Repowering of steam power plants for medium-high increase of power generated

The repowering of steam power plants by overlapped gas turbine units is discussed. The analysis has been performed on a particular arrangement of the two power plants: steam at medium pressure is produced in a proper recovery boiler fed by the exhaust of a gas turbine unit and it expands into the medium-low pressure steam turbine of the pre-existing plant in addition to the main steam flow. This solution involves a simple layout, very few modifications to the pre-existing components and a good compromise between performance and efficiency for medium-high increase of power generated (power ratio GT/ST about 0.3 ÷ 0.5). The thermodynamic effects produced by these operations and the behavior and performance of the repowered plant are analyzed, and the results are presented and discussed.