Study on hydrodynamic characteristics of a subcritical CFB boiler with natural circulation at low loads

Circulating fluidized bed (CFB) boilers are expected to operate at very low loads thus meeting the deep peaking needs of the grid. At this time, the hydrodynamic safety of boilers at low loads is a key concern. In this study, hydrodynamic characteristics of the evaporating heating surface of a 300 MW subcritical CFB boiler were studied under wide loads, based on establishing a hydrodynamic calculation model. The results show that as the boiler load decreases from 100% to 1%, the total circulating water volume shows different trends in the constant- and sliding-pressure load ranges. With the load decreasing from 30% to 1%: the circulating water velocity of loops decreases from 1.01–1.41 m/s to 0.30–0.45 m/s, the circulation ratio increases from 14.17–28.48 to 160.36–289.31; the coefficient of mass flow rate deviation enlarges from 0.848–1.180 to 0.798–1.221. At extremely low load of 1% BMCR, the circulating water velocity in the water wall falls below the minimum value (0.40 m/s) to prevent dirt deposition, and 64% of loops fail to meet the safety verification to prevent water circulation stagnation, indicating that circulation stagnation and dirt deposition should be concerned as boiler load decreases to a very low level.