9.4 Combined-cycle plant
The term 'combined cycle' implies any heat and power producing process where the prime movers employ more than one working fluid in a combination of turbines. The most common and practical form of such plant is the combination of one or more gas turbines with a steam turbine; this section will deal with variants of this basic theme.
Figure 1.132 shows a combined-cycle plant in its simplest form with the heat from the gas-turbine exhaust utilised to generate steam in a heat-recovery steam generator. This cycle makes use of the inherent characteristics of the gas turbine process, where combustion takes place and, following expansion in the turbine, heat is rejected at a relatively high temperature suitable for steam generation. The complementary steam turbine is able to make use of this because of the low temperature exhaust made possible by its condenser. The condensate is returned to the steam generator, via a single combined feedheater and de-aerator, followed by a pump. Typically the steam turbine output will be about 50% of the gas turbine output.
The advantages of such an arrangement are most applicable to countries having an abundant supply of oil or natural gas where there is a need to rapidly extend the utilisation of these resources to meet the populations' demands for electrical power. A phased development could be implemented, the first stage being the installation of gas-turbine generators to meet the immediate needs for electrical power, thus exploiting the potential short delivery times and simple installation of packaged units associated with gas turbine technology. The second stage would be the installation of the steam generators, steam turbine and auxiliaries, thus providing a further increase in power output at a high overall thermal efficiency of about 45%.
A third stage might be considered if a further increase in output for short periods during load peaks is desired. Supplementary firing would be fitted to the boiler to generate more steam to meet the full capability of the steam turbine in respect of steam inlet temperature. Because gas turbines are normally operated with a high excess air factor, there would be sufficient air in the gas turbine exhaust to support the combustion of the additional fuel.
Plant using a high proportion of supplementary firing is sometimes constructed making use of the gas turbine primarily as an air supply for the main combustion process. In this type of plant, the steam turbine may generate as much as eight times the power of the gas turbine and employ a multistage feedheating system to maximise efficiency. This may be more thermally efficient than a steam turbine on its own, but is prone to giving a lower availability unless a means of changeover from gas turbine to forced-draught fan is provided.
A more common variant is to use one or more gas turbines in a dual-pressure heat-recovery steam generator. This permits more heat extraction from the gas turbine exhaust since the low pressure circuit heat transfer can take place at a lower temperature than in the high pressure circuit. A typical arrangement is shown in Fig 1.133. If more than one gas turbine is used, additional flexibility of steam turbine operation is possible, since shutdown of one gas turbine will not prevent steam turbine operation. More efficient part-load operation can be obtained in this way.
Combined-cycle plant developments are likely to follow the development of the gas turbine. Higher combustion and exhaust temperatures will lead to higher overall efficiencies. As the practicability of burning a wider range of fuels (including coal) in the gas turbine improves, the application of combined-cycle plant will become more widespread. With the current state of the art, there are no limitations to such further development in respect of the steam turbine. A wide range of outputs is generally offered by manufacturers, using various numbers and size-ranges of standard gas turbines in combination with steam turbines of modular design.