Cascade co-optimization of the load ramp-down capability considering the transient stored energy of subcritical CFB power units

Coal-fired power plants (CFPPs) play a crucial role in ensuring a stablepower supply and maintaining stability within China's evolving electricity system. As one type of CFPP, subcritical circulating fluidized bed (CFB) power units face challenges in improving their operational flexibility. However, constraints on load regulation in subcritical CFB boilers due to inertia. By studying the transient status of internally stored energy, this paper finds that the compensation phenomenon of drum-stored energy, the insufficient difference between the thermal and chemical energy density of bed materials, and the slow variation of stored energy on the furnace side all restrict load ramp-down. Based on this evidence, this study adopts transient stored energy as a feedforward input, in addition to the passive control of water, air, and slag, to achieve active regulation of the stored energy status. Considering the adverse effect of rapid slag discharge on boiler thermal efficiency, priority is given to the active control of water and air, thereby improving the ultimate load ramp-down rate to 3 % Pe/min. With the further addition of active slag discharge control, the load ramp-down rate reaches 5 % Pe/min, although the unit's energy efficiency is reduced by approximately 0.8 %.