A Proposed New Sulfur-Sulfur Thermochemical Cycle

Of the many thermochemical cycles proposed for the production of hydrogen from water, the Sulfur-Iodine cycle is regarded as highly promising.  In order to enable large scale implementation of this cycle, approaches that simplify product stream management following the Bunsen reaction are particularly important, especially with regards to the management of excess water and excess iodine.  Excess water requires energy-intensive distillation schemes for the concentration of both H₂SO₄ and HI, the latter of which forms an azeotrope in water and requires reactive distillation.[1]  Further, the global supply of iodine would need to grow by orders of magnitude in order for widespread implementation of the sulfur-iodine cycle.[2]  In order to attempt to circumvent some of these problems, we have started to investigate possible process advantages derived from carrying out the Bunsen reaction in an ionic liquid as a low vapor pressure medium.[3]  Beyond examining the kinetics of the Bunsen reaction itself and the evolution of Hydrogen Iodide and Sulfuric Acid from the reaction mixture, we identified conditions under which the reaction system may be biased to intentionally produce a stream of hydrogen sulfide based on a normally undesired side-reaction: [4]

Since the weakly acidic H₂S exists in a non-dissociated, molecular state in the ionic liquid solvent, it is released as a gas from the reaction mixture (b.p. -82 ˚C), giving rise to a system in which a catalytic amount of iodine can be used to produce streams of hydrogen sulfide and sulfuric acid.  From these products, hydrogen can be generated through steam reforming of the hydrogen sulfide at temperatures attainable by nuclear power or solar concentrators, regenerating sulfur dioxide, and the sulfuric acid stream can be processed as in the various sulfur based cycles.  This novel cycle shown in the scheme below, which we term the Sulfur-Sulfur cycle, contains reactant streams that are strictly fluids, like the Sulfur-Iodine cycle, and may be amenable to simpler implementation than other current cycles. 

The presentation will focus on the new cycle concept, conditions favorable to the production of H₂S and the feasibility of steam reforming of the H₂S; in particular, results of hydrogen sulfide production in various ionic liquids under various conditions and reactant concentrations will be shown. 

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[1] Giaconia, A., et al. “Experimental study of two phase separation in the Bunsen section of the sulfur–iodine thermochemical cycle” Intl. Jour. Hydrogen Energy 2005, 30, 619-626.

[2] Vitart X, Carles P, Anzieu P. “A general survey of the potential and the main issues assoiciated with the sulfur iodine thermochemical cycle for hydrogen production using nuclear heat” Prog Nuclear Energy 2008, 50, 401-409.

[3] A. Yokochi, N. AuYeung, “Initial studies of the feasibility of performing the Bunsen reaction in a low vapor pressure liquid medium” 2008 Meeting of the AIChE.

[4] Sakurai, M., et al. “Experimental study on side-reaction occurrence condition in the iodine-sulfur thermochemical hydrogen production process” International Journal of Hydrogen Energy 2000, 25, 613-619.