Introduction to Heat Recovery Steam Generated (HRSG) Technology

The acronym HRSG (Heat Recovery Steam Generated) is in different sources describing the operation of cogeneration and heating plants, but what does it mean? Heat Recovery Steam Generated (HRSG) technology is a recycling steam generator which uses the heat of exhaust from a gas turbine to generate steam for a steam turbine generating electricity.

The simplest scheme of a Combined Cycle Gas Turbine (CCGT) is presented in Figure 1.

The simplest scheme of CCGT
Figure 1: The simplest scheme of CCGT.

In Figure 1, the exhaust flue gases temperature on the outlet of the turbine is equal to 551.709 ℃. This is a too high a temperature to release the gasses into the environment. The excess heat is able to be disposed of while receiving additional electric power which is approximately equivalent to 30% of the capacity of a gas turbine.

To reach the maximum economical and eco-friendly criteria possible for the installation, many pieces of equipment are used including: a waste heat boiler (HRSG); turbines with a selection for a deaerator (Turbine With Extraction, Deaerator); feed and condensate pumps (PUMP2, PUMP); a condenser (Condenser); and a generator (Generator 2). Exhaust gases entering into the HRSG transfer heat to water which is supplied by the condensate pump from the steam turbine condenser to the deaerator and further by the feed pump to the HRSG. Here boiling of water and overheating of the steam occurs. Moving further, the steam enters the turbine where it performs useful work.

Thanks to the use of such devices, the thermal efficiency of the installation increases, and as a result, this positively affects the economic component of enterprises using them.

For our example in Figure 1, the gas turbine has a capacity of 300 MW, the establishment of HRSG allows you to increase the efficiency of the installation.

In addition to the economic benefits, the ecological benefits should also be noted, since the flue gases at the exit from the gas turbine contain unburned fuel components such as nitrogen oxides NOx, CO, and others. To neutralize NOx, a SCR (Selective Catalytic Reduction) catalyst is used, which neutralizes and converts nitrogen oxides to nitrogen (N2) and water (H2O) by using NH3 injection [Figure 2].

Key elements for flue gas cleaning - Fig 2
Figure 2: Key elements for flue gas cleaning.

For example, the exhaust gases of energy gas turbines have a fairly high temperature, and have a volume concentration of O2 in them is at 13-16%, which means this gas can be used as a low-activity oxidizer of the combustion process. Burning a certain amount of fuel is recommended to increase power and stabilize the parameters of the generated steam in the boiler.

Design Component of a HRSG System

A successful HRSG is comprised of the following components.

  1. Duct (Duct-burner): Provides transfer of flue gases through the HRSG path (installation of an auxiliary burner is possible here)
  2. Superheater: Used for the overheating of steam inside it.
  3. Evaporator: Can be either a drum or direct-flow type.
    • – Drum typeThe drum has three major functions. Firstly, a drum is needed to supply saturated water to the evaporator circuit. Secondly, a separation device installed inside it separates water from water vapor and prevents transfer water droplets to the superheater and then to the steam turbine. And thirdly, continuous blowing takes place from the boiler drum. As a result, the process of removing part of the boiler water from the upper water level takes place. Due to this, the salt balance of the boiler water is maintained. Therefore, the requirements for the quality of the feed water at the inlet to the boiler drum may be decreased. It should also be noted that this type of boiler can have up to three circuits (pressures). HRSG with two or more pressures may have an intermediate superheater. [Figure 3]
      Two Pressure
      Figure 3: Two pressure HRSG
    • – Direct-flow type: A direct-flow boiler does not include a drum in its design. Water passes through evaporation tubes once, gradually turning into steam. The region where vaporization finishes is called transitional. After the evaporation tubes, the steam-water mixture (steam) enters the superheater. Once-through, boilers very often have an intermediate superheater. Such boilers operate at both subcritical and supercritical pressures.
  4. Economizer: is designed for heating water.­

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 There are three general configurations for HRSG.

  1. Single-circuit (1-pressure) configurations are generally used at small and medium capacities, with an exhaust gas temperature equal to 450 – 550 ℃.   The simplest circuit of a single-circuit CCGT unit was considered above in Figure 1.
  2. HRSG two-loop circuits (two pressure) are the most common [Figure 3], but regulating of them are much more difficult to achieve. There is a pinch economizer, evaporator, superheater for each circuit. There is the smallest temperature difference between heating gases and water. Usually, the smallest temperature difference is at the outlet of the evaporator. The temperature difference is quite significant at the exit from the HRSG block, and if you need to dispose of it, another block is added. In order to organize the same pinch in the next block, the mass flow rate through it must be different. The efficiency of a CCGT with two circuits increases by increasing the temperature and the high-pressure steam. The limit value of this temperature is determined by the temperature of the flue gases. These circuits are used when deeper cooling is needed than with single-circuit HRSG can provide.
  3.  HRSG three-loop circuits (three pressure) is able to increase plant efficiency further. These configurations are used for powerful energy gas turbines with high exhaust gas parameters where temperatures exceed 580 ℃.

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­Conclusion: 

HRSG use allows for up to a 30% increase in the power from a gas turbine. Additionally, HRSG can be equipped with different devices which eliminate harmful substances in flue gases. But  including these environmental safeguards creates additional hydraulic resistance at the outlet of the GT. This can significantly reduce its efficiency.

The decision to add HRSG can be made only after a careful analysis. To do this critical calculation, use proven software products, such as AxCYCLE ™, which greatly simplify this complicated task. For more information, contact us at Info@SoftInWay.com 

 

References:

  1. http://www.ccj-online.com/take-control-of-your-catalyst-management/ (Figure 2)
  2. Stepanov DV Boiler installations of industrial enterprises / Stepanov DV, Korzhenko ES, Bodnar LA – Vinnitsa, 2011, VNTU
  3. https://en.wikipedia.org/wiki/Heat_recovery_steam_generator (Figure 3)