414
11 Power Gas Turbines
reheat cycle, the steam cycle efficiency is about 38.5 % at gas inlet temperature of
527 °C [5]. The corresponding pressure levels are about 120, 15 and 4 bar. The efficiency values are 40 % at 577 °C and 40.8 % at 647 °C. The efficiency drops to 34 %
at 477 °C and 31.5 % at 427 °C. These efficiencies are based on the heat available in
the gas by cooling it down to 20 °C. The importance of a high inlet temperature at
the gas side is obvious. This effect is similar as with a steam cycle with combustion.
The efficiency does not attain the 43–44 % level as with a fossil fuel plant, due to
impossibility of incorporating regenerative feed water preheating and combustion
air preheating with combined cycles. Feed water preheating by extraction steam
could, in principle, be applied, but would result in a higher outlet temperature at
the gas side. Since there is no combustion, the remaining heat cannot be used for
preheating combustion air. This makes feed water preheating useless. The cited efficiencies can be improved somewhat by optimisation of the pressure levels [5], but
the highest level then becomes much higher (the optimum levels are about 220, 28,
2.2 bar), which means a higher cost of the steam generator and the steam turbine.
Figure 11.22 shows the efficiency obtained with simulation of the combined
cycles, with the efficiencies of the steam part as given above. A 96 % efficiency of
the recovery boiler is assumed. The obtained efficiency is in the order of 57 % for
pressure ratio above 20 and TIT around 1350 °C. This efficiency value is typical for
current gas turbines of the F-class (see explanation hereafter). The figure shows that
the efficiency is almost not dependent on pressure ratio. So, a high pressure ratio is
not useful, but it is not harmful either. The efficiency improves by a higher turbine
inlet temperature. This is different from the simple cycle (Fig. 11.14), where the efficiency does not benefit from higher TIT. This observation demonstrates the effect
of the better efficiency of the steam cycle with higher inlet temperature. With this
observation, we also understand the benefit from the reheat as with the GT24/26
machine of Alstom. The combined-cycle efficiency may be further improved by
closed loop steam cooling. The heat removed from the gas turbine by cooling is then
7KHUPDOHIILFLHQF\
3UHVVXUHUDWLR
Fig. 11.22 Efficiency of combined gas and steam cycles (CH 4 )
11 Power Gas Turbines
reheat cycle, the steam cycle efficiency is about 38.5 % at gas inlet temperature of
527 °C [5]. The corresponding pressure levels are about 120, 15 and 4 bar. The efficiency values are 40 % at 577 °C and 40.8 % at 647 °C. The efficiency drops to 34 %
at 477 °C and 31.5 % at 427 °C. These efficiencies are based on the heat available in
the gas by cooling it down to 20 °C. The importance of a high inlet temperature at
the gas side is obvious. This effect is similar as with a steam cycle with combustion.
The efficiency does not attain the 43–44 % level as with a fossil fuel plant, due to
impossibility of incorporating regenerative feed water preheating and combustion
air preheating with combined cycles. Feed water preheating by extraction steam
could, in principle, be applied, but would result in a higher outlet temperature at
the gas side. Since there is no combustion, the remaining heat cannot be used for
preheating combustion air. This makes feed water preheating useless. The cited efficiencies can be improved somewhat by optimisation of the pressure levels [5], but
the highest level then becomes much higher (the optimum levels are about 220, 28,
2.2 bar), which means a higher cost of the steam generator and the steam turbine.
Figure 11.22 shows the efficiency obtained with simulation of the combined
cycles, with the efficiencies of the steam part as given above. A 96 % efficiency of
the recovery boiler is assumed. The obtained efficiency is in the order of 57 % for
pressure ratio above 20 and TIT around 1350 °C. This efficiency value is typical for
current gas turbines of the F-class (see explanation hereafter). The figure shows that
the efficiency is almost not dependent on pressure ratio. So, a high pressure ratio is
not useful, but it is not harmful either. The efficiency improves by a higher turbine
inlet temperature. This is different from the simple cycle (Fig. 11.14), where the efficiency does not benefit from higher TIT. This observation demonstrates the effect
of the better efficiency of the steam cycle with higher inlet temperature. With this
observation, we also understand the benefit from the reheat as with the GT24/26
machine of Alstom. The combined-cycle efficiency may be further improved by
closed loop steam cooling. The heat removed from the gas turbine by cooling is then
7KHUPDOHIILFLHQF\
3UHVVXUHUDWLR
Fig. 11.22 Efficiency of combined gas and steam cycles (CH 4 )