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Publication Title | Thermodynamic Design Considerations Steam injected gas turbines

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S The Society shall not be responsible for statements or opinions advancedin papers or discussion at meetings of the Society or of its Divisions or Sections,
Steam injection in gas turbines (steam raised from the energy of the exhaust and injected into one or more of the turbine stages) is an attractive option for cogeneration applications. From a thermodynamic point of view, however, there is little information available about methods for optimizing the use of the steam for injection into a gas turbine.
A computer model for an aeroderivative gas turbine is used to analyze the effect of steam injection on net power output and overall efficiency. The effects of varying the quantity of steam injected, the stations at which the steam is injected, and the temperature of the steam that is injected are assessed on a normalized basis, with the turbine-inlet temperature maintained from the simple-cycle design point.
The energy balance between the exhaust of the gas turbine and the flow of steam to be injected is the final constraint in selecting a steam-injected design point to maximize performance. For the engine in this study, increases of over 64% in net power output and 23% in overall efficiency can be achieved with roughly 16% steam/inlet air by mass, which represents all of the steam that can be produced by the exhaust stream for the given conditions.
Injecting steam into gas turbines is not a new concept. From the early days of jet-powered aviation, when water was used to enhance thrust output (Williams and Larson, 1988), to the more recent days of environmental awareness, in which steam is used to control the production of
pollutants in the combustion process (Lefebvre, 1983), there has been some reason to mix the vapor and gas cycles. The thermodynamic effects of steam injection in a gas turbine are increases in net power output and overall efficiency. When steam is injected into the turbine at a constant turbine-inlet temperature, the benefits realized from the added flow of mass and energy in the turbine and the higher pressure-ratio in the compressor outweigh the drawbacks resulting from the increased fuel consumption and compressor-power requirements (Larson and Williams, 1987). From a practical standpoint, steam injection bolsters the lower-cost but less-efficient simple-cycle gas turbine as a power-generation option by making use of the available energy in the exhaust stream. With a properly sized steam-injected gas-turbine system, either process- steam requirements can be met while operating at peak efficiency (in "dry" operation) or excess steam can be injected (for "wet" operation); the excess power can be sold to the local utility (Larson and Williams, 1987). Additionally, the modification of a gas turbine for use with steam injection is relatively simple (Thames and Coleman, 1989).
Because of these merits, steam injection in gas turbines is a topic deserving of study. In fact, many studies have been performed already; however, these typically have dealt only with a final design point, while making no mention of the general criteria used to select that point. Such considerations include: the thermodynamic stations at which steam should be injected, the amounts of steam to be injected at these locations, and the thermodynamic state of the steam (temperature and pressure) at injection. These
all have a significant effect on the performance of a steam-
® or printed in its publications. Discussion is printed only if the paper is pub- • lished in an ASME Journal.. Papers are available from ASME for 15 months
after the meeting. Printed in U.S.A.
Copyright © 1993 by ASME
Peter D. Noymer
Preliminary Design GE Aircraft Engines- Lynn, Massachusetts
David Gordon Wilson Department of Mechanical Engineering Massachusetts Institute of Technology Cambridge, Massachusetts
Presented at the International Gas Turbine and Aeroengine Congress and Exposition Cincinnati, Ohio — May 24-27, 1993
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