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Figure 6. Combined Cycle Cogeneration Power Plant.

All electricity generated in the power plant of the figure can be considered CHP Electricity, since there is no auxiliary cooler or condensing tail in the steam turbine.

A characteristic feature to all combined cycle power plants is, that the heat contained in the gas turbine flue gases is recovered into the steam-water cycle of the power plant in a heat recovery boiler. The heat recovery boiler can be equipped with supplementary firing using the gas turbine flue gases as combustion air. The fuel used in the burners of the supplementary firing system can be different from the gas turbine fuel. In a combined cycle condensing power plant the total efficiency lies in the range of 50 %.

A combined cycle process can be applied, for example, when repowering existing power plants. For example, it is possible to utilise the flue gases as combustion air in a normal power boiler, which may use coal or some other solid fuel as main fuel. Another solution could be to replace existing high pressure feed water heaters with heat exchangers, where gas turbine flue gases are used as heating medium. These and similar solutions do not necessarily mean, that the complete power plant would be converted into a combined cycle power plant. In this case, it would be possible to speak about combined cycle power generation covering only one part of the power plant electricity production. This part of the generation may have the same efficiency as that of the combined cycle power plant, namely about 50 %.