Extension of the Control Range of Power Units: Problems and Their Solution

Abstract

The modern structure of energy consumption enhances the nonuniformity of electrical load curves. With the more pronounced nonuniformity of daily and weekly electrical energy consumption, the requirements for the maneuverable characteristics of power units, which include the control range of the power unit load (technological minimum) and the minimum safe load of the power unit (technical minimum), become more demanding. Due to the problem of maintenance and adequate passing of the minimum of electrical loads during nighttime periods and nonworking days, large supercritical pressure (SCP) condensing power units had to be engaged in controlling the loads. This situation is topical for the Russian power industry in the absence of semipeak power units. For SCP power units, it is advisable to perform unloading under sliding pressure conditions throughout the entire steam-water path. The depth of unloading depends mainly on the reliability of the boilers, the hydraulic design of whose heating surfaces had been performed without considering operation at subcritical pressure. The possibility of application of sliding pressure unloading for SCP units was determined by ensuring reliable temperature and hydraulic conditions of the boiler heating surfaces, in which the state of the working fluid changed from subcooled water to slightly superheated steam. Unloading of drum boilers requires maintenance of reliable circulation in the furnace waterwalls and safe temperature conditions of the steam superheating surfaces. The results of the tests of various types of gas-and-oil fired once-through and drum boilers with unloading at sliding or rated subcritical pressures are presented. The reliability indicators of the hydraulic paths of the boilers and the factors limiting deep unloading of power units have been analyzed. The minimum safe loads were determined. Technical solutions for deep unloading were proposed for the hydraulic circuits of the steam-generating part of the flow path of SCP boilers.