Several factors contribute to the current difficulty of plastic pipes in penetrating the large and medium diameter, medium and low pressure pipe market:

　　I. Solid wall plastic pipes cannot fully meet pipeline performance requirements

　　1. Polyvinyl chloride (PVC) pipes

　　Polyvinyl chloride resin has poor fluidity, making it difficult to manufacture large-diameter solid wall pipes. The diameter of most widely used rigid polyvinyl chloride (PVC-U) pressure pipes globally is mostly around 1200mm. Currently, the maximum diameter of PVC-O pipes produced abroad is 800mm.

　　 2. High-density polyethylene (HDPE) pipes

　　Polyethylene resin has good fluidity and can be used to manufacture large-diameter solid wall pipes. The maximum diameter of HDPE pipes produced abroad reaches 2500mm, while the maximum diameter of HDPE pipes produced domestically is 1600mm. However, because HDPE pipe materials require a larger wall thickness to withstand internal pressure, larger diameter HDPE pipes are currently mostly used in low-pressure applications.

　 　II. Large diameter HDPE pipe connections are inconvenient

　　The vast majority of HDPE pipes use butt fusion welding or electrofusion fittings. Fusion welding has many advantages, but it is inconvenient for large-diameter HDPE pipes, and its reliability is also reduced. The electrofusion fitting connection method requires an appropriate and uniform gap between the fitting and the pipe, and to ensure welding quality, the oxide layer on the pipe surface must also be removed, which is difficult to achieve with large-diameter pipes and fittings.

　　Butt fusion welding requires heavy welding equipment and strict welding processes. Completing a weld point on a large-diameter pipe often takes a considerable amount of time. Another problem is that the quality of the weld is difficult to non-destructively test. Although non-destructive testing technologies using ultrasound and microwaves have been explored and developed in recent years, they have not yet been widely adopted. Currently, large and medium diameter, medium and low pressure pipelines of ductile iron pipes, welded steel pipes, PCCP pipes, glass fiber reinforced plastic sand pipes (RPMP), etc., mostly use rubber ring sealed spigot and socket connections, especially for larger diameters.

　　 III. HDPE solid wall pipes use a lot of materials and are expensive

　　Larger wall thickness HDPE solid wall pipes will increase the cost of pipelines.

　　In short, it is unrealistic to rely solely on non-reinforced plastic solid wall pipes to open up the large and medium diameter pressure pipe market. The opportunities for non-reinforced plastic solid wall pipes in this area are only in low-pressure situations (such as sea discharge pipelines) or as anti-corrosion lining pipes for traditional pipe materials.

　　The global plastic pipe industry has been working to bring plastic pipes into the large and medium diameter, medium and low pressure pipe market.

　　IV. Exploring improved resin performance, improved processes and equipment, and the development of large and medium diameter pressure pipes

　　 1. The global plastic raw material industry has been exploring ways to improve resin performance.

The hope is to produce higher-strength resins, allowing plastic pipes to successfully enter the large and medium diameter pressure pipe market. In the early development of HDPE resin for pipes, the strength grade increased rapidly, from PE40, PE63, PE80 to PE100 in just 30-40 years. However, for the past two decades, it has remained at PE100, and the long-awaited PE125 has yet to enter the stage of large-scale engineering applications. The minimum required strength MRS of HDPE pipes remains at the 10MPa level.

　　In recent years, the development of low melt flow resin PE100 has allowed the pipe wall thickness to be increased to around 100mm, expanding the diameter and pressure range of HDPE solid wall pipes. Although domestic and foreign production lines for manufacturing DN2500mm diameter HDPE solid wall pipes already exist, in practice, the application of larger diameter pipes is still limited to low-pressure situations. The strength of polyvinyl chloride pipes has not increased in recent years, and the MRS of PVC-U pipes remains at 25 MPa, but the pipe diameter has increased to 1200mm or even larger. In recent years, the highest MRS grade of PVC-O pipes has reached 50 MPa, but the current maximum diameter is only 800mm. Analyzing the current situation, the hope of thermoplastic pipes opening up the large and medium diameter pressure pipe market through improvements in plastic resin strength and solid wall pipe processing equipment is not high.

　　 2. Exploring large-diameter steel-reinforced polyethylene pressure pipes

　　Compared to steel prices and labor costs, the price of plastic raw materials in China has been relatively high for a long time, unlike in other countries. China adopted steel wire reinforced polyethylene pipes in the medium and low pressure pipeline field very early on. The main purpose was to reduce resin consumption and lower costs. Among them, steel wire reinforced polyethylene pipes (the construction industry standard is CJ/T 187) were the earliest and most widely used, mainly in the small and medium diameter range. In recent years, the maximum diameter has reached 1000mm. However, only electrofusion socket connections can be used, and the larger the diameter, the greater the difficulty. Domestic companies are also exploring and developing other types of large and medium diameter steel-reinforced polyethylene pressure pipes, but there are not many mature products. The reason is that steel and polyethylene do not bond well. Using steel wire reinforcement and adhesive resin as an intermediary can achieve relatively reliable bonding, but the performance indicators of the interface are limited, and there are problems such as the influence of the use temperature on the bonding strength and corrosion protection.

　　 3. Exploring short glass fiber reinforcement, using winding fusion molding instead of direct extrusion

　　Foreign companies have been exploring for over a decade the method of manufacturing large and medium diameter polyethylene pressure pipes using polyethylene winding fusion molding inner lining + glass fiber reinforced layer + polyethylene winding outer protective layer. This glass fiber reinforced polyethylene (glass fibre reinforcedpolyethylene) pipe (referred to as PE-GF pipe) has already been used in engineering projects abroad. Domestic companies have also begun research and development.

　　PE-GF pipes have a three-layer structure: inner and outer layers of PE100, and the middle layer is short glass fiber reinforced polyethylene, using winding fusion molding. The maximum inner diameter can reach 4m, and the wall thickness of PE-GF pipes can be reduced by nearly 50% compared to PE100 pipes of the same pressure rating. PE-GF pipes can be connected using butt fusion welding and spigot and socket electrofusion methods. This pipe material has been used in recent years in Belgium, Turkey, Colombia, and Japan [2], and has been included in DIN and ASTM standards (DIN SPEC 19674 [3], ASTM F2720/F2720M [4]).

　　However, PE-GF pipes are not widely used at present. It is understood that there are only 9 PE-GF pipe production lines worldwide. The reason is that the reinforcement effect using short glass fiber is not ideal: the MRS value of the highest grade PE-GF 200 material is only 20 MPa, and the design stress is 12.5 MPa (DIN SPEC 19674 [3]). Compared to the design stress of 8 MPa for PE100 material, it is only 1.56 times higher. Therefore, the effect on reducing material consumption and lowering costs is not very significant.

　 　4. Exploring continuous glass fiber reinforced polyethylene pressure pipes

　　Currently, continuous fiber-reinforced thermoplastic (CFRT) is becoming one of the hotspots in reinforced plastics. High-performance/weight ratio and high-performance/price ratio CFRT materials are rapidly developing and are being promoted from fields such as aerospace and automobiles to many industries, including the pipeline production industry.

　　The materials of CFRT pipes include various high-strength fibers (glass fiber, aramid fiber, carbon fiber...) and various thermoplastics (polyethylene, polypropylene, polyamide, PVDF...). Due to cost-effectiveness and simplicity of the process, the most commonly used material is glass fiber-reinforced polyethylene.

　　Glass fiber is abundant, inexpensive, and has high strength, making it an excellent reinforcing material. However, glass fiber is a brittle silicate material with poor wear resistance, flexural resistance, and torsional resistance, and must be pre-impregnated and coated in a polymer material. One of the reasons why glass fiber-reinforced thermoplastics were difficult to promote in the past is that thermoplastics are also highly viscous in the molten state, making it difficult to impregnate glass fibers. Currently, CFRT has overcome this difficulty and can perfectly coat continuously and parallel-aligned glass fibers in thermoplastic plastics. The intermediate product of CFRT is a reinforcing belt.

　　Currently, some enterprises in China have been able to produce CFRT reinforcing belts of glass fiber-reinforced HDPE and PP through technology introduction or independent research and development. The performance is close to the international level. Glass fiber-reinforced PE belts have a glass fiber content of 70%, a thickness of 0.28-0.3mm, a tensile strength of 505MPa, a tensile modulus of 25.7GPa, and an elongation at break of 2.75%.

　　Glass fiber belt-reinforced polyethylene pressure pipes usually have a three-layer composite structure. The inner lining is a solid wall layer of PE100, the middle layer is a reinforcing layer formed by winding and fusion of glass fiber reinforced belts, used to withstand internal pressure loads. The outermost layer is the PE100 outer protective layer, which protects the pipe reinforcement layer (including UV resistance).

　　In recent years, China has made rapid progress in composite materials. It has not only become the world's largest producer of glass fiber, but the basic material for continuous glass fiber-reinforced thermoplastic pipes—glass fiber belts—has also been industrialized and its performance is close to the international level. This lays the foundation for us to develop high-strength large and medium-diameter plastic pressure pipes using new materials.

　　Preliminary exploratory tests in China have shown that the method of using CFRT reinforcement and winding fusion to manufacture large-diameter thermoplastic pressure pipes is feasible in terms of process routes and reliable in terms of structural mechanics, and can significantly reduce wall thickness and material consumption. Currently, the tensile strength of the reinforcing material used in exploratory development—domestically produced continuous glass fiber-reinforced polyethylene belts—is above 500 MPa (higher ones can reach above 700 MPa), which is about 20 times the tensile strength of PE100 (25 MPa). Therefore, in the experiment, a pipe with a diameter of 1000 mm and a pressure of 1 MPa only needs a composite layer of about 3 mm thick glass fiber belt to ensure that the burst pressure exceeds 3 MPa. Economically, the cost of a 3 mm thick glass fiber belt composite layer is approximately equivalent to 9 mm of PE100, while the strength is equivalent to 60 mm thick PE100 material. According to product tests, compared with PE100 pipes of the same diameter and pressure, the wall thickness of large-diameter polyethylene pressure pipes reinforced with glass fiber belts can be reduced by more than 50%, and the cost can be saved by about 40%.

　　 V. Pre-impregnated Continuous Glass Fiber Reinforced Thermoplastic Composite Pipes

　　1. Pre-impregnated continuous glass fiber reinforced thermoplastic composite pipes are currently the most advantageous and cost-effective products on the market. They have unparalleled advantages such as ultra-high pressure resistance, thin walls, light weight, corrosion resistance, temperature resistance, easy construction, and complete recyclability. They are widely used in the petroleum, natural gas, chemical, pharmaceutical, hot spring, and water supply industries. They are particularly advantageous in high-pressure requirements such as high mountains, deep seas, and long-distance transportation. They can also easily solve the problem of old pipe repair using the inner liner drag pipe method.

　　Currently, well-known companies at home and abroad are developing such products. With the establishment of (National Petroleum and Natural Gas Pipeline Company), the release of the industry standard "Non-metallic Composite Pipes for Petroleum and Natural Gas Industry, Part 2: Flexible Composite High-pressure Transportation Pipes" (Note: under review), and the expansion of urban intelligent pipe networks, it is believed that Pre-impregnated continuous glass fiber reinforced thermoplastic composite pipes will be the most influential in the future market.

　　Hydrogen Energy Technology (Jiangsu) Co., Ltd. has a team of elites with rich practical experience and strong technical strength, who are unwilling to lag behind and dare to struggle. They regard the market as their goal and difficulties as motivation. Through countless unremitting efforts, they have finally developed CFRTP pipe extrusion production equipment and matching glass fiber reinforced pipe fittings.

　　The company's developed CFRTP pipe extrusion production equipment Beautiful appearance—jointly designed with the School of Mechanical Engineering, Jiangsu University of Science and Technology; large pipe diameter range—high-pressure pipes (up to 30 MPa) from ￠80-￠800, medium and low-pressure pipes (0.4-1.6 MPa) from ￠1000-￠4000; high degree of automation—its unique automatic continuous belt technology without stopping the machine allows for automatic replacement of the belt on the glass fiber disc without stopping the machine, greatly improving production efficiency; strong internal structure—the main machine uses the most advanced high-speed and high-efficiency screw, European technology high-torque gearbox, high-energy-efficient long-life heater, contactless solid-state relay, high-strength metal coating, Siemens PLC software; the vacuum chamber uses vacuum frequency conversion control, automatic water level and temperature control system, the pipeline uses European oversized water filter to ensure that the pipeline is not blocked for a long time; the traction machine uses multi-power constant torque servo motor traction, multiple tracks enter and exit at the same time, which increases the traction torque and ensures the roundness of the pipe; the cutting machine uses chip-free cutting technology, which is not only silent and chip-free, but also makes the port smooth and beautiful.

　　 CFRTP pipe extrusion production equipment The inner layer of the produced pipes is produced continuously using a continuous extrusion method, the middle glass fiber reinforced layer is produced using a multi-layer cross-online winding method, and then the outer layer is continuously extruded on the outside using a continuous extrusion method. The product can be customized to the required length (6M, 9M, 12M), or it can be continuously produced to the desired length without stopping the machine.

　　The company also owns a number of invention patents (under application), including bonding technology of glass fiber belt and plastic layer, glass fiber winding technology, online automatic continuous belt technology for glass fiber, automatic circularity technology, online pipe making technology, pipe fitting production technology, and electrofusion pipe technology.

　　 1 Preface

　　Hydrogen energy, as a green, environmentally friendly, abundant, and widely used emerging energy source, has become a powerful means for the end-use sector to achieve green and low-carbon transformation. As a strategic emerging industry and the direction of future new energy industry development, the hydrogen energy industry has profound significance for achieving the goals of "carbon peak and carbon neutrality" and promoting the revolution in energy production and consumption. According to the International Hydrogen Energy Council's prediction, by the middle of this century, 18% of global energy end-use demand will be met by hydrogen energy. Currently, the global community is accelerating the layout of the hydrogen energy industry. Japan, which started developing hydrogen energy earliest, established the Hydrogen Energy Association in 1973 and released the world's first national hydrogen energy strategy, the "Basic Hydrogen Energy Strategy," in 2017. The United States released the "Comprehensive Energy Strategy" in 2014, having previously defined a four-stage roadmap for national hydrogen energy development, and it is expected that by 2050, hydrogen energy will account for 14% of the United States' end-use energy demand. In addition, developed countries and regions such as Russia, Germany, the European Union, and South Korea have also issued corresponding policies to promote the development of the hydrogen energy industry.

　　Currently, in the entire hydrogen energy industry chain of "production, storage, transportation, and utilization," the cost of storage and transportation accounts for approximately 30%~40% of the total cost, which is the main bottleneck restricting the large-scale development of hydrogen energy. A thorough review of the current status of hydrogen energy storage and transportation technology and an analysis of future development trends are of profound significance for the large-scale application of hydrogen energy.

　　 2 Current Status of the Hydrogen Energy Industry

　　2.1 Current Status and Future Development Forecast of Global Hydrogen Energy Applications

　　Major developed countries around the world attach great importance to the development of the hydrogen energy industry. Key core technologies in the global hydrogen energy industry chain are maturing, and the construction of hydrogen energy infrastructure is accelerating significantly.

　　Developed countries that attach importance to the development of the hydrogen energy industry, such as the United States, Japan, and Germany, have successively introduced relevant policies to encourage the development of hydrogen energy. The United States' "Hydrogen Economy Roadmap" proposes that the number of hydrogen fuel cell vehicles in operation in the United States will reach 200,000 in 2025 and 530,000 in 2030. The European Union's "European Hydrogen Roadmap: A Sustainable Path for Europe's Energy Transition" proposes that the number of hydrogen fuel cell passenger cars in the European Union will reach 370,000 in 2030. Japan's "Basic Hydrogen Energy Strategy" proposes that the annual production of hydrogen fuel cell passenger cars in Japan should reach 200,000 in 2025 and 800,000 in 2030.

　　From a domestic perspective, China, as the world's largest hydrogen producer, with an annual hydrogen production of approximately 3,300,000 tons, has enormous potential in the supply of clean and low-carbon hydrogen. According to the China Hydrogen Energy Alliance's forecast, the output value of China's hydrogen energy industry will reach 1 trillion yuan by 2025; the demand will reach 3,500,000 tons in 2030, accounting for more than 5% of China's terminal energy system; the demand will approach 6,000,000 tons in 2050, achieving a reduction of approximately 700 million tons of carbon dioxide emissions, accounting for more than 10% of the terminal energy system, and the annual output value of the industry chain will reach 12 trillion yuan. On April 10, 2023, Sinopec announced its plan to build a "West-to-East Hydrogen Transmission" pilot project with a total length of over 400 kilometers, incorporating it into the "National One-Network Construction Implementation Plan" for petroleum and natural gas, marking the launch of the country's first long-distance pure hydrogen transmission pipeline project. The pipeline will be over 400 kilometers long, with an initial capacity of 100,000 tons per year and a long-term capacity of 500,000 tons per year. Upon completion, it will be used to replace existing fossil fuel hydrogen production and transportation hydrogen in the Beijing-Tianjin-Hebei region, significantly alleviating the mismatch between the supply and demand of green hydrogen in China. On April 16 of the same year, PetroChina achieved a significant breakthrough in the technical aspect of long-distance transportation of hydrogen using existing natural gas pipelines, effectively supporting China's future large-scale, low-cost, and long-distance hydrogen transportation. PetroChina conducted on-site tests on the Ningdong natural gas hydrogen blending pipeline demonstration project in Ningxia. The hydrogen blending ratio in this 397-kilometer pipeline has gradually reached 24%, achieving safe and stable operation for 100 consecutive days.

　　According to statistics, the total length of planned hydrogen transmission pipelines in China (including those already built) exceeds 1,800 kilometers. According to the hydrogen energy industry development plan, by 2030, the total mileage of long-distance hydrogen pipelines will reach 3,000 kilometers.

　　2.2 China's Hydrogen Energy Industry Standard System

　　China's hydrogen energy development and construction started later than that of developed countries, and relevant regulations and standards still need to be improved. In 2023, China issued the "Construction Guide for the Hydrogen Energy Industry Standard System (2023 Edition)", which is the first national-level construction guide for a hydrogen energy industry chain standard system. The hydrogen energy industry standard system is based on basic and safety standards, supporting key technical standards for the entire industry chain of hydrogen production, storage and transportation, refueling, and application. Basic and safety standards are at the top level of the hydrogen energy industry standard system structure and are the basic support for hydrogen supply and hydrogen application standards. Hydrogen production standards, hydrogen storage and transportation standards, and hydrogen refueling standards constitute hydrogen supply standards and are the basic guarantee for hydrogen application standards. The structure of the hydrogen energy industry standard system is shown in Figure 1.

 

 

　　The framework of the hydrogen energy industry standard system consists of five parts: basic and safety, hydrogen production, hydrogen storage and transportation, hydrogen refueling, and hydrogen energy application, as shown in Figure 2.

 

 

　　 3 Hydrogen Storage Technology

　　Hydrogen storage technology is a key link in hydrogen energy application systems and is one of the key factors restricting the large-scale application of hydrogen. In the process of hydrogen energy application, providing a stable and safe hydrogen storage solution is the primary guarantee for meeting the current and future large-scale application of hydrogen energy. According to the physical properties of hydrogen, scholars currently divide the mainstream hydrogen storage methods into four categories: high-pressure gaseous hydrogen storage, low-temperature liquid hydrogen storage, organic liquid hydrogen storage, and solid-state hydrogen storage. The advantages and disadvantages of various hydrogen storage technologies are shown in Table 1.

　　3.1 Gaseous Hydrogen Storage

　　Due to hydrogen's very small relative molecular mass and low density, high-quality storage is impossible at room temperature and in a gaseous state. Therefore, gaseous hydrogen storage mainly refers to high-pressure gaseous hydrogen storage. Under high pressure, gaseous hydrogen molecules are compressed to high density, and then the high-density gas is stored in high-pressure resistant containers. This is the most widely used method in hydrogen energy storage and has become a relatively competitive means of on-board hydrogen storage. The pressure range of containers storing high-pressure hydrogen is 15.2~70.9 MPa, and the technologies are relatively mature. Table 2 shows the types of high-pressure gaseous hydrogen storage pressure vessels that have been developed. Type I and Type II containers have low hydrogen storage density and serious hydrogen embrittlement problems. Type III and Type IV containers have relatively higher hydrogen storage density and are often used in on-board hydrogen storage. Currently, most hydrogen refueling stations in China mainly use high-pressure gaseous hydrogen storage.

 

 

 

　　3.2 Liquid Hydrogen Storage

　　When hydrogen is stored in liquid form, the commonly used methods can be divided into two categories: low-temperature liquid hydrogen storage and organic liquid hydrogen storage.

　　3.2.1 Low-Temperature Liquid Hydrogen Storage

　　The method of storing gaseous hydrogen by compressing and deep-cooling it to below -253°C to turn it into liquid, and then storing it in an insulated vacuum storage container, is called cryogenic liquid hydrogen storage, which is a physical storage method. Liquid hydrogen has a high hydrogen storage density, reaching 70.9kg/m3 at atmospheric pressure, which is 856 times the density of hydrogen under standard conditions. It has a large volumetric capacity and offers significant advantages for large-scale, long-distance hydrogen energy storage and transportation. However, the liquefaction process of hydrogen consumes a large amount of energy. It is estimated that liquefying 1kg of hydrogen consumes 4~10 kWh of electricity. Moreover, due to the very low boiling point of hydrogen, it is prone to volatilization by absorbing heat during storage. Therefore, special containers that can withstand ultra-low temperatures, maintain ultra-low temperatures, withstand pressure, and have strong sealing properties are required during liquid hydrogen storage. These containers are difficult and costly to manufacture, which is also a major problem limiting cryogenic liquid hydrogen storage.

　　Liquid hydrogen, as the primary fuel for large rockets, is currently often used in the aerospace industry. Due to the immaturity of research and high application costs in this field in China, it is currently rarely used in the civilian sector. With the development of technology, since 2021, the country has successively issued three national standards related to liquid hydrogen: GB/T 40045-2021 "Liquid Hydrogen Fuel for Hydrogen Energy Vehicles", GB/T 40060-2021 "Technical Requirements for Liquid Hydrogen Storage and Transportation", and GB/T 40061-2021 "Technical Specifications for Liquid Hydrogen Production Systems". These standards have achieved a breakthrough from "zero" in civil standards for China's liquid hydrogen industry, providing strong support for the marketization of the liquid hydrogen industry.

　　3.2.2 Organic Liquid Hydrogen Storage

　　The proposal of organic liquid hydrogen storage can be traced back to 1975. Organic liquid hydrogen storage is a method that uses organic hydrogen storage liquids (LOHC) capable of reacting with hydrogen to form stable hydrogen energy carriers for storage. This method utilizes the reversible hydrogenation and dehydrogenation processes of liquid unsaturated organic compounds to achieve hydrogen storage and release. The reference process is shown in Figure 3. Organic liquid hydrogen storage has a mass hydrogen storage density of about 5%~10%, with a large hydrogen storage capacity. Moreover, the storage medium is liquid organic matter, which can also be transported at room temperature and pressure during storage, making it safer than gaseous storage. Commonly studied liquid hydrogen storage media include hydrocarbons such as benzene, toluene, naphthalene, and organic liquid hydrogen storage materials like ethylcarbazole. Related information and properties are shown in Table 3. This technology is still in the research stage in China and has not yet been widely popularized.

 

 

 

　　3.3 Solid-State Hydrogen Storage

　　"High hydrogen storage density, fast hydrogen absorption/desorption performance, and long-cycle stability" are the basic requirements for ideal hydrogen storage materials. Compared with several other hydrogen storage methods, solid-state hydrogen storage has a relatively larger hydrogen storage density and higher safety, possessing great potential to meet the goals of the International Energy Agency (IEA). According to the different forces between the adsorbent and adsorbate, solid-state hydrogen storage materials can be divided into two main categories: physical adsorption and chemical adsorption. Among them, physical adsorption materials include traditional carbon-based porous materials, mesoporous materials, metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), etc. Physical adsorption mainly achieves hydrogen storage through relatively weak Van der Waals forces, requiring higher adsorption pressure and only achieving hydrogen storage at lower temperatures (77K). Carbon-based hydrogen storage materials mainly include activated carbon, activated carbon fibers, carbon nanofibers, carbon nanotubes, and carbon aerogels, etc. Table 4 lists the hydrogen storage performance of four types of carbon-based materials.

 

 

　　As can be seen from Table 4, at room temperature, the mass hydrogen storage density of graphitic carbon nanofibers is the highest among the other three materials, exceeding 10%, which is due to their special structure. However, carbon-based materials require relatively harsh environmental conditions for the hydrogen adsorption process, have significant limitations in industrial use, and have not yet been widely popularized.

　　Chemical adsorption achieves hydrogen storage by forming hydrides through metallic bonds, covalent bonds, coordinate bonds, etc. Chemical adsorption materials include metal hydrides, complex hydrides, hydrogen clathrates, etc. Among them, metal hydrides such as magnesium-based alloys, rare-earth-based alloys, and titanium alloys often use Pressure-Composition-Temperature curves (PCT curves) to characterize their hydrogen absorption and desorption thermodynamic properties, as shown in Figure 4. In the figure, the x-axis represents the ratio of hydrogen atoms to metal atoms, and the y-axis represents hydrogen pressure. The segment AB in the figure represents the effective hydrogen storage capacity. Under certain temperature conditions, the equilibrium hydrogen pressure is approximately constant. As the temperature increases, the AB segment gradually shortens, indicating that excessively high temperatures are not conducive to the occurrence of hydrogen absorption reactions. The PCT curve is an important indicator for evaluating hydrogen storage performance, directly reflecting the reversible hydrogen storage capacity, equilibrium hydrogen pressure, plateau slope, and hysteresis effect of hydrogen storage materials.

 

 

　　Unlike metal hydrides, complex hydrides are salts where hydrogen atoms are covalently bonded to the central atom of a complex anion to form a complex anion, which then combines with metal ions through ionic bonds to form the hydride. Hydrides are generally represented by the chemical formula AMeH, where A is usually a Group 1 or Group 2 element of the periodic table, and Me is usually B, Al, or N. The theoretical gravimetric hydrogen storage density is 5.5%~21%. Hydrogen release from complex hydrides typically occurs via two methods: hydrolysis or pyrolysis.

　　 4 Hydrogen Energy Pipeline Transportation Technology

　　As a key direction for the future development of new energy, the safety, efficiency, and cost of hydrogen energy storage and transportation are major bottlenecks limiting its large-scale development, and are also hot and key research topics domestically and internationally. Given that pipeline transportation has long been considered a low-cost method for large-scale media transportation, this analysis will focus solely on several possible pipeline transportation methods for hydrogen energy.

　　4.1 High-Pressure Hydrogen Pipeline Transportation

　　Hydrogen and methane have similar properties, and high-pressure hydrogen pipeline transportation can draw on the experience of natural gas pipeline construction. However, compared to a natural gas environment, a hydrogen environment can lead to the degradation of the mechanical properties of pipeline steel. Many research and experimental projects have found that the dissolution and diffusion of hydrogen atoms in the metal lattice can cause degradation of metal material properties, decarburization, and even damage such as blistering or cracking. Compared to the natural gas pipeline industry, there are no unified design and material selection standards for room-temperature high-pressure hydrogen-containing pipelines. However, with the development of the times, countries worldwide have gradually begun to plan hydrogen energy storage and transportation technologies in recent years, proposing design schemes for long-distance pipelines transporting pure hydrogen and blended hydrogen at pressures above 10MPa.

　　To ensure the safety of high-pressure hydrogen pipeline transportation, hydrogen embrittlement countermeasures must be adopted, such as selecting low-strength pipeline steel with low hydrogen embrittlement sensitivity or reducing hydrogen transmission pressure. This also leads to higher construction costs and lower economic benefits for hydrogen pipeline systems compared to natural gas pipeline systems. Some literature indicates that the construction cost of hydrogen pipelines is approximately US$620,000/km, while the construction cost of natural gas pipelines is only US$190,000/km, making the cost of hydrogen pipelines about three times that of natural gas pipelines. At the same time, due to the high cost of hydrogen production and low energy density, it is temporarily less competitive than traditional fossil fuels. Currently, the price of hydrogen at operating hydrogen refueling stations in China is generally between 60 and 80 yuan/kg, which is not significantly advantageous compared to gasoline, diesel, and pure electric vehicles. In the short term, it can only rely on subsidies, and in the long run, it cannot solve the fundamental problems of industrial development. It is estimated that only when the price of hydrogen at hydrogen refueling stations falls below 40 yuan/kg can hydrogen energy truly move towards a 'market-driven' model.

　　Despite the aforementioned problems, high-pressure hydrogen pipeline transportation technology has a low threshold and is the most mature, so it is still the preferred hydrogen transportation method for newly constructed long-distance hydrogen pipelines. Globally, according to incomplete statistics, the current construction scale of hydrogen pipelines is close to 4700 km, with relevant statistical data shown in Table 5. The United States has already built 2720 km of hydrogen pipelines, while Europe has more than 1500 km of hydrogen pipelines.

 

 

　　4.2 Hydrogen Blending in Natural Gas Pipelines

　　Blending a certain proportion of hydrogen into natural gas transmission pipelines is considered one of the currently viable methods for large-scale hydrogen transportation. Some scholars believe that, referring to the current several types of hydrogen transportation methods, long-distance pure hydrogen pipelines and hydrogen blending in natural gas pipelines are the most promising technologies for realizing large-scale networked hydrogen transportation, especially for existing natural gas pipelines. Minor modifications to achieve hydrogen blending in natural gas pipelines can save a large amount of infrastructure construction costs. One of the mid-term (2020-2030) tasks for the development of the hydrogen energy industry in China is to demonstrate the application of hydrogen-blended natural gas pipeline transportation technology, laying the foundation for entering a non-carbon "hydrogen energy era." Table 6 lists some typical domestic and international natural gas pipeline hydrogen blending projects.

　　Hydrogen blending in natural gas pipelines can utilize existing natural gas pipeline facilities, reducing the upfront construction cost of hydrogen pipelines. This is expected to compensate for the disadvantages brought about by the low volumetric energy density of gaseous hydrogen and explore other issues of gaseous hydrogen pipeline transportation in practical applications. Currently, some scholars have studied the feasibility of hydrogen blending in natural gas pipelines, and Academician Yi Baolian of the Chinese Academy of Engineering is also very optimistic about hydrogen blending in natural gas pipelines. However, in addition to addressing user applicability, hydrogen blending in natural gas pipelines also needs to study the influence of the hydrogen blending ratio on the flow state of the fluid in the pipeline and its supporting technologies, explore the adaptability of pipeline materials and facilities to hydrogen blending transportation, and improve the safety and economic benefits of hydrogen blending in natural gas pipelines.

　　4.3 Liquid Hydrogen Pipeline Transportation

　　Liquid hydrogen transportation, as the name suggests, is the high-density transportation of gaseous hydrogen under normal temperature and pressure through cooling and pressurization. In the selection of pipe materials for liquid hydrogen pipelines, to ensure the pipeline's insulation effect and the low-temperature performance of the pipe materials, materials such as stainless steel, aluminum alloy, titanium alloy, and composite materials with excellent hydrogen embrittlement resistance, good low-temperature performance, weldability, and corrosion resistance are mainly used. Currently, 300-series austenitic stainless steel is widely used in liquid hydrogen storage and transportation containers. The 300 m³ liquid hydrogen transport tanker at the Hainan Aerospace Launch Site uses 321 stainless steel as the container material. Compared with stainless steel, aluminum alloy has more advantages in terms of quality, formability, welding performance, and corrosion resistance, and has also been applied in domestic and foreign rocket liquid hydrogen storage tanks.

　　Due to limitations in the construction cost of liquid hydrogen pipelines and backward technology, liquid hydrogen is almost impossible to achieve long-distance, large-scale pipeline transportation. Currently, long-distance transportation of liquid hydrogen can only be achieved by placing liquid hydrogen in special storage devices such as highly low-temperature insulated hydrogen storage tanks and liquid hydrogen transport tank trucks, relying on tank trucks, trains, and barges for transportation. There are no examples of long-distance liquid hydrogen pipeline transportation. How to reduce the construction cost of liquid hydrogen transportation pipelines and achieve rapid pre-cooling of long-distance liquid hydrogen pipelines are the main problems of current low-temperature liquid hydrogen pipeline transportation. Low-temperature liquid hydrogen pipeline transportation is promising in the future. However, at present, it is more suitable for scenarios where liquid hydrogen is used as a high-energy fuel in aerospace, etc., as a short-distance, point-to-point transportation method, and cannot be considered a reasonable choice for large-scale hydrogen pipeline transportation.

 

 

　　 Conclusion

　　The development of hydrogen energy is an important direction for the global energy revolution, an important means to achieve the goals of "carbon peaking and carbon neutrality," and a strategic choice for the clean energy supply of the country. Countries around the world are promoting the rapid development of hydrogen energy, and increasing the layout of the hydrogen energy industry has become a consensus. A comparison of the development of hydrogen energy applications at home and abroad shows that China's hydrogen energy industry started late, and its core technologies lag behind those of foreign countries. China's national layout for the development of hydrogen energy needs to be improved, and a national hydrogen energy market system needs to be built as soon as possible. One of the key links in the development of the hydrogen energy industry is the storage and transportation of hydrogen energy. Hydrogen storage technology is an important bottleneck restricting the large-scale industrial development of hydrogen energy. Hydrogen transportation technology is an important means to connect hydrogen resources to the hydrogen market and is a major link in reducing the cost of hydrogen use.

　　Source: National Pipe Network Group Engineering and Technology Innovation Co., Ltd., Maid Engineering Editing

　　Author: Cui Zhenying

　　Editor: FAN | Reviewer: HOHO