Enhancing direct-fired power plants performance by use of gas turbine technology

The maximum temperature of a gas turbine cycle is considerably higher than that of a steam turbine cycle; however, the gas turbine rejects heat at temperatures far above the ambient level. By combining the high-temperature gas turbine cycle with the medium-temperature steam cycle, a high overall efficiency can be reached. Modern combined-cycle plants obtain a lower heating value efficiency of almost 60% with relatively moderate parameters of the steam bottoming cycle, whereas direct-fired steam plants with supercritical pressures and temperatures still operate under the 50% efficiency level. Ways of improving the performance of direct-fired steam plants by integrating a gas turbine within an existing steam plant are addressed. These include utilization of the gas turbine exhaust as a heating medium, as combustion air, and as fuel. Repowering schemes for a waste incineration plant are given, along with schemes, for repowering by the use of a partial oxidation gas turbine and on externally fired gas turbine. The results of a performance analysis are presented.