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. 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 CO2 emissions
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 a combined
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 anelectric
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.
Efficiency
The energy efficiency of
a conventional thermal power station, considered as salable energy as a percent
of theheating value of the fuel consumed,
is typically 33% to 48%. This efficiency is limited as all heat engines are
governed by the laws of thermodynamics. The
rest of the energy must leave the plant in the form of heat. Thiswaste
heat can go through a condenser and
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 ofnatural
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 M Pa) and multiple stage reheat
reaching about 48% efficiency. Above the critical point for water of 705
°F (374 °C) and 3212 psi (22.06 M Pa), 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.
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 make-up
water in a 500 MW plant amounts to perhaps 120 US gallons per minute (7.6
L/s) to replace water drawn off from the boiler drums for water purity
management, and to also 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 is pressurized
in two stages, and 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, in the middle of
this series of feed water heaters, and before the second stage of
pressurization, the condensate plus the makeup water flows through a deaerator[7][8] 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).[vague] It
is also dosed with pH control agents such asammonia 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 through burners located
at the four corners, or along one wall, or two opposite walls, and it is
ignited to rapidly burn, 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. As the water in the boiler circulates
it absorbs heat and changes into steam. 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
The water enters the
boiler through a section in the convection pass called the economizer. From
the economizer it passes to the steam
drum and from there it goes through downcomers to inlet
headers at the bottom of the water walls. From these headers the water rises
through the water walls of the furnace where some of it is turned into steam
and the mixture of water and steam then re-enters the steam drum. This process
may be driven purely by natural circulation (because the water
is the downcomers is denser than the water/steam mixture in the water walls) or
assisted by pumps. In the steam drum, the water is returned to the downcomers
and the steam is passed through a series of steam
separators and dryers that remove water droplets from the steam.
The dry steam then flows into the super heater coils.
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.
Superheater
Fossil fuel power plants often have a superheater section in the
steam generating furnace. The steam passes through drying equipment inside the
steam drum on to the superheater, a set of tubes in the furnace. Here the steam
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.[ 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.
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.
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.[citation needed]
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
