【Haichuan Hydrogen Energy】Current Status and Trends in Organic Liquid Hydrogen Storage
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The basic principle of Organic Liquid Hydrogen Storage (LOHC) technology is to use liquid organic compounds containing unsaturated carbon bonds as hydrogen storage carriers, enabling reversible storage of hydrogen and recycling of the carriers through hydrogenation-dehydrogenation reactions. Unsaturated liquid organic compounds before hydrogenation are commonly referred to as liquid hydrogen storage carriers, while the products after hydrogenation are called hydrogen-storing organic liquids. Typical liquid hydrogen storage carrier materials include benzene, toluene, xylene, carbazole, nitroethylcarbazole, etc. The working principle of organic liquid hydrogen storage technology can be divided into three steps: 1. Hydrogenation: Hydrogen is added to the liquid hydrogen storage medium through a catalytic reaction, resulting in organic liquid compounds that can store hydrogen stably under normal temperature and pressure conditions (this process can be carried out in specialized hydrogenation plants) ； 2. Transportation: The hydrogen-rich organic liquid (hydrogen oil) produced through hydrogenation is transported to the end users using ordinary tank trucks. It is then filled into the storage tanks for hydrogen-rich organic liquids in a manner similar to filling up diesel or gasoline, ensuring a safe, simple, and rapid process of filling ； 3. Dehydrogenation: The hydrogen-storing organic liquid (hydrogen oil) is pumped to the dehydrogenation reactor using a metering pump, where it undergoes catalytic dehydrogenation under specific temperature conditions. After gas-liquid separation of the reaction products, the hydrogen is sent to applications that require hydrogen, such as fuel cells. The liquid carrier after dehydrogenation is subjected to heat exchange before being recovered and reused.
Compared with other hydrogen storage technologies, organic liquid hydrogen storage has the following advantages: (1) It offers a large hydrogen storage capacity and high storage density. Liquid organic hydrogen storage materials that use novel fused heterocyclic organic molecules as hydrogen carriers can achieve a volume hydrogen storage density of up to 60 g/L, with a reversible hydrogen storage capacity of around 5.6 wt%, which is significantly higher than that of traditional alloy-based and high-pressure hydrogen storage methods. (2) Safe storage and transportation: Hydrogen storage carriers and organic liquid materials for hydrogen storage are in liquid state at normal temperature and pressure, making storage safe. Ordinary pipelines, tank trucks and other equipment can be used to rapidly replenish the materials, and no hydrogen is generated nor is there any energy loss during the entire transportation and replenishment process. (3) The dehydrogenation (hydrogen supply) response is fast, making it suitable for use in combination with fuel cells. Organic liquids used for hydrogen storage exhibit a rapid dehydrogenation response, allowing hydrogen to be produced and used immediately, which facilitates good compatibility with fuel cell systems. (4) High purity of hydrogen and no exhaust emissions: The hydrogen obtained through the dehydration of organic liquid materials for hydrogen storage has a high purity (>99.99%), meeting the quality requirements for hydrogen used in fuel cell systems; moreover, there are no pollutants such as exhaust gases emitted during the dehydration process. (5) Liquid hydrogen storage carriers can be reused; the hydrogenation and dehydrogenation reactions of organic liquids used for hydrogen storage have high conversion rates, and the reaction process is highly reversible under certain conditions, allowing these liquid hydrogen storage carriers to be recycled. Research on organic liquid hydrogen storage technology dates back to 1975, when O. Sultan and M. Shaw first proposed using liquid aromatic compounds such as toluene and benzene as hydrogen storage carriers. By employing cyclic hydrogenation and dehydrogenation reactions with these storage carriers, reversible hydrogen storage became possible, thus opening up a new research direction for this type of hydrogen storage technology.
Currently, demonstration units of Germany’s HT company’s hydrogen storage system (Storage Box) and hydrogen release system (Release Box), which use dibenzyltoluene (with a material hydrogen storage density of 6.23 wt%) as the hydrogen storage medium, are in operation in Germany, while project testing is being carried out in the United States； On February 8, 2022, the Japanese company Chiyoda officially announced that it had achieved the \"world’s first\" large-scale offshore demonstration of hydrogen storage and transportation using methylcyclohexane (MCH) as an organic liquid for storing hydrogen.
The main domestic research institutions include Wuhan Huyang Energy Co., Ltd., China University of Geosciences (Wuhan), Zhejiang University, Xi’an Jiaotong University, and the 712 Research Institute of CSSC. In recent years, the 712th Research Institute of China State Shipbuilding Corporation has carried out engineering design and development work on organic liquid hydrogen storage technology to meet the hydrogen supply needs in areas such as green ships, fuel cell power plants, and defense equipment. In July 2020, the hydrogen source technology team of this institution completed the development of the first prototype in China of an organic liquid dehydrogenation unit using catalytic combustion for heating; this solution addressed the technical challenge of high energy consumption in organic liquid dehydrogenation, resulting in 11 pieces of proprietary intellectual property. In March 2022, China’s first 120-kilowatt-class hydrogen catalytic combustion heating system for organic liquid hydrogen supply, independently developed by the 712th Institute, was installed and tested successfully; it was able to supply hydrogen to fuel cell systems, further improving the technical maturity of hydrogen storage using organic liquids. At present, the organization is gradually promoting the demonstration application of organic liquid hydrogen storage technology in fields such as marine hydrogen energy, green ships, and high-power fuel cell power plants.
In summary, organic liquid hydrogen storage is an important development direction among the various hydrogen storage and transportation technologies currently available. Through continuous efforts by relevant domestic research institutions and enterprises, organic liquid hydrogen storage technology has been successfully applied on a pilot basis in areas such as fuel cell vehicles, distributed energy storage, urban solid waste utilization, and fuel cell power plants. The maturity of this technology is constantly improving, and it is currently in the early stages of industrial application. To address the current issues associated with organic liquid hydrogen storage materials, such as high costs, the need for large amounts of precious metal catalysts in hydrogenation/dehydrogenation processes, and high energy consumption during dehydrogenation, researchers can take the following measures: 1. By scaling up the production of raw materials, it is possible to gradually reduce the cost of liquid hydrogen storage materials; at the same time, efforts should be made to develop new types of liquid hydrogen storage materials with high hydrogen storage capacity and low costs ； 2. Developing highly active and long-lasting non-precious metal catalysts can significantly reduce the costs of materials used for hydrogenation and dehydrogenation catalysts, thereby lowering the production cost of hydrogen ； 3. Further explore application scenarios for organic liquid hydrogen storage, make full use of external waste heat, and increase the effective hydrogen storage density of organic liquids.