Most compressors in the hydrogen industry rely on classic positive displacement technology. Screw compressors and piston compressors work according to this principle. In a piston compressor, hydrogen gas is drawn into a cylinder and compressed by a piston. The piston’s movement is generated via a crankshaft driven by an electric motor. Hydraulically driven, dry-running piston compressors also use the movement of a piston to compress gas. A special design of the piston compressor is the diaphragm compressor. In this type, the gas is displaced by a set of at least three metallic diaphragms. These are moved by an oil column that is driven by the piston movement of a classic gear unit.
Final pressure is the central criterion for compressor selection Despite the similar basic principle of these compressor types, their fields of application in the hydrogen industry differ. The most important points are the available suction pressure, the desired final pressure for the hydrogen application and the quantity of hydrogen gas and thus the capacity of the electrolysis. Further factors are the required purity of the hydrogen or the operating mode intended for the compressor.
For pure hydrogen, hydrogen lines and pipelines with a typical operating pressure of between 25 bar and 100 bar are at the lower end. In tanker vehicles, so-called tube trailers, hydrogen is transported at pressures of 200 to 380 bar, and newer developments with TYPE IV CFRP tanks are even moving towards 680 bar. For the storage of large quantities of hydrogen, salt deposits are being developed or tube bundle storage systems are used. Here, according to Neuman & Esser, storage pressures range from 200 bar to 700 bar. The refueling of cars also requires pressures of up to 700 bar.
In derivative production, methanol synthesis in the low-pressure process at 50 bar is quite modest in terms of pressure. If the hydrogen is to be used in ammonia synthesis, the Haber-Bosch process requires pressures of between 250 and 350 bar. In the future, hydrogen will play a major role in the production of sustainable aviation fuels (SAF), according to Jan Gehrmann, product manager at compressor manufacturer Aerzen. Here too, different pressures are required depending on the process.
Screw compressors for large volume flows The starting point of compression is the suction pressure provided by the electrolysis. Low-pressure electrolysis operates in the range of atmospheric pressure up to a slight overpressure. A large electrolysis plant with a capacity of 500 megawatts, for example, generates a volume flow of up to 120,000 m3 of hydrogen gas per hour. In such a case, one or several parallel screw compressors can be used. According to Gehrmann, screw compressors can easily handle such large volumes. In a typical application, the screw compressor compresses the hydrogen gas from 1 to 6 bar and reduces the volume flow from, for example, 100,000 to around 16,500 m3 per hour. This volume flow can then be further compressed by a piston compressor in the following stage. “The screw compressor is always only a pre-compressor,” says Gehrmann.
Compressors for higher pressures Piston compressors provide higher pressures. According to Neuman & Esser, oil-free piston compressors in the latest development stage achieve final pressures of 300 to 500 bar. With correspondingly large pistons, high delivery capacities can be achieved. It is also possible to integrate several pressure stages in one machine, so that high final pressures can be achieved even at low suction pressures. Hydraulically driven piston compressors achieve very high final pressures of up to 4,000 bar. Diaphragm compressors can also reach final pressures of up to 3,000 bar. Neuman & Esser manufactures large piston compressors such as those needed for the storage of hydrogen in salt caverns or for feeding hydrogen into pipelines. The company’s portfolio also includes diaphragm compressors and hydraulically driven piston compressors.
Oil-free compression chambers for high-purity hydrogen In the diaphragm compressor, the gas chamber is hermetically separated by the metallic, gas-impermeable diaphragms. This prevents any contamination of the hydrogen in the compressor. In dry-running piston compressors, contamination of the gas by lubricating oil is also ruled out, however, dynamic seals are required, in which a certain amount of abrasion is unavoidable.
Aerzen also produces oil-free compressors for the hydrogen industry. A special feature of this type of compressor is that the compression can be cooled by injecting small quantities of water. This reduces the outlet temperature and benefits the maximum possible pressure increase. According to Gehrmann, the water injection is not a problem, because the hydrogen gas from the electrolysis is already saturated with water vapor anyway and because a drying unit is usually installed downstream.
Compressors for volatile power sources In the production of hydrogen from renewable energy sources, it is important that the compressors can react to the volatility of the energy availability. Smaller sizes of piston compressors and especially the hydraulically driven piston compressors are more suitable for start-stop operation. Large compressors are designed for continuous operation. If the delivered gas volume has to be adjusted for process-related reasons, this can, according to Neuman & Esser, be realized by valve lifting or bypass operation. In diaphragm compressors, the delivery rate can be regulated by frequency converter operation.
To account for planned or unplanned downtimes, compressors are usually designed with redundancy. Typical redundant configurations for piston compressors are, according to the manufacturer Neuman & Esser, 2 × 100 %, 2 × 50 % or also 3 × 50 %. The 2 × 100 % configuration ensures complete redundancy; with 2 × 50 %, a certain redundancy is available in the event of unexpected shutdowns, in order to transport half of the possible gas volume flow. With 3 × 50 %, at least half of the delivery capacity can be maintained during maintenance and a simultaneous failure of another compressor.
With the ramp-up of the hydrogen industry, a strong increase in demand for compressors is to be expected. Many manufacturers are already prepared for this today and offer project-specific customized systems.
