A supercritical steam (or Rankine) cycle is used today for more most of the new coal-fired power plants. More recently, it has been proposed as well for future water-cooled nuclear reactors to enhance their efficiency and to reduce their costs. This chapter provides the technical background explaining this technology. Some criteria for boiler design and operation, like drum or once-through boiler design, fixed or sliding pressure operation and coolant mixing, are discussed in general to explain the particular challenges of supercritical steam cycles. Examples of technical solutions are given for two large-scale applications: a coal-fired power plant and a supercritical water-cooled reactor, both producing around 1000 MW electric power.
TopIntroduction
Coal fired power plants are among the most prominent examples of energy conversion using a supercritical steam cycle. They were the result of a continuous development process, starting from sub-critical Rankine cycles in the early 20th century, which were fired with moving grates, up to supercritical steam cycles in the 1990s, which were fired with pulverized coal. Termuehlen and Emsperger (2003), who took part in this development as senior engineering experts at SIEMENS, report about the history of these coal fired power plant and summarize the status reached by the end of the century.
In the 1990s, SIEMENS took significant effort to increase power and efficiency and, simultaneously, to decrease the specific plant erections costs per kW installed power, as reported by Kefer and Seiter (1996). The supercritical steam cycle became the standard for all new built power plants and the specific costs of new power plants built around 2010 were finally down at around 1100 €/kW, including flue gas cleaning systems for dust, NOx and SO2.
Figure 1. Power and efficiency of coal fired power plants in Germany
The history of power and efficiency of power plants built in Germany since the 1950s is shown exemplarily in Figure 1. Within 50 years, the power was increased by a factor of 5, and the net efficiency by more than 10% points. The most important driver for power and efficiency improvements was the live steam temperature, i.e. the steam temperature at the inlet of the high pressure turbine, as shown in Figure 2. It is exceeding now 600°C with a potential to reach 700°C in near future, enabling 50% net efficiency.
Figure 2. Net efficiency of coal fired power plants, shown in Figure 1, increasing with live steam temperature
Among the latest power plants in Germany, using such technologies are:
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Unit 8 of Rheinhafendampfkraftwerk (RDK) Karlsruhe, with 912 MW electric power, firing hard coal with more the 46% efficiency, using live steam of 600°C at 275 bar and reheat steam of 620°C
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Unit 9 of GKM Mannheim, with 911 MW electric power, firing hard coal with 46.4% efficiency, using live steam of 600°C at 285 bar and reheat steam of 610°C
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Datteln Unit 4, with 1055 MW electric power, firing hard coal with more the 45% efficiency, using live steam of 600°C at 275 bar and reheat steam of 620°C
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Hamburg Moorburg, a twin unit with 1654 MW, firing hard coal with more the 46% efficiency, using live steam of 600°C at 276 bar and reheat steam of 620°C
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Niederaussem Unit K, with 1000 MW electric power, firing brown coal with more than 43% efficiency, using live steam of 580°C at 290 bar and reheat steam of 600°C
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Neurath Units F and G, two units with 1100 MW electric power each, firing brown coal with more than 43% efficiency, using live steam of 600°C at 275 bar and reheat steam of 620°C
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Boxberg Unit R with 675 MW electric power each, firing brown coal with 43.7% efficiency, using live steam of 600°C at 285 bar and reheat steam of 620°C
A coal fired power with 700°C live steam temperature, reaching 50% net efficiency, was planned to be built in Wilhelmshaven by 2020, but plans were postponed recently because coal is still too cheap to justify the increased costs of nickel-base alloys needed for this future technology.