My experience is that the compressor is on the same shaft as the gas turbine, and a rotary compressor is integrated into the turbine system. Basically modern gas fired turbines are aeroderivative designs based on jet (aircraft) engines. See GE turbine page:
http://www.gepower.com/prod_serv/products/aero_turbines/en/index.htm - look at the both the LMS and PGT series' performance characteristics.
It may be possible that a reciprocating pump is used in the fuel supply system. But look at GE's Compressor products page for some performance characteristics of different designs.
From GE - http://www.gepower.com/prod_serv/products/compressors/en/index.htm
Axial compressors are used for very high flow and low pressure applications. Our product range covers extensive customer requirements, from highly standardized equipment to cutting-edge compression solutions for liquefied natural gas plants.
Performance
• Inlet volume from 100,000 to 600,000 m³/h
• Discharge pressure up to 25 bar-absolute
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Vertically split compressors are designed to cover a wide range of applications and pressures. They are used primarily for high pressure applications such as ammonia, urea and methanol synthesis, refinery recycle and natural gas compression and injection.
Performance for High Pressure Applications
• Capacity range: up to 15000 m³/h (3,531,700 ft³/h)
• Pressure: up to 700 bar (10,157 psi)
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Horizontally split compressors are used for high flow and low pressure applications. The GE Oil & Gas range of products covers all market needs from standardized equipment to cutting-edge compression solutions such as state-of-the-art compressors for liquefied natural gas plants.
Performance
• Capacity range: up to 500,000 m³/h (17,658,000 ft³/h)
• Pressure: up to 40 bar (580 psi)
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Steam turbines are used in a closed (Rankine) cycle where the discharge is condensed and recycle back to the boiler, which in a combined cycle plant can be based on the discharge of the gas turbine. The CO2 cannot be condensed under the same conditions, and is therefore discharge to the atmosphere.
Another factor is that centrifugal pumps operate at lower speeds, e.g. 1200 rpm as compared to >3600 rpm power a gas turbine shaft. Gearing down would just add to capital and maintenance costs.
