| CaO has advantages of stoichiometric reactive sorption capacity of CO2, low cost, and the abundance of its natural precursors, the calcium looping of CaO carbonation-calcinations process becomes the most promising process for CO2capture process in many industrial processes, such as CO2separation from flue gas and hydrogen production of reactive sorption enhanced reforming. However, during calcium looping process, CaO-based CO2adsorbent suffers from the rapid decay of CO2sorption capacity during multiple cycles, which is the key problem restricting industrial application. Therefore, the study on the improvement of durability of CO2sorption capacity during multiple carbonation-calcination cycles have significant meaning in the environmental, chemical and petroleum industry both in theoretical and practical.In this thesis, nano CaCO3coated with TiO2, MgO and Al2O3were prepared respectively through adsorption phase reaction technique as the precursor to prepare coated modified nano CaO-based CO2adsorbent. The properties of modified nano CaCO3and the methods and mechanism of improvement of the cyclic durability of nano CaO-based CO2adsorbent were studied.Firstly, TiO2coated nano CaCO3was prepared through adsorption phase reaction technique. The effects of TiO2content and preparation temperature on cyclic durability of modified adsorbents were studied. The results showed that when the content of TiO2was10wt%, the adsorbent demonstrated both large sorption capacity and good cyclic durability. Its sorption capacity kept at4.8mol/kg after30cycles. It was found that lower temperature of0-20℃was conducive to form a relative uniform coating layer of TiO2which can improve the stability of adsorbent.MgO coated nano CaCO3was prepared through adsorption phase reaction technique. The effects of precursor of MgO, MgO content, heat treatment temperature of modified adsorbents on cyclic durability were studied. The results showed that the optimal MgO content was12wt%, when the adsorption temperature was650℃-700℃, both the stability and the sorption capacity of the adsorbent showed a significant increase. Compared with other adsorbents prepared with direct mixing or precipitation methods, the adsorbent coated with MgO prepared by adsorption phase reaction technique showed the best durability, in which, the decay rate of sorption capacity after30cycles was reduced from60%-70%to30%.In addition, Al2O3coated nano CaCO3was prepared also through adsorption phase reaction technique. The effects of Al2O3content, and heat treatment temperature of modified adsorbent on cyclic durability were studied. Although pretreated under different calcination temperatures, the modified adsorbents contained with20wt%Al2O3all showed the highest sorption capacity and relatively good stability in the cyclic adsorption-desorption process. XRD tests showed that the Ca12Al14O33was formed, which as a stable support in the adsorbent was the main reason for increased stability. Compared to adsorbents added with Al2O3by different methods, Al2O3coated adsorbents showed the best cyclic properties, not only large sorption capacity but also good stability.The compactness of coating layer was quantitatively measured through XPS method. Associate adsorption stability and coating uniformity, when compact factor was1.0, the adsorbent showed best durability. Uniformity of the coating layer was confirmed by SEM testing results. The results showed that all the TiO2, MgO and Al2O3modified nano CaCO3had reached a good coating results since the coating material relative uniformly distributed on the surface of the nano CaCO3. Although the extents of durability increased of adsorbents coated with different materials were different, the mechanism of improved stability was the same and the mechanism model was proposed. It was thought that the inert material coated on the surface of nano CaCO3that formed a support frame to prevent CaO agglomeration and sintering during calcinations, maintaining the structural stability of modified adsorbent, therefore improving the stability of the adsorbent. |
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