Monday, September 22, 2008

Combustion Turbine-Heat Recovery

Combustion turbines generate a large volume of very hot air. The exhaust is also high in oxygen content as compared to other combustion exhaust streams, as only a small amount of oxygen is required by the combustor relative the total volume available.

Depending on how much thermal energy is required for the application, the turbine exhaust may be supplemented by a duct burner.

A duct burner is a direct fired gas burner located in the turbine exhaust stream. It has a very high efficiency due to the high inlet air temperature, and is used to boost the total available thermal energy. The turbine exhaust boosted by the duct burner is directed into the waste heat boiler, called the Heat Recovery Steam Generator, or HRSG commonly pronounced as 'HerSig'.

Turbine exhaust can also be ducted directly into hot air processes, such as kilns and material drying systems. This is the least costly first cost, as there is no boiler or steam system to purchase. Turbine exhaust can also be ducted directly into absorption chillers for large cooling loads.

The system will also include a diverter for times when waste heat is not needed. The diverter vents the turbine exhaust to atmosphere; this substantially reduces the system efficiency, as only the electric energy is being used. Single or Simple Cycle electric plants (typical of peaker plants) dump all of their turbine exhaust, as they have no thermal requirements. These plants generally use turbines with recuperators to maximize their electrical efficiency.

The higher the electrical efficiency of the turbine, the lower the available thermal energy in the exhaust. Newer turbines with recuperators, and larger sized turbines, tend to have higher efficiencies.

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