| Hydrogen energy,as a new password of modern energy industry system,has attracted extensive attention in recent years due to its advantages of cleanliness,renewability,storage and wide application.Among them,thermochemical processes,especially the Iodine–Sulfur（IS）cycle is considered to be one of the most promising hydrogen production methods owing to its advantages of low cost,high efficiency,easily scale up.The IS cycle consists of three sections:Bunsen reaction section,H2SO4 decomposition section and HI decomposition section,which corresponds to the following three-step thermochemical reactions:Bunsen reaction:2H 2O+I2+SO2→H 2SO4+2HI HI catalytic decompostion reaction:2HI→H2+I2H2SO4 catalytic decompostion reaction:H 2SO4→H 2O+SO2+1/2O2After decades of development,the technology of hydrogen production from IS cycle has made great progress,but it is still far from large-scale industrial application.In 2015,our team built a small-scale IS cycle with a hydrogen production rate of 50 L/h,and obtained valuable data and experience on this system.On this basis,a pilot-scale system with a hydrogen production rate of 5 Nm3/h was constructed by engineering materials in 2019.This study is devoted to realize the continuous and stable operation of 5 Nm3/h IS pilot-scale system and discuss the development path of IS cycle.Again,the suitable heat sources,which could be coupled to IS cycle,are also investigated.The recommended operating range of Bunsen reaction given by previous studies was wide,and the stable stratification time was not considered,which is difficult to guide the debugging of the pilot-scale system.In this paper,the recommended working conditions are further refined,and the effects of iodine,water and temperature on the characteristics of liquid-liquid stratification are explored with the least amount of raw materials.After comprehensive analysis,the initial H2O/I2/HI/H2SO4 molar ratio of 12/2.4/2/1,with the temperature of 353 K,and the initial H2O/I2/HI/H2SO4 molar ratio of 12/2.8/2/1,with the temperature of 363 K are considered to be the best Bunsen reaction conditions,and complete separation of the two phases could be achieved in 25 minutes.For HI decomposition,the activated carbon could achieve better catalytic effect after retorting at 850℃and steam activation at 650℃.It was found that activated carbon was not suitable for pickling-activation.Although some ash could be removed,the specific surface area and pore volume would decreaseed,which affected the catalytic effect.The amount of water in HI solution had little effect on the decomposition efficiency,but the decomposition efficiency decreased significantly with the increasing iodine content.Based on this,the reaction mechanism of iodine-containing hydroiodic acid decomposition on activated carbon was revealed.A new method of primary decomposition at low temperature and secondary decomposition at high temperature was proposed to improve the overall conversion rate of HI.For H2SO4 decomposition,Fe2O3 is a stable and reliable catalyst,but its low specific surface area and pore volume limit the further improvement of sulfuric acid decomposition rate.So,it is necessary to further develop high-efficiency,low-cost,corrosion-resistant sulfuric acid decomposition catalysts.The process simulation of 5Nm3/h IS pilot-scale system was carried out by Aspen Plus software,and the key reaction units were designed based on above calculation results.After fully considering the mechanical properties,processing performance and price,TC4 was selected as the material of Bunsen reaction section and HI decomposition section,Incoloy 800H was selected as the material of H2SO4 decomposer,and Hastelloy C276 was selected as the material of H2SO4purification tower,flash tower and cooler.After finishing the independent debugging of every functional unit,an electric heating rod was installed in the HI decomposition reactor to increase heat transfer,and the material of the sulfuric acid decomposition reactor was replaced with Si C to avoid corrosion.On this basis,the system ran continuously for 4 h at a hydrogen production rate of 80 L/h.Finally,based on the problems existing in debugging,an optimization scheme is proposed,and Fluent software is used to compare the sulfuric acid decomposers of various structures.It is found that the simultaneous heating of the inner heating rod and the outer wall can enhance the heat transfer performance and significantly reduce the length of the preheating section.The continuous operation of the system indicates that the production of hydrogen from IS cycle is one step closer to commercial applications.In this paper,the two most promising development directions of IS cycle,closed-loop coupled to high-temperature gas-cooled reactors（Case A,B,C）and the open-loop cycle coupled to sulfuric acid plant（Case D）were introduced in detail,then above four cases were comprehensively evaluated from the aspects of thermal efficiency,economy and environmental impact.In order to make full use of the internal energy,the internal heat exchange network was reasonably constructed,and the temperature difference of hot and cold streams was set to 5 oC.The thermal efficiency of the closed-loop cycle and the open-loop cycle was calculated as 50.94%and 81.9%respectively.With market electricity price of0.075 US$/k Wh（0.51 CNY/k Wh）and sulfuric acid price of 45 US$/t（306 CNY/t）,the levelized cost of Case A,B,C and D was 2.26,1.82,1.33 and 1.64 US$/kg（15.37,12.38,7.82and 11.15 CNY/kg）H2respectively.Therefore,the closed-loop coupled to high-temperature gas-cooled reactors with more power generation and less hydrogen production and the open-loop cycle coupled to sulfuric acid plant have more advantages in levelized cost.During the life cycle assessment,it was found that the closed-loop coupling system for hydrogen production only and the open-loop coupling system had little effect on the environment.It is considered that the closed-loop coupling system with half heat for hydrogen production and half heat for power generation and the open-loop coupling system for co-production of hydrogen and sulfuric acid are more promising.Aiming at 1000 Nm3/h hydrogen production,the coupling modes of the closed-loop hydrogen production system with the fourth-generation nuclear power reactors and the tower photothermal system were explored.Based on the principle of energy cascade utilization,the heat transfer process of the coupling system was reasonably designed.The amount of heat source was reasonably calculated,the temperature drop corresponding to each reaction unit of the IS cycle was given and the heat balance diagram was drawn.Finally,the thermal efficiency of the coupling system was evaluated.The results show that the high-temperature gas-cooled reactor,the gas-cooled fast reactor and the tower photothermal system coupled to IS cycle have more application prospects owing to the absolute advantages in process structure,heat source medium usage and system efficiency,meanwhile,the supercritical water reactor coupled to IS cycle can be regarded as an effective supplement. |
