Then
steam is super heated and passed through the high pressure turbine (HPT).
4. After discharging from HPT the temperature and
pressure decreases and again reheated to get proper temperature.
5. Then
it passes through the intermediate and low pressure turbine. It rotates the
turbine at a high speed.
6. The
shaft of the turbines is coupled with the shaft of generator. Since the turbine
rotates, the generator shaft also rotates and produces electricity.
7. Then
the steam is condensed at condenser by river water. After condensing the steam
turns into water. Then the water is pumped to the low pressure heater (LPH).
8. Then
the water is pumped into deareator and removed oxide particle and Oxygen gas.
9. Then
the water is pumped into Feed Water Tank (FWT) and passes through the High
Pressure Heater (HPT).
10. Finally
the water is feed into boiler drum and the process circulated in cyclic order
11. All the
units are controlled by manually except the unit-5. It follows DCS (Digital
Control System).
Figure: Digital Control System
Combine Cycle Power Plant (CCPP):
Combine cycle power plant is
modernly emerged power plant which uses the heat from flue gas exhausted by gas
turbine to produce steam to operate the steam turbine as well. In another way
CCPP may be defined as a combine cycle power plant including both GT and ST.
There are actually two separate
units under CCPP.
1.
GT-1 (Combine cycle mode)
2.
GT-2 (Open cycle mode)
Figure:
Gas Turbine
Gas Turbine Unit:
In gas turbine unit both GT-1 and
GT-2 is same except that the flue gas of GT-1 is used to produce steam in
combine cycle. The flow diagram of gas turbine units is shown in below.
Figure:
Flow diagram of gas turbine unit
Starting Sequence:
Condition:
|
Situation:
|
0 rpm
|
Gas turbine starts
|
750 rpm
|
Ignition starts
|
1800 rpm
|
Diesel engine off.
|
2300 rpm
|
Excitation on
|
3000 rpm
|
No load running
|
After synchronizing
|
Breaker on
|
Diesel engine:
A
diesel engine is an internal combustion engine. It is also known as compression ignition engine. It consists of 16 chambers and it’s
power is about 1870 HP. The engine shaft is coupled with the turbine shaft.
Since it rotates at certain speed hence the turbine also rotates and
electricity is produce by generator. Actually it is used as initial power
source.
Condition for running:
1.
Shaft coupling will be connected until engine rpm is
greater than turbine rpm.
2.
Shaft coupling
will be disconnected until engine rpm is less than turbine rpm.
Exciter:
A dc supply is
always connected with the generator for
excitation and to produce magnetic field. If there is no excitation no flux
will produce. Hence no electricity will generate. So a dc exciter is very
necessary for excitation.
Air filter:
A particular air filter is a device
composed of fibrous materials which removes solid particles such as dust,
pollen, mold and bacteria from the air. The filter pad absorbs the small
particles by viscous fluids and the metallic filter removes the large
particles. The combustion air filter prevents abrasive particles from entering
the engine’s cylinders, where it would cause mechanical wear and oil
contamination known as “Engine air induction system.”
So this air must be dust free for efficient
operation. Hence air filter is used.
Three types of filters are used:
1.
Turbine filter
2.
Generator cooling filter
3.
Ventilation filter
Figure: Air Filter
Figure: Air Filter
Compressor:
Here
multistage type rotary compressor is used to compress the air at high pressure
and supply to combustor. This compressed air is used to ignite the fuel. But
its major amount is used to cool the generator.
Combustor:
A combustor is a component or area of a
gas turbine, ramjet, or scramjet engine where combustion takes place. It is
also known as a burner or flame can. A combustion chamber has
ten burners. In a gas turbine engine,
the main combustor or
combustion chamber is fed high pressure air by the compression system and feeds
the hot, high pressure exhaust into the turbine components of the engine. Combustors
are designed to contain and control the combustion of the fuel-air mixture. The
combustor consists of several major components such as:
|
Case:
The case is the outer shell of the combustor, and
is a fairly simple structure. The casing generally requires little maintenance.
The case is protected from thermal loads by the air flowing in it, so thermal
performance is of limited concern. However, the casing serves as a pressure
vessel that must withstand the difference between the high pressures inside the
combustor and the lower pressure outside.
Diffuser:
The purpose of the diffuser is to
slow the high speed, highly compressed, air from the compressor to a velocity
optimal for the combustor. Reducing the velocity results in an unavoidable loss
in total pressure, so one of the design challenges is to limit the loss of
pressure as much as possible. Furthermore, the diffuser must be designed to
limit the flow distortion as much as possible by avoiding flow effects like boundary
layer separation. Like most other gas turbine engine components, the diffuser
is designed to be as short and light as possible.
Liner:
The liner contains the combustion process and
introduces the various airflows (intermediate, dilution, and cooling air) into
the combustion zone. The liner must be designed and built to withstand extended
high temperature cycles. Furthermore, even though high performance alloys are
used, the liners must be cooled with air flow. Some combustors also
make use of thermal barrier coatings. However, air cooling is still required.
In general, there are two main types of liner cooling; film cooling and
transpiration cooling. Film cooling works by injecting (by one of several
methods) cool air from outside of the liner to just inside of the liner. This
creates a thin film of cool air that protects the liner, reducing the
temperature at the liner from around 1800 K to around 830 K, for example. The
other type of liner cooling, transpiration cooling, is a more modern approach
that uses a porous material for the liner. The porous liner allows a small
amount of cooling air to pass through it, providing cooling benefits similar to
film cooling.
Snout:
The snout is an extension of the dome that acts
as an air splitter, separating the primary air from the secondary air flows
(intermediate, dilution, and cooling air).
Dome or swirler:
The dome and swirler are the part of the
combustor that the primary air flows through as it enters the combustion zone.
Their role is to generate turbulence in the flow to rapidly mix the air with
fuel. Early combustors tended to use bluff body domes (rather than swirler), which used a simple
plate to create wake turbulence to mix the fuel and air. The swirler
establishes a local low pressure zone that forces some of the combustion
products to re-circulate, creating the high turbulence. However, the higher the
turbulence, the higher the pressure loss will be for the combustor, so the dome
and swirler must be carefully designed so as not to generate more turbulence
than is needed to sufficiently mix the fuel and air.
Fuel injector:
The fuel injector is responsible for introducing
fuel to the combustion zone and, along with the swirler, is responsible for
mixing the fuel and air. There are four primary types of fuel injectors:
1. pressure-atomizing
2. air blast
3. vaporizing
4. premixing injectors
Pressure
atomizing fuel injectors rely on high fuel pressures. This type of fuel
injector has the advantage of being very simple, but it has several
disadvantages. The fuel system must be robust enough to withstand such high
pressures, and the fuel tends to be heterogeneously atomized, resulting in
incomplete or uneven combustion which has more pollutants and smoke.
The second type of fuel injector is the air blast
injector. This injector "blasts" a sheet of fuel with a stream of
air, atomizing the fuel into homogeneous droplets. This type of fuel injector
led to the first smokeless combustors.
The vaporizing fuel injector, the third type, is
similar to the air blast injector in that primary air is mixed with the fuel as
it is injected into the combustion zone. Heat from the combustion zone is
transferred to the fuel-air mixture, vaporizing some of the fuel (mixing it
better) before it is combusted. This method allows the fuel to be combusted
with less thermal radiation, which helps protect the liner
The premixing injectors work by mixing or
vaporizing the fuel before it reaches the combustion zone. This method allows
the fuel to be very uniformly mixed with the air, reducing emissions from the
engine. One disadvantage of this method is that fuel may auto-ignite. If this
happens the combustor can be seriously damaged.
Igniter:
Most igniters in gas turbine applications are
electrical spark igniters, similar to automotive spark plugs. The igniter needs
to be in the combustion zone where the fuel and air are already mixed, but it
needs to be far enough upstream so that it is not damaged by the combustion
itself. Once the combustion is initially started by the igniter, it is self
sustaining and the igniter is no longer used.
Waste Heat
Recovery Unit (WHRU):
After
combustion some flue gases are produced having temperature about 5000C
which exits through the chimney. This will be great loss if the flue gases are
not used properly. For this a combine cycle plant is introduced which use the
flue gas as working fluid. Here damper is used to recover the waste heat.
.
Damper blocks the chimney and
passes the flue gas to boiler and produce steam. This steam is used to generate
electricity at steam turbine. Since both of steam and gas turbines are used it
is called combine cycle power plant (CCPP).
Figure: Waste Heat Recovery
Benefits of Waste Heat
Recovery:
Benefits of ‘waste heat
recovery’ can be broadly classified in two categories:
Direct Benefits:
Recovery of waste heat has
a direct effect on the efficiency of the process. This is reflected by
reduction in the utility consumption & costs, and process cost.
Indirect Benefits:
a) Reduction
in pollution: A number of toxic combustible wastes such as carbon monoxide
gas, sour gas, carbon black off gases, oil sludge and other plastic chemicals etc,
releasing to atmosphere when burnt in the incinerators serves dual purpose i.e.
recovers heat and reduces the environmental pollution levels.
b) Reduction
in equipment sizes: Waste heat recovery reduces the fuel consumption, which
leads to reduction in the flue gas produced. This results in reduction in equipment
sizes of all flue gas handling equipments such as fans, stacks, ducts, burners,
etc.
c) Reduction
in auxiliary energy consumption: Reduction in equipment sizes gives
additional benefits in the form of reduction in auxiliary energy consumption
like electricity for fans, pumps etc.
Utilization of waste heat:
This waste heat of flue gases is
used to produce steam at steam boiler of CCPP. The water and steam flow diagram
is shown in below:
Figure: Water and steam flow diagram
Boiler function:
There
are many tubes of the boiler. The tubes are named as shown in figure due to
there position and rate of heat contact. In super heater side the temperature
is very high and it’s gradually decrease to upward direction.
LP Evaporator:
It
works like a economizer. It increases the water temperature slightly.
HP evaporator:
Water
inters into the evaporator from HP drum at 2200C. Here water is
converted into saturated steam by reducing its latent heat. When water out from
evaporator its temperature lies at 2300C.
Super heater:
Super
heater makes the saturated steam into super heated steam. This super heated
steam is used to rotate the turbine. Hence the electricity is produced by the
rotation of turbine.
Here some chemicals are added to
purify the water. The process of mixing additives with water is known as
“dosing.”
1.
NH3 is added in deareator to increase the pH
level and dissolve O2.
2.
N2H4 is used to remove dissolved
O2 which may occur oxide with copper and iron as such:
4Cu + 2O2
= 4CuO
4Fe + 3O2
= 2Fe2O3
3.
PO4 is added to remove sand particles.
Overall working principle
of CCPP:
At first air is extracted from
the atmosphere & then filtered by the air filters. The filtered air is then
sent to the air compressor in order to compress at high pressure. After that
compressed air and gas are sent to the combustion chamber which is ignited by
the spark plug. Due to this combustion of the gas high temperature is produced
in the combustion chamber and lies between 8000C to 8500C.
The hot flue gas produced in combustion chamber then passes through the turbine
blades of 2 stages. The produced hot flue gas first enters into the static
blade of turbine which acts as a nozzle to increase the velocity of the flue
gas. This hot flue gas having high velocity strikes the rotating blade of
turbine which in turn rotates the shaft connecting to the rotating blade. This
shaft is also connected with the compressor as well as generator. Hence it is called
single shaft turbine unit.
Due to the rotation of the shaft, magnet existing in the rotor of generator
rotates and produces magnetic flux. This magnetic flux cuts the conductor
situated in the stator which eventually produces electricity. This turbine has two sections. One is called
HP (High Pressure) turbine and other is called LP (Low Pressure) turbine. While
passing through the HP turbine, the temperature & pressure of the flue gas
drops to some extent. The flue gas then
passes through the LP turbine and temperature reduces to 5000C. Consequently the flue gas goes to the damper
as exhaust & is released to the atmosphere through stock.
In case of GT-1, this exhaust gas
is passed through a waste heat recovery unit (WHRU) where a boiler which uses
heat from flue gases to produce steam. WHRU consists of a column of evaporator
through which water is circulated to produce steam. It also consists of an Hp
drum, deareator and some centrifugal pumps. At first, water is pumped into the deareator
to remove all kind of dissolved gases such as O2, CO2
etc. From deareator water is pumped by LP pump to LP evaporator to take some
heat from exhaust gas and then again sent to the deareator. Next water is pumped to the forced flow
section by feed pump to take more heat from exhaust gas and sent to the HP
steam drum. In HP steam drum there resides actually a mixture of steam and
water. This saturated steam from HP steam drum is then sent to the HP
evaporator by HP pump to pressurize it. This high pressure steam is then sent
to the super heater to become superheated steam. This superheated steam is then
sent to turbine by an arrangement of pipe system through CIES valve. The
thermal energy contained in the steam rotates the steam turbine blades. The
shaft connected to the turbine blades also rotates the magnet of generator. Due
to the rotation of electromagnet, magnetic flux cuts on the generator winding.
In this way, a voltage is induced in the conductor and electricity is
produced. When steam is passed through
the HP turbine section the pressure and temperature of the steam becomes very
high and then gradually decreases while passing through the LP turbine. The
exhaust low pressure steam came out from the LP turbine is condensed in the
condenser by the help of river water. This condensed steam is termed as
condensate. This condensate is then again passed to the deareator by condensate
pumps. If there is any shortage in condensate then some make up water is pumped
by makeup pump to compensate the loss.
Production report:
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