Mechanism Investigation Of Homogeneous And Catalytic By Activated Carbon Ash Decomposition Of HI In Sulfur-iodine Cycle

Hydrogen is considered as an ideal energy vector owing to abundant resource and does not produce greenhouse gases in the combustion process. Therefore, the large scale hydrogen production technology has greatly attracted worldwide attention. One of the best candidates is the thermochemical sulfur-iodine cycle, which consists of three reactions:Bunsen reaction:xI2+SO2+2H2O=2HIx+H2SO4HI decomposition:2HI=H2+I2H2SO4decomposition:2H2SO4=2SO2+2H2O+O2HI decomposition process is the unique procedure for generating hydrogen in the entire cycle. And its conversion rate ratio directly influences systemic thermal efficiency and hydrogen production rate. The investigation for HI decomposition nowadays is focused on the developing of steady catalysts, however, the homogeneous HI decomposition is rarely reported. A new, detailed kinetic model was developed for the homogeneous decomposition of HI-H2O and HI-H2O-I2solutions in vapor phase in the sulfur-iodine cycle. The thermodynamic software FactSage and dynamic experiments was used to validate the correctness the mechanism. Compared the thermodynamic and kinetic results with the modeling results calculated by the current mechanism, it concluded that the mechanism can accurately predict the HI decomposition.The modeling results of HI-H2O decomposition shows that the water in the hydroiodic acid played a detrimental role in HI decomposition at a lower temperature range but facilitate HI decomposition at temperatures. The role of residence time was influenced by the reaction temperature, extending the residence time at lower temperatures can promote the HI decomposition, but the HI can achieve the equilibrium decomposition rate within a shorter time at higher temperatures. The sensitivity analysis showed that the elementary reaction HI+HI=H2+I2was the major way to produce hydrogen.The modeling results of HI-H2O-I2decomposition shows that the iodine decreases the HI conversion at all the temperature range. Besides, the HI conversion calculated from HI/I2vapor is higher at600℃to850℃, but lower at850℃to1000℃at each HI/I2ratio. The role of residence time was also influenced by the reaction temperature, its influence weakens with the increase of temperature. Although the pressure did not change the thermodynamic decomposition equilibrium, but the greater pressure could promote the decomposition rate in the kinetic process. The sensitivity analysis showed that the elementary reaction HI+H=H2+I changed to be the major way to produce hydrogen at higher temperature.The catalytic decomposition shows that the activated carbon ash has very little catalytic effect. The most effective active site in activated carbon was the carbon skeleton or some kind of functional groups, the acitivated ingredients in the ash may be one of the inorganic salts as follows:orthoclase, albite, nepheline, aegirine, spinel. Besides, the water and iodine were all inhibit the HI catalytic decomposition, and the increase of water lead to a unstable catalytic curve.