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Combustion Turbines / Simple Cycle / Combined Cycle

A combustion turbine is an internal combustion engine that operates with rotary rather than reciprocating motion. In stationary applications, hot combustion gases are passed across the blades of a turbine wheel at high velocity to generate shaft horsepower. The power output supplied by the rotary motion of the turbine blades is uniform, as opposed to the pulsating power associated with reciprocating engines. The primary components of a combustion turbine include the compressor, the combustor, and the turbine.

The Compressor
The turbine compressor draws in ambient air and compresses it to increased pressure ratios ranging from 5 to 30. Additional air for cooling the hot sections of the gas turbine is also drawn in by the compressor. An axial or centrifugal compressor is utilized to increase the inlet air pressure. Most combustion turbine designs incorporate axial compressors, rather than centrifugal compressors, due to their higher efficiencies and higher capacities; however, centrifugal compressors are used in some small combustion turbine models. As the air is compressed, the increasing pressure also serves to increase the air temperature. The compressed air is then directed to the combustor section, where fuel is introduced, ignited, and burned.

As shown in the figure, all combustors have four basic zones:

1. The Inlet Transition Zone
2. The Primary Combustion Zone
3. The Secondary Combustion Zone
4. The Outlet Transition Zone

The Combustor
The air exiting the compressor section first passes through the inlet transition zone (diffuser) to reduce the velocity and allow sufficient residence time for complete combustion. The air is combined with high-pressure fuel (typically 150 to 200 psi) in the primary combustion zone. First, the air is heated by passing it through the area between the combustor liner and the shroud. This configuration also serves the necessary function of cooling the combustor liner. The air enters the combustor through holes in the liner which control the amount and direction of air flow, maximizing air-fuel mixing, while minimizing pressure drop. The air-fuel ratio is maintained at near-stoichiometric ratios throughout the primary combustion zone. Additional air enters the combustor in the secondary combustion zone to complete the combustion process and to quickly cool the combustion gas to avoid thermal damage of the combustor liner. The outlet transition zone serves as an accelerator to increase the velocity of the hot combustion gas before it enters the turbine.

Types of Combustors
Three basic combustor configurations are used for combustion turbines: annular, can- annular, and silo.

Annular
An annular combustor is a single, continuous chamber roughly the shape of a doughnut that rings the turbine in a plane perpendicular to the air flow. Fuel and air enter the annulus through a number of short nozzles, which allow good distribution of temperature. Combustion takes place in the single annulus.

Can-Annular
The can-annular combustor configuration is similar to an annular combustor. However, combustion takes place in a number of can-shaped chambers arranged in an annular fashion around the turbine, rather than in a single combustion chamber.

Silo
The silo configuration refers to turbine designs in which the combustion chamber is mounted external to the main body. One or more can-shaped chambers may be mounted in this fashion in a vertical or horizontal arrangement.

The Turbine
The turbine section converts the thermal and kinetic energy contained in the hot combustion gas leaving the combustors into shaft power (mechanical energy). The hot gases are expanded through a series of blades mounted on the turbine shaft. The turbine section is generally divided into two sections according to function: the gas-producer (compressor) turbine and the power turbine. The shaft of the gas-producer turbine is connected directly to the compressor and drives all auxiliary devices. The power turbine provides the power to drive the external load (i.e., generator, compressor, etc.). The shaft of the power turbine may be an extension of the gas-producer turbine shaft, or may be independent. Single shaft turbines are generally limited to electric power generation applications where there is little need for speed variation.

Operating Cycles of Combustion Turbines
A combustion turbine is designed to operate in four different configurations, or operating cycles: simple cycle, cogeneration, combined cycle, and regenerative cycle.

Simple Cycle
A combustion turbine that recovers no energy other than turbine shaft power is called a simple cycle combustion turbine. It consists of only the three basic components: the compressor, the combustor, and the turbine. Shaft power may be used to drive a pump, a compressor, or an electrical generator. Typical cycle efficiency is in the 30 to 35 percent range based on the lower heating value (LHV) of natural gas. This cycle offers the lowest installed capital cost but also provides the least efficient use of fuel.

Cogeneration Cycle
In a cogeneration cycle, shaft power is produced as in the simple-cycle configuration; however, energy contained in the exhaust gas is recovered in a heat exchanger to produce process steam. When steam is generated, the exhaust heat exchanger is called a heat recovery steam generator (HRSG). To increase steam capacity, a supplementary duct burner can be placed in the duct upstream of the HRSG to increase the exhaust heat energy. Capital costs associated with cogeneration systems are higher than for simple-cycle turbines; however, total cycle efficiency can be as high as 75 percent (based on the LHV of natural gas).

Combined Cycle
A combined-cycle combustion turbine is very similar to a cogeneration unit; however, the steam produced by the HRSG is directed to a steam turbine. Both the combustion turbine and the steam turbine are used to produce electricity. Supplementary firing of the HRSG with the duct burners may be done to increase steam generation. Cycle efficiency can exceed 55 percent (based on the LHV of natural gas).

Regenerative Cycle
The regenerative cycle combustion turbine is a simple-cycle turbine, with a regenerative heat exchanger used to preheat the combustion air. Thermal energy from the exhaust gas is transferred to the compressor discharge air just prior to the combustion chamber, thereby, reducing the amount of fuel required to reach design combustor temperatures. Regenerators are typically applied to combustion turbines that have moderate pressure ratios (6:1 to 10:1). At higher pressure ratios, the temperature differential between the compressor discharge air and the turbine exhaust becomes minimal and efficient heat transfer becomes economically unattractive.