As our nation moves to increase the usage of hydrogen as an energy carrier, the ability to produce hydrogen as a primary product will depends on new technologies that will provide greater efficiency and lower overall cost. Additionally, the growing interest in finding ways to provide with lower emissions of greenhouse gases, in particular carbon dioxide makes a technology such as Calcium-Based Looping technologies for hydrogen production well suited to further investigation and demonstration, because it generates a nearly pure, sequestration-ready CO2 stream while capturing sulfur impurities. With the potential for capturing CO2 and sulfur, while simultaneously producing high purity hydrogen at a very high efficiency from the input coal feedstock, this process translates to an attractive technology to pursue further.
Limestone is abundant and cheap materials and calcium oxide (CaO) derived from naturally-occurring limestone are microporous in nature. However, due to the high calcination temperature, pore filling and pore closure of the sorbents limit the CO2 sorption capacity for long term uses. To date, steam/water hydration is found to be capable of restoring the reactivity and durability of calcium-based sorbents for multiple calcination-carbonation reaction cycles. 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. Further, the changes in morphological properties of calcium-based sorbents are characterized by Brunauer-Emmett-Teller, Scanning Electron Microscopy and X-Ray Diffraction. 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.
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 CO2 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.