Other Advanced Cycles

There are a number of other advanced cycles that can be applied to gas turbine or combined cycle systems:

The steam injection cycle is similar in many respects to a combined-cycle plant. These plants generate steam from recovered heat from the basic gas turbine cycle. However, instead of using the steam to drive a second power cycle, steam injection-cycle plants inject the steam directly into the gas turbine. The increased total mass flow and energy input to the gas turbine result in significantly enhanced system capacity and overall cycle efficiency. Since steam is used in an open cycle as opposed to a closed condensing steam turbine cycle, the overall efficiency will be slightly lower than that achieved with a combined-cycle. Water usage is also considerably greater.

A regenerator cycle uses a heat exchanger, or recuperator, which transfers heat from turbine exhaust to compressor discharge air prior to combustion of fuel. Recovered heat displaces a portion of fuel that would otherwise be required, thereby enhancing overall cycle efficiency.

The reheat cycle uses an additional combustor or reheat element in which additional fuel is combusted using the oxygen present in the exhaust gas. The reheat cycle increases the thermal efficiency of the turbine cycle by increasing the average temperature of the gases doing expansion work in the turbine section.

The intercooling cycle is used to decrease the work of compression required by the gas turbine cycle by cooling the air in the middle of its compression cycle. For this purpose, two or more compressor sections are used. The intercooler is a heat exchanger through which air exiting the low-pressure compressor passes prior to entering the high-pressure compressor. Intercooling results in lower high-pressure compressor exit temperature, which allows for higher pressure ratios and, therefore, a significant increase in turbine capacity.

The humid air turbine (HAT) cycle is currently under development and is expected to be in production within a few years. In the HAT cycle, exhaust heat from the gas turbine is used to heat and humidify the combustion air. The HAT cycle will operate with intercooling and high-pressure ratios and is expected to offer a thermal efficiency of 45% (LHV) or greater. The HAT cycle is also being designed to operate with fuel from a coal gasifier.

There have also been advancements in coal-fired steam cycle plants. Development has been driven by the demand to minimize air emissions and waste production as much as for improved cycle efficiency. Fluidized-bed combustion (FBC) is a technology that has widespread appeal because of its low emission characteristics.

In an FBC unit, solid, liquid, or gaseous fuels, together with inert materials such as sand, silica, or alumina, and/or sorbents such as limestone are kept suspended through the action of primary air distributed below the combustor floor. Fluidization promotes turbulence, which makes the mass of solids behave more like a liquid. The results of FBC are lower and more uniform distribution of temperature. Fluidized-bed configurations include bub-bling-bed and circulating-fluidized-bed designs, with atmospheric- or elevated-pressure operations.

Research and development for enhancing cycle efficiency of both conventional steam and gas turbine cycle plants is largely focused on improved materials. Given that specific capacity and cycle efficiency are closely tied to increased firing temperatures, materials technology has become a limiting factor. Component development technology has centered on ceramics and advanced alloys for high temperature. Increasing pressure and temperature tolerance due to improved component material strength and turbine blade cooling will allow for improved cycle efficiency.

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