Turbines / Simple Cycle / Combined Cycle
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 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.
shown in the figure, all combustors have four basic zones:
1. The Inlet
2. The Primary Combustion Zone
3. The Secondary Combustion Zone
4. The Outlet Transition Zone
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
Three basic combustor configurations are used for combustion turbines: annular,
can- annular, and silo.
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.
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.
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 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.
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
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.
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).
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).
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.