Observations on the Hydration Behaviour of a Selection of Bed and Fly Ashes From FBC Installations

The behaviour of FBC ash/water systems is complex and the hydration behaviour of FBC ashes attracts attention both for environmental reasons and because hydration could be used to reactivate the ashes for further use in SO₂ capture. In a recent study, hydration of 16 FBC bed and fly ashes from industrial installations firing high-ash coal and mine wastes was studied. Saturated steam at ∼165°C was employed; samples were analyzed chemically and investigated by X-ray diffraction (XRD) and thermal analysis (TGA). One of the more important results was that in some of the ashes there was no unreacted CaO and no Ca(OH)₂ after hydration, and they could even consume CaO added to them before hydration. XRD evidence was also obtained to show that a hydrated calcium silico-aluminate (katoite) was formed during the hydration of ashes high in excess, unreacted CaO. The same ashes were used in the present study. The methods used were the same, but the bed ashes were divided into three size fractions and hydration by saturated steam at 100°C was employed. The results generally confirmed the earlier findings. Differences were small but could be significant. First, no katoite could be detected after hydration, so its formation (and possibly that of other compounds of that type) may require more drastic hydration conditions. Second, small quantities of Ca(OH)₂ were detected in most hydrated samples. Third, systematic differences between bed ash size fractions were found. In particular, the proportion of anhydrite and the degree of CaO to CaSO₄ conversion steadily increased with decreasing particle size fraction, but in the corresponding fly ash was lower and for lime-rich ashes, much lower.