A kinetic study of CFB boiler separated bottom ashes during the steam hydration reaction

Abstract Although petcokes/coals can be efficiently combusted in a circulating fluidized bed (CFB) boiler with in-situ limestone desulfurization, only 20%–40% calcium of limestone particles is typically sulfated. Here, the CFB boiler separated bottom ashes are activated by 200 °C steam in a 15 rpm rotary drum to be recycled as the secondary desulfurization sorbent. Calcium oxide hydration reaction effectively cracks the ash calcium sulfate shell by the formation of calcium hydroxide from ash calcium oxide core and steam. The maximum conversion reaches 76% after ca. 5 min hydration time for 260 µm BA particles with an initial loading of 2 kg. The reaction kinetic study shows that the reaction has an apparent reaction order of 3.4 ± 0.5 to the concentration of core calcium oxide. The water vapor diffusion through the calcium sulfate layer is the likely to be the reaction rate-limiting step as predicted by the shrinking core model.