HRSG Design for Flexibility – Switch-Over at Full Load from Simple Cycle to Combined Cycle Operation for F-Class Gas Turbines

With the growing share of fluctuating renewable energy sources such as wind and solar, conventional power plants are increasingly expected to run in cycling operation mode. Due to the increased demand for flexibility from power plants, simple cycle (SC) and combined cycle (CC) plants are being designed for fast load ramps, shorter start-up and shutdown times, and a higher number of starts and stops. Consequently, the HRSG must also be designed for increased flexibility with an acceptable impact on the lifetime of components. A CCGT plant with a bypass stack and a diverter damper provides the plant operator with the option to start-up and operate the plant over the bypass stack in SC mode or over the HRSG in CC mode. Conventionally, large gas turbines such as the F-class are required to ramp down in GT load before switching over from a SC to CC operation. Such a procedure is necessary to protect the HRSG from very high temperature transients, to minimize low cycle fatigue in thick-walled components such as the HP superheater headers, main steam line and the HP drum and to limit expansion differences. High temperature transients also increase the risk of magnetite cracking in the HP drum, inducing growth of cracks in the drum wall. For a cold HRSG start, switch-over from SC (at base load) operation to CC operation could take approximately an hour. In this new design, a switch-over from SC to CC at 100% GT load was investigated for a F-class GT. The considered HRSG design is a 3-pressure + reheat system equipped with the NEM - patented DrumPlus™ technology. The latest DrumPlus™ design has been optimized for lower wall thickness and has demonstrated the capability to support unrestricted GT ramp-up. The results of the design for the SC to CC switch-over at full GT load, dealing with specific aspects such as row and bundle expansion and impact on fatigue sensitive critical components are presented. Next to an optimized design, an optimized start-up procedure was set up as well, finally enabling the HRSG to handle a switch-over at full load. The increased operation flexibility of the DrumPlus™ design is a valuable benefit to plant operators that need their plant to respond fast to maintain grid stability and increase plant efficiency by switching from a simple cycle to combined cycle operation.