HRSG Design to Meet Next Generation Cycling and Efficiency Requirements

The need for fast start up and cycling of combined cycle power plants in response to the growth of renewables is well understood in the power industry. Both trends have been widely discussed. This need is supported with the Rapid Response combined cycle system employing the new 7HA and 9HA gas turbines and others with hot start times around 30 minutes and operational efficiencies as high as 62 percent.hrsg boiler design
The need for both high efficiency and cycling puts conflicting requirements on Heat Recovery Steam Generation (HRSG) pressure parts. Application of new or higher-grade materials is an option. However, considering the long industry experience and cost of the plant, exploring the capacity of the existing materials, with improved design and analysis, is desired to meet the current challenge. For existing materials to endure higher pressure and temperature, thicker components are needed. The thicker components result in higher through wall temperatures and thermal stress, which is further increased due to faster cycling.
Heat Recovery Steam Generators must be designed to accommodate fast and increased cycling over the life of a power plant. This should include a design life assessment that accounts for fatigue caused by cycling and creep from long term operation at high temperature and pressure.
Critical to designing HRSG pressure parts is a life assessment analysis that takes into account all aspects of imposed steady state and transient thermal and pressure loads along with material and geometry of components. Both the EN and ASME codes provide guidance on life assessment approaches, however the simplified methods provided can result in either over or anticonservative predictions of component life. GE uses an approach generally based on EN 12952-3 and 12952-4 for fatigue and creep assessment along with robust transient modeling to define transient operating conditions. This article outlines the approach used by GE for HRSG life assessment.
It is necessary to define the transient operating conditions that the HRSG pressure parts must endure. For units that are not yet in operation, this requires a dynamic model to predict steam/water flow pressure and temperature in the pressure parts as a function of time. Additionally, the heat transfer to the pressure parts must be determined as a function of the flow conditions as this defines the thermal transient that the component is subjected to. The primary inputs to the dynamic model are the gas turbine flow and temperature curves as these provide the energy into the HRSG. If the HRSG has a duct burner the ramp rate of that heat input must also be included. The dynamic model must include the operational logic and constraints for the various control valves, bypasses and attemperators. Figure 1 shows typical transients for an operating cycle.