Thermochemical assessment of chemical looping assisted by oxygen uncoupling with a MnFe-based oxygen carrier

A previously developed manganese-iron mixed oxide doped with TiO2 shows promising properties for the combustion of solid fuels with intrinsic CO2 capture by Chemical Lopping assisted by Oxygen Uncoupling (CLaOU) owing to its oxygen uncoupling capability, i.e. ability to generate gaseous oxygen at high temperature, and to oxidize gaseous fuels such as H-2, CO or CH4 by reacting with lattice oxygen. In this work, a window of suitable operating conditions was determined to facilitate the use of this material in a CLaOU unit. For this purpose, mass and enthalpy balances were performed for the whole CLaOU unit considering theoretical values of thermochemical parameters, such as the equilibrium constant of relevant reactions in CLaOU and the formation enthalpies of reacting solids. An air reactor temperature of 1148 K and a high air excess ratio are suggested in order to promote the transfer of oxygen via the oxygen uncoupling mechanism, which would be preferable for the combustion of solid fuels. Auto-thermal operation of the CLaOU unit is guaranteed for air excess ratio values lambda < 3. The fuel reactor temperature is highly influenced by the oxygen carrier-to-fuel ratio (phi) and the oxygen carrier regeneration in the air reactor. Assuming a 50-75% regeneration of the oxygen uncoupling capability, and phi = 4-6, the fuel reactor temperature would be between 35 and 50 K above the air reactor temperature. The higher temperature in the fuel reactor could promote the fuel conversion, while oxygen carrier regeneration would be guaranteed by the lower air reactor temperature.