Investigation on the SO2 uptake by hydration products of fluidized bed combustion waste

The behaviour of synthetic preparations of ettringite, Ca(OH)2 and their combinations, in terms of in situ SO2 uptake and comminution propensity in FBC reactors, was investigated. The porosimetric characterization on the synthetic materials, when compared with that of commonly-used limestones, shed light on their good potential for SO2 uptaking. The desulphurizing efficiency was higher when the ettringite content in the sorbent was lower, in accordance with the lower free-CaO content attainable from ettringite. The attrition/fragmentation tendency of sorbents richer in ettringite was larger, consistently with the greater dehydration-induced steam release related to the higher content of bound water into ettringite. XRD data on sulphated samples of 100% ettringite and 100% Ca(OH)2 showed that CaSO4 was the dominant phase and no residual lime was observed. The porosimetric characterization on the same samples led to very interesting results: in the case of ettringite the sulphation reaction primarily caused the plugging of the >100 nm-pores, while in the case of Ca(OH)2 the particles showed a negligible porosity in the whole pore radius range investigated. When comparing the SO2 uptake of the sorbents employed in this work, it has to be borne in mind that the desulphurization efficiency is not only related to the degree of calcium conversion, which is independent of the free-CaO content of the sorbent and is instead connected with its structure. The relationships among pore structure, comminution tendency (fragmented non-elutriated particles are more prone to SO2 capture) and degree of calcium conversion are intriguing matters which will be further investigated.