This paper characterizes the performances of once-through heat recovery steam generators in combined cycle applications. Although the concept of once-through boilers has been extensively used for fired boilers, this circulation mode is rather new in combined cycles.

Once-through circulation is required at very high pressures, including supercritical conditions, for which there is a possible advantage in terms of the plant efficiency. The concept is also certainly the best solution for high subcritical pressures, such as those required by repowering applications.

In addition to cycle performance, this paper describes the advantages and disadvantages of the once-through in heat recovery applications. The operational behaviour of such heat recovery steam generators is discussed, together with the main design implications.

A prototype plant running in supercritical steam conditions is also described, as well as some of its possible applications.

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