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.

Simple cycle gas turbines have long been used to provide peaking power generation due in part to their fast response time. Some owners have operated their combined cycle units in a similar mode and many of these units undergo hundreds of starts a year. In addition, nitrous oxide (NOx) emissions limits often require that the units be ramped quickly to approximately 50% of full load, or higher, imposing thermal strains on the HRSG.

HRSGs, sometimes referred to as waste heat boilers, have long been used in industrial applications to recover heat from exothermic processes and to generate steam for use elsewhere in the process. The industrial units differ from their power generating counterparts. The industrial HRSG is typically in constant operation (does not cycle) and the steam is ultimately used for plant processes or to feed a non-condensing steam turbine before use in the process.

The HRSGs used for power generation, often more complex and operating at higher pressures than their industrial counterparts, may have up to three steam drums each operating at different pressures in addition to superheat and reheat steam sections. The newer units may also have significant makeup water demands caused by 1) the unrecoverable use of steam in NOx-emissions control; 2) the use of high purity water for air cooling and direct injection into the gas turbine; 3) the loss of steam that may be exported at a cogeneration facility.

Closely spaced finned heat exchange elements in the HRSG produce exit gas temperatures that are much lower than those from a conventional boiler. The final tube bundles typically act as a feedwater heater but, at those low temperatures, some or all deaeration must be accomplished in the condenser under vacuum. If a discrete deaerator (DA) is provided, it is often located directly on top of the low pressure (LP) drum, in effect using the LP drum as the DA storage tank. In these cases, steam for deaeration is provided by the HRSG itself. This arrangement can result in excessive dissolved oxygen concentrations in the feedwater during startup when neither pressure nor vacuum deaeration is yet possible. Another configuration employs a completely separate pressure deaerator and feedwater storage tank with steam provided from the LP drum.

Combined cycle plants designed strictly for power generation typically are constructed with all- ferrous materials such as carbon steel, low alloy steel and stainless steel. Cogeneration plants often have copper alloys as well as ferrous alloys in the steam/condensate cycle. Plants with a mixture of ferrous and copper-bearing alloys in the steam/water cycle are referred to as mixed metallurgy cycles. Since the optimal treatment regime is different for all-ferrous and mixed metallurgy systems, chemistry limits are generally segregated by these two generalized categories of system materials. Titanium may be present in any of these configurations without affecting the choice of the treatment regime.

Though originally designed to absorb heat from gas turbine exhaust only, many HRSGs are being built for the regular use of duct burners that can generate significantly higher temperatures than those typical of turbine exhaust gas. The use of duct burners can change the operating pressure of the HRSG and more than double the steaming rate.

Considering their relatively short operating history, HRSGs have seen more than their share of failures. While a few of the common failure mechanisms are strictly mechanical, most have a significant water/ steam chemistry component. The reliable operation of the combined cycle power plant is predicated on stringent control of water and steam chemistry throughout the steam cycle.

 
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