Study On Thermal Preactivation And Hydration Of CaO/Al₂O₃and CaO/MgO

It is believed that climate change is being exacerbated by increased concentrationsof CO₂. Calcium looping cycle is an extremely promising method of removing CO₂from flue gas. Unfortunately, the reactivity of calcium based sorbents decrease rapidlyover a number of cycles. Thermal activation and hydration with steam are two mostpromising methods to reduce the rate of decay in reactivity. In this paper, the synthesisof calcium based sorbent CaO/Al₂O₃and CaO/MgO are pretreated at high temperatureand hydrated with steam respectively. Thermogravimetric analyzer and scanningelectron microscope were used to study the influence of heat treatment conditions on thecyclic calcination／carbonation reactions，and the calcination conditions on circulationcharacteristics of preheated sorbents and the influence of hydrated conditions on thecyclic reactions.It is found that self-activation happened under various experimental conditions forCaO/Al₂O₃, while CaO/MgO was quit the contrary. To explain the result, sorbentskeleton changes need to be considered. Because of the existence of Al₂O₃in thesorbent, an inward hard skeleton grew up through thermal pretreating which keeps theparticle morphology stable. Although the inward hard skeleton was formed inCaO/MgO, this kind of skeleton structure is not stable in the subsequent cyclicabsorption reaction process. The CaO/Al₂O₃showed best cyclic characteristics whenpreheated under1000℃for2h in air and the maximum cyclic conversion ratio is87%.For the CaO/Al₂O₃, varying degrees of self-reactivation happened in100%N₂,100%CO₂and air preheating atmosphere.After pretreating deactive CaO/Al₂O₃and CaO/MgO with steam, both cyclicconversion can restore to as high as the fresh sample. The best hydration temperature isassociated with the decomposition temperature of Ca（OH）₂, sorbents showed best cycliccharacteristics when the steam temperature is100℃. The higher the frequency, thehigher of the average cycle conversion rate, but the increase of the hydration frequencywill lead to the increase of the system energy consumption. Research results show thatthe reason why the deactivation absorbent showed reactivation has nothing to do withthe sorbent additives, but because the releases of H2O improved the internal porestructure of the sorbent particles when Ca（OH）₂decompose, and sorbent activityrecovery.