Copper in Chemical-Looping Combustion (CLC) and Chemical-Looping with Oxygen Uncoupling (CLOU)

Licentiatavhandling, 2012

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, reacts with the oxygen carrier. The feasibility of Al2O3 and MgAl2O4-supported CuO oxygen carriers for CLC and CLOU processes are investigated in this work. The reactivity of these oxygen carriers was evaluated in a laboratory-scale fluidized-bed at 900 and 925°C under alternating reducing and oxidizing conditions. For Al2O3-supported oxygen carriers, CuO reacted with the support forming copper(II) aluminate (CuAl2O4), which is also a viable oxygen carrier, although it has no oxygen uncoupling properties. In the case of MgAl2O4 as support, the oxygen carrier exhibited stable oxygen release due to the presence of intact CuO. 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 thermogravimetry and differential scanning calorimetry (TGA/DSC) due to discrepancy in thermodynamic databases. The reducing and oxidizing pathways in the Cu−Al−O system and the reversibility of the phases during the redox process were also investigated. Here, the phase transformations were examined as a function of duration of the reduction period and oxygen concentration during the re-oxidation period. It was found that the CuAl2O4 is reduced to copper(I) aluminate (CuAlO2; delafossite), Cu2O and elemental Cu. The CuAlO2 phase is characterized by slow kinetics for re-oxidation into CuO and CuAl2O4. The rate of oxygen release and the rate of oxidation of the MgAl2O4-supported CuO oxygen carrier were determined in the temperature range of 850−900°C. Devolatilized wood char was used to facilitate oxygen release from the oxygen carrier in N2-fluidization by maintaining low oxygen concentration around the particles. The Avrami-Erofeev mechanism was used to model the rates of oxygen release. However, during oxidation it was observed that the rate is limited by the oxygen supply, indicating rapid conversion of the oxygen carrier. From the obtained reaction rates, the total amount of the investigated oxygen carrier needed in the air and the fuel reactor is estimated to be between 73−147 kg MW_th^(-1).