Investigation of Nickel- and Iron-Based Oxygen Carriers for Chemical-Looping Combustion

Doctoral thesis, 2010

Carbon capture and storage has the potential of reducing emissions of CO2, generated by combustion of fossil fuels. Chemical-looping combustion is a method intended to capture CO2 without an energy consuming gas separation process. Gas separation is avoided by using circulating oxygen carriers to transfer oxygen from an air reactor to a fuel reactor. Thus, the fuel and the combustion air are never mixed. A thermal analysis of the process identified several oxygen carrier systems with properties suitable for chemical-looping combustion applications. Such properties include high ability to convert different fuels, stability in air and sufficiently high melting temperature. Systems fulfilling these criteria were; metal oxides based on Ni, Cu, Fe, Mn, Co, W and sulphates of Ba, Sr and Ca. In the experimental part of this work, Ni- and Fe-based particles were analyzed with respect to both chemical and physical properties important for oxygen carriers in chemical-looping combustion, as well as gas conversion in a fluidized bed. Oxygen carriers of NiO, supported by NiAl2O4, are suitable for converting gaseous fuels with a high content of CH4. The main reasons are their high reactivity and high melting temperature. The Ni-based oxygen carriers investigated here were prepared from commercially available raw materials in contrast to the pure chemicals which have generally been used before. Oxygen carriers prepared by spray-drying, a production method suitable for large-scale particle preparation, displayed similar properties as oxygen carriers produced by the small-scale freeze-granulation method. Thus, up-scaling of particle production is not expected to present any difficulties. To reduce the risk of fragmentation and attrition of Ni-based oxygen carriers in a circulating chemical-looping combustion system, the strength can be improved by an addition of Ca(OH)2, by increasing the sintering temperature or by extending the sintering time. Materials with MgO added during particle preparation or with MgAl2O4 as supporting agent resulted in a considerably increased CH4 conversion. All oxygen carriers showed high reactivity with CH4 and O2 and a for a promising oxygen carrier of NiO/NiAl2O4, it was concluded that in an ideal reactor without gas solid-phase mass transfer limitations, full CH4 yield should be reached with a solids inventory in the fuel reactor of less than 10-20 kg/MW at 950°C. Fe-based oxygen carriers are cheap, abundant and environmentally sound and therefore well suited for chemical-looping combustion with solid fuels, where the expected lifetime of the oxygen carriers is comparatively short. Several industrial iron-based materials and a natural iron ore with properties well suited for chemical-looping combustion with solid fuels were identified. Generally, these materials displayed a high conversion of syngas, the main intermediate when solid fuels are gasified by steam, in combination with a high mechanical strength. The investigation of synthetically produced ilmenites, FeTiO3 in their reduced form, revealed that an increased Fe:Ti ratio generally improves the total conversion of CO although the initial maximum conversion is relatively constant.