Experimental and numerical investigation on heat transfer characteristics of a circulating fluidized bed boiler during long-term hot standby operation

Harnessing the peak regulation capabilities of coal-fired power plants is essential for enhancing the consumption of renewable energy. The circulating fluidized bed (CFB) boiler has a unique feature that allows it to operate in hot standby mode, enabling the system to maintain a near-zero power output for an extended period. In this work, a field test conducted on a 300 MW subcritical CFB boiler shows that the unit could operate stably for 100 min at an ultra-low load of 1-2 MW, even when the fuel supply is interrupted. Upon reintroducing air and coal, the power output increases rapidly, achieving a load increase rate of 4.7% of its rated power (Pe) per minute. A numerical model is established to analyze temperature field and heat storage capacity. The bed temperature is found to be maintained above an ignition point of the fuel. However, temperature and pressure of superheated steam gradually decreased after 100 min, which failed to meet safety requirements, indicating that the duration of hot standby primarily depends on its ability to maintain steam parameters above safe levels. Throughout the hot standby operation, a total of 20.21 GJ and 81.90 GJ of heat are respectively transferred via radiation and conduction to the working medium. The release of stored heat occurs in the following order: refractory materials, bed materials, and heating surface metals. This highlights the crucial role of refractory materials and particles in sustaining the hot standby state. Additionally, increasing the bed temperature before hot standby operation can effectively extend duration.