HRSG characteristics:
The HRSG performance characteristics were developed using the HRSGS program (described elsewhere) from unfired to max fired conditions.As seen the fuel efficiency increases with load.The exit gas temperature decreases with load. The reasons for these are explained in detail in my books.In simple terms,the excess air is reduced because of supplementary firing(which increases the efficiency). Also,the ratio of gas/steam varies significantly with load in a HRSG unlike in a steam generator and hence the exit gas temperature for a single pressure HRSG could actually decrease with increase in load. Hence the efficiency increases as we fire more. Note that we are NOT adding air and using only the oxygen in the turbine exhaust gases. The fuel efficiency is nearly 100 % and all of the fuel energy goes into steam. In the case of the steam generator,the efficiency varies with load as shown. The exit gas temperature increases as the load increases,due to the higher flue gas quantity.However the casing heat losses are lower at higher load and the combination of these two major losses result in the parabolic curve. The maximum efficiency is reached at a load of 60 to 70 %. The LHV efficiency is about 91-93 % on natural gas,compared to 100 % for HRSGs.Armed with this information,the plant engineer can compute the total steam generation vs fuel input curve with different boilers and see which option workes efficienctly. More information can be found in my papers,books. Hence the HRSG should be utilized to the maximum extent before loading the steam generators.