## EFFECT OF GAS TURBINE LOAD ON HRSG PERFORMANCE

Generally gas turbines perform poorly at low loads, which affect not only their [11, 12] performance but also that of the HRSG located behind them. Because of the low exit gas temperature at lower loads, the HRSG generates less steam and also has the potential for steaming in the economizer. Table 1.4 Shows the exhaust flow and temperature of a small gas turbine as a function of load. It should be noted that the data are typical, presented to illustrate the point that at low gas

Table 1.3 Gas Turbine Performance at Selected Ambient Temperatures

 20° F 40° F 60° F 80° F 100° F 120° F Power, kW 38,150 38,600 35,020 30,820 27,360 24,040 Heat rate, Btu/kWh 9,384 9,442 9,649 9,960 10,257 10,598 Exhaust temp, °F 734 780 797 820 843 870 Exhaust flow, Ib/h 1,123,200 1,094,400 1,029,600 950,400 878,400 810,000 VoI% CO2 2.7 2.9 2.8 2.8 2.7 2.7 H2O 7.6 8.2 8.5 9.2 10.5 12.8 O2 14.6 14.3 14.3 14.2 14.0 13.7 N2 75.1 74.7 74.4 73.8 72.8 70.8

 FIgure 1.11 HRSG performance versus ambient temperature. Gas flow shown has a multiplication factor of 0.1.

Table 1.4 Typical Gas Turbine Performance at Low Loads

 Load (%) 10 20 30 40 100
 Generator kW 415 830 1244 1659 4147 Heat rate, Btu/kWh 48,605 28,595 21,960 18,649 12,882 Efficiency, % 7 12 15.54 18.3 26.5 Exhaust gas, Ib/h 147,960 148,068 148,170 148,320 148,768 Exhaust temp, °F 562 612 662 712 1019 VoI% CO2 1.18 1.38 1.59 1.79 3.04 H2O 3.76 4.14 4.53 4.93 7.33 O2 18.18 17.78 17.28 16.88 14.13 N2 76.9 76.7 76.6 76.4 75.5

Turbine loads the HRSG performance will be poor. Note that at low loads the exhaust temperature is lower but the mass flow changes little.

The HRSG performance at 100% and 40% loads is given iN Figs. 1.12a And 1.12b. The HRSG was designed for the 100% case, and its performance was checked at 40% load using the ‘‘HRSGS’’ program. It may be seen that the economizer generates some steam. Also, the exit gas temperature from the HRSG at low load is very high compared to the normal case. This is due to the fact that less steam is generated in the evaporator and hence the flow through the economizer is also small, resulting in only a small gas temperature drop; the heat sink at the economizer is not large enough to cool the gases to a low temperature. Thus it is recommended that the HRSG not be operated at low loads of the gas turbine for long durations. If it is absolutely required, then a gas bypass damper should be used, or methods suggested in Q8.41, may be tried to minimize economizer steaming.

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