The combustion (gas) turbines being installed in many of today's
natural-gas-fueled power plants are complex machines, but they
basically involve three main sections:
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The compressor, which draws air into the engine, pressurizes it, and feeds it to the
combustion chamber at speeds of hundreds of miles per hour.
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The combustion system, typically made up of a ring of fuel injectors that inject a steady
stream of fuel into combustion chambers where it mixes with the air.
The mixture is burned at temperatures of more than 2000 degrees F. The
combustion produces a high temperature, high pressure gas stream that
enters and expands through the turbine section.
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The turbine is an
intricate array of alternate stationary and rotating aerofoil-section
blades. As hot combustion gas expands through the turbine, it spins the
rotating blades. The rotating blades perform a dual function: they
drive the compressor to draw more pressurized air into the combustion
section, and they spin a generator to produce electricity.
Land based gas turbines are of two types: (1) heavy frame engines
and (2) aeroderivative engines. Heavy frame engines are characterized
by lower pressure ratios (typically below 15) and tend to be physically
large. Pressure ratio is the ratio of the compressor discharge pressure
and the inlet air pressure. Aeroderivative engines are derived from jet
engines, as the name implies, and operate at very high compression
ratios (typically in excess of 30). Aeroderivative engines tend to be
very compact and are useful where smaller power outputs are needed. As
large frame turbines have higher power outputs, they can produce larger
amounts of emissions, and must be designed to achieve low emissions of
pollutants, such as NOx.
One key to a turbine's fuel-to-power efficiency is the temperature
at which it operates. Higher temperatures generally mean higher
efficiencies, which in turn, can lead to more economical operation. Gas
flowing through a typical power plant turbine can be as hot as 2300
degrees F, but some of the critical metals in the turbine can withstand
temperatures only as hot as 1500 to 1700 degrees F. Therefore, air from
the compressor might be used for cooling key turbine
components, reducing ultimate thermal efficiency.
One of the major achievements of the Department of Energy's advanced
turbine program was to break through previous limitations on turbine
temperatures, using a combination of innovative cooling technologies
and advanced materials. The advanced turbines that emerged from the
Department's research program were able to boost turbine inlet
temperatures to as high as 2600 degrees F - nearly 300 degrees hotter
than in previous turbines, and achieve efficiencies as high as 60
percent.
Another way to boost efficiency is to install a recuperator or heat
recovery steam generator (HRSG) to recover energy from the turbine's
exhaust. A recuperator captures waste heat in the turbine exhaust
system to preheat the compressor discharge air before it enters the
combustion chamber. A HRSG generates steam by capturing heat from the
turbine exhaust. These boilers are also known as heat recovery steam
generators. High-pressure steam from these boilers can be used to
generate additional electric power with steam turbines, a configuration
called a combined cycle.
A simple cycle gas turbine can achieve energy conversion
efficiencies ranging between 20 and 35 percent. With the higher
temperatures achieved in the Department of Energy's turbine program,
future hydrogen and syngas fired gas turbine combined cycle plants are
likely to achieve efficiencies of 60 percent or more. When waste heat
is captured from these systems for heating or industrial purposes, the
overall energy cycle efficiency could approach 80 percent.
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