The Study On The Preparation And Performance Of The Synthetic CaO-based Sorbents For CO₂Capture

The increasing concentration of CO₂in the atmosphere due to fossil fuel combustionhas been identified as a major contributor to global warming. Governments and experts inthe world all devote themselves to decrease CO₂emission amount into atmosphere. VariousCO₂capture technologies have been researched and developed. The CyclicalCarbonation-Calcination Reactions （CCRs） technology based on CaO-based absorbents hasbeen proved to be a very promising and effective CO₂capture technology. However, theCO₂capture capacity of CaO decreases rapidly after multiple cycles in the technology.Therefore, doping CaO-based sorbents with some metal oxides were synthesized toimprove the stability of CO₂capture capacity of sorbents in this thesis. Meanwhile, thesintering mechanisms of CaO was explored based on XRD analysis and FSEMmeasurement.Different kinds of CaO-based absorbents were prepared by several methods, and aseries of CaO/MgO sorbents were also synthesized using a Sol-Gel combustion synthesisprocess under various pH conditions （pH=2,4,6,8and10, respectively）. There multicyclicCO₂capture capacity was investigated by conducting many carbonation/calcinationreaction cycles in a fixed bed reactor system. The results show that the performance of theCaO/MgO sorbents made by Sol-Gel combustion synthesis methods are much better thanthat of the sorbents made by dry physical mixing and wet chemistry methods. ForCaO/MgO absorbers made by the Sol-Gel combustion synthesis method using urea, thelower the precursor solution pH value, the smaller prepared CaO/MgO absorbent particleis，also the larger surface area and higher CO₂absorption capacity.In the study of the reactivity and microstructure of synthetic CaO-based absorbents forCO₂capture, a class of synthetic CaO/La₂O₃and CaO/MgO sorbents have been prepared bySol-Gel method. The multicyclic CO₂capture capacity of the synthesized CaO-basedabsorbents and the effect of the additive La³⁺or Mg²⁺were investigated in a fixed bedreactor system. The results showed that the additive of La₂O₃or MgO plays a positive rolein the multiple carbonation-and-calcination cycles. On the basis of the XRD analysis, it is concluded that the addition of La³⁺or Mg²⁺significantly reduced the crystalline size ofCaO. Furthermore, it is also showed from FSEM images analysis that the synthesizedCaO-based absorbents with addition of La³⁺also have a certain degree of sinteringphenomenon. However, the La³⁺doped CaO crystals can activate its lattice, resulting inincreased cation vacancy, which plays a positive role in the carbonation process, therebygreatly improve the reactivity of the sorbents.A wide range of metallic oxide dopants （ZrO₂, Y₂O₃, MgO, La₂O₃, Al₂O₃, CeO, BaO,ZnO, Fe₂O₃and CoO, respectively） were employed to prepare synthetic CaO-basedabsorbents by Sol-Gel method. The cyclic characteristics of them were also investigated inthe fixed bed reactor system. The results indicate that the additive of ZrO₂, Y₂O₃, MgO,La₂O₃or Al₂O₃plays a positive role in the cyclical carbonation-calcination reactions, duringthe cyclic reactions, the additive of these metallic oxide dopants can inhibit grain boundarymigration, slow down the rate of the grain growth, so the average size of CaO crystalline isrelatively small. Simultaneously, it can increase the number of pores in the CaO particles,these hollow structure particles are beneficial to the gas-phase diffusion on the surface,improve the reactivity of the sorbents.In the fixed bed reactor system under different calcination temperatures and CO₂concentrations in calcination atmosphere, CO₂capture capacity of different CaO-basedabsorbents were investigated for multicyclic carbonation-calcination reaction processes.The experimental result has confirmed that, in the multicyclic carbonation-calcinationreactions, under the same carbonation reaction condition, when calcination condition wasselected in the pure CO₂atmosphere or higher calcination temperature, the absorbents ismore prone to sintering. For CaO/ZrO₂absorbent, due to uniform dispersion of ZrO₂dopedwith absorbent, inhibit the rate of grain boundary movement and hinder grain abnormalgrowth, and thus the grain is smaller, preventing the occurrence of secondaryrecrystallization.