碘硫循环制氢是高温气冷堆的工艺热利用的重要途径,可实现大规模、近零碳排放制氢,契合中国的双碳战略目标。碘硫循环工艺中的硫酸分解环节涉及复杂的物理化学过程,且需要在高温和腐蚀环境下进行,保障该环节的分解效率对提高工艺整体制氢效率十分关键,因此建立硫酸分解全过程的耦合计算模型对研究硫酸分解率具有重要意义。本文建立了硫酸相变与两步分解反应的耦合模型,采用数值计算的方法对碘硫循环制氢中硫酸分解的全过程进行模拟,并重点分析了刺刀管式换热器内的催化剂颗粒尺寸对分解率的影响。结果表明,刺刀管式换热器内部的催化反应区域温度满足硫酸核心反应需求,内外管的明显温差促进了热量回收。硫酸相变过程虽较为短暂,但可以强化局部换热,此外两相段长度随流量增大而增大。硫酸的第1步分解与相变几乎同步进行,反应较为彻底,转化率较高。硫酸的第2步分解贯穿催化反应区域全程,该区域前半段的转化率较高。研究结果还表明,催化剂颗粒比表面积较大时,硫酸的综合分解率明显提升,最高可达85%左右。
Abstract
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%.
关键词
碘硫循环 /
高温气冷堆 /
硫酸分解 /
多过程耦合 /
数值模拟
Key words
iodine-sulfur cycle /
high-temperature gas-cooled reactor /
sulfuric acid decomposition /
multi-process coupling /
numerical simulation
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基金
彭威,副教授,E-mail:pengwei@tsinghua.edu.cn
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