A hydrogen economy requires primary sources to provide the power required to produce hydrogen from water, fossil fuels, biomass or similar compounds. Water-splitting processes are particularly attractive, since water is abundant around the planet, the unique products from water splitting are hydrogen and oxygen and a variety of primary energy sources can be used to produce the hydrogen. External electric power can be used to produce hydrogen by conventional water electrolysis [1], but it firstly requires the production of electricity to realize the electrochemical splitting of the water molecule. A more efficient and more cost-effective approach is to use thermal power to drive a thermochemical water splitting process. Such cycles break the hydrogen-oxygen bounds through a series of heat-driven chemical reactions, with recirculation of intermediate substances of different type. By this approach the external heat which drives the chemical reactions is converted into chemical energy of the hydrogen produced from water splitting.

Among the various water splitting processes, the sulfur-based cycles and in particular the thermo-electrochemical Hybrid Sulfur (HyS) is one of the most appealing cycles. This process has only fluid reactants and is comprised of only two global reaction steps: a low temperature electrochemical exothermic section, operating at temperatures on the order of 100 °C, and a high temperature thermal section operating at max temperatures of about 800 °C. In the electrochemical section SO₂ and H₂O are combined together to produce electrochemically H₂ at the electrolyzer cathode and H₂SO₄ at the electrolyzer anode. Sulfuric acid is recycled inside the plant, concentrated and decomposed, in the high temperature thermal section, into SO₂, O₂ and H₂O. Oxygen is then separated from SO₂ and water and extracted as byproduct of the process.

Solar driven HyS process is being studied by the Greenway Energy and the University of South Carolina within the DOE HydroGEN program (part of the DOE Energy Material Network initiative) [2], partnering with the Idaho National Laboratory, the Savannah River National Laboratory and the National Renewable Energy Laboratory. The project focuses on the high temperature sulfuric acid thermal decomposition section, developing new optimized catalyst formulations (i.e. achieving higher activity, lower performance degradation and lower costs), to be integrated in novel solar reactor systems. A new bimetallic catalyst formulation has been developed and synthesized, based on novel composition and support. Compared with the current state of the art sulfuric acid decomposition catalysts, developed by Idaho National Laboratory during the DOE Nuclear Hydrogen Initiative [3], the proposed catalyst shows the potential for higher SO₃ conversion yields and very limited sintering effects. Test results, under sulfuric acid environment and operating temperatures on the order of 800 °C, will be shown and discussed.

The new catalyst will be placed in a novel solar reactor, being developed within the HydroGEN project. The proposed concept is based on a solar cavity receiver system, realizing the sulfuric acid decomposition and internal heat recovery in a single unit, directly heated by the incident solar radiation. This system, along with the actual H₂SO₄ decomposition into SO₂, allows: (1) efficient internal heat recovery from the decomposition products and (2) connections with the metallic interfaced HyS equipment (e.g. tubing, valves etc) at low temperatures. A novel reactive computational fluid dynamic (CFD) model is presented, integrating mass, energy and momentum balance equations. A new kinetics expression has also been developed to model the behavior of the novel catalytic formulation. The model has been developed for both the high temperature catalytic decomposition process and for the vaporization/condensation of the sulfur mixture process. A novel two-phase model has been developed with the properties of the mixtures (SO₂, SO₃, H₂O, H₂SO₄, O₂) extracted from process models and integrated in the CFD code using numerical modeling approximations. Results are presented and discussed.

References

[1] J. Ivy, Summary of Electrolytic hydrogen production. Milestone Report. NREL/MP-560-36734; 2004.

[2] https://www.h2awsm.org/ (Accessed February 2018).

[3] Petkovic, L.M. et al. Applied Catalysis A: General 338 (2008) 27.