L.O. Tomlinson and S. McCullough GE Power Systems Schenectady, NY

Combined-cycle power plants have demonstrated high thermal efficiency, high reliability/availability and economic power generation for application in base load and cyclic duty utility service. The single-shaft combined-cycle power generation system; in which the gas turbine, steam turbine and generator (STAG™) are installed in a tandem arrangement on a single shaft; has emerged as a preferred configuration resulting from simplicity of controls and operation and demonstrated high reliability.  This paper describes the design features, experience and operating characteristics of the General Electric pre-engineered product line of single-shaft STAG combined-cycle systems for power generation applications.

Heat recovery type steam and gas turbine combined-cycle systems are the economic choice for gas- or oil-fired power generation. Integration with environmentally clean gasification systems is extending their economic application to low cost solid fuel utilization. The features contributing to their outstanding generation economics are:

  • High thermal efficiency
  • Low installed cost
  • Fuel flexibility – wide range of gas and liquid fuels
  • Low operation and maintenance cost
  • Operating flexibility – base, mid-range, daily start
  • High reliability
  • High availability
  • Short installation time 
  • High efficiency in small capacity increments
  • Minimum environmental impact – low stack gas emissions and heat rejection

Combined-cycle power generation equipment is manufactured by GE in two basic configurations, single-shaft and multi-shaft. The single-shaft combined cycle system consists of one gas turbine, one steam turbine, one generator and one heat recovery steam generator (HRSG), with the gas turbine and steam turbine coupled to a single generator in a tandem arrangement. Multi-shaft combined-cycle systems have one or more gas turbine generators and HRSGs that supply steam through a common header to a separate single steam turbine generator unit. Both configurations perform their specific functions, but the singleshaft configuration excels in the base load and mid-range power generation applications.

Figure 1. H technology single-shaft combined-cycle
Figure 1. H technology single-shaft combined-cycle

The multi-shaft combined-cycle system configuration is most frequently applied in phased installations in which the gas turbines are installed and operated prior to the steam cycle installation and where it is desired to operate the gas turbines independent of the steam system. The multi-shaft configuration was applied most widely in the early history of heat recovery combined-cycles primarily because it was the least departure from the familiar conventional steam power plants. The singleshaft combined-cycle system has emerged as the preferred configuration for single phase applications in which the gas turbine and steam turbine installation and commercial operation are concurrent.

The first GE single-shaft combined-cycle generation units entered service in 1968. The early units demonstrated the excellent generation economics that could be achieved by these simple, efficient, reliable and compact systems. The single-shaft combined-cycle applications are increasing as a result of: the power generation industry acceptance of GE’s demonstrated outstanding operating characteristics; the development of larger gas turbines making possible large single combined-cycle units; the application of efficient but complex steam cycles (multiple pressure, reheatand steam cooling of the gas turbine) that are simplified by a unitized system. Also, the single-shaft system complements the unitized plant configuration that is most advantageous for solid fuel fired integrated gasification combined-cycle (IGCC) generation systems. Table 1, which shows the experience of GE single-shaft heat recovery combined-cycle generation systems, illustrates theincreasing application of this type of equipment.

The H Technology combined cycles, Figure 1, are the most efficient power generation equipment available today. High efficiency is achieved by the high gas turbine firing temperature with minimum use of cycle air for cooling that is enabled by closed circuit steam cooling of the turbine. The single shaft configuration is ideally suit-ed to this large, highly integrated combined cycle system.


A single-shaft STAG combined-cycle system has been optimized for each of the heavy duty gas turbines in the GE gas turbine product line. The performance ratings for the base configurations burning natural gas fuel are presented in Table 2.

In the STAG system designation, the first digit designates a single gas turbine and the third digit with the following letters designates the gas turbine frame size. The base configuration for the GE single-shaft product line is summarized in Table 3. The three pressure non-reheat steam cycle with natural circulation HRSG for the gas turbines with exhaust gas temperature of 1000 F/538 C is shown in Figure 2. The three-pressure reheat cycle that is applied with gas turbines with exhaust gas temper-ature higher than 1000 F/538 C is shown in Figure 3. Figure 4 shows the three pressure reheat steam cycle that is integrated with the steam cooling system for the H technology gas turbine. The nominal steam conditions at rated output are presented in Table 4 and the major equipment for the single-shaft product line is presented in Table 5.

The single-shaft combined-cycle units follow the typical ambient temperature effects for GE STAG combined-cycle generation systems as shown in Figure 5. The heat rate does not vary significantly with ambient air temperature but the power output increases significantly at low ambi-ent air temperature. Figure 6 illustrates typical part load performance for a single-shaft combined-cycle unit. The heat rate is near the rated value from approximately 80% to 100% load, while the gas turbine inlet guide vanes are modulated to maintain high cycle temperature. At lower load, the output is varied by variation in fuel flow with varying cycle temperature so the heat rate increases as load is reduced.

Minimum environmental impact is a hallmark of all combined-cycle systems. Exhaust emissions are low as a result of high quality combustion in the gas turbines, and heat rejected to cooling water is low since the power produced by the steam cycle is one-third of the unit power output. The Dry Low NOx combustion systems achieve NOx emissions of 25 ppmvd at 15% oxygen (43 g/GJ) or less. Water or steam injection abates NOx emissions to 25 ppmvd at 15% oxygen (43 g/GJ) when burning distillate oil fuel. IGCC systems can achieve NOx emissions as low as 9 ppmvd at 15% oxygen (16 g/GJ) with nitrogen injection or fuel moisturization by saturation.

For environmentally sensitive applications where extremely low NOx emissions are required, selective catalytic reduction (SCR) can be readily adapted to single-shaft combined-cycle systems.

SCRs require a gas temperature range lower than the gas turbine exhaust gas temperature so they are installed in the HRSG in the appropriate zone to suit their operating temperature range. The single-shaft combined-cycle does not have an exhaust gas bypass stack, so the exhaust gas passes through the SCR for reducing NOx emissions at all times. The single-shaft combined-cycle is also ideally suited for incorporation into IGCC systems which can burn coal in an environmentally acceptable.

Figure 2. Single-shaft, combined-cycle, three-pressure non-reheat steam cycle
Figure 2. Single-shaft, combined-cycle, three-pressure non-reheat steam cycle

To Read More Please Download PDF from GE Energy.

Leave a Reply