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Waste from fluidized-bed combustion (FBC) has a low potential for reuse. One possibility for its recycling lies in a hydration process aimed at reactivating the SO 2 sorption ability of the unconverted lime. The formation of ettringite, as well as calcium hydroxide, in the hydrated FBC fly ash to be reinjected into the reactor could be of importance because ettringite is able to play a chemical and physical role in SO 2 capture. The aim of this paper is to investigate the conditions under which ettringite is formed by the liquid-phase hydration of FBC waste. To this end, two industrial FBC fly ashes were hydrated at temperatures of 20 and 70 °C, for curing times ranging from 30 min to 96 h. Ettringite concentrations and hydration levels of up to 50 and 74%, respectively, were measured. The experimental data were also employed to set up a simple kinetic model for ettringite synthesis. With respect to the CaSO 4 concentration, this reaction was of first order for hydration times up to 8 h and of second order thereafter. In this work, the main characterization techniques employed were X-ray fluorescence, X-ray diffraction, and differential thermal and thermogravimetric analyses. Thermogravimetric analysis was also used for quantitative purposes.

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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.