18

Gas turbine Generator Clutch

Steam turbine Condenser HRSG Air intake Stack

Auxiliary block, GT Generator bus duct Generator breaker Fuel oil block Fuel gas block Main transformer Auxiliary transformer Lube oil system Feedwater tank Electric and Control modules

Fig. 12-4 Layout of 251 MW Combined-Cycle System with Single-Shaft Design and Triple-Pressure Steam Cycle. Source: ABB

summer load with condensing steam turbine-driven chillers, or a combination of back-pressure driven chillers and absorption chillers, is a way to level loads and create a more optimal combined-cycle load profile.

turbine or combustion engine can be used for mechanical drive service. This is more prevalent in the smaller capacity applications. Many facility plants or district heating/cooling plants that have been built around the use of steam turbines to drive chillers, pumps, etc., have been retrofitted with gas turbines upstream of the existing plant and have become combined-cycle plants. Building

Stack Heat-Recovery Boiler Gas Turbine Generator Steam Turbine

Plant Gross Output = 253.8 MW Plant Gross Efficiency = 57.9%

Fig. 12-5 Basic Heat Balance for 254 MW Combined-Cycle System with Single-Shaft Design and Triple-Pressure Steam Cycle. Source: Siemens Power Corp.

Plant Gross Output = 253.8 MW Plant Gross Efficiency = 57.9%

Fig. 12-5 Basic Heat Balance for 254 MW Combined-Cycle System with Single-Shaft Design and Triple-Pressure Steam Cycle. Source: Siemens Power Corp.

Because a condensing turbine produces more power than a non-condensing turbine, a combined-cycle cogen-eration configuration using a non-condensing turbine has a lower power generating thermal efficiency than a condensing application. However, the total system efficiency and fuel chargeable-to-power (FCP) will be higher when

Figure 12-7 shows, schematically, a large 199 MW combined-cycle cogeneration system using a three-pressure waste heat boiler (WHB) at pressures of 1400, 450, and 5 psig (97.6, 32.0, and 1.4 bar), with gas-turbine steam injection. In this case, the steam turbine comprises 20% of the total system capacity.

Fuel

Compressor

Combustor

Heat Recovery Steam Generator to Stack

( Condenser J

Fig. 12-6 Combined-Cycle Cogeneration System.

steam energy that is passed on to process is credited to the energy input in the calculations.

While an extremely efficient combined-cycle plant can reach thermal efficiencies in excess of 50% (LHV), combined-cycle cogeneration plants can achieve net thermal fuel efficiencies higher than 80%. Consider the system described in the previous examples, except that 12 MW is produced from the steam turbine (with a reduced auxiliary requirement) and 360,000 MBtu/h (379,764 MJ/h) is passed on to process. In this case, the net heat rate, or FCP, is calculated as follows:

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