Theoretical research on the thermo-economics of regenerative turbine for the reheat unit

Reheating and regenerating technologies are widely used in modern thermal power units with large capacity and high steam parameters. However, with the increasing reheat steam temperature, the superheat degree of the steam extracted after the reheater increases, so that the irreversible loss inside the corresponding regenerators becomes larger and the regenerative effectiveness is weakened. Therefore, the retrofitting scheme by installing a regenerative back pressure turbine has been proposed in order to improve the system thermo-economics. In this paper, on a 1 000 MW ultra-supercritical unit was studied based on the simple calculation method for thermal system and equivalent enthalpy drop method. By installing the regenerative back pressure turbine, the steam from the high-pressure cylinder of the unit will continue to expand in the regenerative turbine to provide extracted steam for the regenerative heaters. With lower superheat degree in the extraction steam, the irreversible loss inside regenerators is reduced and the efficiency of the unit is improved. Calculation results show that the standard coal consumption rate could be decreased by 0.38 g·(kW·h)^-1 when the relative internal efficiency of the regenerative turbine is 0.86.The regenerative turbine can be used to drive the feed water pump, replacing the original feed water pump turbine. The benefits include the reduction the costs of devices and additional power available. When the relative internal efficiency of the regenerative turbine is not high enough, adopting the NO. 8 extraction as regenerative turbine's gas source could improve the operation reliability of feed water pump.