Detailed Information - Combined Cycle Power Plants

Detailed Information - Combined Cycle Power Plants

Detailed Information - Combined Cycle Power Plants

The location of a deaerator in the water/steam cycle of a waste heat boiler is fundamentally different to that of a direct fired power plant boiler or industrial boiler installation. In order to obtain optimal efficiency, the stack temperature of the boiler must be as low as possible. This can be achieved in different ways:

Direct fired installation:

The feedwater is preheated and a low stack temperature can be achieved by using an air heater.

Exhaust gas boiler:

If a gas turbine is used as a source of heat for the boiler, neither combustion air nor an air heater is needed. The water inlet temperature of the boiler should then be as low as possible, and thus no external water pre-heaters are used. The condensate pre-heater is usually integrated into the boiler and the deaerator itself can also be incorporated in the exhaust gas boiler. The performance of the boiler, the gas turbine load, the degree of supplementary firing, and type of boiler all influence the design of the condensate pre heaters, and the deaerator. The arrangement of the condensate pre-heater and deaerator in the system is also referred to as “cold end connection”. The heating of the condensate consists of 2 steps:

  • Heating in an external bundle
  • Heating in the deaerator (mix pre-heating) for the deaeration process.

The choice of a specific cold end connection is to a large extent determined by the necessary investments and the output to be achieved. It is beneficial for the output if the heat is drawn from the flue gases at the lowest temperature possible. On the other hand this demands higher investments. The most common cold end connection approach is listed below.

The inlet temperature of the water in the boiler must be as low as possible because the flue gases in the boiler will contain a certain concentration of water vapor and SO3, depending on the fuel type. Subsequently, depending on the concentrations of these two components and the temperature of the pipe wall in the boiler this can lead to formation of water (water dew point) and sulfuric acid (sulfur dew point)!

Because the temperature of the outer surface of the tubes at the cold end of the boiler is not more than 1 to 3 °C higher than the temperature of the medium transported in them, the water/condensate inlet temperature must be approx. 5 °C higher than the relevant dew point.

Common cold end connections

There are multiple cold end connection solutions. The following are described:

No heat extraction from flue gases

In this system the condensate from the system will be fed directly to the deaerator. LP steam from the steam turbine is used as heating medium

Condensate pre-heater in the boiler

This system is often being used in 1-stage, 2-stage and 3-stage pressure systems. Condensate to the deaerator will be routed via the pre-heater in the boiler. Steam from the boiler is used as heating medium

Condensate pre-heater external to the boiler

In this system the pre-heater is located outside of the boiler resulting in a more compact design. The condensate is fed to an external pre-heater and routed to the deaerator. Steam from the boiler is used as steam source.

Deaerator – evaporator without separate condensate pre-heater

This system uses the flue gases to extract heat for pre-heating the condensate. Condensate is fed into the deaerator directly. The evaporator produces a water/steam mixture which is used as heating source.

Deaerator – evaporator with separate condensate pre-heater

All the heat required for heating up the condensate is obtained from the flue gases. The condensate is pre-heated in the condensate pre-heater. A water/steam mixture coming from the evaporator is used as heating medium.

Deaerator – evaporator with steam export and separate condensate pre-heater

This system combines the deaerator and LP steam drum in one vessel. A portion of the steam from the LP evaporator is used as heating medium. The balance of the steam is used for export (to a steam turbine or to a process).

Economizer circulation

Also here, all heat required for heating-up the condensate is extracted from the flue gases. First the condensate is pre-heated in the boiler. Hot water from the economizer is used as a heating medium, thus controlling the condensate temperature.