Calcium Based CO2 Acceptors for Sorption Enhanced Steam Methane Reforming
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Conversion of natural gas and light hydrocarbons via steam reforming, partial oxidation and electrolysis of water are the major commercial hydrogen production technologies. However, these methods are not economically viable, mainly due to small-scale hydrogen production and high processing cost. Sorption enhanced steam reforming (SESR) is a novel technology for reforming and in situ CO2 removal using metal oxide acceptors such as Ca-based oxides. CO2 capture capacity of natural Ca-based acceptors (e.g. dolomite, limestone) decline rapidly in multi-cycle carbonation/decarbonation processes. This thesis aims to develop Ca-based nano-acceptors with enhanced multi-cycle durability and CO2 capture capacity. A comprehensive experimental study on synthesis and characterization of the nano-acceptors and their stability, reaction kinetics and performance in sorption enhanced steam methane reforming (SESMR) has been conducted. Spray drying and soft chemistry route are applied to develop high temperature Ca-based CO2 acceptors. Spray drying technique is employed for the synthesis of Ca-based mixed oxides. Among the prepared mixed oxide acceptors, CaCe (10:1), CaZr (8:1 and 10:1) and CaCeZr (10:1:1) have larger surface areas (20.6 m2/g, 17.3 m2/g, 18.6 m2/g and 31.2 m2/g) and total pore volume (0.08, 0.07, 0.08 and 0.14 cm3/g). In multi-cycle carbonation/decarbonation, the nano-acceptors demonstrate relatively stable, reversible and high CO2 uptake capacity compared to naturally occurring dolomite and CaCO3 nano-particles. Experimental data demonstrates that capture capacity of the mixed oxides increases with increasing CaO content, which indicates that CaO phase plays as an active role in CO2 capture. The observed capacities of the synthetic acceptors, CaCe (10:1), CaZr (8:1) and CaCeZr (10:1:1) are 0.53, 0.46, and 0.45 g -CO2/g-acceptor, respectively. Formation of nano-inert metal oxides (CeO2, CaZrO3 and Ce0.5Zr0.5O2) on CaO surface effectively mitigates the sintering in high temperature carbonation/decarbonation by blocking the direct contact between the CaO nanoparticles. The stability and enhanced reaction kinetics of the mixed oxide acceptors are due to high Tammann temperature and oxygen vacancies present in the inert dopant metal oxides, respectively. In soft chemistry route multi-cycle sorption stability of nano-CaO acceptors is enhanced by sol-gel and incipient wet impregnation coating. TGA of the multi-cyclic CO2 carbonation/decarbonation indicates that the nano-CaO acceptors with sol-gel coatings and wet impregnation have higher CO2 capture capacity and a longer life time than the uncoated ones. A layer of CaZrO3 is formed in sol-gel and wet impregnation coating which prevents the nano-CaO particles from mutual interaction under high temperatures. Combined Raman spectroscopy, high-resolution electron microscopy, and energy dispersive spectroscopy (EDS) analysis confirm the formation of core-shell, CaO/CaZrO3 composite structure in sol-gel coated nano-CaO acceptors leading to stable capture stability. Whereas an uneven coating of CaO surface by the incipient wet impregnation led to less stability. The mixed oxides, CaCeZr (10:1:1) and CaZr (10:1) are tested in SESMR conditions. The in situ removal of CO2 provides high hydrogen concentration for CaCeZr and CaZr are 96.7% and 98%, respectively. The CO2 capture capacity of both the acceptors is better than dolomite; despite a moderate decrease in sorption capacity. In addition, an increase in methane residence time increases the CO2 capture capacity in SESMR process. The sol-gel coated acceptors are also tested under wet condition and SESMR process. Capture capacity increases with number of carbonation/decarbonation cycles due to structural changes of CaZrO3 coated layers around the nano-CaO acceptors.