Flexibility enhancement for CFB power plants by coupling external ash storage system: Control strategies and performances evaluation

The rapid growth of renewables like wind and solar necessitates flexible operation of circulating fluidized bed (CFB) power plants to maintain power grid stability with accommodating variable output. Enhancement of CFB power plant flexibility via circulating ash regulation has gained wide recognition, yet the corresponding control strategies during load cycling dynamic processes remain underdeveloped. In this study, a dynamic model of the subcritical CFB power plant coupled with the external ash storage system was developed, and dynamic responses to step changes in ash storage and release flowrates were obtained. Following the heat transfer mechanism of boiler, mathematical relationships for ash storage and release flowrates with ash parameter and heat deviations in furnace during transient processes were derived, and optimal ash storage and release flowrates were mathematically obtained. Then, novel control strategies during load decreasing and increasing processes were proposed and quantitatively evaluated. It was found that the maximum load cycling rate during load decreasing processes could be enhanced from 1.0% to 2.0% Pe min−1, and from 1.5% to 2.5% Pe min−1 during the load increasing processes. With the proposed control strategies, the load cycling rate of CFB power plants was still constrained by the live steam pressure deviation, and the fuel saving was dynamically observed during the load increasing processes.