The desulphurizing ability of ettringite formed during the hydration/reactivation of FBC coal fly ash

The performance of a hydrated FBC coal fly ash as sorbent for in situ desulphurization has been investigated and compared with those of synthetic calcium hydroxide and ettringite. The sulphur capture ability was assessed in experiments carried out in a laboratory-scale fluidized bed reactor operating at 850°C under simulated desulphurization conditions. X-ray diffraction analysis, simultaneous differential thermal/thermogravimetric analyses and mercury intrusion porosimetry were used to characterize the materials tested. The hydration/reactivation of FBC residue was carried out at 70°C for 48 h with a water/solid mass ratio equal to 100, giving a hydration product mainly containing ettringite. The hydrated FBC coal fly ash showed a good desulphurizing ability due to calcium oxide generated during thermal decomposition of ettringite. When compared to synthetic ettringite, the hydrated FBC waste showed a higher degree of calcium conversion and a larger elutriation rate. Further comparison between synthetic ettringite and calcium hydroxide suggested that the progress of sulphation was reflected by a different behaviour as far as the pore size distribution is concerned.