Date of Award
Lowering dependence on fossil-based fuels has become a major area of focus in research, especially in transportation fuels. One particularly interesting field is that of hydrogen production. Hydrogen is promising because it has the potential to emit no greenhouse gases when used as fuel and is an excellent vessel for energy storage. Because traditional methods for its generation are inefficient and require nonrenewable fuel use, researchers have begun to focus on thermochemical cycles.
Thermochemical cycles are systems that require heat transfer intrinsically and do not consume fossil fuels nor emit greenhouse gases if renewable energy is used for heating. The hybrid sulfur process (HyS) has become a premiere cycle recently, as it is one of the simplest hydrolysis cycles with only two steps in the reaction and just two inputs, sulfuric acid and water, and still provides high purity hydrogen. The first reaction is endothermic, the high-temperature decomposition of sulfuric acid (H2SO4) into sulfur dioxide and water. Two different sources for the heat energy are viable, nuclear and solar power. These products are then fed to an electrolyzer in which sulfuric acid and hydrogen protons are produced at the anode, then the protons are passed across the membrane to form hydrogen. H2 is removed as the desired product and H2SO4 is recycled to the decomposition step5.
The Sandia National Laboratory (SNL) has been focusing research on the reactor needed for the high temperature acid decomposition step of the reaction. In particular, the materials needed to accommodate such high temperatures are difficult to design and implement into the needed equipment. SNL has developed the innovative bayonet decomposition reactor that minimizes these concerns. The reactor consists of two concentric flow paths, one closed and one open tube. High temperatures are applied at the closed end while the sulfuric acid enters in the open end where it is vaporized before passing through a catalyst bed where the decomposition takes place. For the HyS process to be viable, it is very important for the high-temperature reaction step to consume as little thermal energy as possible, therefore increasing thermal efficiency. Previous pinch analyses performed by Savannah River National Laboratory (SRNL) determined a feasible minimum energy requirement of 328.6 kJ/mol H2 based on a feed concentration of 75% sulfuric acid4.
SRNL has been researching the SO2-depolarized electrolyzer (SDE), which applies a potential to the process fluid to separate water into oxygen, hydrogen protons, and electrons, which recombine to form hydrogen gas and reform the sulfuric acid1. Engineers at SRNL have developed a flowsheet for the HyS process integrating the bayonet reactor heat source with the SDE using ASPEN Plus. It was found that the two areas of the process that require heat input are the Bayonet Reactor and the Vacuum Column reboiler, used for feed concentration, at 340.3 kJ/mol H2 and 75.5 kJ/mol H2, respectively. The total amount of electric energy required was found to be 120.9 kJ/mol H2, used primarily by the SDE. Therefore, the total energy requirement of the process is 685.8 kJ/mol H2, with 252.9 kJ/mol H2 rejected to cooling water. Thermal efficiencies were found to be in the range of 33-39%2.
The HyS process can only be a viable option if it has the potential to outperform alternate hydrogen production techniques. Water electrolysis is one established technology with which the HyS must be able to compete. According to the Nernst equation, standard cell potential for the SDE is lower than that of water electrolysis with traditional water at -1.229V and the HyS process at -0.158; therefore, considerably less electricity per mole of hydrogen product is consumed in the HyS process6. The research discussed above has found that the HyS process has the potential to perform at efficiencies higher than that of water electrolysis, making it a promising area of research. The HyS process also has an advantage over other potential thermochemical cycles due to its relatively simple and few reaction steps. The HyS process also has an advantage over natural gas reforming, which currently produces a large amount of hydrogen currently, in that it is a much cleaner energy source and emits no greenhouse gases. However, further research must be performed to greater explore the economics of the process.
Isenhower, John Jacob; Henderson, Katherine Cross; and Eigenbrot, Nicholas Joseph, "Hybrid Sulfur Process for Solar Production of Hydrogen" (2016). Senior Theses. 73.