Fresh Air Firing and HRSG EFficiency
HRSGs often operate on fresh air when the gas turbine is not in service and have to meet the desired steam generation.However there are two important variables,which can impact the performance; one is the mass flow of air and the other is the firing temperature.When the mass of air reduces naturally the firing temperature increases in order to generate the same steam flow and vice versa.How does this affect the efficiency? The following table based on results from the  HRSG simulation program  tells us why it is better to go for a smaller air flow and higher firing temperature.

Example:A simple HRSG consisting of an evaporator and economizer generates 22,000 lb/h of saturated steam at 300 psig sat when 150,000 lb/h of turbine exhaust at 900 F enters the HRSG.A 20 F pinch and approach was used in the design mode.Feed water temperature is 230 F and 2 % blow down is used. The off-design mode evaluates the firing temperature,fuel input and gas temperature at exit at various air flows as shown in the following table,while generating 30,000 lb/h of steam at the same pressure.

   Table 1: Effect of air flow on firing temperature,fuel input
item  gas turbine  fresh air  fresh air   fresh air
gas /air  to HRSG,lb/h  150,000 150,000 100,000 70,000
inlet gas/air temp,F  900  80  80  80
firing temp,F  900  1084 1402  1800
gas temp to eco,F  441  447  443  438
gas temp lvg eco,F  337  324  300   282
fuel input,MM Btu/h  0  40.0  36.3  34.55
[ fuel input on LHV basis]
Conclusions: It may be seen that as the air flow is reduced,the firing temperature increases,which is obvious.However the efficiency also increases as the fuel input decreases.This is due to the reduced heat losses associated with lower mass flow of air and also the lower exit gas temperature,which is the result of the lower flue gas/steam ratio.(This subject has been discussed in earlier articles on HRSG performance and also in my book on Waste Heat Boilers).Hence it is better to go for a lower mass flow of air and higher firing temperature if the HRSG and the burner design can handle it. A radiant water cooled furnace design may be required if the firing temperature is high,say exceeding 1600-1700 F.Though the initial cost may be higher,the benefit of lower fuel costs in the long run may offset it provided the operating hours in that mode are significant.An economic evaluation based on fuel costs,operating hours and HRSG cost may have to be done. Also,when the air flow is reduced significantly,the gas velocities at various sections including the burner are reduced,which may affect the performance of the burner and the HRSG and this aspect also has to be reviewed.

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