A new control strategy of direct exergy balance for coal-fired power plants: Fundamentals and performance evaluation

In order to facilitate the increasing penetration of grid-connected renewable power, coal-fired power plants should operate more and more flexibly. Most of the-state-of-the-art control strategies, which are mainly based on the idea of direct energy balance, cannot perform well when coal-fired power plants change load with high load cycling ramp rate. To enhance the operational flexibility of coal-fired power plants, the control idea of the direct exergy balance is proposed in this study, which directly characterizes the work capacity balance between the boiler and the steam turbine. The new control strategy of direct exergy balance emphasizes the feedback of the deviation between the turbine exergy demand signal and the boiler exergy signal. Dynamic models of a reference power plant were developed and validated, and performances of control strategies based on direct energy balance and direct exergy balance were evaluated and compared. In the 50 %–75 % load cycling process of the reference power plant, the control strategy of direct exergy balance demonstrated markedly improvements in the load cycling ramp rate (29.2 % relatively), the load cycling comprehensive index K_p (39.4 %), and reduction in the maximum deviations (28.2 %), overshoots (59.8 %), and cumulative deviations (28.7 %) of the key thermal parameters. In conclusion, the control strategy of direct exergy balance significantly strengthens the control performance during rapid load cycling processes and attains a notable enhancement in operational flexibility and a slight improvement in energy efficiency of coal-fired power plant.