Tradeoff Assessments for Part Load Controlled Cooling Air in Stationary Gas Turbines

The demand for flexible part load operation of stationary gas turbines requires the simultaneous design for sufficient efficiency and lifetime. Both can be addressed by the secondary air system. This paper presents investigations on the concepts of cooling air reduction in off-design, aiming for tradeoffs between fuel burn and turbine blade life. The considered lifetime mechanisms are creep and oxidation. In addition, the effects on emissions from the combustion are outlined. The reference gas turbine is a generic gas turbine in the 300MW power output segment. The focus is on the first two stages of the four-stage turbine. All simulations are performed by application of a coupled model that essentially connects gas turbine performance with a secondary air system network model. This coupled model is now extended with blade life evaluation and emission models. The results contain tradeoffs for operating points at base and part load. For example, the combined cooling air control of stage 1 rotor blade and stage 2 vane offers savings up to 0.5% fuel flow at 60% of base load in a combined cycle application. This saving is at the expense of creep life. However, some operating points could even operate at higher blade temperatures in order to improve life regarding hot corrosion. Furthermore, generic sensitivities of controlled secondary air supply to cooling layers and hot gas ingestion are discussed. Overall, the presented trades mark promising potentials of modulated secondary air system concepts from a technical point of view.