Projected Costs of Generating Electricity - OECD Nuclear Energy ...

Gas-fired power plants
Combined cycle gas turbines
All gas-fired power plants presented in this study are based upon the use of combined cycle gas turbine.
Gas turbines, also known as combustion turbines, have been in existence since World War II, when they
were developed for use as aircraft engines. The engine exhaust stream is sufficiently hot that it may be
used to raise steam for electricity production from a steam turbine. The combination of gas and steam
turbines is called a combined cycle gas turbine (CCGT). Gas turbine technology benefited greatly during
the 1980s from developments in military jet aircraft engines and the increased availability of gas and
liberalisation of gas markets in many countries. Currently, CCGT plants have thermal efficiencies of 50
to 60% (LHV).
More than other power plant types, gas turbines thermal efficiencies are affected by ambient temperatures.
As ambient air temperature increases, plant output decreases due to reduced mass flow through the
turbine itself. As with other plant types, the design point efficiencies are typically higher than the average
efficiency obtainable on an annually averaged basis because of variations in ambient conditions and in
off-design point operating regimes.
Heat recovery steam generators have typically used two pressure levels of steam in order to maximise
the heat recovery from the gas turbine exhaust stream. Boilers on advanced turbines will take advantage
of higher exhaust stream temperatures by using three pressure levels of steam.
Gas turbines are compact devices and are produced in factory series. The boilers used to recover heat
from the turbine exhaust (heat recovery steam generators) and the steam turbines are also relatively standardised.
The use of standardised components allows manufacturers to market modular power plants with
reduced design and construction costs.
Pollution control systems
Natural gas normally has little or no sulphur and little or no particulates. Therefore flue gas desulphurisation
systems are not needed. However, as with coal-fired plants, nitrogen oxides are produced during
the combustion process. Low-NO X burners can partially reduce the production of NO X in gas turbine
combustors and are now almost standard on new turbines. Injection of steam or water into the combustors
can also be used to reduce NO X production, but this reduces thermal efficiency and so is less common
in new machines. In areas where strict NO X emission regulations are in effect, additional measures
are normally needed. Post-combustion systems, mainly selective catalytic reduction, can be used.
Advanced gas turbine plants
Advanced gas turbines currently under development, such as so-called “G” designs, will have
efficiencies approaching 60% through the use of high combustion temperatures, steam-cooled turbine
blading, and more complex steam cycles. A number of advanced power cycles involving gas turbines are
under development or being demonstrated. These aim to maximise the efficiency of the steam cycle or to
integrate more tightly the gas and steam cycles. Examples are the HAT and CHAT cycles (humid-air and
cascaded humid-air turbine); intercooled, reheat turbine cycles; STIG cycle (steam injected), Kalina cycle
(ammonia/water steam cycle); and thermochemical exhaust heat recovery.
CO 2 capture and storage
CO 2 capture and storage (CCS) involves three distinct processes: first, capturing CO 2 from the gas
steams emitted during electricity production; second, transporting the captured CO 2 by pipeline or in
tankers; and third storing CO 2 underground in deep saline aquifers, depleted oil and gas reservoirs or
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