Heat Recovery Steam Generators for large combined cycle plants (250 Mwe GT output): experiences with different design options and promising improvements by once-through technology development

摘要

Ansaldo Caldaie has gained extensive experience in designing, manufacturing and installing HRSGs for gas turbine sizes ranging from 40 MWe to 250 MWe. The purpose of this paper is to focus on large size HRSG units suitable for Siemens GT V94.3A or GE 9FA with 250 MWe output. These HRSG units are designed under our proprietary technology based on two design options: horizontal gas flow with natural circulation and vertical gas flow with assisted circulation. Most of the units have been successfully tested and entered into commercial operation during the last five years. The most relevant projects of these types of units are herein presented and described along with their distinguishing features; specifically, the vertical HRSG design suitable for insertion into the existing steel structure of old fired boilers in re-powered power stations is described. A comparison between the horizontal and vertical design options is provided on the basis of functional, constructive and operating data collected from a combined cycle power plant where two units with the same operating characteristics and output, but with different configuration (horizontal and vertical), have been delivered. Following a market trend towards extensive cycling operation and combined cycles of increasing size with gas turbines of improved technology having higher exhaust gas temperatures and flowrates, a new design option has become interesting in the recent years and has been taken into account as a possible advancement for the HRSG technology: once-through mode in the high pressure levels of the unit. For this design option, we, - as Siemens licensee for Benson Once Through HRSG technology - have developed our own standard project. This paper describes the main features of this project and highlights the most relevant advantages that HRSG units based on this concept offer compared to Drum Type units.