No U.S. funding is planned to resolve these remaining issues, since other advanced power systems now offer comparable or superior performance with higher reliability, lower projected cost, lower emissions, and a much lower level of technical risk.

COMBUSTION TURBINES

Background

The combustion turbine is the key power generation component in most advanced coal-based systems. The turbine system consists of a compressor to take combustion air from atmospheric pressure to a pressure of 8 to 16 atmospheres; a combustor burning a fossil fuel (natural gas, light refined petroleum fractions, or coal-derived fuel gas) to produce hot combustion gases; and an expansion turbine to extract work as the high-temperature, high-pressure gas is reduced to ambient conditions. This system is referred to as the Brayton cycle. About two-thirds of the shaft work of the expansion turbine drives the compressor, and the remainder drives a generator to produce electricity. The net power output depends strongly on the turbine inlet temperature, which is limited primarily by materials considerations. Combustion turbines can be designed to burn any of the above-mentioned fuels (provided they are adequately free of contaminants) and to switch from one fuel to another in service.

Two types of combustion turbines are used for electric power generation, namely, heavy-frame and aeroderivative turbines, the latter derived from jet engine technology. Historically, major evolutionary improvements in aircraft jet engine technology have been adapted to heavy-frame utility combustion turbines. In addition, utilities have used aeroderivative combustion turbines for smaller-capacity generation applications. Thus, forecasting the evolution of combustion turbines for power generation is a relatively straightforward matter of assessing current jet airplane engine technology.

State of the Art

Current commercial gas turbine systems offered by U.S. and foreign manufacturers achieve firing temperatures up to 1300 °C (2350 °F), with unit sizes up to about 250 MW. Natural gas and light petroleum liquids are the fuels currently employed for power generation, typically for peak or intermediate loads. The simplest, lowest-cost-per-kilowatt, fossil power plant for peaking duty is the simple Brayton cycle combustion turbine described above. Both aeroderivative and heavy-frame turbines are used this way. Aeroderivative turbines are more efficient in simple-cycle operation because jet aircraft engines are intended to extract maximum energy from the hot combustion gases during the turbo-expansion.



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