Operational flexibility and operation optimization of CHP units supplying electricity and two-pressure steam

Combined heat and power (CHP) is an effective way of enhancing the energy efficiency of thermal power plants. Double-extraction CHP, a typical kind of CHP unit, can supply two-pressure steam to district heating and industry users. The feasible operation domain of a double-extraction CHP unit, which has many constraints, is essential for the flexible operation enhancement and operation optimization of CHP units. In this study, a method is proposed to express the feasible operation domain of the double-extraction CHP unit with a 3D operation domain with six boundary surfaces, and the operating boundary constraints of the double-extraction unit are elaborated. Then, a reference 1000-MW double-extraction CHP case is analyzed, and the operation domains and energy consumption characteristics of three typical heating modes are analyzed. The optimal scheme of heating modes is obtained under different power loads. When the power load is between 783.09 MW and 936.06 MW, Heating Mode 4 (cold reheat steam for HLS1, hot reheat steam for HLS2) has the highest exergy efficiency. This research provides theoretical guidance and a basis for the operation of the CHP unit supplying power load and two heat loads simultaneously.