As renewable energy penetration continues to increase, combined cycle power plants are no longer operating exclusively as baseload facilities. Even plants originally designed for baseload duty are increasingly required to operate flexibly, with more frequent start-ups and shutdowns, faster ramping, and extended periods at part load. As larger shares of renewable generation are connected to the grid, operational flexibility will become a standard requirement rather than an exception. Consequently, HRSGs must be designed to withstand these demanding operating profiles while maintaining reliability and performance.

Flexibility is no longer an optional feature for new-build plants; it is a fundamental design requirement for long-term success.

This changing operating profile places new demands on Heat Recovery Steam Generators (HRSGs), which must provide greater operational flexibility while maintaining the integrity of critical components.

At the same time, advances in gas turbine technology have increased overall plant efficiency. Larger gas turbines operate at higher exhaust temperatures and flows, resulting in higher pressures and temperatures in the bottoming cycle. For the HRSG, this means thicker pressure parts to withstand these conditions. However, thicker components experience higher thermal stresses during cycling operation, making flexibility more challenging. The high-pressure drum, with its large wall thickness, is one of the most critical components affecting lifetime under cycling conditions.

To address this challenge, NEM Energy developed its patented DrumPlus™ design. Rather than eliminating the steam drum, DrumPlus™ retains a natural circulation concept while significantly reducing the size of the drum. The design replaces the conventional high-pressure drum with a compact knockout drum for primary steam-water separation and separator bottles for secondary separation. The reduced drum diameter and wall thickness result in lower thermal stresses during operation.

Thanks to the reduced drum wall thickness, DrumPlus™ supports unrestricted gas turbine start-up from the HRSG. The technology has already proven itself in commercial operation. At the El Segundo FlexPlant™ 10 in California, operational since 2013, the two SGT6- 5000F gas turbines can ramp up without restrictions from the HRSG, enabling approximately 300 MW to be delivered to the grid within ten minutes from a cold start, with the remaining output available in less than one hour.

NEM has further increased plant flexibility by combining DrumPlus™ with a diverter damper. Combined cycle plants equipped with a diverter damper can operate in both simple cycle and combined cycle mode. Traditionally, however, switching from simple cycle to combined cycle operation requires the gas turbine to ramp down in load while the HRSG is heated gradually. For a cold HRSG, this conventional procedure can take approximately one hour and is necessary to minimise thermal stresses in critical HRSG components.

With DrumPlus™ and an optimised diverter damper operating philosophy, the switch from simple cycle to combined cycle can be performed without reducing gas turbine load. By controlling the exhaust gas flow admitted to the HRSG, NEM developed an optimised switching procedure that balances thermal stresses, pressure transients and component temperatures while protecting critical HRSG components. Compared with the conventional switching procedure, operators can significantly reduce the transition time, while also saving operational costs and reducing NOₓ emissions associated with part-load operation.

This technology is currently being implemented at the Jizzakh Combined Cycle Power Plant in Uzbekistan, where an F-class gas turbine is combined with a DrumPlus™ HRSG, bypass stack and diverter damper. Engineering studies were used to optimise the switch-over procedure across the complete cycling load range while maintaining the integrity of the HRSG.

As combined cycle power plants continue to support power systems with increasing shares of renewable energy, operational flexibility becomes increasingly important. DrumPlus™ enables faster starts and rapid switching between simple cycle and combined cycle operation while reducing thermal stresses in critical HRSG components, allowing operators to respond more quickly to changing grid demands.

Visit www.nem-energy.com to learn more about NEM's flexible HRSG solutions.