A Study on Power Uprating of a Steam Injected Gas Turbine Cogeneration System by Compressor Discharge Air Bypass

Gas turbines are widely used for cogeneration systems. In general, electricity and heat demands are not constant throughout the year. In cooling seasons, generally, heat demand decreases but electricity demand increases. In small gas turbine cogeneration systems, steam injection is a good way to respond to the demand variation. However, steam injection causes compressor discharge pressure to rise. This means a reduction in compressor surge margin, which is a critical operational parameter. Hence, even though thermal energy demand decreases considerably as is the case in cooling seasons, the surplus exhaust heat cannot be utilized for the steam injection in the conventional operation. In this study, a modified steam injected operation is suggested, which uprates electric power output without damaging a minimum allowable surge margin. This can be realized by extracting some of compressor discharge air and supplying it to the turbine exhaust side. The modified operation allows more steam to be injected into the combustor in comparison to the conventional steam injected operation while it guarantees the same compressor surge margin. The modified operation concept provides another merit of modulating the heat to power generation ratio by controlling both the amount of air bypass and the steam injection rate. In particular, pure power generating operation, where full amount of generated steam is injected and thus no heat output is available, is possible without decreasing the surge margin below a desired minimum value.Copyright © 2013 by ASME