Indirectly-fired cycles provide one means of using a fuel other than natural gas or distillates of various purities to generate power using a gas turbine. In a closed cycle, the fuel typically is used to heat a clean working fluid which is then expanded through a gas turbine, after which it is cooled and recompressed before being recirculated through the heating circuit. In an open cycle, the heated working fluid (usually air) is exhausted to the atmosphere after expansion in the turbine and passage through heat recovery devices. In both cases, the temperature of the working fluid may be boosted before entry to the turbine by supplementary firing of a premium fuel such as natural gas in a topping combustor. A major advantage of such indirectly-fired cycles is that the concerns arising from the use of a dirty fuel in other advanced cycles are confined to the fireside surfaces of the heat exchange equipment, whereas the gas turbine is exposed to a relatively benign environment. One limitation of such systems is that the emissions problems are the same as for a conventional coal-fired boiler although, on an power output-normalized basis, the emissions from an indirectly-fired cycle may be lower. The requirements of the potential candidate materials for the various components in the circuit are discussed, and the critical issues for each are identified.

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