KEY POINTS TO FOCUS - HOW TO COMMISSION A THERMAL POWER PLANT
A. Electrical Engineer
- HT panel Inspection, testing and commissioning
- LT panel inspection . testing and commissioning
Both the above takes quite a bit of time depending on how well the erection had been made, predominately this is done
- Testing of power cables (if not charged yet)
- Testing of Control system interlocks in the switchgear
- HT Breaker/LTbreaker / MCC module healthiness checking
- Testing of proper feed back in the electrical SCADA system (open/close, power, current, voltage values etc ...)
- Panel Relay setting, inspection and testing
- Testing of CT's PT's and HT gear transducers
- generator and auxiliaries inspection, testing
- Turbine axillary drives testing
- Boiler/HRSG axillary drives testing
- generator control panel testing and commissioning
- ECP or electrical control panel for plant HT and LT switchgear testing and commissioning
- Generator relay panel testing, setting revision and commissioning
- Plant DC system testing and commissioning
- Plant UPS system testing and commissioning
- Generator AVR testing, tuning and commissioning
-
B. Instrumentation Engineer
- Turbine control system testing, tweaking and commissioning
- Plant DCS system (Boiler and BOP) testing, tweaking and commissioning
- Plant SCADA system (electrical switchgear control) testing and commissioning
All the above work involves more or less the same type of work
- Testing (occasional calibration and replacement) of Plant field instrumentation
- Testing and commissioning of plant control valves (loop checking and ensuring proper feedback)
- Testing commissioning of plant solenoid valves (proper operation and feedback
- Testing and commissioning of plant MOV (with electrical
engineer, this by far in my opinion is the most back breaking
work and in the quest of complete automation there are literally 50-100
MOV's in each co generation plant, i don't know MOV's
and me do not go well )
- Testing and commissioning of plant servo's (a very subtle art :) )
- Testing and calibration of plant measurement circuits, pressure,
temperature, flow, speed, position and vibration probes. They
also take a lot of time mainly because they form a major quantity of the
plant instrumentation.
- Testing and commissioning of plant analyzers, In boilers, it will be
PH, silica, conductivity, phosphate, SOx, NOx, CO, unburnt
fuel and oxygen analysers. This work as far as i have seen is
exclusively done by the vendors. all you have to do is to verify the
operation and sign :).
- Testing and tweaking of graphics, minor modifications in consultation with plant operating personnel.
- testing and tweaking of control constants and minor (to sometimes
major) logic modifications in application code to the controllers.
- hanging around a lot of time in marshaling units, RTU's, in
the field and in front of the controller, thinking, scratching your
head and desperately wising to god that atleast this should work.
-
Testing and commissioning of other plant systems, which do not form the
part of the typical turbine and DCS control. They include
- Turbine and generator CO2 release system
- Plant and Trubine heat sensors and fire detectors panel
- MCP (manual call points) system and associated auxiliaries
- Plant communication system (another exclusive vendor area)
Thermal power station over view
A thermal
power station is a power plant in
which the prime mover is steam driven. Water is heated, turns into steam and spins a steam turbine which
drives an electrical generator. After it passes through the turbine, the steam is condensed in
a condenser and
recycled to where it was heated; this is known as a Rankine cycle.
The greatest variation in the design of thermal power stations is due to the
different fuel sources. Some prefer to use the term energy center because
such facilities convert forms of heat energy into electricity.[1] Some
thermal power plants also deliver heat energy for industrial purposes,
for district heating, or
for desalination of
water as well as delivering electrical power. A large part of human CO2emissions
comes from fossil fueled thermal power plants; efforts to reduce these outputs
are various and widespread.
Introductory
overview
Almost
all coal, nuclear, geothermal, solar thermal electric, and waste incineration plants, as well as many natural gas power plants are thermal.Natural gas is
frequently combusted in gas turbines as
well as boilers. The waste heat from
a gas turbine can be used to raise steam, in acombined cycle plant
that improves overall efficiency. Power plants burning coal, fuel oil, or
natural gas are often called fossil-fuel power plants. Some biomass-fueled
thermal power plants have appeared also. Non-nuclear thermal power plants,
particularly fossil-fueled plants, which do not use co-generation are
sometimes referred to as conventional power plants.
Commercial electric utility power
stations are usually constructed on a large scale and designed for continuous
operation. Electric power plants typically use three-phase electrical generators to produce alternating current (AC) electric power at a frequency of 50 Hz or 60 Hz. Large companies or institutions may have their own power plants to
supply heating or
electricity to their facilities, especially if steam is created anyway for
other purposes. Steam-driven power plants have been used in various large
ships, but are now usually used in large naval ships. Shipboard power plants usually directly couple the turbine
to the ship's propellers through gearboxes. Power plants in such ships also
provide steam to smaller turbines driving electric generators to supply
electricity. Shipboard steam power plants can be either fossil fuel or
nuclear. Nuclear marine propulsion is, with few exceptions, used only in naval vessels. There have
been perhaps about a dozen turbo-electric ships
in which a steam-driven turbine drives an electric generator which powers
an electric motor for propulsion.
combined
heat and power (CH&P) plants, often called co-generation plants, produce both electric
power and heat for process heat or space heating. Steam and hot water lose
energy when piped over substantial distance, so carrying heat energy by steam
or hot water is often only worthwhile within a local area, such as a ship,
industrial plant, or district heating of
nearby buildings.
History
Reciprocating
steam engines have been used for mechanical power sources since the 18th
Century, with notable improvements being made by James Watt. The very
first commercial central electrical generating stations in the Pearl Street Station, New York and the Holborn Viaduct power station, London, in
1882, also used reciprocating steam engines. The development of the steam turbine allowed
larger and more efficient central generating stations to be built. By 1892 it
was considered as an alternative to reciprocating engines [2] Turbines
offered higher speeds, more compact machinery, and stable speed regulation
allowing for parallel synchronous operation of generators on a common bus. Turbines
entirely replaced reciprocating engines in large central stations after about
1905. The largest reciprocating engine-generator sets ever built were completed
in 1901 for the Manhattan Elevated Railway. Each of seventeen units weighed
about 500 tons and was rated 6000 kilowatts; a contemporary turbine-set of
similar rating would have weighed about 20% as much.[3]
Efficiency
The energy
efficiency of a conventional thermal power station, considered as salable
energy as a percent of the heating value of
the fuel consumed, is typically 33% to 48%. This efficiency is limited as all
heat engines are governed by the laws ofthermodynamics.
The rest of the energy must leave the plant in the form of heat. This waste heat can
go through a condenserand be disposed of with cooling water or
in cooling towers. If
the waste heat is instead utilized for district heating, it
is called co-generation. An
important class of thermal power station are associated with desalination facilities;
these are typically found in desert countries with large supplies of natural gas and
in these plants, freshwater production and electricity are equally important
co-products.
The Carnot efficiency dictates
that higher efficiencies can be attained by increasing the temperature of the
steam. Sub-critical fossil fuel power plants can achieve 36–40%
efficiency. Super critical designs have efficiencies in the low to mid 40% range, with new
"ultra critical" designs using pressures of 4400 psi (30.3 MPa) and
multiple stage reheat reaching about 48% efficiency. Above the critical
point for water of 705
°F (374 °C) and 3212 psi (22.06 MPa), there is no phase transition from
water to steam, but only a gradual decrease in density.
Current nuclear power plants must operate below the temperatures and pressures that coal-fired
plants do, since the pressurized vessel is very large and contains the entire
bundle of nuclear fuel rods. The size of the reactor limits the pressure that
can be reached. This, in turn, limits their thermodynamic efficiency to 30–32%.
Some advanced reactor designs being studied, such as the Very high temperature reactor, Advanced gas-cooled reactor and Super critical water reactor, would operate at temperatures and pressures similar to current coal
plants, producing comparable thermodynamic efficiency.
Electricity
cost
The direct
cost of electric energy produced by a thermal power station is the result of
cost of fuel, capital cost for the plant, operator labour, maintenance, and
such factors as ash handling and disposal. Indirect, social or environmental
costs such as the economic value of environmental impacts, or environmental and
health effects of the complete fuel cycle and plant decommissioning, are not
usually assigned to generation costs for thermal stations in utility practice,
but may form part of an environmental impact assessment.
Diagram of a
typical coal-fired thermal power station
Typical diagram of a coal-fired thermal
power station
For units
over about 200 MW capacity,
redundancy of key components is provided by installing duplicates of the forced
and induced draft fans, air preheaters, and fly ash collectors. On some units
of about 60 MW, two boilers per unit may instead be provided.
Boiler and
steam cycle
In
fossil-fueled power plants, steam generator refers to a furnace that burns the fossil fuel to boil water to
generate steam.
In
the nuclear plant field, steam
generator refers to a specific type of large heat exchanger used
in a pressurized water reactor (PWR) to thermally connect the primary (reactor plant) and secondary
(steam plant) systems, which generates steam. In a nuclear reactor called
a boiling water reactor (BWR), water is boiled to generate steam directly in the reactor
itself and there are no units called steam generators.
In some
industrial settings, there can also be steam-producing heat exchangers
called heat recovery steam generators (HRSG) which utilize
heat from some industrial process. The steam generating boiler has to produce
steam at the high purity, pressure and temperature required for the steam
turbine that drives the electrical generator.
Geothermal plants need
no boiler since they use naturally occurring steam sources. Heat exchangers may
be used where the geothermal steam is very corrosive or contains excessive
suspended solids.
A fossil
fuel steam generator includes an economizer, a steam drum, and
the furnace with
its steam generating tubes and superheater coils. Necessary safety valves are
located at suitable points to avoid excessive boiler pressure. The air
and flue gas path
equipment include: forced draft (FD) fan, Air Preheater (AP), boiler furnace, induced draft (ID) fan, fly ash collectors (electrostatic precipitator or baghouse) and the flue gas stack.
Feed water heating and deaeration
The feed
water used in the steam boiler is a means of transferring heat energy from the burning
fuel to the mechanical energy of the spinning steam turbine.
The total feed water consists of recirculated condensate water
and purified makeup water. Because the metallic materials it
contacts are subject to corrosion at high temperatures and pressures, the makeup water is highly
purified before use. A system of water softeners and ion exchange demineralizers
produces water so pure that it coincidentally becomes an electrical insulator, with conductivity in the range of 0.3–1.0 microsiemens per centimeter. The makeup
water in a 500 MWe plant amounts to perhaps 20 US gallons per minute (1.25
L/s) to offset the small losses from steam leaks in the system.
The feed
water cycle begins with condensate water being pumped out of the condenser after
traveling through the steam turbines. The condensate flow rate at full load in
a 500 MW plant is about 6,000 US gallons per minute (400 L/s).
The water
flows through a series of six or seven intermediate feed water
heaters, heated up at each point with steam extracted from
an appropriate duct on the turbines and gaining temperature at each stage.
Typically, the condensate plus the makeup water then flows through a deaerator that
removes dissolved air from the water, further purifying and reducing its
corrosiveness. The water may be dosed following this point with hydrazine, a
chemical that removes the remaining oxygen in the water to below 5 parts per billion (ppb). It is also dosed with pH control agents such as ammonia or morpholine to
keep the residual acidity low and thus non-corrosive.
Boiler operation
The boiler
is a rectangular furnace about
50 feet (15 m) on a side and 130 feet (40 m) tall. Its walls are made
of a web of high pressure steel tubes about 2.3 inches (58 mm) in
diameter.
Pulverized coal is
air-blown into the furnace from fuel nozzles at the four corners and it rapidly
burns, forming a large fireball at the center. The thermal radiation of
the fireball heats the water that circulates through the boiler tubes near the
boiler perimeter. The water circulation rate in the boiler is three to four
times the throughput and is typically driven by pumps. As the water in
the boiler circulates it absorbs heat and changes into steam at 700
°F (370 °C) and 3,200 psi (22,000 kPa). It is separated
from the water inside a drum at the top of the furnace. The saturated steam is
introduced into superheat pendant
tubes that hang in the hottest part of the combustion gases as they exit the
furnace. Here the steam is superheated to 1,000
°F (540 °C) to prepare it for the turbine.
Plants
designed for lignite (brown
coal) are increasingly used in locations as varied as Germany, Victoria, Australia and North Dakota. Lignite
is a much younger form of coal than black coal. It has a lower energy density
than black coal and requires a much larger furnace for equivalent heat output.
Such coals may contain up to 70% water and ash, yielding
lower furnace temperatures and requiring larger induced-draft fans. The firing
systems also differ from black coal and typically draw hot gas from the
furnace-exit level and mix it with the incoming coal in fan-type mills that
inject the pulverized coal and hot gas mixture into the boiler.
Plants that
use gas turbines to heat the water for conversion into steam use boilers known
as heat recovery steam generators (HRSG). The exhaust heat from the gas turbines is used to make
superheated steam that is then used in a conventional water-steam generation
cycle, as described in gas turbine combined-cycle
plants section below.
Boiler furnace and steam drum
Once water
inside the boiler or steam generator,
the process of adding the latent heat of vaporization or enthalpy is
underway. The boiler transfers energy to the water by the chemical reaction of
burning some type of fuel.
The water
enters the boiler through a section in the convection pass called the economizer. From the
economizer it passes to the steam drum. Once the
water enters the steam drum it goes down to the lower inlet water wall headers.
From the inlet headers the water rises through the water walls and is
eventually turned into steam due to the heat being generated by the burners
located on the front and rear water walls (typically). As the water is turned
into steam/vapor in the water walls, the steam/vapor once again enters the
steam drum. The steam/vapor is passed through a series of steam and water
separators and then dryers inside the steam drum. The steam separators and
dryers remove water droplets from the steam and the cycle through the water
walls is repeated. This process is known as natural circulation.
The boiler
furnace auxiliary equipment includes coal feed nozzles and igniter guns, soot blowers, water
lancing and observation ports (in the furnace walls) for observation of the
furnace interior. Furnace explosions due to any accumulation of combustible gases after a trip-out are
avoided by flushing out such gases from the combustion zone before igniting the
coal.
The steam
drum (as well as the super heater coils and headers) have air vents and drains
needed for initial start up. The steam drum has internal devices that removes
moisture from the wet steam entering the drum from the steam generating tubes.
The dry steam then flows into the super heater coils.
Superheater
Fossil fuel
power plants can have a superheater and/or re-heater section in the steam
generating furnace. In a fossil fuel plant, after the steam is conditioned by
the drying equipment inside the steam drum, it is piped from the upper drum
area into tubes inside an area of the furnace known as the superheater, which
has an elaborate set up of tubing where the steam vapor picks up more energy
from hot flue gases outside the tubing and its temperature is now superheated
above the saturation temperature. The superheated steam is then piped through
the main steam lines to the valves before the high pressure turbine.
Nuclear-powered
steam plants do not have such sections but produce steam at essentially
saturated conditions. Experimental nuclear plants were equipped with
fossil-fired super heaters in an attempt to improve overall plant operating
cost.
Steam condensing
The
condenser condenses the steam from the exhaust of the turbine into liquid to
allow it to be pumped. If the condenser can be made cooler, the pressure of the
exhaust steam is reduced and efficiency of the cycle increases.
The surface
condenser is a shell and tube heat exchanger in which cooling water is circulated through the tubes.[5][9][10][11] The
exhaust steam from the low pressure turbine enters the shell where it is cooled
and converted to condensate (water) by flowing over the tubes as shown in the
adjacent diagram. Such condensers use steam ejectors or rotary motor-driven exhausters for continuous removal of air and gases from the steam side to
maintain vacuum.
For best
efficiency, the temperature in the condenser must be kept as low as practical
in order to achieve the lowest possible pressure in the condensing steam. Since
the condenser temperature can almost always be kept significantly below 100 °C
where the vapor pressure of
water is much less than atmospheric pressure, the condenser generally works
under vacuum. Thus leaks of non-condensible air into the closed loop must be
prevented.
Typically
the cooling water causes the steam to condense at a temperature of
about 35 °C (95 °F) and that creates an absolute pressure in
the condenser of about 2–7 kPa (0.59–2.1 inHg), i.e. a vacuum of about −95 kPa (−28.1 inHg) relative to atmospheric
pressure. The large decrease in volume that occurs when water vapor condenses
to liquid creates the low vacuum that helps pull steam through and increase the
efficiency of the turbines.
The
limiting factor is the temperature of the cooling water and that, in turn, is
limited by the prevailing average climatic conditions at the power plant's
location (it may be possible to lower the temperature beyond the turbine limits
during winter, causing excessive condensation in the turbine). Plants operating
in hot climates may have to reduce output if their source of condenser cooling
water becomes warmer; unfortunately this usually coincides with periods of high
electrical demand for air conditioning.
The
condenser generally uses either circulating cooling water from a cooling tower to
reject waste heat to the atmosphere, or once-through water from a river, lake
or ocean.
A Marley
mechanical induced draft cooling tower
The heat
absorbed by the circulating cooling water in the condenser tubes must also be
removed to maintain the ability of the water to cool as it circulates. This is
done by pumping the warm water from the condenser through either natural draft,
forced draft or induced draft cooling towers (as
seen in the image to the right) that reduce the temperature of the water by
evaporation, by about 11 to 17 °C (20 to 30
°F)—expelling waste heat to
the atmosphere. The circulation flow rate of the cooling water in a 500 MW unit
is about 14.2 m³/s (500 ft³/s or 225,000 US gal/min) at full load.[12]
The
condenser tubes are made of brass or stainless steel to
resist corrosion from either side. Nevertheless they may become internally
fouled during operation by bacteria or algae in the cooling water or by mineral
scaling, all of which inhibit heat transfer and reduce thermodynamic efficiency. Many plants include an automatic cleaning system that circulates
sponge rubber balls through the tubes to scrub them clean without the need to
take the system off-line.
The cooling
water used to condense the steam in the condenser returns to its source without
having been changed other than having been warmed. If the water returns to a
local water body (rather than a circulating cooling tower), it is tempered with
cool 'raw' water to prevent thermal shock when discharged into that body of
water.
Another
form of condensing system is the air-cooled condenser. The process is similar
to that of a radiator and
fan. Exhaust heat from the low pressure section of a steam turbine runs through
the condensing tubes, the tubes are usually finned and ambient air is pushed
through the fins with the help of a large fan. The steam condenses to water to
be reused in the water-steam cycle. Air-cooled condensers typically operate at
a higher temperature than water-cooled versions. While saving water, the
efficiency of the cycle is reduced (resulting in more carbon dioxide per
megawatt of electricity).
From the
bottom of the condenser, powerful condensate pumps recycle
the condensed steam (water) back to the water/steam cycle.
Reheater
Power plant
furnaces may have a reheater section containing tubes heated by hot flue gases
outside the tubes. Exhaust steam from the high pressure turbine is passed
through these heated tubes to collect more energy before driving the
intermediate and then low pressure turbines.
Air path
External
fans re provided to give sufficient air for combustion. The Primary air fan
takes air from the atmosphere and, first warming it in the air preheater for
better combustion, injects it via the air nozzles on the furnace wall.
The induced
draft fan assists the FD fan by drawing out combustible gases from the furnace,
maintaining a slightly negative pressure in the furnace to avoid backfiring
through any closing.
Steam
turbine generator
Rotor of a
modern steam turbine, used in a power station
The turbine
generator consists of a series of steam turbines interconnected
to each other and a generator on a common shaft. There is a high pressure
turbine at one end, followed by an intermediate pressure turbine, two low
pressure turbines, and the generator. As steam moves through the system and
loses pressure and thermal energy it expands in volume, requiring increasing
diameter and longer blades at each succeeding stage to extract the remaining
energy. The entire rotating mass may be over 200 metric tons and 100 feet
(30 m) long. It is so heavy that it must be kept turning slowly even when
shut down (at 3 rpm) so that the shaft will not bow even slightly and become unbalanced.
This is so important that it is one of only five functions of blackout
emergency power batteries on site. Other functions are emergency lighting, communication,
station alarms and turbogenerator lube oil.
Superheated
steam from the boiler is delivered through 14–16-inch (360–410 mm) diameter
piping to the high pressure turbine where it falls in pressure to 600 psi
(4.1 MPa) and to 600 °F (320 °C) in temperature
through the stage. It exits via 24–26-inch (610–660 mm) diameter cold reheat
lines and passes back into the boiler where the steam is reheated in special
reheat pendant tubes back to 1,000 °F (500 °C). The hot reheat
steam is conducted to the intermediate pressure turbine where it falls in
both temperature and pressure and
exits directly to the long-bladed low pressure turbines and finally exits to
the condenser.
The
generator, 30 feet (9 m) long and 12 feet (3.7 m) in diameter,
contains a stationary stator and a spinning rotor, each
containing miles of heavycopper conductor—no permanent magnets here. In operation it generates up to 21,000 amperes at 24,000 volts AC (504 MWe) as it spins at either 3,000 or 3,600 rpm, synchronized to the power grid. The rotor
spins in a sealed chamber cooled with hydrogen gas,
selected because it has the highest known heat transfer coefficient of any gas and for its low viscosity which
reduces windage losses.
This system requires special handling during startup, with air in the chamber
first displaced by carbon dioxide before
filling with hydrogen. This ensures that the highly explosivehydrogen–oxygen environment is not created.
The power grid frequency is 60 Hz across North America and
50 Hz in Europe, Oceania, Asia (Korea and parts of Japan are notable exceptions) and parts of Africa.
The
electricity flows to a distribution yard where transformers increase
the voltage for transmission to its destination.
The steam turbine-driven generators have auxiliary systems enabling them to work satisfactorily and
safely. The steam turbine generator being rotating equipment generally has a
heavy, large diameter shaft. The shaft therefore requires not only supports but
also has to be kept in position while running. To minimize the frictional
resistance to the rotation, the shaft has a number ofbearings. The bearing shells, in which the shaft rotates, are lined with a low
friction material like Babbitt metal.
Oil lubrication is provided to further reduce the friction between shaft and
bearing surface and to limit the heat generated.
Stack gas
path and cleanup
See
also: Flue-gas emissions from fossil-fuel combustion and Flue-gas desulfurization
As the
combustion flue gas exits
the boiler it is routed through a rotating flat basket of metal mesh which
picks up heat and returns it to incoming fresh air as the basket rotates, This
is called theair preheater.
The gas exiting the boiler is laden with fly ash, which are
tiny spherical ash particles. The flue gas contains nitrogen along
with combustion products carbon dioxide, sulfur dioxide,
and nitrogen oxides.
The fly ash is removed by fabric bag filters or electrostatic precipitators. Once removed, the fly ash byproduct can sometimes be used in the
manufacturing of concrete. This cleaning
up of flue gases, however, only occurs in plants that are fitted with the
appropriate technology. Still, the majority of coal-fired power plants in the
world do not have these facilities.[citation needed] Legislation
in Europe has been efficient to reduce flue gas pollution. Japan has been using
flue gas cleaning technology for over 30 years and the US has been doing the
same for over 25 years. China is now beginning to grapple with the pollution
caused by coal-fired power plants.
Where
required by law, the sulfur and nitrogen oxide pollutants are
removed by stack gas scrubbers which use a pulverized limestone or
other alkaline wet
slurry to remove those pollutants from the exit stack gas. Other devices use
catalysts to remove Nitrous Oxide compounds from the flue gas stream. The gas
travelling up the flue gas stack may
by this time have dropped to about50 °C (120 °F). A typical flue gas
stack may be 150–180 metres (490–590 ft) tall to disperse the remaining flue
gas components in the atmosphere. The tallest flue gas stack in the world is
419.7 metres (1,377 ft) tall at the GRES-2 power plant in Ekibastuz, Kazakhstan.
In the
United States and a number of other countries, atmospheric
dispersion modeling[13] studies
are required to determine the flue gas stack height needed to comply with the
local air pollutionregulations.
The United States also requires the height of a flue gas stack to comply with
what is known as the "Good Engineering Practice (GEP)" stack height.[14][15] In
the case of existing flue gas stacks that exceed the GEP stack height, any air
pollution dispersion modeling studies for such stacks must use the GEP stack
height rather than the actual stack height.
Fly ash collection
Fly ash is
captured and removed from the flue gas by electrostatic precipitators or fabric
bag filters (or sometimes both) located at the outlet of the furnace and before
the induced draft fan. The fly ash is periodically removed from the collection
hoppers below the precipitators or bag filters. Generally, the fly ash is
pneumatically transported to storage silos for subsequent transport by trucks
or railroad cars .
Bottom ash collection and disposal
At the
bottom of the furnace, there is a hopper for collection of bottom ash. This
hopper is always filled with water to quench the ash and clinkers falling down
from the furnace. Some arrangement is included to crush the clinkers and for
conveying the crushed clinkers and bottom ash to a storage site . Ash extractor
is used to discharge ash from Municipal solid waste fired boilers.
Auxiliary
systems
Boiler
make-up water treatment plant and storage
Since there
is continuous withdrawal of steam and continuous return of condensate to
the boiler, losses due to blowdown and leakages have to be made up to maintain
a desired water level in the boiler steam drum. For this, continuous make-up
water is added to the boiler water system. Impurities in the raw water input to
the plant generally consist of calcium and magnesium salts
which impart hardness to
the water. Hardness in the make-up water to the boiler will form deposits on
the tube water surfaces which will lead to overheating and failure of the
tubes. Thus, the salts have to be removed from the water, and that is done by a
water demineralising treatment plant (DM). A DM plant generally consists of
cation, anion, and mixed bed exchangers. Any ions in the final water from this
process consist essentially of hydrogen ions and hydroxide ions, which
recombine to form pure water. Very pure DM water becomes highly corrosive once
it absorbs oxygen from the atmosphere because of its very high affinity for
oxygen.
The
capacity of the DM plant is dictated by the type and quantity of salts in the
raw water input. However, some storage is essential as the DM plant may be down
for maintenance. For this purpose, a storage tank is installed from which DM
water is continuously withdrawn for boiler make-up. The storage tank for DM
water is made from materials not affected by corrosive water, such as PVC. The piping and valves are generally of stainless steel. Sometimes, a
steam blanketing arrangement or stainless steel doughnut float is provided on
top of the water in the tank to avoid contact with air. DM water make-up is
generally added at the steam space of the surface condenser (i.e.,
the vacuum side). This arrangement not only sprays the water but also DM water
gets deaerated, with the dissolved gases being removed by a de-aerator through
an ejector attached to the condenser.
Fuel preparation system
In coal-fired
power stations, the raw feed coal from the coal storage area is first crushed
into small pieces and then conveyed to the coal feed hoppers at the boilers.
The coal is next pulverizedinto a very fine powder. The
pulverizers may be ball mills, rotating
drum grinders, or other
types of grinders.
Some power
stations burn fuel oil rather
than coal. The oil must kept warm (above its pour point) in the
fuel oil storage tanks to prevent the oil from congealing and becoming
unpumpable. The oil is usually heated to about 100 °C before being pumped
through the furnace fuel oil spray nozzles.
Boilers in
some power stations use processed natural gas as their main fuel. Other power stations may use processed natural
gas as auxiliary fuel in the event that their main fuel supply (coal or oil) is
interrupted. In such cases, separate gas burners are provided on the boiler
furnaces.
Barring gear
Barring gear (or "turning gear") is the mechanism provided to rotate
the turbine generator shaft at a very low speed after unit stoppages. Once the
unit is "tripped" (i.e., the steam inlet valve is closed), the
turbine coasts down towards standstill. When it stops completely, there is a
tendency for the turbine shaft to deflect or bend if allowed to remain in one
position too long. This is because the heat inside the turbine casing tends to
concentrate in the top half of the casing, making the top half portion of the
shaft hotter than the bottom half. The shaft therefore could warp or bend by
millionths of inches.
This small
shaft deflection, only detectable by eccentricity meters, would be enough to
cause damaging vibrations to the entire steam turbine generator unit when it is
restarted. The shaft is therefore automatically turned at low speed (about one
percent rated speed) by the barring gear until it has cooled sufficiently to
permit a complete stop.
Oil system
An
auxiliary oil system pump is used to supply oil at the start-up of the steam
turbine generator. It supplies the hydraulic oil system required for steam
turbine's main inlet steam stop valve, the governing control valves, the
bearing and seal oil systems, the relevant hydraulic relays and other
mechanisms.
At a preset
speed of the turbine during start-ups, a pump driven by the turbine main shaft
takes over the functions of the auxiliary system.
Generator cooling
While small
generators may be cooled by air drawn through filters at the inlet, larger
units generally require special cooling arrangements. Hydrogen gas
cooling, in an oil-sealed casing, is used because it has the highest
known heat transfer coefficient of any gas and for its low viscosity which
reduces windage losses.
This system requires special handling during start-up, with air in the
generator enclosure first displaced by carbon dioxide before
filling with hydrogen. This ensures that the highly flammable hydrogen
does not mix with oxygen in the air.
The
hydrogen pressure inside the casing is maintained slightly higher than atmospheric pressure to avoid outside air ingress. The hydrogen must be sealed against
outward leakage where the shaft emerges from the casing. Mechanical seals
around the shaft are installed with a very small annular gap to avoid rubbing
between the shaft and the seals. Seal oil is used to prevent the hydrogen gas
leakage to atmosphere.
The
generator also uses water cooling. Since the generator coils are at a potential
of about 22 kV, an
insulating barrier such as Teflon is used to interconnect the water line and
the generator high voltage windings. Demineralized water of low conductivity is
used.
Generator high voltage system
The
generator voltage for modern utility-connected generators ranges from 11
kV in smaller units to 22 kV in larger units. The generator high
voltage leads are normally large aluminium channels because of their high
current as compared to the cables used in smaller machines. They are enclosed
in well-grounded aluminium bus ducts and are supported on suitable insulators.
The generator high voltage leads are connected to step-up transformers for
connecting to a high voltage electrical substation (usually in the range of 115 kV to 765 kV) for further
transmission by the local power grid.
The necessary protection
and metering devices are included for
the high voltage leads. Thus, the steam turbine generator and the transformer
form one unit. Smaller units,may share a common generator step-up transformer
with individual circuit breakers to connect the generators to a common bus.
Monitoring and alarm system
Most of the
power plant operational controls are automatic. However, at times, manual
intervention may be required. Thus, the plant is provided with monitors and
alarm systems that alert the plant operators when certain operating parameters
are seriously deviating from their normal range.
Battery supplied emergency lighting and
communication
A central
battery system consisting of lead acid cell units
is provided to supply emergency electric power, when needed, to essential items
such as the power plant's control systems, communication systems, turbine lube
oil pumps, and emergency lighting. This is essential for a safe, damage-free
shutdown of the units in an emergency situation.
Transport
of coal fuel to site and to storage
Most
thermal stations use coal as the main fuel. Raw coal is transported from coal mines to a
power station site by trucks, barges, bulk cargo ships or railway cars.
Generally, when shipped by railways, the coal cars are sent as a full train of
cars. The coal received at site may be of different sizes. The railway cars are
unloaded at site by rotary dumpers or side tilt dumpers to tip over onto
conveyor belts below. The coal is generally conveyed to crushers which crush
the coal to about 3⁄4 inches (19 mm) size.
The crushed coal is then sent by belt conveyors to a storage pile. Normally,
the crushed coal is compacted by bulldozers, as compacting of highly volatile
coal avoids spontaneous ignition.
The crushed
coal is conveyed from the storage pile to silos or hoppers at the boilers by
another belt conveyor system.
