1 Introduction

The Combined Cycle Task Group of the ASME Research and Technology Committee on Water and Steam in Thermal Systems has prepared the following set of chemical operating limits specifically to address the requirements for heat recovery steam generators.

A common configuration for power generation utilizes a natural gas-fired combustion turbine (CT) coupled with a heat recovery steam generator (HRSG) that extracts heat from the gas turbine exhaust to produce steam for a separate steam turbine/generator set. Often multiple CT/HRSG units feed a single steam turbine. Some combustion turbines can use fuel oil as either the primary or back up fuel. If steam or hot water is exported to an industrial user, the plant is typically known as a cogeneration facility.

Combined cycle (CT plus HRSG) systems are extremely efficient, with the newest configurations approaching twice the heat rate of conventional fossil fuel-fired power plants. Since the plants are typically powered by natural gas, the need for sulfur emission control technologies is almost always eliminated. For these reasons, the combined cycle power plant has become an extremely common configuration for new generating capacity. Though such arrangements were first conceived and built in the mid-1950s, it was not until the mid-1970s that advances in high-temperature materials and air cooling of gas turbine blades made the combined cycle power plant commercially viable.