Thermochemical two-step water splitting by internally circulating fluidized bed of NiFe2O4 particles: Successive reaction of thermal-reduction and water-decomposition steps

Thermochemical two-step water splitting using a redox system of iron-based oxides or ferrites is a promising process for producing hydrogen without CO2 emission by the use of high-temperature solar heat as an energy source and water as a chemical source. In this study, thermochemical hydrogen production by two-step water splitting was demonstrated on a laboratory scale by using a single reactor of an internally circulating fluidized bed. This involved the successive reactions of thermal-reduction (T-R) and water-decomposition (W-D). The internally circulating fluidized bed was exposed to simulated solar light from Xe lamps with an input power of 2.4–2.6 kWth for the T-R step and 1.6–1.7 kWth for the subsequent W-D step. The feed gas was switched from an inert gas (N2) in the T-R step to a gas mixture of N2 and steam in the W-D step. NiFe2O4/m-ZrO2 and unsupported NiFe2O4 particles were tested as a fluidized bed of reacting particles, and the production rate and productivity of hydrogen and the reactivity of reacting particles were examined.

► Successive two-step reactions of thermochemical water-splitting cycle are demonstrated using a single reactor of internally circulating fluidized bed by switching a passing gas. ► NiFe2O4/m-ZrO2 and unsupported NiFe2O4 are tested for successive reactions to produce hydrogen. ► Hydrogen productivity reaches about 1000 N cm3, and the ferrite conversion is 5–35%. ► Reacting particles did not sinter and coagulate throughout successive reactions. ► Reactor can split steam without needing a pulverization process after each step.