Whole process simulation method of sulfuric acid decomposition in the iodine-sulfur cycle for hydrogen production

The high-temperature gas-cooled reactor is a typical fourth-generation nuclear reactor. It has a high core outlet temperature and great potential for process heat utilization. Thermochemical iodine-sulfur cycle hydrogen production is an essential method of process heat utilization of high-temperature gas-cooled reactors that can achieve large-scale, low-carbon hydrogen production and is consistent with the two-carbon strategic goal of China. Furthermore, the development and utilization of clean energy can effectively alleviate the global energy crisis, and hydrogen energy is considered the most promising source of energy in this century and is receiving continuous attention from the industry. The iodine-sulfur cycle includes three chemical reactions: Bunsen, sulfuric acid decomposition, and hydroiodic acid decomposition. Sulfuric acid decomposition is carried out under high temperature and strong corrosive environment, and involves multiple physical and chemical processes such as flow, heat transfer, phase transition and reaction. Therefore, obtaining the thermal and reaction details of this link is critical for improving the efficiency of the iodine-sulfur cycle. Furthermore, it is crucial to study the thermal and decomposition reaction laws of the fluid in the bayonet sulfuric acid decomposition heat exchanger to improve the decomposition rate of sulfuric acid. In this study, the classical Lee model was improved by analogy to the phase transition mass transfer equation and the component transport equation using the phase transition mass transfer rate constant instead of the chemical reaction rate constant, and a coupled model of the sulfuric acid phase transition and two-step decomposition reaction was established. The whole process of sulfuric acid decomposition was simulated, and the effect of the specific surface area of catalyst particles on the decomposition was analyzed. The results show that the temperature of the catalytic reaction zone inside the bayonet heat exchanger meets the requirements of the sulfuric acid core reaction. The phase transition process is relatively brief, yet it can effectively enhance the direct heat exchange between sulfuric acid and helium. As the sulfuric acid flow increases, the length of the two-phase section also increases. The first decomposition and phase transition of sulfuric acid occur almost simultaneously. The reaction is complete, the conversion rate is high, and the molar fraction of sulfur trioxide is up to 46%. The second-step decomposition of sulfuric acid permeates the entire zone of catalytic activity. The first half zone has a high conversion rate, and the sulfur dioxide molar fraction is up to 33%. Since the gas mixture in the inner tube continuously transfers heat to the sulfuric acid in the annulus, the temperature of the gas mixture at the outlet is lowered, and a small amount of sulfuric acid is produced. The research results also show that when the specific surface area of the catalyst particles is large, the overall rate of sulfuric acid decomposition is significantly improved. The highest rate of sulfuric acid decomposition under the design conditions is about 85%.