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 points 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 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 preferential oxidation of Fe is critical for the ferrites. For processes based on Ni/Co aluminum spinel, reduction to metals is the critical part of the cyclic process. strongly reducing conditions, at high CO concentrations/pressures, formation of nickel carbide (Ni3C) before cobalt carbide Co2C is thermodynamically favored. Pressurized conditions during the reduction step with CO/CO2 containing gases enhance the formation of soot and carbon containing carbide and/or carbonate compounds.