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STIGs-STEAM INJECTED GAS TURBINES Steam may be injected to suppress air pollutants arising from combustion of fuel or to augment the gas turbine power output. A STIG is any gas turbine into which steam is injected primarily for power augmentation and is the subject of this paper. Steam is normally injected after the compressor, is heated in the combustor along with the air, and is expanded through the expander. All the power produced by the expansion of the steam is useful, compared to one-third to one-half of the power produced by expansion of air which must be compressed. Injected steam thus has a dramatic effect on the power output of the gas turbine. Injected steam equal to one percent of airflow can increase power by about four percent. Without supplementary firing, the heat recovery boiler of a STIG will raise steam equal to 15 to 20% of the airflow of its gas turbine by weight. A normal heavy gas turbine can accept one third to one half that quantity of injection steam but with some loss of surge margin and flexibility. To increase power instantaneously in the event of an aborted landing, aircraft engines have wider surge margins to allow large step increases in firing temperature. When adapted to stationary applications, smaller surge margins are adequate. Thus aircraft engines are able to accommodate greater proportions of injected steam while maintaining adequate surge margin for stationary applications. This accounts for the successful use of steam injection with the Allison 501 and the General Electric LM 2500 and 5000. With these engines, the quantity of steam that is injected into the combustors is limited to about 7'l. of airflow. In the LM 5000 additional steam is injected after the high expander. The steam injected into a gas turbine increases the power of the expanders without increasing the power absorbed by the compressors. For best results, the increased expander power should be transmitted directly to the power output shaft. This requires a single shaft gas turbine. If steam is injected into a multi-shaft gas turbine, the match of power and speed of the compressors and expanders is changed. This may limit the quantity and pressure of steam which can be injected into the combustor and may require some of the steam to be injected at lower pressure. These limitations of multi-shaft gas turbines result in less than optimum efficiency and consideration of the Turbo- STIG concept thus has been confined to single shaft gas turbines. The injection into the combustor of all the steam which can be produced by recovered heat results in the highest efficiency of power generation. The injection of additional steam produced by supplementary firing reduces efficiency. Steam raised at a pressure above the gas turbine combustor pressure and expanded in a steam turbine before injection into the combustor produces an increment of power at essentially 100% efficiency. With the higher main steam pressure, optimum heat recovery and best cycle efficiency require that additional steam be raised at a lower pressure and directly injected into the combustor. The use of the steam turbine to drive a topping air compressor to increase the pressure ratio of the gas turbine further increases efficiency. This is the Turbo-STIG system. The Turbo-STIG Cycle The following description can be followed more easily by using the diagram of a Turbo-STIG, figure 1. The gas turbine exhausts into an unfired heat recovery boiler in which steam is raised for injection into the gas turbine. A portion of the steam can be exported for cogeneration applications. Maximum steam is raised at the highest practical pressure and expanded in a back pressure steam turbine before injection into the gas turbine combustor. The back pressure steam turbine drives an additional topping air compressor which raises the gas cycle pressure ratio to the higher pressure required to pass the additional flow. Additional steam is raised at lower pressure to recover more exhaust heat and this steam is injected directly into the combustor. The configuration of the gas turbine is that of a recuperated machine with the recuperator replaced in the cycle by the steam driven topping compressor. For when all HP steam is being exported or during startup without a supply of steam a non return valve is provided to allow air to pass by the topping compressor. If steam is available the gas turbine can be started by applying steam to the steam turbine to drive the topping compressor. This will create a flow of air through the machine, crank the gas turbine and allow the combustor to be lit. A mechanical cranking system can thus be omitted and some cost saved. The power from expansion of the steam is sufficient to drive a topping compressor with a pressure ratio of about 1.2. The air volume flow through the topping compressor is small relative to the inlet flow of the main compressor and a small diameter one or two stage axial or a centrifugal high speed compressor would be suitable. Direct connection of the topping compressor to the steam turbine in a configuration somewhat like a diesel turbocharger is most appropriate. A Turbo-STIG with standard compressor and expander aerodynamics can accommodate all the steam raised by heat recovered from its exhaust without decrease of surge margin. Any required modifications will be less extensive than are required for previously proposed full injection STIG cycles. The Turbo STIG system improves the cycle efficiency relative to normal STIGs, by increasing the overall pressure ratio (of both steam and gas), by steam reheat and by better matching of components when some or all the steam is exported and not injected. The low capital cost common to all STIG systems is retained. 2 Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1989/79160/V004T09A002/4456981/v004t09a002-89-gt-100.pdf by guest on 21 October 2020PDF Image | The Turbocharged Steam Injected Gas Turbine Cycle
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