Reactivation Mechanism Studies on Calcium-Based Sorbents and its Applications for Clean Fossil Energy Conversion Systems

This thesis is intended to systematically explore the fundamental mechanism underlining reactivation of calcium-based sorbents through hydration with water. The CO2 sorption characteristics of calcium-based sorbents before and after the hydration, from a given calcination condition, are examined using a Thermogravimetric Analyzer (TGA). Further, the changes in morphological properties of calcium-based sorbents are characterized by Brunauer-Emmett-Teller (BET), Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). It is found that the hydration changes not only the macrostructure of calcium-based sorbents but also their microstructure. CaO sorbents with a higher surface area, a higher pore volume and a predominantly mesoporous structure are obtained after the hydration. The concentrations of a CaO (111) plane and a CaO (100) plane of CaO sorbents are found to increase simultaneously after the hydration. The changes in sorbent properties at both macroscopic and microscopic levels are considered to be responsible ultimately for the superior and sustained activities of the sorbents for the CO2 capture.;The influence of reactivation by hydration on the performance of a high-purity limestone during the CO2 capture cycle is further investigated in the study. Of particular interest is the variation of the microstructural properties for CaO obtained from hydrated lime. A density functional theory (DFT) calculation to illustrate the behavior of CO2 adsorption on the CaO surface is conducted. Three dominating planes of the CaO surface, i.e. CaO (100), CaO (110) and CaO (111), are considered in the calculation. It is shown that the most stable adsorption configuration on a given surface for CO2 molecules is that the adsorption takes place with the C atom of CO2 molecules adsorbed on the O lattice sites of CaO with the O atom of CO2 molecules pointed to the Ca atoms of CaO. That is, the CO2 molecule does not adsorb either via the O atom of CO 2 molecules or on the Ca sites of CaO. Further, the results show that the CO2 adsorption is more favorable on a CaO (100) surface and a CaO (111) surface compared to a CaO (110) surface based on their corresponding adsorption energy. These calculation results are consistent with the experimental XRD studies.;Besides, the high-pressure carbonation kinetics of calcium oxide (CaO) derived from three calcium-based sorbents, i.e. limestone (CaCO3), calcium hydroxide (Ca(OH)2) and Precipitated Calcium Carbonate (PCC), being used in the Calcium Looping Process (CLP) system, were studied using a Magnetic Suspension Balance (MSB) analyzer. Different total pressures (1000~15000 torrs) and concentrations of CO2 (10∼30%) were tested to determine their effects on the carbonation reaction rate. The carbonation reaction rate was found to increase with an increase in the concentrations of CO2 at a constant total pressure. However, the total pressure has an effect on the carbonation reaction rate only at lower pressures. With a 20% CO2 stream, the reaction rate was observed to increase until the pressure reached 4000 torrs but after that an increase in total pressures has a negligible effect on the rate of carbonation reaction of CaO derived from all three precursors. Further, the carbonation reaction has a different reaction order with respect to the partial pressure of CO2. It was found that the reaction is first-order at lower CO2 partial pressures but changes to zero-order when the CO2 partial pressure exceeds 800 torrs. In addition, the reaction rate of the carbonation conducted at high pressures is greater as compared to that at atmospheric pressure, under cyclic testing. The results also showed that there is no significant difference in the behavior of the carbonation reaction of CaO at elevated pressures regardless of different precursors. (Abstract shortened by UMI.)...