Flowsheeting a Solar-driven Sulfur-Ammonia with ZnSO4-ZnO Water Splitting Cycle

  • Suzanne Fenton, Florida Solar Energy Center, University of Central Florida, United States
  • Cunping Huang, Florida Solar Energy Center, University of Central Florida, United States
  • Nazim Muradov, Florida Solar Energy Center, University of Central Florida, United States
  • Ali T-Raissi, Florida Solar Energy Center, University of Central Florida, United States

    • This paper contains a description of the initial chemical process flowsheet for a sulfur-ammonia solar-driven hydrogen production cycle with ZnSO4-ZnO sub-cycle. The flowsheet was used to provide a “first round” efficiency evaluation of the cycle. The proposed cycle is novel and consists of the following reaction steps:

    1) Solar photocatalytic: (NH4)2SO3(aq) + H2O(l) > (NH4)2SO4(aq) + H2(g)
    2) Thermocatalytic at 250oC: (NH4)2SO4(s) + ZnO(s) > ZnSO4(s) + 2NH3(g) + H2O(g)
    3) Solar Thermal at 900oC: ZnSO4(s) > ZnO(s) +SO2(g) + 1/2O2(g)
    4) Exothermic sulfite regeneration: SO2(g) + 2NH3(g) + H2O(l) > (NH4)2SO3(aq)

    The unique features of this cycle include: a) largely solid/gas phase reactants and products favoring low energy separation, b) participating chemical species that are non-corrosive and non-toxic, and c) a photocatalytic hydrogen production step utilizing photonic portion of sunlight while the thermal component of solar energy is used separately to evolve oxygen.

    Aspen Plus was used to simulate the sulfur-ammonia cycle with ZnSO4-ZnO sub-cycle. This report presents a description of the process design including thermodynamic property methods, unit models and parameters, calculated outputs and key assumptions. The first round efficiency predicted by the simulation was 38% (HHV). The initial simulations provided reasonable estimates of the expected cycle efficiency and will be used to direct efficiency improvement and target areas requiring additional experimental data.