Paper ID : 20100706 Study of Alternative Route in Sulphur-Iodine Closed Loop Thermochemical Cycle for Hydrogen Production

Hydrogen is one among the various clean fuel options which are being researched globally. High temperature closed loop thermo-chemical cycles are one of the strongest candidates for mass scale hydrogen production by splitting water. Quoted efficiencies of these processes vary from 35-58%. Sulfur-Iodine watersplitting cycle is one of the most promising candidates for thermochemical hydrogen production. It consists of three chemical reactions: H2SO 4 � SO 2 + H 2O + ½O 2 (A) xI 2 + SO 2 + 2H 2O � 2HIx + H 2SO 4 (B) 2HIxxI 2 + H 2 (C) These reactions take place at temperatures ~850°C, 120°C and 450°C respectively. HI formed according to chemical equation (B) is an aqueous solution with a large excess of water and iodine. Capital and energy-intensive processes are involved in its concentration/separation, before it is thermally decomposed to H 2 and I 2. An extractive distillation using phosphoric acid and a reactive distillation under pressurized condition have so far been proposed. Since the volatility of HI steeply increases in the HIx solution of higher molality than the quasi-azeotropic one, pre-concentration of the feed HIx solution enables to obtain a distillate of pure HI easily. Such a concentration can be carried out by Electro-Electro-Dialysis technique. Further, HI decomposition is an equilibrium reaction. Conversion efficiency of this reaction is ~22% even in presence of catalyst. Efficiency of this reaction will have impact on overall efficiency of the process. An alternative path to the reaction (C) has been reported wherein a metal is reacted with HI solution to produce metal iodide and hydrogen. Metal iodide is decomposed to iodine and metal. Solvents have also been used to extract HI in order to simplify further HI decomposition process. This paper reviews various alternative routes used in reaction a (C), their pros and cons and also presents future direction of work for this reaction.