The steam parameters of combined cycle and cogeneration plants differ signifi­cantly.

Combined cycle plants typically use unfired HRSGs and generate multiple — pressure-level steam with a complex arrangement of heating surfaces to maximize energy recovery. Fired HRSGs in combined cycle plants are often the exception to the rule owing to their impact on cycle efficiency as discussed above.

In cogeneration plants, a large amount of steam is required and hence supplementary or furnace-fired HRSGs are common. With a high gas inlet temperature, a single-pressure HRSG can often cool the gases to a reasonably low temperature, so single-pressure steam generation is often adequate.

In cogeneration plants, saturated steam is often imported from other boilers to the HRSG to be superheated; steam may also be exported from the HRSG to other plants.

Combined cycle plant HRSGs often operate at steady loads, cogeneration plant steam demand often fluctuates and is a function of the process. Given below is an example of an HRSG simulation in a cogeneration plant. Note the effect on steam temperature with and without the export steam.