Comparison of the Dynamic Behaviour between Bubbling and Circulating Fluidized Bed Combustors

This work compares the dynamic behaviour of the flue gas side of large-scale bubbling and circulating fluidized bed (BFB and CFB respectively) boilers. For this, a dynamic model is developed and presented. The model is used to simulate two industrial units and is validated against steady-state operational data. The model is applied to investigate and compare the transient behaviour of BFB and CFB boilers of the same size given step changes in load and moisture content. Results show that for a certain load change the heat transfer to the waterwalls stabilizes faster in BFB units, yielding larger relative changes in temperature once a new steady state is established. 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 it is found to be less sensitive to changes and slower to respond, with stabilization times of around 15 minutes in contrast to the 1-8 minutes observed for the upper furnace. This behaviour is accentuated in the BFB case where all the solids remain in the dense bottom region