Startup time improvement of combined cycle power plants based on stream splitting concept for heat recovery steam generators

Abstract

One of the main components in combined cycles is the heat recovery steam generator (HRSG) which is responsible for energy recovery from gas turbine exhaust stream and produce superheated steam. In this paper, driving force plot and exergy analysis is used to study the behavior of heat exchangers of an HRSG in base case and off design conditions. The results indicated that performance of high pressure super heaters is far from their ideal operation. Based on this conclusion, a new configuration for the HRSG is proposed to boost its performance. This new configuration is based on split concept where it splits the hot flue gas to bypass the high pressure superheats. Therefore more energy is delivered to the high pressure evaporator and consequently steam production rate increases. Another advantage of this configuration is that it reduces start up time of the HRSG and consequently decreases start up time of the plant which results in higher flexibility of the system. To examine the applicability of the proposed approach, it is used to improve start up time in an existing combined cycle power plant. The results indicate that pressure and mass flow rate of HP stream reaches to its design point almost 20 minutes sooner. Finally, a sensitivity analysis is performed on the effect of split ratio on the power production. The results show that there is an optimum value for split ratio in which power production of steam turbine is maximized.