Enhancement of the bottoming cycle in a gas/steam combined cycle power plant

Safyel, Zerrin
A combined cycle gas/steam power plant combines a gas turbine (topping cycle) with a steam power plant (bottoming cycle) through the use of a heat recovery steam generator. It uses the hot exhaust of the gas turbine to produce steam which is used to generate additional power in the steam power plant. The aim of this study is to establish the different bottoming cycle performances in terms of the main parameters of heat recovery steam generator and steam cycle for a chosen gas turbine cycle. First of all; for a single steam power cycle, effect of main cycle parameters on cycle performance are analyzed based on first law of thermodynamics. Also, case of existence of a reheater section in a steam cycle is evaluated. For a given gas turbine cycle, three different bottoming cycle configurations are chosen and parametric analysis are carried out based on energy analysis to see the effects of main cycle parameters on cycle performance. These are single pressure cycle, single pressure cycle with supplementary firing and dual pressure cycle. Also, effect of adding a single reheat to single pressure HRSG is evaluated. In single pressure cycle, HRSG generates steam at one pressure level. In dual pressure cycle, HRSG generates steam at two different pressure levels. i.e. high pressure and low pressure. In single pressure cycle with supplementary firing excess oxygen in exhaust gas is fired before entering HRSG by additional fuel input. So, temperature of exhaust gas entering the HRSG rises. Second law analysis is performed to able to see exergy distribution throughout the bottoming plant; furthermore second law efficiency values are obtained for single and dual pressure bottoming cycle configurations as well as basic steam power cycle with and without reheat. It is shown that maximum lost work due to irreversibility is in HRSG for a bottoming cycle in a single pressure gas /