Combined-cycle power plant with double intermediate superheating of steam in a single-circuit heat recovery boiler

One of the priority tasks of improving combined-cycle gas recovery plants is to determine the optimal parameters of the gas turbine unit (GTU) cycle and the rational degree of complexity of the technological scheme of the combined plant. The most promising options in terms of thermal efficiency are binary combined-cycle gas turbines (CCGTs) of the recycling type, which include one or two GTUs with heat recovery boilers and a steam turbine. At the same time, it is important to enhance the reliability and efficiency of the steam turbine unit operating in the CCGT cycle, as the turbine efficiency does not exceed 33–36% due to low initial steam parameters, the absence of a regeneration system and the intermediate superheating of steam that has partially passed through the turbine. Equally significant is the task of increasing the efficiency of the heat recovery boiler. To improve the reliability and efficiency of a combined-cycle gas recovery unit, it is proposed to replace the low-pressure circuit in a two-circuit recovery boiler with a two-stage intermediate superheater for the secondary superheating of steam that has been spent in the turbine's high-pressure cylinder (HPC) and low-pressure cylinder (LPC). In this configuration, the steam turbine is designed with three cylinders, and additional heat exchange surface for preheating the source (or mains) water is placed in the tail section of the recovery boiler. An analysis of the operation of the PGU-200 combined-cycle gas unit at the Syzran thermal power plant was conducted, comparing performance with and without the use of two-stage intermediate superheating of steam. Additionally, the study analyzed the impact of changes in the temperature of the outside air and the pressure of the steam spent in the turbine’s HPC and LPC and discharged to intermediate overheating on the main performance indicators of a combined-cycle gas installation. Calculations indicate that double intermediate superheating of steam can increase the power and reliability of the steam turbine by improving the efficiency and dryness of the steam exiting the turbine, while also reducing the specific consumption of conventional fuel for electricity generation by 1.16% (from 231.99 to 229.29 g/(kW·h).