This paper challenges the conventional method of fuel-based bottoming cycle power augmentation in a combined cycle plant, in which a fuel source is combusted in the hot flue gas stream internal to a combined cycle HRSG - also known as supplementary firing or duct firing. Although duct firing is an effective means of increasing plant capacity, it significantly reduces the plant efficiency. Additionally, as the world fuel markets continue to incur a substantial increase in demand, power plant owners and operators are more actively seeking plant solutions that provide better performance flexibility.
To provide a solution that would allow plant owners better dispatch options, a system was developed that provides base load outputs with maximum efficiencies as well as incrementally selectable peaking outputs with high plant efficiencies. Termed as Complementary Fired Combined Cycle (CFCC), this system is predicated on the use of fractionally sized gas turbines, with their exhaust ducted into the HRSG(s) associated with their base GT(s). This configuration offers very high peak loading efficiency as well as the possibility to increase the level of power augmentation due to its unique impact on the HRSG. This system can be applied to new unit construction, and also has the potential to be retrofitted into plants with and without existing duct firing systems.
This paper explains the Complementary Fired Combined Cycle plant design concept and compares its plant performance characteristics with conventional duct fired plants. Retrofitting applications are also explored. Ancillary advantages of the CFCC plant are enumerated, along with economic comparisons of plant Life Cycle Costs.