Enhancement of sulfur uptake by hydration of spent limestone for fluidized-bed combustion application

The reactivation of spent limestone generated by fluidized-bed desulfurization of simulated flue combustion gases was studied. The regeneration technique consisted of spent sorbent hydration by immersion in water under controlled conditions. The effectiveness of the reactivation process was assessed by re-injection of the hydrated material in a fluidized-bed reactor under simulated desulfurization conditions. At the same time, the influence of hydration on the propensity of limestone to undergo attrition was evaluated by following the changes of particle size distribution and by collection of elutriated fines during sorbent utilization in the fluidized-bed reactor. The total calcium sulfation degree of hydrated samples was nearly twice that of the original spent sorbent. Apparently, reactivation did not bring about any significant enhancement of limestone attrition upon re-injection into the fluidized bed. Mechanistic aspects of sorbent reactivation were investigated with the aid of computer-aided scanning electron microscope-energy-dispersive X-ray analysis mapping of polished cross sections of sulfated, hydrated, and resulfated limestone particles. This analysis highlighted a pronounced redistribution of sulfur in the particles upon hydration that apparently provides the key pathway to enhanced sulfur uptake.