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

EFFECT OF GAS TURBINE LOAD ON HRSG PERFORMANCE

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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