|The Hybrid Sulfur Process has two stages. First, the electrolysis of sulfur dioxide and water to generate hydrogen and sulfuric acid, followed by the thermochemical conversion of the sulfuric acid back to sulfur dioxide. Click to enlarge.|
The US Department of Energy’s Savannah River National Laboratory (SRNL) recently successfully completed a 100-hour long demonstration of a sulfur dioxide depolarized electrolyzer (SDE), designed and fabricated by SRNL, to produce hydrogen from water. The SDE is a core component of the Hybrid Sulfur Process.
The demonstration, which showed that the electrolyzer can successfully operate continuously without significant loss of performance, represents a milestone in the development of an efficient, economical process for generating large quantities of hydrogen using advanced nuclear reactors. In previous demonstrations, the electrolyzer had only been operated for short durations.
The Hybrid Sulfur Process (HyS) is one of the variants on sulfur-based thermochemical cycles for the production of hydrogen and is derived from a Westinghouse process. The electrolyzer oxidizes sulfur dioxide to form sulfuric acid (H2SO4) at the anode and reduces protons to form hydrogen at the cathode. The overall electrochemical cell reaction consists of the production of H2SO4 and H2:
SO2 + 2H2O → H2SO4 + H2
The initial electrolysis reaction of sulfur dioxide and water occurs at low temperature. The resulting sulfuric acid is decomposed into steam and sulfur trioxide, which is then further decomposed into sulfur dioxide and oxygen at high temperature (850-950 °C) with heat obtained from the nuclear reactor.
The sulfur dioxide in the electrolyzer reduces the required electrode potential well below that required for electrolysis of pure–water, thus reducing the total energy consumed by the electrolyzer. An electrolyzer operating in the range of 500-600 mV per cell can lead to an overall HyS cycle efficiency in excess of 50%, which is superior to all other currently proposed thermochemical cycles, according to the researchers at SRNL.
An important factor in the efficiency of the Hybrid Sulfur Process is the low amount of cell voltage required by the electrolyzer, which determines the amount of electricity needed. In the 100-hour test, SRNL’s electrolyzer required about 0.8 volts per cell, leaving researchers optimistic that the commercial goal of 0.6 volts per cell can be achieved when operating the electrolyzer at higher temperature and pressure.
Future work will seek to further improve the cell performance and extend its operational durability. SRNL is currently building a larger, multi-cell electrolyzer. Plans call for beginning construction of an integrated labscale Hybrid Sulfur Process, including the larger electrolyzer, during the next fiscal year.
The long-term goal is to build an engineering demonstration of the HyS Process that can be operated in conjunction with DOE’s planned Next Generation Nuclear Plant, scheduled for operation after 2017 at the Idaho National Laboratory.