Nickel oxide redox processes with oxide ion conductor-supported nickel oxide in dry and humidified methane: Effect of oxide ion conductors on induction period in nickel oxide reduction and subsequent hydrogen production

NiO redox processes and hydrogen production from dry and humidified methane were investigated by chemical looping combustion (CLC) and chemical looping reforming (CLR) using NiO as an oxygen carrier. To achieve a lower operating temperature in the NiO reduction process, oxide ion conductors such as gadolinia doped ceria (GDC) and strontium doped lanthanum aluminate (LSA) were investigated as the support materials of NiO. Hydrogen rich gas products were obtained by the CLC and CLR processes but a prolonged induction period for the NiO reduction by methane (i.e., induction period for CO2 formation from methane) was observed at relatively low operating temperatures in both processes. NiO/GDC and NiO/LSA lead to a decrease in the induction period of NiO reduction by comparison with the reference NiO/ZrO2. Upon repeating the NiO reduction–oxidation cycles, we also found that the induction period for NiO/GDC became shorter with an increase in the redox cycle number. However, for NiO/LSA this time became longer, which may result from the strong interaction between Ni and the support materials. This NiO reduction behavior on the oxide ion conductors is discussed together with the autocatalytic reaction mechanism for NiO reduction, which provides new insights into the efficient use of lattice oxygen in CLC and CLR.