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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 siliconchip.com.au 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. December 2015  35