Comparison of the Transient Behaviors of Bubbling and Circulating Fluidized Bed Combustors

This work compares the transient behaviors of the flue gas sides of large-scale bubbling and circulating fluidized bed (BFB and CFB, respectively) boilers. For this purpose, a dynamic model of the in-furnace side of fluidized bed combustors presented and validated by the authors in a former work is used to simulate two industrial units. The results show that for load changes the heat transfer to the waterwalls stabilizes more rapidly in BFB units. Differences in stabilization time between the dense bed and the top of the furnace are observed in both units, caused by the distribution of solids along the combustor: the dense bed contains more solids than regions located higher up in the furnace and, therefore slower to respond, with stabilization times of around 15 minutes, as compared to stabilization times in the range of 1-8 minutes for the upper furnace. This behavior is accentuated in the BFB, where all the solids remain in the dense bottom region. The effect of the characteristic times of the main in-furnace mechanisms (fluid-dynamics, fuel conversion, and heat transfer) on the dynamic performance of BFB and CFB units has been explored and expressed through proposed mathematical relationships.