Mechanism of Particle Breakage during Reactivation of CaO-Based Sorbents for CO2 Capture

The calcium looping cycle is being developed as a method for capturing CO2 from both flue and fuel gases. It works by using CaO as a CO2 carrier and through repeated cycles of carbonation and calcination can extract CO2 from a gas with a lower partial pressure of CO2 (e.g., exhaust stream from a power station) and provide a pure stream of CO2 suitable for sequestration. A key problem in the development of calcium looping technology is the decrease in reactivity of the sorbent with an increasing number of cycles of carbonation and calcination. The hydration of calcined sorbent has been shown to be a promising way of periodically regenerating the sorbent, so that its reactivity can be recovered, reducing the requirement to purge material from the cycle. In previous work, the reactivity of sorbents after hydration has been mainly studied by thermogravimetric analysis or in a fluidized bed with an unrealistically low calcination temperature. For this work, a laboratory-scale reactor capable of operation under more realistic conditions has been designed, built, and commissioned. It consists of a computer-controlled, resistance-heated, fluidized-bed reactor capable of temperature cycling, allowing the sorbent to be exposed to repeated cycles of carbonation and calcination within the same vessel. The sorbent is “reactivated” by hydration after a number of cycles and then exposed to further cycles of CO2 capture and release. The reactivity of the sorbent is measured from the CO2 uptake and release during successive cycles of carbonation and calcination. Preliminary tests have been completed, and these show that, for limestone reacted under mild calcination conditions, the ultimate uptake of CO2 (the carrying capacity) of cycled Havelock limestone can be more than doubled upon hydration. As the calcination conditions before hydration become harsher (the temperature is increased), the regeneration technique becomes less effective. This is also observed, although to differing extents, with La Blanca and Purbeck limestones. This is shown to be due to mass loss from the fluidized bed because of the increased friability of the hydrated sorbent. A particle breakage model has been developed to describe this phenomenon.