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Chapter 10
Closed-Cycle Gas Turbine Process with Steam Injection

Excerpt

Besides the combined cycle systems, which consist of a cascade of gas and steam turbines and offer the highest efficiency for thermal electric generation, there is also the principle of steam injection into the gas turbine itself of interest. Steam is generated in a heat recovery steam generator, which is then injected superheated behind the compressor, in other words in front of the combustion chamber, as indicated in the previous section. The steam injected in this way performs additional work without extra loading the compressor. When the steam generation potential of turbine waste heat is used to the full, the output at the generator terminals can be roughly doubled. However, the steam, which is only partly superheated with waste heat in the heat recovery steam generator, must be further superheated to the hot gas temperature using additional fuel in the combustion chamber. Although the additional power achieved by steam injection is around twice as much as in case of a combined cycle plant, but the increase in efficiency is only half as much.

A major drawback of the steam injection gas turbine (STIG) is the loss of water with the waste heat. Condensation of the water vapor under low partial pressure is enormously complicated and therefore hardly feasible.

However, if the recuperator in a closed-cycle gas turbine is replaced with a heat recovery steam generator, and the steam thus generated injected after the compressor, the following advantages result. Firstly, the steam does not come into contact with the flue gases, which means that it stays clean, and secondly, the steam in the pre cooler of a charged, closed gas turbine process is at a much higher partial pressure, which means that it can be largely condensed out using a natural heat sink. This condensate only needs to be mechanically cleaned before it is fed back to the heat recovery steam generator. In this way the water or steam passes through a closed process and is not lost. Only a small amount of waste sludge water has to be constantly replaced, as is usual with steam power plants. However, there is no need for continual demineralization.

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