COMBINED CYCLE OVERVIEW
This article reviews the main components in a combined cycle plant, namely, gas turbine, steam turbine generator, and HRSG.
A combined cycle facility consists of one or more gas turbines, one or more heat recovery steam generators (HRSGs), and one or more steam turbines. These can be combined in many different ways. The most common is to have two gas turbines, two HRSGs, and one steam turbine. This mode is referred to as a 2 x 1 combination. There are facilities designed in 1 x 1, 3 x 1, and even 4 x 1. There is really no limitation on the combinations.
'''Figure 1''' is a block diagram of a typical combined cycle facility. '''Figure 2''' shows an "F" series combined cycle facility. The term ''combined cycle'' refers to a facility that incorporates both the Brayton and Rankine cycles.
The '''Brayton cycle''' refers to the energy cycle that includes power generation from the operation of the gas turbine generator (GTG). During GTG operation, combustion air is compressed in the GTG compressor section. The air is mixed with fuel in the combustion section and ignited. The resulting hot gases are expanded through the GTG turbine section, driving the generator and converting rotational energy to electrical power. This is referred to as a "simple cycle" arrangement. In a simple cycle machine, a large amount of energy (hot exhaust gases) is lost to the atmosphere.
The '''Rankine cycle''' refers to a typical steam/water cycle. Heat input to the steam/water cycle is derived from the hot exhaust gases discharged from the GTG. The HRSG transfers the exhaust heat to feedwater, which is converted to steam. The steam is directed to the steam turbine generator (STG) where it is expanded, driving a generator and converting rotational energy to electrical power. The exhausted steam is condensed in the condenser and the condensate pumps return the condensate to the HRSG. Heat is removed from the cycle in the condenser by the circulating water system.
'''Figure 1: Basic Combined Cycle Facility'''
'''Figure 2: 2 X 1 Combined Cycle Plant'''
This section describes the following main components within a combined cycle facility:
'''Figure 3''' shows a simplified block diagram of a gas turbine.
'''Figure 3: Simplified Block Diagram of a Gas Turbine'''
Each gas turbine and generator typically uses separate and packaged off-base modular skids for the accessory systems. A summary of the various prepackaged skids may include:
Additional support systems may be used for the operation of each gas turbine. The support systems are installed on off-base skids located adjacent to the gas turbine and are required to be in service during unit operation:
'''Figure 4''' depicts a typical drawing of a large steam turbine.
Each ST typically uses separate and packaged off-base modular skids for the accessory systems. A summary of the various prepackaged skids may include:
'''Figure 4: Typical Steam Turbine Cutaway'''
Each facility is equipped with an individual heat recovery steam generator (HRSG), located at the exhaust end of each gas turbine. During plant operation, the gas turbine discharges a high volume of exhaust flue gas containing a considerable amount of thermal energy. The HRSG reclaims the exhausted thermal energy for the purpose of generating superheated steam for use in the steam turbine generator. Efficient reclamation of the gas turbine exhaust prevents wasted energy, resulting in a significant increase of overall plant efficiency. The installation of the HRSG gives the plant its ’combined cycle’ status, in that it represents a major component of the Rankine cycle portion of the power plant. '''Figure 5''' shows a typical HRSG.
'''Figure 5: Typical HRSG Diagram'''
Additional components of a combined cycle facility include or may include: