Optimization of steam cycles with respect to supercritical parameters

Abstract

Abstract Description of supercritical power stationsInvestigations on the increasing of efficiency of steam turbine cycle have been carried out by many research centres of universities and turbine production concerns already for a few dozen of years. In the case of thermodynamic cycles with applied steam turbines the investigations have dealt with steam pressure and temperature values greater than: the 22.12MPa critical pressure and 647.28K critical temperature, respectively. Over the critical point the difference between liquid phase and gas phase disapears [20], [25], [43]. Already in the 1950s the first attempts to developing the turbine power units based on supercritical parameters have been made. In 1957 the first in the world turbine power unit of supercritical parameters was put in motion in the electric power station, Philo, USA. The unit of 314bar/621oC steam parameters developed 125MW output power. Two years later 325MW power unit of even higher steam parameters (345bar/649oC/566oC/566oC) was built in the Eddystone I electric power station. For the high values of steam parameters special materials suitable to work in high temperatures were required. High investment cost, inadequate quality of materials, faults in assembling and operational problems forced the producers to resign from building the units for so high steam parameters. As late as in 1969 the unit designed for the supercritical parameters with double secondary superheating of 241bar/538oC/552oC/566oC, was set working. The application of such solution to the steam cycle has improved operation of the unit. Therefore in the 1970s, apart from the units with single superheating of 241bar/538/538oC steam parameters, were built the units with double superheating, which developed output power reaching from 350 do 1100 MW. Progress in material technology, started in the 1980s, has made it possible to apply higher steam parameters (310bar/593oC/593oC/593oC) to steam cycle. The application of better and better materials suitable for very high operational temperatures makes it possible to build the units for the supercritical parameters as well as ultrasupercritical ones (400bar/760oC). Modern 3-D calculation programs used in designing fluid flow systems as well as highly efficient devices included into equipment of such units have guaranteed to achieve their high reliability and efficiency. In the 1960÷1990s in USA 159 units of the power range of 300÷1400MW, pressure range of 230÷260bar and temperature of 540÷590oC, including 14 units of double secondary superheating, were set working. In the years 1990÷1998 in Japan and China total output power installed in coal electric power stations has increased threefold. The growth was achieved mainly by applying the units of output power in the range of 400÷700MW, working with the supercritical steam parameters of 255bar/570÷590oC and the secondary superheating of 570÷595oC temperature range. In the years 2000÷2003 three 700MW units based on the 246bar/593oC/593oC steam parameters, two 900MW units based on the 241bar/593oC/593oC steam parameters, two 900MW units based on the 241bar/600oC/610oC steam parameters, three 1000MW units based on the 245bar/600oC/600oC steam parameters and two 1050MW units based on the 250bar/600oC/610oC steam parameters, were put in operation. In Europe significant achievements in building the power units with supercritical parameters can be noted in Denmark and Germany. In 1984 in Denmark the first unit with supercritical parameters was set working in the Studstrupvaerket electric power station. At the beginning of the 21st century two 411MW units with the 290bar/582oC/580oC/580oC steam parameters, including one coal-fired and the other gas-fired, as well as 530MW unit with the 300bar/580oC/600oC steam parameters, working on combusted biomass, were put in operation in Avedore electric power station. Germans, basing on the achievements and operational experience of Danes in the area of building coal electric power plants with high steam parameters, have built mainly 800÷900MW power units fitted with brown coal-fired boilers. In 2002, 1012MW power unit based on the 274bar/580oC/600oC steam parameters was built in the Niederaussem electric power station. A list of selected power units (presently installed or planned ones) intended for the operating with supercritical parameters is presented in Tab. 1. Pątnow II electric power station is the first Polish electric power plant based on supercritical