416
11 Power Gas Turbines
the SGT5-8000H. The combined-cycle efficiency is raised by the higher TIT (about
1500 °C) obtained with better TBC and improved cooling and optimisation of the
triple-pressure reheat steam cycle (170 bar/600 °C; 35 bar/600 °C; 5 bar/300 °C).
The power is 400 MW in simple-cycle mode. In combined-cycle mode, currently
(2014), the power is 600 MW and the efficiency is 60.75 %. The most recent gas turbine by Mitsubishi (2014) is the M701J, called of J-class, with power 470 MW and
efficiency 41.0 % in simple-cycle operation and 680 MW and 61.7 % in combinedcycle operation. It has a steam-cooled combustor, but air cooling in the turbine part.
The TIT is 1600 °C.
11.4.5 Steam Injection
In a combined-cycle system, steam is generated in the recovery boiler and applied
in a steam turbine. But steam injection into the gas turbine is possible as well.
Injection of steam into the combustion chamber is the most efficient way, as the
mixture of combustion gas and steam then enters the turbine at the original TIT.
This requires an increase of the fuel flow rate in order to heat the steam until the
TIT. Efficiency with steam injection is lower than with combined cycles, as the
steam cycle is less favourable, being identical to the gas cycle. The highest pressure in the cycle is far too low for the steam (30 bar compared to 250 bar), the
lowest pressure is far too high (1 bar compared to almost vacuum). The much higher
temperature does not compensate for this. Figure 11.23 renders the efficiency attained with the same assumed characteristics of the recovery boiler as with combined cycles, with the application of one pressure level within the recovery boiler
at a pressure 3 bar above the gas pressure in the combustion chamber and assuming
10 °C temperature difference at the pinch point. The gain in efficiency with respect
7KHUPDOHIILFLHQF\
3UHVVXUHUDWLR
Fig. 11.23 Efficiency of a steam-injected gas turbine (CH 4 )
11 Power Gas Turbines
the SGT5-8000H. The combined-cycle efficiency is raised by the higher TIT (about
1500 °C) obtained with better TBC and improved cooling and optimisation of the
triple-pressure reheat steam cycle (170 bar/600 °C; 35 bar/600 °C; 5 bar/300 °C).
The power is 400 MW in simple-cycle mode. In combined-cycle mode, currently
(2014), the power is 600 MW and the efficiency is 60.75 %. The most recent gas turbine by Mitsubishi (2014) is the M701J, called of J-class, with power 470 MW and
efficiency 41.0 % in simple-cycle operation and 680 MW and 61.7 % in combinedcycle operation. It has a steam-cooled combustor, but air cooling in the turbine part.
The TIT is 1600 °C.
11.4.5 Steam Injection
In a combined-cycle system, steam is generated in the recovery boiler and applied
in a steam turbine. But steam injection into the gas turbine is possible as well.
Injection of steam into the combustion chamber is the most efficient way, as the
mixture of combustion gas and steam then enters the turbine at the original TIT.
This requires an increase of the fuel flow rate in order to heat the steam until the
TIT. Efficiency with steam injection is lower than with combined cycles, as the
steam cycle is less favourable, being identical to the gas cycle. The highest pressure in the cycle is far too low for the steam (30 bar compared to 250 bar), the
lowest pressure is far too high (1 bar compared to almost vacuum). The much higher
temperature does not compensate for this. Figure 11.23 renders the efficiency attained with the same assumed characteristics of the recovery boiler as with combined cycles, with the application of one pressure level within the recovery boiler
at a pressure 3 bar above the gas pressure in the combustion chamber and assuming
10 °C temperature difference at the pinch point. The gain in efficiency with respect
7KHUPDOHIILFLHQF\
3UHVVXUHUDWLR
Fig. 11.23 Efficiency of a steam-injected gas turbine (CH 4 )