Addition of auxilliary fuel reduces the effective excess air in the exhaust gases,as no air is added. The fuel utilizes only the excess oxygen in the turbine exhaust. This is opposite to what happens in a steam generator,where with increase in excess air,the heat losses are more and thus efficiency is reduced.

With increased steam generation,usually the exhaust gas temperature decreases in a single pressure system. This is due to the increased ratio of steam/gas. In a conventional steam generator,the gas/steam ratio is nearly constant,while in a HRSG,the exhaust gas flow remains the same,while the steam generation increases due to auxilliary firing. The increased water flow through the economizer(with gas flow remaining same) can pull the gas temperature further down due to the increased duty.

The following table shows the performance of a saturated steam HRSG at different loads.The data were generated using the HRSGS simulation program described elsewhere.HRSG Simulation

Effect of auxilliary firing on HRSG efficiency

item	case 1	case 2	case3
gas flow,lb/h	150,000	150,000	150,000
inlet gas temp,F	900	900	900
firing temp,F	900	1290	1715
fuel,MM Btu/h(L)	0	17.3	37.6
steam flow,lb/h	22,780	40,000	60,000
steam press,psig	200	200	200
feed water in,F	240	240	240
exit gas temp,F	327	315	310
duty,MM Btu/h	22.67	39.9	59.9
efficiency,%	68.7	79.2	84.9

Note that duty refers to energy absorbed by steam.
Exhaust gas analysis=% vol CO₂=3,H₂O=7,N₂=75,O₂=15.
blow down=3%. fuel input is on LHV basis.
It may be seen that the exit gas temperature decreases as steam generation increases. Also,the additional fuel input is nearly equal to the additional duty.In case 2,the fuel input=17.3 MM Btu/h ,while the additional steam duty=17.3 MM Btu/h.
This means that the fuel to steam efficiency is 100 %. In a conventional steam generator this efficiency is only about 92-93 % on LHV basis. Engineers have to understand this important difference between HRSGS and steam generators.This concept can be used to generate steam efficiently in cogeneration plants.

GENERATING STEAM EFFICIENTLY IN COGENERATION PLANTS
In order to generate steam efficiently in plants which have both gas turbine HRSGs and packaged boilers,one should first understand the load versus performance curves for both types of boilers.

STEAM GENERATOR CHARACTERISTICS
It can be seen from the figure that:
Exit gas temperature increases with load. This is due to the increased flue gas flow through the boiler with load.
Efficiency increases and then falls off slightly. This is due to the combination of flue gas heat losses and casing losses. The casing loss remains constant in Btu/h(for a given ambient condition).However as a % of load,it decreases with load.If the casing loss is say 1 % at 100 % load,it will be 4 % at 25 % load.The flue gas losses (see ASME PTC 4.1) increase with load due to the higher exit gas temperature. The combination of these losses thus results in a parabolic shape for the efficiency curve. The efficiency peaks in the range of 60-70 % load typically.
Also note that the difference in efficiency between say 25 and 100 % loads is small,about 1.5 %.

HRSG CHARACTERISTICS
As seen earlier,the efficiency increases with load significantly.
The exit gas temperature decreases as load increases due to reasons given earlier.
Using these characteristics,the plant engineer can compute the fuel consumption at various loads for each type of boiler and load each in such a way that the total fuel consumption is minimized to the extent possible.Maximizing the HRSG steam output first will help minimize the total fuel consumption.
A curve showing steam output versus fuel consumption may be developed for different combinations of the HRSG and steam generators. This will give an idea of how to load or use the various boilers.

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Books,Software on Boilers,HRSGS by V.Ganapathy
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