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The world’s first operational steam cooled gas turbine, built by MHI, was
commissioned in Japan in 1998 and installed in Unit 4 at the Higashi Niigata
Power Station. In the United States, it was not until early 2001 that Siemens-
Westinghouse achieved commercial operation with a 360 MWe power plant
located in Massachusetts followed immediately by a second 249 MWe plant in
Florida. The gas turbine installed in the American plants is considered
“partially” steam cooled, with just the first stage vanes being incorporated
within a closed loop. Whilst specific teething problems were found during
start up conditions of the W105G gas turbine, a cycle efficiency of ~58%
(LHV) was achieved with this technology.
More recently General Electric installed their first H-class unit at the Baglan
Energy Park in South Wales. This unit, which features steam-cooled rotor and
stator vanes, is currently under commissioning tests with engineers aiming to
break the 60% (LHV) cycle efficiency barrier.
4.3.2 Fuel Heating
In the late 90’s methods of heating fuel prior to combustion in the gas turbine
were being introduced to the combined cycle in order to enhance efficiency.
Preheating of the fuel results in a reduction in the amount of fuel needed to
achieve a given firing temperature in the gas turbine. However, whilst the
efficiency of the cycle improves, the plant power output is found to reduce
slightly. This originates from the fact that when a gas turbine is fed warmer
fuel, it requires less mass flow of that fuel to obtain the previous cold-fuel
firing temperatures, thus the exhaust mass flow and water vapour content of
the combustion products is lower. Less power is therefore obtained from the
combustion gas expansion through the turbine. Furthermore the HRSG
generates a little less steam from the decline in gas turbine exhaust mass flow
and hence a drop in steam turbine power also occurs. However, the overall
improvement in the cycle efficiency results from the fact that the energy
diverted from producing steam power is of relatively low grade, and is better
employed as a heating medium for the fuel.
The fuel heating source may be either steam or water. For the case of steam
this can originate from the steam turbine bleed or directly from one of the
HRSG pressure level circuits. For the case of water heating, the hot water is
drawn from the HRSG economisers.
There is a threshold at which the benefits associated with the increase in
efficiency are found to be at the expense of the level of electrical power
produced [62] . For example fuel heating to around 200°C from an intermediate
pressure economiser exit water source on a typical three pressure reheat
combined cycle, results in a net heat rate gain of about 0.6% with a
corresponding net power loss of about 0.3%. If the temperature of the fuel
was raised to around 300°C by a high pressure economiser exit water source,
the loss of power becomes more evident at 0.75% whereas the noted increase
in the heat rate gain is less apparent as it only rises to 0.8%.
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