Exergetic Analysis of Triple-Level Pressure Combined Power Plant with Supplementary Firing

Abstract

This work presents a thermodynamic analysis of a triple-level pressure combined power plant with supplementary firing. The supplementary firing is a technological development for improving and increasing the output power of a combined-cycle (CC) power plant. In Mexico, there is a plant operating with this configuration, which generates a net power of 1,170 MW, with a power ratio of the gas turbine (GT) to the steam turbine (ST) of 1.35∶1. This plant is composed of four combined-cycle units with supplementary firing, each providing an output power of 292.5 MW. The energetic and exergetic analyses of the combined cycle with and without supplementary firing are pursued, taking into account the combustion process of natural gas and humid air. These analyses are also carried out for the heat recovery steam generator, considering the waste gases of the gas turbine and of the supplementary firing as the energy sources. The obtained results are fuel, steam, and humid air flows; excess of air; gas and steam turbines output works and powers; thermal efficiencies; irreversibilities; and exergetic efficiencies of the main components of the plant. This analysis is carried out, considering the following ambient conditions: 23°C, 101.3 kPa (1.013 bar), and 90% relative humidity. The computed results show that the CC with supplementary firing increased the output power by 12.2% and the efficiency of the heat recovery steam generator by 2.07% although the thermal efficiency of the combined cycle decreased by 1.4% compared with the scenario without supplementary firing.