Hybrid sulfur cycle

The NHI approach for developing high-temperature thermochemical cycles is to develop baseline cycles while also determining the feasibility of alternate cycles. The baseline thermochemical cycles being considered are the sulfur-iodine and hybrid sulfur cycles. Laboratory-scale experiments will be developed for these technologies, as will any additional cycles that are identified and show enough potential to justify that level of investment. The sulfur-iodine and hybrid sulfur cycles are both baseline cycles because they are the most promising mature cycles and potentially have different failure mechanisms. 

William A. Summers et al., The Hybrid Sulfur Cycle for Nuclear Hydrogen Production, Proceedings of GLOBAL 2005, Paper 097, Tsukuba, Japan, October 9013, 2005 (in progress). 

Several studies of H2 production by thermochemical processes have been presented recently, including reports of several cycle statues such as sulfur—iodine (S—I) cycle, ISPRA Mark 9 cycle, hybrid sulfur cycle, Ca—Br cycle, Cu—Cl cycle, and adiabatic UT-3 cycle (Rosen, 2010). Many of these cycles are driven by nuclear or solar energy sources. H2O thermal decomposition generally holds three distinct steps production of H2, production of O2, and material regeneration. In recent decades, thermochemical cycles have been used for H2O decomposition, because they allow appreciable amounts of H2 and O2 to be attained at lower temperatures (usually less than 1000 °C) than are needed for one-step fliermochemical H2O decomposition (Rosen, 2008, Rosen, 2010). 

O Briena, J. A., Hinkley, J. T., Donne, S. W., Lindquist, S. E. (2010). The electrochemical oxidation of aqueous sulfur dioxide a critical review of work with respect to the hybrid sulfur cycle. Electrochimica Acta, 55, 573—591. http //dx.doi.org/10.1016/ j.electacta.2009.09.067. 

Electrochemical oxidation of aqueous SO2 on the electrode surface has also aroused interest in the large-scale hydrogen production processes via a hybrid sulfur cycle which was patented as Westinghouse Process. The electrochemical oxidation of aqueous sulfur dioxide with respect to the hybrid sulfur cycle has recently been reviewed and shown the importance of the mechanism of the oxidation of sulfur dioxide on the electrode surface. Platinum, gold, and carbon materials as electrocatalysts have been reviewed to compare the catalytic activity for SO2 oxidation [16]. The first step in the Westinghouse Process is the dissociation of H2SO4 into SO2 and O2 by thermal cracking at about 1,000 °C ... 

Of all these methods indirect oxidation of SO2 using redox mediators and electrochemically promoted oxidation seem to be the nearest alternatives to the commercial applications. The removal of SO2 by using redox mediators has already been tested in commercial scale in the processes named Peracidox and Ispra Mark HI. Westinghouse Process is another commercial application in terms of hydrogen production via hybrid sulfur cycle. 

While many hydrogen processes have been proposed, the most studied include the copper—chlorine cycle in the process temperature range of 200—600°C (Orhan et al., 2012), the iodine—sulfur (IS) process of 450—850°C (Kasahara et al., 2014), and the hybrid sulfur cycle of 600—850°C (Gorensek and Summers, 2011). 

Brown, N.R., et al. (2009), Analysis Model for Sulfur-Iodine and Hybrid Sulfur Thermochemical Cycles , Journal of Nuclear Technology, 166,43-55. 

A recent screening of several hundred possible reactions (Besenbruch et al 2001) has identified two candidate thermochemical cycles for hydrogen production from water (i.e., cycles that enable chemical reactions to take place at high temperatures) with high potential for efficiency and practical applicability to nuclear heat sources. These are the sulfur-iodine (S-I) and calcium-bromine-iron (Ca-Br) cycles. Also, Argonne National Laboratory (ANL) has identified the copper-chlorine (Cu-Cl) thermochemical cycle for this purpose (Doctor et al 2002). A hybrid sulfur-based process that does not require iodine but has a single electrochemical... 

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 results of our evaluations for the other hybrid cycles are summarized in Table 3. The advantages of the Cu-Cl cycle are its low maximum temperature requirement and its relatively high efficiencies. The Level 1 and 2 efficiencies exceed 40% (LHV). All reactions in this cycle have been proven [1, 9, 10]. The Mg-Cl cycle has a low maximum temperature requirement but comparatively low Level 1 and 2 efficiencies. The chemistry for the thermal reactions has not been demonstrated. The low maximum temperatures, 550 and 600°C, respectively, for the two chloride cycles provide more flexibility in coupling them with heat sources other than the VHTGR. The advantages of the metal sulfate cycles are their lower corrosivity and the common high temperature decomposition reaction with the sulfur cycles. Of the two metal sulfate cycles, the copper sulfate cycle appears more promising because the copper sulfate cycle requires somewhat lower temperatures than the zinc sulfate cycle. The copper sulfate cycle has also been demonstrated with recycled materials, but no experimental work has been reported for the zinc sulfate cycle [1]. 

The Hybrid Sulfur (HyS) Process is one of the two baseline thermochemical cycles identified for development in the NHI program. (The sulfur-iodine cycle is the other). HyS is an all-fluids, two-step hybrid thermochemical cycle, involving a single thermochemical reaction and a single electrochemical reaction. The chemical reactions are shown below ... 

Several alternatives to the sulfiir-iodine process and steam electrolysis are being considered. Thermo-electrochemical cycles at various stages of development are being studied, including two hybrid sulfur-based cycles, the copper-chloride cycle, the magnesium-chloride cycle, the copper ferrite cycle,. Screening tools have been developed to rapidly assess less mature thermo-electrochemical cycles to help decide whether further research is warranted. 

The hybrid sulfur process (also known as Westinghouse GA-22 and Ispra Mark 11) has a single electrochemical step that completes the cycle ... 

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 oxygen analog of pyrrole is furan. In this case, one pair of electrons on the oxygen is part of an aromatic sextet while the other is in an sp2 hybrid AO that lies in the plane of the ring and is not part of the aromatic cycle. The sulfur analog, thiophene, has a similar structure. Both furan and thiophene are aromatic compounds that exhibit sub-... 

Figure 34. Comparison of the flexural strengths of unidirectional carbon/carbon composites (left-hand side) with those of hybrid composites in which the final impregnation is made with an epoxy resin (34) The composites were fabricated with high-modulus fibers rigidized with phenolic resin, and subjected to four densification cycles with coal-tar pitch plus sulfur.

Figure 35. Mechanical properties of carbon/carbon epoxy-resin hybrid composites, compared with the properties of the composite skeletons before resin impregnation (61,62). The composite skeletons were prepared from Sigrafil HM 3 PAN-based fiber, rigidized with a phenolic resin, and densified by four cycles with coal-tar pitch plus sulfur the carbonization temperature was 1000°C. (a) Young s modulus.

---