Sulfuric acid hybrid cycle

Fig. 2-10 Diagram of the Westinghouse sulfuric acid hybrid cycle, from [20]... 

In the Westinghouse sulfuric acid hybrid cycle shown in Fig. 2-10 (see also appendix A.3.2.), HTGR heat will be used for the H2SO4 decomposition step. Both high-temperature and electric steps have been experimentally investigated at the Research Center Julich. The above hybrid cycle has undergone a detailed balance and cost analysis already in a plant design based on nuclear power [8]. 

Research work has been conducted at the Research Center Julich on the Westing-house sulfuric acid hybrid cycle. The heat consuming step of sulfuric acid splitting was successfully realized in bench-scale experiments under HTGR conditions at 4 MPa and with 950 °C heat from a furnace. 

The sulfuric acid hybrid cycle (Westinghouse process) is given by the reaction equations... 

Struck BD, Junginger R, Boltersdorf D, Gehrmann J (1980) The anodic oxidation of sulfur dioxide in the sulfuric acid hybrid cycle. Int J Hydrogen Energy 5 487 97. doi 10.1016/0360-3199(80)90055-5... 

Due to high T and 2 law efficiencies of sulfuric acid based cycles, to date, more than 20 sulfuric acid and/or metal sulfate decomposition based TCWSCs have been reported. Despite difficulties that challenge efficient electrolytic oxidation of sulfur dioxide (SO2), the Westinghouse hybrid cycle still remains as one of the most studied TCWSCs. The Westinghouse cycle is as follows [14] ... 

Thermochemical hybrid cycles offer the possibility of running low-temperature reactions on electricity. The expectations for realization of hybrid processes are similar to those for purely thermochemical processes. Various hybrid processes are energetically possible, but not always practicable. Important criteria are the minimum voltage for the electrolysis step, realizability, efficiency. The sulfuric acid hybrid or Westinghouse process is expected to reduce material streams as compared with the IS process. The electric energy demand here is a factor of 7 (in reality 3 - 4) lower than in the electrolytic water splitting process. Technological improvements are still possible. 

The reversible potential for the sulfur dioxide electrolysis is only 0.17 V, less than 10% that of water electrolysis (minimum of 1.23V at 298K and 1 bar) [65,69]. However corrosion problems in the electrolysis step are severe due to the presence of high concentration (about 50%) sulfuric acid. The overall thermal efficiency of the process, considering both thermal and electrical energy input derived from the same heat source, is estimated as 48.8% [116]. However, in terms of economics and process complexity the hybrid cycles face tough competition from advanced water electrolyzers. 

The Hybrid Sulfur (HyS) thermochemical cycle task addresses the key technology issues involved in the development of a hybrid sulfur hydrogen production system - including the SO2 - H2O electrolyzer design, SO2/O2 separation, and the unique materials and process issues associated with the acid decomposition section. An electrolyser is being developed that can be used in conjunction with the sulfuric acid decomposition section being developed for the S-I cycle in a Hybrid Sulfur Integrated Laboratory-Scale Experiment. 

The thermochemical and electrolytic hybrid hydrogen production process has been developed by Japan Nuclear Cycle Development Institute (JNC). The process is based on sulfuric acid (H2SO4) syntliesis and decomposition process developed earlier (Westinghouse process) and sulfur trioxide (SO,) decomposition process is facilitated by electrolysis with ionic oxygen conductive solid electrolyte at 500°C-550°C. Stable hydrogen and oxygen production for several hours by the process was already confirmed in the experiments performed by JNC. 

The Westinghouse cycle is a variant of sulfur—iodine and is a hybrid concept that requires both thermal and electrical energy. When the product of reaction (4.31) is cooled, sulfurous acid is formed and this is electrolyzed at around 80 °C to produce hydrogen and sulfuric acid for recycling, i.e.,... 

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