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Making thermal power stations much more efficient . . .
Super-critical & UL
STEAM POWER STATION
Coal-fired power stations are out of favour in much of the Western world
because of carbon dioxide emissions but there is a way in which they can
be made more efficient, ie, to use less coal and emit less CO2. This article is
mainly about super-critical and ultra-supercritical steam power stations but
does include other technical improvements to thermal power stations.
W
hile there is much emphasis on “green” or
renewable power sources, they are much more
expensive than conventional sources such as
coal-fired or nuclear power stations which are the only
practical and economic way to provide base load power
(the minimum amount of power drawn through the power
grid 24 hours a day), unless a country has enormous dams
and the accompanying huge hydroelectric power stations.
Intermittent sources of power such as solar or wind must
be backed up at all times by either fossil fuel, nuclear or
hydroelectric plants to whatever the capacity is of the solar
26 Silicon Chip
or wind plants, as at any time the wind might stop blowing
or clouds might cover the sun and the loss of power must
be rapidly made up. This causes conventional base load
plants to be constantly varying their output resulting in
extra wear and tear as well as network management issues.
The sudden loss of production can also be made up with
gas turbine “peaking” generators but that tends to be very
expensive, especially at times of peak demand.
Sources of electricity in Australia
Table 1 and Figure 1 show the actual sources of electricsiliconchip.com.au
TRA-Super-critical
NS
By Dr DAVID MADDISON
ity that are used in the National Electricity Market (NEM)
which is Australia’s wholesale electricity marketplace and
its associated transmission grid. It is the largest interconnected power grid in the world with an end to end distance
of over 5,000km and 40,000km of circuit in the grid.
Eleven billion dollars worth of electricity are traded
each year to 19 million consumers, however it currently
excludes WA and the NT.
GENERATOR CAPACITY
TYPE (% of total generation)
Black coal
39.2
Brown coal
14.3
Gas
20.1
Hydroelectric
16.5
Wind
6.6
Liquid fuel
1.7
Other
1.5
PRODUCTION TYPE
OUTPUT
(% of total generation)
50.8
25.7
11.6
6.6
4.8
0
0.5
OUTPUT PRODUCED
(percent of total production)
Fossil plus hydro
(traditional 24/7 steady state production)
94.7
Fossil only
88.1
Existing traditional renewable
(hydro)
6.6
Table 1: Data from the Australian Energy Regulator for financial year 2014/15 showing source of electricity and
contribution to total generation capacity in the wholesale National Electricity Market (NEM). Note that the contribution of
solar and other sources is so small in the wholesale market that it does not have a separate category. It can be seen that a
vast majority of power in the NEM is derived from fossil and hydro production. The proportion of fossil fuel production
(brown and black coal, gas) is also shown with the contribution due to traditional renewable hydro.
siliconchip.com.au
December 2015 27
Fig.1: graphical representation of data in Table1.
Our requirement for base load production from fossil
fuels and hydro is not going to go away and it can be seen
from Table 1 that 94.7% of wholesale electricity comes from
coal, gas and hydro generation. Coal itself is responsible
for 76.5% of Australia’s total power contribution to the
national grid.
New coal-fired technology
While coal-fired power stations are an established technology, engineers have been working to improve their
efficiency so that they use less coal to produce the same
amount of electricity and as a consequence, produce less
carbon dioxide. The benefit to the consumer is potentially
lower prices due to the consumption of less fuel to make
electricity.
The new developments in thermal power stations are
super-critical and ultra-supercritical steam technology,
fluidised bed combustion and integrated combined cycle
gas turbine technology.
A 1% improvement in the thermal efficiency of a conventional coal-fired power plant actually gives a 2-3%
reduction in carbon dioxide emissions so for this reason
alone the idea is saleable to politicians and certain voters
who believe in “anthropogenic global warming” but the
economic justification is less fuel consumption.
In fact, if the average efficiency of coal plants worldwide
could be increased from 33% to 40%, two fewer gigatonnes
of carbon dioxide emissions would be emitted (or produced)
worldwide. This amounts to about what India emits.
Super-critical steam technology
In a typical coal power plant as shown in Figure 2, coal is
pulverised to the consistency of talcum powder and blown
into a “pulverised coal (PC)” burner in the boiler. The heat
from the burner converts water to steam to drive a turbine
which spins a generator. Once the steam has been through
the turbine it is much cooler and is condensed, then goes
back to the boiler to be reheated.
In reality, the path of the steam is more complicated but
that is the basic principle. Steam turbines are massive,
weighing hundreds of tons and spinning at 3000 RPM for
50Hz systems.
Fig.2: a typical sub-critical coal fired power station. Coal enters via a conveyor belt (14) and into
a hopper (15) and is pulverised to a talcum powder-like consistency in a mill (16). The powder
is mixed with air and blown into the furnace where it is burned, heating water or steam in the
furnace tubes whereby it is passed to the boiler drum (17) where any water is separated from
the steam. Steam from the boiler drum is then passed to the superheater (19) where it is rapidly
heated to 540°C and around 165bar (16.5MPa, 2400 psi) of pressure. This steam then goes through
the high pressure turbine (11) and then is returned to the reheater (21) after which is passed to
an intermediate pressure turbine (9) and from there to the low pressure turbine (6). The steam
is then passed to the condensor (8) which is cooled by water from the cooling tower whereby it
rapidly condenses. The water is then pumped to the economiser (23) where it is preheated before
returning to the boiler drum. Exhaust from the boiler passes through an electrostatic precipitator
(25) and possibly other pollution controls before being vented into the chimney stack (27).
Acknowledgement for graphic: By BillC under GNU Free Documentation License.
28 Silicon Chip
siliconchip.com.au
BOILING
PRESSURE
LATENT HEAT OF
TEMPERATURE (atmospheres or bar) VAPORISATION (kJ/kg)
100°C 0
2256
150°C
4
2110
200°C
14
1942
254°C
41
1691
304°C
90
1356
351°C
165
884
374°C
220
0
Table 3: data from Lalonde Systhermique saturated steam
table showing how the boiling point of water increases
with increasing pressure and how the amount of energy
required to vaporise water diminishes with increasing
pressure until it gets to zero at the supercritical point.
It’s all about temperature and pressure
Depending upon their operating temperatures and pressures, coal-fired power plants are classified as sub-critical
(traditional plants), super-critical or ultra-supercritical.
Super-critical technology involves the use of steam at
a temperature and pressure above its so-called “critical
point”. The critical point of a fluid such as water is that point
at which there is no distinct liquid or gaseous (steam) states.
A super-critical fluid is a special state of matter beyond
the familiar solid, liquid and gas phases. For water, this occurs at a temperature of 374°C and a pressure of 22.31MPa
or 220.15 atmospheres (3,235 psi) (the quoted pressure
varies a little for some reason).
A power plant can operate more efficiently with supercritical steam because the additional energy required to
achieve the higher operating temperatures is proportionally
less than that required to reach sub-critical temperatures.
(More details in the panel on page 35).
To understand the advantage of operation under supercritical conditions consider what happens to water when it
is heated at normal atmospheric pressure. It will heat until
it gets to the boiling point of 100°C. At that point, bubbles
Efficiency
Efficiency if CO2 capture employed
A pulverised coal burner in action!
Want to see what a pulverised coal burner in the open looks like in
action? It’s worth a look! The still above is from a video in a Third World
country (OH&S rules not in force!). See “Coal Powder Burner Part 1”
https://youtu.be/XitLs7y5P78; also see “Pulverised Coal Burner”
https://youtu.be/s0Ntd84EhfU
of steam start to form and are released into the atmosphere
but the temperature of the water does not increase. The
temperature of boiling can only be increased in a pressure vessel which allows the steam to be “superheated”
beyond 100°C.
As the pressure is increased, the boiling temperature increases but the energy required for boiling (the latent heat of
vaporisation) becomes less (see Table 3). A point is reached
where the energy required for vaporisation diminishes to
zero. This is the super-critical temperature and pressure.
The advantages of super-critical steam in power plants
have been known for a long time but it has not been possible to fully implement the technology due to the special
materials required to withstand the high temperatures and
pressures.
Conventional steam power plants operate at a pressure
of around 165bar (16.5MPa or 2393 psi) and are called
sub-critical.
New generation super-critical power plants operate at
pressures of around 243bar (24.3MPa or 3530 psi) and steam
Sub-critical Supercritical
33-37%, 34% typical
37-40%, 38% typical
43% with up to 46% being targeted.
25%
29%
34%
Steam temperature
Below 550°C, typical 540°C
565°C
Steam pressure
Coal consumption
Ultra-supercritical
Below 22MPa or 3200 psi, 24.3MPa or 3530 psi
typical 16.5MPa or 2400 psi
Above 565°C, up to 610°C;
700-720°C being targeted.
To 32MPa, 4640 psi; 36.5-38.5MPa,
5300-5600 psi being targeted.
208,000 kg/h
185,000kg/h
164,000kg/h
2,500,000 kg/h
N/A
1,940,000kg/h
Ash produced
22,800kg/h
N/A
N/A
Desulphurisation products
41,000kg/h
N/A
N/A
2,770,000kg/h <at> 55°C
N/A
2,200,000kg/h
466,000kg/h
415,000kg/h
369,000kg/h
4.84c/kWh
4.78c/kWh
4.69c/kWh
Air consumption
(used for building materials)
Stack gas
CO2 emitted
Representative electricity cost (US$)
Table 4: some performance figures for a typical 500MW pulverised coal plant with HHV (higher heating value) coal for
various technologies. If carbon dioxide capture is employed, efficiencies drop dramatically. The massive flow of materials
through the plant is obvious.
siliconchip.com.au
December 2015 29
Other types of heat generation used in power stations
Fluidised bed combustion
Typical coal-fired power stations use pulverised coal in their
furnaces, as mentioned above. An alternative is fluidised bed
combustion (FBC).
A fluidised bed is formed where particulate matter such as
powder or sandy material is subject to conditions that make it
act like a fluid. Typically this is done by a forcing a liquid or a gas
through the particulate medium. In nature, quicksand is a type of
fluidised bed. For a video of a fluidised bed see “Fluidised Bed:
Floating Duck” https://youtu.be/3BqVFGCUviY
In FBC, coal or some type of biomass such as wood waste or any
type of combustible rubbish is burned in a fluidised bed process.
Unlike the pulverised coal (PC) process which requires high quality feedstock, FBC allows the burning of much lower quality fuel,
including abandoned coal waste which contains non-combustible
material such as dirt and rock.
FBC can either be non-pressurised or pressurised (PFBC). FBC
systems are the most common but PFBC offer the advantage of
producing a gas stream that can also be used to drive a gas turbine,
in addition to heating steam, thus enabling a type of combined
cycle system with steam and gas turbines.
Circulating Fluidised Bed (CFB) is another variant in which pollution reducing agents such as limestone are added to the fuel to
minimise sulphur dioxide production. The lower operating temperatures of this process also minimise production of nitrogen oxides.
CFB can also efficiently burn low value “opportunity fuels” such
as waste from bituminous coal mines, anthracite coal mine waste
or petroleum coke. A video of related interest is “Alstom Introduces
the ultra-supercritical circulated fluidised bed (CFB) boiler” https://
youtu.be/pDKvyUroaC8
Combined cycle gas turbine technology
(CCGT)
One way to improve efficiency is to use two sets of turbines so
that waste heat from the first turbine can be captured and used a
second time. There are two types depending on the source of the
fuel, either those that generate gas from coal “Integrated gasification combined cycle (IGCC)” plants or those that run on natural
gas “Natural gas combined cycle (NGCC)” plants. The overall
thermodynamic efficiency of such systems can reach 50-60%.
Integrated gasification combined cycle
(IGCC) plants
In IGCC plants coal is turned into a synthetic gas or “syngas”
by combining it with oxygen and steam and heating it in much the
same way as “town gas” (or producer gas) used to be produced.
The resulting gas comprises mainly hydrogen and carbon monoxide and this is purified and used to drive a gas turbine which
turns a generator.
Waste heat from the gas turbine exhaust is then used to generate
steam which then passes to a turbo-alternator to generate more
electricity. The term combined cycle refers to the combination of
both gas and steam turbines. Efficiencies in the mid forty% and
possibly up to fifty% are possible but reliability issues inhibit
commercialisation. A typical design is shown below.
Natural gas combined cycle (NGCC) plants
An NGCC plant is much the same as an IGCC plant but uses
natural gas as the fuel instead of gasified coal. Up to 60% efficiency
is possible. Claimed advantages of NGCC plants are: less than half
the capital cost of coal-fired plant; relatively short construction
times and less than half the CO2 emissions of coal plants.
In addition, natural gas can be piped and does not need a lot of
handling infrastructure as does coal. See the video “Natural Gas
Combined Cycle (NGCC) plants” https://youtu.be/D406Liwm1Jc
Combined heat and power (CHP) cogeneration
CHP produces steam to generate electricity and also provide
steam or hot water for distribution to the local community or
industry for heating purposes. CHP plants may burn coal, gas or
any other suitable fuels, including waste products.
CHP plants are designed for flexibility of operation due to varying
demands in summer and winter. High efficiencies of up to 80% are
possible. Typically, these plants are only used in extremely cold
areas such as Scandinavia and Eastern Europe.
See video “CHP - Combined Heat and Power” https://youtu.
be/2Kc6xKQlDtU
A total of 95% efficiency is claimed for that plant.
Typical IGCC power
plant schematic.
Image credit:
Stan Zurek
30 Silicon Chip
siliconchip.com.au
The John W. Turk Jr. Coal Plant, the first
ultra-supercritical plant in the United
States which came online in 2012.
temperatures of around 565°C (those figures vary a little
depending on source). Note that steel starts to glow red at
around 480°C so this ultra-hot steam is causing the metal
to glow! Some performance specifications of plants with
various steam technologies are shown in Table 4.
Super-critical steam plants generally use a different type
of boiler (or more correctly, steam generator as no actual
boiling takes place in the super-critical condition). This is
known as a “once through” steam generator instead of the
more traditional drum or recirculation type boiler generally
used by sub-critical power stations (once through the boiler
before reaching the turbines, although the water does pass
back through the boiler after it is condensed).
A drum boiler operates below the super-critical pressure and water is recirculated through it and a “steam
drum” is used to separate water from steam. The steam
is removed for power generation and any water separated
by the steam drum is recirculated through the boiler to be
turned into steam.
A super-critical once-through steam generator operates
Japan’s coal-fired power plants are
some of the most efficient in the world.
This is the steam turbine at J-Power’s
ultra-supercritical Isogo plant.
siliconchip.com.au
December 2015 31
Alstom have recently announced a 1200MW ultra-supercritical plant for Dubai, scheduled to come on line in 2021.
above the critical temperature and pressure and no steam
drum is required because the super-critical steam is a single
phase with no separation of water and steam necessary.
Drum boilers have a greater wall thickness than oncethrough steam generators, making them slower to start or
change operating conditions. Once-through boilers also
have less working fluid in them which again makes them
more responsive to changes in operating conditions.
Once-through boilers require more sophisticated controls
than drum boilers as changing load demand is met by varying both fuel and feed water flow simultaneously, while
in drum boilers only the fuel flow needs to be controlled.
Other improvements in efficiency are also possible such
as with the use of reheat technology whereby steam from
the first stage of the steam turbine is fed back to the steam
generator for reheating a second time and also heat extrac-
tion from exhaust gases. Siemens say that their turbines
can approach 50% efficiency with reheat stages.
Pressurisation of boilers or steam generators is maintained via the boiler feedwater pump which returns condensate back to the boiler at high pressure.
Turbines and generators designed for super-critical steam
technology are much the same as with subcrititcal designs
but consideration must be made for the much higher steam
pressures and temperatures and the ability to alter conditions to accommodate for varying loads, which is less possible than for sub-critical designs as the sub-critical drum
type boilers take longer to ramp their output up or down.
Steam turbines usually consist of three main sections:
high pressure, intermediate pressure and low pressure.
These consist of sets of blades similar to what is found in
a jet engine. As steam expands through the sets of blades
it causes rotation of the turbine about its axis.
In the high pressure section, steam from the steam generator enters the turbine, expands causing the turbine to
rotate and then, in reheat installations, is returned to the
steam generator for further heating before being passed into
the intermediate pressure section.
In the intermediate section the steam further expands
causing further rotation of the turbine assembly when it
is finally passed to the low pressure section. After the low
pressure section, the spent steam and condensate is passed
through to the condenser where remaining steam, which is
much below atmospheric pressure, is converted to liquid
and then it is returned to the steam generator.
Life cycle costs of super-critical steam plants are only 2%
higher than for sub-critical but their fuel costs are much
less than that, so it is an economic proposition to invest
in this technology.
CSIRO’s Super-critical Solar Thermal Power Plant
Super-critical steam is not only of benefit in fossil fuel
plants but can also be utilised elsewhere where efficient
production of steam is required. Australia’s CSIRO is developing a solar thermal power station that uses 600 suntracking mirrors (heliostats) to direct solar energy from the
sun into a “receiver” containing steam tubes at the top of a
tower as shown below.
The steam generated is used to drive a turbo-alternator.
The steam produced, being at a super-critical pressure of
235Bar (23.5MPa or 3408 psi) and a temperature of 570°C
is a world record for super-critical steam production outside
of fossil fuel thermal plants and enables more power to be
produced for the same amount of sun compared to similar
sub-critical plants.
For a video on this plant see “Super-critical solar steam”
https://youtu.be/P4mFJG2f5bA
“Solar Tower 2”
at the CSIRO Energy
Centre in Newcastle,
NSW. It is a solar
thermal plant that
generates super-critical
steam to drive turbines
to produce electricity.
Image credit: CSIRO
32 Silicon Chip
siliconchip.com.au
First super-critical plant in 1957
The first super-critical steam power plant was built in
1957 in Ohio and was called Philo Unit 6.
You can read about the history of this unit and download
a brochure at www.asme.org/about-asme/who-we-are/
engineering-history/landmarks/228-philo-6-steam-electric-generating-unit
After the Ohio plant, super-critical steam cycles became
more widely used in the US in the late 1960s and units
were built through the 1970s and 1980s.
However, these were pushing materials technology of the
time to the limit and problems were encountered such as
boiler tube fatigue and creep of metal in the steam headers,
steam lines and the turbines.
These problems caused a return to sub-critical technology
with no incentive to return to super-critical technology due
to the low price of coal and the extra construction cost of
super-critical plant not being justifiable.
Conditions are different now and the materials problems
have been solved, hence a greater incentive to use supercritical technology. There are over 400 super-critical units
in use throughout the world at the present time.
Super-critical steam nuclear plants
Super-critical steam can also be used to improve the
thermal efficiency of nuclear power plants; however the
design of nuclear plants is extremely conservative and this
technology is not commercially implemented at the moment.
Nevertheless the super-critical water reactor (SCWR) is
under active investigation worldwide as an advanced reactor
technology as it offers a thermal efficiency of around 45%
compared with around 33% for conventional commercial
reactor designs.
In a nuclear reactor super-critical steam offers many
advantages. Since there is no chaotic boiling of water with
super-critical steam, the internal reactor environment is
much more uniform with no bubbles so this allows much
better heat and fluid flow.
Also, because there is no longer a mixture of steam and
water in the reactor, many steam-related components can
be eliminated such as the pressuriser, steam generator,
various pumps, steam separator and driers.
Super-critical steam is also less of a neutron moderator
(meaning faster neutrons) than water allowing for the possibility, in some designs, of a fast neutron reactor which could
utilise Uranium-238 (which comprises 99.3% of the uranium
present in nature) instead of the much rarer Uranium-235
(0.7% present in nature).
The better heat flow and faster neutrons with super-critical
(Left): design of supercritical water
reactor showing how steam from the
reactor core is utilised
directly in the steam turbine.
Image source: US Department of
Energy Nuclear Energy Research
Advisory Committee.
Image source: US Department of Energy
Nuclear Energy Research Advisory
Committee
siliconchip.com.au
steam allows a smaller core and an overall smaller reactor
reducing construction costs. A fast neutron reactor also allows for long-lived radioactive products to be “transmuted”
to shorter-lived ones.
Due to the greater efficiency of an SCWR more power
can be produced with the same amount of nuclear fuel as
a conventional reactor meaning a greater fuel economy
and lower costs.
Finally, in an SCWR super-critical steam from the reactor
is fed directly to the steam turbine much as in the straight
through steam generator previously mentioned, unlike conventional reactors where the steam from the reactor heats
a secondary steam circuit connected to the steam turbine.
This results in a much more simple and lower cost design.
Of course, there are also some challenging design issues with the SCWR.
Among these are the development of materials that can
reliably withstand the high pressures and temperatures of
the super-critical steam in a radioactive environment; less
cooling fluid in the reactor which reduces the ability to absorb heat from transient events and due to coolant loss in
a malfunction; and a change in the moderating properties
of the coolant between the steam outlet and the steam
inlet due to it being cooler and more dense upon its return.
Solutions to all these problems are under development.
18
19
Above: a typical boiling water pressurised reactor.
Note the primary (18) and secondary (19) steam cycles.
In contrast, in an SCWR, super-critical steam is sent
directly from the reactor to the steam turbine.
Image: Steffen Kuntoff
December 2015 33
Australia’s largest
electricity plants
In Australia, when comparing the size of electricity generation
projects, reference is sometimes made to the Bayswater Power
Station (above) in the Upper Hunter Region of NSW. This is a
coal-fired power station that was commissioned from 1985, with
four 660 megawatt generators for a total capacity of 2,640MW. It
produces about 17,000GWh of electricity per year and its expected
service life is 50 years.
In comparison to the Bayswater plant, Australia’s (and the
Southern Hemisphere’s) largest wind plant is the Macarthur Wind
Farm, in western Victoria. It has a 420MW “nameplate capacity”
but a 35% capacity factor as the wind does not blow all the time,
hence an average power output of 147MW. It, like most wind
plants has an expected service life of 25 years and is expected to
produce 1,250GWh of electricity per year.
Development of China’s coal thermal power station technologies. Image source: “Current Status and Outlook of SC &
USC Power Generation Technology in China”, Electric Power
Planning and Engineering Institute, 23 February 2012.
All of Australia’s coal-fired power stations are sub-critical
but Bayswater has plans for a conversion to super-critical
technology, although those plans seem to be on hold at
the moment. There is also development approval for a
Bayswater B power station which would be 2000MW and
use either combined-cycle gas turbine technology (CCGT)
or ultra-supercritical coal however this approval has been
pending since 2009 and there is no construction yet.
Similarly, there is development approval for the Mt
Piper (NSW) Power Station Extension for CCGT or ultrasupercritical coal technology, also for a 2000MW station
which was approved 2010 but again, there is apparently
no action.
Australia’s 22 sub-critical power stations have a total
generating capacity of 24,608MW with an average age of
30 years.
Worldwide, the focus on new coal-fired power stations,
where they are permitted to be built, is for super-critical and
ultra-supercritical operation due to greater fuel economy
and lower CO2 and other emissions.
The John W Turk Jr. Coal Plant in Arkansas, US was
finished in 2012 and was the first ultra-supercritical coal
plant in the USA. It is rated at 600MW and runs at a steam
pressure of 31MPa or 4500 psi and a temperature of over
600°C. Compared with an equivalent sub-critical plant it
uses 163,000 tonnes less coal and produces 290,000 fewer
tonnes of CO2 per year.
Unfortunately, because of restrictive and ever-changing
environmental laws in the USA, it may be the last. See YouTube video “Arkansas Ultra Supercritical Coal Plant Technology Faces Extinction” https://youtu.be/QIXiGI-CSYM
For a look at a German super-critical steam plant see
“RDK 8 (Germany) supercritical steam power plant” https://
youtu.be/fJVhwg5o0vA
China’s thermal power station development
Another large scale alternative energy plant is Australia’s largest
solar array near Nyngan, NSW, which has a capacity of 102MW at
full power and is expected to generate 235GWh of energy per year.
34 Silicon Chip
While the regulators and activists of the Western world
are increasingly opposed to coal, the developing world
such as China and India have no such inhibitions. China
is currently building the equivalent of two 500MW coal
plants each week and adding about the capacity of the UK
power grid each year.
siliconchip.com.au
Inside the turbine hall of China’s Waigaoqiao No.3 ultra-supercritical power station in Shaghai, with two Siemensdesigned 1000MW ultra-supercritical generators, with the steam generation plant designed by Alstom.
As well, they are they are now the biggest suppliers of
thermal power station equipment in the world. In 2014
alone they added an astonishing 101GW of generating
capacity, more than the total installed capacity of all but
ten nations.
Interestingly, China acquires their thermal coal technology via license arrangements or joint ventures with Western
and Japanese companies such as Alstom, BHK, Siemens,
Mitsui-Babcock, Mitsubishi and Toshiba. The graph opposite shows the extraordinary development of China’s coal
thermal power station technology in terms of unit capacity,
steam temperature and steam pressure.
China is focusing on super-critical and ultra-supercritical
power stations, an example of which is the Waigaoqiao
No. 3 plant with two 1000MW ultra-supercritical units.
In this plant the Shanghai Electric Co. supplied the steam
generation under license from Alstom and the turbines
under license from Siemens.
This plant (shown above and on pages 26 & 27) operates at 600°C and 276bar (27.6MPa or 4,000 psi) and had a
thermal efficiency of 42.7% when opened in 2008 but that
increased to 44.5% in 2011 due to plant improvements and
is now one of the most efficient coal plants in the world.
Conclusions
Despite claims to the contrary, the age of fossil fuel is not
over yet; at the moment there appears to be no genuinely
economic alternative to our cheap and reliable base power
from fossil fuels or nuclear in some countries (and an even
more questionable need to replace it).
New technologies such as super-critical and ultra-supercritical steam are significantly improving the efficiency of
coal plant, while new developments in fossil fuel production are able to provide us with cheap and reliable electrical
energy with less fuel use and lower emissions for many
decades into the future.
SC
Super-critical efficiency gains
To understand why a super-critical plant is around 4% more
efficient than a sub-critical plant we need to look at the losses in
the system.
There are five main losses in a coal-fired power plant: incomplete
coal combustion, energy lost transferring the heat of combustion to
the working fluid (water/steam), heat energy which escapes from
the working fluid in the boiler or piping, turbine inefficiencies and
electrical losses in the alternators and wiring.
The first and last steps, coal combustion losses and electrical
losses, are much the same in sub-critical and super-critical plants.
The single biggest improvement in a super-critical plant is in the
turbines.
Only about 1/3 the chemical energy in the coal burned is ultimately converted to electrical energy and of the 2/3 of the original
energy lost, roughly half (or 1/3 of the total) is in the turbines, due
to either friction or heat remaining in the exhaust.
The turbines in a super-critical plant operate at around 50%
efficiency compared to 46% for a sub-critical plant or 54% for an
ultra-supercritical plant. While the maximum theoretical (Carnot
cycle) efficiency for super-critical temperatures is only ~1% higher,
steam turbines are better approximated using the Rankine cycle
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where the much higher pressures lead to the 4% improvement,
for an overall plant efficiency improvement of around 2%. This
therefore explains about half the overall improvement (ie, from
34% to 38%).
Note that the fact that the fluid entering the turbines is in a
super-critical state is only incidental, as it quickly turns to regular
steam as the pressure drops through the turbine.
It’s simply the higher input temperature and pressure which
yields the higher efficiency.
The other 2% worth of efficiency gains are due to multiple
factors. One is that the steam generator in a super-critical plant
is much smaller than the drum boiler in a sub-critical plant. It
therefore has less surface area and fewer pipes and protrusions
and so loses less heat.
To give an idea of the contribution of the boiler/steam generator to
overall efficiency, a typical boiler is around 86-88% efficient. About
40% of these losses are due to heat carried away in the flue gasses
while some of the remainder is due to incomplete coal combustion.
The fact that the working fluid is heated closer to the coal combustion temperature (of over 1000°C) also means that more of the
combustion energy is transferred to the working fluid.
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