Reuse of Fly Ash from a Fluidized Bed Combustor for Sulfur Uptake:  The Role of Ettringite in Hydration-Induced Reactivation

This study addresses the effectiveness of hydration-induced reactivation of fly ash from an industrial FBC to produce a sorbent suitable for further SO2 uptake during fluidized bed combustion. Ash reactivation was accomplished by water hydration at 70 °C for 48 h. Maximum extent and rate of SO2 sorption as well as attrition/fragmentation behavior of the reactivated ash were assessed. Testing of sulfur sorption capacity was carried out at 850 °C for 100 min in a laboratory-scale fluidized bed reactor under simulated combustion/desulfurization conditions. Characterization of the tested materials was based on X-ray diffraction and simultaneous differential thermal/thermogravimetric analyses. It was found that ettringite was extensively produced during water hydration and that effective enhancement of the sulfur uptake ability of the fly ash was achieved. The conversion degree of calcium to CaSO4 of the reactivated ash was as high as 92%. The very favorable performance of reactivated ash as a sulfur sorbent was mostly related to the large amount of free lime formed during thermal decomposition of ettringite. Compared with other sorbent materials, the reactivated ash material was characterized by a rather large attrition rate.