10.2 Supercritical plant
One effective way of achieving increased thermal efficiency is to increase steam pressure. The limits of a natural circulation boiler are around 180 bar, and although assisted circulation may be used at higher pressures, an overall improvement in station efficiency is not achieved unless the pressure is advanced to about 240 bar, i.e., above the critical pressure of water/steam (221.2 bar).
Although the use of supercritical pressure requires special consideration in the design of the boiler, the implications for the turbine only concern the higher pressure as such.
A further improvement may be obtained by increasing steam temperature. Most of the supercritical plant in service worldwide operates at 538°C, although some 565°C plant exists, and there are some pioneering units with temperatures as high as 650°C. At the higher temperatures, the efficiency is often boosted still further by using double reheat. Apart from the efficiency benefit, this has the merit of reducing the turbine exhaust wetness from the high level that such advanced initial conditions would otherwise entail.
The use of supercritical plant has varied considerably through the world. In both the USA and Japan, it has been used to a considerable degree for large units for base-load operation, typically up to 700 MW tandem-compound and 1300 MW cross-compound. The initial steam conditions have normally been around 240 bar, 538°C. In Germany, supercritical plant has been in the smaller range, up to about 200 MW. In the United Kingdom, two prototype units were built at Drakelow С with steam conditions of 240 bar, 593°C.
At temperatures up to 565°C, low-alloy creep-resistant steels such as 0.5%Cr Mo V and 2.25Cr Mo are used for the high temperature components. The pressure determines the thickness of pressure-containing sections such as steam chests and pipework. The combination of steam conditions, material, thickness, and operating regime determines the plant life in terms of creep and thermal fatigue. At pressures of 240 bar and above, and particularly at the higher temperatures or where reasonable operational flexibility is required, other high temperature materials are used, such as high-Chromium (9-12%) ferritic alloys, or austenitic alloys. Austenitic alloys have some adverse properties, such as poor thermal conductivity and high thermal expansion, so the current trend is to develop the ferritic alloys for use in the large cast and forged components. Development programmes are under way in the USA, Europe, and Japan for designs and materials for the so-called 'ultra-supercritical' plants of 350-1000 MW with steam conditions such as 310 bar 590°C, and later up to 350 bar 650°C, all with double-reheat cycles. These plants are not, however, likely to be in service until after the year 2000.
The use of the double-reheat cycle introduces additional complexity. First, additional boiler controls are required for steam temperature, and secondly the turbine must either have an extra cylinder or it must use a combined cylinder for the first two expansions. The extra cylinder increases machine length and cost, while the combined cylinder may give the possibility of problems due to sealing between the two expansions or due to the close proximity of sections at hot and cold reheat temperatures. Combined HP/ IP cylinders have, however, been widely used by American companies and their associates in machines up to 700 MW.
None of these developments presents technical problems, given sufficient time and resources. Their application in practice depends on potential customers being satisfied that the potential return in improved efficiency is not accompanied by additional risk either to plant life, operational flexibility, or availability. To this end, the development programmes embody the full range of research, design, rig testing, and prototype component testing, which, coupled with the first full-size prototype unit, will give the necessary assurance.
The rate at which such plant will be introduced is however uncertain, depending as it does on factors such as electricity demand, fuel costs, the economic environment, the extent of alternative energy sources, and the refurbishment of existing plant for extended life.