A Study of Steam-Cooled Humidified Gas Turbine Cycles

Abstract

Humidified Gas Turbine (HGT) cycles such as the Evaporative Gas Turbine (EGT) and the Steam-Injected Gas Turbine (STIG) using humid air as the working medium do not require a complete steam turbine bottoming cycle; thus, their initial capital costs are not as high as those for the conventional combined cycles. The performance of a HGT cycle could be comparable to a state-of-the-art combined cycle for small loads. The availability of the steam from a HGT cycle presents opportunities for designing steam-cooled blades. Since the specific heat capacity for steam is higher than that for air, steam could potentially be a better coolant for turbine blades than air, resulting in higher cycle efficiency. In this study, three known HGT cycles are evaluated in terms of their electrical efficiencies and power outputs: the STIG, the Part-flow Evaporative Gas Turbine (PEvGT), and the combined STIG cycles. All the three HGT cycles are analyzed in two cooling options: steam and air coolings. The HGT cycles will be evaluated using consistent thermodynamic properties and assumptions. Like a simple gas turbine cycle, the HGT cycles are based on the well-known Brayton cycle whose performance is dictated by the cycle pressure ratio and turbine inlet temperature. Therefore, the electrical efficiencies and power outputs of the HGT cycles will be calculated as a function of the cycle pressure ratio and turbine inlet temperature. The steam-cooled cycles provide advantages over the air-cooled cycles in the electrical efficiency, power output, and combustion stability. The steam cooling improves the electrical efficiency by approximately 1.4 percentage points for the STIG cycle, by approximately 1.7 percentage points for the PEvGT cycle, and by approximately 1 percentage point for the combined STIG cycle. The maximum electrical efficiency of the steam-cooled PEvGT cycle is 54.6%, only 0.2 percentage points higher than that for the steam-cooled combined STIG cycle. The steam cooling generally results in more power output than the air cooling does for all the HGT cycles at most operating conditions. In addition, the steam cooling reduces the water content of the humid air entering the combustor, leading to significantly improved combustion stability.