Thermodynamic Possibilities and Constraints for Pure Hydrogen Production by a Nickel and Cobalt-based Chemical Looping Process at Lower Temperatures

The reduction of nickel and cobalt oxides by hydrogen, CO, CH4 and model syngas (mixtures of CO + H2 or H2 + CO + CO2) and oxidation by water vapour has been studied from the thermodynamic and chemical equilibrium point of view. Attention was concentrated not only on convenient conditions for reduction of the relevant oxides to metals at temperatures in the range 400 – 1000 K, but also on the possible formation of undesired soot, carbides and carbonates as precursors for carbon monoxide and carbon dioxide formation in the steam oxidation step. Reduction of nickel and cobalt oxides (NiO , CoO and Co3O4) by hydrogen or CO at such temperatures is feasible. The oxidation of Ni and Co by steam and simultaneous production of hydrogen is thermodynamically the more difficult step at temperatures of 400 – 900 K. For the Ni-NiO and Co-CoO systems the formation of corresponding Ni/Co-ferrite or Ni/Co-aluminum spinel could be used for a higher hydrogen equilibrium yield. Only such Ni – NiO and Co-CoO systems with the support of ferrite and aluminum spinel formation could be suitable systems for the chemical looping production of hydrogen by the chemical looping redox process. Oxidation of mixed Ni/Co-Fe metals or alloys by steam without segregation caused by preferrential oxidation of Fe is critical for the ferrites. For process based on Ni/Co-aluminum spinel reduction to metals is critical part of the cyclic process.JRC.F.2-Cleaner energie