Fundamental Research Of Open-loop Sulfur-iodine Thermochemical Cycle For The Production Of Hydrogen And Sulfuric Acid

Hydrogen has ideal characteristics as an energy carrier. Therefore, the concept of a hydrogen energy system has attracted worldwide interest. Huge hydrogen demand is expected in order to develop the hydrogen energy system. Thermochemical splitting of water has been proposed as a clean method for hydrogen production. Hydrogen is obtained by decomposition of water by using heat energy through a chemical cycle process that consists of several reactions. Among the large scale, cost effective and environmentally attractive hydrogen production processes, the sulfur-iodine (SI or IS) thermochemical cycle is a quite promising one.The selection of SI cycle to run in an open-loop fashion for China is tied-up with two important facts: (1) sulfur iron ore as SO₂ source is inexpensive and abundantly available; (2) The product sulfuric acid, in addition to hydrogen, is valuable and marketable.The open-loop SI cycle consists of following two reactions:Bunsen reaction: xI₂ + SO₂ + 2H₂O = 2HIx + H₂SO₄Hydrogen iodide (HI) decomposition reaction: 2HIx = xI₂ + H₂The mass and heat balance of the open-loop SI cycle were calculated with the optimized conditions. Thermal efficiency for hydrogen production was 63.1% with ideal operating conditions.The Bunsen reaction for the production of hydriodic and sulfuric acids from water, iodine and sulfur dioxide has been studied with the evaluation of the effect of solution temperature and composition of initial solution on separation characteristics of two liquid phase. The effect of solution temperature and composition of initial solution on side-reactions of the Bunsen reaction were also investigated. The separation characteristics were found to improve with the increase in iodine fraction and the decrease of water. The side-reactions can be controlled with the increase in iodine fraction and water. Results show that operative temperature has a minor effect on the phase separation, but the increase of temperature can enhance the side-reactions.Thermodynamics simulation of HI decomposition without and with water is investigated under different temperature by FactSage. Results show that the HI decomposition reaction is sensitive to temperature, but not to pressure. Vapor can improve HI decomposition as temperature increased above 700℃. Detailed kinetic modeling and sensitivity analysis for HI homogeneous decomposition with and without O were investigated. The kinetics model of HI decomposition without O was composed of 11 elemental reactions and 5 typical middle species. The result shows that the reactions HI+HI=H₂+I₂ and HI+I=I₂+H play a major role. The kinetics model of HI decomposition with O was composed of 43 elemental reactions and 12 typical middle species. According to the results, the reactions HI+I=I₂+H and HI+O=OH+I are the most important steps, especially the reaction HI+I=I₂+H. The presence of O can obviously promotes the HI decomposition reaction rate, but simultaneously consume H contained in HI. Kinetic calculations of HI decomposition without O are also compared with the experimental data, and all trends of the experiment can be reproduced by the model. The HI decomposition reaction path diagram was constructed in this dissertation.The conversion of HI homogeneous decomposition is rather low. The use of catalyst allows a substantial reduction in temperature to achieve workable reaction rates. We developed different kinds of catalysts to improve HI decomposition.CeO₂ which acts not only a catalyst but also an oxidant with different preparation methods and calcination temperatures have been tested to evaluate their effect on HI decomposition at various temperatures. According to the results of TG-FTIR, BET, XRD, TEM and TPR, the CeO₂ catalyst synthesized by citric-aided sol-gel method and calcined at low temperature(300 and 500℃) shows more lattice defects, smaller crystallites, larger surface area and better reducibility. Lattice defects, especially the reduced surface sites, i.e., Ce³⁺ and oxygen vacancy, and O species play the dominant role in surface reactions of HI decomposition. An original reaction mechanism for HI catalytic decomposition on CeO₂ catalyst is proposed.The Pt/CeO₂ catalysts with different preparation methods, calcination temperatures and oxidative/reductive treatments have been tested to evaluate their effect on HI decomposition. The Pt/CeO₂ catalysts strongly enhance the decomposition of HI. According to the results of TG-FTIR, BET, XRD, TEM,TPR and XPS, it was found that, through sol-gel method, the Pt ions could be inserted into the ceria lattice. This brought about a different synergistic effect between the Pt and Ce components, such as increased the oxygen mobility in ceria support and improved the thermal stability of catalyst. A mechanism is supposed to exist in the Pt/CeO₂ catalysts synthesized by sol-gel at high calcination temperature. It involves the complex conversion such as Ce⁴⁺+Pt→Ce³⁺+Pt²⁺ and oxygen-vacancy diffusion in the Pt-Ce-O system. The oxidative /reductive atmosphere affected the structure and performance of the catalyst by the strong metal-support interaction (SMSI). The activity of the reduced and re-oxidized samples are better than the as-received and oxidized samples. Models were constructed to describe the diffusion of Ce⁴⁺ and oxygen vacancies as well as the possible shell-core structure of Pt crystallites and the decoration/encapsulation by ceria support. Compared to pure ceria, inserting ZrO₂ into the ceria lattice to form ceria-zirconia solid solutions has improved the thermal stability and oxygen storage capacity. Pt/Ce_0.8Zr_0.2 showed the best catalytic performance as a combined result of high oxygen storage capacity of ceria in Ce-ZrO₂, strong interaction between Ni and Ce-ZrO₂ and basic property of the catalyst. The Ni/CeO₂ catalysts with different preparation methods, calcination temperatures and oxidative/reductive treatments have been tested to evaluate their effect on HI decomposition. The Ni/CeO₂ catalysts also strongly enhance the decomposition of HI. According to the results of TG-FTIR, BET, XRD, HRTEM/TPR and XPS, it was found that the Ni²⁺ ions could be inserted into the ceria lattice. This brought about the strong interaction between Ni and CeO₂ and the generation of oxygen vacancies. An original reaction mechanism of HI catalytic decomposition on Ni/CeO₂ has been constructed. We believe that there are three important reactive sites for HI catalytic decomposition. One is the surface site which exhibits in the Ni-Ce interphase, where interfacial Ni sites are located and the strong interaction between Ni and CeO₂ occurs. The other is oxygen vacancy related to the reduced surface sites of CeO₂ support, i.e., Ce³⁺ and Ni²⁺ ions dissolved into the ceria lattice instead of the Ce⁴⁺ ions. The third reactive sites is reduced Ni surface.The Ni/Ce_xZr_1-x(x=1, 0.8, 0.5, 0.2 and 0) synthesized by citric-aided sol-gel method and calcined at 700℃have been tested to evaluate their effect on HI decomposition. Ni/Ce_0.8Zr_0.2 showed the best catalytic performance. Then Ni/Ce_0.8Zr_0.2 with different preparation methods and calcination temperatures have also been tested to evaluate their effect on HI decomposition. Compared to pure ceria, inserting ZrO₂ into the ceria lattice to form ceria-zirconia solid solutions has improved the thermal stability and oxygen storage capacity. Ni/Ce_0.8Zr_0.2 synthesized by citric-aided sol-gel method and calcined at 700℃showed the best catalytic performance due to its high degrees of metal dispersion, high oxygen storage capacity of ceria in Ce-ZrO₂, surface oxygen mobility and strong interaction between Ni and Ce-ZrO₂.Based on our fundamental research and the support of National High Technology Research and Development Program of China (863 Program) and State Key Laboratory of Clean Energy Utilization. We try to construct a bench-scale open-loop SI cycle for continuous hydrogen production. The system design and mass balance of the open-loop SI cycle have already been investigated with the optimized conditions. Designed H₂ production rate is 1 L/h. The Bunsen reaction and HI decomposition reaction are operated at 60℃and 500℃. The catalyst developed by ourself will be selected for HI decomposition. Continuous operation for hydrogen production in a stable state is one of the challenges.