Advanced Technology Combined Cycles
By: R.W. Smith, P. Polukort, C.E. Maslak, C.M. Jones, B.D. Gardiner, GE Power Systems Schenectady, NY

Introduction
The General Electric H technology combined cycles represent the most advanced power generation systems available today. These combined-cycle power generation systems can achieve 60% net thermal efficiency burning natural gas. Their environmental impact per kilowatt-hour is the lowest of all fossil-fired generation equipment. These highly efficient combined cycles integrate the advanced technology, closed-circuit steam-cooled 60 Hz MS7001H and 50 Hz MS9001H gas turbines with reliable steam cycles using state-of-the-art steam turbines and unfired, multi-pressure, reheat heat recovery steam generators (HRSGs).

Figure 1. Advanced technology combined-cycle unit
Figure 1. Advanced technology combined-cycle unit

System Integration Overview
The STAG 107H and STAG 109H are offered in a single-shaft combined-cycle configuration. Figure 1 is a conceptual presentation of an outdoor power plant with this equipment. This configuration complements the cycle integration between the steam-cooled gas turbine and the steam bottoming cycle.

A diagram of the cycle (Figure 2) shows an overview of the three-pressure, reheat steam cycle and its integration with the gas turbine cooling system. Gas turbine cooling steam is supplied from the intermediate pressure (IP) superheater and the high pressure (HP) steam turbine exhaust to the closed circuit system that cools the gas turbine stage 1 and 2 nozzles and buckets. The cooling system operates in series with the reheater, with gas turbine cooling steam returned to the steam cycle cold reheat line. Air extracted from the compressor discharge is cooled using water from the IP economizer.

The cooled air is readmitted to the gas turbine and compressor to cool compressor wheels and selected turbine gas path components. The energy extracted from the compressor discharge air is returned to the steam cycle by generating IP steam. A fuel gas heating system utilizes low grade energy from the HRSG to improve combined-cycle thermal efficiency. Water extracted from the discharge of the HRSG IP economizer is supplied to the fuel gas heater to pre-heat the fuel gas supplied to the combustion system. The water leaving the fuel heater is returned to the cycle through the condensate receiver to the condenser.

STAG 107H/109H cycle diagram
STAG 107H/109H cycle diagram

Performance
The rating point thermal and environmental performance is summarized in Table 1 for the
advanced technology single-shaft combinedcycle units burning natural gas fuel. The graphs in Figure 3 show the ambient temperature effect on performance for the STAG 107H as well as the part load performance. The graphs in Figure 4 show the ambient temperature effect on performance for the STAG 109H as well as the part load performance. The low NOx emissions are achieved by a Dry Low NOx (DLN) combustion system. The closed circuit steam-cooling system also contributes to the low NOx emissions because a minimum of air bypasses the combustors for cooling the gas turbine hot gas path parts.

Table 1. Advanced technology combined cycle – thermal and environmental performance
Table 1. Advanced technology combined cycle – thermal and environmental performance

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