in the Pictures above, it can be seen that the exhaust of a gas turbine is sent towards a particular arrangement of water tubes (boiler).

 

The waste heat boilers catch this energy through a particular design and from this energy produce steam.

 

Waste heat boilers are also referred as "Heat Recovery Steam Generators", HRSG, or simply "Heat Recovery Boilers".

 

Large HRSGs are generally the convective type. But they can resemble large utility boilers with radiant
furnaces if the firing temperature is above 1,600°F to 1,700°F. The duct burner is located ahead of the HRSG,
which consists of a superheater, evaporator and economizer. Additional modules may be required in
multi-pressure units. If a constant steam temperature is required at all loads, the design philosophy is to
make sure that the steam temperature is achieved at unfired conditions.

 

In the fired condition, although steam generation is larger, steam temperature is also higher due to a higher inlet gas temperature.

 

Gas turbine HRSGs have fewer options for controlling emissions because exhaust gases are generated at the turbine. Modifications in gas-turbine combustors and steam/water injection have resulted in a low-NO, exhaust, around 40 ppmv.

 

Gas turbine HRSGs refer to NOx and CO at 15% oxygen dry volume whereas
packaged steam generators use 3% oxygen as the basis.

 

In HRGs, It is important to select a forced-draft fan to handle the desired combustion air mass flow at the
lowest density case, which results in the largest volume of air. Control methods, such as inlet vane modulation, adjust the combustion air flow to maintain desired excess air or air/fuel ratio.