Simple cycle gas turbine engines suffer from limited efficiencies and consequential dominance of fuel prices on generation costs. Combined cycles, however, exploit the waste heat from exhaust gases to boost power output, resulting in overall efficiencies around 50%, which are significantly above those of steam power plants. This paper reviews various types of combined cycles, including repowering, integrated gasification and other advanced systems.

Combined cycles with gas turbine engines

In a combined cycle system (10) which includes a gas turbine (12), a steam turbine (20); and at least one heat recovery steam generator (34) including evaporator(s) (38, 42, 46) operating at one or more pressures, and where condensate from the steam turbine (20) is evaporated by exhaust gas from the gas turbine (12) and returned to the steam turbine (20), the improvement comprising a closed loop cooling air circuit arranged such that hot cooling air from the gas turbine (12) is passed in heat exchange relationship with water from that one or more of the evaporators (46) in the heat recovery steam generator (34).

Closed circuit air cooled gas turbine combined cycle

Use of LHV gas in a gas turbine

Combustion turbine combined cycle plants continue to increase their role in worldwide power generation. Advanced combined cycle plants provide:• high efficiency, often in excess of 50%• short project schedules• reasonable cost with minimum environmental considerationsFor reliable operation of these facilities, it is essential the Heat Recovery Steam Generator (HRSG) design adequately address issues to include:• required load rangeability• transient operation, particularly start-up and shutdown• low life cycle costs through high availabilityThis paper will address the more detailed aspects of the design configuration of large HRSG units.Copyright © 1993 by ASME

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Large combined cycle hrsg units impact of operating considerations

En la actualidad, el uso de ciclos combinados para la produccion de energia electrica se esta extendiendo en mayor medida que otras tecnologias. Esto sucede en parte, por un precio moderado del gas natural y debido a que son sistemas mas eficientes y menos contaminantes que las centrales termicas convencionales. Sin embargo, estos sistemas no tienen un funcionamiento estable a lo largo del ano, ya que las variaciones en los parametros climaticos condicionan su operacion. Por este motivo, conocer el grado de dependencia que el ciclo combinado tiene respecto al clima es un factor adicional a tener en cuenta a la hora de elegir el emplazamiento mas conveniente.

El coste de las condiciones ambientales en los ciclos combinados

Combined cycle plants are becoming the most efficient power plants in our grid systems. Their net plant efficiencies will be in excess of 50% relating to net plant heat rates below 6800 Btu/kWh. Such excellent thermal performance would put them into the base-load plant status for most load dispatchers. This excellent performance requires, however, clean fuel such as natural gas or {number sign}2 fuel oil. Since these fuels might not be readily available and most probably will become more expensive in the future, it is our recommendation to build all combined cycle plants as base-load units with the capability of load cycling and two-shift duty operation. Building combined cycle plants with integrated coal-gasification systems eliminates the need for clean fuels, but the efficiency of coal-gasification plants is about seven percentage points lower than the plant efficiency of combined cycle plants operating with natural gas or {number sign}2 fuel oil. This lower efficiency level becomes comparable with the performance of large conventional coal fired plants. Therefore, considerations must also be made to provide these plants with at least the capability to cycle load, but not necessarily to operate as two-shift units.

H. Termuehlen

Papers

Combined cycle power plants for load cycling duties.