Modeling of a Bayonet Reactor for Sulfurinc Acid Decomposition in Thermo-Electrochemical Hydrogen Production Processes

Abstract

Hydrogen is the most abundant element on Earth. However, being a very reactive element, it does not occur in significant quantities in a pure state. Most hydrogen is combined with other elements and is found in water, fossil fuels, biomass and similar compounds. A hydrogen economy requires primary energy sources to provide the energy to release the hydrogen from these 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 power (e.g. nuclear power, solar power, etc.) can be used to produce hydrogen by conventional water electrolysis [1], which first requires the production of electricity. A more efficient and potentially more cost effective approach is to use external heat to power a thermochemical water splitting process, which breaks 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 thermochemical water splitting processes, the Hybrid Sulfur (HyS) is one of the most appealing cycles. It 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 temperatures of about 800 °C.In the electrochemical section SO2 and H2O combine together to produce electrochemically H2 at the electrolyzer cathode and H2SO4 at the electrolyzer anode. Sulfuric acid is recycled inside the plant and decomposed, in the high temperature thermal section, into SO2, O2 and H2O. The endothermic decomposition of sulfuric acid takes place in two main reactions: H2SO4 --> SO3 + H2O (1) SO3 --> SO2 + 0.5 O2 (2)The required thermal power is provided by external source and by internal heat recovery from the SO2-O2 product of the decomposition reaction. An efficient heat transfer system to operate Reactions 1 and 2 is based on the bayonet reactor concept. This system allows: (1) efficient internal heat recovery from the decomposition products and (2) low temperature connections with the interfaced HyS equipment (e.g. tubing, valves etc). The concept has been demonstrated only as electrically heated lab scale component and most of the modeling analysis has been carried out adopting lumped parameter models [2,3,4]. In order to achieve a conceptual design of the component for larger scale hydrogen production the assessment of the reaction yield and the gradients of temperature and concentration inside the component is of primary importance. A detailed transport model has been developed at University of South Carolina, in conjunction with the Savannah River National Laboratory, within the DOE-EERE Solar Thermochemical Hydrogen (STCH) program. The model is comprised of mass, energy and momentum balance equations, as well as SO3 reaction decomposition kinetics. The model not only evaluates the performance of the internal recovery heat transfer process, but also analyzes the heat exchange with external fluid. A baseline configuration has been established with helium as the baseline heat transfer fluid, leveraging experience from nuclear applications [3,4]. The sulfuric acid mixture flows at 14 bar, while helium flows outside the reactor tubing at 40 bar. The Figure shows temperature profile for the Reaction (2). A 2D radial section of a single tube (length = 2 m) is shown in the Figure, with SO3 mixture flowing in the internal tube and helium flowing in the external annular region. Results demonstrate the effective heat transfer between helium and the reacting mixture, reaching final temperatures on the order of 750 °C. Suitable heat transfer coefficient correlations have also been established based on the results obtained, accounting for convective heat transfer processes (both helium and reacting mixture side) and conductive heat transfer (internal tube walls).