Hydration of Partially Sulphated CFBC Ash With Saturated Steam

Abstract

The hydration of partially sulphated fluidized bed combustion (FBC) ash with saturated steam was carried out in the laboratory. The ash samples were obtained from a commercial-scale 165 MWe circulating fluidized bed combustor (CFBC) firing a petroleum coke and coal blend. Both bottom ash and fly ash were tested, and in addition the bottom ash was also separated into five size fractions and tested. These solid streams and the “as-received” fly ashes were hydrated by steam produced in a pressure bomb for different lengths of time at different saturated temperatures. Samples of the ashes were analyzed for free lime and calcium hydroxide content before and after the hydration process. Scanning electron microscopy (SEM) with an energy dispersive X-ray system (EDX) was employed to determine physical characteristics of the samples. X-ray diffractograms (XRD) were also used to determine the phase composition. These results show that after hydration treatment with saturated steam at elevated pressures, the unreacted CaO in the partially sulphated material can be quantitatively converted to Ca(OH)2. However, the free lime content is also observed to change throughout the hydration process, which indicates that the hydration of CaO is not the only reaction occurring in this system. It is also clear that for fines, i.e., fly ash and <75 μm size fraction bottom ash, the effectiveness of the hydration depends much more strongly on hydration time and temperature than for coarser ashes and it is also clear that the behaviour of each particle size fraction is different.