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The gas turbine (GT) in conjunction with a heat recovery steam generator (HRSG) and a steam turbine (ST), makes combined cycle power plants (CCPP) to the most efficient power generation facilities. Existing direct-fired plants can utilize this advanced cycle concept by adding a GT and a HRSG. This so-called repowering scheme makes the existing power generation facility equally efficient as modern combined cycle power plant.
There are several alternatives to combine and integrate a gas turbine into an existing steam power plant. A detailed assessment of the existing plant equipment as well as technical and economic boundary conditions of a specific project determines the optimal approach and the selection of the repowering concept.
Full repowering replaces the original boiler with a topping cycle consisting of one or more GTs and HRSGs. As the repowered cycle is similar to a modern combined cycle, this concept promises the best efficiency compared to all possible repowering schemes. It is widely used with old plants when the boiler has reached the end of its lifetime.
In the parallel repowering concept, the original boiler will remain in operation. Additionally to the original steam source, a topping cycle will be added. Compared to the full repowering concept, this repowering scheme achieves slightly lower efficiency. Due to the two independent steam sources for the steam turbine, this concept provides a higher fuel flexibility and also greater flexibility in respect to load variations.