The main application of the (Heat Recovery Steam Generators) HRSG is in Combined Cycle power plants. In these plants the power generation from the Rankine cycle part, ie. the steam turbine, is around one third of the total power generated . HRSG’s produce the steam for this.
Classification of HRSG’s is on application, design, or operation. Some of the types are described below.
Fired and Unfired
One way is to classify it based on the heat input.
Normally HRSG’s do not have any additional heat input. The performance and output of the HRSG is dependent on the exhaust heat of the gas turbine. At part loads, this leads to reduced output from the HRSG’s. In addition, ambient conditions also affect the Gas Turbine performance. This could affect the downstream process were the steam is used. To avoid such situations supplemental firing of oil or gas takes place. Even though this may not be an efficient process, it avoids costly production disturbances.
Supplemental Firing takes place in burners in the gas duct at HRSG inlet. Oil or gas is the supplementary fuel. Since the flue gas at exhaust of a Gas Turbine is high in Oxygen content, additional air is not required for combustion. This eliminates the need of Forced draft or Induced Draft fans.
Vertical and Horizontal Types
Another classification is on the construction or design of the HRSG. Based on the gas flow it can be vertical or horizontal.
- Vertical types have gas flow vertically upward with coils placed horizontally.
- Horizontal types have gas flows horizontal with coils placed vertically.
From the performance and cost point of view both are the same. More than the technical issues it is a proprietary design of individual manufacturers or client preferences. Some of the differences are :
- Horizontal types require a 30 % larger footprint area.
- More expansion joints are required in horizontal units.
- Structural requirements are higher in vertical types.
- Horizontal types are more difficult for maintenance and inspections.
- Overall cost may be same in both types.
Multiple Pressure Operation.
Yet another classification is on the operation pressure.
- Smaller HRSG units operate on single pressure. The water to steam conversion takes place in one single pressure circuit. This is similar to conventional combustion boilers.
- In larger units, for optimizing the performance of the HRSG, steam generation takes place in multi pressure circuits. The current optimum is to use three pressure levels. (Details in the next article)