Reactivation Studies on Six Fly Ashes From Commercial CFB Boilers

Six different fly ashes from commercial-scale circulating fluidized bed combustion (CFBC) boilers and the carbon-free residues of these ashes were hydrated with liquid water or steam to determine whether hydration could improve sorbent utilization in these samples under fluidized bed combustion conditions. After hydration, for two fly ashes (FA1 and FA6) and three carbon-free samples (FA2-A, FA3-A and FA6-A), the capacity for taking up SO₂ showed limited or medium improvement; however, hydration was evidently ineffective in reactivating the remaining samples. It is believed that the reason samples FA6 and FA6-A show a relatively high improvement in SO₂ absorption capacity is that these ashes had a larger particle size than any of the other fly ashes examined here. In general, even for these “reactivatable” fly ashes, reactivation by hydration with either liquid water or steam appeared far less promising than for bed ashes, which have been shown to exhibit significant improvement in sulphur capture during re-sulphation. Hydration, whether by steam or liquid water, is not recommended for fly ash, which has a very limited residence time in the boiler due to its small particle size and instead this paper recommends alternative strategies.