Copper and Manganese-based Oxygen Carriers in Chemical-Looping Combustion (CLC) and Chemical-Looping with Oxygen Uncoupling (CLOU)
Doctoral thesis, 2014
The chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) processes are attractive solutions for efficient combustion with inherent separation of carbon dioxide. These processes use a metal oxide as an oxygen carrier to transfer oxygen from an air to a fuel reactor where the fuel, or gasification products of the fuel, is converted. When solid fuel is used in CLC, the char needs to be gasified, by e.g. steam, to form H2 and CO that can subsequently be oxidized to H2O and CO2 by the oxygen carrier. In the case of CLOU, the oxygen carrier releases gas-phase oxygen in the fuel reactor. This enables a high rate of conversion of char from solid fuels, as CLOU eliminates the need for the gasification step required in normal CLC with solid fuels.
In this work, copper- and manganese-based oxygen carriers were investigated for the CLC and the CLOU processes. Their chemical-looping performance was examined in a laboratory-scale fluidized-bed reactor under alternating reducing and oxidizing conditions at different temperatures. The materials were generally evaluated with respect to oxygen release, reactivity and performance. Detailed material analysis and characterization of various oxygen carriers were carried out. Different gaseous fuels such as methane and synthesis gas (50% CO in H2) as well as solid fuels such as petroleum coke and wood char were used.
As copper-based materials, freeze-granulated Al2O3 and MgAl2O4-supported CuO oxygen carriers were investigated for CLC and CLOU applications using methane. In order to establish the phase relationships in the Cu–Al–O system, the standard enthalpy of formation, ΔH_f^0, of CuAl2O4 was reassessed using differential scanning calorimetry (DSC). The reducing and oxidizing pathways in the Cu−Al−O system and the reversibility of the phases during the redox process were also studied.
As manganese-based materials, various manganese-based perovskite-type oxygen carriers produced by extrusion and spray-drying and different manganese ores were studied. The effect of dopants, operating temperature and calcium content on the reactivity of oxygen carriers towards methane and the stability of the perovskite-structured materials as well as tolerance towards deactivation with sulphur were examined for the CLOU process. The reactivity of manganese ores with gaseous fuels and their influence on the rate of char gasification was scrutinized for CLC application with gasification of solid fuels.
A method for obtaining the rate of oxygen release for CLOU by utilizing a devolatilized wood char has been developed and was applied to the MgAl2O4-supported CuO oxygen carrier and the extruded perovskite-type materials. Furthermore, the oxidation rate of the MgAl2O4-supported CuO oxygen carrier for CLOU was studied. Based on the obtained rates, the minimum solids inventories using these oxygen carriers were determined for a CLOU unit.